276 research outputs found
Radiative corrections to the lightest KK states in the T^2/(Z_2\times Z_2') orbifold
We study radiative corrections localized in the fixed points of the orbifold
for the field theory in six dimensions with two dimensions compactified on the
orbifold in a specific realistic model for low energy
physics that solves the proton decay and neutrino mass problem. We calculate
corrections to the masses of the lightest stable KK modes, which could be the
candidates for the dark matter.Comment: 14 pages, 2 figure
The extent of strangeness in equilibration in quark-gluon plasma
The evolution and production of strangeness from chemically equilibrating and
transversely expanding quark gluon plasma which may be formed in the wake of
relativistic heavy ion collisions is studied with initial conditions obtained
from the Self Screened Parton Cascade (SSPC) model. The extent of partonic
equilibration increases almost linearly with the square of the initial energy
density, which can then be scaled with number of participants.Comment: 4 pages including three figures, talk given at ICPAQGP'01, Jaipur,
India, to appear in Pramana - Journal of Physics, Indian Academy of Scienc
Flavor Phenomenology in General 5D Warped Spaces
We have considered a general 5D warped model with SM fields propagating in
the bulk and computed explicit expressions for oblique and non-oblique
electroweak observables as well as for flavor and CP violating effective
four-fermion operators. We have compared the resulting lower bounds on the
Kaluza-Klein (KK) scale in the RS model and a recently proposed model with a
metric modified towards the IR brane, which is consistent with oblique
parameters without the need for a custodial symmetry. We have randomly
generated 40,000 sets of O(1) 5D Yukawa couplings and made a fit of the quark
masses and CKM matrix elements in both models. This method allows to identify
the percentage of points consistent with a given KK mass, which in turn
provides us with a measure for the required fine-tuning. Comparison with
current experimental data on Rb, FCNC and CP violating operators exhibits an
improved behavior of our model with respect to the RS model. In particular,
allowing 10% fine-tuning the combined results point towards upper bounds on the
KK gauge boson masses around 3.3 TeV in our model as compared with 13 TeV in
the RS model. One reason for this improvement is that fermions in our model are
shifted, with respect to fermions in the RS model, towards the UV brane thus
decreasing the strength of the modifications of electroweak observables.Comment: 28 pages, 7 figures, 4 table
Equilibration in Quark Gluon Plasma
The hydrodynamic expansion rate of quark gluon plasma (QGP) is evaluated and
compared with the scattering rate of quarks and gluons within the system.
Partonic scattering rates evaluated within the ambit of perturbative Quantum
Choromodynamics (pQCD) are found to be smaller than the expansion rate
evaluated with ideal equation of state (EoS) for the QGP. This indicate that
during the space-time evolution the system remains out of equilibrium.
Enhancement of pQCD cross sections and a more realistic EoS keep the partons
closer to the equilibrium.Comment: To be published in the Quark Matter 2008 poster proceeding
Localized anomalies in orbifold gauge theories
We apply the path-integral formalism to compute the anomalies in general
orbifold gauge theories (including possible non-trivial Scherk-Schwarz boundary
conditions) where a gauge group G is broken down to subgroups H_f at the fixed
points y=y_f. Bulk and localized anomalies, proportional to \delta(y-y_f), do
generically appear from matter propagating in the bulk. The anomaly zero-mode
that survives in the four-dimensional effective theory should be canceled by
localized fermions (except possibly for mixed U(1) anomalies). We examine in
detail the possibility of canceling localized anomalies by the Green-Schwarz
mechanism involving two- and four-forms in the bulk. The four-form can only
cancel anomalies which do not survive in the 4D effective theory: they are
called globally vanishing anomalies. The two-form may cancel a specific class
of mixed U(1) anomalies. Only if these anomalies are present in the 4D theory
this mechanism spontaneously breaks the U(1) symmetry. The examples of five and
six-dimensional Z_N orbifolds are considered in great detail. In five
dimensions the Green-Schwarz four-form has no physical degrees of freedom and
is equivalent to canceling anomalies by a Chern-Simons term. In all other
cases, the Green-Schwarz forms have some physical degrees of freedom and leave
some non-renormalizable interactions in the low energy effective theory. In
general, localized anomaly cancellation imposes strong constraints on model
building.Comment: 30 pages, 4 figures. v2: reference adde
On the Equation of State of Nuclear Matter in 158A GeV Pb+Pb Collisions
Within a hydrodynamical approach we investigate the sensitivity of single
inclusive momentum spectra of hadrons in 158A GeV Pb+Pb collisions to three
different equations of state of nuclear matter. Two of the equations of state
are based on lattice QCD results and include a phase transition to a
quark-gluon plasma. The third equation of state has been extracted from the
microscopic transport code RQMD under the assumption of complete local
thermalization. All three equations of state provide reasonable fits to data
taken by the NA44 and NA49 Collaborations. The initial conditions before the
evolution of the fireballs and the space-time evolution pictures differ
dramatically for the three equations of state when the same freeze-out
temperature is used in all calculations. However, the softest of the equations
of state results in transverse mass spectra that are too steep in the central
rapidity region. We conclude that the transverse particle momenta are
determined by the effective softness of the equation of state during the
fireball expansion.Comment: 4 pages, including 4 figures and 2 tables. For a PostScript file of
the manuscript, you can also goto http://t2.lanl.gov/schlei/eprint.htm
Gauge coupling renormalization in RS1
We compute the 4D low energy effective gauge coupling at one-loop order in
the compact Randall-Sundrum scenario with bulk gauge fields and charged matter,
within controlled approximations. While such computations are subtle, they can
be important for studying phenomenological issues such as grand unification.
Ultraviolet divergences are cut-off using Pauli-Villars regularization so as to
respect 5D gauge and general coordinate invariance. The structure of these
divergences on branes and in the bulk is elucidated by a 5D position-space
analysis. The remaining finite contributions are obtained by a careful analysis
of the Kaluza-Klein spectrum. We comment on the agreement between our results
and expectations based on the AdS/CFT correspondence, in particular logarithmic
sensitivity to the 4D Planck scale.Comment: 17 pages, Latex2e, uses axodraw.sty, new references added. To be
published in Nucl. Phys.
Renormalization-Group Analysis of Layered Sine-Gordon Type Models
We analyze the phase structure and the renormalization group (RG) flow of the
generalized sine-Gordon models with nonvanishing mass terms, using the
Wegner-Houghton RG method in the local potential approximation. Particular
emphasis is laid upon the layered sine-Gordon (LSG) model, which is the
bosonized version of the multi-flavour Schwinger model and approaches the sum
of two ``normal'', massless sine-Gordon (SG) models in the limit of a vanishing
interlayer coupling J. Another model of interest is the massive sine-Gordon
(MSG) model. The leading-order approximation to the UV (ultra-violet) RG flow
predicts two phases for the LSG as well as for the MSG, just as it would be
expected for the SG model, where the two phases are known to be separated by
the Coleman fixed point. The presence of finite mass terms (for the LSG and the
MSG) leads to corrections to the UV RG flow, which are naturally identified as
the ``mass corrections''. The leading-order mass corrections are shown to have
the following consequences: (i) for the MSG model, only one phase persists, and
(ii) for the LSG model, the transition temperature is modified. Within the
mass-corrected UV scaling laws, the limit of J -> 0 is thus nonuniform with
respect to the phase structure of the model. The modified phase structure of
general massive sine-Gordon models is connected with the breaking of symmetries
in the internal space spanned by the field variables. For the LSG, the
second-order subleading mass corrections suggest that there exists a cross-over
regime before the IR scaling sets in, and the nonlinear terms show explicitly
that higher-order Fourier modes appear in the periodic blocked potential.Comment: 27 pages, 7 figure
Suppressing Electroweak Precision Observables in 5D Warped Models
We elaborate on a recently proposed mechanism to suppress large contributions
to the electroweak precision observables in five dimensional (5D) warped
models, without the need for an extended 5D gauge sector. The main ingredient
is a modification of the AdS metric in the vicinity of the infrared (IR) brane
corresponding to a strong deviation from conformality in the IR of the 4D
holographic dual. We compute the general low energy effective theory of the 5D
warped Standard Model, emphasizing additional IR contributions to the wave
function renormalization of the light Higgs mode. We also derive expressions
for the S and T parameters as a function of a generic 5D metric and zero-mode
wave functions. We give an approximate formula for the mass of the radion that
works even for strong deviation from the AdS background. We proceed to work out
the details of an explicit model and derive bounds for the first KK masses of
the various bulk fields. The radion is the lightest new particle although its
mass is already at about 1/3 of the mass of the lightest resonances, the KK
states of the gauge bosons. We examine carefully various issues that can arise
for extreme choices of parameters such as the possible reintroduction of the
hierarchy problem, the onset of nonperturbative physics due to strong IR
curvature or the creation of new hierarchies near the Planck scale. We conclude
that a KK scale of 1 TeV is compatible with all these constraints.Comment: 44 pages, 11 figures, references adde
The Higgs mechanism as a cut-off effect
We compute the Coleman-Weinberg potential with a finite cut-off for pure
SU(2) and SU(3) five-dimensional gauge theories compactified on an interval. We
show that besides the expected Coulomb phase located at and in the vicinity of
the free infrared stable or "trivial" fixed point, the theory possesses also a
Higgs phase. We compare the results from the potential computation with lattice
data from simulations.Comment: 22 pages, 5 figures; introduction improved, conclusions added,
published in JHE
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