296 research outputs found
Top Quark Production Cross Section at the Tevatron
An overview of the preliminary results of the top quark pair production cross
section measurements at a center-of-mass energy of 1.96 TeV carried out by the
CDF and D0 collaborations is presented. The data samples used for the analyses
are collected in the current Tevatron run and correspond to an integrated
luminosity from 360 pb-1 up to 760 pb-1.Comment: 4 pages, 5 figures, Proceedings of 41th Rencontres De Moriond: QCD
And Hadronic Interactions, 18-25 Mar 2006, La Thuile, Ital
Top Quark Mass Measurements at CDF
The mass of the top quark M_top is interesting both as a fundamental
parameter of the standard model and as an important input to precision
electroweak tests. The Collider Detector at Fermilab (CDF) has a robust program
of top quark mass analyses, including the most precise single measurement,
M_top = 173.4 +/- 2.8 GeV/c^2, using 680 pb^-1 of ppbar collision data. A
combination of current results from CDF gives M_top = 172.0 +/- 2.7 GeV/c^2,
surpassing the stated goal of 3 GeV/c^2 precision using 2 fb^-1 of data.
Finally, a combination with current D0 results gives a world average top quark
mass of 172.5 +/- 2.3 GeV/c^2.Comment: 8 pages, Contribution to Proceedings of the 41st Rencontres de
Moriond: Electroweak Interactions and Unified Theories, La Thuile, Italy,
11-18 March 200
Stacked Weak Lensing Mass Calibration: Estimators, Systematics, and Impact on Cosmological Parameter Constraints
When extracting the weak lensing shear signal, one may employ either locally
normalized or globally normalized shear estimators. The former is the standard
approach when estimating cluster masses, while the latter is the more common
method among peak finding efforts. While both approaches have identical
signal-to-noise in the weak lensing limit, it is possible that higher order
corrections or systematics considerations make one estimator preferable over
the other. In this paper, we consider the efficacy of both estimators within
the context of stacked weak lensing mass estimation in the Dark Energy Survey
(DES). We find the two estimators have nearly identical statistical precision,
even after including higher order corrections, but that these corrections must
be incorporated into the analysis to avoid observationally relevant biases in
the recovered masses. We also demonstrate that finite bin-width effects may be
significant if not properly accounted for, and that the two estimators exhibit
different systematics, particularly with respect to contamination of the source
catalog by foreground galaxies. Thus, the two estimators may be employed as a
systematics cross-check of each other. Stacked weak lensing in the DES should
allow for the mean mass of galaxy clusters to be calibrated to about 2%
precision (statistical only), which can improve the figure of merit of the DES
cluster abundance experiment by a factor of ~3 relative to the self-calibration
expectation. A companion paper (Schmidt & Rozo, 2010) investigates how the two
types of estimators considered here impact weak lensing peak finding efforts.Comment: 14 pages, 9 figures; comments welcom
Improved Limits on Spin-Dependent WIMP-Proton Interactions from a Two Liter CFI Bubble Chamber
Data from the operation of a bubble chamber filled with 3.5 kg of CFI
in a shallow underground site are reported. An analysis of ultrasound signals
accompanying bubble nucleations confirms that alpha decays generate a
significantly louder acoustic emission than single nuclear recoils, leading to
an efficient background discrimination. Three dark matter candidate events were
observed during an effective exposure of 28.1 kg-day, consistent with a neutron
background. This observation provides the strongest direct detection constraint
to date on WIMP-proton spin-dependent scattering for WIMP masses
GeV/c.Comment: 4 pages, 4 figures V2 submitted to match journal versio
Constraints on B and Higgs Physics in Minimal Low Energy Supersymmetric Models
We study the implications of minimal flavor violating low energy
supersymmetry scenarios for the search of new physics in the B and Higgs
sectors at the Tevatron collider and the LHC. We show that the already
stringent Tevatron bound on the decay rate B_s -> mu+ mu- sets strong
constraints on the possibility of generating large corrections to the mass
difference Delta M_s of the B_s eigenstates. We also show that the B_s -> mu+
mu- bound together with the constraint on the branching ratio of the rare decay
b -> s gamma has strong implications for the search of light, non-standard
Higgs bosons at hadron colliders. In doing this, we demonstrate that the former
expressions derived for the analysis of the double penguin contributions in the
Kaon sector need to be corrected by additional terms for a realistic analysis
of these effects. We also study a specific non-minimal flavor violating
scenario, where there are flavor changing gluino-squark-quark interactions,
governed by the CKM matrix elements, and show that the B and Higgs physics
constraints are similar to the ones in the minimal flavor violating case.
Finally we show that, in scenarios like electroweak baryogenesis which have
light stops and charginos, there may be enhanced effects on the B and K mixing
parameters, without any significant effect on the rate of B_s -> mu+ mu-.Comment: 40 pages, 14 figures; added references and note about recent
measurement
Primordial non-Gaussianity and Dark Energy constraints from Cluster Surveys
Galaxy cluster surveys will be a powerful probe of dark energy. At the same
time, cluster abundance is sensitive to any non-Gaussianity of the primordial
density field. It is therefore possible that non-Gaussian initial conditions
might be misinterpreted as a sign of dark energy or at least degrade the
expected constraints on dark energy parameters. To address this issue, we
perform a likelihood analysis of an ideal cluster survey similar in size and
depth to the upcoming South Pole Telescope/Dark Energy Survey (SPT-DES). We
analyze a model in which the strength of the non-Gaussianity is parameterized
by the constant fNL; this model has been used extensively to derive Cosmic
Microwave Background (CMB) anisotropy constraints on non-Gaussianity, allowing
us to make contact with those works. We find that the constraining power of the
cluster survey on dark energy observables is not significantly diminished by
non-Gaussianity provided that cluster redshift information is included in the
analysis. We also find that even an ideal cluster survey is unlikely to improve
significantly current and future CMB constraints on non-Gaussianity. However,
when all systematics are under control, it could constitute a valuable cross
check to CMB observations.Comment: 10 pages, 4 figures. Corrected a minor discrepancy between our
earlier definition of fNL and CMB constraints. References adde
Non-Gaussianity from Broken Symmetries
Recently we studied inflation models in which the inflaton potential is
characterized by an underlying approximate global symmetry. In the first work
we pointed out that in such a model curvature perturbations are generated after
the end of the slow-roll phase of inflation. In this work we develop further
the observational implications of the model and compute the degree of
non-Gaussianity predicted in the scenario. We find that the corresponding
nonlinearity parameter, , can be as large as 10^2.Comment: 7 pages, 1 figur
A Method for the Precision Mass Measurement of the Stop Quark at the International Linear Collider
Many supersymmetric models predict new particles within the reach of the next
generation of colliders. For an understanding of the model structure and the
mechanism(s) of symmetry breaking, it is important to know the masses of the
new particles precisely. In this article the measurement of the mass of the
scalar partner of the top quark (stop) at an e+e- collider is studied. A
relatively light stop is motivated by attempts to explain electroweak
baryogenesis and can play an important role in dark matter relic density. A
method is presented which makes use of cross-section measurements near the
pair-production threshold as well as at higher center-of-mass energies. It is
shown that this method not only increases the statistical precision, but also
greatly reduces the systematic uncertainties, which can be important. Numerical
results are presented, based on a realistic event simulation, for two signal
selection strategies: using conventional selection cuts, and using an Iterative
Discriminant Analysis (IDA). Our studies indicate that a precision of
\Delta\mstop = 0.42 GeV can be achieved, representing a major improvement
over previous studies. While the analysis of stops is particularly challenging
due to the possibility of stop hadronization, the general procedure could be
applied to the mass measurement of other particles as well. We also comment on
the potential of the IDA to discover a stop quark in this scenario, and we
revisit the accuracy of the theoretical predictions for the neutralino relic
densityComment: 41 pages, 14 figures, in JHEP forma
Can Cosmic Structure form without Dark Matter?
One of the prime pieces of evidence for dark matter is the observation of
large overdense regions in the universe. Since we know from the cosmic
microwave background that the regions that contained the most baryons when the
universe was ~400,000 years old were overdense by only one part in ten
thousand, perturbations had to have grown since then by a factor greater than
where is the epoch of recombination. This enhanced
growth does not happen in general relativity, so dark matter is needed in the
standard theory. We show here that enhanced growth can occur in alternatives to
general relativity, in particular in Bekenstein's relativistic version of
MOdified Newtonian Dynamics (MOND). The vector field introduced in that theory
for a completely different reason plays a key role in generating the
instability that produces large cosmic structures today.Comment: 5 pages, 3 figure
What Can Gamma Ray Bursts Teach Us About Dark Energy?
It has been suggested that Gamma Ray Bursts (GRB) may enable the expansion
rate of our Universe to be measured out to very high redshifts (z \gsim 5)
just as type Ia supernovae have done at 1--1.5. We explore this
possibility here, and find that GRB have the potential to detect dark energy at
high statistical significance, but they are unlikely to be competitive with
future supernovae missions, such as SNAP, in measuring the properties of the
dark energy. The exception to this conclusion is if there is appreciable dark
energy at early times, in which case the information from GRB's will provide an
excellent complement to the information from supernovae.Comment: 5 pages, 9 figure
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