301 research outputs found
Ground state non-universality in the random field Ising model
Two attractive and often used ideas, namely universality and the concept of a
zero temperature fixed point, are violated in the infinite-range random-field
Ising model. In the ground state we show that the exponents can depend
continuously on the disorder and so are non-universal. However, we also show
that at finite temperature the thermal order parameter exponent one half is
restored so that temperature is a relevant variable. The broader implications
of these results are discussed.Comment: 4 pages 2 figures, corrected prefactors caused by a missing factor of
two in Eq. 2., added a paragraph in conclusions for clarit
Properties of the random field Ising model in a transverse magnetic field
We consider the effect of a random longitudinal field on the Ising model in a
transverse magnetic field. For spatial dimension , there is at low
strength of randomness and transverse field, a phase with true long range order
which is destroyed at higher values of the randomness or transverse field. The
properties of the quantum phase transition at zero temperature are controlled
by a fixed point with no quantum fluctuations. This fixed point also controls
the classical finite temperature phase transition in this model. Many critical
properties of the quantum transition are therefore identical to those of the
classical transition. In particular, we argue that the dynamical scaling is
activated, i.e, the logarithm of the diverging time scale rises as a power of
the diverging length scale
Neutralino-Nucleon Cross Section and Charge and Colour Breaking Constraints
We compute the neutralino-nucleon cross section in several supersymmetric
scenarios, taking into account all kind of constraints. In particular, the
constraints that the absence of dangerous charge and colour breaking minima
imposes on the parameter space are studied in detail. In addition, the most
recent experimental constraints, such as the lower bound on the Higgs mass, the
branching ratio, and the muon are considered. The
astrophysical bounds on the dark matter density are also imposed on the
theoretical computation of the relic neutralino density, assuming thermal
production. This computation is relevant for the theoretical analysis of the
direct detection of dark matter in current experiments. We consider first the
supergravity scenario with universal soft terms and GUT scale. In this scenario
the charge and colour breaking constraints turn out to be quite important, and
\tan\beta\lsim 20 is forbidden. Larger values of can also be
forbidden, depending on the value of the trilinear parameter . Finally, we
study supergravity scenarios with an intermediate scale, and also with
non-universal scalar and gaugino masses where the cross section can be very
large.Comment: Final version to appear in JHE
The three-dimensional randomly dilute Ising model: Monte Carlo results
We perform a high-statistics simulation of the three-dimensional randomly
dilute Ising model on cubic lattices with . We choose a
particular value of the density, x=0.8, for which the leading scaling
corrections are suppressed. We determine the critical exponents, obtaining , , , and ,
in agreement with previous numerical simulations. We also estimate numerically
the fixed-point values of the four-point zero-momentum couplings that are used
in field-theoretical fixed-dimension studies. Although these results somewhat
differ from those obtained using perturbative field theory, the
field-theoretical estimates of the critical exponents do not change
significantly if the Monte Carlo result for the fixed point is used. Finally,
we determine the six-point zero-momentum couplings, relevant for the
small-magnetization expansion of the equation of state, and the invariant
amplitude ratio that expresses the universality of the free-energy
density per correlation volume. We find .Comment: 34 pages, 7 figs, few correction
Likelihood Functions for Supersymmetric Observables in Frequentist Analyses of the CMSSM and NUHM1
On the basis of frequentist analyses of experimental constraints from
electroweak precision data, g-2, B physics and cosmological data, we
investigate the parameters of the constrained MSSM (CMSSM) with universal soft
supersymmetry-breaking mass parameters, and a model with common non-universal
Higgs masses (NUHM1). We present chi^2 likelihood functions for the masses of
supersymmetric particles and Higgs bosons, as well as b to s gamma, b to mu mu
and the spin-independent dark matter scattering cross section. In the CMSSM we
find preferences for sparticle masses that are relatively light. In the NUHM1
the best-fit values for many sparticle masses are even slightly smaller, but
with greater uncertainties. The likelihood functions for most sparticle masses
are cut off sharply at small masses, in particular by the LEP Higgs mass
constraint. Both in the CMSSM and the NUHM1, the coannihilation region is
favoured over the focus-point region at about the 3-sigma level, largely but
not exclusively because of g-2. Many sparticle masses are highly correlated in
both the CMSSM and NUHM1, and most of the regions preferred at the 95% C.L. are
accessible to early LHC running. Some slepton and chargino/neutralino masses
should be in reach at the ILC. The masses of the heavier Higgs bosons should be
accessible at the LHC and the ILC in portions of the preferred regions in the
(M_A, tan beta) plane. In the CMSSM, the likelihood function for b to mu mu is
peaked close to the Standard Model value, but much larger values are possible
in the NUHM1. We find that values of the DM cross section > 10^{-10} pb are
preferred in both the CMSSM and the NUHM1. We study the effects of dropping the
g-2, b to s gamma, relic density and M_h constraints.Comment: 34 pages, 24 figure
The three-dimensional random field Ising magnet: interfaces, scaling, and the nature of states
The nature of the zero temperature ordering transition in the 3D Gaussian
random field Ising magnet is studied numerically, aided by scaling analyses. In
the ferromagnetic phase the scaling of the roughness of the domain walls,
, is consistent with the theoretical prediction .
As the randomness is increased through the transition, the probability
distribution of the interfacial tension of domain walls scales as for a single
second order transition. At the critical point, the fractal dimensions of
domain walls and the fractal dimension of the outer surface of spin clusters
are investigated: there are at least two distinct physically important fractal
dimensions. These dimensions are argued to be related to combinations of the
energy scaling exponent, , which determines the violation of
hyperscaling, the correlation length exponent , and the magnetization
exponent . The value is derived from the
magnetization: this estimate is supported by the study of the spin cluster size
distribution at criticality. The variation of configurations in the interior of
a sample with boundary conditions is consistent with the hypothesis that there
is a single transition separating the disordered phase with one ground state
from the ordered phase with two ground states. The array of results are shown
to be consistent with a scaling picture and a geometric description of the
influence of boundary conditions on the spins. The details of the algorithm
used and its implementation are also described.Comment: 32 pp., 2 columns, 32 figure
Squark-, Slepton- and Neutralino-Chargino coannihilation effects in the low-energy effective MSSM
Within the low-energy effective Minimal Supersymmetric extension of the
Standard Model (effMSSM) we calculate the neutralino relic density taking into
account slepton-neutralino, squark-neutralino and neutralino/chargino-
neutralino coannihilation channels. By including squark (stop and sbottom)
coannihilation channels we extend our comparative study to all allowed
coannihilations and obtain the general result that all of them give sizable
contributions to the reduction of the neutralino relic density. Due to these
coannihilation processes some models (mostly with large neutralino masses)
enter into the cosmologically interesting region for relic density, but other
models leave this region. Nevertheless, in general, the predictions for direct
and indirect dark matter detection rates are not strongly affected by these
coannihilation channels in the effMSSM.Comment: 14 pages, 10 figures, corrected and to be published in Phys. Rev.
Dynamic structure factor of the Ising model with purely relaxational dynamics
We compute the dynamic structure factor for the Ising model with a purely
relaxational dynamics (model A). We perform a perturbative calculation in the
expansion, at two loops in the high-temperature phase and at one
loop in the temperature magnetic-field plane, and a Monte Carlo simulation in
the high-temperature phase. We find that the dynamic structure factor is very
well approximated by its mean-field Gaussian form up to moderately large values
of the frequency and momentum . In the region we can investigate,
, , where is the correlation
length and the zero-momentum autocorrelation time, deviations are at
most of a few percent.Comment: 21 pages, 3 figure
Slepton and Neutralino/Chargino Coannihilations in MSSM
Within the low-energy effective Minimal Supersymmetric extension of Standard
Model (effMSSM) we calculated the neutralino relic density taking into account
slepton-neutralino and neutralino-chargino/neutralino coannihilation channels.
We performed comparative study of these channels and obtained that both of them
give sizable contributions to the reduction of the relic density. Due to these
coannihilation processes some models (mostly with large neutralino masses)
enter into the cosmologically interesting region for relic density, but other
models leave this region. Nevertheless, in general, the predictions for direct
and indirect dark matter detection rates are not strongly affected by these
coannihilation channels in the effMSSM.Comment: 12 pages, 9 figures, revte
Implications of LHC Searches on SUSY Particle Spectra: The pMSSM Parameter Space with Neutralino Dark Matter
We study the implications of LHC searches on SUSY particle spectra using flat
scans of the 19-parameter pMSSM phase space. We apply constraints from flavour
physics, g_mu-2, dark matter and earlier LEP and Tevatron searches. The
sensitivity of the LHC SUSY searches with jets, leptons and missing energy is
assessed by reproducing with fast simulation the recent CMS analyses after
validation on benchmark points. We present results in terms of the fraction of
pMSSM points compatible with all the constraints which are excluded by the LHC
searches with 1 fb^{-1} and 15 fb^{-1} as a function of the mass of strongly
and weakly interacting SUSY particles. We also discuss the suppression of Higgs
production cross sections for the MSSM points not excluded and contrast the
region of parameter space tested by the LHC data with the constraints from dark
matter direct detection experiments.Comment: 14 pages, 13 figures. v2: increased statistics, to appear in EPJ
- …