1,530 research outputs found
Supersymmetric NLO QCD Corrections to Resonant Slepton Production and Signals at the Tevatron and the LHC
We compute the total cross section and the transverse momentum distribution
for single charged slepton and sneutrino production at hadronic colliders
including NLO supersymmetric and non-supersymmetric QCD corrections. The
supersymmetric QCD corrections can be substantial. We also resum the gluon
transverse momentum distribution and compare our results with two Monte Carlo
generators. We compute branching ratios of the supersymmetric decays of the
slepton and determine event rates for the like-sign dimuon final state at the
Tevatron and at the LHC.Comment: 14 pages, LaTeX, 8 figures, uses REVTex
Closing the Symmetry at Electroweak Scale
We show that some models with gauge
symmetry can be realized at the electroweak scale and that this is a
consequence of an approximate global symmetry. This symmetry
implies a condition among the vacuum expectation value of one of the neutral
Higgs scalars, the 's coupling constant, , the sine of the weak
mixing angle , and the mass of the boson, . In the limit
in which this symmetry is valid it avoids the tree level mixing of the
boson of the Standard Model with the extra boson. We have verified
that the oblique parameter is within the allowed range indicating that the
radiative corrections that induce such a mixing at the 1-loop level are small.
We also show that a custodial symmetry implies that in some of
the models we have to include sterile (singlets of the 3-3-1 symmetry)
right-handed neutrinos with Majorana masses, being the see-saw mechanism
mandatory to obtain light active neutrinos. Moreover, the approximate
symmetry implies that the extra non-standard
particles of these 3-3-1 models can be considerably lighter than it had been
thought before so that new physics can be really just around the corner.Comment: 32 pages, no figure, RevTeX. Some typos correcte
Measuring a Light Neutralino Mass at the ILC: Testing the MSSM Neutralino Cold Dark Matter Model
The LEP experiments give a lower bound on the neutralino mass of about 46 GeV
which, however, relies on a supersymmetric grand unification relation. Dropping
this assumption, the experimental lower bound on the neutralino mass vanishes
completely. Recent analyses suggest, however, that in the minimal
supersymmetric standard model (MSSM), a light neutralino dark matter candidate
has a lower bound on its mass of about 7 GeV. In light of this, we investigate
the mass sensitivity at the ILC for very light neutralinos. We study slepton
pair production, followed by the decay of the sleptons to a lepton and the
lightest neutralino. We find that the mass measurement accuracy for a few-GeV
neutralino is around 2 GeV, or even less if the relevant slepton is
sufficiently light. We thus conclude that the ILC can help verify or falsify
the MSSM neutralino cold dark matter model even for very light neutralinos.Comment: 7 pages, 1 figure; references adde
A screening mechanism for extra W and Z gauge bosons
We generalize a previous construction of a fermiophobic model to the case of
more than one extra and gauge bosons. We focus in particular on the
existence of screening configurations and their implication on the gauge boson
mass spectrum. One of these configurations allows for the existence of a set of
relatively light new gauge bosons, without violation of the quite restrictive
bounds coming from the parameter. The links with Bess and
degenerate Bess models are also discussed. Also the signal given here by this
more traditional gauge extension of the SM could help to disentangle it from
the towers of Kaluza-Klein states over and gauge bosons in extra
dimensions.Comment: 23 pages, 1 figure, extended discussion on precision tests. To appear
in International Journal of Modern Physics
Natural media workshop
This workshop will examine what our current imaging and sensing technologies do to our perception. We will examine, using practical examples, the potential to develop more 'Natural Media’ and technologies by broadening the focus of attention to the whole visual, auditory, tactile and sensual field. The aim is to re-incorporate peripheral awareness into our experience using these multiple sense inputs
Gravitational radiation in d>4 from effective field theory
Some years ago, a new powerful technique, known as the Classical Effective
Field Theory, was proposed to describe classical phenomena in gravitational
systems. Here we show how this approach can be useful to investigate
theoretically important issues, such as gravitational radiation in any
spacetime dimension. In particular, we derive for the first time the
Einstein-Infeld-Hoffman Lagrangian and we compute Einstein's quadrupole formula
for any number of flat spacetime dimensions.Comment: 32 pages, 10 figures. v2: Factor in eq. (3.11) fixed. References
adde
Unitarity and Bounds on the Scale of Fermion Mass Generation
The scale of fermion mass generation can, as shown by Appelquist and
Chanowitz, be bounded from above by relating it to the scale of unitarity
violation in the helicity nonconserving amplitude for fermion-anti-fermion
pairs to scatter into pairs of longitudinally polarized electroweak gauge
bosons. In this paper, we examine the process t tbar -> W_L W_L in a family of
phenomenologically-viable deconstructed Higgsless models and we show that scale
of unitarity violation depends on the mass of the additional vector-like
fermion states that occur in these theories (the states that are the
deconstructed analogs of Kaluza-Klein partners of the ordinary fermions in a
five-dimensional theory). For sufficiently light vector fermions, and for a
deconstructed theory with sufficiently many lattice sites (that is,
sufficiently close to the continuum limit), the Appelquist-Chanowitz bound can
be substantially weakened. More precisely, we find that, as one varies the mass
of the vector-like fermion for fixed top-quark and gauge-boson masses, the
bound on the scale of top-quark mass generation interpolates smoothly between
the Appelquist-Chanowitz bound and one that can, potentially, be much higher.
In these theories, therefore, the bound on the scale of fermion mass generation
is independent of the bound on the scale of gauge-boson mass generation. While
our analysis focuses on deconstructed Higgsless models, any theory in which
top-quark mass generation proceeds via the mixing of chiral and vector fermions
will give similar results.Comment: 12 pages, 11 eps figures included, revtex. Refrences added; wording
modified slightly to emphasize focus on top-quar
Scales of Fermion Mass Generation and Electroweak Symmetry Breaking
The scale of mass generation for fermions (including neutrinos) and the scale
for electroweak symmetry breaking (EWSB) can be bounded from above by the
unitarity of scattering involving longitudinal weak gauge bosons or their
corresponding would-be Goldstone bosons. Including the exact n-body phase space
we analyze the 2 --> n () processes for the fermion-(anti)fermion
scattering into multiple gauge boson final states. Contrary to naive energy
power counting, we demonstrate that as becomes large, the competition
between an increasing energy factor and a phase-space suppression leads to a
{\it strong new upper bound} on the scale of fermion mass generation at a
finite value , which is {\it independent of the EWSB scale,} . For quarks, leptons and Majorana neutrinos, the
strongest 2 --> n limits range from about 3TeV to 130-170TeV (with ), depending on the measured fermion masses. Strikingly, given
the tiny neutrino masses as constrained by the neutrino oscillations,
neutrinoless double-beta decays and astrophysical observations, the unitarity
violation of scattering actually occurs at a scale no
higher than ~170 TeV. Implications for various mechanisms of neutrino mass
generation are analyzed. On the other hand, for the 2 --> n pure
Goldstone-boson scattering, we find that the decreasing phase space factor
always dominates over the growing overall energy factor when becomes large,
so that the best unitarity bound on the scale of EWSB remains at n=2.Comment: 67pp, to match PRD (minor typos fixed
The Equivalence Theorem and Effective Lagrangians
We point out that the equivalence theorem, which relates the amplitude for a
process with external longitudinally polarized vector bosons to the amplitude
in which the longitudinal vector bosons are replaced by the corresponding
pseudo-Goldstone bosons, is not valid for effective Lagrangians. However, a
more general formulation of this theorem also holds for effective interactions.
The generalized theorem can be utilized to determine the high-energy behaviour
of scattering processes just by power counting and to simplify the calculation
of the corresponding amplitudes. We apply this method to the phenomenologically
most interesting terms describing effective interactions of the electroweak
vector and Higgs bosons in order to examine their effects on vector-boson
scattering and on vector-boson-pair production in annihilation. The
use of the equivalence theorem in the literature is examined.Comment: 20 pages LaTeX, BI-TP 94/1
Non locality and causal evolution in QFT
Non locality appearing in QFT during the free evolution of localized field
states and in the Feynman propagator function is analyzed. It is shown to be
connected to the initial non local properties present at the level of quantum
states and then it does not imply a violation of Einstein's causality. Then it
is investigated a simple QFT system with interaction, consisting of a classical
source coupled linearly to a quantum scalar field, that is exactly solved. The
expression for the time evolution of the state describing the system is given.
The expectation value of any arbitrary ``good'' local observable, expressed as
a function of the field operator and its space and time derivatives, is
obtained explicitly at all order in the field-matter coupling constant. These
expectation values have a source dependent part that is shown to be always
causally retarded, while the non local contributions are source independent and
related to the non local properties of zero point vacuum fluctuations.Comment: Submitted to Journal of Physics B: 16 pages: 1 figur
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