10 research outputs found
Large multiplicity fluctuations and saturation effects in onium collisions
This paper studies two related questions in high energy onium-onium
scattering: the probability of producing an unusually large number of particles
in a collision, where it is found that the cross section for producing a
central multiplicity proportional to should decrease exponentially in
. Secondly, the nature of gluon (dipole) evolution when dipole
densities become so high that saturation effects due to dipole-dipole
interactions become important: measures of saturation are developed to help
understand when saturation becomes important, and further information is
obtained by exploiting changes of frame, which interchange unitarity and
saturation corrections.Comment: 30 pages LaTeX2e, 11 figures included using epsfig. Compressed
postscript of whole paper also available at
http://www.hep.phy.cam.ac.uk/theory/papers
OEDIPUS: Onium Evolution, Dipole Interaction and Perturbative Unitarisation Simulation
A Monte Carlo simulation program is presented which can be used to determine
the small- evolution of a heavy onium using Mueller's colour dipole
formulation, giving the full distribution of dipoles in rapidity and impact
parameter. Routines are also provided which calculate onium-onium scattering
amplitudes between individual pairs of onium configurations, making it possible
to establish the contribution of multiple pomeron exchange terms to onium-onium
scattering (the unitarisation corrections).Comment: 21 pages LaTeX2e. Postscript available from
http://www.hep.phy.cam.ac.uk/theory/papers and program available from
ftp://axpf.hep.phy.cam.ac.uk/pub/theory/oedipus.tar.g
Ultra-High Energy Neutrinos: A Review of Theoretical and Phenomenological Issues
We review the phenomenology of ultra-high energy (UHE) neutrino detection.
The motivations for looking for such neutrinos stemming from observational
evidence and the potential for new physics discoveries are enumerated, and
their expected sources and fluxes are given. Cross-sections with nucleons all
the way upto neutrino energies of 10^20 eV, and the attenuation of the fluxes
in the earth are discussed. Finally, sample event-rates for existing and future
Water/Ice Cerenkov detectors are provided.Comment: Invited Talk at Neutrino 2000, XIX International Conference on
Neutrino Physics and Astrophysics, Sudbury, Canada, June 16-21, 2000. 12
pages. References updated; minor changes to tex
Studies of Unitarity at Small~ Using the Dipole Formulation
Mueller's dipole formulation of onium-onium scattering is used to study
unitarity corrections to the BFKL power growth at high energies. After a short
discussion of the spatial distribution of colour dipoles in a heavy quarkonium
and the associated fluctuations, results are presented showing that the one and
two-pomeron contributions to the total cross section are the same at a rapidity
. Above this rapidity the large fluctuations in the onium wave
function cause the multiple pomeron series to diverge. Resumming the series
allows one to show that unitarity corrections set in gradually for the total
cross section, which is dominated by rare, large, configurations of the onia.
The elastic cross section comes mostly from much smaller impact parameters and
has significant unitarity corrections starting at a rapidity .Comment: 30 pages LaTeX2e with 12 figures as appended as uuencoded eps. Uses
epsfig. Postscript file for complete paper available from
http://www.hep.phy.cam.ac.uk/theory/papers/index.htm
Instanton-induced Effects in QCD High-Energy Scattering
We evaluate a number of new instanton-induced phenomena in QCD, starting with
static dipole-dipole potentials, and proceeding to quark-quark and
dipole-dipole scattering at high energy. We use a non-perturbative formulation
of the scattering amplitude in terms of a correlator of two Wilson-lines
(quarks) or Wilson-loops (dipoles) and analyze the Euclidean amplitudes with
both perturbative gluons and instantons. The results are analytically continued
to Minkowski geometry, by interpreting the angle between the Wilson lines as
rapidity. We discuss the relevance of our results for the phenomenology of
near-forward hadronic processes at high energy, especially for processes with
multiple color exchanges
The Gluon Impact Factors
We calculate in the next-to-leading approximation the non-forward gluon
impact factors for arbitrary color state in the -channel. In the case of the
octet state we check the so-called "second bootstrap condition" for the gluon
Reggeization in QCD, using the integral representation for the impact factors.
The condition is fulfilled in the general case of an arbitrary space-time
dimension and massive quark flavors for both helicity conserving and
non-conserving parts.Comment: 32 pages, LaTeX, 1 EPS figure, uses epsf.sty and axodraw.st
NLO BFKL Equation, Running Coupling and Renormalization Scales
I examine the solution of the BFKL equation with NLO corrections relevant for
deep inelastic scattering. Particular emphasis is placed on the part played by
the running of the coupling. It is shown that the solution factorizes into a
part describing the evolution in Q^2, and a constant part describing the input
distribution. The latter is infrared dominated, being described by a coupling
which grows as x decreases, and thus being contaminated by infrared
renormalons. Hence, for this part we agree with previous assertions that
predictive power breaks down for small enough x at any Q^2. However, the former
is ultraviolet dominated, being described by a coupling which falls like
1/(\ln(Q^2/\Lambda^2) + A(\bar\alpha_s(Q^2)\ln(1/x))^1/2)with decreasing x, and
thus is perturbatively calculable at all x. Therefore, although the BFKL
equation is unable to predict the input for a structure function for small x,
it is able to predict its evolution in Q^2, as we would expect from the
factorization theory. The evolution at small x has no true powerlike behaviour
due to the fall of the coupling, but does have significant differences from
that predicted from a standard NLO in alpha_s treatment. Application of the
resummed splitting functions with the appropriate coupling constant to an
analysis of data, i.e. a global fit, is very successful.Comment: Tex file, including a modification of Harvmac, 46 pages, 8 figures as
.ps files. Correction of typos, updating of references, very minor
corrections to text and fig.
Probing Nucleon Spin Structure
One of the important questions in high energy physics is the relation of
quark and gluon spin to that of the nucleons which they comprise. Polarization
experiments provide a mechanism to probe the spin properties of elementary
particles and provide crucial tests of Quantum Chromodynamics (QCD). The
theoretical and experimental status of this fundamental question will be
reviewed in this paper.Comment: 65 pages, 3 Postscript figures, LaTeX. To be published in "Progress
in Particle and Nuclear Physics
Particle Production in a Hadron Collider Rapidity Gap: The Higgs Case
Production of rare particles within rapidity gaps has been proposed as a
background-free signal for the detection of new physics at hadron colliders. No
complete formalism accounts for such processes yet. We study a simple
lowest-order QCD model for their description. Concentrating on Higgs
production, we show that the calculation of the cross section pp -> pp H can be
embedded into existing models which successfully account for diffractive data.
We extend those models to take into account single and double diffractive cross
sections pp -> H X1 X2 with a gap between the fragments X1 and X2. Using
conservative scenarios, we evaluate the uncertainties in our calculation, and
study the dependence of the cross section on the gap width. We predict that
Higgs production within a gap of 4 units of rapidity is about 0.3 pb for a 100
GeV Higgs at the Tevatron, and almost 2 pb for a 400 GeV Higgs within a gap of
6 units at the LHC with 14 TeV beams.Comment: LaTeX file, 30 pages, 12 figures and psfig.sty included in a second
uufile. The full ready-to-print postscript manuscript is available by
anonymous ftp at ftp://lpsvsh.lps.umontreal.ca/ in theorie/hep-ph/Rapidity.ps
(it's a VAX so you'll have to use the format theorie.hep-ph if you change by
more than one directory at a time
A Consistent Next-to-Leading-Order QCD Calculation of Hadronic Diffractive Scattering
We calculate the order alpha_s^2 and order alpha_s^3 QCD contributions to
colour-singlet exchange in the leading log s approximation. We implement the
resulting amplitude at the hadronic level and thus construct the QCD pomeron
and odderon to this order of perturbation theory. We show that the structure of
the hadronic form factors provides a natural mechanism through which the
odderon gets suppressed at t=0 whereas it dominates the elastic cross section
at large t. We also demonstrate that the inclusion of nonperturbative effects
through a modification of the gluon propagator accelerates greatly the
convergence of the log s expansion, although not enough to provide agreement
with the data.Comment: 26 pages, McGill/93-2