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Vector and scalar form factors for K- and D-mesonsemileptonic decays from twisted mass fermionswith Nf = 2
http://hdl.handle.net/2307/271
<Title>Vector and scalar form factors for K- and D-mesonsemileptonic decays from twisted mass fermionswith Nf = 2</Title>
<Authors>Di Vita, Stefano; Haas, Benjamin; Lubicz, Vittorio; Mescia, Federico; Simula, Silvano; Tarantino, Cecilia</Authors>
<Issue Date>2009</Issue Date>
<Pages>257-264</Pages>
<Abstract>We present lattice results for the formfactors relevant in the K→p ℓnℓ and D→p ℓnℓ semileptonic
decays, obtained from simulations with two flavors of dynamical twisted-mass fermions and pion
masses as light as 260 MeV. For K →p ℓn decays we discuss the estimates of the main sources
of systematic uncertainties, including the quenching of the strange quark, leading to our final result
f+(0) = 0.9560(57)stat.(62)syst.. Combined with the latest experimental data, our value of
f+(0) implies for the CKMmatrix element |Vus| the value 0.2267(5)exp.(20) f+(0) consistent with
the first-row CKM unitarity. For D →p ℓnℓ decays the application of Heavy Meson Chiral Perturbation
Theory allows to extrapolate our results for both the scalar and the vector form factors
at the physical point with quite good accuracy, obtaining a nice agreement with the experimental
data. In particular at zero-momentumtransfer we obtain f+(0) = 0.64(5). A preliminary analysis
of the discretization effects is presented and discussed.</Abstract>Wed, 31 Dec 2008 23:00:00 GMThttp://hdl.handle.net/2307/2712008-12-31T23:00:00ZNLO QCD corrections to CP-even and CP-odd Higgs boson production via gluon fusion in the MSSM
http://hdl.handle.net/2307/4508
<Title>NLO QCD corrections to CP-even and CP-odd Higgs boson production via gluon fusion in the MSSM</Title>
<Authors>Di Vita, Stefano</Authors>
<Issue Date>2012-02-28</Issue Date>
<Abstract>The calculation of the gluon fusion production cross section for the MSSM Higgs bosons
is not quite as advanced as in the SM. Indeed, despite valiant efforts, a full computation of
the two-loop quark-squark-gluino contributions, valid for arbitrary values of all the relevant
particle masses, has not been made publicly available so far. Approximate analytic results,
however, can be derived if the Higgs bosons are somewhat lighter than the squarks and
the gluinos. In the MSSM this condition almost certainly applies to the lightest scalar h. Moreover, recent results from SUSY searches at the LHC set preliminary lower bounds
on the squark and gluino masses just below the TeV (albeit for specific models of SUSY
breaking), suggesting that there might be wide regions of the MSSM parameter space in
which the condition also applies to the heavy scalar H and to the pseudoscalar A.
In this thesis we presented an original calculation of the two-loop quark-squark-gluino
contributions to the cross section for the gluon fusion processes gg → ø (where ø is a CP-
even or CP-odd Higgs) in the limit of large supersymmetric particles masses. We exploited
techniques previously developed for the computations of the MSSM CP-even Higgs boson
production cross section, where the cases m2 « m2t and m2 » m2b
were considered, in order
to compute the cross section for MSSM CP-odd Higgs production to the same accuracy.
We also extended the above mentioned techniques in such a way that no specific hierarchy is assumed between m2ø
and m2t. This allowed us to obtain analytic formulae for CP-even
and CP-odd Higgs production which are expected to give a better approximation of the
full result when the Higgs mass is not too far from the top-pair production threshold.
Our computation relies on an extensive use of the last generation of symbolic manipulation software like Mathematica and FORM. We generated the relevant two-loop diagrams
through FeynArts, using a modified version of the MSSM model file, which implements
the Background Field Method. Such a choice turns out to be important in the use of
Pauli-Villars regularization (PVREG). The asymptotic expansions procedure requires the
exact evaluation of two-loop disconnected diagrams, i:e: two loop integrals in which propagators involving both integration momenta are absent, while scalar products of the two integration momenta, raised to integer powers, can occur. For the reduction in Dimensional Regularization (DREG) of this class of integrals we developed a FORM code which
performs an Integration By Parts (IBP) reduction by efficiently importing and enforcing
the IBP identities generated with the software REDUZE.
In the case of CP-odd Higgs production, we performed the computation by regularizing
the loop integrals both in DREG and PVREG. While in the former inconsistencies are
known to arise in connection with the de nition of the Dirac matrix
ᵞ5 in d 6 ≠ 4 dimensions,
the latter allows to avoid such di culties, at the price of a somehow greater computational
effort. We obtained identical results in the two approaches. No such problems affect the
computation of the contributions to CP-even Higgs productions, so in that case we adopted
DREG only.
For what concerns the top-stop-gluino contributions to CP-even Higgs production, we
provided an original result based on an asymptotic expansion in the superparticle masses
(which we generically denote by M), up to and including terms of O(m2
A / M2), O(m2t / M2)
and O(m2
Z=M2), for which no results have been made available so far. A numerical study
of the result of this asymptotic expansion based evaluation and its renormalization scheme
dependence, as well as a quantitative comparison against the Taylor series based result, is
underway and will be presented in a forthcoming publication.
Regarding the contributions to CP-odd Higgs production cross section, we provided
both the result of a Taylor expansion in the pseudoscalar mass, up to and including terms of
O(m2
A=m2t) and O(m2
A=M2), and the result of an asymptotic expansion in the superparticle
masses, up to and including terms of O(m2
A=M2) and O(m2t
=M2). The latter can be
easily adapted to the case of the bottom-sbottom-gluino contributions, which allowed us
to provide a result valid up to and including terms of O(m2b=m2
A) and O(mb=M). We
discussed how the tan β-enhanced terms in the bottom-sbottom-gluino contributions can be
eliminated via an appropriate choice of renormalization scheme for the parameters entering
the one-loop part of the calculation, and compared our results with those obtained in the
effective-Lagrangian approximation.
In both cases we obtained explicit and compact analytic results based on asymptotic
expansions. All of our results can be easily implemented in computer codes for an efficient
and accurate determination of the cross section for scalar and pseudoscalar production at
hadron colliders.
Finally, the results derived in this work for the production cross section can be straight-
forwardly adapted to the NLO computation of the gluonic and photonic decay widths.</Abstract>Mon, 27 Feb 2012 23:00:00 GMThttp://hdl.handle.net/2307/45082012-02-27T23:00:00Z