Left-right Symmetric Extension Research Articles

Fatjet signatures of heavy neutrinos and heavy leptons in a left-right model with universal seesaw at the HL-LHC

We perform a collider search for fatjet signals originating from boosted heavy neutral and charged leptons with masses between a few hundred GeV to a TeV. These heavy leptons originate from the decay of heavy gauge bosons with masses above 4 TeV in a left-right symmetric extension of the Standard Model (SM), which considers a universal seesaw mechanism for the generation of all the SM fermion masses. The fatjet signals arise naturally in this model due to the presence of heavy seesaw partners of the SM fermions which decay to SM gauge bosons carrying large boosts. We employ substructure based variables lepton sub-jet fraction (LSF) and lepton mass drop (LMD) together with kinematic variables of fatjets to look for fatjet signals associated with non-isolated leptons. These variables help in reducing the SM backgrounds while retaining enough statistics for signal events, which leads to a robust discovery potential at the high-luminosity Large Hadron Collider (HL-LHC).

SU(2)R and its axion in cosmology: A common origin for inflation, cold sterile neutrinos, and baryogenesis

We introduce an axion-inflation model embedded in the Left-Right symmetric extension of the SM in which $W_R$ is coupled to the axion. This model merges three milestones of modern cosmology, i.e., inflation, cold dark matter, and baryon asymmetry. Thus, it can naturally explain the observed coincidences among cosmological parameters, i.e., $\eta_{B}\approx P_{\zeta}$ and $\Omega_{DM} \simeq 5~\Omega_{B}$. The source of asymmetry is spontaneous CP violation in the physics of inflation, and the lightest right-handed neutrino is the cold dark matter candidate with mass $m_{N_1}\sim 1~GeV$. The introduced mechanism does not rely on the largeness of the unconstrained CP-violating phases in the neutrino sector nor fine-tuned masses for the heaviest right-handed neutrinos. It has two unknown fundamental scales, i.e. scale of inflation $\Lambda_{\rm inf}=\sqrt{HM_{Pl}}$ and left-right symmetry breaking $\Lambda_{F}$. Sufficient matter asymmetry demands $\Lambda_{\rm inf}\approx\Lambda_{F}$. The baryon asymmetry and dark matter today are remnants of a pure quantum effect (chiral anomaly) in inflation, which, thanks to flavor effects, are memorized by cosmic evolution.

Chiral anomaly in SU(2)R-axion inflation and the new prediction for particle cosmology

Upon embedding the axion-inflation in the minimal left-right symmetric gauge extension of the SM with gauge group SU(2)L × SU(2)R × U(1)B−L, [1] proposed a new particle physics model for inflation. In this work, we present a more detailed analysis. As a compelling consequence, this setup provides a new mechanism for simultaneous baryogenesis and right-handed neutrino creation by the chiral anomaly of WR in inflation. The lightest right-handed neutrino is the dark matter candidate. This setup has two unknown fundamental scales, i.e., the scale of inflation and left-right symmetry breaking SU(2)R × U(1)B−L→ U(1)Y. Sufficient matter creation demands the left-right symmetry breaking scale happens shortly after the end of inflation. Interestingly, it prefers left-right symmetry breaking scales above 1010 GeV, which is in the range suggested by the non-supersymmetric SO(10) Grand Unified Theory with an intermediate left-right symmetry scale. Although WR gauge field generates equal amounts of right-handed baryons and leptons in inflation, i.e. B − L = 0, in the Standard Model sub-sector B − LSM ≠ 0. A key aspect of this setup is that SU(2)R sphalerons are never in equilibrium, and the primordial B − LSM is conserved by the Standard Model interactions. This setup yields a deep connection between CP violation in physics of inflation and matter creation (visible and dark); hence it can naturally explain the observed coincidences among cosmological parameters, i.e., ηB ≃ 0.3Pζ and ΩDM ≃ 5ΩB. The new mechanism does not rely on the largeness of the unconstrained CP-violating phases in the neutrino sector nor fine-tuned masses for the heaviest right-handed neutrinos. The SU(2)R-axion inflation comes with a cosmological smoking gun; chiral, non-Gaussian, and blue-tilted gravitational wave background, which can be probed by future CMB missions and laser interferometer detectors.

Constraining the gauge and scalar sectors of the doublet left-right symmetric model

We consider a left-right symmetric extension of the Standard Model where the spontaneous breakdown of the left-right symmetry is triggered by doublets. The electroweak ρ parameter is protected from large corrections in this Doublet Left-Right Model (DLRM), contrary to the triplet case. This allows in principle for more diverse patterns of symmetry breaking. We consider several constraints on the gauge and scalar sectors of DLRM: the unitarity of scattering processes involving gauge bosons with longitudinal polarisations, the radiative corrections to the muon ∆r parameter and the electroweak precision observables measured at the Z pole and at low energies. Combining these constraints within the frequentist CKMfitter approach, we see that the fit pushes the scale of left-right symmetry breaking up to a few TeV, while favouring an electroweak symmetry breaking triggered not only by the SU(2)L×SU(2)R bi-doublet, which is the case most commonly considered in the literature, but also by the SU(2)L doublet.

Search for heavy neutrinos and third-generation leptoquarks in hadronic states of two \u03c4 leptons and two jets in proton-proton collisions at sqrt{s}=13 TeV

A search for new particles has been conducted using events with two high transverse momentum τ leptons that decay hadronically and at least two energetic jets. The analysis is performed using data from proton-proton collisions at sqrt{s}=13 TeV, collected by the CMS experiment at the LHC in 2016 and corresponding to an integrated luminosity of 35.9 fb−1. The observed data are consistent with standard model expectations. The results are interpreted in the context of two physics models. The first model involves right-handed charged bosons, WR, that decay to heavy right-handed Majorana neutrinos, Nℓ (ℓ = e, μ, τ), arising in a left-right symmetric extension of the standard model. The model considers that Ne and Nμ are too heavy to be detected at the LHC. Assuming that the Nτ mass is half of the WR mass, masses of the WR boson below 3.50 TeV are excluded at 95% confidence level. Exclusion limits are also presented considering different scenarios for the mass ratio between Nτ and WR, as a function of WR mass. In the second model, pair production of third-generation scalar leptoquarks that decay into ττbb is considered, resulting in an observed exclusion region with leptoquark masses below 1.02 TeV, assuming a 100% branching fraction for the leptoquark decay to a τ lepton and a bottom quark. These results represent the most stringent limits to date on these models.

Neutrino predictions from a left-right symmetric flavored extension of the standard model

We propose a left-right symmetric electroweak extension of the Standard Model based on the Δ (27) family symmetry. The masses of all electrically charged Standard Model fermions lighter than the top quark are induced by a Universal Seesaw mechanism mediated by exotic fermions. The top quark is the only Standard Model fermion to get mass directly from a tree level renormalizable Yukawa interaction, while neutrinos are unique in that they get calculable radiative masses through a low-scale seesaw mechanism. The scheme has generalized μ − τ symmetry and leads to a restricted range of neutrino oscillations parameters, with a nonzero neutrinoless double beta decay amplitude lying at the upper ranges generically associated to normal and inverted neutrino mass ordering.

Neutrino and collider implications of a left-right extended Zee model

We study a simple left-right symmetric (LRS) extension of the Zee model for neutrino mass generation. An extra $SU(2)_{L/R}$ singlet charged scalar helps in generating a loop-induced Majorana mass for neutrinos in this model. The right-handed neutrinos in this case are very light of the order of a few eV to a few MeV which makes this scenario quite different from other LRS models. We have analyzed the scalar potential and Higgs spectrum in detail, which also play an important role for the neutrino phenomenology. We identified the parameter regions in the model which satisfy the experimentally observed neutrino masses and mixings along with other experimental constraints. We have then studied the collider signatures of the charged scalar at $e^+e^-$ colliders with different benchmark points. It is possible to get a huge enhancement in the production cross-section of the charged scalar at lepton collider compared to the hadron colliders, resulting in a much stronger signal which can be easily observed at the upcoming ILC or CLIC experiments.

Doubly-charged scalars in the type II seesaw mechanism: Fundamental symmetry tests and high-energy searches

We analyze the sensitivity of low-energy fundamental symmetry tests to interactions mediated by doubly-charged scalars that arise in type II seesaw models of neutrino mass and their left-right symmetric extensions. We focus on the next generation measurement of the parity-violating asymmetry in M\o{}ller scattering planned by the MOLLER collaboration at Jefferson Laboratory. We compare the MOLLER sensitivity to that of searches for charged lepton flavor violation (CLFV) and neutrinoless double beta-decay ($0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$-decay) as well as present and possible future high-energy collider probes. We show that for the simplest type-II seesaw scenario, CLFV searches have the greatest sensitivity. However, in a left-right symmetric extension where the scale of parity-breaking is decoupled from the $SU(2{)}_{R}$-breaking scale, the MOLLER experiment will provide a unique probe of scalar triplet interactions in the right-handed sector for a doubly-charged scalar mass up to $\ensuremath{\sim}10\text{ }\text{ }\mathrm{TeV}$ and help elucidate the mechanism of $0\ensuremath{\nu}\ensuremath{\beta}\ensuremath{\beta}$-decay.

Measurement of the double-β decay half-life and search for the neutrinoless double-β decay of 48Ca with the NEMO-3 detector

Neutrinoless double-β decay is a powerful probe of lepton number violating processes that may arise from Majorana terms in neutrino masses, or from supersymmetric, left-right symmetric, and other extensions of the Standard Model. Of the candidate isotopes for the observation of this process, 48Ca has the highest Qββ-value, resulting in decays with energies significantly above most naturally occurring backgrounds. The nucleus also lends itself to precise matrix element calculations within the nuclear shell model. We present the world’s best measurement of the two-neutrino double-β decay of 48Ca, obtained by the NEMO-3 collaboration using 5.25 yr of data recorded with a 6.99 g sample of isotope, yielding ≈ 150 events with a signal to background ratio larger than 3. Neutrinoless modes of double-β decay are also investigated, with no evidence of new physics. Furthermore, these results indicate that two-neutrino double-β decay would be the main source of background for similar future searches using 48Ca with significantly larger exposures.

Search for third-generation scalar leptoquarks and heavy right-handed neutrinos in final states with two tau leptons and two jets in proton-proton collisions at sqrt{s}=13 TeV

A search is performed for third-generation scalar leptoquarks and heavy right-handed neutrinos in events containing one electron or muon, one hadronically decaying τ lepton, and at least two jets, using a sqrt{s}=13 TeV pp collision data sample corresponding to an integrated luminosity of 12.9 fb-1 collected with the CMS detector at the LHC in 2016. The number of observed events is found to be in agreement with the standard model prediction. A limit is set at 95% confidence level on the product of the leptoquark pair production cross section and β2, where β is the branching fraction of leptoquark decay to a τ lepton and a bottom quark. Assuming β = 1, third-generation leptoquarks with masses below 850 GeV are excluded at 95% confidence level. An additional search based on the same event topology involves heavy right-handed neutrinos, NR, and right-handed W bosons, WR, arising in a left-right symmetric extension of the standard model. In this search, WR bosons are assumed to decay to a tau lepton and NR followed by the decay of the NR to a tau lepton and an off-shell WR boson. Assuming the mass of the right-handed neutrino to be half of the mass of the right-handed W boson, WR boson masses below 2.9 TeV are excluded at 95% confidence level. These results improve on the limits from previous searches for third-generation leptoquarks and heavy right-handed neutrinos with τ leptons in the final state.

Search for heavy neutrinos or third-generation leptoquarks in final states with two hadronically decaying \u03c4 leptons and two jets in proton-proton collisions at sqrt{s}=13 TeV

A search for new particles has been conducted using events with two high transverse momentum (pT) τ leptons that decay hadronically, at least two high-pT jets, and missing transverse energy from the τ lepton decays. The analysis is performed using data from proton-proton collisions, collected by the CMS experiment in 2015 at sqrt{s}=13 TeV, corresponding to an integrated luminosity of 2.1 fb−1. The results are interpreted in two physics models. The first model involves heavy right-handed neutrinos, Nℓ (ℓ = e, μ, τ), and right-handed charged bosons, WR, arising in a left-right symmetric extension of the standard model. Masses of the WR boson below 2.35 (1.63) TeV are excluded at 95% confidence level, assuming the Nτ mass is 0.8 (0.2) times the mass of the WR boson and that only the Nτ flavor contributes to the WR decay width. In the second model, pair production of third-generation scalar leptoquarks that decay into ττbb is considered. Third-generation scalar leptoquarks with masses below 740 GeV are excluded, assuming a 100% branching fraction for the leptoquark decay to a τ lepton and a bottom quark. This is the first search at hadron colliders for the third-generation Majorana neutrino, as well as the first search for third-generation leptoquarks in the final state with a pair of hadronically decaying τ leptons and jets.

Minimal left-right symmetric intersecting D-brane model

We investigate left-right symmetric extensions of the standard model based on open strings ending on D-branes, with gauge bosons due to strings attached to stacks of D-branes and chiral matter due to strings stretching between intersecting D-branes. The left-handed and right-handed fermions transform as doublets under Sp(1)_L and Sp(1)_R, and so their masses must be generated by the introduction of Higgs fields in a bi-fundamental (2,2) representation under the two Sp(1) gauge groups. For such D-brane configurations the left-right symmetry must be broken by Higgs fields in the doublet representation of Sp(1)_R and therefore Majorana mass terms are suppressed by some higher physics scale. The left-handed and right-handed neutrinos pair up to form Dirac fermions which control the decay widths of the right-handed W' boson to yield comparable branching fractions into dilepton and dijets channels. Using the most recent searches at LHC13 Run II with 2016 data we constrain the (g_R, m_{W'}) parameter space. Our analysis indicates that independent of the coupling strength g_R, gauge bosons with masses m_{W'} \agt 3.5~{\rm TeV} are not ruled out. As the LHC is just beginning to probe the TeV-scale, significant room for W' discovery remains.

A consistent model for leptogenesis, dark matter and the IceCube signal

We discuss a left-right symmetric extension of the Standard Model in which the three additional right-handed neutrinos play a central role in explaining the baryon asymmetry of the Universe, the dark matter abundance and the ultra energetic signal detected by the IceCube experiment. The energy spectrum and neutrino flux measured by IceCube are ascribed to the decays of the lightest right-handed neutrino $N_1$, thus fixing its mass and lifetime, while the production of $N_1$ in the primordial thermal bath occurs via a freeze-in mechanism driven by the additional $SU(2)_R$ interactions. The constraints imposed by IceCube and the dark matter abundance allow nonetheless the heavier right-handed neutrinos to realize a standard type-I seesaw leptogenesis, with the $B-L$ asymmetry dominantly produced by the next-to-lightest neutrino $N_2$. Further consequences and predictions of the model are that: the $N_1$ production implies a specific power-law relation between the reheating temperature of the Universe and the vacuum expectation value of the $SU(2)_R$ triplet; leptogenesis imposes a lower bound on the reheating temperature of the Universe at $7\times10^9\,\mbox{GeV}$. Additionally, the model requires a vanishing absolute neutrino mass scale $m_1\simeq0$.

Right-handed lepton mixings at the LHC

We study how the elements of the leptonic right-handed mixing matrix can be determined at the LHC in the minimal Left-Right symmetric extension of the standard model. We do it by explicitly relating them with physical quantities of the Keung-Senjanovic process and the lepton number violating decays of the right doubly charged scalar. We also point out that the left and right doubly charged scalars can be distinguished at the LHC, without measuring the polarization of the final state leptons coming from their decays.

Polarized window for left-right symmetry and a right-handed neutrino at the Large Hadron-Electron Collider

The breaking of parity, a fundamental symmetry between left and right is best understood in the framework of left-right symmetric extension of the standard model. We show that the production of a heavy right-handed neutrino at the proposed Large Hadron-Electron Collider (LHeC) could give us the most simple and direct hint of the scale of this breaking in left-right symmetric theories. This production mode gives a lepton number violating signal with $\Delta L=2$ which is very clean and has practically no standard model background. We highlight that the right-handed nature of $W_R$ exchange which defines the left-right symmetric theories can be confirmed by using a polarized electron beam and also enhance the production rates with relatively lower beam energy.

Intersecting branes, SUSY breaking and the 2 TeV excess at the LHC

Intersecting D-brane models in string theory can naturally support the gauge and matter content of left-right symmetric extensions of the Standard Model with gauge symmetry SU(3)_c x SU(2)_L x SU(2)_R x U(1)_{B-L}. Considering such models as candidates for explaining the 2TeV excesses seen in Run-1 by both ATLAS and CMS, the minimal possible scale of supersymmetry breaking is determined by the requirement of precise one-loop gauge coupling unification. For the vector-like, bifundamental and (anti-)symmetric Higgs content of such brane configurations, this comes out fairly universally at around 19TeV. For the SU(2)_R gauge coupling one finds values 0.48<g_R(M_R)<0.6. Threshold corrections can potentially lower the scale of supersymmetry breaking.

Neutrinoless double beta decay in type I+II seesaw models

We study neutrinoless double beta decay in left-right symmetric extension of the standard model with type I and type II seesaw origin of neutrino masses. Due to the enhanced gauge symmetry as well as extended scalar sector, there are several new physics sources of neutrinoless double beta decay in this model. Ignoring the left-right gauge boson mixing and heavy-light neutrino mixing, we first compute the contributions to neutrinoless double beta decay for type I and type II dominant seesaw separately and compare with the standard light neutrino contributions. We then repeat the exercise by considering the presence of both type I and type II seesaw, having non-negligible contributions to light neutrino masses and show the difference in results from individual seesaw cases. Assuming the new gauge bosons and scalars to be around a TeV, we constrain different parameters of the model including both heavy and light neutrino masses from the requirement of keeping the new physics contribution to neutrinoless double beta decay amplitude below the upper limit set by the GERDA experiment and also satisfying bounds from lepton flavor violation, cosmology and colliders.

Time-reversal symmetry violation in several Lepton-Flavor-Violating processes

We compute a T-odd triple vector correlation for the $\mu\rightarrow e\gamma $ decay and the $\mu\rightarrow e$ conversion process. We find simple results in terms of the CP violating phases of the effective Hamiltonians. Then we focus on the minimal Left-Right symmetric extension of the Standard Model, which can lead to an appreciable correlation. We show that under rather general assumptions, this correlation can be used to discriminate between Parity or Charge-conjugation as the discrete Left-Right symmetry.

Neutron Majorana mass from exotic instantons in a Pati-Salam model

We show how exotic stringy instantons can generate an effective interaction between color diquark sextets in a Pati-Salam model, inducing a Majorana mass term for the neutron. In particular, we discuss a simple quiver theory for a Pati-Salam like model, as an example in which the calculations of exotic instanton effects are simple and controllable. We discuss some different possibilities in order to generate $$ n-\overline{n} $$ oscillations testable in the next generation of experiments, Majorana mass matrices for neutrini and a Post-Sphaleron Baryogenesis scenario. Connections with Dark Matter issues and the Higgs mass Hierarchy problem are discussed, in view of implications for LHC and rare processes physics. The model may be viewed as a completion of a Left-Right symmetric extension of the Standard Model, alternative to a GUT-inspired scenario. Combined measures in Neutron-Antineutron physics, FCNC, LHC, Dark Matter could rule out the proposed model or uncover aspects of physics at the Planck scale!

Search for heavy neutrinos and [Formula: see text] bosons with right-handed couplings in proton-proton collisions at [Formula: see text

A search for heavy, right-handed neutrinos, mathrm {N}_{ell } (ell = mathrm {e}, mu ), and right-handed mathrm {W}_{mathrm {R}} bosons, which arise in the left-right symmetric extensions of the standard model, has been performed by the CMS experiment. The search was based on a sample of two lepton plus two jet events collected in proton–proton collisions at a center-of-mass energy of 8,text {TeV} corresponding to an integrated luminosity of 19.7 mathrm{fb}^{-1}. For models with strict left-right symmetry, and assuming only one mathrm {N}_{ell } flavor contributes significantly to the mathrm {W}_{mathrm {R}} decay width, the region in the two-dimensional (M_{mathrm {W}_{mathrm {R}}}, M_{mathrm {N}_{ell }}) mass plane excluded at a 95 % confidence level extends to approximately M_{mathrm {W}_{mathrm {R}}} = 3.0,text {TeV} and covers a large range of neutrino masses below the mathrm {W}_{mathrm {R}} boson mass, depending on the value of M_{mathrm {W}_{mathrm {R}}}. This search significantly extends the (M_{mathrm {W}_{mathrm {R}}}, M_{mathrm {N}_{ell }}) exclusion region beyond previous results.Electronic supplementary materialThe online version of this article (doi:10.1140/epjc/s10052-014-3149-z) contains supplementary material, which is available to authorized users.