Standard Model Cross Section Research Articles

Searches for neutral Higgs bosons in e +e − collisions at centre-of-mass energies from 192 to 202 GeV

Searches for neutral Higgs bosons are performed with the 237 pb −1 of data collected in 1999 by the ALEPH detector at LEP, for centre-of-mass energies between 191.6 and 201.6 GeV. These searches apply to Higgs bosons within the context of the Standard Model and its minimal supersymmetric extension (MSSM) as well as to invisibly decaying Higgs bosons. No evidence of a signal is seen. A lower limit on the mass of the Standard Model Higgs boson of 107.7 GeV/ c 2 at 95% confidence level is set. In the MSSM, lower limits of 91.2 and 91.6 GeV/ c 2 are derived for the masses of the neutral Higgs bosons h and A, respectively. For a Higgs boson decaying invisibly and produced with the Standard Model cross section, masses below 106.4 GeV/ c 2 are excluded.

Estimate of the three-loop [formula omitted] contribution to σ( WL+WL−→ ZLZL)

The three-loop contribution to the MS single-Higgs-doublet standard-model cross-section σ( W L + W L −→ Z L Z L ) at s=(5 M H ) 2 is estimated via least-squares matching of the asymptotic Padé-approximant prediction of the next order term, a procedure that has been previously applied to QCD corrections to correlation functions and decay amplitudes. In contrast to these prior applications, the expansion parameter for the W L + W L −→ Z L Z L process is the non-asymptotically-free quartic scalar-field coupling of the standard model, suggesting that the least-squares matching be performed over the “infrared” μ 2⩽ s region of the scale parameter. All three coefficients of logarithms within the three-loop term obtained by such matching are found to be within 6.6% relative error of their true values, as determined via renormalization-group methods. Surprisingly, almost identical results are obtained by performing the least squares matching over the μ 2⩾ s region.

Search for γγ decays of a Higgs boson produced in association with a fermion pair in e +e − collisions at LEP

A search for γγ decays of a Higgs boson is performed in the data sample collected at LEP with the ALEPH detector between 1991 and 1999. This corresponds to an integrated luminosity of 672 pb −1 at centre-of-mass energies ranging from 88 to 202 GeV. The search is based on topologies arising from a Higgs boson produced in association with a fermion pair via the Higgs-strahlung process e + e −→Hf f ̄ , with f f ̄ =ν ν ̄ , e + e −,μ +μ −,τ +τ − or q q ̄ . Twenty-two events are selected in the data, while 28 events are expected from standard model processes. An upper limit is derived, as a function of the Higgs boson mass, on the product of the e + e −→Hf f ̄ cross section and the H→γγ branching fraction. In particular, a fermiophobic Higgs boson produced with the standard model cross section is excluded at 95% confidence level for all masses below 100.7 GeV/c 2 .

Direct collider signatures of large extra dimensions

The realization of low (TeV) scale strings usually requires the existence of large (TeV) extra dimensions where gauge bosons live. The direct production of Kaluza–Klein excitations of the photon and Z-boson at present and future colliders is studied in this work. At the LEPII, NLC and Tevatron colliders, these Kaluza–Klein modes lead to deviations from the standard model cross-sections, which provide lower bounds on their mass. At the LHC the corresponding resonances can be produced and decay on-shell, triggering a characteristic pattern in the distribution of dilepton invariant mass.

Very high Q2 physics at HERA

Recent measurements from the H1 and ZEUS experiments at HERA in deep-inelastic ep scattering at very high values of the squared 4-momentum transfer Q 2 are discussed and compared to predictions of the Standard Model. Both experiments observe a large excess of events in neutral current (NC) scattering, ep → eX, at values of Q 2 > 15 000 GeV 2 and a small excess of events in charged current (CC) scattering, ep → νX. The probability that the observed event rates correspond to a statistical fluctuation compatible with the Standard Model cross sections is about 1% for NC reactions. This observation has driven speculations about possible physics beyond the Standard Model.

Large uncertainties in the cross section of elastic WL+WL- scattering.

The amplitudes for $2\rightarrow 2$ scattering processes involving longitudinally polarized gauge bosons $( W_L^\pm, Z_L )$ and the Higgs boson are analyzed up to two loops. Assuming $M_H >> M_W$, the trilinear Higgs coupling, $\lambda v$, is dominant for energies of $\sqrt{s}$ < 1.5 -- 2 $M_H$. For larger values of $\sqrt{s}$, the quartic coupling, $\lambda$, becomes dominant, allowing for a simpler calculation of higher-order corrections. The high-energy amplitudes display a large logarithmic dependence on $\sqrt{s}$ which can be resummed using renormalization group techniques. The resummation of leading-log terms is sufficient for Higgs masses of less than 350 GeV. For 350 < $M_H$ < 450 GeV, a next-to-leading-log resummation is necessary. For even larger values of $M_H$, the perturbative approach fails completely since two-loop terms become in magnitude larger than one-loop terms. Choosing the $\overline{{\rm MS}}$ renormalization scheme instead of the OMS scheme, the coefficients of the perturbative series increase in magnitude, making the breakdown of perturbation theory even more apparent. In conclusion, the Standard Model cross sections presented here have very large uncertainties if $M_H\gtrsim 450$ GeV and $\sqrt{s} \gtrsim 2 M_H$, reducing the sensitivity to contributions from new physics significantly.

Single lepton production at large transverse momentum

We study the production of single isolated leptons at large transverse momentum, p T ℓ> 1 2 M w . The dominant source of such leptons is production of an on-shell W boson recoiling against a hard jet. Vetoing this jet forces the W boson to be produced off resonance and significantly reduces the standard model cross section, thereby enhancing the discovery prospects for non standard model physics. A significant number of events have the topologically interesting signature of a large p T lepton balancing a jet with little measured missing transverse energy.

Minimal total cross section fore + e −→W + W −

The total cross section with the most general form for the gauge boson vertices is given at tree level for the processe + e −→W + W −. Each vertex is parametrized with 7 couplings and the minimum of the total cross section is studied in a wide energy range. Although there are large cancellations between the neutrino exchange diagram and the photon andZ annihilation diagrams, it is surprisingly not possible to get significantly below the standard model cross section for any value of the parameters.