883 research outputs found
Completing the Calculation of BLM corrections to B -> Xs gamma
Perturbative O(alpha_s^2) corrections to BR(B -> Xs gamma) in the BLM
approximation receive contributions from two-, three- and four-body final
states. While all the two-body results are well established by now, the other
ones have remained incomplete for several years. Here, we calculate the last
contribution that has been missing to date, namely the one originating from
interference of the current-current and gluonic dipole operators
(K_18^{(2)beta_0} and K_{28}^{(2)beta_0}). Moreover, we confirm all the
previously known results for BLM corrections to the photon energy spectrum that
involve the current-current operators (e.g., K_{22}^{(2)beta_0} and
K_{27}^{(2)beta_0}). Finally, we also confirm the recent findings of Ferroglia
and Haisch on self-interference of the gluonic dipole operator
(K_{88}^{(2)beta_0}).Comment: 5 pages, 2 ps figure
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
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
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.
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
Tree-level contributions to B -> Xs gamma
Weak radiative decay B -> X_s gamma is known to be a loop-generated process.
However, it does receive tree-level contributions from CKM-suppressed b -> u
ubar s gamma transitions. In the present paper, we evaluate such contributions
together with similar ones from the QCD penguin operators. For a low value of
the photon energy cutoff E_0 ~ m_b/20 that has often been used in the
literature, they can enhance the inclusive branching ratio by more than 10%.
For E_0 = 1.6 GeV or higher, the effect does not exceed 0.4%, which is due to
phase-space suppression. Our perturbative results contain collinear logarithms
that depend on the light quark masses m_q (q=u,d,s). We have allowed m_b/m_q to
vary from 10 to 50, which corresponds to values of m_q that are typical for the
constituent quark masses. Such a rough method of estimation may be improved in
the future with the help of fragmentation functions once the considered effects
begin to matter in the overall error budget for BR(B -> X_s gamma).Comment: v3: Corrected factors of 2 in the subdominant T_3 contributions.
Minor changes in the numerical results (Table II
Supersymmetric Musings on the Predictivity of Family Symmetries
We discuss the predictivity of family symmetries for the soft supersymmetry
breaking parameters in the framework of supergravity. We show that unknown
details of the messenger sector and the supersymmetry breaking hidden sector
enter into the soft parameters, making it difficult to obtain robust
predictions. We find that there are specific choices of messenger fields which
can improve the predictivity for the soft parameters.Comment: 20 pages, 5 figure
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