Lepton Angular Distributions Research Articles

Effects of top-quark anomalous decay couplings at γγ colliders

Most general tbW couplings were investigated in the process γγ → t t → l ± X for unpolarized photon beams. The double angular and energy distribution of the lepton was calculated and an optimal-observable analysis on it was carried out for the SM t t production mechanism. It was also shown that the leptonic angular distribution is insensitive to non-standard tbW vertex. That means that observation of non-standard effects indicates existence of some new physics in the production part.

Angular distribution of leptons in general tt̄ production and decay

Angular distribution of the secondary lepton in top-quark production followed by subsequent semi-leptonic decay is studied assuming general top-quark couplings. It is shown that the distribution does not depend on any possible anomalous tbW couplings and is determined only by the standard V-A decay vertex for any production mechanism if certain well-justified conditions are satisfied.

CPphases in correlated production and decay of neutralinos in the minimal supersymmetric standard model

We investigate the associated production of neutralinos $e^+e^-\to\tilde{\chi}^0_1\tilde{\chi}^0_2$ accompanied by the neutralino leptonic decay $\tilde{\chi}^0_2\to\tilde{\chi}^0_1 \ell^+\ell^-$, taking into account initial beam polarization and production-decay spin correlations in the minimal supersymmetric standard model with general CP phases but without generational mixing in the slepton sector. The stringent constraints from the electron EDM on the CP phases are also included in the discussion. Initial beam polarizations lead to three CP--even distributions and one CP--odd distribution, which can be studied independently of the details of the neutralino decays. We find that the production cross section and the branching fractions of the leptonic neutralino decays are very sensitive to the CP phases. In addition, the production--decay spin correlations lead to several CP--even observables such as lepton invariant mass distribution, and lepton angular distribution, and one interesting T--odd (CP--odd) triple product of the initial electron momentum and two final lepton momenta, the size of which might be large enough to be measured at the high--luminosity future electron--positron collider or can play a complementary role in constraining the CP phases with the EDM constraints.

New hints for testing anomalous top-quark interactions at future linear colliders

Angular and energy distributions for leptons and bottom quarks in the process e +e −→t t ̄ →ℓ ±/ b (−) ⋯ have been calculated assuming the most general top-quark couplings. The double distributions depend both on modification of the t t ̄ production and t (−) → b (−) W decay vertices. However, the leptonic angular distribution turned out to be totally insensitive to non-standard parts of Wtb vertex. Distributions of decay products for polarized top quark in its rest frame have been also calculated. It has been found that the factorization of energy and angular dependence for the double leptonic distribution noticed earlier for the Standard Model survives even if one allows for deviations from the V–A interactions, and the SM angular leptonic distribution turned out to be preserved by anomalous decay vertex.

New physics intt¯spin correlations at the Fermilab Tevatron

We show that the angular distributions of leptons or jets due to ttbar spin correlations allow a determination of the top chromomagnetic moment kappa with an accuracy of order 0.1. The method is quite insensitive to background, event reconstruction, and other experimental uncertainties. The total event number is important, so we suggest the inclusion of all-hadronic events.

Angular distribution of neutron inverse beta decay,ν¯e+p→e++n

The reaction $\bar{\nu}_e + p \to e^+ + n$ is very important for low-energy ($E_\nu \lesssim 60$ MeV) antineutrino experiments. In this paper we calculate the positron angular distribution, which at low energies is slightly backward. We show that weak magnetism and recoil corrections have a large effect on the angular distribution, making it isotropic at about 15 MeV and slightly forward at higher energies. We also show that the behavior of the cross section and the angular distribution can be well-understood analytically for $E_\nu \lesssim 60$ MeV by calculating to ${\cal O}(1/M)$, where $M$ is the nucleon mass. The correct angular distribution is useful for separating $\bar{\nu}_e + p \to e^+ + n$ events from other reactions and detector backgrounds, as well as for possible localization of the source (e.g., a supernova) direction. We comment on how similar corrections appear for the lepton angular distributions in the deuteron breakup reactions $\bar{\nu}_e + d \to e^+ + n + n$ and $\nu_e + d \to e^- + p + p$. Finally, in the reaction $\bar{\nu}_e + p \to e^+ + n$, the angular distribution of the outgoing neutrons is strongly forward-peaked, leading to a measurable separation in positron and neutron detection points, also potentially useful for rejecting backgrounds or locating the source direction.

Spin correlations in the production and subsequent decay of neutralinos

We study the process ${e}^{\ensuremath{-}}{e}^{+}\ensuremath{\rightarrow}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{1}^{0}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{2}^{0}$ with the subsequent decay ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{2}^{0}\ensuremath{\rightarrow}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\chi}}}_{1}^{0}{l}^{+}{l}^{\ensuremath{-}}$ taking into account the complete spin correlations. We give the analytical formulas for the differential cross section and present numerical results for the lepton angular distribution and the distribution of the opening angle between the outgoing leptons for the CERN LEP2 energy of $\sqrt{s}=193\mathrm{GeV}$ and for $\sqrt{s}=500\mathrm{GeV}.$ We examine three representative mixing scenarios in the MSSM and also study the influence of the common scalar mass parameter ${m}_{0}$ on the shape of the distributions. For the lepton angular distribution, the effect of the spin correlations amounts to up to 20% for lower energies. We find that the opening angle distribution is suitable for distinguishing between Higgsino-like and gaugino-like neutralinos. The shape of the lepton angular distribution is very sensitive to the mixing in the gaugino sector and to the value of ${m}_{0}.$ For higher energies, it is also suitable for distinguishing between Higgsino-like and gaugino-like neutralino.

Spin correlations in production and decay of charginos

We study the production of charginos e^+ e^- -> chi^+_i chi^-_j, (i,j=1,2) with polarized beams and the subsequent decays chi^+_i -> chi^0_k ell^+ nu and chi^-_j -> chi^0_l ell^- nu, (k,l=1,...,4), including the complete spin correlations between production and decay. Analytical formulae are presented for the joint spin-density matrix of the charginos, for the chargino decay matrix and for the differential cross section of the combined processes of production and decays. We present numerical results for pair production of the lighter chargino with unpolarized beams and the leptonic decay of chi^+_1 into the lightest neutralino chi^0_1. The lepton angular distribution and the forward-backward asymmetry are studied in four representative scenarios for sqrt{s}=192 GeV and sqrt{s}=200 GeV.

Relativistic and binding energy corrections to heavy quark fragmentation to polarised quarkonium

Abstract We calculate the fragmentation function for a charm quark to decay inclusively into transversely and longitudinally polarised J / ψ , including relativistic and binding energy corrections in powers of the quark relative velocity v . We use these fragmentation functions to calculate the asymmetry in the angular distribution of leptons produced in the subsequent decay of J / ψ . We point out that the inclusion of the said corrections reduces this asymmetry from 5% to about 2%.

Angular distributions of Drell-Yan lepton pairs at the Fermilab Tevatron: Order alpha s2 corrections and Monte Carlo studies.

We investigate the angular distribution of the lepton pair in the process $p \bar{p} \rightarrow \gamma^{\ast} + X \rightarrow \ell^+\ell^- + X$, where the virtual photon is produced at high transverse momentum. The angular distribution of the leptons is very sensitive to possible nonperturbative effects, such as a nontrivial vacuum structure of QCD, and offers a good chance to test such effects. We present complete ${\cal O}(\alpha_s^2)$ calculations of the decay lepton distributions in the lepton pair rest frame. An order ${\cal O}(\alpha_s)$ Monte Carlo study of the lepton angular distributions, with acceptance cuts and energy resolution smearing applied to the leptons, is also presented.

W and Z polarization effects in hadronic collisions.

A Monte Carlo study of the polar and azimuthal angular distributions of the lepton pair arising from the decay of a $W$ or $Z$ boson produced at high transverse momentum in hadronic collisions is presented. In the absence of cuts on the final state leptons, the lepton angular distribution in the gauge boson rest frame is determined by the gauge boson polarization. Numerical results for the lepton angular distributions in the Collins--Soper frame with acceptance cuts and energy resolution smearing applied to the leptons are presented. In the presence of cuts, the lepton angular distributions are dominated by kinematic effects rather than polarization effects, however, some polarization effects are still observable on top of the kinematic effects. Polarization effects are highlighted when the experimental distributions are divided by the Monte Carlo distributions obtained using isotropic gauge boson decay.

Analyzing thee + e ??e + e ? angular distibution at LEP

The analysis of the lepton angular distribution at LEP is reviewed and the specific concerns in the case ofe + e − final state are singled out. Studies of the physical information contained in thee + e − angular distribution are shown to justify the need for a new approach. Two different solutions that enable the complete use of the angular distribution information for the extraction of theZ parameters are presented. We show that these approaches have accuracies well above the future experimental needs.

Parton-model relation without quantum-chromodynamic modifications in lepton pair production

It is pointed out that, in contrast to the recently discovered large quantum-chromodynamics (QCD) correction to the Drell-Yan formula, a certain quark-parton-model relation between structure functions for lepton pair production is not subject to any first-order QCD modification. Both parallelism and contrast to the Callan-Gross relation in deep-inelastic scattering are spelled out. Implications on the lepton angular distribution for both low and high ${q}_{\ensuremath{\perp}}$ are discussed. The case is made that this relation provides a unique opportunity to test the "QCD-improved quark-parton model."

Structure function relations at large transverse momenta in Lepton-pair production processes

Features of the lepton angular distributions due to hard parton sub-processes in lepton-pair production at relatively large q ⊥ are studied. Structure function relations which characterize these mechanisms are derived. Comparisons of these features with those derived from the Drell-Yan mechanism are made.