85 research outputs found
Decoupling heavy particles simultaneously
The renormalization group is extended to cases where several heavy particles
are decoupled at the same time. This involves large logarithms which are
scale-invariant and so cannot be eliminated by a change of renormalization
scheme. A set of scale-invariant running couplings, one for each heavy
particle, is constructed without reference to intermediate thresholds. The
entire heavy-quark correction to the axial charge of the weak neutral current
is derived to next-to-leading order, and checked in leading order by evaluating
diagrams explicitly. The mechanism for cancelling contributions from the top
and bottom quarks in the equal-mass limit is surprisingly non-trivial.Comment: 6 pages, 4 figures. Talk presented at the "QCD Down Under" Workshop,
Barossa Valley and Adelaide, Australia, 10-19 March 2004, with ref 8 now
linked to hep-ph/050727
The relevance of polarized bZ production at LHC
We consider the Z polarization asymmetry
A_Z=(sigma(Z_R)-sigma(Z_L))/(sigma(Z_R)+sigma(Z_L)) in the process of
associated bZ production at the LHC. We show that in the Standard Model (SM)
this quantity is essentially given by its Born approximation, remaining almost
unaffected by QCD scales and parton distribution functions variations as well
as by electroweak corrections. The theoretical quantity that appears in A_Z is
the same that provides the LEP1 Z -> b bbar forward-backward asymmetry, the
only measured observable still in some contradiction with the SM prediction. In
this sense, A_Z would provide the possibility of an independent verification of
the possible SM discrepancy, which could reach, if consistency with LEP1
measurements is imposed, values of the relative ten percent size.Comment: 10 pages, 5 eps figure
Running couplings for the simultaneous decoupling of heavy quarks
Scale-invariant running couplings are constructed for several quarks being
decoupled together, without reference to intermediate thresholds.
Large-momentum scales can also be included. The result is a multi-scale
generalization of the renormalization group applicable to any order.
Inconsistencies in the usual decoupling procedure with a single running
coupling can then be avoided, e.g. when cancelling anomalous corrections from
t,b quarks to the axial charge of the proton.Comment: 12 pages, 1 figure, version to appear in PLB. Pages 8-11 and Fig. 1
are new, with consequent changes to the abstract, page 2, and the references.
We show that our multi-scale renormalization group is needed to achieve
anomaly cancellation in t,b decoupling from the weak neutral current, and
extend it to include large moment
Excited Leptons at the CERN Large Hadron Collider
We analyze the potential of the CERN Large Hadron Collider (LHC) to search
for excited spin-1/2 electrons and neutrinos. Assuming a SU(2)_L X U(1)_Y
invariant model, we study in detail the single production of excited electrons
and neutrinos and respective backgrounds through the reactions p p -> e+ e- V
and e+- \nu V with V=photon, W, or Z. We show that the LHC will be able to
tighten considerably the direct constraints on these possible new states,
probing excited lepton masses up to 1-2 TeV depending on their couplings to
fermions and gauge bosons.Comment: 15 pages, 7 figure
Evading the CKM Hierarchy: Intrinsic Charm in B Decays
We show that the presence of intrinsic charm in the hadrons' light-cone wave
functions, even at a few percent level, provides new, competitive decay
mechanisms for B decays which are nominally CKM-suppressed. For example, the
weak decays of the B-meson to two-body exclusive states consisting of strange
plus light hadrons, such as B\to\pi K, are expected to be dominated by penguin
contributions since the tree-level b\to s u\bar u decay is CKM suppressed.
However, higher Fock states in the B wave function containing charm quark pairs
can mediate the decay via a CKM-favored b\to s c\bar c tree-level transition.
Such intrinsic charm contributions can be phenomenologically significant. Since
they mimic the amplitude structure of ``charming'' penguin contributions,
charming penguins need not be penguins at all.Comment: 28 pages, 6 figures, published version. References added, minor
change
SU(4) Chiral Quark Model with Configuration Mixing
Chiral quark model with configuration mixing and broken SU(3)\times U(1)
symmetry has been extended to include the contribution from c\bar c
fluctuations by considering broken SU(4) instead of SU(3). The implications of
such a model have been studied for quark flavor and spin distribution functions
corresponding to E866 and the NMC data. The predicted parameters regarding the
charm spin distribution functions, for example, \Delta c, \frac{\Delta
c}{{\Delta \Sigma}}, \frac{\Delta c}{c} as well as the charm quark distribution
functions, for example, \bar c, \frac{2\bar c}{(\bar u+\bar d)}, \frac{2 \bar
c}{(u+d)} and \frac{(c+ \bar c)}{\sum (q+\bar q)} are in agreement with other
similar calculations. Specifically, we find \Delta c=-0.009, \frac{\Delta
c}{{\Delta \Sigma}}=-0.02, \bar c=0.03 and \frac{(c+ \bar c)}{\sum (q+\bar
q)}=0.02 for the \chiQM parameters a=0.1, \alpha=0.4, \beta=0.7,
\zeta_{E866}=-1-2 \beta, \zeta_{NMC}=-2-2 \beta and \gamma=0.3, the latter
appears due to the extension of SU(3) to SU(4).Comment: 10 RevTeX pages. Accepted for publication in Phys. Rev.
Vector mesons in a relativistic point-form approach
We apply the point form of relativistic quantum mechanics to develop a
Poincare invariant coupled-channel formalism for two-particle systems
interacting via one-particle exchange. This approach takes the exchange
particle explicitly into account and leads to a generalized eigenvalue equation
for the Bakamjian-Thomas type mass operator of the system. The coupling of the
exchange particle is derived from quantum field theory. As an illustrative
example we consider vector mesons within the chiral constituent quark model in
which the hyperfine interaction between the confined quark-antiquark pair is
generated by Goldstone-boson exchange. We study the effect of retardation in
the Goldstone-boson exchange by comparing with the commonly used instantaneous
approximation. As a nice physical feature we find that the problem of a too
large - splitting can nearly be avoided by taking the dynamics of
the exchange meson explicitly into account.Comment: 14 pages, 1 figur
Real and Virtual Compton Scattering off the Nucleon
A review is given of the very recent developments in the fields of real and
virtual Compton scattering off the nucleon. Both real and virtual Compton
scattering reactions are discussed at low outgoing photon energy where one
accesses polarizabilities of the nucleon. The real Compton scattering at large
momentum transfer is discussed which is asymptotically a tool to obtain
information on the valence quark wave function of the nucleon. The rapid
developments in deeply virtual Compton scattering and associated meson
electroproduction reactions at high energy, high photon virtuality and small
momentum transfer to the nucleon are discussed. A unified theoretical
description of those processes has emerged over the last few years, which gives
access to new, generalized parton distributions. The experimental status and
perspectives in these fields are also discussed.Comment: 25 pages, 17 figure
Parton interactions in the Bjorken limit of QCD
We consider the Bjorken limit in the framework of the effective action
approach and discuss its similarities to the Regge limit. The proposed
effective action allows for a rather simple calculation of the known evolution
kernels. We represent the result in terms of two-parton interaction operators
involving gluon and quark operators depending on light-ray position and
helicity and analyze their symmetry properties.Comment: 32 pages LaTex, 4 eps-figures, comments added, minor correction
Interactions between proteins bound to biomembranes
We study a physical model for the interaction between general inclusions
bound to fluid membranes that possess finite tension, as well as the usual
bending rigidity. We are motivated by an interest in proteins bound to cell
membranes that apply forces to these membranes, due to either entropic or
direct chemical interactions. We find an exact analytic solution for the
repulsive interaction between two similar circularly symmetric inclusions. This
repulsion extends over length scales of order tens of nanometers, and contrasts
with the membrane-mediated contact attraction for similar inclusions on
tensionless membranes. For non circularly symmetric inclusions we study the
small, algebraically long-ranged, attractive contribution to the force that
arises. We discuss the relevance of our results to biological phenomena, such
as the budding of caveolae from cell membranes and the striations that are
observed on their coats.Comment: 22 pages, 2 figure
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