Large transverse momentum dilepton and photon production by photoproduction processes

We calculate the production of large transverse momentum dileptons and photons by using direct and resolved photoproduction processes in relativistic heavy ion collisions. Considering the central collisions of heavy nuclei at Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies, we find that the photoproduction processes modify the dilepton and photon production in the large transverse momentum region.     

Modification of the spectra of correlated charm-anticharm quark pairs in the quark-gluon plasma

Heavy quarks play an important role in probing the properties of strongly interacting quark-gluon plasma(QGP)created in ultra-relativistic heavy-ion collisions.We study the interactions of single heavy(charm)quarks and correlated charm and anticharm(ccˉ)quark pairs with the medium constituents of QGP by performing fireball+Langevin simulations of the pertinent Brownian motion with elastic collisions.Besides studying the traditional observables,the nuclear modification factor and the elliptic flow of single heavy quarks in QGP for different thermal relaxation rates,we also study the broadening of the azimuthal correlations of charm and anticharm quark pairs in the QGP medium for different relaxation rates and transverse momenta classes.We quantified the smearing of ccˉpair azimuthal correlations with an increasing thermal relaxation rate:while the(nearly)back-to-back correlations among ccˉpairs are almost completely washed out at low transverse momentum(pT),these correlations at high pT largely survive the pair diffusion.This provides a novel observable for diagnosing the properties of QGP.     

pp和pÞ碰撞中J/ψ的光生过程(英文)

计算了pp和pÞ碰撞中J/ψ的领头阶光生过程的产额。运用非相对论量子色动力学回顾了重夸克偶素产生的色单态机制与色八重态机制,并将它们分别用于处理直接光子过程和分解光子过程。通过与J/ψ产生的领头阶结果的对比可以看出,光生过程对J/ψ产额的修正在大横动量区域变得明显。The production of J/ψ originating from photoproduction processes in pp and pÞ collisions at leading order is calculated. The color singlet and color octet mechanisms for heavy quarkonium production are reviewed within nonrelativistic QCD, and be used to deal with the direct photon and resolved photon processes respectively. Comparing with the leading order results of J/ψ production, the numerical results show that the modification of photoproduction processes for J/ψ production become obvious at large p-T region.     

Effects of inelastic (re)scattering processes ss→gg and gg→ss on strange hadron production in pp collisions at RHIC and LHC energies

The rapidity densities at mid-rapidity and the transverse momentum distributions for strange hadrons produced in pp collisions are analyzed using the modi ed PACIAE model by considering the effect of inelastic (re)scattering processes ss→gg and gg→ss in parton (re)scattering. The calculated results of the transverse momentum spectra of the strangeness fitting with data measured by STAR and ALICE Collabora- tions can be improved, especially at large transverse momentum levels. This demonstrates that the effect of inelastic (re)scattering processes of ss→gg and gg→ss is not negligible at RHIC and LHC energy levels.     

Effects of inelastic (re)scattering processes s→gg and gg→s on strange hadron production in pp collisions at RHIC and LHC energies

The rapidity densities at mid-rapidity and the transverse momentum distributions for strange hadrons produced in pp collisions are analyzed using the modified PACIAE model by considering the effect of inelastic (re)scattering processes s→gg and gg→s in parton (re)scattering. The calculated results of the transverse momentum spectra of the strangeness fitting with data measured by STAR and ALICE Collaborations can be improved, especially at large transverse momentum levels. This demonstrates that the effect of inelastic (re)scattering processes of s→gg and gg→s is not negligible at RHIC and LHC energy levels.     

Jet-Photon Production at RHIC and LHC

We calculate the production of prompt and thermal photons which includes the contribution of gluons in relativistic heavy ion collisions with the equilibrium and non-equilibrium quark-gluon plasma. We develop a new thermal jet-photon conversion mechanism which plays a vital role in the low transverse momentum region. The effect of the non-equilibrium quark-gluon plasma enhances the contribution of the thermal photons. The shadowing and iso-spin of the nucleus which can properly estimate the prompt photon production are also considered in our calculation.     

色玻璃凝聚近似下极端相对论重离子碰撞中的双轻子和光子产生(英文)

在色玻璃凝聚胶子饱和框架下,研究了相对论重离子碰撞中的双轻子和光子产生。在胶子饱和区域,在微扰近似(kT-因子化近似) 下低转移动量双轻子和光子的主要产生机制是胶子-胶子相互作用。在RHIC 和LHC 能量区域的相对论重离子碰撞中,饱和动量的值远远大于量子色动力学禁闭标度ΛQCD,这使得αs?1。此时,当转移动量小于饱和动量Qs 时质子和原子核的胶子密度值就会很高,双轻子和光子的不变产生横截面会由于饱效应而得到增强。数值结果给出在RHIC 和LHC 能量区域的pp, pA 和AA碰撞中,来源于色玻璃凝聚的低转移动量双轻子和光子产生贡献是显著的。We investigate inclusive dileptons and photons production in relativistic heavy ion collisions based on the idea of gluon saturation in the color glass condensate (CGC) framework. In the gluon saturation region, the dominant mechanism for low-pT dileptons and photons production in the perturbative approach (the kT-factorization approach) is gluon-gluon interaction. At Relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) energies, the value of saturation momentum becomes larger than the Quantum Chromodynamics (QCD) confinement scale ΛQCD for relativistic heavy ion collisions, which implies that αs?1. In this state, the gluon density for proton and nucleus with transverse momentum less than the saturation momentum Qs will reach a high value, and the invariant cross-section for dileptons and direct photons is further enhanced by saturation effects. The numerical results indicate that the production of low-pT dileptons and photons from the color glass condensate becomes prominent in pp, pA, and AA collisions at RHIC and LHC  Energies.     

在RHIC-PHENIX中(s_(NN))~(1/2)=200GeV Au Au碰撞中直接光子的椭圆流测量(英文)

Azimuthal anisotropy of direct photon is measured in S_(NN)~(1/2)=200GeV Au Au collisions at RHIC- PHENIX.Direct photon is one of the most effective probes to study properties of hot dence medium at initial state(also QGP state)of heavy ion collisions because photons almost do not interact strongly with any other particles caused by its long mean free path and they keep their conditions when they axe created.Within statistical and systematic errors,the elliptic flow parameter(v_2)of direct photon is consistent with zero. Direct photon v_2 is estimated by hadron decay photon contamination are subtracted from inclusive photon v_2 in intermediate to high transverse momentum(p_T)region(0 to 10GeV/c)for 3 centrality selections(20% steps)and minimum bias.     

Rayleigh-like distribution of particle transverse momenta in collisions at high energies

The transverse momentum distributions of final-state particles produced in collisions at high en-ergies are studied by using a two-component Rayleigh-like distribution.This representation is based on Liu＇s multisource ideal gas model which describes protons and fragments in high energy nucleus-nucleus collisions.The calculated results are in good agreement with the experimental data of Au-Au,Cu-Cu,d-Au,and pp collisions at the relativistic heavy ion collider energies.The experimental particle momentum distributions of p-Be collisions at 6.4,12.3,and 17.5 GeV/c,as well as Au-Au collisions at 1.5 AGeV are well described by a model based on a single Rayleigh-like distribution of particle transverse momenta.     

Exploring J/ψ Production Mechanism at the Future Electron-Ion Collider

We propose to use transverse momentum p_T distribution of J/ψ production at the future Electron Ion Collider(EIC) to explore the production mechanism of heavy quarkonia in high energy collisions.We apply QCD and QED collinear factorization to the production of a cc pair at high p_T,and non-relativistic QCD factorization to the hadronization of the pair to a J/ψ.We evaluate J/ψ p_T-distribution at both leading and next-to-leading order in strong coupling,and show that production rates for various color-spin channels of a cc pair in electronhadron collisions are very different from that in hadron-hadron collisions,which provides a strong discriminative power to determine various transition rates for the pair to become a J/ψ.We predict that the J/ψ produced in electron-hadron collisions is likely unpolarized,and the production is an ideal probe for gluon distribution of colliding hadron(or nucleus).We find that the J/ψ production is dominated by the color-octet channel,providing an excellent probe to explore the gluon medium in large nuclei at the EIC.     

Large transverse momentum lepton pair production at the Large Hadron Collider

We study the large transverse momentum distribution of lepton pairs produced in heavy-ion collisions, making use of the perturbative QCD. Referring to the calculation of the parton-parton production process into lepton pairs at the Relativistic Heavy Ion Collider (RHIC), the production of lepton pairs at large transverse momentum is extended to the Large Hadron Collider (LHC). The contribution of the parton-parton production process into lepton pairs in Pb-Pb collisions at the LHC is calculated, including the complete processes at large transverse momentum. Lepton pair production with the direct single photon process and the resolved single photon process are considered and confirmed to be significant at the LHC.     

Tau lepton production in ep collisions at HERA

The production of tau leptons in ep collisions is investigated using data recorded by the H1 detector at HERA in the period 1994–2000. Tau leptons are identified by detecting their decay products, using leptonic and hadronic decay modes. The cross section for the production of tau lepton pairs is measured for the first time at HERA. Furthermore, a search for events with an energetic isolated tau lepton and with large missing transverse momentum is performed. The results are found to be in agreement with the Standard Model predictions.     

Photoproduction of large transverse momentum mesons and production of large-pT photons and leptons in proton-proton collisions

The photoproduction of large transverse momentum mesons is examined in a leading particle model using the techniques of the covariant parton model. In the leading particle model the large-p_T meson is produced in a high-energy, wide-angle scattering: γ + q → M + q. Numerical results such as inclusive cross sectioons, angular distributions and particle ratios are presented for the pseudoscalar mesons. The Landshoff-Polking-horne quark-fusion model is then applied to the production of large transverse momentum photons and leptons in proton-proton collisions. In the lepton case this model differs from the usual Drell-Yan mechanism in that the lepton pair has a small invariant mass but comes from a large-p_T virtual photon.     

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.     

Angular distribution of lepton pairs in perturbative QCD

We compute to first order in perturbative QCD the decay angular distribution of the lepton pairs produced in high energy hadronic collisions. The results, integrated over the transverse momentum of the pair, are found in reasonable agreement with experimental data.     

Search for Down-Type Fourth Generation Quarks with the ATLAS Detector in Events with One Lepton and Hadronically Decaying W Bosons

This Letter presents a search for pair production of heavy down-type quarks decaying via b^{'}→Wt in the lepton+jets channel, as b^{'}b[over ˉ]^{'}→W^{-}tW^{+}t[over ˉ]→bb[over ˉ]W^{+}W^{-}W^{+}W^{-}→l^{±}νbb[over ˉ]qq[over ˉ]qq[over ˉ]qq[over ˉ]. In addition to requiring exactly one lepton, large missing transverse momentum, and at least six jets, the invariant mass of nearby jet pairs is used to identify high transverse momentum W bosons. In data corresponding to an integrated luminosity of 1.04 fb^{-1} from pp collisions at sqrt[s]=7 TeV recorded with the ATLAS detector, a heavy down-type quark with mass less than 480?GeV can be excluded at the 95% confidence level.     

在RHIC和LHC中光子-核子碰撞产生的大横动量双轻子

研究了RHIC和LHC能区的Au-Au和Pb-Pb周边重离子碰撞中，来自光子-核子相互作用产生的大横动量双轻子。利用微扰QCD 参数化和Weizs?cker-Williams近似计算了双轻子的产率。经过与领头阶和碎裂过程的双轻子产生数值计算结果相比较，光-核碰撞过程产生的大横动量双轻子在RHIC能区是可忽略的。但是在LHC能区，光-核碰撞在大横动量区域是周边重离子碰撞的一个重要的双轻子源。     

Processes with heavy ion collisions

Some Coulomb effects in heavy ion collisions are considered. Among them the process of muon and lepton pair creation, Coulomb and unitary corrections, statistics of multiple pair production. Effects of multiple photon exchange in process of lepton pair production by linearly polarized photon on nuclei are considered. This process is used for measuring the polarization of initial photon. Relativistic muon energy loss due to the light lepton pair production in the Coulomb field are calculated. Also we consider the effects of multiple photon exchange in elastic lepton scattering on unscreened atomic field and discuss the possible experimental testing.     

Probing gluonic spin-orbit correlations in photon pair production

We consider photon pair production in hadronic collisions at large mass and small transverse momentum of the pair, assuming that factorization in terms of transverse-momentum dependent parton distributions applies. The unpolarized cross section is found to have azimuthal angular dependencies that are generated by a gluonic version of the Boer-Mulders function. In addition, the single transversely polarized cross section is sensitive to the gluon Sivers function. We present simple numerical estimates for the Boer-Mulders and Sivers effects in diphoton production at RHIC and find that the process would offer unique opportunities for exploring transverse-momentum dependent gluon distributions.