3,737 research outputs found
Recent Developments in Physics Beyond the Standard Model
In this talk I discuss some recent developments in physics beyond the
Standard Model. After some initial comments on neutrino masses, I discuss the
status of low-energy supersymmetry and finally turn to describing some recent
work in theories with extra spatial dimensions. Plenary talk at the XIX
International Symposium on Lepton and Photon Interactions at High Energies,
Stanford University, 9-14 August 1999.Comment: 25 pages, 2 eps figure
Split Supersymmetry
The naturalness criterion applied to the cosmological constant implies a
new-physics threshold at 10^-3 eV. Either the naturalness criterion fails, or
this threshold does not influence particle dynamics at higher energies. It has
been suggested that the Higgs naturalness problem may follow the same fate. We
investigate this possibility and, abandoning the hierarchy problem, we use
unification and dark matter as the only guiding principles. The model recently
proposed by Arkani-Hamed and Dimopoulos emerges as a very interesting option.
We study it in detail, analysing its structure, and the conditions for
obtaining unification and dark matter.Comment: 29 pages, comments, corrections and references adde
R-Parity Violation and Unification
The reported anomaly in deep-inelastic scattering at HERA has revived
interest in the phenomenology of R-parity violation. From the theoretical point
of view, the existence of R-violating interactions poses two considerable
problems. The first one concerns the flavour structure of the interactions and
the origin of an appropriate suppression of flavour-changing neutral-current
processes and lepton-family transitions. The second one concerns the way of
embedding R-violating interactions in a grand unified theory (GUT) without
introducing unacceptable nucleon decay rates. We show that the second problem
can be solved by a mechanism which is purely group theoretical and does not
rely on details of the flavour theory. We construct explicit GUT models in
which our mechanism can be realized.Comment: Flipped SU(5) example modified. Conclusions unchange
Nearly Degenerate Gauginos and Dark Matter at the LHC
Motivated by dark-matter considerations in supersymmetric theories, we
investigate in a fairly model-independent way the detection at the LHC of
nearly degenerate gauginos with mass differences between a few GeV and about 30
GeV. Due to the degeneracy of gaugino states, the conventional leptonic signals
are likely lost. We first consider the leading signal from gluino production
and decay. We find that it is quite conceivable to reach a large statistical
significance for the multi-jet plus missing energy signal with an integrated
luminosity about 50 pb^-1 (50 fb^-1) for a gluino mass of 500 GeV (1 TeV). If
gluinos are not too heavy, less than about 1.5 TeV, this channel can typically
probe gaugino masses up to about 100 GeV below the gluino mass. We then study
the Drell-Yan type of gaugino pair production in association with a hard QCD
jet, for gaugino masses in the range of 100-150 GeV. The signal observation may
be statistically feasible with about 10 fb^-1, but systematically challenging
due to the lack of distinctive features for the signal distributions. By
exploiting gaugino pair production through weak boson fusion, signals of large
missing energy plus two forward-backward jets may be observable at a 4-6\sigma
level above the large SM backgrounds with an integrated luminosity of 100-300
fb^-1. Finally, we point out that searching for additional isolated soft muons
in the range p_T ~3-10 GeV in the data samples discussed above may help to
enrich the signal and to control the systematics. Significant efforts are made
to explore the connection between the signal kinematics and the relevant masses
for the gluino and gauginos, to probe the mass scales of the superpartners, in
particular the LSP dark matter.Comment: 35 pages, 32 figure
B -> X_s gamma in supersymmetry: large contributions beyond the leading order
We discuss possible large contributions to B -> X_s gamma, which can occur at
the next-to-leading order in supersymmetric models. They can originate from
terms enhanced by tan(beta) factors, when the ratio between the two Higgs
vacuum expectation values is large, or by logarithm of M_{susy}/M_W, when the
supersymmetric particles are considerably heavier than the W boson. We give
compact formulae which include all potentially large higher-order
contributions. We find that tan(beta) terms at the next-to-leading order do not
only appear from the Hall-Rattazzi-Sarid effect (the modified relation between
the bottom mass and Yukawa coupling), but also from an analogous effect in the
top-quark Yukawa coupling. Finally, we show how next-to-leading order
corrections, in the large tan(beta) region, can significantly reduce the limit
on the charged-Higgs mass, even if supersymmetric particles are very heavy.Comment: 18 pages, 5 figs, extended discussion of light stop case, notational
improvement
The -Problem in Theories with Gauge-Mediated Supersymmetry Breaking
We point out that the -problem in theories in which supersymmetry
breaking is communicated to the observable sector by gauge interactions is more
severe than the one encountered in the conventional gravity-mediated scenarios.
The difficulty is that once is generated by a one-loop diagram, then
usually \bmu is also generated at the same loop order. This leads to the
problematic relation \bmu \sim \mu \Lambda, where 10--100 TeV
is the effective supersymmetry-breaking scale. We present a class of theories
for which this problem is naturally solved. Here, without any fine tuning among
parameters, is generated at one loop, while \bmu arises only at the
two-loop level. This mechanism can naturally lead to an interpretation of the
Higgs doublets as pseudo-Goldstone bosons of an approximate global symmetry.Comment: 18 pages, 2 figure
Gauge Coupling Unification in MSSM + 5 Flavors
We investigate gauge coupling unification at 2-loops for theories with 5
extra vectorlike SU(5) fundamentals added to the MSSM. This is a borderline
case where unification is only predicted in certain regions of parameter space.
We establish a lower bound on the scale for the masses of the extra flavors, as
a function of the sparticle masses. Models far outside of the bound do not
predict unification at all (but may be compatible with unification), and models
outside but near the boundary cannot reliably claim to predict it with an
accuracy comparable to the MSSM prediction. Models inside the boundary can work
just as well as the MSSM.Comment: 28 pages, 13 figures. Added references, fixed minor typos. No changes
to content. Page count was incorrect in v1 Comment
Indication for Light Sneutrinos and Gauginos from Precision Electroweak Data
The present Standard Model fit of precision data has a low confidence level,
and is characterized by a few inconsistencies. We look for supersymmetric
effects that could improve the agreement among the electroweak precision
measurements and with the direct lower bound on the Higgs mass. We find that
this is the case particularly if the 3.6 sigma discrepancy between sin^2
theta_eff from leptonic and hadronic asymmetries is finally settled more on the
side of the leptonic ones. After the inclusion of all experimental constraints,
our analysis selects light sneutrinos, with masses in the range 55-80 GeV, and
charged sleptons with masses just above their experimental limit, possibly with
additional effects from light gauginos. The phenomenological implications of
this scenario are discussed.Comment: 17 pages LaTex, 9 figures, uses epsfi
Thermal and Non-Thermal Production of Gravitinos in the Early Universe
The excessive production of gravitinos in the early universe destroys the
successful predictions of nucleosynthesis. The thermal generation of gravitinos
after inflation leads to the bound on the reheating temperature, T_{RH}< 10^9
GeV. However, it has been recently realized that the non-thermal generation of
gravitinos in the early universe can be extremely efficient and overcome the
thermal production by several orders of magnitude, leading to much tighter
constraints on the reheating temperature. In this paper, we first investigate
some aspects of the thermal production of gravitinos, taking into account that
in fact reheating is not instantaneous and inflation is likely to be followed
by a prolonged stage of coherent oscillations of the inflaton field. We then
proceed by further investigating the non-thermal generation of gravitinos,
providing the necessary tools to study this process in a generic time-dependent
background with any number of superfields. We also present the first numerical
results regarding the non-thermal generation of gravitinos in particular
supersymmetric models.Comment: 31 pages, 7 Postscript figures. New references adde
Non-Thermal Production of Dangerous Relics in the Early Universe
Many models of supersymmetry breaking, in the context of either supergravity
or superstring theories, predict the presence of particles with weak scale
masses and Planck-suppressed couplings. Typical examples are the scalar moduli
and the gravitino. Excessive production of such particles in the early Universe
destroys the successful predictions of nucleosynthesis. In particular, the
thermal production of these relics after inflation leads to a bound on the
reheating temperature, T_{RH} < 10^9 GeV. In this paper we show that the
non-thermal generation of these dangerous relics may be much more efficient
than the thermal production after inflation. Scalar moduli fields may be
copiously created by the classical gravitational effects on the vacuum state.
Consequently, the new upper bound on the reheating temperature is shown to be,
in some cases, as low as 100 GeV. We also study the non-thermal production of
gravitinos in the early Universe, which can be extremely efficient and overcome
the thermal production by several orders of magnitude, in realistic
supersymmetric inflationary models.Comment: 21 pages, 4 Postscript figure
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