arXiv subjects

Rohini M. Godbole

Publications and source records attributed to Rohini M. Godbole.

At least 109 records · Page 6Linked to original sources

arXiv2001

Total Photonic and Hadronic Cross-sections

We discuss total cross-sections within the context of the QCD calculable mini-jet model, highlighting its successes and failures. In particular we show its description of $γγ\to hadrons$ and compare it with OPAL and L3 data. We extrapolate this result to $γp$ total cross-sections and propose a phenomenological ansätz for virtual photon cross-sections. We point out that the good agreement with data obtained with the Eikonal Minijet Model should not hide the many uncertainties buried in the impact parameter distribution. A model obtained from Soft Gluon Summation is briefly discussed and its application to hadronic cross-sections is shown.

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arXiv2001

$γγ$ cross-sections and $γγ$ colliders

We summarize the predictions of different models for total $γγ$ cross-sections. The experimentaly observed rise of $σ_{γγ}$ with $\sqrt{s_{γγ}}$, faster than that for $σ_{\bar p p}$, $σ_{γp}$ is in agreement with the predictions of the Eikonalized Minijet Models as opposed to those of the Regge-Pomeron models. We then show that a measurement of $σ_{γγ}$ with an accuracy of $\lessapprox 8-9% (6-7%)$ is necessary to distinguish among different Regge-Pomeron type models (among the different parametrisations of the EMM models) and a precision of $\lessapprox$ 20% is required to distinguish among the predictions of the EMMs and of those models which treat like 'photon like a proton', for the energy range $300 < \sqrt{s_{γγ}} < 500$ GeV. We further show that the difference in model predictions for $σ_{γγ}$ of about a factor 2 at $\sqrt{s_{γγ}} = 700 $ GeV reduces to $\sim$ 30% when folded with bremsstrahlung $γ$ spectra to calculate $σ({e^+e^- \to e^+e^-γγ\to e^+e^-X})$. We point out then the special role that $γγ$ colliders can play in shedding light on this

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arXiv2000

Physics potential of the LHC

The basic aim of physics studies at the LHC is to unravel the mechanism responsible for the spontaneous symmetry breaking in the Standard Model (SM). In the currently accepted theoretical picture, this translates into finding `direct' experimental evidence for the Higgs sector. TeV scale supersymmetry (SUSY) provides a very attractive solution to the `naturalness' problem that theories with elementary scalar fields have. Hence in this talk I will summarise the physics potential of the LHC for searching for Higgs and Supersymmetry as well as for measurement of the parameters of the Higgs sector and the SUSY model. Theories with localised gravity (and large extra dimensions) give a credible option to have Standard Model without the attendant `naturalness' problems. I will therefore also summarise the potential of LHC to probe these `large' extra dimensions.

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arXiv2000

SUSY at the Linear Colliders: Working Group Summary

I summarise the activities of the different members of the SUSY working group. There have been two major areas of activity: 1) precision measurement of the SUSY particle masses/couplings and hence those of the SUSY model parameters, 2) investigations into SUSY searches at \eplem, \gamgam, $\game$ and $\emem$ colliders, in the nonstandard scenarios such as explicit CP violation, R-parity violation and Anomaly Mediated Supersymmtery Breaking. In addition there have been studies which looked at the effect of `large' extra dimensions at the various colliders mentioned above.

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arXiv2000

Hadronic cross-sections in gamma gamma processes and the next linear collider

In this note we address the issue of theoretical estimates of the hadronic cross-sections for \gamgam processes. We compare the predictions of the minijet model with data as well as other models, highlighting the band of uncertainties in the theoretical predictions as well as those in the final values of the $σ(\gamgam \go {\rm hadrons})$ extracted from the data. We find that the rise of $σ_{γγ}^{\rm tot}$ with energy shown in the latest \gamgam data is in tune with the faster rise expected in the Eikonal Minijet Models (EMM). We present an estimate of the accuracy with which this cross-section needs to be measured, in order to distinguish between the different theoretical models which try to `explain ' the rise of total cross-sections with energy. We find that the precision of measurement required to distinguish the EMM type models from the proton-like models, for $300 < \sqrt{s}_{\gamgam} < 500$ GeV, is $\lsim 20 %$, whereas to distinguish between various proton-like models or among the different parametrizations of the EMM, a precision of $\lsim 8-9 %$ or $\lsim 6-7%$ respectively, is required. We also comment briefly on the implications of these predictions for hadronic backgrounds at the next linear collider (NLC) to be run in the \gamgam mode or \eplem mode.

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arXiv1999

Signals for R-parity-violating Supersymmetry at a 500 GeV e+ e- Collider

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.

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arXiv1998

Observing Doubly Charged Higgs Bosons in Photon-Photon Collisions

We discuss the possibility of observing doubly charged Higgs bosons in $γγ$ collisions. We find that one can increase the range of observability close to the kinematic limit by a judicious choice of the polarisations of the initial $e^-/ e^+$ beams as well as the initial laser beam photons which are made to back-scatter from the former. We also note that, in a large region of parameter space, the generally used lepton number violating decay mode is dominated by the decay into a singly charged Higgs and a real or virtual $W$-boson, giving rise to a large number of fermions in the final state. This can qualitatively alter the strategy for discovering doubly-charged scalars.

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arXiv1996

Can the Aleph Four-Jet Excess be Explained in a Supersymmetric Model with R-Parity Violation?

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.

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arXiv1996

Unusual Charged Higgs Signals at LEP-2

We have made a detailed study of the signals produced at LEP-2 from charged scalar bosons whose dominant decay channels are into four fermions. The event rates as well as kinematics of the final states are discussed when such scalars are either pair-produced or are generated through a tree-level interaction involving a charged scalar, the W and the Z. The backgrounds in both cases are discussed. We also suggest the possibility of reconstructing the mass of such a scalar at LEP-2.

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arXiv1996

Photon Structure Function

After briefly explaining the idea of photon structure functions (\f2gam\ , \flgam) I review the current theoretical and experimental developements in the subject of extraction of \qvph\ from a study of the Deep Inelastic Scattering (DIS). I then end by pointing out recent progress in getting information about the parton content of the photon from hard processes other than DIS.

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arXiv1995

Resolved Photon Processes

We review the present level of knowledge of the hadronic structure of the photon, as revealed in interactions involving quarks and gluons ``in" the photon. The concept of photon structure functions is introduced in the description of deep--inelastic $e γ$ scattering, and existing parametrizations of the parton densities in the photon are reviewed. We then turn to hard \gamp\ and \gaga\ collisions, where we treat the production of jets, heavy quarks, hard (direct) photons, \jpsi\ mesons, and lepton pairs. We also comment on issues that go beyond perturbation theory, including recent attempts at a comprehensive description of both hard and soft \gamp\ and \gaga\ interactions. We conclude with a list of open problems.

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arXiv1995

Modification of the Equivalent Photon Approximation for Resolved Photon Processes

We propose a modification of the equivalent photon approximation (EPA) for processes which involve the parton content of the photon, to take into account the suppression of the photonic parton fluxes due to the virtuality of the photon. We present simple, physically motivated ansätze to model this suppression and show that even though the parton content of the electron no longer factorizes into an electron flux function and a photon structure function, it is still possible to express it as a single integral. We also show that for the TRISTAN experiments its effect can be numerically of the same size as that of the NLO corrections. Further, we discuss a possible measurement at HERA, which can provide an experimental handle on the effect we model through our ansätze.

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arXiv1995

Structure of Light

In this talk I briefly explain the concept of the structure function of a photon (the best known boson). Then I review some of the current experimental evidence which confirms the existence of `strong' interactions of photon suggested by this idea. I end by pointing out how the photon `structure' has important implications for the interactions of high energy photons and hence for the design of the next generation of the high energy $e^+e^-$ (linear) colliders which are absolutely essential for locating the missing links in our knowledge of fundamental particles and interactions among them.

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arXiv1994

Structure of proton and nuclei

We discuss in this set of lectures the structure of proton/neutron as revealed through a study of form-factors. This is followed by a discussion of the structure functions of proton/nuclear targets as measured in the deep inelastic scattering (DIS) of leptons off these targets. We discuss the parton model in DIS as well as outline the usage of parton model in processes other than the DIS. We then go on to dicsuss the EMC effect: the nuclear dependence of the structure functions. After a discussion of different models of the EMC effect we end by pointing out the possibility of distinguishing between these different models by studying the correlation between the $A$--dependence of different hard processes and the EMC effect.

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arXiv1994

Photon structure and the $μ$ puzzle

In this talk I first briefly explain the puzzle presented by the high muon content observed in air-showers which point to the ultra high-energy $γ$ sources like the HER-X-1. Since it had been suggested that a possible explanation of the puzzle might come from the effects of the hadronic structure of the photon, I briefly explain the concept of photon structure function and comment on the uncertainties in the predictions of the \sigtot. Then I show that while the current experiments at the \eplem\ (TRISTAN and LEP) and $ep$ (HERA) colliders have seen clear evidence for the hadronic structure of the photon, the observed muon excess in the air shower experiments, if confirmed by other experiments to be at the same high level, can not be explained in terms of the photon structure function.

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arXiv1994

Virtual Photon Strucutre Functions and the Parton Content of the Electron

We point out that in processes involving the parton content of the photon the usual effective photon approximation should be modified. The reason is that the parton content of virtual photons is logarithmically suppressed compared to real photons. We describe this suppression using several simple, physically motivated ansätze. Although the parton content of the electron in general no longer factorizes into an electron flux function and a photon structure function, it can still be expressed as a single integral. Numerical examples are given for the \eplem\ collider TRISTAN as well as the $ep$ collider HERA.

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arXiv1993

Resolved Photon Processes

We review high--energy scattering processes that are sensitive to the hadronic structure of the photon, describing theoretical predictions as well as recent experimental results. These processes include deep--inelastic electron--photon scattering at \eplem\ colliders; and the production of jets, heavy quarks and isolated photons in the collision of real photons at \eplem\ colliders, as well as in photon--proton collisions at $ep$ colliders. We also comment on ``minijet'' based calculations of total $γp$ and \gamgam\ cross--sections, and discuss the possibility that future linear \eplem\ colliders might produce very large photon fluxes due to the ``beamstrahlung'' phenomenon; in the most extreme cases, we predict more than one hadronic \gamgam\ event to occur at every bunch crossing.

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arXiv1992

Debajyoti Choudhury

Gluon fusion into a very heavy neutrino pair by Higgs exchange is shown to lead to substantial production cross sections at $pp$ supercolliders even without any extra generation of quarks. Rates are calculated for scalar as well as pseudoscalar Higgs. The angular correlation between dileptons emerging from the decays of the neutrinos shows distinctive features for Dirac and Majorana neutrinos as well as for scalar and pseudoscalar Higgs.

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