237 research outputs found
Mass dependence of the hairpin vertex in quenched QCD
The pseudoscalar ``hairpin'' vertex (i.e. quark-disconnected vertex) plays a
key role in quenched chiral perturbation theory. Direct calculations using
lattice simulations find that it has a significant dependence on quark mass. I
show that this mass dependence can be used to determine the quenched
Gasser-Leutwyler constant L5. This complements the calculation of L5 using the
mass dependence of the axial decay constant of the pion. In an appendix, I
discuss power counting for quenched chiral perturbation theory and describe the
particular scheme used in this paper.Comment: 12 pages, 4 figures. Version to appear in Phys. Rev. D. Central
result unchanged, but explanation of calculation improved and minor errors
corrected. New appendix discusses power counting schemes in quenched chiral
perturbation theor
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
Strongly Interacting W's and Z's and the Existence of a Heavy Fourth Generation of Fermions
By employing the dictum that axiomatic principles are devoid of predictive
power, we find that the elastic unitarity constraint, applied to strong
WW scattering, does not alter the assumed spectrum of intermediate
states. We consider intermediate states involving a heavy Higgs and heavy
fermions of a hypothetical fourth generation doublet. In contrast to recent
studies, we find no p-wave resonance, and therefore no violation of the S
parameter upper bound. We conclude that the elastic unitarity constraint sheds
no light on the existence of a heavy fourth generation.Comment: 8 pages including 4 uuencoded, tarred, and compressed postscript
figures, CPP-93-0
Comment on "Evolution of a Quasi-Stationary State"
Approximately forty years ago it was realized that the time development of
decaying systems might not be precisely exponential. Rolf Winter (Phys. Rev.
{\bf 123}, 1503 (1961)) analyzed the simplest nontrivial system - a particle
tunneling out of a well formed by a wall and a delta-function. He calculated
the probability current just outside the well and found irregular oscillations
on a short time scale followed by an exponential decrease followed by more
oscillations and finally by a decrease as a power of the time. We have
reanalyzed this system, concentrating on the survival probability of the
particle in the well rather than the probability current, and find a different
short time behavior.Comment: 8 pages, 6 figures, RevTex
Time Evolution of Unstable Particle Decay Seen with Finite Resolution
Time evolution of the decay process of unstable particles is investigated in
field theory models. We first formulate how to renormalize the non-decay
amplitude beyond perturbation theory and then discuss short-time behavior of
very long-lived particles. Two different formalisms, one that does and one that
does not, assume existence of the asymptotic field of unstable particles are
considered. The non-decay amplitude is then calculated by introducing a finite
time resolution of measurement, which makes it possible to discuss both
renormalizable and non-renormalizable decay interaction including the nucleon
decay. In ordinary circumstances the onset of the exponential decay law starts
at times as early as at roughly the resolution time, but with an enhanced
amplitude which may be measurable. It is confirmed that the short-time formula
of the exponential decay law may be used to set limits on the
nucleon decay rate in underground experiments. On the other hand, an
exceptional example of S-wave decay of very small Q-value is found, which does
not have the exponential period at all.Comment: 26 pages, LATEX file with 8 PS figure
Axial anomaly in the reduced model: Higher representations
The axial anomaly arising from the fermion sector of \U(N) or \SU(N)
reduced model is studied under a certain restriction of gauge field
configurations (the ``\U(1) embedding'' with ). We use the
overlap-Dirac operator and consider how the anomaly changes as a function of a
gauge-group representation of the fermion. A simple argument shows that the
anomaly vanishes for an irreducible representation expressed by a Young tableau
whose number of boxes is a multiple of (such as the adjoint
representation) and for a tensor-product of them. We also evaluate the anomaly
for general gauge-group representations in the large limit. The large
limit exhibits expected algebraic properties as the axial anomaly.
Nevertheless, when the gauge group is \SU(N), it does not have a structure
such as the trace of a product of traceless gauge-group generators which is
expected from the corresponding gauge field theory.Comment: 21 pages, uses JHEP.cls and amsfonts.sty, the final version to appear
in JHE
Heavy Quark Photoproduction in Ultra-peripheral Heavy Ion Collisions
Heavy quarks are copiously produced in ultra-peripheral heavy ion collisions.
In the strong electromagnetic fields, c c-bar and b b-bar are produced by
photonuclear and two-photon interactions; hadroproduction can occur in grazing
interactions. We present the total cross sections, quark transverse momentum
and rapidity distributions, as well as the Q Q-bar invariant mass spectra from
the three production channels. We consider AA and pA collisions at the
Relativistic Heavy Ion Collider and Large Hadron Collider. We discuss
techniques for separating the three processes and describe how the AA to pA
production ratios might be measured accurately enough to study nuclear
shadowing.Comment: Minor changes to satisfy referees and typo fixes; 52 pages including
17 figure
Heavy quarkonium: progress, puzzles, and opportunities
A golden age for heavy quarkonium physics dawned a decade ago, initiated by
the confluence of exciting advances in quantum chromodynamics (QCD) and an
explosion of related experimental activity. The early years of this period were
chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in
2004, which presented a comprehensive review of the status of the field at that
time and provided specific recommendations for further progress. However, the
broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles
could only be partially anticipated. Since the release of the YR, the BESII
program concluded only to give birth to BESIII; the -factories and CLEO-c
flourished; quarkonium production and polarization measurements at HERA and the
Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the
deconfinement regime. All these experiments leave legacies of quality,
precision, and unsolved mysteries for quarkonium physics, and therefore beg for
continuing investigations. The plethora of newly-found quarkonium-like states
unleashed a flood of theoretical investigations into new forms of matter such
as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the
spectroscopy, decays, production, and in-medium behavior of c\bar{c}, b\bar{b},
and b\bar{c} bound states have been shown to validate some theoretical
approaches to QCD and highlight lack of quantitative success for others. The
intriguing details of quarkonium suppression in heavy-ion collisions that have
emerged from RHIC have elevated the importance of separating hot- and
cold-nuclear-matter effects in quark-gluon plasma studies. This review
systematically addresses all these matters and concludes by prioritizing
directions for ongoing and future efforts.Comment: 182 pages, 112 figures. Editors: N. Brambilla, S. Eidelman, B. K.
Heltsley, R. Vogt. Section Coordinators: G. T. Bodwin, E. Eichten, A. D.
Frawley, A. B. Meyer, R. E. Mitchell, V. Papadimitriou, P. Petreczky, A. A.
Petrov, P. Robbe, A. Vair
Non-exponential decay in quantum field theory and in quantum mechanics: the case of two (or more) decay channels
We study the deviations from the exponential decay law, both in quantum field
theory (QFT) and quantum mechanics (QM), for an unstable particle which can
decay in (at least) two decay channels. After a review of general properties of
non-exponential decay in QFT and QM, we evaluate in both cases the decay
probability that the unstable particle decays in a given channel in the time
interval between and An important quantity is the ratio of the
probability of decay into the first and the second channel: this ratio is
constant in the Breit-Wigner limit (in which the decay law is exponential) and
equals the quantity , where and
are the respective tree-level decay widths. However, in the full
treatment (both for QFT and QM) it is an oscillating function around the mean
value and the deviations from this mean value can be
sizable. Technically, we study the decay properties in QFT in the context of a
superrenormalizable Lagrangian with scalar particles and in QM in the context
of Lee Hamiltonians, which deliver formally analogous expressions to the QFT
case.Comment: 32 pages, 10 figures. To appear in "Foundations of Physics
Protein Kinase Activity and Identification of a Toxic Effector Domain of the Target of Rapamycin TOR Proteins in Yeast
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