384 research outputs found
Quenched chiral logarithms in lattice QCD with exact chiral symmetry
We examine quenched chiral logarithms in lattice QCD with overlap Dirac
quark. For 100 gauge configurations generated with the Wilson gauge action at on the lattice, we compute quenched quark
propagators for 12 bare quark masses. The pion decay constant is extracted from
the pion propagator, and from which the lattice spacing is determined to be
0.147 fm. The presence of quenched chiral logarithm in the pion mass is
confirmed, and its coefficient is determined to be , in agreement with the theoretical estimate in quenched chiral perturbation
theory. Further, we obtain the topological susceptibility of these 100 gauge
configurations by measuring the index of the overlap Dirac operator. Using a
formula due to exact chiral symmetry, we obtain the mass in quenched
chiral perturbation theory, Mev, and an estimate
of , which is in good agreement with that
determined from the pion mass.Comment: 24 pages, 6 EPS figures; v2: some clarifications added, to appear in
Physical Review
Neuro-ophthalmic complications of vestibular schwannoma resection : current perspectives
Vestibular schwannomas (VSs), also called acoustic neuromas, are benign intracranial neoplasms of the vestibulocochlear (VIII) cranial nerve. Management options include “wait-and-scan,” stereotactic radiosurgery and surgical resection. Due to the proximity of the VIII nerve to the facial (VII) nerve in the cerebello-pontine angle, the VII nerve is particularly vulnerable to the effects of surgical resection. This can result in poor eye closure, lagophthalmos and resultant corneal exposure post VS resection. Additionally, compression from the tumor or resection can cause trigeminal (V) nerve damage and a desensate cornea. The combination of an exposed and desensate cornea puts the eye at risk of serious ocular complications including persistent epithelial defects, corneal ulceration, corneal vascularization, corneal melting and potential perforation. The abducens (VI) nerve can be affected by a large intracranial VS causing raised intracranial pressure (a false localizing sign) or as a result of damage to the VI nerve at the time of resection. Other types of neurogenic strabismus are rare and typically transient. Contralaterally beating nystagmus as a consequence of vestibular dysfunction is common post-operatively. This generally settles to pre-operative levels as central compensation occurs. Ipsilaterally beating nystagmus post-operatively should prompt investigation for post-operative cerebrovascular complications. Papilledema (and subsequent optic atrophy) can occur as a result of a large VS causing raised intracranial pressure. Where papilledema follows surgical resection of a VS, it can indicate that cerebral venous sinus thrombosis has occurred. Poor visual function following VS resection can result as a combination of all these potential complications and is more likely with larger tumors
Deuteron Photodissociation in Ultraperipheral Relativistic Heavy-Ion on Deuteron Collisions
In ultraperipheral relativistic deuteron on heavy-ion collisions, a photon
emitted from the heavy nucleus may dissociate the deuterium ion. We find
deuterium breakup cross sections of 1.38 barns for deuterium-gold collisions at
a center of mass energy of 200 GeV per nucleon, as studied at the Relativistic
Heavy Ion Collider, and 2.49 barns for deuterium-lead collisions at a center of
mass energy of 6.2 TeV, as proposed for the Large Hadron Collider. This cross
section includes an energy-independent 140 mb contribution from hadronic
diffractive dissociation. At the LHC, the cross section is as large as that of
hadronic interactions. The estimated error is 5%. Deuteron dissociation could
be used as a luminosity monitor and a `tag' for moderate impact parameter
collisions.Comment: Final version, to appear in Phys. Rev. C. Diffractive dissociation
included 10 pages with 3 figure
Charmonium Spectrum from Quenched QCD with Overlap Fermions
We present preliminary results using overlap fermions for the charmonium
spectrum, in particular for hyperfine splitting. Simulations are performed on
lattices, with Wilson gauge action at .
Depending on how the scale is set, we obtain 104(5) MeV (using
) or 88(4) MeV (using =0.5 fm) for the hyperfine
splitting.Comment: 3 pages, 5 fiugres. Talk presented at Lattice 2004 (heavy
Chiral Logs in Quenched QCD
The quenched chiral logs are examined on a lattice with
Iwasaki gauge action and overlap fermions. The pion decay constant is
used to set the lattice spacing, . With pion mass as low
as , we see the quenched chiral logs clearly in
and , the pseudoscalar decay constant. We analyze the data
to determine how low the pion mass needs to be in order for the quenched
one-loop chiral perturbation theory (PT) to apply. With the constrained
curve-fitting method, we are able to extract the quenched chiral log parameter
together with other low-energy parameters. Only for do we obtain a consistent and stable fit with a constant
which we determine to be 0.24(3)(4) (at the chiral scale ). By comparing to the lattice, we estimate the
finite volume effect to be about 2.7% for the smallest pion mass. We also
fitted the pion mass to the form for the re-summed cactus diagrams and found
that its applicable region is extended farther than the range for the one-loop
formula, perhaps up to MeV. The scale independent
is determined to be 0.20(3) in this case. We study the quenched
non-analytic terms in the nucleon mass and find that the coefficient
in the nucleon mass is consistent with the prediction of one-loop PT\@.
We also obtain the low energy constant from . We conclude from
this study that it is imperative to cover only the range of data with the pion
mass less than in order to examine the chiral behavior of
the hadron masses and decay constants in quenched QCD and match them with
quenched one-loop PT\@.Comment: 37 pages and 24 figures, pion masses are fitted to the form for the
re-summed cactus diagrams, figures added, to appear in PR
Semileptonic decays and Charmonium distribution amplitude
In this paper we study the semileptonic decays of the meson in the
Light-Cone Sum Rule (LCSR) approach. The result for each channel depends on the
corresponding distribution amplitude of the final meson. For the case of
decaying into a pseudoscalar meson, to twist-3 accuracy only the leading twist
distribution amplitude (DA) is involved if we start from a chiral current. If
we choose a suitable chiral current in the vector meson case, the main twist-3
contributions are also eliminated and we can consider the leading twist
contribution only. The leading twist distribution amplitudes of the charmonium
and other heavy mesons are given by a model approach in the reasonable way.
Employing this charmonium distribution amplitude we find the cross section
which is consistent with Belle
and BaBar's data. Based on this model, we calculate the form factors for
various decay modes in the corresponding regions. Extrapolating the form
factors to the whole kinetic regions, we get the decay widths and branching
ratios for various decay modes including their modes when they are
kinematically accessible.Comment: Changed content partially, Added references, 16 pages,2 figure
Light-cone QCD Sum Rules for the Baryon Electromagnetic Form Factors and its magnetic moment
We present the light-cone QCD sum rules up to twist 6 for the electromagnetic
form factors of the baryon. To estimate the magnetic moment of the
baryon, the magnetic form factor is fitted by the dipole formula. The numerical
value of our estimation is , which is in
accordance with the experimental data and the existing theoretical results. We
find that it is twist 4 but not the leading twist distribution amplitudes that
dominate the results.Comment: 13 page, 7 figures, accepted for publication in Euro. Phys. J.
Knowledge-based energy functions for computational studies of proteins
This chapter discusses theoretical framework and methods for developing
knowledge-based potential functions essential for protein structure prediction,
protein-protein interaction, and protein sequence design. We discuss in some
details about the Miyazawa-Jernigan contact statistical potential,
distance-dependent statistical potentials, as well as geometric statistical
potentials. We also describe a geometric model for developing both linear and
non-linear potential functions by optimization. Applications of knowledge-based
potential functions in protein-decoy discrimination, in protein-protein
interactions, and in protein design are then described. Several issues of
knowledge-based potential functions are finally discussed.Comment: 57 pages, 6 figures. To be published in a book by Springe
Heavy Quarks and Heavy Quarkonia as Tests of Thermalization
We present here a brief summary of new results on heavy quarks and heavy
quarkonia from the PHENIX experiment as presented at the "Quark Gluon Plasma
Thermalization" Workshop in Vienna, Austria in August 2005, directly following
the International Quark Matter Conference in Hungary.Comment: 8 pages, 5 figures, Quark Gluon Plasma Thermalization Workshop
(Vienna August 2005) Proceeding
Single Electrons from Heavy Flavor Decays in p+p Collisions at sqrt(s) = 200 GeV
The invariant differential cross section for inclusive electron production in
p+p collisions at sqrt(s) = 200 GeV has been measured by the PHENIX experiment
at the Relativistic Heavy Ion Collider over the transverse momentum range $0.4
<= p_T <= 5.0 GeV/c at midrapidity (eta <= 0.35). The contribution to the
inclusive electron spectrum from semileptonic decays of hadrons carrying heavy
flavor, i.e. charm quarks or, at high p_T, bottom quarks, is determined via
three independent methods. The resulting electron spectrum from heavy flavor
decays is compared to recent leading and next-to-leading order perturbative QCD
calculations. The total cross section of charm quark-antiquark pair production
is determined as sigma_(c c^bar) = 0.92 +/- 0.15 (stat.) +- 0.54 (sys.) mb.Comment: 329 authors, 6 pages text, 3 figures. Submitted to Phys. Rev. Lett.
Plain text data tables for the points plotted in figures for this and
previous PHENIX publications are (or will be) publicly available at
http://www.phenix.bnl.gov/papers.htm
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