Search Results (404 total)

Measurements of the azimuthal anisotropy of high-p_T neutral pion (π⁰) production in Au+Au collisions at sqrt[s_NN]=200 GeV by the PHENIX experiment are presented. The data included in this article were collected during the 2004 Relativistic Heavy Ion Collider running period and represent approximately an order of magnitude increase in the number of analyzed events relative to previously published results. Azimuthal angle distributions of π⁰ mesons detected in the PHENIX electromagnetic calorimeters are measured relative to the reaction plane determined event-by-event using the forward and backward beam-beam counters. Amplitudes of the second Fourier component (v₂) of the angular distributions are presented as a function of π⁰ transverse momentum (p_T) for different bins in collision centrality. Measured reaction plane dependent π⁰ yields are used to determine the azimuthal dependence of the π⁰ suppression as a function of p_T, R_AA(Δϕ,p_T). A jet-quenching motivated geometric analysis is presented that attempts to simultaneously describe the centrality dependence and reaction plane angle dependence of the π⁰ suppression in terms of the path lengths of hypothetical parent partons in the medium. This set of results allows for a detailed examination of the influence of geometry in the collision region and of the interplay between collective flow and jet-quenching effects along the azimuthal axis.

We present the results of a search for Higgs bosons predicted in two-Higgs-doublet models, in the case where the Higgs bosons decay to tau lepton pairs, using 1.8  fb^-1 of integrated luminosity of pp̅ collisions recorded by the CDF II experiment at the Fermilab Tevatron. Studying the mass distribution in events where one or both tau leptons decay leptonically, no evidence for a Higgs boson signal is observed. The result is used to infer exclusion limits in the two-dimensional space of tan⁡β versus m_A (the ratio of the vacuum expectation values of the two Higgs doublets and the mass of the pseudoscalar boson, respectively).

We present results on the electroexcitation of the low mass resonances Δ(1232)P₃₃, N(1440)P₁₁, N(1520)D₁₃, and N(1535)S₁₁ in a wide range of Q². The results were obtained in the comprehensive analysis of data from the Continuous Electron Beam Accelerator Facility (CEBAF) large acceptance spectrometer (CLAS) detector at the Thomas Jefferson National Accelerator Facility (JLab) on differential cross sections, longitudinally polarized beam asymmetries, and longitudinal target and beam-target asymmetries for π electroproduction off the proton. The data were analyzed using two conceptually different approaches—fixed-t dispersion relations and a unitary isobar model—allowing us to draw conclusions on the model sensitivity of the obtained electrocoupling amplitudes. The amplitudes for the Δ(1232)P₃₃ show the importance of a meson-cloud contribution to quantitatively explain the magnetic dipole strength, as well as the electric and scalar quadrupole transitions. They do not show any tendency of approaching the pQCD regime for Q²≤6 GeV². For the Roper resonance, N(1440)P₁₁, the data provide strong evidence that this state is a predominantly radial excitation of a three-quark (3q) ground state. Measured in pion electroproduction, the transverse helicity amplitude for the N(1535)S₁₁ allowed us to obtain the branching ratios of this state to the πN and ηN channels via comparison with the results extracted from η electroproduction. The extensive CLAS data also enabled the extraction of the γ^*p→N(1520)D₁₃ and N(1535)S₁₁ longitudinal helicity amplitudes with good precision. For the N(1535)S₁₁, these results became a challenge for quark models and may be indicative of large meson-cloud contributions or of representations of this state that differ from a 3q excitation. The transverse amplitudes for the N(1520)D₁₃ clearly show the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q²>1 GeV², confirming a long-standing prediction of the constituent quark model.

A combined mass and particle identification fit is used to make the first observation of the decay B̅ _s⁰→D_s^±K^∓ and measure the branching fraction of B̅ _s⁰→D_s^±K^∓ relative to B̅ _s⁰→D_s⁺π^-. This analysis uses 1.2  fb^-1 integrated luminosity of pp̅ collisions at sqrt[s]=1.96  TeV collected with the CDF II detector at the Fermilab Tevatron collider. We observe a B̅ _s⁰→D_s^±K^∓ signal with a statistical significance of 8.1σ and measure B(B̅ _s⁰→D_s^±K^∓)/B(B̅ _s⁰→D_s⁺π^-)=0.097±0.018(stat)±0.009(syst).

High-statistics differential cross sections for the reactions γp→pη and γp→pη^' have been measured using the CEBAF large acceptance spectrometer (CLAS) at Jefferson Lab for center-of-mass energies from near threshold up to 2.84 GeV. The η^' results are the most precise to date and provide the largest energy and angular coverage. The η measurements extend the energy range of the world’s large-angle results by approximately 300 MeV. These new data, in particular the η^' measurements, are likely to help constrain the analyses being performed to search for new baryon resonance states.

The exclusive reaction γp→pπ⁺π^- was studied in the photon energy range 3.0–3.8 GeV and the momentum transfer range 0.4<-t<1.0  GeV². Data were collected with the CLAS detector at the Thomas Jefferson National Accelerator Facility. In this kinematic range, the integrated luminosity was about 20  pb^-1. The reaction was isolated by detecting the π⁺ and proton in CLAS, and reconstructing the π^- via the missing-mass technique. Moments of the di-pion decay angular distributions were derived from the experimental data. Differential cross sections for the S, P, and D-waves, in the M_π⁺π^- mass range 0.4–1.4 GeV, were derived performing a partial wave expansion of the extracted moments. Beside the dominant contribution of the ρ(770) meson in the P-wave, evidence for the f₀(980) and the f₂(1270) mesons was found in the S and D-waves, respectively. The differential production cross sections dσ/dt for individual waves in the mass range of the above-mentioned mesons were extracted. This is the first time the f₀(980) has been measured in a photoproduction experiment.

We present a search for associated production of the standard model Higgs boson and a Z boson where the Z boson decays to two leptons and the Higgs decays to a pair of b quarks in pp̅ collisions at the Fermilab Tevatron. We use event probabilities based on standard model matrix elements to construct a likelihood function of the Higgs content of the data sample. In a CDF data sample corresponding to an integrated luminosity of 2.7  fb^-1 we see no evidence of a Higgs boson with a mass between 100  GeV/c² and 150  GeV/c². We set 95% confidence level upper limits on the cross section for ZH production as a function of the Higgs boson mass m_H; the limit is 8.2 times the standard model prediction at m_H=115  GeV/c².

The two-body photodisintegration of ⁴He into a proton and a triton has been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Real photons produced with the Hall-B bremsstrahlung-tagging system in the energy range from 0.35 to 1.55 GeV were incident on a liquid ⁴He target. This is the first measurement of the photodisintegration of ⁴He above 0.4 GeV. The differential cross sections for the γ⁴He→pt reaction were measured as a function of photon-beam energy and proton-scattering angle and are compared with the latest model calculations by J.-M. Laget. At 0.6-1.2 GeV, our data are in good agreement only with the calculations that include three-body mechanisms, thus confirming their importance. These results reinforce the conclusion of our previous study of the three-body breakup of ³He that demonstrated the great importance of three-body mechanisms in the energy region 0.5-0.8 GeV.

We report the observation of the bottom, doubly-strange baryon Ω_b^- through the decay chain Ω_b^-→J/ψΩ^-, where J/ψ→μ⁺μ^-, Ω^-→ΛK^-, and Λ→pπ^-, using 4.2  fb^-1 of data from pp̅ collisions at sqrt[s]=1.96  TeV, and recorded with the Collider Detector at Fermilab. A signal is observed whose probability of arising from a background fluctuation is 4.0×10^-8, or 5.5 Gaussian standard deviations. The Ω_b^- mass is measured to be 6054.4±6.8(stat)±0.9(syst)  MeV/c². The lifetime of the Ω_b^- baryon is measured to be 1.13_-0.40^+0.53(stat)±0.02(syst)  ps. In addition, for the Ξ_b^- baryon we measure a mass of 5790.9±2.6(stat)±0.8(syst)  MeV/c² and a lifetime of 1.56_-0.25^+0.27(stat)±0.02(syst)  ps. Under the assumption that the Ξ_b^- and Ω_b^- are produced with similar kinematic distributions to the Λ_b⁰ baryon, we find σ(Ξ_b^-)B(Ξ_b^-→J/ψΞ^-) / σ(Λ_b⁰)B(Λ_b⁰→J/ψΛ)=0.167_-0.025^+0.037(stat)±0.012(syst) and σ(Ω_b^-)B(Ω_b^-→J/ψΩ^-) / σ(Λ_b⁰)B(Λ_b⁰→J/ψΛ)=0.045_-0.012^+0.017(stat)±  0.004(syst) for baryons produced with transverse momentum in the range of 6–20  GeV/c.

We present an analysis of the mass of the X(3872) reconstructed via its decay to J/ψπ⁺π^- using 2.4  fb^-1 of integrated luminosity from pp̅ collisions at sqrt[s]=1.96  TeV, collected with the CDF II detector at the Fermilab Tevatron. The possible existence of two nearby mass states is investigated. Within the limits of our experimental resolution the data are consistent with a single state, and having no evidence for two states we set upper limits on the mass difference between two hypothetical states for different assumed ratios of contributions to the observed peak. For equal contributions, the 95% confidence level upper limit on the mass difference is 3.6  MeV/c². Under the single-state model the X(3872) mass is measured to be 3871.61±0.16(stat)±0.19(syst)  MeV/c², which is the most precise determination to date.

Bose-Einstein correlations of charged kaons are used to probe Au+Au collisions at sqrt[s_NN]=200  GeV and are compared to charged pion probes, which have a larger hadronic scattering cross section. Three-dimensional Gaussian source radii are extracted, along with a one-dimensional kaon emission source function. The centrality dependences of the three Gaussian radii are well described by a single linear function of N_part^1/3 with a zero intercept. Imaging analysis shows a deviation from a Gaussian tail at r≳10  fm, although the bulk emission at lower radius is well described by a Gaussian. The presence of a non-Gaussian tail in the kaon source reaffirms that the particle emission region in a heavy-ion collision is extended, and that similar measurements with pions are not solely due to the decay of long-lived resonances.

Inclusive electron-proton and electron-deuteron inelastic cross sections have been measured at Jefferson Lab (JLab) in the resonance region, at large Bjorken x, up to 0.92, and four-momentum transfer squared Q² up to 7.5 GeV² in the experiment E00-116. These measurements are used to extend to larger x and Q² precision, quantitative, studies of the phenomenon of quark-hadron duality. Our analysis confirms, both globally and locally, the apparent “violation” of quark-hadron duality previously observed at a Q² of 3.5 GeV² when resonance data are compared to structure function data created from CTEQ6M and MRST2004 parton distribution functions (PDFs). More importantly, our new data show that this discrepancy saturates by Q²~4 GeV², becoming Q² independent. This suggests only small violations of Q² evolution by contributions from the higher-twist terms in the resonance region that is confirmed by our comparisons to ALEKHIN and ALLM97. We conclude that the unconstrained strength of the CTEQ6M and MRST2004 PDFs at large x is the major source of the disagreement between data and these parametrizations in the kinematic regime we study and that, in view of quark-hadron duality, properly averaged resonance region data could be used in global quantum chromodynamics fits to reduce PDF uncertainties at large x.

We report measurements of the beam spin asymmetry in deeply virtual Compton scattering (DVCS) at an electron beam energy of 4.8 GeV using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The DVCS beam spin asymmetry has been measured in a wide range of kinematics, 1.0<Q²<2.8 (GeV/c)², 0.12<x_B<0.48, and 0.1<-t<0.8 (GeV/c)², using the reaction e→p→e^'pX. The number of H(e,e^'γp) and H(e,e^'π⁰p) events are separated in each (Q²,x_B,t) bin by a fit to the line shape of the H(e,e^'p)X M_x² distribution. The validity of the method was studied in detail using experimental and simulated data. It was shown that with the achieved missing mass squared resolution and the available statistics, the separation of DVCS–Bethe-Heitler and π⁰ events can reliably be done with less than 5% uncertainty. Also, the Q² and t dependences of the sinϕ moments of the asymmetry are extracted and compared with theoretical calculations.

We report on a search for the process pp̅ →γ+W/Z with W/Z→qq̅ in events containing two jets and a photon at the center-of-mass energy sqrt[s]=1.96  TeV, using 184  pb^-1 of data collected by the CDF II detector. A neural network event selection has been developed to optimize the rejection of the large QCD production background; it is shown that this method gives a significant improvement in both signal-to-noise ratio and signal sensitivity, as compared with an event selection based on conventional cuts. An upper limit is presented for the γ+W/Z production cross section with the W and Z decaying hadronically.

The process ep→epπ⁰ has been measured at Q²=6.4 and 7.7 (GeV/c²)² in Jefferson Lab's Hall C. Unpolarized differential cross sections are reported in the virtual photon-proton center-of-mass frame considering the process γ^*p→pπ⁰. Various details relating to the background subtractions, radiative corrections, and systematic errors are discussed. The usefulness of the data with regard to the measurement of the electromagnetic properties of the well-known Δ(1232) resonance is covered in detail. Specifically considered are the electromagnetic and scalar-magnetic ratios R_EM and R_SM along with the magnetic transition form factor G_M^*. It is found that the rapid falloff of the Δ(1232) contribution continues into this region of momentum transfer and that other resonances may be making important contributions in this region.

We present a signature-based search for the anomalous production of events containing a photon, two jets, of which at least one is identified as originating from a b quark, and missing transverse energy (E̸_T). The search uses data corresponding to 2.0  fb^-1 of integrated luminosity from pp̅ collisions at a center-of-mass energy of sqrt[s]=1.96  TeV, collected with the CDF II detector at the Fermilab Tevatron. From 6.697 47×10⁶ events with a photon candidate with transverse energy E_T>25  GeV, we find 617 events with E̸_T>25  GeV and two or more jets with E_T>15  GeV, at least one identified as originating from a b quark, versus an expectation of 607±113 events. Increasing the requirement on E̸_T to 50 GeV, we find 28 events versus an expectation of 30±11 events. We find no indications of non-standard-model phenomena.

We have used the Collider Detector at Fermilab (CDF-II) to search for the flavor-changing neutral-current (FCNC) top-quark decay t→Zc using a technique employing ratios of W and Z production, measured in pp̅ data corresponding to an integrated luminosity of 1.52  fb^-1. The analysis uses a comparison of two decay chains, pp̅ →tt̅ →WbWb→ℓνbjjb and pp̅ →tt̅ →ZcWb→ℓℓcjjb, to cancel systematic uncertainties in acceptance, efficiency, and luminosity. We validate the modeling of acceptance and efficiency for lepton identification over the multiyear data set using another ratio of W and Z production, in this case the observed ratio of inclusive production of W to Z bosons. To improve the discrimination against standard model backgrounds to top-quark decays, we calculate the top-quark mass for each event with two leptons and four jets assuming it is a tt̅ event with one of the top quarks decaying to Zc. For additional background discrimination we require at least one jet to be identified as originating from a b quark. No significant signal is found and we set an upper limit on the FCNC branching ratio Br(t→Zc) using a likelihood constructed from the ℓℓcjjb top-quark mass distribution and the number of ℓνbjjb events. Limits are set as a function of the helicity of the Z boson produced in the FCNC decay. For 100% longitudinally-polarized Z bosons we find limits of 8.3% and 9.3% (95% C.L.) depending on the assumptions regarding the theoretical top-quark pair production cross section.

We present the first measurement of the mass of the top quark in a sample of tt̅ →ℓν̅ bb̅ qq̅ events (where ℓ=e,μ) selected by identifying jets containing a muon candidate from the semileptonic decay of heavy-flavor hadrons (soft muon b tagging). The pp̅ collision data used correspond to an integrated luminosity of 2  fb^-1 and were collected by the CDF II detector at the Fermilab Tevatron Collider. The measurement is based on a novel technique exploiting the invariant mass of a subset of the decay particles, specifically the lepton from the W boson of the t→Wb decay and the muon from a semileptonic b decay. We fit template histograms, derived from simulation of tt̅ events and a modeling of the background, to the mass distribution observed in the data and measure a top quark mass of 180.5±12.0(stat)±3.6(syst)  GeV/c², consistent with the current world average value.

We report on the first direct search for charged Higgs bosons decaying into cs̅ in tt̅ events produced by pp̅ collisions at sqrt[s]=1.96  TeV. The search uses a data sample corresponding to an integrated luminosity of 2.2  fb^-1 collected by the CDF II detector at Fermilab and looks for a resonance in the invariant mass distribution of two jets in the lepton+jets sample of tt̅ candidates. We observe no evidence of charged Higgs bosons in top quark decays. Hence, 95% upper limits on the top quark decay branching ratio are placed at B(t→H⁺b)< 0.1 to 0.3 for charged Higgs boson masses of 60 to 150  GeV/c² assuming B(H⁺→cs̅ )=1.0. The upper limits on B(t→H⁺b) are also used as model-independent limits on the decay branching ratio of top quarks to generic scalar charged bosons beyond the standard model.

We present a search for a standard model Higgs boson produced in association with a W boson using 2.7  fb^-1 of integrated luminosity of pp̅ collision data taken at sqrt[s]=1.96  TeV. Limits on the Higgs boson production rate are obtained for masses between 100 and 150  GeV/c². Through the use of multivariate techniques, the analysis achieves an observed (expected) 95% confidence level upper limit of 5.6 (4.8) times the theoretically expected production cross section for a standard model Higgs boson with a mass of 115  GeV/c².

We present inclusive charged hadron elliptic flow (v₂) measured over the pseudorapidity range |η|<0.35 in Au+Au collisions at sqrt[s_NN]=200 GeV. Results for v₂ are presented over a broad range of transverse momentum (p_T=0.2-8.0 GeV/c) and centrality (0–60%). To study nonflow effects that are correlations other than collective flow, as well as the fluctuations of v₂, we compare two different analysis methods: (1) the event-plane method from two independent subdetectors at forward (|η|=3.1-3.9) and beam (|η|>6.5) pseudorapidities and (2) the two-particle cumulant method extracted using correlations between particles detected at midrapidity. The two event-plane results are consistent within systematic uncertainties over the measured p_T and in centrality 0–40%. There is at most a 20% difference in the v₂ between the two event-plane methods in peripheral (40–60%) collisions. The comparisons between the two-particle cumulant results and the standard event-plane measurements are discussed.

We report the observation at the Relativistic Heavy Ion Collider of suppression of back-to-back correlations in the direct photon+jet channel in Au+Au relative to p+p collisions. Two-particle correlations of direct photon triggers with associated hadrons are obtained by statistical subtraction of the decay photon-hadron (γ-h) background. The initial momentum of the away-side parton is tightly constrained, because the parton-photon pair exactly balance in momentum at leading order in perturbative quantum chromodynamics, making such correlations a powerful probe of the in-medium parton energy loss. The away-side nuclear suppression factor, I_AA, in central Au+Au collisions, is 0.32±0.12^stat±0.09^syst for hadrons of 3<p_T^h<5 in coincidence with photons of 5<p_T^γ<15 GeV/c. The suppression is comparable to that observed for high-p_T single hadrons and dihadrons. The direct photon associated yields in p+p collisions scale approximately with the momentum balance, z_T≡p_T^h/p_T^γ, as expected for a measurement of the away-side parton fragmentation function. We compare to Au+Au collisions for which the momentum balance dependence of the nuclear modification should be sensitive to the path-length dependence of parton energy loss.

We present the first observation in hadronic collisions of the electroweak production of vector boson pairs (VV, V=W, Z) where one boson decays to a dijet final state. The data correspond to 3.5  fb^-1 of integrated luminosity of pp̅ collisions at sqrt[s]=1.96  TeV collected by the CDF II detector at the Fermilab Tevatron. We observe 1516±239(stat)±144(syst) diboson candidate events and measure a cross section σ(pp̅ →VV+X) of 18.0±2.8(stat)±2.4(syst)±1.1(lumi)  pb, in agreement with the expectations of the standard model.

We report the observation of single top-quark production using 3.2  fb^-1 of pp̅ collision data with sqrt[s]=1.96  TeV collected by the Collider Detector at Fermilab. The significance of the observed data is 5.0 standard deviations, and the expected sensitivity for standard model production and decay is in excess of 5.9 standard deviations. Assuming m_t=175  GeV/c², we measure a cross section of 2.3_-0.5^+0.6(stat+syst)  pb, extract the CKM matrix-element value |V_tb|=0.91±0.11(stat+syst)±0.07(theory), and set the limit |V_tb|>0.71 at the 95% C.L.