6,644 research outputs found
The Z^0-tagged jet event asymmetry in heavy-ion collisions at the CERN Large Hadron Collider
Tagged jet measurements provide a promising experimental channel to quantify
the similarities and differences in the mechanisms of jet production in
proton-proton and nucleus-nucleus collisions. We present the first calculation
of the transverse momentum asymmetry of Z^0/gamma^*-tagged jet events in
sqrt{s}=2.76$ TeV reactions at the LHC. Our results combine the
O(G_F\alpha_s^2) perturbative cross sections with the radiative and collisional
processes that modify parton showers in the presence of dense QCD matter. We
find that a strong asymmetry is generated in central lead-lead reactions that
has little sensitivity to the fluctuations of the underlying soft hadronic
background. We present theoretical model predictions for its shape and
magnitude.Comment: 4 pages, 4 figures, as published in PR
Thermal field theory derivation of the source term induced by a fast parton from the quark energy-momentum tensor
I derive the distribution of energy and momentum transmitted from a fast
parton to a medium of thermalized quarks, or the source term, in perturbative
thermal field theory directly from the quark energy-momentum tensor. The fast
parton is coupled to the medium by adding an interaction term to the
Lagrangian. The thermal expectation value of the energy-momentum tensor source
term is then evaluated using standard Feynman rules at finite temperature. It
is found that local excitations, which are important for exciting an observable
Mach cone structure, fall sharply as a function of the energy of the fast
parton. This may have implications for the trigger dependence of
measurements of azimuthal dihadron particle correlations in heavy-ion
collisions. In particular, a conical emission pattern would be less likely to
be observed for increasing trigger . I show that the results presented in
this paper can be generalized to more realistic modeling of fast parton
propagation, such as through a time dependent interaction term, in future
studies.Comment: Version as accepted by Physical Review D. New version has several
clarifications and added references. 5 pages, 3 figure
Sonic Mach Cones Induced by Fast Partons in a Perturbative Quark-Gluon Plasma
We derive the space-time distribution of energy and momentum deposited by a
fast parton traversing a weakly coupled quark-gluon plasma by treating the fast
part on as the source of an external color field perturbing the medium. We then
use our result as a source term for the linearized hydrodynamical equations of
the medium. We show that the solution contains a sonic Mach cone and a
dissipative wake if the parton moves at a supersonic speed.Comment: Final version accepted for publicatio
Explicit generating functional for pions and virtual photons
We construct the explicit one-loop functional of chiral perturbation theory
for two light flavours, including virtual photons. We stick to contributions
where 1 or 2 mesons and at most one photon are running in the loops. With the
explicit functional at hand, the evaluation of the relevant Green functions
boils down to performing traces over the flavour matrices. For illustration, we
work out the pi+ pi- -> pi0 pi0 scattering amplitude at threshold at order p^4,
e^2p^2.Comment: 20 pages, 2 figures; version accepted for publication, minor
typographical changes, acknowledgments adde
Interstellar Hydrides
Interstellar hydrides -- that is, molecules containing a single heavy element
atom with one or more hydrogen atoms -- were among the first molecules detected
outside the solar system. They lie at the root of interstellar chemistry, being
among the first species to form in initially-atomic gas, along with molecular
hydrogen and its associated ions. Because the chemical pathways leading to the
formation of interstellar hydrides are relatively simple, the analysis of the
observed abundances is relatively straightforward and provides key information
about the environments where hydrides are found. Recent years have seen rapid
progress in our understanding of interstellar hydrides, thanks largely to
far-IR and submillimeter observations performed with the Herschel Space
Observatory. In this review, we will discuss observations of interstellar
hydrides, along with the advanced modeling approaches that have been used to
interpret them, and the unique information that has thereby been obtained.Comment: Accepted for publication in Annual Review of Astronomy and
Astrophysics 2016, Vol. 5
Convective shutdown in a porous medium at high Rayleigh number
Convection in a closed domain driven by a dense buoyancy source along the upper boundary soon starts to wane owing to the increase of the average interior density. In this paper, theoretical and numerical models are developed of the subsequent long period of shutdown of convection in a two-dimensional porous medium at high Rayleigh number Ra\mathit{Ra}. The aims of this paper are twofold. Firstly, the relationship between this slowly evolving ‘one-sided’ shutdown system and the statistically steady ‘two-sided’ Rayleigh–Bénard (RB) cell is investigated. Numerical measurements of the Nusselt number Nu\mathit{Nu} from an RB cell (Hewitt et al., Phys. Rev. Lett., vol. 108, 2012, 224503) are very well described by the simple parametrization Nu=2.75+0.0069Ra\mathit{Nu}= 2. 75+ 0. 0069\mathit{Ra}. This parametrization is used in theoretical box models of the one-sided shutdown system and found to give excellent agreement with high-resolution numerical simulations of this system. The dynamical structure of shutdown can also be accurately predicted by measurements from an RB cell. Results are presented for a general power-law equation of state. Secondly, these ideas are extended to model more complex physical systems, which comprise two fluid layers with an equation of state such that the solution that forms at the (moving) interface is more dense than either layer. The two fluids are either immiscible or miscible. Theoretical box models compare well with numerical simulations in the case of a flat interface between the fluids. Experimental results from a Hele-Shaw cell and numerical simulations both show that interfacial deformation can dramatically enhance the convective flux. The applicability of these results to the convective dissolution of geologically sequestered CO2{\mathrm{CO} }_{2} in a saline aquifer is discussed
Parton showers as sources of energy-momentum deposition in the QGP and their implication for shockwave formation at RHIC and at the LHC
We derive the distribution of energy and momentum transmitted from a primary
fast parton and its medium-induced bremsstrahlung gluons to a thermalized
quark-gluon plasma. Our calculation takes into account the important and thus
far neglected effects of quantum interference between the resulting color
currents. We use our result to obtain the rate at which energy is absorbed by
the medium as a function of time and find that the rate is modified by the
quantum interference between the primary parton and secondary gluons. This
Landau-Pomeranchuk-Migdal type interference persists for time scales relevant
to heavy ion phenomenology. We further couple the newly derived source of
energy and momentum deposition to linearized hydrodynamics to obtain the bulk
medium response to realistic parton propagation and splitting in the
quark-gluon plasma. We find that because of the characteristic large angle
in-medium gluon emission and the multiple sources of energy deposition in a
parton shower, formation of well defined Mach cones by energetic jets in heavy
ion reactions is not likely.Comment: 8 pages, 4 figure
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