864 research outputs found
Longitudinal top polarisation measurement and anomalous coupling
Kinematical distributions of decay products of the top quark carry
information on the polarisation of the top as well as on any possible new
physics in the decay of the top quark. We construct observables in the form of
asymmetries in the kinematical distributions to probe their effects.
Charged-lepton angular distributions in the decay are insensitive to anomalous
couplings to leading order. Hence these can be a robust probe of top
polarisation. However, these are difficult to measure in the case of highly
boosted top quarks as compared to energy distributions of decay products. These
are then sensitive, in general, to both top polarisation and top anomalous
couplings. We compare various asymmetries for their sensitivities to the
longitudinal polarisation of the top quark as well as to possible new physics
in the vertex, paying special attention to the case of highly boosted top
quarks. We perform a - analysis to determine the regions in the
longitudinal polarisation of the top quark and the couplings of the
vertex constrained by different combinations of the asymmetries. Moreover, we
find that use of observables sensitive to the longitudinal top polarisation can
add to the sensitivity to which the vertex can be probed.Comment: significantly revised version, clarifications on the term
'polarisation' added, new references added, the title modified, 41 figures.
Accepted for publication in EPJ
Fermion Dipole Moments in Supersymmetric Models with Explicitly Broken R-parity
We present a simple analysis that allows us to extract the leading mass
dependence of the dipole moment of matter fermions that might be induced by new
physics. We present explicit results for the supersymmetric model with broken
R-parity as an illustration. We show that the extra contributions to the
electric dipole moment (edm) of fermions from R-parity violating interactions
can occur only at two loop level, contrary to claims in the literature. We
further find that unlike the generic lepto-quark models, the extra
contributions to the dipole moments of the leptons can only be enhanced by
m_b/m_l and not by m_t/m_l relative to the expectations in the Standard Model.
An interesting feature about this enhancement of these dipole moments is that
it does not involve unknown mixing angles. We then use experimental constraints
on the electric dipole moments of electron and neutron to obtain bounds on (the
imaginary part of) products of R-parity violating couplings, and show that
bounds claimed in the literature are too stringent by many orders of magnitude.Comment: 16 pages latex, 7 figure
Gauge boson fusion as a probe of inverted hierarchies in supersymmetry
Supersymmetric scenarios with inverted mass hierarchy can be hard to observe
at a hadron collider, particularly for the non-strongly interacting sector. We
show how the production of stau-pairs via gauge boson fusion, along with hard
jets in the high rapidity region, can be instrumental in uncovering the
signatures of such scenarios. We demonstrate this both in a model-independent
way and with reference to some specific, well-motivated models.Comment: RevTeX4, 4 pages, 2 figures. Final version to appear in Phys.Rev.D
Changes in context, figures modified. References added. Conclusions unchange
Processes at High Energy pp Colliders
In this note we investigate the production of charged heavy particles via
\gaga\ fusion at high energy pp colliders. We revise previous claims that the
\gaga\ cross section is comparable to or larger than that for the corresponding
Drell-Yan process at high energies. Indeed we find that the \gaga\ contribution
to the total production cross section at pp is far below the Drell-Yan cross
section. As far as the individual elastic, semi-elastic and inelastic
contributions to the \gaga\ process are concerned we find that they are all of
the same order of magnitude.Comment: REVTEX, 12 pages, two uuencoded figures appended at the end of the
fil
Top polarisation studies in and production
The polarisation of top quarks produced in high energy processes can be a
very sensitive probe of physics beyond the Standard Model. The kinematical
distributions of the decay products of the top quark can provide clean
information on the polarisation of the produced top and thus can probe new
physics effects in the top quark sector. We study some of the recently proposed
polarisation observables involving the decay products of the top quark in the
context of and production. We show that the effect of the top
polarisation on the decay lepton azimuthal angle distribution, studied recently
for these processes at leading order in QCD, is robust with respect to the
inclusion of next-to-leading order and parton shower corrections. We also
consider the leptonic polar angle, as well as recently proposed energy-related
distributions of the top decay products. We construct asymmetry parameters from
these observables, which can be used to distinguish the new physics signal from
the background and discriminate between different values of
and in a general type II two-Higgs doublet model. Finally, we show
that similar observables may be useful in separating a Standard Model
signal from the much larger QCD induced top pair production background.Comment: 33 pages, 35 figures, references adde
Boosting Higgs CP properties via VH Production at the Large Hadron Collider
We consider ZH and WH production at the Large Hadron Collider, where the
Higgs decays to a bb pair. We use jet substructure techniques to reconstruct
the Higgs boson and construct angular observables involving leptonic decay
products of the vector bosons. These efficiently discriminate between the
tensor structure of the HVV vertex expected in the Standard Model and that
arising from possible new physics, as quantified by higher dimensional
operators. This can then be used to examine the CP nature of the Higgs as well
as CP mixing effects in the HZZ and HWW vertices separately.Comment: 5 pages, 3 figures. v2: two references added and typo correcte
Probing CP-violating Higgs contributions in gamma-gamma -> f anti-f through fermion polarization
We discuss the use of fermion polarization for studying neutral Higgs bosons
at a photon collider. To this aim we construct polarization asymmetries which
can isolate the contribution of a Higgs boson in , , from that of the QED continuum. This can help in getting
information on the coupling in case is a CP
eigenstate. We also construct CP-violating asymmetries which can probe CP
mixing in case has indeterminate CP. Furthermore, we take the MSSM with
CP violation as an example to demonstrate the potential of these asymmetries in
a numerical analysis. We find that these asymmetries are sensitive to the
presence of a Higgs boson as well as its CP properties over a wide range of
MSSM parameters. In particular, the method suggested can cover the region where
a light Higgs boson may have been missed by LEP due to CP violation in the
Higgs sector, and may be missed as well at the LHC.Comment: 14 pages, 14 figures, typeset in revtex4. Version which has appeared
in Physical Review D; typos in two references correcte
Survey of vector-like fermion extensions of the Standard Model and their phenomenological implications
With the renewed interest in vector-like fermion extensions of the Standard
Model, we present here a study of multiple vector-like theories and their
phenomenological implications. Our focus is mostly on minimal flavor conserving
theories that couple the vector-like fermions to the SM gauge fields and mix
only weakly with SM fermions so as to avoid flavor problems. We present
calculations for precision electroweak and vector-like state decays, which are
needed to investigate compatibility with currently known data. We investigate
the impact of vector-like fermions on Higgs boson production and decay,
including loop contributions, in a wide variety of vector-like extensions and
their parameter spaces.Comment: 43 pages, 17 figures; v2: text modified to improve readability,
references added, journal versio
Why the angular distribution of the top decay lepton is unchanged by anomalous coupling
We give a simple physical argument to understand the observation that the
angular distribution of the top decay lepton depends only on the polarisation
of the top and is independent of any anomalous coupling to linear order.Comment: 4 pages, 2 figures. Added a footnote and consequently a few
reference
Jet substructure and probes of CP violation in Vh production
We analyse the hVV (V = W, Z) vertex in a model independent way using Vh
production. To that end, we consider possible corrections to the Standard Model
Higgs Lagrangian, in the form of higher dimensional operators which parametrise
the effects of new physics. In our analysis, we pay special attention to linear
observables that can be used to probe CP violation in the same. By considering
the associated production of a Higgs boson with a vector boson (W or Z), we use
jet substructure methods to define angular observables which are sensitive to
new physics effects, including an asymmetry which is linearly sensitive to the
presence of CP odd effects. We demonstrate how to use these observables to
place bounds on the presence of higher dimensional operators, and quantify
these statements using a log likelihood analysis. Our approach allows one to
probe separately the hZZ and hWW vertices, involving arbitrary combinations of
BSM operators, at the Large Hadron Collider.Comment: 37 pages, 17 figures; v3 matches published versio
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