S. Bar-Shalom

Heavy Majorana Neutrinos in the Effective Lagrangian Description: Application to Hadron Colliders

We consider the effects of heavy Majorana neutrinos N with sub-TeV masses. We argue that the mere presence of these particles would be a signal of physics beyond the minimal seesaw mechanism and their interactions are, therefore, best described using an effective Lagrangian. We then consider the complete set of leading effective operators (up to dimension 6) involving the N and Standard Model fields and show that these interactions can be relatively easy to track at high-energy colliders. For example, we find that an exchange of a TeV-scale heavy vector field can yield thousands of characteristic same-sign lepton number violating l + l + jj events (j = light jet) at the LHC if mN < ∼ 600 GeV, which can also have a distinctive forward-backward asymmetry signal; even the Tevatron has good prospects for this signature if mN < 300 GeV.

Scalar flavor changing neutral currents and rare top quark decays in a two Higgs doublet model 'for the top quark'

In the so called two Higgs doublet model for the top-quark (T2HDM), first suggested by Das and Kao, the top quark receives a special status, which endows it with a naturally large mass, and also potentially gives rise to large flavor changing neutral currents (FCNC) only in the up-quark sector. In this paper we calculate the branching ratio (BR) for the rare decays t → ch and h → t̄c (h is a neutral Higgs) in the T2HDM, at tree level and at 1-loop when it exceeds the tree-level. We compare our results to predictions from other versions of 2HDM’s and find that the scalar FCNC in the T2HDM can play a significant role in these decays. In particular, the 1-loop mediated decays can be significantly enhanced in the T2HDM compared to the 2HDM of types I and II, in some instances reaching BR ∼ 10−4 which is within the detectable level at the LHC.

Charged Higgs boson effects in the production and decay of a heavy Majorana neutrino at the CERN LHC

We consider a new interaction between a heavy Majorana neutrino (N) and a charged Higgs boson (H{sup {+-}}), and show that it can have drastic implications on lepton-number-violating (LNV) signal of same-sign dileptons at the CERN LHC. The LNV signal of heavy Majorana neutrinos previously considered at the LHC, pp{yields}l{sup +}N{yields}l{sup +}l{sup +}W{sup -}, may be overwhelmed by pp{yields}l{sup +}N{yields}l{sup +}l{sup +}H{sup -}. With the subsequent decays H{sup -}{yields}tb or H{sup -}{yields}W{sup -}H{sup 0}, the heavy Majorana neutrino production leads to the spectacular events of l{sup +}l{sup +}bb+2 jets. We also explore the case m{sub N}<m{sub H{sup +}}, where the decay H{sup +}{yields}l{sup +}N can become the dominant N-production mechanism at the LHC. In particular, we show that the process gb{yields}tH{sup -} followed by t{yields}bW{sup +} and H{sup -}{yields}l{sup -}N{yields}l{sup -}l{sup -}W{sup +} could lead to another type of spectacular events of l{sup -}l{sup -}b+4 jets.

Implications of aW+W−(ZZ)-Higgs boson- tc¯interaction fore+e−→tc¯νeν¯e,tc¯e+e−, tc¯Z and fort→cW+W−,cZZin a two Higgs doublet model

The Standard Model with one extra Higgs doublet may give rise to enhanced TREE-LEVEL flavor-changing-scalar coupling of a neutral Higgs to a pair of top-charm quarks. This coupling may drive a large TREE-LEVEL effective W^+W^-(ZZ) - Higgs - t c-bar interaction. As a result we find that the reactions e^+e^- -> t c-bar \nu_e \nu_e-bar, t c-bar e^+ e^-, t c-bar Z and the two rare top decays t -> cW^+W^-, t -> cZZ become very sensitive probes of such an effective interaction. The most promising ones, e^+e^- -> t c-bar \nu_e \nu_e-bar, t c-bar e^+ e^-, may yield several hundreds and up to thousands of such events at the Next Linear Collider with a center of mass energy of \sqrt{s}=0.5 - 2 TeV if the mass of the lightest neutral Higgs is a few hundred GeV. The rare decays t -> cW^+W^- and t -> cZZ may be accessible at the LHC if the mass of the lightest neutral Higgs lies in the narrow window 150 GeV < m_h < 200 GeV.

Collider signals of maximal flavor violation : Same-sign leptons from same-sign top quarks at the Fermilab Tevatron

In models of maximal flavor violation (MxFV) suggested in [S. Bar-Shalom and A. Rajaraman, Phys. Rev. D 77, 095011 (2008).] there is at least one new scalar {phi}{sub FV} which couples to the quarks via {phi}{sub FV}q{sub i}q{sub j}{proportional_to}{xi}{sub ij} where {xi}{sub i3},{xi}{sub 3i}{approx}V{sub tb} for i=1, 2 and {xi}{sub 33}{approx}V{sub td} and V is the CKM matrix. In this article, we explore the potential phenomenological implications of MxFV for collider experiments. We study MxFV signals of same-sign leptons from same-sign top-quark pair production at the Tevatron and at the LHC. We show that the current Tevatron data set has strong sensitivity to this signature, for which there are no current limits. For example, if m{sub {phi}{sub FV}}{approx}200 GeV and the MxFV coupling {xi} has a natural value of {approx}1, we expect {approx}12 MxFV events to survive a selection requiring a pair of same-sign leptons, a tagged b-jet and missing transverse energy, over a background of approximately 4-5 events.

Large tree-level {ital CP} violation in {ital e}{sup +}{ital e}{sup {minus}}{r_arrow}{ital t{bar t}H}{sup 0} in the two-Higgs-doublet model

We find a large CP violation effect within the two-Higgs-doublet model for the reaction

Two Higgs doublets models and CP violating Higgs exchange in e+ e- ---> t anti-t Z

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R-parity violation and uses of the rare decay sneutrino-->gamma+gamma in hadron and photon colliders

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R-parity violation, sneutrino mixing phenomena and CP-violation in p \bar p -> sneutrino -> l^+ l^- + X

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