arXiv subjects

Rathin Adhikari

Publications and source records attributed to Rathin Adhikari.

At least 37 records · Page 2Linked to original sources

arXiv2015

Nonthermal $CP$ violation in soft leptogenesis

Soft leptogenesis is a mechanism which generates the matter-antimatter asymmetry of the Universe via the out-of-equilibrium decays of heavy sneutrinos in which soft supersymmetry breaking terms play two important roles: they provide the required $CP$ violation and give rise to the mass splitting between otherwise degenerate sneutrino mass eigenstates within a single generation. This mechanism is interesting because it can be successful at the lower temperature regime $T \lesssim 10^9$ GeV in which the conflict with the overproduction of gravitinos can possibly be avoided. In earlier works, the leading $CP$ violation is found to be nonzero only if finite temperature effects are included. By considering generic soft trilinear couplings, we find two interesting consequences: (1) the leading $CP$ violation can be nonzero even at zero temperature realizing nonthermal $CP$ violation, and (2) the $CP$ violation is sufficient even far away from the resonant regime allowing soft supersymmetry breaking parameters to assume natural values at around the TeV scale. We discuss phenomenological constraints on such scenarios and conclude that the contributions to charged lepton flavor violating processes are close to the sensitivities of present and future experiments.

hep-ph↗

arXiv2015

The Discovery reach of $CP$ violation in neutrino oscillation with non-standard interaction effects

We have studied the $CP$ violation discovery reach in neutrino oscillation experiment with superbeam, neutrino factory and monoenergetic neutrino beam. For NSI satisfying model-dependent bound for shorter baselines (like CERN-Fréjus set-up ) there is insignificant effect of NSI on the the discovery reach of $CP$ violation due to $δ$. Particularly, for superbeam and neutrino factory we have also considered relatively longer baselines for which there could be significant NSI effects on $CP$ violation discovery reach for higher allowed values of NSI. For monoenergetic beam only shorter baselines are considered to study $CP$ violation with different nuclei as neutrino sources. Interestingly for non-standard interactions - $\varepsilon_{eμ}$ and $\varepsilon_{eτ}$ of neutrinos with matter during propagation in longer baselines in superbeam, there is possibility of better discovery reach of $CP$ violation than that with only Standard Model interactions of neutrinos with matter. For complex NSI we have shown the $CP$ violation discovery reach in the plane of Dirac phase $δ$ and NSI phase $ϕ_{ij}$. The $CP$ violation due to some values of $δ$ remain unobservable with present and near future experimental facilities in superbeam and neutrino factory . However, in presence of some ranges of off-diagonal NSI phase values there are some possibilities of discovering total $CP$ violation for any $δ_{CP}$ value at $5 σ$ for neutrino factory. Our analysis indicates that for some values of NSI phases total $CP$ violation may not be at all observable for any values of $δ$. Using a neutrino beam from electron capture process for nuclei $^{110}_{50}$Sn and $^{152}$Yb, we have shown the discovery reach of $CP$ violation in neutrino oscillation experiment.

hep-ph↗

arXiv2014

Common Origin of Active and Sterile Neutrino Masses with Dark Matter

We propose an abelian extension of the Standard Model which can explain the origin of eV scale masses and mixing for active and sterile neutrinos and at the same time providing a natural cold dark matter candidate. One of the three active neutrinos acquires mass at tree level through seesaw mechanism whereas the other two active neutrinos and one sterile neutrino acquire eV scale masses at one-loop level. The model also allows non-trivial mixing between active and sterile neutrinos at one-loop level which could have interesting signatures at neutrino experiments. After the abelian gauge symmetry gets spontaneously broken down to a $Z_2$ symmetry, the lightest $Z_2$ odd particle can naturally give rise to the cold dark matter of the Universe. The phenomenology of both fermionic and scalar dark matter is briefly discussed by incorporating latest experimental constraints.

hep-ph↗

arXiv2014

Discovering $CP$ violation in neutrino oscillation experiment using neutrino beam from electron capture

Considering the recently obtained value of $θ_{13}$ from Daya Bay and other reactor experiments we have studied the prospects of considering mono-energetic neutrino beam in studying $CP$ violation in the leptonic sector. Using a neutrino beam from electron capture process for nuclei $^{110}_{50}$Sn and $^{152}$Yb and considering two baselines - 130 Km and 250 Km with Water Cherenkov detector, we have shown the discovery reach of $CP$ violation in neutrino oscillation experiment. Particularly for $^{110}_{50}$Sn nuclei $CP$ violation could be found for about 80% of the possible $δ$ values for a baseline of 130 km with boost factor $γ= 500$. This result is obtained with conservative choice of neutrino energy resolution using the possible vertex resolution at the detector and taking into account beam spreading. The nuclei $^{152}$Yb is although more suitable technically for the production of mono-energetic beam, but is found to be not so suitable for good discovery reach of $CP$ violation.

hep-ph↗

arXiv2014

Non-standard interaction effect on $CP$ violation in neutrino oscillation with super-beam

We have studied the $CP$ violation discovery reach in neutrino oscillation experiment with superbeam in presence of non-standard interactions of neutrinos with matter for both short and long baselines. For the most important channel of oscillation ($ν_μ\rightarrow ν_e$) for $CP$ violation discovery there is significant effect in the oscillation probability particularly due to NSIs' $\varepsilon_{eμ}$ and $\varepsilon_{eτ}$ for longer baseline and higher energy in comparison to other NSIs'. Interestingly for these two NSIs' (for real and higher allowed values) there is possibility of better discovery reach of $CP$ violation than that with only Standard Model interactions of neutrinos with matter provided that NSI values are known. For complex NSIs' we have shown the $CP$ violation discovery reach in the plane of Dirac phase $δ$ and NSI phase $ϕ_{ij}$. Our analysis indicates that for some values of some NSI phases total $CP$ violation may not be observable for any values of $δ$.

hep-ph↗

arXiv2014

Discovery reach of $CP$ violation and non-standard interactions in low energy neutrino factory

In low energy neutrino factory ($E_μ< 10$GeV) using MIND detector, we have studied the optimization of $CP$ violation discovery reach in the leptonic sector for different baselines and different parent muon energy considering only Standard Model interactions of neutrinos with matter. Considering such optimized experimental set-up of baseline and energy we have addressed the question of how $CP$ violation discovery reach could get affected by the presence of non-standard interactions of neutrinos with matter during the propagation of neutrinos. For off diagonal NSI elements there could be complex phases $ϕ_{ij}$ which could also lead to $CP$ violation. In presence of these complex phases we have shown the contours showing the discovery reach of $δ$ and $ϕ_{ij}$. We have also shown the discovery reach of NSIs in the same experimental set-up which is optimized for discovery of $CP$ violation in the leptonic sector.

hep-ph↗

arXiv2014

Common Radiative Origin of Active and Sterile Neutrino Masses

Sterile neutrinos with sub-electron volt (eV) masses have recently received serious attention due to the tantalizing hints from reactor neutrino experiments as well as cosmology. While the nine year old Wilkinson Mass Anisotropy Probe experiment suggests the effective number of relativistic degrees of freedom to be $N_{\text{eff}} = 3.84 \pm 0.40$, recently reported Planck collaboration results show more preference towards the standard three light neutrino scenario $ N_{\text{eff}} = 3.30^{+0.54}_{-0.51}$. Keeping in mind that the issue of existence or non-existence of sub-eV scale sterile neutrinos is not yet settled, here we outline a mechanism to generate sub-eV scale masses for three active and one sterile neutrinos simultaneously. The model is based on an abelian extension of standard model where the fermion and scalar fields are charged under the additional $U(1)$ gauge group in such an anomaly free way that it allows one eV scale neutrino and three massless neutrinos at tree level. However, at one loop level, this model naturally allows three active and one sterile neutrino with mass at the sub-eV scale. The model also allows for mixing between active and sterile neutrinos at one loop level which can have interesting signatures in reactor neutrino experiments.

hep-ph↗

arXiv2012

Non-standard interaction in neutrino oscillations and recent Daya Bay, T2K experiments

We study the possible constraints on non-standard interaction(NSIs) in a model independent way by considering the recent results from T2K and Daya Bay neutrino oscillations experiments. Using perturbation method we present generic formulas (suitable for T2K baseline and for large $θ_{13}$ as evident from Daya Bay) for the probability of oscillation for $ν_μ\rightarrow ν_e$, taking into account NSIs at source ($ε^s$), detector ($ε^d$) and during propagation ($ε^m$) of neutrinos through matter. Two separate cases of perturbation with small (slightly large) NSI ($ε_{αβ}^m \sim 0.03(0.18)$) are discussed in detail. Using various possible presently allowed NSI values we reanalyze numerically the $θ_{13} - δ$ allowed region given by recent T2K experimental data. We obtain model independent constraints on NSIs in the $δ-ε_{αβ}^m$ plane using the $θ_{13}$ value as measured by Daya Bay, where $δ$ is the CP violating phase. Depending on $δ$ values significant constraints on $ε_{eτ}$ and $ε_{ττ}$, in particular, are possible for both hierarchies of neutrino masses. Corresponding to T2K's 66% confidence level result, the constraints on $ε_{ττ}$ is shown to be independent of any $δ$ value.

hep-ph↗

arXiv2012

Abelian Gauge Extension of Standard Model: Dark Matter and Radiative Neutrino Mass

We study a simple extension of Standard Model where the gauge group is extended by an additional $U(1)_X$ gauge symmetry. Neutrino mass arise both at tree level as well as radiatively by the anomaly free addition of one singlet fermion $N_R$ and two triplet fermions $Σ_{1R}, Σ_{2R}$ with suitable Higgs scalars. The spontaneous gauge symmetry breaking is achieved in such a way which results in a residual $Z_2$ symmetry and hence providing a stable cold dark matter candidate. We study the possible dark matter candidates in this model by incorporating the constraints from cosmology as well as direct detection experiments. We discuss both low and high mass (from GeV to the TeV scale) regimes of fermionic and scalar dark matter candidates in the model. We show that scalar dark matter relic density, although not significantly affected by the presence or absence of annihilation into $U(1)_X$ gauge boson pairs, is however affected by choice of $U(1)_X$ gauge charges. We discuss the neutrino mass phenomenology and its compatibility with the allowed dark matter mass ranges and also comment on the implications of the model on Higgs signatures at colliders including those related to fourth fermion generation.

hep-ph↗

arXiv2012

Non-standard interactions and bimagic baseline for neutrino oscillations

For standard interactions of neutrinos with matter bimagic baseline of length about 2540 Km is known to be suitable for getting good discovery limits of neutrino mass hierarchy, $\sin^2 θ_{13}$ and CP violation in the $ν_e \rightarrow ν_μ$ oscillation channel. We discuss how even in presence of non-standard interactions (NSIs) of neutrinos with matter this baseline is found to be suitable for getting these discovery limits. This is because even in presence of NSIs one could get the $ν_e \rightarrow ν_μ$ oscillation probability to be almost independent of CP violating phase $δ$ and $θ_{13}$ for one hierarchy and highly dependent on these two for the other hierarchy over certain parts of neutrino energy range. For another certain part of the energy range the reverse of this happens with respect to the hierarchies. We present the discovery limits of NSIs also in the same neutrino energy range. However, as with the increase of neutrino energy the NSI effect in the above oscillation probability gets relatively more pronounced in comparison to the vacuum oscillation parameters, so we consider higher neutrino energy range also for getting better discovery limits of NSIs. Analysis presented here for 2540 Km could also be implemented for longer bimagic baseline $> 6000$ Km.

hep-ph↗

arXiv2011

Light neutrinos from massless texture and below TeV seesaw scale

We present general conditions on Dirac and Majorana mass terms under which a type-I seesaw mechanism can lead to three exactly massless neutrinos at the tree level. We depict several examples where the conditions are satisfied and relate some of them to an underlying U(1) symmetry. We show that higher order corrections may generate the small observed masses and this may be achieved even when the heavy Majorana neutrinos are at the electroweak scale or a little higher.

hep-ph↗

arXiv2011

Magic baseline and magic energy in neutrino oscillation with non-standard interactions

We have discussed conditions under which probability of oscillation ($ν_e\rightarrow ν_μ$) is independent of CP violating phase $δ$. The condition of magic baseline on its length is well-known. We have proposed another condition which is on neutrino energy. We have shown that magic baseline condition is not possible in general, for small $θ_{13}$ with non-standard interaction and for large $θ_{13}$ with both standard and non-standard interactions. However, neutrino energy condition is possible for such cases as well as for cases where magic baseline condition is applicable. We have discussed how one may resolve hierarchy problem for neutrino masses by using such energy condition. For a baseline of length 650 Km, using this energy condition we discuss the possible number of $μ^-$ events at the detector for a period of 5 years and also the sensitivity in measurement of $\cos^2 θ_{13}$.

hep-ph↗

arXiv2009

Neutrino oscillation with flavor non-eigenstates and CP-violating Majorana phases

We analyze neutrino oscillation for the general case when the initial neutrino is not in a pure flavor state. We show that, after such a neutrino beam propagates for a while, the probability of detecting any pure flavor state depends even on the CP-violating Majorana phases in the mixing matrix. The dependence remains even when energy spectrum of the initial beam is taken into account. We discuss various implications of this dependence.

hep-ph↗

arXiv2009

Seesaw Neutrino Mass and New U(1) Gauge Symmetry

The three electroweak doublet neutrinos $ν_{e,μ,τ}$ of the Standard Model may acquire small seesaw masses, using either three Majorana fermion singlets $N$ or three Majorana fermion triplets $(Σ^+,Σ^0,Σ^-)$. It is well-known that the former accommodates the U(1) gauge symmetry $B-L$. It has also been shown some years ago that the latter supports a new $U(1)_X$ gauge symmetry. Here we study two variations of this $U(1)_X$, one for two $N$ and one $Σ$, the other for one $N$ and two $Σ$. Phenomenological consequences are discussed.

hep-ph↗

arXiv2007

The neutrino mass scale and the mixing angle $θ_{13}$ for quasi-degenerate Majorana neutrinos

Considering a general mass matrix for quasi-degenerate neutrinos and treating the experimentally known oscillation parameters as inputs, we study the correlation between the degenerate mass scale $(m)$ and the mixing angle $θ_{13}$. We find that, corresponding to different values of $m$, there exist upper bounds on $θ_{13}$, so that a precise determination of the latter in future may put upper limit on the former, and vice versa. One can also find a possible correlation between $m$ and lower bound of $θ_{13}$, depending on the relative strength of the unperturbed degenerate mass matrix and the perturbation. The possible constraints on the parameters of few models of quasi-degenerate neutrinos are briefly discussed.

hep-ph↗

arXiv2006

Can R-parity violating supersymmetry be seen in long baseline beta-beam experiments?

Long baseline oscillation experiments may well emerge as test beds for neutrino interactions as are present in R-parity violating supersymmetry. We show that flavour diagonal (FDNC) and flavour changing (FCNC) neutral currents arising therefrom prominently impact a neutrino $β$-beam experiment with the source at CERN and the detector at the proposed India-based Neutrino Observatory. These interactions may preclude any improvement of the present limit on $θ_{13}$ and cloud the hierarchy determination unless the upper bounds on ${\not R}$ couplings, particularly $λ^{\prime}$, become significantly tighter. If ${\not R}$ interactions are independently established then from the event rate a lower bound on $θ_{13}$ may be set. We show that there is scope to see a clear signal of non-standard FCNC and FDNC interactions, particularly in the inverted hierarchy scenario and also sometimes for the normal hierarchy. In favourable cases, it may be possible to set lower and upper bounds on $λ^{\prime}$ couplings. FCNC and FDNC interactions due to $λ$ type ${\not R}$ couplings are unimportant.

hep-ph↗

arXiv2004

Constancy of any signal velocity in all inertial frames

If isotropy of space and homogeneity of space and time are the valid laws of nature then one can show that velocity of any signal (whether it is light in a vacuum or in a medium or something having constant velocity in the rest frame of the observer and by using which we can perform all (but not partial) measurements of all physical quantities related with the particular experiment) is constant in all inertial frames. To verify this constancy for other signals apart from light in vacuum (which is well-known) we have proposed particularly one experiment in which all measurements can be done using light in water as signal. If we perform experiment in a medium then the signal connecting the measurements of an event in two inertial frames can not be light in vacuum rather it is light in the medium. In such cases where signal used for measurement is not light in vacuum, relativistic relationships of various physical quantities in Special Theory of Relativity (in which light in vacuum has been used as signal) require modifications. This work might be verified from the analysis related with the number of cosmic muons reaching earth through atmosphere or that related with the amount of oscillation of atmospheric neutrinos as in both these cases signal for our measurement can not be light in vacuum but it is light in air.

physics.gen-ph↗

arXiv2001

Baryon Number Violation in Particle Decays

It has been argued in the past that in baryogenesis via out-of-equilibrium decays one must consider loop diagrams that contain more than one baryon number violating coupling. In this note we argue that the requirement with regard to baryon number violating couplings in loop diagrams is that the interaction between the intermediate on-shell particles and the final particles should correspond to a net change in baryon number and that this can be satisfied even if the loop diagram contains only one baryon number violating coupling. Put simply, we show that to create a baryon asymmetry there should be net B violation to the right of the `cut' in the loop diagram. This is of relevance to some earlier works involving the out-of-equilibrium decay scenario.

hep-ph↗