2,507 research outputs found
Relating the description of gluon production in pA collisions and parton energy loss in AA collisions
We calculate the classical gluon field of a fast projectile passing through a
dense medium. We show that this allows us to calculate both the initial state
gluon production in proton-nucleus collisions and the final state gluon
radiation off a hard parton produced in nucleus-nucleus collisions. This
unified description of these two phenomena makes the relation between the
saturation scale and the transport coefficient more transparent.
Also, we discuss the validity of the eikonal approximation for gluon
propagation inside the nucleus in proton-nucleus collisions at RHIC energy.Comment: 18 pages, 3 figure
Elastic energy loss and longitudinal straggling of a hard jet
The elastic energy loss encountered by jets produced in deep-inelastic
scattering (DIS) off a large nucleus is studied in the collinear limit. In
close analogy to the case of (non-radiative) transverse momentum broadening,
which is dependent on the medium transport coefficient , a class of
medium enhanced higher twist operators which contribute to the non-radiative
loss of the forward light-cone momentum of the jet () are identified and
the leading correction in the limit of asymptotically high is isolated.
Based on these operator products, a new transport coefficient is
motivated which quantifies the energy loss per unit length encountered by the
hard jet. These operator products are then computed, explicitly, in the case of
a similar hard jet traversing a deconfined quark-gluon-plasma (QGP) in the
hard-thermal-loop (HTL) approximation. This is followed by an evaluation of
sub-leading contributions which are suppressed by the light-cone momentum
, which yields the longitudinal "straggling" i.e., a slight change in
light cone momentum due to the Brownian propagation through a medium with a
fluctuating color field.Comment: 5 pages, 1 figure, Revtex
Collinear Photon Emission from the Quark-Gluon Plasma: The Light-Cone Path Integral Formulation
We give a simple physical derivation of the photon emission rate from the
weakly coupled quark-gluon plasma connected with the collinear processes and . The analysis is based on the light-cone
path integral approach to the induced radiation. Our results agree with that by
Arnold, Moore and Yaffe obtained using the real-time thermal perturbation
theory. It is demonstrated that the solution of the AMY integral equation is
nothing but the time-integrated Green's function of the light-cone path
integral approach written in the momentum representation.Comment: 12 pages, 2 figure
Long-distance radiative corrections to the di-pion tau lepton decay
We evaluate the model-dependent piece of O(alpha) long-distance radiative
corrections to tau^- \to \pi^- \pi^0\nu_{\tau} decays by using a meson
dominance model. We find that these corrections to the di-pion invariant mass
spectrum are smaller than in previous calculations based on chiral perturbation
theory. The corresponding correction to the photon inclusive rate is tiny
(-0.15%) but it can be of relevance when new measurements reach better
precision.Comment: 4 pages, 2 figures. An estimate of the shift produced in the
evaluation of the h.v.p. contribution to the muon anomalous magnetic moment
is added. Version to appear in Phys. Rev.
Massive Cosmologies
We explore the cosmological solutions of a recently proposed extension of
General Relativity with a Lorentz-invariant mass term. We show that the same
constraint that removes the Boulware-Deser ghost in this theory also prohibits
the existence of homogeneous and isotropic cosmological solutions.
Nevertheless, within domains of the size of inverse graviton mass we find
approximately homogeneous and isotropic solutions that can well describe the
past and present of the Universe. At energy densities above a certain crossover
value, these solutions approximate the standard FRW evolution with great
accuracy. As the Universe evolves and density drops below the crossover value
the inhomogeneities become more and more pronounced. In the low density regime
each domain of the size of the inverse graviton mass has essentially non-FRW
cosmology. This scenario imposes an upper bound on the graviton mass, which we
roughly estimate to be an order of magnitude below the present-day value of the
Hubble parameter. The bound becomes especially restrictive if one utilizes an
exact self-accelerated solution that this theory offers. Although the above are
robust predictions of massive gravity with an explicit mass term, we point out
that if the mass parameter emerges from some additional scalar field
condensation, the constraint no longer forbids the homogeneous and isotropic
cosmologies. In the latter case, there will exist an extra light scalar field
at cosmological scales, which is screened by the Vainshtein mechanism at
shorter distances.Comment: 21 page
String Theory in Polar Coordinates and the Vanishing of the One-Loop Rindler Entropy
We analyze the string spectrum of flat space in polar coordinates, following
the small curvature limit of the cigar CFT. We first
analyze the partition function of the cigar itself, making some clarifications
of the structure of the spectrum that have escaped attention up to this point.
The superstring spectrum (type 0 and type II) is shown to exhibit an involution
symmetry, that survives the small curvature limit. We classify all marginal
states in polar coordinates for type II superstrings, with emphasis on their
links and their superconformal structure. This classification is confirmed by
an explicit large analysis of the partition function. Next we compare
three approaches towards the type II genus one entropy in Rindler space: using
a sum-over-fields strategy, using a Melvin model approach and finally using a
saddle point method on the cigar partition function. In each case we highlight
possible obstructions and motivate that the correct procedures yield a
vanishing result: . We finally discuss how the QFT UV divergences of the
fields in the spectrum disappear when computing the free energy and entropy
using Euclidean techniques.Comment: 58 pages + appendices, v2: typos corrected, matches published versio
Revisiting noninteracting string partition functions in Rindler space
We revisit non-interacting string partition functions in Rindler space by
summing over fields in the spectrum. In field theory, the total partition
function splits in a natural way in a piece that does not contain surface terms
and a piece consisting of solely the so-called edge states. For open strings,
we illustrate that surface contributions to the higher spin fields correspond
to open strings piercing the Rindler origin, unifying the higher spin surface
contributions in string language. For closed strings, we demonstrate that the
string partition function is not quite the same as the sum over the partition
functions of the fields in the spectrum: an infinite overcounting is present
for the latter. Next we study the partition functions obtained by excluding the
surface terms. Using recent results of JHEP 1505 (2015) 106, this construction,
first done by Emparan, can be put on much firmer ground. We generalize to type
II and heterotic superstrings and demonstrate modular invariance. All of these
exhibit an IR divergence that can be interpreted as a maximal acceleration
close to the black hole horizon. Ultimately, since these partition functions
are only part of the full story, divergences here should not be viewed as a
failure of string theory: maximal acceleration is a feature of a faulty
treatment of the higher spin fields in the string spectrum. We comment on the
relevance of this to Solodukhin's recent proposal. A possible link with the
firewall paradox is apparent.Comment: 33 pages, v2: added several clarifications including a section on the
difference between closed strings and the sum-of-fields approach, matches
published versio
Near-Hagedorn Thermodynamics and Random Walks - Extensions and Examples
In this paper, we discuss several explicit examples of the results obtained
in JHEP 1402 (2014) 127. We elaborate on the random walk picture in these
spacetimes and how it is modified. Firstly we discuss the linear dilaton
background. Then we analyze a previously studied toroidally compactified
background where we determine the Hagedorn temperature and study the random
walk picture. We continue with flat space orbifold models where we discuss
boundary conditions for the thermal scalar. Finally, we study the general link
between the quantum numbers in the fundamental domain and the strip and their
role in thermodynamics.Comment: 34 pages, v2: matches published versio
On the Relevance of the Thermal Scalar
We discuss near-Hagedorn string thermodynamics in general spacetimes using
the formalism of the thermal scalar. Building upon earlier work by Horowitz and
Polchinski, we relate several properties of the thermal scalar field theory
(i.e. the stress tensor and U(1) charge) to properties of the highly excited or
near-Hagedorn string gas. We apply the formulas on several examples. We find
the pressureless near-Hagedorn string gas in flat space and a non-vanishing
(angular) string charge in . We also find the thermal stress tensor for
the highly excited string gas in Rindler space.Comment: 36 pages, v2: section on correlators rewritten and clarifications
added, matches published versio
The Thermal Scalar and Random Walks in AdS3 and BTZ
We analyze near-Hagedorn thermodynamics of strings in the WZW model.
We compute the thermal spectrum of all primaries and find the thermal scalar
explicitly in the string spectrum using CFT twist techniques. Then we use the
link to the Euclidean WZW BTZ black hole and write down the Euclidean BTZ
spectrum. We give a Hamiltonian interpretation of the thermal partition
function of angular orbifolds where we find a reappearance of discrete states
that dominate the partition function. Using these results, we discuss the
nature of the thermal scalar in the WZW BTZ model. As a slight generalization
of the angular orbifolds, we discuss the string gas with a non-zero
chemical potential corresponding to angular momentum around the spatial cigar.
For this model as well, we determine the thermal spectrum and the Hagedorn
temperature as a function of chemical potential. Finally the nature of
corrections to the thermal scalar action is analyzed and we
find the random walk behavior of highly excited strings in this particular
background.Comment: 74 pages, v2: version accepted for publication in JHE
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