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Shireen Gangal

Publications and source records attributed to Shireen Gangal.

17 recordsLinked to original sources
arXiv2025

The Drell-Yan Process at NNLL$^\prime$+NNLO using Rapidity Dependent Jet Vetoes

We present results for the Drell-Yan process $pp \to Z/γ^* + X \to l^+l^- + X$ in the presence of the rapidity dependent jet vetoes $\mathcal{T}_{Bj}$ and $\mathcal{T}_{Cj}.$ These observables provide a tighter veto at central rapidities than at forward rapidities. Our predictions contain a resummation of large logarithms of the veto scale over the hard scale, matched to fixed order in perturbation theory; we present results at both NLL$^\prime$+NLO and NNLL$^\prime$+NNLO. Uncertainty estimates are provided that contain both resummation and fixed-order uncertainties; we find that using a standard profile scale variation procedure for the former seems to underestimate the uncertainty, particularly at NLL$^\prime$+NLO. Consequently, we modify the procedure to avoid cancellation of scale variations between partonic channels, and verify that the uncertainties from this method are reasonable using a simplified version of the Theory Nuisance Parameter (TNP) method. We then find that the uncertainty decreases going from the NLL$^\prime$+NLO to the NNLL$^\prime$+NNLO predictions, and obtain an uncertainty at the level of $1-3\%$ in the NNLL$^\prime$+NNLO predictions when the veto scale is of $\mathcal{O}(10 \text{ GeV})$.

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arXiv2025

Quantum coherence in neutrino spin-flavor oscillations

Coherence, which represents the superposition of orthogonal states, is a fundamental concept in quantum mechanics and can also be precisely defined within quantum resource theory. Thus exploring quantum coherence in neutrino oscillations can not only help in examining the intrinsic quantum nature but can also explore their potential applications in quantum information technologies. Previous studies on quantum coherence have focused on neutrino flavor oscillations (FO). However, FO imply that neutrinos have mass and this can lead to the generation of a tiny but finite magnetic dipole moment of neutrinos through quantum loop diagrams at higher orders of perturbative expansion of the interaction. This electromagnetic property of neutrinos can induce spin flavor oscillations (SFO) in the presence of an external magnetic field and hence is expected to enrich the study of coherence. In this work, we investigate quantum coherence in neutrino SFO with three flavor mixing within the interstellar as well as intergalactic magnetic fields, quantified by the $l_1$ norm and the relative entropy of coherence, and express these measures in terms of neutrino SFO probabilities. For FO, coherence measures can sustain higher values (say, within 50% of the maximum) over distances of several kilometers, which are relevant for terrestrial experiments like reactor and accelerator neutrinos. However, for SFO, we find that the coherence scale can extend to astrophysical distances, spanning from kiloparsecs to gigaparsecs.

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arXiv2023

Leptonic Operators for Cabbibo Angle Anomaly with SMEFT RG Evolution

The measurements of the Cabibbo--Kobayashi--Maskawa (CKM) elements can be contaminated by new-physics effects. We point out that purely leptonic operators at the high scale can influence semileptonic $K$ decays and nuclear beta decay through renormalization group (RG) running, and hence can influence the measurements of $V_{us}$. Interestingly, through this mechanism, a single six-dimensional effective operator $O_{\ell\ell}$ at the high scale can alleviate the tension due to the Cabibbo angle anomaly, by generating the desired operators at the low scale through RG running. When generated as a result of a $Z'$ model, the non-universal leptonic couplings of this operator can also contribute to the lepton flavor universality violating ratios such as $R_{K^{(*)}}$, which would act as stringent constraints on such scenarios. By performing a global fit of the $Z'$ model, we find that it is essential to have non-universal couplings of such a $Z'$ boson to all three generations of leptons.

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arXiv2022

A global analysis of $b \to s \ell \ell$ data in heavy and light $Z'$ models

We perform a model-independent global fit to all $b \to s \ell \ell$ data in the light of recent measurements of the lepton flavour universality violating (LFUV) observables $R_{K_S^0}$ and $R_{K^{*+}}$ as well as the updated measurements of observables in $B_s \to ϕμ^+ μ^-$ decay, by the LHCb collaboration. We obtain new physics (NP) solutions to the current anomalies in the data, assuming NP in the muon sector only. We find that the 1D NP scenarios $C_9^{\rm NP} <0 $ and $C_{9}^{\rm NP}=-C_{10}^{\rm NP}$ continue to be the most favoured ones. However, the significance of the then favoured scenario $C_{9}^{\rm NP}=-C'_{9}$ has reduced and the updated data now marginally prefers $C_{10}^{\rm NP}$ scenario over $C_{9}^{\rm NP}=-C'_{9}$. The 2D scenarios $(C_9^{\rm NP}, C_{10}^{\prime} )$, $(C_9^{\rm NP}, C_{9}^{\prime} )$ and $(C_9^{\rm NP}, C_{10}^{\rm NP} )$, continue to be favoured by the data in the listed order. We also analyse generic TeV scale $Z'$ models which can generate the favored 1D scenarios, $C_9^{\rm NP} $ and $C_9^{\rm NP} = -C_{10}^{\rm NP}$ along with the 2D NP scenarios $(C_9^{\rm NP}, C_{9}^{\prime} )$ and $(C_9^{\rm NP}, C_{10}^{\rm NP} )$. Using the additional constraints from $B_s -\bar{B_s}$ mixing and neutrino trident data, we find that all four models provide an equally good fit to the data. Further, we consider a model with a 25 MeV $Z'$ that couples to muons and has a $q^2$ dependent $b - s$ coupling. We also study the implications of the current data on the LFUV observable $R_ϕ$, $Q_{4,5}$ along with $R_{K^{(*)}}$ in the high $q^2$. We find that a precise measurements of these observables can provide a good discrimination between a few favored model-independent solutions, and have a potential to disentangle different heavy and light $Z'$ scenarios considered in this work.

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arXiv2022

The Forward-Backward Asymmetry in $B\to D^{*}\ellν$: One more hint for Scalar Leptoquarks?

Experimental data have provided intriguing hints for the violation of lepton flavour universality (LFU), including $B\to D^{(*)}τν/B\to D^{(*)}\ellν$, the anomalous magnetic moment of the muon and $b\!\to\! s\ell^+\ell^-$ with a significance of $\!>3\,σ$, $>\!4\,σ$ and $>\!5\,σ$, respectively. Furthermore, in a recent re-analysis of 2018 Belle data, it was found that the forward-backward asymmetry ($ΔA_{\rm FB}$) of $B \to D^{*}μ\bar ν$ vs $B\to D^{*}e\bar ν$ disagrees with the SM prediction by $\approx\!\!4\,σ$, providing an additional sign of LFU violation. We show that a tensor operator is necessary to significantly improve the agreement with data in $ΔA_{\rm FB}$ while respecting the bounds from other $b\to c\ellν$ observables. Importantly, this tensor operator can only be induced (at tree-level within renormalizable models) by a scalar leptoquark. Furthermore, among the two possible representations, the $SU(2)_L$-singlet $S_1$ and the doublet $S_2$, which can interestingly both also account for the anomalous magnetic moment of the muon, only $S_1$ can provide a good fit. Even though the constraints from (differences of) other angular observables prefer a smaller value of $ΔA_{\rm FB}$ than the current central one, this scenario is significantly preferred (nearly $4 σ$) over the SM hypothesis, and is compatible with constraints such as $B\to K^*νν$ and electroweak precision bounds. Therefore, if the $ΔA_{\rm FB}$ anomaly is confirmed, it would provide circumstantial evidence for scalar leptoquarks and pave the way for a natural connection with all other anomalies pointing towards LFU violation.

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arXiv2021

On the effective lifetime of $B_s \to μμγ$

We consider the $B^0_s \to μ^{+} μ^{-} γ$ effective lifetime, and the related CP-phase sensitive quantity $A_{ΔΓ_s}^{μμγ}$, as a way to obtain qualitatively new insights on the current $B$-decay discrepancies. Through a fit comparing pre- to post-Moriond-2021 data we identify a few theory benchmark scenarios addressing these discrepancies, and featuring large CP violation in addition. We then explore the possibility of telling apart these scenarios with $A_{ΔΓ_s}^{μμγ}$, once resonance-modeling and form-factor uncertainties are taken into account. We do so in both regions of low and high invariant di-lepton mass-squared $q^2$. For low $q^2$, we show how to shape the integration range in order to reduce the impact of the $ϕ$-resonance modelling on the $A_{ΔΓ_s}^{μμγ}$ prediction. For high $q^2$, we find that the corresponding pollution from broad-charmonium resonances has a surprisingly small effect on $A_{ΔΓ_s}^{μμγ}$. This is due to a number of cancellations, that can be traced back to the complete dominance of semi-leptonic operator contributions for high $q^2$ -- at variance with low $q^2$ -- and to $A_{ΔΓ_s}^{μμγ}$ behaving like a ratio-of-amplitudes observable. Our study suggests that $A_{ΔΓ_s}^{μμγ}$ is -- especially at high $q^2$ -- a potentially valuable probe of short-distance CP-violating effects in the very same Wilson coefficients that are associated to current $b \to s$ discrepancies. Its discriminating power, however, relies on progress in form-factor uncertainties. Interestingly, high $q^2$ is the region where $B^0_s \to μ^{+} μ^{-} γ$ is already being accessed experimentally, and the region where form factors are more accessible through non-perturbative QCD methods.

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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

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

Higgs Production at NNLL$'$+NNLO using Rapidity Dependent Jet Vetoes

The rapidity-dependent jet veto observables $\mathcal{T}_{Bj}$ and $\mathcal{T}_{Cj}$ provide a tight jet veto at central rapidity, gradually transitioning to a loose veto at forward rapidities. They divide the phase space into exclusive jet bins in a different way to the traditional jet veto observable $p_{Tj}$, and are advantageous to use under harsh pile-up conditions. We obtain predictions for the $0$-jet gluon-fusion (ggF) Higgs cross section using both of these veto observables at NNLL$'+$NNLO, and compare these predictions to the prior state-of-the-art of NLL$'+$NLO. A significant reduction in perturbative uncertainty is observed going from NLL$'+$NLO to NNLL$'+$NNLO, with the NNLL$'+$NNLO predictions lying inside the uncertainty band of the NLL$'+$NLO predictions. We also investigate the relative sensitivities of ggF Higgs cross sections with $\mathcal{T}_{Bj}$, $\mathcal{T}_{Cj}$ and $p_{Tj}$ jet vetoes to underlying event and hadronisation effects using an NLO+parton shower calculation. We find that the cross sections with $\mathcal{T}_{Bj}$ and $\mathcal{T}_{Cj}$ vetoes have a reduced sensitivity to underlying event and hadronisation effects compared to that with a $p_{Tj}$ veto.

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arXiv2020

Two-Loop Beam and Soft Functions for Rapidity-Dependent Jet Vetoes

Jet vetoes play an important role in many analyses at the LHC. Traditionally, jet vetoes have been imposed using a restriction on the transverse momentum $p_{Tj}$ of jets. Alternatively, one can also consider jet observables for which $p_{Tj}$ is weighted by a smooth function of the jet rapidity $y_j$ that vanishes as $|y_j| \to \infty$. Such observables are useful as they provide a natural way to impose a tight veto on central jets but a looser one at forward rapidities. We consider two such rapidity-dependent jet veto observables, $\mathcal{T}_{Bj}$ and $\mathcal{T}_{Cj}$, and compute the required beam and dijet soft functions for the jet-vetoed color-singlet production cross section at two loops. At this order, clustering effects from the jet algorithm become important. The dominant contributions are computed fully analytically while corrections that are subleading in the limit of small jet radii are expressed in terms of finite numerical integrals. Our results enable the full NNLL' resummation and are an important step towards N3LL resummation for cross sections with a $\mathcal{T}_{Bj}$ or $\mathcal{T}_{Cj}$ jet veto.

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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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arXiv2017

Probing new physics through $B^*_s \rightarrow μ^+ μ^-$ decay

We perform a model independent analysis of new physics in $B^*_s \rightarrow μ^+ μ^-$ decay. We intend to identify new physics operator(s) which can provide large enhancement in the branching ratio of $B^*_s \rightarrow μ^+ μ^-$ above its standard model prediction. For this, we consider new physics in the form of vector, axial-vector, scalar and pseudoscalar operators. We find that scalar and pseudoscalar operators do not contribute to the branching ratio of $B^*_s \rightarrow μ^+ μ^-$. We perform a global fit to all relevant $b \to s μ^+ μ^-$ data for different new physics scenarios. For each of these scenarios, we predict $Br(B^*_s \rightarrow μ^+ μ^-)$. We find that a significant enhancement in $Br(B^*_s \rightarrow μ^+ μ^-)$ is not allowed by any of these new physics operators. In fact, for all new physics scenarios providing a good fit to the data, the branching ratio of $B^*_s \rightarrow μ^+ μ^-$ is suppressed as compared to the SM value. Hence the present $b \to s μ^+ μ^-$ data indicates that the future measurements of $Br(B^*_s \rightarrow μ^+ μ^-)$ is expected to be suppressed in comparison to the standard model prediction.

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arXiv2014

Rapidity-Dependent Jet Vetoes

Jet vetoes are a prominent part of the signal selection in various analyses at the LHC. We discuss jet vetoes for which the transverse momentum of a jet is weighted by a smooth function of the jet rapidity. With a suitable choice of the rapidity-weighting function, such jet-veto variables can be factorized and resummed allowing for precise theory predictions. They thus provide a complementary way to divide phase space into exclusive jet bins. In particular, they provide a natural and theoretically clean way to implement a tight veto on central jets with the veto constraint getting looser for jets at increasingly forward rapidities. We mainly focus our discussion on the 0-jet case in color-singlet processes, using Higgs production through gluon fusion as a concrete example. For one of our jet-veto variables we compare the resummed theory prediction at NLL'+NLO with the recent differential cross section measurement by the ATLAS experiment in the $H\toγγ$ channel, finding good agreement. We also propose that these jet-veto variables can be measured and tested against theory predictions in other SM processes, such as Drell-Yan, diphoton, and weak diboson production.

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arXiv2013

NLO Uncertainties in Higgs + 2 jets from Gluon Fusion

A central ingredient in establishing the properties of the newly discovered Higgs-like boson is to isolate its production via vector boson fusion (VBF). With the typical experimental selection cuts, the VBF sample is contaminated by a 25% fraction from Higgs + 2 jet production via gluon fusion (ggF) which has large perturbative uncertainties. We perform a detailed study of the perturbative uncertainties in the NLO predictions for pp -> H+2 jets via ggF used by the ATLAS and CMS collaborations, with the VBF selection cuts of their current H -> gamma gamma analyses. We discuss in detail the application of the so-called "ST method" for estimating fixed-order perturbative uncertainties in this case, and also consider generalizations of it. Qualitatively, our results apply equally to other decay channels with similar VBF selection cuts. Typical VBF selections include indirect restrictions or explicit vetoes on additional jet activity, primarily to reduce non-Higgs backgrounds. We find that such restrictions have to be chosen carefully and are not necessarily beneficial for the purpose of distinguishing between the VBF and ggF production modes, since a modest reduction in the relative ggF contamination can be easily overwhelmed by its quickly increasing perturbative uncertainties.

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arXiv2012

b -> s Decays in a model with Z-mediated flavor changing neutral current

In the scenario with Z mediated flavor changing neutral current occurring at the tree level due to the addition of a vector-like isosinglet down-type quark d' to the SM particle spectrum, we perform a chi square fit using the flavor physics data and obtain the best fit value along with errors of the tree level Zbs coupling, U_{sb}. The fit indicates that the new physics coupling is constrained to be small: we obtain |U_{sb}| <= 3.40 X 10^{-4} at 3 sigma. Still this does allow for the possibility of new physics signals in some of the observables such as semileptonic CP asymmetry in Bs decays.

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