arXiv subjects
Publications and source records attributed to B. G. Zakharov.
We study the radiative correction to $p_T$-broadening of a fast quark in a quark-gluon plasma beyond the soft gluon approximation. We find that the radiative processes can suppress considerably $p_T$-broadening. This differs dramatically from previous calculations to logarithmic accuracy in the soft gluon approximation, predicting a considerable enhancement of $p_T$-broadening.
We study the color randomization of two-gluon states produced after splitting of a primary fast gluon in the quark-gluon plasma. We find that for the LHC conditions the color randomization of the $gg$ pairs is rather slow. At jet energies $E=100$ and $500$ GeV, for typical jet path length in the plasma in central Pb+Pb collisions, the $SU(3)$-multiplet averaged color Casimir of the $gg$ pair differs considerably from its value $2N_c$ for a fully randomized $gg$ state. Our calculations of the energy dependence for generation of the nearly collinear decuplet $gg$ states, that can lead to the baryon jet fragmentation, show that the contribution of the anomalous decuplet color states to the baryon production should become small at $p_T\gtrsim 10$ GeV.
We study, within the Monte-Carlo Glauber model, centrality dependence of the midrapidity charged multiplicity density $dN_{ch}/dη$ and of the anisotropy coefficients $\varepsilon_{2,3}$ in Pb+Pb collisions at $\sqrt{s}=5.02$ TeV and in Xe+Xe collisions at $\sqrt{s}=5.44$ TeV. Calculations are performed for versions with and without nucleon meson cloud. The fraction of the binary collisions, $α$, has been fitted to the data on $dN_{ch}/dη$ in Pb+Pb collisions. We obtain $α\approx 0.09(0.13)$ with (without) meson cloud. The effect of meson cloud on the $dN_{ch}/dη$ is relatively small. For Xe+Xe collisions for $0$-$5$\% centrality bin we obtain $dN_{ch}/dη\approx 1149$ and $1134$ with and without meson cloud, respectively. We obtain $\varepsilon_2(\mbox{Xe})/\varepsilon_2(\mbox{Pb})\sim 1.45$ for most central collisions, and $\varepsilon_2(\mbox{Xe})/\varepsilon_2(\mbox{Pb})$ close to unity at $c\gtrsim 20$\%. We find a noticeable increase of the eccentricity in Xe+Xe collisions at small centralities due to the prolate shape of the Xe nucleus. The triangularity in Xe+Xe collisions is bigger than in Pb+Pb collisions at $c\lesssim 70$\%. We obtain $\varepsilon_3(\mbox{Xe})/\varepsilon_3(\mbox{Pb})\sim 1.3$ at $c\lesssim 1$\%.
The incorporation of running $α_s$ for the gluon emission vertex in calculations of radiative parton energy loss in a quark-gluon plasma is discussed. It is argued that the virtuality scale for running $α_s$ for induced gluon emission is determined by the square of the transverse momentum of the emitted gluon rather than by the square of the invariant mass of the final two-parton state often used in the literature.
We perform quantum calculations of fluctuations of the electromagnetic fields in $AA$ collisions at RHIC and LHC energies. We find that in the quantum picture the field fluctuations are much smaller than predictions of the classical Monte-Carlo simulation with the Woods-Saxon nuclear density.
We study the role of running coupling and the effect of variation of the thermal quark mass on contribution of the collinear bremsstrahlung and annihilation to photon emission in $AA$ collisions in a scheme similar to that used in our previous jet quenching analyses. We find that for a scenario with the thermal quark mass $m_q\sim 50-100$ MeV contribution of the higher order collinear processes summed with the $2\to 2$ processes can explain a considerable part ($\sim 50$\%) of the experimental photon spectrum at $k_T\sim 2-3$ GeV for Au+Au collisions at $\sqrt{s}=0.2$ TeV. But for $m_q=300$ MeV and for the thermal quark mass predicted by the HTL scheme the theoretical predictions underestimate considerably the experimental spectrum.
We study nuclear modification of the photon-tagged jets in $AA$ collisions within the jet quenching scheme based on the light-cone path integral approach to the induced gluon emission. The calculations are performed for running coupling. Collisional energy loss is treated as a perturbation to the radiative mechanism. We obtain a reasonable agreement with the recent data from the STAR Collaboration on the mid-rapidity nuclear modification factor $I_{AA}$ for Au+Au collisions at $\sqrt{s}=200$ GeV for parametrization of running $α_s$ consistent with that necessary for description of the data on suppression of the high-$p_T$ spectra.
We perform quantum calculations of fluctuations of the electromagnetic fields in $AA$ collisions at RHIC and LHC energies. The analysis is based on the fluctuation-dissipation theorem. We find that in the quantum picture the field fluctuations are very small. They turn out to be much smaller than the predictions of the classical Monte-Carlo simulation with the Woods-Saxon nuclear density.
We study the effect of the nucleon meson cloud on centrality dependence of the jet nuclear modification factor $R_{pA}$. We find that the meson-baryon Fock components may lead to a noticeable deviation of $R_{pA}$ from unity. Our results for $R_{pA}$ show the same tendency as that observed by ATLAS in $p+Pb$ collisions at $\sqrt{s}=5.02$ TeV. The meson cloud suppresses the central jet events and enhances the peripheral jet events. But quantitatively the effect is somewhat smaller than in the data.
We study the effect of the nucleon meson cloud on predictions of the Monte Carlo Glauber wounded nucleon model for $AA$, $pA$, and $pp$ collisions. From the analysis of the data on the charged multiplicity density in $AA$ collisions we find that the meson-baryon Fock component reduces the required fraction of binary collisions by a factor of $\sim 2$ for Au+Au collisions at $\sqrt{s}=0.2$ TeV and $\sim 1.5$ for Pb+Pb collisions at $\sqrt{s}=2.76$ TeV. For central $AA$ collisions the meson cloud can increase the multiplicity density by $\sim 16-18$\%. We give predictions for the midrapidity charged multiplicity density in Pb+Pb collisions at $\sqrt{s}=5.02$ TeV for the future LHC run 2. We find that the meson cloud has a weak effect on the centrality dependence of the ellipticity $ε_2$ in $AA$ collisions. For collisions of the deformed uranium nuclei at $\sqrt{s}=0.2$ TeV we find that the meson cloud may improve somewhat agreement with the data on the dependence of the elliptic flow on the charged multiplicity for very small centralities defined via the ZDCs signals. We find that the meson cloud may lead to a noticeable reduction of $ε_2$ and the size of the fireball in $pA$ and $pp$ collisions.
We develop a formalism for the photon emission from the quark-gluon plasma with an external electromagnetic field. We then use it to investigate the effect of magnetic field on the photon emission from the quark-gluon plasma created in $AA$ collisions. We find that even for very optimistic assumption on the magnitude of the magnetic field generated in $AA$ collisions its effect on the photon emission rate is practically negligible. For this reason the magnetic field cannot generate a significant azimuthal asymmetry in the photon spectrum.
We study the influence of the magnetic field on the photon emission from the quark-gluon plasma created in $AA$ collisions. We find that even for very optimistic assumption on the magnitude of the magnetic field for noncentral $AA$ collisions the effect of magnetic field is very small.
We study the influence of the meson cloud of the nucleon on predictions of the Monte Carlo Glauber model for the charged particle multiplicity density at mid-rapidity in $AA$ collisions. We find that for central $AA$ collisions the meson cloud can increase the multiplicity density by $\sim 16-18$\%. The meson-baryon Fock component reduces the required fraction of the binary collisions by a factor of $\sim 2$ for Au+Au collisions at $\sqrt{s}=0.2$ TeV and $\sim 1.5$ for Pb+Pb collisions at $\sqrt{s}=2.76$ TeV.
We study the flavor dependence of the medium modification factor $R_{pp}$ for $pp$ collisions for scenario with formation of a small-size quark-gluon plasma (QGP) for RHIC ($\sqrt{s}=0.2$ TeV) and LHC ($\sqrt{s}=2.76$ TeV) energies. We find that at $p_T\sim 10$ GeV the pion spectrum is suppressed by $\sim 20-30\,(25-35)$\% for RHIC (LHC), for $D$~($B$) mesons the suppression effect is smaller by a factor of $\sim 0.7-0.8$ ($0.5$). The flavor hierarchy $R_{pp}^π<R_{pp}^D<R_{pp}^B$ is held at $p_T\lesssim 20$ GeV for RHIC and at $p_T\lesssim 70$ GeV for LHC. This gives a sizeable reduction of the heavy-to-light ratios of the nuclear modification factors $R_{AA}$ as compared to that in the standard scenario without the QGP production in $pp$ collisions.
We study radiative parton energy loss in an expanding quark-gluon plasma with magnetic monopoles. We find that for realistic number density of thermal monopoles obtained in lattice simulations parton rescatterings on monopoles can considerably enhance energy loss for plasma produced in $AA$ collisions at RHIC and LHC energies. However, contrary to previous expectations, monopoles do not lead to the surface dominance of energy loss.
We study jet quenching in $pp$ and $pA$ collisions in the scenario with formation of a mini quark-gluon plasma. We find a significant suppression effect. For light hadrons at $p_{T}\sim 10$ GeV we obtained the reduction of the spectra by $\sim [20-30,25-35,30-40]$\% in $pp$ collisions at $\sqrt{s}=[0.2, 2.76,7]$ TeV. We discuss how jet quenching in $pp$ collisions may change the predictions for the nuclear modification factors in $AA$ collisions for light and heavy flavors. We also give predictions for modification of the photon-tagged and inclusive jet fragmentation functions in high multiplicity $pp$ events.
We study the electromagnetic response of the quark-gluon plasma in AA-collisions at RHIC and LHC energies for a realistic space-time evolution of the plasma fireball. We demonstrate that for a realistic electric conductivity the electromagnetic response of the plasma is in a quantum regime when the induced electric current does not generate a classical electromagnetic field, and can only lead to a rare emission of single photons.
We evaluate the medium suppression of light hadron spectra in $pp$ collisions at RHIC and LHC energies in the scenario with formation of a mini quark-gluon plasma. We find a significant suppression effect. For $p_{T}\sim 10$ GeV we obtained the reduction of the spectra by $\sim [20-30,25-35,30-40]$ % at $\sqrt{s}=[0.2, 2.76,7]$ TeV. We also discuss how this phenomenon may change the predictions for the nuclear modification factors for $AA$ and $pA$ collisions.