Direct Photon Processes Research Articles

Inclusive Photoproduction of Υ(1s) and J/ψ in Ultra-peripheral p-Pb Collisions at TeV**Supported by the National Natural Science Foundation of China under Grant Nos. 11465021, 11065010

With a photon spectrum of high energy charged partons, both differential and total cross sections of J/ψ and Υ(1s) inclusive photoproduction in ultra-peripheral p-Pb Collisions (UPCs) at TeV are given. A direct photon process and a resolved photon process including fragment and non-fragment contributions are considered. The total cross section is compared to the inelastic production results in coherent p-Pb collisions at the same center-of-mass energy. Our results seem non-negligible.

Photoproduction of J/ψ in pp and PbPb Collisions at Leading Order

The production of J/ψ originating from photoproduction processes in pp and PbPb collisions at leading order is calculated. We use the color singlet and color octet mechanisms for heavy quarkonium production to deal with the direct photon processes, and consider fragmentation processes. The shadowing and iso-spin of the nucleus are also considered in our calculation. The numerical results show that the modification of photoproduction processes for J/ψ production becomes obvious in the large pT region at LHC energy.

Three-gluon contribution to the single spin asymmetry in Drell-Yan and direct-photon processes

We derive the single-spin-dependent cross section for the Drell-Yan lepton-pair production and the direct-photon production in the $pp$-collision induced by the twist-3 three-gluon correlation functions in the transversely polarized nucleon in the leading order with respect to the QCD coupling constant. Combined with the contribution from the twist-3 quark-gluon correlation functions in the literature, this completes the twist-3 cross section for these processes. We also present a model calculation of the asymmetry for the direct photon production at the RHIC energy, demonstrating the sensitivity of the asymmetry to the form of the three-gluon correlation functions. In particular, we show that the asymmetry in the backward direction of the polarized nucleon is determined by the small-$x$ behavior of the correlation functions.

Dijet photoproduction of massless charm jets at next-to-leading order of QCD

We compute the charm dijet photoproduction cross section at next-to-leading order of QCD in the zero-mass variable flavour number scheme, i.e. with active charm quarks in the proton and photon. The results are compared to recent measurements from the ZEUS experiment at HERA. The predictions for various distributions agree well with the data, in particular for large momentum fractions of the the partons in the photon, where direct photon processes dominate. At low momentum fractions, the predictions are quite sensitive to the charm content in the photon. The experimental data are shown to favour parameterizations with a substantial charm quark density such as the one proposed by Cornet et al.

Study of direct photon plus jet production in CMS experiment at $\sqrt{s}$ = 14 TeV

We present simulation results of γ+jet analysis using CMS (compact muon solenoid) object-oriented software at the large hadron collider (LHC) center of mass energy \(\sqrt{s}\) = 14 TeV. The study of direct photon production helps in validating the perturbative quantum chromodynamics (pQCD) and providing information on the gluon distribution in the nucleons. Direct photon processes also constitute a major background to several other standard model (SM) processes and signals of new physics. Thus these processes need to be understood precisely in the new energy regime. In this work, we have done a detailed study of the GEANT4 simulated γ+jet events generated with Pythia, and the related background processes. Isolation cuts have been optimized for direct photon which improves the signal over background ratio by ∼25% as compared to previous studies done in CMS. The inclusion of a large Δφ cut between the photon and the leading jet at 40° in the analysis leads to a further increase of ∼15% in S/B, thus giving an overall gain of ∼42% in S/B ratio.

ηc Diffractive Production in the Direct Photon Process**The project supported by the State Key Basic Research Development Program under Grant No. G20000774, the Knowledge Innovation Project of the Chinese Academy of Sciences under Grant No. CJKCX2-SW-N02, National Natural Science Foundation of China under Grant Nos. 19835010, 19673002, and 10175074, Natural Science Foundation of Hebei Province of China under Grant No.

Based on the factorization theorem for lepton induced hard diffractive scattering and color octet heavy quarkonium production mechanism, ηc diffractive production in the direct photon process is studied. The results show that this process can be measured at DESY HERA, and ηc production has different features from J/ψ production, which is weakly affected by the initial and final state gluon radiation. Therefore, ηc photoproduction can be viewed as reliable estimate. The experimental study of this process can give valuable insight in the color octet heavy quarkonium production mechanism.

Inelastic photoproduction of $J/\psi$ mesons at HERA

An analysis of inelastic photoproduction of J/Psi mesons is presented using data collected at the ep collider HERA corresponding to an integrated luminosity of above 80pb-1. Differential and double differential cross sections are measured in a wide kinematic region: 60<W_gammap<260 GeV, 1<p_t^2< 60 GeV^2 and 0.05<z<0.9, where z is the fraction of the energy of the exchanged photon transferred to the J/Psi meson in the rest frame of the target proton. Cross sections at z<0.3 are presented for the first time. Theoretical calculations within the Colour Singlet Model at NLO for direct photon processes are shown to give a good description of the data in the medium z region (0.3<z<0.9) up to the highest p_t^2 values. A calculation using a k_t factorisation approach in LO in the Colour Singlet Model is also able to describe these data. The data in the full z range are also compared to LO calculations within a non-relativistic QCD framework including color octet and colour singlet contributions for direct and resolved photons. It seems possible to reconcile data and theory with modest contributions from colour octet processes. The polarisation of the J/Psi meson is measured as a function of z and p_t,psi and is reasonably described by the theoretical predictions.

$ J/\psi +{\mathrm{jet}} $ diffractive production in the direct photon process at HERA

We present a study of $J/\psi + jet$ diffractive production in the direct photon process at HERA based on the factorization theorem for lepton-induced hard diffractive scattering and the factorization formalism of the nonrelativistic QCD (NRQCD) for quarkonia production. Using the diffractive gluon distribution function extracted from HERA data on diffractive deep inelastic scattering and diffractive dijet photon production, we show that this process can be studied at HERA with present integrated luminosity, and can give valuable insights in the color-octet mechanism for heavy quarkonia production.

Measurement of jet shapes in high- $Q^2$ deep inelastic scattering at HERA

The shapes of jets with transverse energies, $E_{\rm T}^{\rm jet}$ , up to 45 GeV produced in neutral- and charged-current deep inelastic $e^+p$ scattering (DIS) at $Q^2 > 100$ GeV $^2$ have been measured with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the $\eta$ - $\phi$ plane with a cone radius of one unit. The jets become narrower as $E^{\rm jet}_{\rm T}$ increases. The jet shapes in neutral- and charged-current DIS are found to be very similar. The jets in neutral-current DIS are narrower than those in resolved processes in photoproduction and closer to those in direct-photon processes for the same ranges in $E_{\rm T}^{\rm jet}$ and jet pseudorapidity. The jet shapes in DIS are observed to be similar to those in $e^+e^-$ interactions and narrower than those in $\bar{p}p$ collisions for comparable $E_{\rm T}^{\rm jet}$ . Since the jets in $e^+e^-$ interactions and $e^+p$ DIS are predominantly quark initiated in both cases, the similarity in the jet shapes indicates that the pattern of QCD radiation within a quark jet is to a large extent independent of the hard scattering process in these reactions.

Measurement of jet shapes in high-

The shapes of jets with transverse energies, $E_{\rm T}^{\rm jet}$ , up to 45 GeV produced in neutral- and charged-current deep inelastic $e^+p$ scattering (DIS) at $Q^2 > 100$ GeV $^2$ have been measured with the ZEUS detector at HERA. Jets are identified using a cone algorithm in the $\eta$ - $\phi$ plane with a cone radius of one unit. The jets become narrower as $E^{\rm jet}_{\rm T}$ increases. The jet shapes in neutral- and charged-current DIS are found to be very similar. The jets in neutral-current DIS are narrower than those in resolved processes in photoproduction and closer to those in direct-photon processes for the same ranges in $E_{\rm T}^{\rm jet}$ and jet pseudorapidity. The jet shapes in DIS are observed to be similar to those in $e^+e^-$ interactions and narrower than those in $\bar{p}p$ collisions for comparable $E_{\rm T}^{\rm jet}$ . Since the jets in $e^+e^-$ interactions and $e^+p$ DIS are predominantly quark initiated in both cases, the similarity in the jet shapes indicates that the pattern of QCD radiation within a quark jet is to a large extent independent of the hard scattering process in these reactions.

Testing quarkonium production with photoproducedJ/ψ+γ

I compute the leading color-octet contributions to the process $\gamma + p \to J/\psi + \gamma (+ X)$ within the non-relativistic QCD (NRQCD) factorization formalism. In the color-singlet model, $J/\psi + \gamma$ can only be produced when the photon interacts through its structure function, while the color-octet mechanism allows for production of $J/\psi + \gamma$ via direct photon-gluon fusion. Resolved photon processes can be easily be distinguished from direct photon processes by examining the fraction of the incident photon energy carried away by the $J/\psi$ in the event. Therefore, this process provides a conclusive test of the color-octet mechanism. $J/\psi + \gamma$ production is particularly sensitive to the NRQCD matrix element which figures prominently in the fragmentation production of $J/\psi$ at large $p_{\perp}$ in hadron colliders. I also examine the predictions of the color evaporation model (CEM) of quarkonium production and find that this process can easily discriminate between the NRQCD factorization formalism and the CEM.