CT14QED parton distribution functions from isolated photon production in deep inelastic scattering

Abstract

We describe the implementation of Quantum Electrodynamic (QED) evolution at Leading Order (LO) along with Quantum Chromodynamic (QCD) evolution at Next-to-Leading Order (NLO) in the CTEQ-TEA Global analysis package. The inelastic contribution to the photon Parton Distribution Function (PDF) is described by a two-parameter ansatz, coming from radiation off the valence quarks, and based on the CT14 NLO PDFs. Setting the two parameters to be equal allows us to completely specify the inelastic photon PDF in terms of the inelastic momentum fraction carried by the photon, p γ 0 , at the initial scale Q 0 = 1.295 GeV. We obtain constraints on the photon PDF by comparing with ZEUS data [10] on the production of isolated photons in deep inelastic scattering, ep → eγ + X. For this comparison we present a new perturbative calculation of the process that consistently combines the photon-initiated contribution with the quark-initiated contribution. Comparison with the data allows us to put a constraint at the 90% confidence level of p γ 0 0.14% for the inelastic photon PDF at the initial scale of Q 0 = 1.295 GeV in the oneparameter radiative ansatz. The resulting inelastic CT14QED PDFs will be made available to the public. In addition, we also provide CT14QEDinc PDFs, in which the inclusive photon PDF at the scale Q 0 is defined by the sum of the inelastic photon PDF and the elastic photon distribution obtained from the Equivalent Photon Approximation.

The production of a diphoton resonance via photon-photon fusion

“…This result is consistent with the CT14 [36] and MRST2004 [37] QED PDF sets, which include some theoretical guidance for the form of the photon PDF. Although the NNPDF2.3QED [35] set, which takes a completely free parameterisation of the photon PDF, prefers a lower central value ∼ 2.1, this is in a region where the corresponding photon PDF in this approach is relatively unconstrained, and the uncertainty on the ratio is quite large.…”

“…We find that generally the predicted ratios in table 1 are consistent with our results, with the exception of the NNPDF2.3 set, which predicts a somewhat lower ratio. Thus those sets which include theoretical guidance for the photon PDF appear to prefer a somewhat higher value of this ratio, compared to the NNPDF case (this is also clear from figure 11 of [36], where the NNPDF set shows a flatter behaviour with decreasing x). However, when uncertainties are accounted for the tension is not too dramatic.…”

“…However, in the CT14 set the photon momentum fraction p γ 0 is largely insensitive to the precise form of the photon PDF in the x region relevant to 750 GeV resonance production (x ∼ 0.09 (0.06) at √ s = 8 (13) TeV), where it is small in size. In [36], the form of the photon PDF in this x region is driven by the assumption of a purely incoherent input, while the ZEUS data [38] which they fit to does not directly constrain the photon PDF for x above ∼ 0.02. Thus, while for our set we find p γ 0 = 0.2 % at the CT14 starting scale, any apparent tension here and in the predicted luminosity should not be taken literally.…”

Two-loop QED corrections to the Altarelli-Parisi splitting functions

“…The EW corrections to PDFs need to be carefully studied for precise predictions at the LHC, as concluded from modern analysis performed up to NNLO in QCD and LO in QED. In fact, it was shown that the corrections induced are non negligible and, moreover, become crucial at higher energies [14][15][16][17].…”

Next-to-leading order QCD and electroweak corrections to Higgs-strahlung processes at the LHC

“…Ref. [10] studied the impact of the CMS measurements of W boson pair production at the LHC at 8 TeV on the NNPDF2.3qed, NNPDF3.0qed [11], CT14QEDinc [12], and LUXqed [13] photon PDFs, and found that the data strongly suggest that the NNPDF photon error band should be significantly reduced. Therefore, it is necessary to re-examine the physics effects induced by photon-initiated process.…”