3,152 research outputs found
Higgs decays in supersymmetric models with light neutralinos
In the Minimal Supersymmetric Standard Model, neutralinos lighter than 50 GeV
are compatible with all accelerator, precision, and cosmological bounds. Such
neutralinos might constitute a relevant decay channel for the Higgs boson,
modifying its expected signatures at hadron colliders. We study the branching
ratio h --> \chi\chi and determine the region in the supersymmetric parameter
space where it is sizable. We have found that, in fact, the Higgs may
dominantly decay into neutralino pairs. Besides, as a result of this new
channel, the branching ratio into visible modes, such as h --> \gamma\gamma,
gets suppressed.Comment: 9 pages, 5 figure
Light mixed sneutrinos as thermal dark matter
In supersymmetric models with Dirac neutrino masses, a left-right mixed
sneutrino can be a viable dark matter candidate. We examine the
MSSM+ parameter space where this is the case with particular
emphasis on light sneutrinos with masses below 10 GeV. We discuss implications
for direct and indirect dark matter searches, including the relevant
uncertainties, as well as consequences for collider phenomenology.Comment: 33 pages, 14 figures; one figure and references adde
Equilibrium random-field Ising critical scattering in the antiferromagnet Fe(0.93)Zn(0.07)F2
It has long been believed that equilibrium random-field Ising model (RFIM)
critical scattering studies are not feasible in dilute antiferromagnets close
to and below Tc(H) because of severe non-equilibrium effects. The high magnetic
concentration Ising antiferromagnet Fe(0.93)Zn(0.07)F2, however, does provide
equilibrium behavior. We have employed scaling techniques to extract the
universal equilibrium scattering line shape, critical exponents nu = 0.87 +-
0.07 and eta = 0.20 +- 0.05, and amplitude ratios of this RFIM system.Comment: 4 pages, 1 figure, minor revision
How light can the lightest neutralino be?
In this talk we summarize previous work on mass bounds of a light neutralino
in the Minimal Supersymmetric Standard Model. We show that without the GUT
relation between the gaugino mass parameters M_1 and M_2, the mass of the
lightest neutralino is essentially unconstrained by collider bounds and
precision observables. We conclude by considering also the astrophysics and
cosmology of a light neutralino.Comment: 6 pages, 3 figures, to appear in the proceedings of the 16th
International Symposium on Particles, Strings and Cosmology (PASCOS2010),
Valencia (Spain), July 19th - 23rd, 201
Relic density of neutralino dark matter in the MSSM with CP violation
We calculate the relic density of dark matter in the MSSM with CP violation.
We analyse various scenarios of neutralino annihilation: the cases of a bino,
bino-wino and bino-Higgsino LSP, annihilation through Higgs, as well as
sfermion coannihilation scenarios. Large phase effects are found, on the one
hand due to shifts in the masses, on the other hand due to modifications of the
couplings. Taking special care to disentangle the effects in masses and
couplings, we demonstrate that the presence of CP phases can have a significant
influence on the neutralino relic abundance. Typical variations in \Omega h^2
solely from modifications in the couplings are O(10%-100%), but can reach an
order of magnitude in some cases.Comment: 36 pages, 21 figures (low resolution). A version with high-resolution
figures can be downloaded from http://cern.ch/kraml/papers/omc
Events with Isolated Charged Leptons and Large Missing Transverse Momentum at HERA
Striking events with isolated charged leptons, large missing transverse
momentum and large transverse momentum of the hadronic final state were
observed at the electron proton collider HERA in a data sample corresponding to
a luminosity of about 130 pb-1. The H1 collaboration observed 11 events with
isolated electrons or muons and with transverse momentum above 25 GeV. Only
3.4+-0.6 events were expected from Standard Model (SM) processes. Six of these
events have a transverse momentum of greater than 40 GeV, while 1.3+-0.3 events
were expected. The ZEUS collaboration observed good agreement with the SM.
However, ZEUS found two events with a similar event topology, but tau leptons
instead of electrons or muons in the final state. Only 0.2+-0.05 events were
expected from SM processes. For various hypotheses the compatibility of the
experimental results was investigated with respect to the SM and with respect
to possible explanations beyond the SM. Prospects for the high-luminosity
HERA-II data taking period are given
Destruction of first-order phase transition in a random-field Ising model
The phase transitions that occur in an infinite-range-interaction Ising
ferromagnet in the presence of a double-Gaussian random magnetic field are
analyzed. Such random fields are defined as a superposition of two Gaussian
distributions, presenting the same width . Is is argued that this
distribution is more appropriate for a theoretical description of real systems
than its simpler particular cases, i.e., the bimodal () and the
single Gaussian distributions. It is shown that a low-temperature first-order
phase transition may be destructed for increasing values of , similarly
to what happens in the compound , whose
finite-temperature first-order phase transition is presumably destructed by an
increase in the field randomness.Comment: 13 pages, 3 figure
Tricritical Points in the Sherrington-Kirkpatrick Model in the Presence of Discrete Random Fields
The infinite-range-interaction Ising spin glass is considered in the presence
of an external random magnetic field following a trimodal (three-peak)
distribution. The model is studied through the replica method and phase
diagrams are obtained within the replica-symmetry approximation. It is shown
that the border of the ferromagnetic phase may present first-order phase
transitions, as well as tricritical points at finite temperatures. Analogous to
what happens for the Ising ferromagnet under a trimodal random field, it is
verified that the first-order phase transitions are directly related to the
dilution in the fields (represented by ). The ferromagnetic boundary at
zero temperature also exhibits an interesting behavior: for , a single tricritical point occurs, whereas if
the critical frontier is completely continuous; however, for
, a fourth-order critical point appears. The stability
analysis of the replica-symmetric solution is performed and the regions of
validity of such a solution are identified; in particular, the Almeida-Thouless
line in the plane field versus temperature is shown to depend on the weight
.Comment: 23pages, 7 ps figure
Critical dynamics and effective exponents of magnets with extended impurities
We investigate the asymptotic and effective static and dynamic critical
behavior of (d=3)-dimensional magnets with quenched extended defects,
correlated in dimensions (which can be considered as the
dimensionality of the defects) and randomly distributed in the remaining
dimensions. The field-theoretical renormalization group
perturbative expansions being evaluated naively do not allow for the reliable
numerical data. We apply the Chisholm-Borel resummation technique to restore
convergence of the two-loop expansions and report the numerical values of the
asymptotic critical exponents for the model A dynamics. We discuss different
scenarios for static and dynamic effective critical behavior and give values
for corresponding non-universal exponents.Comment: 12 pages, 6 figure
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