46 research outputs found
Lattice Theories with Nonlinearly Realized Chiral Symmetry
We present the lattice formulation of effective Lagrangians in which chiral
symmetry is realized nonlinearly on the fermion fields. In this framework both
the Wilson term removing unphysical doubler fermions and the fermion mass term
do not break chiral symmetry. Our lattice formulation allows us to address
non-perturbative questions in effective theories of baryons interacting with
pions and in models involving constitutent quarks interacting with pions and
gluons. With the presented methods, a system containing a non-zero density of
static baryons interacting with pions can be studied on the lattice without
encountering a complex action problem. This might lead to new insights into the
phase diagram of strongly interacting matter at non-zero chemical potential.Comment: 3 pages, Lattice2003(chiral
Emission of thermal photons and the equilibration time in Heavy-Ion collisions
The emission of hard real photons from thermalized expanding hadronic matter
is dominated by the initial high-temperature expansion phase. Therefore, a
measurement of photon emission in ultrarelativistic heavy-ion collisions
provides valuable insights into the early conditions realized in such a
collision. In particular, the initial temperature of the expanding fireball or
equivalently the equilibration time of the strongly interacting matter are of
great interest. An accurate determination of these quantities could help to
answer the question whether or not partonic matter (the quark gluon plasma) is
created in such collisions. In this work, we investigate the emission of real
photons using a model which is based on the thermodynamics of QCD matter and
which has been shown to reproduce a large variety of other observables. With
the fireball evolution fixed beforehand, we are able to extract limits for the
equilibration time by a comparison with photon emission data measured by WA98.Comment: 12 pages, 5 figures, accepted for publication at Phys. Rev.
Gravitino Dark Matter Scenarios with Massive Metastable Charged Sparticles at the LHC
We investigate the measurement of supersymmetric particle masses at the LHC
in gravitino dark matter (GDM) scenarios where the next-to-lightest
supersymmetric partner (NLSP) is the lighter scalar tau, or stau, and is stable
on the scale of a detector. Such a massive metastable charged sparticle would
have distinctive Time-of-Flight (ToF) and energy-loss () signatures. We
summarise the documented accuracies expected to be achievable with the ATLAS
detector in measurements of the stau mass and its momentum at the LHC. We then
use a fast simulation of an LHC detector to demonstrate techniques for
reconstructing the cascade decays of supersymmetric particles in GDM scenarios,
using a parameterisation of the detector response to staus, taus and jets based
on full simulation results. Supersymmetric pair-production events are selected
with high redundancy and efficiency, and many valuable measurements can be made
starting from stau tracks in the detector. We recalibrate the momenta of taus
using transverse-momentum balance, and use kinematic cuts to select
combinations of staus, taus, jets and leptons that exhibit peaks in invariant
masses that correspond to various heavier sparticle species, with errors often
comparable with the jet energy scale uncertainty.Comment: 23 pages, 10 figures, updated to version published in JHE
Signatures of Axinos and Gravitinos at Colliders
The axino and the gravitino are well-motivated candidates for the lightest
supersymmetric particle (LSP) and also for cold dark matter in the Universe.
Assuming that a charged slepton is the next-to-lightest supersymmetric particle
(NLSP), we show how the NLSP decays can be used to probe the axino LSP scenario
in hadronic axion models as well as the gravitino LSP scenario at the Large
Hadron Collider and the International Linear Collider. We show how one can
identify experimentally the scenario realized in nature. In the case of the
axino LSP, the NLSP decays will allow one to estimate the value of the axino
mass and the Peccei-Quinn scale.Comment: 20 pages, 5 figures, revised version as published in Phys.Lett.B
(comments on the experimental feasibility added
St\"uckelino Dark Matter in Anomalous U(1)' Models
We study a possible dark matter candidate in the framework of a minimal
anomalous extension of the MSSM. It turns out that in a suitable
decoupling limit the St\"uckelino, the fermionic degree of freedom of the
St\"uckelberg multiplet, is the lightest supersymmetric particle (LSP). We
compute the relic density of this particle including coannihilations with the
next to lightest supersymmetric particle (NLSP) and with the next to next to
lightest supersymmetric particle (NNLSP) which are assumed almost degenerate in
mass. This assumption is needed in order to satisfy the stringent limits that
the Wilkinson Microwave Anisotropy Probe (WMAP) puts on the relic density. We
find that the WMAP constraints can be satisifed by different NLSP and NNLSP
configurations as a function of the mass gap with the LSP. These results hold
in the parameter space region where the model remains perturbative.Comment: 21 pages, 6 figures, improved section 5, revised version published on
EPJ
SO(10) unified models and soft leptogenesis
Motivated by the fact that, in some realistic models combining SO(10) GUTs
and flavour symmetries, it is not possible to achieve the required baryon
asymmetry through the CP asymmetry generated in the decay of right-handed
neutrinos, we take a fresh look on how deep this connection is in SO(10). The
common characteristics of these models are that they use the see-saw with
right-handed neutrinos, predict a normal hierarchy of masses for the neutrinos
observed in oscillating experiments and in the basis where the right-handed
Majorana mass is diagonal, the charged lepton mixings are tiny.
In addition these models link the up-quark Yukawa matrix to the neutrino
Yukawa matrix Y^\nu with the special feature of Y^\nu_{11}-> 0 Using this
condition, we find that the required baryon asymmetry of the Universe can be
explained by the soft leptogenesis using the soft B parameter of the second
lightest right-handed neutrino whose mass turns out to be around 10^8 GeV. It
is pointed out that a natural way to do so is to use no-scale supergravity
where the value of B ~1 GeV is set through gauge-loop corrections.Comment: 26 pages, 2 figures. Added references, new appendix of a relevant fit
and improved comment
Gauge vs. Gravity mediation in models with anomalous U(1)'s
In an attempt to implement gauge mediation in string theory, we study string
effective supergravity models of supersymmetry breaking, containing anomalous
gauge factors. We discuss subtleties related to gauge invariance and the
stabilization of the Green-Schwarz moduli, which set non-trivial constraints on
the transmission of supersymmetry breaking to MSSM via gauge interactions.
Given those constraints, it is difficult to obtain the dominance of gauge
mediation over gravity mediation. Furthermore, generically the gauge
contributions to soft terms contain additional non-standard terms coming from
D-term contributions. Motivated by this, we study the phenomenology of recently
proposed hybrid models, where gravity and gauge mediations compete at the GUT
scale, and show that such a scenario can respect WMAP constraints and would be
easily testable at LHC.Comment: 40 pages, 5 figure
Constraining warm dark matter with cosmic shear power spectra
We investigate potential constraints from cosmic shear on the dark matter
particle mass, assuming all dark matter is made up of light thermal relic
particles. Given the theoretical uncertainties involved in making cosmological
predictions in such warm dark matter scenarios we use analytical fits to linear
warm dark matter power spectra and compare (i) the halo model using a mass
function evaluated from these linear power spectra and (ii) an analytical fit
to the non-linear evolution of the linear power spectra. We optimistically
ignore the competing effect of baryons for this work. We find approach (ii) to
be conservative compared to approach (i). We evaluate cosmological constraints
using these methods, marginalising over four other cosmological parameters.
Using the more conservative method we find that a Euclid-like weak lensing
survey together with constraints from the Planck cosmic microwave background
mission primary anisotropies could achieve a lower limit on the particle mass
of 2.5 keV.Comment: 26 pages, 9 figures, minor changes to match the version accepted for
publication in JCA
Are direct photons a clean signal of a thermalized quark gluon plasma?
Direct photon production from a quark gluon plasma (QGP) in thermal
equilibrium is studied directly in real time. In contrast to the usual S-matrix
calculations, the real time approach is valid for a QGP that formed and reached
LTE a short time after a collision and of finite lifetime ( as expected at RHIC or LHC). We point out that during such
finite QGP lifetime the spectrum of emitted photons carries information on the
initial state. There is an inherent ambiguity in separating the virtual from
the observable photons during the transient evolution of the QGP. We propose a
real time formulation to extract the photon yield which includes the initial
stage of formation of the QGP parametrized by an effective time scale of
formation . This formulation coincides with the S-matrix approach
in the infinite lifetime limit. It allows to separate the virtual cloud as well
as the observable photons emitted during the pre- equilibrium stage from the
yield during the QGP lifetime. We find that the lowest order contribution
which does \emph{not} contribute to the S-matrix
approach, is of the same order of or larger than the S-matrix contribution
during the lifetime of the QGP for a typical formation time . The yield for momenta features a
power law fall-off and is larger than that obtained
with the S-matrix for momenta . We provide a
comprehensive numerical comparison between the real time and S-matrix yields
and study the dynamics of the build-up of the photon cloud and the different
contributions to the radiative energy loss. The reliability of the current
estimates on photon emission is discussed.Comment: 31 pages, 12 eps figures, version to appear in PR
Dark matter in supersymmetric models with axino LSP in Randall-Sundrum II brane model
The axino dark matter hypothesis in RSII brane model is studied. Within the
framework of CMSSM we assume that the lightest neutralino or stau is the NLSP,
and that the axino production has a single contribution from the NLSP decay. It
is found that the axino can play the role of dark matter in the universe and we
determine what the axino mass should be for different values of the
five-dimensional Planck mass. An upper bound is obtained for the latterComment: 12 pages,3 figures, accepted in JHE