A clear test of perturbative QCD in large-pT photoproduction

It is shown that measurement of the cross-section differences $\text{E}\text{d}\text{σ}\text{d}{}^3\text{p(γ}\text{p}\text{→}\text{h}{}^{\mathrm{+}}\text{X}\text{) ?}\text{E}\text{d}\text{σ}\text{d}{}^3\text{p (γ}\text{p}\text{→}\text{h}{}^{\mathrm{?}}\text{X}\text{)}$ at large transverse momentum provides a good test of perturbative QCD. Such differences are insensitive to the parton Fermi motion and to the poorly known sea-quark and gluon distribution functions. Moreover the hadronized photon component is shown to contribute little compared to the point-like one. Presently available photon beams at the CERN-SPS and FNAL are sufficient to test QCD in large-p_T photoproduction.     

Photoproduction of μ pairs and large-pT particles in QCD

We calculate cross sections for $\text{γ}\text{N}\text{→ μ}\text{μ}\text{X}\text{, γ}\text{N}\text{→ (}\text{jet)X and}\text{γ}\text{N}\text{→ π}{}^{\mathrm{+}}\text{X}$ using the distribution functions for quarks in the photon predicted by QCD. The Drell-Yan contribution to the μ-pair cross section is masked by the Bethe-Heitler process in the region where it is interesting. However, the characteristic cross section for photoproduction of large-p_T particles, which is predicted by QCD, is measurable.     

Trigger bias in large pT reactions

We explore how particle multiplicities in large transverse momentum reactions are affected by the way in which the detection apparatus is triggered. For example, we predict that the multiplicity of particles accompanying a large p_T hadron is increased when it is required that there be a second large p_T hadron in the opposite hemisphere. The mechanism responsible for this effect is shown also to make correlations between pairs of large p_T pions produced on opposite sides depend only weakly on their charges.We include a list of further experimental tests, feasible with available apparatus, that are suggested by the two-jet picture of large p_T reactions.     

A recombination model for large-PT reactions

A difficult theoretical problem in large P_T reactions is to reproduce simultaneously the observed single-particle distributions and the distribution of 〈P_out〉² versus X²_E for different values of $\text{s}$ and P^trig_T. The purpose of this letter is to report successful fits to these distribution in a “recombination” model and to present the predictions of the model at $\text{s}\text{=540}\text{GeV}$, corresponding to the CERN PP? collider.     

Jets and correlations in large pT reactions

Recent experimental data on correlations in large transverse momentum reactions are analyzed in a two-jet picture. Scaling in jet fragmentation is shown to account well for the existing data. The formulae that are given can be used as a basis for a more detailed analysis of the dynamics. A particular result is that we predict that a two-jet structure can also be produced in events where it is not required that any single hadron has large p_T, with a cross section as large as $\text{1}\text{10}$ mb or more (at ISR energies with the total transverse momentum of each jet ?2.5 GeV/c).     

Photoproduction of large transverse momentum mesons and production of large-pT photons and leptons in proton-proton collisions

The photoproduction of large transverse momentum mesons is examined in a leading particle model using the techniques of the covariant parton model. In the leading particle model the large-p_T meson is produced in a high-energy, wide-angle scattering: γ + q → M + q. Numerical results such as inclusive cross sectioons, angular distributions and particle ratios are presented for the pseudoscalar mesons. The Landshoff-Polking-horne quark-fusion model is then applied to the production of large transverse momentum photons and leptons in proton-proton collisions. In the lepton case this model differs from the usual Drell-Yan mechanism in that the lepton pair has a small invariant mass but comes from a large-p_T virtual photon.     

Kinematical scale breaking in high pT interactions

Assuming scaling in the longitudinal reduced momentum in the overall c.m. for the parton fragmentation function, there are, in any hard scattering model, at least three possible sources of a lack of scaling in $\text{(}\text{1}\text{σ}\text{)}\text{d}\text{σ}\text{d}\text{x}{}_{\mathrm{<mtext>e</mtext>}}$. In this paper we analyse these causes emphasising one of them, “the skew mechanism”, which occurs because the axes of the partons are not the x-axis used to express the reduced momenta of the hadrons and because, simultaneously, the transverse momentum of hadrons inside partons is not negligible. A particular result of our study is that the lack of scaling in $\text{(}\text{1}\text{σ}\text{)}\text{d}\text{σ}\text{d}\text{x}{}_{\mathrm{<mtext>e</mtext>}}$ observed by CCHK and BFS collaborations is not necessarily in contradiction with the scaling in the longitudinal reduced momentum of the jet.     

QCD three-jet production in large-p⊥ processes

The contribution of the hard scattering subprocess quark + quark → quark + quark + gluon to two-particle inclusive distributions at large p_⊥ is studied. It is shown to be accessible experimentally, and agrees with recent data on azimuthal distributions in the adequate kinematical region.     

Spin asymmetries for large-pt jet production in two-photon processes

Relying on the hard scattering formalism we estimate cross sections and spin asymmetries for large-p_t jet production from two-photon processes in collisions of electrons and positrons of definite helicity states. We use distribution functions of polarized partons in polarized electrons obtained in a modified leading logarithmic approximation as well as spin-dependent parton cross sections according to lowest-order QCD calculations. Detecting at least one forward or backward hadronic jet, topologies of jets can be observed different from the 2 large p_t production.     

The transverse momentum of partons in large pT processes

An approximate method is used to investigate the effects of parton transverse momentum in large p_T particle production within the framework of hard scattering models. We derive an approximate expression for the mean bias towards the trigger of each of the two participating partons and find that event one of the partons is biased more than the other, even with a 90° trigger. We treat the transverse momentum of partons and their closely related off-mass-shell behaviour as a perturbation in the equation for the single-particle inclusive cross section, and then expand in a Taylor series. We calculate the first non-zero correction term and find that to this order, the cross section is increased by parton transverse momentum effects by typically a factor of 1.7 for p_T = 3 GeV/c with 〈k_T〉 = 500 MeV/c, or 1.25 for p_T = 3 GeV/c with 〈k_T〉 = 300 MeV/c, and that the correction decreases rapidly with increasing p_T.     

Scale violating effects in large qT muon pair production

We compare the hard scattering and DDT approaches to large q_T and large M muon pair production, the former being valid at all q_T but only to first order in α_s, the latter including a sum over all orders but being valid only for q_T << M. The DDT formula is rederived in the common domain of applicability of the two approaches. A numerical study indicates that the higher order effects are important for comparison with experiment.     

Weak and electromagnetic effects in large pT proton-proton collisions

The weak and electromagnetic contributions to the inclusive cross section for hadron production at large p_T in proton ISR experiments are calculated using the parton model. When compared with extrapolations of recent strong interaction data, it is found that they overtake the strong cross section at large p_T for beam energies of several hundred GeV at a cross section which might be accessible (assuming a reasonable W boson mass in the weak case). Further, the parity violating effects of the weak interaction are found to be large and thus in principle could give an unambigous signature to weakly produced events, using polarised beams.     

Inclusive η production at high pT at the ISR

The inclusive η production cross section at the CERN ISR has been measured for p_T values of up to 11 GeV/c. We find that the $\text{η}\text{π}{}^0$ cross-section ratio has an average value of 0.55 ± 0.07 and varies little with p_T.     

QCD calculation of large pT hadronic collision jet structure

We define an analogue of ”thrust” which enables the investigation of the jet structure of large p_T hadronic collisions via a perturbative QCD calculation. In this paper we present the result for the single subprocess qq → qq + X, where the quark flavours are distinct, to O(g⁶) in the strong coupling constant.     

Study of resolved high-pT neutral pions at the CERN ISR

The inclusive production of π⁰ at large values of p_T in pp collisions at the ISR has been studied. In this experiment the two photons are resolved and separately measured for pT values of up to 6 GeV/c, giving confidence that the desired signal has been separated from various backgrounds.     

QCD calculation of expected large xT 3-jet structure at Isabelle

We investigate in perturbative QCD the magnetude and shape of the large x_T(?0.5) 3-transverse jet thrust spectrum from pp collisions. We show that in this valence subprocess dominated region the shape of the spectrum is determined unambiguously by QCD. At Isabelle energies and the lowest acceptable x_T the magnitude of the cross section should allow observation of such 3-jet events but with rather low statistics.     

The needle in the large pT haystack: Higgs versus quark or gluon jets together with Z0 in hadronic reactions

The possibility of observing a Higgs particle via high p_T bremsstrahlung from a weak Z₀ in p$\text{p}$ and pp collisions is examined. It appears that, although very weak, some signal might be observed in the future p$\text{p}$ collision experiments.     

Opposite-side angular correlation of high-pT particles as a probe of gluons

Opposite-side angular distributions of high-p_T particles in the jet trigger experiments are studied in the lowest-order QCD. Gluon jets play a crucial role in understanding correlations between two high-p_T jets. The angular distribution of the positive-to-negative particle ratio and that of charmed particles can be good probes of the gluon density inside a nucleon.