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Saurabh D. Rindani

Publications and source records attributed to Saurabh D. Rindani.

At least 19 recordsLinked to original sources
arXiv2025

T-odd observables from anomalous $tbW$ couplings in single-top W associated production at the LHC

We investigate the possibility that an imaginary anomalous $tbW$ coupling can be measured in the process $pp \to tW^-X$ by means of T-odd observables. One such observable is the polarization of the top quark transverse to the production plane. Another is the asymmetry between the cross sections with a positive and with a negative value of a certain T-odd correlation constructed out of observable momenta when the top quark decays leptonically. The mean value of this correlation, which is also T-odd, is the third observable. These three T-odd observables are shown to be proportional to the imaginary part of only one of the $tbW$ anomalous couplings, the other couplings giving vanishing contributions. This imaginary part could signal either a CP-odd coupling, or an absorptive part in the effective coupling. We estimate the 1-$σ$ limits that might be derived in the case of each of these observables for centre-of-mass energies 7, 8, 13 and 14 TeV at the LHC.

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arXiv2024

T-odd observables from anomalous $tbW$ couplings in single-top production at an $ep$ collider

We investigate the possibility that an imaginary anomalous $tbW$ coupling can be measured in the process $e^-p \to ν_e \overline t X$ by means of T-odd observables. One such observable considered here is the polarization of the top antiquark transverse to the production plane. The other is a T-odd correlation constructed out of observable momenta when the top quark decays leptonically. Both these T-odd observables are shown to be proportional to the imaginary part of only one of the $tbW$ anomalous couplings, the other couplings giving either vanishing or negligible contribution. This imaginary part could signal either a CP-odd coupling, or an absorptive part in the effective coupling, or both. We estimate the 1-$σ$ limits that might be derived in the case of each of these observables for a collider with a proton energy of 7 TeV and an electron energy of 60 GeV and also in the case of a higher electron energy of 150 GeV.

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arXiv2024

Determining the structure of $Hqt$ couplings at the LHC

Experiments at the Large Hadron Collider (LHC) have found that the Higgs boson discovered in 2012 has properties largely consistent with predictions from the standard model (SM). However, the determination of the couplings of the Higgs boson to various other SM particles is not very precise, and it is possible that future experiments with enhanced accuracy may be able to detect deviations from the SM if any. One category of couplings studied at the LHC is Higgs flavour-changing couplings to a top-quark and a light-quark ($u$ or $c$) combination, which vanishes at tree level in the SM, but could be induced by interactions beyond the SM. Experiments have put a limit on the magnitude of these couplings, only one coupling assumed nonzero at a time. We investigate here to what extent the relative magnitudes of the $Hut$ and $Hct$ couplings and their chirality structure can be determined by means of more detailed kinematics at a future higher-luminosity version of the LHC in the process of $tH$ production. In particular, we find that the top-quark polar-angle distribution in $tH$ production could reveal the relative magnitudes of the $Hut$ and $Hct$ couplings, while the azimuthal distribution of the charged leptons arising from top decay can be used for a determination of the chiral structure of the $Hut$ coupling.

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arXiv2023

$Z$ polarization at an $e^+e^-$ collider and properties of decay-lepton angular asymmetries

$Z$ production at an $e^+e^-$ collider, associated with production of other particles, can be an accurate source of information of details of electroweak interactions, including possible interactions beyond the standard model. We discuss from a general physical point of view the properties of the density matrix as well as lepton angular asymmetries. While most considerations will be applicable to processes of $Z$ production associated with any other particle or particles, for some discussions, we specialize to a $HZ$ final state. While many of the results can be found in earlier literature, especially for the process $e^+e^- \to HZ$, we give details of the reasoning, which are not always found. We discuss the properties of the spin density matrix under C, P and T transformations, and combinations thereof, as also the predictions of these for the corresponding leptonic asymmetries. The specific transformations P, CP, T and CPT are of special importance and we discuss the consequences of these symmetries or their absence for the leptonic asymmetries. A specific issue which has been given attention to is the role of beam polarization, and how one can infer on general grounds which asymmetries get enhanced by the use of beam polarization. Similarly, we also discuss which asymmetries would be sensitive to the measurement of tau polarization in $Z$ decay in to $τ^+τ^-$.

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arXiv2023

Polarized $Z$ cross sections in Higgsstrahlung for the determination of anomalous $ZZH$ couplings

The production of a Higgs boson in association with a $Z$ at an electron-positron collider is one of the cleanest methods for the measurement of the couplings of the Higgs boson. In view of the large production cross section at energies a little above the threshold, it seems feasible to make a more detailed study of the process by measuring the cross sections for polarized $Z$ in order to measure possible anomalous $ZZH$ couplings. We show that certain combinations of cross sections in $e^+e^- \to ZH$ with different $Z$ polarizations help to enhance or isolate the effect of one of the two kinds of anomalous $ZZH$ couplings possible on general grounds of CP and Lorentz invariance. These combinations can be useful to get information on the $ZZH$ coupling in the specific contexts of an effective field theory, two-Higgs-doublet models, and composite Higgs models, in a relatively model-independent fashion. We find in particular that the longitudinal helicity fraction of the $Z$ is expected to be insensitive to anomalous couplings, and would be close to its value in the standard model in the scenarios we consider. We also discuss the sensitivity of the proposed measurements to the anomalous couplings, including longitudinal beam polarizations, which suppress backgrounds, and can improve the sensitivity if appropriately chosen.

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arXiv2022

Polarized $Z$ cross sections in Higgsstrahlung for the determination of anomalous $ZZH$ couplings

The production of a Higgs boson in association with a $Z$ at an electron-positron collider is one of the cleanest methods for the measurement of the couplings of the Higgs boson. In view of the large production cross section at energies a little above the threshold, it seems feasible to make a more detailed study of the process by measuring the cross sections for polarized $Z$ in order to measure possible anomalous $ZZH$ couplings. We show that certain combinations of cross sections in $e^+e^- \to ZH$ with different $Z$ polarizations help to enhance or isolate the effect of one of the two kinds of $ZZH$ couplings possible on general grounds of CP and Lorentz invariance. These combinations can be useful to get information on the $ZZH$ coupling in the specific contexts of an effective field theory, two-Higgs-doublet models, and composite Higgs models, in a relatively model-independent fashion. We find in particular that the longitudinal helicity fraction of the $Z$ is expected to be insensitive to anomalous couplings, and would be close to its value in the standard model in the scenarios we consider. We also discuss the sensitivity of the proposed measurements to the anomalous couplings.

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arXiv2022

Use of $Z$ polarization in $e^+e^- \to ZH$ to measure the triple-Higgs coupling

It is shown that a certain angular asymmetry of charged leptons produced in the decay of $Z$ in the process $e^+e^- \to ZH$, related to a tensor polarization component of the $Z$, can be used to constrain the anomalous triple-Higgs coupling, independent of the other anomalous couplings like the $ZZH$ coupling which dominate at tree level. This is because the angular asymmetry is odd under na\" ive time reversal, and hence dependent on loop-level contributions. At a future $e^+e^-$ collider like the International Linear Collider (ILC), for example, a limit of about $3.4$ might be placed on the ratio of the actual triple Higgs coupling to that predicted in the standard model for a centre-of-mass energy of 500 GeV and an integrated luminosity of 30 fb$^{-1}$ with electron and positron beams having longitudinal polarization of $-80\%$ and $+30\%$, respectively.

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arXiv2021

Indirect measurement of triple-Higgs coupling at an electron-positron collider with polarized beams

We examine the possibility of using single-Higgs production at an $e^+e^-$ collider with polarized beams to measure, or constrain, indirectly a possible anomalous triple-Higgs coupling, which can contribute to the process via one-loop diagrams. In the dominant process $e^+e^- \to ZH$, longitudinally polarized beams can lead to an improvement in the cross section by 50\% for $e^-$ and $e^+$ polarizations of $-0.8$ and $+0.3$, respectively. This corresponds to an improvement in the sensitivity to the triple-Higgs coupling of about 18\% for a centre-of-mass energy of 250 GeV and an integrated luminosity of 2 ab$^{-1}$, making a strong case of beam polarization. This also implies that with polarized beams, the luminosity needed to get a particular sensitivity is less by about 33\% as compared to that needed with unpolarized beams. Even when only the $e^-$ beam is polarized $-0.8$, the improvement in the sensitivity is about 8\%. We also study the effect of longitudinal beam polarization on the sensitivity to the triple-Higgs coupling of Higgs production through the subdominant process $e^+e^- \to H ν\bar ν$ occurring through $WW$ fusion.

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arXiv2021

Study of anomalous gauge-Higgs couplings using $Z$ boson polarization at LHC

We estimate model-independent bounds that could be obtained on the anomalous $ZZH$ vertex using polarization parameters of the $Z$ boson produced in the Higgstrahlung process at the LHC. We calculate the eight independent polarization parameters from the spin density matrix elements of the $Z$, which can probe underlying new physics contributions to $ZH$ production. By using the approach that connects these polarization observables to the coefficients in the angular distribution of the decay products of the $Z$, we estimate the limits on the anomalous $ZZH$ coupling that can be obtained at the 14 TeV LHC.

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arXiv2019

Probing anomalous gauge-Higgs couplings using $Z$ boson polarization at $e^+ e^-$ colliders

We study possible new physics interactions in the $ZZH$ vertex contributing to the Higgsstrahlung process $e^+ e^- \to ZH$ at proposed future $e^+e^-$ colliders using the polarization of the $Z$ as a probe. We calculate the spin density matrix of the $Z$ for the process and determine the eight independent polarization parameters of the $Z$ boson which have the potential to constrain the anomalous couplings. We study angular asymmetries using the decay leptons from the $Z$ boson which are simply related to the polarization observables. We also estimate the limits that can be placed on the anomalous couplings using measurements of these angular asymmetries at centre of mass energies of 250 GeV and 500 GeV and various combinations of polarized $e^+$ and $e^-$ beams.

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arXiv2018

Probing CP nature of a mediator in associated production of dark matter with single top quark

We consider associated production of dark matter with single top quark, in a simplified dark matter model with spin-0 mediators. The produced top quark is polarized and the polarization depends on the CP of the mediator. We calculate both the cross-section and top polarization for these processes. We compute angular asymmetries which demonstrate the difference between the polarization expected for the scalar or pseudoscalar mediator. Both the cross section and top polarization are sensitive to the CP property of the mediator, depending on the mediator mass. We find that these polarization asymmetries add value to the determination of the CP property of the mediator particularly in the case of a state with indeterminate CP.

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arXiv2018

Inclusive spin-momentum analysis and new physics at a polarized electron-positron collider

We consider the momentum distribution and the polarization of an inclusive heavy fermion in a process assumed to arise from standard-model (SM) $s$-channel exchange of a virtual $γ$ or $Z$ with a further contribution from physics beyond the standard model involving $s$-channel exchanges. The interference of the new physics amplitude with the SM $γ$ or $Z$ exchange amplitude is expressed entirely in terms of the space-time signature of such new physics. Transverse as well as longitudinal polarizations of the electron and positron beams are taken into account. Similarly, we consider the cases of the polarization of the observed final-state fermion along longitudinal and two transverse spin-quantization axes which are required for a full reconstruction of the spin dependence of the process. We show how these model-independent distributions can be used to deduce some general properties of the nature of the interaction and some of their properties in prior work which made use of spin-momentum correlations.

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arXiv2016

Effective fermion-Higgs interactions at an $e^+e^-$ collider with polarized beams

We consider the possibility of new physics giving rise to effective interactions of the form $e^+e^-Hf \bar f$, where $f$ represents a charged lepton $\ell$ or a (light) quark $q$, and $H$ the recently discovered Higgs boson. Such vertices would give contributions beyond the standard model to the Higgs production processes $e^+e^- \to H\ell^+\ell^-$ and $e^+e^- \to H q \bar q$ at a future $e^+e^-$ collider. We write the most general form for these vertices allowed by Lorentz symmetry. Assuming that such interactions contribute in addition to the standard model production processes, where the final-state fermion pair comes from the decay of the $Z$ boson, we obtain the differential cross section for the processes $e^+e^- \to H\ell^+\ell^-$ and $e^+e^- \to Hq\bar q$ to linear order in the effective interactions. We propose several observables with differing CP and T properties which, if measured, can be used to constrain the couplings occurring in interaction vertices. We derive possible limits on these couplings that may be obtained at a collider with centre-of-mass energy of 500 GeV and an integrated luminosity of 500 fb$^{-1}$. We also carry out the analysis assuming that both the electron and positron beams can be longitudinally polarized, and find that the sensitivity to the couplings can be improved by a factor of 2-4 by a specific choice of the signs of the polarizations of both the electron and positron beams for the same integrated luminosity.

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arXiv2016

Unraveling the CP phase of top-Higgs coupling in associated production at the LHC

We study the sensitivity of top polarization observables to the CP phase $ζ_t$ in the top Yukawa coupling in the process $pp\to thj$ at the 14 TeV high-luminosity run of the Large Hadron Collider (HL-LHC). We calculate the top polarization in this process as well as an azimuthal asymmetry of the charged lepton arising from the decay of the top in the lab frame. We find that the dependence of this lab-frame azimuthal asymmetry on the phase $ζ_t$ closely resembles the dependence of the top polarization on $ζ_t$. As compared to the cross section, which is sensitive to $ζ_t$ for larger values, the lepton azimuthal asymmetry can provide a sensitive measurement of $ζ_t$ for smaller values.

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arXiv2015

Unravelling the non-standard top and Higgs couplings in associated top-Higgs production at the High-luminosity LHC

We study the sensitivities of non-standard top and Higgs couplings in the $pp\to thj$ process at the 14 TeV high-luminosity run of Large Hadron Collider (HL-LHC). We calculate top polarization and construct various lab-frame observables to study their sensitivities on anomalous couplings at HL-LHC. Both anomalous Wtb and hVV couplings contribute in the production process. However, only the former contributes in top decay and only the latter takes part in Higgs decay. We study this interesting interplay of couplings in the production and decay observables. In production, these couplings significantly affect the top polarization. As a measure of top polarization, we look at decay-lepton angular distributions in the laboratory frame and study the effect of anomalous couplings on these distributions. We construct certain asymmetries to study the sensitivity of these distributions to top quark couplings. In particular, real and imaginary parts of an hVV coupling and real part of a Wtb coupling f2R significantly alter the top polarization and, in turn, affect the decay-lepton distributions in laboratory frame.

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arXiv2015

Polarization of the top quark as a probe of its chromomagnetic and chromoelectric couplings in single-top production at the Large Hadron Collider

We study the sensitivity of the Large Hadron Collider (LHC) to top quark chromomagnetic (CMDM) and chromoelectric (CEDM) dipole moments and $Wtb$ effective couplings in single-top production in association with a $W^-$ boson, followed by semileptonic decay of the top. We calculate the top polarization and the effects of these anomalous couplings on it at two centre-of-mass (cm) energies, 8 TeV and 14 TeV. As a measure of top polarization, we look at decay-lepton angular distributions in the laboratory frame, without requiring reconstruction of the rest frame of the top, and study the effect of the anomalous couplings on these distributions. We construct certain asymmetries to study the sensitivity of these distributions to top-quark couplings. The Wt single-top production mode helps to isolate the anomalous $ttg$ and $Wtb$ couplings, in contrast to top-pair production and other single-top production modes, where other new-physics effects can also contribute. We determine individual limits on the dominant couplings, viz., the real part of the CMDM $Reρ_2$, the imaginary part of the CEDM $Imρ_3$, and the real part of the tensor Wtb coupling $Ref_{2r}$, which may be obtained by utilizing these asymmetries at the LHC. We also obtain simultaneous limits on pairs of these couplings taking two couplings to be non-zero at a time.

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arXiv2015

Polarization of top quark as a probe of its chromomagnetic and chromoelectric couplings in $tW$ production at the Large Hadron Collider

We study the sensitivity of the Large Hadron Collider (LHC) to top quark chromomagnetic (CMDM) and chromoelectric (CEDM) dipole moments and $Wtb$ effective couplings in single-top production in association with a $W^-$ boson, followed by semileptonic decay of the top. The $Wt$ single-top production mode helps to isolate the anomalous $ttg$ and $Wtb$ couplings, in contrast to top-pair production and other single-top production modes, where other new-physics effects can also contribute. We calculate the top polarization and the effects of these anomalous couplings on it at two centre-of-mass (cm) energies, 8 TeV and 14 TeV. As a measure of top polarization, we look at decay-lepton angular distributions in the laboratory frame, without requiring reconstruction of the rest frame of the top, and study the effect of the anomalous couplings on these distributions. We construct certain asymmetries to study the sensitivity of these distributions to top-quark couplings. We determine individual limits on the dominant couplings, viz., the real part of the CMDM $\mathrm{Re}ρ_2$, the imaginary part of the CEDM $\mathrm{Im}ρ_3$, and the real part of the tensor $Wtb$ coupling $\mathrm{Ref_{2R}}$, which may be obtained by utilizing these asymmetries at the LHC. We also obtain simultaneous limits on pairs of these couplings taking two couplings to be non-zero at a time.

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