arXiv subjects
Publications and source records attributed to Gautam Bhattacharyya.
We derive stringent upper bounds on all the $(λ''_{ijk} μ_l)$-type combinations from the consideration of proton stability, where $λ''_{ijk}$ are baryon-number-violating trilinear couplings and $μ_l$ are lepton-number-violating bilinear mass parameters in a R-parity-violating supersymmetric theory.
We have derived new upper limits on several products (two at a time) of lepton number violating $λ'$-type couplings from the consideration of $ΔS = 2$ and $ΔB = 2$ box graphs. Each box contains one scalar lepton and one W-boson or one charged-Higgs-boson as internal lines. Most of these bounds are more stringent than previously obtained. Some of these product couplings drive $K_L$ (and some other $B_d$) decays to two charged leptons at enhanced rates. Some of them can explain the rare $K^+ \to π^+ ν\barν$ event recently observed at BNL.
If R-parity violation turns out to be a true aspect of Nature, a speculation about its possible origin could add a new dimension to the supersymmetric flavour problem. It has been shown in the past by Barbieri, Hall and their collaborators that the small breaking parameters of an approximate non-abelian flavour symmetry could govern the light quark and lepton masses and at the same time could account for the near degeneracies of squarks and sleptons. A possible connection of the above feature to the natural suppressions of R-parity-violating couplings has been investigated here. With some modifications of the approximate flavour symmetry, a supersymmetric theory without R-parity has been motivated that has testable experimental signatures.
I review the upper limits on the R-parity-violating Yukawa couplings from indirect searches. Some limits have been updated using recent data.
The question of naturalness is addressed in the context of gauge-mediated supersymmetry breaking models. Requiring that $M_Z$ arises naturally imposes upper limits on the right-handed selectron mass in these models that are stronger than in the Minimal Supersymmetric Standard Model and are interesting from the point of view of searches at the current and future colliders.
I review the bounds on the $R$-parity-violating supersymmetric Yukawa couplings from the considerations of proton stability, $n$--$\bar{n}$ oscillation, $ν_e$-Majorana mass, neutrino-less double $β$ decay, charged-current universality, $e$--$μ$--$τ$ universality, $ν_μ$--$e$ scattering, atomic parity violation, $ν_μ$ deep-inelastic scattering, $K^{+}$-decays, $τ$-decays, $D$-decays and from the precision LEP electroweak observables. I also mention about the sparticle bounds at colliders when the assumption of $R$-parity-conservation is relaxed. Finally, I mention how $R$-parity-violating models have been invoked in an attempt to explain the reported excess in ALEPH 4-jet events.
The $\eegg$ event reported recently by the CDF Collaboration has been interpreted as a signal of supersymmetry in several recent papers. In this article, we report on an alternative non-supersymmetric interpretation of the event using an extension of the standard model which contains new physics at the electroweak scale that does not effect the existing precision electroweak data. We extend the standard model by including an extra sequential generation of fermions, heavy right-handed neutrinos for all generations and an extra singly charged SU(2)-singlet Higgs boson. We discuss possible ways to discriminate this from the standard supersymemtric interpretations.
From non-observation at LEP1, a lower mass limit of 45 GeV has been established on any additional sequential fermion beyond the three generations. Precision measurements have further constrained the number of such additional generations, either degenerate or nearly so, to be at most one. LEP2, an energy-upgraded version of LEP1, would provide greater mass-reach in the search for such particles. We study the pair-production of fourth-generation leptons for various LEP2 energy options. We find that in most cases such particles could be discovered/ruled out up to the kinematic limit.
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 ($\sim$ 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.
The experimental values of $R_b$ and $R_c$ are the only data which do not seem to agree with Standard Model predictions. Although it is still premature to draw any definite conclusions, it is timely to look for new physics which could explain the excess in $R_b$ and deficit in $R_c$. We investigate this problem in a simple extension of the Standard Model, where a charge $+2/3$ isosinglet quark is added to the standard spectrum. Upon the further introduction of an extra scalar doublet, one finds a solution with interesting consequences.
Electroweak precision data at LEP are used to place bounds on some $λ'$-type ($L$-violating) and $λ''$-type ($B$-violating) $R$-parity violating supersymmetric Yukawa couplings. We consider one-loop corrections (triangles and self-energies) induced by these couplings to the $Z$-partial widths. The couplings involving the top quark induce stronger loop effects and hence are more strongly constrained than others. On the other hand, information on $D$- and $τ$-decays are used to constrain some of the $λ'$-type couplings which are complementary to the ones constrained from LEP. Among the bounds we have derived, many are new from a phenomenological view-point.
In the context of the left--right symmetric model, the decay $b \rightarrow s γ$ receives contributions from the gauge interactions mediated mainly by the $W_L$, through $W_L$--$W_R$ mixing and also from the Yukawa interactions of the charged and the neutral (flavour-changing) scalars (the latter type of Yukawa interaction has been overlooked in the previous literature). Following the recent CLEO measurement of the inclusive $b \rightarrow s γ$ process and the measurement of the top-quark mass by the CDF and D0 collaborations, the parameter space of the left--right symmetric model is more squeezed than before.
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.
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 the $τ$, since previous constraints on them were very mild.
$D$- 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.
In the Standard Model (SM), dilepton production in hadron--hadron collisions proceeds through the conventional Drell--Yan mechanism $q \bar{q} \rightarrow l^+ l^-$ with the exchange of a gauge boson. Some extensions of the SM contain a quark--lepton contact interaction {\it via} a $q l ϕ$ 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 $q\bar{q} \rightarrow l^+l^-$ 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).
We point out that $R \equiv Br(b\rightarrow d γ)/Br(b\rightarrow s γ)$ 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.
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 $Z \rightarrow l^+l^-$ 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 $A_{LR}$.