arXiv subjects

Shaouly Bar-Shalom

Publications and source records attributed to Shaouly Bar-Shalom.

At least 37 records · Page 2Linked to original sources

arXiv2014

The 125 GeV Higgs in the context of four generations with 2 Higgs doublets

We interpret the recent discovery of a 125 GeV Higgs-like state in the context of a two Higgs doublets model with a heavy 4th sequential generation of fermions, in which one Higgs doublet couples only to the 4th generation fermions, while the second doublet couples to the lighter fermions of the 1st-3rd families. This model is designed to accommodate the apparent heaviness of the 4th generation fermions and to effectively address the low-energy phenomenology of a dynamical electroweak symmetry breaking scenario. The physical Higgs states of the model are, therefore, viewed as composites primarily of the 4th generation fermions. We find that the lightest Higgs, h, is a good candidate for the recently discovered 125 GeV spin-zero particle, when tanβ~ O(1), for typical 4th generation fermion masses of M_{4G} = 400 -600 GeV, and with a large t - t' mixing in the right-handed quarks sector. This, in turn, leads to BR(t' -> t h) ~ O(1), which drastically changes the t' decay pattern. We also find that, based on the current Higgs data, this two Higgs doublet model generically predicts an enhanced production rate (compared to the SM) in the pp -> h -> tau tau channel and a reduced VV -> h -> gamma gamma and pp -> V -> Vh -> Vbb ones. Finally, the heavier CP-even Higgs is excluded by the current data up to m_H ~ 500 GeV, while the pseudoscalar state, A, can be as light as 130 GeV. These heavier Higgs states and the expected deviations from the SM in some of the Higgs production channels can be further excluded or discovered with more data.

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arXiv2014

Higgs-radion unification: radius stabilization by an SU(2) bulk doublet and the 126 GeV scalar

We investigate a Randall-Sundrum model with an SU(2) doublet propagating in the bulk. Upon calculating its gravitational effect we find that a stabilized radius can be generated without the use of an additional scalar, as needed for example in the Goldberger-Wise (GW) mechanism, and with no additional fine-tuning other than the inescapable one due to the cosmological constant; similar tuning is also present in the GW mechanism. The lowest scalar excitation in this scenario, the counterpart of the radion of the GW mechanism, has both radion-like and Higgs-like couplings to the SM fields. It, thus, plays a dual role and we, therefore, denote it as the "Higgs-radion" ($h_r$). As opposed to the GW radion case, our Higgs-radion is found to be compatible with the 126 GeV scalar recently discovered at the LHC, at the level of $1σ$, with a resulting $95\%$ CL bound on the KK-gluon mass of: $4.48~TeV<M_{KKG}< 5.44~TeV$. An important consequence of our setup should be accentuated: the radion of the traditional RS scenarios simply does not exist, so that our Higgs-radion is not the conventional mixed state between the GW radion and the Higgs.

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arXiv2013

Hybrid dynamical electroweak symmetry breaking with heavy quarks and the 125 GeV Higgs

Existing models of dynamical electroweak symmetry breaking (EWSB) find it very difficult to get a Higgs of mass lighter than $m_t$. Consequently, in light of the LHC discovery of the ~125 GeV Higgs, such models face a significant obstacle. Moreover, with three generations those models have a superheavy cut-off around $10^{17}$ GeV, requiring a significant fine-tuning. To overcome these twin difficulties, we propose a hybrid framework for EWSB, in which the Higgs mechanism is combined with a Nambu-Jona-Lasinio mechanism. The model introduces a strongly coupled doublet of heavy quarks with a mass around 500 GeV, which forms a condensate at a compositeness scale $Λ$ about a few TeV, and an additional unconstrained scalar doublet which behaves as a "fundamental" doublet at $Λ$. This "fundamental"-like doublet has a vanishing quartic term at $Λ$ and is, therefore, not the SM doublet, but should rather be viewed as a pseudo-Goldstone boson of the underlying strong dynamics. This setup is matched at the compositeness scale $Λ$ to a tightly constrained hybrid two Higgs doublet model, where both the composite and unconstrained scalars participate in EWSB. This allows us to get a good candidate for the recently observed 125 GeV scalar which has properties very similar to the Standard Model Higgs. The heavier (mostly composite) CP-even scalar has a mass around 500 GeV, while the pseudoscalar and the charged Higgs particles have masses in the range 200 -300 GeV.

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arXiv2013

Dynamical Origin for the 125 GeV Higgs; a Hybrid setup

We describe a hybrid framework for electroweak symmetry breaking (EWSB), in which the Higgs mechanism is combined with a Nambu-Jona-Lasinio mechanism. The model introduces an unconstrained scalar (i.e., acts as "fundamental" but not the SM field) and a strongly coupled doublet of heavy quarks with a mass around 500 GeV, which forms a condensate at a compositeness scale Λ~ O(1) TeV. This setup is matched at that scale to a tightly constrained hybrid two Higgs doublet model, where both the composite and unconstrained scalars participate in EWSB. This allows us to get a good candidate for the recently observed 125 GeV scalar which has properties very similar to the Standard Model Higgs. The heavier (mostly composite) CP-even scalar has a mass around 500 GeV, while the pseudoscalar and the charged Higgs particles have masses in the range 200 -300 GeV.

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arXiv2012

Two Higgs doublets, a 4th generation and a 125 GeV Higgs: a review

We review the possible role that multi-Higgs models may play in our understanding of the dynamics of a heavy 4th sequential generation of fermions. We describe the underlying ingredients of such models, focusing on two Higgs doublets, and discuss how they may effectively accommodate the low energy phenomenology of such new heavy fermionic degrees of freedom. We also discuss the constraints on these models from precision electroweak data as well as from flavor physics and the implications for collider searches of the Higgs particles and of the 4th generation fermions, bearing in mind the recent observation of a light Higgs with a mass of ~125 GeV.

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arXiv2012

The Need for New Search Strategies for Fourth Generation Quarks at the LHC

Most limits on the fourth generation heavy top quark (the t') are based on the assumed dominance of t' -> Wb, which is expected to be case in the minimal fourth generation framework with a single Higgs (the so called SM4). Here we show, within a variant of a Two Higgs Doublet Model with four generations of fermions, that, in general, a different t' detection strategy is required if the physics that underlies the new heavy fermionic degrees of freedom goes beyond the "naive" SM4. We find that the recent CMS lower bounds: m_{t'}< 450 GeV in the semi-leptonic channel pp -> t't' -> lνqqbb and m_{t'}< 557 GeV in the dilepton channel pp -> t't' ->llννbb, that were obtained using the customary (SM4-driven) detection strategies, do not apply. In particular, we demonstrate that if the decay t' -> ht dominates, then applying the "standard" CMS search tools leads to a considerably relaxed lower bound: m_{t'} >~350 GeV. We, therefore, suggest an alternative search strategy that is more sensitive to beyond SM4 dynamics of the fourth generation fermions.

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arXiv2011

Muon g-2 and lepton flavor violation in a two Higgs doublets model for the fourth generation

In the minimal Standard Model (SM) with four generations (the so called SM4) and in "standard" two Higgs doublets model (2HDM) setups, e.g., the type II 2HDM with four fermion generations, the contribution of the 4th family heavy leptons to the muon magnetic moment is suppressed and cannot accommodate the measured $ \sim 3 σ$ access with respect to the SM prediction. We show that in a 2HDM for the 4th generation (the 4G2HDM), which we view as a low energy effective theory for dynamical electroweak symmetry breaking, with one of the Higgs doublets coupling only to the 4th family leptons and quarks (thus effectively addressing their large masses), the loop exchanges of the heavy 4th generation neutrino can account for the measured value of the muon anomalous magnetic moment. We also discuss the sensitivity of the lepton flavor violating decays $μ\to e γ$ and $τ\to μγ$ and of the decay $B_s \to μμ$ to the new couplings which control the muon g-2 in our model.

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arXiv2011

Two Higgs doublets with 4th generation fermions - models for TeV-scale compositeness

We construct a class of two Higgs doublets models with a 4th sequential generation of fermions that may effectively accommodate the low energy characteristics and phenomenology of a dynamical electroweak symmetry breaking scenario which is triggered by the condensates of the 4th family fermions. In particular, we single out the heavy quarks by coupling the "heavier" Higgs doublet (Φ_h) which possesses a much larger VEV only to them while the "lighter" doublet (Φ_l) couples only to the light fermions. We study the constraints on these models from precision electroweak data as well as from flavor data. We also discuss some distinct new features that have direct consequences on the production and decays of the 4th family quarks and leptons in high energy colliders; in particular the conventional search strategies for t' and b' may need to be significantly revised.

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arXiv2010

Second order direct CP asymmetry in B_(s) -> X l nu

A direct CP asymmetry in inclusive semileptonic B_(s) decays vanishes by CPT to lowest order in weak interactions. Calculating the asymmetry at second order weak interactions in the Cabibbo-Kobayashi-Maskawa framework we find A_{sl} = (-3.2 \pm 0.9) x 10^{-9}. A maximal asymmetry which is two orders of magnitude larger is estimated in a left-right symmetric model. This quite generic upper bound implies negligible effects on wrong-sign lepton asymmetries in B^0 and B_s decays.

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arXiv2010

Collider signals of a composite Higgs in the Standard Model with four generations

Recent fits of electroweak precision data to the Standard Model (SM) with a 4th sequential family (SM4) point to a possible "three-prong composite solution": (1) the Higgs mass is at the TeV-scale, (2) the masses of the 4th family quarks t',b' are of O(500) GeV and (3) the mixing angle between the 4th and 3rd generation quarks is of the order of the Cabibbo angle, θ_{34} ~ O(0.1). Such a manifestation of the SM4 is of particular interest as it may suggest that the Higgs is a composite state, predominantly of the 4th generation heavy quarks. Motivated by the above, we show that the three-prong composite solution to the SM4 can have interesting new implications for Higgs phenomenology. For example, the Higgs can decay to a single heavy 4th generation quark via the 3-body decays (through an off-shell t' or b') H -> t'(bar) t'* -> t'(bar) b W+ and H -> b'(bar) b'* -> b'(bar) t W-. These flavor diagonal decays can be dramatically enhanced at the LHC (by several orders of magnitudes) due to the large width effects of the resonating heavy Higgs in the processes gg -> H -> t'(bar) t'* -> t'(bar) b W+ and gg -> H -> b'(bar) b'* -> b'(bar) t W-, thus yielding a viable signal above the corresponding continuum QCD production rates. In addition, the Higgs can decay to a single t' and b' in the loop-generated flavor changing (FC) channels H -> b' b(bar), t' t(bar). These FC decays are essentially "GIM-free" and can, therefore, have branching ratios as large as 10^{-4} - 10^{-3}.

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arXiv2010

Direct Mass Limits for Chiral Fourth-Generation Quarks in All Mixing Scenarios

Present limits on chiral fourth-generation quark masses $m_{b'}$ and $m_{t'}$ are broadly generalized and strengthened by combining both $t'$ and $b'$ decays and considering the full range of $t'$ and $b'$ flavor-mixing scenarios (with the lighter generations). Various characteristic mass-splitting choices are considered. With $m_{t'} > m_{b'}$ we find that CDF limits on the $b'$ mass vary by no more than 10-20% with any choice of flavor-mixing, while for the $t'$ mass, we typically find stronger bounds, in some cases up to $m_{t'} > 430$ GeV. For $m_{b'} > m_{t'}$ we find $m_{b'} > 380 - 430$ GeV, depending on the flavor-mixing and the size of the $m_{t'} - m_{b'}$ mass splitting.

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arXiv2009

Extended Friedberg Lee hidden symmetries, quark masses and CP-violation with four generations

Motivated in part by the several observed anomalies involving CP asymmetries of B and B_s decays, we consider the Standard Model with a 4th sequential family (SM4) which seems to offer a rather simple resolution. We initially assume T-invariance by taking the up and down-quark 4x4 mass matrix to be real. Following Friedberg and Lee (FL), we then impose a "hidden" symmetry on the unobserved ("hidden") up and down-quark SU(2) states. The hidden symmetry for four generations ensures the existence of two zero-mass eigenstates, which we take to be the (u,c) and (d,s) states in the up and down-quark sectors, respectively. Then, we simultaneously break T-invariance and the hidden symmetry by introducing two phase factors in each sector. This breaking mechanism generates the small quark masses m_u, m_c and m_d, m_s which, along with the orientation of the hidden symmetry, determine the size of CP-violation in the SM4. For illustration we choose a specific physical picture for the hidden symmetry and the breaking mechanism that reproduces the observed quark masses, mixing angles and CP-violation, and at the same time allows us to further obtain very interesting relations/predictions for the mixing angles of t and t'. For example, with this choice we get V_{td} ~ (V_{cb}/V_{cd} - V_{ts}/V_{us}) + O(λ^2) and V_{t'b} ~ V_{t'd}x(V_{cb}/V_{cd}), V_{tb'} ~ V_{t'd}x(V_{ts}/V_{us}), implying that V_{t'd} > V_{t'b},V_{tb'}. We furthermore find that the Cabibbo angle is related to the orientation of the hidden symmetry and that the key CP-violating quantity of our model at high-energies, J_{SM4} = Im[V_{tb} V_{t'b}^* V_{t'b'} V_{tb'}^*], which is the high-energy analogue of the Jarlskog invariant of the SM, is proportional to the light-quark masses and the measured CKM angles.

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arXiv2008

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^+ j j events (j=light jet) at the LHC if m_N < 600 GeV, which can also have a distinctive forward-backward asymmetry signal; even the Tevatron has good prospects for this signature if m_N < 300 GeV.

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arXiv2008

Models and Phenomenology of Maximal Flavor Violation

We consider models of maximal flavor violation (MxFV), in which a new scalar mediates large q_3 <-> q_1 or q_3 <-> q_2 flavor changing transitions (q_i is an i'th generation quark), while q_3 <-> q_3 transitions are suppressed, e.g., ξ_{31}, ξ_{13} ~ V_{tb} and ξ_{33} ~ V_{td}, where ξ_{ij} are the new scalar couplings to quarks and V is the CKM matrix. We show that, contrary to the conventional viewpoint, such models are not ruled out by the existing low energy data on K^0, B^0 and D^0 oscillations and rare K and B-decays. We also show that these models of MxFV can have surprising new signatures at the LHC and the Tevatron.

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arXiv2008

Collider Signals of Maximal Flavor Violation: Same-Sign Leptons from Same-Sign Tops at the Tevatron

In models of maximal flavor violation (MxFV) there is at least one new scalar $Φ_{FV}$ which couples to the quarks via $Φ_{FV} q_i q_j \propto ξ_{ij}$ where $ξ_{i3},ξ_{3i} \sim V_{tb}$ for $i = 1,2$ and $ξ_{33} \sim V_{td}$ 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 dataset has strong sensitivity to this signature, for which there are no current limits. For example, if $m_{Φ_{FV}} \sim 200$ GeV and the MxFV coupling $ξ$ has a natural value of $\sim 1$, we expect $\sim 12$ 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.

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arXiv2008

Charged Higgs Boson Effects in the Production and Decay of a Heavy Majorana Neutrino at the LHC

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

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arXiv2008

Scalar FCNC and rare top decays in a two Higgs doublet model "for the top"

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->tc (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.

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arXiv2007

Signature of heavy Majorana neutrinos at a linear collider: Enhanced charged Higgs pair production

A charged Higgs pair can be produced at an ee collider through a t-channel exchange of a heavy neutrino (N) via e^+ e^- -> H^+ H^- and, if N is a Majorana particle, also via the lepton number violating (LNV) like-sign reaction e^\pm e^\pm \to H^\pm H^\pm. Assuming no a-priori relation between the effective eNH^+ coupling (ξ) and light neutrino masses, we show that this interaction vertex can give a striking enhancement to these charged Higgs pair production processes. In particular, the LNV H^-H^- signal can probe N at the ILC in the mass range 100 GeV < m_N < 10^4 TeV and with the effective mixing angle, ξ, in the range 10^{-4} < ξ^2 < 10^{-8} - well within its perturbative unitarity bound and the neutrinoless double beta decay (ββ_{0ν}) limit. The lepton number conserving (LNC) e^+ e^- \to H^+ H^- mode can be sensitive to, e.g., an O(10) TeV heavy Majorana neutrino at a 500 GeV International Linear Collider (ILC), if ξ^2 > 0.001.

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