R. Arcidiacono

A sample of 1.69×107 fully reconstructed π0→γe+e− decay candidates collected by the NA48/2 experiment at CERN in 2003–2004 is analyzed to search for the dark photon (A′) production in the π0→γA′ decay followed by the prompt A′→e+e− decay. No signal is observed, and an exclusion region in the plane of the dark photon mass mA′ and mixing parameter ε2 is established. The obtained upper limits on ε2 are more stringent than the previous limits in the mass range 9MeV/c2<mA′<70MeV/c2. The NA48/2 sensitivity to the dark photon production in the K±→π±A′ decay is also evaluated.

The direct CP violation parameter Re(epsilon'/epsilon) has been measured from the decay rates of neutral kaons into two pions using the NA48 detector at the CERN SPS. The 2001 running period was devoted to collecting additional data under varied conditions compared to earlier years (1997-99). The new data yield the result: Re(epsilon'/epsilon) = (13.7 +/- 3.1) times 10^{-4}. Combining this result with that published from the 1997, 98 and 99 data, an overall value of Re(epsilon'/epsilon) = (14.7 +/- 2.2) times 10^{-4} is obtained from the NA48 experiment.

In this paper we report on the timing resolution obtained in a beam test with pions of 180 GeV/c momentum at CERN for the first production of 45 µm thick Ultra-Fast Silicon Detectors (UFSD). UFSD are based on the Low-Gain Avalanche Detector (LGAD) design, employing n-on-p silicon sensors with internal charge multiplication due to the presence of a thin, low-resistivity diffusion layer below the junction. The UFSD used in this test had a pad area of 1.7 mm2. The gain was measured to vary between 5 and 70 depending on the sensor bias voltage. The experimental setup included three UFSD and a fast trigger consisting of a quartz bar readout by a SiPM. The timing resolution was determined by doing Gaussian fits to the time-of-flight of the particles between one or more UFSD and the trigger counter. For a single UFSD the resolution was measured to be 34 ps for a bias voltage of 200 V, and 27 ps for a bias voltage of 230 V. For the combination of 3 UFSD the timing resolution was 20 ps for a bias voltage of 200 V, and 16 ps for a bias voltage of 230 V.

Low-Gain Avalanche Diodes (LGAD) are silicon detectors with output signals that are about a factor of 10 larger than those of traditional sensors. In this paper we analyze how the design of LGAD can be optimized to exploit their increased output signal to reach optimum timing performances. Our simulations show that these sensors, the so-called Ultra-Fast Silicon Detectors (UFSD), will be able to reach a time resolution factor of 10 better than that of traditional silicon sensors.

A precision measurement of the ratio RK of the rates of kaon leptonic decays K±→e±ν and K±→μ±ν with the full data sample collected by the NA62 experiment at CERN in 2007–2008 is reported. The result, obtained by analysing ∼150000 reconstructed K±→e±ν candidates with 11% background contamination, is RK=(2.488±0.010)×10−5, in agreement with the Standard Model expectation.

The CMS detector team describes their experiment and observation of decay products from a standard model Higgs boson, allowing its mass to be determined.

In this paper, we report on the radiation resistance of 50-micron thick LGAD detectors manufactured at the Fondazione Bruno Kessler employing several different doping combinations of the gain layer. LGAD detectors with gain layer doping of Boron, Boron low-diffusion, Gallium, Carbonated Boron and Carbonated Gallium have been designed and successfully produced. These sensors have been exposed to neutron fluences up to $\phi_n \sim 3 \cdot 10^{16}\; n/cm^2$ and to proton fluences up to $\phi_p \sim 9\cdot10^{15}\; p/cm^2$ to test their radiation resistance. The experimental results show that Gallium-doped LGADs are more heavily affected by initial acceptor removal than Boron-doped LGAD, while the presence of Carbon reduces initial acceptor removal both for Gallium and Boron doping. Boron low-diffusion shows a higher radiation resistance than that of standard Boron implant, indicating a dependence of the initial acceptor removal mechanism upon the implant width. This study also demonstrates that proton irradiation is at least twice more effective in producing initial acceptor removal, making proton irradiation far more damaging than neutron irradiation.

The NA62 experiment at CERN reports searches for $K^+\to\mu^+N$ and $K^+\to\mu^+\nu X$ decays, where $N$ and $X$ are massive invisible particles, using the 2016-2018 data set. The $N$ particle is assumed to be a heavy neutral lepton, and the results are expressed as upper limits of ${\cal O}(10^{-8})$ of the neutrino mixing parameter $|U_{\mu4}|^2$ for $N$ masses in the range 200-384 MeV/$c^2$ and lifetime exceeding 50 ns. The $X$ particle is considered a scalar or vector hidden sector mediator decaying to an invisible final state, and upper limits of the decay branching fraction for $X$ masses in the range 10-370 MeV/$c^2$ are reported for the first time, ranging from ${\cal O}(10^{-5})$ to ${\cal O}(10^{-7})$. An improved upper limit of $1.0\times 10^{-6}$ is established at 90% CL on the $K^+\to\mu^+\nu\nu\bar\nu$ branching fraction.

We report the results from a study of a partial sample of ∼2.3×107 K±→π±π0π0 decays recorded by the NA48/2 experiment at the CERN SPS, showing an anomaly in the π0π0 invariant mass (M00) distribution in the region around M00=2m+, where m+ is the charged pion mass. This anomaly, never observed in previous experiments, can be interpreted as an effect due mainly to the final state charge exchange scattering process π+π−→π0π0 in K±→π±π+π− decay [N. Cabibbo, Phys. Rev. Lett. 93 (2004) 121801]. It provides a precise determination of a0−a2, the difference between the ππ scattering lengths in the isospin I=0 and I=2 states. A best fit to a rescattering model [N. Cabibbo, G. Isidori, JHEP 0503 (2005) 21] corrected for isospin symmetry breaking gives (a0−a2)m+=0.268±0.010(stat)±0.004(syst), with additional external uncertainties of ±0.013 from branching ratio and theoretical uncertainties. If the correlation between a0 and a2 predicted by chiral symmetry is taken into account, this result becomes (a0−a2)m+=0.264±0.006(stat)±0.004(syst)±0.013(ext).

We report results from the analysis of the $\mbox {$\mathrm {K}^{\pm}$}\rightarrow \pi^{+} \pi^{-} \mathrm{e}^{\pm} \nu$ ( $\mbox {$\mathrm {K}_{\mathrm {e}4}$}$ ) decay by the NA48/2 collaboration at the CERN SPS, based on the total statistics of 1.13 million decays collected in 2003–2004. The hadronic form factors in the S- and P-wave and their variation with energy are obtained. The phase difference between the S- and P-wave states of the ππ system is accurately measured and allows a precise determination of $\mbox {$a_{0}^{0}$}$ and $\mbox {$a_{0}^{2}$}$ , the I = 0 and I = 2 S-wave ππ scattering lengths: $\mbox {$a_{0}^{0}$}= 0.2220 \pm 0.0128 \mbox {$\mathrm {_{stat}}$}\pm 0.0050 \mbox {$\mathrm {_{syst}}$}\pm 0.0037\mbox {$\mathrm {_{th}}$},\mbox {$a_{0}^{2}$}= -0.0432 \pm 0.0086 \mbox {$\mathrm {_{stat}}$}\pm 0.0034 \mbox {$\mathrm {_{syst}}$}\pm 0.0028\mbox {$\mathrm {_{th}}$}$ . Combination of this result with the other NA48/2 measurement obtained in the study of $\mbox {$\mbox {$\mathrm {K}^{\pm}$}\rightarrow \mbox {$\pi ^{0}$}\mbox {$\pi ^{0}$}\mbox {$\scriptstyle \pi ^{\pm }$}$}$ decays brings an improved determination of $\mbox {$a_{0}^{0}$}$ and the first precise experimental measurement of $\mbox {$a_{0}^{2}$}$ , providing a stringent test of Chiral Perturbation Theory predictions and lattice QCD calculations. Using constraints based on analyticity and chiral symmetry, even more precise values are obtained: $\mbox {$a_{0}^{0}$}=0.2196 \pm 0.0028\mbox {$\mathrm {_{stat}}$}\pm 0.0020 \mbox {$\mathrm {_{syst}}$}$ and $\mbox {$a_{0}^{2}$}= -0.0444 \pm 0.0007 \mbox {$\mathrm {_{stat}}$}\pm 0.0005 \mbox {$\mathrm {_{syst}}$}\pm 0.0008 \mbox {$\mathrm {_{ChPT}}$}$ .

We report results from a new measurement of the Ke4 decay K±→π+π-e±ν by the NA48/2 collaboration at the CERN SPS, based on a partial sample of more than 670 000 Ke4 decays in both charged modes collected in 2003. The form factors of the hadronic current (F,G,H) and ππ phase difference (δ=δs-δp) have been measured in ten independent bins of the ππ mass spectrum to investigate their variation. A sizeable acceptance at large ππ mass, a low background and a very good resolution contribute to an improved experimental accuracy, a factor two better than in the previous measurement, when extracting the ππ scattering lengths a0 0 and a0 2. Under the assumption of isospin symmetry and using numerical solutions of the Roy equations, the following values are obtained in the plane (a0 0,a0 2): a0 0=0.233±0.016stat±0.007syst,a0 2=-0.0471±0.011stat±0.004syst. The presence of potentially large isospin effects is also considered and will allow comparison with precise predictions from Chiral Perturbation Theory.

In this contribution we will review the progresses toward the construction of a tracking system able to measure the passage of charged particles with a combined precision of ∼10 ps and ∼10 μm, either using a single type of sensor, able to concurrently measure position and time, or a combination of position and time sensors.

A search for heavy neutral lepton production in K+ decays using a data sample collected with a minimum bias trigger by the NA62 experiment at CERN in 2015 is reported. Upper limits at the 10−7 to 10−6 level are established on the elements of the extended neutrino mixing matrix |Ue4|2 and |Uμ4|2 for heavy neutral lepton mass in the ranges 170–448 MeV/c2 and 250–373 MeV/c2, respectively. This improves on the previous limits from HNL production searches over the whole mass range considered for |Ue4|2, and above 300 MeV/c2 for |Uμ4|2.

The NA62 experiment at the CERN SPS reports the first search for $K^+ \rightarrow \pi^+ \nu \bar{\nu}$ using the decay-in-flight technique, based on a sample of $1.21\times10^{11}$ $K^+$ decays collected in 2016. The single event sensitivity is $3.15\times 10^{-10}$, corresponding to 0.267 Standard Model events. One signal candidate is observed while the expected background is 0.152 events. This leads to an upper limit of $14 \times 10^{-10}$ on the $K^+ \rightarrow \pi^+ \nu \bar{\nu}$ branching ratio at 95\% CL.

A search for heavy neutral lepton (N) production in K+→e+N decays using the data sample collected by the NA62 experiment at CERN in 2017–2018 is reported. Upper limits of the extended neutrino mixing matrix element |Ue4|2 are established at the level of 10−9 over most of the accessible heavy neutral lepton mass range 144–462 MeV/c2, with the assumption that the lifetime exceeds 50 ns. These limits improve significantly upon those of previous production and decay searches. The |Ue4|2 range favoured by Big Bang Nucleosynthesis is excluded up to a mass of about 340 MeV/c2.

The results of a search for $\pi^0$ decays to a photon and an invisible massive dark photon at the NA62 experiment at the CERN SPS are reported. From a total of $4.12\times10^8$ tagged $\pi^0$ mesons, no signal is observed. Assuming a kinetic-mixing interaction, limits are set on the dark photon coupling to the ordinary photon as a function of the dark photon mass, improving on previous searches in the mass range 60--110 MeV/$c^2$. The present results are interpreted in terms of an upper limit of the branching ratio of the electro-weak decay $\pi^0 \to \gamma \nu \overline{\nu}$, improving the current limit by more than three orders of magnitude.

This paper presents the principles of operation of Resistive AC-Coupled Silicon Detectors (RSDs) and measurements of the temporal and spatial resolutions using a combined analysis of laser and beam test data. RSDs are a new type of n-in-p silicon sensor based on the Low-Gain Avalanche Diode (LGAD) technology, where the n+ implant has been designed to be resistive, and the read-out is obtained via AC-coupling. The truly innovative feature of RSD is that the signal generated by an impinging particle is shared isotropically among multiple read-out pads without the need for floating electrodes or an external magnetic field. Careful tuning of the coupling oxide thickness and the n+ doping profile is at the basis of the successful functioning of this device. Several RSD matrices with different pad width-pitch geometries have been extensively tested with a laser setup in the Laboratory for Innovative Silicon Sensors in Torino, while a smaller set of devices have been tested at the Fermilab Test Beam Facility with a 120 GeV/c proton beam. The measured spatial resolution ranges between 2.5μm for 70–100 pad-pitch geometry and 17μm with 200–500 matrices, a factor of 10 better than what is achievable in binary read-out (binsize∕12). Beam test data show a temporal resolution of ∼40ps for 200 μm pitch devices, in line with the best performances of LGAD sensors at the same gain.

The DC-coupled Resistive Silicon Detectors (DC-RSD) are the evolution of the AC-coupled RSD (RSD) design, both based on the Low-Gain Avalanche Diode (LGAD) technology. The DC-RSD design concept intends to address a few known issues present in RSDs (e.g., baseline fluctuation, long tail-bipolar signals) while maintaining their advantages (e.g., signal spreading, 100% fill factor). The simulation of DC-RSD presents several unique challenges linked to the complex nature of its design and the large pixel size. The defining feature of DC-RSD, charge sharing over distances that can be as large as a millimetre, represents a formidable challenge for Technology-CAD (TCAD), the standard simulation tool. To circumvent this problem, we have developed a mixed-mode approach to the DC-RSD simulation, which exploits a combination of two simulation tools: TCAD and Spice. Thanks to this hybrid approach, it has been possible to demonstrate that, according to the simulation, the key features of the RSD, excellent timing and spatial resolutions (few tens of picoseconds and few microns), are maintained in the DC-RSD design. In this work, we present the developed models and methodology, mainly showing the results of device-level numerical simulation, which have been obtained with the state-of-the-art Synopsys Sentaurus TCAD suite of tools. Such results will provide all the necessary information for the first batch of DC-RSD produced by Fondazione Bruno Kessler (FBK) foundry in Trento, Italy.

A measurement of the direct CP violating charge asymmetries of the Dalitz plot linear slopes Ag=(g+-g-)/(g++g-) in K±→π±π+π- and K±→π±π0π0 decays by the NA48/2 experiment at CERN SPS is presented. A new technique of asymmetry measurement involving simultaneous K+ and K- beams and a large data sample collected allowed a result of an unprecedented precision. The charge asymmetries were measured to be Ac g=(-1.5±2.2)×10-4 with 3.11×109K±→π±π+π- decays, and An g=(1.8±1.8)×10-4 with 9.13×107K±→π±π0π0 decays. The precision of the results is limited mainly by the size of the data sample.

We report the results from a study of the full sample of ∼6.031×107 K ±→π ± π 0 π 0 decays recorded by the NA48/2 experiment at the CERN SPS. As first observed in this experiment, the π 0 π 0 invariant mass (M 00) distribution shows a cusp-like anomaly in the region around M 00=2m +, where m + is the charged pion mass. This anomaly has been interpreted as an effect due mainly to the final state charge exchange scattering process π + π −→π 0 π 0 in K ±→π ± π + π − decay. Fits to the M 00 distribution using two different theoretical formulations provide the presently most precise determination of a 0−a 2, the difference between the π π S-wave scattering lengths in the isospin I=0 and I=2 states. Higher-order π π rescattering terms, included in the two formulations, allow also an independent, though less precise, determination of a 2.

Is it possible to design a detector able to concurrently measure time and position with high precision? This question is at the root of the research and development of silicon sensors presented in this contribution. Silicon sensors are the most common type of particle detectors used for charged particle tracking, however their rather poor time resolution limits their use as precise timing detectors. A few years ago we have picked up the gantlet of enhancing the remarkable position resolution of silicon sensors with precise timing capability. I will be presenting our results in the following pages.

We designed, produced, and tested RSD (Resistive AC-Coupled Silicon Detectors) devices, an evolution of the standard LGAD (Low-Gain Avalanche Diode) technology where a resistive n-type implant and a coupling dielectric layer have been implemented. The first feature works as a resistive sheet, freezing the multiplied charges, while the second one acts as a capacitive coupling for readout pads. We succeeded in the challenging goal of obtaining very fine pitch (50, 100, and 200 um) while maintaining the signal waveforms suitable for high timing and 4D-tracking performances, as in the standard LGAD-based devices.

In this paper we report results from a neutron irradiation campaign of Ultra-Fast Silicon Detectors (UFSD) with fluences of 1e14, 3e14, 6e14, 1e15, 3e15 and 6e15 neq/cm2. The UFSD used in this study are circular 50 μ m thick Low-Gain Avalanche Detectors (LGAD), with a 1.0 mm diameter active area. Hamamatsu Photonics (HPK), Japan, produced the UFSD with pre-irradiation internal gain in the range 5–70 depending on the bias voltage. The sensors were tested pre-irradiation and post-irradiation with minimum ionizing particles (MIPs) from a 90Sr β-source. The leakage current, internal gain and the timing resolution were measured as a function of bias voltage at −20 °C and −30 °C. The timing resolution of each device under test was extracted from the time difference with a second calibrated UFSD in coincidence, using the constant fraction discriminator (CFD) method for both. The dependence of the gain upon the irradiation fluence is consistent with the acceptor removal mechanism; at −20 °C the highest gain decreases from 70 before radiation to 2 after a fluence of 6e15 n/cm2. Consequently, the timing resolution was found to deteriorate from 20 ps to 50 ps. The results indicate that the most accurate time resolution is obtained varying with fluence the CFD value used to determine the time of arrival, from 0.1 for pre-irradiated sensors to 0.6 at the highest fluence. Key changes to the pulse shape induced by irradiation, i.e. (i) the contribution of charge multiplication not limited to the gain layer zone, (ii) the shortening of the rise time and (iii) the reduced pulse height, were compared with the WF2 simulation program and were found to be in agreement.

Fondazione Bruno Kessler (FBK, Trento, Italy) has recently delivered its first 50 $\mu$m thick production of Ultra-Fast Silicon Detectors (UFSD), based on the Low-Gain Avalanche Diode design. These sensors use high resistivity Si-on-Si substrates, and have a variety of gain layer doping profiles and designs based on Boron, Gallium, Carbonated Boron and Carbonated Gallium to obtain a controlled multiplication mechanism. Such variety of gain layers will allow identifying the most radiation hard technology to be employed in the production of UFSD, to extend their radiation resistance beyond the current limit of $\phi \sim$ 10$^{15}$ n$_{eq}$/cm$^2$. In this paper, we present the characterisation, the timing performances, and the results on radiation damage tolerance of this new FBK production.

The NA62 experiment at CERN reports a search for the lepton number violating decays K+→π−e+e+ and K+→π−μ+μ+ using a data sample collected in 2017. No signals are observed, and upper limits on the branching fractions of these decays of 2.2×10−10 and 4.2×10−11 are obtained, respectively, at 90% confidence level. These upper limits improve on previously reported measurements by factors of 3 and 2, respectively.

The NA62 experiment at the CERN SPS reports a study of a sample of $4 \times10^{9}$ tagged $\pi^0$ mesons from $K^+ \to \pi^+ \pi^0 (\gamma)$, searching for the decay of the $\pi^0$ to invisible particles. No signal is observed in excess of the expected background fluctuations. An upper limit of $4.4 \times10^{-9}$ is set on the branching ratio at 90% confidence level, improving on previous results by a factor of 60. This result can also be interpreted as a model-independent upper limit on the branching ratio for the decay $K^+ \to \pi^+ X$, where $X$ is a particle escaping detection with mass in the range 0.110-0.155 GeV$/c^2$ and rest lifetime greater than 100 ps. Model-dependent upper limits are obtained assuming $X$ to be an axion-like particle with dominant fermion couplings or a dark scalar mixing with the Standard Model Higgs boson.

Abstract This paper presents the measurements on first very thin Ultra-Fast Silicon Detectors (UFSDs) produced by Fondazione Bruno Kessler; the data have been collected in a beam test setup at the CERN PS, using beam with a momentum of 12 GeV/c. UFSDs with a nominal thickness of 25 and 35 $$\mu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> m and an area of 1 $$\times$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mo>×</mml:mo> </mml:math> 1 $$\text {mm}^2$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mtext>mm</mml:mtext> <mml:mn>2</mml:mn> </mml:msup> </mml:math> have been considered, together with an additional HPK 50- $$\mu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> m thick sensor, taken as reference. Their timing performances have been studied as a function of the applied voltage and gain. A time resolution of about 25 ps and of 22 ps at a voltage of 120 and 240 V has been obtained for the 25 and 35 $$\mu$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>μ</mml:mi> </mml:math> m thick UFSDs, respectively.

A sample of 7253 $K^\pm\toπ^\pm e^+e^-(γ)$ decay candidates with 1.0% background contamination has been collected by the NA48/2 experiment at the CERN SPS, allowing a precise measurement of the decay properties. The branching ratio in the full kinematic range was measured to be ${\rm BR}=(3.11\pm0.12)\times 10^{-7}$, where the uncertainty includes also the model dependence. The shape of the form factor $W(z)$, where $z=(M_{ee}/M_K)^2$, was parameterized according to several models, and, in particular, the slope $δ$ of the linear form factor $W(z)=W_0(1+δz)$ was determined to be $δ=2.32\pm0.18$. A possible CP violating asymmetry of $K^+$ and $K^-$ decay widths was investigated, and a conservative upper limit of $2.1\times 10^{-2}$ at 90% CL was established.

A study of the dynamics of the rare decay K±→π±γγ has been performed on a sample of 232 decay candidates, with an estimated background of 17.4±1.1 events, collected by the NA62 experiment at CERN in 2007. The results are combined with those from a measurement conducted by the NA48/2 Collaboration at CERN. The combined model-independent branching ratio in the kinematic range z=(mγγ/mK)2>0.2 is BMI(z>0.2)=(0.965±0.063)×10−6, and the combined branching ratio in the full kinematic range assuming a Chiral Perturbation Theory description is B(Kπγγ)=(1.003±0.056)×10−6. A detailed comparison of the results with the previous measurements is performed.

A bstract A search for the K + → π + X decay, where X is a long-lived feebly interacting particle, is performed through an interpretation of the K + → $$ {\pi}^{+}\nu \overline{\nu} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>ν</mml:mi> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> analysis of data collected in 2017 by the NA62 experiment at CERN. Two ranges of X masses, 0–110 MeV /c 2 and 154–260 MeV /c 2 , and lifetimes above 100 ps are considered. The limits set on the branching ratio, BR( K + → π + X ), are competitive with previously reported searches in the first mass range, and improve on current limits in the second mass range by more than an order of magnitude.

A precision test of lepton flavour universality has been performed by measuring the ratio RK of kaon leptonic decay rates K+ --> e+nu and K+ --> mu+nu in a sample of 59813 reconstructed K+ --> e+nu candidates with (8.71 +- 0.24)% background contamination. The result RK = (2.487 +- 0.013) * 10^{-5} is in agreement with the Standard Model expectation.

The NA48/2 experiment at CERN collected a large sample of charged kaon decays to final states with multiple charged particles in 2003–2004. A new upper limit on the rate of the lepton number violating decay K±→π∓μ±μ± is reported: B(K±→π∓μ±μ±)<8.6×10−11 at 90% CL. Searches for two-body resonances X in K±→πμμ decays (such as heavy neutral leptons N4 and inflatons χ) are also presented. In the absence of signals, upper limits are set on the products of branching fractions B(K±→μ±N4)B(N4→πμ) and B(K±→π±X)B(X→μ+μ−) for ranges of assumed resonance masses and lifetimes. The limits are in the (10−11,10−9) range for resonance lifetimes below 100 ps.

The NA62 experiment collected a large sample of charged kaon decays in 2007 with a highly efficient trigger for decays into electrons. A measurement of the $\pi^0$ electromagnetic transition form factor slope parameter from $1.11\times10^{6}$ fully reconstructed $K^\pm \to \pi^\pm \pi^0_D, \pi^0_D \to e^+ e^-\gamma$ events is reported. The measured value $a = (3.68 \pm 0.57)\times10^{-2}$ is in good agreement with theoretical expectations and previous measurements, and represents the most precise experimental determination of the slope in the time-like momentum transfer region.

The NA62 experiment recorded a large sample of $K^+ \rightarrow \mu^+ \nu_{\mu}$ decays in 2007. A peak search has been performed in the reconstructed missing mass spectrum. In the absence of a signal, limits in the range $2 \times 10^{-6}$ to $10^{-5}$ have been set on the squared mixing matrix element $|U_{\mu4} |^2$ between muon and heavy neutrino states, for heavy neutrino masses in the range 300-375 MeV/$c^2$. The result extends the range of masses for which upper limits have been set on the value of $|U_{\mu4} |^2$ in previous production search experiments.

In this paper we present the numerical simulation of silicon detectors with internal gain as the main tool for 4-dimensional (4D) particle trackers design and optimization. The Low-Gain Avalanche Diode (LGAD) technology and its present limitations are reviewed with the aim of introducing the Resistive AC-Coupled Silicon Detectors (RSD) paradigm as a case study of our investigation. Authors here present Spice-like and 2D/3D Technological Computer-Aided Design (TCAD) simulations to characterize sensors in terms of both their electrostatic behavior, capacitive (dynamic) coupling and radiation-hardness performances, showing the methodological approach used in order to extract the set of layout rules allowing the release of RSD1, the incoming production run at Fondazione Bruno Kessler (FBK) of next-generation silicon detectors for 4D tracking with intrinsic 100% fill-factor.

Several future high-energy physics facilities are currently being planned. The proposed projects include high energy e+e− circular and linear colliders, hadron colliders, and muon colliders, while the Electron–Ion Collider (EIC) is expected to construct at the Brookhaven National Laboratory in the future. Each proposal has its advantages and disadvantages in terms of readiness, cost, schedule, and physics reach, and each proposal requires the design and production of specific new detectors. This paper first presents the performances necessary for future silicon tracking systems at the various new facilities. Then it illustrates a few possibilities for the realization of such silicon trackers. The challenges posed by the future facilities require a new family of silicon detectors, where features such as impact ionization, radiation damage saturation, charge sharing, and analog read-out are exploited to meet these new demands.

Low Gain Avalanche Diodes (LGADs) are now considered a viable solution for 4D-tracking thanks to their excellent time resolution and good resistance to high radiation fluence. However, the currently available LGAD technology is well suited only for applications that require coarse space precision, pixels with pitch in the range 500 µm–1 mm, due to the presence of a no-gain region between adjacent pixels of about 50μm, in which the gain is completely suppressed. In this paper, we will discuss the segmentation issues in the LGAD technology and we will present two new segmentation strategies aimed at producing LGADs with high spatial resolution and high fill factor. The first presented design is the so-called Trench-Isolated LGAD (TI-LGAD). Here, the pixel isolation is provided by trenches, physically etched in the silicon and then filled with silicon oxide. The second design is the Resistive AC-coupled Silicon Detector (RSD), an evolution of LGADs, where the segmentation is obtained by means of AC-coupled electrodes. Prototypes of both designs have been produced at FBK and characterized at the Laboratories for Innovative Silicon Sensors (INFN and University of Turin) by means of a laser setup to estimate the space resolution and the fill factor. The functional characterization shows that both the technologies yield fully working small pixel LGADs (down to 50 µm), providing the first examples of sensors able to concurrently measure space and time with excellent precision.

A search for the decay KS→π0e+e− has been made by the NA48/1 experiment at the CERN SPS accelerator. Using data collected during 89 days in 2002 with a high-intensity KS beam, 7 events were found with a background of 0.15 events. The branching fraction BR(KS→π0e+e−,mee>0.165 GeV/c2)=(3.0+1.5−1.2(stat)±0.2(syst))×10−9 has been measured. Using a vector matrix element and a form factor equal to one, the measurement gives BR(KS→π0e+e−)=(5.8+2.9−2.4)×10−9.

A bstract The NA62 experiment reports an investigation of the $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>K</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>ν</mml:mi> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> mode from a sample of K + decays collected in 2017 at the CERN SPS. The experiment has achieved a single event sensitivity of (0 . 389 ± 0 . 024) × 10 − 10 , corresponding to 2.2 events assuming the Standard Model branching ratio of (8 . 4 ± 1 . 0) × 10 − 11 . Two signal candidates are observed with an expected background of 1.5 events. Combined with the result of a similar analysis conducted by NA62 on a smaller data set recorded in 2016, the collaboration now reports an upper limit of 1 . 78 × 10 − 10 for the $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>K</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>ν</mml:mi> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> branching ratio at 90% CL. This, together with the corresponding 68% CL measurement of ( $$ {0.48}_{-0.48}^{+0.72} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msubsup> <mml:mn>0.48</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.48</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.72</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) × 10 − 10 , are currently the most precise results worldwide, and are able to constrain some New Physics models that predict large enhancements still allowed by previous measurements.

Abstract RSDs (Resistive AC-Coupled Silicon Detectors) are n-in-p silicon sensors based on the LGAD (Low-Gain Avalanche Diode) technology, featuring a continuous gain layer over the whole sensor area. The truly innovative feature of these sensors is that the signal induced by an ionising particle is seen on several pixels, allowing the use of reconstruction techniques that combine the information from many read-out channels. In this contribution, the first application of a machine learning technique to RSD devices is presented. The spatial resolution of this technique is compared to that obtained with the standard RSD reconstruction methods that use analytical descriptions of the signal sharing mechanism. A Multi-Output regressor algorithm, trained with a combination of simulated and real data, leads to a spatial resolution of less than 2 μm for a sensor with a 100 μm pixel. The prospects of future improvements are also discussed.

The book describes the development of innovative silicon sensors known as ultra-fast silicon detectors for use in the space-time tracking of charge particles. The first comprehensive collection of information on the topic, otherwise currently scattered in existing literature, this book presents a comprehensive introduction to the development of ultra-fast silicon detectors with the latest technology and applications from the field. It will be an ideal reference for graduate and postgraduates studying high energy and particle physics and engineering, in addition to researchers in the area. Key features Authored by a team of subject area specialists, whose research group first invented ultra-fast silicon detectors The first book on the topic to explain the details of the design of silicon sensors for 4-dimensional tracking Presents state-of-the-art results, and prospects for further performance evolutions &nbsp; The Open Access version of this book, available at&nbsp;www.taylorfrancis.com/books/oa-mono/10.1201/9781003131946/, has been made available under a Creative Commons Attribution-Non Commercial-No Derivatives 4.0 license. Cover image credit goes to Marta Tornago &nbsp; &nbsp;

The past ten years have seen the advent of silicon-based precise timing detectors for charged particle tracking. The underlying reason for this evolution is a design innovation: the Low-Gain Avalanche Diode (LGAD). In its simplicity, the LGAD design is an obvious step with momentous consequences: low gain leads to large signals maintaining sensors stability and low noise, allowing sensor segmentation. Albeit introduced for a different reason, to compensate for charge trapping in irradiated silicon sensors, LGAD found fertile ground in the design of silicon-based timing detectors. Spurred by this design innovation, solid-state-based timing detectors for charged particles are going through an intense phase of R&D, and hybrid and monolithic sensors, with or without internal gain, are being explored. This contribution offers a review of this booming field.

The NA62 experiment at CERN, designed to study the ultra-rare decay $K^+ \to \pi^+\nu\overline{\nu}$, has also collected data in beam-dump mode. In this configuration, dark photons may be produced by protons dumped on an absorber and reach a decay volume beginning 80 m downstream. A search for dark photons decaying in flight to $\mu^+\mu^-$ pairs is reported, based on a sample of $1.4 \times 10^{17}$ protons on dump collected in 2021. No evidence for a dark photon signal is observed. A region of the parameter space is excluded at 90% CL, improving on previous experimental limits for dark photon masses between 215 and 550 MeV$/c^2$.

A bstract The NA62 experiment at CERN, designed to study the ultra-rare decay K + → π + $$ \nu \overline{\nu} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>ν</mml:mi> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> , has also collected data in beam-dump mode. In this configuration, dark photons may be produced by protons dumped on an absorber and reach a decay volume beginning 80 m downstream. A search for dark photons decaying in flight to μ + μ − pairs is reported, based on a sample of 1 . 4 × 10 17 protons on dump collected in 2021. No evidence for a dark photon signal is observed. A region of the parameter space is excluded at 90% CL, improving on previous experimental limits for dark photon masses between 215 and 550 MeV /c 2 .

The response of the CMS barrel calorimeter (electromagnetic plus hadronic) to hadrons, electrons and muons over a wide momentum range from 2 to 350 GeV/c has been measured. To our knowledge, this is the widest range of momenta in which any calorimeter system has been studied. These tests, carried out at the H2 beam-line at CERN, provide a wealth of information, especially at low energies. The analysis of the differences in calorimeter response to charged pions, kaons, protons and antiprotons and a detailed discussion of the underlying phenomena are presented. We also show techniques that apply corrections to the signals from the considerably different electromagnetic (EB) and hadronic (HB) barrel calorimeters in reconstructing the energies of hadrons. Above 5 GeV/c, these corrections improve the energy resolution of the combined system where the stochastic term equals 84.7±1.6% and the constant term is 7.4±0.8%. The corrected mean response remains constant within 1.3% rms.

Segmented silicon sensors with internal gain, the so called Ultra-FAST Silicon Detectors (UFSD), have been produced at FBK for the first time. UFSD are based on the concept of Low-Gain Avalanche Detectors (LGAD), which are silicon detectors with an internal, low multiplication mechanism (gain ∼ 10). This production houses two main type of devices: one type where the gain layer is on the same side of the read-out electrodes, the other type where the gain layer is on the side opposite to the pixellated electrodes (reverse-LGAD). Several technological splits have been included in the first production run, with the aim to tune the implantation dose of the multiplication layer, which controls the gain value of the detector. An extended testing on the wafers has been performed and the results are in line with simulations: the fabricated detectors show good performances, with breakdown voltages above 1000 Volts, and gain values in the range of 5–60 depending on the technological split. The detectors timing resolution has been measured by means of a laboratory setup based on an IR picosecond laser. The sample with higher gain shows time resolution of 55 ps at high reverse bias voltage, indicating very promising performance for future particle tracking applications.

In the past few years, there has been growing interest in the development of silicon sensors able to simultaneously measure accurately the time of passage and the position of impinging charged particles. In this contribution, a review of the progresses in the design of UFSD (Ultra-Fast Silicon Detectors) sensors, manufactured at the FBK (Fondazione Bruno Kessler) Foundry, aiming at tracking charged particles in 4 dimensions, is presented. The state-of-the-art UFSD sensors, with excellent timing capability, are planned to be used in both ATLAS and CMS experiments detector upgrade, in order to reduce the background due to the presence of overlapping events in the same bunch crossing. The latest results on sensors characterization including time resolution, radiation resistance and uniformity of the response are here summarized, pointing out the interplay between the design of the gain layer and the UFSD performances. The research is now focusing on the maximization of the sensor fill factor, to be able to reduce the pixel size, exploring the implementation of shallow trenches for the pixel isolation and the development of resistive AC-coupled UFSD sensors. In conclusion, a brief review on research paths tailored for detection of low energy X-rays or for low material budget applications is given.

Searches for the lepton number violating K þ → π -μ þ e þ decay and the lepton flavor violatingand π 0 → μ -e þ decays are reported using data collected by the NA62 experiment at CERN in 2017-2018.No evidence for these decays is found and upper limits of the branching ratios are obtained at 90% confidence level:and Bðπ 0 → μ -e þ Þ < 3.2 × 10 -10 .These results improve by 1 order of magnitude over previous results for these decay modes.

In the past few years, the need of measuring accurately the spatial and temporal coordinates of the particles generated in high-energy physics experiments has spurred a strong R&D in the field of silicon sensors. Within these research activities, the so-called Ultra-Fast Silicon Detectors (UFSDs), silicon sensors optimized for timing based on the Low-Gain Avalanche Diode (LGAD) design, have been proposed and adopted by the CMS and ATLAS collaborations for their respective timing layers. The defining feature of the Ultra-Fast Silicon Detectors (UFSDs) is the internal multiplication mechanism, determined by the gain layer design. In this paper, the performances of several types of gain layers, measured with a telescope instrumented with a 90Sr β-source, are reported and compared. The measured sensors are produced by Fondazione Bruno Kessler (FBK) and Hamamatsu Photonics (HPK). The sensor yielding the best performance, both when new and irradiated, is an FBK 45 μm-thick sensor with a carbonated deep gain implant, where the carbon and the boron implants are annealed concurrently with a low thermal load. This sensor is able to achieve a time resolution of 40 ps up to a radiation fluence of 2.5⋅1015 neq/cm2, delivering at least 5 fC of charge.

A search for the decay KS→π0μ+μ− has been made by the NA48/1 Collaboration at the CERN SPS accelerator. The data were collected during 2002 with a high-intensity KS beam. Six events were found with a background expectation of 0.22−0.11+0.18 events. Using a vector matrix element and unit form factor, the measured branching ratio is B(KS→π0μ+μ−)=[2.9−1.2+1.5(stat)±0.2(syst)]×10−9.

Calibration of the relative response of the individual channels of the barrel electromagnetic calorimeter of the CMS detector was accomplished, before installation, with cosmic ray muons and test beams. One fourth of the calorimeter was exposed to a beam of high energy electrons and the relative calibration of the channels, the intercalibration, was found to be reproducible to a precision of about 0.3%. Additionally, data were collected with cosmic rays for the entire ECAL barrel during the commissioning phase. By comparing the intercalibration constants obtained with the electron beam data with those from the cosmic ray data, it is demonstrated that the latter provide an intercalibration precision of 1.5% over most of the barrel ECAL. The best intercalibration precision is expected to come from the analysis of events collected in situ during the LHC operation. Using data collected with both electrons and pion beams, several aspects of the intercalibration procedures based on electrons or neutral pions were investigated.

Ultra Fast Silicon Detectors (UFSD) are sensors optimized for timing measurements employing a thin multiplication layer to increase the output signal. A multipurpose read-out board hosting a low-cost, low-power fast amplifier was designed at the University of Kansas and tested at the European Organization for Nuclear Research (CERN) using a 180 GeV pion beam. The amplifier has been designed to read out a wide range of detectors and it was optimized in this test for the UFSD output signal. In this paper we report the results of the experimental tests using 50 μm thick UFSD with a sensitive area of 1.4mm2. A timing precision below 30 ps wasachieved.

We review the progress toward the development of a novel type of silicon detectors suited for tracking with a picosecond timing resolution, the so called Ultra-Fast Silicon Detectors. The goal is to create a new family of particle detectors merging excellent position and timing resolution with GHz counting capabilities, very low material budget, radiation resistance, fine granularity, low power, insensitivity to magnetic field, and affordability. We aim to achieve concurrent precisions of ∼ 10 ps and ∼ 10 μm with a 50 μm thick sensor. Ultra-Fast Silicon Detectors are based on the concept of Low-Gain Avalanche Detectors, which are silicon detectors with an internal multiplication mechanism so that they generate a signal which is factor ∼ 10 larger than standard silicon detectors.

The GigaTracKer (GTK) is the beam spectrometer of the CERN NA62 experiment. The detector features challenging design specifications, in particular a peak particle flux reaching up to 2.0 MHz/mm$^2$, a single hit time resolution smaller than 200 ps and, a material budget of 0.5% X$_0$ per tracking plane. To fulfill these specifications, novel technologies were especially employed in the domain of silicon hybrid time-stamping pixel technology and micro-channel cooling. This article describes the detector design and reports on the achieved performance.

In this work, we introduce a new design concept: the DC-coupled Resistive Silicon Detectors, based on the LGAD technology.This new design intends to address a few known drawbacks of the first generation of AC-coupled Resistive Silicon Detectors (RSD).The sensor behaviour is simulated using a fast hybrid approach based on a combination of two packages, Weightfield2 and LTSpice.The simulation demonstrates that the key features of the RSD design are maintained, yielding excellent space and time resolutions: a few tens of ps and a few microns.In this report, we will outline the optimization methodology and the results of the simulation.We will also present detailed studies on the effects induced by the choice of key design parameters on the space and time resolutions provided by this sensor.

A search for the $K^+\to\mu^-\nu e^+e^+$ decay, forbidden within the Standard Model by either lepton number or lepton flavour conservation depending on the flavour of the emitted neutrino, has been performed using the dataset collected by the NA62 experiment at CERN in 2016--2018. An upper limit of $8.1\times 10^{-11}$ is obtained for the decay branching fraction at 90% CL, improving by a factor of 250 over the previous search.

The basic principle of operation of silicon sensors with resistive read-out is built-in charge sharing. Resistive Silicon Detectors (RSD, also known as AC-LGAD), exploiting the signals seen on the electrodes surrounding the impact point, achieve excellent space and time resolutions even with very large pixels. In this paper, a TCT system using a 1064 nm picosecond laser is used to characterize RSD sensors produced by Fondazione Bruno Kessler. The paper first introduces the parametrization of the errors in the determination of the position and time coordinates in RSD, then outlines the reconstruction method, and finally presents the results. Three different pixel pitches are used in the analysis: 200 × 340, 450 × 450, and 1300 × 1300 μm2. At gain = 30, the 450 × 450 μm2 pixel achieves a time jitter of 20 ps and a spatial resolution of 15 μm concurrently, while the 1300 × 1300 μm2 pixel achieves 30 ps and 30 μm, respectively. The implementation of cross-shaped electrodes improves considerably the response uniformity over the pixel surface.

Abstract The recently developed Low-Gain Avalanche Diode (LGAD) technology has gained growing interest within the high-energy physics (HEP) community, thanks to its capability of internal signal amplification that improves the particle detection. Since the next generation of HEP experiments will require tracking detectors able to efficiently operate in environments where expected fluences will exceed 1 × 10 17 1 MeV n eq /cm 2 , the design of radiation-resistant particle detectors becomes of utmost importance. To this purpose, Technology Computer-Aided Design (TCAD) simulations are a relevant part of the current detector R&amp;D, not only to support the sensor design and optimization, but also for a better understanding and modelling of radiation damage. In this contribution, the recent advances in the TCAD modelling of non-irradiated and irradiated LGAD sensors are presented, whose validation relies on the agreement between the simulated and experimental data — in terms of current-voltage (I-V), capacitance-voltage (C-V), and gain-voltage (G-V) characteristics, coming from devices manufactured by Hamamatsu Photonics (HPK), and accounting for different irradiation levels and temperatures.

Abstract RSDs are LGAD silicon sensors with 100% fill factor, based on the principle of AC-coupled resistive read-out. Signal sharing and internal charge multiplication are the RSD key features to achieve picosecond-level time resolution and micron-level spatial resolution, thus making these sensors promising candidates as 4D-trackers for future experiments. This paper describes the use of a neural network to reconstruct the hit position of ionizing particles, an approach that can boost the performance of the RSD with respect to analytical models. The neural network has been trained in the laboratory and then validated on test beam data. The device-under-test in this work is a 450 μm-pitch matrix from the FBK RSD2 production, which achieved a resolution of about 65 μm at the DESY Test Beam Facility, a 50% improvement compared to a simple analytical reconstruction method, and a factor two better than the resolution of a standard pixel sensor of equal pitch size with binary read-out. The test beam result is compatible with the laboratory ones obtained during the neural network training, confirming the ability of the machine learning model to provide accurate predictions even in environments very different from the training one. Prospects for future improvements are also discussed.

This paper reports on the spatial and temporal resolutions of an RSD 450 microns pitch pixels array measured at the DESY test beam facility. RSDs, Resistive Silicon Detectors, also known as AC-LGAD, achieve excellent position and temporal resolution by exploiting charge sharing among neighboring electrodes. The RSD matrix used in this study is part of the second FBK RSD production, RSD2, composed of 450-micron pitch pixels with cross-shaped electrodes. A 7-pixel matrix was read out by the FAST2 ASIC, a 16-channel amplifier fully custom ASIC developed by INFN Torino using the 110 nm CMOS technology. The total area covered by the matrix is about 1.5 mm2. The position resolution reached in this test is 15 microns, about 3.4% of the pitch. The temporal resolution achieved in this work is 60 ps, dominated by the FAST2 resolution. The work also demonstrates that RSD sensors achieve 100% fill factor and homogenous resolutions over the whole matrix surface, making them suitable for 4D tracking applications.

This paper reports on a measurement campaign to characterize the inter-pad region of Ultra-Fast Silicon Detectors (UFSDs) manufactured by Fondazione Bruno Kessler. The devices under test are either pixel or strip arrays, featuring a large number of different inter-pad layouts; both pre-irradiation and irradiated sensors have been measured. The aim of the study is to link the design parameters of the inter-pad region to the operation of the sensors, providing insights into the design of UFSD arrays with narrow inter-pad gaps. We concluded that, in the UFSD design, the doping level and the area of the p-stop should be kept low, in order to avoid the early breakdown of the device and the micro-discharges effect; UFSDs with such characteristics proved also rather insensitive to floating pads and irradiation. Thanks to these findings, it was possible to design a UFSD array that yields the expected performance with an inter-pad width as small as 25 μm, significantly improving its fill factor with respect to standard designs. Two innovative experimental techniques are presented in this work: the first one is based on a TCT setup, the second makes use of an ultra-low light CCD camera.

The GigaTracKer is a hybrid silicon pixel detector designed for the fixed-target experiment NA62 at the CERN SPS aiming to measure the branching ratio of the very rare kaon decay $K^+ \rightarrow \pi^+\nu \bar{\nu}$ with 10\% precision. The detector has to provide measurements of momentum, direction and time of beam particles arriving at a rate of 750 MHz. The tracking system consists of four stations installed in vacuum ($\sim10^{-6}$ mbar), $60.8 \times 27\ \text{mm}^2$ each, with a total material budget of less than 2X$_0$. Each station is cooled with a microchannel cooling plate used for the first time in a high energy physics experiment. The beam particles are tracked in 4 dimensions by means of time-stamping pixels ($300\times300\ \mu \text{m}^{2}$) with the single hit time resolution reaching 115 ps. This performance has to be maintained despite the beam irradiation amounting to a yearly fluence of $4.5 \times 10^{14}\ 1MeV\ n_{\text{eq}}/200\ \text{days}$. The detector has been fully operational since 2016. We describe the GigaTracKer design and performance in the 2016-2022 years of NA62 data taking.

A bstract The NA48/2 experiment at CERN reports the first observation of the K ± → π 0 π 0 μ ± ν decay based on a sample of 2437 candidates with 15% background contamination collected in 2003–2004. The decay branching ratio in the kinematic region of the squared dilepton mass above 0.03 GeV 2 / c 4 is measured to be (0.65 ± 0.03) × 10 − 6 . The extrapolation to the full kinematic space, using a specific model, is found to be (3.45 ± 0.16) × 10 − 6 , in agreement with chiral perturbation theory predictions.

Abstract EXFLU1 is a new batch of radiation-resistant silicon sensors manufactured at Fondazione Bruno Kessler (FBK, Italy). The EXFLU1 sensors utilize thin substrates that remain operable even after extensive irradiation. They incorporate Low-Gain Avalanche Diode (LGAD) technology, enabling internal multiplication of charge carriers to boost the small signal produced by a particle crossing their thin active thicknesses, ranging from 15 to 45 μ m. To address current challenges related to acceptor removal, the EXFLU1 production incorporates improved defect engineering techniques. This includes the so called carbonated LGADs, where carbon doping is implanted alongside boron in the gain layer. This contribution focuses on evaluating the performances of thin sensors with carbonated gain layer from the EXFLU1 production, before and after irradiation up to 2.5· 10 15 n 1 Mev eq. /cm 2 . The conducted tests involve static and transient characterizations, including I-V and C-V measurements, as well as laser and β -source tests. This work aims to present the state of the art in LGAD sensor technology with a carbonated gain layer and shows the characterization of the most radiation-resistant LGAD sensors produced to date.

This paper summarizes the beam test results obtained with a Resistive Silicon Detector (RSD) (also called AC-Low Gain Avalanche Diode, AC-LGAD) pixel array tested at the DESY beam test facility with a 5 GeV/c electron beam. Furthermore, it describes in detail the simulation results of DC-RSD, an evolution of the RSD design. The simulations campaign described in this paper has been instrumental in the definition of the structures implemented in the Fondazione Bruno Kessler FBK first DC-RSD production. The RSD matrix used in this study is part of the second FBK RSD production, RSD2. The best position resolution reached in this test is σx=15 μm, about 3.4% of the pitch. DC-RSD LGAD, are an evolution of the AC-coupled design, eliminating the dielectric and using a DC-coupling to the electronics. The concept of DC-RSD has been finalized using full 3D Technology-CAD simulations of the sensor behavior. TCAD simulations are an excellent tool for designing this innovative class of detectors, enabling the evaluation of different technology options (e.g., the resistivity of the n+ layer, contact materials) and geometrical layouts (shape and distance of the read-out pads).

Abstract The NA62 experiment at CERN utilises a differential Cherenkov counter with achromatic ring focus (CEDAR) for tagging kaons within an unseparated monochromatic beam of charged hadrons. The CEDAR-H detector was developed to minimise the amount of material in the path of the beam by using hydrogen gas as the radiator medium. The detector was shown to satisfy the kaon tagging requirements in a test-beam before installation and commissioning at the experiment. The CEDAR-H performance was measured using NA62 data collected in 2023.

We report on the measurement of the direct emission (DE) and interference (INT) terms of the K\pm -> \pi\pm\pi^0 g decay by the NA48/2 experiment at the CERN SPS. From the data collected during 2003 and 2004 about 600k such decay candidates have been selected. The relative amounts of DE and INT with respect to the internal bremsstrahlung (IB) contribution have been measured in the range 0<T*\pi<80 MeV: Frac_{DE} (0<T*\pi<80 MeV) = (3.32\pm 0.15_{stat} \pm 0.14_{sys})x10^{-2} Frac_{INT} (0<T*\pi<80 MeV) = (- 2.35\pm 0.35_{stat} \pm 0.39_{sys})x10^{-2}, where T*pi is the kinetic energy of the charged pion in the kaon rest frame. This is the first observation of an interference term in T*\pi decays. In addition, a limit on the CP violating asymmetry in the K^+ and K^- branching ratios for this channel has been determined to be less than 1.5x10^{-3} at 90% confidence level.

In this paper we report measurements of the uniformity of time resolution, signal amplitude, and charged particle detection efficiency across the sensor surface of low-gain avalanche detectors (LGAD). Comparisons of the performance of sensors with different doping concentrations and different active thicknesses are presented, as well as their temperature dependence and radiation tolerance up to 6×1014 n/cm2. Results were obtained at the Fermilab test beam facility using 120 GeV proton beams, and a high precision pixel tracking detector. LGAD sensors manufactured by the Centro Nacional de Microelectrónica (CNM) and Hamamatsu Photonics (HPK) were studied. The uniformity of the sensor response in pulse height before irradiation was found to have a 2% spread. The signal detection efficiency and timing resolution in the sensitive areas before irradiation were found to be 100% and 30–40 ps, respectively. A “no-response” area between pads was measured to be about 130 μm for CNM and 170μm for HPK sensors. After a neutron fluence of 6×1014 n/cm2 the CNM sensor exhibits a large gain variation of up to a factor of 2.5 when comparing metalized and non-metalized sensor areas. An irradiated CNM sensor achieved a time resolution of 30 ps for the metalized area and 40 ps for the non-metalized area, while a HPK sensor irradiated to the same fluence achieved a 30 ps time resolution.

The properties of 50 um thick Low Gain Avalanche Diode (LGAD) detectors manufactured by Hamamatsu photonics (HPK) and Fondazione Bruno Kessler (FBK) were tested before and after irradiation with 1 MeV neutrons. Their performance were measured in charge collection studies using b-particles from a 90Sr source and in capacitance-voltage scans (C-V) to determine the bias to deplete the gain layer. Carbon infusion to the gain layer of the sensors was tested by FBK in the UFSD3 production. HPK instead produced LGADs with a very thin, highly doped and deep multiplication layer. The sensors were exposed to a neutron fluence from 4e14 neq/cm2 to 4e15 neq/cm2. The collected charge and the timing resolution were measured as a function of bias voltage at -30C, furthermore the profile of the capacitance over voltage of the sensors was measured.

From 56 days of data taking in 2002, the NA48/1 experiment observed 6316 Ξ0→Σ+e−ν¯e candidates (with the subsequent Σ+→pπ0 decay) and 555 Ξ0¯→Σ+¯e+νe candidates with background contamination of 215±44 and 136±8 events, respectively. From these samples, the branching ratios BR(Ξ0→Σ+e−ν¯e)=(2.51±0.03stat±0.09syst)×10−4 and BR(Ξ0¯→Σ+¯e+νe)=(2.55±0.14stat±0.10syst)×10−4 were measured allowing the determination of the CKM matrix element |Vus|=0.209−0.028+0.023. Using the Particle Data Group average for |Vus| obtained in semileptonic kaon decays, we measured the ratio g1/f1=1.20±0.05 of the axial-vector to vector form factors.

Ensuring the radiation hardness of PbWO4 crystals was one of the main priorities during the construction of the electromagnetic calorimeter of the CMS experiment at CERN. The production on an industrial scale of radiation hard crystals and their certification over a period of several years represented a difficult challenge both for CMS and for the crystal suppliers. The present article reviews the related scientific and technological problems encountered.

The properties of 60-μm thick Ultra-Fast Silicon Detectors (UFSD) detectors manufactured by Fondazione Bruno Kessler (FBK), Trento (Italy) were tested before and after irradiation with minimum ionizing particles (MIPs) from a 90Sr β-source. This FBK production, called UFSD2, has UFSDs with gain layer made of Boron, Boron low-diffusion, Gallium, carbonated Boron and carbonated Gallium. The irradiation with neutrons took place at the TRIGA reactor in Ljubljana, while the proton irradiation took place at CERN SPS. The sensors were exposed to a neutron fluence of 4⋅1014, 8⋅1014, 1.5⋅1015, 3⋅1015, 6⋅ 1015 neq/cm2 and to a proton fluence of 9.6⋅ 1014 p/cm2, equivalent to a fluence of 6⋅ 1014 neq/cm2. The internal gain and the timing resolution were measured as a function of bias voltage at -20oC. The timing resolution was extracted from the time difference with a second calibrated UFSD in coincidence, using the constant fraction method for both.

The upgrades of the CMS and ATLAS experiments for the high luminosity phase of the Large Hadron Collider will employ precision timing detectors based on Low Gain Avalanche Detectors (LGADs). We present a suite of results combining measurements from the Fermilab Test Beam Facility, a beta source telescope, and a probe station, allowing full characterization of the HPK type 3.1 production of LGAD prototypes developed for these detectors. We demonstrate that the LGAD response to high energy test beam particles is accurately reproduced with a beta source. We further establish that probe station measurements of the gain implant accurately predict the particle response and operating parameters of each sensor, and conclude that the uniformity of the gain implant in this production is sufficient to produce full-sized sensors for the ATLAS and CMS timing detectors.

The KS→γγ decay rate has been measured with the NA48 detector using a high intensity short neutral beam from the CERN SPS. The measured branching ratio BR(KS→γγ)=(2.78±0.06stat±0.04syst)×10−6, obtained from 7461±172 KS→γγ events, is significantly higher than the O(p4) prediction of chiral perturbation theory. Using a KL beam the ratio Γ(KL→γγ)Γ(KL→π0π0π0)=(2.81±0.01stat±0.02syst)×10−3 has been measured.

Measured ratios of decay rates for ${\mathcal{R}}_{K e 3 / K2\pi}$ , ${\mathcal{R}}_{K \mu3 / K2\pi}$ and ${\mathcal{R}}_{K \mu3 / Ke3}$ are presented. These measurements are based on K± decays collected in a dedicated run in 2003 by the NA48/2 experiment at CERN. The results obtained are ${\mathcal{R}}_{K e 3 / K2\pi} = 0.2496\pm0.0009 ({\text{stat}})\pm0.0004 ({\text{syst}})$ and ${\mathcal{R}}_{K \mu3 / K2\pi} = 0.1637\pm0.0006 ({\text{stat}})\pm0.0003 ({\text{syst}})$ . Using the PDG average for the K±→π±π0 normalisation mode, both values are found to be larger than the current values given by the particle data book and lead to a larger magnitude of the |Vus| CKM element than previously accepted. When combined with the latest particle data book value of |Vud|, the result is in agreement with unitarity of the CKM matrix. In addition, a new measured value of ${\mathcal{R}}_{K \mu3 / Ke3} = 0.656\pm0.003({\text{stat}})\pm0.001({\text{syst}})$ is compared to the semi-empirical predictions based on the latest form factor measurements.

The NA48/2 experiment at CERN collected two data samples with minimum bias trigger conditions in 2003 and 2004. A measurement of the rate and dynamic properties of the rare decay K±→π±γγ from these data sets based on 149 decay candidates with an estimated background of 15.5±0.7 events is reported. The model-independent branching ratio in the kinematic range z=(mγγ/mK)2>0.2 is measured to be BMI(z>0.2)=(0.877±0.089)×10−6, and the branching ratio in the full kinematic range assuming a particular Chiral Perturbation Theory description to be B(Kπγγ)=(0.910±0.075)×10−6.

The LHCb Vertex Locator (VELO) is used to reconstruct beam-gas interaction vertices which allows one to obtain precise profiles of the LHC beams.In LHCb, this information is combined with the profile of the reconstructed beam-beam collisions and with the LHC beam currents to perform precise measurements of the luminosity.This beam-gas imaging (BGI) method also allows one to study the transverse beam shapes, beam positions and angles in real time.Therefore, a demonstrator beam-gas vertex detector (BGV) based on scintillating fibre modules has been built and installed in LHC Ring 2 at point 4.

This paper describes the performance of a prototype timing detector, based on 50 μm thick Ultra Fast Silicon Detector, as measured in a beam test using a 180 GeV/c momentum pion beam. The dependence of the time precision on the pixel capacitance and bias voltage is investigated in this paper. A timing precision from 30 ps to 100 ps (RMS), depending on the pixel capacitance, has been measured at a bias voltage of 180 V.

The performance of the Ultra-Fast Silicon Detectors (UFSD) after irradiation with neutrons and protons is compromised by the removal of acceptors in the thin layer below the junction responsible for the gain. This effect is tested both with capacitance–voltage, C–V, measurements of the doping concentration and with measurements of charge collection, CC, using charged particles. We find a perfect linear correlation between the bias voltage to deplete the gain layer determined with C–V and the bias voltage to collect a defined charge, measured with charge collection. An example for the usefulness of this correlation is presented.

The distribution of the K±→π±π+π− decays in the Dalitz plot has been measured by the NA48/2 experiment at the CERN SPS with a sample of 4.71×108 fully reconstructed events. With the standard Particle Data Group parameterization the following values of the slope parameters were obtained: g=(−21.134±0.017)%, h=(1.848±0.040)%, k=(−0.463±0.014)%. The quality and statistical accuracy of the data have allowed an improvement in precision by more than an order of magnitude, and are such as to warrant a more elaborate theoretical treatment, including pion–pion rescattering, which is in preparation.

Using the full data set of the NA48/2 experiment, the decay K+- -> pi+- e+ e- gamma is observed for the first time, selecting 120 candidates with 7.3 +- 1.7 estimated background events. With K+- -> pi+- pi0D as normalisation channel, the branching ratio is determined in a model-independent way to be Br(K+- -> pi+- e+ e- gamma, m_eegamma > 260 MeV/c^2) = (1.19 +- 0.12_stat +- 0.04_syst) x 10^-8. This measured value and the spectrum of the e+ e- gamma invariant mass allow a comparison with predictions of Chiral Perturbation Theory.

As first observed by the NA48/2 experiment at the CERN SPS, the π0π0 invariant mass (M00) distribution from K±→π±π0π0 decay shows a cusp-like anomaly at M00=2m+, where m+ is the charged pion mass. An analysis to extract the ππ scattering lengths in the isospin I=0 and I=2 states, a0 and a2, respectively, has been recently reported. In the present work the Dalitz plot of this decay is fitted to a new empirical parameterization suitable for practical purposes, such as Monte Carlo simulations of K±→π±π0π0 decays.

The NA48/2 experiment at CERN reports the first observation of the K±→π±π0e+e− decay from an exposure of 1.7×1011 charged kaon decays recorded in 2003–2004. A sample of 4919 candidates with 4.9% background contamination allows the determination of the branching ratio in the full kinematic region, BR(K±→π±π0e+e−)=(4.24±0.14)×10−6. The study of the kinematic space shows evidence for a structure dependent contribution in agreement with predictions based on chiral perturbation theory. Several P- and CP-violating asymmetries are also evaluated.

The performance of electromagnetic calorimeter modules made of proton-irradiated PbWO4 crystals has been studied in beam tests. The modules, similar to those used in the Endcaps of the CMS electromagnetic calorimeter (ECAL), were formed from 5×5 matrices of PbWO4 crystals, which had previously been exposed to 24 GeV protons up to integrated fluences between 2.1× 1013 and 1.3× 1014 cm−2. These correspond to the predicted charged-hadron fluences in the ECAL Endcaps at pseudorapidity η = 2.6 after about 500 fb−1 and 3000 fb−1 respectively, corresponding to the end of the LHC and High Luminosity LHC operation periods. The irradiated crystals have a lower light transmission for wavelengths corresponding to the scintillation light, and a correspondingly reduced light output. A comparison with four crystals irradiated in situ in CMS showed no significant rate dependence of hadron-induced damage. A degradation of the energy resolution and a non-linear response to electron showers are observed in damaged crystals. Direct measurements of the light output from the crystals show the amplitude decreasing and pulse becoming faster as the fluence increases. The latter is interpreted, through comparison with simulation, as a side-effect of the degradation in light transmission. The experimental results obtained can be used to estimate the long term performance of the CMS ECAL.

The combination of precision space and time information in particle tracking, the so called 4D tracking, is being considered in the upgrade of the ATLAS, CMS and LHCb experiments at the High-Luminosity LHC, set to start data taking in 2024–2025. Regardless of the type of solution chosen, space–time tracking brings benefits to the performance of the detectors by reducing the background and sharpening the resolution; it improves tracking performances and simplifies tracks combinatorics. Space–time tracking also allows investigating new physics channels, for example it opens up the possibilities of new searches in long-living particles by measuring accurately the time of flight between the production and the decay vertexes. The foreseen applications of 4D tracking in experiments with very high acquisition rates, for example at HL-LHC, add one more dimension to the problem, increasing dramatically the complexity of the read-out system and that of the whole detector design: we call 5D tracking the application of 4D tracking in high rate environments.

In this contribution we describe the second run of RSD (Resistive AC-Coupled Silicon Detectors) designed at INFN Torino and produced by Fondazione Bruno Kessler (FBK), Trento. RSD are n-in-p detectors intended for 4D particle tracking based on the LGAD technology that get rid of any segmentation implant in order to achieve the 100% fill-factor. They are characterized by three key-elements, (i) a continuous gain implant, (ii) a resistive n-cathode and (iii) a dielectric coupling layer deposited on top, guaranteeing a good spatial reconstruction of the hit position while benefiting from the good timing properties of LGADs. We will start from the very promising results of our RSD1 batch in terms of tracking performances and then we will move to the description of the design of the RSD2 run. In particular, the principles driving the sensor design and the specific AC-electrode layout adopted to optimize the signal confinement will be addressed.

A bstract The NA62 experiment at CERN targets the measurement of the ultra-rare $$ {K}^{+}\to {\pi}^{+}\nu \overline{\nu} $$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msup> <mml:mi>K</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mo>→</mml:mo> <mml:msup> <mml:mi>π</mml:mi> <mml:mo>+</mml:mo> </mml:msup> <mml:mi>ν</mml:mi> <mml:mover> <mml:mi>ν</mml:mi> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> decay, and carries out a broad physics programme that includes probes for symmetry violations and searches for exotic particles. Data were collected in 2016–2018 using a multi-level trigger system, which is described highlighting performance studies based on 2018 data.

Using data taken during the year 2000 with the NA48 detector at the CERN SPS, a search for the CP violating decay K_S -> 3 pi0 has been performed. From a fit to the lifetime distribution of about 4.9 million reconstructed K0/K0bar -> 3 pi0 decays, the CP violating amplitude eta_000 = A(K_S -> 3 pi0)/A(K_L -> 3 pi0) has been found to be Re(eta_000) = -0.002 +- 0.011 +- 0.015 and Im(eta_000) = -0.003 +- 0.013 +- 0.017. This corresponds to an upper limit on the branching fraction of Br(K_S -> 3 pi0) < 7.4 x 10^-7 at 90% confidence level. The result is used to improve knowledge of Re(epsilon) and the CPT violating quantity Im(delta) via the Bell-Steinberger relation.

We report a measurement of the direct CP violation asymmetry parameter $A_g$ in charged kaon decays $K^\pm\to\pi^\pm\pi^+\pi^-$ by the NA48/2 experiment at the CERN SPS. The experiment has been designed not to be limited by systematics in the asymmetry measurement. Using $1.67\times 10^9$ such decays collected during the 2003 run, the charge asymmetry in the Dalitz plot linear slope parameter $g$ has been measured to be $A_g=(1.7\pm2.9)\times 10^{-4}$. The precision of the result is limited by the statistics used.

In this contribution we present our most recent numerical investigations towards the development of silicon particle detectors able to provide accurate measurements in both space and time (4D tracking). In particular, we discuss the performances of different Low-Gain Avalanche Diode (LGAD) detectors, by presenting comparisons between measurements and TCAD (Technology Computer-Aided Design) simulations, performed on several detectors fabricated by Fondazione Bruno Kessler (FBK, Italy), Centro Nacional de Microelectrónica (CNM, Spain) and Hamamatsu Photonics K.K. (HPK, Japan). To have a satisfactory timing resolution, carriers multiplication in LGAD has to be properly controlled through the implantation of a specific highly-dopedp-type layer underneath the n-cathode. This internal multiplication process is so crucial in view of having large output signals for accurate time measurements, that numerical simulation turns out to be one of the main tools in designing LGADs. For this reason, in this paper we present a simulation framework, where the most robust avalanche models - Massey, van Overstraeten-de Man and Okuto-Crowell - have been tested. Thus, at the end, we propose a reliable designing tool which is highly predictive in the field of research and development of LGADs.

The decay asymmetries of the weak radiative hyperon decays Ξ0→Λγ and Ξ0→Σ0γ have been measured with high precision using data of the NA48/1 experiment at CERN. From about 52 000 Ξ0→Λγ and 15 000 Ξ0→Σ0γ decays, we obtain for the decay asymmetries αΞ0→Λγ=−0.704±0.019stat±0.064syst and αΞ0→Σ0γ=−0.729±0.030stat±0.076syst, respectively. These results are in good agreement with previous experiments, but more precise.

Abstract Beam monitoring in particle therapy is a critical task that, because of the high flux and the time structure of the beam, can be challenging for the instrumentation. Recent developments in thin silicon detectors with moderate internal gain, optimized for timing applications (Ultra Fast Silicon Detectors, UFSD), offer a favourable technological option to conventional ionization chambers. Thanks to their fast collection time and good signal-to-noise ratio, properly segmented sensors allow discriminating and counting single protons up to the high fluxes of a therapeutic beam, while the excellent time resolution can be exploited for measuring the proton beam energy using time-of-flight techniques. We report here the results of the first tests performed with UFSD detector pads on a therapeutic beam. It is found that the signal of protons can be easily discriminated from the noise, and that the very good time resolution is confirmed. However, a careful design is necessary to limit large pile-up inefficiencies and early performance degradation due to radiation damage.

The Ultra Fast Silicon Detectors (UFSD) are a novel concept of silicon detectors based on the Low Gain Avalanche Diode (LGAD) technology, which are able to obtain time resolution of the order of few tens of picoseconds. First prototypes with different geometries (pads/pixels/strips), thickness (300 and 50 μm) and gain (between 5 and 20) have been recently designed and manufactured by CNM (Centro Nacional de Microelectrónica, Barcelona) and FBK (Fondazione Bruno Kessler, Trento). Several measurements on these devices have been performed in laboratory and in beam test and a dependence of the gain on the temperature has been observed. Some of the first measurements will be shown (leakage current, breakdown voltage, gain and time resolution on the 300 μm from FBK and gain on the 50 μm-thick sensor from CNM) and a comparison with the theoretically predicted trend will be discussed.

A search for direct CP-violation in K±→π±π0π0 decay based on 47.14 million events has been performed by the NA48/2 experiment at the CERN SPS. The asymmetry in the Dalitz plot linear slopes Ag=(g+−g−)/(g++g−) is measured to be Ag=(1.8±2.6)×10−4. The design of the experiment and the method of analysis provide good control of instrumental charge asymmetries in this measurement. The precision of the result is limited by statistics and is almost one order of magnitude better than that of previous measurements by other experiments.

A sample of 65210 K ± → π 0 π 0 e ± ν (K e4 00 ) decay candidates with 1% background contamination has been collected in 2003-2004 by the NA48/2 collaboration at the CERN SPS. A study of the differential rate provides the first measurement of the hadronic form factor variation in the plane (M 2 , M 2 ) and brings evidence for a cusp-like structure in the distribution of the squared π 0 π 0 invariant mass around $$ 4{m}_{\pi^{+}}^2 $$ . Exploiting a model independent description of this form factor, the branching ratio, inclusive of radiative decays, is obtained using the K ± → π 0 π 0 π ± decay mode as normalization. It is measured to be BR(K e4 00 ) = (2.552 ± 0.010stat ± 0.010syst ± 0.032ext) × 10−5, which improves the current world average precision by an order of magnitude while the 1.4% relative precision is dominated by the external uncertainty from the normalization mode. A comparison with the properties of the corresponding mode involving a π + π − pair (K e4 + − ) is also presented.

Detectors able to measure the time of flight with very high accuracy (∼10 ps RMS) are becoming fundamental in the design of new High Energy Physics experiments, where accurate time measurements will be used to mitigate pileup effects. The development of such detectors has spurred intense R&D in both silicon sensors and the associated readout electronics, aiming at obtaining silicon-based detectors with a time resolution in the few tens-of-picosecond range. This work presents FAST, a family of three different 20 channel amplifier-comparator chips, tailored to the readout of Ultra Fast Silicon Detectors. These ASICs have been designed optimizing the sensor-readout interplay with the aim of reaching the smallest possible jitter term. The three chips of the FAST family differ in the architecture of the front-end while sharing the channel back-end, consisting of a leading-edge discriminator and a LVDS driver. The goal of these front-ends is to achieve a time resolution of about 30 ps RMS while coupled to a sensor with a few pico-Farad capacitance, keeping the power budget of the single channel below 1.3 mW. This paper reports the description of the FAST design architecture and summarizes the results on the initial characterization of one chip of the FAST family, in a stand-alone test structure and when coupled to a UFSD.

A bstract A measurement of the form factors of charged kaon semileptonic decays is presented, based on 4.4 × 10 6 K ± → π 0 e ± ν e ( K e 3 ± ) and 2.3 × 10 6 K ± → π 0 μ ± ν μ ( K μ 3 ± ) decays collected in 2004 by the NA48/2 experiment. The results are obtained with improved precision as compared to earlier measurements. The combination of measurements in the K e 3 ± and K μ 3 ± modes is also presented.

Using the NA48 detector at the CERN SPS, 31 KS->pi0 gamma gamma candidates with an estimated background of 13.7 +- 3.2 events have been observed. This first observation leads to a branching ratio of BR(KS->pi0 gamma gamma) = (4.9 +- 1.6(stat) +- 0.9(syst)) x 10^-8 in agreement with Chiral Perturbation theory predictions.