arXiv subjects

Amol Dighe

Publications and source records attributed to Amol Dighe.

At least 55 records · Page 3Linked to original sources

arXiv2015

Enhancing sensitivity to neutrino parameters at INO combining muon and hadron information

The proposed ICAL experiment at INO aims to identify the neutrino mass hierarchy from observations of atmospheric neutrinos, and help improve the precision on the atmospheric neutrino mixing parameters. While the design of ICAL is primarily optimized to measure muon momentum, it is also capable of measuring the hadron energy in each event. Although the hadron energy is measured with relatively lower resolution, it nevertheless contains crucial information on the event, which may be extracted when taken concomitant with the muon data. We demonstrate that by adding the hadron energy information to the muon energy and muon direction in each event, the sensitivity of ICAL to the neutrino parameters can be improved significantly. Using the realistic detector response for ICAL, we present its enhanced reach for determining the neutrino mass hierarchy, the atmospheric mass squared difference and the mixing angle theta23, including its octant. In particular, we show that the analysis that uses hadron energy information can distinguish the normal and inverted mass hierarchies with Deltachi^2 approx 9 with 10 years exposure at the 50 kt ICAL, which corresponds to about 40% improvement over the muon-only analysis.

hep-ph↗

arXiv2014

Nonuniversality of indirect CP asymmetries in $D \to ππ, KK$ decays

We point out that, if the direct CP asymmetries in the $D \to π^+ π^-$ and $D \to K^+ K^-$ decays are unequal, the indirect CP asymmetries as measured in these modes are necessarily unequal. This nonuniversality of indirect CP asymmetries can be significant with the right amount of new physics contributions, a scenario that may be fine-tuned, but is still viable. A model-independent fit to the current data allows different indirect CP asymmetries in the above two decays. This could even be contributing to the apparent tension between the difference CP asymmetries $ΔA_{\rm CP}$ measured through the pion-tagged and muon-tagged data samples at the LHCb. This also implies that the measurements of $A_Γ$ and $y_{\rm CP}$ in the $π^+ π^-$ and $K^+ K^-$ decay modes can be different, and averaging over these two modes should be avoided. In any case, the complete analysis of CP violation measurements in the $D$ meson sector needs to take into account the possibility of different indirect CP asymmetries in the $π^+π^-$ and $K^+ K^-$ channels.

hep-ph↗

arXiv2013

Hadron energy response of the Iron Calorimeter detector at the India-based Neutrino Observatory

The results of a Monte Carlo simulation study of the hadron energy response for the magnetized Iron CALorimeter detector, ICAL, proposed to be located at the India-based Neutrino Observatory (INO) is presented. Using a GEANT4 modeling of the detector ICAL, interactions of atmospheric neutrinos with target nuclei are simulated. The detector response to hadrons propagating through it is investigated using the hadron hit multiplicity in the active detector elements. The detector response to charged pions of fixed energy is studied first, followed by the average response to the hadrons produced in atmospheric neutrino interactions using events simulated with the NUANCE event generator. The shape of the hit distribution is observed to fit the Vavilov distribution, which reduces to a Gaussian at high energies. In terms of the parameters of this distribution, we present the hadron energy resolution as a function of hadron energy, and the calibration of hadron energy as a function of the hit multiplicity. The energy resolution for hadrons is found to be in the range 85% (for 1GeV) -- 36% (for 15 GeV).

physics.ins-det↗

arXiv2013

The Reach of INO for Atmospheric Neutrino Oscillation Parameters

The India-based Neutrino Observatory (INO) will host a 50 kt magnetized iron calorimeter (ICAL@INO) for the study of atmospheric neutrinos. Using the detector resolutions and efficiencies obtained by the INO collaboration from a full-detector GEANT4-based simulation, we determine the reach of this experiment for the measurement of the atmospheric neutrino mixing parameters ($\sin^2 θ_{23}$ and $|Δm_{32}^2 |$). We also explore the sensitivity of this experiment to the deviation of $θ_{23}$ from maximal mixing, and its octant.

hep-ph↗

arXiv2013

New Physics in b -> s mu+ mu-: CP-Conserving Observables

We perform a comprehensive study of the impact of new-physics operators with different Lorentz structures on decays involving the b -> s mu+ mu- transition. We examine the effects of new vector-axial vector (VA), scalar-pseudoscalar (SP) and tensor (T) interactions on the differential branching ratios and forward-backward asymmetries (A_{FB}'s) of Bsbar -> mu+ mu-, Bdbar -> Xs mu+ mu-, Bsbar -> mu+ mu- gamma, Bdbar -> Kbar mu+ mu-, and Bdbar -> K* mu+ mu-, taking the new-physics couplings to be real. In Bdbar -> K* mu+ mu-, we further explore the polarization fraction f_L, the angular asymmetry A_T^{(2)}, and the longitudinal-transverse asymmetry A_{LT}. We identify the Lorentz structures that would significantly impact these observables, providing analytical arguments in terms of the contributions from the individual operators and their interference terms. In particular, we show that while the new VA operators can significantly enhance most of the asymmetries beyond the Standard Model predictions, the SP and T operators can do this only for A_{FB} in Bdbar -> Kbar mu+ mu-.

hep-ph↗

arXiv2013

Testing Times for Supersymmetry: Looking Under the Lamp Post

We make a critical study of two highly-constrained models of supersymmetry --- the constrained minimal supersymmetric standard model (cMSSM), and the non-universal Higgs mass model (NUHM) --- in the light of the 125-126 GeV Higgs boson, the first observation of $B_s \to μμ$ at the LHCb, and the updated $B \to τν$ branching ratio at BELLE. It turns out that these models are still allowed by the experimental data, even if we demand that there be a light stop with mass less than 1.5 TeV. The only significant effects of all these constraints are to push the mass of the light stop above $\sim 500$ GeV, and to prefer the universal trilinear coupling $A_0$ to be large and negative. We calculate the Higgs boson branching ratios to $WW, ZZ, ττ$ and $γγ$ in these models and show that improved experimental limits on these could put them to the most stringent experimental tests yet.

hep-ph↗

arXiv2012

Optimization of the baseline and the parent muon energy for a low energy neutrino factory

We discuss the optimal setup for a low energy neutrino factory in order to achieve a 5σ-discovery of a nonzero mixing angle θ_{13}, a nonzero CP phase δ_{CP}, and the mass hierarchy. We explore parent muon energies in the range 5--16 GeV, and baselines in the range 500--5000 km. We present the results in terms of the reach in sin^2θ_{13}, emphasizing the dependence of the optimal baseline on the true value of δ_{CP}. We show that the sensitivity of a given setup typically increases with parent muon energy, reaching saturation for higher energies. The saturation energy is larger for longer baselines; we present an estimate of this dependence. In the light of the recent indications of a large θ_{13}, we also determine how these preferences would change if indeed a large θ_{13} is confirmed. In such a case, the baselines ~2500 km (~1500 km) may be expected to lead to hierarchy determination (δ_{CP} discovery) with the minimum exposure.

hep-ph↗

arXiv2012

How large can the branching ratio of $B_s \to τ^+ τ^-$ be ?

Motivated by the large like-sign dimuon charge asymmetry observed recently, whose explanation would require an enhanced decay rate of $B_s \to τ^+ τ^-$, we explore how large a branching ratio of this decay mode is allowed by the present constraints. We use bounds from the lifetimes of $B_d$ and $B_s$, constraints from the branching ratios of related $b \to s τ^+ τ^-$ modes, as well as measurements of the mass difference, width difference and CP-violating phase in the $B_s$-$\bar{B}_s$ system. Using an effective field theory approach, we show that a branching ratio as high as 15% may be allowed while being consistent with the above constraints. The model with a scalar leptoquark cannot increase the branching ratio to a per cent level. However, an enhancement up to 5% is possible in the model with an extremely light $Z'$ with flavor-dependent interactions, even after all the couplings are taken to be perturbative. This however cannot account for the dimuon anomaly completely by itself.

hep-ph↗

arXiv2012

Large mass splittings for fourth generation fermions allowed by LHC Higgs exclusion

In the context of the Standard model with a fourth generation, we explore the allowed mass spectra in the fourth generation quark and lepton sectors as functions of the Higgs mass. Using the constraints from unitarity and oblique parameters, we show that a heavy Higgs allows large mass splittings in these sectors, opening up new decay channels involving W emission. A Higgs heavier than $ \sim 800 GeV $ would in fact necessitate either a heavy quark decay channel t'-> b'W/b'-> t' W or a heavy lepton decay channel τ' -> ν' W as long as the mixing between the third and fourth generations is small. This mixing tends to suppress the mass splittings and hence the W-emission channels. The possibility of the W-emission channel could substantially change the search strategies of fourth generation fermions at the LHC and impact the currently reported mass limits.

hep-ph↗

arXiv2012

The next-generation liquid-scintillator neutrino observatory LENA

We propose the liquid-scintillator detector LENA (Low Energy Neutrino Astronomy) as a next-generation neutrino observatory on the scale of 50 kt. The outstanding successes of the Borexino and KamLAND experiments demonstrate the large potential of liquid-scintillator detectors in low-energy neutrino physics. LENA's physics objectives comprise the observation of astrophysical and terrestrial neutrino sources as well as the investigation of neutrino oscillations. In the GeV energy range, the search for proton decay and long-baseline neutrino oscillation experiments complement the low-energy program. Based on the considerable expertise present in European and international research groups, the technical design is sufficiently mature to allow for an early start of detector realization.

astro-ph.IM↗

arXiv2011

New Physics in b -> s mu+ mu-: CP-Violating Observables

We perform a comprehensive study of the impact of new-physics operators with different Lorentz structures on CP-violating observables involving the b -> s mu+ mu- transition. We examine the effects of new vector-axial vector (VA), scalar-pseudoscalar (SP) and tensor (T) interactions on the CP asymmetries in the branching ratios and forward-backward asymmetries of Bs -> mu mu, B -> Xs mu mu, Bs -> mu mu gamma, B -> K mu mu, and B -> K* mu mu. In B -> K* mu mu, we also explore the direct CP asymmetries in the longitudinal polarization fraction f_L and the angular asymmetries A_T^{(2)} and A_{LT}, as well as the triple-product CP asymmetries A_T^{(im)} and A^{(im)}_{LT}. We find that, in almost all cases, the CP-violating observables are sensitive only to new physics which involves VA operators. The VA new physics may therefore be unambiguously identified by a combined analysis of future measurements of these CP-violating observables.

hep-ph↗

arXiv2011

Reconciling anomalous measurements in $B_s-\bar{B}_s$ mixing: the role of CPT-conserving and CPT-violating new physics

Recently observed anomalies in the $B_s \to J/ψϕ$ decay and the like-sign dimuon asymmetry $A^b_{sl}$ hint at possible new physics (NP) in the $\bsbsbar$ mixing. We parameterize the NP with four model-independent quantities: the magnitudes and phases of the dispersive part $M_{12}$ and the absorptive part $Γ_{12}$ of the NP contribution to the effective Hamiltonian. We constrain these parameters using the four observables $ΔM_s$, $ΔΓ_s$, the mixing phase $β_s^{J/ψϕ}$, and $A^b_{sl}$. Our quantitative fit indicates that the NP should contribute a significant dispersive as well as absorptive part. In fact, models that do not contribute a new absorptive part are disfavored at more than 99% confidence level. We extend this formalism to include CPT violation, and show that CPT violation by itself, or even in presence of CPT-conserving new physics without an absorptive part, helps only marginally in the simultaneous resolution of these anomalies. The NP absorptive contribution to $\bsbsbar$ mixing therefore seems to be essential, and would imply a large branching fraction for channels like $B_s \to τ^+ τ^-$.

hep-ph↗

arXiv2011

Linearized flavor-stability analysis of dense neutrino streams

Neutrino-neutrino interactions in dense neutrino streams, like those emitted by a core-collapse supernova, can lead to self-induced neutrino flavor conversions. While this is a nonlinear phenomenon, the onset of these conversions can be examined through a standard stability analysis of the linearized equations of motion. The problem is reduced to a linear eigenvalue equation that involves the neutrino density, energy spectrum, angular distribution, and matter density. In the single-angle case, we reproduce previous results and use them to identify two generic instabilities: The system is stable above a cutoff density ("cutoff mode"), or can approach an asymptotic instability for increasing density ("saturation mode"). We analyze multi-angle effects on these generic types of instabilities and find that even the saturation mode is suppressed at large densities. For both types of modes, a given multi-angle spectrum typically is unstable when the neutrino and electron densities are comparable, but stable when the neutrino density is much smaller or much larger than the electron density. The role of an instability in the SN context depends on the available growth time and on the range of affected modes. At large matter density, most modes are off-resonance even when the system is unstable.

hep-ph↗

arXiv2011

Do new data on [B+ -> tau+ nu_tau] decays point to an early discovery of supersymmetry at the LHC?

The recent Belle and BaBar measurements of the branching ratio of $B^+ \to τ^+ ν_τ$ indicate a significant deviation from the Standard Model prediction. We demonstrate that this measurement has a serious impact on models with minimal flavor violation involving a charged Higgs boson, ruling out a large portion of the currently-allowed parameter space. In the constrained minimal supersymmetric standard model, this creates a tension between the measurements of $B^+ \to τ^+ ν_τ$ and the anomalous magnetic moment of the muon, unless $\tanβ$ is small, $μ> 0$, and $A_0$ takes a large negative value. In fact, a very small region of the parameter space of this model, with small values of $m_0$ and $m_{1/2}$, survives all the constraints at 95% C.L.. It is remarkable that this specific region is still consistent with the lightest supersymmetric particle as the dark matter. Moreover, it predicts observable SUSY signals in the early runs of the LHC, even perhaps at 7 TeV. We also show that a consistent explanation for the deviation of the $B^+ \to τ^+ ν_τ$ branching ratio from the Standard Model can be achieved in a non-universal Higgs mass model, which could also predict early signals of supersymmetry at the LHC.

hep-ph↗

arXiv2011

Constraints on the Four-Generation Quark Mixing Matrix from a Fit to Flavor-Physics Data

In the scenario with four quark generations, we perform a fit using flavor-physics data and determine the allowed values -- preferred central values and errors -- of all of the elements of the 4X4 quark mixing matrix. In addition to the direct measurements of some of the elements, we include in the fit the present measurements of several flavor-changing observables in the K and B systems that have small hadronic uncertainties, and also consider the constraints from the vertex corrections to Z -> b bbar. The values taken for the masses of the fourth-generation quarks are consistent with the measurements of the oblique parameters and perturbativity of the Yukawa couplings. We find that |{\tilde V}_{tb}| >= 0.98 at 3sigma, so that a fourth generation cannot account for any large deviation of |{\tilde V}_{tb}| from unity. The fit also indicates that all the new-physics parameters are consistent with zero, and the mixing of the fourth generation with the other three is constrained to be very small: we obtain |{\tilde V}_{ub'}| < 0.06, |{\tilde V}_{cb'}| < 0.027, and |{\tilde V}_{tb'}| < 0.31 at 3sigma. Still, this does allow for the possibility of new-physics signals in Bd, Bs and rare K decays.

hep-ph↗

arXiv2010

2540 km: Bimagic baseline for neutrino oscillation parameters

We show that a source-to-detector distance of 2540 km offers multiple advantages for a low energy neutrino factory with a detector that can identify muon charge. At this baseline, for any neutrino hierarchy, the wrong-sign muon signal is almost independent of CP violation and $θ_{13}$ in certain energy ranges. This reduces the uncertainties due to these parameters and allows the identification of the hierarchy in a clean way. In addition, part of the muon spectrum is also sensitive to the CP violating phase and $θ_{13}$, so that the same setup can be used to probe these parameters as well.

hep-ph↗

arXiv2010

New-physics contributions to the forward-backward asymmetry in B -> K* mu+ mu-

We study the forward-backward asymmetry (AFB) and the differential branching ratio (DBR) in B -> K* mu+ mu- in the presence of new physics (NP) with different Lorentz structures. We consider NP contributions from vector-axial vector (VA), scalar-pseudoscalar (SP), and tensor (T) operators, as well as their combinations. We calculate the effects of these new Lorentz structures in the low-q^2 and high-q^2 regions, and explain their features through analytic approximations. We find two mechanisms that can give a significant deviation from the standard-model predictions, in the direction indicated by the recent measurement of AFB by the Belle experiment. They involve the addition of the following NP operators: (i) VA, or (ii) a combination of SP and T (slightly better than T alone). These two mechanisms can be distinguished through measurements of DBR in B -> K* mu+ mu- and AFB in B -> K mu+ mu-.

hep-ph↗

arXiv2010

Signatures of collective and matter effects on supernova neutrinos at large detectors

We calculate the expected galactic supernova neutrino signal at large next-generation underground detectors. At different epochs after the explosion, the primary fluxes can be quite different. For these primary neutrino fluxes, spectral splits induced by collective neutrino flavor transformations can arise for either mass hierarchy in both neutrino and antineutrino channels. We classify flux models according to the nature and number of these splits, and calculate the observable electron-neutrino and electron-antineutrino spectra at Earth, taking into account subsequent matter effects. We find that some of the spectral splits could occur sufficiently close to the peak energies to produce significant distortions in the observable SN neutrino signal. The most striking signature of this effect would be presence of peculiar energy dependent modulations associated with Earth matter crossing, present only in portions of the SN neutrino energy spectra demarcated by spectral splits. These signatures at proposed large water Cherenkov, scintillation, and liquid Argon detectors could give hints about the primary SN neutrino fluxes, as well as on the neutrino mass hierarchy and the mixing angle theta_{13}.

hep-ph↗