272 research outputs found
Complete NNLO QCD Analysis of B -> X_s l^+ l^- and Higher Order Electroweak Effects
We complete the next-to-next-to-leading order QCD calculation of the
branching ratio for B -> X_s l^+ l^- including recent results for the
three-loop anomalous dimension matrix and two-loop matrix elements. These new
contributions modify the branching ratio in the low-q^2 region, BR_ll, by about
+1% and -4%, respectively. We furthermore discuss the appropriate normalization
of the electromagnetic coupling alpha and calculate the dominant higher order
electroweak effects, showing that, due to accidental cancellations, they change
BR_ll by only -1.5% if alpha(mu) is normalized at mu = O(m_b), while they shift
it by about -8.5% if one uses a high scale normalization mu = O(M_W). The
position of the zero of the forward-backward asymmetry, q_0^2, is changed by
around +2%. After introducing a few additional improvements in order to reduce
the theoretical error, we perform a comprehensive study of the uncertainty. We
obtain BR_ll(1 GeV^2 <= q^2 <= 6 GeV^2) = (1.57 +- 0.16) x 10^-6 and q_0^2 =
(3.76 +- 0.33) GeV^2 and note that the part of the uncertainty due to the
b-quark mass can be easily reduced.Comment: 26 pages, 7 figures; v5: corrected normalisation in Eq. (5),
numerical results unchange
Is Vtb=1 ?
The strongest constraint on Vtb presently comes from the 3 x 3 unitarity of
the CKM matrix, which fixes Vtb to be very close to one. If the unitarity is
relaxed, current information from top production at Tevatron still leaves open
the possibility that Vtb is sizably smaller than one. In minimal extensions of
the standard model with extra heavy quarks, the unitarity constraints are much
weaker and the EW precision parameters entail the strongest bounds on Vtb. We
discuss the experimental perspectives of discovering and identifying such new
physics models at the Tevatron and the LHC, through a precise measurement of
Vtb from the single top cross sections and by the study of processes where the
extra heavy quarks are produced.Comment: 19 pages, 8 figure
Dynamics of a classical gas including dissipative and mean field effects
By means of a scaling ansatz, we investigate an approximated solution of the
Boltzmann-Vlasov equation for a classical gas. Within this framework, we derive
the frequencies and the damping of the collective oscillations of a
harmonically trapped gas and we investigate its expansion after release of the
trap. The method is well suited to studying the collisional effects taking
place in the system and in particular to discussing the crossover between the
hydrodynamic and the collisionless regimes. An explicit link between the
relaxation times relevant for the damping of the collective oscillations and
for the expansion is established.Comment: 4 pages, 1 figur
Mean field effects in a trapped classical gas
In this article, we investigate mean field effects for a bosonic gas
harmonically trapped above the transition temperature in the collisionless
regime. We point out that those effects can play also a role in low dimensional
system. Our treatment relies on the Boltzmann equation with the inclusion of
the mean field term.
The equilibrium state is first discussed. The dispersion relation for
collective oscillations (monopole, quadrupole, dipole modes) is then derived.
In particular, our treatment gives the frequency of the monopole mode in an
isotropic and harmonic trap in the presence of mean field in all dimensions.Comment: 4 pages, no figure submitted to Phys. Rev.
More Model-Independent Analysis of b->s Processes
We study model-independently the implications of non-standard scalar and
pseudoscalar interactions for the decays b ->s gamma, b -> s g, b -> s l^+l^-
(l=e,mu) and B_s -> mu^+ mu^-. We find sizeable renormalization effects from
scalar and pseudoscalar four-quark operators in the radiative decays and at
O(alpha_s) in hadronic b decays. Constraints on the Wilson coefficients of an
extended operator basis are worked out. Further, the ratios R_H = BR(B -> H
mu^+ mu^-)/BR(B -> H e^+ e^-), for H=K^(*), X_s, and their correlations with
B_s -> mu^+ mu^- decay are investigated. We show that the Standard Model
prediction for these ratios defined with the same cut on the dilepton mass for
electron and muon modes, R_H= 1 + O(m^2_mu/m^2_b), has a much smaller
theoretical uncertainty (<1%) than the one for the individual branching
fractions. The present experimental limit R_K < 1.2 puts constraints on scalar
and pseudoscalar couplings, which are similar to the ones from current data on
BR(B_s -> mu^+ mu^-). We find that new physics corrections to R_{K*} and
R_{X_s} can reach 13% and 10%, respectively.Comment: 28 pages, 6 figures; Table 1 updated, two refs added (to appear in
PRD
Ground state and elementary excitations of single and binary Bose-Einstein condensates of trapped dipolar gases
We analyze the ground-state properties and the excitation spectrum of
Bose-Einstein condensates of trapped dipolar particles. First, we consider the
case of a single-component polarized dipolar gas. For this case we discuss the
influence of the trapping geometry on the stability of the condensate as well
as the effects of the dipole-dipole interaction on the excitation spectrum. We
discuss also the ground state and excitations of a gas composed of two
antiparallel dipolar components.Comment: 12 pages, 9 eps figures, final versio
Charmless hadronic decays and new physics effects in the general two-Higgs doublet models
Based on the low-energy effective Hamiltonian with the generalized
factorization, we calculate the new physics contributions to the branching
ratios of the two-body charmless hadronic decays of and mesons
induced by the new gluonic and electroweak charged-Higgs penguin diagrams in
the general two-Higgs doublet models (models I, II and III). Within the
considered parameter space, we find that: (a) the new physics effects from new
gluonic penguin diagrams strongly dominate over those from the new -
and - penguin diagrams; (b) in models I and II, new physics contributions
to most studied B meson decay channels are rather small in size: from -15% to
20%; (c) in model III, however, the new physics enhancements to the
penguin-dominated decay modes can be significant, , and
therefore are measurable in forthcoming high precision B experiments; (d) the
new physics enhancements to ratios {\cal B}(B \to K \etap) are significant in
model III, , and hence provide a simple and plausible new
physics interpretation for the observed unexpectedly large B \to K \etap
decay rates; (e) the theoretical predictions for and
in model III are still consistent with the data
within errors; (f) the significant new physics enhancements to the
branching ratios of and decays are helpful to improve the
agreement between the data and the theoretical predictions; (g) the theoretical
predictions of in the 2HDM's are generally
consistent with experimental measurements and upper limits ()Comment: 55 pages, Latex file, 17 PS and EPS figures. With minor corrections,
final version to be published in Phys.Rev. D. Repot-no: PKU-TH-2000-4
Fermionic NNLL corrections to b -> s \gamma
In this paper we take the first step towards a complete
next-to-next-to-leading logarithmic (NNLL) calculation of the inclusive decay
rate for . We consider the virtual corrections of order
\alphas^2 n_f to the matrix elements of the operators , and
and evaluate the real and virtual contributions to . These
corrections are expected to be numerically important. We observe a strong
cancelation between the contributions from the current-current operators and
and obtain, after applying naive non-abelianization, a reduction of the
branching ratio of 3.9% (for GeV) and an increase of 3.4% (for
GeV).Comment: 38 pages, result extended to allow for an explicit photon energy
cut-off, appendix and references added, final result and conlclusions
unchange
Lower limit on the neutralino mass in the general MSSM
We discuss constraints on SUSY models with non-unified gaugino masses and R_P
conservation. We derive a lower bound on the neutralino mass combining the
direct limits from LEP, the indirect limits from gmuon, bsgamma, Bsmumu and the
relic density constraint from WMAP. The lightest neutralino (mneutralino=6GeV)
is found in models with a light pseudoscalar with MA<200GeV and a large value
for . Models with heavy pseudoscalars lead to mneutralino>18(29)GeV
for . We show that even a very conservative bound from the
muon anomalous magnetic moment can increase the lower bound on the neutralino
mass in models with mu<0 and/or large values of . We then examine
the potential of the Tevatron and the direct detection experiments to probe the
SUSY models with the lightest neutralinos allowed in the context of light
pseudoscalars with high . We also examine the potential of an e+e-
collider of 500GeV to produce SUSY particles in all models with neutralinos
lighter than the W. In contrast to the mSUGRA models, observation of at least
one sparticle is not always guaranteed.Comment: 37 pages, LateX, 16 figures, paper with higher resolution figures
available at
http://wwwlapp.in2p3.fr/~boudjema/papers/bound-lsp/bound-lsp.htm
SUSY breaking mediation mechanisms and (g-2)_\mu, B -> X_s \gamma, B -> X_{s} l^+ l^- and B_s -> \mu^+ \mu^-
We show that there are qualitative differences in correlations among
, , and in various SUSY breaking mediation mechanisms: minimal supergravity
(mSUGRA), gauge mediation (GMSB), anomaly mediation (AMSB), gaugino mediation
(MSB), weakly and strongly interacting string theories, and
brane models. After imposing the direct search limits on the Higgs boson and
SUSY particle search limits and branching ratio, we find all
the scenarios can accommodate the in the range of
(a few tens), and predict that the branching ratio for can differ from the standard model (SM) prediction by
but no more. On the other hand, the is sensitive to the
SUSY breaking mediation mechanisms through the pseudoscalar and stop masses
( and ), and the stop mixing angle. In the GMSB with a
small messenger number, the AMSB, the MSB and the noscale scenarios,
one finds that , which is
below the search limit at the Tevatron Run II. Only the mSUGRA or string
inspired models can generate a large branching ratio for this decay.Comment: 40 pages, 21 figures (to appear in JHEP
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