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Register a new userLepton Pair Production Through Two Photon Process in Heavy Ion Collisions
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This paper investigates the electromagnetic production of lepton pairs with low transverse momentum in relativistic heavy ion collisions. We estimate the initial photons transverse momentum contributions by employing models where the average transverse momentum squared of the incoming photon can be calculated in the equivalent photon approximation. We further derive an all order QED resummation for the soft photon radiation, which gives an excellent description of the ATLAS data in ultra-peripheral collisions at the LHC. For peripheral and central collisions, additional $p_T$-broadening effects from multiple interaction with the medium and the magnetic field contributions from the quark-gluon plasma are also discussed.
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We investigate the azimuthal angular correlation between the lepton transverse momentum $P_perp$ and the impact parameter $b_perp$ in non-central heavy-ion collisions, where the leptons are produced through two-photon scattering. Among the Fourier harmonic coefficients, a significant $v_4$ asymmetry is found for the typical kinematics at RHIC and LHC with a mild dependence on the $P_perp$, whereas $v_2$ is power suppressed by the lepton mass over $P_perp$. This unique prediction, if confirmed from the experiments, shall provide crucial information on the production mechanism for the dilepton in two-photon processes.
Hadron spectroscopy provides direct physical measurements that shed light on the non-perturbative behavior of quantum chromodynamics (QCD). In particular, various exotic hadrons such as the newly observed $T_{cc}^+$ by the LHCb collaboration, offer unique insights on the QCD dynamics in hadron structures. In this letter, we demonstrate how heavy ion collisions can serve as a powerful venue for hadron spectroscopy study of doubly charmed exotic hadrons by virtue of the extremely charm-rich environment created in such collisions. The yields of $T_{cc}^+$ as well as its potential isospin partners are computed within the molecular picture for Pb-Pb collisions at center-of-mass energy $2.76~mathrm{TeV}$. We find about three-order-of-magnitude enhancement in the production of $T_{cc}^+$ in Pb-Pb collisions as compared with the yield in proton-proton collisions, with a moderately smaller enhancement in the yields of the isospin partners $T_{cc}^0$ and $T_{cc}^{++}$. The $T_{cc}^+$ yield is comparable to that of the $X(3872)$ in the most central collisions while shows a considerably stronger decrease toward peripheral collisions, due to a threshold effect of the required double charm quarks for $T_{cc}^+$. Final results for their rapidity and transverse momentum $p_T$ dependence as well as the elliptic flow coefficient are reported and can be tested by future experimental measurements.
We study the charged particle and transverse energy production mechanism from AGS, SPS, RHIC to LHC energies in the framework of nucleon and quark participants. At RHIC and LHC energies, the number of nucleons-normalized charged particle and transverse energy density in pseudorapidity, which shows a monotonic rise with centrality, turns out to be an almost centrality independent scaling behaviour when normalized to the number of participant quarks. A universal function which is a combination of logarithmic and power-law, describes well the charged particle and transverse energy production both at nucleon and quark participant level for the whole range of collision energies. Energy dependent production mechanisms are discussed both for nucleonic and partonic level. Predictions are made for the pseudorapidity densities of transverse energy, charged particle multiplicity and their ratio (the barometric observable, $frac{dE_{rm{T}}/deta}{dN_{rm{ch}}/deta} ~equiv frac{E_{rm{T}}}{N_{rm{ch}}}$) at mid-rapidity for Pb+Pb collisions at $sqrt{s_{rm{NN}}}=5.5$ TeV. A comparison with models based on gluon saturation and statistical hadron gas is made for the energy dependence of $frac{E_{rm{T}}}{N_{rm{ch}}}$.
We provide a simple derivation for particle production in heavy-ion collisions that is proportional to the rate of entropy production. We find that the particle production depends only on the power of the centre-of-mass collision energy $sqrt{s_{rm NN}}$ and the effective phase-space/volume (e.g. geometry of the collision approximated by the number of nucleons participating in the collision $N_{rm part}$). We show that at low-energies the pseudo-rapidity density of particles per participating nucleon pair scales linearly with $sqrt{s_{rm NN}}$ while at high-energies with $sqrt{s_{rm NN}}^{1/3}$. The $sqrt{s_{rm NN}}^{1/3}$ region is directly related to sub-nucleon degrees of freedom and creation of a quark-gluon plasma (QGP). This picture explains experimental observation that the shape of the distributions of pseudorapidity-density per nucleon pair of charged particles does not depend on $sqrt{s_{rm NN}}$ over a large span of collision energies. We provide an explanation of the scaling and connect it with the maximum rate per unit time of entropy production. We conclude with remarks on the hadron-parton phase transition. In particular, our considerations suggest that the pseudo-rapitidy density of the produced particles per $N_{rm part}/2$ larger than approximately 1 (excluding particles from jet fragmentation) is a signature of a QGP formation.
We model effects of color fluctuations (CFs) in the light-cone photon wave function and for the first time make predictions for the distribution over the number of wounded nucleons $ u$ in the inelastic photon-nucleus scattering. We show that CFs lead to a dramatic enhancement of this distribution at $ u=1$ and large $ u > 10$. We also study the implications of different scales and CFs in the photon wave function on the total transverse energy $Sigma E_T$ and other observables in inelastic $gamma A$ scattering with different triggers. Our predictions can be tested in proton-nucleus and nucleus-nucleus ultraperipheral collisions at the LHC and will help to map CFs, whose first indications have already been observed at the LHC.
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