P. Salvini

The layout of the RPCs, used in the Argo-YBJ experiment to image with a high space-time granularity the atmospheric shower, is described in this paper. The detector has been assembled to provide both digital and analog informations in order to cover a wide particle density range with a time accuracy of 1 ns. The experimental results obtained operating the chambers in streamer mode at sea level with a standard gas mixture are presented.

The measurement of the cosmic ray energy spectrum, in particular for individual species of nuclei, is an important tool to investigate cosmic ray production and propagation mechanisms. The determination of the ``knees'' in the spectra of different species remains one of the main challenges in cosmic ray physics. In fact, experimental results are still conflicting. In this paper we report a measurement of the mixed proton and helium energy spectrum, obtained with the combined data of the ARGO-YBJ experiment and a wide field of view Cherenkov telescope, a prototype of the future LHAASO experiment. By means of a multiparameter technique, we have selected a high-purity proton plus helium sample. The reconstructed energy resolution is found to be about 25% throughout the investigated energy range from 100 TeV to 3 PeV, with a systematic uncertainty in the absolute energy scale of 9.7%. The found energy spectrum can be fitted with a broken power-law function, with a break at the energy ${\mathrm{E}}_{k}=700\ifmmode\pm\else\textpm\fi{}230(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}70(\mathrm{sys})\text{ }\text{ }\mathrm{TeV}$, where the spectral index changes from $\ensuremath{-}2.56\ifmmode\pm\else\textpm\fi{}0.05$ to $\ensuremath{-}3.24\ifmmode\pm\else\textpm\fi{}0.36$. The statistical significance of the observed spectral break is 4.2 standard deviations.

The events recorded by ARGO-YBJ in more than five years of data collection have been analyzed to determine the diffuse gamma-ray emission in the Galactic plane at Galactic longitudes 25° < l < 100° and Galactic latitudes . The energy range covered by this analysis, from ∼350 GeV to ∼2 TeV, allows the connection of the region explored by Fermi with the multi-TeV measurements carried out by Milagro. Our analysis has been focused on two selected regions of the Galactic plane, i.e., 40° < l < 100° and 65° < l < 85° (the Cygnus region), where Milagro observed an excess with respect to the predictions of current models. Great care has been taken in order to mask the most intense gamma-ray sources, including the TeV counterpart of the Cygnus cocoon recently identified by ARGO-YBJ, and to remove residual contributions. The ARGO-YBJ results do not show any excess at sub-TeV energies corresponding to the excess found by Milagro, and are consistent with the predictions of the Fermi model for the diffuse Galactic emission. From the measured energy distribution we derive spectral indices and the differential flux at 1 TeV of the diffuse gamma-ray emission in the sky regions investigated.

The extended TeV gamma-ray source ARGO J2031+4157 (or MGRO J2031+41) is positionally consistent with the Cygnus Cocoon discovered by $Fermi$-LAT at GeV energies in the Cygnus superbubble. Reanalyzing the ARGO-YBJ data collected from November 2007 to January 2013, the angular extension and energy spectrum of ARGO J2031+4157 are evaluated. After subtracting the contribution of the overlapping TeV sources, the ARGO-YBJ excess map is fitted with a two-dimensional Gaussian function in a square region of $10^{\circ}\times 10^{\circ}$, finding a source extension $\sigma_{ext}$= 1$^{\circ}$.8$\pm$0$^{\circ}$.5. The observed differential energy spectrum is $dN/dE =(2.5\pm0.4) \times 10^{-11}(E/1 TeV)^{-2.6\pm0.3}$ photons cm$^{-2}$ s$^{-1}$ TeV$^{-1}$, in the energy range 0.2-10 TeV. The angular extension is consistent with that of the Cygnus Cocoon as measured by $Fermi$-LAT, and the spectrum also shows a good connection with the one measured in the 1-100 GeV energy range. These features suggest to identify ARGO J2031+4157 as the counterpart of the Cygnus Cocoon at TeV energies. The Cygnus Cocoon, located in the star-forming region of Cygnus X, is interpreted as a cocoon of freshly accelerated cosmic rays related to the Cygnus superbubble. The spectral similarity with Supernova Remnants indicates that the particle acceleration inside a superbubble is similar to that in a SNR. The spectral measurements from 1 GeV to 10 TeV allows for the first time to determine the possible spectrum slope of the underlying particle distribution. A hadronic model is adopted to explain the spectral energy distribution.

The ARGO-YBJ detector is an extensive air shower array that has been used to monitor the northern $\gamma$-ray sky at energies above 0.3 TeV from 2007 November to 2013 January. In this paper, we present the results of a sky survey in the declination band from $-10^{\circ}$ to $70^{\circ}$, using data recorded over the past five years. With an integrated sensitivity ranging from 0.24 to $\sim$1 Crab units depending on the declination, six sources have been detected with a statistical significance greater than 5 standard deviations. Several excesses are also reported as potential $\gamma$-ray emitters. The features of each source are presented and discussed. Additionally, $95\%$ confidence level upper limits of the flux from the investigated sky region are shown. Specific upper limits for 663 GeV $\gamma$-ray AGNs inside the ARGO-YBJ field of view are reported. The effect of the absorption of $\gamma$-rays due to the interaction with extragalactic background light is estimated.

Measuring the anisotropy of the arrival direction distribution of cosmic rays provides important information on the propagation mechanisms and the identification of their sources.In fact, the flux of cosmic rays is thought to be dependent on the arrival direction only due to the presence of nearby cosmic ray sources or particular magnetic-field structures.Recently, the observation of unexpected excesses at TeV energy down to an angular scale as narrow as $10 raised the possibility that the problem of the origin of Galactic cosmic rays may be addressed by studying the anisotropy.The ARGO-YBJ experiment is a full-coverage extensive air showers array, sensitive to cosmic rays with the energy threshold of a few hundred GeV.Searching for small-size deviations from the isotropy, the ARGO-YBJ Collaboration explored the declination region $ À20 -80 , making use of about 3:7 Â 10 11 events collected from November 2007 to May 2012.In this paper, the detection of different significant (up to 13 standard deviations) medium-scale anisotropy regions in the arrival directions of cosmic rays is reported.The observation was performed with unprecedented detail.The relative excess intensity with respect to the

This paper reports on the measurement of the large-scale anisotropy in the distribution of cosmic-ray arrival directions using the data collected by the air shower detector ARGO-YBJ from 2008 January to 2009 December,during the minimum of solar activity between cycles 23 and 24. In this period, more than 200 billion showers were recorded with energies between 1 and 30 TeV. The observed two-dimensional distribution of cosmic rays is characterized by two wide regions of excess and deficit, respectively, both of relative intensity 0.001 with respect to a uniform flux, superimposed on smaller size structures. The harmonic analysis shows that the large-scale cosmic-ray relative intensity as a function of R.A. can be described by the first and second terms of a Fouries series. The high event statistics allow the study of the energy dependence of the anistropy, showing that the amplitude increases with energy, with a maximum intensity at 10 TeV, and then decreases while the phase slowly shifts toward lower values of R.A. with increasing energy. The ARGO-YBJ data provide accurate observations over more than a decade of energy around this feature of the anisotropy spectrum.

We report on the extensive multi-wavelength observations of the blazar Markarian 421 (Mrk 421) covering radio to gamma-rays, during the 4.5 year period of ARGO-YBJ and Fermi common operation time, from August 2008 to February 2013. In particular, thanks to the ARGO-YBJ and Fermi data, the whole energy range from 100 MeV to 10 TeV is covered without any gap. In the observation period, Mrk 421 showed both low and high activity states at all wavebands. The correlations among flux variations in different wavebands were analyzed. Seven large flares, including five X-ray flares and two GeV gamma-ray flares with variable durations (3-58 days), and one X-ray outburst phase were identified and used to investigate the variation of the spectral energy distribution with respect to a relative quiescent phase. During the outburst phase and the seven flaring episodes, the peak energy in X-rays is observed to increase from sub-keV to few keV. The TeV gamma-ray flux increases up to 0.9-7.2 times the flux of the Crab Nebula. The behavior of GeV gamma-rays is found to vary depending on the flare, a feature that leads us to classify flares into three groups according to the GeV flux variation. Finally, the one-zone synchrotron self-Compton model was adopted to describe the emission spectra. Two out of three groups can be satisfactorily described using injected electrons with a power-law spectral index around 2.2, as expected from relativistic diffuse shock acceleration, whereas the remaining group requires a harder injected spectrum. The underlying physical mechanisms responsible for different groups may be related to the acceleration process or to the environment properties.

ARGO-YBJ is an air shower detector array with a fully covered layer of resistive plate chambers. It is operated with a high duty cycle and a large field of view. It continuously monitors the northern sky at energies above 0.3 TeV. In this paper, we report a long-term monitoring of Mrk 421 over the period from 2007 November to 2010 February. This source was observed by the satellite-borne experiments Rossi X-ray Timing Explorer and Swift in the X-ray band. Mrk 421 was especially active in the first half of 2008. Many flares are observed in both X-ray and γ-ray bands simultaneously. The γ-ray flux observed by ARGO-YBJ has a clear correlation with the X-ray flux. No lag between the X-ray and γ-ray photons longer than 1 day is found. The evolution of the spectral energy distribution is investigated by measuring spectral indices at four different flux levels. Hardening of the spectra is observed in both X-ray and γ-ray bands. The γ-ray flux increases quadratically with the simultaneously measured X-ray flux. All these observational results strongly favor the synchrotron self-Compton process as the underlying radiative mechanism.

Cosmic rays are hampered by the Moon and a deficit in its direction is expected (the so-called Moon shadow). The Moon shadow is an important tool to determine the performance of an air shower array. Indeed, the westward displacement of the shadow center, due to the bending effect of the geomagnetic field on the propagation of cosmic rays, allows the setting of the absolute rigidity scale of the primary particles inducing the showers recorded by the detector. In addition, the shape of the shadow permits to determine the detector point spread function, while the position of the deficit at high energies allows the evaluation of its absolute pointing accuracy. In this paper we present the observation of the cosmic ray Moon shadowing effect carried out by the ARGO-YBJ experiment in the multi-TeV energy region with high statistical significance (55 standard deviations). By means of an accurate Monte Carlo simulation of the cosmic rays propagation in the Earth-Moon system, we have studied separately the effect of the geomagnetic field and of the detector point spread function on the observed shadow. The angular resolution as a function of the particle multiplicity and the pointing accuracy have been obtained. The primary energy of detected showers has been estimated by measuring the westward displacement as a function of the particle multiplicity, thus calibrating the relation between shower size and cosmic ray energy. The stability of the detector on a monthly basis has been checked by monitoring the position and the deficit of the Moon shadow. Finally, we have studied with high statistical accuracy the shadowing effect in the day/''night'' time looking for possible effect induced by the solar wind.

We report the observation of TeV gamma-rays from the Cygnus region using the ARGO-YBJ data collected from 2007 November to 2011 August. Several TeV sources are located in this region including the two bright extended MGRO J2019+37 and MGRO J2031+41. According to the Milagro data set, at 20 TeV MGRO J2019+37 is the most significant source apart from the Crab Nebula. No signal from MGRO J2019+37 is detected by the ARGO-YBJ experiment, and the derived flux upper limits at 90% confidence level for all the events above 600 GeV with medium energy of 3 TeV are lower than the Milagro flux, implying that the source might be variable and hard to be identified as a pulsar wind nebula. The only statistically significant (6.4 standard deviations) gamma-ray signal is found from MGRO J2031+41, with a flux consistent with the measurement by Milagro.

As one of the brightest active blazars in both X-ray and very high energy $\gamma$-ray bands, Mrk 501 is very useful for physics associated with jets from AGNs. The ARGO-YBJ experiment is monitoring it for $\gamma$-rays above 0.3 TeV since November 2007. Starting from October 2011 the largest flare since 2005 is observed, which lasts to about April 2012. In this paper, a detailed analysis is reported. During the brightest $\gamma$-rays flaring episodes from October 17 to November 22, 2011, an excess of the event rate over 6 $\sigma$ is detected by ARGO-YBJ in the direction of Mrk 501, corresponding to an increase of the $\gamma$-ray flux above 1 TeV by a factor of 6.6$\pm$2.2 from its steady emission. In particular, the $\gamma$-ray flux above 8 TeV is detected with a significance better than 4 $\sigma$. Based on time-dependent synchrotron self-Compton (SSC) processes, the broad-band energy spectrum is interpreted as the emission from an electron energy distribution parameterized with a single power-law function with an exponential cutoff at its high energy end. The average spectral energy distribution for the steady emission is well described by this simple one-zone SSC model. However, the detection of $\gamma$-rays above 8 TeV during the flare challenges this model due to the hardness of the spectra. Correlations between X-rays and $\gamma$-rays are also investigated.

The proton-air cross section in the energy range 1--100 TeV has been measured by the ARGO-YBJ cosmic ray experiment. The analysis is based on the primary cosmic ray flux attenuation for different atmospheric depths (i.e. zenith angles) and exploits the detector capabilities of selecting the shower development stage by means of hit multiplicity, density and lateral profile measurements at ground. The effects of shower fluctuations, the contribution of heavier primaries and the uncertainties of the hadronic interaction models, have been taken into account. The results have been used to estimate the total proton-proton cross section at center-of-mass energies between 70 and 500 GeV, where no accelerator data are currently available.

The ARGO-YBJ experiment has been taking data for nearly 2 years. In order to monitor continuously the performance of the Resistive Plate Chamber detectors and to study the daily temperature effects on the detector performance, a cosmic ray muon telescope was setup near the carpet detector array in the ARGO-YBJ laboratory. Based on the measurements performed using this telescope, it is found that, at the actual operating voltage of 7.2 kV, the temperature effect on the RPC time resolution is about 0.04ns/∘C and on the particle detection efficiency is about 0.03%/∘C. Based on these figures we conclude that the environmental effects do not affect substantially the angular resolution of the ARGO-YBJ detector.

ð5 Ä 200Þ TeV is reported.The method exploited to analyze the experimental data is based on a Bayesian procedure.The measured intensities of the light component are consistent with the recent CREAM results and higher than that obtained adding the proton and helium spectra reported by the RUNJOB experiment.

In 2008, the blazar Markarian 421 entered a very active phase and was one of the brightest sources in the sky at TeV energies, showing frequent flaring episodes. Using the data of ARGO-YBJ, a full coverage air shower detector located at Yangbajing (4300 m a.s.l., Tibet), we monitored the source at gamma-ray energies E>0.3 TeV during the whole year. The observed flux was variable, with the strongest flares in March and June, in correlation with X-ray enhanced activity. While during specific episodes the TeV flux could be several times larger than the Crab Nebula one, the average emission from day 41 to 180 was almost twice the Crab level, with an integral flux of (3.6 ± 0.6) × 10−11 photons cm−2 s−1 for energies E>1 TeV, and decreased afterward. This Letter concentrates on the flares that occurred in the first half of June. This period has been deeply studied from optical to 100 MeV gamma rays, and partially up to TeV energies, since the moonlight hampered the Cherenkov telescope observations during the most intense part of the emission. Our data complete these observations, with the detection of a signal with a statistical significance of 3.8 standard deviations on June 11–13, corresponding to a gamma-ray flux about 6 times larger than the Crab one above 1 TeV. The reconstructed differential spectrum, corrected for the intergalactic absorption, can be represented by a power law with an index α = −2.1+0.7−0.5 extending up to several TeV. The spectrum slope is fully consistent with previous observations reporting a correlation between the flux and the spectral index, suggesting that this property is maintained in different epochs and characterizes the source emission processes.

The extended gamma-ray source MGRO J1908+06, discovered by the Milagro air shower detector in 2007, has been observed for ∼4 years by the ARGO-YBJ experiment at TeV energies, with a statistical significance of 6.2 standard deviations. The peak of the signal is found at a position consistent with the pulsar PSR J1907+0602. Parameterizing the source shape with a two-dimensional Gauss function, we estimate an extension of σext = 049 ± 022, which is consistent with a previous measurement by the Cherenkov Array H.E.S.S. The observed energy spectrum is dN/dE = 6.1 ± 1.4 × 10−13 (E/4 TeV)−2.54 ± 0.36 photons cm−2 s−1 TeV−1, in the energy range of ∼1–20 TeV. The measured gamma-ray flux is consistent with the results of the Milagro detector, but is ∼2–3 times larger than the flux previously derived by H.E.S.S. at energies of a few TeV. The continuity of the Milagro and ARGO-YBJ observations and the stable excess rate observed by ARGO-YBJ and recorded in four years of data support the identification of MGRO J1908+06 as the steady powerful TeV pulsar wind nebula of PSR J1907+0602, with an integrated luminosity over 1 TeV ∼ 1.8 times the luminosity of the Crab Nebula.

We report on a measurement of thermal neutrons, generated by the hadronic component of extensive air showers (EAS), by means of a small array of EN-detectors developed for the PRISMA project (PRImary Spectrum Measurement Array), novel devices based on a compound alloy of ZnS(Ag) and 6LiF. This array has been operated within the ARGO-YBJ experiment at the high altitude Cosmic Ray Observatory in Yangbajing (Tibet, 4300 m a.s.l.). Due to the tight correlation between the air shower hadrons and thermal neutrons, this technique can be envisaged as a simple way to estimate the number of high energy hadrons in EAS. Coincident events generated by primary cosmic rays of energies greater than 100 TeV have been selected and analyzed. The EN-detectors have been used to record simultaneously thermal neutrons and the air shower electromagnetic component. The density distributions of both components and the total number of thermal neutrons have been measured. The correlation of these data with the measurements carried out by ARGO-YBJ confirms the excellent performance of the EN-detector.

The ARGO-YBJ experiment is a full-coverage air shower detector located at the Yangbajing Cosmic Ray Observatory (Tibet, People's Republic of China, 4300 m a.s.l.). The high altitude, combined with the full-coverage technique, allows the detection of extensive air showers in a wide energy range and offer the possibility of measuring the cosmic ray proton plus helium spectrum down to the TeV region, where direct balloon/space-borne measurements are available. The detector has been in stable data taking in its full configuration from November 2007 to February 2013. In this paper the measurement of the cosmic ray proton plus helium energy spectrum is presented in the region 3-300 TeV by analyzing the full collected data sample. The resulting spectral index is $\gamma = -2.64 \pm 0.01$. These results demonstrate the possibility of performing an accurate measurement of the spectrum of light elements with a ground based air shower detector.

The p̄p total annihilation cross section has been measured, with the Obelix apparatus at LEAR, at ten values of the antiproton incident momentum between 43 and 175 MeV/c. The values of the cross section show that the well known 1p behaviour of the annihilation cross section is drastically modified at very low momenta, which demonstrates the important role of the Coulomb force in low energy p̄p interaction. Moreover, they do not present any explicit resonant behaviour. Finally, when compared to potential model calculations, the data suggest that the percentage of P-wave in p̄p interaction around 50 MeV/c antiproton incident momentum is less than 5%.

We report stopping powers of hydrogen and helium for antiprotons of kinetic energies ranging from about 0.5 keV to 1.1 MeV. The Barkas effect, i.e., a difference in the stopping power for antiprotons and protons of the same energy in the same material, shows up clearly in either of the gases. Moreover, below \ensuremath{\approx}0.5 keV there is indirect evidence for an increase of the antiproton stopping power. This "nuclear" effect, i.e., energy losses in quasimolecular interactions, shows up in fair agreement with theoretical predictions.

The ARGO-YBJ air shower detector monitored the Crab Nebula gamma ray emission from 2007 November to 2013 February. The integrated signal, consisting of $\sim$3.3 $\times$ 10$^5$ events,reached the statistical significance of 21.1 standard deviations. The obtained energy spectrum in the energy range 0.3-20 TeV can be described by a power law function dN/dE = I$_0$ (E / 2 TeV)$^{-\alpha}$, with a flux normalization I$_0$ = (5.2 $\pm$ 0.2) $\times$ 10$^{-12}$ photons cm$^{-2}$ s$^{-1}$ TeV$^{-1}$ and $\alpha$ = 2.63 $\pm$ 0.05, corresponding to an integrated flux above 1 TeV of 1.97 $\times$ 10$^{-11}$ photons cm$^{-2}$ s$^{-1}$. The systematic error is estimated to be less than 30$\%$ for the flux normalization and 0.06 for the spectral index. Assuming a power law spectrum with an exponential cutoff dN/dE = I$_0$ (E / 2 TeV)$^{-\alpha}$ $\exp$ (-E / E$_{cut}$), the lower limit of the cutoff energy E$_{cut}$ is 12 TeV, at 90$\%$ confidence level. Our extended dataset allows the study of the TeV emission over long timescales. Over five years, the light curve of the Crab Nebula in 200-day bins is compatible with a steady emission with a probability of 7.3 $\times$ 10$^{-2}$. A correlated analysis with Fermi-LAT data over $\sim$4.5 years using the light curves of the two experiments gives a Pearson correlation coefficient $r$ = 0.56 $\pm$ 0.22. Concerning flux variations on timescales of days, a "blind" search for flares with a duration of 1-15 days gives no excess with a significance higher than four standard deviations. The average rate measured by ARGO-YBJ during the three most powerful flares detected by Fermi-LAT is 205 $\pm$ 91 photons day$^{-1}$, consistent with the average value of 137 $\pm$ 10 day$^{-1}$.

The energy spectrum of cosmic Hydrogen and Helium nuclei has been measured, below the so-called "knee", by using a hybrid experiment with a wide field-of-view Cherenkov telescope and the Resistive Plate Chamber (RPC) array of the ARGO-YBJ experiment at 4300 m above sea level. The Hydrogen and Helium nuclei have been well separated from other cosmic ray components by using a multi-parameter technique. A highly uniform energy resolution of about 25% is achieved throughout the whole energy range (100 TeV - 700 TeV). The observed energy spectrum is compatible with a single power law with index gamma=-2.63+/-0.06.

Abstract This paper reports on the observation of the sidereal large-scale anisotropy of cosmic rays using data collected by the ARGO-YBJ experiment over 5 years (2008–2012). This analysis extends previous work limited to the period from 2008 January to 2009 December, near the minimum of solar activity between cycles 23 and 24. With the new data sample, the period of solar cycle 24 from near minimum to maximum is investigated. A new method is used to improve the energy reconstruction, allowing us to cover a much wider energy range, from 4 to 520 TeV. Below 100 TeV, the anisotropy is dominated by two wide regions, the so-called “tail-in” and “loss-cone” features. At higher energies, a dramatic change of the morphology is confirmed. The yearly time dependence of the anisotropy is investigated. Finally, no noticeable variation of cosmic-ray anisotropy with solar activity is observed for a median energy of 7 TeV.

The pp total annihilation cross section has been measured at four values of the p̄ incident momentum, between 70 MeV/c and 38 MeV/c, with the Obelix apparatus at LEAR. The new measurements are in agreement with the trend of previous measurements of the pp total annihilation cross section at low energy, performed by the Obelix experiment [A. Bertin et al., Phys. Lett. B 369 (1996) 77; A. Benedettini et al., Nucl. Phys. B (Proc. Suppl.) 56A (1997) 58], as well as with a fit of the latter data based on a low energy expansion of the scattering amplitude [J. Carbonell, K.V. Protasov, A. Zenoni, Phys. Lett. B 397 (1997) 345]. The departure of the annihilation cross section from a smooth behaviour, suggested by a previous measurement of the cross section around 44 MeV/c [A. Bertin et al., Phys. Lett. B 369 (1996) 77], is not confirmed by the new data.

The ARGO-YBJ experiment is mainly devoted to search for astronomical gamma sources. The arrival direction of air showers is reconstructed thanks to the times measured by the pixels of the detector. Therefore, the timing calibration of the detector pixels is crucial in order to get the best angular resolution and pointing accuracy. Because of the large number of pixels a hardware timing calibration is practically impossible. Therefore an off-line software calibration has been adopted. Here, the details of the procedure and the results are presented.

We report the observation of a very high energy γ-ray source whose position is coincident with HESS J1841−055. This source has been observed for 4.5 years by the ARGO-YBJ experiment from 2007 November to 2012 July. Its emission is detected with a statistical significance of 5.3 standard deviations. Parameterizing the source shape with a two-dimensional Gaussian function, we estimate an extension , which is consistent with the HESS measurement. The observed energy spectrum is dN/dE = (9.0 ± 1.6) × 10−13(E/5 TeV)−2.32 ± 0.23 photons cm−2 s−1 TeV−1, in the energy range 0.9–50 TeV. The integral γ-ray flux above 1 TeV is 1.3 ± 0.4 Crab, which is 3.2 ± 1.0 times the flux derived by HESS. The differences in the flux determination between HESS and ARGO-YBJ and possible counterparts at other wavelengths are discussed.

The ARGO-YBJ experiment has been in stable data taking from November 2007 till February 2013 at the YangBaJing Cosmic Ray Observatory (4300 m a.s.l.). The detector consists of a single layer of Resistive Plate Chambers (RPCs) (6700 m2) operated in streamer mode. The signal pick-up is obtained by means of strips facing one side of the gas volume. The digital readout of the signals, while allows a high space–time resolution in the shower front reconstruction, limits the measurable energy to a few hundred TeV. In order to fully investigate the 1–10 PeV region, an analog readout has been implemented by instrumenting each RPC with two large size electrodes facing the other side of the gas volume. Since December 2009 the RPC charge readout has been in operation on the entire central carpet (∼5800 m2). In this configuration the detector is able to measure the particle density at the core position where it ranges from tens to many thousands of particles per m2. Thus ARGO-YBJ provides a highly detailed image of the charge component at the core of air showers. In this paper we describe the analog readout of RPCs in ARGO-YBJ and discuss both the performance of the system and the physical impact on the EAS measurements.

The ARGO-YBJ detector, located at the Yangbajing Cosmic Ray Laboratory (4300 m a. s. l., Tibet, China), was a "full coverage" air shower array. The high altitude location and the frequent occurrence of thunderstorms, made ARGO-YBJ suitable to study the effects of atmospheric electric fields (AEF) on secondary cosmic rays. By analyzing the data of the ARGO-YBJ detector recorded during thunderstorms, significant variations of the rate of detected showers have been observed. During 20 thunderstorm episodes in 2012, the variations of the shower rates (both increases and decreases of amplitudes up to a few percent) are found to be correlated to the intensity and polarity of the AEF, and strongly dependent on the primary zenith angle. To understand the observed behavior, Monte Carlo simulations have been performed with corsika and g4argo (a code based on geant4). We found that the data are well consistent with simulations, assuming the presence of a uniform electric field in a layer of thickness of 500 m in the atmosphere above the observation level. Due to the AEF accelerates/decelerates and deflects the secondary charged particles (mainly electrons and positrons) according to their charge, modifying the number and position of particles with energy exceeding the detector threshold. For the differences in electron and positron flux, spectrum, and lateral distribution, the AEF has an asymmetric effect on the shower particles, producing significant variations of the particle pattern on the ground, and, consequently, on the rate of detected showers, consistent with observations.2 MoreReceived 8 February 2022Accepted 5 July 2022DOI:https://doi.org/10.1103/PhysRevD.106.022008© 2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCosmic ray & astroparticle detectorsCosmic rays & astroparticlesGravitation, Cosmology & Astrophysics

The simultaneous measurement of the spatial coordinates and times of p\ifmmode\bar\else\textasciimacron\fi{}s annihilating at rest in a ${\mathrm{H}}_{2}$ target at very low density \ensuremath{\rho} (\ensuremath{\rho}/${\mathrm{\ensuremath{\rho}}}_{0}$${10}^{\mathrm{\ensuremath{-}}2}$, ${\mathrm{\ensuremath{\rho}}}_{0}$ being the STP density) gives the possibility of evaluating the behavior of the p\ifmmode\bar\else\textasciimacron\fi{} stopping power in ${\mathrm{H}}_{2}$ at low energies (below 120 keV). It is different from that of protons (the Barkas effect). Moreover, it is shown that a rise at low-energy values (\ensuremath{\lesssim}1 keV) is needed to agree with experimental data.

The φπ+/ωπ+ ratio from n¯p annihilations on a liquid hydrogen target, for n¯ momenta between 64 and 297 MeV/c, was measured using the OBELIX spectrometer at LEAR. The ratio R(ϕπ/ωπ)=σ(n¯p→ϕπ+)/σ(n¯p→ωπ+) turned out 0.110±0.015stat±0.006syst. Implications of this result on the OZI rule are discussed.

The ratio Rab between the cross sections for the annihilation of p on n and on p bound in the 4He nucleus at four p momenta (at rest, 192.8, 306.2 and 607.7 MeV/c) has been obtained by the analysis of p4He annihilation events detected with a self-shunted streamer chamber exposed to the LEAR p beams. Rab increases from 0.42 ± 0.05 at rest up to 0.66 ± 0.09 at 607.7 MeV/c. The results are discussed in the light of Glauber theory analyses of p2H and p-nucleus data.

The Λ0 and K0s production cross sections have been measured at 607 MeV/c for the reaction p̄+20Ne→V0+X (V0=Λ, 0, K0s). The Λ0/K0s production ratio is found to be (2.3±0.7). The data are consistent with an evaporative character of the emission mechanism from a multinucleon system for Λ0 and from one nucleon for K0s. The rapidity distributions of negative tracks in K0x events are different from those in Λ0 events without V0.

Cosmic ray antiprotons provide an important probe to study the cosmic ray propagation in the interstellar space and to investigate the existence of dark matter. Acting the Earth-Moon system as a magnetic spectrometer, paths of primary antiprotons are deflected in the opposite sense with respect to those of the protons in their way to the Earth. This effect allows, in principle, the search for antiparticles in the direction opposite to the observed deficit of cosmic rays due to the Moon (the so-called `Moon shadow'). The ARGO-YBJ experiment, located at the Yangbajing Cosmic Ray Laboratory (Tibet, P.R. China, 4300 m a.s.l., 606 g/cm$^2$), is particularly effective in measuring the cosmic ray antimatter content via the observation of the cosmic rays shadowing effect due to: (1) good angular resolution, pointing accuracy and long-term stability; (2) low energy threshold; (3) real sensitivity to the geomagnetic field. Based on all the data recorded during the period from July 2006 through November 2009 and on a full Monte Carlo simulation, we searched for the existence of the shadow cast by antiprotons in the TeV energy region. No evidence of the existence of antiprotons is found in this energy region. Upper limits to the $\bar{p}/p$ flux ratio are set to 5 % at a median energy of 1.4 TeV and 6 % at 5 TeV with a confidence level of 90%. In the TeV energy range these limits are the lowest available.

The p̄D and p̄4He total annihilation cross sections have been measured with the Obelix apparatus at LEAR at, respectively, three and two values of the p̄ incident momentum between 70 MeV/c and 36 MeV/c. The values of the p̄D annihilation cross section, at such low energies, are in agreement with a surprising result obtained in a recent low statistics measurement of the antiprotonic deuterium Kα X-rays [M. Augsburger et al., Phys. Lett. B 461 (1999) 417] (see following article), concerning the unexpected narrow width of the antiprotonic deuterium 1s level.

First results on meson spectroscopy using antineutrons as projectiles are reported. Three-body (π+π+π−) and five-body (π+π+π+π−π−) final states following the annihilation of 100 to 290 MeV/c antineutrons on hydrogen were analyzed. The (π+π−) invariant mass spectrum in the three-body reaction is accounted for by the ϱ0 anf f2(1270) resonances, together with an enhancement around 1540 MeV, in the region of the AX (1565) and the f2(1515) resonances, whereas the best fit for the (π+π+π−π−) invariant mass in five-body events shows the already reported structure ζ(1480) and suggests the presence of a second structure around 1650 MeV.

The results of the analysis of 3940 (KKπ)ππ events from a sample of 18 millions pp annihilations in the liquid hydrogen target of the OBELIX spectrometer at LEAR (CERN) are presented. The presence of two pseudoscalar states at 1.416±0.002GeV/c2, with width 0.050±0.004GeV/c2, and at 1.46±0.01GeV/c2, with width 0.105±0.015GeV/c2, is established. The lighter mass resonance decays mainly to KKπ, possibly with final state interactions and a small contribution coming from a0π. The higher mass 0−+ state, which is seen for the first time in pp annihilation at rest, decays only to K∗K. Masses, widths and decay modes are in agreement with the analysis of JΨ radiative decay performed by the Mark III Collaboration. From the fit the G-parity was determined to be +1. The dominant three-body decay mode of the lighter pseudoscalar is observed for the first time in this measurement.

The p¯4He annihilation cross section averaged over the interval 40–50 MeV/c has been measured using a streamer chamber in a magnetic field. The measured value is 1342±250 mb. It agrees with a behaviour like 1/p of the annihilation cross section. Our result has been obtained at the lowest momentum achieved till now in measurements of antiproton annihilation in flight.

The production rate of3H in $$\bar p^4 $$ He annhilation between 0 and 600 MeV/c is deduced in an approximate way from that of3He, and the lower and upper limits for the production rate of p and2H are measured. The momentum distributions for the different particles are given.

The ARGO-YBJ experiment at YangBaJing in Tibet (4300 m a.s.l.) has been taking data with its full layout since October 2007. Here we present a few significant results obtained in gamma-ray astronomy and cosmic-ray physics. Emphasis is placed on the analysis of gamma-ray emission from point-like sources (Crab Nebula, MRK 421), on the preliminary limit on the antiproton/proton flux ratio, on the large-scale cosmic-ray anisotropy and on the proton–air cross-section. The performance of the detector is also discussed, and the perspectives of the experiment are outlined.

The Sun blocks cosmic-ray particles from outside the solar system, forming a detectable shadow in the sky map of cosmic rays detected by the ARGO-YBJ experiment in Tibet. Because the cosmic-ray particles are positively charged, the magnetic field between the Sun and the Earth deflects them from straight trajectories and results in a shift of the shadow from the true location of the Sun. Here, we show that the shift measures the intensity of the field that is transported by the solar wind from the Sun to the Earth.

Results obtained by the RPC ECOgas@GIF++ Collaboration, using Resistive Plate Chambers operated with new, eco-friendly gas mixtures, based on Tetrafluoropropene and carbon dioxide, are shown and discussed in this paper. Tests aimed to assess the performance of this kind of detectors in high-irradiation conditions, analogous to the ones foreseen for the coming years at the Large Hadron Collider experiments, were performed, and demonstrate a performance basically similar to the one obtained with the gas mixtures currently in use, based on Tetrafluoroethane, which is being progressively phased out for its possible contribution to the greenhouse effect. Long term aging tests are also being carried out, with the goal to demonstrate the possibility of using these eco-friendly gas mixtures during the whole High Luminosity phase of the Large Hadron Collider.

In view of the High Luminosity upgrade of the CERN LHC, the forward CMS Muon spectrometer will be extended with two new stations of improved Resistive Plate Chambers (iRPC) covering the pseudorapidity range from 1.8 to 2.4. Compared to the present RPC system, the gap thickness is reduced to lower the avalanche charge, and an innovative 2D strip readout geometry is proposed. These improvements will allow iRPC detector to cope with higher background rates. A new Front-End-Board (FEB) is designed to readout iRPC signals with a threshold as low as 30 fC and an integrated Time Digital Converter with a resolution of 30 ps. In addition, the communication bandwidth is significantly increased by using optical fibers. The history, final design, certification, and calibration of this FEB are presented.

The charge readout of Resistive Plate Chambers (RPCs) is implemented in the ARGO-YBJ experiment to measure the charged particle density of the shower front up to 104/m2, enabling the study of the primary cosmic rays with energies in the “knee” region. As the first time for RPCs being used this way, a telescope with RPCs and scintillation detectors is setup to calibrate the number of charged particles hitting a RPC versus its charge readout. Air shower particles are taken as the calibration beam. The telescope was tested at sea level and then moved to the ARGO-YBJ site for coincident operation with the ARGO-YBJ experiment. The charge readout shows good linearity with the particle density in the dynamic range (up to 200/m2).

A correlation between the secondary cosmic ray flux and the near-earth electric field intensity, measured during thunderstorms, has been found by analyzing the data of the ARGO-YBJ experiment, a full coverage air shower array located at the Yangbajing Cosmic Ray Laboratory (4300 m a. s. l., Tibet, China). The counting rates of showers with different particle multiplicities, have been found to be strongly dependent upon the intensity and polarity of the electric field measured during the course of 15 thunderstorms. In negative electric fields (i.e. accelerating negative charges downwards), the counting rates increase with increasing electric field strength. In positive fields, the rates decrease with field intensity until a certain value of the field EFmin (whose value depends on the event multiplicity), above which the rates begin increasing. By using Monte Carlo simulations, we found that this peculiar behavior can be well described by the presence of an electric field in a layer of thickness of a few hundred meters in the atmosphere above the detector, which accelerates/decelerates the secondary shower particles of opposite charge, modifying the number of particles with energy exceeding the detector threshold. These results, for the first time, give a consistent explanation for the origin of the variation of the electron/positron flux observed for decades by high altitude cosmic ray detectors during thunderstorms.

The measurement of different reactions of pd annihilation at rest in a gaseous target has been performed using the OBELIX spectrometer at LEAR (CERN). A strong deviation from the OZI-rule prediction was found from the measurement of the ratio R = φπωπ in two regions of proton momenta, P < 200 MeV/c and P > 400 MeV/c: R(φπ−ωπ−) = (133 ± 26) × 10−3 and (113 ± 30) × 10−3, respectively. These values are about 30 times greater than the theoretical prediction. For the first time the excitation of the †-resonance was observed among the final-state products of pd annihilation. The existence of a broad enhancement in the 4π invariant mass at m ≈ 1480 MeV, seen in previous experiments, was confirmed. A ≈ 100 MeV downward shift of the bump position, when the proton momentum increased up to P > 400 MeV/c, was also observed, while the positions of ω, ϱ and f2(1270) did not change with the proton momentum. The following branching ratios were measured: BR(pd → π−φp) = (6.62 ± 0.49) × 10−4, for P < 200 MeV/c; BR(pd → π−φp) = (0.95 ± 0.22) × 10−4, for P > 400 MeV/c; BR(pd → π−ωp) = (49.7 ± 8.9) × 10−4, for P < 200 MeV/c; BR(pd → π−ωp) = (8.38 ± 1.09) × 10−4, for P > 400 MeV/c; BR(pd → 2π−π+p) = (150 ± 6) × 10−4, for P < 200 MeV/c; BR(pd → 2π−π+p) = (16.6 ± 0.9) × 10−4, for P > 400 MeV/c; BR(pd → 3π−2π+p) = (326 ± 12) × 10−4, for P < 200 MeV/c; BR(pd → 3π−2π+p) = (44 ± 7) × 10−4, for P > 400 MeV/c; BR(pd → ΛK+π−) = (0.96 ± 0.19) × 10−4, for P > 400 MeV/c; BR(pd → ΛK+π−π0) = (3.5 ± 0.8) × 10−4, for P > 400 MeV/c; BR(pp → 2π−2π+) = (540 ± 20) × 10−4; BR(pp → 3π−3π+) = (251 ± 21) × 10−4.

The partial wave analysis of the reaction pp → π+π−π0 at rest has been performed for the first time by using high-statistics data sets collected in hydrogen targets at three different densities. The different mixtures of partial waves corresponding to our samples allow a reliable determination of each contribution. This technique is crucial in the identification of the exotic candidate f2(1565) which can be produced by 1S0, 3P1 and 3P2 initial states. The amplitude analysis was performed in the frame of the K-Matrix and P-Vector approach. It requires presence of the exotic candidates f0(1500) and f2(1565) with the following masses and total widths: (1449 ± 20)MeV, (114 ± 30)MeV and (1507 ± 15)MeV, (130 ± 20)MeV respectively. In addition to f0(980), f0(1300) f2(1270) and ϱ(770), a clear evidence of I = 1, JP = 1− signal, which we identify with ϱ(1450), is obtained.

The ARGO-YBJ (Astrophysical Radiation Ground-based Observatory at YangBaJing) experiment is designed for very high energy γ-astronomy and cosmic ray researches. Due to the full coverage of a large area (5600 m2) with resistive plate chambers at a very high altitude (4300 m a.s.l.), the ARGO-YBJ detector is used to search for transient phenomena, such as Gamma-ray bursts (GRBs). Because the ARGO-YBJ detector has a large field of view (∼2 sr) and is operated with a high duty cycle (>90%), it is well suited for GRB surveying and can be operated in searches for high energy GRBs following alarms set by satellite-borne observations at lower energies. In this paper, the sensitivity of the ARGO-YBJ detector for GRB detection is estimated. Upper limits to fluence with 99% confidence level for 26 GRBs inside the field of view from June 2006 to January 2009 are set in the two energy ranges 10–100 GeV and 10 GeV–1 TeV.

Abstract The ARGO-YBJ detector, located at the Yangbajing Cosmic Ray Laboratory (4300 m a. s. l., Tibet, China), was a “full coverage” (central carpet with an active area of ∼93%) air shower array dedicated to gamma-ray astronomy and cosmic-ray studies. The wide field of view (∼2 sr) and high duty cycle (&gt;86%), made ARGO-YBJ suitable to search for short and unexpected gamma-ray emissions like gamma-ray bursts (GRBs). Between 2007 November 6 and 2013 February 7, 156 satellite-triggered GRBs (24 of them with known redshift) occurred within the ARGO-YBJ field of view (zenith angle θ ≤ 45°). A search for possible emission associated with these GRBs has been made in the two energy ranges 10–100 GeV and 10–1000 GeV. No significant excess has been found in time coincidence with the satellite detections nor in a set of different time windows inside the interval of one hour after the bursts. Taking into account the EBL absorption, upper limits to the energy fluence at a 99% confidence level have been evaluated, with values ranging from ∼10 −5 erg cm −2 to ∼10 −1 erg cm −2 . The Fermi -GBM burst GRB 090902B, with a high-energy photon of 33.4 GeV detected by Fermi -LAT, is discussed in detail.

New measurements of the pp total annihilation cross-section at very low momenta (around 50 MeV/c) confirm the previous results obtained by the OBELIX collaboration. The measured values are in general agreement with a scattering length approximation of the annihilation cross-section where the pp scattering lengths are obtained from measurement of the protonium shifts and widths and which accounts for the Coulomb interaction. Partial annihilation cross-sections and annihilation frequencies for some channels have been measured too. The values of the total and partial cross sections at the lowest p momentum show an unexpected behaviour

Abstract A preliminary final state analysis of a sample of n p in-flight annihilations into three and five charged pions is presented. The partial-wave analysis of three-prong data confirms previous interpretations of the Ax ƒ 2 (1520) resonance, from p p annihilations at rest. Concerning five-prong events our analysis favours spin 0 for the already reported ζ (1480) state, but more refined calculations are underway. At high n momenta, the five-prong data show some evidence for a possible spin 2 state at 1640 MeV/c 2 decaying in ρ ° ρ °.

The study of the K±Kmiss0π∓π+π− channel (7016 events), from pp annihilations in a gaseous hydrogen target at NTP, with the OBELIX spectrometer at LEAR (CERN), is presented. A spin-parity analysis provides evidence for two axial vectors, one in the low KKπ mass region, the well established f1(1285), decaying to a0(980)π, the second, the 1++ component of the old puzzling El, the f1(1420), decaying mainly to K∗K, seen for the first time in pp annihilation at rest. The analysis confirms the dominant production of the pseudoscalar η(1405), decaying mainly to (Kπ)SK, and the existence of a second pseudoscalar in the same mass region, the η(1460), decaying mainly to K∗K.

The study of the I=1,JPC=0++ states with the OBELIX detector in the channel p̄p→K±K0Sπ∓ at three different target densities is reported. The data show the evidence for an extra scalar state with mass 1.29±0.01 GeV/c2 and width 0.080±0.005 GeV/c2.

A spin-parity analysis of a sample of p̄p→K+K−π+π−π0 annihilation events taken at rest in gaseous H2 at NTP is reported. The invariant mass spectrum of the K+K−π0 system shows the presence of the E/ι resonance pattern at 1.42 GeV. The analysis of this signal confirms the results obtained by the OBELIX Collaboration in previous works, namely: the existence of two pseudoscalar states at 1.413 ± 0.002 and 1.460±0.012 GeV with widths 0.051 ± 0.004 and 0.120±0.015 GeV, respectively. The p̄p system in the 3P1 wave shows also a signal corresponding to the axial vector f1(1420) decaying to K∗K̄ with mass 1.420±0.003 GeV and width 0.061±0.005 GeV. A study of the incoherent phase space background shows that its origin is mostly due to the reflection of a resonant state in the K+K−π+π− system. The isobar decomposition of this resonant state is mainly f0(1370)(ππ)S, its parameters are: JPC=0++, m=1.670±0.02 GeV and width 0.267±0.036 GeV. This signal can be probably identified with another decay mode of the f0(1710).

The elastic scattering data of p̄ on 12C, 16O and 40Ca nuclei at incident energies of 47 and 179 MeV are fitted with a Glauber model. The best fit procedure determines both the parameters of the p̄-neutron scattering amplitude and the degree of accuracy of the model. It is found that the theory holds well at 179 MeV, whereas it seems to be slightly inaccurate at 47 MeV. The comparison with the results on the p̄-neutron interaction obtained in other works is also performed.

The analysis is based upon p̄Ne annihilation events at rest and at 607 MeV/c detected with a self-shunted streamer chamber exposed to the LEAR beams. The main measured quantities are the following: charged-prong multiplicity, negative pion multiplicity; momentum, angular and rapidity distributions. General distributions are given as well as distributions as functions of charged prong multiplicity. A correlation is found between high multiplicity and depth of the annihilation point. The data are compared with recent theoretical analyses.

The search for Gamma Ray Burst (GRB) emission in the energy range 1-100 GeV in coincidence with the satellite detection has been carried out using the Astrophysical Radiation with Ground-based Observatory at YangBaJing (ARGO-YBJ) experiment. The high altitude location (4300 m a.s.l.), the large active surface ($\sim$ 6700 m$^2$ of Resistive Plate Chambers), the wide field of view ($\sim 2~$sr, limited only by the atmospheric absorption) and the high duty cycle ($>$ 86 %) make the ARGO-YBJ experiment particularly suitable to detect short and unexpected events like GRBs. With the scaler mode technique, i.e., counting all the particles hitting the detector with no measurement of the primary energy and arrival direction, the minimum threshold of $\sim$ 1 GeV can be reached, overlapping the direct measurements carried out by satellites. During the experiment lifetime, from December 17, 2004 to February 7, 2013, a total of 206 GRBs occurring within the ARGO-YBJ field of view (zenith angle $\theta$ $\le$ 45$^{\circ}$) have been analyzed. This is the largest sample of GRBs investigated with a ground-based detector. Two lightcurve models have been assumed and since in both cases no significant excess has been found, the corresponding fluence upper limits in the 1-100 GeV energy region have been derived, with values as low as 10$^{-5}~$erg cm$^{-2}$. The analysis of a subset of 24 GRBs with known redshift has been used to constrain the fluence extrapolation to the GeV region together with possible cutoffs under different assumptions on the spectrum.

The antineutron–proton total cross section has been measured in the low momentum range 50–400 MeV/c (below 100 MeV/c for the first time). The measurement was performed at LEAR (CERN) by the OBELIX experiment, thanks to its unique antineutron beam facility. A thick target transmission technique has been used. The measured total cross section shows an anomalous behaviour below 100 MeV/c. A dominance of the isospin I=0 channel over the I=1 one at low energy is clearly deduced.

The results of the spin-parity analysis of pp → 2π+2π− annihilations at very low momentum p (≈ 50 MeV/c) are reported. To describe the data the production of the ϱ, f2, a2 and a1 mesons and the presence of the ππ interaction in S-wave (the σ term) in the final state are necessary. The best fit solution requires also the presence of a ϱ′ state of mass and width M = 1.282 ± 0.037, Γ = 0.236 ± 0.036 GeV/c2 and of a heavy pion π(1300) of mass and width M = 1.275 ± 0.015, Γ = 0.218 ± 0.100 GeV/c2. The measured fraction of the annihilation cross section into 2π+2π− is (7.61 ± 0.35) · 10−2.

New data are reported on antiproton annihilations at rest with production of Λ and Ks0, using a streamer chamber with 3He, 4He and 20Ne as gas targets. The data include Λ, Ks0, ΛKs0 and Ks0KS0 production rates and momentum distributions, π− momentum spectra, mean numbers of charged particles generally and of negatively charged particles separately for different reaction channels. The yields are compared to simple combinatorial calculations based on the extreme assumptions of Λ production via B = 1 or via B = 0 (K̄ rescattering) annihilations. Λ and Ks0 momentum spectra are compared to simple model calculations where B = 0 and B = 1 annihilations with and without final-state interactions are considered. A review of existing data on Λ and Ks0 production is presented, showing the dependence on the p̄ momentum and on the mass number of the target.

Antiproton-3 He annihilation events at rest have been detected using a self-shunted streamer chamber. The ratio of the cross section for annihilation on neutrons and on protons has been measured (0.467 ± 0.035). It is compared with other results from annihilation on free nucleons, deuterium, 3He and 4He. The low value of the ratio seems to indicate a strong isospin dependence of the antinucleon-nucleon P-wave amplitude.

At LEAR of CERN the annihilation of antiprotons, stopping in 3He and 4He filling a self-shunted streamer chamber in a magnetic field, has been studied. The charged-particle multiplicities have been measured and the relative probabilities of π− production in p3He and p4He annihilation events have been obtained. The ratio between the p annihilation probability on the neutron and the proton for 3He and 4He has been deduced to be about half the value obtained for 2H in bubble chamber experiments. The analysis of the results shows that this difference cannot be due only to the pion final-state interaction or to the shadow effect of the nucleons of the nuclei. The probability of p annihilation at rest on a proton bound in the nucleus results to be twice as high as that on a bound neutron, showing the dominance of annihilation in the I = 0 isospin states.

The hadronic interaction of cosmic rays with solar atmosphere can produce high energy gamma-rays. The gamma-ray luminosity is correlated both with the flux of primary cosmic rays and the intensity of the solar magnetic field. The gamma-rays below 200 GeV have been observed by Fermi without any evident energy cutoff. The bright gamma-ray flux above 100 GeV has been detected only during solar minimum. The only available data in the TeV range come from the HAWC observations, however, outside the solar minimum. The ARGO-YBJ data set has been used to search for sub-TeV/TeV gamma-rays from the Sun during the solar minimum from 2008 to 2010, the same time period covered by the Fermi data. A suitable model containing the Sun shadow, solar disk emission, and inverse-Compton emission has been developed, and the chi-square minimization method was used to quantitatively estimate the disk gamma-ray signal. The result shows that no significant gamma-ray signal is detected and upper limits to the gamma-ray flux at 0.3–7 TeV are set at the 95% confidence level. In the low energy range these limits are consistent with the extrapolation of the Fermi-LAT measurements taken during solar minimum and are compatible with a softening of the gamma-ray spectrum below 1 TeV. They also provide an experimental upper bound to any solar disk emission at TeV energies. Models of dark matter annihilation via long-lived mediators predicting gamma-ray fluxes >10−7 GeV cm−2 s−1 below 1 TeV are ruled out by the ARGO-YBJ limits.

A confirming evidence for two pseudoscalar states with masses M1=1405±5, Γ1=50±4 MeV/c2 and M2=1500±10, Γ2=100±20 MeV/c2 is reported. The second pseudoscalar fills the nonet and leaves the first as an outsider supporting the hypothesis for its exotic nature.

The study of CPT invariance with the highest achievable precision in all particle sectors is of fundamental importance for physics. Equally important is the question of the gravitational acceleration of antimatter. In recent years, impressive progress has been achieved in capturing antiprotons in specially designed Penning traps, in cooling them to energies of a few milli-electron volts, and in storing them for hours in a small volume of space. Positrons have been accumulated in large numbers in similar traps, and low energy positron or positronium beams have been generated. Finally, steady progress has been made in trapping and cooling neutral atoms. Thus the ingredients to form antihydrogen at rest are at hand. Once antihydrogen atoms have been captured at low energy, spectroscopic methods can be applied to interrogate their atomic structure with extremely high precision and compare it to its normal matter counterpart, the hydrogen atom. Especially the 1S–2S transition, with a lifetime of the excited state of 122 msec and thereby a natural linewidth of 5 parts in 1016, offers in principle the possibility to directly compare matter and antimatter properties at a level of 1 part in 1018.

The use of low-density hydrogen targets is demonstrated experimentally to be a new way of selecting protonium (pp atom) annihilations with initial angular momentum L = 1. The mean value and the width of the time distributions which occur between protonium formation and annihilation have been measured at room temperature at 8.2, 4.1, 2.1 mbar. They are (110±10; 45±5), (145±15; 82±9) and (210±25;125±9) ns, respectively.

Abstract The p annihilation at rest in a NTP 4He-gas target has been studied by means of the Obelix spectrometer installed at the LEAR accelerator of CERN. Reactions with production of 2π−2π+, 2π−3π+, 2π−2π+p, 2π−π+2p, 2π−3p (with pπ>80 MeV/c and pp>300 MeV/c) have been selected and single- and multinucleon annihilations identified. For the first time some known mesonic and baryonic resonances have been observed in p 4He annihilations and branching ratios are evaluated for a number of reaction channels.

Events with production of two K+ are observed in p¯He4 annihilations at rest. They are detected in annihilation reactions with four and five prongs and are interpreted to proceed via the following intermediate states: 2K+Σ−Σ−ps→2K+2π−(2nps), 2K+Σ−Σ+nπ−→2K+π+2π−(3n), 2K+Σ−Λn→2K+p2π−(2n) and 2K+K−Λ2n→2K+K−pπ−(2n) with yields (lower limits) (0.17±0.04)×10−4, (2.71±0.47)×10−4, (1.21±0.29)×10−4 and (0.28±0.14)×10−4, respectively. They could be the signature of different production mechanisms like rescattering cascades and formation of quark–gluon plasma and of bound K−K−-few nucleon systems. Even though the statistics is very small, signatures compatible with (K−K−nn) and (K−K−nnp) bound systems are discussed.

The ARGO-YBJ experiment, a full coverage extensive air shower (EAS) detector located at high altitude (4300 m a.s.l.) in Tibet, China, has smoothly taken data, with very high stability, since November 2007 to the beginning of 2013. The array consisted of a carpet of about 7000 m2 Resistive Plate Chambers (RPCs) operated in streamer mode and equipped with both digital and analog readout, providing the measurement of particle densities up to few particles per cm2. The unique detector features (full coverage, readout granularity, wide dynamic range, ..) and location (very high altitude) allowed a detailed study of the lateral density profile of charged particles at ground very close to the shower axis and its description by a proper lateral distribution function (LDF). In particular, the information collected in the first 10 m from the shower axis have been shown to provide a very effective tool for the determination of the shower development stage (“age”) in the energy range 50 TeV - 10 PeV. The sensitivity of the age parameter to the mass composition of primary Cosmic Rays is also discussed.

The reaction p̄p→K+K−π0 was analysed for antiproton annihilations at rest at three hydrogen target densities. A strong dependence of the p̄p→φπ0 yield on the quantum numbers of the initial state is observed. The branching ratio of the φπ0 channel from the 3S1 initial state is more than 15 times larger than the one from the 1P1 state. A large apparent violation of the OZI rule for tensor meson production from p̄p-annihilations from the P-waves (1+++2++) is observed: Rexp(f′2π0/f2π0)=(149±20)·10−3, significantly exceeding the OZI-rule prediction R=16·10−3.

Data of antiproton annihilation at rest in a hydrogen gas target at NTP have been collected with the Obelix spectrometer exposed to the beam extracted from the LEAR accelerator of CERN. The reactions pp → φπ0 and pp → φη have been studied. The analysis of the angular distribution of the kaons emitted by the φ decay has shown that the reaction pp → φπ0 occurs essentially from 3S1 states with a production rate RNTP = (2.46 ± 0.23 ± 0.07) × 10−4, while the rate from 1P1 states is compatible with zero. Because of acceptance limitations, 3S1 and 1P1 contributions to the reaction pp → φη could not be resolved and lower and upper limits for the production rate have been measured with the middle value RNTP = (0.87 ± 0.21) × 10−4. Also, for the phase-space K+K−η events the production rate RNTP = (4.67 ± 0.35) × 10−4 has been measured. The φπ0 and φη branching ratios in S- and P-waves (defined in the text) have been estimated in a model dependent way. For the evaluation of the φη rate, a combination of our production rate and of data obtained with a liquid target has been used. The φπ0 production rate, compared to our preliminary value for the ωπ0 production, confirms the strong violation of the OZI rule observed in other experiments. No violation is apparent in the φη production.

The spin-parity analysis of the $\mathrm{n\ifmmode \bar{}\else \={}\fi{}}\stackrel{\ensuremath{\rightarrow}}{p}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}$ exclusive reaction in flight is presented. The main aim is to study the $({\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}})$ invariant mass spectrum in the region around 1500 MeV. The analysis was performed with a Breit-Wigner parametrization for all the resonant states and, for the scalar sector in the mass region below 1.2 GeV, by means of a K-matrix-like treatment. It clearly shows the need for two states, a scalar one ${(0}^{++})$ with mass and width $(1522\ifmmode\pm\else\textpm\fi{}25) \mathrm{MeV}$ and $(108\ifmmode\pm\else\textpm\fi{}33) \mathrm{MeV}$, and a tensorial one ${(2}^{++})$ with mass $(1575\ifmmode\pm\else\textpm\fi{}18) \mathrm{MeV}$ and width $(119\ifmmode\pm\else\textpm\fi{}24) \mathrm{MeV}$, respectively. In addition, the analysis requires the presence of a scalar state at $(1280\ifmmode\pm\else\textpm\fi{}55) \mathrm{MeV}$, $(323\ifmmode\pm\else\textpm\fi{}13) \mathrm{MeV}$ broad, and of a second vectorial one, in addition to the ${\ensuremath{\rho}}^{0}(770)$ signal, with mass and width $(1348\ifmmode\pm\else\textpm\fi{}33) \mathrm{MeV}$ and $(275\ifmmode\pm\else\textpm\fi{}10) \mathrm{MeV}$, respectively.

Data about antiprotons slowing down in gaseous targets at very low energies (E1 keV) show that the stopping power in ${\mathrm{D}}_{2}$ is lower than in ${\mathrm{H}}_{2}$; the right way to explain this behavior seems to be through a nuclear stopping power derived from the classical Rutherford formula. \textcopyright{} 1996 The American Physical Society.

From the study of the annihilation at rest pp → K±Kmiss0π∓π+π− the production rates of the η(1440) → K±KL0π∓ were obtained for three different hydrogen target densities: liquid, gaseous at NTP and at low pressure (5 mbar). The rate values are: fη(1440) (LH) = (6.0±0.5) · 10−4, fη(1440) (NTP) = (2.9±0.4) · 10−4 and fη(1440) (5mbar) = (1.0±0.2) · 10−4. From these results, the density dependence of the annihilation fraction from the 1S0 protonium level can be extracted directly.

The pp annihilation cross-sections at 66.4–68.9–71.4 MeV/c have been measured using the presently instrumented part of the OBELIX spectrometer. The results have been obtained at p energies ( ≃ 2.5 MeV) by one order of magnitude smaller than in any existing experiment concerning the in-flight antiproton-proton annihilation. The average value obtained over the total interval 65.1–72.5 MeV/c, βσanT = 33.2 ± 1.7 (stat.) ±1.8 (syst.) mb, in agreement with a specific calculation, would favour for the spin-averaged S-wave scattering length the value (−0.7 + i0.8) fm. The branching ratios for the annihilations in two, four and six charged mesons turn out to be quite similar to those for annihilation at rest in liquid hydrogen.

We have measured a value of 15.5±2.9 mb for the p-4He break-up cross section at 179.6 MeV. This low value is in agreement with the prediction of the Glauber model and is due to the large probability for the antiproton to annihilate into the nuclei, which makes only a surface layer of the nuclear volume effective for the break-up process.

Abstract The dependence of the strangeness production on the number of nucleons involved in the annihilation process is investigated experimentally in p ¯ p and p ¯ 4 He annihilation at rest. In He, annihilations with the involvement of one and several nucleons ( B = 0 and B ⩾ 1 baryonic number, respectively) are identified. Strangeness enhancement factors, i.e., ratios of K ± , K S 0 , K 0 ∗ and ϕ meson yields in 4 He to the same yields in H, are derived for final states containing 4 charged mesons ( 2 π + 2 π − , π + π − π ∓ K ± , π + π − K + K − ). This work completes a previous investigation concerning charged kaon production in meson final states without neutral mesons. The results are compared with our previous ones and with results found in experiments on heavy-ion collisions. It is put in evidence that the strangeness content in the energy blob created by the annihilation has a lower or higher intensity depending on the reaction channel: for instance, π + π − π ∓ K ± production increases with B and π + π − K + K − production decreases. The maximum enhancement factor (about 22) is found for π + π − π − K + without neutral mesons. This value (as well as that concerning π + π + π − K − ) is definitely higher than the values predicted by theoretical investigations based on the formation of a highly excited hadronic gas in the annihilation on few nucleons and values found in heavy-ion collisions. Is it the signature of the formation of quark–gluon plasma?

Abstract The present generation of Micro-Pattern Gaseous Detectors (MPGDs) are radiation hard detectors, capable of detecting effciently particle rates of several MHz/cm 2 , while exhibiting good spatial resolution (≤ 50 µm) and modest time resolution of 5-10 ns, which satisfies the current generation of experiments (High Luminosity LHC upgrades of CMS and ATLAS) but it is not sufficient for bunch crossing identification of fast timing systems at FCC-hh. Thanks to the application of thin resistive films such as Diamond-Like Carbon (DLC) a new detector concept was conceived: Fast Timing MPGD (FTM). In the FTM the drift volume of the detector has been divided in several layers each with their own amplification structure. The use of resistive electrodes makes the entire structure transparent for electrical signals. After some first initial encouraging results, progress has been slowed down due to problems with the wet-etching of DLC-coated polyimide foils. To solve these problems a more in-depth knowledge of the internal stress of the DLC together with the DLC-polyimide adhesion is required. We will report on the production of DLC films produced in Italy with Ion Beam Sputtering and Pulsed Laser Deposition, where we are searching to improve the adhesion of the thin DLC films, combined with a very high uniformity of the resistivity values.

The CMS experiment, located at the Large Hadron Collider (LHC) in CERN, has a redundant muon system composed by three different gaseous detector technologies: Cathode Strip Chambers (in the forward regions), Drift Tubes (in the central region), and Resistive Plate Chambers (both its central and forward regions). All three are used for muon reconstruction and triggering. The CMS RPC system confers robustness and redundancy to the muon trigger. The RPC system operation in the challenging background and pileup conditions of the LHC environment is presented. The RPC system provides information to all muon track finders and thus contributing to both muon trigger and reconstruction. The summary of the detector performance results obtained with proton-proton collision at √s = 13 TeV during 2016 and 2017 data taking have been presented. The stability of the system is presented in terms of efficiency and cluster size vs time and increasing instantaneous luminosity. Data-driven predictions about the expected performance during High Luminosity LHC (HL-LHC) stage have been reported.

Abstract A new generation of resistive plate chambers, capable of withstanding high particle fluxes (up to 2000 Hz · cm -2 ) and instrumented with precise timing readout electronics is proposed to equip two of the four high pseudorapidity stations of the CMS muon system. Double-gap RPC detectors, with each gap made of two 1.4 mm High Pressure Laminate electrodes and separated by a gas gap of the same thickness, are proposed. The new layout reduces the amount of the avalanche charge produced by the passage of a charged particle through the detector. This improves the RPC rate capability by reducing the needed time to collect this charge. To keep the RPC efficiency high, a sensitive, low-noise and high time resolution front-end electronics is needed to cope with the lower charge signal of the new RPC. An ASIC called PETIROC that has all these characteristics has been selected to read out the strips of new chambers. Thin (0.6 mm) printed circuit board, 160 cm long, equipped with pickup strips of 0.75 cm average pitch, will be inserted between the two new RPC's gaps. The strips will be read out from both ends, and the arrival time difference of the two ends will be used to determine the hit position along the strip. Results from the improved RPC equipped with the new readout system and exposed to cosmic muons in the high irradiation environment at CERN GIF++ facility are presented in this work.

The stopping power of H, for pS was measured in the p energy interval 1 keV + 120 keV using a low density gas target (ppρ ≈ l0−3 + 10−2 ÷ pρ being the Hz density at STP) with the OBELIX spectrometer at the LEAR of CEBN. Furthermore, the formation of metastable states in the p 4He interaction at rest, having a mean life of the order of 3 μs, was investigated at six target gas pressures (3 atm and 400,250,200,150,50 mbar). The influence of contaminants (like Ha, Ne, Ar, N2 and Xe) on the life of such metastable states was examined at the target gas pressure of 3 atm.

We have measured the K+K-/π+π- ratio R from p annihilations in gaseous D2 and H2 at atmospheric pressure. The measurement was performed with the OBELIX spectrometer. From the measured value in gaseous D2 (0.27±0.02) we infer a P wave contribution to p−p annihilation in D2 of (18±7)%.

The frequency of the protonium annihilation channel pp → KSKL has been measured at three different target densities: liquid hydrogen (LH), gaseous hydrogen at NTP conditions and gaseous hydrogen at low pressure (5 mbar). The obtained results are: f(pp → KSKL, LH) = (7.8 ± 0.7stat ± 0.3sys) × 10−4, f(pp → KSKL, NTP) = (3.5 ± 0.5stat ± 0.2sys) × 10−4 and f(pp → KSKL, 5 mbar) = (1.0 ± 0.3stat ± 0.1sys) × 10−4. Since the KSKL final stat and be originated only from the 3S1 initial state, these values give direct information on the scaling of the protonium spin-triplet S-wave annihilation probability with the density.

The pionic annihilation of antiprotons stopped on 3He nuclei in a self-shunted streamer chamber exposed to the antiproton beam of LEAR is studied. The data concern charged-particle multiplicity distributions, branching ratios for different final states, the probability of final-state interaction, π−, π+, p and d momentum spectra, like and unlike pion angular correlations, π and other charged-particle angular correlations. The comparison of the 3He data with those obtained on 1H, 2H and 4He does not reveal relevant effects due to the increase of the nucleon number; the small differences can be seen as due to a weak final-state interaction.

The measurement of angles and momenta, as well as the identification of the masses of the products of p4He annihilation events detected in a self-shunted streamer chamber are described.

Abstract The dependence of the K + and K − production on the number of nucleons involved in the annihilation process is investigated experimentally in the p ¯ annihilation at rest on hydrogen, deuterium, 3He and 4He gas targets. Annihilations with any number of prongs (charged pions and kaons, protons and deuterons) are analyzed. Events with and without production of neutral mesons and with and without emission of fast neutrons (that is neutrons involved in the annihilation process) are recognized. The results are consistent with our previous ones on a more restricted sample of annihilation reactions and put in evidence that the strangeness production is lower or higher depending on the reaction channel. As a general trend, the strangeness production is higher in events without neutral mesons and still higher in events with the involvement of a higher number of nucleons. Both K + and K − productions increase with the number of involved nucleons, but K + much more. The maximum K + production is observed in the reaction K + 2 π + 2 π − 3 n on 4He (with the involvement of 3–4 nucleons); compared with the production on hydrogen in the reaction K + π + 2 π − , the production on 4He is higher by a factor of 31.7 ± 5.5 . In the light of some theoretical speculations, this enhancement factor is too high to be explainable in terms of hadronic interactions and could be interpreted as a signature of quark deconfinement and of formation of a quark–gluon plasma.

During 2013 and 2014 (Long Shutdown LS1) the CMS experiment is upgrading the forward region installing a fourth layer of RPC detectors in order to complete and improve the muon system performances in the view of the foreseen high luminosity run of LHC. The new two endcap disks consists of 144 double-gap RPC chambers assembled at three different production sites: CERN, Ghent (Belgium) and BARC (India). The chamber components as well as the final detectors are subjected to full series of tests established in parallel at all the production sites.

The Resistive Plate Chambers (RPCs) are employed in the CMS Experiment at the LHC as dedicated trigger system both in the barrel and in the endcap. This article presents results of the radiation background measurements performed with the 2011 and 2012 proton-proton collision data collected by CMS. Emphasis is given to the measurements of the background distribution inside the RPCs. The expected background rates during the future running of the LHC are estimated both from extrapolated measurements and from simulation.

During Phase-2 of the LHC, known as the High Luminosity LHC (HL-LHC), the accelerator will increase its instantaneous luminosity to 5 × 1034 cm−2 s−1, delivering an integrated luminosity of 3000 fb−1 over 10 years of operation starting from 2027. In view of the HL-LHC, the CMS muon system will be upgraded to sustain efficient muon triggering and reconstruction performance. Resistive Plate Chambers (RPCs) serve as dedicated detectors for muon triggering due to their excellent timing resolution, and will extend the acceptance up to pseudorapidity values of |η|=2.4. Before Long Shutdown 3 (LS3), the RE3/1 and RE4/1 stations of the endcap will be equipped with new improved Resistive Plate Chambers (iRPCs) having different design and geometry than the present RPC system. The iRPC geometry configuration improves the detector's rate capability and its ability to survive the harsh background conditions of the HL-LHC . Also, new electronics with excellent timing performances (time resolution of less than 150 ps) are developed to read out the RPC detectors from both sides of the strips to allow for good spatial resolution along them. The performance of the iRPC has been studied with gamma radiation at the Gamma Irradiation Facility (GIF++) at CERN. Ongoing longevity studies will help to certify the iRPCs for the HL-LHC running period. The main detector parameters such as the current, rate and resistivity are regularly monitored as a function of the integrated charge. Preliminary results of the detector performance will be presented.

Antinucleon-nucleus annihilations into two-body final states containing only one or no meson are unusual annihilations (Pontecorvo reactions), practically unexplored experimentally, with the exception of the channel p d → π− p, for which only two low-statistics measurements exist. Their physical interest lies in the possibility of exploring small-distance nuclear dynamics, in which an important role can be played by non-nucleonic degrees of freedom. A new measurement of the p d → π− p reaction rate at rest, performed with the OBELIX spectrometer at LEAR, with the best statistics up to now and a careful evaluation of systematic effects is reported, together with a critical analysis of the existing theoretical models. The measured branching ratio, which confirms the previous results, can represent a reference point for the studies in the field.

The results of measuring the ratio R = Y(φπ+π−)Y(ωπ+π−) for antiproton annihilation at rest in a gaseous and a liquid hydrogen target are presented. It was found that the value of this ratio increases with the decrease of the dipion mass, which demonstrates the difference between the φ and ω production mechanisms. An indication of the momentum transfer dependence of the apparent OZI rule violation for the φ production from the 3S1 initial state was found.

This paper reports new experimental results on the annihilation of antiprotons stopped in a4He gas target. The measurement was carried out with the Obelix spectrometer installed at the LEAR machine of CERN. Main results can be summarized as follows, (i) The invariant mass distribution of 2π+2π− produced at rest is similar to those from hydrogen and deuterium targets: the peak seen in recent experiments around 1150 MeV/c2 is absent, (ii) 2π angular correlations are very close to those found for hydrogen. The nuclear structure does not modify remarkably the pion production and emission, (iii) πp and pp angular correlations are strongly peaked close to 180°, in qualitative agreement with INC model predictions, (iv) The pp invariant mass distribution reveals events which may be due to η(ω) absorption; the estimated upper limit for probability of π and ω absorption is about 3 × 10−3. (v) Events having features compatible with two-nucleon annihilations with production of heavy mesonic systems are detected both in the 2π+ 2π− and the 2π−π+ channels. The production rate of the 2π+ 2π− channel is of the order of 5 × 10−3.

The ARGO-YBJ experiment, located at Yangbajing, Tibet, China, performed by a wide Sino-Italian collaboration, is designed to study cosmic rays, sub-TeV gamma ray sources and GeV Gamma Ray Burst (GRB) emission in the northern hemisphere, by means of detecting small size EAS (Extensive Air Shower) using a full coverage RPC (Resistive Plate Chamber) carpet. The central carpet of the detector is installed and put into operation to date, with 1900m2 of the carpet already operating since December 2004. With a trigger multiplicity of ≥60 hits, corresponding to a primary mode energy of 2 TeV, the angular resolution of EAS measurements is < 1° for showers with more than 500 recorded hits. We report the preliminary results of data taking performed during 2005: the all-sky survey for gamma ray sources and the search for GRBs, as well as the cosmic ray spectrum up to about 100TeV. The Forbush decrease of the cosmic ray flux during January, 2005 is observed using the ARGO data.

The HL-LHC phase is designed to increase by an order of magnitude the amount of data to be collected by the LHC experiments. To achieve this goal in a reasonable time scale the instantaneous luminosity would also increase by an order of magnitude up to 6 · 1034 cm−2s−1. The region of the forward muon spectrometer (|η| > 1.6) is not equipped with RPC stations. The increase of the expected particles flux up to 2 kHz/cm2 (including a safety factor 3) motivates the installation of RPC chambers to guarantee redundancy with the CSC chambers already present. The current CMS RPC technology cannot sustain the expected background level. The new technology that will be chosen should have a high rate capability and provide a good spatial and timing resolution. A new generation of Glass-RPC (GRPC) using low-resistivity glass is proposed to equip at least the two most far away of the four high η muon stations of CMS. First the design of small size prototypes and studies of their performance in high-rate particles flux are presented. Then the proposed designs for large size chambers and their fast-timing electronic readout are examined and preliminary results are provided.

We report on a systematic study of double-gap and four-gap phenolic resistive plate chambers (RPCs) for the Phase-2 upgrade of the CMS muon system at high η. In the present study, we constructed real-sized double-gap and four-gap RPCs with gap thicknesses of 1.6 and 0.8 mm, respectively, with 2-mm-thick phenolic high-pressure-laminated (HPL) plates. We examined the prototype RPCs with cosmic rays and with 100-GeV muons provided by the SPS H4 beam line at CERN. To examine the rate capability of the prototype RPCs both at Korea University and at the CERN GIF++ facility, the chambers were irradiated with 137Cs sources providing maximum gamma rates of about 1.5 kHz cm−2. For the 1.6-mm-thick double-gap RPCs, we found the relatively high threshold on the produced detector charge was conducive to effectively suppressing the rapid increase of strip cluster sizes of muon hits with high voltage, especially when measuring the narrow-pitch strips. The gamma-induced currents drawn in the four-gap RPC were about one-fourth of those drawn in the double-gap RPC. The rate capabilities of both RPC types, proven through the present testing using gamma-ray sources, far exceeded the maximum rate expected in the new high-η endcap RPCs planned for future phase-II runs of the Large Hadron Collider (LHC).