5,830 research outputs found

### Topological Charge Screening and the `Proton Spin' Beyond the Chiral Limit

The theory of the `proton spin' effect proposed in our earlier papers is
extended to include the chiral SU(3) symmetry breaking and flavour mixing
induced by non-vanishing quark masses in QCD. The theoretical basis is the
derivation of exact, unified Goldberger-Treiman (GT) relations valid beyond the
chiral limit. The observed suppression in the flavour singlet axial charge
$a^0(Q^2)$ is explained by an anomalously small value for the slope of the
singlet current correlation function <0|T~\pl^\m J_{\m 5}^0 ~ \pl^\n J_{\n
5}^0 |0>, a consequence of the screening of topological charge in the QCD
vacuum. Numerical predictions are obtained by evaluating the current
correlation functions using QCD spectral sum rules. The results, $a^0(Q^2) =
0.31 \pm 0.02$ and $\int dx ~g_1^p(x,Q^2) = 0.141 \pm 0.005$ (at Q^2=10
\~\GV^2), are in good agreement with current experimental data on the
polarised proton structure function $g_1^p$.Comment: 44 pages, plain TeX, 2 ps figure

### CFT, Holography, and Causal Entropy Bound

The causal entropy bound (CEB) is confronted with recent explicit entropy
calculations in weakly and strongly coupled conformal field theories (CFTs) in
arbitrary dimension $D$. For CFT's with a large number of fields, $N$, the CEB
is found to be valid for temperatures not exceeding a value of order
$M_P/N^{{1\over D-2}}$, in agreement with large $N$ bounds in generic cut-off
theories of gravity, and with the generalized second law. It is also shown that
for a large class of models including high-temperature weakly coupled CFT's and
strongly coupled CFT's with AdS duals, the CEB, despite the fact that it
relates extensive quantities, is equivalent to (a generalization of) a purely
holographic entropy bound proposed by E. Verlinde.Comment: 14 pages, 2 figure

### Radiation Problem in Transplanckian Scattering

We investigate hard radiation emission in small-angle transplanckian
scattering. We show how to reduce this problem to a quantum field theory
computation in a classical background (gravitational shock wave). In momentum
space, the formalism is similar to the flat-space light cone perturbation
theory, with shock wave crossing vertices added. In the impact parameter
representation, the radiating particle splits into a multi-particle virtual
state, whose wavefunction is then multiplied by individual eikonal factors. As
a phenomenological application, we study QCD radiation in transplanckian
collisions of TeV-scale gravity models. We derive the distribution of initial
state radiation gluons, and find a suppression at large transverse momenta with
respect to the standard QCD result. This is due to rescattering events, in
which the quark and the emitted gluon scatter coherently. Interestingly, the
suppression factor depends on the number of extra dimensions and provides a new
experimental handle to measure this number. We evaluate the leading-log
corrections to partonic cross-sections due to the initial state radiation, and
prove that they can be absorbed into the hadronic PDF. The factorization scale
should then be chosen in agreement with an earlier proposal of Emparan, Masip,
and Rattazzi. In the future, our methods can be applied to the gravitational
radiation in transplanckian scattering, where they can go beyond the existing
approaches limited to the soft radiation case.Comment: 41 pp, v2: minor changes and added refs, conforms with published
versio

### The cosmology with the Dp-brane gas

We study the effect of the Dp-brane gas in string cosmology. When one kind of
Dp-brane gas dominates, we find that the cosmology is equivalent to that of the
Brans-Dicke theory with the perfect fluid type matter. We obtain $\gamma$, the
equation of state parameter, in terms of p and the space-time dimension.Comment: 12 pages, 3 figure

### Perturbations in a Bouncing Brane Model

The question of how perturbations evolve through a bounce in the Cyclic and
Ekpyrotic models of the Universe is still a matter of ongoing debate. In this
report we show that the collision between boundary branes is in most cases
singular even in the full 5-D formalism, and that first order perturbation
theory breaks down for at least one perturbation variable. Only in the case
that the boundary branes approach each other with constant velocity shortly
before the bounce, can a consistent, non singular solution be found. It is then
possible to follow the perturbations explicitly until the actual collision. In
this case, we find that if a scale invariant spectrum developed on the hidden
brane, it will get transferred to the visible brane during the bounce.Comment: 15 pages, minor modifications, a few typos correcte

### Target Independence of the Emc-SMC Effect

An approach to deep inelastic scattering is described in which the matrix
elements arising from the operator product expansion are factorised into
composite operator propagators and proper vertex functions. In the case of
polarised \m p scattering, the composite operator propagator is identified
with the square root of the QCD topological susceptibility
$\sqrt{\chi^{\prime}(0)}$, while the corresponding proper vertex is a
renormalisation group invariant. We estimate $\chi^{\prime}(0)$ using QCD
spectral sum rules and find that it is significantly suppressed relative to the
OZI expectation. Assuming OZI is a good approximation for the proper vertex,
our predictions, \int_{0}^{1}dx g_1^p (x;Q^2=10\GV^2)= 0.143 \pm 0.005 and
$G^{(0)}_A \equiv \Delta \Sigma = 0.353 \pm 0.052$, are in excellent agreement
with the new SMC data. This result, together with one confirming the validity
of the OZI rule in the \hp radiative decay, supports our earlier conjecture
that the suppression in the flavour singlet component of the first moment of
$g_1^p$ observed by the EMC-SMC collaboration is a target-independent feature
of QCD related to the $U(1)$ anomaly and is not a property of the proton
structure. As a corollary, we extract the magnitude of higher twist effects
from the neutron and Bjorken sum rules.Comment: 22 pages, 8 figures available on request

### The explicit Mordell conjecture for families of curves

In this article we prove the explicit Mordell Conjecture for large families of curves. In addition, we introduce a method, of easy application, to compute all rational points on curves of quite general shape and increasing genus. The method bases on some explicit and sharp estimates for the height of such rational points, and the bounds are small enough to successfully implement a computer search. As an evidence of the simplicity of its application, we present a variety of explicit examples and explain how to produce many others. In the appendix our method is compared in detail to the classical method of Manin-Demjanenko and the analysis of our explicit examples is carried to conclusion

### The eta-prime propagator in quenched QCD

The calculation of the eta-prime hairpin diagram is carried out in the
modified quenched approximation (MQA) in which the lattice artifact which
causes exceptional configurations is removed by shifting observed poles at
kappa<kappa_c in the quark propagators to the critical value of hop ping
parameter. By this method, the eta-prime propagator can be accurately
calculated even for very light quark mass. A determination of the topological
susceptibility for quenched QCD is also obtained, using the fermionic method of
Smit and Vink to calculate winding numbers.Comment: 3 pages, 3 postscript figure

### Trialogue on the number of fundamental constants

This paper consists of three separate articles on the number of fundamental
dimensionful constants in physics. We started our debate in summer 1992 on the
terrace of the famous CERN cafeteria. In the summer of 2001 we returned to the
subject to find that our views still diverged and decided to explain our
current positions. LBO develops the traditional approach with three constants,
GV argues in favor of at most two (within superstring theory), while MJD
advocates zero.Comment: Version appearing in JHEP; 31 pages late

### A Stringy Correspondence Principle in Cosmology

We study a d-dimensional FRW universe, containing a perfect fluid with p = w
\rho and \frac{1} {d - 1} \le w \le 1, and find a correspondence principle
similar to that of Horowitz and Polchinski in the black hole case. This
principle follows quite generally from thermodynamics and the conservation of
energy momentum tensor, and can be stated along similar lines as in the black
hole case: ``When the temperature T of the universe becomes of order string
scale the universe state becomes a highly excited string state. At the
transition, the entropies and energies of the universe and strings differ by
factors of {\cal O}(1).'' Such a matching is absent for w \ne 1 if the
transition is assumed to be when the curvature or the horizon length is of
order string scale.Comment: 14 pages. V2: More references added and some minor textual
modifications mad

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