We report results on rho(770)^0 -> pi+pi- production at midrapidity in p+p and peripheral Au+Au collisions at sqrt(s_NN) = 200 GeV. This is the first direct measurement of rho(770)^0 -> pi+pi- in heavy-ion collisions. The measured rho^0 peak in the invariant mass distribution is shifted by ~40 MeV/c^2 in minimum bias p+p interactions and ~70 MeV/c^2 in peripheral Au+Au collisions. The rho^0 mass shift is dependent on transverse momentum and multiplicity. The modification of the rho^0 meson mass, width, and shape due to phase space and dynamical effects are discussed.
The raw $\pi^{+} \pi^{-}$ invariant mass distributions after subtraction of the like-sign reference distribution for minimum bias p+p (top) and peripheral Au+Au (bottom) interactions.
The raw $\pi^{+} \pi^{-}$ invariant mass (solid line) and the like-sign reference distributions (open circles) for peripheral Au+Au collisions.
The $\rho^{0}$ mass as a function of $p_{T}$ for minimum bias $p$+$p$ (filled circles), high multiplicity $p$+$p$ (open triangles), and peripheral Au+Au (filled squares) collisions. The error bars indicate the systematic uncertainty. Statistical errors are negligible. The $\rho^{0}$ mass was obtained by fitting the data to the BW×PS functional form described in the text. The dashed lines represent the average of the $\rho^{0}$ mass measured in $e^{+} e^{−}$. The shaded areas indicate the ρ0 mass measured in $p$+$p$ collisions. The open triangles have been shifted downward on the abscissa by $50$ MeV/$c$ for clarity.
A large solid angle detector has been used to observe two body events produced by electron-positron collisions in the Orsay storage ring. From the π + π − excitation curve in the ϱ region we have deduced the amplitude and the phase of the ω-ϱ interference, and the ϱ resonance paramaters: M ϱ = (775.4±7.3) MeV, Γ ϱ = (149.6 ± 23.2) MeV, √ B ( ω → π + π − ) = 0.19 ± 0.05, φ = (85.7 ± 15.3) 0 , σ ( e + e − → ϱ ) = (1.00 ± 0.13) μ b at S = M ϱ 2 , B ( ϱ → e + e − = (4.1 ± 0.5) × 10 −5 , Γ ( ϱ → e + e − ) = (6.1 ± 0.7) keV, ( g ϱ 2 /4 π ) = 2.26 ± 0.25, ( g ϱππ 2 /4 π ) = 2.84 ± 0.50.
STATISTICAL ERRORS ONLY. CROSS SECTION AT RHO0 PEAK IS 1.00 +- 0.13 MUB FROM FIT.
Production of φ mesons in e+e− annihilation at a center-of-mass energy of 29 GeV has been observed with the time-projection chamber detector at the PEP storage ring. The φ production rate has been measured in the energy range 0.075<x<0.55 (x=2Eφs), giving 0.077±0.012(stat)±0.016(syst) φ's per event. The average value of pt2 relative to the thrust axis is 1.0±0.4 (GeV/c)2.
No description provided.
EXTRAPOLAATION TO ALL X USES LUND MONTE CARLO PREDICTIONS.
ERRORS ARE BOTH STATISTICAL AND SYSTEMATIC. PT IS MEASURED RELATIVE TO THE EVENT THRUST AXIS, AND IS FOUND TO HAVE A MEAN VALUE OF 1.0 +- 0.4 GEV.
We present a study of Z +gamma + X production in p-bar p collisions at sqrt{S}=1.8 TeV from 97 (87) pb^{-1} of data collected in the eegamma (mumugamma) decay channel with the D0 detector at Fermilab. The event yield and kinematic characteristics are consistent with the Standard Model predictions. We obtain limits on anomalous ZZgamma and Zgammagamma couplings for form factor scales Lambda = 500 GeV and Lambda = 750 GeV. Combining this analysis with our previous results yields 95% CL limits |h{Z}_{30}| < 0.36, |h{Z}_{40}| < 0.05, |h{gamma}_{30}| < 0.37, and |h{gamma}_{40}| < 0.05 for a form factor scale Lambda=750 GeV.
CONST(NAME=SCALE) is the model parameter, used in the modification of the couplings as follows: h = hi0/(1 + M(gamma Z)**2/CONT(NAME=SCALE)**2)**n. See article for details.
A study of Z boson pair production in e+e- annihilation at center-of-mass energies near 183 GeV and 189 GeV is reported. Final states containing only leptons, (l+l-l+l- and l+l-nu nubar), quark and lepton pairs, (q qbar l+l-, q qbar nu nubar) and the all-hadronic final state (q qbar q qbar) are considered. In all states with at least one Z boson decaying hadronically, q qbar and b bbar final states are considered separately using lifetime and event-shape tags, thereby improving the cross-section measurement. At sqrt(s) = 189 GeV the Z-pair cross section was measured to be 0.80 (+0.14-0.13, stat.) (+0.06-0.05, syst.) pb, consistent with the Standard Model prediction. At sqrt(s) = 183 GeV the 95% C.L. upper limit is 0.55 pb. Limits on anomalous ZZgamma and ZZZ couplings are derived.
Measured cross sections for Z0 pair production.
We report measurements of Xi and Xi-bar hyperon absolute yields as a function of rapidity in 158 GeV/c Pb+Pb collisions. At midrapidity, dN/dy = 2.29 +/- 0.12 for Xi, and 0.52 +/- 0.05 for Xi-bar, leading to the ratio of Xi-bar/Xi = 0.23 +/- 0.03. Inverse slope parameters fitted to the measured transverse mass spectra are of the order of 300 MeV near mid-rapidity. The estimated total yield of Xi particles in Pb+Pb central interactions amounts to 7.4 +/- 1.0 per collision. Comparison to Xi production in properly scaled p+p reactions at the same energy reveals a dramatic enhancement (about one order of magnitude) of Xi production in Pb+Pb central collisions over elementary hadron interactions.
From a data sample of 183 pb^-1 recorded at a center-of-mass energy of roots = 189 GeV with the OPAL detector at LEP, 3068 W-pair candidate events are selected. Assuming Standard Model W boson decay branching fractions, the W-pair production cross section is measured to be sigmaWW = 16.30 +- 0.34(stat.) +- 0.18(syst.) pb. When combined with previous OPAL measurements, the W boson branching fraction to hadrons is determined to be 68.32 +- 0.61(stat.) +- 0.28(syst.) % assuming lepton universality. These results are consistent with Standard Model expectations.
Total W+ W- pair production cross section.
Cross sections for the individual decay modes of the W+ W-.
A study of W-pair production in e+e- annihilations at Lep2 is presented, based on 877 W+W- candidates corresponding to an integrated luminosity of 57 pb-1 at sqrt(s) = 183 GeV. Assuming that the angular distributions of the W-pair production and decay, as well as their branching fractions, are described by the Standard Model, the W-pair production cross-section is measured to be 15.43 +- 0.61 (stat.) +- 0.26 (syst.) pb. Assuming lepton universality and combining with our results from lower centre-of-mass energies, the W branching fraction to hadrons is determined to be 67.9 +- 1.2 (stat.) +- 0.5 (syst.)%. The number of W-pair candidates and the angular distributions for each final state (qqlnu,qqqq,lnulnu) are used to determine the triple gauge boson couplings. After combining these values with our results from lower centre-of-mass energies we obtain D(kappa_g)=0.11+0.52-0.37, D(g^z_1)=0.01+0.13-0.12 and lambda=-0.10+0.13-0.12, where the errors include both statistical and systematic uncertainties and each coupling is determined setting the other two couplings to the Standard Model value. The fraction of W bosons produced with a longitudinal polarisation is measured to be 0.242+-0.091(stat.)+-0.023(syst.). All these measurements are consistent with the Standard Model expectations.
Total W+ W- cross section measurement. The DSYS error corresponds to the total systematic error.
Cross section for W+ W- production in different decay channels. The DSYS error corresponds to the total systematic error.
A search for the leptonic decays of W bosons produced in the reaction e^+ p\to e^+ W^\pm X at a centre-of-mass energy of 300 GeV has been performed with the ZEUS detector at HERA using an integrated luminosity of 47.7 pb^-1 . Three events consistent with W\to e\nu decay are found, giving a cross section of 0.9 +1.0 -0.7 \pm 0.2 pb, in good agreement with the Standard Model prediction. The corresponding 95% C.L. upper limit on the cross section is 3.3 pb. A search for the decay W\to \mu\nu has a smaller selection efficiency and yields no candidate events. Limits on the cross section for W production with large hadronic transverse momentum have been obtained. A search for high-transverse-momentum isolated tracks in events with large missing transverse momentum yields results in good agreement with Standard Model expectations, in contrast to a recent report by the H1 collaboration of the observation of an excess of such events.
Measured cross section from three events.
95 PCT CONFIDENCE UPPER LIMIT TO THE PROCESS.
Elements of the spin density matrix for W bosons in e+e- -> W+W- -> qqln events are measured from data recorded by the OPAL detector at LEP. This information is used calculate polarised differential cross-sections and to search for CP-violating effects. Results are presented for W bosons produced in e+e- collisions with centre-of-mass energies between 183 GeV and 209 GeV. The average fraction of W bosons that are longitudinally polarised is found to be (23.9 +- 2.1 +- 1.1)% compared to a Standard Model prediction of (23.9 +- 0.1)%. All results are consistent with CP conservation.
The fraction of longitudinal polarization for leptonically and hadronically decaying W bosons. The average values for all the centre of mass energies and for both lepton and hadron decay combined are also given.
The luminosity weighted average over all the centre of mass energies of the diagonal elements of the RHO++ and RHO-- SDM as a function of the cosine of the angle of the W- boson for the leptonic decay channel.
The luminosity weighted average over all the centre of mass energies of the diagonal element of the RHO00 SDM as a function of the cosine of the angle of the W- boson for both leptonic and hadronic decay channels, and combined.
Forum