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Gad Eilam

Publications and source records attributed to Gad Eilam.

At least 19 recordsLinked to original sources

Search for like-sign dileptons plus two jets signal in the framework of the manifest left-right symmetric model

The left-right symmetric model may present evidence of new physics at the era of the Large Hadron Collider (LHC). We use its framework to investigate the lepton number violating process $pp \to e^\pm e^\pm jj+X$ at the $\unit[14]{TeV}$ LHC. We show that for an integrated luminosity of $\unit[300]{fb^{-1}}$, the discovery contour of the right-handed boson $W_R$ and the right-handed electron neutrino $N_e$ as a result of the $pp \to W^\pm_R \to e^\pm N \to e^\pm e^\pm jj$ process can be expanded upon considering an additional channel mediated by the right-handed doubly charged Higgs $δ^{\pm\pm}_R$, i.e. $pp\to W^\pm_R \to W^{\mp*}_R δ^{\pm\pm*}_R \to e^\pm e^\pm jj$.

hep-ph

R-Parity Violating Supersymmetry and the 125 GeV Higgs signals

We study the impact of R-parity violating Supersymmetry (RPV SUSY) on the 125 GeV Higgs production and decay modes at the LHC. We assume a heavy SUSY spectrum with multi-TeV squarks and SU(2) scalar singlets as well as the decoupling limit in the SUSY Higgs sector. In this case the lightest CP-even Higgs is SM-like when R-parity is conserved. In contrast, we show that when R-parity violating interactions are added to the SUSY framework, significant deviations may occur in some production and decay channels of the 125 GeV Higgs-like state. Indeed, we assume a single-flavor (mostly third generation) Bilinear RPV (BRPV) interactions, which generate Higgs-sneutrino mixing, lepton-chargino mixing and neutrino-neutralino mixing, and find that notable deviations of ${\cal O}(20-30\%)$ may be expected in the Higgs signal strength observables in some channels, e.g., in $p p \to h \to μ^+ μ^-, τ^+ τ^-$. Moreover, we find that new and detectable signals associated with BRPV Higgs decays to gauginos, $h\rightarrowν_τ\tildeχ_{2}^{0}$ and $h\rightarrowτ^{\pm}χ_{2}^{\mp}$, may also arise in this scenario. These decays yield a typical signature of $h \to τ^\pm \ell^\mp + {\not\!\! E_T}$ ($\ell =e,μ,τ$) that can be much larger than in the SM, and may also be accompanied by an ${\cal O}(20-30\%)$ enhancement in the di-photon signal $pp \to h \to γγ$. We also examine potential interesting signals of Trilinear R-parity violation (TRPV) interactions in the production and decays of the Higgs-sneutrino BRPV mixed state (assuming it is the 125 GeV scalar) and show that, in this case also, large deviations up to ${\cal O}(100\%)$ are expected in e.g., $p p \to h \to μ^+ μ^-, τ^+ τ^-$, which are sensitive to the BRPV$\times$TRPV coupling product.

hep-ph

Light-quarks Yukawa couplings and new physics in exclusive high-$p_T$ Higgs + jet and Higgs + $b$-jet events

We suggest that the exclusive Higgs + light (or b)-jet production at the LHC, $pp \to h+j(j_b)$, is a rather sensitive probe of the light-quarks Yukawa couplings and of other forms of new physics (NP) in the Higgs-gluon $hgg$ and quark-gluon $qqg$ interactions. We study the Higgs $p_T$-distribution in $pp \to h+j(j_b) \to γγ+ j(j_b)$, i.e., in $h+j(j_b)$ production followed by the Higgs decay $h \to γγ$, employing the ($p_T$-dependent) signal strength formalism to probe various types of NP which are relevant to these processes and which we parameterize either as scaled Standard Model (SM) couplings (the kappa-framework) and/or through new higher dimensional effective operators (the SMEFT framework). We find that the exclusive $h+j(j_b)$ production at the 13 TeV LHC is sensitive to various NP scenarios, with typical scales ranging from a few TeV to ${\cal O}(10)$ TeV, depending on the flavor, chirality and Lorentz structure of the underlying physics.

hep-ph

Implementation of the left-right symmetric model in FeynRules

We present an implementation of the manifest left-right symmetric model in FeynRules. The different aspects of the model are briefly described alongside the corresponding elements of the model file. The model file is validated and can be easily translated to matrix element generators such as MadGraph5_aMC@NLO, CalcHEP, Sherpa, etc. The implementation of the left-right symmetric model is a useful step for studying new physics signals with the data generated at the LHC.

hep-ph

Contact Interactions in Higgs-Vector Boson Associated Production at the ILC

We explore new physics (NP) effects in Higgs-Vector boson associated production at a future International Linear Collider (ILC) via e+e- -> Zh,Zhh, using effective field theory (EFT) techniques. In particular, we focus on a certain class of dimension 6 operators, which are generated by tree-level exchanges of a new heavy vector field in the underlying theory. These operators induce new contact terms of the form ψψϕDϕ, involving the Standard Model (SM) fermions (ψ), gauge-bosons (D is the covariant derivative) and the SM Higgs field (ϕ). We investigate the high-energy behaviour of these new effective interactions in e+e- -> Zh,Zhh, imposing bounds from electroweak precision measurements, and show that the ILC is an excellent testing ground for probing this type of NP via e+e- -> Zh,Zhh. We also address the validity of the EFT expansion and we study the correlation between the hZ and hhZ signals, which can be utilized in future searches for NP in these channels.

hep-ph

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.

hep-ph

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.

hep-ph

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.

hep-ph

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}.

hep-ph

Neutrinoless double beta decays of the top quark and other effects of heavy Majorana neutrinos

We discuss the rare decay of the top quark into a pair of same charge leptons (with identical or different flavors), a b quark and a (real or virtual) W-. The above process proceeds only if the exchanged neutrino N is of the Majorana type. This decay is the neutrinoless double b decay of the top. We find measurable values for its rate at the LHC with luminosity of 100 inverse fb. Furthermore, we consider an interaction of charged Higgs bosons with N which leads to lepton number violating processes such as pp to l+ N to l+ l+ H-, exhibiting spectacular events of the type: l+ l+ b b(bar) + 2 jets.

hep-ph

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.

hep-ph

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.

hep-ph

Single Top Production via Gluon Fusion at CERN LHC

We calculate the one-loop flavor violating top quark decay t -> cgg in the Minimal Supersymmetric Standard Model. We discuss the branching ratios obtained with minimal flavor violation, as well as with soft-supersymmetry induced general flavor violation. Based on this rate we calculate the cross section for the single top quark production via gluon fusion, gg -> t cbar, and evaluate its contribution to the cross section for single top quark production in pp collisions at the Large Hadron Collider. We calculate all contributions coming from the standard model and charged Higgs loops, as well as gluino (and neutralino)-up-type squarks, and chargino-down-type squarks loops. Our numerical results show that the gluino and the chargino contributions are largest over the whole parameter range in the unconstrained Minimal Supersymmetric Standard Model. While in general the gluino contributions dominate the cross section, this result depends on the supersymmetric flavor violating parameters in the up and down squark sector, the relative mass of the gauginos, and whether or not the Grand Unified Theory relationships between gaugino masses are satisfied. In the most promising scenarios, the pp -> t cbar + tbar c + X cross section at the Large Hadron Collider can reach a few hundreds fb.

hep-ph

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.

hep-ph

Experimental Constraints on Scharm-Stop Flavor Mixing and Implications in Top-quark FCNC Processes

We examine experimental constraints on scharm-stop flavor mixing in the minimal supersymmetric standard model, which arise from the experimental bounds on squark and Higgs boson masses, the precision measurements of W-boson mass and the effective weak mixing angle, as well as the experimental data on B_s-\bar B_s mixing and b -> s gamma. We find that the combined analysis can put rather stringent constraints on \tilde{c}_L-\tilde{t}_L and \tilde{c}_L-\tilde{t}_R mixings. As an illustration for the effects of such constraints, we examine various top-quark flavor-changing neutral-current processes induced by scharm-stop mixings at the LHC and find that their maximal rates are significantly lowered.

hep-ph

Rare Decay of the Top t -> cgg in the Standard Model

We calculate the one-loop flavor changing neutral current top quark decay t -> cgg in the Standard Model. We demonstrate that the rate for t -> cgg exceeds the rate for a single gluon emission t -> cg by about two orders of magnitude, while the rate for t -> cq barq (q=u) is slightly smaller than for t -> cg.

hep-ph

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.

hep-ph

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.

hep-ph