D. Treille

DELPHI data collected at centre-of-mass energies up to 208 GeV have been analysed to search for charginos, neutralinos and sfermions in the framework of the Minimal Supersymmetric Standard Model (MSSM) with R-parity conservation. No evidence for a signal was found in any of the channels. The results of each search were used to derive limits on production cross-sections and particle masses. In addition, the combined result of all searches excludes regions in the parameter space of the constrained MSSM, leading to limits on the mass of the Lightest Supersymmetric Particle and other supersymmetric particles.

From the study of the reaction π−p→pFppπ− using a fast proton (pF) trigger device in the CERN Omega spectrometer, we find evidence for two narrow pp states produced mainly in association with a Δ° (1232) and a N° (1520). The statistical significance of each peak is greater than 6 standard deviations. Masses and natural widths of these resonances are respectively M = 2020 ± 3 MeV, Λ1 = 24 ± 12 MeV and M2 = 2204 ± 5 MeV, Λ2 = 16−16+20 MeV. Our data are consistent with a small production of the narrow ∼ 1935 MeV resonance already reported. Production cross sections for these new pp resonances are given.

Results of the three Orsay experiments are analyzed in the vector meson dominance hypothesis. The coupling constants of ϱo, ωo and φo mesons to the photons are determined, and the mixing angle θY and θN of the ωo and φo mesons are calculated. Our values are compared to various theoretical predictions.

We present results of measurements of K±p and pp elastic scattering and of the annihilation reactions pp→π+π− and pp→K+K− at an incident laboratory momentum of 5 GeV/c. Nearly complete angular distributions were obtained. Results are also presented for π-meson proton elastic scattering in the momentum transfer ranges 2 < −t < 8 (GeV/c)2 (for π+) and 0.16 < −t < 7 (GeV/c)2 (for π−). All measurements were done in one experimental geometry. The measured differential cross sections range from 10 to 10−5 mb/(GeV/c)2. For the first time, backward peaks have been observed in K−p and pp elastic scattering. In K+p elastic scattering the forward peak shows a break at −t ≈ 0.8 (GeV/c)2 and the backward peak shows a structure at −u ≈ 1.2 (GeV/c)2. Large angle πp scattering oscillates around an average differential cross section of 0.1 μb/(GeV/c)2. In addition to the well-known dip at −t ≈ 3 (GeV/c)2, we observe further dips at −t ≈ 4.6 and 5.7 (GeV/c)2. The annihilation reactions show both forward and backward peaks.

The differential cross section for the reaction $\ensuremath{\gamma}+p\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}n$ was measured at 19 photon energies between 300 and 750 MeV in the laboratory frame, for pion angles between 0\ifmmode^\circ\else\textdegree\fi{} and 130\ifmmode^\circ\else\textdegree\fi{} in the c.m. system. The pions were analyzed in angle and momentum with a magnetic spectrometer and detected by a counter telescope. The 0\ifmmode^\circ\else\textdegree\fi{} measurements could be achieved, in spite of the excessive positron rate, owing to a mass-spectrometer arrangement. No direct indication for the electromagnetic excitation of the ${P}_{11}$ resonance (1466 MeV) was found. Comparison is made with theoretical calculations of ${\ensuremath{\pi}}^{+}$ photoproduction.

Measurements have been made of relative production cross sections of the Jψ by π±, K±, p and p̄ at 39.5 GeV/c incident on copper. Jψ production rates from π−, K− and p̄ are similar. The Jψ relative particle/anti-particle production cross sections for x>0 are σ(π+)σ(π−)=(0.87±0.14), σ(K+)/σ(K−)=(0.85±0.5) and σ(p)σ(p̄)=(0.15 ±0.08). The small p/p̄ cross section ratio disagrees with models of J/ψ production by gluon amalgamation.

We present results on photoproduction of ϱ0 and ω in the reactions γp→π+π−p and γp→π+π−π0p by tagged photons in the energy ranges 20 to 70 GeV and 20 to 45 GeV, respectively. The production of the ϱ0 shows dominantly the characteristics of a diffractive process with respect to the Eγ and t dependence of the cross section and the spin density matrix. The ϱ0 photoproduction yields on average over the photon energy range a total cross section of σ(γp→ϱ0p) = 9.4±0.1 μb with an additional systematic error of ±1 μb, and average slope parameters of the t distribution dσ/dt≈exp(−b|t| + ct2), of b=9.1±0.1 GeV−2 and c = 3.1 ±±0.2 GeV−4. The shape of the ϱ0 peak in the π+π− invariant spectra shows a skewing similar to that observed at lower energies. The photoproduction of ω is also consistent with a diffractive process and has a cross section of σ(γp→ωp) = 1.2± 0.1 μb with an additional systematic error of ±0.2 μb. The average slope parameters of the t distribution are b=8.3 ± 1.3 GeV−2 and c = 3.4±2.6 GeV−4.

The elastic scattering of K± mesons on protons has been studied at 5 GeV/c. A total of about 500 000 events have been measured in the c.m. angular range 17° < θcm < 165° corresponding to 0.2 < −t < (GeV/c)2. We observed a K−p backward peak which we have parametrized as dσ/du = (0.6 ± 0.2) exp [(3.3 ± 0.6)u] μb/(GeV/c)2, while for the K+p backward peak we find dσ/du = (17.5 ± 1) exp [(3.6 ± 0.2)u] μb/(GeV/c)2. The K−p cross-section falls to about 0.03 μb(GeV/c)2 around −t = 5 (GeV/c)2, while the K+p cross-section stays in the vicinity of 0.3 μb(GeV/c)2 in the same t-region. The K+p and K−p differential cross-sections have cross-over points at −t = 0.2, 1.1 and about 3.5 (GeV/c)2.

We have measured the e+e−→φ reaction by the KS0KL0 and 3π decay modes of the φ. We have deduced Γ(φ→ all), Γ(φ→e+e−), as well as B(φ→KS0KL0), B(φ→K+K−) and B(φ→π+π−π0).

We have measured the e+e− → ø reaction by its K+K− decay mode. Using our previous results on KOKO and the 3π decay mode of the ø mesons, we compute Γø → e+e− and then compare the whole Orsay results to theoretical predictions.

Measurements of the reaction γp → pπ+π−π+π− are presented, in which π+π−π+π− systems with masses up to 3 GeV are produced from fragmentation of the incident photon. The reaction is dominated by production of the large peak of the ϱ′(1600) meson and, at higher masses ≳2 GeV, y production of jet-like 4π systems. The ϱ′(1600) meson is produced by a predominantly s-channel helicity conserving mechanism. At higher masses there are also indications of ϱπ peaks, of masses 1.3 GeV (the A2 meson) and 1.75 GeV, produced with a recoiling π meson by a mechanism consistent with the Deck effect.

The differential cross-section for 5 GeV/cπ+p and π−p elastic scattering have been measured in the c.m. angular region 27° < θcm < 130° corresponding to 0.5 < |t| < 7.8 (GeV/c)2. Dips are observed in both reactions at −t = 2.8 and 4.8 (GeV/c)2 where the cross-sections are approximately 0.1 μb/(GeV/c)2.

Angular distributions of π+ and K+p elastic scattering have been measured for an incident beam momentum of 10.0 GeV/c. For π+p elastic scattering almost the complete angular distribution was measured. The angular distribution of proton-proton elastic scattering was measured for an incident momentum of 9.0 GeV/c in the interval of the four-momentum transfer squared from 0.7 (GeV/c)2 to 5.0 (GeV/v)2. For π+p elastic scattering the structures at −t = 2.8 (GeV/c)2 and −t = 4.8 (GeV/c)2 are less pronounced than at lower momenta. The cross section for scattering at 90° in the c.m. system is of the order of 1 nb/GeV/c)2. For K+p elastic scattering is a break in the angular distribution around −t = 3 (GeV/c)2. The differential cross sections for proton-proton elastic scattering decrease smoothly with increasing momentum transfers.

The DELPHI Silicon Tracker, an ensemble of microstrips, ministrips and pixels, was completed in 1997 and has accumulated over 70pb−1 of high-energy data. The Tracker is optimised for the LEP2 physics programme. It consists of a silicon microstrip barrel and endcaps with layers of silicon pixel and ministrip detectors. In the barrel part, three-dimensional b tagging information is available down to a polar angle of 25°. Impact parameter resolutions have been measured of 28μm ⊕71/(psin3/2θ)μm in Rφ and 34μm ⊕69/pμm in Rz, where p is the track momentum in GeV/c. The amount of material has been kept low with the use of double-sided detectors, double-metal readout, and light mechanics. The pixels have dimensions of 330×330μm2 and the ministrips have a readout pitch of 200μm. The forward part of the detector shows average efficiencies of more than 96%, has signal-to-noise ratios of up to 40 in the ministrips, and noise levels at the level of less than one part per million in the pixels. Measurements of space points with low backgrounds are provided, leading to a vastly improved tracking efficiency for the region with polar angle less than 25°.

This report summarizes a study of the physics potential of the CLIC e+e- linear collider operating at centre-of-mass energies from 1 TeV to 5 TeV with luminosity of the order of 10^35 cm^-2 s^-1. First, the CLIC collider complex is surveyed, with emphasis on aspects related to its physics capabilities, particularly the luminosity and energy, and also possible polarization, \gamma\gamma and e-e- collisions. The next CLIC Test facility, CTF3, and its R&D programme are also reviewed. We then discuss aspects of experimentation at CLIC, including backgrounds and experimental conditions, and present a conceptual detector design used in the physics analyses, most of which use the nominal CLIC centre-of-mass energy of 3 TeV. CLIC contributions to Higgs physics could include completing the profile of a light Higgs boson by measuring rare decays and reconstructing the Higgs potential, or discovering one or more heavy Higgs bosons, or probing CP violation in the Higgs sector. Turning to physics beyond the Standard Model, CLIC might be able to complete the supersymmetric spectrum and make more precise measurements of sparticles detected previously at the LHC or a lower-energy linear e+e- collider: \gamma\gamma collisions and polarization would be particularly useful for these tasks. CLIC would also have unique capabilities for probing other possible extensions of the Standard Model, such as theories with extra dimensions or new vector resonances, new contact interactions and models with strong WW scattering at high energies. In all the scenarios we have studied, CLIC would provide significant fundamental physics information beyond that available from the LHC and a lower-energy linear e+e- collider, as a result of its unique combination of high energy and experimental precision.

A dipion enhancement of mass 1.59 GeV and width 0.23 GeV is observed in the channel γp→π+π−p. The spin-parity of the enhancement is consistent with being 1−.

Antiproton-proton elastic scattering has been measured at 5 GeV/c. A total of 30 000 events were observed in the angular range 17° < θcm < 136°, corresponding to 0.3 < −t < 7.7 (GeV/c2). In addition to the known dip at −t = 0.5 (GeV/c)2, we observe a structure at about −t = 2 (GeV/c)2 and a backward peak with a slope4.1 ± 0.6 (GeV/c)2. The extrapolated differential cross-section at u = 0 is 1.3 ± 0.8 μb/(GeV/c)2.

A new fixed-target experiment at the CERN SPS accelerator is proposed that will use decays of charm mesons to search for Heavy Neutral Leptons (HNLs), which are right-handed partners of the Standard Model neutrinos. The existence of such particles is strongly motivated by theory, as they can simultaneously explain the baryon asymmetry of the Universe, account for the pattern of neutrino masses and oscillations and provide a Dark Matter candidate. Cosmological constraints on the properties of HNLs now indicate that the majority of the interesting parameter space for such particles was beyond the reach of the previous searches at the PS191, BEBC, CHARM, CCFR and NuTeV experiments. For HNLs with mass below 2 GeV, the proposed experiment will improve on the sensitivity of previous searches by four orders of magnitude and will cover a major fraction of the parameter space favoured by theoretical models. The experiment requires a 400 GeV proton beam from the SPS with a total of 2x10^20 protons on target, achievable within five years of data taking. The proposed detector will reconstruct exclusive HNL decays and measure the HNL mass. The apparatus is based on existing technologies and consists of a target, a hadron absorber, a muon shield, a decay volume and two magnetic spectrometers, each of which has a 0.5 Tm magnet, a calorimeter and a muon detector. The detector has a total length of about 100 m with a 5 m diameter. The complete experimental set-up could be accommodated in CERN's North Area. The discovery of a HNL would have a great impact on our understanding of nature and open a new area for future research.

We present the first results on inclusive photo-production of prompt photons at high transverse momenta. The data were taken in an open spectrometer at CERN using a high intensity photon beam energy between 50 and 150 GeV. After subtracting the yield of photons from indirect sources, a clear excess is observed for transverse momenta above 2.5 GeVc Deep inelastic Compton scattering with appropriate QCD corrections account for this excess. The data disfavour the gauge integer charge quark models so far proposed.

The first results on inclusive photoproduction of π0 at transverse momenta up to 4 GeV/c, using incident γ energies between 50 and 150 GeV are presented. A comparison is made with inclusive π0 production obtained, in the same experiment, with incident π−. Using the π− data to parametrize the hadronic behaviour of the photon, significant differences are observed in quantitative agreement with QCD Compton scattering and corrections thereof.

The angular distributions for the two annihilation channels p̄p→π+π− and p̄p→ K+K− have been measured at 6.2 GeV/c. The two-pion channel shows peripheral peaks for π+ and π− going forward, and the two-kaon channel shows a peripheral peak for the K− going forward. The results have been compared with the line-reversed elastic backward scattering reactions and also with a constituent interchange model.

This report emphasizes the comparison between supersymmetric models and experiments. A minimal theoretical introduction is included as a guide to the interpretation of results. The existing constraints from low energy measurements, accelerator searches (LEP, Tevatron and HERA) and non-accelerator searches for neutralinos are presented. Prospects for upgrades of these facilities and for the LHC and linear collider are summarized. Most discussions are made in the framework of the minimal supersymmetric standard model inspired by supergravity (MSUGRA). But alternatives such as gauge mediated supersymmetry breaking (GMSB), anomaly mediated supersymmetry breaking (AMSB), models with R-parity violation and even alternatives to supersymmetry are also briefly considered.

Antiproton-proton elastic scattering data at 6.2 GeV/c in the range 0.3 (GeV/c)2 ⩽ −t ⩽ 10.0 (GeV/c)2 is presented. The experiment, using spark chambers and proportional chambers, was performed at the CERN Proton Synchroton. The dip at about −t = 0.5 and the structure at about −t = 2.2 (GeV/c)2 are equally strong at 6.2 GeV/c as they are at 5.0 GeV/c. The differential cross sections around 90° c.m. are not very different at 5.0 GeV/c and at 6.2 GeV/c, whereas in the backward region there is a decrease of the order of one magnitude.

The differential cross-sections for the annihilation processes pp→π−π+ and pp→K−K+ have been measured at an incident laboratory momentum at 5 GeV/c. Strong backward and forward peaks are observed in the π+π− differential cross-sections while the K+K− cross-section is shown to have a peak only for K− going forward. The annihilation cross-sections are compared with the cross-sections for the crossed channel backward processes π±p→pπ± and K±p→pK±.

This paper presents results on charm photoproduction in the energy interval 40 to 160 GeV, obtained from the high-statistics charm samples of the NA 14/2 experiment at CERN. We measure the charm cross-section, the distributions inx F andp 2 and various production ratios and charge asymmetries. The total non-diffractive open-charm cross-section per nucleon is measured to be $$\sigma _{(\gamma N \to c\bar cX)} $$ at 〈E γ〉 =100 GeV. We discuss the photoproduction of charm in terms of theoretical and phenomenological models. We compare the measuredp 2 andx F distributions with first-order QCD calculations of photon-gluon fusion and obtain a value for the charm-quark mass ofm c =1.5 +0.2 −0.1 GeV/c2.

The production and decay of a D0 have been observed in a nuclear emulsion exposed to a photon beam from the CERN super proton synchroton in conjunction with the Omega spectrometer. All the decay products have been detected and measured in the emulsion and in the spectrometer. The mass of the charmed particle is 1866 ± 8 MeV/c2 and its decay time is (2.26 ± 0.05) × 10−14 s.

The absolute cross-section σ of Bhabha scattering e+e− → e+e− has been measured at a total center of mass energy of 1020 MeV to test quantum electrodynamics. The result is compared to the theoretical cross-section modified by adding a positive or a negative metric pole to the photon propagator: 1q2 → 1q2 ± 1(q2−Λ2). A lower limit of Λ is obtained: Λ > 3.8 GeV (positive metric) or Λ > 2.0 GeV (negative metric).

The reaction γp→K+K−p has been investigated with photons in the energy range of 20<Eγ<36 GeV and with K+K− pairs in the mass range of MK+K−<2.0 GeV. The production of the φ(1019) contributes with a cross section σ(γp → φp) × BR(φ→K+K−) = 240±6 nb with an additional systematic error of ±20 nb. In the higher mass range of 1.05<MK+K−<2.0 GeV the production of K+K− pairs yields a cross section σ(γp→K+K−p) = 160±8 nb with an additional systematic error of +40−30 nb.

Exclusive and inclusive pp photoproduction (with ⩾ 2 accompanying charged particles) have been measured in the photon energy range 44–70 GeV. The exclusive process shows no prominent features; the inclusive process shows evidence for S(1936) production as a resonant-like structure with mass 1930 ± 2 MeV and width 12 ± 7 MeV.

K+p elastic scattering has been measured over nearly the whole angular range at an incident momentum of 10 GeV/c. The differential cross-section is found to decrease smoothly in the forward direction to -t ≈ 2 (GeV/c)2, where there is a change in slope, followed by a further decrease to -t ≈ 6 (GeV/c)2. Around 90° c.m. the cross-section is approximately 1 nb/(GeV/c)2, which is more than two orders of magnitude lower than at 5 GeV/c. The backward peak has no structure.

The operation and performance of a silicon vertex detector are described. The detector, which consists of a silicon multilayer active target followed by a microstrip chamber, was operated in a 1.2 T magnetic field and exposed to the NA14 high intensity photon beam. Results on the precision and efficiency of the detector for reconstruction of the interaction products are presented.

We have analyzed backward meson production in the reaction π−p → pfπ+π−π− at 9 GeV/c and 12 GeV/c incident π− momenta, from an experiment performed at the CERN Ω Spectrometer using a fast proton (pf) trigger device. We find strong production of quasi-two-body processes N∗ϱ and N∗f with a production mechanism consistent with u-channel nucleon exchange. At a lower level, we observe N∗π processes with a 3-body baryon decay through Δ(1232)π. In the (3π)− system, we find evidence for A1− and clear A2− backward production with similar cross sections (≈0.5 μb).

Negative results on backward production, via baryon exchange, of exotic non-strange mesons are presented. The reactions π−p→pforwardX−and π−n→ pforwardX−− have been studied with a 12 GeV/c π− beam in the Omega spectrometer at CERN. No resonant peak in X→ppπ−, ppπ−π−, ppπ−π0,π+π−π−π0 has been seen. The upper limits obtained on cross sections for exotic meson production X → NNπ, NNπ, 4π are lower than the ϱ− backward production cross section in the π−p→pϱ− reaction; this result seems to contradict the predictions of the two-component duality model. Compared to already published experiments in the search for exotics produced via baryon exchange, the sensitivity of this experiment is increased by an order of magnitude.

Measurements of ω and η production in the reaction γp→π+π−π+π−π0p are presented, showihg that the ωπ+π− channel is made up of an enhancement at ∼1.7 GeV of width ∼0.5 GeV, which is consistent with being a 1− state, together with a distribution at higher masses containing higher angular momentum terms. The ηπ+π− final state is produced with a similar cross section and also shows a low-mass enhancement.

From an experiment done with the CERN Omega spectrometer, triggered by a fast forward proton device, we present results on the differential cross section dσdu for π−p backward elastic scattering. The dσdu distribution agrees with an AeBu law. The compilation of existing results shows a discrepancy between results but the (dσdu)u=0 data fit perfectly an s2α0−2 dependence, as predicted by a single Δδ Regge trajectory exchange. A search for the reaction π−p→dp, with a fast forward deuteron, which can be produced by a double-baryon exchange mechanism, gives cross-section upper limits of ∼1% of the backward elastic cross section.

We have observed φ production in π−p interactions at 19 GeV/c with (44±10) events in the final state φπ+π−π−p and (45±9) events in φK+K−π−p. The production ratios φπ+π−π−p/ωπ+π −π−p ≈ 0.005 and φK+K−π−p/ϱ°K+K−π−p ≈ 0.45 agree with Zweig-rule expectations.

The angular distributions for positive pion photoproduction from hydrogen have been measured in the energy region 300–750 MeV, for pion angles between 50° and 130° in the center of mass system. The pions were analyzed in angle and momentum with a magnetic spectrometer and detected by a counters telescope.

This paper describes the trigger system of the DELPHI detector at LEP. It reports on the most relevant aspects of the hardware and shows the software strategies that have been developed to optimize its use. In 1993 the structure of the trigger in four decision levels has become fully operational and data collected during this period have been used to study the trigger performance. Various final state channels such as μ+μ−, e+e− and hadronic events were selected and their trigger efficiencies were calculated as a function of the polar angle θ. The results obtained indicate that, for any of the event topologies considered, the DELPHI trigger efficiency is independent of the θ angle, and, furthermore, the attained efficiency values are determined to be very close to 100% within an extremely good precision. This is a consequence of the high redundancy presently provided by all the DELPHI subdetectors. In addition to this analysis, events containing isolated particles either in the barrel or in the forward regions have been selected to evaluate the trigger response to single particles. Hence, trigger efficiencies for single particles have also been computed for charged tracks as a function of the momentum and for photons as a function of the deposited electromagnetic energy.

Evidence is presented for inclusive photoproduction of F± mesons in three decay modes, ηπ±, ηπ±π+π− and ηπ±π+π+π−π−. The average mass of the F± is found to be 2.020±0.010 GeV.

An unsuccessful search has been made for the reaction π−p→pFp̄n to find an M− state in p̄n corresponding to the M0 (2020) and M0 (2204) found earlier in the baryon exchange process π−p → Δ0 (or N0 (1520)) + M0. From this negative result, it is concluded that the previous reaction proceeds mainly by nucleon exchange and it is suggested that the NN̄ coupling for the M0 is more important than the Δ&#x0304;N coupling. Some indications are also given for the NN̄ mode as the main decay mode for these M0 states.

General features of the inclusive hadroproduction of muon pairs of mass greater than 1.5 GeV/c2 are shown. For incident π−, the data extend up to M=5.7 GeV/c2 (i.e.M2/s≈0.4). The scaling cross section M3dσ/dM for π−N reactions is much flatter than that for pN reactions and exceeds the latter by two to three orders of magnitude at large M2/s.

Results are presented on the inclusive photoproduction of λ and λ for incident photon energies between 25 and 70 GeV. The slope parameter of the pT2 distribution is found to be 2.83±0.1 GeV−2 for λ and 3.28±0.25 GeV−2 for λ. The xF distributions, measured in the range −0.2 to 0.7, show that while λ are produced centrally, λ production extends to more negative values of xF; the shapes show no energy dependence and are similar to those in pion-induced reactions. The polarization of the produced λ is less than 10%. The results are discussed in terms of vector dominance and quark fusion models.

The reaction γp → K+K− p has been investigated with tagged photons in the energy range of 20 < Eγ < 70 GeV. A structure in the 1.7 GeV mass region is observed and interpreted in terms of a recurrence of the ø.

An analysis is presented of the channel γp→K+K−π+π−p. Evidence is found for a K+Kπ enhancement of mass ≈1.9 GeV and width ≈0.4 GeV. The spin-parity of the enhancement is consistent with being 1−.

Data on 6.2 GeV/c π−p and K−p elastic scattering cross sections are presented in the range 0.3 < −t < 10.7 (GeV/c)2. Large-angle π−p scattering differential cross sections oscillate around an average differential cross section of 50 nb/(GeV/c)2, with minima at cos θcm ≈ 0 and −0.4 The well-known dip at −t = 2.8 (GeV/c)2 remains constant in t. In the backward direction there is a dip at −u ≈ 1 GeV/c)2. K−p elastic scattering has a diffraction peak with a slope of 7.30 ± 0.08 (GeV/c)−2 in the region 0.3 < −t < 0.6 (GeV/c)2. The structure at about −t = 1 (GeV/c)2 is less pronounced than at 5 GeV/c. The large-angle region is characterized by a very fast decrease of the differential cross section with increasing energy.

In π−p interactions at 9 GeV/c and 12 GeV/c, the forward production of N∗1680 and N∗1520 has been observed with features of nucleon exchange. The production of Δ1232 is strongly suppressed.

The differential cross section for the reaction π−p → pd has been measured at 5 GeV/c in the interval 0.55 < −t < 2.5 (GeV/c)2, corresponding to the c.m. angular range 17o < θπ−p < 87o. The cross section is found to be roughly constant with an average value of 0.1 μb/(GeV/c)2. Assuming isotropy over the whole angular range, aa total cross section 0.6 ± 0.2 μb is deduced. This agrees with the extrapolated cross section value for the inverse reaction pd → pπ−.

Properties ofD mesons produced in the photo-production experiment NA 14/2 at CERN are reported. The following ratios of branching fractions were measured: $$\begin{gathered} Br(D^0 \to K^ + K^ - )/Br(D^0 \to K^ - \pi ^ + ) = 0.16 \pm 0.05, \hfill \\ Br(D^0 \to K^ - \pi ^ + \pi ^0 )/Br(D^0 \to K^ - \pi ^ + ) = 4.0 \pm 0.09 \pm 1.0, \hfill \\ Br(D^0 \to K^ - \pi ^ + \pi ^ + \pi ^ - )/Br(D^0 \to K^ - \pi ^ + ) \hfill \\ = 1.9 \pm 0.25 \pm 0.20 \hfill \\ \end{gathered} $$ . TheD 0→K − π + π + π − decay was analysed and the contribution of resonant subprocesses found consistent with 1ρ 0 and 0.23 $$\bar K$$ *0 per event. The angular distributions ofD 0→ $$\bar K$$ *0 showed parity violation and destructive interference betweenS- andD-waves. From a Dalitz-plot analysis of the decayD +→K − π + π + the contribution from the quasi two-body decayD +→ $$\bar K$$ *0 π + was measured to be 0.14±0.04±0.04.

Results on backward (3π)- system produced in π-p→pfπ+π-π- reaction at 9 and 12 GeV/c are given. The ϱ0π- mass spectra show two clear signals at 1050 MeV (A1 region) and 1303 MeV (A-2). The width of the enhancement in the A1 region (195±32 MeV) is narrower than found in diffractive experiments. Total backward cross sections for those signals are of the same order of magnitude (∼0.5 μb).

Kinematic correlations between the charmed D and D mesons produced by a photon beam of mean energy 100GeVc have been measured by the NA142 experiment at CERN using a sample of almost background-free fully reconstructed DD events. The observed D and DD distributions are compared to the predictions of production models using different parameters for the charm fragmentation function and for the intrinsic transverse momentum of the partons.

The NA14 photoproduction experiment, carried out at the CERN SPS, has measured the inclusive cross section of events with a prompt photon at high pT in the final state. Here we present a study of the topology of the final state of these events. The topology differs from that observed for events with an identified π0 in the same kinematical domain. Using these differences we have extracted the contribution from the Born term to the inclusive prompt photon cross section. The result is in good agreement with theoretical predictions and supports the assignment of a fractional electric charge to the quarks.

The CERN charm photoproduction experiment NA14/2 has studied the K+K−π+, φπ+, φπ+π0, and φπ+π+π− decay modes of the Ds+ and D+ charmed mesons, and the corresponding charge conjugate states. We measure branching ratios: Br(D+→φπ+)Br(D+→K−π+π+)=0.098±0.032±0.014, Br(Ds+→K∗0K+)Br(Ds+→φπ+)=0.85±0.34±0.20, and obtain upper for the other channels. Contrary to the prediction of some theoretical models, we do noy observe any decay of Ds into φρ. Using the Lund Monte Carlo to hadronize the final-state partons we derive a value for the absolute branching fraction Br(Ds+ → φπ+) =4.8±1.9%.

Results are presented on inclusive φ photoproduction for incident energies between 25 and 70 GeV. Distributions in Feynman x, pT and decay angles are presented and interpreted in terms of quark fusion models. We demonstrate a similarity with the corresponding pion-induced reaction and obtain information on the hadronic nature of the photon.

The aim of this work was to measure π+ photoproduction at zero degree and small angles in the unknown region between the highest measurement of Knapp et al. (Eγ = 290 MeV) and the lowest measurement of Beneventano et al. (Eγ = 550 MeV).

If neutrinos are Dirac particles, their right-handed partners must be present in the theory. Once introduced in the Standard Model (SM), the difference between the baryon number B and the lepton number L can be promoted to a local U(1)B−L symmetry since the corresponding gauge anomalies can be canceled by the right-handed neutrinos. Furthermore, the extremely small neutrino mass can be explained naturally if it is originated from the quantum correction. In this work, we propose simplest models of radiative Dirac neutrino mass using only U(1)B−L symmetry, and without introducing additional fermions and without imposing ad hoc symmetries. In this simple framework, we provide minimal models where Dirac neutrino mass appears at the (i) one-loop, (ii) two-loop and (iii) three-loop. By performing systematic analysis, we show that the minimal one-loop model requires three beyond SM scalar multiplets, whereas minimal two-loop and three-loop models require five. The presented two-loop and three-loop Dirac mass models have not appeared in the literature before.

We report on diffractive production of 6π systems with a continuum of masses from 2 to 5 GeV/c2 in the reaction γp→π+π−π+π−π+π-p. The event structure is compared in detail with Monte Carlo studies of pT limited phase space, and with the jets observed in hadronic systems produced by electron-positron annihilation. Strong similarities are found. For this jet-like structure of the 6π system the axis is consistent with being along the incident photon direction.

After one year of physics at LEP and ∼ 700k Z0 decays registered by its four experiments, no new particle nor new phenomenon has been unveiled. At the per cent level the Z0 is standard. Several potentially interesting channels have been explored to the 10−4 level or less without success.

In a photoproduction experiment using a mean photon energy of 100 GeV we have observed 29±8 Λc(Λc) charmed-baryon and antibaryon decays in the pK−π+ (pK+π−) final state. Quasi two-body final states do not contribite significantly to this channel. The mass of the Λc was measured to be 2281.7±2.7±2.6 MeV/c2 and its lifetime 0.18±0.03±0.03 ps. The ratio of ΛcD production, measured in this experiment, is significantly greater than that predicted by photon-gluon fusion and using a Lund model to describe the hadronization. This excess cannot be completely accounted for in this model, even using a Λc branching fraction in pKπ as high as 5%.

A quantitative test of QCD from high energy photoproduction of large transverse momentum charged hadrons, using incident γ energies between 50 and 150 GeV, is presented. The inclusive hadron P1 and Pt distributions show a clear excess over the VDM contribution. This excess is found to be in good agreement with second-order QCD calculations. This agreement does not depend critically on the choice of the gluon fragmentation function, and is observed over the large kinematical domain covered by this experiment.

A search for photoproduction of F mesons in decay modes containing a φ meson gives upper limits for B·σ, at the three standard deviation level, of 4.0 nb for σπ± and 15 nb for φπ+π−π±. A three standard deviation signal observed in the φϱ± mode corresponds to a B·σ of 33 ± 10 nb.

A measurement of the pion electroproduction cross section from protons, at a transfer q2 = −3.4 fm−2 and a πN mass of 1136 MeV, gave the values σL = 27.0 ± 902 μb,σT=81.8 ± 4.3μb,σLσT = 0.33 ± 0.13 . It is shown that this result is very sensitive to a correct application of radiative corrections but rather insensitive to the exact analytic expressions used.

Using a high-statistics sample of more than 1000 reconstructed charmed-particle decays, the lifetimes of the weakly decaying charmed mesons and of theΛ + baryon have been measured: $$\begin{gathered} \tau _{D^ + } = 1.03 \pm 0.08 + 0.06 ps,\tau _{D^0 } = 0.417 \pm 0.018 \pm 0.015 ps \hfill \\ \tau _{D_s^ + } = 0.33_{ - 0.08}^{ + 0.12} \pm 0.03 ps,\tau _{\Lambda _c^ + } = 0.18 \pm 0.03 \pm 0.03 ps. \hfill \\ \end{gathered} $$

After a brief and personal account of LEP/SLC achievements, I recall the landmarks of their history, extending over the last 25 years. In particular I describe some attempts made at LEP1, like the use of longitudinal polarization and of a multibunch scheme. I compare the final results of LEP electroweak measurements and their accuracy to what was anticipated in various prospective studies, with the conclusion that LEP did always much better than expected. Focussing on Higgs search, I review the main steps of this adventure and confront what was achieved to what one could have done.

Abstract We have measured the differential cross-section for the reaction p p → π + Λ − at 5 GeV /c , the π+ being in t he cm angular range 0.47 cos θ p π + cm , corresponding to 0.12 d σ d ω = 4.1 ± 1.6 μ b / sr for 0.94 cos θ p π + cm .

A search for the decay Ds+→η′π+;η′→ϱγ has been performed by the NA14/2 photoproduction experiment at CERN. A signal compatible with the Ds+ has been observed, leading to a ratio of branching fractions of Br(Ds+→η′π+)Br(Ds+→φπ+)=2.5±1.0-0.4+1.5. A value of Br(D+→η′π+)Br(D+→K−π+π+)<0.1 is set at 90% CL.

Results are presented on inclusive photoproduction in hydrogen of Σ0, Σ1385±, Ξ−0, and their antiparticles for incident energies between 20 and 70 GeV. Hyperons and antihyperons are mainly measured in the kinematical range of central and forward production (xF > −0.3). We find the cross-section ratios σ(Σ1385+)σ(Σ1385−) = 1.4 ± 0.1, σ(Σ1385+)σ(Σ1385−) = 1.3 ± 0.2 and an antiparticle/particle ratio of about 0.3 for the three sigma states. The results suggest an important role of the valence quarks of both the beam projectile and target in the production process in qualitative agreement with current ideas of quark fusion models. The inclusive production of Σ1385± is also observed in a small subsample of the events containing ΛΛ pairs. The xF distribution of Λ (Λ) in ΛΛ events as well as those in Λp (pΛ) is the same as those in single Λ(Λ) inclusive events. Weak signals in the K− pπ+ and Λπ+ π+ π− mass spectra are observed around a mass of 2.26 GeV.

A search for charmed particles produced in π−p exclusive reactions at 19 GeV/c incident beam momentum has been carried out with the Omega spectrometer at CERN. Three million interactions were recorded with a trigger which required a forward K− or p with transverse momentum greater than 0.5 GeV/c. An additional two million triggers were recorded with a forward K+ or proton, with no transverse momentum restriction. Analysis of the four-constraint final states K+K−π−p and K+K−π+π−π−p shows no evidence for the associated production of charmed particles. The upper limits for the product of production cross sections and the two charmed particle decay branching ratios are below 100 nb for most of the channels. The experiment was sensitive over a wide range of production angles.

The physical motivations to search for new particles and new effects are reviewed. The status of searches, direct and indirect, performed at accelerators, is described, as well as the foreseeable future at upgraded and planned machines. The main emphasis of the review is put on the study of electroweak symmetry breaking, in particular the search for Higgs bosons, and on supersymmetry.

Data taken by DELPHI during the 1995 and 1996 LEP runs have been used to search for the supersymmetric partners of electron, muon and tau leptons and of top and bottom quarks. The observations are in agreement with standard model predictions. Limits are set on sfermion masses. Searches for long lived scalar leptons from low scale supersymmetry breaking models exclude stau masses below 55 GeV/c2 at the 95% confidence level, irrespective of the gravitino mass.

The results on the impact parameter for the reactions π−p→pπ+π−π−, pK+K−π−, with a fast secondary forward proton are given at 9 and 12 GeV/c. For the reaction π−p→pπ+π−π− our results corresponding to the small |u| region (|u| < 1 GeV2) are compared to the existing data for the small |t| region. We show that small |u| events are more central than small |t| events.

Abstract We have studied the backward production of ω0 mesons in the u-channel I u = 1 2 exchange reaction π−p → N0(1680)ω0 at 9 GeV/c and 12 GeV/c incident momenta. The data come from an experiment performed at the CERN Omega Spectrometer using a fast proton trigger device. The backward production of the η0 meson has also been observed and the coupling constant ratio gηNN/gπNN has been estimated.

The LEP collider and the performances which have been achieved are presented in simple terms. Some basic facts of electron circular machine physics are recalled. The ambitious and very successful programmes undertaken to maximize LEP luminosity and energy are described in detail. To cite this article: R. Bailey et al., C. R. Physique 3 (2002) 1107–1120. Le collisionneur LEP et ses performances sont présentés en termes simples. Quelques éléments de la physique machine d'un collisionneur circulaire à électrons sont rappelés. Les deux batailles, pour la luminosité et pour l'énergie maximale, que le LEP a menées et gagnées sont décrites en détail. Pour citer cet article : R. Bailey et al., C. R. Physique 3 (2002) 1107–1120.

The authors describe in detail the basic concepts of the design, the composition of the data flow, and the tools developed to monitor the trigger in real time. The results of the trigger performance in terms of efficiencies achieved during the first two and a half years of operation in the LEP (Large Electron Positron Collider) machine are also shown. So far, a total of approximately 10/sup 6/ hadronic Z/sup 0/s have been collected from 21 million recorded events. The DELPHI trigger system provides an efficiency nearly 100% over the covered solid angle for all particles and physical channels. The system has also proved to be stable despite very different beam conditions. This is a consequence of the high redundancy provided by the different detectors for all e/sup +/e/sup -/ final state processes, which also ensures a method for calculating the absolute trigger efficiencies using data only.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

I will review some of the challenges set by the LHC machine, the LHC detectors and the extraction of LHC physics. I will also consider briefly another question: which road should one follow after the first phase of LHC?

Delphi (DEtector with Lepton, Photon and Hadron Identification) is one of the four experiments of the LEP (Large Electron Positron) collider at CERN. The detector is laid out to provide a nearly 4/spl pi/ coverage for charged particle tracking, electromagnetic, hadronic calorimetry and extended particle identification. The trigger system consists of four levels. The first two are synchronous with the BCO (beam cross over) and rely on hardwired control units, while the last two are performed asynchronously with respect to the BCO and are driven by the Delphi host computers. The aim of this paper is to give a comprehensive global view of the trigger system architecture, presenting in detail the first two levels, their various hardware components and the latest modifications introduced in order to improve their performance and make more user friendly the whole software user interface.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The DELPHI detector installed in the LEP (Large Electron-Positron Collider) is equipped with RICH (ring imaging Cherenkov) counters. The barrel part incorporates a liquid (C/sub 6/F/sub 14/) and a gaseous (C/sub 5/F/sub 12/) radiator, providing particle identification up to 20 GeV/c. The Cherenkov photons of both radiators are detected by TPC (time projection chamber)-like photon detectors. The drift gas (75% CH/sub 4/+25% C/sub 2/H/sub 6/) is doped with TMAE (tetrakis-dimethylamine-ethylene) by which the ultraviolet Cherenkov photons are converted into single free photoelectrons. These are drifted towards multiwire proportional chambers at the end of the drift tubes, and the space coordinates of the conversion point are determined. One half of the barrel RICH is now equipped with drift tubes and has provided results from the liquid radiator since spring 1990. The gas radiator has been tested with C/sub 2/F/sub 6/ as a preliminary filling since August 1990. The data obtained demonstrate the good particle identification potential. For the liquid radiator, the number of detected photons per ring in hadron jets is N=8, whereas for muon pairs (single tracks) N=10 has been obtained. For the gas radiator, 2.1 photons per track were observed, which demonstrates good functioning of the focussing mirrors.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A brief overview of fifty years of particle physics at CERN and elsewhere is given.

A brief overview of fifty years of particle physics at CERN and elsewhere is given.

An overview of the physics potential of the RICH technique in present and future programmes is given.

The title may seem a bit presumptuous, but the goal is simply to underline what is actually known experimentally about the Universe, how sound is our understanding at various moments of its evolution and at which point hypotheses start to replace certainties. To do so, we will travel backward in time along the timeline of the Universe in the expanding model which is now the prevailing one. Due to size constraints, this expose is a short version of a more complete one, containing figures and references.

We have analysed about 85 000 fast Λ0 events, obtained in a fast proton triggered experiment performed at the CERN-Ω spectrometer at 9 and 12 GeV/c incident π− beam. Nearly 2500 Λ0K+π− events have been isolated. We find strong production of quasi-two-body processes Λ0K∗0 and ∑∗−K+ consistent with u-channel hyperon exchange. Results on Λ0 polarization, K∗0 decay parameters and differential cross sections are given for Λ0K∗0(892) and Λ0K∗0(1430) final states. A comparison is made with the associated backward Λ0(1520)K∗0 production seen in the four-prong reaction π−p→pK−K+π− obtained in the same experiment.

The electron-deuteron elastic scattering cross section was measured for an energy transfer momentum equal to 1 fermi/sup -1/ and for three angles (60, 90, and 130 deg ). The results were analyzed with the nid of recent Glendenning and Kramer calculations in order to deduce the neutron form factors. (tr-auth)