949 research outputs found
QCD challenges in radiative B decays
Radiative decays of the B meson are known to provide important constraints on
the MSSM and many other realistic new physics models in the sub-TeV range. The
inclusive branching ratio BR(B -> Xs gamma) being the key observable is
currently measured to about 7% accuracy. Reaching a better precision on the
theory side is a challenge both for the perturbative QCD calculations and for
analyses of non-perturbative hadronic effects. The current situation is briefly
summarized here.Comment: Overview talk presented at the International Workshop on QCD, Theory
and Experiment "QCD@Work'', Martina Franca, Italy, June 20th-23rd, 201
Four-Loop Anomalous Dimensions for Radiative Flavour-Changing Decays
We evaluate the complete four-loop anomalous dimension matrix that is
necessary for determining the effective flavour-changing neutral current
couplings qbar-q'-gamma and qbar-q'-g at the next-to-next-to-leading order in
QCD. The resulting O(alpha_s^2(mu_b)) correction to the B -> X_s gamma
branching ratio amounts to around -2.9% for mu_b = 5 GeV, and -4.4% for mu_b =
2.5 GeVComment: 19 page
in the Two Higgs Doublet Model up to Next-to-Next-to-Leading Order in QCD
We compute three-loop matching corrections to the Wilson coefficients
and in the Two Higgs Doublet Model by applying expansions for small,
intermediate and large charged Higgs boson masses. The results are used to
evaluate the branching ratio of to next-to-next-to
leading order accuracy, and to determine an updated lower limit on the charged
Higgs boson mass. We find \mhplus \ge 380 GeV at 95% confidence level when
the recently completed BABAR data analysis is taken into account. Our results
for the charged Higgs contribution to the branching ratio exhibit considerably
weaker sensitivity to the matching scale , as compared to previous
calculations.Comment: 20 pages, 15 figures; v2: minor modifications, matches published
version in JHE
Exclusive rare B -> K*e+e- decays at low recoil: controlling the long-distance effects
We present a model-independent description of the exclusive rare decays B->
K* e+e- in the low recoil region (large lepton invariant mass q^2 ~ m_b^2). In
this region the long-distance effects from quark loops can be computed with the
help of an operator product expansion in 1/Q, with Q={m_b, \sqrt{q^2}}.
Nonperturbative effects up to and including terms suppressed by Lambda/Q and
mc^2/mb^2 relative to the short-distance amplitude can be included in a
model-independent way. Based on these results, we propose an improved method
for determining the CKM matrix element |V{ub}| from a combination of rare and
semileptonic B and D decays near the zero recoil point. The residual
theoretical uncertainty from long distance effects in this |V{ub}|
determination comes from terms in the OPE of order alpha_s(Q)\Lambda/mb,
alpha_s^2(Q), mc^4/mb^4$ and duality violations and is estimated to be below
10%.Comment: 21 pages RevTex, 2 figures; v3: extensive numerical changes in the
NLL analysis, with improved stability under scale dependence. Typos fixed,
version to appear in Phys.Rev.
The electromagnetic dipole operator effect on B -> Xs gamma at O(alpha_s^2)
The flavor-changing electromagnetic dipole operator O_7 gives the dominant
contribution to the B -> Xs gamma decay rate. We calculate two-loop QCD
corrections to its matrix element together with the corresponding
bremsstrahlung contributions. The optical theorem is applied, and the relevant
imaginary parts of three-loop diagrams are computed following the lines of our
recent t -> Xb W calculation. The complete result allows us to test the
validity of the naive non-abelianization (NNA) approximation that has been
previously applied to estimate the NNLO QCD correction to Gamma(B -> Xs
gamma)/Gamma(B -> Xu e nu). When both decay widths are normalized to m^5_{b,R}
in the same renormalization scheme R, the calculated O(alpha_s^2) correction is
sizeable (~ 6%), and the NNA estimate is about 1/3 too large. On the other
hand, when the ratio of the decay widths is written as
S*(m_b(m_b)/m_{b,pole})^2, the calculated O(alpha_s^2) correction to S is at
the level of 1% for both the complete and the NNA results.Comment: Corrected pi^2 terms in the description of intermediate steps in
Section II. Final results unchange
The B -> Xs gamma Rate and CP Asymmetry within the Aligned Two-Higgs-Doublet Model
In the two-Higgs-doublet model the alignment of the Yukawa matrices in
flavour space guarantees the absence of flavour-changing neutral currents at
tree level, while introducing new sources for CP violation parametrized in a
very economical way. This implies potentially large influence in a number of
processes, b -> s gamma being a prominent example where rather high
experimental and theoretical precision meet. We analyze the CP rate asymmetry
in this inclusive decay and determine the resulting constraints on the model
parameters. We demonstrate the compatibility with previously obtained limits.
Moreover we extend the phenomenological analysis of the branching ratio, and
examine the influence of resulting correlations on the like-sign dimuon charge
asymmetry in B decays.Comment: 12 pages, 6 figures. Submitted to Phys.Rev.
Flavor changing effects on single charged Higgs boson production associated with a bottom-charm pair at CERN Large Hadron Collider
We study flavor changing effects on the \ppbchT process at the Large Hadron
Collider(LHC), which are inspired by the left-handed up-type squark mixings in
the Minimal Supersymmetric Standard Model(MSSM). We find that the SUSY QCD
radiative corrections to coupling can significantly enhance the cross
sections at the tree-level by a factor about with our choice of
parameters. We conclude that the squark mixing mechanism in the MSSM makes the
\ppbchT process a new channel for discovering a charged Higgs boson and
investigating flavor changing effects.Comment: One Latex file, 27 pages, 8 figures,to be appeared in PR
Complete One-Loop MSSM Predictions for B --> lepton lepton' at the Tevatron and LHC
During the last few years the Tevatron has dramatically improved the bounds
on rare B-meson decays into two leptons. In the case of B_s --> mu+ mu-, the
current bound is only ten times greater than the Standard Model expectation.
Sensitivity to this decay is one of the benchmark goals for LHCb performance
and physics. The Higgs penguin dominates this rate in the region of large
tan(beta) of the MSSM. This is not necessarily the case in the region of low
tan(beta), since box and Z-penguin diagrams may contribute at a comparable
rate. In this article, we compute the complete one-loop MSSM contribution to B
--> l+l'- for l,l' = e, mu. We study the predictions for general values of
tan(beta) with arbitrary flavour mixing parameters. We discuss the possibility
of both enhancing and suppressing the branching ratios relative to their
Standard Model expectations. In particular, we find that there are
"cancellation regions" in parameter space where the branching ratio is
suppressed well below the Standard Model expectation, making it effectively
invisible to the LHC.Comment: 30 pages, 4 figures; v.3: corrected factors of (2 pi) in (2.11),
(3.1), (A.11), (A.13-14
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