147 research outputs found
Calculating two- and three-body decays with FeynArts and FormCalc
The Feynman diagram generator FeynArts and the computer algebra program
FormCalc allow for an automatic computation of 2->2 and 2->3 scattering
processes in High Energy Physics. We have extended this package by four new
kinematical routines and adapted one existing routine in order to accomodate
also two- and three-body decays of massive particles. This makes it possible to
compute automatically two- and three-body particle decay widths and decay
energy distributions as well as resonant particle production within the
Standard Model and the Minimal Supersymmetric Standard Model at the tree- and
loop-level. The use of the program is illustrated with three standard examples:
h->b\bar{b}, \mu->e\bar{\nu}_e\nu_\mu, and Z->\nu_e\bar{\nu}_e.Comment: 8 pages, 1 figur
Counting dark matter particles in LHC events
We suggest trying to count the number of invisible particles produced in
missing energy events at the LHC, arguing that multiple production of such
particles provides evidence that they constitute stable Dark Matter and that
counting them could yield further insights into the nature of Dark Matter. We
propose a method to count invisible particles, based on fitting the shapes of
certain transverse- or invariant-mass distributions, discuss various effects
that may affect the measurement, and simulate the use of the method to count
neutrinos in Standard Model processes and Dark Matter candidates in new physics
processes.Comment: 18 pages, 13 figures, revtex4 forma
ppbar enhancement in B and J/Psi decay
The near-threshold enhancement in the ppbar invariant mass spectrum from the
B^+ -> K^+ ppbar decay reported recently by the BaBar Collaboration is studied
within the J\"ulich NNbar model. We illustrate that the invariant mass
dependence of the ppbar spectrum close to the threshold can be reproduced by
the final state interactions. This explanation is in line with our previous
analysis of the ppbar invariant mass spectrum from the J/Psi -> gamma ppbar
decay measured by the BES Collaboration. We also comment on a structure found
recently in the pi^+ pi^- eta' mass spectrum of the radiative J/Psi decay by
the BES Collaboration. In particular we argue that one should be rather
cautions in bringing this structure in connection with the enhancement found in
the ppbar invariant mass spectrum or with the existence of NNbar bound states.Comment: 4 pp, 2 figs, uses revtex
Probing resonance decays to two visible and multiple invisible particles
We consider the decay of a generic resonance to two visible particles and any
number of invisible particles. We show that the shape of the invariant mass
distribution of the two visible particles is sensitive to both the mass
spectrum of the new particles, as well as the decay topology. We provide the
analytical formulas describing the invariant mass shapes for the nine simplest
topologies (with up to two invisible particles in the final state). Any such
distribution can be simply categorized by its endpoint, peak location and
curvature, which are typically sufficient to discriminate among the competing
topologies. In each case, we list the effective mass parameters which can be
measured by experiment. In certain cases, the invariant mass shape is
sufficient to completely determine the new particle mass spectrum, including
the overall mass scale.Comment: Added new figures, conclusions unchanged, published versio
Thermalization of gluons in ultrarelativistic heavy ion collisions by including three-body interactions in a parton cascade
We develop a new 3+1 dimensional Monte Carlo cascade solving the kinetic
on-shell Boltzmann equations for partons including the inelastic gg ggg
pQCD processes. The back reaction channel is treated -- for the first time --
fully consistently within this scheme. An extended stochastic method is used to
solve the collision integral. The frame dependence and convergency are studied
for a fixed tube with thermal initial conditions. The detailed numerical
analysis shows that the stochastic method is fully covariant and that
convergency is achieved more efficiently than within a standard geometrical
formulation of the collision term, especially for high gluon interaction rates.
The cascade is then applied to simulate parton evolution and to investigate
thermalization of gluons for a central Au+Au collision at RHIC energy. For this
study the initial conditions are assumed to be generated by independent
minijets with p_T > p_0=2 GeV. With that choice it is demonstrated that overall
kinetic equilibration is driven mainly by the inelastic processes and is
achieved on a scale of 1 fm/c. The further evolution of the expanding gluonic
matter in the central region then shows almost an ideal hydrodynamical
behavior. In addition, full chemical equilibration of the gluons follows on a
longer timescale of about 3 fm/c.Comment: 121 pages with 55 figures, revised version. Two eps-figures and
comments are added. Formula (54) which has typo in journal version is given
correctl
Proton Lifetime and Baryon Number Violating Signatures at the LHC in Gauge Extended Models
There exist a number of models in the literature in which the weak
interactions are derived from a chiral gauge theory based on a larger group
than SU(2)_L x U(1)_Y. Such theories can be constructed so as to be
anomaly-free and consistent with precision electroweak measurements, and may be
interpreted as a deconstruction of an extra dimension. They also provide
interesting insights into the issues of flavor and dynamical electroweak
symmetry breaking, and can help to raise the mass of the Higgs boson in
supersymmetric theories. In this work we show that these theories can also give
rise to baryon and lepton number violating processes, such as nucleon decay and
spectacular multijet events at colliders, via the instanton transitions
associated with the extended gauge group. For a particular model based on
SU(2)_1 x SU(2)_2, we find that the violating scattering cross sections
are too small to be observed at the LHC, but that the lower limit on the
lifetime of the proton implies an upper bound on the gauge couplings.Comment: 36 page
A Numerical Unitarity Formalism for Evaluating One-Loop Amplitudes
Recent progress in unitarity techniques for one-loop scattering amplitudes
makes a numerical implementation of this method possible. We present a
4-dimensional unitarity method for calculating the cut-constructible part of
amplitudes and implement the method in a numerical procedure. Our technique can
be applied to any one-loop scattering amplitude and offers the possibility that
one-loop calculations can be performed in an automatic fashion, as tree-level
amplitudes are currently done. Instead of individual Feynman diagrams, the
ingredients for our one-loop evaluation are tree-level amplitudes, which are
often already known. To study the practicality of this method we evaluate the
cut-constructible part of the 4, 5 and 6 gluon one-loop amplitudes numerically,
using the analytically known 4, 5 and 6 gluon tree-level amplitudes.
Comparisons with analytic answers are performed to ascertain the numerical
accuracy of the method.Comment: 29 pages with 8 figures; references updated in rsponse to readers'
suggestion
Dimuon production by laser-wakefield accelerated electrons
We analyze pair production generated by high-energy electrons
emerging from a laser-wakefield accelerator. The pairs are created
in a solid thick high- target, following the electron accelerating plasma
region. Numerical estimates are presented for electron beams obtained presently
in the LBL TW laser experiment \cite{C2} and possible future developments.
Reactions induced by the secondary bremsstrahlung photons dominate the dimuon
production. According to our estimates, a 20 pC electron bunch with energy of 1
(10) GeV may create about 200 (6000) muon pairs. The produced can be
used in studying various aspects of muon-related physics in table top
installations. This may be considered as an important step towards the
investigation of more complicated elementary processes induced by laser driven
electrons.Comment: 14 pages, 5 figure
TeV Scale Seesaw and a flavorful Z' at the LHC
Small neutrino masses and their large mixing angles can be generated at the
TeV scale by augmenting the Standard Model with an additional generation
dependent, anomaly-free U(1)_{nu} symmetry, in the presence of three
right-handed neutrinos. The Z' gauge boson associated with the breaking of the
U(1)_{nu} symmetry can be produced at the LHC. The flavorful nature of the Z'
can be established by measuring its non-universal couplings to the charged
leptons as determined by the lepton's U(1)_{nu} charges, which also govern the
neutrino flavor structure. While the LHC has the potential of discovering the
Z' up to M_{Z'} = 4.5 TeV with 100 fb^(-1) data at the center of mass energy
sqrt{s} = 14 TeV, to establish the flavorful nature of the Z' requires much
higher integrated luminosity. For our bench mark parameters that are consistent
with neutrino oscillation data, at sqrt{s} = 14 TeV, a 5 sigma distinction
between the dielectron and dimuon channels for M_{Z'} = 3 TeV requires 500
fb^(-1) of data. We find that the forward backward asymmetry distributions can
also be useful in distinguishing the dielectron and dimuon channels in the low
invariant mass and transverse momentum regions.Comment: 9 pages, 13 figures; v2: version to appear in Phys. Rev.
Heavy boson production through the collision of an ultrahigh-energy neutrino on a target nucleon
We discuss W and Z production through the deep inelastic neutrino-nucleon
scattering in the context of the standard model SU(3)x SU(2)x U(1) of the
strong and electroweak interactions. We find the cross section rates for the
process neutrino + nucleon --> lepton(-) + W(+) + X for the case of
ultrahigh-energy neutrinos colliding on a target nucleon.Comment: 20 pages, 6 figure
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