Sreerup Raychaudhuri
My doctoral
research was carried out at the Department of Physics of the University
of Calcutta, under the supervision of Amitava Raychaudhuri (no relation)
who now holds the prestigious Palit Professorship of Physics, a chair formerly
held by stalwarts such as C.V. Raman and M.N. Saha. The specific area in
which I worked is a branch of electroweak phenomenology which deals with
a class of particles called Higgs bosons. In the standard electroweak theory
of Glashow, Salam and Weinberg, a single massive scalar boson is predicted,
which goes by the name of the Higgs boson (after Peter Higgs, a British
scientist, who first worked out its significance in electroweak theory).
There are many forms of electroweak theory which go beyond the Standard
Model, and most of these predict not one but several Higgs bosons. Sometimes,
these are carry electric charge, unlike the one in the Standard Model,
which is uncharged. Such charged Higgs bosons exist, for example,
in the minimal supersymmetric version of the Standard Model (which balances
the bosonic and fermionic degrees of freedom in the Standard Model) which
is one of the most popular ways of going beyond the Standard Model. None
of these Higgs bosons have been discovered yet. It is assumed, therefore,
that if these particles exist, then they must be very massive, so that
it is not possible to produce them at laboratory energies. There
are two routes to discovery:
-
to build
bigger particle accelerators with higher energies and actually produce
the particles;
-
to look
for rare events, allowed in quantum field theory, in which such a particle
appears and disappears as an intermediate (virtual) state and modifies
the frequency with which the events occur.
The first
option is clearly not suitable for a Ph.D. project, since a typical accelerator
costs billions of dollars and requires huge teams of scientists just to
analyse the experimental data. Such projects are actually on, financed
by the USA, Japan and the EC. However, the second option is obviously easier,
since all it requires is to be able to do a type of computation in quantum
field theory called one-loop Feynman diagram calculations. It was this
skill which I learnt and applied to various processes during my doctoral
work.
This work was
carried out under a Junior Research Fellowship (upgraded to Senior Research
Fellowship after two years) awarded by the University Grants Commission
(UGC), Government of India on the basis of my performance in the National
Efficiency Test (NET) conducted in 1986.
A
more technical account
My
doctoral work concerned processes involving one-loop diagrams leading to
signals for Higgs bosons in models beyond the Standard Model at various
energies. In pursuit of this, I first investigated bounds on charged Higgs
bosons in supersymmetric models from the low-energy processs B ->
K*
and B0-anti
B0
mixing. Bounds on the parameters of these models were obtained. After submission
of my thesis, the process B -> K*
was indeed observed and today provides the most stringent bounds on models
with charged Higgs bosons. I also investigated the rare process Z0
-> b anti s at LEP energies and found bounds from this too.
At the same LEP energies I studied the possibility of producing and detecting
singlet Higgs bosons coupling to vectorlike fermions and showed that such
processes are very little constrained by LEP. Finally I made a detailed
calculation of the rare process H±
->
W±
in a supersymmetric model which involved the evaluation of more than 100
one-loop diagrams. The net result could be useful in detecting a charged
Higgs boson heavier than the top quark at the high energies and luminosities
of the CERN Large Hadron Collider.