Measurements of inclusive charged-hadron transverse-momentum and pseudorapidity distributions are presented for proton-proton collisions at sqrt(s) = 0.9 and 2.36 TeV. The data were collected with the CMS detector during the LHC commissioning in December 2009. For non-single-diffractive interactions, the average charged-hadron transverse momentum is measured to be 0.46 +/- 0.01 (stat.) +/- 0.01 (syst.) GeV/c at 0.9 TeV and 0.50 +/- 0.01 (stat.) +/- 0.01 (syst.) GeV/c at 2.36 TeV, for pseudorapidities between -2.4 and +2.4. At these energies, the measured pseudorapidity densities in the central region, dN(charged)/d(eta) for |eta| < 0.5, are 3.48 +/- 0.02 (stat.) +/- 0.13 (syst.) and 4.47 +/- 0.04 (stat.) +/- 0.16 (syst.), respectively. The results at 0.9 TeV are in agreement with previous measurements and confirm the expectation of near equal hadron production in p-pbar and pp collisions. The results at 2.36 TeV represent the highest-energy measurements at a particle collider to date.
A measurement of the underlying event (UE) activity in proton-proton collisions is performed using events with charged-particle jets produced in the central pseudorapidity region (abs(eta[jet]) < 2) and with transverse momentum 1<= pt[jet] < 100 GeV. The analysis uses a data sample collected at a centre-of-mass energy of 2.76 TeV with the CMS experiment at the LHC. The UE activity is measured as a function of pt[jet] in terms of the average multiplicity and scalar sum of transverse momenta (pt) of charged particles, with abs(eta) < 2 and pt > 0.5 GeV, in the azimuthal region transverse to the highest pt jet direction. By further dividing the transverse region into two regions of smaller and larger activity, various components of the UE activity are separated. The measurements are compared to previous results at 0.9 and 7 TeV, and to predictions of several Monte Carlo event generators, providing constraints on the modelling of the UE dynamics.
We report a new measurement of the pseudorapidity (eta) and transverse-energy (Et) dependence of the inclusive jet production cross section in pbar b collisions at sqrt(s) = 1.8 TeV using 95 pb**-1 of data collected with the DZero detector at the Fermilab Tevatron. The differential cross section d^2sigma/dEt deta is presented up to |eta| = 3, significantly extending previous measurements. The results are in good overall agreement with next-to-leading order predictions from QCD and indicate a preference for certain parton distribution functions.
The production cross sections for Υ’s, ψ’s and χ c ’s in high energy pp̄ collisions have been measured using the CDF detector at Fermilab. Heavy quarkonia production involves a variety of mechanisms, and the data is used to disentangle various components. Large prompt ψ‐cross sections were observed, contrary to conventional expectations.
We establish the existence of the top quark using a 67 pb^-1 data sample of Pbar-P collisions at Sqrt(s) = 1.8 TeV collected with the Collider Detector at Fermilab (CDF). Employing techniques similar to those we previously published, we observe a signal consistent with t-tbar decay to WW b-bbar, but inconsistent with the background prediction by 4.8 sigma. Additional evidence for the top quark is provided by a peak in the reconstructed mass distribution. We measure the top quark mass to be 176 +/-8(stat) +/- 10(sys.) GeV/c^2, and the t-tbar production cross section to be 6.8 +3.6 -2.4 pb.
The first observation of $Z$ boson production in proton-lead collisions at a centre-of-mass energy per proton-nucleon pair of $\sqrt{s_{NN}}=5~\text{TeV}$ is presented. The data sample corresponds to an integrated luminosity of $1.6~\text{nb}^{-1}$ collected with the LHCb detector. The $Z$ candidates are reconstructed from pairs of oppositely charged muons with pseudorapidities between 2.0 and 4.5 and transverse momenta above $20~\text{GeV}/c$. The invariant dimuon mass is restricted to the range $60-120~\text{GeV}/c^2$. The $Z$ production cross-section is measured to be \begin{eqnarray*} \sigma_{Z\to\mu^+\mu^-}(\text{fwd})&=&13.5^{+5.4}_{-4.0}\text{(stat.)}\pm1.2\text{(syst.)}~\text{nb} \end{eqnarray*} in the direction of the proton beam and \begin{eqnarray*} \sigma_{Z\to\mu^+\mu^-}(\text{bwd}) & =&10.7^{+8.4}_{-5.1}\text{(stat.)}\pm1.0\text{(syst.)}~\text{nb} \end{eqnarray*} in the direction of the lead beam, where the first uncertainty is statistical and the second systematic.
We present the first observation of exclusive $e^+e^-$ production in hadron-hadron collisions, using $p\bar{p}$ collision data at \mbox{$\sqrt{s}=1.96$ TeV} taken by the Run II Collider Detector at Fermilab, and corresponding to an integrated luminosity of \mbox{532 pb$^{-1}$}. We require the absence of any particle signatures in the detector except for an electron and a positron candidate, each with transverse energy {$E_T>5$ GeV} and pseudorapidity {$|\eta|<2$}. With these criteria, 16 events are observed compared to a background expectation of {$1.9\pm0.3$} events. These events are consistent in cross section and properties with the QED process \mbox{$p\bar{p} \to p + e^+e^- + \bar{p}$} through two-photon exchange. The measured cross section is \mbox{$1.6^{+0.5}_{-0.3}\mathrm{(stat)}\pm0.3\mathrm{(syst)}$ pb}. This agrees with the theoretical prediction of {$1.71 \pm 0.01$ pb}.
We have searched for exclusive 2-photon production in proton-antiproton collisions at sqrt{s} = 1.96 TeV, using 532/pb of integrated luminosity taken by the Run II Collider Detector at Fermilab. The event signature requires two electromagnetic showers, each with transverse energy E_T > 5 GeV and pseudorapidity |eta|<1.0, with no other particles detected in the event. Three candidate events are observed. We discuss the consistency of the three events with gamma-gamma, pi0-pi0, or eta-eta production. The probability that other processes fluctuate to 3 events or more is 1.7x10^-4. An upper limit on the cross section of p+pbar --> p+gamma-gamma+pbar is set at 410 fb with 95% confidence level.
Three-jet production in deep inelastic ep scattering and photoproduction was investigated with the ZEUS detector at HERA using an integrated luminosity of 127 pb-1. Measurements of differential cross sections are presented as functions of angular correlations between the three jets in the final state and the proton-beam direction. These correlations provide a stringent test of perturbative QCD and show sensitivity to the contributions from different colour configurations. Fixed-order perturbative QCD calculations assuming the values of the colour factors C_F, C_A and T_F as derived from a variety of gauge groups were compared to the measurements to study the underlying gauge group symmetry. The measured angular correlations in the deep inelastic ep scattering and photoproduction regimes are consistent with the admixture of colour configurations as predicted by SU(3) and disfavour other symmetry groups, such as SU(N) in the limit of large N.
The first measurement of two-pion Bose-Einstein correlations in central Pb-Pb collisions at $\sqrt{s_{\rm NN}} = 2.76$ TeV at the Large Hadron Collider is presented. We observe a growing trend with energy now not only for the longitudinal and the outward but also for the sideward pion source radius. The pion homogeneity volume and the decoupling time are significantly larger than those measured at RHIC.
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