Study of taupair production at HERA Aggarwal, R.; Antonelli, S.; Antonov, A. ...
The journal of high energy physics,
02/2011, Letnik:
2011, Številka:
2
Journal Article
Recenzirano
Odprti dostop
A study of events containing two tau leptons with high transverse momentum has been performed with the ZEUS detector at HERA, using a data sample corresponding to an integrated luminosity of 0.33 fb
...−1
. The tau candidates were identified from their decays into electrons, muons or hadronic jets. The number of tau-pair candidates has been compared with the prediction from the Standard Model, where the largest contribution is expected from Bethe-Heitler processes. The total visible cross section was extracted. Standard Model expectations agree well with the measured distributions, also at high invariant mass of the tau pair.
Jet substructure and differential cross sections for jets produced in the photoproduction and deep inelastic
ep scattering regimes have been measured with the ZEUS detector at HERA using an ...integrated luminosity of
82.2
pb
−
1
. The substructure of jets has been studied in terms of the jet shape and subjet multiplicity for jets with transverse energies
E
T
jet
>
17
GeV
. The data are well described by the QCD calculations. The jet shape and subjet multiplicity are used to tag gluon- and quark-initiated jets. Jet cross sections as functions of
E
T
jet
, jet pseudorapidity, the jet–jet scattering angle, dijet invariant mass and the fraction of the photon energy carried by the dijet system are presented for gluon- and quark-tagged jets. The data exhibit the behaviour expected from the underlying parton dynamics. A value of
α
s
(
M
Z
)
of
α
s
(
M
Z
)
=
0.1176
±
0.0009
(
stat.
)
−
0.0026
+
0.0009
(
exp.
)
−
0.0072
+
0.0091
(
th.
)
was extracted from the measurements of jet shapes in deep inelastic scattering.
A search for lepton-flavor-violating interactions \(e p \to \mu X \) and \(e p\to \tau X \) has been performed with the ZEUS detector using the entire HERA I data sample, corresponding to an ...integrated luminosity of \(130 \text{pb}^{-1}\). The data were taken at center-of-mass energies, \(\sqrt{s}\), of 300 and \(318 \text{Ge}\;\text{V\/}\). No evidence of lepton-flavor violation was found, and constraints were derived on leptoquarks (LQs) that could mediate such interactions. For LQ masses below \(\sqrt{s}\), limits were set on \(\lambda_{eq_1} \sqrt{\beta_{\ell q}}\), where \(\lambda_{eq_1}\) is the coupling of the LQ to an electron and a first-generation quark q1, and \(\beta_{\ell q}\) is the branching ratio of the LQ to the final-state lepton \(\ell\) (μ or \(\tau\)) and a quark q. For LQ masses much larger than \(\sqrt{s}\), limits were set on the four-fermion interaction term \(\lambda_{e q_\alpha} \lambda_{\ell q_\beta} / M_{\mathrm{LQ}}^2\) for LQs that couple to an electron and a quark \(q_\alpha\) and to a lepton \(\ell\) and a quark \(q_\beta\), where \(\alpha\) and \(\beta\) are quark generation indices. Some of the limits are also applicable to lepton-flavor-violating processes mediated by squarks in R-Parity-violating supersymmetric models. In some cases, especially when a higher-generation quark is involved and for the process \(e p\to \tau X \), the ZEUS limits are the most stringent to date.
Diffractive photoproduction of vector mesons, \(\gamma p \to V Y\), where Y is a proton-dissociative system, has been measured in e+p interactions with the ZEUS detector at HERA using an integrated ...luminosity of 25\(\pbi\). The differential cross section, \(d\sigma/d t\), is presented for \(1.2 < -t<12{\rm GeV}^2\), where t is the square of the four-momentum transferred to the vector meson. The data span the range in photon-proton centre-of-mass energy, W, from 80 GeV to 120 GeV. The t distributions are well fit by a power law, \(d\sigma/d t \propto (-t)^{-n}\). The slope of the effective Pomeron trajectory, measured from the W dependence of the \(\rho^0\) and \(\phi\) cross sections in bins of t, is consistent with zero. The ratios \(d\sigma_{\gamma p \to \phi Y}/d t\) to \(d\sigma_{\gamma p \to \rho^0 Y}/d t\) and \(d\sigma_{\gamma p \to J/\psi Y}/d t\) to \(d\sigma_{\gamma p \to \rho^0 Y}/d t\) increase with increasing -t. Decay-angle analyses for \(\rho^0\), \(\phi\) and \(J/\psi\) mesons have been carried out. For the \(\rho^0\) and \(\phi\) mesons, contributions from single and double helicity flip are observed. The results are compared to expectations of theoretical models.
A search for physics beyond the Standard Model has been performed with high-Q2 neutral current deep inelastic scattering events recorded with the ZEUS detector at HERA. Two data sets, e+p→e+X and ...e−p→e−X, with respective integrated luminosities of 112 pb−1 and 16 pb−1, were analyzed. The data reach Q2 values as high as 40000 GeV2. No significant deviations from Standard Model predictions were observed. Limits were derived on the effective mass scale in eeqq contact interactions, the ratio of leptoquark mass to the Yukawa coupling for heavy leptoquark models and the mass scale parameter in models with large extra dimensions. The limit on the quark charge radius, in the classical form factor approximation, is 0.85×10−16 cm.
Measurements of the cross sections for charged current deep inelastic scattering in e+p collisions with a longitudinally polarised positron beam are presented. The measurements are based on a data ...sample with an integrated luminosity of 132 pb−1 collected with the ZEUS detector at HERA at a centre-of-mass energy of 318 GeV. The total cross section is presented at positive and negative values of the longitudinal polarisation of the positron beams. The single-differential cross-sections dσ/dQ2, dσ/dx and dσ/dy are presented for Q2>200 GeV2. The reduced cross-section \(\tilde{\sigma}\) is presented in the kinematic range 200<Q2<60 000 GeV2 and 0.006<x<0.562. The measurements agree well with the predictions of the Standard Model. The results are used to determine a lower limit on the mass of a hypothetical right-handed W boson.
A search for pentaquarks decaying to Ξ−π− (Ξ−π+) and corresponding antiparticles has been performed with the ZEUS detector at HERA. The data sample consists of deep inelastic ep scattering events at ...centre-of-mass energies of 300 and 318 GeV, and corresponds to 121 pb−1 of integrated luminosity. A clear signal for Ξ0(1530)→Ξ−π+ was observed. However, no signal for any new baryonic state was observed at higher masses in either the Ξ−π− or Ξ−π+ channels. The searches in the antiparticle channels were also negative. Upper limits on the ratio of a possible Ξ3/2−− (Ξ3/20) signal to the Ξ0(1530) signal were set in the mass range 1650–2350 MeV.
The production of beauty quarks with a \(D^{\ast\pm}\) and a muon in the final state has been measured with the ZEUS detector at HERA using an integrated luminosity of 114 pb-1. Low ...transverse-momentum thresholds for the muon and D* meson allow for a measurement of beauty production closer to the production threshold than previous measurements. The beauty signal was extracted using the charge correlations and angular distributions of the muon with respect to the D* meson. Cross sections for photoproduction and deep inelastic scattering are somewhat higher than, but compatible with, next-to-leading-order QCD predictions, and compatible with other measurements.
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