arXiv subjects

Shaouly Bar-Shalom

Publications and source records attributed to Shaouly Bar-Shalom.

At least 55 records · Page 3Linked to original sources

arXiv2006

Massive neutrinos in a grounds-up approach with quark-lepton similarity

We examine neutrino oscillations in a two Higgs doublet model (2HDM) in which the second doublet couples only to the third generation right-handed up-fermions, i.e., to t_R and to N_3 which is the heaviest right-handed Majorana neutrino. The inherently large tan(beta) of this model can naturally account for the large top mass and, based on a quark-lepton similarity ansatz, when embedded into a seesaw mechanism it can also account for the observed neutrino masses and mixing angles giving a very small theta_13: -0.017 < theta_13 < 0.021 at 99% CL, and a very restrictive prediction for the atmospheric mixing angle: 42.9(deg) < theta_atm < 45.2(deg) at 99% CL. The large value of tan(beta) also sets the mass scale of the heaviest right-handed Majorana neutrino N_3 and triggers successful leptogenesis through a CP-asymmetry in the decays of the N_1 (lightest right-handed Majorana) which is tan^2(beta) enhanced compared to the CP-asymmetry obtained in models for leptogenesis with one Higgs doublet or in the MSSM. This enhancement allows us to relax the lower bound on M_{N_1} and consequently also the lower bound on the reheating temperature of the early universe.

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arXiv2006

Majorana neutrinos and lepton-number-violating signals in top-quark and W-boson rare decays

We discuss rare lepton-number-violating top-quark and W-boson four-body decays to final states containing a same-charge lepton pair, of the same or of different flavors: t -> b W- li^+ lj^+ and W+ -> J J' li^+ lj^+, where i \ne j or i=j and J J' stands for two light jets originating from a (u-bar d) or a (c-bar s) pair. These ΔL=2 decays are forbidden in the Standard Model and may be mediated by exchanges of Majorana neutrinos. We adopt a model independent approach for the Majorana neutrinos mixing pattern and calculate the branching ratios (BR) for these decays. We find, for example, that for O(1) mixings between heavy and light Majorana neutrinos (not likely but not ruled out) and if at least one of the heavy Majorana neutrinos has a mass of ~100 GeV, then the BR's for these decays are: BR(t -> b li^+ lj^+ W-) ~ 10^{-4} and BR(W+ -> li^+ lj^+ J J') ~ 10^{-7} if m_N ~ 100 GeV and BR(t -> b li^+ lj^+ J J') ~ BR(W+ -> li^+ lj^+ J J') ~ 0.01 if m_N < 50 GeV. Taking into account the present limits on the neutrino mixing parameters, we obtain more realistic values for these BR's: BR(t -> b li^+ lj^+ W-) ~ 10^{-6} and BR(W+ -> li^+ lj^+ J J') ~ 10^{-10} for m_N ~ 100 GeV and BR(t -> b li^+ lj^+ J J') ~ BR(W+ -> li^+ lj^+ J J') ~ 10^{-6} for m_N < 50 GeV.

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arXiv2006

Neutrino masses, mixing and leptogenesis in a two Higgs doublet model for the third generation

We examine neutrino oscillations in a two Higgs doublet model (2HDM) in which the second doublet couples only to the third generation right-handed up-fermions, i.e., to t_R and to N_3 which is the heaviest right-handed Majorana neutrino. The inherently large tan(beta) of this model can naturally account for the large top mass and, based on a quark-lepton similarity ansatz, when embedded into a seesaw mechanism it can also account for the observed neutrino masses and mixing angles giving a very small θ_{13}: -0.017 < θ_{13} < 0.021 at 99% CL, and a very restrictive prediction for the atmospheric mixing angle: 42.9^0 < θ_{atm} < 45.2^0 at 99% CL. The large value of tan(beta) also sets the mass scale of the heaviest right-handed Majorana neutrino N_3 and triggers successful leptogenesis through a CP-asymmetry in the decays of the N_1 (lightest right-handed Majorana) which is tan^2(beta) enhanced compared to the CP-asymmetry obtained in models for leptogenesis with one Higgs doublet or in the MSSM. This enhancement allows us to relax the lower bound on M_{N_1} and consequently also the lower bound on the reheating temperature of the early universe.

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arXiv2005

"Seesawing" away the hierarchy problem

We describe a model for the scalar sector where all interactions occur either at an ultra-high scale L_U ~ 10^16 - 10^19 GeV or at an intermediate scale L_I=10^9 - 10^11 GeV. The interaction of physics on these two scales results in an SU(2) Higgs condensate at the electroweak (EW) scale, L_EW, through a seesaw-like Higgs mechanism, L_EW ~ (L_I)^2/L_U, while the breaking of the SM SU(2)XU(1) gauge symmetry occurs at the intermediate scale L_I. The EW scale is, therefore, not fundamental but is naturally generated in terms of ultra-high energy phenomena and so the hierarchy problem is alleviated. We show that our ``seesaw-Higgs'' model predicts the existence of sub-eV neutrino masses which are generated through a ``two-step'' seesaw mechanism in terms of the same two ultra-high scales: m_nu ~ (L_I)^4/(L_U)^3 ~ (L_EW)^2/L_U. We also show that our seesaw Higgs model can be naturally embedded in theories with tiny extra dimensions of size R ~ 1/L_U ~ 10^{-16} fm, where the seesaw induced EW scale arises from a violation of a symmetry at a distant brane if there are 7 tiny extra dimensions.

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arXiv2005

Width Effects on Near Threshold Decays of the Top Quark t-> cWW,cZZ and of Neutral Higgs Bosons

The nonzero widths of heavy particles become significant when they appear in the final state of any decay occurring just around its kinematical threshold. To take into account such effects, a procedure, called the convolution method, was proposed by Altarelli, Conti and Lubicz. We expand their study which included only threshold effects for t-> bWZ in the standard model. We discuss finite width effects in the three body decays t-> cWW,cZZ and A^0(h^0)->tbW in the type III version of a two Higgs doublet model. In particular, we find a substantial enhancement in the decay t->cZZ, which brings its branching ratio to BR(t->cZZ)~10^-3, and in the decay A^0->tbW, which, unlike the h^0 case, becomes competitive with the A^0 two-body decay modes.

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arXiv2005

Seesaw induced electroweak scale, the hierarchy problem and sub-eV neutrino masses

We describe a model for the scalar sector where all interactions occur either at an ultra-high scale L_U ~ 10^{16}-10^{19} GeV or at an intermediate scale L_I ~ 10^{9}-10^{11} GeV. The interaction of physics on these two scales results in an SU(2) Higgs condensate at the electroweak (EW) scale, L_{EW}, through a seesaw-like Higgs mechanism, L_{EW} ~ L_I^2/L_U, while the breaking of the SM SU(2)XU(1) gauge symmetry occurs at the intermediate scale L_I. The EW scale is, therefore, not fundamental but is naturally generated in terms of ultra-high energy phenomena and so the hierarchy problem is alleviated. We show that the class of such ``seesaw Higgs'' models predict the existence of sub-eV neutrino masses which are generated through a ``two-step'' seesaw mechanism in terms of the same two ultra-high scales: m_nu ~ L_I^4/L_U^3 ~ L_{EW}^2/L_U. The neutrinos can be either Dirac or Majorana, depending on the structure of the scalar potential. We also show that our seesaw Higgs model can be naturally embedded in theories with tiny extra dimensions of size R ~ 1/L_U ~ 10^{-16} fm, where the seesaw induced EW scale arises from a violation of a symmetry at a distant brane; in particular, in the scenario presented there are 7 tiny extra dimensions.

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arXiv2004

B -> X_s gamma gamma and B_s -> gamma gamma in supersymmetry with broken R-parity

We examine the effects of R-parity violating (RPV) supersymmetry on the two-photon B decays B -> X_s gamma gamma and B_s -> gamma gamma. We find that, although there are many one-loop RPV diagrams that can contribute to these two-photon B decays, the RPV effect is dominated by a single diagram. This diagram, named here lambda-irreducible, has a distinct topology which is irrelevant for the b -> s gamma amplitude at one-loop and has thus a negligible effect on the one-photon decay B -> X_s gamma. We show that the lambda-irreducible RPV diagram can give BR(B_s -> gamma gamma) ~ 5*10^(-6) and BR(B -> X_s gamma gamma) ~ 6*10^(-7), which is about 16 and 5 times larger than the SM values, respectively. Although the enhancement to the decay width of B -> X_s gamma gamma is not that dramatic, we find that the energy distribution of the two photons is appreciably different from the SM, due to new threshold effects caused by the distinct topology of the RPV lambda-irreducible diagram. Moreover, this diagram significantly changes the forward-backward asymmetry with respect to the softer photon in B -> X_s gamma gamma. Thus, the RPV effect in B -> X_s gamma gamma can be discerned using these observables.

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arXiv2004

Three heavy jet events at hadron colliders as a sensitive probe of the Higgs sector

Assuming that a non-standard neutral Higgs with an enhanced Yukawa coupling to a bottom quark is observed at future hadron experiments, we propose a method for a better understanding of the Higgs sector. Our procedure is based on "counting" the number of events with heavy jets (where "heavy" stands for a c or b jet) versus b jets, in the final state of processes in which the Higgs is produced in association with a single high p_T c or b jet. We show that an observed signal of the type proposed, at either the Tevatron or the LHC, will rule out the popular two Higgs doublet model of type II as well as its supersymmetric version - the Minimal Supersymmetric Standard Model (MSSM), and may provide new evidence in favor of some more exotic multi Higgs scenarios. As an example, we show that in a version of a two Higgs doublet model which naturally accounts for the large mass of the top quark, our signal can be easily detected at the LHC within that framework. We also find that such a signal may be observable at the upgraded Tevatron RunIII, if the neutral Higgs in this model has a mass around 100 GeV and \tanβ> 50 and if the efficiency for distinguishing a c jet from a light jet will reach the level of 50%.

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arXiv2004

Effective R-parity violation from supersymmetry breaking

We present a scenario in which Yukawa-like R-parity violating (RPV) couplings are naturally suppressed. In our framework, RPV is assumed to originate from the SUSY breaking mechanism and then transmitted into the SUSY Lagrangian only through soft SUSY breaking operators in the scalar potential. The RPV Yukawa-like operators of the superpotential, conventionally parametrized by the couplings λ, λ' and λ'', are then generated through loops containing the SUSY scalars, the gauginos and the soft RPV interactions and are, therefore, manifest as effective operators with a typical strength of order 10^{-3}.

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arXiv2003

Lepton-flavor-violating decays L->l gamma gamma as a new probe of supersymmetry with broken R-parity

Lepton-flavor-violating decays of the type L->l gamma gamma (mu->e gamma gamma, tau->e gamma gamma, and tau->mu gamma gamma) are proposed as new probes of R-parity-violating supersymmetry. Non-penguin diagrams with a sneutrino that decays into two photons via a triangle graph might trigger such decays even in the absence of the corresponding radiative decays into one photon only, e.g. mu->e gamma. Thus, processes of the type L->l gamma gamma may provide an independent probe of new flavor physics.

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arXiv2002

Flavor changing Z-decays from scalar interactions at a Giga-Z Linear Collider

The flavor changing decay Z -> d_I \bar{d}_J is investigated with special emphasis on the b \bar{s} final state. Various models for flavor violation are considered: two Higgs doublet models (2HDM's), supersymmetry (SUSY) with flavor violation in the up and down-type squark mass matrices and SUSY with flavor violation mediated by R-parity-violating interaction. We find that, within the SUSY scenarios for flavor violation, the branching ratio for the decay Z -> b \bar{s} can reach 10^{-6} for large \tanβvalues, while the typical size for this branching ratio in the 2HDM's considered is about two orders of magnitudes smaller at best. Thus, flavor changing SUSY signatures in radiative Z decays such as Z -> b \bar{s} may be accessible to future ``Z factories'' such as a Giga-Z version of the TESLA design.

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arXiv2002

$B\to ϕπ$ and $B^0 \to ϕϕ$ in the Standard Model and new bounds on R parity violation

We study the pure penguin decays $B \to ϕπ$ and $B^0 \to ϕϕ$. Using QCD factorization, we find ${\cal B}(B^\pm \toϕπ^{\pm} )=2.0^{+0.3}_{-0.1}\times 10^{-8}$. For the pure penguin annihilation process $B^0 \to ϕϕ$, analyzed here for the first time, ${\cal B}(B^0 \toϕϕ)=2.1^{+1.6}_{-0.3}\times 10^{-9}$. The smallness of these decays in the Standard Model makes them sensitive probes for new physics. From the upper limit of $B\to ϕπ$,we find constraints on R parity violating couplings, $| λ{''}_{i23}λ{''}_{i21}|<6\times10^{-5}$, $| λ'_{i23}λ'_{i21}|<4\times10^{-4}$ and $| λ'_{i32}λ'_{i12}|<4\times10^{-4}$ for $i=1,2,3$. Our new bounds on $|λ{''}_{i23}λ{''}_{i21}|$ are one order of magnitude stronger than before. Within the available upper bounds for $| λ{''}_{i23}λ{''}_{i21}|$, $|λ'_{i23}λ'_{i21}|$ and $|λ'_{i32}λ'_{i12}|$, we find that ${\cal B}(B\toϕϕ)$ could be enhanced to $10^{-8}\sim 10^{-7}$. Experimental searches for these decays are strongly urged.

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arXiv2000

CP Violation in Top Physics

CP violation in top physics is reviewed. The Standard Model has negligible effects, consequently CP violation searches involving the top quark may constitute the best way to look for physics beyond the Standard Model. Non-standard sources of CP violation due to an extended Higgs sector with and without natural flavor conservation and supersymmetric theories are discussed. Experimental feasibility of detecting CP violation effects in top quark production and decays in high energy e+ e-, gamma-gamma, mu+ mu-, pp and p-bar p colliders are surveyed. Searches for the electric, electro-weak and the chromo-electric dipole moments of the top quark in e+ e- -> t-bar t and in p p -> t-bar t X are descibed. In addition, other mechanisms that appear promising for experiments, e.g., tree-level CP violation in e+ e- -> t-bar t h, t-bar t Z, t-bar t nu_e-bar nu_e and in the top decay t -> b tau nu_tau and CP violation driven by s-channel Higgs exchanges in p p, gamma gamma, mu+ mu- -> t-bar t etc., are also discussed.

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arXiv2000

Gauge Boson - Gauge Boson Scattering in Theories with Large Extra Dimensions

We consider the scattering amplitudes of the form V_1 V_2 -> V_3 V_4, where V_i=γ, Z, W or g(=gluon) are the Standard Model gauge bosons, due to graviton exchange in Kaluza-Klein theories with large extra dimensions. This leads to a number of experimentally viable signatures at high energy leptonic and hadronic colliders. We discuss the observability or future limits on the scale of the gravitational interactions (m_D), that may be obtained at an e+e- Next Linear Collider (NLC) and at the LHC, by studying some of these type of gauge boson scattering processes. We find that the attainable limits through these type of processes are: m_D > 3 TeV at the NLC and m_D > 6 TeV at the LHC.

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arXiv1999

Dijet Production at Hadron Colliders in Theories with Large Extra Dimensions

We consider the production of high invariant mass jet pairs at hadron colliders as a test for TeV scale gravitational effects. We find that this signal can probe effective Planck masses of about 10 TeV at the LHC with center of mass energy of 14 TeV and 1.5 TeV at the Tevatron with center of mass energy of 2 TeV. These results are compared to analogous scattering processes at leptonic colliders.

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arXiv1999

Graviton Production By Two Photon and Electron-Photon Processes In Kaluza-Klein Theories With Large Extra Dimensions

We consider the production of gravitons via two photon and electron-photon fusion in Kaluza-Klein theories which allow TeV scale gravitational interactions. We show that at electron-positron colliders, the processes l+l- -> l+ l- graviton, with l=e, mu, can lead to a new signal of low energy gravity of the form l+l- -> l+l- + missing energy which is well above the Standard Model background. For example, with two extra dimensions at the Next Linear Collider with a center of mass energy of 500 or 1000 GeV, hundreds to thousands such l+ l- graviton events may be produced if the scale of the gravitational interactions, M_D, is around a few TeV. At a gamma-electron collider, more stringent bounds may be placed on M_D via the related reaction e^-γ-> e^- graviton. For instance, if a 1TeV electron positron collider is converted to an electron-photon collider, a bound of ~10TeV may be placed on the scale M_D if the number of extra dimensions delta=2 while a bound of ~4TeV may be placed if delta=4.

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arXiv1999

Graviton Production By Two Photon Processes In Kaluza-Klein Theories With Large Extra Dimensions

We consider the production of gravitons via two photon fusion in Kaluza-Klein theories which allow TeV scale gravitational interactions. We find that the processes l+ l- to l+ l- + graviton, with l=electron or muon can put quite stringent bounds on such theories. For example, with two extra dimensions at the Next Linear Collider with a center of mass energy of 500 (1000) GeV attainable bounds on the scale of the gravitational interactions can reach about 6 (9) TeV.

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arXiv1997

Probing New Higgs-Top Interactions at the Tree-Level in a Future e^+e^- Collider

The possibility of observing large signatures of new CP-violating and flavor-changing Higgs-Top couplings in a future e^+e^- collider experiments such as e^+e^- -> t bar-t h, t bar-t Z and e^+e^- -> t bar-c ν_e bar-ν_e, t bar-c e^+ e^- is discussed. Such, beyond the Standard Model, couplings can occur already at the tree-level within a class of Two Higgs Doublets Models. Therefore, an extremely interesting feature of those reactions is that the CP-violating and flavor-changing effects are governed by tree-level dynamics. These reactions may therefore serve as unique avenues for searching for new phenomena associated with Two Higgs Doublets Models and, as is shown here, could yield statistically significant signals of new physics. We find that the CP-asymmetries in e^+e^- -> t bar-t h, t bar-t Z can reach tens of percents, and the flavor changing cross-section of e^+e^- -> t bar-c ν_e bar-ν_e is typically a few fb's, for light Higgs mass around the electroweak scale.

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