V. Ravindran


Research Summary:

Higher order QCD corrections to various strong interaction processes at hadron colliders are very important to reduce theoretical uncertainties of various cross sections measured. These experimental results are used to study the structure of hadrons and also to discover/rule out the existence of new particles. For example, the RHIC at BNL is going to do experiments with polarised hadron beams to study the spin structure of the proton and similarly Tevetron at Fermilab and LHC at CERN are going to look out for the most important particle of Standard Model, namely Higgs which is responsible for spontaneous symmetry breaking mechanism. It is also important to study the effect of higher QCD corrections to beyond standard model predictions.

For spin-RHIC experiment, we computed the full next-to-leading order (NLO) corrected inclusive cross section $d^3\Delta \sigma/dQ^2/dy/dp_T$ for massive lepton pair production in longitudinally polarized proton-proton collisions $p + p\to l^+l^- + 'X'$. Here $'X'$ denotes any inclusive hadronic state and Q represents the invariant mass of the lepton pair which has transverse momentum $p_T$ and rapidity y. All QCD partonic subprocesses have been included provided the lepton pair is created by a virtual photon, which is a valid approximation for $Q<50{\rm GeV}$. Like in unpolarized proton-proton scattering the dominant subprocess is given by $q(\bar q) + g \to \gamma^* + 'X'$ so that massive lepton pair production provides us with an excellent method to measure the spin density of the gluon. Our calculations are carried out using the method of n-dimensional regularization by making a special choice for the $\gamma_5$-matrix. Like in the case of many other prescriptions evanescent counter terms appear. They are determined by computing the NLO coefficient functions for $d\Delta\sigma/dQ^2$ and the polarized cross section for Higgs production using both n-dimensional regularization and a four dimensional regularization technique in which the $\gamma_5$-matrix is uniquely defined. Our calculations reveal that the non-singlet polarized coefficient function equals the unpolarized one up to a minus sign. We give predictions for double longitudinal spin asymmetry measurements at the RHIC.

For Higgs physics at Tevetron and LHC, we computed the next-to-next-to-leading order (NNLO) corrections to the total cross section for (pseudo-) scalar Higgs boson production using an alternative method than those used in previous calculations. All QCD partonic subprocesses have been included and the computation is carried out in the effective Lagrangian approach which emerges from the standard model by taking the limit $m_t \to \infty$ where $m_t$ denotes the mass of the top quark. Our results agree with those published earlier in the literature. We estimate the theoretical uncertainties by comparing the $K$-factors and the variation with respect to the mass factorization/renormalization scales with the results obtained by lower order calculations. We also investigate the dependence of the cross section on several parton density sets provided by different groups. Further we study which part of the coefficient functions dominates the cross section. This is of interest for the resummation of large corrections which occur near the boundary of phase space. It turns out that depending on the definition of the total cross section the latter is dominated by the soft-plus-virtual gluon corrections represented by $\delta(1-x)$ and $(\ln^i(1-x)/(1-x))_+$ terms.

In the beyond standard model physics, we considered resonant production of sneutrino and slepton at hadronic colliders such as the Tevatron and the LHC within the context of a R-parity violating supersymmetric model. We present 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 cross sections as compared to the LO. The results obtained in this article are also applicable to resonant production of any color-neutral scalar.


Publications:


1) NLO Differential Distributions of Massive Lepton Pair Production in Longitudinally polarized Proton Proton collisions.

by V. Ravindran, J. Smith, W.L. van Neerven, YITP-SB-02-32, INLO-PUB-4-02, Jul 2002. 54pp. Nucl.Phys.B647:275-318,2002, e-Print Archive: hep-ph/0207076

2) QCD Corrections to Resonant slepton production in hadron colliders.

By Debajyoti Choudhury, Swapan Majhi, V. Ravindran MRI-P-020704, Jul 2002. 21pp. Nucl.Phys.B660:343-361,2003 e-Print Archive: hep-ph/0207247

3) NNLO corrections to the total cross-section for Higgs boson production in hadron hadron collisions.

By V. Ravindran, J. Smith, W. L. van Neerven YITP-SB-03-02, INLO-PUB-01-03, Feb 2003. 58pp. Nucl. Phys.B(in print) e-Print Archive: hep-ph/0302135

4) Working Group Report: Quantum Chromodynamics and Hadronic Structure.

By R. Basu, V. Ravindran. 2003. Published in Pramana 60:401-404,2003


Conference/Workshops Attended:

1) Linear Collider Meeting held at TIFR Mumbai, January 1-2, 2003.

2) IX International Conference on Particles, Strings and Cosmology, PASCOS'03,TIFR, Mumbai, January 3-8, 2003


Visits to other Institutes:

1) Instituut-Lorentz, University of Leiden, The Netherlands, June, 2002.

2) CERN, Geneva, Switzerland, July, 2002.

3) ICTP, Trieste, Italy, August 2002.

4) DESY-Zeuthen, Germany, September-November 2002.


Invited Lectures/Seminars:

1) "Differential distribution for Higgs production at LHC and Tevetron" DESY-Zeuthen, Germany.

2) "Polarised Observables at Linear Colliders" Linear Collider Meeting at TIFR, Mumbai.

3) "NNLO QCD corrections to total Higgs production cross section at LHC" PASCOS'03, TIFR, Mumbai.



© Harish-Chandra Research Institute