M. Della Negra

We report the observation of four electron-positron pairs and one muon pair which have the signature of a two-body decay of a particle of mass ∼ 95 GeV/c2. These events fit well the hypothesis that they are produced by the process p̄+ p→Z0+X(with Z0→ ℓ++ ℓ−), where Z0 is the Intermediate Vector Boson postulated by the electroweak theories as the mediator of weak neutral currents.

The general characteristics of inelastic proton-antiproton collisions at the CERN SPS Collider are studied with the UA1 detector using magnetic and calorimetric analysis. Results are presented on charged particle multiplicities and transverse and longitudinal momenta, and on total transverse energy distributions at centre of mass energies ranging from 0.2 to 0.9 TeV.

We have measured transverse momentum spectra up to 10 GeV/c for charged particles produced centrally in proton-antiproton collisions at 540 GeV in the centre of mass at the CERN collider. Our results are compared with data at ISR energies and with the predictions of a QCD model. The charged particle spectrum shows a clear dependence on charged track multiplicity.

We report the observation of five events in which a missing transverse energy larger than 40 GeV is associated with a narrow hadronic jet and of two similar events with a neutral electromagnetic cluster (either one or more closely spaced photons). We cannot find an explanation for such events in terms of backgrounds or within the expectations of the Standard Model.

A clear signal is observed for the production of an isolated large-transverse-momentum lepton in association with two or three centrally produced jets. The two-jet events cluster around the W± mass, indicating a novel decay of the Intermediate Vector Boson. The rate and features of these events are not consistent with expectations of known quark decays (charm, bottom). They are, however, in agreement with the process W→ tb followed by t → bℓv, where t is the sixth quark (top) of the weak Cabibbo current. If this is indeed so, the bounds on the mass of the top quark are 30 GeV/c2 < mt <550 GeV/c2.

A sample of 52 Intermediate Vector Boson decays in the (vee) channel is described. They were produced at the CERN SPS Collider for an integrated luminosity of 0.136 pb−1. Both production and decay properties fit well with expectations from the Standard Model of weak interactions. An improved value for the W mass is given and compared with the previously published value for the Z0 mass.

We report on a search for B0↔B¯0 oscillations (mixing) using events with two identified muons from data collected at the CERN p¯p collider. In the absence of B0↔B¯0 oscillations, dimuons coming directly from decays of beauty-antibeauty paris must have opposite signs. Like-sign dimuons are expected from events where one muon arises from beauty decay and the other from the charm decay of the associated beauty-charm cascade. Taking these processes into account, together with the contribution from charm production, the predicted ratio of like-sign to unlike-sign muon pairs is 0.26±0.03. Experimentally we measure 0.42±0.07±0.03. A natural explanation for the excess of like-sign events is the existence of a significant amount of B0↔B¯0 transitions. The fraction of beauty particles that produce first-generation decay muons with the opposite electric charge from that expected without mixing is deduced to be: χ = 0.121±0.047. Combined with the null result from searches for B0↔B¯0 oscillations at e+e− colliders, our results are consistent with transitions inthe B0s system, as favoured theoretically.

With a segmented total absorption calorimeter of large acceptance, we have measured the total transverse energy spectrum for pp̄ collisions at s12 = 540 GeV up to ΣET= 130 GeV in the pseudo-rapidity range |η|< 1.5. Using two different algorithms, we have looked for localized depositions of transverse energy (jets). For ΣET > 40 GeV, the fraction of events with two jets increases with ΣET; this event structure is dominant for ΣET > 100 GeV. We measure the inclusive jet cross section up to ET(jet) = 60 GeV and the two-jets mass distribution to 120 GeV/c2. The measured cross sections are compatible with the predictions of hard scattering models based on QCD.

The production of transverse energy clusters in minimum bias proton-antiproton collisions at the CERN SPS Collider is studied with the UA1 detector over a new range of centre of mass energies (√s = 0.2−0.9 TeV). This study is intended to investigate how low in transverse momentum perturbative QCD is able to describe the dynamics of hadron collisions. We observe that clusters with transverse energy in excess of a few GeV exhibit properties in agreement with QCD expectations for parton scattering, supporting their interpretation in terms of jet production. We find that the jet-event rate represents a sizeable fraction of the inelastic rate and is increasing with √s over the measured energy range.

Events with a charged particle of high transverse momentum were studied in pp collisions at √s = 52.5 GeV with the Split Field magnet (SFM) at the CERN-ISR, triggering on a charged particle of pT > 2 GeV/c, in the c.m. angular region θ ≅ 20° and θ ≅ 45°. The results provide further evidence for jet-like structure in high pT reactions. One of two produced jets is associated with the triggering hadron whereas the other is produced at an azimuthal angle that differs from that of the trigger by roughly 180°. The main features of the data are found to agree with the predictions of a simple parton-parton hard scattering model with vector gluon exchange, provided that one allows for partons in the proton to have transverse momentum. The ingredients of the model are all derived from data on deep inelastic lepton-proton scattering and from hadron production in e+e− annihilation. The model retains a simple pT−4 dependence for the parton-parton scattering. However, for pT ≲ 4GeV/c, the transverse movement of partons yields an approximate pT−8 scaling for the inclusive hadron distribution in agreement with observations. For the understanding of our data, it is found to be essential to take into account the soft component arising from the fragmentation of the spectator partons into hadrons.

An extensive study of production and decay properties of charged and neutral Intermediate Vector Bosons (IVB) at the CERN proton-antiproton collider is presented. Intermediate Vector Bosons were detected in the electron, muon, and tau decay modes at centre-of-mass energies of 0.546 and 0.630 TeV. This paper is a summary, based on all the available data from the UA1 experiment from the running periods 1982–1985. Results are presented and compared with expectations of the Standard Eletroweak Model and QCD-improved Drell-Yan annihilation processes. The general conclusion is that there is an excellent agreement between the predictions of the Standard Model and our measurements.

A sample of events with large missing transverse energy from 715 nb−1 of data from the UA1 experiment at the CERN proton-antiproton collider is used to search for evidence of supersymmetric particle production. Assuming that the photino is the lightest supersymmetric particle and that it is massless, we find a limit on the squark mass of mq>45 GeV/c2 at 90% CL, independently of the gluino mass. Similarly, we find a limit on the gluino mass of mg>53 GeV/c2 (at 90% CL) independently of the squark mass, provided that the gluino is not long-lived (i.e. provided that the squark is not too heavy, mq⪅1 TeV/c2). For equal squark and gluino masses we find a limit mq=mg>75 GeV/c2 at 90% CL. The effect of a non-zero photino mass on these limits is studied.

We report evidence for beauty particle production through the observation of dimuon events from proton-antiproton collisions at energies of √s=546 GeV and √s=630 GeV at the CERN collider. Our data indicate that semi-leptonic decays of beauty particles are the dominant source of pairs of high-pT muons. The beauty flavour creation (gg or q¯q→b¯b) cross-section needed to explain the dimuon rate is σ{p¯p→b¯b+X, pbT5 GeV/c, |η|<2.0}=(1.1±0.1±0.4) μb, which is in good agreement with QCD calculations. We also observe clear signals for ϒ→μ+μ− (hidden beauty) and high-pT J/ψ→μ+μ−, well above the backgraound of continuum muon pairs from the Drell-Yan mechanism.

We present a detailed study of hadronic jets obtained in a data sample taken in the UA1 detector with a localized transverse energy trigger. We discuss the average shape of jets in terms of energy and charged particle content, and compare this to data generated in Monte Carlo programs. We further extend the previously reported inclusive jet cross section to the region of ET = 100 GeV. A comparison with theoretical models of cross sections for events with more than two jets is also given.

The two-jet cross section measured in the UA1 apparatus at the CERN pp Collider has been analysed in terms of the centre-of-mass scattering angle θ and the scaled longitudinal parton momenta x1 and x2. The angular distribution dσ/d cos σ rises rapidly as cos → 1, independent of x2 and x2, as expected in vector gluon theories (QCD). The differential cross section in x1 and x2 is consistent with factorization and provides a measurement of the proton structure function F(x) = G(x) + 49[Q(x) + Q(x)] at values of the four-momentum transfer squared, -t̂ ≈ 2000 GeV2. Over the range x = 0.10−0.80 the structure function shows an exponential x dependence and may be parametrized by the form F(x) = 6.2 exp (−9.5x).

We report measurements of b-quark and B-hadron production in pp̄ collisions at √s=630 GeV. We use muon samples to extract beauty production cross-sections over a wide range of transverse momentum in the central rapidity range |y| < 1.5. We compare our results to an O(αs3) QCD prediction and find good agreement over the measured b-quark transverse momentum range 6 GeV/c to 54 GeV/c. Using the shape of the pT and y distribution predicted by QCD to extrapolate our data, we infer a total cross-section for b-quark production at √s=630GeV ofσ(pp̄→bb̄+X)=19.3±7(exp.)±9(th.μb.

The inclusive jet cross section has been measured in the UA1 experiment at the CERN pp Collider at centre-of-mass energies √s = 546 GeV and √s = 630 eV. The cross sections are found to be consistent with QCD predictions, The observed change in the cross section with the centre-of-mass energy √s is accounted for in terms of xT scaling.

Track information from the central detector in the UA1 experiment at the CERN proton-antiproton collider (√s = 540 GeV) is used to determine the charged particle multiplicity distribution for the pseudorapidity range ∣η∣ < 1.3. The mean value for this multiplicity per unit of η is 3.9 ± 0.3 for events having at least one track in this range. Data from the central electromagnetic and hadron calorimeters are used to examine the correlation between the transverse energy measured in the collisions and the multiplicity in the central region.

Proton-antiproton elastic scattering at CM energy 540 GeV has been studied in the t-range 0.04 < −t < 0.45 GeV2. The data are well fitted by the form exp (bt) with b = 17.1 ± 1.0 GeV−2 for |t| = 0.04 − 0.18 GeVsu2 and b = 13.7 ± 0.2 ± 0.2 GeV−2 for |t| = 0.21−0.45 GeV2. A luminosity measurement combined with the optical theorem gives σtot = 67.6 ± 5.9 ± 2.7 mb and σe1/σtot = 0.209 ± 0.018 ± 0.008.

We report on a search for new heavy quarks using data collected by the UA1 experiment during 1983, 1984 and 1985 at the CERN proton-antiproton collider, corresponding to an integrated luminosity of approximately 700 nb−1. Studying events with a muon or an isolated electron, accompanied by one or more jets, we find good agreement between our data and Monte Carlo predictions for the production of charm and beauty, without the need for a new quark. A top quark model, involving the decayW→t $$\bar b$$ and directt $$\bar t$$ production via the strong interaction, is used to determine our detection efficiency for top. This allows us to place an upper limit on the cross section for producing top quarks as a function of the top quark mass. Our analysis is not sensitive to theW→t $$\bar b$$ process alone. By comparing our limit with a calculation of thet $$\bar t$$ cross section, added to theW→t $$\bar b$$ cross section derived from our own measurements ofW →lv, we are able to place a lower limit on the mass of the top quark. From the lowest order (α 2 ) calculation, using the choice of structure functions andQ 2 scale that give the lowest cross section, we find: $$m_{top} > 44GeV/c^2 \left( {95\% c.l.} \right).$$ Including an estimate of the next higher order (α 2 ) and calculating the cross section with the EUROJET QCD Monte Carlo program using a less extreme choice for the structure functions andQ 2 scale gives: $$m_{top} > 56GeV/c^2 \left( {95\% c.l.} \right).$$ A search has also been made for a fourth generation, charge 1/3 quark (b'). Assuming that theb' mass is smaller than that of the top quark and that it can-not be produced inW decays, the mass limits, using the above procedures, are respectivelym b' >32 GeV/c2 andm b' >44 GeV/c2, both at 95% confidence level.

The properties of a sample of 172 charged intermediate vector bosons decaying in the (eve) channel and 16 neutral intermediate vector bosons decaying in the (e+e−) channel are summarized. Masses, decay widths, decay angular distributions, and production cross sections are given, and a limit is put on the number of light neutrino types Nv ⩽ 10 at 90% CL.

We summarize the results obtained in the UA1 experiment on the production of bottom quarks in proton-antiproton collisions at √s=0.63 TeV. Independent muon data samples are used to determine the bottom quark production cross section in different transverse momentum ranges from 6 to 30 GeV. A recent theoretical calculation to O(αs3) of the inclusive bottom quark transverse momentum spectrum in hadronic collisions shows reasonable agreement with the data. We extrapolate the integral PT distribution to PT=0 and in rapidity to estimate the total cross section forthe production of bottom quark pairs. Assuming the shape in PT and rapidity given by the O(αs3) calcultaion, we obtain σ(pp→bb+X) = 10.2 ±3.3 μb.

We report on further analysis of events with large missing transverse energy from 715 nb−1 of data from the UA1 experiment at the CERN proton-antiproton collider. Measured W and Z0 rates and heavy flavour cross sections are used to predict rates of missing transverse energy event from all known standard model process. Limits are derived on two possible new sources, namely: (1) semi-hadronic decays of a fourth sequential charged heavy lepton coupling to the W boson, (2) Z0 decays into additional pairs of non-interacting particles. After taking into additional pairs of non-interacting particles. After taking into account all known sources of missing transverse energy events, we find a mass limit on a fourth-generation charged lepton of mL>41 GeV/c2 (90% CL) and a limit on the number of additional neutrino species of n⩽7 (90% CL).

We present updated results from the UA1 experiment on cross sections times leptonic branching ratios for intermediate vector bosons produced in proton-antiproton collisions at a centre-of-mass energy of 0.630 TeV. We find that σ·B(W→lβn)=0.63±0.04±0.10nb and σ·B(Z0→l+l−=0.071±0.011nb where l denotes an average of electron and muon channels. An average over all data (electron and muon channels at 0.546 and 0.630 TeV) gives a ratio of R≡σ·(W→lν)/σ·B(Z0→l+l−) = 9.1+1.7−1.2. Using current theoretical predictions for this quantity, limits are extracted on the number of light neutrino types. Combining UA1 and UA2 data and imposing the bound Nν⩾3 we find that Nν⩽5.9 at 90% CL. Constraints on the mass of the top quark are also discussed.

Evidence for the forward production of D+ mesons has been found in pp collisions at a c.m. energy of 52.5 GeV. A signal of 92 ± 18 events above background has been observed in the decay channel D+ → K∗oπ+ → K−π+π+. The cross section in the phase space region of our experiment is given and implications for the total cross section for charmed D meson production are discussed.

Results on charged particle production in pp̄ collision at s12 = 540 GeV are presented. The data were obtained at the CERN pp̄ collider using the UA1 detector, operated without magnetic field. The central particle density is 3.3 + - 0.2 per unit o pseudo-rapidity for non-diffractive events. KNO scaling of the multiplicity distributions withresults from ISR energies is observed.

The production of a Λ+c charmed baryon has been observed in pp collisions at the CERN ISR. A sharp peak at 2.26 GeV/c2 was found in the decay mode Λ+c → K∗0p in events triggered with a forward K− meson. Also the decay mode Λ+c → K− Δ++ seems to be present. The estimation of the total cross section for Λ+c production is given. However, the resulting value depends on the assumed form of the differential cross section. In addition, Λ+c production in events triggered by the presence of a direct electron is discussed.

In the course of an extensive study of pp annihilations at rest involving KK pairs in the final state, 2000 events pp → K10K±K±π± and 364 events pp → K10K10π0 have been analyzed. This analysis shows that the K∗(891) is strongly produced. In addition, we observe a KK JP = 2+ resonance at 1280 MeV and a significant enhancement in the KK system at threshold, in I = 1. We also need S-wave (Kπ) amplitudes to reproduce the distribution of events on the Dalitz plot.

We summarize the present experimental evidence for an I=1 (KK¯ enhancement in pp¯ annihilations at rest and at 1.2 GeV/c. We propose different possible interpretations for the enhancement.

We report on the first observation of the beauty baryon Λb in an exclusive decay channel at the CERN pp collider. Using 4.7 pb−1 of muon data collected in the 1988/89 collider runs we reconstruct 16±5 Λb's in the decay mode Λb→JψΛ above a background of 9±1 events, corresponding to a significance of about five standard deviations. We measure the Λb mass to be mΛb=5640±50±30 MeV/c2. Using the beauty cross-section measured by UA1 we deduce for the product of the production fraction and branching ratio fΛb Br (Λb→JψΛ)=(1.8±1.0)×10-10. Our sample contains three-muon event in which the beauty particle opposite to the Λb is tagged by the third muon. We also observe an indication of a signal in the decay channel B0→JψK0∗ with a significance of three standard deviations.

A significant intermittency signal is observed in 630 GeV pp collisions measured in the UA1 central detector. It occurs with similar magnitude in different variables: pseudorapidity, rapidity and azimuthal angle. The signal increases with decreasing charged particle multiplicity in the event. Its strength in a sample of low p⊥ tracks and its multiplicity dependence are not reproduced by commonly used Monte Carlo models of high energy interactions.

Isolated photons, produced directly by a scattering process, have been observed in the UA1 experiment at the CERN pp collider at centre-of-mass energies √=546 GeV and √=630 GeV. Single and double photon differential cross sections have been determined and found to be consistent with the expectations of QCD.

Results are presented on two-jet and three-jet cross sections, measured in the UA1 experiment at the CERN Super Proton Synchrotron (SPS) pp̄ Collider, at the highest available subprocess cms energies (ŝ>150 GeV). Precise measurements of the two-jet angular distribution are consistent with previous results but show significant scale-breaking effects. The three-jet Dalitz plot and the three-jet angular distributions show evidence for final- and initial-state bremsstrahlung processes, in agreement with the leading-order QCD predictions. A comparison of the yield of wide-angle three-jet events with the yield of two-jet events at smaller scattering angles gives for the strong interaction coupling constant: αs(K3JK2J)=0.16±0.02±0.03 at Q2≈4000 GeV2, where the factor K3JK2J may plausibly be assumed to be close to unity.

We report results based on a sample of events with large missing transverse energy corresponding to 715 nb−n of data from the UA1 experiment at the CERN proton-antiproton collider. High transverse-momentum tau-leptons from W decays are observed for the first time through their semi-hadronic decay modes and associated missing transverse energy. The measured relative rates of W→eν, W→μν, and W→τν provide the first direct tests of the electron-muon-tau universality of the weak charged couplings at Q2=mw2. We determine gτge=1.01±0.09±0.05 and gμge=1.05±0.07±0.08.

Angular distributions of high-mass jet pairs (180<m2J<350 GeV) have been measured in the UA1 experiment at the CERN pp̄ Collider (s=630GeV). We show that angular distributions are independent of the subprocess centre-of-mass (CM) energy over this range, and use the data to put constraints on the definition of the Q2 scale. The distribution for the very high mass jet pairs (240<m 2J<300 GeV) has also been used to obtain a lower limit on the energy scale Λc of compositeness of quarks. We find Λc>415 GeV at 95% confidence level.

Inelastic events with an observed charged multiplicity nobs ⩾ 7 have been studied at the CERN ISR using the split field magnet detector. Correlations of particle densities, charges and transverse momenta have been determined and a phenomenological analysis in the framework of cluster models has been performed. A good description of all results is obtained if one allows for charged clusters, produced in a mechanism with limited charge exchange. The mean transverse momentum of clusters is 0.65 ± 0.10 GeV/c. Short-range correlations of particles of like charge due to the Bose-Einstein effect can be described as a second-order interferrence phenomenon with a radius R = 1.34 ± 0.31 fm and a lifetime of cτ = 1.38 ± 0.60 fm for the pion source.

Inclusive production of ϱ0,f, andg 0 mesons and ofK 0 ,K *0 (892), ϕ andK *0(1430)mesons has been measured at <y>∼2.6 and <p T >∼1.1 GeV/c in proton-proton interactions at $$\sqrt s = 52.5$$ GeV. The negative particle from the two-body decays of these resonances were identified by a threshold Cerenkov counter and used for triggering. Starting from the measured differential cross section, total inclusive cross sections for the vector and tensor mesons were determined using various parametrizations for they andp T dependence of the differential cross section. The experimental results are discussed in the framework of production models based on the parton picture. The strangeness suppresion factor λ=(0.30±0.10) due toSU(3) symmetry breaking of the quark sea is derived.

We have measured the production cross-section times branching ratio for J/ψ→μ+μ− in pp̄ interactions at √s = 630 GeV in the kinematic range |y|<2.0 and pT>5 GeV/c, BR(J/ψ→μ+μ−)σ(pp̄→J/ψ)=6.18±0.24±0.81 nb. The data sample collected in 1988 and 1989 for an integrated luminosity of 4.7 pb−1 represents a fivefold improvement over the statistics in our earlier study of the J/ψ production process, and the pT distribution which is measured extends to 28 GeV/c. Using event topology we show that the rate for the direct production of J/ψ, via radiative decays of χ states, is larger than that for production via B-hadrons. Production of ψ′ is also studied using the decay modes <ψ′→μ+μ− and ψ′→J/ψπ+ψ−.

We report on a search for the decays B0→ μ+μ−, B→ μ+ μ−X and Bd0 → μ+ μ−K0∗, which are expected to be rare if mediated by flavor changing neutral currents. Using data collected during the 1984–1989 CERN pp̄ Collider runs, the UA1 search was carried out using μ+ μ− events with 3.9<Mμμ < 5.5 GeV/c2. We find 90% confidence level upper limits on the branching for BR → μ+μ− of 8.3 × 10−6, for B → μ+μ−X of 5.0 × 10−5, and for B0d→μ+μ−K0∗ of of 2.3 × 10−5. Implicatitions for upper limits on the t-quark mass are discussed.

Bose-Einstein correlations have been measured at the SPS collider at centre-of-mass energies from √s=0.2 to 0.9 TeV. The size and the incoherence parameter of the particle emitting region have been determined as a function of charged particle density in pseudorapidity Δn/Δη, centre-of-mass energy and the direction of emission of the charged particles. For fixed values of Δn/Δη the size of the interaction region and the incoherence parameter remain constant with rising √s. At fixed √s, the size of the interaction region increases with rising charged particle density while the incoherence parameter decreases.

Two-jet mass distributions have been measured as a function of centre-of-mass scattering angle for high-mass jet pairs produced in proton-antiproton collisions at the CERN collider operating at a centre-of-mass energy of 630 GeV. The agreement between QCD expectations and the experimental measurements has been used to place limits on the production cross section of an object X decaying into two jets. In particular we consider the existence of a massive colour octet of vector gauge bosons (axigluons). We exclude axigluons with a width ΛA < 0.4mA and a mass mA in the range 150 < mA < 310 GeV/c2 (95% CL).

The observation of an apparent excess of radiative Z0 decays into e+e−γ and μ+μ−γ has prompted the search for massive eνγ and μνγ final states containing an energetic photon. No events were found other than those consistent with QED radiative effects in leptonic W decays. The data sample corresponds to an integrated luminosity of 0.136 pb−1 produced at the CERN super proton synchrotron (SPS) collider. An upper limit on the occurrence of such events is given.

We have measured the W transverse momentum distribution (pTW) using a sample of 323 W → eν and W → μν events produced in proton-antiproton collisions at the CERN collider. In the present letter we extend the study of the distribution up to pTW ∼- m W and compare to leading and higher order QCD. This comparison is a precise test of QCD with hadron colliders and the inclusive spectrum gives good agreement over a large range of pTW. However we observed two events at very large pTW (∼- 100 GeV/c) in which the W candidate recoils against an energetic di-jet system. Both events have a very large missing transverse energy and a jet-jet mass compatible with the W mass. In a separate analysis, a topologically similar event has been observed in which a high-mass di-jet system is balanced by a large missing transverse energy which could be interpreted as Z0 → νν decay. We cannot easily explain these three events in terms of explicit second-order QCD calculations. However we cannot exclude at this stage the possibility that they are the result of non-gaussian fluctuations in the response of UA1 calorimetry or a statistical fluctuation in the data.

We present a study of low mass dimuon events [m(μμ) < 6 GeV/c2] from the UA1 experiment at the CERN pp collider. Contributions from semileptonic decays of heavy flavour particles, Drell-Yan type processes, J/ψ decays and leptonic decays of light mesons are extracted, and cross sections for high pT beauty and Drell-Yan production are derived. A limit for the branching fraction for the exotic decay B0→μ+μ− is also obtained. The cross section for section for low mass, high pT Drell-Yan prod uction is compared to the measured direct photon cross section using QED and QCD derived relationship. This relationship is used to infer a measurement of the single photon cross section at lower values of transverse momentum, where photons cannot be unambigously identified.

We have studied high transverse momentum J/ψ production in pp interactions at s=630 GeV. The measured cross section times branching fraction for J/ψ→μ+μ− production with pT⩾5 GeV/c and |y|<2.0 is 7.5±0.7±1.2 nb. The event topology is used to establish that there is J/ψ production from both the direct production of cc bound states and the decay of B hadrons. The inferred yield of J/ψ from beauty decays is compatible with our estimates of beauty production in other channels.

We report the observation of five muonic Z0 decays. The mass and cross section times branching ratio is consistent with the previous measurements of Z0→e+e−. Three of the muonic decays have unexpected features. One event is of the type Z0→μ+μ−γ. Two of the Z0→μ+μ− decays are accommpanied by several (>4) energetic (ET > 10 GeV) jets which are difficult to explain within the framework of standard QCD corrections.

A search for events having the characteristics of cosmic ray Centauros has been made in 540 GeV centre of mass proton-antiproton collisions, using information on charged particle multiplicities and transverse momenta from our central detector image chamber, together with energy deposition in our calorimeters. No such events were found in 48 000 low bias events.

A sample of two-jet events from the UA1 experiment at the CERN pp̄ Collider has been used to study the fragmentation of high-energy quark and gluon jets into charged hadrons. Compared with lower-energy jets observed in e+e− and pp collisions, the fragmentation function measured in the present experiment is softer (i.e. peaked to smaller values of z) and the mean internal transverse momentum is larger, mainly because of the effects of the QCD scaling violations. Using our knowledge of the quark and gluon structure functions in the proton, together with the QCD matrix elements, a statistical separation of quark and gluon jets is achieved within the present experiment. The fragmentation function for the gluon jets is found to be softer, and the angular spread of the fragmentation products larger, than is the case for quark jets.

The production properties of a sample of 172 charged intermediate vector bosons produced at the CERN Super Proton Synchrotron Collider are described. The production cross-section, the longitudinal- and transverse-momentum distributions and the properties of hadronic jet activity produced in association with the weak bosons are in agreement with the expectations of the QCD-improved Drell-Yan mechanism.

The search for the standard model Higgs boson at the Large Hadron Collider (LHC) started more than two decades ago. Much innovation was required and diverse challenges had to be overcome during the conception and construction of the LHC and its experiments. The ATLAS and CMS Collaboration experiments at the LHC have discovered a heavy boson that could complete the standard model of particle physics.

An analysis of W and Z boson production at UA1, using 4.66 pb−1 of data from the 1988 and 1989 CERN pp Collider runs at s=0.63 TeV, yields R≡σWBr(W→μv)/σzBr(Z→μμ)=10.4−1.5+1.8stat.±0.8(syst.) We find R=9.5−1.0+1.1(stat.+syst.) when combining all available UA1 data, in both the electron and muon channel, taken in the period 1983–1989. In the framework of the standard model, the value of R is used to infer the total width of the W boson, ΓWtot=2.18−0.24+0.26 (exp.)±0.04(theory) GeV/c2.

Charge correlations of particles in an event with a large pT triggering particle have been investigated applying a new method, which has been developed for non-diffractive inelastic hadronic events. The correlation length for charge compensation of the fragments of the hard scattered partons and of the spectators, respectively, are equal to that one in normal inelastic events. Part of the charge of the high pT trigger particle is compensated by the soft particles of the “away jet”. These results support the idea that the rearrangement of quantum numbers in quark fragmentation shows a universal behaviour.

Evidence from the KK decay mode for the splitting of the A2 meson is presented.

A novel method was developed to obtain precise timing of muon hits in drift tubes at the first trigger level, and hence to associate a detected muon with the bunch crossing in which it originated. A very good time resolution of ∼ 2 ns was obtained. Some other topics related to muon detection were investigated.

Inclusive fragmentation of jets into charged particles has been studied in the UA1 experiment at the CERN Super Proton Synchrotron (SPS) pp̄ Collider at s = 540 GeV for jets having a transverse energy above 30 GeV. The observed fragmentation function is very similar to that seen in e+e−jets and is therefore indicative of similar hadronization processes for gluons and quarks. Scaling with the energy of the jet is valid to a good approximation, although slight deviations are suggested by the data. The average transverse momentum with respect to the jet axis for jets with ET > 30 GeV is 600 MeV/c for particles having z > 0.1 and increases slowly with the jet energy.

We report evidence for the production of the charged D∗ mesons in pp̄ collisions at s = 540 GeV. The search was confined to the charged particle fragments of hadronic jets, which are expected to be predominantly gluon jets in this experiment. The fragmentation function and production rate for D∗ in jets of average transverse momentum of 28 GeV/c are given.

3roton-antiproton elastic scattering at cm energy 540 GeV has been studied in the t range 0.14 ⩽ −t ⩽ 0.26 GeV2. The data is well fitted by an exponential form exp(bt) with b = 13.3 ± 1.5 GeV−2.

In events at centre of mass energy 540 GeV from the CERN pp collider, we have found clear evidence for correlations in rapidity and azimuthal angle between high transverse momentum charged particles. These correlations increase with transverse momentum and are much stronger than the general two-particle correlations in minimum bias events. By analogy to ISR results, a qualitative interpretation in terms of hard scattering and fragmentation of partons is discussed.

In an experiment performed with the SFM facility at the CERN ISR, we have studied events with a large-pT particle (π+, π-, K-, p) produced at polar angle settings of 20° and 45°. The longitudinal momentum distributions of leading fragments in the spectator jet of the same rapidity hemisphere as the trigger are strongly correlated to the nature of the trigger particle involved. These distributions are compared with predictions from dimensional counting rules, if parton scattering via vector gluon exchange is assumed for the large-pT process. Given slight modifications of the original counting rules, we observe a fair agreement. As to the fastest particle (jet leader) in the “away” jet, its average charge is about 13 independent of the charge of the trigger; this value is expected for the case of three valence quarks participating in the hard scattering process. From the analysis of the trigger jet we find ϱ0/π+ and ϱ0/π- production ratios near unity at a value of the transverse momentum of about 2 GeV/c.

We report on a new measurement of B0−B0 mixing at the CERN Spp̄S collider. Mixing is measured in the non-isolated high mass dimoun sample using data from the 1988–1989 collider runs. The measured value of the mixing parameter, χ, is 0.145±0.035(stat.) ± 0.014(syst.). The average of this measurement and that from our 1984–1985 data is ξ= 0.14 ± 0.029 (stat.) ± 0.017 (syst.) assuming fully correlated errors. Using the measurement of ξd from ARGUS and CLEO , we obtain χ = 0.50 ± 0.20, which gives a limit of χs > 0.17 (0.12) at 90% (95%) CL. Including the measurements of χ from the ALEPH and L3 experiments gives χs = 0.53 ± 0.15, and a limit of χs > 0.27 (0.23) at 90% (95%) CL.

We report on a search for new heavy quarks decaying semileptonically into muons, using 4.7 pb−1 of data taken during the CERN $$p\bar p$$ collider runs in 1988 and 1989. The properties of isolated muons accompanied by jets are consistent with the predictions from the Standard Model but do not show a signal for a new heavy quark. Combining all the UA1 data (5.4 pb−1), lower mass limits are obtained at 60 GeV/c2 (95% CL) for thet-quark and 43 GeV/c2 (95% CL) for a fourth-generation charge-1/3 quark (b′-quark).

We report the final state charged-hadron multiplicities and topological cross sections for inelastic scattering of 16 GeV μ−,s from hydrogen in the SLAC 40-inch bubble chamber. We find that for 1.8 ⩽ W ⩽ 5.0 GeV the charged hadron multiplicity is generally lower at Q2 = 1.5 GeV2 than for photoproduction.

Events with a positive particle of large transverse momentum emitted at medium angles have been studied using the split field magnet spectrometer at the CERN-ISR at a c.m. energy of √s = 52.5 GeV. Positive particles with large longitudinal momentum, |x| > 0.5, i.e., leading particles, associated to large pT events, are analysed.

We report on measurements of correlated $$b\bar b$$ production in $$p\bar p$$ collisions at $$\sqrt s = 630GeV$$ , using dimuon data to tag both theb and $$\bar b$$ quarks. Starting from an inclusive dimuon sample we obtain improved cross-sections for single inclusive beauty production and confirm our earlier results on $$B^0 - \bar B^0$$ mixing. From a study of $$b\bar b$$ correlations we derive explicit cross-sections for semi-differential $$b\bar b$$ production. We compare the measured cross-sections and correlations to $$\mathcal{O}\left( {\alpha _s^3 } \right)$$ QCD predictions and find good quantitative agreement. From the measured angular distributions we establish a size-able contribution from higher order QCD processes with a significance of about seven standard deviations. A large nonperturbative contribution to these higher order corrections is excluded.

We report the observation of 21 dimuon events at the CERN pp Collider with the UA1 detector. The events range in invariant dimuon mass from 2 to 22 GeV / c2. The properties of these events are given. The bulk of the events are consistent with heavy-flavour production (mainly bb) with a few candidates for Drell-Yan production. There remain a few events which are difficult to interpret in terms of these processes, in particular two events with isolated, like-sign muons.

A recent publication by Basile et al. claims evidence for a hadronic state with beauty flavour, Λb → D0pπ−, produced in pp collisions at √s=63 GeV. We report a negative result from an experiment which has used the same detector and has a significantly higher sensitivity. In addition we show that neither experiment can observe such a signal, the data available being about two orders of magnitude below the minimum required.

Using the split field magnet (SFM) of the CERN ISR, we have investigated the main features of the reaction pp → ppπ+π− in the kinematical region where double pomeron exchange (DPE) is supposed to play an important rôle. Using kinematically fitted events at the five standard ISR energies from √s = 23.4 GeV to √s = 63.4 GeV, we have studied different data subsamples as a means of distinguishing DPE from the tail of single diffraction (SD). Our results are compatible with a significant DPE contribution; in particular there is no correlation between the two protons and the s- and t-dependences follow the expected behaviour.

The reaction pp→ppπ+π− at √s=31GeV was studied at the Split Field Magnet Facility of the CERN-ISR. Selecting events with two leading protons of x>0.9 and a rapidity gap Δy>2 between the protons and both pions a sample of 720 events is obtained representing a cross section of 25±10μb. The mass distribution of the dipion system, x and pT distributions for the protons are presented and analyzed for the presence of double pomeron exchange.

The inclusive spectrum of positive particles has been measured at the ISR energy √s = 52.5 GeV as function of transverse momentum, pT for large pT values using the Split Field Magnet facility. The angular dependence of the distribution is observed and discussed in the range 9° ⩽ θ∗ ⩽ 21°. An upper limit for the production of ∅ meson with pT ⩾ 3 GeV/c is derived from the same sample of data.

We present the performance of a hodoscope made of resistive plate chambers (RPC) installed in the RD5 experiment at CERN and exposed to beams of high energy muons and pions. The efficiency as a function of the particle flux is studied for different resistivities. A substantial improvement is obtained with plates of lower resistivity. Tracking muons through the RPC and using the bending power of a 2.7 Tm iron toroid, a simple trigger algorithm is used to select large momentum muons.

We present a study of heavy flavour (charm and beauty) production using data collected with the UA1 detector at the CERN Proton-Antiproton collider at a centre-of-mass energy of 630 GeV for an integrated luminosity of 556 nb−1. This follows our earlier study ofb $$\bar b$$ andc $$\bar c$$ production in dimuon events and the evidence for mixing betweenB 0 and $$\bar B^0 $$ states. Properties of an inclusive sample of events containing a muon withp T >6 GeV/c are compared with QCD predictions for heavy flavour production, using the ISAJET Monte Carlo program. The predicted largep T muon yield from heavy flavour production and decay agrees with the data to within 30%. For events with a muon ofp T >10 GeV/c and at least one jet withE T >10 GeV, we measure the ratio charm/(charm+beauty) to be (24±8±9)%. We also show that the correlations observed between the two heavy quark jets expected by flavour conservation are consistent with the mixture of lowest and higher order QCD processes. We study in detail the topological and kinematic properties of a subsample of events containing a highp T muon and one or more jets. The good agreement of the theoretical predictions with our data in a region dominated by the production of charm and beauty shows that one of the main background sources to a new heavy quark signature is well understood.

We present results on vector-meson leptoproduction in the SLAC fast-cycling 40-in. hydrogen bubble chamber exposed to 16-GeV/c muons. From 3644 inelastic events we obtained 495 events of ${\ensuremath{\mu}}^{\ensuremath{-}}p\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}{\ensuremath{\pi}}^{+}{\ensuremath{\pi}}^{\ensuremath{-}}p$, of which approximately 160 are $\ensuremath{\rho}$ events with $W&gt;2$ GeV and ${Q}^{2}&gt;0.05$ ${\mathrm{GeV}}^{2}$. Analyzing these data in the same way as in our previous photoproduction experiment, we find the ${\ensuremath{\rho}}^{0}$ contribution to ${\ensuremath{\sigma}}_{\mathrm{tot}}$ decreases by about 35% between ${Q}^{2}=0 \mathrm{and} 1$ ${\mathrm{GeV}}^{2}$, and the exponential slope parameter becomes approximately 20% smaller, although within errors it is still consistent with the photoproduction value. The decay angular distribution of the ${\ensuremath{\rho}}^{0}$ shows a substantial ${cos}^{2}\ensuremath{\theta}$ component which can be interpreted as evidence for production by longitudinal photons assuming $s$-channel helicity is conserved. Data from 39 $\ensuremath{\omega}$-production events are also presented. Upper limits for $\ensuremath{\varphi}$ and ${\ensuremath{\rho}}^{\ensuremath{'}}$ production are given.

Experimental results are presented on the reactions pp → K1OK1Oπ+π− and pp → K1OK2Oπ+π− at 0.7 GeV/c. The production of K∗, S∗, fO, ϕ, ϱ is observed, as well as the recently proposed F1 meson: its spin-parity quantum numbers are discussed.

A search for multifractal structures, in analogy with multifractal theories, is performed on UA1 minimum bias events. A downward concave multifractal spectral function,f(α) (where α is the Lipschitz-Hölder exponent), indicates that there are self-similar cascading processes, governing the evolution from the quark to the hadron level, in the final states of hadronic interactions.f(α) is an accurate measure of the bin to bin fluctuations of any observable. It is shown that the most sensitive comparison between data and the Monte Carlo models, GENCL and PYTHIA 4.8 can be made usingf(α). It is found that these models do not fully reproduce the behaviour of the data.

The general scheme for the fast, pipelined first level trigger on high pt muons in the CMS detector at LHC is presented. The prototype PACT system was tested in the high momentum muon beams in the RD5 experiment during 1993/94 runs. The obtained efficiency curves are shown.

A search for t-quark decay into charged Higgs, t→H+b, with the charged Higgs decaying via the H+→τ+vτ mode, has been done in proton-antiproton collisions at √2 = 0.63 TeV with the UA1 experiment. No evidence for this decay das been observed. Mass limits at 95% and 90% CL for the t-quark and the charged Higgs are presented. The results have also been interpreted in terms of the tan β parameter of a two-doublet Higgs sector.

It is probable that LEP detectors will often include 4π calorimeters. Since this is a novel technique, not much expertise exists yet in the field of pattern recognition for large calorimeter systems. A fast method to simulate calorimeter signals, based on an analytical parameterization of electromagnetic and hadronic showers, developed by the UA1 software group on calorimetry, is presented. Some reconstruction problems are discussed, in particular the question of disentangling individual showers within an energetic jet.

Experimental results are presented and discussed for the K01K01 system threshold, with evidence for the presence of the S∗ and the f0 messons.

The UA1 Collaboration has recently improved its measurement of the beauty production cross-section by including explicit measurements of bb correlations. Using these data we have determined the strong coupling constant αs. The comparison of the measured cross-section for 2-body final states with O(αs3) QCD predictions yields a measurement of αs(20 GeV) = 0.145−0.010 exp −0.016 th+0.012 +0.013, corresponding to αs(Mz) = 0.113−0.006 exp −0.009 th+0.007 +0.008. This is the first theoretically well-defined measurement of αs from a purely hadronic production process. Evaluating αs from cross-sections at different Q2-values we find that the running of αs is needed for internal consistency of the UA1 data.

This is the first full solid angle analysis of large transverse energy events in $$p\bar p$$ collisions at the CERN collider. Events with transverse energies in excess of 200 GeV at $$\sqrt s = 630 GeV$$ are studied for any non-standard physics and quantitatively compared with expectations from perturbative QCD Monte Carlo models. A corrected differential cross section is presented. A detailed examination is made of jet profiles, event jet multiplicities and the fraction of the transverse energy carried by the two jets with the highest transverse jet energies. There is good agreement with standard theory for events with transverse energies up to the largest observed values $$( \approx \sqrt {s/2} )$$ and the analysis shows no evidence for any non-QCD mechanism to account for the event characteristics.

Design and construction details are presented of a four-layer, position sensitive, cathode strip chamber and a low cost, highly multiplexed readout system based on monolithic circuit technology that are well suited for a muon detector at future hadron colliders. Track location is determined by interpolation of the cathode induced charge, using a new design with intermediate strips between readout nodes to reduce the number of channels and improve position resolution and linearity. Results are reported from tests with an 55Fe source and a 300 GeV/c muon beam in RD5 at CERN. The beam test demonstrated position resolution of 40 μm per layer which is less than 1% of the readout pitch. The timing resolution for the entire four-layer detector was 3.6 ns r.m.s. which is adequate for fully efficient beam crossing identification in an LHC experiment.

The split field magnet facility of the CERN ISR has been used to measure events with a secondary particle of large transverse momentum. The selection was done by triggering on positive particles of pT > 1.5 GeV/c emitted near the forward direction. Inclusive distributions of the other charged secondaries have been studied over almost the full kinematical range as a function of their y, pT, ø and charge. The particle density shows a broad increase in the ø hemisphere opposite to the selected large pT particle. This increase occurs in a wide rapidity interval centered at y = 0, although the trigger is at y = 2.

The relative yields of pions of both charges have been measured in the c.m angular 5.5° ⩽θ∗⩽21° in collisions at s=52.5 GeV. The transverse momentum and x-dependence of these relative yields are presented and discussed.

The total punchthrough probability of showers produced by negatively charged pions of momenta 30, 40, 50, 75, 100, 200 and 300 GeV/c, has been measured in the RD5 experiment at CERN using a toroidal spectrometer. The range of the measurement extends to 5.3 m of equivalent iron. Our results have been obtained by two different analysis methods and are compared with the resutls of a previous experiment.

Abstract Single layer honeycomb strip chambers have been constructed for the tracking calorimeter TRACAL. This detector is a component of the RD5 experimental test setup at CERN, where aspects of muon detection in LHC conditions are studied. The construction of the chambers is described and the excellent spatial resolution obtained from strip signals (σ

We have measured the rate of D∗± meson production inside the jets produced in pp collisions at √s = 630 GeV. For jets in the transverse energy range 15<ET<60 GeV we find a production rate of 0.10±0.04±0.03 D∗± per jet, which is in good agreement with perturbative QCD calculations. In addition, we find that the D∗± fragmentation distribution is strongly peaked towards low z consistent with gluon splitting as the dominant production mechanism.

Abstract The parallel plate chamber technique is summarized together with recent results of tests concerning efficiency, time resolution, high rate capability and radiation resistance.

The article describes the early years of the two large general-purpose experiments, ATLAS and CMS, at CERN's Large Hadron Collider (LHC). It covers the early conception of the detector designs to achieve the physics goals, the subsequent building of the worldwide collaborations, the evolution of the designs incorporating advances in technology and other considerations, and the painstaking global construction efforts. A detailed technical description of the detectors is beyond the scope of this review. This article also describes the development and deployment of the software and computing systems, by both the collaborations and the LHC Worldwide Computing Grid, in order to extract the physics results.

A search for long-lived particles (LLPs) produced in association with a Z boson is presented. The study is performed using data from proton-proton collisions with a center-of-mass energy of 13 TeV recorded by the CMS experiment during 2016-2018, corresponding to an integrated luminosity of 117 fb$^{-1}$. The LLPs are assumed to decay to a pair of standard model quarks that are identified as displaced jets within the CMS tracker system. Triggers and selections based on Z boson decays to electron or muon pairs improve the sensitivity to light LLPs (down to 15 GeV). This search provides sensitivity to beyond the standard model scenarios which predict LLPs produced in association with a Z boson. In particular, the results are interpreted in the context of exotic decays of the Higgs boson to a pair of scalar LLPs (H $\to$ SS). The Higgs boson decay branching fraction is constrained to values less than 6% for proper decay lengths of 10-100 mm and for LLP masses between 40 and 55 GeV. In the case of low-mass ($\approx$15 GeV) scalar particles that subsequently decay to a pair of b quarks, the search is sensitive to branching fractions $\mathcal{B}$(H $\to$ SS) $\lt$ 20% for proper decay lengths of 10-50 mm. The use of associated production with a Z boson increases the sensitivity to low-mass LLPs of this analysis with respect to gluon fusion searches. In the case of 15 GeV scalar LLPs, the improvement corresponds to a factor of 2 at a proper decay length of 30 mm.

A measurement of inclusive four-jet production in proton-proton collisions at a center-of-mass energy of 13\TeV is presented. The transverse momenta of jets within $\lvert\eta\rvert \lt$ 4.7 reach down to 35, 30, 25, and 20 GeV for the first-, second-, third-, and fourth-leading jet, respectively. Differential cross sections are measured as functions of the jet transverse momentum, jet pseudorapidity, and several other observables that describe the angular correlations between the jets. The measured distributions show sensitivity to different aspects of the underlying event, parton shower, and matrix element calculations. In particular, the interplay between angular correlations caused by parton shower and double-parton scattering contributions is shown to be important. The double-parton scattering contribution is extracted by means of a template fit to the data, using distributions for single-parton scattering obtained from Monte Carlo event generators and a double-parton scattering distribution constructed from inclusive single-jet events in data. The effective double-parton scattering cross section is calculated and discussed in view of previous measurements and of its dependence on the models used to describe the single-parton scattering background.

We present a search for the decay W±→π±γ, using data taken at the CERN proton-antiproton Collider with the UA1 detector and corresponding to an integrated luminosity of 0.7 pb−1. We do not observe a signal for such a process: hence we obtain an upper limit on the π±γ decay width of 5.8×10−2Γ (W±→e±v) (95%CL).

A novel muon detector concept for LHC was studied in a test beam. The application of mean-timer technique to arrays of drift tubes provides the space and time resolution needed for first level trigger and track reconstruction using only the drift time information. A complete study of the performance of this new detector was done.