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Electroweak Measurements in Electron-Positron Collisions at W-Boson-Pair Energies at LEP

The ALEPH Collaboration , The DELPHI Collaboration , The L3 Collaboration , The OPAL Collaboration , The LEP Electroweak Working Group
14 Feb 2013 hep-ex hep-ph arxiv.org/abs/1302.3415

Electroweak measurements performed with data taken at the electron-positron collider LEP at CERN from 1995 to 2000 are reported. The combined data set considered in this report corresponds to a total luminosity of about 3 fb$^{-1}$ collected by the four LEP experiments ALEPH, DELPHI, L3 and OPAL, at centre-of-mass energies ranging from 130 GeV to 209 GeV. Combining the published results of the four LEP experiments, the measurements include total and differential cross-sections in photon-pair, fermion-pair and four-fermion production, the latter resulting from both double-resonant WW and ZZ production as well as singly resonant production. Total and differential cross-sections are measured precisely, providing a stringent test of the Standard Model at centre-of-mass energies never explored before in electron-positron collisions. Final-state interaction effects in four-fermion production, such as those arising from colour reconnection and Bose-Einstein correlations between the two W decay systems arising in WW production, are searched for and upper limits on the strength of possible effects are obtained. The data are used to determine fundamental properties of the W boson and the electroweak theory. Among others, the mass and width of the W boson, $M_W$ and $\Gamma_W$, the branching fraction of W decays to hadrons, $B(W\to\mathrm{had})$, and the trilinear gauge-boson self-couplings $g^Z_1$, $\kappa_\gamma$ and $\lambda_\gamma$ are determined to be: M_W = 80.376 \pm 0.033 ~GeV \Gamma_W = 2.195 \pm 0.083 ~GeV B(W\to\mathrm{had}) = 67.41 \pm 0.27 % g^Z_1 = 0.984 ^{+0.018}{-0.020} \kappa\gamma = 0.982 \pm 0.042 \lambda_\gamma = -0.022 \pm 0.019

Measurements of CP-conserving Trilinear Gauge Boson Couplings WWV (V = gamma,Z) in e+e- Collisions at LEP2

The DELPHI Collaboration , Abdallah J.
03 Feb 2010 hep-ex arxiv.org/abs/1002.0752

The data taken by DELPHI at centre-of-mass energies between 189 and 209 GeV are used to place limits on the CP-conserving trilinear gauge boson couplings Delta_g1z, lambda_gamma and Delta_kappag associated to W+W- and single W production at LEP2. Using data from the jjlv, jjjj, jjX and lX final states, where j, l and X represent a jet, a lepton and missing four-momentum, respectively, the following limits are set on the couplings when one parameter is allowed to vary and the others are set to their Standard Model values of zero: Delta_g1z = -0.025^{+0.033}{-0.030}, lambda_gamma = 0.002^{+0.035}{-0.035} and Delta_kappag = 0.024^{+0.077}_{-0.081} . Results are also presented when two or three parameters are allowed to vary. All observations are consistent with the predictions of the Standard Model and supersede the previous results on these gauge coupling parameters published by DELPHI.

Correlations between Polarisation States of W Particles in the Reaction e-e+ -> W-W+ at LEP2 Energies 189-209 GeV

The DELPHI Collaboration , Abdallah J.
07 Aug 2009 hep-ex arxiv.org/abs/0908.1023

In a study of the reaction e-e+ -> W-W+ with the DELPHI detector, the probabilities of the two W particles occurring in the joint polarisation states transverse-transverse (TT), longitudinal-transverse plus transverse-longitudinal (LT) and longitudinal-longitudinal (LL) have been determined using the final states WW -> l nu q qbar (l = e, mu). The two-particle joint polarisation probabilities, i.e. the spin density matrix elements rho_TT, rho_LT, rho_LL, are measured as functions of the W- production angle, theta_W-, at an average reaction energy of 198.2 GeV. Averaged over all cos(theta_W-), the following joint probabilities are obtained: rho_TT = (67 +/- 8)%, rho_LT = (30 +/- 8)%, rho_LL = (3 +/- 7)% . These results are in agreement with the Standard Model predictions of 63.0%, 28.9% and 8.1%, respectively. The related polarisation cross-sections sigma_TT, sigma_LT and sigma_LL are also presented.

Inclusive single-particle production in two-photon collisions at LEP II with the DELPHI detector

The DELPHI Collaboration , Abdallah J.
29 Jun 2009 hep-ex arxiv.org/abs/0906.5302

A study of the inclusive charged hadron production in two-photon collisions is described. The data were collected with the DELPHI detector at LEP II. Results on the inclusive single-particle p_T distribution and the differential charged hadrons dsigma/dp_T cross-section are presented and compared to the predictions of perturbative NLO QCD calculations and to published results.

Di-jet production in gamma-gamma collisions at LEP2

The DELPHI Collaboration , Abdallah J.
28 Jan 2009 hep-ex arxiv.org/abs/0901.4500

The production of two high-p_T jets in the interactions of quasi-real photons in e+e- collisions at sqrt{s_ee} from 189 GeV to 209 GeV is studied with data corresponding to an integrated e+e- luminosity of 550 pb^{-1}. The jets reconstructed by the k_T cluster algorithm are defined within the pseudo-rapidity range -1 < eta < 1 and with jet transverse momentum, p_T, above 3 GeV/c. The differential di-jet cross-section is measured as a function of the mean transverse momentum ptmean of the jets and is compared to perturbative QCD calculations.

Observation of the Muon Inner Bremsstrahlung at LEP1

The DELPHI Collaboration , Abdallah J.
28 Jan 2009 hep-ex arxiv.org/abs/0901.4488

Muon bremsstrahlung photons converted in front of the DELPHI main tracker (TPC) in dimuon events at LEP1 were studied in two photon kinematic ranges: 0.2 < E_gamma <= 1 GeV and transverse momentum with respect to the parent muon p_T < 40 MeV/c, and 1 < E_gamma <= 10 GeV and p_T < 80 MeV/c . A good agreement of the observed photon rate with predictions from QED for the muon inner bremsstrahlung was found, contrary to the anomalous soft photon excess that has been observed recently in hadronic Z^0 decays. The obtained ratios of the observed signal to the predicted level of the muon bremsstrahlung are 1.06 +/- 0.12 +/- 0.07 in the photon energy range 0.2 < E_gamma <= 1 GeV and 1.04 +/- 0.09 +/- 0.12 in the photon energy range 1 < E_gamma <= 10 GeV. The bremsstrahlung dead cone is observed for the first time in the direct photon production at LEP.

Study of b-quark mass effects in multijet topologies with the DELPHI detector at LEP

The DELPHI Collaboration , Abdallah J.
24 Apr 2008 hep-ex arxiv.org/abs/0804.3883

The effect of the heavy b-quark mass on the two, three and four-jet rates is studied using LEP data collected by the DELPHI experiment at the Z peak in 1994 and 1995. The rates of b-quark jets and light quark jets (l = uds) in events with n = 2, 3, and 4 jets, together with the ratio of two and four-jet rates of b-quarks with respect to light-quarks, R_n^bl, have been measured with a double-tag technique using the CAMBRIDGE jet-clustering algorithm. A comparison between experimental results and theory (matrix element or Monte Carlo event generators such as PYTHIA, HERWIG and ARIADNE) is done after the hadronisation phase. Using the four-jet observable R_4^bl, a measurement of the b-quark mass using massive leading-order calculations gives: m_b(M_Z) = 3.76 +/- 0.32 (stat) +/- 0.17 (syst) +/- 0.22 (had) +/- 0.90 (theo) GeV/c^2 . This result is compatible with previous three-jet determinations at the M_Z energy scale and with low energy mass measurements evolved to the M_Z scale using QCD Renormalisation Group Equations.

Measurement of the Mass and Width of the W Boson in e+e- Collisions at sqrt{s} = 161 - 209 GeV

The DELPHI Collaboration , Abdallah J.
17 Mar 2008 hep-ex arxiv.org/abs/0803.2534

A measurement of the W boson mass and width has been performed by the DELPHI collaboration using the data collected during the full LEP2 programme (1996-2000). The data sample has an integrated luminosity of 660 pb^-1 and was collected over a range of centre-of-mass energies from 161 to 209 GeV. Results are obtained by applying the method of direct reconstruction of the mass of the W from its decay products in both the W+W- -> lvqq and W+W- -> qqqq channels. The W mass result for the combined data set is M_W = 80.336 +/- 0.055 (Stat.) +/- 0.028 (Syst.) +/- 0.025 (FSI) +/- 0.009 (LEP) GeV/c^2, where FSI represents the uncertainty due to final state interaction effects in the qqqq channel, and LEP represents that arising from the knowledge of the collision energy of the accelerator. The combined value for the W width is Gamma_W = 2.404 +/- 0.140 (Stat.) +/- 0.077 (Syst.) +/- 0.065 (FSI) GeV/c^2. These results supersede all values previously published by the DELPHI collaboration. This paper is dedicated to the memory of Carlo Caso.

Study of W boson polarisations and Triple Gauge boson Couplings in the reaction e+e- -> W+W- at LEP 2

The DELPHI Collaboration , Abdallah J.
08 Jan 2008 hep-ex arxiv.org/abs/0801.1235

A determination of the single W Spin Density Matrix (SDM) elements in the reaction e+e- -> W+W- -> l nu q qbar (l=e/mu) is reported at centre-of-mass energies between 189 and 209 GeV. The data sample used corresponds to an integrated luminosity of 520 pb^{-1} taken by DELPHI between 1998 and 2000. The single W SDM elements, rho_{tau tau'}^{W+-} (tau,tau' = +/-1 or 0), are determined as a function of the W- production angle with respect to the e- beam direction and are obtained from measurements of the W decay products by the application of suitable projection operators, Lambda_{tau tau'}, which assume the V-A coupling of the W boson to fermions. The measured SDM elements are used to obtain the fraction of longitudinally polarised Ws, with the result: sigma_L/sigma_tot = 24.9 +/- 4.5(stat) +/- 2.2(syst) % at a mean energy of 198 GeV. The SDM elements are also used to determine the Triple Gauge Couplings Delta g_1^Z, Delta kappa_gamma, lambda_gamma and g_4^Z, kappa_Z and lambda_Z. For the CP-violating couplings the results of single parameter fits are: g_4^Z = -0.39 +0.19 -0.20 kappa_Z = -0.09 +0.08 -0.05 lambda_Z = -0.08 +/- 0.07 . The errors are a combination of statistical and systematic errors. All results are consistent with the Standard Model.

Search for Pentaquarks in the Hadronic Decays of the Z Boson with the DELPHI Detector at LEP

The DELPHI Collaboration , Abdallah J.
02 Aug 2007 hep-ex arxiv.org/abs/0708.0415

The quark model does not exclude states composed of more than three quarks, like pentaquark systems. Controversial evidence for such states has been published in the last years, in particular: for a strange pentaquark Theta(1540)+; for a double-strange state, the Xi(1862)--, subsequently called Phi(1860)--; and for a charmed state, the Theta_c(3100)^0. If confirmed, a full pentaquark family might exist; such pentaquark states could be produced in e+e- annihilations near the Z energy. In this paper a search for pentaquarks is described using the DELPHI detector at LEP, characterized by powerful particle identification sub-systems crucial in the separation of the signal from the background for these states. At 95% CL, upper limits are set on the production rates of such particles and their charge-conjugate state per Z decay: <N_Theta+> x Br(Theta+ -> pK0_S) < 5.1 x 10^{-4} <N_Theta++> < 1.6 x 10^{-3} <N_Phi(1860)-- > x Br(Phi(1860)-- -> Xi- pi-) < 2.9 x 10^{-4} <Theta_c(3100)^0> x Br(Theta_c(3100)^0 -> D*+ pbar) < 8.8 x 10^{-4} .