CERN Proton-antiproton Collider Research Articles

The physics of hadronic jets

This article summarizes the main results obtained in the study of hard collisions at the CERN proton–antiproton collider. These results include the first, unambiguous observation of jet production at high transverse momenta in hadronic collisions, as expected from parton–parton scattering at large angles. Other types of hard parton collisions, such as the production of direct photons and of heavy flavours, and the measurement of W and Z bosons with high transverse momenta, are also reviewed in this article. All of these processes can be described in the framework of perturbative QCD.

Experimental limit on the decay W ±→π ±γ at the cern proton-antiproton collider

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 fast charge integrating and shaping circuit

The development of a low cost fast charge integrating and shaping circuit (FCISC) was motivated by the need for an interface between the photomultipliers of an existing hadronic calorimeter and recently developed new readout electronics designed to match the output of small ionization chambers for the upgraded UA1 detector at the CERN proton-antiproton collider. This paper describes the design principles of gated and ungated charge integrating and shaping circuits. An FCISC prototype using discrete components was made and its properties were determined with a computerized test setup. Finally an SMD implementation of the FCISC is presented and the performance is reported.

Soft quark annihilation contribution to the low mass dimuon production in high-p T jet events

The soft quark annihilation contribution to the low-mass, unlike-sign dimuon production in high-p T jet events, which has recently been observed by the UA1 collaboration at the CERN protonantiproton collider, is estimated within a model that successfully describes the anomalous dilepton production in soft hadronic collisions. A method is proposed for disentangling the various production mechanisms, based on comparing the mean multiplicity of hadrons accompanying a dimuon with a quantity calculated from the lowest multiplicity moments in events without dimuons.

Construction and performance of a position detector for the UA1 uranium-TMP calorimeter

A full-scale prototype position detector has been built and tested for the uranium-tetramethylpentane (TMP) calorimeter which is presently being constructed for the UA1 experiment at the CERN proton-antiproton collider. The position detector consists of a sealed stainless steel box containing TMP and three electrode planes, where the middle plane is used for energy measurement, and the other two are highly segmented to enable a precise measurement of the position of electromagnetic showers. The position detector has been operated in electron and muon test beams at CERN. The detector performs well, both as an element of the calorimeter and as a device for measuring shower positions, and yields a spatial resolution of about 1 mm for high energy electron showers.

Recent results from UA1 at the CERN proton-antiproton collider

UA1 has completed the analysis of all the data taken over the part six years. In particular I would like to report here on our latest results in the study of heavy flavours and show the importance of the muon channel. The results of our search for new heavy quarks (t and b′) are given. The beam tests of the first module of our new uranium-TMP calorimeter are described. Finally I will mention our plans for the CERN Proton-Antiproton Collider run from September to December this year.

Studies of intermediate vector boson production and decay in UA1 at the CERN proton-antiproton collider

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.

Study of heavy flavour production in events with a muon accompanied by jet(s) at the CERN proton-antiproton collider

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.

Direct photon production at the CERN proton-antiproton collider

Isolated photons, produced directly by a scattering process, have been observed in the UA1 experiment at the CERN p p 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.

The UA1 VME data acquisition system

The data acquisition system of a large-scale experiment such as UA1, running at the CERN proton-antiproton collider, has to cope with very high data rates and to perform sophisticated triggering and filtering in order to analyze interesting events. These functions are performed by a variety of programmable units organized in a parallel multiprocessor system whose central architecture is based on the industry-standard VME/VMXbus.

Search for new heavy quarks at the CERN proton-antiproton collider

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 s ) 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 s ) 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.

Events with large missing transverse energy at the CERN collider: III. Mass limits on supersymmetric particles

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 m q >45 GeV/c 2 at 90% CL, independently of the gluino mass. Similarly, we find a limit on the gluino mass of m g >53 GeV/c 2 (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, m q ⪅1 TeV/c 2 ). For equal squark and gluino masses we find a limit m q =m g >75 GeV/c 2 at 90% CL. The effect of a non-zero photino mass on these limits is studied.

Production of W's with large transverse momentum at the CERN proton-antiproton collider

We have measured the W transverse momentum distribution ( p T W ) 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 p T W ∼- 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 p T W . However we observed two events at very large p T W (∼- 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 Z 0 → ν ν 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.

Large transverse momentum W production at Hadron colliders

Motivated by the recent observations of events in which a large transverse momentum W has been produced together with two energetic jets at the CERN proton—antiproton collider, we consider standard model processes in which a W is created in association with a second weak boson which then decays hadronically into two jets. We find that the production and decay of heavy quark—antiquark pairs can give rise to an observable number of (WW) events in existing collider data samples if the heavy-quark mass is close to the W mass. We calculate the expected event rates for proton—antiproton colliders operating in the energy range 0.6–2.0 TeV, and discuss the kinematic properties of the events, and the identity of the heavy quark Q which could be either the top quark, or a down-type quark from a new (fourth) generation.

Events with large missing transverse energy at the cern collider: II. search for the decays of W ± into heavy leptons and of Z 0 into non-interacting particles

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 Z 0 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) Z 0 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 m L>41 GeV/ c 2 (90% CL) and a limit on the number of additional neutrino species of n⩽7 (90% CL).

Events with large missing transverse energy at the cern collider: I.W→ τν decay and test of τ μe universality at Q2= mw2

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→e ν, W→ μν, and W→ τν provide the first direct tests of the electron-muon-tau universality of the weak charged couplings at Q 2= m w 2. We determine g τ g e =1.01±0.09±0.05 and g μ g e =1.05±0.07±0.08 .

A microvertex detector for experiment UA1 at the CERN [formula omitted] collider

The design and operation of a small pressurized drift chamber is described. The chamber provides 16 space points between 3 and 8 cm from the beam axis of the CERN proton—antiproton collider.

Model of new scalar bosons as the source of one-jet events at the $$\bar p$$ p collider

A model involving color octet and singlet scalar bosons is studied as a possible candidate to account for the one-jet events with large missing transverse energy observed at the CERN protonantiproton collider. A distinguishing characteristic of this model, as compared to various models based on supersymmetry, is that the heavy particle being produced is assumed to be also the carrier of the missing momentum.

The central electromagnetic calorimeter of UA1

We describe the construction, calibration and performance of the central electromagnetic calorimeter of the UA1 experiment at the CERN proton-antiproton collider. The calorimeter is of the lead-scintillator sandwich type. It is 26.4 radiation lengths thick and covers a surface of about 50 m 2. We estimate the resolution of the calorimeter for electrons of energy greater than 1 GeV to be the sum in quadrature of 15% √E ( E in GeV) and a constant 3%. The first term comes from the inherent resolution of the calorimeter due to sampling fluctuations and photostatistics. The second term comes from uncertainties in the calibration procedure and dominates the resolution for electrons from W and Z 0 decay. The uncertainty in the overall energy scale also reflects the uncertainties in the calibration procedure and is estimated to be 3%.

A microprocessor-controlled readout system for imaging drift chambers

A multihit compact readout system for drift chambers is described. This system records the drift time, the charge division, and the energy loss for up to 16 hits per wire, giving a three-dimensional representation of multitract events. A total of 1000 channels is used to read the imaging drift chambers in the forward arms in the UA1 apparatus at the CERN proton-antiproton collider.