arXiv subjects

Saurabh D. Rindani

Publications and source records attributed to Saurabh D. Rindani.

At least 73 records · Page 4Linked to original sources

arXiv2002

QCD corrections to decay-lepton polar and azimuthal angular distributions in e+e- -> t tbar in the soft-gluon approximation

QCD corrections to order alpha_s in the soft-gluon approximation to angular distributions of decay charged leptons in the process e+e- -> t tbar followed by semileptonic decay of t or tbar, are obtained in the e+e- centre-of-mass frame. As compared to distributions in the top rest frame, these have the advantage that they would allow direct comparison with experiment without the need to reconstruct the top rest frame. The results also do not depend on the choice of a spin quantization axis for t or tbar. Analytic expression for the triple distribution in the polar angle of t and polar and azimuthal angles of the lepton is obtained. Analytic expression is also derived for the distribution in the charged-lepton polar angle. Numerical values are discussed for total c.m. energies of 400 GeV, 800 GeV and 1500 GeV.

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arXiv2002

A predictive scheme for neutrino masses

The solar and atmospheric data and possibly large value for the effective neutrino mass in neutrinoless double beta decay experiment together indicate that all the three neutrinos are nearly degenerate. A verifiable texture for the neutrino mass matrix is proposed to accommodate these results. This texture allows almost degenerate neutrino masses two of which are exactly degenerate at tree level. The standard model radiative corrections lift this degeneracy and account for the solar deficit. The solar scale is correlated with the effective neutrino mass $m_{ee}$ probed in neutrinoless double beta decay experiments. The model can accommodate a large value of O(eV) for $m_{ee}$. Six observables corresponding to three neutrino masses and three mixing angles are determined in terms of only three unknown parameters within the proposed texture.

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arXiv2002

CP violation in open ttbar production at a linear collider

CP-violating asymmetries in the processes e+e- -> t tbar and gamma gamma -> ttbar can provide information about CP-violating couplings of the top quark, whose presence would signal physics beyond the standard model. Work on studies of different asymmetries has been described. Special emphasis has been laid on asymmetries in charged lepton distributions arising from top decay. The effect of longitudinal beam polarization has been discussed. Limits possible on the electric dipole moment of the top quark from different angular asymmetries have been obtained.

hep-ph↗

arXiv2001

Decay-lepton angular asymmetries in polarized e+e- -> t tbar as a measure of CP-violating dipole couplings of the top quark

In the presence of an electric dipole coupling of the top quark to a photon, and an analogous "weak" dipole coupling to the Z, CP violation in the process e+e- -> t tbar leads to the polarization of the top and anti-top. This polarization can be analyzed by studying the angular distributions of decay charged leptons when the top or anti-top decays leptonically. We have obtained analytic expressions for these distributions when either t or tbar decays leptonically, including O(alpha_s) QCD corrections in the soft-gluon approximation. The angular distributions are insensitive to anomalous interactions in top decay. We study two types of simple CP-violating polar-angle asymmetries and two azimuthal asymmetries which do not need the full reconstruction of the t or tbar. We have evaluated independent 90% CL limits that may be obtained on the real and imaginary parts of the electric and weak dipole couplings at a linear collider operating at centre-of-mass energy 500 GeV with integrated luminosity 200 inverse fb and also at 1000 GeV with integrated luminosity 1000 inverse fb. The effect of longitudinal beam polarization has been included.

hep-ph↗

arXiv2001

Decay-lepton angular distributions in e+e- -> t tbar to O(alpha_s) in the soft-gluon approximation

Order-alpha_s QCD corrections in the soft-gluon approximation to angular distributions of decay charged leptons in the process e+e- -> t tbar, followed by semileptonic decay of t or tbar, are obtained in the e+e- centre-of-mass frame. As compared to distributions in the top rest frame, these have the advantage that they would allow direct comparison with experiment without the need to reconstruct the top rest frame or a spin quantization axis. Analytic expressions for the distribution in the charged-lepton polar angle and triple distribution in the polar angle of t and polar and azimuthal angles of the lepton are obtained. Numerical values are discussed for total c.m. energies of 400 GeV and 800 GeV.

hep-ph↗

arXiv2000

Phenomenology of Pseudo Dirac Neutrinos

We formulate general conditions on $3\times 3$ neutrino mass matrices under which a degenerate pair of neutrinos at a high scale would split at low scale by radiative corrections involving only the standard model fields. This generalizes the original observations of Wolfenstein on pseudo Dirac neutrinos to three generations. A specific model involving partially broken discrete symmetry and solving the solar and atmospheric anomalies is proposed. The symmetry pattern of the model naturally generates two large angles one of which can account for the large angle MSW solution to the solar neutrino problem.

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arXiv2000

Effect of anomalous tbW vertex on decay-lepton distributions in e+e- -> t tbar and CP-violating asymmetries

We obtain analytic expressions for the energy and polar-angle double differential distributions of a secondary lepton l+ (l-) arising from the decay of t (tbar) in e+e- -> t tbar with an anomalous tbW decay vertex. We also obtain analytic expressions for the various differential cross sections with the lepton energy integrated over. In this case, we find that the angular distributions of the secondary lepton do not depend on the anomalous coupling in the decay, regardless of possible anomalous couplings occurring in the production amplitude for e+e- -> t tbar. Our study includes the effect of longitudinal e- and e+ beam polarization. We also study the lepton energy and beam polarization dependence of certain CP-violating lepton angular asymmetries arising from an anomalous tbW decay vertex and compare them with the asymmetries arising due to CP violation in the production process due to the top electric or weak dipole moment.

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arXiv1999

Neutrino Anomalies in an Extended Zee Model

We discuss an extended $SU(2)\times U(1)$ model which naturally leads to mass scales and mixing angles relevant for understanding both the solar and atmospheric neutrino anomalies. No right-handed neutrinos are introduced in the model.The model uses a softly broken $L_e-L_μ-L_τ$ symmetry. Neutrino masses arise only at the loop level. The one-loop neutrino masses which arise as in the Zee model solve the atmospheric neutrino anomaly while breaking of $L_e-L_μ-L_τ$ generates at two-loop order a mass splitting needed for the vacuum solution of the solar neutrino problem. A somewhat different model is possible which accommodates the large-angle MSW resolution of the solar neutrino problem.

hep-ph↗

arXiv1999

Lepton Flavour Violation in a Left-Right Symmetric Model

We consider in this paper a Left-Right symmetric gauge model in which a global lepton-number-like symmetry is introduced and broken spontaneously at a scale that could be as low as 10^4 GeV or so. The corresponding physical Nambu-Goldstone boson, which we call majoron and denote J, can have tree-level flavour-violating couplings to the charged fermions, leading to sizeable majoron-emitting lepton-flavour-violating weak decays. We consider explicitly a leptonic variant of the model and show that the branching ratios for μ-> e+J, τ-> e + J and τ-> μ+ J decays can be large enough to fall within the sensitivities of future μand τfactories. On the other hand the left-right gauge symmetry breaking scale may be as low as few TeV.

hep-ph↗

arXiv1999

Longitudinal quark polarization in e+e- --> t tbar and chromoelectric and chromomagnetic dipole couplings of the top quark

The effect of anomalous chromomagnetic (mu) and chromoelectric couplings (d) of the gluon to the top quark are considered in e+ e- --> t tbar, with unpolarized and longitudinally polarized electron beams. The total cross section, as well as t and tbar polarizations are calculated to order alpha_s in the presence of the anomalous couplings. One of the two linear combinations of t and tbar polarizations is CP even, while the other is CP odd. The limits that could be obtained at a typical future linear collider with an integrated luminosity of 50 1/fb and a total c.m. energy of 500 GeV on the most sensitive CP-even combination of anomalous couplings are estimated as -3 < Re(mu) < 2 for Im(mu) = 0 = d, and sqrt{Im(mu)^2 + |d|^2} < 2.25 for Re(mu) = 0. There is an improvement by roughly a factor of 2 at 1000 GeV. On the other hand, from the CP-odd combination, we derive the possible complementary bounds as -3.6 < Im(mu^* d) < 3.6 for Im(d) = 0, and -10 < Im(d) < 10 for Im(mu^* d) = 0, at a c.m. energy of 500 GeV. The corresponding limit for 1000 GeV is almost an order of magnitude better for Im(mu^* d), though somewhat worse for Im(d). Results for the c.m. energies 500 GeV and 1000 GeV, if combined, would yield independen limits on the two CP-violating parameters of -0.8 < Im(mu^* d) < 0.8 and -11 < Im(d) < 11.

hep-ph↗

arXiv1999

CP-violating Z-gamma-gamma and top-quark electric dipole couplings in gamma gamma -> t bar{t}

An effective anomalous CP-violating Z-gamma-gamma coupling can give rise to observable CP-odd effects in gamma gamma -> t bar{t}. We study certain asymmetries in the decay lepton distributions in gamma gamma -> t bar{t} arising from top decay in the presence of a CP-violating Z-gamma-gamma coupling as well as a top-quark electric dipole coupling. We find that a photon linear collider with geometric luminosity of 20 fb^{-1} can put limits of the order of 0.1 on the imaginary part of the CP-violating anomalous Z-gamma-gamma coupling using these asymmetries.

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arXiv1998

Vacuum Solutions of Neutrino Anomalies Through a Softly Broken U(1) symmetry

We discuss an extended SU(2)XU(1) model which naturally leads to mass scales and mixing angles relevant for understanding both the solar and atmospheric neutrino anomalies in terms of the vacuum oscillations of the three known neutrinos. The model uses a softly broken L_e-L_μ-L_τ symmetry and contains a heavy scale M_H\sim 10^{15} GeV. The L_e-L_μ-L_τ symmetric neutrino masses solve the atmospheric neutrino anomaly while breaking of L_e-L_μ-L_τ generates highly suppressed radiative mass scale Δ_S\sim 10^{-10} eV^2 needed for the vacuum solution of the solar neutrino problem. All the neutrino masses in the model are inversely related to M_H, thus providing seesaw-type of masses without invoking any heavy right-handed neutrinos. Possible embedding of the model into an SU(5) grand unified theory is discussed.

hep-ph↗

arXiv1997

Single-quark spin asymmetries in e+ e- ---> t tbar and anomalous gluon couplings

The effect of anomalous chromoelectric couplings of the gluon to the top quark are considered in e+ e- --> t tbar. The total cross section, as well as t and tbar polarizations are calculated to order alpha_s in the presence of the anomalous couplings. One of the two linear combinations of t and tbar polarizations is CP even, while the other is CP odd. Limits that could be obtained at a future linear collider on CP-odd combinations of anomalous couplings are determined.

hep-ph↗

arXiv1997

Forward-backward asymmetry in e+e- -> nu nubar Z from anomalous CP-odd WWZ couplings

Anomalous CP-odd W+W-Z couplings can give rise to a forward-backward asymmetry in the Z angular distribution in the e+e- centre-of-mass frame in the process e+e- -> nu nubar Z. Of the three CP-odd couplings possible, only the imaginary part of one of the couplings, f_4^Z, which is C odd and P even, contributes to the forward-backward asymmetry. It is found that a limit of the order of 0.1 can be placed on this coupling at a Next Linear Collider with centre-of-mass energy of 500 GeV and integrated luminosity of 50 fb^{-1}.

hep-ph↗

arXiv1997

Electric dipole moment of top quark and CP-violating asymmetries in gamma gamma -> t tbar

CP-violating asymmetries due to a possible electric dipole interaction of the top quark in the production and subsequent decay of top quark-top antiquark pair in photon-photon collisions are studied. The asymmetries defined can be used to determine the imaginary part of the electric dipole form factors. A gamma-gamma collider with photon beams generated from laser back-scattering off electron beams with an integrated geometric luminosity of 20 fb^{-1} can put a limit of the order of 10^{-17} e cm on the imaginary part of the electric dipole form factor of the top quark if the electron beams have longitudinal polarization and the laser beams have circular polarization.

hep-ph↗

arXiv1997

CP-violating dipole form factors of the top quark and tau lepton in scalar leptoquark models

We calculate the CP-violating electric and weak dipole form factors of the top quark and the tau lepton in models with scalar leptoquarks coupling only to the third generation of quarks and leptons. We obtain numerical values of the real and imaginary parts of these form factors at various energies for different values of leptoquark masses and couplings. The existing limits on the tau electric and weak dipole form factors allows us to put a limit on the masses and couplings of such leptoquarks and therefore on the top electric and weak dipole form factors. We also discuss constraints on the form factors coming from indirect limits on leptoquark masses and couplings deduced from LEP results on Z properties.

hep-ph↗

arXiv1996

Simple decay-lepton asymmetries in polarized e+e- -> t tbar and CP-violating dipole couplings of the top quark

We study two simple $CP$-violating asymmetries of leptons coming from the decay of $t$ and $øt$ in $e^+e^- \ra \tt$, which do not need the full reconstruction of the $t$ or $øt$ for their measurement. They can arise when the top quark possesses nonzero electric and weak dipole form factors in the couplings to the photon and $Z$, respectively. Together, these two asymmetries can help to determine the electric and weak dipole form factors independently. If longitudinal beam polarization is available, independent determination of form factors can be done by measuring only one of the asymmetries. We obtain estimates of 90\% confidence limits that can be put on these form factors at a future linear $\ee$ collider operating at $\sqrt{s}=500$ GeV.

hep-ph↗

arXiv1995

Decay-lepton angular distribution in polarized $e^+e^- \to \tt$ and $CP$-violating dipole couplings of the top quark

In the presence of an electric dipole coupling of $t \bar t$ to a photon, and an analogous ``weak" dipole coupling to the $Z$, $CP$ violation in the process $e^+e^- \rightarrow t \bar t$ leads to the polarization of the top and anti-top. This polarization can be analyzed by studying the energy and angular distributions of a decay charged lepton (anti-lepton) when the top (anti-top) decays leptonically. We have obtained analytic expressions for these distributions when either $t$ or $\overline t$ decays leptonically. We study two types of simple $CP$-violating angular asymmetries which do not need tof simple $CP$-violating angular asymmetries which do not need the full reconstruction of the $t$ or $\bar t$. These together can help to determine the electric and weak dipole form factors independently. We have also shown how the use of longitudinal beam polarization can help to do this with only one asymmetry measurement, and to imprhelp to do this with only one asymmetry measurement, and to improve the sensitivity.

hep-ph↗