1,891 research outputs found
Supersymmetric Correction to Top Quark Pair Production near Threshold
We studied the leading supersymmetric contribution to top-antitop threshold
production using the NRQCD framework. The one-loop matching to the potential
and the Wilson coefficient of the leading production current were considered.
We point out that the leading correction to the potential is zero due to
SU(3)_c gauge invariance. This is true in general for any new physics that
enters above the electroweak scale. The shape of the top quark pair production
cross section is therefore almost unaffected near threshold, allowing a precise
determination of the top quark mass based on the Standard Model calculations.
The supersymmetric correction to the Wilson coefficient c_1 of the production
current decouples for heavy super particles. Its contribution is smaller than
the Standard Model next-to-next-leading-log results.Comment: 7 pages, 2 figure
Exotic QQ\qbar\qbar States in QCD
We show that QCD contains stable four-quark QQ\qbar\qbar hadronic states in
the limit where the heavy quark mass goes to infinity. (Here Q denotes a heavy
quark, \qbar a light antiquark and the stability refers only to the strong
interactions.) The long range binding potential is due to one pion exchange
between ground state Q\qbar mesons, and is computed using chiral perturbation
theory. For the Q=b, this long range potential may be sufficiently attractive
to produce a weakly bound two meson state.Comment: (21 pages, 1 figure; uses harvmac.tex, tables.tex and uufiles),
CERN-TH.6744/92, CALT-68-186
The Nucleon-Nucleon Potential in the 1/N_c Expansion
The nucleon-nucleon potential is analysed using the 1/N_c expansion of QCD.
The NN potential is shown to have an expansion in 1/N_c^2, and the strengths of
the leading order central, spin-orbit, tensor, and quadratic spin-orbit forces
(including isospin dependence) are determined. Comparison with a successful
phenomenological potential (Nijmegen) shows that the large-N_c analysis
explains many of the qualitative features observed in the nucleon-nucleon
interaction. The 1/N_c expansion implies an effective Wigner supermultiplet
symmetry for light nuclei. Results for baryons containing strange quarks are
presented in an appendix.Comment: 17 pages, 3 figures, TeX, macros harvmac and eps
Factorization Structure of Gauge Theory Amplitudes and Application to Hard Scattering Processes at the LHC
Previous work on electroweak radiative corrections to high energy scattering
using soft-collinear effective theory (SCET) has been extended to include
external transverse and longitudinal gauge bosons and Higgs bosons. This allows
one to compute radiative corrections to all parton-level hard scattering
amplitudes in the standard model to NLL order, including QCD and electroweak
radiative corrections, mass effects, and Higgs exchange corrections, if the
high-scale matching, which is suppressed by two orders in the log counting, and
contains no large logs, is known. The factorization structure of the effective
theory places strong constraints on the form of gauge theory amplitudes at high
energy for massless and massive gauge theories, which are discussed in detail
in the paper. The radiative corrections can be written as the sum of
process-independent one-particle collinear functions, and a universal soft
function. We give plots for the radiative corrections to q qbar -> W_T W_T, Z_T
Z_T, W_L W_L, and Z_L H, and gg -> W_T W_T to illustrate our results. The
purely electroweak corrections are large, ranging from 12% at 500 GeV to 37% at
2 TeV for transverse W pair production, and increasing rapidly with energy. The
estimated theoretical uncertainty to the partonic (hard) cross-section in most
cases is below one percent, smaller than uncertainties in the parton
distribution functions (PDFs). We discuss the relation between SCET and other
factorization methods, and derive the Magnea-Sterman equations for the Sudakov
form factor using SCET, for massless and massive gauge theories, and for light
and heavy external particles.Comment: 44 pages, 30 figures. Refs added, typos fixed. ZL ZL plots removed
because of a possible subtlet
Flavor Changing Neutral Currents, an Extended Scalar Sector, and the Higgs Production Rate at the LHC
We study extensions of the standard model with additional colored scalar
fields which can couple directly to quarks. Natural suppression of flavor
changing neutral currents implies minimal flavor violation, and fixes the
scalars to transform as (8,2)_1/2 under the SU(3) X SU(2) X U(1) gauge
symmetry. We explore the phenomenology of the standard model with one
additional (8,2)_1/2 scalar, and discuss how this extension can modify flavor
physics and the Higgs boson production rate at the LHC. Custodial SU(2)
symmetry can be implemented for the octet scalars since they transform as a
real color representation. Additional weak scale degrees of freedom needed for
gauge unification are discussed.Comment: Minor change
Renormalization of the Vector Current in QED
It is commonly asserted that the electromagnetic current is conserved and
therefore is not renormalized. Within QED we show (a) that this statement is
false, (b) how to obtain the renormalization of the current to all orders of
perturbation theory, and (c) how to correctly define an electron number
operator. The current mixes with the four-divergence of the electromagnetic
field-strength tensor. The true electron number operator is the integral of the
time component of the electron number density, but only when the current
differs from the MSbar-renormalized current by a definite finite
renormalization. This happens in such a way that Gauss's law holds: the charge
operator is the surface integral of the electric field at infinity. The theorem
extends naturally to any gauge theory.Comment: 9 pages. Corresponds to published version (Phys. Rev. D), including
appendix about Weeks's parado
Chiral Perturbation Theory for phi -> rho gamma gamma and phi -> omega gamma gamma
We predict differential decay distributions for phi->rho gamma gamma and phi
-> omega gamma gamma using chiral perturbation theory. We also consider the
isospin violating decay phi -> omega pi^0. Experimental information on these
decays can be used to determine couplings in the heavy vector meson chiral
Lagrangian.Comment: It was shown by P. Ko et al., hep-ph/9510205 (Phys. Lett. B366 (1996)
287), that there is a etaprime pole contribution that dominates over what we
calculate
Analysis of General Power Counting Rules in Effective Field Theory
We derive the general counting rules for a quantum effective field theory
(EFT) in dimensions. The rules are valid for strongly and weakly
coupled theories, and predict that all kinetic energy terms are canonically
normalized. They determine the energy dependence of scattering cross sections
in the range of validity of the EFT expansion. We show that the size of cross
sections is controlled by the power counting of EFT, not by chiral
counting, even for chiral perturbation theory (PT). The relation between
and is generalized to dimensions. We show that the
naive dimensional analysis counting is related to counting. The
EFT counting rules are applied to PT, low-energy weak interactions,
Standard Model EFT and the non-trivial case of Higgs EFT.Comment: V2: more details and examples added; version published in journal. 17
pages, 4 figures, 2 table
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