136 research outputs found
The Fermion Self-Energy during Inflation
We compute the one loop fermion self-energy for massless Dirac + Einstein in
the presence of a locally de Sitter background. We employ dimensional
regularization and obtain a fully renormalized result by absorbing all
divergences with BPHZ counterterms. An interesting technical aspect of this
computation is the need for a noninvariant counterterm owing to the breaking of
de Sitter invariance by our gauge condition. Our result can be used in the
quantum-corrected Dirac equation to search for inflation-enhanced quantum
effects from gravitons, analogous to those which have been found for massless,
minimally coupled scalars.Comment: 63 pages, 3 figures (uses axodraw.sty), LaTeX 2epsilon. Revised
version (to appear in Classical and Quantum Gravity) corrects some typoes and
contains some new reference
Universal extra dimensions and Z->b bar-b
We study, at the one loop level, the dominant contributions from a single
universal extra dimension to the process (Z\to b\bar{b}). By resorting to the
gaugeless limit of the theory we explain why the result is expected to display
a strong dependence on the mass of the top-quark, not identified in the early
literature. A detailed calculation corroborates this expectation, giving rise
to a lower bound for the compactification scale which is comparable to that
obtained from the parameter. An estimate of the subleading corrections
is furnished, together with a qualitative discussion on the difference between
the present results and those derived previously for the non-universal case.Comment: 16 pages, 4 figures, revtex
Production of para-- and orthopositronium at relativistic heavy ion colliders
We consider the ortho-- and parapositronium production in the process Ps where A is a nucleus with the charge number Z. The inclusive cross
section and the energy distribution of the relativistic Ps are calculated which
are of primary interest from the experimental point of view. The accuracy of
the corresponding cross sections is given by omitting terms for the para--Ps and for the ortho--Ps production
where and 16 for the RHIC and the LHC. Within this
accuracy the multiphoton (Coulomb) corrections are taken into account. We show
that the RHIC and the LHC will be Ps factories with a productions rate of about
relativistic Ps per day. The fraction of the ortho--Ps is
expected to be of the same order as that of the para--Ps for Au--Au and Pb--Pb
collisions.Comment: 22 pages, 5 figures, RevTeX, misprint correcte
Production of relativistic positronium in collisions of photons and electrons with nuclei and atoms
We consider the production of ultrarelativistic positronium (Ps) in and processes where is an atom or a nucleus
with charge . For the photoproduction of para- and ortho-Ps and the
electroproduction of para-Ps we obtain the most complete description compared
with previous works. It includes high order corrections and
polarization effects. The accuracy of the obtained cross sections is determined
by omitted terms of the order of the inverse Ps Lorentz factor squared. The
studied high order multi-photon electroproduction of ortho-Ps dominates for the
collision of electrons with heavy atoms over the bremsstrahlung production from
the electron via a virtual photon proposed by Holvik and Olsen. Our results
complete and correct the studies of those authors.Comment: 19 pages, 9 figures, RevTex; v2: minor corrections for the accuracy
of the results, a discussion of the literature added in a footnote, one
additional reference; v3: diagram of Fig.2 correcte
Inverse bremsstrahlung contributions to Drell-Yan like processes
The contribution of the sub-process in
hadron-hadron interactions is considered. It is a part of one-loop electroweak
radiative corrections for the Drell-Yan production of lepton pairs at hadron
colliders. It is shown that this contribution should be taken into account
aiming at the 1% accuracy of the Drell-Yan process theoretical description.
Both the neutral and charged current cases are evaluated. Numerical results are
presented for typical conditions of LHC experiments.Comment: 11 pages, 8 figure
Applications of quark-hadron duality in F2 structure function
Inclusive electron-proton and electron-deuteron inelastic cross sections have
been measured at Jefferson Lab (JLab) in the resonance region, at large Bjorken
x, up to 0.92, and four-momentum transfer squared Q2 up to 7.5 GeV2 in the
experiment E00-116. These measurements are used to extend to larger x and Q2
precision, quantitative, studies of the phenomenon of quark-hadron duality. Our
analysis confirms, both globally and locally, the apparent violation of
quark-hadron duality previously observed at a Q2 of 3.5 GeV2 when resonance
data are compared to structure function data created from CTEQ6M and MRST2004
parton distribution functions (PDFs). More importantly, our new data show that
this discrepancy saturates by Q2 ~ 4 Gev2, becoming Q2 independent. This
suggests only small violations of Q2 evolution by contributions from the
higher-twist terms in the resonance region which is confirmed by our
comparisons to ALEKHIN and ALLM97.We conclude that the unconstrained strength
of the CTEQ6M and MRST2004 PDFs at large x is the major source of the
disagreement between data and these parameterizations in the kinematic regime
we study and that, in view of quark-hadron duality, properly averaged resonance
region data could be used in global QCD fits to reduce PDF uncertainties at
large x.Comment: 35 page
Pion and Sigma Polarizabilities and Radiative Transitions
Fermilab E781 plans measurements of gamma-Sigma and -pion
interactions using a 600 GeV beam of Sigmas and pions, and a virtual photon
target. Pion polarizabilities and radiative transitions will be measured in
this experiment. The former can test a precise prediction of chiral symmetry;
the latter for a_1(1260) ----> pi + gamma is important for understanding the
polarizability. The experiment also measures polarizabilities and radiative
transitions for Sigma hyperons. The polarizabilities can test predictions of
baryon chiral perturbation theory. The radiative transitions to the
Sigma*(1385) provide a measure of the magnetic moment of the s-quark. Previous
experimental and theoretical results for gamma-pi and gamma-Sigma interactions
are given. The E781 experiment is described.Comment: 13 pages text (tex), Tel Aviv U. Preprint TAUP 2204-94, uses
Springer-Verlag TEX macro package lecproc.cmm (appended at end of tex file,
following \byebye), which requires extracting lecproc.cmm and putting this
file in your directory in addition to the tex file (mmcd.tex) before tex
processing. lecproc.cmm should be used following instructions and guidelines
available from Springer-Verlag. Submitted to the Proceedings of Workshop on
Chiral Dynamics, Massachusetts Institute of Technology, July 1994, Eds. A.
Bernstein, B. Holstein. Replaced Oct. 4 to add TAUP preprint number. Replaced
Oct. 12 to correct Pb target thickness from 1.3% interaction to 0.3
Quantum Gravitational Corrections to the Nonrelativistic Scattering Potential of Two Masses
We treat general relativity as an effective field theory, obtaining the full
nonanalytic component of the scattering matrix potential to one-loop order. The
lowest order vertex rules for the resulting effective field theory are
presented and the one-loop diagrams which yield the leading nonrelativistic
post-Newtonian and quantum corrections to the gravitational scattering
amplitude to second order in G are calculated in detail. The Fourier
transformed amplitudes yield a nonrelativistic potential and our result is
discussed in relation to previous calculations. The definition of a potential
is discussed as well and we show how the ambiguity of the potential under
coordinate changes is resolved.Comment: 27 pages, 17 figure
Information and Discrimination from b Quark Production on Z Resonance
We introduce and define operatively in a model independent way a new ``heavy"
b-vertexparameter, , that can be derived from the measurement of a
special polarization asymmetry for production of b-quarks on Z resonance. We
show that the combination of the measurement of with that of a second
and previously defined ``heavy" b-vertex parameter can
discriminate a number of models of New Physics that remain associated to
different ``trajectories" in the plane of the variations of the two parameters.
This is shown in particular for some popular SUSY and technicolor-type models.
In general, this discrimination is possible if a measurement of
\underline{both} parameters is performed.Comment: 22 pages, 6 figures available by air mail upon request, (e-mail
[email protected] PM/94-04, UTS-DFT-94-02 .( revised version
with corrected references
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