Evidence for PartonkTEffects in High-pTParticle Production

Abstract

Inclusive π 0 and direct-photon cross sections in the kinematic range 3.5< p T <12 GeV/c with central rapidities (y cm ) are presented for 530 and 800 GeV/c proton beams and a 515 GeV/c π − beam incident on Be targets. Current Next-to-Leading-Order perturbative QCD calculations fail to adequately describe the data for conventional choices of scales. Kinematic distributions from these hard scattering events provide evidence that the interacting partons carry significant initial-state parton transverse momentum (k T ). Incorporating these k T effects phenomenologically greatly improves the agreement between calculations and the measured cross sections.

Unintegrated parton distributions and prompt photon hadroproduction

“…Of course there could have been a large effect if q t ∼ p tγ which would have allowed the singularity of the hard subprocess amplitude to be approached. However the strong ordering of transverse momenta must be incorporated in the formalism, via θ(|t| − q Figure 4: The scale dependence of the predictions for production of prompt photons in pp collisions at √ s = 24.3 GeV, pBe collisions at √ s = 38.8 GeV and pp collisions at √ s = 1.8 TeV shown together with UA6 [8], E706 [9] and CDF [10] data respectively. The continuous curves are the predictions with the incoming partonic transverse momentum q t included, whereas the dashed curves correspond to the unsmeared (q t = 0) results in which the integrated partons are sampled at the hard scale µ.…”

“…Thus part of the observed p tγ comes from the initial partonic k t such that the hard subprocess singularity dσ/dt ∼ 1/p 4 tγ is approached more closely, and hence leads to a steeper p tγ spectrum. However in order to describe the observed spectra it is necessary to introduce a k t spectrum with an average value which increases from k t ∼ 0.5 GeV to more than 2 GeV [6] as the collision energy √ s increases from UA6, E706 [8,9] to Tevatron [10] energies. Such large partonic k t cannot originate solely from the large distance confinement domain, but must also have a significant perturbative QCD component.…”

A critical phenomenological study of inclusive photon production in hadronic collisions

“…An attractive possibility made use of the intrinsic ambiguities of fixed order perturbation theory to define the various unphysical scales entering the theoretical predictions by means of various "optimisation" prescriptions [15,16,17,18]: an excellent agreement [12,19,20,21] between theory and experiments over the whole available range of energy and transverse momentum was thus obtained with a single set of structure functions and a unique value of Λ QCD . More recently however it has been proposed to fix the unphysical scales at some arbitrary "physical" values and to introduce an extra non-perturbative parameter, called the "intrinsic transverse momentum" of the partons in the hadrons which is fitted to the data at each energy [8,22,23,24,25]. This parameter increases with energy (technically speaking it is therefore not an "intrinsic" momentum) and this is interpreted as taking into account the effects of multiple soft gluon emission associated to the hard partonic scattering.…”

Operator analysis of pT -widths of TMDs

“…Of course, in practice the isolation of the average partonic transverse momentum from experimental measurements is extremely challenging due to the multitude of contributing effects. For example, the transverse momentum imbalance of dijet, dimuon and diphoton pairs in hadron-hadron collisions has been used to extract the average partonic transverse momentum [33,34]. Dimuons and diphotons are primarily sensitive to quark contributions, but under scale changes also gluon contributions enter.…”