arXiv subjects

T. Stelzer

Publications and source records attributed to T. Stelzer.

29 records · Page 2Linked to original sources

arXiv1996

Gluon Radiation in $t\bar t$ Production and Decay at the LHC

Understanding the pattern of gluon radiation in $t \bar t$ production and decay processes is important for making an accurate determination of the top mass from the momenta of its decay products. The larger energy of the LHC $pp$ collider boosts the top cross section by a factor of 100 compared to that at the Tevatron, but it also increases the amount of additional gluon radiation. We calculate the cross section for gluon radiation in top production and decay at the LHC. The distributions of this radiation are presented and the exact matrix-element results are compared with results from the HERWIG parton-shower Monte Carlo.

hep-ph↗

arXiv1996

Spin Correlation in Top-Quark Production at Hadron Colliders

We propose techniques to observe the correlation of the spins of top quarks and antiquarks at the Tevatron and the LHC. Observation of the spin correlation would confirm that the top quark decays before its spin flips, and would place a lower bound on the top-quark width and V_tb. The spin correlation may also be a useful tool to study the weak decay amplitude of the top quark.

hep-ph↗

arXiv1995

Single-top-quark production via q qbar -> t bbar

We consider single-top-quark production via the weak process $q\bar q to t\bar b$ at hadron colliders. This process may provide the best measurement of the magnitude of the Cabbibo-Kobayashi-Maskawa matrix element $V_{tb}$. We show that a signal can potentially be observed at the Fermilab Tevatron with 3 $fb^{-1}$ of integrated luminosity. In contrast, the signal is masked at the CERN Large Hadron Collider by top-quark pair production and single-top-quark production via $W$-gluon fusion.

hep-ph↗

arXiv1995

On Gluon Radiation in $t \bar t$ Production and Decay

Understanding the pattern of gluon radiation in $t \bar t$ production processes is important for making an accurate determination of the top mass from the measurement of its decay products. In a recent paper we showed that the exact matrix element and parton shower (HERWIG) calculations gave very different results for the distribution of gluon jets in $t \bar t$ production at the Tevatron $p \bar p$ collider. By repeating the calculation for the simpler $e^+e^- \to b \bar b W^+W^-g$ process, we reveal even more dramatic differences between the two approaches. We conclude that there are significant differences in gluon radiation between HERWIG and the matrix element calculation in regions of phase space where one would expect agreement.

hep-ph↗

arXiv1995

SIGNATURES OF CP-VIOLATION IN THE PRESENCE OF MULTIPLE B-PAIR PRODUCTION AT HADRON COLLIDER

We calculate the production of 2 b-quark pairs in hadron collisions. Sources of multiple pairs are multiple interactions and higher order perturbative QCD mechanisms. We subsequently investigate the competing effects of multiple b-pair production on measurements of CP-violation: i) the increase in event rate with multiple b-pair cross sections which may reach values of order 1 barn in the presence of multiple interactions and ii) the dilution of $b$ versus $\bar b$ tagging efficiency because of the presence of events with 4 $B$-mesons. The impact of multiple $B$-meson production is small unless the cross section for producing a single pair exceeds 1~mb. We show that even for larger values of the cross section the competing effects i) and ii) roughly compensate so that there is no loss in the precision with which CP-violating CKM angles can be determined.

hep-ph↗

arXiv1994

Gluon Radiation in t\bar{t} Production at the Tevatron p\bar{p} Collider

We present a complete calculation of the matrix elements for the processes $q \bar q, gg \to b W^+ \bar b W^- g$ and $qg \to b W^+ \bar b W^- q$ which are relevant for the study of events with an additional jet in $t \bar t$ production at the Tevatron $p \bar p $ collider. Our calculation includes (i) the contributions from gluons emitted during the top production and decay stages and the interference between these, and (ii) the complete set of Feynman diagrams corresponding to both resonant and non-resonant top production. We study the distribution in phase space of the additional parton jet and make comparisons with previous studies based on the soft-gluon approximation and with results from parton-shower Monte Carlo simulations. The implications for top mass measurements are briefly discussed.

hep-ph↗

arXiv1994

Z Plus Four Jets Production in Hadronic Collisions

We present first results of $Z+4$ jet cross sections at the Tevatron $p\bar p$ collider with heavy quark flavor identification. The $Z + 4$ jet channel is of particular interest as a normalizer for the $W +4$ jet background to top quark signals, as a background to a possible $t\to cZ$ flavor-changing neutral-current (FCNC) decay signal, and as a background to missing-$p_T$ signals from gluino pairs. We also calculate the different heavy flavor contributions to $W+4$ jet production. The MADGRAPH program is used to generate all leading order subprocess helicity amplitudes. We present Monte Carlo studies with separation and acceptance criteria suitable for the Tevatron experimental analyses.

hep-ph↗

arXiv1994

Calculation of Z Plus Four Jet Production at the Tevatron

We present the first calculation of $Z+4$ jet production with heavy quark flavor identification at the Tevatron $p\bar p$ collider. The $Z + 4$ jet channel is especially interesting as a normalizer for the $W +4$ jet background to top quark signals, as a background to a possible $t\to cZ$ flavor-changing neutral-current (FCNC) decay signal, and as a background to missing-$p_T$ signals from gluino pairs. We also calculate the contributions to $W+4$ jet production from all the different heavy-flavor final states. The MADGRAPH program is used to generate all leading order subprocess helicity amplitudes. We present Monte Carlo results with separation and acceptance criteria suitable for the Tevatron experimental analyses. The dependence of the cross sections on experimental cuts and the theoretical ambiguities due to the scale dependence are discussed. The predicted ($W+4$ jet)/($Z+4$ jet) ratio is insensitive to most of these choices.

hep-ph↗

arXiv1994

Singe Top Production at LEP 200

We present exact tree level cross sections for the single top production process $e^- e^+ \rightarrow e^- \barν_e t \bar{b} $ at LEP~200. The results reproduce roughly those obtained earlier by using the equivalent real photon approximation and we confirm the observation that detecting a top heavier than half the c.m.~energy is not feasible at LEP~200. The calculation has been performed by a new automatic Feynman amplitude generator MadGraph which produces HELAS code for the helicity amplitudes.

hep-ph↗

arXiv1994

Automatic Generation of Tree Level Helicity Amplitudes

The program MadGraph is presented which automatically generates postscript Feynman diagrams and Fortran code to calculate arbitrary tree level helicity amplitudes by calling HELAS[1] subroutines. The program is written in Fortran and is available in Unix and VMS versions. MadGraph currently includes standard model interactions of QCD and QFD, but is easily modified to include additional models such as supersymmetry.

hep-ph↗

arXiv1993

Rapidity gap signals in Higgs production at the SSC

We examine the structure of the underlying event in neutral Higgs production at the Superconducting-Supercollider (SSC). Gaps, regions of rapidity containing no soft particle production, can provide a clean signature for $W$ boson fusion to the heavy Higgs. We first examine the physical basis of gap production and estimate the survival probability of gaps in the minijet model. Then, using PYTHIA, and HERWIG we compare gap events to $W$ pair production from top decay and $q\bar{q}$ fusion. We find that, if experimental problems can be overcome, gaps should provide a small, but clean, signal for heavy Higgs production at the SSC.

hep-ph↗