170 research outputs found
Forward--Backward Asymmetry in B -> X_s l^+ l^- at the NNLL Level
We report the results of a new calculation of soft-gluon corrections in B ->
X_s l^+ l^- decays. In particular, we present the first calculation of
bremsstrahlung and corresponding virtual terms to the lepton forward-backward
asymmetry, which allows us to systematically include all contributions to this
observable beyond the lowest non-trivial order. The new terms are important,
for instance the position of the zero of the asymmetry receives corrections of
O(10%). Using a different method, we also provide an independent check of
recently published results on bremsstrahlung and infrared virtual corrections
to the dilepton-invariant mass distribution.Comment: 20 pages, 5 figures. v2: typo in (4.26) corrected; discussion on
mu-dependence modifie
On the possibility of a very light A^0 at low \tan\beta
The searches at LEP II for the processes e^+e^-\to h^0Z and e^+e^-\to h^0A^0
in the Minimal Supersymmetric Standard Model (MSSM) fail to exclude regions of
the m_h,m_A plane where \tan\beta <1, thus allowing a very light A^0 (m_A< 20
GeV). Such a parameter choice would predict a light H^\pm with m_{H^\pm}< m_W.
Although the potentially large branching ratio for H^\pm \to A^0 W^* would
ensure that H^\pm also escaped detection in direct searches at LEP II and the
Tevatron Run I, we show that this elusive parameter space is overwhelmingly
disfavoured by electroweak precision measurements.Comment: 11 pages, 2 figures, Revtex, references added, minor additions to
tex
Supersymmetric contributions to B -> D K and the determination of angle \gamma
We analyze supersymmetric contributions to B^- -> D^0 K^- and B^- ->\bar{D}^0
K^- processes. We investigate the possibility that supersymmetric CP violating
phases can affect our determination for the angle \gamma in the unitary
triangle of Cabibbo-Kobayashi-Maskaw mixing matrix. We calculate the gluino and
chargino contributions to b--> u(\bar{c}s) and b-->c(\bar{u}s) transitions in a
model independent way by using the mass insertion approximation method. We also
revise the D^0 - \bar{D}^0 mixing constraints on the mass insertions between
the first and second generations of the up sector. We emphasize that in case of
negligible D^0 -\bar{D}^0 mixing, one should consider simultaneous
contributions from more than one mass insertion in order to be able to obtain
the CP asymmetries of these processes within their 1\sigma experimental range.
However, with a large D^0-\bar{D}^0 mixing, one finds a significant deviation
between the two asymmetries and it becomes natural to have them of order the
central values of their experimental measurements.Comment: 20 page
NNLL QCD Corrections to the Decay
We briefly discuss the status of the NNLL QCD calculations in the inclusive
rare B decay . Two important ingredients, the two-loop
matrix elements of the four quark operator and the bremsstrahlung
contributions, were quite recently finalised. The new contributions
significantly improve the sensitivity of the inclusive decay decay in testing extensions of the standard model in the sector of flavour
dynamics; for instance the two-loop calculation cuts the low- scale uncertainty
in half and the bremsstrahlung calculation leads to a 10% shift of the position
of the zero of the forward- backward asymmetry.Comment: 7 pages, latex, 5 figures, references adde
NNLL QCD Contribution of the Electromagnetic Dipole Operator to Gamma(anti-B -> X(s) gamma)
We present an independent calculation of that part of the O(\alpha_s^2)
contribution to the decay width \Gamma(\bar B -> X_s \gamma) which arises from
the self-interference term of the electromagnetic dipole operator O_7. Using a
different method, we find complete agreement with a previous calculation. This
NNLL contribution is an important ingredient for the complete NNLL prediction
of \Gamma(\bar B -> X_s \gamma) which will resolve the charm quark mass
ambiguity appearing at NLL accuracy.Comment: 15 pages, uses axodraw.st
Flavor changing Z-decays from scalar interactions at a Giga-Z Linear Collider
The flavor changing decay Z -> d_I \bar{d}_J is investigated with special
emphasis on the b \bar{s} final state. Various models for flavor violation are
considered: two Higgs doublet models (2HDM's), supersymmetry (SUSY) with flavor
violation in the up and down-type squark mass matrices and SUSY with flavor
violation mediated by R-parity-violating interaction. We find that, within the
SUSY scenarios for flavor violation, the branching ratio for the decay Z -> b
\bar{s} can reach 10^{-6} for large \tan\beta values, while the typical size
for this branching ratio in the 2HDM's considered is about two orders of
magnitudes smaller at best. Thus, flavor changing SUSY signatures in radiative
Z decays such as Z -> b \bar{s} may be accessible to future ``Z factories''
such as a Giga-Z version of the TESLA design.Comment: 27 pages, 15 figures, REVTeX4. A new section added and a few minor
corrections were made in the tex
SUSY-Induced Top Quark FCNC Processes at Linear Colliders
In the Minimal Supersymmetric Model (MSSM) the hitherto unconstrained flavor
mixing between top-squark and charm-squark will induce the flavor-changing
neutral-current (FCNC) interaction between top quark and charm quark, which
then give rise to various processes at the next generation linear collider
(NLC), i.e., the top-charm associated productions via ,
and collisions as well as the top quark rare decays
(, or ). All these processes involve the same part of the
parameter space of the MSSM. Through a comparative analysis for all these
processes at the NLC, we found the best channel to probe such SUSY-induced top
quark FCNC is the top-charm associated production in collision,
which occurs at a much higher rate than or collision and
may reach the detectable level for some part of the parameter space. Since the
rates predicted by the Standard Model are far below the detectable level, the
observation of such FCNC events would be a robust indirect evidence of SUSY.Comment: 12 pages, 8 figures (more refs added, discussions extended
SUSY GUT Model Building
I discuss an evolution of SUSY GUT model building, starting with the
construction of 4d GUTs, to orbifold GUTs and finally to orbifold GUTs within
the heterotic string. This evolution is an attempt to obtain realistic string
models, perhaps relevant for the LHC. This review is in memory of the sudden
loss of Julius Wess, a leader in the field, who will be sorely missed.Comment: 24 pages, 14 figures, lectures given at PiTP 2008, Institute for
Advanced Study, Princeton, to be published in the European Physical Journal
Flavor Physics in an SO(10) Grand Unified Model
In supersymmetric grand-unified models, the lepton mixing matrix can possibly
affect flavor-changing transitions in the quark sector. We present a detailed
analysis of a model proposed by Chang, Masiero and Murayama, in which the
near-maximal atmospheric neutrino mixing angle governs large new b -> s
transitions. Relating the supersymmetric low-energy parameters to seven new
parameters of this SO(10) GUT model, we perform a correlated study of several
flavor-changing neutral current (FCNC) processes. We find the current bound on
B(tau -> mu gamma) more constraining than B(B -> X_s gamma). The LEP limit on
the lightest Higgs boson mass implies an important lower bound on tan beta,
which in turn limits the size of the new FCNC transitions. Remarkably, the
combined analysis does not rule out large effects in B_s-B_s-bar mixing and we
can easily accomodate the large CP phase in the B_s-B_s-bar system which has
recently been inferred from a global analysis of CDF and DO data. The model
predicts a particle spectrum which is different from the popular Constrained
Minimal Supersymmetric Standard Model (CMSSM). B(tau -> mu gamma) enforces
heavy masses, typically above 1 TeV, for the sfermions of the degenerate first
two generations. However, the ratio of the third-generation and
first-generation sfermion masses is smaller than in the CMSSM and a (dominantly
right-handed) stop with mass below 500 GeV is possible.Comment: 44 pages, 5 figures. Footnote and references added, minor changes,
Fig. 2 corrected; journal versio
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