The Majorana nature of neutrinos is one of the most fundamental questions in particle physics.It is directly related to the violation of accidental lepton number symmetry. This motivated enormous efforts into the search of such process and among them, one conventional experiment is the neutrinoless double-beta decay (0νββ). On the other hand, there have been proposals of future electron-positron colliders as "Higgs factory" for the precise measurement of Higgs boson properties and it has been proposed to convert such machine into an electron-electron collider. This option enables a new way to probe TeV Majorana neutrino via the inverse 0νββ decay process (e − e − → W − W − ) as an alternative and complementary test to the conventional 0νββ decay experiments. In this paper, we investigate the collider search for e − e − → W − W − in different decay channels at future electron colliders. We find the pure hadronic channel, semi-leptonic channel with muon and pure leptonic channel with dimuon have the most discovery potential. 1Enormous neutrino oscillation experiments in the last two decades have provided definite evidence for non-zero neutrino masses and the mixing between different flavors [1][2][3]. Even though the recent discovery of a Higgs-like boson has significantly improved our knowledge over generation of SM fermion masses, being tiny but electric neutral, the origin of neutrino mass may remain an open question. Firs of all, if neutrino masses arise from Yukawa couplings as the same mechanicsm as quarks and charged leptons, one immediately encounters the O(10 −12 ) hierarchy in y ν /y t . A second argument arises from the prediction of electric charge quantization. Anomaly-free conditions determine U (1) Y as the unique U (1) gauge symmetry in SM up to a normalization factor [7]. Though extending SM with milli-charged Dirac neutrino does not explicitly violate the anomaly-free conditions, the hyper-charge assignment is no longer uniquely determined unless the neutrino is a Majorana particle [8]. On the other hand, the bound on neutrino electric charge Q ν is |Q ν | (0.5 ± 2.9) × 10 −21 e (68% CL) by assuming charge conservation in β-decay n → p+e − +ν e [4,5], and |Q ν | < 2×10 −15 e from SN1987A astrophysics observation [6]. These facts motivate the study of Majorana neutrinos.Taking the effective theory approach, Majorana mass term is from the non-renormalizablewith dimensionless coupling y ij . This dimensionfive operator breaks lepton number by two units (∆L = 2) and indicates new physics at some specific Λ However, they're still weaker than the electroweak precision observable(EWPO) bound from constraining the non-unitarity of leptonic mixing matrix [37]More detailed analyses are available in [38][39][40].As an alternative, e − e − → W − W − scattering process in Fig.1 mediated by Majorana neutrino exchange is sensitive to the TeV-seesaw scenario. The intriguing feature of this process is that it could be regarded as the inverse of 0νββ decay with LNV but could occur at colliders. In addition...
The precision measurement of the WWγ vertex at the future Large Hadron electron Collider (LHeC) at CERN is discussed in this paper. We propose to measure this vertex in the e − p → e − W AE j channel as a complement to the conventional charged current ν e γj channel. In addition to the cross section measurement, χ 2 method studies of angular variables provide powerful tools to probe the anomalous structure of triple gauge boson couplings. We study the distribution of the well-known azimuthal angle between the final state forward electron and jet in this vector-boson fusion process. On the other hand, full reconstruction of leptonic W decay opens a new opportunity to measure W polarization that is also sensitive to the anomalous triple gauge boson couplings. Taking into consideration the superior determination of parton distribution functions based on future LHeC data, the constraints of λ γ and Δκ γ might reach up to Oð10 −3 Þ level in the most ideal case with the 2-3 ab −1 data set, which shows a potential advantage compared to those from LHC and Large Electron-Positron Collider (LEP) data.
Motivated by the recent di-photon excess by both ATLAS and CMS collaborations at the LHC, we systematically investigate the production and di-photon decay of onia formed by pair of all possible color exotic scalars in minimal extension. When such scalar massive meta-stable colored and charged (MMCC) particles are produced in pair near threshold, η onium can be formed and decay into di-photon through annihilation as pp → η → γγ.Squarkonium is formed by meta-stable squarks in supersymmetric models such as stoponium. Diquarkonium is formed by meta-stable color sextet diquarks which may be realized in the Pati-Salam model. Octetonium is formed by color octet scalars bosons as in the Manohar-Wise model. Stoponium prediction is much smaller than the required signal to account for the di-photon excess. Due to the enhancement factor from color and electric charge, predictions of diquarkonium and octetonium are of O(10 fb) which are significantly greater than the stoponium prediction. Since the color enhancement also results in large production at the colliders, such light color exotic states of O(375 GeV) suffer from severe direct search constraints. On the other hand, if their dominant decay mode involve top quark, they may be buried in the tt plus jets samples and can potentially be searched via
An on-shell effective theory (OSET) approach has been widely used in searches of various supersymmetric signals, in particular, gluino/squark pairs with long cascade decay chains in which complete matrix element calculations may encounter high dimensional integrations. On the other hand, leptons from polarized chargino decays may show a significant boost effect in some scenarios and simulation without polarization information may underestimate or overestimate the lepton p T cut efficiencies in the first place. We study the polarization effects in leptonic decaying charginos from squarks or gluinos. Taking the polarization effects into account, we find it still justifiable to take only the OSET approach for a large parameter region, for instance, the first two generation squarks due to indistinguishable final states as well as a flat angular distribution in the motion of the lepton. On the other hand, we use the leptonic stop to illustrate the feature and find that the lepton p T cut efficiencies in cross section measurements can have maximally 25 % reduction or maximally 17 % enhancement in comparison with the kinematics-only approach. The signal rates after the cuts simulated by OSET are then underestimated/overestimated and the real bound on the squark/gluino should be more stringent or loose for a specific choice of the chargino and one can take the simulated efficiencies as a fast-simulation factor to multiply to the OSET simulated results.
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