461 research outputs found
Nearby resonances beyond the Breit-Wigner approximation
We consider a description of propagators for particle resonances which takes
into account the quantum mechanical interference due to the width of two or
more nearby states that have common decay channels, by incorporating the
effects arising from the imaginary parts of the one-loop self-energies.
Depending on the couplings to the common decay channels, the interference
effect, not taken into account in the usual Breit-Wigner approximation, can
significantly modify the cross section or make the more long-lived resonance
narrower. We give few examples of New Physics models for which the effect is
sizable, namely a generic two and multiple Higgs model and neutral vector
resonances in Higgsless models. Based on these results we suggest the
implementation of a proper treatment of nearby resonances into Monte Carlo
generators.Comment: 14 pages, 3 figure
Interference effects of neutral MSSM Higgs bosons with a generalised narrow-width approximation
Mixing effects in the MSSM Higgs sector can give rise to a sizeable
interference between the neutral Higgs bosons. On the other hand, factorising a
more complicated process into production and decay parts by means of the
narrow-width approximation (NWA) simplifies the calculation. The standard NWA,
however, does not account for interference terms. Therefore, we introduce a
generalisation of the NWA (gNWA) which allows for a consistent treatment of
interference effects between nearly mass-degenerate particles. Furthermore, we
apply the gNWA at the tree and 1-loop level to an example process where the
neutral Higgs bosons and are produced in the decay of a heavy
neutralino and subsequently decay into a fermion pair. The propagator
mixing is found to agree well with the approximation of Breit-Wigner
propagators times finite wave-function normalisation factors, both leading to a
significant interference contribution. The factorisation of the interference
term based on on-shell matrix elements reproduces the full interference result
within a precision of better than 1% for the considered process. The gNWA also
enables the inclusion of contributions beyond the 1-loop order into the most
precise prediction.Comment: 7 pages, 7 figures, Contribution to the proceedings of ICHEP 201
Finite-Width Effects in Top Quark Production at Hadron Colliders
Production cross sections for t\bar{t} and t\bar{t}j events at hadron
colliders are calculated, including finite width effects and off resonance
contributions for the entire decay chain, t --> bW --> b\ell\nu, for both top
quarks. Resulting background rates to Higgs search at the CERN LHC are updated
for inclusive H --> WW studies and for H --> \tau\tau and H --> WW decays in
weak boson fusion events. Finite width effects are large, increasing
t\bar{t}(j) rates by 20% or more, after typical cuts which are employed for
top-background rejection.Comment: 32 pages, 11 figures, 7 tables; minor changes, reference added, to be
published in Phys. Rev.
Spatially Dependent Quantum Interference Effects in the Detection Probability of Charged Leptons Produced in Neutrino Interactions or Weak Decay Processes
Feynman's path amplitude formulation of quantum mechanics is used to analyse
the production of charged leptons from charged current weak interaction
processes. For neutrino induced reactions the interference effects predicted
are usually called `neutrino oscillations'. Similar effects in the detection of
muons from pion decay are here termed `muon oscillations'. Processes considered
include pion decay (at rest and in flight), and muon decay and nuclear
-decay at rest. In all cases studied, a neutrino oscillation phase
different from the conventionally used one is found. A concise critical review
is made of previous treatments of the quantum mechanics of neutrino and muon
oscillations.Comment: 45 pages, 1 table, 3 figures. Supersedes hep-ph/0110064. Consistent
use of MRS matrix to describe charged lepton-neutrino couplings. Unphysical
`lepton flavour eigenstates' removed from the formalis
Gauge-Independent Approach to Resonant Dark Matter Annihilation
In spontaneously broken gauge theories, transition amplitudes describing
dark-matter (DM) annihilation processes through a resonance may become highly
inaccurate close to a production threshold, if a Breit-Wigner (BW) ansatz with
a constant width is used. To partially overcome this problem, the BW propagator
needs to be modified by including a momentum dependent decay width. However,
such an approach to resonant transition amplitudes generically suffers from
gauge artefacts that may also give rise to a bad or ambiguous high-energy
behaviour for such amplitudes. We address the two problems of gauge dependence
and high-energy unitarity within a gauge-independent framework of resummation
implemented by the so-called Pinch Technique. We study DM annihilation via
scalar resonances in a gauged U(1) complex-scalar extension of the Standard
Model that features a massive stable gauge field which can play the role of the
DM. We find that the predictions for the DM abundance may vary significantly
from previous studies based on the naive BW ansatz and propose an alternative
simple approximation which leads to the correct DM phenomenology. In
particular, our results do not depend on the gauge-fixing parameter and are
consistent with considerations from high-energy unitarity.Comment: 29 pages, 9 figures, v2: minor typos corrected, matches published
versio
Interference effects in BSM processes with a generalised narrow-width approximation
A generalisation of the narrow-width approximation (NWA) is formulated which
allows for a consistent treatment of interference effects between nearly
mass-degenerate particles in the factorisation of a more complicated process
into production and decay parts. It is demonstrated that interference effects
of this kind arising in BSM models can be very large, leading to drastic
modifications of predictions based on the standard NWA. The application of the
generalised NWA is demonstrated both at tree level and at one-loop order for an
example process where the neutral Higgs bosons and of the MSSM are
produced in the decay of a heavy neutralino and subsequently decay into a
fermion pair. The generalised NWA, based on on-shell matrix elements or their
approximations leading to simple weight factors, is shown to produce UV- and
IR-finite results which are numerically close to the result of the full process
at tree level and at one-loop order, where an agreement of better than is
found for the considered process. The most accurate prediction for this process
based on the generalised NWA, taking into account also corrections that are
formally of higher orders, is briefly discussed.Comment: 44 pages, 17 figure
Interference effects of neutral MSSM Higgs bosons with a generalised narrow-width approximation
AbstractMixing effects in the MSSM Higgs sector can give rise to a sizeable interference between the neutral Higgs bosons. On the other hand, factorising a more complicated process into production and decay parts by means of the narrow-width approximation (NWA) simplifies the calculation. The standard NWA, however, does not account for interference terms. Therefore, we introduce a generalisation of the NWA (gNWA) which allows for a consistent treatment of interference effects between nearly mass-degenerate particles. Furthermore, we apply the gNWA at the tree and 1-loop level to an example process where the neutral Higgs bosons h and H are produced in the decay of a heavy neutralino and subsequently decay into a fermion pair. The h – H propagator mixing is found to agree well with the approximation of Breit-Wigner propagators times finite wave-function normalisation factors, both leading to a significant interference contribution. The factorisation of the interference term based on on-shell matrix elements reproduces the full interference result within a precision of better than 1% for the considered process. The gNWA also enables the inclusion of contributions beyond the 1-loop order into the most precise prediction
- …