2,510 research outputs found
On the rigidity of back-to-back top quark pairs in e^+e^- annihilation
We consider the effect of gluon radiation on the energy of top/antitop quarks
and on the anticollinearity of top-antitop quark pairs produced in
annihilation. Our results are presented in terms of the -dependence of the
cross section and the dependence on the cosine of the opening angle
between top and antitop for a center of mass energy of
. We then go on to determine mean values for the top
quark's energy as well as its longitudinal and transverse projections, and for
the deviation of and from the
anticollinearity limits and . For a
center of mass energy of we obtain , and . Thus, at this energy gluon radiation causes a total
average energy loss of 0.71% of the top quark's energy. The average energy loss
in the longitudinal direction is 1.06% and the average energy gain in the
transverse direction is 1.88%. These percentage figures go up to 3.77%, 5.19%
and 6.06%, respectively, at 1000\GeV. For the mean of the acollinearity angle
we obtain at
, the value of which goes up to at . From an
analysis of the transverse momentum of the top we find that the mean transverse
momentum of the top stays close to the mean total momentum of the gluon in the
energy range from threshold to showing that the gluon momentum has a
large mean transverse component in this energy range.Comment: 17 pages, 7 postscript figures, to appear in Nucl. Phys.
What if Supersymmetry Breaking Unifies beyond the GUT Scale?
We study models in which soft supersymmetry-breaking parameters of the MSSM
become universal at some unification scale, , above the GUT scale,
\mgut. We assume that the scalar masses and gaugino masses have common
values, and respectively, at . We use the
renormalization-group equations of the minimal supersymmetric SU(5) GUT to
evaluate their evolutions down to \mgut, studying their dependences on the
unknown parameters of the SU(5) superpotential. After displaying some generic
examples of the evolutions of the soft supersymmetry-breaking parameters, we
discuss the effects on physical sparticle masses in some specific examples. We
note, for example, that near-degeneracy between the lightest neutralino and the
lighter stau is progressively disfavoured as increases. This has the
consequence, as we show in planes for several different values
of , that the stau coannihilation region shrinks as
increases, and we delineate the regions of the plane
where it is absent altogether. Moreover, as increases, the focus-point
region recedes to larger values of for any fixed and
. We conclude that the regions of the plane that are
commonly favoured in phenomenological analyses tend to disappear at large
.Comment: 24 pages with 11 eps figures; references added, some figures
corrected, discussion extended and figure added; version to appear in EPJ
Bayesian approach and Naturalness in MSSM analyses for the LHC
The start of LHC has motivated an effort to determine the relative
probability of the different regions of the MSSM parameter space, taking into
account the present, theoretical and experimental, wisdom about the model.
Since the present experimental data are not powerful enough to select a small
region of the MSSM parameter space, the choice of a judicious prior probability
for the parameters becomes most relevant. Previous studies have proposed
theoretical priors that incorporate some (conventional) measure of the
fine-tuning, to penalize unnatural possibilities. However, we show that such
penalization arises from the Bayesian analysis itself (with no ad hoc
assumptions), upon the marginalization of the mu-parameter. Furthermore the
resulting effective prior contains precisely the Barbieri-Giudice measure,
which is very satisfactory. On the other hand we carry on a rigorous treatment
of the Yukawa couplings, showing in particular that the usual practice of
taking the Yukawas "as required", approximately corresponds to taking
logarithmically flat priors in the Yukawa couplings. Finally, we use an
efficient set of variables to scan the MSSM parameter space, trading in
particular B by tan beta, giving the effective prior in the new parameters.
Beside the numerical results, we give accurate analytic expressions for the
effective priors in all cases. Whatever experimental information one may use in
the future, it is to be weighted by the Bayesian factors worked out here.Comment: LaTeX, 19 pages, 3 figure
Higgs Boson Bounds in Three and Four Generation Scenarios
In light of recent experimental results, we present updated bounds on the
lightest Higgs boson mass in the Standard Model (SM) and in the Minimal
Supersymmetric extension of the Standard Model (MSSM). The vacuum stability
lower bound on the pure SM Higgs boson mass when the SM is taken to be valid up
to the Planck scale lies above the MSSM lightest Higgs boson mass upper bound
for a large amount of SUSY parameter space. If the lightest Higgs boson is
detected with a mass M_{H} < 134 GeV (150 GeV) for a top quark mass M_{top} =
172 GeV (179 GeV), it may indicate the existence of a fourth generation of
fermions. The region of inconsistency is removed and the MSSM is salvagable for
such values of M_{H} if one postulates the existence of a fourth generation of
leptons and quarks with isodoublet degenerate masses M_{L} and M_{Q} such that
60 GeV 170 GeV.Comment: 7 pages, 4 figures. To be published in Physical Review
Revisiting the Higgs Mass and Dark Matter in the CMSSM
Taking into account the available accelerator and astrophysical constraints,
the mass of the lightest neutral Higgs boson h in the minimal supersymmetric
extension of the Standard Model with universal soft supersymmetry-breaking
masses (CMSSM) has been estimated to lie between 114 and ~ 130 GeV. Recent data
from ATLAS and CMS hint that m_h ~ 125 GeV, though m_h ~ 119 GeV may still be a
possibility. Here we study the consequences for the parameters of the CMSSM and
direct dark matter detection if the Higgs hint is confirmed, focusing on the
strips in the (m_1/2, m_0) planes for different tan beta and A_0 where the
relic density of the lightest neutralino chi falls within the range of the
cosmological cold dark matter density allowed by WMAP and other experiments. We
find that if m_h ~ 125 GeV focus-point strips would be disfavoured, as would
the low-tan beta stau-chi and stop -chi coannihilation strips, whereas the
stau-chi coannihilation strip at large tan beta and A_0 > 0 would be favoured,
together with its extension to a funnel where rapid annihilation via
direct-channel H/A poles dominates. On the other hand, if m_h ~ 119 GeV more
options would be open. We give parametrizations of WMAP strips with large tan
beta and fixed A_0/m_0 > 0 that include portions compatible with m_h = 125 GeV,
and present predictions for spin-independent elastic dark matter scattering
along these strips. These are generally low for models compatible with m_h =
125 GeV, whereas the XENON100 experiment already excludes some portions of
strips where m_h is smaller.Comment: 24 pages, 9 figure
Physics in the Real Universe: Time and Spacetime
The Block Universe idea, representing spacetime as a fixed whole, suggests
the flow of time is an illusion: the entire universe just is, with no special
meaning attached to the present time. This view is however based on
time-reversible microphysical laws and does not represent macro-physical
behaviour and the development of emergent complex systems, including life,
which do indeed exist in the real universe. When these are taken into account,
the unchanging block universe view of spacetime is best replaced by an evolving
block universe which extends as time evolves, with the potential of the future
continually becoming the certainty of the past. However this time evolution is
not related to any preferred surfaces in spacetime; rather it is associated
with the evolution of proper time along families of world linesComment: 28 pages, including 9 Figures. Major revision in response to referee
comment
Model Independent Properties and Cosmological Implications of the Dilaton and Moduli Sectors of 4-d Strings
We show that if there is a realistic 4-d string, the dilaton and moduli
supermultiplets will generically acquire a small mass O(m_{3/2}), providing the
only vacuum-independent evidence of low-energy physics in string theory beyond
the supersymmetric standard model. The only assumptions behind this result are
(i) softly broken supersymmetry at low energies with zero cosmological
constant, (ii) these particles interact with gravitational strength and the
scalar components have a flat potential in perturbation theory, which are
well-known properties of string theories. (iii) They acquire a of the
order of the Planck scale (as required for the correct value of the gauge
coupling constants and the expected compactification scale) after supersymmetry
gets broken. We explore the cosmological implications of these particles.
Similar to the gravitino, the fermionic states may overclose the Universe if
they are stable or destroy nucleosynthesis if they decay unless their masses
belong to a certain range or inflation dilutes them. For the scalar states it
is known that the problem cannot be entirely solved by inflation, since
oscillations around the minimum of the potential can lead to a huge entropy
generation at late times. We discus some possible ways to alleviate this
entropy problem, that favour low-temperature baryogenesis, and also comment on
the possible role of these particles as dark matter candidates or as sources of
the baryon asymmetry through their decay.Comment: 15 pages,CERN-TH.6958/93,NEIP-93-006, IEM-FT-75/93, Late
Can a wormhole generate electromagnetic field?
We have considered the possibility of a slowly rotating wormhole surrounded
by a cloud of charged particles. Due to slow rotation of the wormhole, the
charged particles are dragged thereby producing an electromagnetic field. We
have determined the strength of this electromagnetic field and the
corresponding flux of radiation.Comment: 9 pages, typos fixe
Constrained Supersymmetric Flipped SU(5) GUT Phenomenology
We explore the phenomenology of the minimal supersymmetric flipped SU(5) GUT
model (CFSU(5)), whose soft supersymmetry-breaking (SSB) mass parameters are
constrained to be universal at some input scale, , above the GUT scale,
. We analyze the parameter space of CFSU(5) assuming that the lightest
supersymmetric particle (LSP) provides the cosmological cold dark matter,
paying careful attention to the matching of parameters at the GUT scale. We
first display some specific examples of the evolutions of the SSB parameters
that exhibit some generic features. Specifically, we note that the relationship
between the masses of the lightest neutralino and the lighter stau is sensitive
to , as is the relationship between the neutralino mass and the masses
of the heavier Higgs bosons. For these reasons, prominent features in generic
planes such as coannihilation strips and rapid-annihilation
funnels are also sensitive to , as we illustrate for several cases with
tan(beta)=10 and 55. However, these features do not necessarily disappear at
large , unlike the case in the minimal conventional SU(5) GUT. Our
results are relatively insensitive to neutrino masses.Comment: 23 pages, 8 figures; (v2) added explanations and corrected typos,
version to appear in EPJ
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