arXiv subjects

Ashutosh Kumar Alok

Publications and source records attributed to Ashutosh Kumar Alok.

At least 37 records · Page 2Linked to original sources

arXiv2022

Neutrino spin-flavour precession in magnetized white dwarf

Due to notoriously small value of the neutrino magnetic moment, the phenomena of neutrino spin flavour precession (SFP) requires very high magnetic field. This makes only a handful of systems suitable to study this phenomena. By the observation of SFP, the Dirac and Majorana nature of neutrinos is expected to be distinguished. In this work, we point out the potential of white dwarf (WD) system in studying the spin-flavour oscillation of neutrinos. From recent analysis, it has been found that young isolated WDs may harbor very strong internal magnetic field, even without exhibiting any surface magnetic field. The presence of magnetic field enhances the cooling process and along with that, renders the spin-flavour oscillation of neutrinos emitted in the neutrino cooling process. Employing the standard WD specifications, we analyse whether a magnetized WD is a suitable environment to distinguish between the Dirac and Majorana nature of neutrino. Lower value of spin flavour transition probability implies reduced active neutrino flux which is possible to be estimated in terrestrial neutrino detectors. We find that the spin flavour transition probability of Dirac neutrinos is much higher in comparison to the Majorana neutrino which converts the active neutrino flavours to sterile in a significant amount. We also examine the sensitivity of the spin flavour transition probability to the neutrino magnetic moment.

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arXiv2022

Can NSI affect non-local correlations in neutrino oscillations?

Non-local correlations in entangled systems are usually captured by measures such as Bell's inequality violation. It was recently shown that in neutrino systems, a measure of non-local advantage of quantum coherence (NAQC) can be considered as a stronger measure of non-local correlations as compared to the Bell's inequality violation. In this work, we analyze the effects of non standard interaction (NSI) on these measures in the context of two flavour neutrino oscillations for DUNE, MINOS, T2K, KamLAND, JUNO and Daya Bay experimental set-ups. We find that even in the presence of NSI, Bell's inequality violation occurs in the entire energy range whereas the NAQC violation is observed only in some specific energy range justifying the more elementary feature of NAQC. Further, we find that NSI can enhance the violation of NAQC and Bell's inequality parameter in the higher energy range of a given experimental set-up; these enhancements being maximal for the KamLAND experiment. However, the possible enhancement in the violation of the Bell's inequality parameter over the standard model prediction can be up to 11% whereas for NAQC it is 7%. Thus although NAQC is a comparatively stronger witness of nonclassicality, it shows lesser sensitivity to NSI effects in comparison to the Bell's inequality parameter.

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arXiv2022

A unique discrimination between new physics scenarios in $b\rightarrow sμ^+μ^-$ anomalies

A number of observables related to the $b \to s \ell^+ \ell^-$ transition show deviations from their standard model predictions. A global fit to the current $b\rightarrow s\ell^+\ell^-$ data suggests several new physics solutions. Considering only one operator at a time and new physics only in the muon sector, it has been shown that the new physics scenarios (I) $C_9^{\rm NP}<0$, (II) $C_{9}^{\rm NP} = -C_{10}^{\rm NP}$, (III) $C_9^{\rm NP} = -C_9^{\prime \rm NP}$ can account for all data. In this work, we develop a procedure to uniquely identify the correct new physics solution. The scenario II predicts a significantly lower value of $\mathcal{B}(B_s\to μ^+μ^-)$ and can be distinguished from the other two scenarios if the experimental uncertainty comes down by a factor of three. On the other hand, a precise measurement of the CP averaged angular observables $S_9$ in high $q^2$ bin of $B\to K^*μ^+μ^-$ decay can uniquely discriminate between the other two scenarios. We propose new methods, in terms of azimuthal angle asymmetries, to measure $S_9$ with the necessary precision.

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arXiv2022

Impact of $b \to s \ell \ell$ anomalies on rare charm decays in non-universal $Z'$ models

In this work, we study the impact of $b \to s \ell \ell$, $B_s - \bar{B_s}$ mixing and neutrino trident measurements on observables in decays induced by $c \to u $ transition in the context of a non-universal $Z'$ model which generates $C^{\rm NP}_{9} <0$ and $C^{\rm NP}_9 = - \,C^{\rm NP}_{10} $ new physics scenarios at the tree level. We inspect the effects on $D^0 \to π^0 ν\barν$, $D^+ \to π^+ ν\barν$ and $B_c \to B^+ ν\barν $ decays which are induced by the quark level transition $c \to u ν\barν$. The fact that the branching ratios of these decays are negligible in the standard model (SM) and the long distance effects are relatively smaller in comparison to their charged dileptons counterparts, they are considered to provide genuine null-tests of SM. Therefore the observation of these modes at the level of current as well as planned experimental sensitivities would imply unambiguous signature of new physics. Using the constraints on $Z'$ couplings coming from a combined fit to $b \to s \ell \ell$, $ΔM_s$ and neutrino trident data, we find that any meaningful enhancement over the SM value is ruled out in the considered framework. The same is true for $D - \bar{D}$ mixing observable $ΔM_D$ along with $D^0 \to μ^+ μ^-$ and $D^+ \to π^+ μ^+ μ^-$ decay modes which are induced through $c \to u μ^+ μ^-$ transition.

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arXiv2021

The role of non-universal $Z$ couplings in explaining the $V_{us}$ anomaly

The tension among measurements of $V_{us}$ from different channels, the so-called Cabibbo Angle Anomaly, can be interpreted as a signal of lepton flavor universality (LFU) violation in the $W$ boson couplings. We investigate this issue in the framework of effective field theory, keeping the gauge structure of the Standard Model (SM) unchanged. We introduce gauge-invariant dimension-6 effective operators that couple the Higgs doublet to leptons, thereby giving non-universal tree-level contributions to the couplings of electroweak gauge bosons. Due to the $SU(2)_L$ gauge symmetry, a tension arises between the $V_{us}$ measurements that are affected by new $W$ couplings, and the electroweak precision measurements, which are also affected by the new $Z$ couplings. We show that this tension can be alleviated by allowing additional sources of gauge-invariant couplings of $Z$ boson to left- or right-handed leptons, and find the optimal regions indicated by the current data in the Wilson-coefficient space. We illustrate our model-independent results with the examples of minimal extensions of the SM involving the vector-like lepton (VLL) models. We point out that dimension-6 operators coupling the Higgs doublet to leptons can affect the rate of $h \to ττ$ decay significantly in general, however this effect is restricted to less than a per cent level for the minimal VLL models.

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arXiv2021

New physics in $b\rightarrow se^+e^-$: A model independent analysis

The lepton universality violating flavor ratios $R_K/R_{K^*}$ indicate new physics either in $b \to s μ^+ μ^-$ or in $b \to s e^+ e^-$ or in both. If the new physics is only $b \to s e^+ e^-$ transition, the corresponding new physics operators, in principle, can have any Lorentz structure. In this work, we perform a model independent analysis of new physics only in $b \to se^+e^-$ decay by considering effective operators either one at a time or two similar operators at a time. We include all the measurements in $b\rightarrow se^+e^-$ sector along with $R_K/R_{K^*}$ in our analysis. We show that various new physics scenarios with vector/axial-vector operators can account for $R_K/R_{K^*}$ data but those with scalar/pseudoscalar operators and with tensor operators can not. We also show that the azimuthal angular observable $P_1$ in $B \to K^* e^+ e^-$ decay is most suited to discriminate between the different allowed solutions.

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arXiv2021

New physics effects on quantum coherence in neutrino oscillations

Several measures of quantum correlations such as Leggett-Garg and Bell-type inequalities have been extensively studied in the context of neutrino oscillations. However these analyses are performed under the assumption of standard model (SM) interactions of neutrinos. In this work we study new physics effects on $l_1$-norm based measure of quantum coherence which quantifies the quantumness embedded in the system and is also intrinsically related to various measures of quantum correlations. Moreover, it is considered to be a resource theoretical tool which can be utilized in quantum algorithms and quantum channel discrimination. The new physics effects are incorporated in a model independent way by using the effective Lagrangian for the neutral current non-standard neutrino interactions (NSI). Bounds on the NSI parameters are extracted from a recent global analysis of oscillation experiments including COHERENT (coherent neutrino-nucleus scattering experiment) data. In the context of upcoming DUNE experimental setup, we find that the most favourable combination of LMA-Light sector of $θ_{12}$ (i.e., $θ_{12}< 45^o$) with normal mass ordering decreases the coherence in the system in comparison to the SM prediction for all values of neutrino energy E and CP violating phase $δ$ (except in the narrow region around E ~ 2 GeV). On the other hand, a large enhancement in the value of coherence parameter in the entire $(E-δ)$ plane is possible for the dark octant of $θ_{12}$ ($θ_{12}> 45^o$) with inverted ordering. For almost all values of CP violating phase, the enhancement is more protuberant in the region around E ~ 4 GeV where maximum neutrino flux is expected in the DUNE experiment. Therefore for the normal mass ordering, the SM interaction provides favourable conditions for quantum information tasks while the NSI favours inverted ordering scenario for such tasks.

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arXiv2021

Continuing search for new physics in $b \to s μμ$ decays: two operators at a time

The anomalies in the measurements of observables involving $b \to s μμ$ decays, namely $R_K$, $R_{K^*}$, $P_5^{\prime}$, and $B_s^ϕ$, may be addressed by adding lepton-universality-violating new physics contributions to the effective operators ${\cal O}_9, {\cal O}_{10}, {\cal O}^\prime_9, {\cal O}^\prime_{10}$. We analyze all the scenarios where the new physics contributes to a pair of these operators at a time. We perform a global fit to all relevant data in the $b \to s$ sector to estimate the corresponding new Wilson coefficients, $C_9^{\rm NP}, C_{10}^{\rm NP}, C_9^\prime, C_{10}^\prime$. In the light of the new data on $R_K$ and $R_{K^*}$ presented in Moriond 2019, we find that the scenarios with new physics contributions to the ($C_9^{\rm NP}$, $C_9^\prime$) or ($C_9^{\rm NP}$, $C_{10}^\prime$) pair remain the most favored ones. On the other hand, though the competing scenario ($C_9^{\rm NP}$, $C_{10}^{\rm NP}$) remains attractive, its advantage above the SM reduces significantly due to the tension that emerges between the $R_K$ and $R_{K^*}$ measurements with the new data. The movement of the $R_K$ measurement towards unity would also result in the re-emergence of the one-parameter scenario $C_9^{\rm NP} = -C_9^\prime$.

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arXiv2020

Predictions for $B_s \to \bar{K}^* \ell \,\ell$ in non-universal $Z'$ models

The lepton flavor universality violating (LFUV) measurements $R_K$ and $R_{K^*}$ in $B$ meson decays can be accounted for in non-universal $Z'$ models. We constrain the couplings of these $Z'$ models by performing a global fit to correlated $b \to s \ell \ell$ and $b \to d \ell \ell $ processes, and calculate their possible implications for $B_s \to \bar{K}^*\ell \ell$ observables. For real new physics (NP) couplings, the 1-$σ$ favored parameters allow the corresponding LFUV ratio $R_{K^*}^{(s)}$ in $B_s \to \bar{K}^*\ell \ell$ to range between 0.8 -- 1.2 at low $q^2$. Complex NP couplings improve the best fit only marginally, however they allow a significant enhancement of the branching ratio, while increasing the range of $R_{K^*}^{(s)}$ at low $q^2$ to 0.8 -- 1.8. We find that NP could cause zero-crossing in the forward-backward asymmetry $A_{FB}$ to shift towards lower $q^2$ values, and enhancement in the magnitude of integrated $A_{FB}$. The $CP$ asymmetry $A_{CP}$ may be suppressed and even change sign. The simultaneous measurements of integrated $R_{K^*}^{(s)}$ and $A_{CP}$ values to 0.1 and 1% respectively, would help in constraining the effective NP Wilson coefficient $C_9$ in $ b \to d μμ$ interactions.

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arXiv2020

New Physics solutions for $b\rightarrow c\,τ\,\barν$ anomalies after Moriond 2019

At Moriond 2019, Belle collaboration has announced new measurements on the flavour ratios $R_D - R_{D^*}$ which are consistent with their Standard Model predictions within $1.2σ$. After inclusion of these measurements, the global tension in $R_D - R_{D^*}$ has reduced from $4.1σ$ to $3.1σ$ which is still significant. The measurements of these ratios indicate towards the violation of lepton flavor universality in $b\rightarrow c\,l\,\barν$ decay. Assuming new physics in $b\rightarrow c\,τ\,\barν$ transition, we have done a global fit to all available data in this sector to identify the allowed new physics solutions. We find that there are seven allowed new physics solutions which can account for all measurements in $b\rightarrow c\,τ\,\barν$ transition. We show that a simultaneous measurement of the $τ$ polarization fraction and forward-backward asymmetry in $B\rightarrow D\,τ\,\barν$, the zero crossing point of forward backward asymmetry in $B\rightarrow D^*τ\barν$ and the branching ratio of $B_c\rightarrow τ\,\barν$ decay can distinguish these seven new physics solutions if they can be measured with a required precision.

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arXiv2019

Solutions to $R_D$-$R_{D^*}$ in light of Belle 2019 data

Earlier this year, the Belle collaboration presented their new measurements of $R_D$ and $R_{D^*}$ using a new method. These measurements are consistent with the Standard Model predictions, whereas the global averages of the earlier measurements had a $4.1σ$ discrepancy. With the inclusion of the new data in the global averages, the discrepancy comes down to $3.1σ$. In this work, we study the study the new physics solutions to the $R_D$-$R_{D^*}$ anomaly allowed by the reduction in the discrepancy. Among the four fermion operators, which arise through a single particle exchange, only the $(V-A)$ operator solution survives. We found three additional solutions with two dis-similar operators. The branching ratio of $B_c\rightarrow τ\,\barν$ is powerful discriminant between these four allowed solutions.

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arXiv2019

A global fit to $b\rightarrow cτ\barν$ anomalies after Moriond 2019

At Moriond 2019, Belle collaboration announced their new measurements on $R_D$ and $R_{D^*}$ which are in agreement with their Standard Model (SM) predictions within $1.2σ$. After inclusion of these measurements, the discrepancy between the world averages and the SM predictions of $R_D$-$R_{D^*}$ comes down from $4.1σ$ to $3.1σ$. Here we do a global fit by taking all measurements in $b\rightarrow cτ\barν$ transition. We find that there are seven allowed new physics solutions, each with different Lorentz structure. Further we show that it is possible to distinguish between them by means of the five observables: the $τ$ polarization fraction in the decay $B \rightarrow D τ\barν$, the precision measurement of $R_D$, the forward-backward asymmetries in the decays $B \rightarrow (D,D^*) τ\barν$ and the branching ratio of $B_c\rightarrow τ\barν$.

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arXiv2019

Lepton flavor non-universality in the B-sector: a global analyses of various new physics models

The measurements of the ratios $R_{K^{(*)}}$ along with $R_{D^{(*)}}$ hint towards lepton flavor non universality which is in disagreement with the standard model. In this work, we reanalyze the four new physics models, which are widely studied in the literature as a candidates for the simultaneous explanations of these measurements. These are, standard model like vector boson (VB), $SU(2)_L$-singlet vector leptoquark ($U_1$), $SU(2)_L$-triplet scalar leptoquark ($S_3$) and $SU(2)_L$ triplet vector leptoquark ($U_3$) models. We assume a coupling only to the third generation in the weak basis, so that the $b \to s μ^+ μ^-$ transition is generated only via mixing effects. Preforming a global fit to all relevant data, we show that the vector boson model violates the current upper bound on ${Br}(τ\to 3μ)$ and hence is inconsistent with the present data. Further, we show that within this framework, the $U_1$ leptoquark model cannot simultaneously accommodate $R_{K^{(*)}}$ and $R_{D^{(*)}}$ measurements. We emphasize that this conclusion is independent of the additional constraints coming from renormaliztion group running effects and high-$p_T$ searches. In addition, we show that the $S_3$ and $U_3$ models are highly disfavored by the constraints coming from $b\to s ν\bar ν$ data. Finally, we find a that hypothesis of two LQ particles is also challenged by $b\to s \bar νν$ data.

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arXiv2019

A quantum information theoretic quantity sensitive to the neutrino mass-hierarchy

In this work, we derive a quantum information theoretic quantity similar to the Leggett-Garg inequality, which can be defined in terms of neutrino transition probabilities. For the case of $ν_μ\to ν_e/\barν_μ\to\barν_e$ transitions, this quantity is sensitive to CP violating effects as well as the neutrino mass-hierarchy, namely which neutrino mass eigenstate is heavier than the other ones. The violation of the inequality for this quantity shows an interesting dependence on mass-hierarchy. For normal (inverted) mass-hierarchy, it is significant for $ν_μ\to ν_e$ ($\barν_μ\to \barν_e$) transitions. This is applied to the two ongoing accelerator experiments T$2$K and NO$ν$A as well as the future experiment DUNE.

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arXiv2019

Resolving $R_D$ and $R_{D^*}$ anomalies

The current world averages of the ratios $R_{D^{(*)}}$ are about $4σ$ away from their Standard Model prediction. These measurements indicate towards the violation of lepton flavor universality in $b\rightarrow c\,l\,\barν$ decay. The different new physics operators, which can explain the $R_{D^{(*)}}$ measurements, have been identified previously. We show that a simultaneous measurement of the polarization fractions of $τ$ and $D^*$ and the angular asymmetries $A_{FB}$ and $A_{LT}$ in $B\rightarrow D^*τ\barν$ decay can distinguish all the new physics amplitudes and hence uniquely identify the Lorentz structure of new physics.

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arXiv2019

Leggett-Garg inequality in the context of three flavour neutrino oscillation

The present work is devoted to the characterization of the Leggett-Garg inequality for three-flavoured neutrino oscillations in presence of both matter and Charge-Conjugation and Parity violating (CP) effects. This study complements and completes the recent one put forward in [17] by relaxing the stationary condition. At variance with the latter case, the LGI contains interference terms which cannot be expressed in terms of experimentally measurable quantities, thus drawing a clear-cut distinction between the two scenarios, as well as highlighting the role of the stationary assumption on such systems. We find that the additional terms are small for high energy neutrino beam compared to the maximum value attained by the Leggett-Garg parameter.

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arXiv2018

Quantum correlations and the neutrino mass degeneracy problem

Many facets of nonclassicality are probed in neutrino system in the context of three flavour neutrino oscillations. The analysis is carried out for parameters relevant to two ongoing experiments NO$ν$A and T2K, and also for the upcoming experiment DUNE. The various quantum correlations turn out to be sensitive to the mass-hierarchy problem in neutrinos. This sensitivity is found to be more prominent in DUNE experiment as compared to NO$ν$A and T2K experiments. This can be attributed to the large baseline and high energy of the DUNE experiment. Further, we find that to probe these correlations, the neutrino (antineutrino) beam should be preferred if the sign of mass square difference $Δ_{31}$ turns out to be positive (negative).

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arXiv2018

New physics effects in radiative leptonic $B_s$ decay

There are several anomalous measurements in the decays induced by the quark level transition $b \to s μ^+ μ^-$, which do not agree with the predictions of the Standard Model. These measurements could be considered as signatures of physics beyond the Standard Model. Working within the framework of effective field theory, several groups have performed global fits to all $b \to s μ^+ μ^-$ data to identify the Lorentz structure of possible new physics. Many new physics scenarios have been suggested to explain the anomalies in the $b \to s μ^+ μ^-$ sector. In this work, we investigate the impact of these new physics scenarios on the radiative leptonic decay of $B_s$ meson. We consider the branching ratio of this decay along with the ratio $R_γ$ of the differential distribution $B_s \to μ^+ μ^- γ$ \& $B_s \to e^+ e^- γ$ and the muon forward-backward asymmetry $A_{FB}$. We find that the predicted values of the branching ratio and $A_{FB}$ are close to the SM results for all allowed new physics solutions whereas some of the solutions allow large deviation in $R_γ$ from its SM prediction.

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