Gautam Bhattacharyya

New LEP bounds on

We use the precision electroweak data at LEP to place bounds on B-violating Yukawa couplings, two theoretically appealing examples being provided by R-parity violating supersymmetry and diquarks. The couplings involving the third generation quarks are most severely constrained. These bounds are complementary to those obtained from low-energy processes.

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D- and τ-decays are used to place bounds on some R-parity-violating λ′-type Yukawa interactions. Some of these bounds are competitive with the existing ones, some are improved while some are new.

violating scalar couplings: R-parity violating supersymmetry or diquarks

We consider one-loop corrections to partial widths of the Z induced by supersymmetric Yukawa interactions that violate R-parity. The precise experimental values of the leptonic Z partial widths bound these Yukawa couplings, with the most interesting constraints being those on couplings involving τ, since previous constraints on them were very mild.

D- AND τ-DECAYS: PLACING NEW BOUNDS ON R-PARITY-VIOLATING SUPERSYMMETRIC COUPLINGS

Abstract The vertex corrections to the leptonic partial widths of the Z induced by leptoquarks that couple leptons to the top quark are considered. We obtain stringent bounds on the parameter space of the masses and Yukawa couplings of these leptoquarks, using the latest information on the Z → l + l − decay widths measured at LEP. Leptoquarks coupling with electroweak strength to top quarks are constrained to be heavier than several hundred GeV, at 95% C.L. As a consequence, such leptoquarks cannot make a significant contribution to lepton asymmetries, τ polarisation asymmetries or A LR .

NEW LEP CONSTRAINTS ON SOME SUPERSYMMETRIC YUKAWA INTERACTIONS THAT VIOLATE R-PARITY

Abstract In the Standard Model (SM), dilepton production in hadron-hadron collisions proceeds through the conventional Drell-Yan mechanism q q → l + l − with the exchange of a gauge boson. Some extensions of the SM contain a quark-lepton contact interaction via a qlΦ Yukawa coupling, where Φ is a scalar. Theories with scalar leptoquarks and R-parity violating SUSY models are the most important examples of such extensions. These Yukawa couplings induce a different dynamical configuration compared to the SM (t-channel vs. s-channel) in the q q → l + l − process and thus offer the possibility of being identifiable upon imposition of suitable kinematic cuts. We discuss these effects in the context of the dilepton production in the CDF experiment, and explore consequences in the forthcoming Large Hadron Collider (LHC).

Bounds on the masses and couplings of leptoquarks from leptonic partial widths of the Z

We consider a minimal extension of the standard model where a real, gauge singlet scalar field is added to the standard spectrum. Introducing the Ansatz of universality of scalar couplings, we are led to a scenario which has a set of very distinctive and testable predictions: (i) the mixing between the standard model Higgs and the new state is near maximal, (ii) the ratio of the two Higgs mass eigenstates is fixed ({approx}{radical}(3)), (iii) the decay modes of each of the two eigenstates are standard model like. We also study how electroweak precision tests constrain this scenario. We predict the lighter Higgs to lie in the range of 114 and 145 GeV, and hence the heavier one between 198 and 250 GeV. The predictions of the model can be tested at the upcoming LHC.

R-parity violating SUSY or leptoquarks: Virtual effects in dilepton production

Review of prospects for discovery of new physics signals at LEP2. The areas covered include SUSY, exotic fermions, BESS models, leptoquarks, virtual effects and CP violating observables.

Universal Doublet-Singlet Higgs Couplings and phenomenology at the CERN Large Hadron Collider

If R-parity is broken and the photino, although unstable, does not decay within the detector, then in new semileptonic B-decay modes a light (∼ 2–3 GeV) photino can be produced carrying missing energy. However, the photino, being massive, arranges a different kinematical configuration for the visible decay products as compared to a standard semileptonic event where the neutrino carries the missing energy. We study the above kinematic distributions in an attempt to explore the above scenario.

Searches for New Physics

Abstract We point out that R ¬ Br(b → dγ) Br(b → sγ) is a sensitive probe of possible violation of CKM unitarity. We compute R in a minimal extension of the Standard Model containing an additional isosinglet charge (− 1 3 ) quark, which leads to a deviation from CKM unitarity.

Searching R-parity-violating supersymmetry in semileptonic B-decays

Abstract We investigate the mixing of an extra Z0′ with the standard Z0 and mixings of exotic fermions with their standard counterparts through some precisely measured electroweak observables. These observables are geared to search for physics beyond the Standard Model. We observe that although most of such mixings are severely constrained by the recent LEP data, some of the mixing angles could still be rather large, awaiting future tests.