124,017 research outputs found
Gauge Coupling Unification from Unified Theories in Higher Dimensions
Higher dimensional grand unified theories, with gauge symmetry breaking by
orbifold compactification, possess SU(5) breaking at fixed points, and do not
automatically lead to tree-level gauge coupling unification. A new framework is
introduced that guarantees precise unification -- even the leading loop
threshold corrections are predicted, although they are model dependent. Precise
agreement with the experimental result, \alpha_s^{exp} = 0.117 \pm 0.002,
occurs only for a unique theory, and gives \alpha_s^{KK} = 0.118 \pm 0.004 \pm
0.003. Remarkably, this unique theory is also the simplest, with SU(5) gauge
interactions and two Higgs hypermultiplets propagating in a single extra
dimension. This result is more successful and precise than that obtained from
conventional supersymmetric grand unification, \alpha_s^{SGUT} = 0.130 \pm
0.004 \pm \Delta_{SGUT}. There is a simultaneous solution to the three
outstanding problems of 4D supersymmetric grand unified theories: a large mass
splitting between Higgs doublets and their color triplet partners is forced,
proton decay via dimension five operators is automatically forbidden, and the
absence of fermion mass relations amongst light quarks and leptons is
guaranteed, while preserving the successful m_b/m_\tau relation. The theory
necessarily has a strongly coupled top quark located on a fixed point and part
of the lightest generation propagating in the bulk. The string and
compactification scales are determined to be around 10^{17} GeV and 10^{15}
GeV, respectively.Comment: 29 pages, LaTe
Mass scale of vectorlike matter and superpartners from IR fixed point predictions of gauge and top Yukawa couplings
We use the IR fixed point predictions for gauge couplings and the top Yukawa
coupling in the MSSM extended with vectorlike families to infer the scale of
vectorlike matter and superpartners. We quote results for several extensions of
the MSSM and present results in detail for the MSSM extended with one complete
vectorlike family. We find that for a unified gauge coupling
vectorlike matter or superpartners are expected within 1.7 TeV (2.5 TeV) based
on all three gauge couplings being simultaneously within 1.5\% (5\%) from
observed values. This range extends to about 4 TeV for . We
also find that in the scenario with two additional large Yukawa couplings of
vectorlike quarks the IR fixed point value of the top Yukawa coupling
independently points to a multi-TeV range for vectorlike matter and
superpartners. Assuming a universal value for all large Yukawa couplings at the
GUT scale, the measured top quark mass can be obtained from the IR fixed point
for . The range expands to any for
significant departures from the universality assumption. Considering that the
Higgs boson mass also points to a multi-TeV range for superpartners in the
MSSM, adding a complete vectorlike family at the same scale provides a
compelling scenario where the values of gauge couplings and the top quark mass
are understood as a consequence of the particle content of the model.Comment: 31 pages, 9 figures, typos corrected, matches the published version.
v4: Appendix correcte
Universal Impedance Fluctuations in Wave Chaotic Systems
We experimentally investigate theoretical predictions of universal impedance
fluctuations in wave chaotic systems using a microwave analog of a quantum
chaotic infinite square well potential. Our approach emphasizes the use of the
radiation impedance to remove the non-universal effects of the particular
coupling from the outside world to the scatterer. Specific predictions that we
test include the probability distribution functions (PDFs) of the real (related
to the local density of states in disordered metals) and imaginary parts of the
normalized cavity impedance, the equality of the variances of these PDFs, and
the dependence of the universal PDFs on a single control parameter
characterizing the level of loss. We find excellent agreement between the
statistical data and theoretical predictions.Comment: 5 pages, 3 figures, submitted to Phys. Rev. Let
Modification of the Unitarity Relation for sin2beta-Vub in Supersymmetric Models
Recently, a more than 2sigma discrepancy has been observed between the well
measured inclusive value of Vub and the predicted value of Vub from the
unitarity triangle fit using the world average value of sin2beta. We attempt to
resolve this tension in the context of grand unified SO(10) and SU(5) models
where the neutrino mixing matrix is responsible for flavor changing neutral
current at the weak scale and the models with non-proportional A-terms (can be
realized simply in the context of intersecting D-brane models) and investigate
the interplay between the constraints arising from B_{s,d}-\bar B_{s,d}
mixings, epsilon_K, Br(tau -> mu gamma), Br(mu -> e gamma) and a fit of this
new discrepancy. We also show that the ongoing measurement of the phase of Bs
mixing will be able to identify the grand unified model. The measurement of
Br(tau -> e gamma) will also be able to test these scenarios, especially the
models with non-proportional A-terms.Comment: 20 pages, 4 figures. Minor corrections, references adde
Systematics of Coupling Flows in AdS Backgrounds
We give an effective field theory derivation, based on the running of Planck
brane gauge correlators, of the large logarithms that arise in the predictions
for low energy gauge couplings in compactified AdS}_5 backgrounds, including
the one-loop effects of bulk scalars, fermions, and gauge bosons. In contrast
to the case of charged scalars coupled to Abelian gauge fields that has been
considered previously in the literature, the one-loop corrections are not
dominated by a single 4D Kaluza-Klein mode. Nevertheless, in the case of gauge
field loops, the amplitudes can be reorganized into a leading logarithmic
contribution that is identical to the running in 4D non-Abelian gauge theory,
and a term which is not logarithmically enhanced and is analogous to a two-loop
effect in 4D. In a warped GUT model broken by the Higgs mechanism in the
bulk,we show that the matching scale that appears in the large logarithms
induced by the non-Abelian gauge fields is m_{XY}^2/k where m_{XY} is the bulk
mass of the XY bosons and k is the AdS curvature. This is in contrast to the UV
scale in the logarithmic contributions of scalars, which is simply the bulk
mass m. Our results are summarized in a set of simple rules that can be applied
to compute the leading logarithmic predictions for coupling constant relations
within a given warped GUT model. We present results for both bulk Higgs and
boundary breaking of the GUT gauge group.Comment: 22 pages, LaTeX, 3 figures. Comments and references adde
Relating Physical Observables in QCD without Scale-Scheme Ambiguity
We discuss the St\"uckelberg-Peterman extended renormalization group
equations in perturbative QCD, which express the invariance of physical
observables under renormalization-scale and scheme-parameter transformations.
We introduce a universal coupling function that covers all possible choices of
scale and scheme. Any perturbative series in QCD is shown to be equivalent to a
particular point in this function. This function can be computed from a set of
first-order differential equations involving the extended beta functions. We
propose the use of these evolution equations instead of perturbative series for
numerical evaluation of physical observables. This formalism is free of
scale-scheme ambiguity and allows a reliable error analysis of higher-order
corrections. It also provides a precise definition for as the pole in the associated 't Hooft scheme. A concrete application to
is presented.Comment: Plain TEX, 4 figures (available upon request), 22 pages,
DOE/ER/40322-17
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