Collisional Energy Loss Research Articles

SU‐E‐I‐56: Computational Internal Dosimetry Methods as Applied to the UF Series of Hybrid Phantoms

Purpose: To compute specific absorbed fractions for monoenergetic photons and electrons for all UF hybrid ICRP‐reference phantoms. Ultimately, radionuclide S values will be calculated for all radionuclides of interest in nuclear medicine. Methods: Voxelized versions of the UF newborn hybrid computational phantoms were used in conjunction with the radiation transport code MCNPX v2.6 to determine the absorbed fraction of energy per unit mass (SAF) for a variety of source‐target organ combinations for 21 photon and electron energies. A method known as a fluence‐to‐dose response function was used, for the first time in a model of the newborn child, to determine the absorbed dose to active marrow and total shallow marrow, the radiosensitive tissues of the skeleton. A novel method for minimizing the poor statistics associated with electron transport was also introduced. First, a simulation was completed which only tracked primary electrons, giving the dose contribution solely attributed to collisional energy losses. Next, a simulation was completed which recorded the initial energy of any photons produced by the primary electrons. Monoenergetic photon SAFs, determined previously, were then weighted according to each monoenergetic electron bremsstrahlung energy spectrum so as to procure low uncertainty photon dose from the primary electrons. Results: The novel method for electron dosimetry was benchmarked and found to be an accurate and effective way of reducing the uncertainty associated with electron cross‐dose. S values were generated for Tc‐99m and compared to estimates provided by OLINDA/EXM 1.0. Noticeable differences were seen for both self‐dose and cross‐dose scenarios. Conclusion: The work performed represents a state‐of‐the‐art internal dosimetry model, and once dosimetry has been performed on the entire UF series of phantoms, the results will be incorporated into an online accessible software package which can be of use in imaging and dose optimization for pediatric nuclear medicine patients.

Heavy quark energy loss in proton proton collisions at LHC

A promising probe to study deconfined matter created in high energy nuclear collisions is the energy loss of (heavy) quarks. Experiments at the Relativistic Heavy Ion Collider (RHIC) have shown that heavy quarks, i.e. charm and bottom quarks show a remarkable high momentum suppression, comparable to light quarks. In this exploratory study we investigate the energy loss of heavy quarks in high multiplicity proton proton collisions at LHC energies. We will find a small, however non-negligible energy loss of high momentum charm quarks.

STAR: Jet reconstruction, direct γ and multi-hadron correlations : Hard Probes of the initial and final state

Recent results from the STAR collaboration focusing on hard probes of the initial and final state are presented. Correlations at forward rapidities in d+Au collisions at low x are utilized to study the possible onset of saturation effects at RHIC energies. Reference measurements of fully reconstructed jets in d+Au collisions as well as jet quantities in p+p collisions will be discussed. In addition, jet-hadron, direct γ-hadron correlations and results from full-jet reconstruction in Au+Au collisions will be presented and discussed with respect to the p+p reference measurements. These measurements can be used to put further constraints on the underlying mechanisms of partonic energy loss in heavy-ion collisions at RHIC.

Color decoherence in in-medium QCD cascades

The talk, based on [1], analyzes the consequences of the combined effect of the consequences of the assumption that the effects of quantum coherence and the resulting angular ordering in QCD cascades are disrupted within the finite-sized hot fireball created in ultrarelativistic heavy ion collisions and of the collisional energy losses of the cascade gluons and final hadronic clusters.

Effect of running coupling on photons from jet–plasma interaction in relativistic heavy-ion collisions

We discuss the role of collisional energy loss on high pT photon data measured by PHENIX collaboration by calculating photon yield in jet–plasma interaction. The phase space distribution of the participating jet is dynamically evolved by solving the Fokker–Planck equation. We treat the strong coupling constant (αs) as a function of momentum and temperature while calculating the drag and diffusion coefficients. It is observed that the quenching factor is substantially modified as compared to the case when αs is taken as constant. It is shown that the data are reasonably well reproduced when contributions from all the relevant sources are taken into account. Predictions at higher beam energies relevant for LHC experiment have been made.

The evolution from quasi-elastic to deep inelastic processes and its connection to fusion

Substantial experimental and theoretical efforts have been made in the last decade to study into detail the transition from quasi-elastic to deep inelastic processes in heavy ion collisions. The understanding of this transition is relevant not only to probe which degrees of freedom play a role in the multinucleon transfer process but also for its connection with near barrier fusion reactions. A boost in the field has been given with the implementation of large solid angle magnetic spectrometers coupled to γ arrays with which extensive investigations have been carried out for nuclear structure and reaction dynamics. In the present paper aspects of these studies will be presented, with emphasis on processes undergoing significant energy loss in binary collisions.

Measurements of the total energy lost per electron–ion pair lost in low-pressure inductive argon, helium, oxygen and nitrogen discharge

Experimental measurements of the total energy lost per electron–ion pair lost, εT, were performed in a low-pressure inductive atomic gases (Ar, He) and molecular gases (O2, N2) discharge. The value of εT was determined from a power balance based on the electropositive global (volume-averaged) model. A floating harmonic method was employed to measure ion fluxes and electron temperatures at the discharge wall. In the pressure range 5–50 mTorr, it was found that the measured εT ranged from about 70 to 150 V for atomic gases, but from about 180 to 1300 V for molecular gases. This difference between atomic and molecular discharge is caused by additional collisional energy losses of molecular gases. For argon discharge, the stepwise ionization effect on εT was observed at relatively high pressures. For different gases, the measured εT was evaluated with respect to the electron temperature, and then compared with the calculation results, which were derived from collisional and kinetic energy loss. The measured εT and their calculations showed reasonable agreement.

Decoherence and energy loss in QCD cascades in nuclear collisions

The paper analyzes the consequences of the assumption that the effects of quantum coherence and the resulting angular ordering in QCD cascades are disrupted within the hot fireball created in ultrarelativistic heavy ion collisions. The effects of decoherence are taken into account by switching off angular ordering for the vertices of MC-generated cascade that lie within the fireball of some given size. We show that decoherence exerts a pronounced influence on the spatiotemporal pattern of a cascade. Related effects due to non-radiative losses of cascade gluons and those of final prehadrons are discussed. We show that decoherence leads to a substantial increase in the yield of cascade particles hadronizing inside the hot zone thus exerting a strong influence on the properties of final hadrons and amplifying the effects of collisional energy losses of cascade partons and final prehadrons that take place inside the fireball.

A stochastic, local mode study of neon–liquid surface collision dynamics

Equations of motion for a fast, light rare gas atom passing over a liquid surface are derived and used to infer the dynamics of neon collisions with squalane and perfluorinated polyether surfaces from experimental data. The equations incorporate the local mode model of a liquid surface via a stochastic process and explicitly account for impulsive collisional energy loss to the surface. The equations predict angular distributions for scattering of neon that are in good quantitative agreement with experimental data. Our key dynamical conclusions are that experimental angular distributions derive mainly from local mode surface topography rather than from structural features of individual surface molecules, and that the available data for these systems can be accounted for almost exclusively by single collisions between neon atoms and the liquid surface.

Photons from jet — plasma interaction in collisional energy loss scenario

We calculate photons from jet plasma interaction in a collisional energy loss scenario. It is shown that the PHENIX photon data is well reproduced when photons from initial hard collisions are taken into account.

Parton energy loss in heavy-ion collisions via direct-photon and charged-particle azimuthal correlations

Charged-particle spectra associated with direct photon ($\gamma_{dir} $) and $\pi^0$ are measured in $p$+$p$ and Au+Au collisions at center-of-mass energy $\sqrt{s_{_{NN}}}=200$ GeV with the STAR detector at RHIC. A hower-shape analysis is used to partially discriminate between $\gamma_{dir}$ and $\pi^0$. Assuming no associated charged particles in the $\gamma_{dir}$ direction (near side) and small contribution from fragmentation photons ($\gamma_{frag}$), the associated charged-particle yields opposite to $\gamma_{dir}$ (away side) are extracted. At mid-rapidity ($|\eta|<0.9$) in central Au+Au collisions, charged-particle yields associated with $\gamma_{dir}$ and $\pi^0$ at high transverse momentum ($8< p_{T}^{trig}<16$ GeV/$c$) are suppressed by a factor of 3-5 compared with $p$ + $p$ collisions. The observed suppression of the associated charged particles, in the kinematic range $|\eta|<1$ and $3< p_{T}^{assoc} < 16$ GeV/$c$, is similar for $\gamma_{dir}$ and $\pi^0$, and independent of the $\gamma_{dir}$ energy within uncertainties. These measurements indicate that the parton energy loss, in the covered kinematic range, is insensitive to the parton path length.

Photons from jet–plasma interaction in relativistic heavy ion collisions

We expose the role of collisional energy loss on high $p_T$ photon data measured by PHENIX collaboration by calculating photon yield in jet plasma interaction. The phase space distribution of the participating jet is dynamically evolved by solving Fokker-Planck equation. It is shown that the data is reasonably well reproduced when contributions from all the relevant sources are taken into account. Predictions at higher beam energies relevant for LHC experiment have been made.

Transport theoretical description of collisional energy loss in infinite quark–gluon matter

We study the time evolution of a high-momentum gluon or quark propagating through an infinite, thermalized, partonic medium utilizing a Boltzmann equation approach. We calculate the collisional energy loss of the parton, study its temperature and flavor dependence as well as the momentum broadening incurred through multiple interactions. Our transport calculations agree well with analytic calculations of collisional energy loss where available, but offer the unique opportunity to address the medium response as well in a consistent fashion.

Competition of heavy quark radiative and collisional energy loss in deconfined matter

We extend our recently advanced model on collisional energy loss of heavy quarks in a quark gluon plasma by including radiative energy loss. We discuss the approach and present first preliminary results, including a comparison of the role of both types of energy loss for experimental data. We draw the conclusion that the present nuclear modification factor RAA data on non-photonic single electrons do not permit us to ‘select’ between these two types of energy-loss mechanisms.

Overview on jet results from STAR

Full jet reconstruction allows access to the parton kinematics over a large energy domain and can be used to constrain the mechanisms of energy loss in heavy-ion collisions. Such measurements are challenging at RHIC, due to the high-multiplicity environments created in heavy-ion collisions. In these proceedings, we report an overview of the results on full jet reconstruction obtained by the STAR experiment. Jet measurements in 200 GeV p+p show that jets are calibrated pQCD probes and provide a baseline for jet measurements in Au+Au collisions. Inclusive differential jet production cross sections and ratios are reported for central 200 GeV Au+Au collisions and compared to p+p. We also present measurements of fully reconstructed di-jets at mid-rapidity, and compare spectra and fragmentation functions in p+p and central Au+Au collisions.

Measurement of the total energy losses per electron-ion lost in various mixed gas inductively coupled plasmas

The total energy loss per electron-ion lost was measured at various gases and mixtures (Ar, He, N2, O2, Ar/N2, Ar/O2, He/N2, and He/O2) in the pressure range of 5–50 mTorr in an inductively coupled plasma. To measure electron temperatures and ion fluxes at a chamber wall, the floating harmonics method was used. The absorbed power was determined by measuring antenna resistances and currents. The total energy losses were obtained from the power balance equation of a global model. In the case of Ar mixture plasma with molecular gas, the total energy loss decreased with fractional Ar flow rate. He mixture plasma decreased more than the decrease in total energy loss of Ar mixture plasma. These experimental results were compared and were consistent with average collisional energy loss.

A statistical approach to energy loss during gas–liquid collisions

A novel theoretical approach to collisional energy exchange during a gas–liquid collision is outlined for the case of a light, fast incoming atom. The method differs from the usual ballistic models of energy exchange in that the energy loss is statistically inferred from the eventual response of the liquid surface to the collision, rather than by directly considering the mechanics of energy transfer into the surface. With this approach, we obtain simple, accurate formulas for the average collisional energy loss and trapping probability of the incoming atom. We also present a graphical method for determining the well depth of the gas–liquid potential.

Peculiarities in charge symmetry of energy loss in ion-atom collisions

Systematic experimental studies on energy loss (−dE/dx) were performed for ions with nuclear charge Z = 2–18 in various gases. The energy range ∼0.001 to ∼1 MeV/amu was considered. Detailed analysis confirmed the oscillating character of the energy loss dependence on the ionic nuclear charge associated with the ions’ electron shell structure. The symmetry of the energy loss dependence on the nuclear charges of projectile Z and target Zt is examined.

Parton energy loss in a classical strongly coupled QGP

We investigate the energy loss of heavy quarks in the gas, liquid and solid phase of a classical quark–gluon plasma (cQGP) using molecular dynamics simulations. The model consists of massive quarks and gluons interacting as a classical non-relativistic colored Coulomb gas. We show that the electric force–force decorrelates on a short time scale causing the energy loss to be mostly diffusive and Langevin-like in the cQGP. We find that the drag coefficient changes with the heavy quark mass, while the diffusion constant does not. The fractional collisional energy loss is much larger than the leading order estimates from a wQGP (weakly coupled QGP) because of the core repulsion. Following recent suggestions, we show how the cQGP results can be translated to the sQGP (strongly coupled QGP) results in the T = ( 1 − 3 ) T c range.

Brownian motion in granular gases of viscoelastic particles

A theory is developed of Brownian motion in granular gases (systems of many macroscopic particles undergoing inelastic collisions), where the energy loss in inelastic collisions is determined by a restitution coefficient ɛ. Whereas previous studies used a simplified model with ɛ = const, the present analysis takes into account the dependence of the restitution coefficient on relative impact velocity. The granular temperature and the Brownian diffusion coefficient are calculated for a granular gas in the homogeneous cooling state and a gas driven by a thermostat force, and their variation with grain mass and size and the restitution coefficient is analyzed. Both equipartition principle and fluctuation-dissipation relations are found to break down. One manifestation of this behavior is a new phenomenon of “relative heating” of Brownian particles at the expense of cooling of the ambient granular gas.