Lepton Forward-backward Asymmetry Research Articles

Publisher’s Note: Measurement of the lepton forward-backward asymmetry inB→Xsℓ+ℓ−decays with a sum of exclusive modes [Phys. Rev. D93, 032008 (2016)

We report the first measurement of the lepton forward-backward asymmetry AFB as a function of the squared four-momentum of the dilepton system, q2, for the electroweak penguin process B→Xsl+l- with a sum of exclusive final states, where l is an electron or a muon and Xs is a hadronic recoil system with an s quark. The results are based on a data sample containing 772×106 BB¯ pairs recorded at the ϒ(4S) resonance with the Belle detector at the KEKB e+e- collider. AFB for the inclusive B→Xsl+l- is extrapolated from the sum of 10 exclusive Xs states whose invariant mass is less than 2 GeV/c2. For q2>10.2 GeV2/c2, AFB<0 is excluded at the 2.3σ level, where σ is the standard deviation. For q2<4.3 GeV2/c2, the result is within 1.8σ of the standard model theoretical expectation.

Measurement of the lepton forward-backward asymmetry inB→Xsℓ+ℓ−decays with a sum of exclusive modes

We report the first measurement of the lepton forward-backward asymmetry ${\cal A}_{\rm FB}$ as a function of the squared four-momentum of the dilepton system, $q^2$, for the electroweak penguin process $B \rightarrow X_s \ell^+ \ell^-$ with a sum of exclusive final states, where $\ell$ is an electron or a muon and $X_s$ is a hadronic recoil system with an $s$ quark. The results are based on a data sample containing $772\times10^6$ $B\bar{B}$ pairs recorded at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. ${\cal A}_{\rm FB}$ for the inclusive $B \rightarrow X_s \ell^+ \ell^-$ is extrapolated from the sum of 10 exclusive $X_s$ states whose invariant mass is less than 2 GeV/$c^2$. For $q^2 > 10.2$ GeV$^2$/$c^2$, ${\cal A}_{\rm FB} < 0$ is excluded at the 2.3$\sigma$ level, where $\sigma$ is the standard deviation. For $q^2 < 4.3$ GeV$^2$/$c^2$, the result is within 1.8$\sigma$ of the Standard Model theoretical expectation.

Comparative analysis of theΛb→Λℓ+ℓ−decay in the SM, SUSY, and RS model with custodial protection

We comparatively analyze the rare $\Lambda _b\rightarrow \Lambda \ell^+ \ell^-$ channel in standard model, supersymmetry and Randall-Sundrum model with custodial protection (RS$_c$). Using the parametrization of the matrix elements entering the low energy effective Hamiltonian in terms of form factors, we calculate the corresponding differential decay width and lepton forward-backward asymmetry in these models. We compare the results obtained with the most recent data from LHCb as well as lattice QCD results on the considered quantities. It is obtained that the standard model, with the form factors calculated in light-cone QCD sum rules, can not reproduce some experimental data on the physical quantities under consideration but the supersymmetry can do it. The RS$_c$ model predictions are roughly the same as the standard model and there are no considerable differences between the predictions of these two models. In the case of differential decay rate, the data in the range $4 $ GeV$^2/$c$^4\leq$ $q^2 \leq 6$ GeV$^2/$c$^4$ can not be described by any of the considered models.

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.

Constraints on tensor and scalar couplings from [Formula: see text] and [Formula: see text

The angular distribution of (ell = e,,mu ,,tau ) depends on two parameters, the lepton forward–backward asymmetry, A_mathrm{FB}^ell , and the flat term, F_H^ell . Both are strongly suppressed in the standard model and constitute sensitive probes of tensor and scalar contributions. We use the latest experimental results for ell = mu in combination with the branching ratio of B_srightarrow bar{mu }mu to derive the strongest model-independent bounds on tensor and scalar effective couplings to date. The measurement of F_H^mu provides a complementary constraint to that of the branching ratio of B_srightarrow bar{mu }mu and allows us – for the first time – to constrain all complex-valued (pseudo-)scalar couplings and their chirality-flipped counterparts in one fit. Based on Bayesian fits of various scenarios, we find that our bounds even become tighter when vector couplings are allowed to deviate from the standard model and that specific combinations of angular observables in B rightarrow K^*bar{ell }ell are still allowed to be up to two orders of magnitude larger than in the standard model, which would place them in the region of LHCb’s sensitivity.

Angular analysis of the B0 → K*0e+e− decay in the low-q2 region

An angular analysis of the $B^0 \rightarrow K^{*0} e^+ e^-$ decay is performed using a data sample, corresponding to an integrated luminosity of 3.0 ${\mbox{fb}^{-1}}$, collected by the LHCb experiment in $pp$ collisions at centre-of-mass energies of 7 and 8 TeV during 2011 and 2012. For the first time several observables are measured in the dielectron mass squared ($q^2$) interval between 0.002 and 1.120${\mathrm{\,Ge\kern -0.1em V^2\!/}c^4}$. The angular observables $F_{\mathrm{L}}$ and $A_{\mathrm{T}}^{\mathrm{Re}}$ which are related to the $K^{*0}$ polarisation and to the lepton forward-backward asymmetry, are measured to be $F_{\mathrm{L}}= 0.16 \pm 0.06 \pm0.03$ and $A_{\mathrm{T}}^{\mathrm{Re}} = 0.10 \pm 0.18 \pm 0.05$, where the first uncertainty is statistical and the second systematic. The angular observables $A_{\mathrm{T}}^{(2)}$ and $A_{\mathrm{T}}^{\mathrm{Im}}$ which are sensitive to the photon polarisation in this $q^2$ range, are found to be $A_{\mathrm{T}}^{(2)} = -0.23 \pm 0.23 \pm 0.05$ and $A_{\mathrm{T}}^{\mathrm{Im}} =0.14 \pm 0.22 \pm 0.05$. The results are consistent with Standard Model predictions.

Anomalous top-Higgs couplings and top polarisation in single top and Higgs associated production at the LHC

In this paper, we put constraints on anomalous $\mathcal{CP}$-violating top-Higgs couplings using the currently available Higgs data and explore the prospect of measuring these couplings at 240 GeV TLEP. We find that the $\mathcal{CP}$-violating phase $\xi$ is currently limited in the range $|\xi|< 0.6\pi$ at 95\% C.L. and may be further constrained to $|\xi| <0.07\pi$ at TLEP. Under this consideration, we further investigate the observability of the scalar ($\xi =0$), pseudoscalar ($\xi =0.5\pi$) and mixed ($\xi =0.25\pi$) top-Higgs interactions through the channel $pp \to t(\to \ell^+\nu_\ell b)h(\to b\overline{b})j$. We find that it is most promising to observe pure pseudoscalar interactions with $y_t=y_t^{SM}$, although this will be challenging due to a low signal to background ratio. We also find that the anomalous top-Higgs couplings can lead to sizeable differences in lepton forward-backward asymmetries and can be distinguished by measuring the lepton angular distributions from polarised top quarks at 14 TeV LHC.

Lepton polarization asymmetries of H -&gt; + - 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.

Forward-backward asymmetry of leptonic decays oftt¯at the Fermilab Tevatron

We report on a study of the measurement techniques used to determine the leptonic forward-backward asymmetry of top-antitop quark pairs in Tevatron experiments with a proton-antiproton initial state. Recently it was shown that a fit of the differential asymmetry as a function of ${q}_{l}{\ensuremath{\eta}}_{l}$ (where ${q}_{l}$ is the charge of the lepton from the cascade decay of the top quarks and ${\ensuremath{\eta}}_{l}$ is the final pseudorapidity of the lepton in the detector frame) to a hyperbolic tangent function can be used to extrapolate to the full leptonic asymmetry. We find this empirical method to well reproduce the results from current experiments and present arguments as to why this is the case. We also introduce two more models, based on Gaussian functions, that better model the ${q}_{l}{\ensuremath{\eta}}_{l}$ distribution. With our better understanding, we find that the asymmetry is mainly determined by the shift of the mean of the ${q}_{l}{\ensuremath{\eta}}_{l}$ distribution, the main contribution to the inclusive asymmetry comes from the region around $|{q}_{l}{\ensuremath{\eta}}_{l}|=1$, and the extrapolation from the detector-covered region to the inclusive asymmetry is stable via a multiplicative scale factor, giving us confidence in the previously reported experimental results.

Analysis of the semileptonicΛb→Λℓ+ℓ−transition in the topcolor-assisted technicolor model

We comparatively analyze the flavor changing neutral current process of the $\Lambda_b \rightarrow \Lambda \ell^+ \ell^-$ in the standard model as well as topcolor-assisted technicolor model using the form factors calculated via light cone QCD sum rules in full theory. In particular, we calculate the decay width, branching ratio and lepton forward-backward asymmetry related to this decay channel. We compare the results of the topcolor-assisted technicolor model with those of the standard model and debate how the results of the topcolor-assisted technicolor model depart from the standard model predictions. We also compare our results on the differential branching ratio with recent experimental data provided by CDF and LHCb Collaborations.

Exclusive [formula omitted] decays

The experimental progress on flavor-changing neutral current decays governed by b→sℓ+ℓ− transitions has grown enormously due to the latest results of BaBar, Belle, CDF and LHCb. Especially the exclusive modes B→K(⁎)ℓ+ℓ−, which have the largest rates, provide a variety of observables which constrain non-standard interactions that would affect them beyond the Standard Model. The theoretical treatment, based on expansions in ΛQCD/mb, focuses on low- and high dilepton invariant mass regions. Notably, form factor symmetries guided the identification of optimized observables in both regions which have small form factor dependences and sensitivity to new physics. Current experimental results of rates and lepton forward-backward asymmetries allow for first global analysis of b→sℓ+ℓ− decays in combination with b→sγ and Bs→μ+μ−. These analyses are now ready to be applied to include high-statistics results from LHCb and Super-Flavor factories within the next years and to profit from optimized observables.

Comparative analysis of the semileptonic Λ b → Λℓ+ℓ− transition in SM and different SUSY scenarios using form factors from full QCD

We work out the semileptonic $\Lambda_b\rightarrow \Lambda \ell^+ \ell^-$ transition in standard as well as different supersymmetric models. In particular, considering the parametrization of the matrix elements entered the low energy effective Hamiltonian in terms of form factors in full QCD, we calculate the amplitude and differential decay rate responsible for this decay channel in supersymmetric models. We then use the form factors calculated via light cone QCD sum rules in full theory to analyze the differential branching ratio and lepton forward-backward asymmetry of this decay channel in different supersymmetric models and compare the obtained results with those of the standard model. We also discuss how the results of different supersymmetric models deviate from the standard model predictions and which SUSY scenarios are favored.

The rare decay B→ Xsℓ +ℓ − to NNLL precision for arbitrary dilepton invariant mass

We present a new phenomenological analysis of the inclusive rare decay B→ X s ℓ +ℓ −. In particular, we present the first calculation of the NNLL contributions due to the leading two-loop matrix elements, evaluated for arbitrary dilepton invariant mass. This allows to obtain the first NNLL estimates of the dilepton mass spectrum and the lepton forward–backward asymmetry in the high M 2 ℓ +ℓ − region, and to provide an independent check of previously published results in the low M 2 ℓ +ℓ − region. The numerical impact of these NNLL corrections in the high-mass region ( M 2 ℓ +ℓ − >14.4 GeV 2) amounts to −13% in the integrated rate, and leads to a reduction of the scale uncertainty to ±3%. The impact of non-perturbative contributions in this region is also discussed in detail.

Tests of the standard model and constraints on new physics from measurements of fermion-pair production at 189-209 GeV at LEP

Production of events with hadronic and leptonic final states has been measured in \({\rm e^+e^-}\) collisions at centre-of-mass energies of 130–172 GeV, using the OPAL detector at LEP. Cross-sections and leptonic forward-backward asymmetries are presented, both including and excluding the dominant production of radiative Z\(\gamma\) events, and compared to Standard Model expectations. The ratio \(R_{\rm b}\) of the cross-section for \({\rm b\bar b}\) production to the hadronic cross-section has been measured. In a model-independent fit to the Z lineshape, the data have been used to obtain an improved precision on the measurement of \(\gamma\)-Z interference. The energy dependence of \(\alpha_{\mathrm{em}}\) has been investigated. The measurements have also been used to obtain limits on extensions of the Standard Model described by effective four-fermion contact interactions, to search for \(t\)-channel contributions from new massive particles and to place limits on gaugino pair production with subsequent decay of the gaugino into a light gluino and a quark pair.

Gauge invariance and form factors for the decayB→γl+l−

We analyze the form factors for the $\stackrel{\ensuremath{\rightarrow}}{B}\ensuremath{\gamma}{l}^{+}{l}^{\ensuremath{-}}$ weak transition. We show that making use of the gauge invariance of the $\stackrel{\ensuremath{\rightarrow}}{B}\ensuremath{\gamma}{l}^{+}{l}^{\ensuremath{-}}$ amplitude, the structure of the form factors in the resonance region, and their relations at large values of the photon energy results in efficient constraints on the behavior of the form factors. Based on these constraints, we propose a simple parametrization of the form factors and apply it to the lepton forward-backward (FB) asymmetry in the $\stackrel{\ensuremath{\rightarrow}}{B}\ensuremath{\gamma}{l}^{+}{l}^{\ensuremath{-}}$ decay. We find that the behavior of the FB asymmetry as a function of the photon energy, as well as the location of its zero, depend only weakly on the $\stackrel{\ensuremath{\rightarrow}}{B}\ensuremath{\gamma}$ form factors, and thus constitutes a powerful tool for testing the standard model.

Exclusive Λb→ Λℓ +ℓ − decay beyond standard model

Using the most general, model independent form of the effective Hamiltonian, the exclusive rare baryonic Λ b → Λℓ +ℓ −(ℓ= μ, τ) decay is analyzed. We study the sensitivity of the branching ratio and lepton forward–backward asymmetry to the new Wilson coefficients. It is shown that these physical quantities are quite sensitive to the new Wilson coefficients. Determination of the position of zero value of the forward–backward asymmetry can serve as a useful tool for establishing new physics beyond the standard model, as well as fixing the sign of the new Wilson coefficients.

Next-to-next-to leading logarithmic corrections to the angular distribution and to the forward-backward asymmetries inb→Xsl+l−

We present next-to-next-to leading logarithmic (NNLL) results for the double differential decay width $d\ensuremath{\Gamma}(\stackrel{\ensuremath{\rightarrow}}{b}{X}_{s}{\mathcal{l}}^{+}{\mathcal{l}}^{\ensuremath{-}})/[d\ifmmode \hat{s}\else \^{s}\fi{}d\mathrm{cos}(\ensuremath{\theta})],$ where $s=\ifmmode \hat{s}\else \^{s}\fi{}{m}_{b}^{2}$ is the invariant mass squared of the lepton pair and \ensuremath{\theta} is the angle between the momentum of the b quark and the ${\mathcal{l}}^{+},$ measured in the rest frame of the lepton pair. From these results we also derive NNLL results for the lepton forward-backward asymmetries, as these quantities are known to be very sensitive to new physics. While the principal steps in the calculation of the double differential decay width are the same as for $d\ensuremath{\Gamma}(\stackrel{\ensuremath{\rightarrow}}{b}{X}_{s}{\mathcal{l}}^{+}{\mathcal{l}}^{\ensuremath{-}})/d\ifmmode \hat{s}\else \^{s}\fi{},$ which is already known to NNLL precision, genuinely new calculations for the combined virtual and gluon bremsstrahlung corrections associated with the operators ${O}_{7},$ ${O}_{9},$ and ${O}_{10}$ are necessary. In this paper, we neglected certain other bremsstrahlung contributions, which are known to have only a small impact on $d\ensuremath{\Gamma}(\stackrel{\ensuremath{\rightarrow}}{b}{X}_{s}{\mathcal{l}}^{+}{\mathcal{l}}^{\ensuremath{-}})/d\ifmmode \hat{s}\else \^{s}\fi{}.$ We find that the NNLL corrections drastically reduce the renormalization scale (\ensuremath{\mu}) dependence of the forward-backward asymmetries. In particular, ${s}_{0},$ the position at which the forward-backward asymmetries vanish, is essentially free of uncertainties due to the renormalization scale at NNLL precision. We find ${s}_{0}^{\mathrm{NNLL}}=0.162\ifmmode\pm\else\textpm\fi{}0.005,$ where the error is dominated by the uncertainty in ${m}_{c}{/m}_{b}.$ This is to be compared with ${s}_{0}^{\mathrm{NLL}}=0.144\ifmmode\pm\else\textpm\fi{}0.020,$ where the error is dominated by uncertainties due to the choice of \ensuremath{\mu}.

Forward–backward asymmetry in B→ Xsℓ +ℓ − at the NNLL level

We report the results of a new calculation of soft-gluon corrections in B→ X s ℓ +ℓ − decays. In particular, we present the first calculation of bremsstrahlung and corresponding virtual terms to the lepton forward–backward asymmetry, which allows us to systematically include all contributions to this observable beyond the lowest non-trivial order. The new terms are important, for instance, the position of the zero of the asymmetry receives corrections of O(10%). Using a different method, we also provide an independent check of recently published results on bremsstrahlung and infrared virtual corrections to the dilepton-invariant mass distribution.

Erratum:b→sll¯in the minimal supergravity model [Phys. Rev. D55,4273 (1997)

The $b \rightarrow s l^+ l^-$ process is studied in the minimal supergravity model in detail. Taking account of the long distance contributions from the $c\overline{c}$ resonances, we calculate the branching ratio and the lepton forward-backward asymmetry in this model. We find that there is a strong correlation between the branching ratios of $b \rightarrow s \gamma$ and $b \rightarrow s l^+ l^-$ processes and that the interference effect can change the $b \rightarrow s l^+ l^-$ branching ratio in the off-resonance regions by up to $\pm 15%$ depending on the relative phase between the long and short distance contributions. Using various phenomenological constraints including the branching ratio of $b \rightarrow s \gamma$, we show that there are regions in the parameter space where the branching ratio of $b \rightarrow s l^+ l^-$ is enhanced by about 50% compared to the SM. We also show that the branching ratio of $b \rightarrow s \nu{\overline \nu}$ is reduced at most by 10% from the SM prediction.

Discovery limits of new extra gauge bosons at the CERN LHC

We study the Drell-Yan production and corresponding decay signatures at the CERN Large Hadron Collider (LHC) of extra Z bosons from a superstring-inspired ${E}_{6}$ model characterized by an extra inert $\mathrm{SU}{(2)}_{I}$ group at electroweak energies, and from the $\mathrm{Sp}{(6)}_{L}\ensuremath{\bigotimes}\mathrm{U}{(1)}_{Y}$ model. We discuss how to distinguish the models by examining the extra Z-boson couplings to fermions through its leptonic decay modes and muon-pair forward-backward asymmetries.