arXiv subjects
Publications and source records attributed to A. Brandenburg.
Future hadron collider experiments will allow for a detailed investigation of $t\bar{t}$ spin correlation effects. In this talk, recent progress in the theoretical description of these effects is reported. In particular, next-to-leading order results for the $t\bar t$ spin correlations in $q\bar{q}\to t\bar{t}X$ are presented, using various spin quantization axes.
Future hadron collider experiments are expected to record large to huge samples of $t\bar t$ events. The analysis of these data with respect to $t\bar t$ spin-spin correlations requires precise predictions of the production of these quark pairs in a general spin configuration. Therefore we have computed, at next-to-leading order (NLO) in the QCD coupling, the spin density matrices describing $t\bar t$ production by quark antiquark annihilation, which is the dominant production process at the Tevatron. Moreover we have computed the strength of the $t\bar t$ spin correlation at NLO, using various spin quantization axes.
We discuss the polarization and spin correlations of top quarks produced above threshold at a future linear collider, including QCD radiative corrections.
We have studied the determination of the running b-quark mass, $m_b(M_Z)$, using $Z^0$ decays into 3 or more hadronic jets. We calculated the ratio of $\geq3$-jet fractions in $e^+e^-\to b\bar{b}$ vs. $e^+e^-\to q_l\bar{q_l}$ ($q_l$ = u or d or s) events at next-to-leading order in perturbative QCD using six different infra-red- and collinear-safe jet-finding algorithms. We compared with corresponding measurements from the SLD Collaboration and found a significant algorithm-dependence of the fitted $m_b(M_Z)$ value. Our best estimate, taking correlations into account, is $m_b(M_Z) = 2.56 \pm 0.27 (stat.) ^{+0.28}_{-0.38} (syst.) ^{+0.49}_{-1.48} (theor.) GeV/c^2$.
A striking manifestation of CP violation in the electroweak symmetry breaking sector would be the existence of neutral Higgs boson(s) with undefined CP parity. We analyse signatures of such a boson, with a mass of about 300 GeV or larger, produced in high energy proton-proton collisions at LHC energies in its top-quark antitop-quark decay channel. The large irreducible $t\bar t$ background is taken into account. We propose, both for the dilepton and the lepton + jets decay channels of $t\bar t$, several correlations and asymmetries with which (Higgs sector) CP violation can be traced. We show that for judiciously chosen cuts on the $t\bar t$ invariant mass these CP observables yield, for an LHC integrated luminosity of 100 $\rm{fb}^{-1}$, statistically significant signals for a range of Higgs boson masses and Yukawa couplings.
We study the changes in the dynamical behaviour of axisymmetric spherical mean-field dynamo models produced by changes in their geometry and topology, by considering a two parameter family of models, ranging from a full sphere to spherical shell, torus and disc-like configurations, within a unified framework. We find that the two parameter space of the family of models considered here separates into at least three different regions with distinct characteristics for the onset of dynamo action. In two of these regions, the most easily excited fields are oscillatory, in one case with dipolar symmetry, and in the other with quadrupolar, whereas in the third region the most easily excited field is steady and quadrupolar. In the nonlinear regime, we find that topological changes can alter significantly the dynamical behaviour, whilst modest changes in geometry can produce qualitative changes, particularly for thin disc-like configurations. This is of potential importance, since the exact shapes of astrophysical bodies, especially accretion discs and galaxies, are usually not precisely known.
Calculations for mean field dynamo models (in both full spheres and spherical shells), with both algebraic and dynamic $α$--quenchings, show qualitative as well as quantitative differences and similarities in the dynamical behaviour of these models. We summarise and enhance recent results with extra examples. Overall, the effect of using a dynamic $α$ appears to be complicated and is affected by the region of parameter space examined.
We calculate the spin density matrix of top quark pairs for the reaction $e^+e^-\to t\bar{t}X$ to order $α_s$. As an application we show next-to-leading order results for a variety of spin observables for the $t\bar{t}$ system. These include the top quark and antiquark polarizations and $t\bar{t}$ spin-spin correlations as a function of the center-of-mass energy and of the top quark scattering angle for arbitrary longitudinal polarization of the electron/positron beam.
The reaction $e^+e^-$ annihilation into three jets was recently computed for massive quarks at next-to-leading order in perturbative QCD. We discuss some results of our calculation for b jets produced at the Z resonance.
We discuss a calculation of the next-to-leading order QCD corrections to the process $e^+e^-\to 3$ jets with massive quarks, and show numerical results for the three jet fraction and the differential two jet rate.
We compute the order $α_s$ QCD corrections to the density matrix for the decay of a neutral Higgs boson $ϕ$ with (in)definite CP parity into a quark antiquark pair, respectively the QED corrections for the decay into a pair of charged leptons. We classify and calculate single spin asymmetries and spin-spin correlations which are generated by the scalar and pseudoscalar Yukawa couplings. These spin effects can be traced in $ϕ\to τ^-τ^+$ and, for heavy Higgs bosons, in $ϕ\to t\bar{t}$. We also calculate resulting correlations among the final states and estimate, for the respective decay modes, the number of events needed to measure the Yukawa couplings with these correlations at the 3 sigma level.
We show that the CP-violating dipole form factors of the tau lepton can be of the order of $α/π$ in units of the length scale set by the inverse $Z$ boson mass. We propose a few observables which are sensitive to these form factors at LEP2 and higher e^+e^- collision energies.
Tests of CP violation by appropriate momentum correlations in $Z\to 3$ jets and in particular in $Z\to b\bar bX$ probe CP-violating effective couplings -- that manifest themselves as form factors -- which conserve the quark chirality and quark flavour. By giving two examples we show that such couplings can be induced at one-loop order in extensions of the Standard Model with CP violation beyond the Kobayashi-Maskawa phase. In one of the models we compute the chirality-conserving part of the CP-violating $Zb{\bar b}$-gluon amplitude for massless $b$ quarks, determine the resulting effective dimension $d=6$ couplings in the local limit, and discuss the possible size of the effects. Finally we show that in models with excited quarks the chiral-invariant CP-violating effective interactions could be quite large if appropriate couplings are of a size characteristic of a strong interaction
We report on an investigation of CP violation in (un)polarized $e^+e^-\to{\bar t}t$ resulting from an extended neutral Higgs sector or from the minimal SUSY extension of the Standard Model (SM). We consider c.m. energies from the ${\bar t}t$ threshold to the TeV range. In addition sensitivity estimates for CP-violating form factors of the top quark are made. Further we discuss the prospects of probing Higgs sector CP violationin $γγ\to{\bar t}t$.
We consider the production of muon pairs from the scattering of pions on longitudinally polarized protons. We calculate the cross section and the single spin asymmetry for this process, taking into account pion bound state effects. We work in the kinematic region where the photon has a large longitudinal momentum fraction, which allows us to treat the bound state problem perturbatively. Our predictions are directly proportional to the pion distribution amplitude. A measurement of the polarized Drell-Yan cross section thus allows the determination of the shape of the pion distribution amplitude.
We investigate for two-Higgs doublet models with intrinsic CP violation in the scalar potential CP-nonconserving effects in unpolarized $γγ\rightarrow t \bar{t} $ for a range of neutral Higgs boson masses which includes resonant $φ$ production and the subsequent decay of $φ\rightarrow t \bar{t}$. The importance of taking into account, even in the resonant case, the interference with the nonresonant background is shown. Further, we propose and calculate three asymmetries which efficiently trace CP-violating effects in $γγ\rightarrow t \bar{t} $ using semileptonic $t \bar{t}$ decays.
We discuss hard rescattering effects that can be measured using CP-even, T$_{\rm N}$-odd triple-product observables in polarized $Z$ decays to three jets. We show that the standard model contributions, from both QCD and electroweak rescattering, are very small. Thus these measurements are potentially sensitive to physics beyond the standard model. We investigate one such contribution which involves a new gauge boson coupling to baryon number.