Debajyoti Choudhury

Research Summary:

Most of my research activity during the last year was concentrated on (a) identifying signals of physics beyond the Standard Model (SM) at colliders and (b) studying cosmological implications of String Theory inspired higher dimensional models.

• Signals for physics beyond the Standard Model
  • 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.[1], 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) × U(1), as well as extra-dimensional models with a non-factorizable geometry. A possible role for the tt‾H couplings is also discussed.

  • Supersummetry

    Although the scalar partners of the leptons are expected to be amongst the lisghtest of the supersymmetric particles, they can be notoriously difficult to detect at hadronic colliders (such as the LHC). In Ref.[2] we demonstrate that charged slepton pairs produced via vector-boson fusion along with two high-mass, high-p_T forward/backward jets (in two opposite hemispheres) can have a higher production cross-section for heavy slepton masses than that from conventional Drell-Yan production at a hadronic collider. We analyse the signal and leading backgrounds in detail in the minimal supersymmetric standard model with conserved baryon and lepton numbers. Our investigation reveals that the mass reach of the vector-boson fusion channel is certainly an improvement over the scope of the Drell-Yan mode.

  • Higher dimensional theories

    The models of large extra compact dimensions, as suggested by Arkani-Hamed, Dimopoulos and Dvali, predict exciting phenomenological consequences with gravitational interactions becoming strong at the TeV scale. Such theories can be tested at the existing and future colliders. In Ref.[1], we study the contribution of virtual Kaluza-Klein excitations in the process e⁺e^- → tt‾H at future linear collider (NLC). We find that the virtual exchange KK gravitons can modify the cross-section σ(e⁺e^- → tt‾H) significantly from its Standard Model value and will allow the effective string scale to be probed up to 7.9 TeV. Furthermore, such a measurement has great potential in identifying the source of the new physics thus observed.

• String Theory inspired Cosmology

  In a previous exercise, we had analysed the effective action of the tachyon field on a D-brane, of both bosonic as well as superstring theory. We had found that the non-standard kinetic term of the tachyon field requires a correction to the Born-Infeld type Lagrangian. The resulting dynamics had a very interesting cosmological significance, particularly in the context of inflation.

  Continuing in the above direction, we next studied a more realistic string theory inspired model for hybrid inflation [3]. The context is provided by a brane-antibrane system partially compactified on a compact submanifold of (a caricature of) a Calabi-Yau manifold. The interbrane distance acts as the inflaton, whereas the end of the inflationary epoch is brought about by the rapid rolling of the tachyon. The number of e-foldings is sufficiently large and is controlled by the initial conditions. The slow roll parameters, however, are essentially determined by the geometry and have little parametric dependence. Primordial density fluctuations can be made consistent with current data at the cost of reducing the string scale.

List of Research Papers written:

• ZZH Coupling: A Probe to the origin of EWSB?
  D. Choudhury, A. Datta and K. Huitu
  Nucl. Phys. B673 385 (2003).

• Slepton production from gauge boson fusion
  D. Choudhury, A. Datta, K. Huitu, P. Konar, S. Moretti and B. Mukhopadhyaya
  Phys. Rev. D68:075007,2003

• Hybrid Inflation and Brane-Antibrane System
  D. Choudhury, D. Ghoshal, D.P. Jatkar and S. Panda
  Jour. Cosmology and Astropart Phys. 07, 009 (2003).

• Higgs production in association with top quark pair at e⁺e^- colliders in theories of higher dimensional gravity
  D. Choudhury, N.G. Deshpande and D.K. Ghosh
  hep-ph/0311284

Conference/Workshops Attended:

• Meeting on Neutrino Physics, Argonne National Lab, Argonne, USA.

Visits to other Institutes:

• High Energy Physics Division, Argonne National Lab, Argonne, USA (May-Jun 2003).

• Theory Division, Fermi National Accelerator Lab, Batavia, USA (Jul 2003).

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).