A.G. Clark

Observation of single isolated electrons of high transverse momentum in events with missing transverse energy at the CERN pp collider

Abstract We report the results of a search for single isolated electrons of high transverse momentum at the CERN p p collider. Above 15 GeV/c, four events are found having large missing transverse energy along a direction opposite in azimuth to that of the high-pT electron. Both the configuration of the events and their number are consistent with the expectations from the process p + p → W ± + anything , with W→e+ν, where W± is the charged Intermediate Vector Boso postulated by the unified electroweak theory.

Evidence for Z0→e+e− at the CERN p collider

Abstract From a search for electron pairs produced in p p collisions at s = 550 GeV we report the observation of eight events which we interpret as resulting from the process p +p→Z 0 + anything, followed by the decay Z0→e++e− or Z0→e++e−+γ, where Z0 is the neutral Intermediate Vector Boson postulated by the unified electroweak theory. We use four of these events to measure the Z0 mass M Z = 91.9 ± 1.3 ± 1.4 (systematic) GeV /c 2 .

Observation of very large transverse momentum jets at the CERN pp collider

The distribution of total tranverse energy ΣET over the pseudorapidity interval −1 < η < 1 and an azimuthal range Δφ=300° has been measured in the UA2 experiment at the CERN pp collider (s = 540 GeV) using a highly segmented total absorption caloriter. In the events with very large ΣET (ΣET⪆60 GeV) most of the transverse energy is found to be contained in small angular regions as expected for high transverse momentum hadron jets. We discuss the properties of a sample of two-jet events with invariant two-jet masses up to 140 GeVc2 and we measure the cross section for inclusive jet production in the range of jet transverse momenta between 15 and 60 GeVc.

Evidence for Transverse Jets in High Mass Diffraction

Abstract Localized energy clusters (jets) with transverse energy between 5 and 13 GeV are observed semi-inclusively in proton-antiproton interactions with√s = 630GeV which also contain a final state proton with more than 90% of the beam protons momentum. The system which recoils against the proton is studied for invariant masses in the range 105–190 GeV. The production rate, angular and energy distributions and profiles of the clusters are in good agreement with predictions of a QCD model of diffractive scattering, in which the clusters result from the hard scattering of gluons in an exchange pomeron with the partons in an incident antiproton. A soft gluon structure function of the pomeron, such as(1 − x) 5 , is preferred.

The central calorimeter of the UA2 experiment at the CERN pp collider

Abstract The highly segmented electromagnetic and hadronic calorimeter of the UA2 experiment has been successfully operated at the CERN p p Collider for more than 3 years. The mechanical design and construction technique of the calorimeter as well as its performance and stability in test beams and at the p p Collider are described in detail.

Electron pair production at the CERN ISR

Abstract The electron pair production cross section from pp collisions at c.m. energies √s = 52 and 63 GeV has been measured for masses above 1.4 GeV/c2. The J/ψ is dominant in this mass range and its production properties have been studied. An estimate of the ψ′ production cross section is given. Limits for ϒ → e+e− and χ → J ψγ productions are presented. The behaviour of the continuum cross section as a function of mass is compared to current theoretical estimates.

Observation of Electrons Produced in Association with Hard Jets and Large Missing Transverse Momentum in p anti-p Collisions at s**(1/2) = 540-GeV

Abstract Using a sample of events collected by UA2 and corresponding to an integrated luminosity of 116 nb −1 . we have searched for electron-“neutrino” pairs in which the transverse momenta of the electron and of the “neutrino” exceed 15 GeV/c and 25 GeV/c respectively. A total of 35 events are observed in low background conditions. Most events can be interpreted in terms of W ± production from QCD processes. Events in which the observation of hard jets makes this interpretation unlikely are described in detail. Possible sources of background contamination are considered.

MEASUREMENT OF THE STANDARD MODEL PARAMETERS FROM A STUDY OF W AND Z BOSONS

Abstract A study has been made of the decays W → ev and Z → e + e − , using the UA2 detector at the CERN pp 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 of the

Abstract The production of very large transverse momentum ( p T ) hadron jets has been measured in the UA2 experiment at the CERN p p Collider for s =630 GeV . The inclusive jet production cross sections exhibit a p T -dependent increase with respect to the s =546 GeV data from previous Collider runs. This increase can be described both by QCD calculations and by approximate x T =2 p T /√ s scaling. No significant deviation of the data from QCD predictions is observed at very large p T , placing a lower limit on the characteristic scale Λ c of a hypothetical superstrong contact interaction responsible for the binding of preons in the quark ( Λ c >370 GeV at 95% CL).

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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.