3,673 research outputs found
Lepton Flavours at the Early LHC Experiments as the Footprints of the Dark Matter Producing Mechanisms
The mSUGRA parameter space corresponding to light sleptons well within the
reach of LHC and relatively light squarks and gluinos (mass 1 TeV) has
three regions consistent with the WMAP data on dark matter relic density and
direct mass bounds from LEP 2. Each region can lead to distinct leptonic
signatures from squark-gluino events during the early LHC experiments
(integrated luminosity or even smaller). In the much studied
stau-LSP coannihilation region with a vanishing common trilinear coupling
() at the GUT scale a large fraction of the final states contain electrons
and / or muons and - - universality holds to a good
approximation. In the not so well studied scenarios with non-vanishing
both LSP pair annihilation and stau-LSP coannihilation could contribute
significantly to the dark matter relic density for even smaller squark-gluino
masses. Our simulations indicate that the corresponding signatures are final
states rich in -leptons while final states with electrons and muons are
suppressed leading to a violation of lepton universality. These features may be
observed to a lesser extent even in the modified parameter space (with non-zero
) where the coannihilation process dominates. We also show that the
generic -leptons + -jets+ signatures without flavour tagging
can also discriminate among the three scenarios. However, the signals become
more informative if the and -jet tagging facilities at the LHC
experiments are utilized.Comment: 28 page
Determining the Mass of Dark Matter Particles with Direct Detection Experiments
In this article I review two data analysis methods for determining the mass
(and eventually the spin-independent cross section on nucleons) of Weakly
Interacting Massive Particles with positive signals from direct Dark Matter
detection experiments: a maximum likelihood analysis with only one experiment
and a model-independent method requiring at least two experiments.
Uncertainties and caveats of these methods will also be discussed.Comment: 24 pages, 10 figures, 1 reference added, typos fixed, published
version, to appear in the NJP Focus Issue on "Dark Matter and Particle
Physics
SUSY darkmatter at the LHC - 7 TeV
We have analysed the early LHC signatures of the minimal supergravity
(mSUGRA) model. Our emphasis is on the 7 - run corresponding to an
integrated luminosity of although we have also discussed
briefly the prospects at LHC-10 . We focus on the parameter space yielding
relatively light squark and gluinos consistent with the darkmatter relic
density data and the LEP bounds on the lightest Higgs scalar mass. This
parameter space is only allowed for non-vanishing trilinear soft breaking term
. A significant region of the parameter space with large to moderate
negative values of consistent with the stability of the scalar potential
and relic density production via neutralino annihilation and/or neutralino -
stau coannihilation yields observable signal via the jets + missing transverse
energy channel. The one lepton + jets + missing energy signal is also viable
over a smaller but non-trivial parameter space. The ratio of the size of the
two signals - free from theoretical uncertainties - may distinguish between
different relic density generating mechanisms. With efficient -tagging
facilities at 7 the discriminating power may increase significantly. We
also comment on other dark matter relic density allowed mSUGRA scenarios and
variants there of in the context of LHC-7 .Comment: Brief comments on signals at 7 TeV in the Higgs funnel region of
mSUGRA, models with non universal scalar and gaugino masses have been added.
Accepted for publication in PR
Probing MeV Dark Matter at Low--Energy Colliders
It has been suggested that the annihilation of Dark Matter particles \chi
with mass between 0.5 and 20 MeV into e^+e^- pairs could be responsible for the
excess flux of 511 keV photons coming from the central region of our galaxy
that has been detected by the IINTEGRAL satellite. The simplest way to achieve
the required cross section for \chi pair annihilation while respecting existing
constraints is to introduce a new vector boson U with mass M_U below a few
hundred MeV. In this Letter we point out that over most of the allowed
parameter space, the process e^+e^- to U \gamma, followed by the decay of U
into either an e^+e^- pair or into an invisible (\nu \bar \nu or \chi \bar
\chi) channel, should lead to signals that can be detected by the B-factory
experiments BaBar and Belle. A smaller, but still substantial, region of
parameter space can also be probed at the \Phi factory DAFNE.Comment: 4 pages, revtex with equation.sty (included), 2 embedded postscript
figures. Minor changes, added a couple of references; version to appear in
PR
The Thermal Abundance of Semi-Relativistic Relics
Approximate analytical solutions of the Boltzmann equation for particles that
are either extremely relativistic or non-relativistic when they decouple from
the thermal bath are well established. However, no analytical formula for the
relic density of particles that are semi-relativistic at decoupling is yet
known. We propose a new ansatz for the thermal average of the annihilation
cross sections for such particles, and find a semi-analytical treatment for
calculating their relic densities. As examples, we consider Majorana- and
Dirac-type neutrinos. We show that such semi-relativistic relics cannot be good
cold Dark Matter candidates. However, late decays of meta-stable
semi-relativistic relics might have released a large amount of entropy, thereby
diluting the density of other, unwanted relics.Comment: 22 pages, 5 figures. Comments and references adde
Abundance of Cosmological Relics in Low-Temperature Scenarios
We investigate the relic density n_\chi of non-relativistic long-lived or
stable particles \chi in cosmological scenarios in which the temperature T is
too low for \chi to achieve full chemical equilibrium. The case with a heavier
particle decaying into \chi is also investigated. We derive approximate
solutions for n_\chi(T) which accurately reproduce numerical results when full
thermal equilibrium is not achieved. If full equilibrium is reached, our ansatz
no longer reproduces the correct temperature dependence of the \chi number
density. However, it does give the correct final relic density, to an accuracy
of about 3% or better, for all cross sections and initial temperatures.Comment: 26 pages, 8 figures, comments added, to appear in Phys. Rev.
Exploring compressed supersymmetry with same-sign top quarks at the Large Hadron Collider
In compressed supersymmetry, a light top squark naturally mediates efficient
neutralino pair annihilation to govern the thermal relic abundance of dark
matter. I study the LHC signal of same-sign leptonic top-quark decays from
gluino and squark production, which follows from gluino decays to top plus stop
followed by the stop decaying to a charm quark and the LSP in these models.
Measurements of the numbers of jets with heavy-flavor tags in the same-sign
lepton events can be used to confirm the origin of the signal. Summed
transverse momentum observables provide an estimate of an effective
superpartner mass, which is correlated with the gluino mass. Measurements of
invariant mass endpoints from the visible products of gluino decays do not
allow direct determination of superpartner masses, but can place constraints on
them, including lower bounds on the gluino mass as a function of the top-squark
mass.Comment: 22 pages. v2: Discussion of competition between 2-body and 4-body
stop decays corrected. References adde
Analysis of the chargino and neutralino mass parameters at one-loop level
In the Minimal Supersymmetric Standard Model (MSSM) the masses of the
neutralinos and charginos depend on the gaugino and higgsino mass parameters M,
M' and . If supersymmetry is realized, the extraction of these parameters
from future high energy experiments will be crucial to test the underlying
theory. We present a consistent method how on-shell parameters can be properly
defined at one-loop level and how they can be determined from precision
measurements. In addition, we show how a GUT relation for the parameters M and
M' can be tested at one-loop level. The numerical analysis is based on a
complete one-loop calculation. The derived analytic formulae are given in the
appendix.Comment: 16 pages, 8 figure
Virtual Photon Strucutre Functions and the Parton Content of the Electron
We point out that in processes involving the parton content of the photon the
usual effective photon approximation should be modified. The reason is that the
parton content of virtual photons is logarithmically suppressed compared to
real photons. We describe this suppression using several simple, physically
motivated ans\"atze. Although the parton content of the electron in general no
longer factorizes into an electron flux function and a photon structure
function, it can still be expressed as a single integral. Numerical examples
are given for the \eplem\ collider TRISTAN as well as the collider HERA.Comment: 16 pages and 3 figures (available from DREES@WSICPHEN as topdraw or
PS files); LaTeX with equation.sty; MAD/PH/819, BU 94-0
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