H. J. Besch

Search for direct single-photon production at large pT in photon-proton collisions at √s = 62.4 GeV

Abstract As part of a study of large p T phenomena in photon-proton collisions at the CERN ISR, a search for direct single photon production has been performed. A statistical division of the data sample into the fraction consistent with single photon and the fraction due to multiphoton decays of neutral hadrons is accomplished by measuring the average conversion probability for the sample in a one radiation length thick converter. The fraction of the sample attributable to direct single photon production is 〈 γ /all〉 = 0.074 ± 0.012 for 6 GeV/ c p T 10 GeV/ c , and 〈 γ /all〉 = 0.26 ± 0.04 for p T > 10 GeV/ c , with an additional systematic uncertainty of ±0.05 for both values.

Determination of the angular and energy dependence of hard constituent scattering from π0 pair events at the CERN intersecting storage rings

We present data on proton-proton collisions, obtained at the CERN Intersecting Storage Rings, in which two roughly back-to-back ..pi../sup 0/s of high transverse momentum (p/sub T/) were produced. The angular distribution of the dipion axis relative to the collision axis is found to be independent of both the effective mass m of the dipion system and the centre-of-mass energy ..sqrt..s of the proton-proton collision. The cross-sections dsigma/dm at the two values of ..sqrt..s satisfy a scaling law of the form dsigma/dm = G(x)/m/sup n/, where x = m(..pi../sup 0/,..pi../sup 0/)/..sqrt..s and n = 6.5 +- 0.5. We show from our data that the leading ..pi../sup 0/ carries most of the momentum of the scattered parton. Given this fact, the axis of the dipion system follows closely the direction of the scattered constituents, and we exploit this to determine the angular dependence of the hard-scattering subprocess. We also compare our data with the lowest order QCD predictions using structure functions as determined in deep-inelastic scattering and fragmentation functions from electron-positron annihilation.

A measurement of the transverse momenta of partons, and of jet fragmentation as a function of √s in p-p collisions

A large solid angle apparatus consisting of a superconducting solenoid magnet, cylindrical drift chambers and two arrays of lead-glass counters was used to examine particles associated with a high transverse momentum trigger in p-p collisions with three ..sqrt.. s values at the CERN ISR. The trigger was given by energy deposition in lead-glass arrays centered at 90/sup 0/. The trigger transverse momentum range covered was 3 < P/sub Ttrig/ < 11 GeV/c. Results are given for P/sub out/ for both individual charged particles, and also for the sum of charged particle momenta in the hemisphere opposite to the trigger. Mean vaues are then deduced for the parton transverse momentum, k/sub T/, and for the jet fragmentation momentum, j/sub T/.

Observation of jet structure in high transverse energy events at the CERN intersecting storage rings

Abstract The spectrum d N d E T 0 ( E T 0 = neutral transverse energy) has been measured with an electromagnetic calorimeter covering 90% of 2 π in azimuth. The measurement, performed in pp collisions at s = 62 GeV , covered the E T 0 range from 10 to 30 GeV. Evidence is presented that a large fraction of these events are jet-like, the fraction increasing with E T 0 .

The total neutral energy spectra in the central region of α−α, d−d and p−p interactions at sNN = 31 GeV per nucleon pair at the CERN ISR

The spectra in total neutral energy, ETOT0, have been measured in pp, dd and αα collisions at centre-of-mass energy per nucleon pair, sNN, of 31 GeV. The range in ETOT0 covered was 1 to 19 GeV in pp, 18 to 24 GeV in dd and 1 to 34 GeV in αα interactions. The ratio of the αα to the pp spectra rises monotonically from 7 at ETOT0 = 1.5 GeV to 105 at ETOT 0 = 19 GeV. Since ETOT0 is the sum over many particles in the detector, it is directly sensitive to multiple nucleon-nucleon collisions in the interacting α particles. The αα spectrum in the range 8 < ETOT0 < 17 GeV is consistent with being described by a simple model of 4 simultaneous nucleon-nucleon collisions with a cross section about equal to 10% of the observed αα interaction cross section. The spectrum of ETOT0 above 17 GeV is even flatter than this extreme case where four nucleon pairs are interacting.

Observation of KNO scaling in the neutral energy spectra from αα and pp collisions at ISR energies

Abstract Neutral transverse energy spectra in pp and αα interactions are analyzed in terms of the wounded nucleon model. Analysis of the αα spectrum by application of a multiple nucleon-nucleon collision mechanism is conveniently performed when a gamma distribution is used to represent the pp spectral shape. The wounded nucleon model provides a reasonable description of the αα spectrum for the first three orders of magnitude, but completly fails to account for the slope of the high energy tail of the distribution. However, the pp and αα spectra can both be fitted to the same gamma distribution when scaled by their respective mean values and thus exhibit KNO scaling.

Jets in high transverse energy events at the CERN intersecting storage rings

Abstract Most events with high neutral transverse energy, ET0, produced in pp colisions at √s = 62.3 GeV, are jet-like. The evidence for this is presented, based on data collected using an electromagnetic calorimeter covering 90% of 2π in azimuth. The spectrum dN/dET0 has been measured over the ET0 range from 10 to 35 GeV. Properties of the observed jets are discussed.

High pT π0 production from αα and αp collisions at the CERN ISR

Abstract The invariant cross sections for π0 meson production in alpha—alpha and alpha—proton collisions at the ISR were meas- ured up to transverse momenta of 7 GeV c and 8 GeV c , respectively. These measurements are compared with π0 production in pp collisions at the same values of s / nucleon, and the variation of the nuclear A-dependence with pT is determined.

A study of particle correlations and charge ratios in direct photon events at the CERN intersecting storage rings

Abstract A search for direct photons produced in proton-proton collisions has been performed at two s values at the CERN Intersecting Storage Rings (ISR). It is found that direct photons are produced preferentially with no accompanying particles on the same side. On the away side, the ratio of positive to negative high- x E particles is found to be higher for high transverse momentum direct-photon events, than for high transverse momentum neutral-meson events.

A study of e+e− pairs in the mass range 11 to 25 GeV/c2 at the CERN intersecting storage rings

Abstract A sample of 58e + e − events with an invariant mass greater than 11 GeV/c 2 produced in pp collisions at a centre-of-mass energy of 62.3 GeV is discussed. The cross sections are presented as a function of the mass and transverse momentum. The electron pairs produced with a mean transverse momentum of 2.50 ± 0.25 GeV/ c .