The list of research topics that I have been working on is:
- Beyond the Standard Model phenomenology at the Large Hadron Collider
(LHC), International Linear Collider (ILC) and Photon Linear Collider
(PLC),
- Characterization of (non-)standard Higgs bosons at various colliders,
- CP violation in various fundamental interactions,
- Polarization/spin measurement of top quark and other fundamental
particles,
- Role of beam polarization at ILC and PLC,
- Phenomenology of KK-excitations in various extra-dimension models.
Introduction
The standard model (SM) of particle physics has been tested to an extremely
high degree of accuracy, reaching its high point in the precision measurements
at the CERN e+e−
collider LEP. However, the bosonic sector of the SM is not
complete, the Higgs boson is yet to be found. A direct experimental
demonstration of the Higgs mechanism of the fermion mass generation still
does not exist. Thus the discovery of the Higgs boson, a study of its
properties and the subsequent study of electroweak symmetry breaking (SSB)
mechanism is one of the prime aims of all the
current and next generation colliders.
The standard model of particles physics is the best tested model, still fails
to answer some fundamental questions.
Even after a great experimental success, the SM suffers from some theoretical
problems such as:
- instability of the Higgs boson mass against radiative correction,
- hierarchy of the fundamental scales,
- hierarchy in the masses of fermions,
- mechanism to break the electroweak symmetry,
- mechanism of CP violation etc.
Some or all of these problems are addressed in various models
of physics beyond the SM, such as models with extra-dimensions,
with supersymmetry, or with other additional symmetries. Each of these model
invariably propose new heavy particles that can potentially be produced at the
upcoming LHC. Thus LHC will be a usefull machine to distinguish among the
various models of new physics.
The approach
The usual approach to new physics is top-down and centered around
a model of choice.
Most of the approaches towards new physics has been
top-down, i.e. model
building beyond the SM respecting the existing experimental bounds and
addressing various issues listed in the previous paragraph. The merit of this
approach is that it provides mechanism to address/solve the problems with the
SM. However, the possible demerits are:
- it may not be a testable theory,
- it may have a rich spectrum with many free parameters,
- it may bring some more un-solved problems than it has solved etc.
Another approach to new physics is
bottom-up, i.e.
phenomenological model building based on fundamental symmetries and
experimental data. Though this method may not give a first principle
understanding of the underlying phenomenon, it provides one with enough
inputs to constrain/direct the
top-down approach.
The bottom-up approach, on the other hand, is process specific and
independent of the model. It can be handy in directing top-down approach.
Unlike the
top-down approach, the issues of pure theoretical nature
are of very less or no importance in a
bottom-up
approach. For example, even the
renormalizibility of the corresponding Quantum Field Theory (QFT) is not
a requisite. Usually the new particle or interaction of interest is
parameterized in terms of additional couplings and form-factors. Then one
constructs a set of observables and develops a strategy to measure the
proposed couplings, which in certain limits may imitate the SM scenario. The
task here is to look for as many observables as possible, which can be
constructed at a given collider. And then use them in a strategic way to
extract maximum information about the underlying dynamics. Some of the
observables are of very generic types and can be used for different
colliders and processes, while some are very specific to either the collider
or the process or both.
I have worked on some observables, like asymmetries sensitive to
polarization of top-quark, which can be used at all the colliders in any
process of top quark production process. While some other observables, as
simple as forward-backward asymmetry, may not be available at symmetric
collider, such as LHC and PLC. Thus the strategy part of the analysis very
much depends upon the choice of the collider, process and, of course, the
nature of the new physics phenomenon one is looking for.
The new physics is expected in Higgs and top sector of the SM.
Since the Higgs boson
of the SM is not yet discovered, the phenomenon of electro-weak symmetry
breaking may, in principle, be more intricate than what is suggested in the SM.
Further the top-quark mass is very close to the electro-weak scale, leads to
the possibility of it having a large sensitivity towards new physics of
electro-weak symmetry breaking. Thus top quark processes are most important for
new physics studies.
Top quark: a looking glass
Decay lepton angular distribution is a pure probe of new physics in production
of top quark independent of possible new physics in the decay process.
The top quark is very heavy and hence decays much before hadronization. Thus
its spin information is translated into the kinematical distribution of its
decay products. At the same time its decay width is very small as compared to
its mass leading to a sharp resonance, which helps reducing the backgrounds.
This also makes it possible to use
narrow width approximation and
simplify the calculations. With the use of narrow width approximation it has
been shown (by me and collaborators) that the angular distribution of the decay
leptons from top quark is insensitive to the anomalous contribution to the
tbW vertex in all processes of top quark production at all colliders.
Hence the lepton distribution is sensitive only to the
distribution and polarization of the produced top quarks, which is decided by
the production dynamics. Thus, use of lepton distribution provides us with a
tool to look at possible new physics in top quark production process without
any contamination from possible new physics in the decay process. This result
is modified only at
per mill level due to radiative correction, hence
a very useful observable for a
bottom-up approach.
The only drawback in using lepton distribution is the low branching ratio of
W-boson into leptonic channel. This can potentially be improved by use of
down-type light quarks in place of leptons. This, however, has two problems:
Hadronic decay channels of top-quark receive large radiative corrections.
- the charge determination of light quarks is not easy and
- the radiative correction to the above mentioned "insensitivity" can be as
large as 7% in the hadronic channels.
The possibility to use hadronic channels is under
investigation. These distribution are a tool to measure the polarization of
the top-quark with a high accuracy and can be employed to analyze CP-violating
and/or P-violating interactions.
P/CP violation and polarization
Polarization and spin correlations are very useful tools to probe and
distinguish among various new physics candidates.
The phenomenon of P/CP violation is expected to appear in possible new physics
as required by the baryon asymmetry of the Universe. The
bottom-up
approach mentioned above includes P/CP violation. Further, in many cases, P/CP
observables involve the polarization and/or spin correlation of some of the
particles. This requires the
measurement of polarization of fundamental particles wherever possible. The
polarization of τ-lepton has been studied in detail and I have worked on
possible measurement of top quark's polarization using as simple observables
as possible. These observables are employed to study P/CP properties of various
interactions in a simple and feasible way. The azimuthal distribution of
secondary lepton from decay of top quark is sensitive its polarization. It
has been used to probe Higgs boson interactions at PLC and also the
interaction of KK-excitations in various extra-dimension models, which have a
P-violating interaction. In fact, using lepton-polarimeter for top quarks it is
possible to study heavy, neutral vector boson and distinguish between various
model candidate, such as ADD model, RS model, little Higgs model, technicolor
models etc.
The beam polarization is an important tool to probe new things or simply
improve the sensitivity.
The polarization is an additional parameter that can be tuned for initial beams
and can be measured for some of the final state particles. This additional
parameter is very useful in providing additional information and in some cases
simply magnifying the sensitivity. I have studied the role of longitudinal and
transverse beam polarization at LC for Higgs boson and triple gauge boson
couplings. It turns out that if it is not possible to study the polarization of
the produced particles (by means of kinematical distribution) then one should
put in beam polarization and measure simple kinematical distributions. A
strategic combination of different initial state polarizations, longitudinal
and/or transverse, is instrumental in giving otherwise unattenable
informations.
Markov-Chain-Monte-Carlo
The MCMC analysis maps the avialable parameter space of a given model in a
top-down approach.
In the
top-down approach to new physics the most prominant models are
Minimal Supersymmetric Standard Model (MSSM) and warped extra dimension model
like Randal-Sundrum (RS). Both this models receive constraints from the
existing collider data and also from B-physics observable and cosmological
observations. It is thus instructive to know the parts of parameter space of
these models that comply with the existing constraints. To this end, recently,
I have been working on
constraining phenomenological MSSM by low energy observables, electro-weak
precision observables in a Markov-Chain-Monte-Carlo approach. The next step in
this analysis will be to look for the (possibly correlated) collider signature
of the constrained parameter space. A similar study has also been performed
for the RS model using a very small set of collider observables. A more
elaborate study of the RS parameter space using MCMC method is still lacking
and it requires a fair bit of research to
set-up such an analysis.
Spin measurements at colliders
Spin measurments can distinguish between SUSY and non-SUSY models having similar
collider signature.
The MSSM and the extra-dimesional models compete with each other not just in
providing the explaination to the un-resolved issues with the SM but also
in having almost identical collider signature for many of its prominant
processes with only difference being the spins of the particles involved. It
is thus very important to have method to measure the spins of the particles at
hadronice colliders like LHC. I have worked in this direction and have found
a set asymmetries that can help one assess the spins of the particle if the
momentum of the particle is reconstructed. This, however, is not always the
case for the various processes at LHC in either MSSM or RS models. More methods
need to be discovered and I am actively persueing this line of research.
Bottom line: In the bottom-up approach
the polarization studies and use of beam polarization at LC and
PLC are very useful in characterization of new particles and the polarization
studies at LHC is very useful to distinguish among various new physics
models. To this end, recently, I have been working on spin/polarization
measurment of heavy particles and constraining a generic SUSY models by low
energy observables, electro-weak precision observables in a
Markov-Chain-Monte-Carlo approach.