Distributions Of Leptons Research Articles

Study of theCPproperty of the Higgs boson at a photon collider usingγγ→tt¯→lX

We study the possible effects of CP violation in the Higgs sector on tt - production at a γγ collider. These studies are performed in a model-independent way in terms of six form factors {R(S γ ),I(S γ ),R(P γ ),I(P γ ),S t ,P t } which parametrize the CP mixing in the Higgs sector, and a strategy for their determination is developed. We observe that the angular distribution of the decay lepton from t/t - produced in this process is independent of any CP violation in the tbW vertex and hence best suited for studying CP mixing in the Higgs sector. Analytical expressions are obtained for the angular distribution of leptons in the c.m. frame of the two colliding photons for a general polarization state of the incoming photons. We construct combined asymmetries in the initial state lepton (photon) polarization and the final state lepton charge. They involve CP even (x's) and odd (y's) combinations of the mixing parameters. We study limits up to which the values of x and y, with only two of them allowed to vary at a time, can be probed by measurements of these asymmetries, using circularly polarized photons. We use the numerical values of the asymmetries predicted by various models to discriminate among them. We show that this method can be sensitive to the loop-induced CP violation in the Higgs sector in the minimal supersymmetric standard model.

Lepton helicity distributions in the polarized Drell-Yan process

The lepton helicity distributions in the polarized Drell-Yan process at RHIC energy are investigated. For the events with relatively low invariant mass of lepton pair in which the weak interaction is negligible, only the measurement of lepton helicity can prove the antisymmetric part of the hadronic tensor. Therefore it might be interesting to consider the helicity distributions of leptons to obtain more information on the structure of nucleon from the polarized Drell-Yan process. We estimate the QCD corrections at O(alpha_s) level to the hadronic tensor including both intermediate gamma and Z bosons. We present the numerical analyses for different invariant masses and show that the u (bar{u}) and d (bar{d}) quarks give different and characteristic contributions to the lepton helicity distributions. We also estimate the lepton helicity asymmetry for the various proton's spin configurations.

Lepton asymmetry in polarized Drell-Yan

The lepton helicity distributions in the polarized Drell-Yan process at RHIC energy are investigated. In the absence of the weak interaction, only the measurement of lepton helicity can prove the antisymmetric part of the hadronic tensor. Therefore it might be interesting to consider the helicity distributions of leptons to obtain more information on the structure of nucleon from the polarized Drell-Yan process. We estimate the QCD corrections at O ( α s ) level to the hadronic tensor including both intermediate γ and Z bosons. We report the numerical analyses on the Z pole and show that the u( u) and d( d) quarks give different and characteristic contributions to the lepton helicity distributions. We also estimate the lepton helicity asymmetry for the various proton's spin configurations.

Single spin asymmetry in open charm photoproduction and decay as a test of pQCD

We analyze the possibility of measuring the single spin asymmetry (SSA) in open charm production by linearly polarized photons in the planned E160/E161 experiments at SLAC using the inclusive spectra of secondary (decay) leptons. In leading order pQCD, the SSA in the azimuthal distribution of the charged decay lepton is predicted to be about 0.2 for SLAC kinematics. Our calculations show that the SSA in the decay lepton distribution is well defined in pQCD: it is stable both perturbatively and parametrically, and practically insensitive to theoretical uncertainties in the charm semileptonic decays. Nonperturbative contributions to the leptonic SSA due to the gluon transverse motion in the target, heavy quark fragmentation and Fermi motion of the c-quark inside the D-meson are predicted to be about 10%. We conclude that measurements of the azimuthal asymmetry in the secondary lepton distribution would provide a good test of the applicability of pQCD to open charm production at energies of fixed target experiments. Our analysis of the SLAC experimental conditions shows that this SSA can be measured in E160/E161 with an accuracy of about ten percent.

QCD corrections to decay-lepton polar and azimuthal angular distributions in e + e − → t $$\overline t $$ in the soft-gluon approximationin the soft-gluon approximation

QCD corrections to order α s in the soft-gluon approximation to angular distributions of decay charged leptons in the process e + e − → t \(\overline t \), followed by semileptonic decay of t \(\overline t \), are obtained in the esu− e − centre-of-mass frame. As compared to distributions in the top rest frame, these have the advantage that they would allow direct comparison with experiment without the need to reconstruct the top rest frame. The results also do not depend on the choice of a spin quantization axis for t or t. Analytic expression for the triple distribution in the polar angle of t and polar and azimuthal angles of the lepton is obtained. Analytic expression is also derived for the distribution in the charged-lepton polar angle. Numerical values are discussed for √s=400, 800 and 1500 GeV.

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.

Identification of a horizontal gauge boson in hadronic collisions

We compare distributions of leptons produced in hadronic collisions, by a horizontal neutral gauge boson, \(Z^{\prime},\) suggested by the \(Sp(6)_{\mathrm{L}}\otimes U(1)_{Y}\) model, to those produced by other theoretically motivated neutral gauge bosons occurring in left–right symmetric models and in superstring-inspired \(E_6\) models. Forward–backward asymmetries in the gauge boson leptonic decay are found to be sensitive to specific forms of the couplings. The asymmetries are expected to be maximal for \(Z^{\prime},\) distinguishing it from the other Z's.

Large electric dipole moments of heavy leptons.

In many models of CP violation, the electric dipole moment (EDM) of a heavy charged or neutral lepton could be very large. In models in which the EDMs scale as the cube of the mass, as well as in vectorlike models, a 100 GeV heavy lepton can have an EDM as large as 0.01e fermi. This could dominate the electromagnetic properties of heavy leptons. The angular distribution and production cross section of charged and neutral heavy leptons with large EDMs is calculated and discussed. The interesting possibility that a heavy neutrino with a large EDM could leave an ionization track in a drift chamber is investigated.

Decay-lepton angular distributions in [formula omitted] to [formula omitted] in the soft-gluon approximation

Order- α s QCD corrections in the soft-gluon approximation to angular distributions of decay charged leptons in the process e + e −→ t t , followed by semileptonic decay of t or t , are obtained in the e +e − centre-of-mass frame. As compared to distributions in the top rest frame, these have the advantage that they would allow direct comparison with experiment without the need to reconstruct the top rest frame or a spin quantization axis. Analytic expressions for the distribution in the charged-lepton polar angle, and triple distribution in the polar angle of t and polar and azimuthal angles of the lepton are obtained. Numerical values for the polar-angle distributions of charged leptons are discussed for s =400 GeV and 800 GeV.

Spin correlations in e+e−→W+W−

We provide a covariant polarization formalism for the spin and angular correlations of the W boson pair in the process e+e−→W+W− so as to probe new physics beyond the Standard Model. The general form of the polarization density matrices is explicitly computed in order to perform a model-independent study. The collinearity angle distribution of the secondary leptons from the subsequent decays of W bosons is presented as well.

Lepton energy asymmetry and precision supersymmetry study at hadron colliders

We study the distribution of lepton pairs from the second lightest neutralino decay \tilde{\chi}^0_2-->\tilde{l}l followed by \tilde{l}\to \tilde{\chi}^0_1 l. The distribution of the ratio of lepton transverse momenta A_T shows peak structure if m_{ll}< m^{max}_{ll}/2 is required. The peak position A_T^{peak} is described by a simple function of the ino and slepton masses in the m_{ll}\sim 0 limit. When a moderate m_{ll} cut is applied, A_T^{peak} depends on the \tilde{\chi}^0_2 velocity distribution, but the dependence would be corrected by studying the lepton P_T distribution. A_T^{peak} and the edge of m_{ll} distributions are used to determine the mass parameters involved in the decay for parameters of interest to LHC experiments. For some cases the ino and slepton masses may be determined within 10% by the lepton distribution only independent of model assumptions. Correct combinations of A_{T}^{peak} and m_{ll}^{edge} would be identified even if different \tilde{\chi}^0_2 decay chains are co-existing. The analysis could be extended to the Tevatron energy scale or other cascade decays.

Effect of anomalous tbW vertex on decay-lepton distributions in e + e − → tt and CP-violating asymmetries

We obtain analytic expressions for the energy and polar-angle double differential distributions of a secondary lepton l + (l −) arising from the decay of t(t) in e + e − → tt with an anomalous tbW decay vertex. We also obtain analytic expressions for the various differential cross-sections with the lepton energy integrated over. In this case, we find that the angular distributions of the secondary lepton do not depend on the anomalous coupling in the decay, regardless of possible anomalous couplings occurring in the production amplitude for e + e − → tt. Our study includes the effect of longitudinal e − and e + beam polarization. We also study the lepton energy and beam polarization dependence of certain CP-violating lepton angular asymmetries arising from an anomalous tbW decay vertex and compare them with the asymmetries arising due to CP-violation in the production process due to the top electric or weak dipole moment.

Trilepton signal for supersymmetry at the Fermilab Tevatron reexamined

Within a wide class of models, the CERN LEP2 lower limit of 95 GeV on the chargino mass implies gluinos are heavier than $\ensuremath{\sim}300 \mathrm{GeV}.$ In this case electroweak ${W}_{1}W{\overline{\ifmmode \tilde{}\else \~{}\fi{}}}_{1}$ production and ${W}_{1}{Z}_{2}$ production are the dominant supersymmetry (SUSY) processes at the Fermilab Tevatron, and the extensively examined isolated trilepton signal from ${W}_{1}{Z}_{2}$ production assumes an even greater importance. We update our previous calculations of the SUSY reach of luminosity upgrades of the Fermilab Tevatron in this channel incorporating (i) decay matrix elements in the computation of the momenta of leptons from chargino and neutralino decays, (ii) the trilepton background from ${W}^{*}{Z}^{*}$ and ${W}^{*}{\ensuremath{\gamma}}^{*}$ production which, though neglected in previous analyses, turns out to be the dominant background, and finally, (iii) modified sets of cuts designed to reduce these new backgrounds and increase the range of model parameters for which the signal is observable. We show our improved projections for the reach for SUSY of both the Fermilab Main Injector and the proposed TeV33 upgrade. We also present opposite sign same flavor dilepton invariant mass distributions as well as the ${p}_{T}$ distributions of leptons in SUSY trilepton events, and comment upon how the inclusion of decay matrix elements impacts upon the Tevatron reach, as well as upon the extraction of neutralino masses.

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 decay leptons from e+e−→ W+W− at threshold

The reaction e + e −→ W + W − produces a W-boson pair with a non-trivial spin correlation at threshold. This correlation leads to a characteristic angular correlation between the leptons produced in W ±→ℓ ± ν (angles relative to the e − beam direction): dσ/d cosθ +d cosθ −∼(1− cosθ +)(1+ cosθ −)(1+ cosθ − cosθ +) . If only the ℓ − is observed, its angular distribution is dσ/d cosθ −∼(1+ cosθ −)(3− cosθ −) , implying a forward-backward asymmetry of 3/8. An analytic result is also given for the azimuthal correlation. These results are reproduced by a Monte Carlo program, that also enables us to study the effects of the W decay width. The threshold behaviour, which stems from the dominance of ν-exchange, is contrasted with that due to γ- and Z-exchange, which is relevant for annihilation in the helicity state e − R e + L .

CP-violating Zγγ and top-quark electric dipole couplings in [formula omitted

An effective anomalous CP-violating Zγγ coupling can give rise to observable CP-odd effects in γγ→t t . We study certain asymmetries in the decay lepton distributions in γγ→t t arising from top decay in the presence of a CP-violating Zγγ coupling as well as a top-quark electric dipole coupling. We find that a photon linear collider with geometric luminosity of 20 fb −1 can put limits of the order of 0.1 on the imaginary part of the CP-violating anomalous Zγγ coupling using these asymmetries.

Quarkonium polarization in nonrelativistic QCD and the quark-gluon plasma

We examine angular distributions of leptons arising from the decay of J/psi in inclusive hadroproduction. Taking into account feed-down contributions from chi_{1,2}, a flat distribution emerges without fine tuning parameters. Changes in the ratio of direct to total J/psi cross sections would change this distribution significantly. Such angular distributions are, therefore, confirmatory tests of the J/psi suppression signal for the production of a quark-gluon plasma. Related effects are predicted for the Upsilon.

Electric dipole moment of the top quark andCP-violating asymmetries inγγ→tt¯

$\mathrm{CP}$ violation can arise in the production of top-quark--top-antiquark pair in photon-photon collisions in the presence of a nonzero electric dipole coupling of the top quark. This can show up in the distribution of leptons produced in the subsequent decay of the quarks. We study $\mathrm{CP}$-violating asymmetries in the final-state leptons under the assumption that $\mathrm{CP}$ violation occurs in the production of the top-quark pair, neglecting $\mathrm{CP}$ violation in the decay. The asymmetries defined can be used to determine the imaginary part of the electric dipole form factors. A \ensuremath{\gamma}\ensuremath{\gamma} collider with photon beams generated from laser backscattering off electron beams with an integrated geometric luminosity of $20{\mathrm{fb}}^{\mathrm{\ensuremath{-}}1}$ can put a limit of the order of ${10}^{\ensuremath{-}17}e\mathrm{cm}$ on the imaginary part of the electric dipole form factor of the top quark if the electron beams have longitudinal polarization and the laser beams have circular polarization.

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%.