Photon production from non-equilibrium disoriented chiral condensates in a longitudinal expansion: A theoretic framework

A theoretical framework is developed for treating the quantization of the photons in a spacetime with a longitudinal expansion. This can be used to study the production of the photons through the non-equilibrium relaxation of a disoriented chiral condensate presumably formed in the expanding hot central region in ultra-relativistic heavy-ion collisions. These photons can be a signature of the formation of disoriented chiral condensates in the direct photon measurements of heavy-ion collisions.     

High energy γ-ray emission in heavy-ion collisions

We calculate double-differential cross sections for energetic photon production in intermediate energy nucleus-nucleus collisions. The production mechanism is assumed to be either that of collective bremsstrahlung or that of electromagnetic transitions between time-dependent single-particle states as emerging from a self-consistent treatment of the heavy-ion dynamics in the one-body limit (TDHF). Effects from residual nucleon-nucleon collisions on the collective current are further taken into account via a relaxation ansatz for the single-particle occupation numbers. The total yields for energetic photon production above 50 MeV in the simple limit considered indicate that up to 10% of the experimental cross sections can be attributed to collective bremsstrahlung and electromagnetic transitions.     

The relevance of {\mathbf{\gamma^*_{\mathrm{L}}}} in hard collisions of virtual photons

We explore the relevance of extending the concept of the partonic structure to the longitudinally polarized virtual photon involved in hard collisions. We show that for moderate photon virtualities and in the kinematical region accessible in current experiments at HERA and LEP, the contributions of its longitudinal polarization to hard collisions are sizable and should be taken into account as part of the resolved photon component. Received: 30 March 2000 / Published online: 6 July 2000     

Photons from quark gluon plasma and hot hadronic matter

The productions of real photons from quark gluon plasma and hot hadronic matter formed after the nucleus-nucleus collisions at ultra-relativistic energies are discussed. The effects of the spectral shift of the hadrons at finite temperature on the production of photons are investigated. On the basis of the present analysis it is shown that the photon spectra measured by WA98 collaboration in Pb + Pb collisions at CERN SPS energies can be explained by both QGP as well as hadronic initial states if the spectral shift of hadrons at finite temperature is taken into account. Several other works on the analysis of WA98 photon data have also been briefly discussed.     

Search for direct photons from S-Au collisions at 200 GeV/nucleon

The CERES experiment has measured inclusive photon production in S-Au collisions of 200 GeV/nucleon at the CERN SPS. No evidence for direct emission of photons was found. For the kinematic region 2.1<y<2.65 and 0.4 GeV/c<p⊥<2.0 GeV/c the yield andp⊥-dependence of the observed photons are well reproduced by hadron decays. Furthermore, their production rate is found to be proportional to the charged particle density. The systematic errors comparing the measured and expected photon yield result in an upper limit of 14% for the emission of direct photons in central S-Au collisions. For a photon source with a yield depending quadratically on the charged particle density the limit can be reduced to 7%.     

Search for direct photons from S-Au collisions at 200 GeV/nucleon

The CERES experiment has measured inclusive photon production in S-Au collisions of 200 GeV/nucleon at the CERN SPS. No evidence for direct emission of photons was found. For the kinematic region 2.1<y<2.65 and 0.4 GeV/c<p??<2.0 GeV/c the yield andp??-dependence of the observed photons are well reproduced by hadron decays. Furthermore, their production rate is found to be proportional to the charged particle density. The systematic errors comparing the measured and expected photon yield result in an upper limit of 14% for the emission of direct photons in central S-Au collisions. For a photon source with a yield depending quadratically on the charged particle density the limit can be reduced to 7%.     

CP Asymmetry in Top Quark-Pair Production via Photon Fusion:(Ⅱ) For Polarized Photon Beams

The effects of CP asymmetry in top-pair production with polarized photon beam collisions,which may arise in two Higgs doublet extensions of the minimal standard model, are studied.Some of the advantages to observe the CP non-invariance in the production are pointed out.The CP asymmetry will be enhanced by polarized photon fusion greatly compared to the unpolarized one and the signal can reach so high up to a few percents.     

QGP tomography with photon tagged jets in ALICE

γ+jet events provide a tomographic measurement of the medium formed in heavy ion collisions at LHC energies.Tagging events with a well identified high p T direct photon and measuring the correlation distribution of hadrons emitted oppositely to the photon in ALICE,allows us to determine,with a good approximation,both the jet fragmentation function and the back-to-back azimuthal alignment of the direct photon and the jet.Comparing these two observables measured in pp collisions with the ones measured in AA collisions will reveal the modifications of the jet structure induced by the medium formed in AA collisions and consequently will infer the medium properties.     

Au-Au碰撞中大横动量双轻子对的产生(英文)

运用微扰QCD讨论了重离子碰撞中大横动量轻子对的分布, 计算了Au-Au碰撞中两个部分子产生的轻子对的贡献。引入了轻子对产生的直接单光子过程和分解单光子过程。大横动量情况下的所有过程都包括在内, 而且考虑了核遮蔽效应和同位旋效应, 作为QGP背景的双轻子信号有了一个好的修正。The large transverse momentum distribution of lepton pairs produced in heavy-ion collisions has been studied, making use of the perturbative QCD. The contribution of the two parton production process into lepton pairs in Au Au collisions is calculated. Lepton pair production with the direct single photon process and the resolved single photon process are introduced. We believe that the photon processes are significant. The complete processes at large transverse momentum are included, and moreover, the effect of shadowing and isospin of nucleus are also considered in heavy ion collisions. Dilepton signals to regard the background of QGP have a good correction.     

Observation of direct photons in central 158A GeV (208)P(208)b+Pb collisions

A measurement of direct photon production in 208Pb+208Pb collisions at 158A GeV has been carried out in the CERN WA98 experiment. The invariant yield of direct photons in central collisions is extracted as a function of transverse momentum in the interval 0.51.5 GeV/c. The result constitutes the first observation of direct photons in ultrarelativistic heavy-ion collisions. It could be significant for diagnosis of quark-gluon-plasma formation.     

Photoproduction of top quarks in peripheral heavy ion collisions

In relativistic heavy ion collisions, top quarks can be produced by photon-gluon fusion when a photon from the Weizs?cker-Williams virtual photon field of one nucleus interacts with a gluon in the other nucleus. Photoproduction with heavy ions at the Large Hadron Collider (LHC) will be the first accessible non-hadronic top production channel. We calculate the photoproduction cross sections, pair mass and top quark rapidity distributions in peripheral lead-lead and oxygen-oxygen collisions. The cross sections are sensitive to the top quark charge and the large- gluon distribution in the nucleus. We find a cross section of 15 pb in oxygen-oxygen collisions, leading to 210 pairs in a one month ( s) LHC run. In pA collisions, the rate is higher, 1100 pairs per month for pO. A comparison of the AA and pA data might allow for a study of gluon shadowing at high Q . Received: 18 May 2001 / Revised version: 13 June 2001 / Published online: 24 August 2001     

On the observability of direct photons in ultrarelativistic nucleus-nucleus collisions by means of inclusive photon and hadron spectra

A method is presented to search for a direct photon signal in collisions between nuclei at ultrarelativistic energies. In the proposed method the analysis is performed in the center of momentum system of the radiating system, being the cms of the colliding nucleons in case of collisions of symmetric systems. This results in a maximal strength of a possible direct photon signal in the observed energy spectra, since the smearing of the signal due to the Lorentz transformation between the cms and the laboratory frame in case of fixed target experiments is absent. The feasibility of detecting a direct photon signal is examined by comparison of calculated photon and hadron spectra for central and peripheral ³²S on S collisions at a nucleon-nucleon cms energy of about 20 GeV, being the energy domain of the CERN-SPS fixed target experiments.     

Interferences of molecular X-rays in heavy ion collisions

The possibility of coherence effects in molecular orbital X-rays emitted during multiple heavy ion collisions in a crystal target is investigated. That X-ray energy region is found, where in spite of the thermal vibrations of the target atoms interferences have to be expected. The dynamical theory for the molecular orbital evolution is used. The 70 MeV Ni — Ni collisions are treated as an example. An experiment for checking the coherent photon emission in multiple molecular scattering processes is suggested, which in turn can eventually be used to measure the ratio of e.g. triple to binary collision.     

Centrality dependence of direct photon production in (square root)S(NN) = 200 GeV Au + Au collisions

The first measurement of direct photons in Au + Au collisions at (square root)S(NN) = 200 GeV is presented. The direct photon signal is extracted as a function of the Au + Au collision centrality and compared to next-to-leading order perturbative quantum chromodynamics calculations. The direct photon yield is shown to scale with the number of nucleon-nucleon collisions for all centralities.     

Stimulated photon echo study of Na(32S12)-CO velocity-changing collisions

Using the recently developed single-state stimulated photon echo technique, we have studied the velocity changes experienced by ground-state Na atoms in collisions with carbon monoxide molecules. We find that the ground-state Na atoms have a total cross section of ≈420 Ų for velocity-changing collisions with CO, and that they experience an average velocity change of ≈600 cm/s. A scattering kernel of exponential form is shown to be consistent with our results. We discuss the essential features of the estimated photon echo technique.     

Photon echo and stimulated photon echo study of various collisional-relaxation channels

Collisional relaxation in SF₆ gas and its mixtures with He and Xe is studied by photon echo and stimulated photon echo methods from the standpoint of the possibility of identifying the contributions of different types of collisions. The nonexponential nature of the kinetic curve of the photon echo is clearly observed for pure SF₆, it is weaker in the mixture SF₆+Xe, and it is virtually completely absent for high degrees of dilution of SF₆ with helium. These features can be explained on the basis of estimates, made from experimental data, of the critical delay between the exciting pulses (for which the nonexponential behavior should be most strongly manifested). In pure SF₆ it is possible to distinguish the contribution of the inelastic channel (rotational relaxation) and the contribution of weak collisions. To distinguish successfully the relaxation channels in mixtures with buffer gases a heavier buffer gas and a much better time resolution must be used. It is shown that data obtained on the orientation and alignment relaxation rates by the stimulated photon echo method can serve as an upper limit for the rates of inelastic processes which cannot be measured by the photon echo method. The combined use of photon echo and stimulated photon echo methods made it possible to obtain data on the cross sections for elastic and inelastic scattering of the collisional pairs SF₆–SF₆, SF₆–Xe, and SF₆–He. Zh. éksp. Teor. Fiz. 116, 47–56 (July 1999)     

Dilepton creation based on an analytic hydrodynamic solution

High-energy collisions of various nuclei, so called “Little Bangs” are observed in various experiments of heavy ion colliders. The time evolution of the strongly interacting quark-gluon plasma created in heavy ion collisions can be described by hydrodynamical models. After expansion and cooling, the hadrons are created in a freeze-out. Their distribution describes the final state of this medium. To investigate the time evolution one needs to analyze penetrating probes, such as direct photon or dilepton observables, as these particles are created throughout the evolution of the medium. In this paper we analyze an 1+3 dimensional analytic solution of relativistic hydrodynamics, and we calculate dilepton transverse momentum and invariant mass distributions. We investigate the dependence of dilepton production on time evolution parameters, such as emission duration and equation of state. Using parameters from earlier fits of this model to photon and hadron spectra, we compare our calculations to measurements as well. The most important feature of this work is that dilepton observables are calculated from an exact, analytic, 1+3D solution of relativistic hydrodynamics that is also compatible with hadronic and direct photon observables.     

Scaling of single photon production in hadronic collisions

Scaling of single photon production in pp and collisions is studied. It is empirically observed that the available data scales for and for larger . The NLO pQCD predictions for pp collisions at an of 200 and 5500 GeV, relevant for RHIC and LHC energies are seen to closely follow this scaling behavior. Implications for single photon production in heavy ion collisions are discussed. Received: 2 February 2001 / Revised version: 28 May 2001 / Published online: 5 October 2001     

Hard thermal photon production in relativistic heavy ion collisions

The recent status of hard thermal photon production in relativistic heavy ion collisions is reviewed and the current rates are presented with emphasis on corrected bremsstrahlung processes in the quark–gluon plasma (QGP) and quark–hadron duality. Employing Bjorken hydrodynamics with an EOS supporting the phase transition from QGP to hot hadron gas (HHG), thermal photon spectra are computed. For SPS 158 GeV Pb + Pb collisions, comparison with other theoretical results and the WA98 direct photon data indicates significant contributions due to prompt photons. Extrapolating the presented approach to RHIC and LHC experiments, predictions of the thermal photon spectrum show a QGP outshining the HHG in the high-p_T-region.     

Large-pT production of single and double photons in proton-proton and pion-proton collisions

Quantum chromodynamic (QCD) predictions are made for the large transverse momentum production of single and double photons in proton-proton, proton-antiproton, and pion-proton collisions. In π^?p collisions at center-of-mass energy W = 27.4 GeV and p_T = 4.0 GeV, it is estimated that about 0.3% of the 90° single photon triggers will be balanced on the “away-side” by a single photon with roughly the same transverse momentum. In π⁺p collisions this fraction drops to about 0.09%. These fractions increase with p_T. In addition to the pure QED annihilation term $\text{q}\text{q}\text{→γγ}$, it is found that the QCD-induced subprocess gg→γγ provides an important source of double photons. Photon bremsstrahlung contributions are also examined. Experimental study of the systematics of single and double photon production in hadron-hadron collisions will provide information on the size of the strong interaction coupling constant, α_s(Q), and on the charges of the quarks. Knowledge of the gluon distributions within hadrons and of the effective transverse momentum of partons in hadrons can also be gained.