Temperature Dependence Of Energy Loss Research Articles

Boltzmann dynamics and temperature dependence of energy loss: Towards an understanding of the RAA and v2 puzzle for D-Mesons

The two key observables related to heavy quarks that have been measured in experiments are the nuclear suppression factor RAA and the elliptic flow v2. Reproducing these two observables simultaneously is a puzzle which have challenged all the existing models. We have studied two ingredients responsible to address a large part of such a puzzle: the temperature dependence of the energy loss and the full solution of the Boltzmann collision integral for the scattering between the heavy quarks and the particle of the bulk. We have considered four different models to evaluate the temperature dependence of drag and diffusion coefficients of the heavy quark. All these four different models are set to reproduce the same RAA(pT) measured in experiments at RHIC and LHC energy. We have found that for the same RAA(pT) one can generate 2-3 times more v2 depending on the temperature dependence of the heavy quarks drag coefficient. Moreover comparing the Fokker-Planck and the Boltzmann approach we have found that the latter seems more efficient into reproducing the elliptic flow for the same RAA. Even a larger difference between the two approaches emerges from the comparison of a more differential observable, the angular correlation.

Sensitivity of the jet quenching observables to the temperature dependence of the energy loss

The quenching of minijets (particles with ${p}_{T}\ensuremath{\gg}T,{\ensuremath{\Lambda}}_{\mathrm{QCD}}$) in ultrarelativistic heavy-ion collisions has been one of the main predictions and discoveries at the BNL Relativistic Heavy Ion Collider. We analyze the correlation between different observables like the nuclear modification factor ${R}_{\mathit{AA}}({p}_{T})$, the elliptic flow, and the ratio of quark to gluon suppressions. We show that the temperature (or entropy density) dependence of the in-medium energy loss strongly affects the relation among these observables. In particular, the large elliptic flow and the nearly equal ${R}_{\mathit{AA}}({p}_{T})$ of quarks and gluons can be accounted for only if the energy loss occurs mainly around ${T}_{c}$ and the $q\ensuremath{\leftrightarrow}g$ conversion is significant. The use of an equation of state fitted to lattice QCD calculations, slowing down the cooling as $T\ensuremath{\rightarrow}{T}_{c}$, seems to contribute to both the enhancement of ${v}_{2}$ and the efficiency of the conversion mechanism.

Vibrational Relaxation and Bond Dissociation of Excited Methylpyrazine in the Collision with HF

Vibrational relaxation and competitive C-<TEX>$H_{methyl}$</TEX> and C-<TEX>$H_{ring}$</TEX> bond dissociations in vibrationally excited methylpyrazine in the collision with HF have been studied by use of classical trajectory procedures. The energy lost by the vibrationally excited methylpyrazine upon collision is not large and it increases slowly with increasing total vibrational energy content between 20,000 and 45,000 <TEX>$cm^{-1}$</TEX>. Above the energy content of 45,000 <TEX>$cm^{-1}$</TEX>, however, energy loss decreases. The temperature dependence of energy loss is negligible between 200 and 400 K, but above 45,000 <TEX>$cm^{-1}$</TEX> the energy loss increases as the temperature is raised. Energy transfer to or from the excited methyl C-H bond occurs in strong collisions with HF, that is, relatively large amount of translational energy is transferred in a single step. On the other hand, energy transfer to the ring C-H bond occurs in a series of small steps. When the total energy content ET of methylpyrazine is sufficiently high, either or both C-H bonds can dissociate. The C-<TEX>$H_{methyl}$</TEX> dissociation probability is higher than the C-<TEX>$H_{ring}$</TEX> dissociation probability. The dissociation of the ring C-H bond is not the result of the direct intermolecular energy flow from the direct collision between the ring C-H and HF but the result of the intramolecular flow of energy from the methyl group to the ring C-H stretch.

Intramolecular Energy Flow and Bond Dissociation in the Collision between Vibrationally Excited Toluene and HF

Intramolecular energy flow and C-H methyl and C-H ring bond dissociations in vibrationally excited toluene in the collision with HF have been studied by use of classical trajectory procedures. The energy lost by the vibrationally excited toluene upon collision is not large and it increases slowly with increasing total vibrational energy content between 20,000 and 45,000 cm -1 . Above the energy content of 45,000 cm -1 , however, energy loss decreases. Furthermore, in the highly excited toluene, toluene gains energy from incident HF. The temperature dependence of energy loss is negligible between 200 and 400 K. Energy transfer to or from the excited methyl C-H bond occurs in strong collisions with HF transferring relatively large amount of its translational energy (»k B T) in a single step, whereas energy transfer to the ring C-H bond occurs in a series of small steps. When the total energy content E T of toluene is sufficiently high, either C-H bond can dissociate. The C-H methyl dissociation probability is higher than the C-H ring dissociation probability. The dissociation of the ring C-H bond is not the result of the intermolecular energy flow from the direct collision between the ring C-H and HF but the intramolecular flow of energy from the methyl group to the ring C-H stretch. The C-H ring ···HF interaction is not important in transferring energy and in turn bond dissociation.

Nonequilibrium Statistical Thermodynamics of Two Dissipative Processes in Solids: Energy Losses of Channeled Particles at Low Velocities and Ultrasound Absorption by Conduction Electrons

We use methods of nonequilibrium statistical thermodynamics to investigate two dissipative processes in solids. We find the electron energy losses of a particle moving in a planar channeling regime and the sound absorption coefficient in metals under electron impurity scattering. The oscillator model is used to analyze the contribution from the effect of electron entrainment by a moving scattering center to the total dissipated energy. We investigate the frequency and temperature dependence of energy losses and also the dependence of the absorption coefficient on the sound wave vector.

Collision-induced intramolecular energy flow and C–H bond dissociation in excited toluene

The collision-induced intramolecular energy flow and C–H bond dissociation in toluene have been studied using classical dynamics procedures. The molecule initially contains high amounts of vibrational excitation in the methyl C–H stretch and the nearby benzene ring C–H stretch and it is in interaction with Ar. The two excited C–H stretches are coupled to each other through two C–C stretching, two H–C–C bending and one C–C–C bending modes, all of which are initially in the ground state. At 300 K, the energy lost by the excited molecule upon collision is not large and it increases slowly with increasing total vibrational energy content between 10 000 and 40 000 cm−1. Above the energy content of 40 000 cm−1, energy loss increases rapidly. Near 65 000 cm−1 energy loss takes a maximum value of about 1000 cm−1. The temperature dependence of energy loss is weak between 200 and 400 K. When the energy content is sufficiently high, either or both C–H bonds can dissociate, producing free radicals, C6H5CH2, C6H4CH3, or C6H4CH2. The ring C–H dissociation occurs almost entirely in a direct-mode mechanism on a subpicosecond time scale. Nearly half of methyl group C–H dissociation events occur on a subpicosecond time scale and the rest through a complex-mode collision in which bond dissociation occurs several picoseconds after the initial impact. In the complex-mode collision, Ar binds to the radical forming a weakly bound benzyl⋯Ar complex. In both dissociative and nondissociative events, intramolecular energy flow is efficient, taking place upon the initial impact on a subpicosecond time scale.

Inelastic losses of low-energy ions transmitted through thin films

Experimental results on inelastic losses by ions of energy 10 3−10 4 eV transmitted through thin solid films are presented and discussed. The C, Cu, Ag, and Ni films (from 35 to 900 Å thick) have been bombarded by He +, Ne + and Ar + ions. The angular and temperature dependence of energy losses of transmitted ions have been studied. Experimentally obtained values of inelastic losses appear to agree with the Lindhard-Scharff-Schott (LSS) theory. A step-like increase in inelastic energy losses has been observed for He + ions transmitted through the Ni films undergoing ferro- to paramagnetic phase transition.