Jet reconstruction and underlying event studies in p+p and d+Au collisions from STAR

Description of parton interaction with the hot and dense nuclear matter created in heavy-ion collisions at high energies is a complex task, which requires a detailed knowledge of jet production in p+p and d+Au collisions. Measurements in these collision systems are therefore essential to disentangle initial state nuclear effects from cold nuclear matter effects, medium-induced $k_T$ broadening and jet quenching. To obtain complete description of the p+p (d+Au) collision it is also important to study particle production in the underlying event. The measured properties of underlying event can be used to tune the QCD based Monte-Carlo models. In this paper some of the recent results on jet and underlying event properties in p+p and d+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV measured by the STAR experiment are presented. In particular, the preliminary results on charged and strange particle fragmentation functions in p+p collisions are discussed and confronted with PYTHIA simulations. Next, the measurement of inclusive jet spectrum and di-jet correlations in d+Au collisions is presented and compared with the measurements in p+p collisions to estimate the size of cold nuclear matter effects. Finally, the study of underlying event properties in p+p collisions is shown and compared to PYTHIA simulation.Comment: 9 pages, 8 figures, proceedings of the 27th Winter Workshop on Nuclear Dynamics, Winter Park, Colorado, US

Jets and jet-like correlations studies from STAR

I present recent results from jets and jet-like correlation measurements from STAR. The pp data are compared to those from Au-Au collisions to attempt to infer information on the medium produced and how hard scattered partons interact with this matter. Results from d-Au events are utilized to investigate the magnitude of cold nuclear matter effects on hard scatterings. The evolution of the underlying event from pp to d-Au collisions is studied. In heavy-ion collisions, background fluctuations are the major source of systematic uncertainties in jet measurements. Detailed studies are therefore being made of these fluctuations and recent progress in our understanding is reported. Jet and jet-hadron correlations results are presented and give clear indications that partonic fragmentation at RHIC is highly modified in the presence of a strongly coupled coloured medium, resulting in a significant broadening and softening of the jet fragments correlation. Finally di-hadron correlations utilizing identified particles as triggers indicate that the "ridge" is stronger for p+K than for pi but that the near-side peak per-trigger yield remains unaltered from d-Au to Au-Au collisions.Comment: Proceedings for QM201

Phenomenology of GUT-less Supersymmetry Breaking

We study models in which supersymmetry breaking appears at an intermediate scale, M_{in}, below the GUT scale. We assume that the soft supersymmetry-breaking parameters of the MSSM are universal at M_{in}, and analyze the morphology of the constraints from cosmology and collider experiments on the allowed regions of parameter space as M_{in} is reduced from the GUT scale. We present separate analyses of the (m_{1/2},m_0) planes for tan(beta)=10 and tan(beta)=50, as well as a discussion of non-zero trilinear couplings, A_0. Specific scenarios where the gaugino and scalar masses appear to be universal below the GUT scale have been found in mirage-mediation models, which we also address here. We demand that the lightest neutralino be the LSP, and that the relic neutralino density not conflict with measurements by WMAP and other observations. At moderate values of M_{in}, we find that the allowed regions of the (m_{1/2},m_0) plane are squeezed by the requirements of electroweak symmetry breaking and that the lightest neutralino be the LSP, whereas the constraint on the relic density is less severe. At very low M_{in}, the electroweak vacuum conditions become the dominant constraint, and a secondary source of astrophysical cold dark matter would be necessary to explain the measured relic density for nearly all values of the soft SUSY-breaking parameters and tan(beta). We calculate the neutralino-nucleon cross sections for viable scenarios and compare them with the present and projected limits from direct dark matter searches.Comment: 35 pages, 9 figures; typos corrected, references adde

Search for Neutral Higgs Bosons in Events with Multiple Bottom Quarks at the Tevatron

The combination of searches performed by the CDF and D0 collaborations at the Fermilab Tevatron Collider for neutral Higgs bosons produced in association with b quarks is reported. The data, corresponding to 2.6 fb-1 of integrated luminosity at CDF and 5.2 fb-1 at D0, have been collected in final states containing three or more b jets. Upper limits are set on the cross section multiplied by the branching ratio varying between 44 pb and 0.7 pb in the Higgs boson mass range 90 to 300 GeV, assuming production of a narrow scalar boson. Significant enhancements to the production of Higgs bosons can be found in theories beyond the standard model, for example in supersymmetry. The results are interpreted as upper limits in the parameter space of the minimal supersymmetric standard model in a benchmark scenario favoring this decay mode.Comment: 10 pages, 2 figure

Search for New Physics in Lepton + Photon + X Events with L=305 pb-1 of ppbar Collisions at roots=1.96 TeV

We present results of a search for anomalous production of events containing a charged lepton (either electron or muon) and a photon, both with high transverse momentum, accompanied by additional signatures, X, including missing transverse energy (MET) and additional leptons and photons. We use the same kinematic selection criteria as in a previous CDF search, but with a substantially larger data set, 305 pb-1, a ppbar collision energy of 1.96 TeV, and the upgraded CDF II detector. We find 42 Lepton+Photon+MET events versus a standard model expectation of 37.3 +- 5.4 events. The level of excess observed in Run I, 16 events with an expectation of 7.6 +- 0.7 events (corresponding to a 2.7 sigma effect), is not supported by the new data. In the signature of Multi-Lepton+Photon+X we observe 31 events versus an expectation of 23.0 +- 2.7 events. In this sample we find no events with an extra photon or MET and so find no events like the one ee+gg+MET event observed in Run I.Comment: 7 pages, 3 figures, 1 table. Accepted to PR

Top Quark Mass Measurement from Dilepton Events at CDF II with the Matrix-Element Method

We describe a measurement of the top quark mass using events with two charged leptons collected by the CDF II detector from $p\bar{p}$ collisions with $\sqrt s = 1.96$ TeV at the Fermilab Tevatron. The likelihood in top mass is calculated for each event by convoluting the leading order matrix element describing $q\bar{q} \to t\bar{t} \to b\ell\nu_{\ell}\bar{b}\ell'\nu_{\ell'}$ with detector resolution functions. The presence of background events in the data sample is modeled using similar calculations involving the matrix elements for major background processes. In a data sample with integrated luminosity of 340 pb$^{-1}$, we observe 33 candidate events and measure $M_{top} = 165.2 \pm 6.1(\textrm{stat.}) \pm 3.4(\textrm{syst.}) \mathrm{~GeV}/c^2.$ This measurement represents the first application of this method to events with two charged leptons and is the most precise single measurement of the top quark mass in this channel.Comment: 21 pages, 14 figure

Measurement of B(t->Wb)/B(t->Wq) at the Collider Detector at Fermilab

We present a measurement of the ratio of top-quark branching fractions R= B(t -> Wb)/B(t -> Wq), where q can be a b, s or a d quark, using lepton-plus-jets and dilepton data sets with integrated luminosity of ~162 pb^{-1} collected with the Collider Detector at Fermilab during Run II of the Tevatron. The measurement is derived from the relative numbers of t-tbar events with different multiplicity of identified secondary vertices. We set a lower limit of R > 0.61 at 95% confidence level.Comment: 7 pages, 2 figures, published in Physical Review Letters; changes made to be consistent with published versio

Top Quark Mass Measurement from Dilepton Events at CDF II

We report a measurement of the top quark mass using events collected by the CDF II Detector from ppbar collisions at \sqrt{s} = 1.96 TeV at the Fermilab Tevatron. We calculate a likelihood function for the top mass in events that are consistent with ttbar->bbar l^- \nubar b l^+ \nu decays. The likelihood is formed as the convolution of the leading-order matrix element and detector resolution functions. The joint likelihood is the product of likelihoods for each of 33 events collected in 340 pb^{-1} of integrated luminosity, yielding a top quark mass M_t = 165.2 +- 6.1 (stat.) +- 3.4(syst.) GeV/c^2. This first application of a matrix-element technique to ttbar->bbar l^- \nubar b l^+ \nu decays gives the most precise single measurement of M_t in dilepton events. Combined with other CDF Run II measurements using dilepton events, we measure M_{t} = 167.9 +- 5.2 (stat.) +- 3.7 (syst.) GeV/c^2.Comment: 7 pages, 2 figures, To be published in Phys. Rev. Lett. Replacement includes minor revisions made prior to acceptance by journa