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Register a new userNLO Calculation of Prompt Photon Production in DIS at HERA
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We present a NLO calculation of prompt photon production in DIS. The calculation involves direct, fragmentation and resolved contributions. It is performed in the virtual-photon proton center-of-mass system. A comparison of the theoretical results with HERA data is carried out.
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We compute the next-to-leading order (NLO) impact factor for inclusive photon $+$dijet production in electron-nucleus (e+A) deeply inelastic scattering (DIS) at small $x$. An important ingredient in our computation is the simple structure of ``shock wave fermion and gluon propagators. This allows one to employ standard momentum space Feynman diagram techniques for higher order computations in the Regge limit of fixed $Q^2gg Lambda_{rm QCD}^2$ and $xrightarrow 0$. Our computations in the Color Glass Condensate (CGC) effective field theory include the resummation of all-twist power corrections $Q_s^2/Q^2$, where $Q_s$ is the saturation scale in the nucleus. We discuss the structure of ultraviolet, collinear and soft divergences in the CGC, and extract the leading logs in $x$; the structure of the corresponding rapidity divergences gives a nontrivial first principles derivation of the JIMWLK renormalization group evolution equation for multiparton lightlike Wilson line correlators. Explicit expressions are given for the $x$-independent $O(alpha_s)$ contributions that constitute the NLO impact factor. These results, combined with extant results on NLO JIMWLK evolution, provide the ingredients to compute the inclusive photon $+$ dijet cross-section at small $x$ to $O(alpha_s^3 ln(x))$. First results for the NLO impact factor in inclusive dijet production are recovered in the soft photon limit. A byproduct of our computation is the LO photon+ 3 jet (quark-antiquark-gluon) cross-section.
A recent H1 measurement of triple differential dijet cross sections in electron-proton interactions in the region of photon virtualities 2<Q2<80 GeV is presented and compared to LO and NLO QCD predictions. Effects that go beyond the fixed-order NLO QCD calculations are identified.
In this work, we will present the first complete calculation of the one-loop longitudinal photon-to-quark-antiquark light cone wave function, with massive quarks. The quark masses are renormalized in the pole mass scheme. The result is used to calculate the next-to-leading order correction to the high energy Deep Inelastic Scattering longitudinal structure function on a dense target in the dipole factorization framework. For massless quarks the next-to-leading order correction was already known to be sizeable, and our result makes it possible to evaluate it also for massive quarks.
Next-to-leading order predictions matched to parton showers are compared with recent ATLAS data on inclusive photon production and CMS data on associated photon and jet production in pp and pPb collisions at different centre-of-mass energies of the LHC. We find good agreement and, as expected, considerably reduced scale uncertainties compared to previous theoretical calculations. Predictions are made for the ratio of inclusive photons over decay photons $R_gamma$, an important quantity to evaluate the significance of additional photon sources, e.g. thermal radiation from a Quark-Gluon-Plasma, and for distributions in the parton momentum fraction in lead ions $x_{rm Pb}^{rm obs}$, that could be determined by ALICE, ATLAS, CMS and LHCb in ongoing analyses of photon+jet production in pPb collisions at $sqrt{s_{NN}}=5.02$ TeV. These data should have an important impact on the determination of nuclear effects such as shadowing at low $x$.
We study the inclusive production of isolated prompt photons within the framework of the quasi-multi-Regge-kinematic approach, applying the quark Reggeization hypothesis. We describe accurately and without free parameters the transverse momentum and pseudorapidity spectra of prompt photons in the inclusive photoproduction at the HERA Collider. It is shown that the main mechanism of the inclusive prompt photon production in the gamma p collisions is the fusion of a Reggeized quark (antiquark) from the proton and a collinear antiquark (quark) from the photon into a photon, via the effective Reggeon-quark-gamma vertex. The fragmentation of the quark, which is produced via the gamma-Reggeon-quark and quark-Reggeon-quark vertices, into a photon is strongly suppressed by the isolation cone condition and it gives a significant contribution in the region of a large negative pseudorapidity only. At the stage of numerical calculations we use the Kimber-Martin-Ryskin prescription for unintegrated quark and gluon distribution functions, with the following collinear parton densities as input: MRST for a proton and GRV for a photon.
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