MIT bag model inspired partonic transverse momentum distribution for prompt photon production inppcollisions

Abstract

We consider the prompt photon production in pp collisions using, within the framework of perturbative QCD, a non-gaussian distribution for the transverse momentum distribution of the partons inside the proton. Our description adopts the widely used in the literature factorization of the partonic momentum distribution into longitudinal and transverse components. It is argued that the non-gaussian distribution of the intrinsic transverse momenta of the partons is dictated by the asymptotic freedom as well as the 3D confinement of the partons in the proton. To make this association more transparent we use the MIT bag model, which plainly incorporates both properties (asymptotic freedom, confinement), in order to determine in a simplified way the partonic transverse momentum distribution. A large set of data from 6 different experiments have been fitted with this simple description using as a single free parameter the mean partonic transverse momentum kT . Surprisingly enough, a perfect fit of the experimental data turns out to require kT values which are compatible with Heisenberg's uncertainty relation for the proton and decrease almost smoothly as a function of the scaled variable z = p T √ s , where pT is the transverse momentum of the final photon and √ s is the beam energy in the center of mass frame. Our analysis indicates that asymptotic freedom and 3D confinement may influence significantly the form of the partonic transverse momentum distribution leaving an imprint on the pp → γ + X cross section.PACS numbers: 13.60. Le,13.85.Ni,12.38.Qk The production of photons with large transverse momentum is an excellent probe of the dynamics in hard scattering processes [1,2]. In particular, the study of direct photon production possesses numerous and well known advantages, both theoretical and experimental [2][3][4][5][6][7][8][9]. In the latter case the main advantage is that photons are easier to detect than jets. From the theoretical point of view the main advantage is the simplicity of the process allowing for an accurate determination of the gluon distribution within the proton. In the lowest order (O(αα s )) only two subprocesses, gq → γq (Compton) and qq → γg (annihilation), contribute to high p T photons. Their characteristic signature is the production of a photon isolated from the hadrons in the event, accompanied by a kinematically balancing high-p T jet appearing on the opposite site. In the next-to-leading order (NLO) the process associated with the production of a photon coming from the collinear fragmentation of a hard parton produced in a short-distance subprocess, constitutes a background to the direct photon production of the same order in α s as the corresponding Born level terms [10] provided that the fragmentation scale is large enough. However, the contribution from fragmentation remains small (less than 10%) for fixed target experiments and becomes significant only in inclusive prompt photon production at higher collider energies [10]. Recently there has been observed a systematic disagreeme...

Partonic transverse momenta in non-relativistic hyper-central quark potential models

“…In fact this process is optimal for this purpose since it is not affected by experimental ambiguities caused by final state hadronic interactions. In order to proceed we use the phenomenological scheme proposed in [15,17] for the calculation of the differential cross-section for inclusive γ-production. This scheme incorporates partonic subprocesses according to perturbative QCD, partonic effects in the proton described through the longitudinal parton distribution functions (PDF) and effects due to the intrinsic transverse momenta of the partons described through g(k T ).…”

“…One possibility for testing the phenomenological relevance of three-body forces is to consider their influence in the description of physical processes, like prompt photon production in pp-collisions, within the framework of perturbative QCD. Our strategy is to find the value of k in (2) for which we achieve the best description of experimental data and then to compare our results with those obtained using g(k T ) determined through the MIT bag model [17] or the corresponding two-body potential [15].…”

“…Interestingly enough, this treatment turned out to be very efficient in the description of the experimental data resolving several unsatisfactory issues present in the usual approach involving a Gaussian form for g(k T ). In [15] the treatment was based on a twobody potential of the form (1) while in a later work the MIT bag model [16] was used in a similar manner to obtain g(k T ) and subsequently to describe successfully the prompt photon production in pp collisions [17]. The results of these two works indicate that confinement, asymptotic freedom and/or relativistic description make an imprint on the intrinsic transverse momentum distribution of the constituent quarks, detectable in the cross section of pp collisions.…”

“…The results of these two works indicate that confinement, asymptotic freedom and/or relativistic description make an imprint on the intrinsic transverse momentum distribution of the constituent quarks, detectable in the cross section of pp collisions. As mentioned in [17] this fact could give an explanation for the systematic discrepancy between theoretical next-to-leading order (NLO) calculations [18] and experimental data [19,20] of inclusive single photon production (pp → γX). The observed gap for this process is particularly significant in fixed target experiments and cannot be filled even after taking into consideration certain large contributions to the partonic hard scattering cross section to all orders in perturbation theory, using the threshold resummation technique [21,22,23].…”

“…Having fixed A k , B k we determine g(k T ) for several values of k (k = 0.1, 0.5, 1, 2) and then we use the standard treatment within perturbative QCD for the calculation of the cross section for prompt photon production in a pp collision experiment. We compare our results with those of [15,17] in order to extract information concerning the presence or not of traces of three-body effects in g(k T ) traceable through the considered experimental data.…”