261 research outputs found
The static PQCD potential with modified boundary conditions
We calculate the potential between two static quarks in QCD using modified
boundary conditions for the perturbative expansion. Through a change of the
Feynman iepsilon prescription we effectively add a "sea" of gluons to the
asymptotic states with energies below a given scale Lambda. We find that the
standard result for the static potential gets corrections of order Lambda^2/Q^2
both at small and large momentum transfers Q^2. After resummation of the
infrared sensitive corrections we find that the running coupling alpha_s(Q^2)
freezes in the infrared and that the exchanged gluon gets an effective
tachyonic mass. We verify that identical results are obtained in the Coulomb
and Feynman gauges.Comment: 13 pages, 3 figures, updated version accepted for publication in
JHEP, additions to introduction and discussion, references added, results
unchange
Conformal Expansions: A Template for QCD Predictions
The use of conformal expansions for predictions in quantum chromodynamics is
discussed as a way to avoid renormalization scheme and scale ambiguities, as
well as factorial growth of perturbative coefficients due to renormalons.
Special emphasis is given to the properties of an assumed skeleton expansion
and its relation to the Banks-Zaks expansion. The relation of BLM scale-setting
to the skeleton expansion is also discussed and new criteria for the
applicability of BLM scale-setting are presented.Comment: 16 pages, Invited talk given at RADCOR-2000, Carmel CA, USA, 11-15
September, 200
Resummation and power corrections by Dressed Gluon Exponentiation
Event-shape distributions in annihilation offer a unique lab oratory
for understanding perturbative and non-perturbative aspects of QCD. Dressed
Gluon Exponentiation is a resummation method designed to evaluate differ ential
cross sections close to a kinematic threshold and provide the basis for
parametrization of power corrections. The method and its application in the
case of the thrust and the heavy jet mass distributions in the two-jet region
are briefly presented.Comment: 5 pages, 3 figures in ps, talk given at XXXI International Symposium
on Multiparticle Dynamics, Sept 1-7, 2001, Datong China. URL
http://ismd31.ccnu.edu.cn
Closing the Window on Light Charged Higgs Bosons in the NMSSM
In the Next-to-Minimal SuperSymmetric Model (NMSSM) the lightest CP-odd Higgs
bosons (a1) can be very light. As a consequence, in addition to the standard
charged Higgs boson (h+) decays considered in the MSSM for a light charged
Higgs (m_h+ < m_t), the branching fraction for h+ to a1 W can be dominant. We
investigate how this signal can be searched for in t tbar production at the
Large Hadron Collider (LHC) in the case that m_a1 \gtrsim 2m_B with the a1
giving rise to a single b bbar-jet and discuss to what extent the LHC
experiments are able to discover such a scenario with an integrated luminosity
\sim 20 fb-1. We also discuss the implications of the possible Higgs-signal
observed at the LHC.Comment: 20 pages, 11 figures, some minor updates and clarifications, version
accepted for publication in Advances in High Energy Physics special issue on
The Next-to-Minimal Supersymmetric Standard Mode
breaking effects in 2-loop RG evolution of 2HDM
We investigate the effects of a symmetry in the CP-conserving
Two-Higgs-Doublet-Model (2HDM); which is often imposed to prevent
Flavor-Changing-Neutral-Currents (FCNCs) at tree-level. Specifically, we
analyze how a breaking of the symmetry spreads during renormalization
group evolution; employing general 2-loop renormalization group equations that
we have derived. Evolving the model from the electroweak to the Planck scale,
we find that while the case of an exact symmetric 2HDM is very
constrained, a soft breaking of the symmetry extends the valid parameter
space regions. The effects of a hard breaking in the scalar sector as
well as the stability of the flavor alignment ansatz are also investigated. We
find that while a hard breaking of the symmetry in the potential is
problematic, since it speeds up the growth of quartic couplings, the generated
FCNCs are heavily suppressed. Conversely, we also find that hard breaking
in the Yukawa sector at most gives moderate breaking in the potential;
whereas the FCNCs can become quite sizable far away from the symmetric
regions.Comment: 33 pages, journal versio
Anomaly-free Model Building with Algebraic Geometry
We present a method to find anomaly-free gauged Froggatt-Nielsen type models
using results from algebraic geometry. These methods should be of general
interest for model building beyond the Standard Model (SM) when rational
charges are required. We consider models with a gauged flavor symmetry
with one flavon and two Higgs doublets and three right-handed SM singlets to
provide three model examples based on different physical assumptions. The
models we study are: anomaly-free with no SM neutral heavy chiral fermions,
anomaly-free with SM neutral heavy chiral fermions, and supersymmetric with SM
neutral heavy chiral fermions where the anomalies cancel via the Green-Schwarz
mechanism. With these different models we show how algebraic methods may be
used in model building; both to reduce the charge constraints by calculation of
Gr\"obner bases, and to find rational solutions to cubic equations using
Mordell-Weil generators.
Using these tools we find three phenomenologically viable models explaining
the observed flavor structure.Comment: Changed to match published versio
Conformal Symmetry as a Template: Commensurate Scale Relations and Physics Renormalization Schemes
Commensurate scale relations are perturbative QCD predictions which relate
observable to observable at fixed relative scale, such as the "generalized
Crewther relation", which connects the Bjorken and Gross-Llewellyn Smith deep
inelastic scattering sum rules to measurements of the e+ e- annihilation cross
section. We show how conformal symmetry provides a template for such QCD
predictions, providing relations between observables which are present even in
theories which are not scale invariant. All non-conformal effects are absorbed
by fixing the ratio of the respective momentum transfer and energy scales. In
the case of fixed-point theories, commensurate scale relations relate both the
ratio of couplings and the ratio of scales as the fixed point is approached. In
the case of the scheme defined from heavy quark interactions,
virtual corrections due to fermion pairs are analytically incorporated into the
Gell-Mann Low function, thus avoiding the problem of explicitly computing and
resumming quark mass corrections related to the running of the coupling.
Applications to the decay width of the Z boson, the BFKL pomeron, and virtual
photon scattering are discussed.Comment: 27 pages, Talk presented at the Les Rencontres de Physique de la
Vallee d'Aoste, La Thuile, Italy, February 28-March 6, 199
Higgs phenomenology in the Stealth Doublet Model
We analyze a model for the Higgs sector with two scalar doublets and a
symmetry that is manifest in the Yukawa sector but broken in the potential.
Thus, one of the doublets breaks the electroweak symmetry and has tree-level
Yukawa couplings to fermions, whereas the other doublet has no vacuum
expectation value and no tree-level couplings to fermions. Since the
parity is broken the two doublets can mix, which leads to a distinct and novel
phenomenology. This Stealth Doublet Model can be seen as a generalization of
the Inert Doublet Model with a broken symmetry. We outline the model and
present constraints from theory, electroweak precision tests and collider
searches, including the recent observation of a Higgs boson at the LHC. The
charged scalar and the CP-odd scalar couple to fermions at one-loop
level. We compute the decays of and and in particular the one-loop
decays , ,
and . We also describe how to calculate and renormalize
such processes in our model. We find that if one of or is the
lightest scalar, or are typically
their respective dominating decay channels. Otherwise, the dominating decays of
and are into a scalar and a vector. Due to the absence of
tree-level fermion couplings for and , we consider pair production
and associated production with vector bosons and scalars at the LHC. If the
parameter space of the model that favors is realized
in Nature, we estimate that there could be a considerable amount of such events
in the present LHC data.Comment: 40 pages, 25 figures. Version 4. Minor updates, references added.
Version to appear in PR
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