Intrinsic transverse momentum in the π−p→γγX reaction at 280 GeV/c

A full next-to-leading order study of direct photon pair production in hadronic collisions

“…Various correlations between the two photons: the distribution of the p T imbalance variable z = −p T (γ 1 ).p T (γ 2 )/p 2 T (γ 1 ) the distribution of the azimuthal angle between the two photons (φ γγ ), the distribution of p out 10 , and the distribution of transverse momentum of diphotons (q T ), have been measured also by the WA70 collaboration [2]. These distributions are infrared sensitive near the elastic boundary of the spectrum (e.g.…”

“…The "two fragmentation" contribution involves two such convolutions, hence one more smearing. This distribution is another interesting infrared sensitive observable, measured by several experiments both at fixed target and collider energies [2,5,6], though less discussed in the literature from the theoretical side. The regime φ γγ → π includes back-to-back photons, a set of configurations which lie at the elastic boundary of the phase space.…”

“…Beside this important motivation, this process deserves interest by its own. The production of such pairs of photons has been experimentally studied in a large domain of energies, from fixed targets [1,2,3] to colliders [4,5,6]. A wide variety of observables has been measured, such as distributions of invariant mass, azimuthal angle and transverse momentum of the pairs of photons, inclusive transverse momentum distributions of each photon, which offer the opportunity to test our understanding of this process.…”

Angular and momentum asymmetry in particle production at high energies

“…To quantify the effects related to the nonvanishing total transverse momentum of the final state one has to use an approach in which the transverse momenta of incoming partons are not neglected. Let us note, that in a number of recent experiments on diphoton [3], π 0 and direct photon [4] production a substantial discrepancy between the data and predictions of collinearly factorized NLO QCD was observed. Taking into account the intrinsic transverse momenta of order of 1 − 2 GeV substantially improves this situation.…”

Evidence for PartonkTEffects in High-pTParticle Production

“…This k T is presumably due to effects of hadron size (primordial k T ) as well as initial-state gluon radiation. Measurements of Drell-Yan pair production [3] and direct di-photon production [4] have demonstrated the presence of substantial effective k T , (significantly larger than can be attributed to primordial k T ), and have revealed a significant √ s dependence of k T . A resummation of soft gluon emissions has recently been used to reproduce the size of the effect observed in the WA70 direct di-photon data [5].…”