Debajyoti Choudhury


Research Summary:

Most of my research activity during the last year was concentrated on (a) calculating radiative (QCD) corrections to electroweak processes and (b) looking for signals of physics beyond the Standard Model (SM) at colliders. It is obvious that the two areas are interrelated, for an accurate calculation of the SM processes is a must for any discovery of new physics to be made.

  • QCD Corrections to electrweak processes

    • A very intriguing channel for the production of supersymeetric particles at the currently operating and forthcoming hadronic colliders consists of resonant processes. In Ref.[3], we calculate the next to leading order QCD corrections to total cross sections which originate from both quark- as well as gluon-initiated processes. For couplings involving only the first generation quarks, the K factor at the Tevatron can be as large as 1.5 for a 100 GeV sfermion and falls to nearly 1.1 as the sfermion mass reaches 1 TeV. At the LHC, the variation is between 1.2 and 1.45 for masses less than 2 TeV. While the dependence on the parton density parametrization is found to be mild, this ceases to be true if the strange quark plays a dominant role in the production process. We also study the renormalization and factorization-scale dependence and find it to be less pronounced for the NLO cros sections as compared to the LO. The results obtained in this article are also applicable to resonant production of any color-neutral scalar.

      This result was recently used by the CDF collaboration at the Tevatron in their search strategies [hep-ex/0307012]. Further experimental work is in progress.

    • The interaction of ultra-high energy neutrinos with matter has, of late, attracted a lot of interest. Several international experiments designed to detect them are already in place and many more are being planned. In Ref.[1], we reconsider the Standard Model interactions of such neutrinos. The next to leading order QCD corrections are presented for charged-current and neutral-current processes. Contrary to popular expectations, these corrections are found to be quite substantial, especially for very large (anti-) neutrino energies. Hence, they need to be taken into account in any search for new physics effects in high-energy neutrino interactions. In our extrapolation of the parton densities to kinematical regions as yet unexplored directly in terrestrial accelerators, we are guided by double asymptotic scaling in the large $Q^2$ and small Bjorken $x$ region and to models of saturation in the low $Q^2$ and low $x$ regime. The sizes of the consequent uncertainties are commented upon. We also briefly discuss some variables which are insensitive to higher order QCD corrections and are hence suitable in any search for new physics.

  • Supersymmetry

    • In Ref.[2], we study the production and decays of top squarks at the Tevatron collider in models of low-energy supersymmetry breaking (which are related to, though not confined to, the models with Gauge Mediated Supersymmetry Breaking). We consider the case where the lightest standard model superpartner is a light bino, which predominantly decays into a photon and a light gravitino. Considering the stops to be the next-to-lightest standard model superpartner, we analyze stop signatures associated with jets, photons and missing energy, which lead to signals naturally larger than the associated SM backgrounds. We consider 2-body and 3-body decays of the top squarks and show that the reach of the Tevatron can be larger than in the standard supergravity models. For a modest projection of the final Tevatron luminosity, ${\cal{L}} \simeq 4$ $fb^{-1}$, stop masses of about 300 GeV are found to be easily accessible at the Tevatron collider in both 2-body and 3-body decay modes.

    • The presence of relatively light top squarks (in fact, even of other squarks) may lead to significant deviations in dilepton production at hadronic colliders, especially if $R$-parity is broken. Concentrating on dimuon production at the LHC, we perform [4] an evaluation of the optimal sensitivity to the $R$-violating coupling through a maximum likelihood analysis. The measurement uncertainties are evaluated through a study of fully generated events processed through a fast simulation of the ATLAS detector. It is found that a host of $R$-violating couplings can be measured to a statistical accuracy of better than 10%, over a significant part of the $m_{\tilde f}$ - $\lambda$ parameter space still allowed by low energy measurements. Since the bounds thus obtained do not simply scale as the squark mass, one can do significantly better at the LHC than at the Tevatron. The same analysis can also be extended to assess the reach of the LHC to effects due to any non-SM structure of the four-fermion amplitude, caused by exchanges of new particles with different spins such as leptoquarks and gravitons that are suggested by various theoretical id

    • In supersymmetrics models, it is sometimes the case that the lightest chargino is nearly degenerate with the lightest neutralino. Popular examples are theories with Anmoaly-Mediated Supersymmetry breaking or supergravity-inspired models wherein the gauginos are much heavier than the higgsinos. In such cases, the chargino tends to decay into the neutralino alongwith a soft pion (or a lepton-neutrino pair). Near degeneracy of the chargino and neutralino masses can cause the other decay products (the pion or the lepton) to be almost invisible, thereby rendering ineffective the usual search strategies. In Ref.[1], we demonstrate that photon-photon colliders offer a possibility of clean detection of such an event through a hard photon tag.

    • Continuing on theeories with anomaly mediated supersymmetry breaking, we examine the capability of an $e^- \gamma$ collider in unravelling such scenarios. We demonstrate [5] that the associated production of a sneutrino with the lightest chargino leads to a substantially large signal size. The background is negligibly small, though. Even more interestingly, a measurement of the fundamental supersymmetry breaking parameters could be possible.

  • Higgs physics
    • At future experimental facilities (the LHC as well as linear colliders), it might be possible to not only discover Higgs particles but to measure their couplings as well. In Ref.[2], we argue that the $ZZH$ coupling constitutes a simple probe of the nature of the scalar sector responsible for electroweak symmetry breaking. We demonstrate the efficacy of this measure through an analysis of four-dimensional models containing scalars in arbitrary representation of $SU(2) \times U(1)$, as well as extra-dimensional models with a non-factorizable geometry. A possible role for the $t \bar t H$ couplings is also discussed.



List of Research Papers written during this period

  1. Radiative Production of Invisible Charginos in Photon Photon Collisions
    D. Choudhury, B. Mukhopadhyaya, S. Rakshit and A. Datta
    hep-ph/0205103.
    Jour. High Energy Phys. 0301, 069 (2003).

  2. Top Squark Searches at the Tevatron in models of low-energy Supersymmetry Breaking
    M. Carena, D. Choudhury, R.A. Diaz, H.E. Logan and C.E.M. Wagner
    hep-ph/0206167.
    Phys. Rev. D66:115010 (2002)

  3. QCD Corrections to Resonant Slepton Production in hadron colliders.
    D. Choudhury, S. Majhi and V. Ravindran
    hep-ph/0207247.
    Nucl. Phys. B660, 343 (2003)

  4. Measuring $R$-parity violating couplings in Dilepton production at the LHC
    D. Choudhury, R.M. Godbole and G. Polesello
    hep-ph/0207248
    Jour. High Energy Phys. 0208, 004 (2002).

  5. NLO Corrections to Ultrahigh-Energy Neutrino Nucleon Scattering, Saturation and Small $x$.
    R. Basu, D. Choudhury and S. Majhi
    hep-ph/0208125.
    Jour. High Energy Phys. 0210, 012 (2002).

  6. Signals of Anomaly Mediated Supersymmetry Breaking in an $e \gamma$ Collider
    D. Choudhury, D.K. Ghosh and S. Roy
    hep-ph/0208240.
    Nucl. Phys. B646, 3, (2002).

  7. $ZZH$ Coupling: A Probe to the origin of EWSB?
    D. Choudhury, A. Datta and K. Huitu
    hep-ph/0302141



Conference/Workshops Attended:

  • 5th Nordic LHC Meeting, Helsinki.
  • Workshop on ``Large Hadron Collider - Linear Collider Comparison'', CERN, Geneva.
  • Meeting of the ``CLIC Physics Study Group'', CERN, Geneva.
  • 2nd Meeting of ``Indian Linear Collider Working Group'', TIFR, Mumbai.
  • PASCOS 2003, TIFR, Mumbai.
  • Miniconf on Linear Colliders, HRI.
  • TP2003 at IACS, Calcutta.

Visits to other Institutes:

  • Helsinki Institute for Physics, U. of Helsinki.
  • Max-Planck-Institut für Physik, Munich.
  • Department of Physics, U. of Wien.
  • Indian Association for the Cultivation of Science, Calcutta.
  • Department of Physics, U. of Delhi.


Invited Lectures/Seminars:

  • ``Heavy Higgs Bosons at a Linear Collider'', U. of. Helsinki.

  • ``Cosmology in Higher Dimensional Theories'', Max-Planck-Institut für Physik, Munich.

  • ``Linear Collider as a Higgs Analyser'', U. of Vienna.

  • ``Magnetic Moment of the Muon : What have we learnt ?'', IACS, Calcutta.

  • ``Heavy Higgs Bosons'', U. of Calcutta.

  • Brane Dynamics in the Randall-Sundrum model, Inflation and Graceful Exit, PASCOS 2003 (Parallel Session).

  • ``Heavy Higgs Bosons at a Linear Collider'', PASCOS 2003 (Parallel Session).

Other Activities:

Courses taught

  • ``Computational Physics and Numerical Analysis''.
  • Part of ``Mathematical Physics''

Students guided

Mr. Swapan K. Majhi is currently pursuing his Ph.D. under my supervision.
Apart from this, I also interact on a regular basis with Mr. Partha Konar (student of Prof. B. Mukhopadhyaya).

Conference organization

Organized ``Miniconf on Linear Colliders'' at HRI.

 



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