4,919 research outputs found

### Next-to-leading Order Calculation of the Color-Octet 3S1 Gluon Fragmentation Function for Heavy Quarkonium

The short-distance coefficients for the color-octet 3S1 term in the
fragmentation function for a gluon to split into heavy quarkonium states is
calculated to order alpha_s^2. The gauge-invariant definition of the
fragmentation function by Collins and Soper is employed. Ultraviolet
divergences are removed using the MS-bar renormalization procedure. The
longitudinal term in the fragmentation function agrees with a previous
calculation by Beneke and Rothstein. The next-to-leading order correction to
the transverse term disagrees with a previous calculation.Comment: 17 pages, 4 figure

### Universality in the physics of cold atoms with large scattering length

Effective field theories exploit a separation of scales in physical systems
in order to perform systematically improvable, model-independent calculations.
They are ideally suited to describe universal aspects of a wide range of
physical systems. I will discuss recent applications of effective field theory
to cold atomic and molecular few-body systems with large scattering length.Comment: Invited talk at Few-Body 17, June 2003, Durham, NC, USA, 5 pages, 3
figures, uses espcrc1.st

### Color and Spin in Quarkonium Production

I describe the NRQCD factorization approach to the inclusive production of
heavy quarkonium, contrasting it with the color-singlet and color evaporation
models. These approaches differ dramatically in their assumptions about the
roles played by color and spin in the production process. They also differ
dramatically in their predictions for the production of charmonium at large
transverse momentum.Comment: 17 pages, LATEX with style file sprocl.st

### Few-Body Effects in Cold Atoms and Limit Cycles

Physical systems with a large scattering length have universal properties
independent of the details of the interaction at short distances. Such systems
can be realized in experiments with cold atoms close to a Feshbach resonance.
They also occur in many other areas of physics such as nuclear and particle
physics. The universal properties include a geometric spectrum of three-body
bound states (so-called Efimov states) and log-periodic dependence of
low-energy observables on the physical parameters of the system. This behavior
is characteristic of a renormalization group limit cycle. We discuss
universality in the three- and four-body sectors and give an overview of
applications in cold atoms.Comment: 7 pages, 3 figures, plenary talk at the 18th International IUPAP
Conference on Few-Body Problems in Physics (FB18), Santos, Brazil, August
200

### Prompt Upsilon and Psi Production at LEP

Color-octet contributions to quarkonia production at LEP are studied herein.
The short distance formation of heavy quark-antiquark pairs in color-octet
configurations via gluon fragmentation processes is significantly enhanced
relative to the creation of color-singlet pairs via heavy quark fragmentation.
But the subsequent long distance hadronization of these colored pairs into
physical quarkonium bound states is suppressed compared to the nonperturbative
evolution of their colorless counterparts. We find that the overall LEP rates
for gluon fragmentation into prompt Upsilon and Psi vector bosons exceed those
from heavy quark fragmentation. Inclusion of the dominant color-octet
quarkonium production channel eliminates sizable discrepancies between previous
predictions and recent measurements of prompt Z \to \Jpsi+X, $Z \to \psi'+X$
and $Z \to \Upsilon+X$ branching fractions.Comment: 15 pages, harvmac, 3 uuencoded figure

### Perturbative QCD and Tau Decay

Sufficiently inclusive observables in the decay of the tau lepton can be
calculated using the methods of perturbative QCD. These include the asymmetry
parameter $A_\tau$ that determines that angular distribution of the total
hadron momentum in the decay of a polarized tau. It should be possible to
measure $A_\tau$ accurately using existing data from LEP. Reliable estimates of
theoretical errors are essential in order to determine whether a given
observable is sufficiently inclusive to be calculated using perturbative
methods. The theoretical uncertainties due to higher orders in $\alpha_s$ can
be estimated using recent calculations to all orders in the large-$(33-2N_f)$
limit. These estimates indicate that tau decay data can be used to determine
$\alpha_s(M_Z)$ to a precision of 2% or better.Comment: 9 pages, LaTeX with espcrc2.sty, invited talk presented at the TAU 96
Workshop, Estes Park, Colorado, in September 199

### Thermalisation of Longitudinal Gluons

In the usual real-time finite-temperature gauge theory both the physical and
the unphysical degrees of freedom are thermalised. We discuss the alternative
approach where only the physical transverse components of the gauge field have
bare thermal propagators, whereas the unphysical degrees of freedom are not
heated. We show how pinch singularities are avoided: sometimes this requires
resummation. If only the hard thermal loop is included in the resummation, the
spatially-longitudinal component of the gauge field, which contains an extra
collective plasmon mode, becomes fully thermalised, though the Faddeev-Popov
ghost and the remaining unphysical component of the gauge field remain frozen.Comment: 10 pages, 1 figure appended as pictex-file, DAMTP 93-06, TUW-93-0

### Gauge Dependence of the Resummed Thermal Gluon Self Energy

The gauge dependence of the hot gluon self energy is examined in the context
of Pisarski's method for resumming hard thermal loops. Braaten and Pisarski
have used the Ward identities satisfied by the hard corrections to the n-point
functions to argue the gauge fixing independence of the leading order resummed
QCD plasma damping rate in covariant and strict Coulomb gauges. We extend their
analysis to include all linear gauges that preserve rotational invariance and
display explicitly the conditions required for gauge fixing independence. It is
shown that in covariant gauges the resummed damping constant is gauge fixing
independent only if an infrared regulator is explicitly maintained throughout
the calculation.Comment: 29 pages, report BI-TP 92/19, LPTHE-Orsay 92/32, WIN-TH-92/02 (June
1992

### Tau Polarimetry with Inclusive Decays

The spin asymmetry parameter $A_\tau$ characterizing the angular distribution
of the total hadron momentum in the decay of a polarized tau can be calculated
rigorously using perturbative QCD and the operator product expansion.
Perturbative QCD corrections to the free quark result $A_\tau = 1/3$ can be
expressed as a power series in $\alpha_s(M_\tau)$ and nonperturbative QCD
corrections can be expanded systematically in powers of $1/M_\tau^2$. The QCD
prediction is $A_\tau = 0.41 \pm 0.02$. In the decay of a high energy tau into
hadrons, the value of the hadronic energy distribution $dR_\tau/dz$ evaluated
at the maximum hadronic energy fraction $z = 1$ can also be calculated
rigorously from QCD.Comment: LateX, 11 pages, no figures, NUHEP-TH-93-

### Hard photon production rate of a quark-gluon plasma at finite quark chemical potential

We compute the photon production rate of a quark-gluon plasma (QGP) at finite
quark chemical potential $\mu$ using the Braaten-Pisarski method, thus
continuing the work of Kapusta, Lichard, and Seibert who did the calculation
for $\mu =0$.Comment: 9 pages, revtex, no figures, error in soft part corrected, figures
available at ftp://theorie.physik.uni-giessen.de/usr/users/ftp/photon

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