Inclusive Prompt Photon Production Research Articles

New Constraint for Isotropic Lorentz Violation from LHC Data

New calculations for the kinematics of photon decay to fermions under an isotropic violation of Lorentz invariance (LV), parametrized by the standard-model extension, are presented in this Letter and used to interpret prompt photon production in LHC data. The measurement of inclusive prompt photon production at the LHC Run 2, with photons observed up to a transverse energy of 2.5 TeV, provides the lower bound κ˜tr>−1.06×10−13 on the isotropic coefficient κ˜tr at 95% confidence level. This result improves over the previous bound from hadron colliders by a factor of 55. The calculations for the kinematics of photon decay have further potential use to constrain LV coefficients from the appearance of fermion pairs. Published by the American Physical Society 2024

Electroweak probes of small and large systems with the ATLAS detector

Measurements of isolated prompt photon and massive electroweak (W and Z) boson production in different collision systems are of great interest to understand the partonic structure of heavy nuclei, and serve as a constraint on the initial state in larger collision systems. These channels are sensitive to a variety of effects such as the modification of the parton densities in nuclei in certain kinematic regions, and the energy loss of partons as they undergo multiple interactions in the nucleus before the hard parton-parton scattering. High-statistics samples of lead–lead and proton–lead collision data at sNN=5.02TeV and8.16TeV, respectively, taken by the ATLAS experiment at the LHC, as well as proton–proton comparison data at analogous collision energies, allow for a detailed study of these phenomena in data and comprehensive comparisons to the predictions of a variety of theoretical approaches. This paper presents the latest ATLAS results in these topics, including updated results on inclusive prompt photon production in proton–lead collisions over a broad kinematic range and high-precision W boson results in lead–lead collisions.

Inclusive prompt photon production in electron-nucleus scattering at small x

We compute the differential cross-section for inclusive prompt photon production in deeply inelastic scattering (DIS) of electrons on nuclei at small x in the framework of the Color Glass Condensate (CGC) effective theory. The leading order (LO) computation in this framework resums leading logarithms in x as well as power corrections to all orders in Qs, A2/Q2, where Qs,A(x) is the nuclear saturation scale. This LO result is proportional to universal dipole and quadrupole Wilson line correlators in the nucleus. In the soft photon limit, the Low-Burnett-Kroll theorem allows us to recover existing results on inclusive DIS dijet production. The k⊥ and collinearly factorized expressions for prompt photon production in DIS are also recovered in a leading twist approximation to our result. In the latter case, our result corresponds to the dominant next-to-leading order (NLO) perturbative QCD contribution at small x. We next discuss the computation of the NLO corrections to inclusive prompt photon production in the CGC framework. In particular, we emphasize the advantages for higher order computations in inclusive photon production, and for fully inclusive DIS, arising from the simple momentum space structure of the dressed quark and gluon “shock wave” propagators in the “wrong” light cone gauge A− = 0 for a nucleus moving with PN+ → ∞.

The effect of neutron skin on inclusive prompt photon production in Pb + Pb collisions at Large Hadron Collider energies

Recent experiments on lead () nuclei have observed the celebrated phenomenon of the neutron skin thickness of low energy nuclear physics. Skin thickness provides a measure of the extension of the spatial distribution of neutrons inside the atomic nucleus than protons. We have studied the effect of neutron skin thickness on inclusive prompt photon production in Pb + Pb collisions at Large Hadron Collider energies. We have calculated the ‘central-to-peripheral ratio’ () of prompt photon production with and without accounting for the neutron skin effect. The neutron skin causes a characteristic enhancement of the ratio, in particular at forward rapidity, which is distinguishable in our calculation. However, a very precise direct photon measurement up to large transverse momenta would be necessary to constrain the feature in experiment.

Reconsideration of the inclusive prompt photon production at the LHC withkT-factorization

We reconsider the inclusive production of isolated prompt photons in $pp$ collisions at the LHC energies in the framework of $k_T$-factorization approach. Our analysis is based on the ${\cal O}(\alpha \alpha_s)$ off-shell (depending on the transverse momenta of initial quarks and gluons) production amplitudes of $q^* g^* \to \gamma q$ and $q^* \bar q^* \to \gamma g$ partonic subprocesses and transverse momentum dependent (or unintegrated) quark and gluon densities in a proton, which are chosen in accordance with the Kimber-Martin-Ryskin prescription. We show that the sub-leading high-order ${\cal O}(\alpha \alpha_s^2)$ contributions, not covered by the non-collinear evolution of parton densities, are important to describe latest LHC data.

Centrality dependence of inclusive prompt photon production in d+Au, Au+Au, p+Pb, and Pb+Pb collisions

We calculate the centrality dependence of the midrapidity nuclear modification for inclusive prompt photon production in d+Au and Au+Au collisions at RHIC and in p+Pb and Pb+Pb collisions at the LHC. Our results, using the recent spatially dependent nuclear PDF set EPS09s, are consistent with the existing high-pT data from the PHENIX and CMS collaborations. The good agreement even in the case of nucleus+nucleus collisions suggests that the high-pT direct photon production is not significantly altered by the strongly interacting medium produced in such collisions. We find the centrality dependence of the nuclear modifications generally rather weak but perhaps measurable at low pT.

EPS09s and EKS98s: Impact parameter dependent nPDF sets

In our recent study we have determined two new spatially dependent nuclear PDF (nPDF) sets, EPS09s and EKS98s. With these, the hard-process cross-sections can be calculated in different centrality classes consistently with the globally analyzed nPDFs for the first time. The sets were determined by exploiting the $A$-systematics of the globally fitted nPDF sets, EPS09 and EKS98. For the spatial dependence of the nPDFs we used a power series ansatz in the nuclear thickness function $T_A$. In this flash talk we introduce the framework, and present our NLO EPS09s-based predictions for the nuclear modification factor in four centrality classes for inclusive neutral pion production in p+Pb collisions at the LHC and for inclusive prompt photon production in d+Au collisions at RHIC at midrapidity.

CGC predictions for [formula omitted] collisions at the LHC and signature of QCD saturation

We present various predictions for the upcoming p+Pb collisions at S=5 TeV within the color glass condensate (CGC) formalism, including single inclusive charged hadron production, single inclusive prompt photon production, direct photon production, charged hadron multiplicity distribution and photon–hadron azimuthal correlations. Using the running-coupling Balitsky–Kovchegov evolution equation for calculating various observables, we show that the main source of uncertainties is due to less constrained initial nuclear saturation scale. This gives rise to rather large theoretical uncertainties for nuclear modification factor RpA at the LHC. Nevertheless, we propose a simple scheme in which one can still test the main dynamics of the CGC/saturation in p+A collisions at the LHC.

Prompt photon production and photon-hadron correlations at RHIC and the LHC from the color glass condensate

We investigate inclusive prompt photon and semi-inclusive prompt photon-hadron production in high energy proton-nucleus collisions using the Color Glass Condensate (CGC) formalism which incorporates non-linear dynamics of gluon saturation at small x via Balitsky-Kovchegov equation with running coupling. For inclusive prompt photon production, we rewrite the cross-section in terms of direct and fragmentation contributions and show that the direct photon (and isolated prompt photon) production is more sensitive to gluon saturation effects. We then analyze azimuthal correlations in photon-hadron production in high energy proton-nucleus collisions and obtain a strong suppression of the away-side peak in photon-hadron correlations at forward rapidities, similar to the observed mono-jet production in deuteron-gold collisions at forward rapidity at RHIC. We make predictions for the nuclear modification factor R_{p(d)A} and photon-hadron azimuthal correlations in proton(deuteron)-nucleus collisions at RHIC and the LHC at various rapidities.

Inclusive prompt photon production in nuclear collisions at RHIC and LHC

Nuclear modification factors of inclusive prompt photon production in d-Au collisions at RHIC and p-Pb collisions at the LHC are provided at different rapidities. The calculations are performed at NLO accuracy using the EPS09 NLO nuclear parton distribution functions (nPDFs) and their error sets. The results are compared to the ones obtained with the nDS and HKN07 NLO nPDFs, and to the corresponding nuclear modification factors of neutral pion production in these collisions. The sensitivity of these results to the scale choice is also investigated. Interestingly, the predictions using the different nPDF sets differ from each other to the extent that this observable can be expected to become very useful for probing nPDFs over a wide range of Bjorken-x. In order to obtain a perturbative QCD baseline in heavy-ion collisions, calculations are carried out for minimum bias Au-Au collisions at RHIC and Pb-Pb collisions at the LHC. We also estimate the maximal possible suppression which the produced QCD matter can be expected to have on inclusive prompt photon production due to the quenching of the fragmentation component. The nuclear modification factor for prompt photon production is thus suggested to be used for gauging both the cold and the hot nuclear matter effects on other hard processes which are expected to be affected by quark-gluon plasma formation, such as large-pT hadron and jet production.

Testing for [formula omitted]-factorization with inclusive prompt photon production at LHC

We study the possibility to analyse the data on inclusive prompt photon production at first LHC runs in the framework of the kT-factorization QCD approach. Our consideration is based on the amplitude for the production of a single photon associated with a quark pair in the fusion of two off-shell gluons. The quark component is taken into account separately using the quark–gluon Compton scattering and quark–antiquark annihilation QCD subprocesses. The unintegrated parton densities in a proton are determined using the Kimber–Martin–Ryskin (KMR) prescription as well as the CCFM evolution equation.

Prompt photon photoproduction at HERA within the framework of the quark Reggeization hypothesis

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 $\ensuremath{\gamma}p$ 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: Martin-Roberts-Stirling-Thorne for a proton and Gl\"uck-Reya-Vogt for a photon.

Deep inelastic scattering and prompt photon production within the framework of the quark Reggeization hypothesis

We study $ep$ deep inelastic scattering and the inclusive production of prompt photon within the framework of the quasi-multi-Regge-kinematic approach, applying the quark Reggeization hypothesis. We describe structure functions ${F}_{2}$ and ${F}_{L}$ supposing that a virtual photon scatters on a Reggeized quark from a proton, via the effective gamma-Reggeon-quark vertex. It is shown that the main mechanism of the inclusive prompt photon production in $p\overline{p}$ collisions is the fusion of a Reggeized quark and a Reggeized antiquark into a photon, via the effective Regeon-Reggeon-gamma vertex. We describe the inclusive photon transverse momentum spectra measured by the CDF and D0 Collaborations within errors and without free parameters, using the Kimber-Martin-Ryskin unintegrated quark and gluon distribution functions in a proton.

Isolated photons at hadron colliders at O( ααS2) (II): Spin-dependent case

The cross section for inclusive prompt photon production with polarized hadron beams is calculated at order αα s 2 using the phase-space slicing or analytic/Monte Carlo method. Isolation cuts are placed on the photon and the results are compared to a previous fully analytic calculation. Numerical results for the isolated cross section are presented for pp → γ + X at RHIC centre-of-mass energies with plausible isolation parameters using the most modern polarized parton densities evolved in next-to-leading order QCD. The perturbative stability of the asymmetries and scale dependence of the results are briefly discussed.

Inclusive prompt photon production in polarized pp collisions at HERA- [formula omitted

We present a NLO study of inclusive polarized prompt photon production in a conceivable fixed target pp mode of HERA with longitudinally polarized protons at s = 39 GeV . We analyze the sensitivity of the corresponding double spin asymmetry to the proton's polarized gluon distribution Δg and estimate the expected statistical precision in its determination. The main theoretical uncertainties in the predictions are examined.

Prompt photon production in γγ collisions and the gluon content of the photon

We calculate the cross section for inclusive prompt photon production in γγ collisions, i.e. the reaction e + e − → γγ → γX, in next-to-leading order QCD. We show that at LEP2 energies this cross section is measurable and is sensitive to the gluon distribution of the photon, g γ , which is currently very poorly constrained by data.

Inclusive prompt photon production in hadronic final states of e+e- annihilation.

We provide complete analytic expressions for the inclusive prompt photon production cross section in hadronic final states of ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ annihilation reactions through one-loop order in quantum chromodynamics perturbation theory. Computed explicitly are direct photon production through first order in the electromagnetic strength ${\mathrm{\ensuremath{\alpha}}}_{\mathrm{em}}$ and the quark-to-photon and gluon-to-photon fragmentation contributions through first order in the strong coupling ${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$. The full angular dependence of the cross sections is displayed, separated into transverse (1+${\mathrm{cos}}^{2}$${\mathrm{\ensuremath{\theta}}}_{\ensuremath{\gamma}}$) and longitudinal (${\mathrm{sin}}^{2}$${\mathrm{\ensuremath{\theta}}}_{\ensuremath{\gamma}}$) components, where ${\mathrm{\ensuremath{\theta}}}_{\ensuremath{\gamma}}$ specifies the direction of the photon with respect to the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ collision axis. We discuss extraction of fragmentation functions from ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ data. \textcopyright{} 1996 The American Physical Society.

Largep T prompt photon production at HERA

We examine the possibility of using direct photon production at HERA as a means of obtaining information about the photon structure and fragmentation functions. Estimates of the cross section for inclusive prompt photon production at theep collider HERA are made completely in next-to-leading order QCD. Using parton distributions for the photon evolved in next-to-leading order QCD for the first time, we show that theO(α s ) corrections to the processes involving these functions are significant in the regions where they dominate. This can have important consequences for the possibility of measuring the gluon content of the photon using these processes at HERA. Moreover, we show that it may not be straightforward to separate the various contributions to the cross section in theep lab frame.

Inclusive prompt photon production inpp and $$\bar p$$ p interactions

High transverse momentum (p T ) production of pions and photons in proton-proton and proton-antiproton interactions are studied within the QCD framework and compared with the presently available experimental data. Scaling violation effects are included, and shown to be important at largep T . Predictions are made for the γ/π0 ratio at collider energies. The possibility of extricating the gluon distribution function from the invariant cross section data for the process\(\bar p\)+p→γ+X is suggested.