arXiv subjects

Baoyi Chen

Publications and source records attributed to Baoyi Chen.

67 records · Page 4Linked to original sources

arXiv2018

Nonplanar On-shell Diagrams and Leading Singularities of Scattering Amplitudes

Bipartite on-shell diagrams are the latest tool in constructing scattering amplitudes. In this paper we prove that a Britto-Cachazo-Feng-Witten (BCFW)-decomposable on-shell diagram process a rational top-form if and only if the algebraic ideal comprised of the geometrical constraints is shifted linearly during successive BCFW integrations. With a proper geometric interpretation of the constraints in the Grassmannian manifold, the rational top-form integration contours can thus be obtained, and understood, in a straightforward way. All rational top-form integrands of arbitrary higher loops leading singularities can therefore be derived recursively, as long as the corresponding on-shell diagram is BCFW-decomposable.

hep-th↗

arXiv2017

Charmonium Coherent Photoproduction and Hadroproduction with Effects of Quark Gluon Plasma

We study the charmonium coherent photoproduction and hadroproduction consistently with modifications from both cold and hot nuclear matters. The strong electromagnetic fields from fast moving nucleus interact with the other target nucleus, producing abundant charmonium in the extremely low transverse momentum region $p_T<0.1$ GeV/c. This results in significative enhancement of $J/ψ$ nuclear modification factor in semi-central and peripheral collisions. In the middle $p_T$ region such as $p_T<3\sim 5$ GeV/c, $J/ψ$ final yield is dominated by the combination process of single charm and anti-charm quarks moving in the deconfined matter, $c+\bar c\rightarrow J/ψ+g$. In the higher $p_T$ region, $J/ψ$ production are mainly from parton initial hard scatterings at the beginning of nucleus-nucleus collisions and decay of B hadrons. We include all of these production mechanisms and explain the experimental data well in different colliding centralities and transverse momentum regions.

nucl-th↗

arXiv2017

Strong Diffusion Effect of Charm Quarks on $J/ψ$ Production in Pb-Pb collisions at the LHC

We study the $J/ψ$ production based on coalescence model at $\sqrt{s_{NN}}$ = 2.76 and 5.02 TeV Pb-Pb collisions. With the colliding energy increasing from 2.76 TeV to 5.02 TeV, the number of charm pairs is enhanced by more than 50%. However, the ratio of $J/ψ$ inclusive nuclear modification factors $R^{5.02TeV}_{AA}/R^{2.76TeV}_{AA}$ is only about 1.1 $\sim$ 1.2. We find that the regeneration of $J/ψ$ is proportional to the densities of charm and anti-charm quarks, instead of their total numbers. The charm quark density is diluted by the strong expansion of quark gluon plasma, which suppresses the combination probability of heavy quarks and $J/ψ$ regeneration. This effect is more important in higher colliding energies where QGP expansion is strong. We also propose the ratio $N_{J/ψ}/(N_D)^2$ as a measurement of $c$ and $\bar c$ coalescence probability, which is only affected by the heavy quark diffusions in QGP, and does not depend on the inputs such as cold nuclear matter effects and cross sections of charm quark production. Further more, we give the predictions at the energy of Future Circular Collider ($\sqrt{s_{NN}}$ = 39 TeV).

nucl-th↗

arXiv2017

Separating metric perturbations in near-horizon extremal Kerr

Linear perturbation theory is a powerful toolkit for studying black hole spacetimes. However, the perturbation equations are hard to solve unless we can use separation of variables. In the Kerr spacetime, metric perturbations do not separate, but curvature perturbations do. The cost of curvature perturbations is a very complicated metric-reconstruction procedure. This procedure can be avoided using a symmetry-adapted choice of basis functions in highly symmetric spacetimes, such as near-horizon extremal Kerr. In this paper, we focus on this spacetime, and (i) construct the symmetry-adapted basis functions; (ii) show their orthogonality; and (iii) show that they lead to separation of variables of the scalar, Maxwell, and metric perturbation equations. This separation turns the system of partial differential equations into one of ordinary differential equations over a compact domain, the polar angle.

gr-qc↗

arXiv2017

Bottomonium Continuous Production from Unequilibrium Bottom Quarks in Ultrarelativistic Heavy Ion Collisions

We employ the Langevin equation and Wigner function to describe the bottom equark dynamical evolutions and their formation into a bound state in the expanding Quark Gluon Plasma (QGP). The additional suppressions from parton inelastic scatterings are supplemented in the regenerated bottomonium. Hot medium modifications on $Υ(1S)$ properties are studied consistently by taking the bottomonium potential to be the color-screened potential from Lattice results, which affects both $Υ(1S)$ regeneration and dissociation rates. Finally, we calculated the $Υ(1S)$ nuclear modification factor $R_{AA}^{\rm rege}$ from bottom quark combination with different diffusion coefficients in Langevin equation, representing different thermalization of bottom quarks. In the central Pb-Pb collisions (b=0) at $\sqrt{s_{NN}}=5.02$ TeV, we find a non-negligible $Υ(1S)$ regeneration, and it is small in the minimum bias centrality. The connections between bottomonium regeneration and bottom quark energy loss in the heavy ion collisions are also discussed.

nucl-th↗

arXiv2017

Elliptic flow as a probe for $ψ(2S)$ production mechanism in relativistic heavy ion collisions

I discuss the elliptic flows of $ψ(2S)$ with different production mechanisms in $\sqrt{s_{NN}}=2.76$ TeV Pb-Pb collisions. If the final $ψ(2S)$s are mainly from the recombination of uncorrelated charm and anticharm quarks at $T\approx T_c$, charm and anticharm quarks will carry large collective flows of the bulk medium, which will be inherited by the regenerated $ψ(2S)$s. This indicates a larger elliptic flow of $ψ(2S)$ than that of $J/ψ$ which can be regenerated at $T\ge T_c$, $v_2^{ψ(2S)}>v_2^{J/ψ}$. However, if the final $ψ(2S)$s are mainly from the transitions of $J/ψ\rightarrow ψ(2S)$ caused by the color screening of QGP, its elliptic flow should be close to the elliptic flow of $J/ψ$, $v_2^{ψ(2S)}\sim v_2^{J/ψ}$. Therefore, $ψ(2S)$ elliptic flow is a sensitive probe for its production mechanisms in relativistic heavy ion collisions.

nucl-th↗

arXiv2016

Internal Charmonium Evolution in the Quark-Gluon Plasma

We employ a time-dependent Schrödinger equation to study the evolution of a $c\bar c$ dipole in a quark-gluon plasma (QGP). Medium effects on the heavy-quark potential in the QGP are found to significantly affect the timescales of the internal evolution of the dipole. Color-screening can enhance the overlap of the expanding wavepackage with excited states at high temperature, while it is reduced at lower temperatures where the dipole favors the formation of the charmonium ground state. We investigate the consequences of this mechanism on the double ratio of charmonium nuclear modification factors, $R_{AA}^{ψ(2S)}/R_{AA}^{J/ψ}$, in heavy-ion collisions. The impact of the transition mechanisms on this ratio turns out to be rather sensitive to the attractive strength of the potential, and to its temperature dependence.

nucl-th↗

arXiv2016

Cold and Hot Nuclear Matter Effects on Charmonium Production in p+Pb Collisions at LHC Energy

We study cold and hot nuclear matter effects on charmonium production in p+Pb collisions at $\sqrt{s_\text{NN}}=5.02$ TeV in a transport approach. At the forward rapidity, the cold medium effect on all the $c\bar c$ states and the hot medium effect on the excited $c\bar c$ states only can explain well the $J/ψ$ and $ψ'$ yield and transverse momentum distribution measured by the ALICE collaboration, and we predict a significantly larger $ψ'$ $p_\text{T}$ broadening in comparison with $J/ψ$. However, we can not reproduce the $J/ψ$ and $ψ'$ data at the backward rapidity with reasonable cold and hot medium effects.

nucl-th↗

arXiv2016

Detailed Rapidity Dependence of $J/ψ$ Production at LHC

$J/ψ$ suppression with rapidity is studied with both (3+1) dimensional transport equation and hydrodynamics in details at $\sqrt{s_{NN}}=2.76$ TeV Pb+Pb collisions. In central rapidity region of the hot medium, $J/ψ$ nuclear modification factor shows weak rapidity dependence due to the boost invariance of the medium. But in forward rapidity region for entire transverse momentum bin, $J/ψ$ regeneration is strongly suppressed due to the decreasing charm pair cross section. This makes $J/ψ$ $R_{AA}(y)$ decrease with rapidity. The ratio $r_{AA}$ of mean transverse momentum square in AA collisions to that in pp collisions increases strongly in forward rapidity region and is more sensitive to the $J/ψ$ production mechanisms.

hep-ph↗

arXiv2015

Charmonium Production with QGP and Hadron Gas Effects at SPS and FAIR

The production of charmonium in heavy-ion collisions is investigated based on Boltzmann-type transport model for charmonium evolution and langevin equation for charm quark evolution. Charmonium suppression and regeneration in both quark-gluon plasma (QGP) and hadron phase are considered. Charm quarks are far from thermalization, and regeneration of charmonium in QGP and hadron gas is neglectable at SPS and FAIR. At peripheral collisions, charmonium suppression with hadron gas explains the experimental data well. But at central collisions, additional suppression from deconfined matter (QGP) is necessary for the data. This means there should be QGP produced at central collisions, and no QGP produced at peripheral collisions at SPS energy. Predictions are also made at FAIR $\sqrt{s_{NN}}=7.7$ GeV Au+Au collisions.

hep-ph↗

arXiv2013

ψ^\prime Production and B Decay in Heavy Ion Collisions at LHC

In comparison with J/ψ, the excited charmonium state ψ^\prime is loosely bounded and its yield is dominantly from the B-hadron decay. Based on the transport approach, we study the double ratio of N(ψ^\prime)/N(J/ψ) from A+A collisions to that from p+p collisions at LHC energy. We found that the primordial production in the initial stage and the regeneration in the hot medium are not significant for ψ^\prime production in heavy ion collisions at LHC and the double ratio in semi-central and central collisions is controlled by the B decay.

nucl-th↗

arXiv2012

Mean Field Effect on J/ψProduction in Heavy Ion Collisions

The mean field effect in quark-gluon plasma on J/ψproduction in heavy ion collisions is perturbatively calculated in a transport approach. While the global nuclear modification factor R_{AA} is not sensitive to the mean field, the reduced threshold for J/ψregeneration leads to a significant enhancement of R_{AA} at low transverse momentum at RHIC and LHC energies.

nucl-th↗

arXiv2011

Upsilon Production as a Probe for Early State Dynamics in High Energy Nuclear Collisions at RHIC

Upsilon production in heavy ion collisions at RHIC energy is investigated. While the transverse momentum spectra of the ground state Upsilon(1s) are controlled by the initial state Cronin effect, the excited $b\bar b$ states are characterized by the competition between the cold and hot nuclear matter effects and sensitive to the dissociation temperatures determined by the heavy quark potential. We emphasize that it is necessary to measure the excited heavy quark states in order to extract the early stage information in high energy nuclear collisions at RHIC.

nucl-th↗