5,671 research outputs found
A model for the spacetime evolution of heavy-ion collisions at RHIC
We investigate the space-time evolution of ultrarelativistic Au-Au collisions
at full RHIC energy using a schematic model of the expansion. Assuming a
thermally equilibrated system, we can adjust the essential scale parameters of
this model such that the measured transverse momentum spectra and Hanbury-Brown
Twiss (HBT) correlation parameters are well described. We find that the
experimental data strongly constrain the dynamics of the evolution of the
emission source although hadronic observables for the most part reflect the
final breakup of the system.Comment: 9 pages, 3 figures, submitted to Phys. Rev.
Yang-Mills Equations of Motion for the Higgs Sector of SU(3)-Equivariant Quiver Gauge Theories
We consider SU(3)-equivariant dimensional reduction of Yang-Mills theory on
spaces of the form R x SU(3)/H, with H equals either SU(2) x U(1) or U(1) x
U(1). For the corresponding quiver gauge theory we derive the equations of
motion and construct some specific solutions for the Higgs fields using
different gauge groups. Specifically we choose the gauge groups U(6) and U(8)
for the space R x CP^2 as well as the gauge group U(3) for the space R x
SU(3)/U(1)xU(1), and derive Yang-Mills equations for the latter one using a
spin connection endowed with a non-vanishing torsion. We find that a specific
value for the torsion is necessary in order to obtain non-trivial solutions of
Yang-Mills equations. Finally, we take the space R x CP^1 x CP^2 and derive the
equations of motion for the Higgs sector for a U(3m+3) gauge theory.Comment: 21 pages, 4 figures; v2: figures added, references updated, published
version (JMP
Hard dihadron correlations in heavy-ion collisions at RHIC and LHC
High transverse momentum (P_T) processes are considered to be an important
tool to probe and understand the medium produced in ultrarelativistic heavy-ion
collisions via the interaction of hard, perturbatively produced partons with
the medium. In this context, triggered hard dihadron correlations constitute a
class of observables set between hard single inclusive hadrons (dominated by
the leading jet fragments) and fully reconstructed jets - while they probe some
features of the perturbative QCD evolution of a parton shower in the medium,
they do not suffer from the problem of finding a suitable separation between
soft perturbative (jet-like) and soft non-perturbative (medium-like) physics as
the identification of full jets does. On the other hand, the trigger
requirement introduces non-trivial complications to the process, which makes
the medium-modification of the correlation pattern difficult and non-intuitive
to understand. In this work, we review the basic physics underlying triggered
dihadron correlations and make a systematic comparison of several combinations
of medium evolution and parton-medium interaction models with the available
data from 200 AGeV Au-Au collisions at RHIC. We also discuss the expected
results for 2.76 ATeV Pb-Pb collisions at the LHC.Comment: 13 pages, 7 figures, submitted to PR
Why a long-lived fireball can be compatible with HBT measurements
The common interpretation of HBT data measured at top SPS energies leads to
apparent source lifetimes of 6-8 fm/c and emission duration of approximately
2-3 fm/c. We investigate a scenario with continuous pion emission from a
long-lived (~17 fm/c) thermalized source in order to show that it is not
excluded by the data. Starting from a description of the source's spacetime
expansion based on gross thermodynamical properties of hot matter (which is
able to describe a number of experimental observables), we introduce the pion
emission function with a contribution from continuous emission during the
source's lifetime and another contribution from the final breakup and proceed
by calculating the HBT parameters R_out and R_side. The results are compared
with experimental data measured at SPS for 158 AGeV central Pb-Pb collisions.
We achieve agreement with data, provided that some minor modifications of the
fireball evolution scenario are made and find that the parameter R_out is not
sensitive to the fireball lifetime, but only to the duration of the final
breakup, in spite of the fact that emission takes place throughout the whole
lifetime. We explicitly demonstrate that those findings do not alter previous
results obtained within this model.Comment: 13 pages, 5 figures, submitted to Phys. Rev. C. (revised description
of fireball expansion
Efficient representation of fully many-body localized systems using tensor networks
We propose a tensor network encoding the set of all eigenstates of a fully
many-body localized system in one dimension. Our construction, conceptually
based on the ansatz introduced in Phys. Rev. B 94, 041116(R) (2016), is built
from two layers of unitary matrices which act on blocks of contiguous
sites.
We argue this yields an exponential reduction in computational time and
memory requirement as compared to all previous approaches for finding a
representation of the complete eigenspectrum of large many-body localized
systems with a given accuracy. Concretely, we optimize the unitaries by
minimizing the magnitude of the commutator of the approximate integrals of
motion and the Hamiltonian, which can be done in a local fashion. This further
reduces the computational complexity of the tensor networks arising in the
minimization process compared to previous work. We test the accuracy of our
method by comparing the approximate energy spectrum to exact diagonalization
results for the random field Heisenberg model on 16 sites. We find that the
technique is highly accurate deep in the localized regime and maintains a
surprising degree of accuracy in predicting certain local quantities even in
the vicinity of the predicted dynamical phase transition. To demonstrate the
power of our technique, we study a system of 72 sites and we are able to see
clear signatures of the phase transition. Our work opens a new avenue to study
properties of the many-body localization transition in large systems.Comment: Version 2, 16 pages, 16 figures. Larger systems and greater
efficienc
Neutrino-Mass Hierarchies and Non-linear Representation of Lepton-Flavour Symmetry
Lepton-flavour symmetry in the Standard Model is broken by small masses for
charged leptons and neutrinos. Introducing neutrino masses via dimension-5
operators associated to lepton-number violation at a very high scale, the
corresponding coupling matrix may still have entries of order 1, resembling the
situation in the quark sector with large top Yukawa coupling. As we have shown
recently, in such a situation one may introduce the coupling matrices between
lepton and Higgs fields as non-linear representations of lepton-flavour
symmetry within an effective-theory framework. This allows us to separate the
effects related to the large mass difference observed in atmospheric neutrino
oscillations from those related to the solar mass difference. We discuss the
cases of normal or inverted hierarchical and almost degenerate neutrino
spectrum, give some examples to illustrate minimal lepton-flavour violation in
radiative and leptonic decays, and also provide a systematic definition of
next-to-minimal lepton-flavour violation within the non-linear framework.Comment: 17 pages, 1 figur
Focal adhesions as mechanosensors: the two-spring model
Adhesion-dependent cells actively sense the mechanical properties of their
environment through mechanotransductory processes at focal adhesions, which are
integrin-based contacts connecting the extracellular matrix to the
cytoskeleton. Here we present first steps towards a quantitative understanding
of focal adhesions as mechanosensors. It has been shown experimentally that
high levels of force are related to growth of and signaling at focal adhesions.
In particular, activation of the small GTPase Rho through focal adhesions leads
to the formation of stress fibers. Here we discuss one way in which force might
regulate the internal state of focal adhesions, namely by modulating the
internal rupture dynamics of focal adhesions. A simple two-spring model shows
that the stiffer the environment, the more efficient cellular force is built up
at focal adhesions by molecular motors interacting with the actin filaments.Comment: Latex, 17 pages, 5 postscript figures include
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