Lepton Polarization Research Articles

Azimuthal distributions in radiative decay of a polarized τ lepton

Various distributions over the angles of the emitted photon, especially over the azimuthal angle, in the one-meson radiative decay of the polarized $\tau$ lepton, $\tau^-\to\pi^-\gamma\nu_{\tau}$, have been investigated. In connection with this, the photon phase space is discussed in more detail since in the case of the polarized $\tau$ lepton it is not trivial. The decay matrix element contains both the inner bremsstrahlung and the resonance (structural) contributions. The azimuthal dependence of some observables have been calculated. They are the asymmetry of the differential decay width caused by the $\tau$ lepton polarization, the Stokes parameters of the emitted photon itself and the correlation parameters describing the influence of $\tau$-lepton polarization on the photon Stokes parameters. The numerical estimation was done in the $\tau$ lepton rest frame for arbitrary direction of the $\tau$ lepton polarization 3-vector. The vector and axial-vector form factors describing the structure-dependent part of the decay amplitude are determined using the chiral effective theory with resonances (R$\chi$T). It was found that the features of the azimuthal distributions allows to separate various terms in the spin-dependent contribution. The so-called up-down and right-left asymmetries are also calculated.

Analysis of forward–backward and lepton polarization asymmetries inB → K1ℓ+ℓ−decays in the two-Higgs-doublet model

The exclusive semileptonic $B\to K_{1}(1270) \ell^{+}\ell^{-}$ ($\ell=\mu , \tau$) decays are analyzed in variants of two Higgs double models (THDMs). The mass eigenstates $K_{1}(1270)$ and $K_{1}(1400)$ are the mixture of two axial-vector SU(3) $^{1}{P}_{1}$ and $^{3}{P}_{1}$ states with the mixing angle $\theta_{K}$. Making use of the form factors calculated in the Light Cone QCD approach and by taking the mixing angle $\theta_{K}=-34^{\circ}$, the impact of the parameters of the THDMs on different asymmetries in above mentioned semileptonic $B$ meson decays are studied. In this context the forward-backward asymmetry and different lepton polarization asymmetries have been analyzed. We have found comprehensive effects of the parameters of the THDMs on the above mentioned asymmetries. Therefore, the precise measurements of these asymmetries at the LHC and different $B$ factories, for the above mentioned processes, can serve as a good tool to put some indirect constraints on the parametric space of the different versions of THDM.

Polarized lepton-nucleon elastic scattering and a search for a light scalar boson

Lepton-nucleon elastic scattering, using the one-photon and one-scalar-boson exchange mechanisms considering all possible polarizations, is used to study searches for a new scalar boson and suggest new measurements of the nucleon form factors. A new light scalar boson, which feebly couples to leptons and nucleons, may account for the proton radius and muon $g-2$ puzzles. We show that the scalar boson produces relatively large effects in certain kinematic region when using sufficient control of lepton and nucleon spin polarization. We generalize current techniques to measure the ratio $G_E/G_M$ and present a new method to separately measure $G_M^2$ and $G_E^2$ using polarized incoming and outgoing muons.

Lepton polarization asymmetries for FCNC decays of the Λb baryon

Branching ratios, lepton forward–backward asymmetries, and lepton polarization asymmetries for the flavor-changing neutral current (FCNC) dileptonic decays of the [Formula: see text] baryon to the ground state and a number of excited state [Formula: see text] baryons are calculated using form factors extracted using wave functions from a constituent quark model. The SM branching ratios for the transition to the ground state calculated using these quark model form factors are consistent with the recent measurement reported by the LHCb collaboration. It is shown that the lepton polarization asymmetries are largely insensitive to the transition form factors and, therefore, may also be insensitive to the effects of QCD in the nonperturbative regime. These observables could therefore provide somewhat model independent ways of extracting various combinations of the Wilson coefficients.

Polarization effects in radiative decay of a polarized τ lepton

The polarization effects in the one-meson radiative decay of the polarized $\tau$ lepton, $\tau^-\to\pi^-\gamma\nu_{\tau}$, are investigated. The inner bremsstrahlung and structural amplitudes are taken into account. The asymmetry of the differential decay width caused by the $\tau$ lepton polarization and the Stokes parameters of the emitted photon itself, are calculated depending on the polarization of the decaying $\tau$ lepton. The numerical estimation of these physical quantities was done for arbitrary direction of the $\tau$ lepton polarization 3-vector in the rest frame. The vector and axial-vector form factors describing the structure-dependent part of the decay amplitude are determined using the chiral effective theory with resonances (R$\chi$T).

Lepton polarization asymmetries of H -> + - decay in the Standard Model

Recently, CMS and ATLAS collaborations at LHC announced a Higgs like particle with mass near $125$GeV. Regarding this, to explore its intrinsic properties, different observables are needed to be measured precisely at the LHC for various decay channels of the Higgs. In this context, we calculate the final state lepton polarization asymmetries, namely, single lepton polarization asymmetries ($P_i$) and double lepton polarization asymmetries ($P_{ij}$) in the SM for radiative semileptonic Higgs decay $H\to\gamma\tau^+\tau^-$. In the phenomenological analysis of these lepton polarization asymmetries both tree and loop level diagrams are considered and it is found that these diagrams give important contributions in the evaluation of said asymmetries. Interestingly, it is found that in $P_{ij}$ the tree level diagrams contribute separately, which however, are missing in the calculations of $P_i$ and the lepton forward-backward asymmetries ($A_{FB}$). Similar to the other observables such as the decay rate and the lepton forward-backward asymmetries, the $\tau$-lepton polarization asymmetries would be interesting observables. The experimental study of these observables will provide a fertile ground to explore the intrinsic properties of the SM Higgs boson and its dynamics as well as help us to extract the signatures of the possible new physics beyond the SM.

The study of rare $ {B_c}\to D_{s,d}^{{\left( * \right)}}l\overline{l} $ decays

In this paper, we study rare decays $B_c\rightarrow D^{(*)}_{s,d}l\bar{l}$ within the Standard Model. The penguin, box, annihilation, color-favored cascade and color-suppressed cascade contributions are included. Based on our calculation, the annihilation and color-favored cascade diagrams play important roles in the differential branching fractions, forward-backward asymmetries, longitudinal polarizations of the final vector mesons and leptonic longitudinal polarization asymmetries. More importantly, color-favored cascade decays largely enhance the resonance cascade contributions. To avoid the resonance cascade contribution pollution, new cutting regions are put forward.

Implication of nonuniversalZ′boson to the lepton polarization asymmetries in theB→K*ℓ+ℓ−decay

In this work we study the implication of the nonuniversal ${Z}^{\ensuremath{'}}$ model to the double lepton polarization asymmetries in the $B\ensuremath{\rightarrow}{K}^{*}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$, $(l=\ensuremath{\mu},\ensuremath{\tau})$ decay. To see the variation in the values of lepton polarization asymmetries from the standard model values, we have taken bounds of the UTfit collaboration, namely, ${S}_{1}$ and ${S}_{2}$ on the values of different parameters of the universal ${Z}^{\ensuremath{'}}$ model such as left-right couplings of the extra gauge boson with leptons and the new weak phase ${\ensuremath{\phi}}_{sb}$. It is found that double lepton polarization asymmetries are sensitive to the coupling parameters of ${Z}^{\ensuremath{'}}$ boson with fermions. Therefore, the measurements of these lepton polarization asymmetries for the above-mentioned decay at current colliders can be helpful to clarify the status of the existence of the extra gauge boson, i.e., ${Z}^{\ensuremath{'}}$.

Application of TauSpinner for Studies on $\tau $-Lepton Polarization and Spin Correlations in $Z$, $W$ and $H$ Decays at the LHC

The tau-lepton plays an important role in the physics program at LHC. Its spin can be used for separation of signal from background or in measuring properties of New Particles decaying to tau leptons. The TauSpinner package represents a tool to modify tau spin effects in any sample containing tau leptons. Generated events, featuring taus produced from intermediate state W, Z, H bosons can be used as an input. The information on the polarization and spin correlations is reconstructed from the kinematics of the tau lepton(s) (nutau in case of W-mediated processes) and tau decay products. By weights, attributed on the event-by-event basis, it enables numerical evaluation and/or modification of the spin effects. We review distributions to monitor spin effects in leptonic and hadronic tau decays with up to three pions, to provide benchmarks for validation of spin content of the event sample and to visualize the tau lepton spin polarization and correlation effects. The demonstration examples for use of TauSpinner libraries, are documented. New validation methods of such an approach are provided. Other topics, like TauSpinner systematic errors or sensitivity of experimental distributions to spin, are addressed in part only. This approach is of interest for implementation of spin effects in embedded tau lepton samples, where Z to mu mu events from data of muons are replaced by simulated tau leptons. Embedding is used at LHC for estimating Z to tau tau background to H to tau tau signatures.

Impact ofZ′and universal extra dimension parameters on different asymmetries inBs→ϕℓ+ℓ−decays

A comprehensive study of the impact of new-physics on different observables for $B_{s}\to \phi \ell^{+}\ell^{-}$ has been done. We examine the new physics models such as $Z^{\prime}$ and UED models, where the effects of new physics coming through the modification of Wilson coefficients. We have analyzed these effects through the theoretical prediction of the branching ratio, the forward-backward asymmetry, lepton polarization asymmetries and the helicity fractions of the final state meson. These all observables will definitely be measured in present and future colliders with great precision. We also pointed out that hadronic uncertainties in various physical observables are small which make them an ideal probe to establish new physics. Therefore, the measurements of these observables for the same decay would permit the detection of physics beyond the Standard Model and will also help us to distinguish between different new physics scenarios.

Lepton polarization asymmetries in B → K 1ℓ+ℓ− decay as a searching tool for new physics

We study the single and double lepton polarization asymmetries in B → K 1l+l− (l = μ, τ ) decay by using most general non-standard local four-fermi interactions, where K 1 is the mixture of axial vector states K 1A and K 1B , which are the 3 P 1 and 1 P 1 states respectively. This study shows that both single and double lepton polarization asymmetries are influenced by new physics couplings. We have also seen the dependence of these asymmetries on mixing angle θ K between K 1A and K 1B flavor states. In addition to these, correlations between the lepton polarization asymmetries and branching ratio are also calculated. We have found that the observation of the above mentioned asymmetries at present and future colliders such as LHC and Super B factories will assist to dig out the status of physics beyond the SM.

Polarized, unpolarized directCPviolation, and single lepton polarization asymmetries ofB→K*ℓ+ℓ−decay inZ′model

In this work we study the implication of the fourth generation standard model (SM4) to the single and double lepton polarization asymmetries in the $B\rightarrow K^{\ast}\ell^{+}\ell^{-}$, $(l=\mu,\tau)$ decay. To see the variation in the values of lepton polarization asymmetries from the standard model values, we have taken lower bound values of CMS and ATLAS for $m_{t^\prime}=404$GeV and 552GeV respectively whereas $\phi_{sb}$ is vary in two allowed regions $75^\circ-115^\circ$ and $240^\circ-290^\circ$ while for the numerical values of these asymmetries $|\lambda_{t^\prime}|$ is set to be $6\times10^{-3}$. It is found that both single and double lepton polarization asymmetries are sensitive to the SM4 parameters. Therefore, the measurements of the lepton polarization asymmetries for the above mentioned decay at current colliders can be helpful to clear the smog on the status of the existence of the fourth generation.

Lepton polarization effects in [formula omitted] decay in family non-universal [formula omitted] model

Possible manifestation of the family non-universal Z′ boson effects in lepton polarization in rare, exclusive baryonic Λb→Λℓ+ℓ− decay is examined. It is observed that the double lepton polarizations PNN, PTT and PTN are sensitive to the Z′ contribution. Moreover, it is found that the values of the polarized forward–backward asymmetry AFBLL are different in Standard Model (SM) and family non-universal Z′ model in different regions of q2, and therefore can serve as an efficient tool for establishing new physics beyond the SM.

COMPLETE STUDY OF THE SEMILEPTONIC B TO $K_2^*(1430)$ DECAYS IN THE STANDARD MODEL WITH FOURTH GENERATION

The [Formula: see text] decays are analyzed in the standard model (SM) extended to fourth generation of quarks (SM4). The decay rate, forward–backward asymmetry, lepton polarization asymmetries, the helicity fractions of the final state [Formula: see text] meson and CP asymmetry are discussed using the form factors calculated in the light-cone sum rules (LCSR) approach. We utilized the constraints on different fourth generation parameters obtained from the experimental information on K, B and D decays and from their recent searches at the ATLAS and CMS experiments to explore their impact on the [Formula: see text] decays. We find that the values of above mentioned physical observables deviate significantly from their minimal SM predications. In the present study, we will also perform the analysis of CP asymmetry in [Formula: see text] decays within SM4, which is one of the most interesting observable in the study of SM4.

Inclusive deep inelastic scattering at high Q 2 with longitudinally polarised lepton beams at HERA

ABSTRACT Inclusive e ± p single and double differential cross sections for neutral and charged current deep inelastic scattering processes are measured with the H1 detector at HERA. The data were taken at a centre-of-mass energy of $ \sqrt {s} = {319} $ GeV with a total integrated luminosity of 333.7 pb−1 shared between two lepton beam charges and two longitudinal lepton polarisation modes. The differential cross sections are measured in the range of negative four-momentum transfer squared, Q 2, between 60 and 50 000 GeV2, and Bjorken x between 0.0008 and 0.65. The measurements are combined with earlier published unpolarised H1 data to improve statistical precision and used to determine the structure function $ xF_{3}^{{\gamma Z}} $ . Ameasurementoftheneutralcurrentparityviolating structure function $ F_{2}^{{\gamma Z}} $ is presented for the first time. The polarisation dependence of the charged current total cross section is also measured. The new measurements are well described by a next-to-leading order QCD fit based on all published H1 inclusive cross section data which are used to extract the parton distribution functions of the proton.

B s → ϕl + l − decays in the topcolor-assisted technicolor model

Using the updated form factors within the light-cone QCD sum rule approach, we calculate the new physics contributions to rare semileptonic $ \overline B {\text{s}} $ → ϕμ + μ − , ϕτ + τ − decays from the new particles appearing in the topcolor-assisted technicolor (TC2) model. In our evaluations, we find that: (i) the branching ratio, normalized forward-backward asymmetry and lepton polarization asymmetries show highly sensitivity to charged toppions contributions and little sensitivity to Z ′ contributions. The TC2 enhancements to the branching ratios of these decays can reach a factor of ~ 2; (ii) the NP enhancement to the forward-backward asymmetry of the decay B s → ϕμ + μ − is in the range −13 % to 3 %, but −9 % to −6 % for decay B s → ϕτ + τ − compared to the SM predictions; (iii) the TC2 model provide an enhancement of about 12 % to the longitudinal polarization asymmetry P L for decay B s → ϕμ + μ −, but a decrease of about 10 % to the transverse polarization asymmetry P T for the decay B s → ϕτ + τ −.

Study ofBc→Ds*ℓ+ℓ−in a single universal extra dimension

The rare semileptonic Bc-> D_s^* \ell^+ \ell^- decay is studied in the scenario of the universal extra dimension model with a single extra dimension in which inverse of the compactification radius R is the only new parameter. The sensitivity of differential branching ratio, total branching ratio, polarization and forward-backward asymmetries of final state leptons, both for muon and tau, to the compactification parameter is presented. For some physical observables uncertainty on the form factors and resonance contributions have been considered in the calculations. Obtained results, compared with the available data, show that there appear new contributions due to the extra dimension.

Constraint on compactification scale via recently observed baryonic Λ b → Λℓ + ℓ − channel and analysis of the Σ b → Σℓ + ℓ − transition in SM and UED scenario

We obtain a lower limit on the compactification scale of extra dimension via comparison of the branching ratio in the baryonic $\Lambda_b\rightarrow \Lambda \mu^+ \mu^-$ decay channel recently measured by CDF collaboration and our previous theoretical study. We also use the newly available form factors calculated via light cone QCD sum rules in full theory to analyze the flavour changing neutral current process of the $\Sigma_b \rightarrow \Sigma \ell^+ \ell^-$ in universal extra dimension scenario in the presence of a single extra compact dimension. We calculate various physical quantities like branching ratio, forward-backward asymmetry, baryon polarizations and double lepton polarization asymmetries defining the decay channel under consideration. We also compare the obtained predictions with those of the standard model.

Analysis of the Λb → Λℓ+ℓ− transition in the SM4 using form factors from full QCD

Using the responsible form factors calculated via full QCD, we analyze the $\Lambda_{b}\rightarrow \Lambda \ell^{+}\ell^{-}$ transition in the standard model containing fourth generation quarks (SM4). We discuss effects of the presence of $t'$ fourth family quark on related observables like branching ratio, forward-backward asymmetry, baryon polarization as well as double lepton polarization asymmetries. We also compare our results with those obtained in the SM as well as with predictions of the SM4 but using form factors calculated within heavy quark effective theory. The obtained results on branching ratio indicate that the $\Lambda_{b}\rightarrow \Lambda \ell^{+}\ell^{-}$ transition is more probable in full QCD comparing to the heavy quark effective theory. It is also shown that the results on all considered observables in SM4 deviate considerably from the SM predictions when $m_{t'}\geq 400 GeV$.

Two-Higgs-doublet model and double-lepton polarization asymmetries inB→K0*(1430)ℓ+ℓ−decay

In this paper, the dependency of ${P}_{ij}$'s on the dilepton invariant mass, ${q}^{2}$, and the model III two-Higgs-doublets model (2HDM) parameters for $B\ensuremath{\rightarrow}{K}_{0}^{*}(1430){\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ decay were investigated, and the results were compared to those of the standard model (SM) and Appelquist, Cheng and Dobrescu model. Also, for this decay, the effects of model III 2HDM parameters on the averages of double-lepton polarization asymmetries, $⟨{P}_{ij}⟩$'s, were studied, and by taking into account the corresponding theoretical and experimental errors in the SM, the results of the SM and 2HDM were compared to each other. In addition, by comparing the averages of double-lepton polarization asymmetries in 2HDM to those of SM4 and obtaining the required number of events for detecting each asymmetry at the LHC or the Super Large Hadron Collider, we present a comprehensive discussion regarding the lepton polarizations of $B\ensuremath{\rightarrow}{K}_{0}^{*}{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ decay. We discovered that the study of the double-lepton polarization asymmetries and the corresponding averages in the $B\ensuremath{\rightarrow}{K}_{0}^{*}(1430){\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ decay can provide good signals for probing new physics beyond the SM in the future $B$-physics experiments.