3,987 research outputs found
Distributions for one-lepton SUSY Searches with the ATLAS Detector
Using ATLAS data corresponding to 70 +- 8 nb^-1 of integrated luminosity from
the 7 TeV proton-proton collisions at the LHC, distributions of relevant
supersymmetry-sensitive variables are shown for the final state containing
jets, missing transverse momentum and one isolated electron or muon. With
increased integrated luminosities, selections based on these distributions will
be used in the search for supersymmetric particles: it is thus important to
show that the Standard Model backgrounds to these searches are under good
control.Comment: 3 pages, to appear in the Proceedings of the Hadron Collider Physics
Symposium 2010, Toronto, Ontario, Canada, 23 - 27 Aug 2010, available on the
CERN document server under the number ATL-PHYS-PROC-2010-07
Extracting constraints from direct detection searches of supersymmetric dark matter in the light of null results from the LHC in the squark sector
The comparison of the results of direct detection of Dark Matter, obtained
with various target nuclei, requires model-dependent, or even arbitrary,
assumptions. Indeed, to draw conclusions either the spin-dependent (SD) or the
spin-independent (SI) interaction has to be neglected. In the light of the null
results from supersymmetry searches at the LHC, the squark sector is pushed to
high masses. We show that for a squark sector at the TeV scale, the framework
used to extract contraints from direct detection searches can be redefined as
the number of free parameters is reduced. Moreover, the correlation observed
between SI and SD proton cross sections constitutes a key issue for the
development of the next generation of Dark Matter detectors.Comment: Figure 3 has been updated. Conclusions unchange
Determining the squark mass at the LHC
We propose a new way to determine the squark mass based on the shape of
di-jet invariant mass distribution of supersymmetry (SUSY) di-jet events at the
Large Hadron Collider (LHC). Our algorithm, which is based on event kinematics,
requires that the branching ratio is
substantial for at least some types of squarks, and that
. We select di-jet events with no
isolated leptons, and impose cuts on the total jet transverse energy,
, on , and on the
azimuthal angle between the two jets to reduce SM backgrounds. The shape of the
resulting di-jet mass distribution depends sensitively on the squark mass,
especially if the integrated luminosity is sufficient to allow a hard enough
cut on and yet leave a large enough signal to obtain the
distribution. We simulate the signal and Standard Model (SM) backgrounds for
100 fb integrated luminosity at 14 TeV requiring GeV.
We show that it should be possible to extract to within about
3% at 95% CL --- similar to the precision obtained using --- from the
di-jet mass distribution if GeV, or to within % if TeV.Comment: 20 pages, 9 figures. Footnote added, updated reference
A vine copula mixed effect model for trivariate meta-analysis of diagnostic test accuracy studies accounting for disease prevalence
A bivariate copula mixed model has been recently proposed to synthesize diagnostic test accuracy studies and it has been shown that it is superior to the standard generalized linear mixed model in this context. Here, we call trivariate vine copulas to extend the bivariate meta-analysis of diagnostic test accuracy studies by accounting for disease prevalence. Our vine copula mixed model includes the trivariate generalized linear mixed model as a special case and can also operate on the original scale of sensitivity, specificity, and disease prevalence. Our general methodology is illustrated by re-analyzing the data of two published meta-analyses. Our study suggests that there can be an improvement on trivariate generalized linear mixed model in fit to data and makes the argument for moving to vine copula random effects models especially because of their richness, including reflection asymmetric tail dependence, and computational feasibility despite their three dimensionality
Search for the minimal universal extra dimension model at the LHC with =7 TeV
Universal Extra Dimension (UED) model is one of the popular extension of the
Standard Model (SM) which offers interesting phenomenology. In the minimal UED
(mUED) model, Kaluza-Klein (KK) parity conservation ensures that KK
states can only be pair produced at colliders and the lightest KK particle is
stable. In most of the parameter space, first KK excitation of SM hypercharge
gauge boson is the lightest one and it can be a viable dark matter candidate.
Thus, the decay of KK particles will always involve missing transverse
energy as well as leptons and jets. The production cross sections of KK
particles are large and such particles may be observed at the Large Hadron
Collider (LHC). We explore the mUED discovery potential of the LHC with
= 7 TeV in the multileptonic final states. Since in the early LHC
run, precise determination of missing transverse energy may not be possible, we
examine the LHC reach with and without using missing transverse energy
information. We observe that missing transverse energy cut will not improve
mUED discovery reach significantly. We have found that opposite sign di-lepton
channel is the most promising discovery mode and with first of
collected luminosity, LHC will be able to discover the strongly interacting
KK particles with masses upto 800 to 900 GeV.Comment: 23 pages, 7 figures and one tabl
Modelling stochastic bivariate mortality
Stochastic mortality, i.e. modelling death arrival via a jump process with stochastic intensity, is gaining increasing reputation as a way to represent mortality risk. This paper represents a first attempt to model the mortality risk of couples of individuals, according to the stochastic intensity approach.
On the theoretical side, we extend to couples the Cox processes set up, i.e. the idea that mortality is driven by a jump process whose intensity is itself a stochastic process, proper of a particular generation within each gender. Dependence between the survival times of the members of a couple is captured by an Archimedean copula.
On the calibration side, we fit the joint survival function by calibrating separately the (analytical) copula and the (analytical) margins. First, we select the best fit copula according to the methodology of Wang and Wells (2000) for censored data. Then, we provide a sample-based calibration for the intensity, using a time-homogeneous, non mean-reverting, affine process: this gives the analytical marginal survival functions. Coupling the best fit copula with the calibrated margins we obtain, on a sample generation, a joint survival function which incorporates the stochastic nature of mortality improvements and is far from representing independency.On the contrary, since the best fit copula turns out to be a Nelsen one, dependency is increasing with age and long-term dependence exists
Signals from R-parity violating top quark decays at LHC
We evaluate the potential of the CERN LHC collider to observe rare decays of
the top quark in channels involving R-parity violating (RPV) interactions. We
stress the importance of calculating top quark production and decay
simultaneously as a true 2->4 process.
The process of tt-bar pair production followed by RPV decay of one of the top
quarks is analyzed with fast detector simulation. We show that intermediate
supersymmetric particles can be observed as resonances even if they are heavier
than the top quark due to the significant off-shell top-quark mass effects. The
approach where the top quark is produced on-mass-shell and then decays into 2-
or 3-body final state would in general lead to incorrect kinematical
distributions and rates. The rates of the 2 -> 4 process with top quark
production and RPV 3-body decay depend on the total width of the heavy
intermediate sfermion which could,therefore, be measured indirectly.
We find that the LHC collider offers a unique potential to study rare top
quark decays in the framework of supersymmetry with broken R-parity for
branching fractions of RPV top decays as low as 10^{-6}Comment: 23 pages, 22 figure
UV laser controlled quantum well intermixing in InAlGaAs/GaAs heterostructures
Abstract : The influence of surface irradiation of GaAs with a KrF excimer laser on the magnitude of the quantum well intermixing (QWI) effect has been investigated on GaAs/AlGaAs and GaAs/AlGaAs/InAlGaAs QWs heterostructures. The selective area irradiation through a SiOx mask was carried out in an atmospheric environment. Following the 1000 pulses irradiation at 100 mJ/cm2, the samples were annealed in a rapid thermal annealing furnace at 900 °C. Photoluminescence mapping and cathodoluminescence measurements show that significant laser-induced suppression of the QWI process can be achieved with lateral resolution of the order of 1μm
Status of the PICASSO Project
The Picasso project is a dark matter search experiment based on the
superheated droplet technique. Preliminary runs performed at the Picasso Lab in
Montreal have showed the suitability of this detection technique to the search
for weakly interacting cold dark matter particles. In July 2002, a new phase of
the project started. A batch of six 1-liter detectors with an active mass of
approximately 40g was installed in a gallery of the SNO observatory in Sudbury,
Ontario, Canada at a depth of 6,800 feet (2,070m). We give a status report on
the new experimental setup, data analysis, and preliminary limits on
spin-dependent neutralino interaction cross section.Comment: 3 pages, 2 figures. To appear in the Proceedings of the TAUP 2003
conference, 5-9 September, 2003, University of Washington, Seattle, US
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