215 research outputs found
Lectures on string theory and cosmology
In these lectures I review recent attempts to apply string theory to
cosmology, including string cosmology and various models of brane cosmology. In
addition, the review includes an introduction to inflation as well as a
discussion of transplanckian signatures. I also provide a critical discussion
of the possible role of holography. The material is based on lectures given in
January 2004 at the RTN String School in Barcelona, but also contain some
additional material.Comment: Lectures given in January 2004 at the RTN Barcelona String School, 50
pages, 9 figure
Noncommutative Field Theory and Spontaneous Symmetry Breaking
We investigate the noncommutative analogue of the spontaneously broken linear
sigma model at the one-loop quantum level. In the commutative case,
renormalization of a theory with a spontaneously broken continuous global
symmetry depends on cancellations that enable the limited set of counterterms
consistent with that symmetry to remove the divergences even after its
spontaneous breaking, while preserving the masslessness of the associated
Goldstone modes. In the noncommutative case, we find that these cancellations
are violated, and the renormalized one-loop correction to the inverse pion
propagator explicitly yields a mass shift which depends on the ultraviolet
cutoff. Thus, we cannot naively take the ultraviolet cutoff to infinity first,
and then take the external momentum to zero to verify Nambu-Goldstone symmetry
realization. However, from the Wilsonian perspective where the cutoff is fixed
and physical, the zero external momentum limit of the inverse pion propagator
still vanishes, and implies the masslessness of the pion fields at one-loop.
This is another demonstration of the failure of ultraviolet and infrared limits
to commute in noncommutative field theories, and signals the incompatibility of
Nambu-Goldstone symmetry realization with the continuum renormalization of
these theories.Comment: 17 pages, LaTeX, uses feynmp package; analyzes general case using all
orderings of star product in quartic vertices; uses symmetrized Feynman
vertex rules; revised combinatorics; reference added; conclusions unaltered;
diagrams now appear correctl
Critical Constraints on Chiral Hierarchies
We consider the constraints that critical dynamics places on models with a
top quark condensate or strong extended technicolor (ETC). These models require
that chiral-symmetry-breaking dynamics at a high energy scale plays a
significant role in electroweak symmetry breaking. In order for there to be a
large hierarchy between the scale of the high energy dynamics and the weak
scale, the high energy theory must have a second order chiral phase transition.
If the transition is second order, then close to the transition the theory may
be described in terms of a low-energy effective Lagrangian with composite
``Higgs'' scalars. However, scalar theories in which there are more than one
coupling can have a {\it first order} phase transition instead, due to
the Coleman-Weinberg instability. Therefore, top-condensate or strong ETC
theories in which the composite scalars have more than one coupling
cannot always support a large hierarchy. In particular, if the
Nambu--Jona-Lasinio model solved in the large- limit is a good
approximation to the high-energy dynamics, then these models will not produce
acceptable electroweak symmetry breaking.Comment: 10 pages, 1 postscript figure (appended), BUHEP-92-35, HUTP-92/A05
The heavy quark decomposition of the S-matrix and its relation to the pinch technique
We propose a decomposition of the S-matrix into individually gauge invariant
sub-amplitudes, which are kinematically akin to propagators, vertices, boxes,
etc. This decompsition is obtained by considering limits of the S-matrix when
some or all of the external particles have masses larger than any other
physical scale. We show at the one-loop level that the effective gluon
self-energy so defined is physically equivalent to the corresponding gauge
independent self-energy obtained in the framework of the pinch technique. The
generalization of this procedure to arbitrary gluonic -point functions is
briefly discussed.Comment: 11 uuencoded pages, NYU-TH-94/10/0
High-Temperature Effective Potential of Noncommutative Scalar Field Theory: Reduction of Degree of Freedom by Noncommutativity
The renormalization of effective potentials for the noncommutative scalar
field theory at high temperature are investigated to the two-loop
approximation. The Feynman diagrams in evaluating the effective potential may
be classified into two types: the planar diagrams and nonplanar diagrams. The
nonplanar diagrams, which depend on the parameter of noncommutativity, do not
appear in the one-loop potential. Despite their appearance in the two-loop
level, they do not have an inclination to restore the symmetry breaking in the
tree level, in contrast to the planar diagrams. This phenomenon is explained as
a consequence of the drastic reduction of the degrees of freedom in the
nonplanar diagrams when the thermal wavelength is smaller than the
noncommutativity scale. Our results show that the nonplanar two-loop
contribution to the effective potential can be neglected in comparsion with
that from the planar diagrams.Comment: Latex, 17 pages, change the conclusion, improve the Englis
Leptonic constants of heavy quarkonia in potential approach of NRQCD
We consider a general scheme for calculating the leptonic constant of heavy
quarkonium QQ-bar in the framework of nonrelativistic quantum chromodynamics,
NRQCD, operating as the effective theory of nonrelativistic heavy quarks. We
explore the approach of static potential in QCD, which takes into account both
the evolution of effective charge in the three-loop approximation and the
linearly raising potential term, which provides the quark confinement. The
leptonic constants of bb-bar and cc-bar systems are evaluated by making use of
two-loop anomalous dimension for the current of nonrelativistic quarks, where
the factor for the normalization of matrix element is introduced in order to
preserve the renormalization group invariance of estimates.Comment: 18 pages, 6 eps-figures, discussion and references added, vNRQCD
analysis considere
Direct Mediation and Metastable Supersymmetry Breaking for SO(10)
We examine a metastable Macroscopic SO(N) SQCD model of
Intriligator, Seiberg and Shih (ISS). We introduce various baryon and meson
deformations, including multitrace operators and explore embedding an SO(10)
parent of the standard model into two weakly gauged flavour sectors. Direct
fundamental messengers and the symmetric pseudo-modulus messenger mediate SUSY
breaking to the MSSM. Gaugino and sfermion masses are computed and compared for
each deformation type. We also explore reducing the rank of the magnetic quark
matrix of the ISS model and find an additional fundamental messenger.Comment: 43 pages, Latex. Version to appear in JHEP
Higgs boson couplings to quarks with supersymmetric CP and flavor violations
In minimal supersymmetric model (SUSY) with a light Higgs sector, explicit CP
violation and most general flavor mixings in the sfermion sector, integration
of the superpartners out of the spectrum induces potentially large
contributions to the Yukawa couplings of light quarks via those of the heavier
ones. These corrections can be sizeable even for moderate values of tan(beta),
and remain nonvanishing even if all superpartners decouple. When the SUSY
breaking scale is close to the electroweak scale, the Higgs exchange effects
can compete with the gauge boson and box diagram contributions to rare
processes, and their partial cancellations can lead to relaxation of the
existing bounds on flavor violation sources. In this case there exist sizeable
enhancements in flavor-changing Higgs decays. When the superpartners completely
decouple, however, the Higgs mediation becomes the dominant SUSY contribution
to rare processes the saturation of which, without a strong suppression of the
flavor mixings, prefers large tan(beta) and certain ranges for the CP-odd
phases. The decay rate of the lightest Higgs into light down quarks become
comparable with that into the bottom quark. Moreover, the Higgs decay into the
up quark is significantly enhanced. There are observable implications for rare
processes, atomic electric dipole moments, and collider searches for Higgs
bosons.Comment: 20 pp. Added references, improved the discussion of FCNC
constraints;journal versio
Primeval Corrections to the CMB Anisotropies
We show that deviations of the quantum state of the inflaton from the thermal
vacuum of inflation may leave an imprint in the CMB anisotropies. The quantum
dynamics of the inflaton in such a state produces corrections to the
inflationary fluctuations, which may be observable. Because these effects
originate from IR physics below the Planck scale, they will dominate over any
trans-Planckian imprints in any theory which obeys decoupling. Inflation sweeps
away these initial deviations and forces its quantum state closer to the
thermal vacuum. We view this as the quantum version of the cosmic no-hair
theorem. Such imprints in the CMB may be a useful, independent test of the
duration of inflation, or of significant features in the inflaton potential
about 60 e-folds before inflation ended, instead of an unlikely discovery of
the signatures of quantum gravity. The absence of any such substructure would
suggest that inflation lasted uninterrupted much longer than
e-folds.Comment: 17 pages, latex, no figures; v3: added references and comments, final
version to appear in Phys. Rev.
General structure of the photon self-energy in non-commutative QED
We study the behavior of the photon two point function, in non-commutative
QED, in a general covariant gauge and in arbitrary space-time dimensions. We
show, to all orders, that the photon self-energy is transverse. Using an
appropriate extension of the dimensional regularization method, we evaluate the
one-loop corrections, which show that the theory is renormalizable. We also
prove, to all orders, that the poles of the photon propagator are gauge
independent and briefly discuss some other related aspects.Comment: 16 pages, revtex4. This is the final version to be published in Phys.
Rev.
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