2,914 research outputs found
QCD corrections to e+ e- --> 4 jets
We report on the next-to-leading order QCD calculation for e+ e- --> 4 jets.
We explain some modern techniques which have been used to calculate the
one-loop amplitudes efficiently. We further report on the general purpose
numerical program ``Mercutio'', which can be used to calculate any infrared
safe four-jet quantity in electron-positron annihilation at next-to-leading
order.Comment: 4 pages, talk given at the UK Phenomenology Workshop on Collider
Physics, Durham, 19-24 September 199
Long distance contribution to , - a searching ground mode for new physics
The decay has been sugested as a test for minimal
supersymmetric standard model and for supersymmetric models with R-parity
violating couplings, in view of its extreme smallnesss in the standard model.
We calculate two long distance contributions to this decay, that associated
with and intermediate states and that induced by virtual ,
mesons. The branching ratio due to these contributions is ,
which is somewhat smaller than the standard model short distance result,
leaving this decay free for the search of new physics.Comment: 13 pages, 2 figures, revised versio
Effect of the Pauli principle on photoelectron spin transport in GaAs
In p+ GaAs thin films, the effect of photoelectron degeneracy on spin
transport is investigated theoretically and experimentally by imaging the spin
polarization profile as a function of distance from a tightly-focussed light
excitation spot. Under degeneracy of the electron gas (high concentration, low
temperature), a dip at the center of the polarization profile appears with a
polarization maximum at a distance of about from the center. This
counterintuitive result reveals that photoelectron diffusion depends on spin,
as a direct consequence of the Pauli principle. This causes a concentration
dependence of the spin stiffness while the spin dependence of the mobility is
found to be weak in doped material. The various effects which can modify spin
transport in a degenerate electron gas under local laser excitation are
considered. A comparison of the data with a numerical solution of the coupled
diffusion equations reveals that ambipolar coupling with holes increases the
steady-state photo-electron density at the excitation spot and therefore the
amplitude of the degeneracy-induced polarization dip. Thermoelectric currrents
are predicted to depend on spin under degeneracy (spin Soret currents), but
these currents are negligible except at very high excitation power where they
play a relatively small role. Coulomb spin drag and bandgap renormalization are
negligible due to electrostatic screening by the hole gas
Probing the Higgs Field Using Massive Particles as Sources and Detectors
In the Standard Model, all massive elementary particles acquire their masses
by coupling to a background Higgs field with a non-zero vacuum expectation
value. What is often overlooked is that each massive particle is also a source
of the Higgs field. A given particle can in principle shift the mass of a
neighboring particle. The mass shift effect goes beyond the usual perturbative
Feynman diagram calculations which implicitly assume that the mass of each
particle is rigidly fixed. Local mass shifts offer a unique handle on Higgs
physics since they do not require the production of on-shell Higgs bosons. We
provide theoretical estimates showing that the mass shift effect can be large
and measurable, especially near pair threshold, at both the Tevatron and the
LHC.Comment: 6 pages, no figures; Version 2 corrects some typographical errors of
factors of 2 in equations 14, 17, 18 and 19 (all of the same origin) and
mentions a linear collider as an interesting place to test the results of
this pape
Measurement of the Higgs Boson Mass with a Linear e+e- Collider
The potential of a linear e+e- collider operated at a centre-of-mass energy
of 350 GeV is studied for the measurement of the Higgs boson mass. An
integrated luminosity of 500 fb-1 is assumed. For Higgs boson masses of 120,
150 and 180 GeV the uncertainty on the Higgs boson mass measurement is
estimated to be 40, 65 and 70 MeV, respectively. The effects of beam related
systematics, namely a bias in the beam energy measurement, the beam energy
spread and the luminosity spectrum due to beamstrahlung, on the precision of
the Higgs boson mass measurement are investigated. In order to keep the
systematic uncertainty on the Higgs boson mass well below the level of the
statistical error, the beam energy measurement must be controlled with a
relative precision better than 10-4.Comment: 19 pages, 10 Figure
Energy dependence of ratios of multiplicities and their slopes for gluon and quark jets
The difference between the ratio of multiplicities and the ratio of their
derivatives on energy for gluon and quark jets is calculated up to
next-to-next-to leading order of perturbative QCD. Its non-zero value is
uniquely defined by the running property of the QCD coupling constant. It is
shown that this difference is rather small compared to values which can be
obtained from experimental data. This disagreement can be ascribed either to
strong non-perturbative terms or to experimental problems with a scale choice,
jets separation and inadequate assignement of soft particles to jets.Comment: 5 pages, LATEX, no Figs; submitted to JETP Let
Phenomenology of non-universal gaugino masses and implications for the Higgs boson decay
Grand unified theories (GUTs) can lead to non-universal gaugino masses at the
unification scale. We study the implications of such non-universal gaugino
masses for the composition of the lightest neutralino in supersymmetric (SUSY)
theories based on SU(5) gauge group. We also consider the phenomenological
implications of non-universal gaugino masses for the phenomenology of Higgs
bosons in the context of Large Hadron Collider.Comment: 4 pages, 8 eps figures, style files included, Talk at LCWS06,
Bangalore, India; typos correcte
Tau and Charm physics highlights
In tau physics, we are at the frontier between the completion of the LEP
program and the start of analyses from b-factories, which are expected to
produce results in the coming years. Nice results from CLEO are steadily
delivered in the meantime. For charm, impressive progress have been achieved by
fixed target experiments in the search for CP violation and D^0 - \bar D^0
oscillations. First results from b-factories demonstrate the power of these
facilities in such areas. The novel measurement of the D* width by CLEO happens
to be rather different from current expectations. The absence of a charm
factory explains the lack or the very slow progress in the absolute scale
determinations for charm decays.Comment: "Typos corrected and references added
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