488 research outputs found
Trouble for MAC
We show that the next-to-leading corrections to the kernel of the gap
equation can be large and of opposite sign to the lowest order kernel, in the
presence of a gauge boson mass. This calls into question the reliability of the
Most Attractive Channel hypothesis.Comment: 8 pages, 1 figure, LaTe
Effects of technicolor on standard model running couplings
We discuss the running couplings in the standard model, SU(3SU(2U(1, when the Higgs sector is replaced by SU(
technicolor. Particular attention is given to the running of the couplings at
momentum scales where technicolor is nonperturbative, and in this region we
apply a relativistic constituent technifermion model. This model has been
tested against the known running of the QED coupling due to nonperturbative
QCD. An understanding of this low momentum running allows the calculation of
the couplings at a higher scale, , where technicolor becomes
perturbative. We provide numerical values for the changes in the three standard
model couplings between and due to technicolor, assuming
separately ``one doublet'' and ``one family'' technicolor models. The
distinction between a running and walking technicolor coupling is also
considered.Comment: 14 pages of LaTeX, UTPT-94-
Symmetry breaking via fermion 4-point functions
We construct the effective action and gap equations for nonperturbative
fermion 4-point functions. Our results apply to situations in which fermion
masses can be ignored, which is the case for theories of strong flavor
interactions involving standard quarks and leptons above the electroweak scale.
The structure of the gap equations is different from what a naive
generalization of the 2-point case would suggest, and we find for example that
gauge exchanges are insufficient to generate nonperturbative 4-point functions
when the number of colors is large.Comment: 36 pages, uses Revtex and eps files for figure
Calculation of the Chiral Lagrangian Coefficients from the Underlying Theory of QCD: A Simple Approach
We calculate the coefficients in the chiral Lagrangian approximately from QCD
based on a previous study of deriving the chiral Lagrangian from the first
principles of QCD in which the chiral Lagrangian coefficients are defined in
terms of certain Green's functions in QCD. We first show that, in the large
N(c)-limit, the anomaly part contributions to the coefficients are exactly
cancelled by certain terms in the normal part contributions, and the final
results of the coefficients only concern the remaining normal part
contributions depending on QCD interactions. We then do the calculation in a
simple approach with the approximations of taking the large-N(c) limit, the
leading order in dynamical perturbation theory, and the improved ladder
approximation, thereby the relevant Green's functions are expressed in terms of
the quark self energy. By solving the Schwinger-Dyson equation for the quark
self energy, we obtain the approximate QCD predicted coefficients and the quark
condensate which are consistent with the experimental values.Comment: Further typos corrected, to appear in Phys. Rev.
Possible Discovery Channel for New Charged Leptons at the LHC
We propose a channel for the possible discovery of new charged leptons at the
Large Hadron Collider. The proposed final state contains three same-sign
leptons, making this new channel practically back- groundless. The method is
illustrated for two different cases: the four-family Standard Model and the
Grand Unified Theory based on the E6 gauge group. An example study taking 250
GeV as the charged lepton mass shows that in both models, about 8 signal events
can be expected at 14 TeV center-of-mass energy with 1 fb^-1 of integrated
luminosity. Although the event yield might not be sufficient for detailed
measurements of the charged lepton properties, it would be sufficient to claim
discovery through a counting experiment.Comment: 8 pages, 4 figures. v2 update includes an estimate of the
backgrounds, consideration of the EW oblique parameters, and minor
improvements. v3 update includes detector acceptance and ttbar backgroun
Are Four-Fermion Operators Relevant for the Fermion Mass Problem?
Four-fermion operators may have large anomalous scaling and become relevant
operators in some strongly interacting gauge theories. We present a detailed
model which illustrates some of the implications of such operators for the
generation of quark and lepton masses. Such operators would originate at high
scales where quarks and leptons experience a new strong interaction, but no
unbroken technicolor interaction is required. The breakdown of both the new
gauge symmetry and electroweak gauge symmetry is associated with a dynamical
TeV mass for fourth family quarks. Among the new physics signatures are
anomalous (chromo)magnetic moments and their flavor-changing counterparts.Comment: 23 pages, postscript, version to be published in PRD, PDF file is
available at http://miteymac.physics.utoronto.ca/bh/papers/papers.htm
Derivation of the Effective Chiral Lagrangian for Pseudoscalar Mesons from QCD
We formally derive the chiral Lagrangian for low lying pseudoscalar mesons
from the first principles of QCD considering the contributions from the normal
part of the theory without taking approximations. The derivation is based on
the standard generating functional of QCD in the path integral formalism. The
gluon-field integration is formally carried out by expressing the result in
terms of physical Green's functions of the gluon. To integrate over the
quark-field, we introduce a bilocal auxiliary field Phi(x,y) representing the
mesons. We then develop a consistent way of extracting the local pseudoscalar
degree of freedom U(x) in Phi(x,y) and integrating out the rest degrees of
freedom such that the complete pseudoscalar degree of freedom resides in U(x).
With certain techniques, we work out the explicit U(x)-dependence of the
effective action up to the p^4-terms in the momentum expansion, which leads to
the desired chiral Lagrangian in which all the coefficients contributed from
the normal part of the theory are expressed in terms of certain Green's
functions in QCD. Together with the existing QCD formulae for the anomaly
contributions, the present results leads to the complete QCD definition of the
coefficients in the chiral Lagrangian. The relation between the present QCD
definition of the p^2-order coefficient F_0^2 and the well-known approximate
result given by Pagels and Stokar is discussed.Comment: 16 pages in RevTex, some typos are corrected, version for publication
in Phys. Rev.
Analysis of the Lambda_b -> Lambda_c + l nu_l decay within a light-front constituent quark model
We present an investigation of the Isgur-Wise form factor relevant for the
semileptonic decay Lambda_b -> \Lambda_c + l nu_l performed within a
light-front constituent quark model. Adopting different baryon wave functions
it is found that the Isgur-Wise form factor depends sensitively on the baryon
structure. It is shown however that the shape of the Isgur-Wise function in the
full recoil range relevant for the Lambda_b -> \Lambda_c + l nu_l decay can be
effectively constrained using recent lattice QCD results at low recoil. Then,
the Lambda_b -> \Lambda_c + l nu_l decay is investigated including both
radiative effects and first-order power corrections in the inverse heavy-quark
mass. Our final predictions for the exclusive semileptonic branching ratio, the
longitudinal and transverse asymmetries, and the longitudinal to transverse
decay ratio are: Br(Lambda_b -> \Lambda_c + l nu_l) = (6.3 +/- 1.6) % |V_bc /
0.040|**2 ~ tau(Lambda_b) / (1.24 ps), a_L = -0.945 +/- 0.014, a_T = -0.62 +/-
0.09 and R_L/T = 1.57 +/- 0.15, respectively. Moreover, both the longitudinal
asymmetry and the (partially integrated) longitudinal to transverse decay ratio
are found to be only marginally affected by the model dependence of the
Isgur-Wise form factor as well as by first-order power corrections; therefore,
their experimental determination might be a very interesting tool for testing
the SM and for investigating possible New Physics.Comment: revised version with inclusion of PQCD corrections and improved
discussion of power corrections, to appear in Physical Review
Narrow Technihadron Production at the First Muon Collider
In modern technicolor models, there exist very narrow spin-zero and spin-one
neutral technihadrons---, and ---with masses of a
few 100 GeV. The large coupling of to , the direct
coupling of and to the photon and , and the superb
energy resolution of the First Muon Collider may make it possible to resolve
these technihadrons and produce them at extraordinarily large rates.Comment: 11 pages, latex, including 2 postscript figure
V,W and X in Technicolour Models
Light techni-fermions and pseudo Goldstone bosons that contribute to the
electroweak radiative correction parameters V,W and X may relax the constraints
on technicolour models from the experimental values of the parameters S and T.
Order of magnitude estimates of the contributions to V,W and X from light
techni-leptons are made when the the techni-neutrino has a small Dirac mass or
a large Majorana mass. The contributions to V,W and X from pseudo Goldstone
bosons are calculated in a gauged chiral Lagrangian. Estimates of V,W and X in
one family technicolour models suggest that the upper bounds on S and T should
be relaxed by between 0.1 and 1 depending upon the precise particle spectrum.Comment: 19 pages + 2 pages of ps figs, SWAT/1
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