arXiv subjects
Publications and source records attributed to Amitava Raychaudhuri.
We study the physics reach of an experiment where neutrinos produced in a beta-beam facility at CERN are observed in a large magnetized iron calorimeter (ICAL) at the India-based Neutrino Observatory (INO). The CERN-INO distance is close to the so-called "magic" baseline which helps evade some of the parameter degeneracies and allows for a better measurement of the neutrino mass hierarchy and $θ_{13}$.
We show that the earth matter effects in the ${\rm {ν_e \to ν_e}}$ survival probability can be used to cleanly determine the third leptonic mixing angle $θ_{13}$ and the sign of the atmospheric neutrino mass squared difference, $Δm^2_{31}$, using a $β$-beam as a $ν_e$ source.
A low intermediate scale within minimal supersymmetric SO(10) GUTs is a desirable feature to accommodate leptogenesis. We explore this possibility in models where the intermediate gauge symmetry breaks spontaneously by (a) doublet Higgs scalars and also (b) by triplets. In both scenarios gauge coupling unification requires the scale of left-right symmetry breaking ($M_R$) to be close to the unification scale. This will entail unnaturally small neutrino Yukawa couplings to avoid the gravitino problem and allow successful leptogenesis. We point out that any one of three options -- threshold corrections due to the mass spectrum near the unification scale, gravity induced non-renormalizable operators near the Planck scale, or presence of additional light Higgs multiplets -- can permit unification along with much lower values of $M_R$ as required for leptogenesis. In the triplet model, independent of these corrections, we find a lower bound on the intermediate scale, $M_R > 10^9$ GeV, arising from the requirement that the theory must remain perturbative at least upto the GUT scale. We show that in the doublet model $M_R$ can even be in the TeV region which, apart from permitting resonant leptogenesis, can be tested at LHC and ILC.
We show that a detector placed near a beta-beam storage ring can probe lepton number violating interactions, as predicted by supersymmetric theories with R-parity non-conservation. In the presence of such interactions, nu_tau can be produced during beta-decay leading to tau leptons through weak interactions. Alternatively, electron neutrinos from beta-decay of radioactive ions can produce tau leptons in a nearby detector through these interactions. The muons from the decay of these tau leptons can be readily identified in a small iron calorimeter detector and will signal violation of R-parity.
We underscore the physics advantage of an experiment where neutrinos produced in a beta-beam facility at CERN are observed in a large magnetized iron calorimeter (ICAL) at the India-based Neutrino Observatory (INO). The CERN-INO distance is close to the so-called "magic" baseline which helps evade some of the parameter degeneracies and allows for a better measurement of the neutrino mass hierarchy and $θ_{13}$. We expound the possibility of using radioactive $^8B$ and $^{8}Li$ as the source isotopes for the $\nue$ and $\anue$ beta-beam, respectively, and show that very good sensitivity to both the mass hierarchy and $θ_{13}$ is possible with a boost $γ$ in the 250-500 ballpark.
We study the power law running of gauge, Yukawa and quartic scalar couplings in the universal extra dimension scenario where the extra dimension is accessed by all the standard model fields. After compactifying on an $S_1 /Z_2$ orbifold, we compute one-loop contributions of the relevant Kaluza-Klein (KK) towers to the above couplings up to a cutoff scale $Λ$. Beyond the scale of inverse radius, once the KK states are excited, these couplings exhibit power law dependence on $Λ$. As a result of faster running, the gauge couplings tend to unify at a relatively low scale, and we choose our cutoff also around that scale. For example, for a radius $R \sim 1 TeV^{-1}$, the cutoff is around 30 TeV. We then examine the consequences of power law running on the triviality and vacuum stability bounds on the Higgs mass. We also comment that the supersymmetric extension of the scenario requires $R^{-1}$ to be larger than $\sim 10^{10}$ GeV in order that the gauge couplings remain perturbative up to the scale where they tend to unify.
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.
We consider 5d SU(5) GUT models based on the orbifold $S^1/(Z_2 \times Z_2')$, and study the different possibilities of placing the SU(5) matter multiplets in three possible locations, namely, the two branes at the two orbifold fixed points and SU(5) bulk. We demonstrate that if flavour hierarchies originate solely from geometrical suppressions due to wavefunction normalisation of fields propagating in the bulk, then it is not possible to satisfy even the gross qualitative behaviour of the CKM and MNS matrices regardless of where we place the matter multiplets.
A high intensity source of a single neutrino flavour with known spectrum is most desirable for precision measurements, the consensus direction for the future. The beta beam is an especially suitable option for this. We discuss the prospects of a very long baseline beta beam experiment with a magnetized iron calorimeter detector. In particular, with the source at CERN and the detector at the proposed India-based Neutrino Observatory (INO) the baseline is near the `magic' value where the effect of the CP phase is small. We observe that this experiment will be well suited to determine the sign of $m_3^2 - m_2^2$ and will be capable of probing $θ_{13}$ down to about 1$^\circ$.
In the context of an universal extra-dimensional scenario, we consider production of the first Kaluza-Klein electron positron pair in an $e^+e^-$ collider as a case-study for the future International Linear Collider. The Kaluza-Klein electron decays into a nearly degenerate Kaluza-Klein photon and a standard electron, the former carrying away missing energy. The Kaluza-Klein electron and photon states are heavy with their masses around the inverse radius of compactification, and their splitting is controlled by radiative corrections originating from bulk and brane-localised interactions. We look for the signal event $e^+e^- +$ large missing energy for $\sqrt s = 1$ TeV and observe that with a few hundred fb$^{-1}$ luminosity the signal will be readily detectable over the standard model background. We comment on how this signal may be distinguished from similar events from other new physics.
We consider the effect of ambient matter on the Majorana phase of neutrinos. We find that this can lead to an observable signal if a neutrino oscillation experiment could be performed where the source and the detector are at appropriately different matter densities. We illustrate the situation using a beta beam neutrino source as an example and show that a 5 sigma signal for the matter modification of the Majorana phase could be possible in a 5-year run.
The K2K experiment has presented the first results on the observation of nu_mu. They show a depletion compared to the expectations and are consistent with neutrino oscillations with a mass-splitting in the range favoured by the Super-Kamiokande atmospheric neutrino measurements. Here we examine the extent by which the range of Delta m^2 obtained from the K2K measurements can vary due to the uncertainties in the flux, cross-section, and detector efficiency.
Neutrino Majorana masses and mixings can be generated from a dimension-5 operator within the standard model particle content. After a review of the mechanism of radiative enhancement of the mixing angle in a two-neutrino case, we consider three-flavour mass matrices of two-zero texture generated from such an operator and investigate the possibility of implementing the mechanism here. We observe that radiative magnification of only the solar angle is consistent with oscillation data on masses and mixings, and that too for nearly degenerate neutrinos, with two of them having opposite CP parities, while for hierarchical masses the mechanism does not work. In supersymmetry or in an extra-dimensional scenario the above features are qualitatively unchanged.
We study the renormalization of the $llHH$-type Majorana neutrino mass operator in a scenario where there is a compactified extra dimension and the fields involved correspond to only the standard model particles and their Kaluza-Klein excitations. We observe that in a two flavour scenario, where one of the neutrinos is necessarily $ν_τ$, it is indeed possible to generate a large mixing at $\sim$ 100 GeV starting from a very small mixing near the ultra-violet cutoff $\sim$ 30 TeV. {\em En passant}, we also derive the Higgs mass upper and lower limits from perturbative unitarity and stability of the potential, respectively.
We consider a 5-dimensional SU(5) model wherein the symmetry is broken to the 4-dimensional Standard Model by compactification of the 5th dimension on an S^1/(Z_2 \times Z^\prime_2) orbifold. We identify the members of all SU(5) representations upto 75 which have zero modes. We examine how these light scalars affect gauge coupling unification assuming a single intermediate scale and present several acceptable solutions. The 5-D compactification scale coincides with the unification scale of gauge couplings and is determined via this renormalization group analysis. When SO(10) is considered as the GUT group there are only two solutions, so long as a few low dimensional scalar multiplets upto 126 are included.
The SNO result on charged current deuteron disintegration, the SuperKamiokande 1258-day data on electron scattering, and other solar neutrino results are used to revisit the model of neutrino oscillations driven by a violation of the equivalence principle. We use a chisq minimization technique to examine oscillation between the nu(e) and another active neutrino, both massless, and find that within the Standard Solar Model the fit to the SNO and SuperKamiokande spectra are moderately good while a very good fit is obtained when the absolute normalizations of the 8B and hep neutrino fluxes are allowed to vary. The best fit prefers large, but not maximal, mixing, essentially no hep neutrinos, and a 40% reduction in the 8B neutrino flux. The fit to the total rates from the different experiments is not encouraging but when the rates and spectra are considerd together the situation is much improved. We remark on the expectations of the VEP model for the neutral current measurements at SNO.
Heavy right-handed neutrinos are of current interest. The interactions and decay of such neutrinos determine their decoupling epoch during the evolution of the universe. This in turn affects various observable features like the energy density, nucleosynthesis, CMBR spectrum, galaxy formation, and baryogenesis. Here, we consider reduction of right-handed electron-type Majorana neutrinos, in the left-right symmetric model, by the WR+ - WR- channel and the channel originating from an anomaly, involving the SU(2)R gauge group, as well as decay of such neutrinos. We study the reduction of these neutrinos for different ranges of left-right model parameters, and find that, if the neutrino mass exceeds the right-handed gauge boson mass, then the neutrinos never decouple for realistic values of the parameters, but, rather, decay in equilibrium. Because there is no out-of-equilibrium decay, no mass bounds can be set for the neutrinos.
We present a solution of the solar neutrino deficit using three flavors of neutrinos within the R-parity non-conserving supersymmetric model. In vacuum, mass and mixing is restricted to the nu(mu)-nu(tau) sector only, which we choose in consistency with the requirements of the atmospheric neutrino anomaly. The nu(e) is massless and unmixed. The flavor changing and flavor diagonal neutral currents present in the model and an energy-dependent resonance-induced nu(e)-nu(mu) mixing in the sun result in the new solution to the solar neutrino problem. The best fit to the solar neutrino rates and spectrum (1258-day SK data) requires a mass square difference of 10^{-5} eV^2 in vacuum between the two lightest neutrinos. This solution cannot accommodate a significant day-night effect for solar neutrinos.