arXiv subjects
Publications and source records attributed to M. Sachwitz.
In order to determine the fundamental MSSM parameters M_1, M_2, μand \tanβthe tau polarization from stau decays can be explored as a `bridge' between the gaugino/higgsino and the stau sector in particular in the high \tanβrange. Even in the case of high \tanβan accuracy of δ(\tanβ)\approx 5% with an simultanous determination of A_τ is possible without assuming a specific SUSY breaking scheme.
The basic parameters of supersymmetric theories can be determined at future e^+e^- linear colliders with high precision. We investigate in this report how polarisation measurements in \tildeτ and \tilde{t} or \tilde{b} decays to τleptons and t quarks plus neutralinos or charginos can be used to measure \tanβ(in particular for large values) and to determine the trilinear couplings A_τ, A_t and A_b in sfermion pair production.
Single top quark cross section evaluations for the complete sets of tree-level diagrams in the $e^+ e^-$, $e^- e^-$, $γe$ and $γγ$ modes of the next linear collider with unpolarized and polarized beams are performed within the Standard Model and beyond. From comparison of all possibilities we conclude that the process $γ_+ e^-_L \to e^- t \bar b$ is extremely favoured due to large cross section, no $t \bar t$ background, high degrees of beam polarization, and exceptional sensitivities to $V_{tb}$ and anomalous $Wtb$ couplings. Similar reasons favour the process $e^- e^- \to e^- ν_e \bar t b$ for probing top quark properties despite a considerably lower cross section. Less favourable are processes like $e^+ e^-, γγ\to e^- ν_e t \bar b$. Three processes were chosen to probe their sensitivity to anomalous $Wtb$ couplings, with best bounds found for $γ_+ e^-_L \to e^- t \bar b$ and $e^+_R e^-_R \to e^- ν_e t \bar b$.
The Wtb vertex can be probed on future colliders in the processes of single top production (LHC, pp mode, NLC, $γe$ mode) and of top pair production (NLC, e^+ e^- mode). We analyse observables sensitive to anomalous Wtb couplings in the top pair production process of e^+ e^- collisions. In particular, forward-backward and spin-spin asymmetries of the top decay products and the asymmetry of the lepton energy spectum are considered. Possible bounds on anomalous couplings obtained are competitive to those expected from the upgraded Tevatron and LHC. The validity of the infinitely small width approximation for the three-body top decay is also studied in detail.
Assuming an integrated luminosity of 500 fb$^{-1}$ and a center-of-mass energy of 350 GeV, we examine the prospects for measuring branching fractions of a Standard Model-like Higgs boson with a mass of 160 GeV at the future linear \ee collider TESLA when the Higgs is produced via the Higgsstrahlung mechanism, \ee \pfr HZ. We study in detail the precisions achievable for the branching fractions of the Higgs into WW$^*$, ZZ$^*$ and \bb. However, the measurement of BF(H \pfr \gaga) remains a great challence. Combined with the expected error for the inclusive Higgsstrahlung production rate the uncertainty for the total width of the Higgs is estimated.
We present the results of calculations for the process gamma gamma -> W + 2 fermions at a future Photon Linear Collider (PLC). The calculations include at the same time the next-to-leading order Higgs signal and the complete tree level gauge boson background. We present numerical results in the intermediate mass Higgs region 140 GeV < M_H < 2M_W. We propose strategies for the determination of Higgs properties using the leptonic and hadronic final states for both polarized and unpolarized photon beams.
Analytic formulae are derived for the complete photon energy spectrum due to QED corrections to fermion pair production in case of a limited angular acceptance for the final state fermions. After a numerical integration over the energy of non-observed photons, this corresponds to typical experimental conditions at LEP/SLC.
We present the convolution integral for fermion pair production in the electroweak standard theory to order O(alpha) including also soft photon exponentiation. The result is complete in the sense that it includes initial and final state radiation and their interference. From the basic result - analytic formulae for the differential cross section - we also derive the corresponding expressions for total cross section and integrated forward-backward asymmetry. The numerical importance of different contributions for the analysis of experiments at LEP/SLC energies is discussed.
Results of complete tree level calculation of the single top and light Higgs production in the reaction $γe \to νb \bar{b} W$ at the Next Linear Collider are presented. In addition, the contributions of anomalous operators to the $Wtb$ and $WWH$ couplings are included into the complete 4-body consideration. The sensitivity for probing the structure of the couplings in a model independent way is analyzed.
We discuss discovery potentials for a 140 GeV Standard Model Higgs boson produced in e+e- collisions at 360 GeV, including all potential irreducible and reducible background contributions. In the second part of the study, we estimate the uncertainties expected for the branching fractions of the Higgs into bb-bar, tau+tau-, WW* and into cc-bar+gg including a realistic error estimation of the inclusive bremsstrahlung Higgs production cross section.
Results of complete tree level calculations of the single top production reaction $γe \to ν\bar{t} b$ at the Next Linear Collider, including the contribution of anomalous operators to the Wtb coupling are presented. The sensitivity for probing the structure of the Wtb coupling in a model independent way is analyzed and found to be significantly higher than for comparable measurements at the Tevatron.
For an electron-photon collider the complete tree-level cross sections of the reaction $γe \rightarrow νb \bar{b} W$ are computed at center-of-mass energies between 0.5 and 2.0 TeV, for top masses of 160 to 200 GeV and Higgs masses between 80 and 140 GeV within the Standard Model. It is shown that most of the $νb \bar{b}$ events are due to Higgs and $Z/γ^*$ production (with $H, Z/γ^* \rightarrow b \bar{b}$ decay) while top production (with $t \rightarrow bW$ decay) is about 50% smaller. Multiperipheral background and interferences are small, respectively negligible, in the energy range studied. By convoluting the basic cross sections with an energy spectrum of the backscattered photon beam, and inserting linear collider luminosities as anticipated in present designs, realistic $νb \bar{b} W$ event rates are estimated. This results in large event rates for $γe \rightarrow νt b$ and $γe \rightarrow νH W$. We estimate that the CKM matrix element $|V_{tb}|$, can be probed from the $νt b$ final state to an accuracy of 1-3% at $\sqrt{s_{e^+e^-}} > 1 TeV$. Assuming an effective Lagrangian based on dimension-6 operators we discuss the sensitivity for detecting deviations of the HWW coupling from the Standard Model in the reaction $γe \rightarrow νH W$.
We describe how to use ZFITTER, a program based on a semi-analytical approach to fermion pair production in e+e- annihilation and Bhabha scattering. A flexible treatment of complete ${\cal O}(α)$ QED corrections, also including higher orders, allows for three calculational {\bf chains} with different realistic sets of restrictions in the photon phase space. {\tt ZFITTER} consists of several {\bf branches} with varying assumptions on the underlying hard scattering process. One includes complete ${\cal O}(α)$ weak loop corrections with a resummation of leading higher-order terms. Alternatively, an ansatz inspired from S-matrix theory, or several model-independent effective Born cross sections may be convoluted. The program calculates cross sections, forward-backward asymmetries, and for $τ$~pair production also the final-state polarization. Various {\bf interfaces} allow fits to be performed with different sets of free parameters.