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Raj Gandhi

Publications and source records attributed to Raj Gandhi.

At least 37 records · Page 2Linked to original sources
arXiv2015

Optimal configurations of the Deep Underground Neutrino Experiment

We perform a comprehensive study of the ability of the Deep Underground Neutrino Experiment (DUNE) to answer outstanding questions in the neutrino sector. We consider the sensitivities to the mass hierarchy, the octant of θ_{23} and to CP violation using data from beam and atmospheric neutrinos. We evaluate the dependencies on the precision with which θ_{13} will be measured by reactor experiments, on the detector size, beam power and exposure time, on detector magnetization, and on the systematic uncertainties achievable with and without a near detector. We find that a 35 kt far detector in DUNE with a near detector will resolve the eight-fold degeneracy that is intrinsic to long baseline experiments and will meet the primary goals of oscillation physics that it is designed for.

hep-ph↗
arXiv2015

The impact of sterile neutrinos on CP measurements at long baselines

With the Deep Underground Neutrino Experiment (DUNE) as an example, we show that the presence of even one sterile neutrino of mass $\sim$1 eV can significantly impact the measurements of CP violation in long baseline experiments. Using a probability level analysis and neutrino-antineutrino asymmetry calculations, we discuss the large magnitude of these effects, and show how they translate into significant event rate deviations at DUNE. Our results demonstrate that measurements which, when interpreted in the context of the standard three family paradigm, indicate CP conservation at long baselines, may, in fact hide large CP violation if there is a sterile state. Similarly, any data indicating the violation of CP cannot be properly interpreted within the standard paradigm unless the presence of sterile states of mass O(1 eV) can be conclusively ruled out. Our work underscores the need for a parallel and linked short baseline oscillation program and a highly capable near detector for DUNE, in order that its highly anticipated results on CP violation in the lepton sector may be correctly interpreted.

hep-ph↗
arXiv2015

The Direct Detection of Boosted Dark Matter at High Energies and PeV events at IceCube

We study the possibility of detecting dark matter directly via a small but energetic component that is allowed within present-day constraints. Drawing closely upon the fact that neutral current neutrino nucleon interactions are indistinguishable from DM-nucleon interactions at low energies, we extend this feature to high energies for a small, non-thermal but highly energetic population of DM particle $χ$, created via the decay of a significantly more massive and long-lived non-thermal relic $ϕ$, which forms the bulk of DM. If $χ$ interacts with nucleons, its cross-section, like the neutrino-nucleus coherent cross-section, can rise sharply with energy leading to deep inelastic scattering, similar to neutral current neutrino-nucleon interactions at high energies. Thus, its direct detection may be possible via cascades in very large neutrino detectors. As a specific example, we apply this notion to the recently reported three ultra-high energy PeV cascade events clustered around $1-2$ PeV at IceCube (IC). We discuss the features which may help discriminate this scenario from one in which only astrophysical neutrinos constitute the event sample in detectors like IC.

hep-ph↗
arXiv2014

Revisiting the implications of CPT and unitarity for baryogenesis and leptogenesis

In the context of GUT baryogenesis models, a well-known theorem asserts that CPT conservation and the unitarity of S-matrix require that the lowest order contribution that leads to the generation of a non-zero net CP-violation via the decay of a heavy particle must be to $\mathcal{O}({α_\slashed{B}}^3)$, where $α_\slashed{B}$ is a baryon number (B) violating coupling. We revisit this theorem (which holds for lepton number (L) violation, and hence for leptogenesis as well) and examine its implications for models where the particle content allows the heavy particle to also decay via modes which conserve B (or L) in addition to modes which do not. We systematically expand the S-matrix order by order in B\slash L-violating couplings, and show, in such cases, that the net CP-violation is non-zero even to $\mathcal{O}({α_\slashed{B}}^2)$, without actually contradicting the theorem. By replacing a B/L violating coupling (usually constrained to be small) by a relatively unconstrained B/L conserving one, our result may allow for sufficient CP violation in models where it may otherwise have been difficult to generate the observed baryon asymmetry. As an explicit application of this result, we construct a model in low-scale leptogenesis.

hep-ph↗
arXiv2014

Probing Lorentz and CPT Violation in a Magnetized Iron Detector using Atmospheric Neutrinos

We study the sensitivity of the Iron Calorimeter (ICAL) at the India-Based Neutrino Observatory (INO) to Lorentz and CPT violation in the neutrino sector. Its ability to identify the charge of muons in addition to their direction and energy makes ICAL a useful tool in putting constraints on these fundamental symmetries. Using resolution, efficiencies, errors and uncertainties obtained from ICAL detector simulations, we determine sensitivities to $δ{b}_{31}$, which parametizes the violations in the muon neutrino sector. We carry out calculations for three generic cases representing mixing in the CPT violating part of the hamiltonian, specifically, when the mixing is 1) small, 2) large, 3) the same as that in the PMNS matrix. We find that for both types of hierarchy, ICAL at INO should be sensitive to $δ{b}_{31}$ $\gtrsim$ $4\times10^{-23}$ GeV at 99% C.L. for 500 kt-yr exposure, unless the mixing in the CPT violation sector is small.

hep-ph↗
arXiv2013

Configuring the Long-Baseline Neutrino Experiment

We study the neutrino oscillation physics performance of the Long-Baseline Neutrino Experiment (LBNE) in various configurations. In particular, we compare the case of a surface detector at the far site augmented by a near detector, to that with the far site detector placed deep underground but no near detector. In the latter case, information from atmospheric neutrino events is also utilized. For values of θ_{13} favored by reactor experiments and a 100 kt-yr exposure, we find roughly equivalent sensitivities to the neutrino mass hierarchy, the octant of θ_{23}, and to CP violation. We also find that as the exposure is increased, the near detector helps increase the sensitivity to CP violation substantially more than atmospheric neutrinos.

hep-ph↗
arXiv2012

On the interpretation of IceCube cascade events in terms of the Glashow resonance

The IceCube experiment (IC) has recently observed 2 cascade events with energies between 1 and 10 PeV. This energy combined with the fact that no muon-track events are observed may be interpreted as a cosmogenic $\bar ν_e$ interacting in IC via the Glashow resonance (GR) $\bar ν_e e \to W^-\to$ (hadrons or $\bar ν_e e$). We point out a unique, background-free signature of the GR, a single isolated muon unaccompanied by any shower activity from the interaction $\bar ν_e e \to W^-\to \bar ν_μμ^-$, and propose it as a test of this interpretation. We calculate the event numbers and find that a single such event is expected over about a three-year period in IC. We also show that, if event rates remain at their current levels then, even with the GR, standard cosmogenic fluxes cannot easily explain the observations. Moreover, if muon-tracks remain conspicuous by their absence, then new physics needs to be invoked. As example scenarios in conformity with the observations, we calculate event rates for neutrino decay and Lorentz-invariance violation.

hep-ph↗
arXiv2012

Neutrino mass hierarchy and octant determination with atmospheric neutrinos

The recent discovery by the Daya-Bay and RENO experiments, that θ_{13} is nonzero and relatively large, significantly impacts existing experiments and the planning of future facilities. In many scenarios, the nonzero value of θ_{13} implies that θ_{23} is likely to be different from π/4. Additionally, large detectors will be sensitive to matter effects on the oscillations of atmospheric neutrinos, making it possible to determine the neutrino mass hierarchy and the octant of θ_{23}. We show that a 50 kT magnetized liquid argon neutrino detector can ascertain the mass hierarchy with a significance larger than 4 sigma with moderate exposure times, and the octant at the level of 2-3 sigma with greater exposure.

hep-ph↗
arXiv2012

Atmospheric neutrinos as a probe of eV^2-scale active-sterile oscillations

The down-going atmospheric ν_μ and {\bar{ν_μ}} fluxes can be significantly altered due to the presence of eV^2-scale active-sterile oscillations. We study the sensitivity of a large Liquid Argon detector and a large magnetized iron detector (like the proposed ICAL at INO) to these oscillations. Such oscillations are indicated by results from LSND, and more recently, from MiniBooNE and from reanalyses of reactor experiments following recent recalculations of reactor fluxes. There are other tentative indications of the presence of sterile states in both the νand {\barν} sectors as well. Using the allowed sterile parameter ranges in a 3+1 mixing framework in order to test these results, we perform a fit assuming active-sterile oscillations in both the muon neutrino and antineutrino sectors, and compute oscillation exclusion limits using atmospheric down-going muon neutrino and anti-neutrino events. We find that (for both ν_μ and {\bar{ν_μ}}) a Liquid Argon detector, an ICAL-like detector or a combined analysis of both detectors with an exposure of 1 Mt yr provides significant sensitivity to regions of parameter space in the range 0.1 < Δm^2 < 5 eV^2 for \sin^2 2Θ_{μμ}\geq 0.08. Thus atmospheric neutrino experiments can provide complementary coverage in these regions, improving sensitivity limits in combination with bounds from other experiments on these parameters. We also analyse the bounds using muon antineutrino events only and compare them with the results from MiniBooNE.

hep-ph↗
arXiv2011

The Glashow resonance at IceCube: signatures, event rates and $pp$ vs. $pγ$ interactions

We revisit the signatures of the Glashow resonance process $\barν_e e \to W$ in the high-energy astrophysical neutrino observatory IceCube. We note that in addition to the standard hadronic and electromagnetic showers produced by an incoming neutrino at the resonance energy of $E_ν\approx 6.3$ PeV, there are two clear signals of the process: the "pure muon" from $\barν_e e \to \barν_μμ$ and the "contained lollipop" from $\barν_e e \to \barν_ττ$. The event rate and the signal-to-background ratio (the ratio of the resonant to concurrent non-resonant processes) are calculated for each type of interaction, based on current flux limits on the diffuse neutrino flux. Because of the low background in the neighborhood of the resonance, the observation of only one pure muon or contained lollipop event essentially signals discovery of the resonance, even if the expected event numbers are small. We also evaluate the total event rates of the Glashow resonance from the extra-galactic diffuse neutrino flux and emphasize its utility as a discovery tool to enable first observations of such a flux. We find that one can expect 3.6 (0.65) events per year for a pure $pp$ ($pγ$) source, along with an added contribution of 0.51 (0.21) from non-resonant events. We also give results as a function of the ratio of $pp$ vs $pγ$ sources.

astro-ph.HE↗
arXiv2010

Ultra-high neutrino fluxes as a probe for non-standard physics

We examine how light neutrinos coming from distant active galactic nuclei (AGN) and similar high energy sources may be used as tools to probe non-standard physics. In particular we discuss how studying the energy spectra of each neutrino flavour coming from such distant sources and their distortion relative to each other may serve as pointers to exotic physics such as neutrino decay, Lorentz symmetry violation, pseudo-Dirac effects, CP and CPT violation and quantum decoherence. This allows us to probe hitherto unexplored ranges of parameters for the above cases, for example lifetimes in the range $ 10^{-3}-10^{4} $ s/eV for the case of neutrino decay. We show that standard neutrino oscillations ensure that the different flavours arrive at the earth with similar shapes even if their flavour spectra at source may differ strongly in both shape and magnitude. As a result, observed differences between the spectra of various flavours at the detector would be signatures of non-standard physics altering neutrino fluxes during propagation rather than those arising during their production at source. Since detection of ultra-high energy (UHE) neutrinos is perhaps imminent, it is possible that such differences in spectral shapes will be tested in neutrino detectors in the near future. To that end, using the IceCube detector as an example, we show how our results translate to observable shower and muon-track event rates.

hep-ph↗
arXiv2010

Diffuse Ultra-High Energy Neutrino Fluxes and Physics Beyond the Standard Model

We study spectral distortions of diffuse ultra-high energy (UHE) neutrino flavour fluxes resulting due to physics beyond the Standard Model (SM). Even large spectral differences between flavours at the source are massaged into a common shape at earth by SM oscillations, thus, any significant observed spectral differences are an indicator of new physics present in the oscillation probability during propagation. Neutrino decay and Lorentz symmetry violation (LV) are examples, and result in significant distortion of the fluxes and the well-known bounds on them, which may allow UHE detectors to probe lifetimes, the mass hierarchy and LV parameters over a broad range.

hep-ph↗
arXiv2010

The Physics Role and Potential of future Atmospheric Detectors

We discuss the physics capabilities of basic types of future atmospheric detectors being considered at present, with their strengths and limitations, and compare them with those of long baseline (LBL) experiments. We also argue that recent studies signal the importance of synergistically combining complementary features of both these classes of experiments in order to accrue maximum benefit towards furthering our goal of building a complete picture of neutrino properties and parameters.

hep-ph↗
arXiv2009

The Role and Detectability of the Charm Contribution to Ultra High Energy Neutrino Fluxes

It is widely believed that charm meson production and decay may play an important role in high energy astrophysical sources of neutrinos, especially those that are baryon-rich, providing an environment conducive to pp interactions. Using slow-jet supernovae (SJS) as an example of such a source, we study the detectability of high-energy neutrinos, paying particular attention to those produced from charmed-mesons. We highlight important distinguishing features in the ultra-high energy neutrino flux which would act as markers for the role of charm in the source. In particular, charm leads to significant event rates at higher energies, after the conventional (pi, K) neutrino fluxes fall off. We calculate event rates both for a nearby single source and for diffuse SJS fluxes for an IceCube-like detector. By comparing muon event rates for the conventional and prompt fluxes in different energy bins, we demonstrate the striking energy dependence in the rates induced by the presence of charm. We also show that it leads to an energy dependant flux ratio of shower to muon events, providing an additional important diagnostic tool for the presence of prompt neutrinos. Motivated by the infusion of high energy anti-electron neutrinos into the flux by charm decay, we also study the detectability of the Glashow resonance due to these sources.

hep-ph↗
arXiv2009

Mass Hierarchy Determination for $θ_{13}=0$ and Atmospheric Neutrinos

We examine the possibility of determining the neutrino mass hierarchy in the limit θ_{13} = 0 using atmospheric neutrinos as the source. In this limit, in which θ_{13} driven matter effects are absent, independent measurements of Δ_{31} and Δ_{32} can, in principle, lead to hierarchy determination. Since the difference between these two is Δ_{21}, one needs an experimental arrangement where Δ_{21} L/E \gtrsim 1 can be achieved. This condition can be satisfied by atmospheric neutrinos since they have a large range of energies and baselines. In spite of this, we find that hierarchy determination in the θ_{13}=0 limit with atmospheric neutrinos is not a realistic possibility, even in conjunction with an apparently synergistic beam experiment like T2K or NOvA. We discuss the reasons for this, and also in the process clarify the conditions that must be satisfied in general for hierarchy determination if θ_{13} = 0.

hep-ph↗
arXiv2008

Resolving the Mass Hierarchy with Atmospheric Neutrinos using a Liquid Argon Detector

We explore the potential offered by large-mass Liquid Argon detectors for determination of the sign of Delta m_{31}^2, or the neutrino mass hierarchy, through interactions of atmospheric neutrinos. We give results for a 100 kT sized magnetized detector which provides separate sensitivity to ν_μ, \barν_μand, over a limited energy range, to ν_e, \barν_e.We also discuss the sensitivity for the unmagnetized version of such a detector. After including the effect of smearing in neutrino energy and direction and incorporating the relevant statistical,theoretical and systematic errors, we perform a binned χ^2 analysis of simulated data. The χ^2 is marginalized over the presently allowed ranges of neutrino parameters and determined as a function of θ_{13}. We find that such a detector offers superior capabilities for hierarchy resolution, allowing a > 4σdetermination for a 100 kT detector over a 10 year running period for values of \sin^2 2θ_{13} \ge 0.05. For an unmagnetized detector, a 2.5σhierarchy sensitivity is possible for \sin^2 2θ_{13} = 0.04.

hep-ph↗
arXiv2007

Mass Hierarchy Determination via future Atmospheric Neutrino Detectors

We study the problem of determination of the sign of Delta m^2_{31}, or the neutrino mass hierarchy, through observations of atmospheric neutrinos in future detectors. We consider two proposed detector types : (a) Megaton sized water Cerenkov detectors, which can measure the survival rates of nu_μ+ \barν_μand nu_e + \barν_e and (b) 100 kton sized magnetized iron detectors, which can measure the survival rates of ν_μand \barν_μ. For energies and path-lengths relevant to atmospheric neutrinos, these rates obtain significant matter contributions from P_{μe}, P_{μμ} and P_{ee}, leading to an appreciable sensitivity to the hierarchy. We do a binned χ^2 analysis of simulated data in these two types of detectors which includes the effect of smearing in neutrino energy and direction and incorporates detector efficiencies and relevant statistical, theoretical and systematic errors. We also marginalize the χ^2 over the allowed ranges of neutrino parameters in order to accurately account for their uncertainties. Finally, we compare the performance of both types of detectors vis a vis the hierarchy determination.

hep-ph↗