The photon content of the proton in the CT18 global analysis

Abstract

Recently, two photon PDF sets based on implementations of the LUX ansatz into the CT18 global analysis were released. In CT18lux, the photon PDF is calculated directly using the LUX master formula for all scales, µ. In an alternative realization, CT18qed, the photon PDF is initialized at the starting scale, µ 0 , using the LUX formulation and evolved to higher scales µ(> µ 0 ) with a combined QED+QCD kernel at O(α), O(αα s ) and O(α 2 ). In the small-x region, the photon PDF uncertainty is mainly induced by the quark and gluon PDFs, through the perturbative DIS structure functions. In comparison, the large-x photon uncertainty comes from various low-energy, nonperturbative contributions, including variations of the inelastic structure functions in the resonance and continuum regions, higher-twist and target-mass corrections, and elastic electromagnetic form factors of the proton. We take the production of doubly-charged Higgs pairs, (H ++ H −− ), as an example of scenarios beyond the Standard Model to illustrate the phenomenological implications of these photon PDFs at the LHC.

Photons in the proton: implications for the LHC

“…The same choice was made in Refs. [21][22][23][24][25], though in Refs. [24,25] a PDF set in which the photon PDF is determined using Eq.…”

“…[21][22][23][24][25], though in Refs. [24,25] a PDF set in which the photon PDF is determined using Eq. (2.18) at all scales was also presented.…”

“…Top right: the relative PDF uncertainties on the photon PDF in these two determinations. Bottom: same as top panels now comparing NNPDF4.0QED with MSHT20QED [23] and CT18QED [24] (all NNLO). Bands correspond to 1σ uncertainties Fig.…”

“…[19,20]), where it was shown that the photon PDF can be computed perturbatively in QED, given as input the proton structure functions at all scales, from the elastic (Q 2 → 0) to the deep-inelastic (Q 2 → ∞) regimes -the so-called LuxQED method. Since then, this LuxQED framework has been the basis of all QED PDF sets [21][22][23][24][25].…”

NNLO constraints on proton PDFs from the SeaQuest and STAR experiments and other developments in the CTEQ-TEA global analysis

“…In addition to this contribution, our group presented other contributions to the DIS'2021 proceedings. A new CT18QED NNLO analysis with two implementations of the photon PDF was released in June 2021 [2,3]. Impact of the combined heavy-quark HERA DIS data on CT18 PDFs is discussed in [4].…”