713 research outputs found
Confronting Electroweak Fine-tuning with No-Scale Supergravity
Applying No-Scale Supergravity boundary conditions at a heavy unification
scale to the Flipped grand unified theory with extra TeV-scale
vector-like multiplets, No-Scale -, we express the
-boson mass as an explicit function of the boundary gaugino mass
, , with implicit dependence upon a
dimensionless ratio of the supersymmetric Higgs mixing parameter and
. Setting the top Yukawa coupling consistent with GeV at
GeV, the value of naturally tends toward , which
indirectly suggests underlying action of the Giudice-Masiero mechanism.
Proportional dependence of all model scales upon the unified gaugino mass
in the No-Scale - model suggests one possible
mechanism of confronting the electroweak fine tuning problem.Comment: Physics Letters B version; 8 Pages, 7 Figures, 1 Tabl
An Alternative Yukawa Unified SUSY Scenario
Supersymmetric SO(10) Grand Unified Theories with Yukawa unification
represent an appealing possibility for physics beyond the Standard Model.
However Yukawa unification is made difficult by large threshold corrections to
the bottom mass. Generally one is led to consider models where the sfermion
masses are large in order to suppress these corrections. Here we present
another possibility, in which the top and bottom GUT scale Yukawa couplings are
equal to a component of the charged lepton Yukawa matrix at the GUT scale in a
basis where this matrix is not diagonal. Physically, this weak eigenstate
Yukawa unification scenario corresponds to the case where the charged leptons
that are in the 16 of SO(10) containing the top and bottom quarks mix with
their counterparts in another SO(10) multiplet. Diagonalizing the resulting
Yukawa matrix introduces mixings in the neutrino sector. Specifically we find
that for a large region of parameter space with relatively light sparticles,
and which has not been ruled out by current LHC or other data, the mixing
induced in the neutrino sector is such that , in
agreement with data. The phenomenological implications are analyzed in some
detail.Comment: 32 pages, 22 Figure
Combined LHC/ILC analysis of a SUSY scenario with heavy sfermions
We discuss the potential of combined analyses at the Large Hadron Collider
and the planned International Linear Collider to explore low-energy
supersymmetry in a difficult region of the parameter space characterized by
masses of the scalar SUSY particles around 2 TeV. Precision analyses of cross
sections for light chargino production and forward--backward asymmetries of
decay leptons and hadrons at the ILC, together with mass information on chi^0_2
and squarks from the LHC, allow us to determine the underlying fundamental
gaugino/higgsino MSSM parameters and to constrain the masses of the heavy,
kinematically inaccessible sparticles. No assumptions on a specific
SUSY-breaking mechanism are imposed. For this analysis the complete spin
correlations between production and decay processes are taken into account.Comment: new figure added, updated to match the published versio
Top Squarks and Bottom Squarks in the MSSM with Complex Parameters
We present a phenomenological study of top squarks (~t_1,2) and bottom
squarks (~b_1,2) in the Minimal Supersymmetric Standard Model (MSSM) with
complex parameters A_t, A_b, \mu and M_1. In particular we focus on the CP
phase dependence of the branching ratios of (~t_1,2) and (~b_1,2) decays. We
give the formulae of the two-body decay widths and present numerical results.
We find that the effect of the phases on the (~t_1,2) and (~b_1,2) decays can
be quite significant in a large region of the MSSM parameter space. This could
have important implications for (~t_1,2) and (~b_1,2) searches and the MSSM
parameter determination in future collider experiments. We have also estimated
the accuracy expected in the determination of the parameters of ~t_i and ~b_i
by a global fit of the measured masses, decay branching ratios and production
cross sections at e^+ e^- linear colliders with polarized beams. Analysing two
scenarios, we find that the fundamental parameters apart from A_t and A_b can
be determined with errors of 1% to 2%, assuming an integrated luminosity of 1
ab^-1 and a sufficiently large c.m.s. energy to produce also the heavier ~t_2
and ~b_2 states. The parameter A_t can be determined with an error of 2 - 3%,
whereas the error on A_b is likely to be of the order of 50%.Comment: 31 pages, 8 figures, comments and references added, conclusions
unchanged; version to appear in Phys. Rev.
Low-scale warped extra dimension and its predilection for multiple top quarks
Within warped extra dimension models that explain flavor through geometry,
flavor changing neutral current constraints generally force the Kaluza-Klein
scale to be above many TeV. This creates tension with a natural electroweak
scale. On the other hand, a much lower scale compatible with precision
electroweak and flavor changing neutral current constraints is allowed if we
decouple the Kaluza-Klein states of Standard Model gauge bosons from light
fermions bulk mass parameters). The main
signature for this approach is four top quark production via the Kaluza-Klein
excitations' strong coupling to top quarks. We study single lepton, like-sign
dilepton, and trilepton observables of four-top events at the Large Hadron
Collider. The like-sign dilepton signature typically has the largest discovery
potential for a strongly coupled right-handed top case (M_{KK} \sim 2-2.5
\TeV), while single lepton is the better when the left-handed top couples most
strongly (M_{KK} \sim 2 \TeV). We also describe challenging lepton-jet
collimation issues in the like-sign dilepton and trilepton channels. An
alternative single lepton observable is considered which takes advantage of the
many bottom quarks in the final state. Although searches of other particles may
compete, we find that four top production via Kaluza-Klein gluons is most
promising in a large region of this parameter space.Comment: 35 pages, 8 figures. discussions improved, references adde
Testing the Nambu-Goldstone Hypothesis for Quarks and Leptons at the LHC
The hierarchy of the Yukawa couplings is an outstanding problem of the
standard model. We present a class of models in which the first and second
generation fermions are SUSY partners of pseudo-Nambu-Goldstone bosons that
parameterize a non-compact Kahler manifold, explaining the small values of
these fermion masses relative to those of the third generation. We also provide
an example of such a model. We find that various regions of the parameter space
in this scenario can give the correct dark matter abundance, and that nearly
all of these regions evade other phenomenological constraints. We show that for
gluino mass ~700 GeV, model points from these regions can be easily
distinguished from other mSUGRA points at the LHC with only 7 fb^(-1) of
integrated luminosity at 14 TeV. The most striking signatures are a dearth of
b- and tau-jets, a great number of multi-lepton events, and either an
"inverted" slepton mass hierarchy, narrowed slepton mass hierarchy, or
characteristic small-mu spectrum.Comment: Corresponds to published versio
Two-photon decays of the lightest Higgs boson of supersymmetry at the LHC
We discuss the production and two-photon decay of the lightest Higgs boson
(h) of the minimal supersymmetric standard model at the CERN Large Hadron
Collider. Since the observability of the signal is quite model dependent, we
conduct a thorough scan of the parameter space of minimal supersymmetry,
including experimental and theoretical constraints. If kinematically allowed,
supersymmetric decay modes of h may be important, and can even dominate all
others. The coupling of h to can be different from that of a
standard model Higgs boson; this can diminish (or enhance, but only if
tan is very large) the h -> signal. We emphasize the
importance of a full treatment of radiative corrections in the Higgs sector for
obtaining the h coupling. If supersymmetric particles are not
too heavy, their contributions in loops can either enhance or suppress both the
production cross-section and the h -> branching fraction. We
discuss the relative importance of these factors in the context of various
scenarios for the discovery of supersymmetry. Even if h is not detected at the
LHC, h may still exist in its expected mass region.Comment: 15 pages plain TeX plus 7 uuencoded figures include
Detecting a light Higgs boson at the Fermilab Tevatron through enhanced decays to photon pairs
We analyze the prospects of the Tevatron for finding a Higgs boson in the two
photon decay mode. We conclude that the Standard Model (SM) Higgs boson will
likely not be discovered in this mode. However, we motivate several theories
beyond the SM, including the MSSM, that predict a Higgs boson with enhanced
branching fractions into photons, and calculate the luminosity needed to
discover a general Higgs boson at the Tevatron by a two-photon invariant mass
peak at large transverse momentum. We find that a high luminosity Tevatron will
play a significant role in discovering or constraining these theories.Comment: 20 pages, latex, 5 figure
Supersymmetry Without Prejudice at the LHC
The discovery and exploration of Supersymmetry in a model-independent fashion
will be a daunting task due to the large number of soft-breaking parameters in
the MSSM. In this paper, we explore the capability of the ATLAS detector at the
LHC ( TeV, 1 fb) to find SUSY within the 19-dimensional
pMSSM subspace of the MSSM using their standard transverse missing energy and
long-lived particle searches that were essentially designed for mSUGRA. To this
end, we employ a set of k previously generated model points in the
19-dimensional parameter space that satisfy all of the existing experimental
and theoretical constraints. Employing ATLAS-generated SM backgrounds and
following their approach in each of 11 missing energy analyses as closely as
possible, we explore all of these k model points for a possible SUSY
signal. To test our analysis procedure, we first verify that we faithfully
reproduce the published ATLAS results for the signal distributions for their
benchmark mSUGRA model points. We then show that, requiring all sparticle
masses to lie below 1(3) TeV, almost all(two-thirds) of the pMSSM model points
are discovered with a significance in at least one of these 11 analyses
assuming a 50\% systematic error on the SM background. If this systematic error
can be reduced to only 20\% then this parameter space coverage is increased.
These results are indicative that the ATLAS SUSY search strategy is robust
under a broad class of Supersymmetric models. We then explore in detail the
properties of the kinematically accessible model points which remain
unobservable by these search analyses in order to ascertain problematic cases
which may arise in general SUSY searches.Comment: 69 pages, 40 figures, Discussion adde
Supersymmetry Without Prejudice
We begin an exploration of the physics associated with the general
CP-conserving MSSM with Minimal Flavor Violation, the pMSSM. The 19 soft SUSY
breaking parameters in this scenario are chosen so as to satisfy all existing
experimental and theoretical constraints assuming that the WIMP is a
conventional thermal relic, ie, the lightest neutralino. We scan this parameter
space twice using both flat and log priors for the soft SUSY breaking mass
parameters and compare the results which yield similar conclusions. Detailed
constraints from both LEP and the Tevatron searches play a particularly
important role in obtaining our final model samples. We find that the pMSSM
leads to a much broader set of predictions for the properties of the SUSY
partners as well as for a number of experimental observables than those found
in any of the conventional SUSY breaking scenarios such as mSUGRA. This set of
models can easily lead to atypical expectations for SUSY signals at the LHC.Comment: 61 pages, 24 figs. Refs., figs, and text added, typos fixed; This
version has reduced/bitmapped figs. For a version with better figs please go
to http://www.slac.stanford.edu/~rizz
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