3,791 research outputs found
Standard model explanation of a CDF dijet excess in Wjj
We demonstrate the recent observation of a peak in the dijet invariant mass
of the Wjj signal observed by the CDF Collaboration can be explained as the
same upward fluctuation observed by CDF in single-top-quark production. In
general, both t-channel and s-channel single-top-quark production produce
kinematically induced peaks in the dijet spectrum. Since CDF used a Monte Carlo
simulation to subtract the single-top backgrounds instead of data, a peak in
the dijet spectrum is expected. The D0 Collaboration has a small upward
fluctuation in their published t-channel data; and hence we predict they would
see at most a small peak in the dijet invariant mass spectrum of Wjj if they
follow the same procedure as CDF.Comment: 3 pg., 2 figs, revtex, minor clarifications, to appear in Phys. Rev.
Stringy origin of Tevatron Wjj anomaly
The invariant mass distribution of dijets produced in association with W
bosons, recently observed by the CDF Collaboration at Tevatron, reveals an
excess in the dijet mass range 120-160 GeV/c^2, 3\sigma beyond Standard Model
expectations. We show that such an excess is a generic feature of low mass
string theory, due to the production and decay of a leptophobic Z', a singlet
partner of SU(3) gluons coupled primarily to the U(1) baryon number. In this
framework, U(1) and SU(3) appear as subgroups of U(3) associated with open
strings ending on a stack of 3 D-branes. In addition, a minimal model contains
two other stacks to accommodate the electro-weak SU(2) \in U(2) and the
hypercharge U(1). Of the three U(1) gauge bosons, the two heavy Z' and Z"
receive masses through the Green-Schwarz mechanism. For a given Z' mass, the
model is quite constrained. Fine tuning three of its free parameters is just
sufficient to simultaneously ensure: a small Z-Z' mixing in accord with the
stringent LEP data on the mass; very small (less than 1%) branching ratio
into leptons; and a large hierarchy between Z" and Z' masses. The heavier
neutral gauge boson Z" is within the reach of LHC.Comment: v1 8 pages revtex; v2 better fit to the data (Z" within LHC reach),
references added; v3 to appear in PL
Prompt Decays of General Neutralino NLSPs at the Tevatron
Recent theoretical developments have shown that gauge mediation has a much
larger parameter space of possible spectra and mixings than previously
considered. Motivated by this, we explore the collider phenomenology of gauge
mediation models where a general neutralino is the lightest MSSM superpartner
(the NLSP), focusing on the potential reach from existing and future Tevatron
searches. Promptly decaying general neutralino NLSPs can give rise to final
states involving missing energy plus photons, Zs, Ws and/or Higgses. We survey
the final states and determine those where the Tevatron should have the most
sensitivity. We then estimate the reach of existing Tevatron searches in these
final states and discuss new searches (or optimizations of existing ones) that
should improve the reach. Finally we comment on the potential for discovery at
the LHC.Comment: 41 pages, minor changes, added refs and discussion of previous
literatur
Searches for Physics Beyond the Standard Model at Colliders
All experimental measurements of particle physics today are beautifully
described by the Standard Model. However, there are good reasons to believe
that new physics may be just around the corner at the TeV energy scale. This
energy range is currently probed by the Tevatron and HERA accelerators and
selected results of searches for physics beyond the Standard Model are
presented here. No signals for new physics have been found and limits are
placed on the allowed parameter space for a variety of different particles.Comment: Proceedings for 2007 Europhysics Conference on High Energy Physics,
Manchester, July 200
Measurements of the Production, Decay and Properties of the Top Quark: A Review
With the full Tevatron Run II and early LHC data samples, the opportunity for
furthering our understanding of the properties of the top quark has never been
more promising. Although the current knowledge of the top quark comes largely
from Tevatron measurements, the experiments at the LHC are poised to probe
top-quark production and decay in unprecedented regimes. Although no current
top quark measurements conclusively contradict predictions from the standard
model, the precision of most measurements remains statistically limited.
Additionally, some measurements, most notably the forward-backward asymmetry in
top quark pair production, show tantalizing hints of beyond-the-Standard-Model
dynamics. The top quark sample is growing rapidly at the LHC, with initial
results now public. This review examines the current status of top quark
measurements in the particular light of searching for evidence of new physics,
either through direct searches for beyond the standard model phenomena or
indirectly via precise measurements of standard model top quark properties
Probabilistic Particle Flow Algorithm for High Occupancy Environment
Algorithms based on the particle flow approach are becoming increasingly
utilized in collider experiments due to their superior jet energy and missing
energy resolution compared to the traditional calorimeter-based measurements.
Such methods have been shown to work well in environments with low occupancy of
particles per unit of calorimeter granularity. However, at higher instantaneous
luminosity or in detectors with coarse calorimeter segmentation, the overlaps
of calorimeter energy deposits from charged and neutral particles significantly
complicate particle energy reconstruction, reducing the overall energy
resolution of the method. We present a technique designed to resolve
overlapping energy depositions of spatially close particles using a
statistically consistent probabilistic procedure. The technique is nearly free
of ad-hoc corrections, improves energy resolution, and provides new important
handles that can improve the sensitivity of physics analyses: the uncertainty
of the jet energy on an event-by-event basis and the estimate of the
probability of a given particle hypothesis for a given detector response. When
applied to the reconstruction of hadronic jets produced in the decays of tau
leptons using the CDF-II detector at Fermilab, the method has demonstrated
reliable and robust performance.Comment: Accepted by Nuclear Instruments and Methods
A Fast Track towards the `Higgs' Spin and Parity
The LHC experiments ATLAS and CMS have discovered a new boson that resembles
the long-sought Higgs boson: it cannot have spin one, and has couplings to
other particles that increase with their masses, but the spin and parity remain
to be determined. We show here that the `Higgs' + gauge boson invariant-mass
distribution in `Higgs'-strahlung events at the Tevatron or the LHC would be
very different under the J^P = 0+, 0- and 2+ hypotheses, and could provide a
fast-track indicator of the `Higgs' spin and parity. Our analysis is based on
simulations of the experimental event selections and cuts using PYTHIA and
Delphes, and incorporates statistical samples of `toy' experiments.Comment: 18 pages, 9 pdf figure
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