The e + e − → μ + μ − reaction has been studied at centre of mass energies ranging between 38.3 abd 46.8 GeV with the CELLO detector at PETRA. We present results on the cross section and the charge asymmetry for this channel. Combining all the data at the average energy 〈 s 〉=43 GeV we obtain R μμ =〈 σ μμ / σ 0 〉=0.98±0.04±0.04, 〈 A μμ 〉=(−14.1±3.7±1.0)%, where σ 0 is the QED cross section and A μμ is the charge asymmetry corrected for pure radiative effects. These results are in good agreement with the expected values of R μμ =1.01 and A μμ =−14.5% at that energy.
Forward-backward asymmetry.
An analysis of the data collected in 1997 and 1998 with the DELPHI detector at e+e- collision energies close to 183 and 189 GeV was performed in order to extract the hadronic and leptonic fermion-pair cross-sections, as well as the leptonic forward-backward asymmetries and angular distributions. The data are used to put limit on contact interactions between fermions, the exchange of R-parity violating SUSY sneutrinos, Z' bosons and the existence of gravity in extra dimensions.
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Production of events with hadronic and leptonic final states has been measured in e^+e^- collisions at centre-of-mass energies of 130-172 GeV, using the OPAL detector at LEP. Cross-sections and leptonic forward-backward asymmetries are presented, both including and excluding the dominant production of radiative Z \gamma events, and compared to Standard Model expectations. The ratio R_b of the cross-section for bb(bar) production to the hadronic cross-section has been measured. In a model-independent fit to the Z lineshape, the data have been used to obtain an improved precision on the measurement of \gamma-Z interference. The energy dependence of \alpha_em has been investigated. The measurements have also been used to obtain limits on extensions of the Standard Model described by effective four-fermion contact interactions, to search for t-channel contributions from new massive particles and to place limits on chargino pair production with subsequent decay of the chargino into a light gluino and a quark pair.
Errors include statistical and systematic effects combined, with the formerdominant.
ASYM(C=MEAS) and ASYM(C=CORR) stand for measured values without (C=MEAS) and with (C=CORR) correction for interference between initial- and final-state radiation.
ASYM(C=MEAS) and ASYM(C=CORR) stand for measured values without (C=MEAS) and with (C=CORR) correction for interference between initial- and final-state radiation.
The production of Z bosons in pPb collisions at sqrt(s[NN]) = 5.02 TeV is studied by the CMS experiment via the electron and muon decay channels. The inclusive cross section is compared to pp collision predictions, and found to scale with the number of elementary nucleon-nucleon collisions. The differential cross sections as a function of the Z boson rapidity and transverse momentum are measured. Though they are found to be consistent within uncertainty with theoretical predictions both with and without nuclear effects, the forward-backward asymmetry suggests the presence of nuclear effects at large rapidities. These results provide new data for constraining nuclear parton distribution functions.
Forward-backward asymmetry (AFB) distribution of the Z bosons in pPb collisions as a function of rapidity in the fiducial region for the combined leptonic decay channel.
The first study of W boson production in pPb collisions is presented, for bosons decaying to a muon or electron, and a neutrino. The measurements are based on a data sample corresponding to an integrated luminosity of 34.6 inverse nanobarns at a nucleon-nucleon centre-of-mass energy of sqrt(s[NN]) = 5.02 TeV, collected by the CMS experiment. The W boson differential cross sections, lepton charge asymmetry, and forward-backward asymmetries are measured for leptons of transverse momentum exceeding 25 GeV, and as a function of the lepton pseudorapidity in the abs(eta[lab]) < 2.4 range. Deviations from the expectations based on currently available parton distribution functions are observed, showing the need for including W boson data in nuclear parton distribution global fits.
Lepton charge asymmetry, $(N_{\ell}^+ - N_{\ell}^-)/(N_{\ell}^+ + N_{\ell}^-)$ as a function of the lepton pseudorapidity.
The reaction e + e − → τ + τ − has been studied at centre of mass energies between 14.0 and 46.8 GeV with the CELLO detector at the PETRA e + e − collider. We present results for the cross section σ τ and the charge asymmetry A τ . The results are in good agreement with the standard model. We have also measured the topological decay rates BR 1 , BR 3 and BR 5 for the inclusive decay of the τ lepton into one, three and five charge particles. The results confirm and improve earlier CELLO measurements at other energies. We find for the combined values at all energies BR 1 = (84.9 ± 0.4 ± 0.3)%, BR 3 = (15.0 ± 0.4 ± 0.3)% and BR 5 = (0.16 ± 0.13 ± 0.04)%.
Corrected for radiative effects and background contributions.
We have studied 419 τ pair events produced in the reactione+e−→τ+ τ− at a c.m. energy of 34.6 GeV. We measure the cross section and angular distribution, as well as the decay branching ratios. The production characteristics are consistent with the Standard Electroweak Model predictions of γ andZ0 interference. The branching ratios are generally consistent with the τ decaying according to standard weak interaction principles, but we observe somewhat more decays resulting in single charged hadrons plus neutrals than are predicted by present theory.
Asymmetry based on fits to angular distribution.
The production and decay of τ-pairs was studied with the JADE detector at PETRA at center-of-mass energies of 30 ⩽√ s ⩽ 46.78 GeV. The total production cross section for τ-pairs agreed with QED predictions to order α 3 . Lower limits on QED cut-off parameters of Λ + > 285 GeV and Λ − > 210 GeV at 95% confidence level were ontained. The decay branching fractions into one and three charged particles were determined to be (86.1 ± 0.5 ± 0.9)% and (13.6±0.5 ±0.80)%. In the angular distributions a forward-backward asymmetry was observed, from which the axial-vector weak charge to the τ was determined to be a τ = −0.74 ± 0.22 in agreement with the standard model. An analysis of the process e + e − → τ + τ − γ showed agreement with QED calculations to O(α 3 ).
Forward-backward asymmetry determined from fit to angular distribution of form N*(1 + cos(theta)**2 + (3/8)*A*cos(theta)).
Study of radiative tau events.
Differential cross section data of the CELLO experiment on pair production of muons, taus, and heavy quarks ine+e−-annihilation are presented and analysed, together with our data on Bhabha scattering, in terms of compositeness effects characterized by the mass scale Λ. We discuss difficulties in the combination of limits Λ from different experiments. The appropriate parameter to combine different results turns out to be ɛ=±1/Λ2, which is in contrast to Λ Gaussian distributed.
Charge asymmetry for charm quarks derived from the differential cross section data.
Charge asymmetry for bottom quarks derived from the differential cross section data.
Differential cross sections fore+e−→e+e−, τ+, τ- measured with the CELLO detector at\(\left\langle {\sqrt s } \right\rangle= 34.2GeV\) have been analyzed for electroweak contributions. Vector and axial vector coupling constants were obtained in a simultaneous fit to the three differential cross sections assuming a universal weak interaction for the charged leptons. The results,v2=−0.12±0.33 anda2=1.22±0.47, are in good agreement with predictions from the standardSU(2)×U(1) model for\(\sin ^2 \theta _w= 0.228\). Combining this result with neutrino-electron scattering data gives a unique axial vector dominated solution for the leptonic weak couplings. Assuming the validity of the standard model, a value of\(\sin ^2 \theta _w= 0.21_{ - 0.09}^{ + 0.14}\) is obtained for the electroweak mixing angle. Additional vector currents are not observed (C<0.031 is obtained at the 95% C.L.).
Combined MU and TAU asymmetry. See PL 114B(1982)282 (<a href=http://durpdg.dur.ac.uk/scripts/reacsearch.csh/TESTREAC/red+1234> RED = 1234 </a>) and ZP C14(1982)283 (<a href=http://durpdg.dur.ac.uk/scripts/reacsearch.csh/TESTREAC/red+1245> RED = 1245 </a>) for individual asymmetry measurements.
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