Ritesh K. Singh

Ritesh K. Singh
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High Energy Physics - Phenomenology (23)
 
High Energy Physics - Experiment (4)
 
Physics - Popular Physics (3)
 
Physics - Physics Education (2)
 
High Energy Astrophysical Phenomena (1)
 
Mathematics - Mathematical Physics (1)
 
Quantum Physics (1)
 
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Publications Authored By Ritesh K. Singh

The anomalous trilinear gauge couplings of $Z$ and $\gamma$ are studied in $e^+e^-\to ZZ/Z\gamma$ with longitudinal beam polarizations using a complete set of polarization asymmetries for $Z$ boson. We quantify the goodness of the beam polarization in terms of the likelihood and find best choice of $e^-$ and $e^+$ polarizations to be ($+0.16$, $-0. Read More

In most of the models beyond the Standard Model, the top quark is expected to be polarized when produced in the decay of some heavier particle, like the gluino or the stop. The polarization is constructed, in an experiment or in simulations, through the distribution of top decay products. Here, we propose an estimator of top quark polarization that depends only on the kinematics of it's mother particle, apart from its decay couplings to top quarks, and is given in terms of the top polarization expected in the rest frame of the decaying particle. Read More

We study the anomalous trilinear gauge couplings of $Z$ and $\gamma$ using a complete set of polarization asymmetries for $Z$ boson in $e^+e^-\to ZZ/Z\gamma$ processes with unpolarized initial beams. We use these polarization asymmetries, along with the cross-section, to obtain a simultaneous limit on all the anomalous couplings using Markov Chain Monte Carlo (MCMC) method. For an $e^+e^-$ collider running at $500$ GeV center-of-mass energy and $100$ fb$^{-1}$ of integrated luminosity the simultaneous limits on the anomalous couplings are $1\sim3\times 10^{-3}$. Read More

The Heun operator has been cast, in terms of the elements of an underlying $su(1,1)$ algebra, under specific parametric conditions, for the purpose of spectrum generation. These elements are differential operators of \emph{degrees} $\pm 1/2$ and $0$. It is found that the regular singularities at $0$ and $\infty$ of the general Heun equation must be \emph{elementary} under the required parametric conditions. Read More

The Tevatron has measured a discrepancy relative to the Standard Model prediction in the forward-backward asymmetry in top quark pair production. This asymmetry grows with the rapidity difference of the two top quarks. It also increases with the invariant mass of the tt pair, reaching, for high invariant masses, 3. Read More

Determining the spin of new particles is an important tool for discriminating models beyond the Standard Model. We show that in case of cascades of subsequent two body decays the existing strategy to extract the spin from lepton and quark spectra can be used without changes even if one allows for dim-5 and dim-6 operators which might be induced by physics just beyond the reach of LHC. We show analytically that these operators do not change the overall structure of these spectra. Read More

In this note we demonstrate the use of top polarization in the study of $t \bar t$ resonances at the LHC, in the possible case where the dynamics implies a non-zero top polarization. As a probe of top polarization we construct an asymmetry in the decay-lepton azimuthal angle distribution (corresponding to the sign of $\cos\phi_\ell$) in the laboratory. The asymmetry is non-vanishing even for a symmetric collider like the LHC, where a positive $z$ axis is not uniquely defined. Read More

Certain supersymmetric grand unified models predict that the coefficients of the quadratic terms in the MSSM Higgs potential should be degenerate at the GUT scale. We discuss some examples for such models, and we analyse the implications of this peculiar condition of a GUT-scale degenerate Higgs mass matrix for low-scale MSSM phenomenology. To this end we explore the parameter space which is consistent with existing experimental constraints by means of a Markov Chain Monte Carlo analysis. Read More

The identification of the correct model for physics beyond the Standard Model requires the determination of the spin of new particles. We investigate to which extent the spin of a new particle $X$ can be identified in scenarios where it decays dominantly in three-body decays $X\to f\bar{f} Y$. Here we assume that $Y$ is a candidate for dark matter and escapes direct detection at a high energy collider such as the LHC. Read More

We investigate the effects of new physics scenarios containing a high mass vector resonance on top pair production at the LHC, using the polarization of the produced top. In particular we use kinematic distributions of the secondary lepton coming from top decay, which depends on top polarization, as it has been shown that the angular distribution of the decay lepton is insensitive to the anomalous tbW vertex and hence is a pure probe of new physics in top quark production. Spin sensitive variables involving the decay lepton are used to probe top polarization. Read More

Since the PAMELA results on the "anomalously" high positron fraction and the lack of antiproton excess in our Galaxy, there has been a tremendous number of studies advocating new types of dark matter, with larger couplings to electrons than to quarks. This raises the question of the production of dark matter particles (and heavy associated coloured states) at LHC. Here, we explore a very simple benchmark dark matter model and show that, in spite of the agreement between the PAMELA antiproton measurements and the expected astrophysical secondary background, there is room for large couplings of a WIMP candidate to heavy quarks. Read More

The CDF and D0 experiments have reported on the measurement of the forward-backward asymmetry of top quark pair production at the Tevatron and the result is that it is more than 2 standard deviations above the predicted value in the Standard Model. This has to be added to the longstanding anomaly in the forward-backward asymmetry for bottom quark production at LEP which is 3 standard deviations different from the Standard Model value. The discrepancy in the bottom asymmetry can be accounted for by the contributions of Kaluza-Klein excitations of electroweak gauge bosons at LEP in warped extra dimensional models in which the fermions are localized differently along the extra dimension so that the gauge interactions of heavy third generation fermions are naturally different from that of light fermions. Read More

Exploiting the azimuthal angle dependence of the density matrices we construct observables that directly measure the spin of a heavy unstable particle. A novelty of the approach is that the analysis of the azimuthal angle dependence in a frame other than the usual helicity frame offers an independent cross-check on the extraction of the spin. Moreover, in some instances when the transverse polarisation tensor of highest rank is vanishing, for an accidental or dynamical reason, the standard azimuthal asymmetries vanish and would lead to a measurement with a wrong spin assignment. Read More

We consider the Randall-Sundrum extra dimensional model with fields propagating in the bulk based on an extended electroweak gauge symmetry with specific fermion charges and localizations that allow to explain the LEP anomaly of the forward--backward asymmetry for $b$--quarks, $A_{FB}^b$. We study the manifestations of the strongly--interacting and electroweak gauge boson Kaluza--Klein excitations $\vkk$ at the LHC, with masses of the order of a few TeV, which dominantly decays into top and bottom quark pairs. We first analyze the two--body tree--level production processes $pp \to t\bar t$ and $b\bar b$ in which the Kaluza--Klein (KK) excitations of gauge bosons are exchanged. Read More

In this note, we consider possible constraints from $t \bar t$ production on the gauge bosons of theories with an extended strong interaction sector such as axigluons or flavour universal colorons. Such constraints are found to be competitive with those obtained from the dijet data. The current $t \bar t$ data from the Tevatron rule out axigluon masses ($m_A$) up to 900 GeV and 850 GeV at 2 $\sigma$ and 4 $\sigma$ levels respectively. Read More

We discuss the use of fermion polarization for studying neutral Higgs bosons at a photon collider. To this aim we construct polarization asymmetries which can isolate the contribution of a Higgs boson $\phi$ in $\gamma\gamma\to f \bar f$, $f=\tau/t$, from that of the QED continuum. This can help in getting information on the $\gamma\gamma\phi$ coupling in case $\phi$ is a CP eigenstate. Read More

We investigate the possibilities of studying new physics in various processes of t-quark production using kinematical distributions of the secondary lepton coming from decay of t quarks. We show that the angular distributions of the secondary lepton are insensitive to the anomalous tbW vertex and hence are pure probes of new physics in a generic process of t-quark production. The energy distribution of the lepton is distinctly affected by anomalous tbW couplings and can be used to analyze them independent of the production process of t quarks. Read More

We examine, in a model independent way, the sensitivity of a Linear Collider to the couplings of a light Higgs boson to gauge bosons. Including the possibility of CP violation, we construct several observables that probe the different anomalous couplings possible. For an intermediate mass Higgs, a collider operating at a center of mass energy of 500 GeV and with an integrated luminosity of 500 fb$^{-1}$ is shown to be able to constrain the $ZZH$ vertex at the few per cent level, and with even higher sensitivity in certain directions. Read More

Unfortunately, we have discovered a bug in the computer program which invalidates the numerical results of our paper. While our formulae and the dependencies explained in the paper are correct, the numerical values of the asymmetries presented in the paper go down by more than two orders of magnitude. The plots and the conclusions of the paper as they stand are hence invalid and will have to be changed. Read More

This is a summary of the projects undertaken by the Working Group I on High Energy Collider Physics at the Eighth Workshop on High Energy Physics Phenomenology (WHEPP8) held at the Indian Institute of Technology, Mumbai, January 5-16, 2004. The topics covered are (i) Higgs searches (ii) supersymmetry searches (iii) extra dimensions and (iv) linear collider. Read More

Unfortunately, we have discovered a bug in the computer program which invalidates the numerical results presented in this talk. While our formulae and the dependencies are correct, the numerical values of the asymmetries presented in the paper go down by more than two orders of magnitude. The new results will be presented elsewhere. Read More

We show that an anomalous CP-violating gamma-gamma-Z vertex gives rise to a novel asymmetry with transversely polarized electron and positron beams in the process e+e- -> gamma Z. This asymmetry, which is odd under naive time reversal, is proportional to the real part of the gamma-gamma-Z CP-violating coupling. This is in contrast to the simple forward-backward asymmetry of the gamma (or Z) with unpolarized or longitudinally polarized beams studied earlier, which is even under naive time reversal, and is proportional to the imaginary part. Read More

The matrix element of the isoscalar axial vector current, $\bar{u}\gamma_\mu\gamma_5u + \bar{d}\gamma_\mu\gamma_5d $, between nucleon states is computed using the external field QCD sum rule method. The external field induced correlator, $<0|\bar{q}\gamma_\mu\gamma_5q|0>$, is calculated from the spectrum of the isoscalar axial vector meson states. Since it is difficult to ascertain, from QCD sum rule for hyperons, the accuracy of validity of flavour SU(3) symmetry in hyperon decays when strange quark mass is taken into account, we rely on the empirical validity of Cabbibo theory to dertermine the matrix element $\bar{u}\gamma_{\mu}\gamma_5 u + \bar{d}\gamma_{\mu}\gamma_5 d - 2 \bar{s}\gamma_{\mu}\gamma_5 s$ between nucleon states. Read More

We study possible effects of CP violation in the Higgs sector on $t\bar t$ production at a $\gamma\gamma$-collider. These studies are performed in a model-independent way in terms of six form-factors $\{\Re(S_{\gamma}), \Im(S_{\gamma}), \Re(P_{\gamma}), \Im(P_{\gamma}), S_t, P_t\}$ which parametrize the CP mixing in the Higgs sector, and a strategy for their determination is developed. We observe that the angular distribution of the decay lepton from $t/\bar t$ produced in this process is independent of any CP violation in the $tbW$ vertex and hence best suited for studying CP mixing in the Higgs sector. Read More

A brief description of the path-breaking evidence for the observation of neutrino oscillations at the Sudbury Neutrino Observatory is presented and the experimental principles and theory thereof are briefly discussed. Read More

This two-part article considers certain fundamental symmetries of nature, namely the discrete symmetries of parity (P), charge conjugation (C) and time reversal (T), and their possible violation. Recent experimental results are discussed in some depth. In the first part of this article we present a general background and discuss parity violation. Read More

This two-part article considers certain fundamental symmetries of nature, namely the discrete symmetries of parity (P), charge conjugation (C) and time reversal (T), and their possible violation. Recent experimental results are discussed in some depth. In this second part, we discussion CP and $T$ violation and arrive at a synthesis. Read More