190 research outputs found
Higgs Mass Textures in Flipped SU(5)
We analyze the Higgs doublet-triplet mass splitting problem in the version of
flipped SU(5) derived from string theory. Analyzing non-renormalizable terms up
to tenth order in the superpotential, we identify a pattern of field vev's that
keeps one pair of electroweak Higgs doublets light, while all other Higgs
doublets and all Higgs triplets are kept heavy, with the aid of the economical
missing-doublet mechanism found in the field-theoretical version of flipped
SU(5). The solution predicts that second-generation charge -1/3 quarks and
charged leptons are much lighter than those in the third generation.Comment: 15 pages LaTe
One-loop Higgs mass finiteness in supersymmetric Kaluza-Klein theories
We analyze the one-loop ultraviolet sensitivity of the Higgs mass in a
five-dimensional supersymmetric theory compactified on the orbifold S^1/Z_2,
with superpotential localized on a fixed-point brane. Four-dimensional
supersymmetry is broken by Scherk-Schwarz boundary conditions. Kaluza-Klein
interactions are regularized by means of a brane Gaussian distribution along
the extra dimension with length l_s\simeq\Lambda^{-1}_s, where \Lambda_s is the
cutoff of the five-dimensional theory. The coupling of the n-mode, with mass
M^{(n)}, acquires the n-dependent factor exp{-(M^{(n)}/\Lambda_s)^2/2}, which
makes it to decouple for M^{(n)}\gg \Lambda_s. The sensitivity of the Higgs
mass on \Lambda_s is strongly suppressed and quadratic divergences cancel by
supersymmetry. The one-loop correction to the Higgs mass is finite and equals,
for large values of \Lambda_s, the value obtained by the so-called
KK-regularization.Comment: 8 pages, 1 figure. The discussion on the distribution giving rise to
couplings suppressed by exp(-M/Lambda) is revised and the result is finite
and equals that of the Gaussian cas
The timescale of low-mass proto-helium white dwarf evolution
A large number of low-mass (< 0.20 M_sun) helium white dwarfs (He WDs) have
recently been discovered. The majority of these are orbiting another WD or a
millisecond pulsar (MSP) in a close binary system; a few examples are found to
show pulsations or to have a main-sequence star companion. There appear to be
discrepancies between the current theoretical modelling of such low-mass He WDs
and a number of key observed cases, indicating that their formation scenario
remains to be fully understood. Here we investigate the formation of detached
proto-He WDs in close-orbit low-mass X-ray binaries (LMXBs). Our prime focus is
to examine the thermal evolution and the contraction phase towards the WD
cooling track and investigate how this evolution depends on the WD mass. Our
calculations are then compared to the most recent observational data. Numerical
calculations with a detailed stellar evolution code were used to trace the
mass-transfer phase in a large number of close-orbit LMXBs. Subsequently, we
followed the evolution of the detached low-mass proto-He WDs, including stages
with residual shell hydrogen burning and vigorous flashes caused by unstable
CNO burning. We find that the time between Roche-lobe detachment until the
low-mass proto-He WD reaches the WD cooling track is typically Delta_t_proto =
0.5 - 2 Gyr, depending systematically on the WD mass and therefore on its
luminosity. The lowest WD mass for developing shell flashes is ~0.21 M_sun for
progenitor stars of mass M2 <= 1.5 M_sun (and ~0.18 M_sun for M2 = 1.6 M_sun).
The long timescale of low-mass proto-He WD evolution can explain a number of
recent observations, including some MSP systems hosting He WD companions with
very low surface gravities and high effective temperatures. We find no evidence
for Delta_t_proto to depend on the occurrence of flashes and thus question the
suggested dichotomy in thermal evolution of proto-WDs.Comment: 6 pages, 6 figures, 2 tables, A&A Letters, accepte
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
Constraints From Gauge Coupling Unification On The Scale Of Supersymmetry Breaking
We reanalyze precision LEP data and coupling constant unification in the
minimal supersymmetric model including the evolution of the gaugino
masses. We derive general bounds on the primordial gaugino
supersymmetry-breaking mass-scale in terms of the various input
parameters. The model cannot accommodate m_{1/2}<1\TeV for values of \as <
0.115, even for extreme values of the other inputs. We emphasize
the sensitivity of this type of calculations to the various input parameters.Comment: 9 pages, 1 figure not included, ACT-10/9
Multi-wavelength studies of pulsars and their companions
Neutron stars are the degenerate relic cores of massive stars formed in the aftermath of a supernova explosion. Matter in their centes is believed to be condensed at densities as high as ten times that found in atomic nuclei. Thus, observational access to their properties provides the means to study the behavior of physical laws in extreme conditions, beyond the reach of terrestrial experiments. Rapidly rotating, highly magnetized neutron stars emit a narrow intense beam of radio emission from their magnetospheric poles. When this pulse happens to intersect our line of sight, it gives rise to the pulsar phenomenon. Regular radio-timing of pulse arrival times on earth, results in some of the most precise measurements in astrophysics. This thesis deals with the study of binary millisecond pulsars with white dwarf companions and is divided in 7 Chapters. Chapters 1 & 2 give a brief introduction to neutron stars, pulsars, and binary pulsars. Chapter 3 describes spectroscopic and optical observations of the low mass white dwarf companion to PSR J1909-3744. For this system, radio observations have yielded a precise mass measurement as well as distance information. Combined with the optical data, these provide the first observational test for theoretical white-dwarf cooling models and spectra. The latter, if reliable, can be used to infer theory-independent masses for similar systems. In Chapter 4, I discuss the measurement of the component masses in the short-orbit PSR J1738+0333 system based on spectroscopy of its white-dwarf companion. This system is particularly important for understanding the physics of pulsar recycling and binary evolution. Moreover, combined with the measurement of the orbital decay from radio-timing, the masses pose the most stringent constraints on Scalar-Tensor gravity. Chapter 5 describes radio and optical observations of PSR J0348+0432, a compact pulsar-white dwarf binary discovered recently with the 100-m Green-Bank Radio Telescope. Spectral observations of its bright white-dwarf companion show that the neutron star in the system is the most massive known to date. This measurement is based on a new set of white-dwarf cooling models, designed to take into account the remaining uncertainties not constrained by PSR J1909-3744. Furthermore, I discuss radio-timing observations that have yielded a significant measurement of the orbital decay which is completely consistent with the General Relativity prediction. This provides a verification of the theory in a highly non-linear gravitational regime, far beyond the reach of previous experiments. PSR J0348+0432 also poses significant constraints on the equation-of-state at supra-nuclear densities and sheds light to the evolution of low-mass X-ray binaries. In Chapter 6, I present the detection of the optical counterpart of the 1 solar mass companion to PSR J1141-6545 that verifies its white-dwarf nature. This simple observation is particularly important for understanding the unique evolutionary history of the binary and justifies the constraints on alternative-gravity theories imposed by the system. Finally, Chapter 7 summarizes the main conclusions of this work
Five-Dimensional Gauged Supergravity and Supersymmetry Breaking in ~Theory
We extend the formulation of gauged supergravity in five dimensions, as
obtained by compactification of ~theory on a deformed Calabi-Yau manifold,
to include non-universal matter hypermultiplets. Even in the presence of this
gauging, only the graviton supermultiplets and matter hypermultiplets can
couple to supersymmetry breaking sources on the walls, though these mix with
vector supermultiplets in the bulk. Whatever the source of supersymmetry
breaking on the hidden wall, that on the observable wall is in general a
combination of dilaton- and moduli-dominated scenarios.Comment: 20 pages, LaTex, corrected typos and notation, added reference
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