arXiv subjects

Rabindra N. Mohapatra

Publications and source records attributed to Rabindra N. Mohapatra.

At least 55 records · Page 3Linked to original sources

arXiv2018

Vacuum structure of the left-right symmetric model

The left-right symmetric model (LRSM), originally proposed to explain parity violation in low energy processes, has since emerged as an attractive framework for light neutrino masses via the seesaw mechanism. The scalar sector of the minimal LRSM consists of an $SU(2)$ bi-doublet, as well as left- and right-handed weak isospin triplets, thus making the corresponding vacuum structure much more complicated than that of the Standard Model. In particular, the desired ground state of the Higgs potential should be a charge conserving, and preferably global, minimum with parity violation at low scales. We show that this is not a generic feature of the LRSM potential and happens only for a small fraction of the parameter space of the potential. We also analytically study the potential for some simplified cases and obtain useful conditions (though not necessary) to achieve successful symmetry breaking. We then carry out a detailed statistical analysis of the minima of the Higgs potential using numerical minimization and find that for a large fraction of the parameter space, the potential does not have a good vacuum. Imposing the analytically obtained conditions, we can readily find a small part of the parameter space with good vacua. Consequences for some scalar masses are also discussed.

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arXiv2018

Minimally Extended Left-Right Symmetric Model for Dark Matter with U(1) Portal

A minimal extension of the left-right symmetric model for neutrino masses that includes a vector-like singlet fermion dark matter (DM) is presented with the DM connected to the visible sector via a gauged U(1) portal. We discuss the symmetry breaking in this model and calculate the mass and mixings of the extra heavy neutral gauge boson at the TeV scale. The extra gauge boson can decay to both standard model particles as well to dark matter. We calculate the relic density of the singlet fermion dark matter and its direct detection cross section and use these constraints to obtain the allowed parameter range for the new gauge coupling and the dark matter mass.

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arXiv2018

Probing TeV scale origin of neutrino mass at future lepton colliders via neutral and doubly-charged scalars

We point out how future lepton colliders can provide unique insight into the scalar sector of TeV scale models for neutrino masses with local $B-L$ symmetry. Our specific focus is on the TeV scale left-right model, which naturally embeds this $B-L$ symmetry. In particular, we make a detailed study of the lepton collider implications of the neutral ($H_3$) and doubly-charged ($H^{\pm\pm}$) scalars from the right-handed triplet Higgs that is responsible for the spontaneous breaking of the $B-L$ symmetry and implementing the seesaw mechanism. Due to mixing with other scalars, the neutral scalar $H_3$ could acquire sizable flavor violating couplings to the charged leptons. Produced on-shell or off-shell at the planned $e^+e^-$ colliders, it would induce distinct lepton flavor violating signals like $e^+e^- \to μ^\pm τ^\mp ~ (+H_3)$, with the couplings probed up to $\sim 10^{-4}$ for a wide range of neutral scalar mass, which is well beyond the reach of current searches for charged lepton flavor violation. The Yukawa couplings of the doubly-charged scalar $H^{\pm\pm}$ to the charged leptons might also be flavor-violating, which is correlated to the heavy right-handed neutrino masses and mixings. With a combination of the pair, single and off-shell production of $H^{\pm\pm}$ like $e^+e^- \to H^{++} H^{--},\, H^{\pm\pm} e^\mp μ^\mp,\, μ^\pm τ^\mp$, the Yukawa couplings can be probed up to $10^{-3}$ at future lepton colliders, which is allowed by current lepton flavor data in a large region of parameter space. For both the neutral and doubly-charged cases, the scalar masses could be probed up to the few-TeV range in the off-shell channel.

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arXiv2018

Natural Alignment of Quark Flavors and Radiatively Induced Quark Mixings

The standard model does not provide an explanation of the observed alignment of quark flavors i.e. why are the up and down quarks approximately aligned in their weak interactions according to their masses? We suggest a resolution of this puzzle using a combination of left-right and Peccei-Quinn (PQ) symmetry. The quark mixings in this model vanish at the tree level and arise out of one loop radiative corrections which explains their smallness. The lepton mixings on the other hand appear at the tree level and are therefore larger. We show that all fermion masses and mixings can be fitted with a reasonable choice of parameters. The neutrino mass fit using seesaw mechanism requires the right handed $W_R$ mass bigger than 18 TeV. Due to the presence of PQ symmetry, this model clearly provides a solution to the strong CP problem.

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arXiv2018

Lepton Flavor Violation Induced by a Neutral Scalar at Future Lepton Colliders

Many new physics scenarios beyond the Standard Model often necessitate the existence of a (light) neutral scalar $H$, which might couple to the charged leptons in a flavor violating way, while evading all existing constraints. We show that such scalars could be effectively produced at future lepton colliders, either on-shell or off-shell depending on their mass, and induce lepton flavor violating (LFV) signals, i.e. $e^+ e^- \to \ell_α^\pm \ell_β^\mp (+H)$ with $α\neq β$. We find that a large parameter space of the scalar mass and the LFV couplings can be probed, well beyond the current low-energy constraints in the lepton sector. In particular, a scalar-loop induced explanation of the longstanding muon $g-2$ anomaly can be directly tested in the on-shell mode.

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arXiv2018

Leptonic $CP$ Violation and Proton Decay in SUSY SO(10)

We study the correlation between proton lifetime and leptonic CP violation in a class of renormalizable supersymmetric SO(10) grand unified theories (GUTs) with {\bf 10}, {\bf 126} and {\bf 120} Higgs fields, which provides a unified description of all fermion masses and possibly resolution of the strong CP problem. This specific model is unique in that it can readily be compatible with current proton lifetime limits for a supersymmetry (SUSY) breaking scale as low as 5 TeV due to the presence of a specific Yukawa texture. Our investigation here reveals that proton partial lifetimes predicted by this class of models will be tested by forthcoming proton decay searches; furthermore, a discovery of leptonic CP violation in neutrino oscillations would also lead to substantial reduction of the parameter space of the model.

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arXiv2018

Leptogenesis Constraints on $B-L$ breaking Higgs Boson in TeV Scale Seesaw Models

In the type-I seesaw mechanism for neutrino masses, there exists a $B-L$ symmetry, whose breaking leads to the lepton number violating mass of the heavy Majorana neutrinos. This would imply the existence of a new neutral scalar associated with the $B-L$ symmetry breaking, analogous to the Higgs boson of the Standard Model. If in such models, the heavy neutrino decays are also responsible for the observed baryon asymmetry of the universe via the leptogenesis mechanism, the new seesaw scalar interactions with the heavy neutrinos will induce additional dilution terms for the heavy neutrino and lepton number densities. We make a detailed study of this dilution effect on the lepton asymmetry in three generic classes of seesaw models with TeV-scale $B-L$ symmetry breaking, namely, in an effective theory framework and in scenarios with global or local $U(1)_{B-L}$ symmetry. We find that requiring successful leptogenesis imposes stringent constraints on the mass and couplings of the new scalar in all three cases, especially when it is lighter than the heavy neutrinos. We also discuss the implications of these new constraints and prospects of testing leptogenesis in presence of seesaw scalars at colliders.

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arXiv2018

Same Sign versus Opposite Sign Dileptons as a Probe of Low Scale Seesaw Mechanisms

We calculate the ratio $R_{\ell\ell}$ of same sign (SS) to opposite sign (OS) dileptons in type I and generalized inverse seesaw models and show that it can be anywhere between 0 and 1 depending on the detailed texture of the right-handed neutrino mass matrix. Measurement of $R_{\ell\ell}$ in hadron colliders can therefore provide a way to probe the nature of seesaw mechanism and also to distinguish between the two types of seesaw mechanisms. We work within the framework of left-right symmetric model as an example. We emphasize that coherence of the final states in the $W_R$ decay is crucial for this discussion and it requires the right-handed neutrinos to be highly degenerate. We isolate the range of parameters in the model where this effect is observable at the LHC and future colliders.

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arXiv2017

Leptogenesis with TeV Scale $W_R$

Successful leptogenesis within the conventional TeV-scale left-right implementation of type-I seesaw has been shown to require that the mass of the right-handed $W_R^\pm$ boson should have a lower bound much above the reach of the Large Hadron Collider. This bound arises from the necessity to suppress the washout of lepton asymmetry due to $W_R^\pm$-mediated $ΔL\neq 0$ processes. We show that in an alternative quark seesaw realization of left-right symmetry, the above bound can be avoided. Lepton asymmetry in this model is generated not via the usual right-handed neutrino decay but rather via the decay of new heavy scalars producing an asymmetry in the $B-L$ carrying Higgs triplets responsible for type-II seesaw, whose subsequent decay leads to the lepton asymmetry. This result implies that any evidence for $W_R$ at the LHC 14 will point towards this alternative realization of left-right symmetry, which is also known to solve the strong CP problem.

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arXiv2017

Constraints on Mirror Models of Dark Matter from Observable Neutron-Mirror Neutron Oscillation

The process of neutron-mirror neutron oscillation, motivated by symmetric mirror dark matter models, is governed by two parameters: $n-n'$ mixing parameter $δ$ and $n-n'$ mass splitting $Δ$. For neutron mirror neutron oscillation to be observable, the splitting between their masses $Δ$ must be small and current experiments lead to $δ\leq 2\times 10^{-27}$ GeV and$Δ\leq 10^{-24}$ GeV. We show that in mirror universe models where this process is observable, this small mass splitting constrains the way that one must implement asymmetric inflation to satisfy the limits of Big Bang Nucleosynthesis on the number of effective light degrees of freedom. In particular we find that if asymmetric inflation is implemented by inflaton decay to color or electroweak charged particles, the oscillation is unobservable. Also if one uses SM singlet fields for this purpose, they must be weakly coupled to the SM fields.

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arXiv2017

Quark Seesaw, Dark $U(1)$ symmetry and Baryon-Dark Matter Coincidence

We attempt to understand the baryon-dark-matter coincidence problem within the quark seesaw extension of the standard model where parity invariance is used to solve the strong CP problem. The $SU(2)_L\times SU(2)_R\times U(1)_{B-L}$ gauge symmetry of this model is extended by a dark $U(1)_X$ group plus inclusion of a heavy neutral vector-like fermion $χ_{L,R}$ charged under the dark group which plays the role of dark matter. All fermions are Dirac type in this model. Decay of heavy scalars charged under $U(1)_X$ leads to simultaneous asymmetry generation of the dark matter and baryons after sphaleron effects are included. The $U(1)_X$ group not only helps to stabilize the dark matter but also helps in the elimination of the symmetric part of the dark matter via $χ-\barχ$ annihilation. For dark matter mass near the proton mass, it explains why the baryon and dark matter abundances are of similar magnitude (the baryon-dark-matter coincidence problem). This model is testable in low threshold (sub-keV) direct dark matter search experiments.

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arXiv2017

Long Lived Light Scalars as Probe of Low Scale Seesaw Models

We point out that in generic TeV scale seesaw models for neutrino masses with local $B-L$ symmetry breaking, there is a phenomenologically allowed range of parameters where the Higgs field responsible for $B-L$ symmetry breaking leaves a physical real scalar field with mass around GeV scale. This particle (denoted here by $H_3$) is weakly mixed with the Standard Model Higgs field ($h$) with mixing $θ_1\lesssim m_{H_3}/m_h$, barring fine-tuned cancellation. In the specific case when the $B-L$ symmetry is embedded into the TeV scale left-right seesaw scenario, we show that the bounds on the $h-H_3$ mixing $θ_1$ become further strengthened due to low energy flavor constraints, thus forcing the light $H_3$ to be long lived, with displaced vertex signals at the LHC. The property of left-right TeV scale seesaw models are such that they make the $H_3$ decay to two photons as the dominant mode. This is in contrast with a generic light scalar that mixes with the SM Higgs boson, which could also have leptonic and hadronic decay modes with comparable or larger strength. We discuss the production of this new scalar field at the LHC and show that it leads to testable displaced vertex signals of collimated photon jets, which is a new distinguishing feature of the left-right seesaw model. We also study a simpler version of the model where the $SU(2)_R$ breaking scale is much higher than the ${\cal O}$(TeV) $U(1)_{B-L}$ breaking scale, in which case the production and decay of $H_3$ proceed differently, but its long lifetime feature is still preserved for a large range of parameters. Thus, the search for such long-lived light scalar particles provides a new way to probe TeV scale seesaw models for neutrino masses at colliders.

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arXiv2017

Displaced Photon Signal from a Light Scalar in Minimal Left-Right Symmetric Model

We point out that in the minimal left-right realization of TeV scale seesaw for neutrino masses, the neutral scalar from the right-handed $SU(2)_R$ breaking sector could be much lighter than the right-handed scale. We discuss for the first time the constraints on this particle from low-energy flavor observables, find that the light scalar is necessarily long-lived. We show that it can be searched for at the LHC via displaced signals of a collimated photon jet, and can also be tested in current and future high-intensity experiments. In contrast to the unique diphoton signal (and associated jets) in the left-right case, a generic beyond Standard Model light scalar decays mostly to leptons or jets. Thus, the diphoton channel proposed here provides a new avenue to test the left-right framework and reveal the underlying neutrino mass generation mechanism.

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arXiv2016

Vector-Like Quarks and Leptons, SU(5) $\otimes$ SU(5) Grand Unification, and Proton Decay

SU(5) $\otimes$ SU(5) provides a minimal grand unification scheme for fermions and gauge forces if there are vector-like quarks and leptons in nature. We explore the gauge coupling unification in a non-supersymmetric model of this type, and study its implications for proton decay. The properties of vector-like quarks and intermediate scales that emerge from coupling unification play a central role in suppressing proton decay. We find that in this model, the familiar decay mode $p \to e^+ π^0$ may have a partial lifetime within the reach of currently planned experiments.

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arXiv2016

Naturally Stable Right-Handed Neutrino Dark Matter

We point out that a class of non-supersymmetric models based on the gauge group $SU(3)_C \times SU(2)_L\times SU(2)_R\times U(1)_{Y_L}\times U(1)_{Y_R}$ possesses an automatic, exact $Z_{2 }$ symmetry under which the fermions in the $SU(2)_R\times U(1)_{Y_R}$ sector (called $R$-sector) are odd and those in the standard model sector (called $L$-sector) are even. This symmetry, which is different from the usual parity symmetry of the left-right symmetric models, persists in the lepton sector even after the gauge symmetry breaks down to $SU(3)_C \times U(1)_{\rm EM}$. This keeps the lightest right-handed neutrino naturally stable, thereby allowing it to play the role of dark matter (DM) in the Universe. There are several differences between the usual left-right models and the model presented here: (i) our model can have two versions, one which has no parity symmetry so that the couplings and masses in the $L$ and $R$ sectors are unrelated, and another which has parity symmetry so that couplings are related; (ii) the $R$-sector fermions are chosen much heavier than the $L$-sector ones in both scenarios; and finally (iii) both light and heavy neutrinos are Majorana fermions with the light neutrino masses arising from a pure type-II seesaw mechanism. We discuss the DM relic density, direct and indirect detection prospects and associated collider signatures of the model. Comparing with current collider and direct detection constraints, we find a lower bound on the DM mass of order of 1 TeV. We also point out a way to relax the DM unitarity bound in our model for much larger DM masses by an entropy dilution mechanism. An additional feature of the model is that the DM can be made very long lived, if desired, by allowing for weak breaking of the above $Z_{2}$ symmetry. Our model also predicts the existence of long-lived colored particles which could be searched for at the LHC.

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arXiv2016

Heavy right-handed neutrino dark matter in left-right models

We show that in a class of non-supersymmetric left-right extensions of the Standard Model (SM), the lightest right-handed neutrino (RHN) can play the role of thermal Dark Matter (DM) in the Universe for a wide mass range from TeV to PeV. Our model is based on the gauge group $SU(3)_c \times SU(2)_L\times SU(2)_R\times U(1)_{Y_L}\times U(1)_{Y_R}$ in which a heavy copy of the SM fermions are introduced and the stability of the RHN DM is guaranteed by an automatic $Z_2$ symmetry present in the leptonic sector. In such models the active neutrino masses are obtained via the type-II seesaw mechanism. We find a lower bound on the RHN DM mass of order TeV from relic density constraints, as well as an unitarity upper bound in the multi-TeV to PeV scale, depending on the entropy dilution factor. The RHN DM could be made long-lived by soft-breaking of the $Z_2$ symmetry and provides a concrete example of decaying DM interpretation of the PeV neutrinos observed at IceCube.

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arXiv2016

Limiting Equivalence Principle Violation and Long-Range Baryonic Force from Neutron-Antineutron Oscillation

We point out that if the baryon number violating neutron-antineutron oscillation is discovered, it would impose strong limits on the departure from Einstein's equivalence principle at a level of one part in $10^{19}$. If this departure owes its origin to the existence of long-range forces coupled to baryon number $B$ (or $B-L$), it would imply very stringent constraints on the strength of gauge bosons coupling to baryon number current. For instance, if the force mediating baryon number has strength $α_B$ and its range is larger than a megaparsec, we find the limit to be $α_B \leq 2\times 10^{-57}$, which is much stronger than all other existing bounds. For smaller range for the force, we get slightly weaker, but still stringent bounds by considering the potential of the Earth and the Sun.

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arXiv2016

Probing the Higgs Sector of the Minimal Left-Right Symmetric Model at Future Hadron Colliders

If neutrino masses arise from a TeV-scale minimal Left-Right seesaw model, the ensuing extended Higgs sector with neutral, singly and doubly-charged scalars has a plethora of implications for new Higgs boson searches beyond the Standard Model at future hadron colliders, such as the $\sqrt s=14$ TeV High-Luminosity Large Hadron Collider (HL-LHC) and the proposed $\sqrt s=100$ TeV collider (FCC-hh or SPPC). In this article, we provide a glimpse of this new physics in the Higgs sector. Our discussion focuses on the minimal non-supersymmetric version of the Left-Right model with high-scale parity breaking but TeV-scale $SU(2)_R$-breaking, a property desirable to suppress the type-II seesaw contribution to neutrino masses. We analyze the masses and couplings of the physical Higgs bosons in this model, and discuss their dominant production and decay modes at hadron colliders. We identify the best discovery channels for each of the non-SM Higgs bosons and estimate the expected SM backgrounds in these channels to derive the sensitivity reaches for the new Higgs sector at future hadron colliders under discussion. Following a rather conservative approach, we estimate that the heavy Higgs sector can be effectively probed up to 15 TeV at the $\sqrt s=100$ TeV machine. We also discuss how the LR Higgs sector can be distinguished from other extended Higgs sectors.

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