Direct Photon Production Research Articles

Challenges of direct photon production at forward rapidities and large pT

Direct photons produced in interactions with nuclear targets represent a cleaner probe for investigation of nuclear effects than hadrons, since photons have no final state interaction and no energy loss or absorption is expected in the produced hot medium. Therefore, besides the Cronin enhancement at medium-high transverse momenta pT and isospin effects at larger pT, one should not expect any nuclear effects. However, this fact is in contrast to the PHENIX data providing an evidence for a significant large-pT suppression at mid rapidities in central d + Au and Au + Au collisions that cannot be induced by coherent phenomena (gluon shadowing, Color Glass Condensate). We demonstrate that such an unexpected results is subject to deficit of energy induced universally by multiple initial state interactions (ISI) towards the kinematic limits (large Feynman xF and/or large ). For this reason, in order to enhance the effects of coherence, one should be cautious going to forward rapidities and higher energies. In the LHC kinematic region ISI corrections are irrelevant at mid rapidities but cause rather strong suppression at forward rapidities and large pT. Numerical calculations of invariant pT spectra and the nuclear modification factor were performed within two different models, the color dipole formalism and the model based on kT-factorization, which are successfully confronted with available data from the RHIC and LHC collider experiments. Finally, we perform also predictions for a strong onset of ISI corrections at forward rapidities and corresponding expected suppression can be verified by the future measurements at LHC.

Direct Photon Production in Pion-Proton Collisions

In this article we calculate the contribution of the higher-twist Feynman diagrams to the large-pT inclusive single direct photon production cross section in πp collisions in case of the running coupling and frozen coupling approaches within holographic QCD. We determined the structure of infrared renormalon singularities of the higher-twist subprocess cross section. Also compared and analyzed the resummed higher-twist cross sections with the ones obtained in the framework of the frozen coupling approach.

Prompt photon production and photon-hadron jet correlations with POWHEG

We present a calculation of direct photon production at next-to-leading order of QCD and a matching of this calculation with parton showers using POWHEG BOX. Based on simulations with POWHEG+PYTHIA, we perform a detailed phenomenological analysis of PHENIX data on prompt photon production and photon-hadron jet correlations in pp collisions at RHIC, considerably improving the description of these data with respect to previous calculations, and we suggest additional interesting analyses.

Direct photon measurement in Pb–Pb collisions at [formula omitted] with ALICE

The ALICE experiment has measured the direct photon spectra in Pb-Pb collisions at sNN=2.76 TeV for three different centrality selections. The measurement was performed emplying a method utilizing conversion of photons into e+e− pairs in the detector material, and a method using the PHOS calorimeter. The two measurements were combined in order to measure direct photons over a broad transverse momentum range of 0.9<pT<14 GeV/c. A direct photon signal was observed in the thermal region 0.9<pT<2.1 GeV/c, with a significance of 2.6σ in the 20% most central collisions. The corresponding direct photon spectrum can be described by an exponential with an inverse slope parameter of (304±11stat±40syst) MeV, without subtracting the hard scattering component, which is somewhat larger than the inverse slope parameter of the direct photon spectrum at RHIC energies. Within uncertainties, the data can be described by different models for photon production in heavy-ion collisions. These proceedings provide a summary of the results published in [J. Adam, others (ALICE Collaboration), Direct photon production in Pb-Pb collisions at sNN=2.76 TeV, arXiv:1509.07324] and [J. Adam, others (ALICE Collaboration), Supplemental figures: Direct photon production in Pb-Pb collisions at sNN=2.76 TeV. URL https://cds.cern.ch/record/2102398].

Direct photon production and jet energy-loss in small systems: The XXVth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions

Two types of penetrating probes, direct photon and QCD jets, are investigated in the background of a small and rapidly expanding droplet of quark-gluon plasma. The additional thermal electromagnetic radiation results in a ∼50% enhancement of the direct photons. In high multiplicity p+Pb collisions, jets can lose a sizeable fraction of their initial energy, leading to a charged hadron RpA of ∼0.8 at a transverse momentum around 10GeV. Those two proposed measurements can help understand the apparent collective behaviour observed in small collision systems.

From soft photon study at Nuclotron and U-70 to NICA

Experimental and theoretical studies of direct photon production in hadronic collisions essentially expand our insights in multiparticle production mechanisms. These photons are useful probes to investigate nuclear matter at all stages of the interaction. Soft photons play a particular role in these studies. Until now we have no explanation for the experimentally observed excess of soft photons. These photons have low transverse momenta \( p_{T} < 0.1\) GeV/c, \( \vert x\vert < 0.01\) . In this domain their yield exceeds the theoretical estimates by 5-8 times. The registration of soft photons at Nuclotron (LHEP, JINR) has been carried out by the electromagnetic calorimeter built by the SVD-2 Collaboration. Soft photon electromagnetic calorimeter was tested at U-70, IHEP (Protvino). For the first time the soft photon yield at interactions of 3.5A GeV/c per nucleon deuterium and lithium beams has been measured. The obtained energy spectra confirm the increased yield of soft photons with their energy less than 50MeV (in the laboratory system) in comparison with theoretical predictions and agree with previous experiments at high-energy interactions. It is planned to continue soft photon study at the future accelerator complex NICA with heavy-ion beams.

FoCal – A high granularity electromagnetic calorimeter for forward direct photon measurements

The measurement of direct photon production at forward rapidity (y∼3−5) at the LHC provides access to the structure of protons and nuclei at very small values of fractional momentum (x∼10−5). FoCal, an extremely-high-granularity Forward Calorimeter covering 3.3<η<5.3 is proposed as a detector upgrade to the ALICE experiment. To facilitate the design of the upgrade and to perform generic R&D necessary for such a novel calorimeter, a compact high-granularity electromagnetic calorimeter prototype has been built. The corresponding R&D studies are the focus of this paper. The prototype is a Si/W sampling calorimeter. It was instrumented with 24 layers of Monolithic Active Pixel Sensors, a total of 39M pixels. We report on performance studies of the prototype with test beams at DESY and CERN in a broad energy range. The results of the measurements demonstrate a very small Molière radius (∼11mm) and good linearity of the response. Unique results on the detailed lateral shower shape, which are crucial for the two-shower separation capabilities, are presented. We compare the measurements to GEANT-based MC simulations, which additionally include a modeling of charge diffusion. The studies demonstrate the feasibility of this high-granularity technology for use in the proposed detector upgrade. They also show the extremely high potential of this technology for future calorimeter development.

Differential measurements of dielectron and direct virtual photon productions at STAR

Dileptons and photons have been serving as penetrating probes in heavy-ion collisions for quite a long time since the strongly interacting matter predicted by QCD theory. We present the mid-rapidity dielectron results in 0-80% Au+Au collisions at sNN=19.6,39,62.4, and 200 GeV, and the direct virtual photon results in 0-80% Au+Au collisions at sNN=200 GeV at STAR. The excess yields of dielectron with respect to hadronic sources at low mass range are presented, which are consistently described by a broadened ρ spectral function plus thermal radiations. Furthermore, the direct virtual photon yield has been measured and found it is consistent with a model that includes thermal radiation and initial hard parton scattering.

Recent developments in the theory of electromagnetic probes in relativistic heavy-ion collisions

The theoretical developments in the study of electromagnetic radiation in relativistic heavy-ion collisions are reviewed. The recent progress in the rates for photon and lepton pair production is discussed. Together with the improvements in the hydrodynamic descriptions of the bulk medium, the combined effort is discussed to resolve the “direct photon flow puzzle” in the RHIC and the LHC experiments. Further prediction of the direct photon production in high multiplicity proton-nucleus collisions at the LHC energy can serve as a signature of the quark gluon plasma formation in these small systems. Phenomenological study of dilepton production at finite net baryon density is highlighted at the collision energies available for the RHIC beam energy scan program.

Direct photon spectra and azimuthal anisotropy

The transverse momentum distribution and azimuthal anisotropy of direct photon production in Au+Au collisions at sNN=200 GeV were measured in the PHENIX experiment at RHIC. Through the external conversion method, earlier yield and v2 measurements were extended down to pT = 0.4 GeV/c. For the first time the third order Fourier coefficient, v3 was measured. The yield shows a large, exponential excess above prompt photon production. v2 is consistent with that of hadrons at low pT. v3 is about 50% the size of v2. Current models cannot describe simultaneously the large observed yield and azimuthal anisotropy.

Consistent simulation of direct-photon production in hadron collisions including associated two-jet production

We have developed an event generator for direct-photon production in hadron collisions, including associated 2-jet production in the framework of the GR@PPA event generator. The event generator consistently combines [Formula: see text] + 2-jet production processes with the lowest-order [Formula: see text] + jet and photon-radiation (fragmentation) processes from quantum chromodynamics (QCD) 2-jet production using a subtraction method. The generated events can be fed to general-purpose event generators to facilitate the addition of hadronization and decay simulations. Using the obtained event information, we can simulate photon isolation and hadron-jet reconstruction at the particle (hadron) level. The simulation reasonably reproduces measurement data obtained at the large hadron collider (LHC) concerning not only the inclusive photon spectrum, but also the correlation between the photon and jet. The simulation implies that the contribution of the [Formula: see text] + 2-jet is very large, especially in low photon-[Formula: see text] ([Formula: see text] 50 GeV) regions. Discrepancies observed at low [Formula: see text], although marginal, may indicate the necessity for the consideration of further higher-order processes. Unambiguous particle-level definition of the photon-isolation condition for the signal events is desired to be given explicitly in future measurements.

Examinations of the early degrees of freedom in ultrarelativistic nucleus-nucleus collisions

The Parton-Hadron-String-Dynamics (PHSD) transport model is used to study the impact on the choice of initial degrees of freedom on the final hadronic and electromagnetic observables in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. We find that a non-perturbative system of massive gluons (scenario I) and a system dominated by quarks and antiquarks (scenario II) lead to different hadronic observables when imposing the same initial energy-momentum tensor $T_{\mu \nu}(x)$ just after the passage of the impinging nuclei. In case of the gluonic initial condition the formation of $s,{\bar s}$ pairs in the QGP proceeds rather slow such that the anti-strange quarks and accordingly the $K^+$ mesons do not achieve chemical equilibrium even in central Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV. Accordingly, the $K^+$ rapidity distribution is suppressed in the gluonic scenario and in conflict with the data from the BRAHMS Collaboration. The proton and antiproton rapidity distributions also disfavor the scenario I. Furthermore, a clear suppression of direct photon and dilepton production is found for the pure gluonic initial conditions which is not so clearly seen in the present photon and dilepton spectra from Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV due to a large contribution from other channels. It is argued that dilepton spectra in the invariant mass range 1.2 GeV $< M <$ 3 GeV will provide a definitive answer once the background from correlated $D$-meson decays is subtracted experimentally.

QCD analysis and effective temperature of direct photons in lead-lead collisions at the LHC

We present a systematic theoretical analysis of the ALICE measurement of low-pT direct-photon production in central lead-lead collisions at the LHC with a centre-of-mass energy of sNN=2.76 TeV. Using next-to-leading order of perturbative QCD, we compute the relative contributions to prompt-photon production from different initial and final states and the theoretical uncertainties coming from independent variations of the renormalisation and factorisation scales, the nuclear parton densities and the fragmentation functions. Based on different fits to the unsubtracted and prompt-photon subtracted ALICE data, we consistently find an exponential, possibly thermal, photon spectrum from the quark-gluon plasma (or hot medium) with slope T=304±58 MeV and 309±64 MeV at pT∈[0.8;2.2] GeV and pT∈[1.5;3.5] GeV as well as a power-law (pT−4) behavior for pT>4 GeV as predicted by QCD hard scattering.

Direct photon production in Pb–Pb collisions atsNN=2.76 TeV

Direct photon production at mid-rapidity in Pb-Pb collisions at $\sqrt{s_{_{\mathrm{NN}}}} = 2.76$ TeV was studied in the transverse momentum range $0.9 < p_\mathrm{T} < 14$ GeV$/c$. Photons were detected with the highly segmented electromagnetic calorimeter PHOS and via conversions in the ALICE detector material with the $e^+e^-$ pair reconstructed in the central tracking system. The results of the two methods were combined and direct photon spectra were measured for the 0-20%, 20-40%, and 40-80% centrality classes. For all three classes, agreement was found with perturbative QCD calculations for $p_\mathrm{T} \gtrsim 5$ GeV$/c$. Direct photon spectra down to $p_\mathrm{T} \approx 1$ GeV$/c$ could be extracted for the 20-40% and 0-20% centrality classes. The significance of the direct photon signal for $0.9 < p_\mathrm{T} < 2.1$ GeV$/c$ is $2.6\sigma$ for the 0-20% class. The spectrum in this $p_\mathrm{T}$ range and centrality class can be described by an exponential with an inverse slope parameter of $(297 \pm 12^\mathrm{stat}\pm 41^\mathrm{syst})$ MeV. State-of-the-art models for photon production in heavy-ion collisions agree with the data within uncertainties.

Novel effects in twist-three SSA

We discuss the role of gluon poles and the gauge invariance for the hadron tensors of Drell-Yan and direct photon production processes with the transversely polarized hadron. These hadron tensors are needed to construct the corresponding single spin asymmetries. For the Drell-Yan process, we perform our analysis within both the Feynman and axial-type (contour) gauges for gluons. In both the Feynman and contour gauges, we demonstrate that the gauge invariance leads to the need of the new (non-standard) diagrams. Moreover, in the Feynman gauge, we argue the absence of gluon poles in the correlators 〈̄ψγ⊥A+ ψ〉 related traditionally to dT(x,x)/dx. As a result, these terms disappear from the final QED gauge invariant Drell-Yan hadron tensor. For the direct photon production, by using the contour gauge for gluon fields, we find that there are new twist-3 terms present in the hadron tensor of the considering process in addition to the standard twist-3 terms.

Recent PHENIX results on hard probes and direct photon production

A hot and dense matter called strongly interacting quark-gluon plasma (sQGP) is created in heavy ion collisions at RHIC energies. Detailed study of the properties of this new state of matter is a driving force of recent research at RHIC. In these proceedings we present most recent PHENIX results for system size and energy dependence of hadron and jet production at high transverse momentum in heavy ion collisions at RHIC. We also report latest results for direct photon production including soft direct photon yields and anisotropic flow.

Systematic study of real photon and Drell-Yan pair production in p+A (d+A) interactions

We study nuclear effects in production of Drell-Yan pairs and direct photons in proton-nucleus collisions. For the first time, these effects are studied within the color dipole approach using the Green function formalism which naturally incorporates the color transparency and quantum coherence effects. The corresponding numerical results for the nuclear modification factor are compared with available data. Besides, we present a variety of predictions for the nuclear suppression as a function of transverse momentum pT, Feynman variable xF and invariant mass M of the lepton pair which can be verified by experiments at RHIC and LHC. We found that the nuclear suppression is caused predominantly by effects of quantum coherence (shadowing corrections) and by the effective energy loss induced by multiple initial state interactions. Whereas the former dominate at small Bjorken x2 in the target, the latter turns out to be significant at large x1 in the projectile beam and is universal at different energies and transverse momenta. (Less)

Factorization theorem, gluon poles and new contributions in semi-inclusive processes

We discuss the role of gluon poles and the gauge invariance for the hadron tensors of Drell-Yan and direct photon production processes with the transversely polarized hadron.

Perspetives of study the direct photon production process at FAIR energy

The modeling of high energy photons production in collisions of antiproton beam having E beam = 15 GeV with the proton target pp→ γ + {ptX} is done using the event sample simulated by PYTHIA6 generator. Such energy is high enough to consider this collision as a relativistic one and being caused by parton-parton scattering. The distribution of the set of kinematic variables and cuts which can be useful for getting the information about proton structure in the available kinematic region is obtained. The contributions of fake photons which can appear from the hadron decays as well as of the background caused by the minimum bias events and other QCD processes are estimated. The set of cuts which can be useful for separation of signal events containing the direct photons from background events is proposed.

PARTON DISTRIBUTION FUNCTIONS WITH QED CORRECTIONS

We present the first unbiased determination of parton distribution functions (PDFs) with electroweak corrections. The aim of this thesis is to provide an exhaustive description of the theoretical framework and the technical implementation which leads to the determination of a set of PDFs which includes the photon PDF and quantum electrodynamics (QED) contributions to parton evolution. First, we introduce and motivate the need of including electroweak corrections to PDFs, providing phenomenological examples and presenting an overview of the current state of the art in PDF fits. The theoretical implications of such corrections are then described through the implementation of the combined QCD+QED evolution in APFEL, a public code for the solution of the PDF evolution developed particularly for this thesis. We proceed by presenting the new structure of the Neural-Network PDF (NNPDF) methodology used for the extraction of this set of PDFs with QED corrections. We then provide a first determination of the full set of PDFs based on deep-inelastic scattering data and LHC data for $W$ and $Z/\gamma^*$ Drell-Yan production, using leading-order QED and NLO or NNLO QCD: the so-called NNPDF2.3QED set of PDFs. We perform a preliminary investigation of the phenomenological implications of NNPDF2.3QED set, in particular, focusing on the photon-induced corrections to direct photon production at HERA, high-mass dilepton and $W$ pair production at the LHC and finally, providing a first determination of lepton PDFs through the APFEL evolution. We conclude with a summary of the technological upgrades required for the improvement of future PDF determinations with electroweak corrections.