36 research outputs found
Closing the window on the axigluon mass using top quark production data
The contribution of axigluons (the massive color-octet gauge bosons in all
chiral color models) to top quark pair production in hadronic collisions is
considered. The agreement between the experimental values of the ttbar
production cross-section at the TEVATRON and recent QCD predictions is used to
discuss limits on the axigluon mass. Specifically,intermediate mass axigluons,
those in the mass range 50 GeV < M_A < 120 GeV which has not already been
excluded, would increase the tree-level qqbar -> ttbar cross-section by a
factor of >= 2, thereby increasing the theoretical predictions for
sigma_{ttbar} by Delta sigma_{ttbar} = 3.2-3.7 pb (2.7-3.1 pb) using
leading-order (next-to-leading order) parton distributions over this mass
range, independent of the axigluon decay width. Such an increase is roughly
1.3-1.6 (0.9-1.2) standard deviations larger than that suggested by the
apparent good agreement between combined experimental results and recent
theoretical calculations and so is not ruled out, but is definitely disfavored.
Future high-statistics top-quark production runs will likely make a more
definitive statement. The forward-backward asymmetry in ttbar production
induced by axigluons in this mass window is also discussed and found to be
quite large and so could provide another constraint.Comment: 9 pages, LaTeX, 2 separate postscript file
Radiation zeros and scalar particles beyond the standard model
Standard radiation zeros arise from the factorization properties of
tree-level amplitudes involving a massless photon and can occur when all
charged particles in the initial and final state have the same sign. We
investigate how several different processes involving new scalar particles
beyond the standard model may exhibit radiation zeros and how this structure
might be exploited to probe their electromagnetic structure. We focus on (i)
unnoticed aspects of angular zeros in the process e- + e- --> Delta-- + gamma
for doubly charged Higgs boson (or any bilepton) production and (ii) the
process gamma + e- --> q + S/V for scalar (S) or vector (V) leptoquarks (LQs).
We also discuss how factorized amplitudes and radiation zeros may appear in the
gauge boson fusion production of non-conjugate leptoquark pairs via gamma + W
--> S_i + S_j* in high energy ee reactions and how the zeros affect the
production cross-sections for various types of scalar leptoquarks.Comment: 18 pages (LaTeX) plus 5 postscript figure
QCD-supression by Black Hole Production at the LHC
Possible consequences of the production of small black holes at the LHC for
different scenarios with large extra dimensions are investigated. The effects
from black hole production on some standard jet observables are examined,
concentrating on the reduction of the QCD cross section. It is found that black
hole production of partons interacting on a short enough distance indeed seem
to generate a drastic drop in the QCD cross section. However from an
experimental point of view this will in most cases be camouflaged by energetic
radiation from the black holes
Zeroing in on more photons and gluons
We discuss radiation zeros that are found in gauge tree amplitudes for
processes involving multi-photon emission. Previous results are clarified by
examples and by further elaboration. The conditions under which such amplitude
zeros occur are identical in form to those for the single-photon zeros, and all
radiated photons must travel parallel to each other. Any other neutral particle
likewise must be massless (e.g. gluon) and travel in that common direction. The
relevance to questions like gluon jet identification and computational checks
is considered. We use examples to show how certain multi-photon amplitudes
evade the zeros, and to demonstrate the connection to a more general result,
the decoupling of an external electromagnetic plane wave in the ``null zone".
Brief comments are made about zeros associated with other gauge-boson emission.Comment: 26 page
Third-generation leptoquark decays and collider searches
Collider searches for first-, second-, and third-generation scalar (S) or
vector (V) leptoquarks (LQs) focus on the quark-lepton decay modes S,V -> q l.
For SU(2)-doublet and -triplet leptoquarks with a sufficiently large splitting
between the components, decays involving real W-boson emission (such as
S_2^{(+5/3)} -> S_2^{(+2/3)} W^{+} and others) become possible and can change
the patterns of leptoquark decays. For third-generation leptoquarks, where
these mass splittings might be large, such modes could dominate certain
leptoquark decays as they are (if kinematically allowed) guaranteed to be of
order g^2 where g is the electroweak coupling. We calculate the decay rates for
all such processes involving SU(2)-doublet and triplet, scalar and vector
leptoquarks. Standard limits on mass splittings from precision electroweak
measurements imply that only such decays involving SU(2)-doublet scalar LQs are
likely kinematically possible.Comment: 13 pages, LaTeX, 2 separate postscript figure
Quantum mechanical analysis of the equilateral triangle billiard: periodic orbit theory and wave packet revivals
Using the fact that the energy eigenstates of the equilateral triangle
infinite well (or billiard) are available in closed form, we examine the
connections between the energy eigenvalue spectrum and the classical closed
paths in this geometry, using both periodic orbit theory and the short-term
semi-classical behavior of wave packets. We also discuss wave packet revivals
and show that there are exact revivals, for all wave packets, at times given by
where and are the length of one side
and the mass of the point particle respectively. We find additional cases of
exact revivals with shorter revival times for zero-momentum wave packets
initially located at special symmetry points inside the billiard. Finally, we
discuss simple variations on the equilateral
() triangle, such as the half equilateral
() triangle and other `foldings', which have
related energy spectra and revival structures.Comment: 34 pages, 9 embedded .eps figure
Wave packet revivals and the energy eigenvalue spectrum of the quantum pendulum
The rigid pendulum, both as a classical and as a quantum problem, is an
interesting system as it has the exactly soluble harmonic oscillator and the
rigid rotor systems as limiting cases in the low- and high-energy limits
respectively. The energy variation of the classical periodicity () is
also dramatic, having the special limiting case of at the
'top' of the classical motion (i.e. the separatrix.) We study the
time-dependence of the quantum pendulum problem, focusing on the behavior of
both the (approximate) classical periodicity and especially the quantum revival
and superrevival times, as encoded in the energy eigenvalue spectrum of the
system. We provide approximate expressions for the energy eigenvalues in both
the small and large quantum number limits, up to 4th order in perturbation
theory, comparing these to existing handbook expansions for the characteristic
values of the related Mathieu equation, obtained by other methods. We then use
these approximations to probe the classical periodicity, as well as to extract
information on the quantum revival and superrevival times. We find that while
both the classical and quantum periodicities increase monotonically as one
approaches the 'top' in energy, from either above or below, the revival times
decrease from their low- and high-energy values until very near the separatrix
where they increase to a large, but finite value.Comment: 27 pages, 8 embedded .eps figures; to appear, Annals of Physic
Heavy quark production via leptoquarks at a neutrino factory
The proposed neutrino factory (NF) based on a muon storage ring (MSR) is an
ideal place to look for heavy quark production via neutral current (NC) and
charged current (CC) interactions. In this article, we address the issue of
contribution coming from mediating leptoquarks (LQ) in interactions leading to the production of at a
MSR and investigate the region where LQ interactions are significant in the
near-site experiments.Comment: 12 pages latex, 10 ps figures, uses axocolour.sty, Slightly revised
version to appear in PR
Bohmian mechanics, the quantum-classical correspondence and the classical limit: the case of the square billiard
Square billiards are quantum systems complying with the dynamical
quantum-classical correspondence. Hence an initially localized wavefunction
launched along a classical periodic orbit evolves along that orbit, the
spreading of the quantum amplitude being controlled by the spread of the
corresponding classical statistical distribution. We investigate wavepacket
dynamics and compute the corresponding de Broglie-Bohm trajectories in the
quantum square billiard. We also determine the trajectories and statistical
distribution dynamics for the equivalent classical billiard. Individual Bohmian
trajectories follow the streamlines of the probability flow and are generically
non-classical. This can also hold even for short times, when the wavepacket is
still localized along a classical trajectory. This generic feature of Bohmian
trajectories is expected to hold in the classical limit. We further argue that
in this context decoherence cannot constitute a viable solution in order to
recover classicality.Comment: Figures downgraded to low resolution; To be published in Found. Phys.
(2009)
Effective Lagrangian description of the lepton flavor violating decays Z-->li lj
A comprehensive analysis of the lepton flavor violating (LFV) decays Z-->li
lj is presented within the effective Lagrangian approach. Both the decoupling
and nondecoupling scenarios are explored. The experimental constraints from li
--> lj lk \bar{lk} and li -->lj gamma as well as some relationships arising
from the gauge invariance of the effective Lagrangian are used to put
constraints on Z-->li lj. It is found that while current experimental data
impose very strong constraints on Z-->mu e, the channel Z --> tau mu (e)still
may be at the reach of the planned TESLA collider.Comment: References added, final version to appear in Physical Review