New parton distribution functions for the photon

Abstract

We present new improved parton distributions for the photon. We fit all available data on the photon structure function, F γ 2 (x, Q 2 ), with Q 2 ≥ 3 GeV 2 , in order to determine the quark distributions. We also pay particular attention to the gluon distribution in the photon, g γ (x, Q 2 ), which has been poorly constrained in earlier analyses which only include structure function data. We use large p T jet production in γγ collisions from TRISTAN to constrain g γ . We also see what information can be gleaned from γp collisions at HERA on g γ and on the quark distributions at large x, where there are no structure function data. We review future prospects of elucidating the parton distributions of the photon.

Constraining gluon shadowing using photoproduction in ultraperipheralpAandAAcollisions

“…Another source of differences is in the choice and scope of experimental data from which F γ 2 is extracted. At present there is an appreciable number of photon parton distribution sets available, both at leading and next-to-leading orders [55][56][57][58][59][60][61][62][63][64][65][66][67][68][69][70]. It should be noted that unlike in the case of a nucleon, there are no valence quarks present in the photon; therefore antiquark distributions are the same as quark distributions.…”

Observation of isolated high-E photons in photoproduction at HERA

“…This result can be compared with NLO calculations at the parton level of Gordon [27] (using an LO radiative contribution), in which the integrated cross section for the process ep → e + γ prompt + jet + X is evaluated under the same kinematic conditions as used above. Using the GS and GRV NLO [28] photon parton densities, integrated cross sections of 14.05 (13.17) pb and 17.93 (16.58) pb respectively are obtained, where the first value is calculated at a QCD scale µ = 0.25 (E γ T ) 2 and the second, parenthesized value at µ = (E γ T ) 2 . These values cover the range of theoretical uncertainty of each calculation.…”

NLO Photon Parton Parametrization Using ee and ep Data

“…It differs from other NLO parametrizations [3][4][5][6][7] in that the data used in the fitting procedure include the expected behaviour of F γ 2 at low-x [8], as derived from F p 2 measurements [9] under Gribov factorization assumption [10] and, in addition, the measurements of the dijet photoproduction cross sections [11].…”