Direct Photon Cross Section Research Articles

Measurement of Direct-Photon Cross Section and Double-Helicity Asymmetry at sqrt[s]=510 GeV in p[over →]+p[over →] Collisions.

We present measurements of the cross section and double-helicity asymmetry A_{LL} of direct-photon production in p[over →]+p[over →] collisions at sqrt[s]=510 GeV. The measurements have been performed at midrapidity (|η|<0.25) with the PHENIX detector at the Relativistic Heavy Ion Collider. At relativistic energies, direct photons are dominantly produced from the initial quark-gluon hard scattering and do not interact via the strong force at leading order. Therefore, at sqrt[s]=510 GeV, where leading-order-effects dominate, these measurements provide clean and direct access to the gluon helicity in the polarized proton in the gluon-momentum-fraction range 0.02<x<0.08, with direct sensitivity to the sign of the gluon contribution.

Low-Mass Dielectron Production in pp, p–Pb and Pb–Pb Collisions with ALICE

The ALICE Collaboration measures the production of low-mass dielectrons in pp, p–Pb and Pb–Pb collisions at the LHC. The main detectors used in the analyses are the Inner Tracking System, Time Projection Chamber and Time-Of-Flight detector, all located at mid-rapidity. The dielectron yield in p–Pb collisions shows an overall agreement with the hadronic cocktail. The pair transverse momentum distributions are sensitive to the contributions from open heavy-flavours. In Pb–Pb collisions, uncorrected background-subtracted yields have been extracted in two centrality classes. In pp collisions the production of virtual photons relative to the inclusive yield is determined by analyzing the dielectron excess with respect to the expected hadronic sources. The direct photon cross section is then calculated and found to be in agreement with NLO pQCD calculations. A feasibility study for LHC Run 3 after the ALICE upgrade indicates the possibility for a future measurement of the early effective temperature.

Low-Mass Dielectron Production in pp, p–Pb and Pb–Pb Collisions with ALICE

The ALICE Collaboration measures the production of low-mass dielectrons in pp, p-Pb and Pb-Pb collisions at the LHC. The main detectors used in the analyses are the Inner Tracking System, Time Projection Chamber and Time-Of-Flight detector, all located around mid-rapidity. The production of virtual photons relative to the inclusive yield in pp collisions is determined by analyzing the dielectron excess with respect to the expected hadronic sources. The direct photon cross section is then calculated and found to be in agreement with NLO pQCD calculations. Results from the invariant mass analysis in p-Pb collisions show an overall agreement between data and hadronic cocktail. In Pb-Pb collisions, uncorrected background- subtracted yields have been extracted in two centrality classes. A feasibility study for LHC run 3 after the ALICE upgrade indicates the possibility for a future measurement of the early effective temperature.

Accessing Gluon Polarization in the Proton with Direct Photons at PHENIX

Direct photon production in polarized pp collisions serves as an excellent probe of the spin structure of the proton. Gluon-quark scattering, g + q → γ + q, dominates direct photon production for p + p collisions. This channel is theoretically clean as it requires no fragmentation function for the photon. In addition, the double longitudinal asymmetry ALLis directly proportional to the quark and gluon polarizations, giving access to both the magnitude and sign of the polarized gluon distribution since the polarized quark distributions are known from polarized DIS. The status of the direct photon cross section measurement and ALLfor center of mass energies 200 and 500 GeV at midrapidity (|η| < 0.35) will be presented.

Inclusiveπ0,η, and direct photon production at high transverse momentum inp+pandd+Aucollisions atsNN=200GeV

We report a measurement of high-p_T inclusive pi^0, eta, and direct photon production in p+p and d+Au collisions at sqrt(s_NN) = 200 GeV at midrapidity (0 < eta < 1). Photons from the decay pi^0 -> gamma gamma were detected in the Barrel Electromagnetic Calorimeter of the STAR experiment at the Relativistic Heavy Ion Collider. The eta -> gamma gamma decay was also observed and constituted the first eta measurement by STAR. The first direct photon cross section measurement by STAR is also presented, the signal was extracted statistically by subtracting the pi^0, eta, and omega(782) decay background from the inclusive photon distribution observed in the calorimeter. The analysis is described in detail, and the results are found to be in good agreement with earlier measurements and with next-to-leading order perturbative QCD calculations.

Measurements of Fragmentation Photons with the PHENIX Detector

Direct photons associated with jets provide a direct measurement of the effects of energy loss on the fragmentation of the parton as it propagates through the medium. Perturbative QCD calculations describe the direct photon cross section well at next-to-leading order, predicting a significant contribution from photons produced through parton fragmentation. Non-perturbative quantities such as the photon fragmentation function, which is poorly constrained, lead to large theoretical uncertainties. The measurement of photons correlated with jets in p + p collisions serves as an important test of these calculations and is an essential baseline measurement for comparison to A + A collisions. A natural way of selecting such photons is to study hadron-photon correlations. Results for the production of photons associated with high p T hadron triggers are presented for PHENIX p + p data at 200 GeV center-of-mass energy.

A systematic study of direct photon production in heavy ion collisions

A theoretical derivation of photon bremsstrahlung, induced by the interactions of an energetic quark in a hot and dense quark–gluon plasma, is given in the framework of the reaction operator approach. For the physically relevant case of hard jet production, followed by few in-medium interactions, we find that the Landau–Pomeranchuk–Migdal suppression of the bremsstrahlung photon intensity is much stronger than in the previously discussed limit of on-shell quarks and a large number of soft scatterings. This result is incorporated in the first systematic study of direct photon production in minimum bias d+Cu and d+Au and central Cu+Cu and Au+Au heavy ion collisions at the Relativistic Heavy Ion Collider at center of mass energies s=62.4 GeV and 200 GeV. We find that the contribution of the photons created via final-state interactions is limited to 35% for 2 GeV<pT<5 GeV and at high transverse momenta the modification of the direct photon cross section is dominated by initial-state cold nuclear matter effects.

Direct photon production in p + p and Au + Au collisions at in the PACIAE model

A parton and hadron cascade model, PACIAE, is used in investigating the PHENIX data of the direct photon and π0 invariant differential cross sections in p + p collisions and the direct photon and π0pT distributions in 0–10% of most central Au + Au collisions at top RHIC energy. It turns out that the above data are possible to be described by this microscopic transport model.

Measurement of direct photon production at Tevatron fixed target energies

Measurements of the production of high-pT direct photons by a 515 GeV/c π- beam and 530 and 800 GeV/c proton beams in interactions with Be and H 2 targets are presented. The data span the kinematic ranges of 3.5 &lt; pT &lt; 12 GeV/c in transverse momentum and 1.5 units in rapidity. The inclusive direct-photon cross sections are compared with NLO pQCD calculations and expectations based on a phenomenological parton-kT model.

Measurement of direct-photon production at the Fermilab Tevatron fixed target energies

Measurements of the production of high transverse momentum direct photons by a 515 GeV/c piminus beam and 530 and 800 GeV/c proton beams in interactions with beryllium and hydrogen targets are presented. The data span the kinematic ranges of 3.5 < p_T < 12 GeV/c in transverse momentum and 1.5 units in rapidity. The inclusive direct-photon cross sections are compared with next-to-leading-order perturbative QCD calculations and expectations based on a phenomenological parton-k_T model.

Direct photon cross section with conversions at CDF

We present a measurement of the isolated direct photon cross section in p-pbar collisions at sqrt(s) = 1.8 TeV and |eta| < 0.9 using data collected between 1994 and 1995 by the Collider Detector at Fermilab (CDF). The measurement is based on events where the photon converts into an electron-positron pair in the material of the inner detector, resulting in a two-track event signature. To remove pi0 -> gamma gamma and eta -> gamma gamma events we use a new background subtraction technique which takes advantage of the tracking information available in a photon conversion event. We find that the shape of the cross section as a function of pT is poorly described by next-to-leading-order QCD predictions, but agrees with previous CDF measurements.

Study of partonkTsmearing effects in direct photon production at the Fermilab Tevatron

Previous detailed studies of direct photon production from both fixed-target and collider experiments have witnessed a pattern of deviation between the measured inclusive cross sections and the corresponding theoretical expectations in the low transverse momentum ${(p}_{T})$ regime. Most data sets display steeper ${p}_{T}$ dependence than the next-to-leading-order (NLO) perturbative QCD (PQCD) calculations with standard choices of scales and parton distribution functions in this region. A simple implementation of higher-order soft-gluon corrections to the NLO PQCD predictions significantly improves the agreement between data and theory. This interesting feature motivated us to investigate the D\O{} and CDF measurements of inclusive photon cross section at $\sqrt{s}=1.8\mathrm{TeV}$ from the run $1b$ and also at $\sqrt{s}=630\mathrm{GeV}.$ We use the latest updated parton distribution function CTEQ6M in the NLO QCD calculations for direct photon cross section to describe the data. The conventional theoretical uncertainties originating from scale dependence and gluon distributions have been illustrated. We estimate the impact of additional soft-gluon radiation on the direct photon production using PYTHIA (LO PQCD), which adds transverse momentum ${k}_{T}$ to initial-state partons through a Gaussian smearing. The impact of ${k}_{T}$ effects on the discrepancy in the low-${p}_{T}$ region is explored using a phenomenological model, wherein we merge the NLO calculations with ${k}_{T}$ correction factors. We show that this approach provides a much more acceptable description of the Fermilab Tevatron data.

Gauge boson production at the tevatron

We present measurements on gauge boson production from data taken during 1994–1996 by the DØ and CDF detectors: the differential production cross section of the W boson as a function of the transverse momentum [1,2], the ratio of W and Z differential cross sections [3,4], direct photon cross-sections at √ s=630 and 1800 GeV [5,6], and studies of Drell-Yan production [7,8]. All measurements are in good agreement with currently available theoretical predictions in most of the measured kinematic range.

Study of direct photon production at the CERN LHC

Study of direct photon production in high-energy hadronic collisions provides a clean tool for testing the essential validity of perturbative quantum chromodynamics (PQCD) predictions as well as for constraining the gluon distribution of nucleons. These attractive considerations prompted us to study the characteristics of direct photons at CERN LHC energy $(\sqrt{s}=14\mathrm{TeV}).$ In order to validate our simulation results, we first describe the direct photon data at $\sqrt{s}=1.8\mathrm{TeV}$ in the central pseudorapidity $(\ensuremath{\eta})$ region. We used next-to-leading-order (NLO) QCD calculations and leading-order (LO) PYTHIA estimates with the latest parton distribution function, CTEQ5M1. At $\sqrt{s}=14\mathrm{TeV},$ the LO and NLO QCD predictions for direct photon cross section are presented as a function of transverse momentum of photon ${(p}_{T})$ in the kinematical region $20\mathrm{GeV}l{p}_{T}l400\mathrm{GeV}$ and $|\ensuremath{\eta}|l3.$ The sensitivity of the theoretical predictions to the choice of renormalization scales and gluon distributions is also demonstrated. The pseudorapidity $(\ensuremath{\eta})$ and cone size dependence of the direct photon cross section is also discussed.

Evidence for PartonkTEffects in High-pTParticle Production

Inclusive pizero and direct-photon cross sections in the kinematic range 3.5 < pT < 12 GeV/c with central rapidities are presented for 530 and 800 GeV/c proton beams and a 515 GeV/c pi- beam incident on beryllium targets. Current Next-to-Leading-Order perturbative QCD calculations fail to adequately describe the data for conventional choices of scales. Kinematic distributions from these hard scattering events provide evidence that the interacting partons carry significant initial-state parton transverse momentum (kT). Incorporating these kT effects phenomenologically greatly improves the agreement between calculations and the measured cross sections.

Direct photon cross sections in proton-proton and antiproton-proton interactions at [formula omitted] GeV

We report results on inclusive direct photon ( γ), π 0, and η production in both pp and p̄p interactions at s =24.3 GeV in the transverse momentum range 4.1≤ p T ≤7.7 GeV/ c and rapidity range −0.1≤ y≤0.9. The data were collected between 1988 and 1990 by the UA6 experiment at CERN, which employed an internal H 2 gas jet target in the Sp̄pS collider. The inclusive direct photon cross sections and the cross section difference σ( pp )−σ( pp) expressed as functions of p T ( γ) are compared with next-to-leading order QCD predictions.

Inclusive dijet production at HERA: direct photon cross sections in next-to-leading order QCD

We have calculated inclusive two-jet cross sections in next-to-leading order QCD for direct photoproduction in low $Q^2$ $ep$ collisions at HERA. Infrared and collinear singularities in real and virtual contributions are cancelled with the phase space slicing method. Analytical formulas for the different contributions giving the dependence on the slicing parameter are presented. Various one- and two-jet distributions have been computed demonstrating the flexibility of the method.

Direct photon production in [formula omitted] and pp interactions at √ s = 24.3 GeV

Inclusive direct photon invariant cross sections have been measured in both pp and pp collisions at √ s = 24.3 GeV at the CERN SPS, permitting the first measurement of the difference of the pp and pp cross sections. The direct photon cross section in pp collisions has been found to be systematically larger than that in pp collisions, which indicates a significant contribution of the qq annihilation term as predicted by theoretical calculations.

Determination of α s and the gluon distribution using direct photon production in [formula omitted] and pp collisions

Direct photon cross sections obtained in pp and pp collisions at √ s = 24.3 GeV at the CERN SPS, are used in a next-to-leading order QCD analysis. From the cross-section difference σ( pp → γX) − σ (pp → γX) , and quark distributions measured in deep inelastic scattering, a determination of the strong coupling constant, α s , is performed via a measurement of Λ (4) MS . This first measurement of Λ (4) MS using direct photon data yields a value Λ (4) MS = 235 ± 79 (stat.) ± 70 (syst.) MeV. The corresponding value of α s expressed at M Z 2 is α s (M Z 2) = 0.112±0.006 (stat.)±0.005 (syst.) −0.001 +0.009 (theor.). The gluon distribution in the proton for x > 0.2 is determined both from the pp and from the pp data, and the results agree with each other and with previous determinations using the same method.

CDF top quark searches

We present a study of low mass dimuon events [m(μμ) < 6 GeV/c2] from the UA1 experiment at the CERN pp collider. Contributions from semileptonic decays of heavy flavour particles, Drell-Yan type processes, J/ψ decays and leptonic decays of light mesons are extracted, and cross sections for high pT beauty and Drell-Yan production are derived. A limit for the branching fraction for the exotic decay B0→μ+μ− is also obtained. The cross section for section for low mass, high pT Drell-Yan prod uction is compared to the measured direct photon cross section using QED and QCD derived relationship. This relationship is used to infer a measurement of the single photon cross section at lower values of transverse momentum, where photons cannot be unambigously identified.