Sensitivity of the LHC isolated-γ + jet data to the parton distribution functions of the proton

Abstract

We study the impact of differential isolated-photon+jet cross sections measured in proton-proton collisions at a center-of-mass energy of √ s = 7 TeV on the parton distribution functions (PDF) of the proton. Next-to-leading-order perturbative QCD (pQCD) calculations complemented with the NNPDF2.1 parton densities, and a Bayesian PDF reweighting method are employed. We find that although the current data provide only mild constraints to the parton densities, future γ-jet measurements with reduced experimental uncertainties can improve our knowledge of the gluon density over a wide range of parton fractional momenta x as well as of the quarks at low-x.

Direct Photon Production at Next-to–Next-to-Leading Order

“…As such they represent critical standard candles for the exploration of the SM at the LHC. For instance, measurements over a wide range of kinematic configurations -corresponding to different rapidities and transverse momenta of the photon -could be used to provide a precision probe of parton distribution functions (pdfs) [5]. However, up to now, the large theoretical uncertainty has meant that this data is not routinely used in fits.…”

Progress in the Determination of the Partonic Structure of the Proton

“…Inclusion of present-day LHC prompt photon data would only lead to a moderate reduction of order of ∼ 20% of the gluon uncertainty [138] at mediumsmall x ∼ 0.02, with photon+jet data having an even milder impact [139], but future data are likely to be much more constraining. Here, the main bottleneck is that theoretical predictions are only available up to NLO, and higher order corrections are likely to be large.…”

Dynamics of isolated-photon plus jet production in pp collisions at s=7 TeV with the ATLAS detector

“…The goal of the analysis presented here is to study the kinematics and dynamics of the isolated-photon plus jet system by measuring the bin-averaged cross sections as functions of the leading-photon transverse energy (E γ T ), the leading-jet transverse momentum (p jet T ) and rapidity (y jet ), the difference between the azimuthal angles of the photon and the jet ( φ γ j ), the photon-jet invariant mass (m γ j ) and cos θ γ j , where the variable θ * is referred to as θ γ j here and henceforth. The photon was required to be isolated by using the same isolation criterion as in previous measurements [5,6,9] based on the amount of transverse energy inside the cone given by (η − η γ ) 2 + (φ − φ γ ) 2 R = 0.4, centred around the photon direction (defined by η γ and φ γ ). The jets were defined using the anti-k t jet algorithm [10] with distance parameter R = 0.6.…”

“…The measurements of angular correlations between the photon and the jet can be used to probe the dynamics of the hard-scattering process. Since the dominant production mechanism in pp collisions at the LHC is through the qg → qγ process, measurements of prompt-photon plus jet production have been used to constrain the gluon density in the proton [1,2]. Furthermore, precise measurements of photon plus jet production are also useful for the tuning of the Monte…”