1,322 research outputs found
Correlator of heavy-quark currents at small q^2 in the large-beta_0 limit
The correlator of vector heavy-quark currents at small q^2 is considered in
the large-beta_0 limit. The leading IR renormalon ambiguity of the sum of the
perturbative series is canceled by the UV renormalon ambiguity of the gluon
condensate. Asymptotic behaviour of the perturbative series is obtained in a
model-independent way, up to a single unknown normalization factor.
Gluon-virtuality distribution functions for the perturbative correction are
calculated.Comment: 8 pages, 3 figure
Boltzmann hierarchy for the cosmic microwave background at second order including photon polarization
Non-gaussianity and B-mode polarization are particularly interesting features
of the cosmic microwave background, as -- at least in the standard model of
cosmology -- their only sources to first order in cosmological perturbation
theory are primordial, possibly generated during inflation. If the primordial
sources are small, the question arises how large is the non-gaussianity and
B-mode background induced in second-order from the initially gaussian and
scalar perturbations. In this paper we derive the Boltzmann hierarchy for the
microwave background photon phase-space distributions at second order in
cosmological perturbation theory including the complete polarization
information, providing the basis for further numerical studies. As an aside we
note that the second-order collision term contains new sources of B-mode
polarization and that no polarization persists in the tight-coupling limit.Comment: LaTeX, 33 page
Conceptual aspects of QCD factorization in hadronic B decays
I review the meaning of ``QCD factorization'' in hadronic two-body B decays
and then discuss recent results of theoretical (rather than phenomenological)
nature: the proof of factorization at two loops; the identification of
``chirally enhanced'' power corrections; and the role of annihilation
contributions.Comment: 10 pages, LaTeX. Based on talks presented at the UK Phenomenology
Workshop on Heavy Flavour and CP Violation, 17 - 22 September 2000, Durham,
proceedings to appear in J. Phys. G; the 5th International Symposium on
Radiative Corrections (RADCOR2000), Carmel, California, September 11 - 15,
2000; the 4th Workshop on Continuous Advances in QCD, Minneapolis, 12-14 May
2000; the Vth International Workshop on Heavy Quark Physics, Dubna, 6-8 April
200
Soft and Coulomb gluon resummation in squark-antisquark production at the LHC
We present results for a combined resummation of soft and Coloumb gluon
corrections in squark-antisquark production at the LHC, including the
non-trivial interference of the two effects and the contribution of
squark-antisquark bound states below threshold. The total correction to the
next-to-leading order approximation is found to be sizeable, and amounts to
5-14 % in the squark mass region 200 GeV-2 TeV. The scale dependence of the
total cross section is also reduced.Comment: 6 pages, 3 figures, to appear in the proceedings of the 9th
International Symposium on Radiative Corrections (RADCOR 2009), October 25-30
2009, Ascona, Switzerlan
Threshold resummation for pair production of coloured heavy (s)particles at hadron colliders
We derive a factorization formula for the production of pairs of heavy
coloured particles in hadronic collisions near the production threshold that
establishes factorization of soft and Coulomb effects. This forms the basis for
a combined resummation of Coulomb and soft corrections, including the
non-trivial interference of the two effects. We develop a resummation formalism
valid at NNLL accuracy using the momentum-space approach to soft gluon
resummation. We present numerical results for the NLL resummed
squark-antisquark production cross section at the LHC and Tevatron, including
also the contribution of squark-antisquark bound states below threshold. The
total correction on top of the next-to-leading order approximation is found to
be sizeable, and amounts to (4-20)% in the squark mass region 200 GeV-3 TeV at
the 14 TeV LHC. The scale dependence of the total cross section is also
reduced.Comment: 71 pages, LaTeX, 18 figures. v2 matches published version (minor
typos corrected, more references added
Soft-Collinear Messengers: A New Mode in Soft-Collinear Effective Theory
It is argued that soft-collinear effective theory for processes involving
both soft and collinear partons, such as exclusive B-meson decays, should
include a new mode in addition to soft and collinear fields. These
"soft-collinear messengers" can interact with both soft and collinear particles
without taking them far off-shell. They thus can communicate between the soft
and collinear sectors of the theory. The relevance of the new mode is
demonstrated with an explicit example, and the formalism incorporating the
corresponding quark and gluon fields into the effective Lagrangian is
developed.Comment: 22 pages, 5 figures. Extended Section 6, clarifying the relevance of
different types of soft-collinear interaction
Heavy neutralino relic abundance with Sommerfeld enhancements - a study of pMSSM scenarios
We present a detailed discussion of Sommerfeld enhancements in neutralino
dark matter relic abundance calculations for several popular benchmark
scenarios in the general MSSM. Our analysis is focused on models with heavy
wino- and higgsino-like neutralino LSP and models interpolating between these
two scenarios. This work is the first phenomenological application of effective
field theory methods that we have developed in earlier work and that allow for
the consistent study of Sommerfeld enhancements in non-relativistic neutralino
and chargino co-annihilation reactions within the general MSSM, away from the
pure-wino and pure-higgsino limits.Comment: 38 pages, 14 figures, 3 table
Non-relativistic pair annihilation of nearly mass degenerate neutralinos and charginos I. General framework and S-wave annihilation
We compute analytically the tree-level annihilation rates of a collection of
non-relativistic neutralino and chargino two-particle states in the general
MSSM, including the previously unknown off-diagonal rates. The results are
prerequisites to the calculation of the Sommerfeld enhancement in the MSSM,
which will be presented in subsequent work. They can also be used to obtain
concise analytic expressions for MSSM dark matter pair annihilation in the
present Universe for a large number of exclusive two-particle final states.Comment: LATeX, 24 pages (+ 25 pages Appendix), 11 figures; v2 - replaced
incorrect version of Fig. 4 and fixed typos listed in the JHEP erratu
Charmless decays and the new physics effects in the minimal supergravity model
By employing the QCD factorization approach, we calculate the new physics
contributions to the branching radios of the two-body charmless
decays in the framework of the minimal supergravity (mSUGRA) model. Within the
considered parameter space, we find that (a) the supersymmetric (SUSY)
corrections to the Wilson coefficients () are very small and can
be neglected safely, but the leading order SUSY contributions to
and can be rather large and even change the
sign of the corresponding coefficients in the standard model; (b) the possible
SUSY contributions to those penguin-dominated decays in mSUGRA model can be as
large as ; (c) for the well measured decays, the
significant SUSY contributions play an important rule to improve the
consistency of the theoretical predictions with the data; (d) for decays, the theoretical predictions of the corresponding branching
ratios become consistent with the data within one standard deviation after the
inclusion of the large SUSY contributions in the mSUGRA model.Comment: 31 pages, Latex file, 4 ps and eps figures, minor corrections, final
version to appear in Physical Review
Threshold expansion of massive coloured particle cross sections
Pair production of massive coloured particles in hadron collisions is
accompanied by potentially large radiative corrections related to the
suppression of soft gluon emission and enhanced Coulomb exchange near the
production threshold. We recently developed a framework to sum both series of
corrections for the partonic cross section using soft-collinear and
non-relativistic effective theory. If it can be argued that the resummed cross
section approximates the complete result over a significant kinematic range, an
improvement of the hadronic cross section results, even when the production is
not kinematically constrained to the threshold. This is discussed here for the
case of top quark production.Comment: 5 pages, to appear in: Proceedings of the 10th DESY Workshop on
Elementary Particle Theory: Loops and Legs in Quantum Field Theory 2010,
W\"orlitz, Germany, 25-30 Apr 201
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