Sensitivity of isolated photon production at TeV hadron colliders to the gluon distribution in the proton

Abstract

We compare the single inclusive spectra of isolated photons measured at high transverse energy in proton-antiproton collisions at √ s = 1.96 TeV with next-to-leading order perturbative QCD predictions with various parametrizations of the parton distribution functions (PDFs). Within the experimental and theoretical uncertainties, the Tevatron data can be reproduced equally well by the recent CTEQ6.6, MSTW08 and NNPDF1.2 PDF sets. We present also the predictions for isolated γ spectra in proton-proton collisions at √ s = 14 TeV for central (y = 0) and forward (y = 4) rapidities relevant for LHC experiments. Different proton PDFs result in maximum variations of order ±30% in the expected E γ T -differential isolated γ cross sections. The inclusion of the isolated photon data in global PDF fits will place extra independent constraints on the gluon density.

Quantitative constraints on the gluon distribution function in the proton from collider isolated-photon data

“…The parton-to-photon fragmentation functions used are the BFG-II ("large gluon") set [31]. The isolated-γ spectrum obtained with the alternative BFG-I ("small gluon") set is just a few percent smaller than the one obtained with BFG-II in the lowest E γ T range [27] since a significant fraction (up to 80%, see Fig. 1) of the fragmentation photons are removed by the application of isolation cuts.…”

“…Such µ scales are found to result in spectra that agree well with the central values of the experimental data. At LHC and Tevatron energies, the sensitivity to changes in the arbitrary theoretical scales was discussed in [27] and found to be of about ±(10-15)% in the measured E γ T ranges. The parton-to-photon fragmentation functions used are the BFG-II ("large gluon") set [31].…”

“…[25,26] for previous studies) and showed that collider isolated photon data was in reasonable agreement with the theoretical predictions. Indeed, as discussed in detail in [27], by (i) increasing the √ s from fixed-target to collider energies, and by (ii) requiring the photon to be isolated from any hadronic activity within a given distance around its direction, one is left with an isolated-photon sample dominated by energy scales away from non-perturbative effects (such as intrinsic-k T broadening [28]), and where the uncertain contribution from photons issuing from the collinear fragmentation of final-state partons [29] are significantly reduced. Figure 1 shows the relative contribution of the three leading diagrams -direct Compton, direct quark-antiquark annihilation, and fragmentation -for inclusive (left) and for isolated (right) photon production in proton-antiproton (p-p) at Tevatron (top) and in proton-proton (p-p) collisions at the nominal LHC energy (bottom).…”

Nuclear modification factors from the CMS experiment

“…In order to suppress the much larger background coming from the electromagnetic decays of neutral mesons (mostly π 0 , η → γ γ) produced in the fragmentation of the hard scattered partons, isolation requirements on the reconstructed photon candidates are imposed. The application of isolation requirements suppresses also the fragmentation photon component and enhances significantly photons coming from the quark-gluon Compton process [5].…”

Measurement of the Isolated Prompt Photon Production Cross Section inppCollisions ats=7 TeV

“…Such a measurement complements deepinelastic scattering, Drell-Yan pair production, and jet production measurements [1][2][3]. At the Large Hadron Collider (LHC) [4], a significant increase of center-ofmass energy with respect to previous collider experiments [5][6][7][8][9] allows the exploration of new kinematic regions in the hard-interaction processes in hadron-hadron collisions [10]. Isolated prompt photon production also represents a background to searches for new phenomena involving photons in the final state.…”