Ritesh K. Singh

The role of polarized positrons and electrons in revealing fundamental interactions at the Linear Collider

The proposed International Linear Collider (ILC) is well-suited for discovering physics beyond the Standard Model and for precisely unraveling the structure of the underlying physics. The physics return can be maximized by the use of polarized beams. This report shows the paramount role of polarized beams and summarizes the benefits obtained from polarizing the positron beam, as well as the electron beam. The physics case for this option is illustrated explicitly by analyzing reference reactions in different physics scenarios. The results show that positron polarization, combined with the clean experimental environment provided by the linear collider, allows to improve strongly the potential of searches for new particles and the identification of their dynamics, which opens the road to resolve shortcomings of the Standard Model. The report also presents an overview of possible designs for polarizing both beams at the ILC, as well as for measuring their polarization.

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 long-standing 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 through exchanges of Kaluza-Klein gluons at the Tevatron due to the indirect constraints originating from precision electroweak data.

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 b 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 → t¯

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

We investigate the possibility of studying new physics in various processes of t-quark production using kinematical distributions of the secondary lepton coming from the decay of t quarks. We show that the angular distribution of the secondary lepton is insensitive to the anomalous tbW vertex and hence is a pure probe 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

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

\gamma \gamma

On measurement of top polarization as a probe of \( t\bar{t} \) production mechanisms at the LHC

collider mediated by a heavy Higgs boson.

Constraining the MSSM with universal gaugino masses and implication for searches at the LHC

Abstract In this Letter, we consider possible constraints from t 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 t ¯ data from the Tevatron rule out axigluon masses ( m A ) up to 910 GeV and 920 GeV at 95% and 90% confidence levels, respectively. For the case of the flavour universal colorons, for cot ξ = 1 , where ξ is the mixing angle, the mass ranges m C ≲ 800 GeV and 895 ≲ m C ≲ 1960 GeV are excluded at 95% confidence level (C.L.), whereas the same at 90% C.L. are m C ≲ 805 GeV and 880 ≲ m C ≲ 2470 GeV . For cot ξ = 2 on the other hand, the excluded range is m C ≲ 955 ( 960 ) GeV and 1030 ≲ m C ≲ 3250 ( 1020 ≲ m C ≲ 3250 ) GeV at 95 % ( 90 % ) C.L., respectively. 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 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.

A model independent spin analysis of fundamental particles using azimuthal asymmetries

In this note we demonstrate the use of top polarization in the study of

Transverse beam polarization and CP-violating triple gauge-boson couplings in e+e−→γZ

t\bar{t}

Top polarization as a probe of new physics

resonances at the LHC, in the possible case where the dynamic s 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 ϕℓ) in the laboratory. The asymmetry is non-vanishing even for a symmetric collider li ke 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 poss ibilities at the LHC with two energy options: