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.

Ultrarelativistic heavy ion collisions recreate in the laboratory the thermodynamical conditions prevailing in the early universe up to 10$^{-6}$ seconds, thereby allowing the study of the quark-gluon plasma (QGP), a state of quantum chromodynamics (QCD) matter with deconfined partons. The top quark, the heaviest elementary particle known, is accessible in nucleus-nucleus collisions at the CERN LHC, and constitutes a novel probe of the QGP. Here, we report the first-ever evidence for the production of top quarks in nucleus-nucleus collisions, using lead-lead collision data at a nucleon-nucleon centre-of-mass energy of 5.02 TeV recorded by the CMS experiment. Two methods are used to measure the cross section for top quark pair production ($\sigma_\mathrm{t\bar{t}}$) via the decay into charged leptons (electrons or muons) and bottom quarks. One method relies on the leptonic information alone, and the second one exploits, in addition, the presence of bottom quarks. The measured cross sections, $\sigma_\mathrm{t\bar{t}} = $ 2.54 $^{+0.84}_{-0.74}$ and 2.03 $^{+0.71}_{-0.64}$ $\mu$b, respectively, are compatible with expectations from scaled proton-proton data and QCD predictions.

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.

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 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.

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}.

The production cross sections of the prompt charmed mesons D$^0$, D$^+$, D$^{*+}$ and D$_s$ were measured at mid-rapidity in p-Pb collisions at a centre-of-mass energy per nucleon pair $\sqrt{s_{\rm NN}}=5.02$ TeV with the ALICE detector at the LHC. D mesons were reconstructed from their decays D$^0\rightarrow{\rm K}^-\pi^+$, D$^+\rightarrow{\rm K}^-\pi^+\pi^+$, D$^{*+}\rightarrow D^0\pi^+$, D$_s^+\rightarrow\phi\pi^+\rightarrow{\rm K}^-{\rm K}^+\pi^+$, and their charge conjugates. The $p_{\rm T}$-differential production cross sections were measured at mid-rapidity in the interval $1<p_{\rm T}<24$ GeV/$c$ for D$^0$, D$^+$ and D$^{*+}$ mesons and in $2<p_{\rm T}<12$ GeV/$c$ for D$_s$ mesons, using an analysis method based on the selection of decay topologies displaced from the interaction vertex. The production cross sections of the D$^0$, D$^+$ and D$^{*+}$ mesons were also measured in three $p_{\rm T}$ intervals as a function of the rapidity $y_{\rm cms}$ in the centre-of-mass system in $-1.26<y_{\rm cms}<0.34$. In addition, the prompt D$^0$ cross section was measured in pp collisions at $\sqrt{s}=7$ TeV and p-Pb collisions at $\sqrt{s_{\rm NN}}=5.02$ TeV down to $p_{\rm T}=0$ using an analysis technique that is based on the estimation and subtraction of the combinatorial background, without reconstruction of the D$^0$ decay vertex. The nuclear modification factor $R_{\rm pPb}(p_{\rm T})$, defined as the ratio of the $p_{\rm T}$-differential D-meson cross section in p-Pb collisions and that in pp collisions scaled by the mass number of the Pb nucleus, was calculated for the four D-meson species and found to be compatible with unity within experimental uncertainties. The results are compared to theoretical calculations that include cold-nuclear-matter effects and to transport model calculations incorporating the interactions of charm quarks with an expanding deconfined medium.

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.

We have measured the differential cross section for the inclusive production of psi(2S) mesons decaying to mu^{+} mu^{-1} that were produced in prompt or B-decay processes from ppbar collisions at 1.96 TeV. These measurements have been made using a data set from an integrated luminosity of 1.1 fb^{-1} collected by the CDF II detector at Fermilab. For events with transverse momentum p_{T} (psi(2S)) > 2 GeV/c and rapidity |y(psi(2S))| < 0.6 we measure the integrated inclusive cross section sigma(ppbar -> psi(2S)X) Br(psi(2S) -> mu^{+} mu^{-}) to be 3.29 +- 0.04(stat.) +- 0.32(syst.) nb.

The production of Upsilon(1S), Upsilon(2S) and Upsilon(3S) mesons decaying into the dimuon final state is studied with the LHCb detector using a data sample corresponding to an integrated luminosity of 3.3 pb^{-1} collected in proton-proton collisions at a centre-of-mass energy of sqrt{s}=2.76 TeV. The differential production cross-sections times dimuon branching fractions are measured as functions of the Upsilon transverse momentum and rapidity, over the ranges p_T<15 GeV/c and 2.0<y<4.5. The total cross-sections in this kinematic region, assuming unpolarised production, are measured to be sigma(pp -> Upsilon(1S) X) x B(Upsilon(1S) -> mu+mu-) = 1.111 +/- 0.043 +/- 0.044 nb, sigma(pp -> Upsilon(2S) X) x B(Upsilon(2S) -> mu+mu-) = 0.264 +/- 0.023 +/- 0.011 nb, sigma(pp -> Upsilon(3S) X) x B(Upsilon(3S) -> mu+mu-) = 0.159 +/- 0.020 +/- 0.007 nb, where the first uncertainty is statistical and the second systematic.

We present a measurement of the transverse momentum distribution of dielectron pairs with invariant mass near the mass of the Z boson. The data were obtained using the DO detector during the 1994-1995 run of the Tevatron Co!lider at Fermilab. The data used in the measurement corresponds to an integrated luminosity of 108.5 $pb^{-1}$ The measurement is compared to current phenomenology for vector boson production in proton-antiproton interactions, and the results are found to be consistent with expectation from Quantum Chromodynamics (QCD).

We have studied single diffraction dissociation ( p p→ p X ) in proton-antiproton collisions at √ s =1.8TeV, covering the ranges 3⪅ M X ⪅200 GeV and 0.05⪅| t |⪅0.11 (GeV/ c ) 2 . Parameterizing the production to be of the form dσ ( d t d M 2 X ) = (M 2 X ) −α exp (bt) , we obtain α = 1.13±0.07 and b = 10.5±1.8(GeV/ c ) −2 . The total single diffraction dissociation cross section is 2 σ SD =8.1±1.7 mb. Comparisons are made to previous lower energy data, and to an earlier measurement by us at the same energy.

The production of top quark-antiquark pair events in $p\bar{p}$ collisions at $\sqrt{s}=1.96$ TeV is studied as a function of the transverse momentum and absolute value of the rapidity of the top quarks as well as of the invariant mass of the $t\bar{t}$ pair. We select events containing an isolated lepton, a large imbalance in transverse momentum, and four or more jets with at least one jet identified to originate from a $b$ quark. The data sample corresponds to 9.7 fb$^{-1}$ of integrated luminosity recorded with the D0 detector during Run II of the Fermilab Tevatron Collider. Observed differential cross sections are consistent with standard model predictions.

We have studied J ψ production in p p collisions at s = 1.8 TeV with the DØ detector at Fermilab using μ + μ − data. We have measured the inclusive J ψ production cross section as a function of J ψ transverse momentum, p T . For the kinematic range p T > 8 GeV/ c and |η| < 0.6 we obtain σ(p p → J ψ + X) · Br ( J ψ → μ + μ − ) = 2.08 ± 0.17( stat) ± 0.46(syst) nb. Using the muon impact parameter we have estimated the fraction of J ψ mesons coming from B meson decays to be f b = 0.35 ± 0.09(stat)±0.10(syst) and inferred the inclusive b production cross section. From the information on the event topology the fraction of nonisolated J ψ events has been measured to be f nonisol = 0.64 ± 0.08(stat)±0.06(syst). We have also obtained the fraction of J ψ events resulting from radiative decays of χ c states, f χ = 0.32 ± 0.07(stat)±0.07(syst). We discuss the implications of our measurements for charmonium production processes.

We present a measurement of the ttbar production cross section in ppbar collisions at root(s) = 1.8TeV by the D0 experiment at the Fermilab Tevatron. The measurement is based on data from an integrated luminosity of approximately 125 pb~-1 accumulated during the 1992-1996 collider run. We observe 39 ttbar candidate events in the dilepton and lepton+jets decay channels with an expected background of 13.7+-2.2 events. For a top quark mass of 173.3GeV/c~2, we measure the ttbar production cross section to be 5.5+-1.8 pb.

We present a study of events with Z bosons and hadronic jets produced in $\overline{p}p$ collisions at a center-of-mass energy of 1.8 TeV. The data consist of 6708 $Z \rightarrow e~+e~-$ decays from 106 pb$~{-1}$ of integrated luminosity collected using the CDF detector at the Tevatron Collider. The Z $+ \ge n$ jet cross sections and jet production properties have been measured for n = 1 to 4. The data compare well to predictions of leading order QCD matrix element calculations with added gluon radiation and simulated parton fragmentation.

Fermilab experiment E735 located at the CO intersection region of the\(\sqrt s= 1.8\) TeV\(p\bar p\) collider analysed over 900 Φ→K+K− events. Measured were the transverse momentum spectrum, the correlation between the average transverse momentum <pt> and the charged particle multiphcityNc, as well as the probability of Φ production per charged track,NΦ/Nc, versusNc. We have also made an estinate of the total inclusive cross section for Φ mesons,\(\sigma (p\bar p \to \phi X) = 7.3 \pm 2.2 mb\).

We present a study of events with W bosons and hadronic jets produced in pbar p collisions at a center of mass energy of 1.8 TeV. The data consist of 51400 W^+/- -> e^+/- nu decay candidates from 108 pb^-1 of integrated luminosity collected with the CDF detector at the Tevatron Collider. The cross sections and jet production properties have been measured for W + \geq 1 to \geq 4 jet events. The data are compared to predictions of leading order QCD matrix element calculations with added gluon radiation and simulated fragmentation.

We report on a search for bottom squarks produced in pbarp collisions at sqrt(s) = 1.8 TeV using the D0 detector at Fermilab. Bottom squarks are assumed to be produced in pairs and to decay to the lightest supersymmetric particle (LSP) and a b quark with branching fraction of 100%. The LSP is assumed to be the lightest neutralino and stable. We set limits on the production cross section as a function of bottom squark mass and LSP mass.

We have searched for the pair production of first generation scalar leptoquarks in the eejj channel using the full data set (123 pb-1) collected with the D0 detector at the Fermilab Tevatron during 1992--1996. We observe no candidates with an expected background of approximately 0.4 events. Comparing the experimental 95% confidence level upper limit to theoretical calculations of the cross section with the assumption of a 100% branching fraction to eq, we set a lower limit on the mass of a first generation scalar leptoquark of 225 GeV/c^2. The results of this analysis rule out the interpretation of the excess of high Q^2 events at HERA as leptoquarks which decay exclusively to eq.

We report on a search for second generation leptoquarks with the D\O\ detector at the Fermilab Tevatron $p\overline{p}$ collider at $\sqrt{s}$ = 1.8 TeV. This search is based on 12.7 pb$~{-1}$ of data. Second generation leptoquarks are assumed to be produced in pairs and to decay into a muon and quark with branching ratio $\beta$ or to neutrino and quark with branching ratio $(1-\beta)$. We obtain cross section times branching ratio limits as a function of leptoquark mass and set a lower limit on the leptoquark mass of 111 GeV/c$~{2}$ for $\beta = 1 $ and 89 GeV/c$~{2}$ for $\beta = 0.5 $ at the 95\%\ confidence level.

If quarks are composite particles then excited states are expected. We have searched in pp¯ collisions for excited quarks (q*) which decay to common quarks by emitting a W boson (q*→qW) or a photon (q*→qγ). The simplest model of excited quarks has been excluded for mass M*<540 GeV/c2 at 95% confidence level.

We report the first measurement of the cross section for Z boson pair production at a hadron collider. This result is based on a data sample corresponding to 1.9 fb-1 of integrated luminosity from ppbar collisions at sqrt{s} = 1.96 TeV collected with the CDF II detector at the Fermilab Tevatron. In the llll channel, we observe three ZZ candidates with an expected background of 0.096^{+0.092}_{-0.063} events. In the llnunu channel, we use a leading-order calculation of the relative ZZ and WW event probabilities to discriminate between signal and background. In the combination of llll and llnunu channels, we observe an excess of events with a probability of $5.1\times 10^{-6}$ to be due to the expected background. This corresponds to a significance of 4.4 standard deviations. The measured cross section is sigma(ppbar -> ZZ) = 1.4^{+0.7}_{-0.6} (stat.+syst.) pb, consistent with the standard model expectation.

The ALICE collaboration at the LHC reports measurement of the inclusive production cross section of electrons from semi-leptonic decays of beauty hadrons with rapidity $|y|<0.8$ and transverse momentum $1<p_{\mathrm{T}}<10$ GeV/$c$, in pp collisions at $\sqrt{s} = $ 2.76 TeV. Electrons not originating from semi-electronic decay of beauty hadrons are suppressed using the impact parameter of the corresponding tracks. The production cross section of beauty decay electrons is compared to the result obtained with an alternative method which uses the distribution of the azimuthal angle between heavy-flavour decay electrons and charged hadrons. Perturbative QCD calculations agree with the measured cross section within the experimental and theoretical uncertainties. The integrated visible cross section, $\sigma_{\mathrm{b} \rightarrow \mathrm{e}} = 3.47\pm0.40(\mathrm{stat})^{+1.12}_{-1.33}(\mathrm{sys})\pm0.07(\mathrm{norm}) \mu$b, was extrapolated to full phase space using Fixed Order plus Next-to-Leading Log (FONLL) predictions to obtain the total b$\bar{\mathrm{b}}$ production cross section, $\sigma_{\mathrm{b\bar{b}}} = 130\pm15.1(\mathrm{stat})^{+42.1}_{-49.8}(\mathrm{sys})^{+3.4}_{-3.1}(\mathrm{extr})\pm2.5(\mathrm{norm})\pm4.4(\mathrm{BR}) \mu$b.

The production of $\phi$ mesons has been studied in pp collisions at LHC energies with the ALICE detector via the dimuon decay channel in the rapidity region $2.5 < y < 4$. Measurements of the differential cross section ${\rm d}^2\sigma/{\rm d}y {\rm d}p_{\rm T}$ are presented as a function of the transverse momentum ($p_{\rm T}$) at the center-of-mass energies $\sqrt{s}=5.02$, 8 and 13 TeV and compared with the ALICE results at midrapidity. The differential cross sections at $\sqrt{s}=5.02$ and 13 TeV are also studied in several rapidity intervals as a function of $p_{\rm T}$, and as a function of rapidity in three $p_{\rm T}$ intervals. A hardening of the $p_{\rm T}$-differential cross section with the collision energy is observed, while, for a given energy, $p_{\rm T}$ spectra soften with increasing rapidity and, conversely, rapidity distributions get slightly narrower at increasing $p_{\rm T}$. The new results, complementing the published measurements at $\sqrt{s}=2.76$ and 7 TeV, allow one to establish the energy dependence of $\phi$ meson production and to compare the measured cross sections with phenomenological models. None of the considered models manages to describe the evolution of the cross section with $p_{\rm T}$ and rapidity at all the energies.

We present the results of a search for the top quark in 19.3 pb−1 of p¯p collisions at √s =1.8 TeV. The data were collected at the Fermilab Tevatron collider using the Collider Detector at Fermilab (CDF). The search includes standard model tt¯ decays to final states eeνν¯, eμνν¯, and μμνν¯ as well as e+ν+jets or μ+ν+jets. In the (e,μ)+ν+jets channel we search for b quarks from t decays via secondary vertex identification and via semileptonic decays of the b and cascade c quarks. In the dilepton final states we find two events with a background of 0.56−0.13+0.25 events. In the e,μ+ν+jets channel with a b identified via a secondary vertex, we find six events with a background of 2.3±0.3. With a b identified via a semileptonic decay, we find seven events with a background of 3.1±0.3. The secondary vertex and semileptonic-decay samples have three events in common. The probability that the observed yield is consistent with the background is estimated to be 0.26%. The statistics are too limited to firmly establish the existence of the top quark; however, a natural interpretation of the excess is that it is due to tt¯ production. We present several cross-checks. Some support this hypothesis; others do not. Under the assumption that the excess yield over background is due to tt¯, constrained fitting on a subset of the events yields a mass of 174±10−12+13 GeV/c2 for the top quark. The tt¯ cross section, using this top quark mass to compute the acceptance, is measured to be 13.9−4.8+6.1 pb.

The $p_{\rm T}$-differential production cross sections of the prompt (B feed-down subtracted) charmed mesons D$^0$, D$^+$, and D$^{*+}$ in the rapidity range $|y|<0.5$, and for transverse momentum $1< p_{\rm T} <12$ GeV/$c$, were measured in proton-proton collisions at $\sqrt{s} = 2.76$ TeV with the ALICE detector at the Large Hadron Collider. The analysis exploited the hadronic decays D$^0 \rightarrow $K$\pi$, D$^+ \rightarrow $K$\pi\pi$, D$^{*+} \rightarrow $D$^0\pi$, and their charge conjugates, and was performed on a $L_{\rm int} = 1.1$ nb$^{-1}$ event sample collected in 2011 with a minimum-bias trigger. The total charm production cross section at $\sqrt{s} = 2.76$ TeV and at 7 TeV was evaluated by extrapolating to the full phase space the $p_{\rm T}$-differential production cross sections at $\sqrt{s} = 2.76$ TeV and our previous measurements at $\sqrt{s} = 7$ TeV. The results were compared to existing measurements and to perturbative-QCD calculations. The fraction of cdbar D mesons produced in a vector state was also determined.

We present a measurement of the b-quark cross section in 1.8 TeV p-p¯ collisions recorded with the Collider Detector at Fermilab using muonic b-quark decays. In the central rapidity region (‖yb‖<1.0), the cross section is 295±21±75 nb (59±14±15 nb) for pTb>21 GeV/c (29 GeV/c). Comparisons are made to previous measurements and next-to-leading order QCD calculations.

We present results of a search for W+W− production through the leptonic decay channel W+W−→l+l−νν¯ in p¯p collisions at s=1.8TeV. In a 108pb−1 data sample recorded with the Collider Detector at Fermilab, five W+W− candidates are found with an expected standard model background of 1.2±0.3 events. The W+W− production cross section is measured to be σ(p¯p→W+W−)=10.2−5.1+6.3(stat)±1.6(syst)pb, in agreement with the standard model prediction. Limits on WWγ and WWZ anomalous couplings are presented.

We present the results of a search for third generation leptoquark (LQ) pairs in 110±8pb−1of p¯p collisions at s=1.8TeV recorded by the Collider Detector at Fermilab. We assume third generation leptoquarks decay to a τ lepton and a b quark with branching ratio β. We observe one candidate event, consistent with standard model background expectations. We place upper limits on σ(p¯p→LQLQ¯)̇β2 as a function of the leptoquark mass MLQ. We exclude at 95% confidence level scalar leptoquarks with MLQ<99GeV/c2, gauge vector leptoquarks with MLQ<225GeV/c2, and nongauge vector leptoquarks with MLQ<170GeV/c2 for β=1.

The inclusive cross sections times leptonic branching ratios for W and Z boson production in PbarP collisions at Sqrt(s)=1.8 TeV were measured using the D0 detector at the Fermilab Tevatron collider: Sigma_W*B(W->e, nu) = 2.36 +/- 0.07 +/- 0.13 nb, Sigma_W*B(W->mu,nu) = 2.09 +/- 0.23 +/- 0.11 nb, Sigma_Z*B(Z-> e, e) = 0.218 +/- 0.011 +/- 0.012 nb, Sigma_Z*B(Z->mu,mu) = 0.178 +/- 0.030 +/- 0.009 nb. The first error is the combined statistical and systematic uncertainty, and the second reflects the uncertainty in the luminosity. For the combined electron and muon analyses we find: [Sigma_W*B(W->l,nu)]/[Sigma_Z*B(Z->l,l)] = 10.90 +/- 0.49. Assuming Standard Model couplings, this result is used to determine the width of the W boson: Gamma(W) = 2.044 +/- 0.093 GeV.

We present a measurement of the cross section and the first measurement of the heavy flavor content of associated direct photon + muon events produced in hadronic collisions. These measurements come from a sample of 1.8 TeV ppbar collisions recorded with the Collider Detector at Fermilab. Quantum chromodynamics (QCD) predicts that these events are primarily due to Compton scattering process charm+gluon -> charm+photon, with the final state charm quark producing a muon. The cross section for events with a photon transverse momentum between 12 and 40 GeV/c is measured to be 46.8+-6.3+-7.5 pb, which is two standard deviations below the most recent theoretical prediction. A significant fraction of the events in the sample contain a final-state bottom quark. The ratio of charm to bottom production is measured to be 2.4+-1.2, in good agreement with QCD models.

We have searched for second generation leptoquark (LQ) pairs in the \mu\mu+jets channel using 94+-5 pb^{-1} of pbar-p collider data collected by the D0 experiment at the Fermilab Tevatron during 1993-1996. No evidence for a signal is observed. These results are combined with those from the \mu\nu+jets and \nu\nu+jets channels to obtain 95% confidence level (C.L.) upper limits on the LQ pair production cross section as a function of mass and $beta, the branching fraction of a LQ decay into a charged lepton and a quark. Lower limits of 200(180) GeV/c^2 for \beta=1(1/2) are set at the 95% C.L. on the mass of scalar LQ. Mass limits are also set on vector leptoquarks as a function of \beta.

We present a study of J/ψ and ψ(2S) production in pp¯ collisions, at s=1.8TeV with the CDF detector at Fermilab. The J/ψ and ψ(2S) mesons are reconstructed using their μ+μ− decay modes. We have measured the inclusive production cross section for both mesons as a function of their transverse momentum in the central region, |η|<0.6. We also measure the fraction of these events originating from b hadrons. We thus extract individual cross sections for J/ψ and ψ(2S) mesons from b-quark decays and prompt production. We find a large excess (approximately a factor of 50) of direct ψ(2S) production compared with predictions from the color singlet model.

None

We present a measurement of the ttbar pair production cross section in ppbar collisions at sqrt(s) = 1.96 TeV utilizing approximately 425 pb-1 of data collected with the D0 detector. We consider decay channels containing two high pT charged leptons (either e or \mu) from leptonic decays of both top-daughter W bosons. These were gathered using four sets of selection criteria, three of which required that a pair of fully identified leptons (i.e., e\mu, ee, or \mu\mu) be found. The fourth approach imposed less restrictive criteria on one of the lepton candidates and required that at least one hadronic jet in each event be tagged as containing a b quark. For a top quark mass of 175 GeV, the measured cross section is 7.4 +/-1.4(stat} +/- 1.0(syst) pb.

We present the results of a search for neutral Higgs bosons produced in association with $b$ quarks in $p\bar{p}\to b\bar{b} \phi\to b\bar{b}b\bar{b}$ final states with $91 \pm 7$ pb$^{-1}$ of $p\bar{p}$ collisions at $\sqrt{s}=1.8$ TeV recorded by the Collider Detector at Fermilab. We find no evidence of such a signal and the data is interpreted in the context of the neutral Higgs sector of the Minimal Supersymmetric extension of the Standard Model. With basic parameter choices for the supersymmetric scale and the stop quark mixing, we derive 95% C.L. lower mass limits for neutral Higgs bosons for $\tb$ values in excess of 35.

None

Results are presented on a measurement of the ttbar pair production cross section in ppbar collisions at sqrt{s} = 1.8 TeV from nine independent decay channels. The data were collected by the Dzero experiment during the 1992-1996 run of the Fermilab Tevatron Collider. A total of 80 candidate events are observed with an expected background of 38.8 +- 3.3 events. For a top quark mass of 172.1 GeV/c^2, the measured cross section is 5.69 +- 1.21 (stat) +- 1.04 (sys) pb.

Light-by-light scattering ($\gamma\gamma\rightarrow\gamma\gamma$) is a quantum-mechanical process that is forbidden in the classical theory of electrodynamics. This reaction is accessible at the Large Hadron Collider thanks to the large electromagnetic field strengths generated by ultra-relativistic colliding lead (Pb) ions. Using 480 $\mu$b$^{-1}$ of Pb+Pb collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, the ATLAS Collaboration reports evidence for the $\gamma\gamma\rightarrow\gamma\gamma$ reaction. A total of 13 candidate events are observed with an expected background of 2.6$\pm$0.7 events. After background subtraction and analysis corrections, the fiducial cross section of the process $\textrm{Pb+Pb}\,(\gamma\gamma)\rightarrow \textrm{Pb}^{(\ast)}\textrm{+}\textrm{Pb}^{(\ast)}\,\gamma\gamma$, for photon transverse energy $E_{\mathrm{T}}>$3 GeV, photon absolute pseudorapidity $|\eta|<$2.4, diphoton invariant mass greater than 6 GeV, diphoton transverse momentum lower than 2 GeV and diphoton acoplanarity below 0.01, is measured to be 70 $\pm$ 24 (stat.) $\pm$ 17 (syst.) nb, which is in agreement with Standard Model predictions.

We present a new measurement of the $B^+$ meson differential cross section $d\sigma/d p_T$ at $\sqrt{s}=1960$ GeV. The data correspond to an integrated luminosity of 739pb$^{-1}$ collected with the upgraded CDF detector (CDF II) at the Fermilab Tevatron collider. $B^+$ candidates are reconstructed through the decay $B^+ \to J/\psi K^+$, with $J/\psi \to \mu^+ \mu^-$. The integrated cross section for producing $B^+$ mesons with $p_T \geq 6$ GeV/c and $|y| \leq 1$ is measured to be $2.78 \pm 0.24 \mu$b

Using the Collider Detector at Fermilab, the W-boson differential cross section dσ/dPT is measured using W→eν events in proton-antiproton collisions at √s =1.8 TeV. A next-to-leading-order theoretical calculation agrees well with the data. The cross section (σ) for PT>50 GeV/c is measured to be 423±58(stat)±108(syst) pb.

We report the full reconstruction of χc mesons through the decay chain χc→J/ψ γ, J/ψ→μ+μ−, using data obtained at the Collider Detector at Fermilab in 2.6±0.2 pb−1 of p¯p collisions at √s =1.8 TeV. This exclusive χc sample is used to measure the χc-meson production cross section times branching fractions. We obtain σ×B=3.2±0.4(stat)−1.1+1.2(syst) nb for χc mesons decaying to J/ψ with pT>6.0 GeV/c and pseudorapidity ‖η‖<0.5. From this and the inclusive J/ψ cross section we calculate the inclusive b-quark cross section to be 12.0±4.5 μb for pTb>8.5 GeV/c and ‖yb‖<1.

We have studied tbar-t production using multijet final states in pbar-p collisions at a center-of-mass energy of 1.8 TeV, with an integrated luminosity of 110.3 pb(-1). Each of the top quarks with these final states decays exclusively to a bottom quark and a W boson, with the W bosons decaying into quark-antiquark pairs. The analysis has been optimized using neural networks to achieve the smallest expected fractional uncertainty on the tbar-t production cross section, and yields a cross section of 7.1 +/- 2.8(stat.) +/- 1.5(syst.) pb, assuming a top quark mass of 172.1 GeV/c^(2). Combining this result with previous D0 measurements, where one or both of the W bosons decay leptonically, gives a tbar t production cross section of 5.9 +/- 1.2(stat) +/- 1.1(syst) pb.

We have searched for first generation scalar leptoquark (LQ) pairs in the enu+jets channel using ppbar collider data (integrated luminosity= 115 pb^-1) collected by the DZero experiment at the Fermilab Tevatron during 1992-96. The analysis yields no candidate events. We combine the results with those from the ee+jets and nunu+jets channels to obtain 95% confidence level (CL) upper limits on the LQ pair production cross section as a function of mass and of beta, the branching fraction to a charged lepton. Comparing with the next-to-leading order theory, we set 95% CL lower limits on the LQ mass of 225, 204, and 79 GeV/c^2 for beta=1, 1/2, and 0, respectively.

A change in estimated integrated luminosity (from 226 pb$^{-1} to 257 pb$^{-1}$ leads to a corrected value for ${\sigma (p \bar p \to Z) \cdot}$Br${(Z \to \tau \tau)}$ of $209\pm13(stat.)\pm16(syst.)\pm13(lum) pb.

This Letter describes a measurement of the muon cross section originating from b quark decay in the forward rapidity range 2.4 < y(mu) < 3.2 in pbarp collisions at sqrt(s) = 1.8 TeV. The data used in this analysis were collected by the D0 experiment at the Fermilab Tevatron. We find that NLO QCD calculations underestimate b quark production by a factor of four in the forward rapidity region. A cross section measurement using muon+jet data has been included in this version of the paper.

We present a search for first generation leptoquark with 110pb^1 of data collected with the CDF detector. We set 95% C.L. cross section limits as a function of the leptoquark mass.

We have measured the cross section of γ+D*± production in p¯p collisions at s=1.8TeV using the Collider Detector at Fermilab. In this kinematic region, the Compton scattering process (gc→γc) is expected to dominate and thus provide a direct link to the charm quark density in the proton. From the 45±18 γ+D*± candidates in a 16.4pb−1 data sample, we have determined the production cross section to be 0.38±0.15(stat)±0.11(syst) nb for the rapidity range |y(D*±)|<1.2 and |y(γ)|<0.9, and for the transverse momentum range pT(D*±)>6GeV/c and 16<pT(γ)<40GeV/c. The measured cross section is compared to a theoretical prediction.

We have used 19 pb**-1 of data collected with the Collider Detector at Fermilab to search for new particles decaying to dijets. We exclude at 95% confidence level models containing the following new particles: axigluons with mass between 200 and 870 GeV, excited quarks with mass between 80 and 570 GeV, and color octet technirhos with mass between 320 and 480 GeV.