176,004 research outputs found
Derivation of a Vacuum Refractive Index in a Stringy Space-Time Foam Model
It has been suggested that energetic photons propagating in vacuo should
experience a non-trivial refractive index due to the foamy structure of
space-time induced by quantum-gravitational fluctuations. The sensitivity of
recent astrophysical observations, particularly of AGN Mk501 by the MAGIC
Collaboration, approaches the Planck scale for a refractive index depending
linearly on the photon energy. We present here a new derivation of this
quantum-gravitational vacuum refraction index, based on a stringy analogue of
the interaction of a photon with internal degrees of freedom in a conventional
medium. We model the space-time foam as a gas of D-particles in the bulk
space-time of a higher-dimensional cosmology where the observable Universe is a
D3-brane. The interaction of an open string representing a photon with a
D-particle stretches and excites the string, which subsequently decays and
re-emits the photon with a time delay that increases linearly with the photon
energy and is related to stringy uncertainty principles. We relate this
derivation to other descriptions of the quantum-gravitational refractive index
in vacuo.Comment: 8 pages, 3 eps figure
Prospects for Discovering Supersymmetry at the LHC
Supersymmetry is one of the best-motivated candidates for physics beyond the
Standard Model that might be discovered at the LHC. There are many reasons to
expect that it may appear at the TeV scale, in particular because it provides a
natural cold dark matter candidate. The apparent discrepancy between the
experimental measurement of g_mu - 2 and the Standard model value calculated
using low-energy e+ e- data favours relatively light sparticles accessible to
the LHC. A global likelihood analysis including this, other electroweak
precision observables and B-decay observables suggests that the LHC might be
able to discover supersymmetry with 1/fb or less of integrated luminosity. The
LHC should be able to discover supersymmetry via the classic missing-energy
signature, or in alternative phenomenological scenarios. The prospects for
discovering supersymmetry at the LHC look very good.Comment: 8 pages, 11 figure
Elements of F-ast Proton Decay
Gauge coupling unification in the Minimal Supersymmetric Standard Model
(MSSM) strongly suggests the existence of a Grand Unified Theory (GUT), which
could be probed by the observation of proton decay. Proton lifetime in the p
\to (e+|mu+) pi0 dimension six mode is proportional in the fourth power to the
GUT mass scale, and inversely proportional in the fourth power to the GUT
coupling. We provide an updated dictionary of solutions for the relevant
unification parameters with generic beta-function coefficients, significantly
upgrading the level of detail with which second order effects are treated, and
correcting subtle published errors. F-lipped SU(5) with strict MSSM field
content is known to survive existing null detection limits for proton decay
approaching 10^34 years, and indeed, the lifetime predicted by prior studies
can be so long that successful detection is not currently plausible. Recently
studied classes of F-theory derived GUT models postulate additional vector-like
multiplets at the TeV scale which modify the renormalization group to yield a
substantial increase in the SU(3)_C X SU(2)_L unified coupling. We find the
conjunction of these models with the F-resh analysis employed to be
comparatively F-ast proton decay, only narrowly evading existing detection
limits, and likely falling within the observable range of proposed next
generation detectors such as DUSEL and Hyper-Kamiokande. The TeV-scale vector
multiplets are themselves suitable for cross correlation by the Large Hadron
Collider. Their presence moreover magnifies the gap between the dual mass
scales of Flipped SU(5), allowing for an elongated second stage
renormalization, pushing grand unification to the doorstep of the reduced
Planck mass.Comment: V2, As published in Nuclear Physics B; 57 pages, 7 figures, 12 table
Lower Limits on Soft Supersymmetry-Breaking Scalar Masses
Working in the context of the CMSSM, we argue that phenomenological
constraints now require the universal soft supersymmetry-breaking scalar mass
m_0 be non-zero at the input GUT scale. This conclusion is primarily imposed by
the LEP lower limit on the Higgs mass and the requirement that the lightest
supersymmetric particle not be charged. We find that m_0 > 0 for all tan beta
if mu 0 only when tan beta sim 8 and
one allows an uncertainty of 3+ GeV in the theoretical calculation of the Higgs
mass. Upper limits on flavour-changing neutral interactions in the MSSM squark
sector allow substantial violations of non-universality in the m_0 values, even
if their magnitudes are comparable to the lower limit we find in the CMSSM.
Also, we show that our lower limit on m_0 at the GUT scale in the CMSSM is
compatible with the no-scale boundary condition m_0 = 0 at the Planck scale.Comment: 11 pages, latex, 6 eps figure
Prospects for Sparticle Discovery in Variants of the MSSM
We discuss the prospects for detecting supersymmetric particles in variants
of the minimal supersymmetric extension of the Standard Model (MSSM), in light
of laboratory and cosmological constraints. We first assume that the lightest
supersymmetric particle (LSP) is the lightest neutralino chi, and present
scatter plots of the masses of the two lightest visible supersymmetric
particles when the input scalar and gaugino masses are constrained to be
universal (CMSSM), when the input Higgs scalar masses are non-universal (NUHM),
and when the squark and slepton masses are also non-universal and the MSSM is
regarded as a low-energy effective field theory valid up to the GUT scale
(LEEST) or just up to 10 TeV (LEEST10). We then present similar plots in
various scenarios when the LSP is the gravitino. We compare the prospects for
detecting supersymmetry at linear colliders (LCs) of various energies, at the
LHC, and as astrophysical dark matter. We find that, whilst a LC with a
centre-of-mass energy E_{CM} <= 1000 GeV has some chance of discovering the
lightest and next-to-lightest visible supersymmetric particles, E_{CM} >= 3000
GeV would be required to `guarantee' finding supersymmetry in the neutralino
LSP scenarios studied, and an even higher E_{CM} might be required in certain
gravitino dark matter scenarios. Direct dark matter experiments could explore
part of the low-mass neutralino LSP region, but would not reveal all the models
accessible to a low-energy LC.Comment: 19 pages, 16 eps figures, as accepted in PL
Stringy Space-Time Foam, Finsler-like Metrics and Dark Matter Relics
We discuss modifications of the thermal Dark Matter (DM) relic abundances in
stringy cosmologies with D-particle space-time foamy backgrounds. As a result
of back-reaction of massive DM on the background space-time, owing to its
interaction with D-particle defects in the foam, quantum fluctuations are
induced in the space-time metric. We demonstrate that these lead to the
presence of extra source terms in the Boltzmann equation used to determine the
thermal dark matter relic abundances. The source terms are determined by the
specific form of the induced metric deformations; the latter depend on the
momentum transfer of the DM particle during its interactions with the
D-particle defects and so are akin to Finsler metrics. In the case of low
string scales arising from large extra dimensions our results may have
phenomenological implications for the search of viable supersymmetric models
Astroparticle Aspects of Supersymmetry
After recalling the motivations for expecting supersymmetry to appear at
energies below 1 TeV, the reasons why the lightest supersymmetric particle is
an ideal candidate for cold dark matter are reviewed from a historical
perspective. Recent calculations of the relic density including coannihilations
and rapid annihilations through direct-channel Higgs boson poles are presented.
The experimental constraints from LEP and elsewhere on supersymmetric dark
matter are reviewed, and the prospects for its indirect or direct detection are
mentioned. The potential implications of a Higgs boson weighing about 115 GeV
and the recent measurement of the anomalous magnetic moment of the muon are
summarized.Comment: 12 pages, 10 eps figures, invited plenary talk at conference on 30
Years Of Supersymmetry, Oct. 2000, Minneapolis, Minnesot
Logarithmic Operators Fold D branes into AdS_3
We use logarithmic conformal field theory techniques to describe recoil
effects in the scattering of two Dirichlet branes in D dimensions. In the
particular case that a D1 brane strikes a D3 brane perpendicularly, thereby
folding it, we find that the recoil space-time is maximally symmetric, with
AdS_3 x E_{D-3} geometry. We comment on the possible applications of this
result to the study of transitions between different background metrics.Comment: 10 pages revtex, one eps figure include
Time-dependent perturbations in two-dimensional String Black Holes
We discuss time-dependent perturbations (induced by matter fields) of a
black-hole background in tree-level two-dimensional string theory. We analyse
the linearized case and show the possibility of having black-hole solutions
with time-dependent horizons. The latter exist only in the presence of
time-dependent `tachyon' matter fields, which constitute the only propagating
degrees of freedom in two-dimensional string theory. For real tachyon field
configurations it is not possible to obtain solutions with horizons shrinking
to a point. On the other hand, such a possibility seems to be realized in the
case of string black-hole models formulated on higher world-sheet genera. We
connect this latter result with black hole evaporation/decay at a quantum
level.}Comment: 11 pages, two figures,UA-NPPS.9/92; CERN-TH.6671/9
On the Higgs Mass in the CMSSM
We estimate 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), subject to the available accelerator and
astrophysical constraints. For m_t = 174.3 GeV, we find that 114 GeV < m_h <
127 GeV and a peak in the tan beta distribution simeq 55. We observe two
distinct peaks in the distribution of m_h values, corresponding to two
different regions of the CMSSM parameter space. Values of m_h < 119 GeV
correspond to small values of the gaugino mass m_{1/2} and the soft trilinear
supersymmetry-breaking parameter A_0, lying along coannihilation strips, and
most of the allowed parameter sets are consistent with a supersymmetric
interpretation of the possibly discrepancy in g_mu - 2. On the other hand,
values of m_h > 119 GeV may correspond to much larger values of m_{1/2} and
A_0, lying in rapid-annihilation funnels. The favoured ranges of m_h vary with
m_t, the two peaks being more clearly separated for m_t = 178 GeV and merging
for m_t = 172.7 GeV. If the g_mu - 2 constraint is imposed, the mode of the m_h
distribution is quite stable, being sim 117 GeV for all the studied values of
m_t.Comment: 14 pages, 12 figure
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