Virtual Photon Production Research Articles

Di-electron production at the LHC: unravelling virtual-photon and heavy-flavour contributions

The production of virtual photons is a very sensitive probe of the properties of the quark-gluon plasma. As they are experimentally detected by lepton pairs, they suffer from a large background arising from hadron decays. Light-flavour hadrons dominate at low invariant masses below mee ~ 0.5 GeV and heavy flavours above. These contributions must therefore also be taken into account in experimental analyses at the LHC. In this paper, we calculate the direct contribution from virtual photons produced in the Drell-Yan process with an additional jet in POWHEG and find that it is significant at low invariant masses. We also simulate the background contributions from cc¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ c\\overline{c} $$\\end{document} and bb¯\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ b\\overline{b} $$\\end{document} production with POWHEG and quantify the theoretical uncertainties due to variations of the perturbative scales and parton distribution functions. We find larger relative and absolute uncertainties for the lighter c quarks than for heavier b quarks.

Direct virtual photon production in Au+Au collisions at [formula omitted

We report the direct virtual photon invariant yields in the transverse momentum ranges 1<pT<3 GeV/c and 5<pT<10 GeV/c at mid-rapidity derived from the dielectron invariant mass continuum region 0.10<Mee<0.28 GeV/c2 for 0–80% minimum-bias Au+Au collisions at sNN=200 GeV. A clear excess in the invariant yield compared to the nuclear overlap function TAA scaled p+p reference is observed in the pT range 1<pT<3 GeV/c. For pT>6 GeV/c the production follows TAA scaling. Model calculations with contributions from thermal radiation and initial hard parton scattering are consistent within uncertainties with the direct virtual photon invariant yield.

Measurement of dielectrons in pp, p–Pb and Pb–Pb collisions with ALICE at the LHC

Dielectrons are penetrating probes which carry unaffected information on the hot and dense medium created in ultra-relativistic heavy-ion collisions. The low-mass region of the dielectron spectrum (mπ0 < mee < mρ) is sensitive to virtual photon production and in-medium modifications of low-mass vector meson spectral functions. These modifications are connected to the restoration of chiral symmetry, which is expected at high temperature. Dielectrons are also sensitive to heavy-flavor production.In these proceedings, we present the preliminary results of dielectron measurements in pp collisions at = 7 TeV, in p–Pb collisions at = 5.02 TeV and in central (0-10%) Pb–Pb collisions at = 2.76 TeV with the ALICE experiment at the LHC. The expected scenario and perspectives for the dielectron measurement after the ALICE upgrade are also discussed.

Parton density constraints from massive vector boson production at the LHC

We demonstrate that not only the production of virtual photons decaying into low- mass lepton pairs, but also the one of weak bosons at large transverse momenta is dominated by quark-gluon scattering. Measurements of these processes at the LHC can therefore provide useful constraints on the parton densities in the proton, in particular the one of the gluon, and their nuclear modifications.

Transverse momentum dependent parton densities in associated real and virtual photon and jet production at the LHC

We study the associated production of real (isolated) or virtual photons (with their subsequent leptonic decay) and hadronic jets in proton-proton collisions at the LHC using the ${k}_{T}$-factorization approach of QCD. The consideration is based on the off-shell quark-gluon QCD Compton scattering subprocesses. In the case of virtual photon production, the contributions from $Z$ boson exchange as well as ${\ensuremath{\gamma}}^{*}\ensuremath{-}Z$ interference with the full spin correlations are included. The transversemomentum-dependent (TMD) quark and gluon densities in a proton are determined from the Kimber-Martin-Ryskin prescription or Catani-Ciafoloni-Fiorani-Marchesini (CCFM) equation. In the latter, we restrict to the case where the gluon-to-quark splitting occurs at the last evolution step and calculate the sea quark density as a convolution of the CCFM-evolved gluon distribution and the TMD gluon-to-quark splitting function. Our numerical predictions are compared with the recent experimental data taken by the ATLAS Collaboration. We discuss the theoretical uncertainties of our calculations and argue that further studies are capable of constraining the TMD parton densities in the proton.

Monte-Carlo simulation of lepton pairs production in "pp̄ → e+e− + X" events at PANDA experiment

Electron-positron pairs production via the quark-antiquark annihilation process in collisions of antiproton beam having Ebeam = 15 GeV with the proton target in PANDA experiment is modeled by using the PYTHIA6.4 Monte-Carlo event generator and PandaRoot framework. The considered quark level subprocess pass through the production of virtual photon which converts into the lepton pair (qq̄ → γ* → e+e−). The spectrum of the final state leptons in this process of quark-antiquark annihilation obviously depends on the form of parton distributions inside colliding protons and may provide an interesting information about the quark dynamics inside the hadron. The measurement of the total transverse momentum of a lepton pair as a whole system may give an important information about the intrinsic transverse momentum kT that appears due to the Fermi motion of quarks inside the nucleon. In the article the distributions of different kinematical variables of final state electrons/positrons are presented. The influence of the detector environment on the presence of fake leptons is demonstrated. The signal leptons registration efficiency is calculated. The background contribution is also discussed.

Monte-Carlo simulation of lepton pairs production in "pp̄ → μ+μ− + X" events at Ebeam = 5 GeV

Muon pairs production via the quark-antiquark annihilation process in collisions of antiproton beam with Ebeam = 5 GeV (which corresponds to the center-of-mass energy of the pp̄ system Ecm = 3.363 GeV) with the proton target in PANDA experiment is modeled by using of PYTHIA6.4 generator. The considered quark level subprocess goes through the production of virtual photon which converts into lepton pair (qq̄ → γ* → μ+μ−). Quark-antiquark annihilation process of hadron-hadron collision may provide an interesting information about the quark dynamics inside the hadron. The measurement of the total transverse momentum of a lepton pair as a whole system may provide an important information about the intrinsic transverse momentum kT that appears due to the Fermi motion of quarks inside the nucleon. The distributions of different kinematical variables of final state muons are presented. The problems due to the presence of fake leptons that appear from meson decays, as well as due to the background caused by minimum bias events are also discussed. The set of cuts which allows to separate the signal events with lepton pairs from the backgrounds is proposed.

Electron distributions in bremsstrahlung and virtual photon production

Analytic expressions for electron distributions in bremsstrahlung and virtual photon radiation are derived and discussed. The results are used to investigate possible electron scattering geometries which maximize the virtual photon production at forward angles while limiting bremsstrahlung to acceptable values.

Initial-state interactions and single-spin asymmetries in Drell–Yan processes

We show that the initial-state interactions from gluon exchange between the incoming quark and the target spectator system lead to leading-twist single-spin asymmetries in the Drell–Yan process H 1 H 2 ↑→ℓ +ℓ − X. The QCD initial-state interactions produce a T-odd spin-correlation S → H 2 · P → H 1 × Q → between the target spin and the virtual photon production plane which is not power-law suppressed in the Drell–Yan scaling limit at large photon virtuality Q 2 at fixed x F . The single-spin asymmetry which arises from the initial-state interactions is not related to the target or projectile transversity distribution δq H ( x, Q). The origin of the single-spin asymmetry in πp ↑→ℓ +ℓ − X is a phase difference between two amplitudes coupling the proton target with J z p=± 1 2 to the same final-state, the same amplitudes which are necessary to produce a nonzero proton anomalous magnetic moment. The calculation requires the overlap of target light-front wavefunctions with different orbital angular momentum: ΔL z =1; thus the SSA in the Drell–Yan reaction provides a direct measure of orbital angular momentum in the QCD bound state. The single-spin asymmetry predicted for the Drell–Yan process πp ↑→ℓ +ℓ − X is similar to the single-spin asymmetries in deep inelastic semi-inclusive leptoproduction ℓ p ↑→ℓ′ πX which arises from the final-state rescattering of the outgoing quark. The Bjorken-scaling single-spin asymmetries predicted for the Drell–Yan and leptoproduction processes highlight the importance of initial- and final-state interactions for QCD observables.

Thermal Photons and Lepton Pairs from Quark Gluon Plasma and Hot Hadronic Matter

The formulation of the real and virtual photon production rate from strongly interacting matter is presented in the framework of finite temperature field theory. The changes in the hadronic spectral function induced by temperature are discussed within the ambit of the Walecka type model, gauged linear and non-linear sigma models, hidden local symmetry approach, and QCD sum rule approach. The possibility of observing the direct thermal photon and lepton pairs from quark gluon plasma has been contrasted with those from hot hadronic matter with and without medium effects for various mass variation scenarios. At SPS energies, in-medium effects of different magnitude on the hadronic properties for the Walecka model, Brown–Rho scaling, and Nambu scaling scenarios are conspicuously visible through the low invariant mass distribution of dileptons and transverse momentum spectra of photons. However, at RHIC energies the thermal photon (dilepton) spectra originating from quark gluon plasma overshines those from hadronic matter for large transverse momentum (invariant mass) irrespective of the models used for evaluating the finite temperature effects on the hadronic properties. It is thus expected that at both RHIC and LHC energies the formation of quark gluon plasma in the initial stages may indeed turn out to be a realistic scenario.

Bremsstrahlung and photon production in thermal QCD

In this paper, we extend the study of bremsstrahlung photon production in a quark-gluon plasma to the cases of soft static photons and hard real photons. The general framework of this study is the effective perturbative expansion based on the resummation of hard thermal loops. Despite the fact that bremsstrahlung only comes at two loops, we find that in both cases it generates contributions of the same order of magnitude as those already calculated by several other groups at one loop. Furthermore, a new process contained in the two-loop diagrams dominate the emission of a very hard real photon. In all cases, the rate of real or virtual photon production in the plasma is appreciably increased compared to the one-loop predictions.

Results of recent bremsstrahlung measurements on few-body systems at KVI

A series of bremsstrahlung measurements have been performed with the superconducting cyclotron, AGOR, at KVI. These measurements, on the proton-proton and proton-deuteron systems, aim to investigate the nucleon-nucleon interaction as the nucleons go off their mass shell. Cross sections and analyzing powers have been measured for all possible exit channels of both systems with an incident proton energy of 190 MeV, which is below the particle-production threshold. Preliminary results of the real-photon as well as virtual-photon production are presented and compared to theoretical predictions.

Soft gluon corrections to large- pT direct photon and dilepton production

QCD O( α s 2) corrections to large- p T real and virtual photon production from scattering in the quark-gluon channel are studied in the soft gluon limit. In this limit we carefully preserve the analytic structure of amplitudes in the small-momenta integration regions. The result is a rather large correction to the Born term which significantly improves the agreement with data on p + p → γ + X. The application of the approach to other processes is discussed and the dominant terms of the O( α s 2) correction to e +e − → three jets are reproduced.

Hadroproduction of quark flavors

We show that the extension of the conserved-vector-current hypothesis to gluons successfully describes the magnitude and energy dependence of the hadroproduction of strangeness, charm, and $b$, $t$ flavors in their manifest ($K, D, B$) or hidden ($\ensuremath{\varphi}, \ensuremath{\psi}, \ensuremath{\Upsilon}$) forms. We investigate the relation between the hadronic excitation of $\ensuremath{\psi}$ and $\ensuremath{\Upsilon}$ particles for different assumptions regarding the quark structure of $\ensuremath{\Upsilon}$. In our approach cross sections are simply obtained by rescaling experimental data on the production of virtual photons by $\frac{{{\ensuremath{\alpha}}_{g}}^{2}(M)}{{\ensuremath{\alpha}}^{2}}$, where ${\ensuremath{\alpha}}_{g}(M)$ is the quark-gluon coupling constant, varying with the gluon mass $M$ in the sense of asymptotic freedom. We propose the powerful phenomenological tool that the total inclusive yield for producing flavor bound states scales in $\frac{\ensuremath{\Gamma}}{{M}^{3}}$ where $M$ and $\ensuremath{\Gamma}$ are respectively the mass and direct hadronic width of the state. We also show that the model successfully describes the relative production rates of dileptons (via $\ensuremath{\psi}$ as well as off-resonance) for different beam particles.

CVC for gluons and hadroproduction of quark flavours

We investigate the phenomenological implications of the straightforward extension of the conserved vector current hypothesis to gluons. The characteristics of strange, charmed particle and ψ production cross sections in hadron collisions are successfully obtained from experimental data on the production of virtual photons, rescaled by a factor α g 2( M)/ α 2· α g( M) is the quark-gluon coupling constant, varying with the gluon mass M in the sense of asymptotic freedom.