Dilepton-tagged $Q\overline{Q}+\mathrm{jet}$ events at the LHC

We propose a new method for identifying and isolating events through semileptonic decays of the pair. Employing these decay dileptons to tag the jet in a specific kinematic region provides a clean signature of jets associated with heavy-quark production. The measurement, in both pp and heavy-ion collisions, is essential for addressing heavy-quark fragmentation in vacuum and in a dense medium. We present next-to-leading order calculations of production (leading order in production) in TeV pp collisions at the LHC and discuss the feasibility of the measurement in heavy-ion collisions at TeV.     

Longitudinal broadening of near-side jets due to parton cascade

Longitudinal broadening along the Δη direction on the near side in the two-dimensional (Δφ×Δη) di-hadron correlation distribution has been studied for central Au+Au collisions at  GeV, within a dynamical multi-phase transport model. It was found that longitudinal broadening is generated by a longitudinal flow induced by a strong parton cascade in central Au+Au collisions, to be compared with p+p collisions at  GeV. The longitudinal broadening may shed light on the strongly interacting partonic matter at RHIC.     

Analysis of non-photonic electrons in Cu + Cu collisions at $\sqrt{s_{NN}}=200$ GeV

Heavy-flavor semileptonic decays are expected to dominate the spectrum of non-photonic electrons produced from collisions at the energies of the Relativistic Heavy Ion Collider. The non-photonic electron yield is suppressed by approximately a factor of 5 in central Au + Au events at  GeV relative to p+p events with the same collision energy. Most theoretical models predict less non-photonic-electron suppression than is observed experimentally. We present a preliminary measurement of the yield of non-photonic electrons in Cu + Cu events at  GeV, as well as the nuclear modification factor.     

Multijet production in neutral current deep inelastic scattering at HERA and determination of $\alpha_s$

Multijet production rates in neutral current deep inelastic scattering have been measured in the range of exchanged boson virtualities . The data were taken at the ep collider HERA with centre-of-mass energy using the ZEUS detector and correspond to an integrated luminosity of . Jets were identified in the Breit frame using the k_T cluster algorithm in the longitudinally invariant inclusive mode. Measurements of differential dijet and trijet cross sections are presented as functions of jet transverse energy ( ), pseudorapidity ( ) and Q² with and . Next-to-leading-order QCD calculations describe the data well. The value of the strong coupling constant , determined from the ratio of the trijet to dijet cross sections, is . Received: 1 April 2005, Revised: 28 June 2005, Published online: 30 August 2005     

Jet properties from two-particle azimuthal correlations in d+Au collisions at $\sqrt{s_{NN}}$ =200 GeV at STAR

We present measurements of azimuthal correlations between photons (from π⁰ decay) and charged hadrons in d+Au collisions at  =200 GeV. We use di-hadron correlations to study parton fragmentation in d+Au collisions at RHIC. Specifically, the near-side and away-side peaks of the azimuthal angular difference distribution are used to measure the root-mean-squared (RMS) fragmentation transverse momentum and the mean intrinsic parton transverse momentum . The measurements with leading photons are compared to results using leading charged particles. PACS  25.75.-q     

Analysis of Ωb? (bss) and Ωc0 (css) with QCD sum rules

We calculate the masses and the pole residues of the heavy baryons Ω _c ⁰(css) and Ω _b ⁻(bss) with the QCD sum rules. The numerical values  GeV (or  GeV) and  GeV (or  GeV) are in good agreement with the experimental data.     

Di-jet production in photon-photon collisionsat $\sqrt{s_{\protect\bf\mathrm{ee}}}$ from 189 to 209 GeV

Di-jet production is studied in collisions of quasi-real photons at e ⁺ e^- centre-of-mass energies from 189 to 209 GeV at LEP. The data were collected with the OPAL detector. Jets are reconstructed using an inclusive -clustering algorithm for all cross-section measurements presented. A cone jet algorithm is used in addition to study the different structure of the jets resulting from either of the algorithms. The inclusive di-jet cross-section is measured as a function of the mean transverse energy of the two leading jets, and as a function of the estimated fraction of the photon momentum carried by the parton entering the hard sub-process, , for different regions of . Angular distributions in di-jet events are measured and used to demonstrate the dominance of quark and gluon initiated processes in different regions of phase space. Furthermore the inclusive di-jet cross-section as a function of and is presented, where is the jet pseudo-rapidity. Different regions of the - -space are explored to study and control the influence of an underlying event. The results are compared to next-to-leading order perturbative QCD calculations and to the predictions of the leading order Monte Carlo generator PYTHIA.Received: 20 December 2002, Published online: 17 October 2003     

A short review on jet identification

Jets can be used to probe the physical properties of the high energy density matter created in collisions at the Relativistic Heavy Ion Collider (RHIC). Measurements of strong suppression of inclusive hadron distributions and di-hadron correlations at high p _T have already provided evidence for partonic energy loss. However, these measurements suffer from well-known geometric biases due to the competition of energy loss and fragmentation. These biases can be avoided if the jets are reconstructed independently of their fragmentation details—quenched or unquenched. In this paper, we discuss modern jet reconstruction algorithms (cone and sequential recombination) and their corresponding background subtraction techniques required by the high multiplicities of heavy ion collisions. We review recent results from the STAR experiment at RHIC on direct jet reconstruction in central Au+Au collisions at  GeV.     

First fragmentation function measurements from full jet reconstruction in heavy-ion collisions at $\sqrt{s_{\mathrm{NN}}}=200$ GeV by STAR

Measurements of inclusive hadron suppression and di-hadron azimuthal correlations in ultra-relativistic nuclear collisions at RHIC have provided important insights into jet quenching in hot QCD matter, but are limited in their sensitivity due to well-known biases. Full jet reconstruction in heavy-ion collisions would conceptually provide a direct measurement of the energy of the scattered parton before energy loss, alleviating such biases and allowing a measurement of the energy loss probability distribution in a model-independent way from hard probes. In these proceedings we utilize recent progress in the reconstruction of jets in the heavy ion environment and present the first measurement of the fragmentation function from fully reconstructed jets in heavy ion collisions. The fragmentation function measured in central Au + Au collisions at  GeV will be presented and discussed with respect to p + p reference measurements.     

Recent high-p_{\rm T} results from STAR

The STAR Collaboration has a broad range of recent results on intermediate and high- phenomena in Au + Au collisions at = 200 and 62 GeV and in d + Au at = 200 GeV. These include new measurements of spectra, azimuthal anisotropies and di-hadron correlations. The comparison of the 62 and 200 GeV Au + Au results indicates that jet quenching, elliptic flow and di-hadron correlation measurements are very similar at the two energies. Meson-baryon differences that have been seen at intermediate in 200 GeV Au + Au collisions are also present in 62 GeV Au + Au collisions and in 200 GeV d + Au collisions. Measurements of backward-forward inclusive hadron yield asymmetries and forward-midrapidity di-hadron correlations in d + Au collisions are consistent with the saturation picture. A brief review of these results is presented. Arrival of the final proofs: 30 June 2005 PACS: 25.75.Dw, 25.75.Ld, 25.75.Gz     

ZZ production in e+ e- int eractions at $\sqrt{s} =$ 183-209 G eV

Measurements of on-shell ZZ production are described, using data from the DELPHI experiment at LEP in e ⁺ e ^- collisions at centre-of-mass energies between 183 and 209 GeV, corresponding to an integrated luminosity of about 665 pb^-1. Results obtained in each of the final states , , , , , l ⁺ l ^- l ⁺ l ^-, and (with ) are presented. The measured production cross-sections are consistent with the Standard Model expectations. These results update and supersede those already published at 183 and 189 GeV.Received: 3 March 2002, Revised: 28 May 2003, Published online: 19 September 2003     

Strange particle production at RHIC

We report STAR measurements of mid-rapidity yields for the Λ , , K _S ⁰ , Ξ ⁻, , Ω ⁻, particles in Cu + Cu and Au + Au  GeV collisions. We show that at a given number of participating nucleons, bulk strangeness production is higher in Cu + Cu collisions compared to Au + Au collisions at the same center of mass energy, counter to predictions from the Canonical formalism. We compare both the Cu + Cu and Au + Au yields to AMPT and EPOS predictions, and find they reproduce key qualitative aspects of the data. Finally, we investigate other scaling parameters and find bulk strangeness production for both the measured data and theoretical predictions, scales better with the number participants that undergo more than one collision.     

Open heavy flavor production from single muons in PHENIX at RHIC

The PHENIX experiment has studied open heavy flavor production in  =200 GeV p+p and d+Au collisions using the semi-leptonic decay into single muons. The results from these measurements and the details of the analysis technique are presented. The results from p+p collisions obtained at mid-rapidity are compared to perturbative QCD calculations. The production of light mesons is the major background source for the open flavor measurement using muons. The nuclear modification factor for light mesons were measured in Cu+Cu collisions at  =200 GeV is presented.     

Heavy flavor production in PHENIX

The PHENIX experiment at RHIC measured single electron spectra in p + p, d + Au and Au + Au collisions at GeV, and in Au + Au collisions at GeV. In these spectra, electrons from semi-leptonic decays of charmed particles are the dominant contribution after subtraction of all 'photonic' sources (photon conversions, Dalitz decays, decays of light vector mesons). The p + p open charm production cross-section is found to be in good agreement with pQCD NLO calculations. The shape of the distributions obtained for p + p interactions is compared with those observed for nucleus-nucleus collisions. From p + p to d + Au and Au + Au interactions, open charm production is found to scale with the number of binary collisions . Au + Au data at GeV is compatible with the ISR p + p results scaled by . The elliptic flow parameter v₂ of heavy flavor electrons has also been measured, and is found to be non-zero in the intermediate p_T range. Arrival of the final proofs: 29 June 2005 PACS: . ^* Deceased Spokesperson     

Investigation of the high-p<Subscript>T</Subscript> strange baryon anomalies at RHIC

Results from RHIC have shown that there is an enhanced baryon/meson ratio in the intermediate transverse momentum range (2<p_T<6 GeV/c) in Au+Au collisions at both  =130 and 200 GeV. This was initially demonstrated by measurements of the p̄/π^- ratio which was then extended in p_T by the Λ/K⁰ _S ratio. The data were successfully described by models utilising different hadronization mechanisms: those having recombination of quarks and others having an interplay between flow, jet quenching and incorporating baryon junction loops. The strange particle data from the first Au+Au run at  =200 GeV gave tantalising hints that the observed enhancement of baryons compared to mesons was diminished by a p_T of 6 GeV/c, but a lack of statistics in this range made a definitive statement impossible. Here we present an extended analysis of identified strange baryons and mesons in Au+Au collisions at  =200 GeV using data obtained by the STAR experiment from the 2004 running period. The increase in statistics extends the measurement of Λ hyperons out to at least 7 GeV/c and K⁰ _S mesons out to 9 GeV/c. This data allows us to place limits on the range where in-vacuum fragmentation functions are applicable and the effect of baryon dominance is reduced. We also discuss the prospects for making these measurements using multiply-strange baryons and mesons (Ω and ϕ).     

Studying the relative bottom contribution via heavy-quark decay electron measurements in STAR

We report measurements of the azimuthal angular correlation distribution of heavy-quark decay electrons and open charmed mesons in pp collisions at  GeV in the STAR experiment at RHIC. This measurement in combination with current theoretical model calculations allows to extract the relative bottom contribution to the heavy-quark decay electrons, which is important for the interpretation of the observed strong suppression of the high-p _T electron yield in central Au+Au collisions.     

Search for narrow resonances lighter than ? mesons

We report a search for narrow resonances, produced in collisions at  TeV, that decay into muon pairs with invariant mass between 6.3 and 9.0 GeV/c ². The data, collected with the CDF II detector at the Fermilab Tevatron collider, correspond to an integrated luminosity of 630 pb⁻¹. We use the dimuon invariant mass distribution to set 90% upper credible limits of about 1% to the ratio of the production cross section times muonic branching fraction of possible narrow resonances to that of the ϒ(1S) meson.     

Associated charged Higgs and $W$ boson production in the MSSM at the CERN large hadron collider

We investigate the viability of observing charged Higgs bosons () produced in association with bosons at the CERN large hadron collider, using the leptonic decay and hadronic decay, within different scenarios of the minimal supersymmetric standard model (MSSM) with both real and complex parameters. Performing a parton level study we show how the irreducible standard model background from jets can be controlled by applying appropriate cuts and find that the size of a possible signal depends on the cuts needed to suppress QCD backgrounds and misidentifications. In the standard maximal mixing scenario of the MSSM we find a viable signal for large and intermediate masses () when using softer cuts (,  50 GeV), whereas for harder cuts (, 100 GeV) we only find a viable signal for very large (). We have also investigated a special class of MSSM scenarios with large mass splittings among the heavy Higgs bosons where the cross-section can be resonantly enhanced by factors up to one hundred, with a strong dependence on the -violating phases. Even so we find that the signal after cuts remains small except for small masses () when using the softer cuts. Finally, in all the scenarios we have investigated we have only found small -asymmetries.     

Medium effects on high p_{\rm T} particle production measured with the PHENIX experiment

Transverse momentum ( ) spectra measured by the PHENIX experiment at RHIC in Au + Au, d + Au and pp collisions at and in Au + Au collisions at are presented. A suppression of the yield of high hadrons in central Au + Au collisions by a factor 4-5 at is found relative to the pp reference scaled by the nuclear overlap function . In contrast, direct photons are not suppressed in central Au + Au collisions and no suppression of high particles can be seen in d + Au collisions. This leads to the conclusion that the dense medium formed in central Au + Au collisions is responsible for the suppression. Arrival of the final proofs: 30 June 2005 PACS: 25.75.Dw     

<Emphasis Type="Italic">ϒ</Emphasis> production in STAR

The study of formation and interaction of heavy quarkonia in relativistic heavy ion collisions provides important insight into the properties of the produced medium. Lattice QCD studies show sequential suppression of quarkonia states with increasing temperature; which affirms that a full spectroscopy, including bottomonium, can provide us a thermometer for the matter produced under extreme conditions in relativistic heavy ion collisions and the most direct probe of deconfinement. With the completion of the STAR Electromagnetic Calorimeter and with the increased luminosity provided by RHIC in Runs 6 and 7, the study of ϒ production via the di-electron channel becomes possible. We present preliminary results on ϒ measurements in pp collisions (from Run 6) along with preliminary results from Au+Au collisions (in Run 7) at  GeV from the STAR experiment.