Breakdown of the Narrow Width Approximation for New Physics

The narrow width approximation is used in high energy physics to reduce the complexity of scattering calculations. It is a fortunate accident that it works so well for the Standard Model, but in general it will fail in the context of new physics. We find numerous examples of significant corrections when the calculation is performed fully off-shell including a finite width, notably from effects from the decay matrix elements. If not taken into account, attempts to reconstruct the Lagrangian of a new physics discovery from data would result in considerable inaccuracies and likely inconsistencies.Comment: 4 p., 3 figs, comments clarified, version to appear in PR

Finite-Width Effects in Top Quark Production at Hadron Colliders

Production cross sections for t\bar{t} and t\bar{t}j events at hadron colliders are calculated, including finite width effects and off resonance contributions for the entire decay chain, t --> bW --> b\ell\nu, for both top quarks. Resulting background rates to Higgs search at the CERN LHC are updated for inclusive H --> WW studies and for H --> \tau\tau and H --> WW decays in weak boson fusion events. Finite width effects are large, increasing t\bar{t}(j) rates by 20% or more, after typical cuts which are employed for top-background rejection

Top Background Extrapolation for H -> WW Searches at the LHC

A leading order (LO) analysis is presented that demonstrates that key top backgrounds to H -> W^+W^- -> l^\pm l^\mp \sla{p}_T decays in weak boson fusion (WBF) and gluon fusion (GF) at the CERN Large Hadron Collider can be extrapolated from experimental data with an accuracy of order 5% to 10%. If LO scale variation is accepted as proxy for the theoretical error, parton level results indicate that the tt~j background to the H -> WW search in WBF can be determined with a theoretical error of about 5%, while the tt~ background to the H -> WW search in GF can be determined with a theoretical error of better than 1%. Uncertainties in the parton distribution functions contribute an estimated 3% to 10% to the total error.Comment: 17 pages, 9 tables, 4 figures; LO caveat emphasized, version to be published in Phys. Rev.

Robust LHC Higgs Search in Weak Boson Fusion

We demonstrate that an LHC Higgs search in weak boson fusion production with subsequent decay to weak boson pairs is robust against extensions of the Standard Model or MSSM involving a large number of Higgs doublets. We also show that the transverse mass distribution provides unambiguous discrimination of a continuum Higgs signal from the Standard Model.Comment: 12p, 2 figs., additional comments on backgrounds, version to appear in PR

NLO QCD predictions for internal jet shapes in DIS at HERA

The transverse momentum flow inside jets is a sensitive measure of internal jet structure. For the current jets in deep inelastic scattering this jet shape measure is determined at order alpha_s^2, i.e. with up to three partons inside a single jet. The scale dependence of jet shapes in various jet algorithms is discussed. Results agree well with recent measurements by the ZEUS Collaboration, without introducing the hadronization parameter R_sep.Comment: 10 pages, LaTeX2.09, REVTeX3.1, 9 postscript figure

A new method for extracting the bottom quark Yukawa coupling at the CERN Large Hadron Collider

We propose a new method for measuring the H -> bb rate at the CERN LHC in a manner which would allow extraction of the b quark Yukawa coupling. Higgs boson production in purely electroweak WHjj events is calculated. The Standard Model signal rate including decays W -> l nu and H -> bb is 11 fb for M_H = 120 GeV. It is possible to suppress the principal backgrounds, Wbbjj and ttjj, to approximately the level of the signal. As the top quark Yukawa coupling does not appear in this process, it promises a reliable extraction of g_Hbb in the context of the Standard Model or some extensions, such as the MSSM.Comment: added background, updated/added references, additional comment

Next-to-leading order jet distributions for Higgs boson production via weak-boson fusion

The weak-boson fusion process is expected to provide crucial information on Higgs boson couplings at the Large Hadron Collider at CERN. The achievable statistical accuracy demands comparison with next-to-leading order QCD calculations, which are presented here in the form of a fully flexible parton Monte Carlo program. QCD corrections are determined for jet distributions and are shown to be modest, of order 5 to 10% in most cases, but reaching 30% occasionally. Remaining scale uncertainties range from order 5% or less for distributions to below +-2% for the Higgs boson cross section in typical weak-boson fusion search regions.Comment: 19 pages, 8 figure

Top Pair Production Beyond Double-Pole Approximation: pp, pp~ --> 6 Fermions and 0, 1 or 2 Additional Partons

Hadron collider cross sections for tt~ production and di-lepton, single-lepton and all-jet decays with up to 2 additional jets are calculated using complete LO matrix elements with 6-, 7- and 8-particle final states. The fixed-width, complex-mass and overall-factor schemes (FWS, CMS & OFS) are employed and the quality of narrow-width and double-pole approximations (NWA & DPA) is investigated for inclusive production and suppressed backgrounds to new particle searches. NWA and DPA cross sections differ by 1% or less. The inclusion of sub- and non-resonant amplitudes effects a cross section increase of 5-8% at pp supercolliders, but only minor changes at the Tevatron. On-shell tt~/Wtb backgrounds for the H --> WW decay in weak boson fusion, the hadronic \tau decay of a heavy H^\pm and the \phi --> hh --> \tau\tau bb~ radion decay at the LHC are updated, with corrections ranging from 3% to 30%. FWS and CMS cross sections are uniformly consistent, but OFS cross sections are up to 6% smaller for some backgrounds.Comment: 20 pages, 6 tables, 1 figur

QCD corrections to vector boson fusion processes

NLO QCD corrections to H, W and Z production via vector boson fusion have recently been calculated in the form of flexible parton level Monte Carlo programs. This allows for the calculation of distributions and cross sections with cuts at NLO accuracy. Some features of the calculation, as well as results for the LHC, are reviewed.Comment: 1 style file, 1 latex file, 5 figures, Contribution to the Proceedings of "Loops and Legs in Quantum Field Theory, 2004", Zinnowitz, Usedom Island, Germany, April, 200

Three Numerical Puzzles and the Top Quark's Chiral Weak-Moment

Versus the standard model's t --> W b decay helicity amplitudes, three numerical puzzles occur at the 0.1 % level when one considers the amplitudes in the case of an additional (f_M + f_E) coupling of relative strength 53 GeV. The puzzles are theoretical ones which involve the t --> W b decay helicity amplitudes in the two cases, the relative strength of this additional coupling, and the observed masses of these three particles. A deeper analytic realization is obtained for two of them. Equivalent realizations are given for the remaining one. An empirical consequence of these analytic realizations is that it is important to search for effects of a large chiral weak-moment of the top-quark, the effective mass-scale is about 53 GeV. A full theoretical resolution would include relating the origin of such a chiral weak-moment and the mass generation of the top-quark, the W-boson, and probably the b-quark.Comment: 18 pages, 1 postscript table (revised to better explain notation, model #1, add a little material...