413 research outputs found

### Semileptonic decays of the standard Higgs boson

The Higgs boson decay into a pair of real or virtual W bosons, with one of
them decaying leptonically, is predicted within the Standard Model to have the
largest branching fraction of all Higgs decays that involve an isolated
electron or muon, for M_h > 120 GeV. We compute analytically the
fully-differential width for this h -> l \nu jj decay at tree level, and then
explore some multi-dimensional cuts that preserve the region of large signal.
Future searches for semileptonic decays at the Tevatron and LHC, employing
fully-differential information as outlined here, may be essential for ruling
out or in the Higgs boson and for characterizing a Higgs signal.Comment: 17 pages, 5 .eps figure

### Singular ways to search for the Higgs boson

The discovery or exclusion of the fundamental standard scalar is a hot topic,
given the data of LEP, the Tevatron and the LHC, as well as the advanced status
of the pertinent theoretical calculations. With the current statistics at the
hadron colliders, the workhorse decay channel, at all relevant H masses, is H
to WW, followed by W to light leptons. Using phase-space singularity
techniques, we construct and study a plethora of "singularity variables" meant
to facilitate the difficult tasks of separating signal and backgrounds and of
measuring the mass of a putative signal. The simplest singularity variables are
not invariant under boosts along the collider's axes and the simulation of
their distributions requires a good understanding of parton distribution
functions, perhaps not a serious shortcoming during the boson hunting season.
The derivation of longitudinally boost-invariant variables, which are functions
of the four charged-lepton observables that share this invariance, is quite
elaborate. But their use is simple and they are, in a kinematical sense,
optimal.Comment: 19 pages, including 21 figure

### Diagnosing Spin at the LHC via Vector Boson Fusion

We propose a new technique for determining the spin of new massive particles
that might be discovered at the Large Hadron Collider. The method relies on
pair-production of the new particles in a kinematic regime where the vector
boson fusion production mechanism is enhanced. For this regime, we show that
the distribution of the leading jets as a function of their relative azimuthal
angle can be used to distinguish spin-0 from spin-1/2 particles. We illustrate
this effect by considering the particular cases of (i) strongly-interacting,
stable particles and (ii) supersymmetric particles carrying color charge. We
argue that this method should be applicable in a wide range of new physics
scenarios.Comment: 5 pages, 4 figure

### Review of Top Quark Physics

We present an overview of Top Quark Physics - from what has been learned so
far at the Tevatron, to the searches that lie ahead at present and future
colliders. We summarize the richness of the measurements and discuss their
possible impact on our understanding of the Standard Model by pointing out
their key elements and limitations. When possible, we discuss how the top quark
may provide a connection to new or unexpected physics.Comment: 84 pp. With permission from the Annual Review of Nuclear & Particle
Science. Final version of this material is scheduled to appear in the Annual
Review of Nuclear & Particle Science Vol. 53, to be published in December
2003 by Annual Reviews (http://www.annualreviews.org

### New Physics Signals in Longitudinal Gauge Boson Scattering at the LHC

We introduce a novel technique designed to look for signatures of new physics
in vector boson fusion processes at the TeV scale. This functions by measuring
the polarization of the vector bosons to determine the relative longitudinal to
transverse production. In studying this ratio we can directly probe the high
energy E^2-growth of longitudinal vector boson scattering amplitudes
characteristic of models with non-Standard Model (SM) interactions. We will
focus on studying models parameterized by an effective Lagrangian that include
a light Higgs with non-SM couplings arising from TeV scale new physics
associated with the electroweak symmetry breaking, although our technique can
be used in more general scenarios. We will show that this technique is stable
against the large uncertainties that can result from variations in the
factorization scale, improving upon previous studies that measure cross section
alone

### Lorentz-Violating Supergravity

The standard forms of supersymmetry and supergravity are inextricably wedded
to Lorentz invariance. Here a Lorentz-violating form of supergravity is
proposed. The superpartners have exotic properties that are not possible in a
theory with exact Lorentz symmetry and microcausality. For example, the bosonic
sfermions have spin 1/2 and the fermionic gauginos have spin 1. The theory is
based on a phenomenological action that is shown to follow from a simple
microscopic and statistical picture.Comment: 15 pages; to be published in Proceedings of Beyond the Desert 2003
(Castle Ringberg, Tegernsee, Germany, 9-14 June 2003), edited by H. V.
Klapdor-Kleingrothau

### Supersymmetric Monojets at the Large Hadron Collider

Supersymmetric monojets may be produced at the Large Hadron Collider by the
process qg -> squark neutralino_1 -> q neutralino_1 neutralino_1, leading to a
jet recoiling against missing transverse momentum. We discuss the feasibility
and utility of the supersymmetric monojet signal. In particular, we examine the
possible precision with which one can ascertain the neutralino_1-squark-quark
coupling via the rate for monojet events. Such a coupling contains information
on the composition of the neutralino_1 and helps bound dark matter direct
detection cross-sections and the dark matter relic density of the neutralino_1.
It also provides a check of the supersymmetric relation between gauge couplings
and gaugino-quark-squark couplings.Comment: 46 pages, 10 figures. The appendix has been rewritten to correct an
error that appears in all previous versions of the appendix. This error has
no effect on the results in the main body of the pape

### Two Simple W' Models for the Early LHC

W' gauge bosons are good candidates for early LHC discovery. We define two
reference models, one containing a W'_R and one containing a W'_L, which may
serve as ``simplified models'' for presenting experimental results of W'
searches at the LHC. We present the Tevatron bounds on each model and compute
the constraints from precision electroweak observables. We find that indirect
low-energy constraints on the W'_L are quite strong. However, for a W'_R
coupling to right-handed fermions there exists a sizeable region in parameter
space beyond the bounds from the Tevatron and low-energy precision measurements
where even 50 inverse picobarns of integrated LHC luminosity are sufficient to
discover the W'_R. The most promising final states are two leptons and two
jets, or one lepton recoiling against a ``neutrino jet''. A neutrino jet is a
collimated object consisting of a hard lepton and two jets arising from the
decay of a highly boosted massive neutrino.Comment: 20 pages, 8 figures. v2: references adde

### Improving the sensitivity of Higgs boson searches in the golden channel

Leptonic decays of the Higgs boson in the ZZ* channel yield what is known as
the golden channel due to its clean signature and good total invariant mass
resolution. In addition, the full kinematic distribution of the decay products
can be reconstructed, which, nonetheless, is not taken into account in
traditional search strategy relying only on measurements of the total invariant
mass. In this work we implement a type of multivariate analysis known as the
matrix element method, which exploits differences in the full production and
decay matrix elements between the Higgs boson and the dominant irreducible
background from q bar{q} -> ZZ*. Analytic expressions of the differential
distributions for both the signal and the background are also presented. We
perform a study for the Large Hadron Collider at sqrt{s}=7 TeV for Higgs masses
between 175 and 350 GeV. We find that, with an integrated luminosity of 2.5
fb^-1 or higher, improvements in the order of 10 - 20 % could be obtained for
both discovery significance and exclusion limits in the high mass region, where
the differences in the angular correlations between signal and background are
most pronounced.Comment: 31 pages, 8 figures. v2: Minus signs in definitions of angles
corrected. Typos fixed. Reference added. Cosmetic changes to Figure 4.
Additional sentence added for clarificatio

### Hints of R-parity violation in B decays into $\tau \nu$

In this article we show that the recently observed enhanced semi-leptonic and
leptonic decay rates of the B meson into \tau \nu modes can be explained within
the frame work of R-parity violating (RPV) MSSM. In particular, RPV
contributions involving the exchange of right-handed down-type squarks give a
universal contribution to the B+ --> \tau \nu, B --> D \tau \nu and the B -->
D* \tau \nu decays. We find that the masses and couplings that explain the
enhanced B decay rates are phenomelogically viable and the squarks can possibly
be observed at the LHC.Comment: 8 pages and 3 figures; Updated section 2 and 3, but results are
unmodifie

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