4,799 research outputs found
First Hints of Jet Quenching at RHIC
At this conference first data from RHIC has been presented. Spectra of
charged hadrons and identified neutral pions obtained in central collisions
exhibit a depletion at large transverse momenta compared to expectations
deduced from and data and lower energy heavy ion data. While
spectra measured in peripheral collisions exhibit the expected power-law shape,
spectra from central collisions are closer to exponential. In addition, a
significant azimuthal anisotropy of high momentum charged particle production
has been found. All observations are in qualitative agreement with theoretical
predictions that quark matter formed in heavy ion collisions quenches jet
production.Comment: Proceedings of a summary talk given at the Quark Matter 2001
conferenc
Mitigation of the LHC Inverse Problem
The LHC inverse problem refers to the difficulties in determining the
parameters of an underlying theory from data (to be) taken by the LHC
experiments: if they find signals of new physics, and an underlying theory is
assumed, could its parameters be determined uniquely, or do different parameter
choices give indistinguishable experimental signatures? This inverse problem
was studied before for a supersymmetric Standard Model with 15 free parameters.
This earlier study found 283 indistinguishable pairs of parameter choices,
called degenerate pairs, even if backgrounds are ignored. We can resolve all
but 23 of those pairs by constructing a true \chi^2 distribution using mostly
counting observables. The elimination of systematic errors would even allow
separating the residual degeneracies. Taking the Standard Model background into
account we still can resolve 237 of the 283 "degenerate" pairs. This indicates
that (some of) our observables should also be useful for the purpose of
determining the values of SUSY parameters.Comment: 32 pages, 13 figures, typo in (3.6) corrected, version to appear in
Phys. Rev.
CP-violating Supersymmetric Higgs at the Tevatron and LHC
We analyze the prospect for observing the intermediate neutral Higgs boson
() in its decay to two lighter Higgs bosons () at the presently
operating hadron colliders in the framework of the CP violating MSSM using the
PYTHIA event generator. We consider the lepton+ 4-jets+ \met channel from
associate production, with W h_2 \ra W h_1 h_1 \ra \ell \nu_\ell b
\bar b b\bar b. We require two, three or four tagged -jets. We explicitly
consider all relevant Standard Model backgrounds, treating -jets separately
from light flavor and gluon jets and allowing for mistagging. We find that it
is very hard to observe this signature at the Tevatron, even with 20 fb
of data, in the LEP--allowed region of parameter space due to the small signal
efficiency, even though the background is manageable. At the LHC, a priori huge
SM backgrounds can be suppressed by applying judiciously chosen kinematical
selections. After all cuts, we are left with a signal cross section of around
0.5 fb, and a signal to background ratio between 1.2 and 2.9. According to our
analysis this Higgs signal should be viable at the LHC in the vicinity of
present LEP exclusion once 20 to 50 fb of data have been accumulated at
TeV.Comment: 27 pages, 12 figure
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
MRI and clinical resolution of a suspected intracranial toxoplasma granuloma with medical treatment in a domestic short hair cat
A two-year-old cat was presented with a left paradoxical vestibular syndrome. MRI of the brain revealed an extra-axial homogenously contrast enhancing mass in the region of the left caudal cerebellar peduncle. Toxoplasma serology was consistent with active infection and the lesion was suspected to be a toxoplasma granuloma. Following eight weeks of tapering oral prednisolone and 11 weeks of oral clindamycin treatment, repeat MRI revealed resolution of the lesion. Eighteen months after initial diagnosis, the cat remained neurologically normal. Differential diagnoses for a solitary, extra-axial, contrast enhancing mass lesion in the feline brain should include toxoplasma granuloma, which can undergo MRI and clinical resolution with medical treatment
Hour-glass magnetic spectrum in a stripe-less insulating transition metal oxide
An hour-glass shaped magnetic excitation spectrum appears to be an universal
characteristic of the high-temperature superconducting cuprates. Fluctuating
charge stripes or alternative band structure approaches are able to explain the
origin of these spectra. Recently, an hour- glass spectrum has been observed in
an insulating cobaltate, thus, favouring the charge stripe scenario. Here we
show that neither charge stripes nor band structure effects are responsible for
the hour-glass dispersion in a cobaltate within the checkerboard charge ordered
regime of La2-xSrxCoO4. The search for charge stripe ordering reflections
yields no evidence for charge stripes in La1.6Sr0.4CoO4 which is supported by
our phonon studies. With the observation of an hour-glass-shaped excitation
spectrum in this stripe-less insulating cobaltate, we provide experimental
evidence that the hour-glass spectrum is neither necessarily connected to
charge stripes nor to band structure effects, but instead, probably intimately
coupled to frustration and arising chiral or non-collinear magnetic
correlations
Weighing the universe with accelerators and detectors
Suppose the lightest superpartner (LSP) is observed at colliders, and WIMPs
are detected in explicit experiments. We point out that one cannot immediately
conclude that cold dark matter (CDM) of the universe has been observed, and we
determine what measurements are necessary before such a conclusion is
meaningful. We discuss the analogous situation for neutrinos and axions; in the
axion case we have not found a way to conclude axions are the CDM even if
axions are detected.Comment: 15 pages, 3 figures; minor changes included and typos fixe
Open charm contribution to dilepton spectra produced in nuclear collisions at SPS energies
Measurements of open charm hadro-production from CERN and Fermilab
experiments are reviewed, with particular emphasis on the absolute cross
sections and on their A and sqrt(s) dependences. Differential pt and xf cross
sections calculated with the Pythia event generator are found to be in
reasonable agreement with recent data. The calculations are scaled to
nucleus-nucleus collisions and the expected lepton pair yield is deduced. The
charm contribution to the low mass dilepton continuum observed by the CERES
experiment is found to be negligible. In particular, it is shown that the
observed low mass dilepton excess in S-Au collisions cannot be explained by
charm enhancement.Comment: 19 pages, 12 eps figures included. To be published in Z.Phys.
Inference on the tail process with application to financial time series modelling
To draw inference on serial extremal dependence within heavy-tailed Markov
chains, Drees, Segers and Warcho{\l} [Extremes (2015) 18, 369--402] proposed
nonparametric estimators of the spectral tail process. The methodology can be
extended to the more general setting of a stationary, regularly varying time
series. The large-sample distribution of the estimators is derived via
empirical process theory for cluster functionals. The finite-sample performance
of these estimators is evaluated via Monte Carlo simulations. Moreover, two
different bootstrap schemes are employed which yield confidence intervals for
the pre-asymptotic spectral tail process: the stationary bootstrap and the
multiplier block bootstrap. The estimators are applied to stock price data to
study the persistence of positive and negative shocks.Comment: 22 page
Higgs funnel region of SUSY dark matter for small and renormalization group effects on pseudoscalar Higgs boson with scalar mass non-universality
A non-universal scalar mass supergravity type of model is explored where the
first two generation of scalars and the third generation of sleptons may be
very massive. Lighter or vanishing third generation of squarks as well as Higgs
scalars at the unification scale cause the radiative electroweak symmetry
breaking constraint to be less prohibitive. Thus, both FCNC/CP-violation
problems as well as the naturalness problem are within control. We identify a
large slepton mass effect in the RGE of (for the down type of
Higgs) that may turn the later negative at the electroweak scale even for a
small . A hyperbolic branch/focus point like effect is found for
that may result in very light Higgs spectra. The lightest stable
particle is dominantly a bino that pair annihilates via Higgs exchange, giving
rise to a WMAP satisfied relic density region for all . Detection
prospects of such LSPs in the upcoming dark matter experiments both of direct
and indirect types (photon flux) are interesting. The Higgs bosons and the
third generation of squarks are light in this scenario and these may be easily
probed besides charginos and neutralinos in the early runs of LHC.Comment: 36 pages and 7 Postscript files. Minor changes in the text. Version
accepted for publication in Phys. Rev.
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