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.Comment: 17 pages, LaTeX, 3 figures in postscript files (included), macros file include

Exploring the Inert Doublet Model through the dijet plus missing transverse energy channel at the LHC

In this study of the Inert Doublet Model (IDM), we propose that the dijet + missing transverse energy channel at the Large Hadron Collider (LHC) will be an effective way of searching for the scalar particles of the IDM. This channel receives contributions from gauge boson fusion, and $t-$channel production, along with contributions from $H^+$ associated production. We perform the analysis including study of the Standard Model (SM) background with assumed systematic uncertainty, and optimise the selection criteria employing suitable cuts on the kinematic variables to maximise the signal significance. We find that with high luminosity option of the LHC, this channel has the potential to probe the IDM in the mass range of up to about 400 GeV, which is not accessible through other leptonic channels. In a scenario with light dark matter of mass about 65 GeV, charged Higgs in the mass range of around 200 GeV provides the best possibility with a signal significance of about $2\sigma$ at an integrated luminosity of about 3000 fb$^{-1}$.Comment: 16 pages, 3 figures, pdflatex; tables modified with new selection criteria to accommodate systematics. version to appear in PL

Probing the indefinite CP nature of the Higgs Boson through decay distributions in the process e+e−→ttˉΦe^+e^-\to t\bar{t}\Phi

The recently discovered scalar resonance at the LHC is now almost confirmed to be a Higgs Boson, whose CP properties are yet to be established. At the ILC with and without polarized beams, it may be possible to probe these properties at high precision. In this work, we study the possibility of probing departures from the pure CP-even case, by using the decay distributions in the process $e^+ e^- \to t \bar{t} \Phi$, with $\Phi$ mainly decaying into a $b\bar b$ pair. We have compared the case of a minimal extension of the SM case (Model I) with an additional pseudoscalar degree of freedom, with a more realistic case namely the CP-violating Two-Higgs Doublet Model (Model II) that permits a more general description of the couplings. We have considered the ILC with $\sqrt{s}=800$\,GeV and integrated luminosity of $300\, {\rm fb}^{-1}$. Our main findings are that even in the case of small departures from the CP-even case, the decay distributions are sensitive to the presence of a CP-odd component in Model II, while it is difficult to probe these departures in Model I unless the pseudoscalar component is very large. Noting that the proposed degrees of beam polarization increases the statistics, the process demonstrates the effective role of beam polarization in studies beyond the Standard Model. Further, our study shows that an indefinite CP Higgs would be a sensitive laboratory to physics beyond the SM.Comment: 14 pages using revtex, 10 figures, corresponds to version accepted for publication in Phys. Rev. D.; compared to v1, discussion extended, figure added, table added, section reorganize

Effects of TeV Scale Gravity on e^+e^- --> W^+ W^-

We study the process e^+e^- --> W^+W^- to probe low-scale gravity at high energy linear colliders. A characteristic signature of extra-dimensions models is the forward-backward asymmetry due to the interference of spin-2 graviton and spin-1 SM gauge boson exchange terms, even when right-polarised electron beam is used. Our analysis shows that larger than 5% asymmetry is possible at a linear collider with \sqrt{s}=500 (800) GeV if the mass scale, M_S is smaller than 2.7 (4.5) TeV. W^- polarisation factors measured with a few percent accuracy will also be able to put similar limits on M_S.Comment: 8 pages including figure

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.Comment: 10 pages, latex, two figures included, Version accepted for publication in PL

CP violation at a linear collider with transverse polarization

We show how transverse beam polarization at $e^+e^-$ colliders can provide a novel means to search for CP violation by observing the distribution of a single final-state particle without measuring its spin. We suggest an azimuthal asymmetry which singles out interference terms between standard model contribution and new-physics scalar or tensor effective interactions in the limit in which the electron mass is neglected. Such terms are inaccessible with unpolarized or longitudinally polarized beams. The asymmetry is sensitive to CP violation when the transverse polarizations of the electron and positron are in opposite senses. The sensitivity of planned future linear colliders to new-physics CP violation in $e^+e^- \to t \bar{t}$ is estimated in a model-independent parametrization. It would be possible to put a bound of $\sim 7$ TeV on the new-physics scale $\Lambda$ at the 90% C.L. for $\sqrt{s}=500$ GeV and $\int dt {\cal L}=500 {\rm fb}^{-1}$, with transverse polarizations of 80% and 60% for the electron and positron beams, respectively.Comment: 15 pages, latex, includes 5 figures. This version (v3) corresponds to publication in Physical Review; extended version of v2 which corresponded to LC note LC-TH-2003-099 with corrected figure caption