84 research outputs found
Next-to-leading order numerical calculations in Coulomb gauge
Calculations of observables in quantum chromodynamics can be performed using
a method in which all of the integrations, including integrations over virtual
loop momenta, are performed numerically. We use the flexibility inherent in
this method in order to perform next-to-leading order calculations for event
shape variables in electron-positron annihilation in Coulomb gauge. The use of
Coulomb gauge provides the potential to go beyond a purely order alpha_s^2
calculation by including, for instance, renormalon or parton showering effects.
We expect that the approximations needed to include such effects at all orders
in alpha_s will be simplest in a gauge in which unphysically polarized gluons
do not propagate over long distances.Comment: 36 pages with 7 figure
New Fermions at ee Colliders: II.~Signals and Backgrounds
We discuss the production, at high--energy ee linear colliders, of
new heavy fermions predicted by extensions of the Standard Model. We analyze in
great details the various signals and the corresponding backgrounds for both
pair production and single production in association with ordinary fermions.
Concentrating on new leptons, we use a model detector for ee collisions
at a center of mass energy of 500 GeV, to illustrate the discovery potential of
the Next Linear Colliders.Comment: 22 pages + 9 figures (not included: available by mail from A.
Djouadi), Preprint UdeM-LPN-TH-93-15
Multiplicity with a Thrust Cut
We evaluate the multiplicity of hadrons in the -annihilation at a
given thrust in the modified leading-log approximation, including
corrections. The calculation is done at a large value of
by the use of the factorisation which takes place in the
one-particle-inclusive cross section at a given . At a small , a
different type of factorisation takes place, which also enable us to evaluate
the multiplicity. Two approaches are compared numerically. Measuring this
quantity near , we can determine the multiplicity ratio between a
gluon-jet and a quark-jet.Comment: OCHA-PP-32, LATEX FILE, 21
A comparison of different methods in the study of dynamical fluctuations in high energy e+e- collisions
Different methods in the study of anomalous scaling of factorial moments in
high energy e+e- collisions are examined in some detail. It is shown that the
horizontal and vertical factorial moments are equivalent only when they are
used in combination with the cummulant variables. The influence of different
reference frames and that of phase space restrictions is also discussed.Comment: 5 pages, 6 figure
Decay of Z into Two Light Higgs Bosons
If the standard electroweak gauge model is extended to include two or more
Higgs doublets, there may be a neutral Higgs boson which is light (with a
mass of say 10 GeV) but the coupling is suppressed so that it has so far
escaped experimental detection. However, the effective coupling is
generally unsuppressed, hence the decay of Z into two light Higgs bosons plus a
fermion-antifermion pair may have an observable branching fraction, especially
if decays invisibly as for example in the recently proposed doublet Majoron
model.Comment: 10 pages, LaTex, figures available upon request to
[email protected]
Bounds on the mass of the b' quark, revisited
Recent results from the DELPHI collaboration led us to review the present
bounds on the b' quark mass. We use all available experimental data for m_b' >
96 GeV to constrain the b' quark mass as a function of the
Cabibbo-Kobayashi-Maskawa elements in a sequential four generations model. We
find that there is still room for a b' with a mass larger than 96 GeV.Comment: 9 pages and 7 figures. REVTEX
Probing quark gluon plasma with jets
We study multiple scatterings of jets on constituents of quark gluon plasma
and introduce energy--energy correlations to quantify their effects. The
effects from a longitudinally expanding plasma on medium as well as high energy
jets are found to be significant at both RHIC and LHC energies. Because jets
escape from the plasma long before the completion of mixed phase, these effects
are free from complications of final state hadronic interactions and decays.
These suggest that jets can be used to probe the plasma that might be created
in future high energy heavy ion collisions.Comment: 15 pages, 6 figures in 5 ps files included, McGill/94-1
Corrections to Angular Ordering in Multiple Hadroprodroproduction
One of the key ideas in describing the multiparticle production at high
energies in the perturbative QCD is the angular ordering in successive
soft-gluon emission. We analyse the angular distribution of particles in a jet,
and investigate the corrections to the angular ordering. At a small angle from
the jet direction, the angular ordering is exact at the next-to-leading order.
At a large angle, the angular ordering overestimates the angular particle
density by a factor , where is the
anomalous dimension of the multiplicity. The correction restores
the boost invariance of the cross section.Comment: Latex fil
Determination of the Strong Coupling \boldmath{\as} from hadronic Event Shapes and NNLO QCD predictions using JADE Data
Event Shape Data from annihilation into hadrons collected by the
JADE experiment at centre-of-mass energies between 14 GeV and 44 GeV are used
to determine the strong coupling . QCD predictions complete to
next-to-next-to-leading order (NNLO), alternatively combined with resummed
next-to-leading-log-approximation (NNLO+NLLA) calculations, are used. The
combined value from six different event shape observables at the six JADE
centre-of-mass energies using the NNLO calculations is
= 0.1210 +/- 0.0007(stat.) +/- 0.0021(expt.) +/- 0.0044(had.)
+/- 0.0036(theo.) and with the NNLO+NLLA calculations the combined value is
= 0.1172 +/- 0.0006(stat.) +/- 0.0020(expt.) +/- 0.0035(had.) +/-
0.0030(theo.) . The stability of the NNLO and NNLO+NLLA results with respect to
missing higher order contributions, studied by variations of the
renormalisation scale, is improved compared to previous results obtained with
NLO+NLLA or with NLO predictions only. The observed energy dependence of
agrees with the QCD prediction of asymptotic freedom and excludes
absence of running with 99% confidence level.Comment: 9 pages, EPHJA style, 4 figures, corresponds to published version
with JADE author lis
On a Light Spinless Particle Coupled to Photons
A pseudoscalar or scalar particle that couples to two photons but not
to leptons, quarks and nucleons would have effects in most of the experiments
searching for axions, since these are based on the coupling.
We examine the laboratory, astrophysical and cosmological constraints on
and study whether it may constitute a substantial part of the dark matter. We
also generalize the interactions to possess gauge
invariance, and analyze the phenomenological implications.Comment: LaTex, 20p., 6 figures. Changes in sections 4, 5 and figure 2, our
bounds are now more stringent. To be published in Physical Review
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