UA2 Detector Research Articles

Preliminary analysis of the performance of a rich counter for low- pT electron identification at the CERN p [formula omitted] collider

A ring-imaging Cherenkov (RICH) counter with a 60 cm long freon (C 2F 6) gas radiator has been used inside the UA2 detector at the CERN SPS p p Collider, the aim being to measure the amount of promptly produced electrons in the p T region 1–3 GeV/ c. About 800 000 events were recorded and the analysis is in progress. Results from a first analysis of a limited event-sample indicate that it will be possible to reach an e π rejection power of at least 10 −4 with the RICH counter.

Jet measures and hadronic event shapes at the CERN $$\bar p$$ p collider

We analyze the energy density distribution in harronic final states as a function of their total transverse energy measured in the segmented central calorimeter of the UA2 detector. The energy dependence of collective shape variables is investigated. The data, collected at the CERN $$\bar p$$ p Collider at $$\sqrt s = 630$$ GeV, exhibit strong variations in all these variables over the transverse energy range between 15 and 210 GeV, corresponding to substantial modifications in the structure of multihadronic final states. The evolution of the energy density distribution and of the collective shape variables shows a clear transition between two extreme dynamical regimes, respectively dominated byp T -limited phase space and by collimated two-jet configurations. A study of the relative populations of two-and three-jet systems reveals two different sources of configurations having three distinct lobes in the pattern of the energy-flow. A first component, steeply falling with energy, receives substantial contributions from soft parton collisions at lower transverse energies. Above 60 GeV a new hard component emerges, characterized by an approximately constant rate with respect to the dominant two-jet structures.

Measurement of the standard model parameters from a study of W and Z bosons

A study has been made of the decays W → ev and Z → e +e −, using the UA2 detector at the CERN p̄p Collider. The data correspond to an integrated luminosity of 142 nb −1 at a centre-of-mass collision energy √ s=546 GeV, and 768 nb −1 at √ s=630 GeV. Measurements of the standard model parameters from samples of 251 W decay and 39 Z decay candidates are compared with expectations of the standard electroweak model.

Measurement ofW ± andZ 0 properties at the CERN $$\bar pp$$ p collider

A study ofW± andZ0 properties has been performed using the UA2 detector at the\(\bar pp\) collider. The data correspond to a total integrated luminosity of 142 nb−1 at\(\sqrt s \)=546 GeV, and of 310 nb−1 at\(\sqrt s \)=630 GeV. The experimental results are compared with the predictions of the Standard Model of the unified electroweak theory.

Inclusive particle production in the transverse momentum range between 0.25 and 40 GeV/c at the CERN $$Sp\bar pS$$ collider

Inclusive particle production cross-sections have been measured at the $$Sp\bar pS$$ collider using the UA2 detector in various ranges of transverse momentum (P T ) and pseudo-rapidity (η). Cross-section measurements are presented forπ 0 production (P T ≦15 GeV/c, |η|≦0.85 andP T ≦40 GeV/c, 1.0≦|η|≦1.8), for η meson production (3≦P T ≦6 GeV/c, |η|≦0.85) and for charged particle production (P T ≦10 GeV/c, 1.0≦|η|≦1.8). Results are compared with the predictions of QCD calculations.

Intermediate-mass dimuon events at the CERN p [formula omitted] collider at √ s = 540 GeV

We report the observation of 21 dimuon events at the CERN p p Collider with the UA1 detector. The events range in invariant dimuon mass from 2 to 22 GeV / c 2. The properties of these events are given. The bulk of the events are consistent with heavy-flavour production (mainly b b ) 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.

Nobel Prize to Rubbia and van der Meer for Finding W and Z

Carlo Rubbia and Simon van der Meer of CERN share the 1984 Nobel prize in physics for “their decisive contributions to the large project which led to the discovery of the field particles W and Z, communicators of weak interaction.” In 1976 Rubbia and his collaborators had the idea of converting the Super Proton Synchrotron at CERN into a proton-antiproton collider, where the W and Z particles could be created. In 1968 van der Meer had invented the method of stochastic cooling, which allowed the dense packing and storage of enough antiprotons to make the W and Z in pp̄ collisions. The CERN pp̄ collider project, the largest ever to appear in the context of the Nobel prize, yielded its first collisions in 1981. Two year later, in 1983, both the W and Z were observed by the UA1 detector group led by Rubbia and by the UA2 detector group. The Royal Swedish Academy of Sciences, in announcing the award, said, “Two persons in the CERN project are outstanding—Carlo Rubbia, who had and developed the idea, and Simon van der Meer, whose invention made it feasible.” This year's prize, which is worth about $193 000, is the first to honor work done at CERN.

Multitube proportional chambers for the localization of electromagnetic showers in the CERN UA2 detector

24 multitube proportional chambers with a total number of 5424 wires have been constructed for the CERN UA2 detector. They serve to localize the electromagnetic showers produced by electrons and photons in a lead-iron converter in front of each chamber, and to reject hadrons. Details of the construction and the operation parameters are given. The response to electrons and to hadrons in the energy range 20–60 GeV is discussed. The chambers and their electronics proved to be very reliable and stable.

Measurement of jet production properties at the CERN [formula omitted] Collider

Jet production properties at s = 540 GeV have been measured in the UA2 detector at the CERN p p Collider. Results on the total transverse momentum of the jet system, on the parton density in the nucleon (structure function) and on the two-jet angular distributions are reported. The data are compared with QCD predictions and extrapolations from lower energy experiments.

Measurement of jet fragmentation properties at the CERN [formula omitted] Collider

Fragmentation properties of a sample of two-jet events measured by the UA2 detector at the CERN p p Collider are described. The energy flow is compared with different model predictions. The charged particle multiplicity in jets is found to exceed extrapolations from lower energy e +e − jet data.

UA1 at CERN says it has candidates for sixth quark, top

Early in July the UA1 detector group working at the CERN proton‐antiproton collider announced that they have found six candidate events suggesting the top quark. According to the standard model, quark flavors come in pairs—up and down, strange and charmed, bottom and top. The fifth quark, the bottom b, whose mass is about 5.2 GeV, was needed to explain the upsilon meson, found in 1977. But the missing sixth quark could not be found. If the mass of the top t had been less than 22 GeV, experiments at PETRA (at the DESY laboratory in Hamburg) with 22‐GeV electrons colliding with 22‐GeV positrons would have produced t and t̄ from the 44‐GeV center‐of‐mass energy available.

The system of forward-backward drift chambers in the UA2 detector

The system of multiplane drift chambers for the forward-backward toroidal spectrometers of the UA2 experiment at the SPS pp̄ collider is described. Details about mechanical design and construction techniques are given. Results on the performance of the chambers are reported.

Observation of the muonic decay of the charged intermediate vector boson

Muons of high transverse momentum p μ T have been observed in the large drift chambers surrounding the UA1 detector at the CERN 540 GeV pp̄ collider. For an integrated luminosity of 108 nb −1, 14 isolated muons have been found with p T > 15 GeV/ c. They are correlated with a large imbalance in total transverse energy, and show a kinematic behaviour consistent with the muonic decay of the Intermediate Vector Boson W ± of weak interactions. The partial cross section is in agreement with previous measurements for electronic decays and with muon-electron universality. The W mass is determined to be m W = 81 +6 −7 GeV/ c 2.

Hadronic jet production at the CERN proton-antiproton collider

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 E T = 100 GeV. A comparison with theoretical models of cross sections for events with more than two jets is also given.

Inclusive charged particle production at the CERN [formula omitted]p collider

Transverse momentum distributions of pions, kaons and protons have been measured around 90° in the UA2 detector at the SPS p p collider, at a CM energy of 540 GeV. All the cross sections have increased by more than a factor of 2 over those measured at ISR energies and exhibit a flatter behaviour with respect to transverse momentum.

Charged particle multiplicity distributions in proton-antiproton collisions at 540 GeV centre of mass energy

Results on charged particle production in pp̄ collision at s 1 2 = 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.