arXiv subjects
Publications and source records attributed to Sreerup Raychaudhuri.
Supersymmetric models without conservation of R-parity are reviewed and low-energy constraints on the extra trilinear couplings listed. Current searches at the LEP, Tevatron and HERA colliders are then summed up. Prospects for further study, especially at future colliders, are briefly touched upon.
Dijet production at the Tevatron including effects of virtual exchanges of spin-2 Kaluza-Klein modes in theories with large extra dimensions is considered. The experimental dijet mass and angular distribution are exploited to obtain stringent limits ($\sim 1.2$ TeV) on the effective string scale $M_S$.
We investigate the production of charginos and neutralinos at a 500 GeV e^+e^- collider (NLC) and study their decays to the lightest neutralino, which then decays into multi-fermion final states through couplings which do not conserve R-parity. These couplings are assumed to affect only the decay of the lightest neutralino. Detailed analyses of the possible signals and backgrounds are performed for five selected points in the parameter space.
We use the invariant mass distribution of Drell-Yan dileptons as measured by the CDF and D0 Collaborations at the Fermilab Tevatron and make a careful analysis to constrain Kaluza-Klein models with large extra dimensions. The combined data from both collaborations lead to a conservative lower bound on the string scale M_S of about 1 TeV at 95% confidence level.
The prospect of large extra dimensions and an effective theory of gravity at around a TeV has interesting experimental consequences. In these models, the Kaluza-Klein modes interact with Standard Model particles and these interactions lead to testable predictions at present and planned colliders. We investigate the effect of virtual exchanges of the spin-2 Kaluza-Klein modes in the production cross-section of $t \bar t$ pairs at the Tevatron and the LHC and find that the $t \bar t$ cross-section can be an effective probe of the large extra dimensions. This enables us to put bounds on the effective low-energy scale.
In scenarios motivated by string theories, it is possible to have extra Kaluza-Klein dimensions compactified to rather large magnitudes, leading to large effects of gravity at scales down to a TeV. The effect of the spin-2 Kaluza-Klein modes on the deep-inelastic cross-section at HERA is investigated. We find that the data can be used to obtain bounds on the effective low energy scale, $M_S$.
At the Fermilab Tevatron, final states with three tagged b-jets could play an important role in searches for a Higgs boson with mass in the range 100-300 GeV. These signals arise from gb fusion and we demonstrate their observability in the limit of a large b-quark Yukawa coupling. Rather promising discovery limits on such a coupling are obtained and consequent effects on the parameter space of the Higgs-boson sector in the MSSM are discussed.
We investigate the effects of R-parity-violating $LL\bar E$ and $LQ\bar D$ operators on the production of fermion pairs at a $μ^+ μ^-$ collider with high energy and luminosity. We show that comparison of angular distributions leads to useful discovery limits for the R-parity-violating couplings, especially if the exchanged sfermions turn out to be heavy. Products of these couplings can also be probed well beyond present experimental limits, especially for the second and third generations. Finally, we compare results at a muon collider with those obtainable from an $e^+ e^-$ linear collider with similar energy and luminosity.
The excess in high-Q^2 events at HERA is introduced and possible explanations within the framework of R-parity violating supersymmetry are discussed.
At an e-γcollider, a selectron \tilde{e}_{L,R} may be produced in association with a (lightest) neutralino \tildeχ_1. Decay of the selectron may be expected to yield a final state with an electron and another \tildeχ_1. If R-parity is violated, these two neutralinos will decay, giving rise to distinctive signatures, which are identified and studied.
A light stop, with an R-parity-violating coupling $λ'_{131}$, has been suggested as an explanation of the excess in high-$Q^2$ neutral current events observed at the HERA collider. We show that in this scheme a corresponding excess in charged current events --- such as that reported by the H1 Collaboration --- can appear naturally, without calling for the presence of light sleptons or additional R-parity-violating couplings, if there exists a chargino lighter than the stop. The predicted event shapes agree well with the data. The relevant region of parameter space is identified, taking into account constraints coming from precision electroweak measurements, atomic parity violation and recent searches for first-generation leptoquarks at the Tevatron collider.
We interpret the reported disagreement between the measured ratio, R_{10}, of the W + 1 jet cross-section to the W + 0 jet cross-section at the Tevatron and the Standard Model (SM) prediction, as the effect of interactions mediated by a colour-octet analogue of the W-boson, the W_8. The presence of a W_8 with mass O(300) GeV, and with couplings to quarks and gluons of the order of electroweak strength, allows the observations of the D0 collaboration to be reproduced quite accurately. Such an interaction is not in contradiction with the present CDF data on W+ W- or dijet production, though higher luminosities may reveal measurable effects.
We examine the possibility that the high-Q^2 events seen at HERA are due to the production and decay of squarks of R-parity violating supersymmetry. The relevant R-parity violating coupling(s) is identified and shown to lie between 0.03 and 0.26. Consequences of such a coupling at other experiments, such as the LEP and Tevatron, are discussed.
We re-examine like-sign dilepton signals at the Fermilab Tevatron assuming that the excess high-$Q^2$ events recently seen at HERA are due to scalar resonances such as squarks of $R$-parity-violating supersymmetry. For gluinos in the mass range of 200--350 GeV, the like-sign dilepton signal can help to make the crucial distinction between the most favoured squark explanation and other proposed solutions.
We investigate the possibility that the excess of four-jet events in $e^+e^-$ collisions at LEP-1.5 which has been reported by the ALEPH Collaboration could be due to the production of charginos or neutralinos, followed by their decay into quark jets through baryon number-violating ($λ^{\prime\prime}$) couplings. An estimate at the parton level shows, however, that these events cannot be due to neutralinos because of the low cross-section, and is unlikely to be due to the production of chargino pairs because of the largely different event shapes.
Under the assumption of gaugino mass unification at a high scale, chargino and neutralino masses depend on the value of the gluino mass, which itself becomes a function of squark masses through self-energy corrections. We demonstrate that this leads to combined bounds on squark and gluino masses from the limits on chargino, neutralino and Higgs boson masses obtained in the CERN LEP-1 and LEP-1.5 runs. These bounds turn out to be comparable to those obtained from direct searches at the Fermilab Tevatron and may be expected to improve as LEP energies go higher.
We study $t \bar t$ production in R-parity violating supersymmetry. The annihilation channel $q \bar q \rightarrow t \bar t$ gets new contributions from $t$-channel exchange of squarks or sleptons. With the data from Tevatron on $t \bar t$ production, we find that the squark- or slepton-exchange processes constrain the $B$-violating $λ''$ couplings or the $L$-violating $λ'$ couplings, respectively. Our bounds are already comparable to the few existing constraints on third-generation $R$-parity violating couplings, and will improve when more precise measurements of the $t \bar t$ production cross-section become available. We also discuss the effects of these couplings for top production at the LHC.
Currently popular search strategies for supersymmetric particles may be significantly affected due to relatively light sneutrinos which decay dominantly into invisible channels. In certain cases the second lightest neutralino may also decay invisibly leading to two extra carriers of missing energy (in addition to the lightest supersymmetric particle (LSP)) -- the virtual LSPs (VLSPs). It is shown that if the sneutrino masses happen to be in the small but experimentally allowed range ~ 45 - 55 GeV, these particles together with neutralino pairs may contribute significantly to the missing energy in the process e+ e- ---> γ+ missing energy at LEP-2 energies as an enhancement over the Standard Model or the conventional MSSM predictions. It is further shown that a much larger region of the parameter space can be scanned at a high luminosity e+ e- collider at 500 GeV like the proposed NLC machine. Moreover this process can play a complementary role to direct chargino searches at LEP-2 and NLC which may fail due to a near mass degeneracy of the chargino and the sneutrino. Formulae for the cross sections taking into account full mixings of the charginos and the neutralinos are derived. The signal remains observable even in the context of more restricted models based on N=1 SUGRA with common scalar and gaugino masses. The effect of soft photon brehmsstrahlung on the signal is also discussed briefly.