The inclusive production of charged hadrons in the collisions of quasi-real photons e+e- -> e+e- +X has been measured using the OPAL detector at LEP. The data were taken at e+e- centre-of-mass energies from 183 to 209 GeV. The differential cross-sections as a function of the transverse momentum and the pseudorapidity of the hadrons are compared to theoretical calculations of up to next-to-leading order (NLO) in the strong coupling constant alpha{s}. The data are also compared to a measurement by the L3 Collaboration, in which a large deviation from the NLO predictions is observed.
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The Mark J Collaboration at the DESY e+e− collider PETRA presents results on the electroweak reactions e+e−→μ+μ−τ+τ−,μ+μ−γ, and e+e−μ+μ−. The c.m. energy range is 12 to 46.78 GeV. In the μ+μ− and τ+τ− channels the total cross sections and the forward-backward asymmetries are reported and compared with other experiments. The results are in excellent agreement with the standard model. The weak-neutral-current vector and axial-vector coupling constants are determined. The values for muons and τ’s are compatible with universality and with the predictions of the standard model. In the μ+μ−γ channel, all measured distributions, including the forward-backward muon asymmetry, are in excellent agreement with the electroweak theory. Our data on the two-photon process, e+e−μ+μ−, agrees with QED to order α4 over the entire energy range and the Q2 range from 0.7 to 166 GeV2.
We report a photon-photon experiment performed at the Orsay storage rings. 300e + e − , μ + μ − and π + π − pairs produced with low invariant masses have been observed. For each event, one or both protons have been tagged at a very small angle. The γγ→μ + μ − and γγ→π + π − cross-sections have been measured near threshold, the γγ→e + e − process being used as a normalization. The observed invariant mass distribution is compared to theoretical calculations for each of the three processes. The μ + μ − data are in good agreement with QED predictions. The π + π − cross-section, in our experimental acceptance, is somewhat larger than the one expected from the Born terms only.
Large transverse energy cross sections of 300 GeV/ c pions and protons on hydrogen have been measured with a segmented calorimeter covering the central rapidity region −0.88 < y < 0.67 and 2 π in azimuth. The selected events show large multiplicities and no jet-like event structure. Processes more complicated than the scattering of two constituents appear to dominate these inelastic collisions.
The charged-particle fractional momentum distribution within jets, D(z), has been measured in dijet events from 1.8-TeV p¯p collisions in the Collider Detector at Fermilab. As expected from scale breaking in quantum chromodynamics, the fragmentation function D(z) falls more steeply as dijet invariant mass increases from 60 to 200 GeV/c2. The average fraction of the jet momentum carried by charged particles is 0.65±0.02(stat)±0.08(syst).
We analyze a sample of W + jet events collected with the Collider Detector at Fermilab (CDF) in ppbar collisions at sqrt(s) = 1.8 TeV to study ttbar production. We employ a simple kinematical variable "H", defined as the scalar sum of the transverse energies of the lepton, neutrino and jets. For events with a W boson and four or more jets, the shape of the "H" distribution deviates by 3.8 standard deviations from that expected from known backgrounds to ttbar production. However this distribution agrees well with a linear combination of background and ttbar events, the agreement being best for a top mass of 180 GeV/c^2.
Momenta of charged particles produced in inelastic αα, αp, andpp collisions were measured using the Split-Field-Magnet detector at the CERN Intersecting Storage Rings. Inclusive and semi-in-clusive spectra are presented as a function of rapidityy, Feynman-x, and transverse momentumpT. The inclusivey distributions agree well with predictions of the dual parton model; the highest particle densities are reached aty≃0 and the momenta of leading protons decrease significantly for increasing total multiplicity. ‘Temperatures’ are equal in αα, αp, andpp interactions. ThepT distributions depend weakly on the multiplicity.
Balance functions have been measured in terms of relative pseudorapidity ($\Delta \eta$) for charged particle pairs at the Relativistic Heavy-Ion Collider (RHIC) from Au+Au collisions at $\sqrt{s_{\rm NN}}$ = 7.7 GeV to 200 GeV using the STAR detector. These results are compared with balance functions measured at the Large Hadron Collider (LHC) from Pb+Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV by the ALICE Collaboration. The width of the balance function decreases as the collisions become more central and as the beam energy is increased. In contrast, the widths of the balance functions calculated using shuffled events show little dependence on centrality or beam energy and are larger than the observed widths. Balance function widths calculated using events generated by UrQMD are wider than the measured widths in central collisions and show little centrality dependence. The measured widths of the balance functions in central collisions are consistent with the delayed hadronization of a deconfined quark gluon plasma (QGP). The narrowing of the balance function in central collisions at $\sqrt{s_{\rm NN}}$ = 7.7 GeV implies that a QGP is still being created at this relatively low energy.
New results on the production of charged pions in p+p interactions are presented. The data come from a sample of 4.8 million inelastic events obtained with the NA49 detector at the CERN SPS at 158 GeV/c beam momentum. Pions are identified by energy loss measurement in a large TPC tracking system which covers a major fraction of the production phase space. Inclusive invariant cross sections are given on a grid of nearly 300 bins per charge over intervals from 0 to 2 GeV/c in transverse momentum and from 0 to 0.85 in Feynman x. The results are compared to existing data in overlapping energy ranges.
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