arXiv subjects

Ritesh K. Singh

Publications and source records attributed to Ritesh K. Singh.

At least 55 records · Page 3Linked to original sources

arXiv2017

Heun polynomials and exact solutions for the massless Dirac particle in the C-metric

The equation of motion of a massless Dirac particle in the C-metric leads to the general Heun equation (GHE) for the radial and the polar variables. The GHE, under certain parametric conditions, has been cast in terms of a new set of $su(1,1)$ generators involving differential operators of \emph{degrees} $\pm 1/2$ and $0$. Additional \emph{Heun polynomials} are obtained using this new algebraic structure and are used to construct some exact solutions for the radial and the polar parts of the Dirac equation.

gr-qc↗

arXiv2016

Polarization of top produced in particle decays in an arbitrary frame

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. This estimator allows one to estimate the top polarization without performing a full simulation. We find this estimator is independent of the production angle of the mother, top decay angle (for unpolarized mother), and the spin of the mother particle. We study the quality of the estimator with finite width of the mother particle via examples of gluinos and stops decaying into top quark at LHC. We also point out how for the mass spectra of gluinos and top squarks currently expected in a `natural' scenario, the polarization of the top quarks produced in the gluino decays can uniquely track the mixing angle in the stop sector.

hep-ph↗

arXiv2016

On Spectrum Generating Algebra of the Heun Operator

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. The spectrum generation has been demonstrated through a set of examples.

quant-ph↗

arXiv2012

New physics contributions to the forward-backward asymmetry at the Tevatron

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, 3.4 standard deviations above the Next to Leading Order prediction for the charge asymmetry of QCD. However, perfect agreement between experiment and the Standard Model was found in both total and differential cross section of top quark pair production. As this result could be a sign of new physics we have parametrized this new physics in terms of a complete set of dimension six operators involving the top quark. We have then used a Markov Chain Monte Carlo approach in order to find the best set of parameters that fits the data, using all available data regarding top quark pair production at the Tevatron. We have found that just a very small number of operators are able to fit the data better than the Standard Model.

hep-ph↗

arXiv2011

Effects of Anomalous Couplings on Spin Determination in Cascade Decays

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.

hep-ph↗

arXiv2011

The forward-backward asymmetry of top quark production at the Tevatron in warped extra dimensional models

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. In this paper, we show that it is more difficult to elaborate a model generating a significant top asymmetry in a similar way -- through exchanges of Kaluza-Klein gluons at the Tevatron -- due to the indirect constraints originating from precision electroweak data.

hep-ph↗

arXiv2010

On measurement of top polarization as a probe of $t \bar t$ production mechanisms at the LHC

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 $\cosϕ_\ell$) 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. The angular distribution of the leptons has the advantage of being a faithful top-spin analyzer, unaffected by possible anomalous $tbW$ couplings, to linear order. We study, for purposes of demonstration, the case of a $Z'$ as might exist in the little Higgs models. We identify kinematic cuts which ensure that our asymmetry reflects the polarization in sign and magnitude. We investigate possibilities at the LHC with two energy options: $\sqrt{s} = 14$ TeV and $\sqrt{s} = 7$ TeV, as well as at the Tevatron. At the LHC the model predicts net top quark polarization of the order of a few per cent for $M_{Z'} \simeq 1200 $ GeV, being as high as $10 %$ for a smaller mass of the $Z'$ of $700$ GeV and for the largest allowed coupling in the model, the values being higher for the $7$ TeV option. These polarizations translate to a deviation from the standard-model value of azimuthal asymmetry of up to about $4%$ ($7%$) for $14$ ($7$) TeV LHC, whereas for the Tevatron, values as high as $12%$ are attained. For the $14$ TeV LHC with an integrated luminosity of 10 fb$^{-1}$, these numbers translate into a $3 σ$ sensitivity over a large part of the range $500 \lesssim M_{Z'} \lesssim 1500 $ GeV.

hep-ph↗

arXiv2010

On SUSY GUTs with a degenerate Higgs mass matrix

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.

hep-ph↗

arXiv2010

Spin Discrimination in Three-Body Decays

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. We show that in the case that all intermediate particles are heavy, one can get information on the spins of $X$ and $Y$ at the LHC by exploiting the invariant mass distribution of the two standard model fermions. We develop a model-independent strategy to determine the spins without prior knowledge of the unknown couplings and test it in a series of Monte Carlo studies.

hep-ph↗

arXiv2010

Implication of the PAMELA antiproton data for dark matter indirect detection at LHC

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. Contrary to what could have been naively anticipated, the PAMELA pbar/p measurements do not challenge dark matter model building, as far as the quark sector is concerned. A quarkophillic species is therefore not forbidden.Owing to these large couplings, one would expect that a new production channel opens up at the LHC, through quark--quark and quark--gluon interactions. Alas, when the PDF of the quark is taken into account, prospects for a copious production fade away.

hep-ph↗

arXiv2009

A model independent spin analysis of fundamental particles using azimuthal asymmetries

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. In a frame such as the one we construct, the correct spin assignment would however still be possible. The method gives direct information about the spin of the particle under consideration and the same event sample can be used to identify the spins of each particle in a decay chain. A drawback of the method is that it is instrumental only when the momenta of the test particle can be reconstructed. However we hope that it might still be of use in situations with only partial reconstruction. We also derive the conditions on the production and decay mechanisms for the spins, and hence the polarisations, to be measured at a collider experiment. As an example for the use of the method we consider the simultaneous reconstruction, at the partonic level, of the spin of both the top and the $W$ in top pair production in $e^+ e^-$ in the semi-leptonic channel.

hep-ph↗

arXiv2007

Top production at the Tevatron/LHC and nonstandard, strongly interacting spin one particles

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 $σ$ and 4 $σ$ levels respectively. For the case of flavour universal colorons the data rule out a mass ($m_C$) below 800 GeV (780 GeV) at the $2 (4) σ$ level and also the mass range between 900 GeV to 2.1 TeV at 2 $σ$ level, for $\cot ξ= 1$, where $ξ$ is the mixing angle. For $\cot ξ=2$ on the other hand, the excluded range is $m_C \lsim 950 (920)$ GeV and $m_C \gsim 1.02 (1.15 \lsim m_C \lsim 1.8)$ TeV at $2 σ$ ($4 σ$) level. We point out that for higher axigluon/coloron masses, even for the dijet channel, the limits on the coloron mass, for $\cot ξ= 1$, may be different than those for the axigluon. We also compute the expected forward-backward asymmetry for the case of the axigluons which would allow it to be discriminated against the SM as also the colorons. We further find that at the LHC, the signal should be visible in the $t \bar t$ invariant mass spectrum for a wide range of axigluon and coloron masses that are still allowed. We point out how top polarisation may be used to further discriminate the axigluon and coloron case from the SM as well as from each other.

hep-ph↗

arXiv2007

Kaluza--Klein excitations of gauge bosons at the LHC

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. We find that the additional channels can lead to a significant excess of events with respect to the Standard Model prediction; characteristic top quark polarization and angular asymmetries are quantitatively studied and turn out to probe the chiral structure of couplings to excited states. We then analyze higher order production processes for the gauge boson excitations which have too weak or no couplings to light quarks and, in particular, the loop induced process $gg \to \vkk \to t\bar t$ and $b\bar b$ in which the anomalous $gg \vkk $ four--dimensional vertex has to be regulated. The RS effects in this process, as well as in the four--body reactions $pp \to t\bar t b\bar b$, $t\bar t t\bar t$, $b\bar b b\bar b$ and in the related three--body reactions $gb \to b t \bar t$, $b b\bar b$, in which the $\vkk$ excitations are mainly radiated off the eavy quarks, are shown to be potentially difficult to test at LHC, due to small hase space and low parton density for $\mkk \gsim 3$ TeV.

hep-ph↗

arXiv2007

Signatures of anomalous VVH interactions at a linear collider

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. However, the lack of sufficient number of observables as well as contamination from the $ZZH$ vertex limits the precision with which the $WWH$ coupling can be measured.

hep-ph↗

arXiv2006

Probing CP-violating Higgs contributions in gamma-gamma -> f anti-f through fermion polarization

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 $ϕ$ in $γγ\to f \bar f$, $f=τ/t$, from that of the QED continuum. This can help in getting information on the $γγϕ$ coupling in case $ϕ$ is a CP eigenstate. We also construct CP-violating asymmetries which can probe CP mixing in case $ϕ$ has indeterminate CP. Furthermore, we take the MSSM with CP violation as an example to demonstrate the potential of these asymmetries in a numerical analysis. We find that these asymmetries are sensitive to the presence of a Higgs boson as well as its CP properties over a wide range of MSSM parameters. In particular, the method suggested can cover the region where a light Higgs boson may have been missed by LEP due to CP violation in the Higgs sector, and may be missed as well at the LHC.

hep-ph↗

arXiv2006

Lepton distribution as a probe of new physics in production and decay of the t quark and its polarization

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. The effects of t polarization on the distributions of the decay lepton are demonstrated for top-pair production process at a gamma-gamma collider mediated by a heavy Higgs boson.

hep-ph↗

arXiv2005

Tau Polarization as a Probe of CP of a Light Higgs Boson

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.

hep-ph↗

arXiv2004

Summary of the Activities of the Working Group I on High Energy and Collider Physics

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.

hep-ph↗