R. D. Field

A Parametrization of the Properties of Quark Jets

A model is analyzed that provides a parametrization of the properties of the jet of mesons generated by a fast outgoing quark. It is assumed that the meson that contains the original quark leaves momentum and flavor to a remaining jet in which the particles are distributed (except for scaling of the energy and possible changes of flavor) like those of the original jet. One function, the probability f(η) that the remaining jet has a fraction η of the momentum of the original jet, is chosen (as a parabola) so the final distribution of charged hadrons agrees with data from lepton experiments. All the properties of quark jets are determined from f(η) and three parameters; the degree that SU(3) is broken in the formation of new quark-antiquark pairs (s[bar]s is taken as half as likely as uu[bar]), the spin nature of the primary mesons (assumed to be vector and pseudoscalar with equal probability), and the mean transverse momentum given to these primary mesons. Monte Carlo methods are used to generate typical jets. Analytic approximations are also given. Many features of quark jets are examined. The distribution of momentum of various hadrons D_q^h(z), the properties of the hadrons of largest momentum in the jet, correlations, rapidity-gap distribution of charge and of transverse momentum are some of the subjects discussed. The appearance of the jets to an instrument sensitive only to particles above some minimum momentum is also described. Although the model is probably not a true description of the physical mechanism responsible for quark jets, many predictions of the model seem quite reasonable, possibly much like real quark jets (except that the possibility of the emission of baryons is disregarded). The purpose of this work is to provide a model useful in the design of experiments in which quark jets may be observed, and further to provide a standard to facilitate the comparison of lepton-generated jets with the high-p⊥ jets found in hadron collisions.

Correlations among particles and jets produced with large transverse momenta

We continue to investigate consequences of the assumption that the high transverse momentum particles seen in hadron-hadron collisions are produced by a single, hard, large-angle elastic scattering of quarks; one from the target and one from the beam. A large p⊥ event consists of four jets (collection of hadrons moving in roughly the same direction). The two outgoing quarks fragment into two jets of hadrons with roughly equal and opposite transverse momentum. The other two jets have small transverse momentum and result from the break up of the beam and target hadrons. We compare the model with existing data on the correlations among particles on the same and opposite (away) side of a large p⊥ trigger. If we allow the quarks within the initial hadrons to have a large mean transverse momentum, 〈k⊥〉_(h→q), then good agreement with data is found. We have taken 〈k⊥〉_(h→q) = 500 MeV; however, even larger values are suggested by certain ISR experiments. A large cross section for producing a jet (quark) is predicted. For instance, σ(pp → jet + X)/σ(pp → π^0 + X) = 370 at x⊥ = 0.4 and θ_(c.m.) = 90°. This is consistent with a recent FNAL jet trigger experiment. Predictions for future FNAL and ISR experiments are made.

Triple Regge and finite mass sum rule analysis of the inclusive reaction pp → pX☆

Abstract We present the results of a comprehensive analysis of inclusive pp → pX data in terms of the triple Regge formalism. We use all available data and in the theory include PPP, PPR, RRP, RRR, ππP, and ππR terms. Further we examine the importance of interference terms (e.g. PRP) and present solutions with the triple pomeron either vanishing or non-vanishing at t = 0. Predictions are made for both pp → pX and pn → pX at NAL and ISR energies. In addition, finite mass sum rules (FMSR), constructed from low-mass diffractive dissociation data, are shown to be in good agreement with the high-energy, high-mass fits. The energy, momentum transfer and moment dependence of the FMSR are discussed in detail.

A QCD Model for e+ e- Annihilation

Abstract A QCD model for e + e − annihilation is presented, and its consequences are compared with experimental data. The model involves production of a shower of partons described by a simple approximation to QCD perturbation theory, and decay of colour singlet clusters of produced partons into hadrons through a simple phase space process. The model reproduces most known theoretical features of QCD, and, with certain choices of parameters, appears to correspond well with experimental results.

Large-pT production of single and double photons in proton-proton and pion-proton collisions

Quantum chromodynamic (QCD) predictions are made for the large transverse momentum production of single and double photons in proton-proton, proton-antiproton, and pion-proton collisions. In π−p collisions at center-of-mass energy W = 27.4 GeV and pT = 4.0 GeV, it is estimated that about 0.3% of the 90° single photon triggers will be balanced on the “away-side” by a single photon with roughly the same transverse momentum. In π+p collisions this fraction drops to about 0.09%. These fractions increase with pT. In addition to the pure QED annihilation term qq→γγ, it is found that the QCD-induced subprocess gg→γγ provides an important source of double photons. Photon bremsstrahlung contributions are also examined. Experimental study of the systematics of single and double photon production in hadron-hadron collisions will provide information on the size of the strong interaction coupling constant, αs(Q), and on the charges of the quarks. Knowledge of the gluon distributions within hadrons and of the effective transverse momentum of partons in hadrons can also be gained.

Beyond leading order QCD perturbative corrections to the pion form factor

Abstract The order α s 2 ( Q 2 ) corrections to the pion form factor, F π ( Q 2 ), are calculated using perturbative quantum chromodynamics and dimensional regularization. In the MS renormalization scheme these corrections are large. This means that reliable perturbative predictions cannot be made until momentum transfers, Q , of about 300–400 GeV are reached or unless one can sum the large perturbative terms to all orders. Attempts to reorganize the perturbation series so that the first term gives a better approximation of the complete sum indicate that at Q = 10 GeV the pion form factor may be about a factor of two larger than the leading order result.

The direct photon spectrum in the inclusive decay of the upsilon

Abstract The effect of the self coupling of gluons on the direct photon spectrum in the decay ϒ(1s) → ggγ is estimated using a parton-shower Monte Carlo approximation to QCD perturbation theory. Outgoing gluons radiate additional gluons thereby developing an invariant mass which distorts the Dalitz plot for the decay resulting in photons of less energy than the “naive” massless gluon prediction. The correction to the “Born term” photon spectrum is sizable and is not very sensitive to the value of the QCD parameter Λ.

BACKWARD np SCATTERING WITH A POLARIZED TARGET.

The authors have measured the polarization parameter P in neutron-proton elastic scattering near the backward direction, using a polarized proton target. Measurements covered the range of incident neutron moment from 1.0 to 5.5 GeV/c and of four-momentum transfer squared u from -0.005 to -0.5 (GeV/c){sup 2}. Forward going protons were detected by means of a wire-spark-chamber spectrometer. Slow neutrons near 90 deg lab angle were detected in coincidence by means of an array of plastic scintillation counters. P was determined from the change in counting rate I of scattered particles upon reversal of the target polarization P{sub T} according to I = I{sub 0}[l + P(P{sub T} {center_dot} k incident neutron x k final neutron/sin {theta} final neutron)]. They find that P is consistently negative and shows no marked structure as a function of u and of incident momentum. The data roughly follow the simple form P = -0.5 {radical}-u/m{sub p}.

The underlying event

I review studies of the underlying event (UE) in hadronic collisions, dating from the first CDF studies in 2000 to the latest LHC findings and surprises. I explain the CDF quantum chromodynamics (QCD) Monte Carlo model tunes and describe how well the Tevatron tunes did at predicting the behavior of the UE at the LHC. In a very short time, the LHC experiments collected a large amount of data at 900 GeV and 7 TeV that can be used to study the UE in great detail. I review the LHC UE results and compare them with one another, with the Tevatron results, and with some of the LHC QCD Monte Carlo model tunes. I also explain the relationship between minimum bias collisions and the UE and discuss new techniques for studying the UE (i.e., techniques beyond the traditional approach).

High energy multi-jets at the CERN pp collider and the SSC

Abstract Multi-jet topologies at large transverse energy in p p collisions at W = 540 GeV are studied using a QCD parton-shower Monte Carlo model. The model includes gluon radiation off partons in both the initial and final state and hadrons are labeled according to whether they result from beam and target fragmentation of from outgoing parton fragmentation. Results of the QCD model are compared with CERN collider data from UA1 and UA2 and with predictions of the naive 4-jet parton model. The data show qualitative agreement with the QCD Monte Carlo and deviate greatly with the predictions of the naive parton model. Estimates of multi-jet production in p p collisions at the SSC energy of W = 40 TeV are presented and discussed.