Department of Physics

Transverse-Momentum and Pseudorapidity Distributions of Charged Hadrons in pp Collisions at √s=7 TeV

B. Agkun et al. — Thu, 07 Feb 2013 13:40:34 PST

Charged-hadron transverse-momentum and pseudorapidity distributions in proton-proton collisions at √s=7 TeV are measured with the inner tracking system of the CMS detector at the LHC. The charged-hadron yield is obtained by counting the number of reconstructed hits, hit pairs, and fully reconstructed charged-particle tracks. The combination of the three methods gives a charged-particle multiplicity per unit of pseudorapidity dN_ch/dη|_|η|<0.5=5.78±0.01(stat)±0.23(syst) for non-single-diffractive events, higher than predicted by commonly used models. The relative increase in charged-particle multiplicity from √s=0.9 to 7 TeV is [66.1±1.0(stat)±4.2(syst)]%. The mean transverse momentum is measured to be 0.545±0.005(stat)±0.015(syst) GeV/c. The results are compared with similar measurements at lower energies.

First Flavor-Tagged Determination of Bounds on Mixing-Induced CP Violation in Bs⁰→J/ψϕ Decays

K. Chung et al. — Thu, 07 Feb 2013 13:40:32 PST

This Letter describes the first determination of bounds on the CP-violation parameter 2β_s using B_s⁰ decays in which the flavor of the bottom meson at production is identified. The result is based on approximately 2000 B_s⁰→J/ψϕ decays reconstructed in a 1.35 fb^-1 data sample collected with the CDF II detector using pp̅ collisions produced at the Fermilab Tevatron. We report confidence regions in the two-dimensional space of 2β_s and the decay-width difference ΔΓ. Assuming the standard model predictions of 2β_s and ΔΓ, the probability of a deviation as large as the level of the observed data is 15%, corresponding to 1.5 Gaussian standard deviations

Measurement of ratios of fragmentation fractions for bottom hadrons in pp̅ collisions at √s=1.96 TeV

K. Chung et al. — Thu, 07 Feb 2013 13:40:31 PST

This paper describes the first measurement of b-quark fragmentation fractions into bottom hadrons in Run II of the Tevatron Collider at Fermilab. The result is based on a 360 pb^-1 sample of data collected with the CDF II detector in pp̅ collisions at √s=1.96 TeV. Semileptonic decays of B̅ ⁰, B^-, and B̅ _s⁰ mesons, as well as Λ_b⁰ baryons, are reconstructed. For an effective bottom hadron p_T threshold of 7 GeV/c, the fragmentation fractions are measured to be f_u/f_d=1.054±0.018(stat)_-0.045^+0.025(sys)±0.058(B), f_s/(f_u+f_d)=0.160±0.005(stat)_-0.010^+0.011(sys)_-0.034^+0.057(B), and f_{Λ_b}/(f_u+f_d)=0.281±0.012(stat)_-0.056^+0.058(sys)_-0.087^+0.128(B), where the uncertainty B is due to uncertainties on measured branching ratios. The value of f_s/(f_u+f_d) agrees within one standard deviation with previous CDF measurements and the world average of this quantity, which is dominated by LEP measurements. However, the ratio f_{Λ_b}/(f_u+f_d) is approximately twice the value previously measured at LEP. The approximately 2σ discrepancy is examined in terms of kinematic differences between the two production environments.

Analytic smearing of SU(3) link variables in lattice QCD

Colin Morningstar et al. — Fri, 01 Feb 2013 13:48:22 PST

An analytic method of smearing link variables in lattice QCD is proposed and tested. The differentiability of the smearing scheme with respect to the link variables permits the use of modern Monte Carlo updating methods based on molecular dynamics evolution for gauge-field actions constructed using such smeared links. In examining the smeared mean plaquette and the static quark-antiquark potential, no degradation in effectiveness is observed as compared to link smearing methods currently in use, although an increased sensitivity to the smearing parameter is found.

Fine Structure of the QCD String Spectrum

Keisuke Jimmy Juge et al. — Fri, 01 Feb 2013 13:48:20 PST

Using advanced lattice methods in quantum chromodynamics, three distinct scales are established in the excitation spectrum of the gluon field around a static quark-antiquark pair as the color source separation R is varied. On the shortest length scale, the excitations are consistent with states created by local gluon field operators arising from a multipole operator product expansion. An intermediate crossover region below 2 fm is identified with a dramatic rearrangement of the level orderings. On the largest length scale of 2–3 fm, the spectrum agrees with that expected for stringlike excitations. The energies nearly reproduce asymptotic π/R string gaps, but exhibit a fine structure, providing important clues for developing an effective bosonic string description.

Scaling and further tests of heavy meson decay constant determinations from nonrelativistic QCD

S. Collins et al. — Fri, 01 Feb 2013 13:48:19 PST

We present results for the B_s meson decay constant f_{B_s} from simulations at three lattice spacings in the range a^-1=1.1 to 2.6 GeV using NRQCD heavy quarks and clover light quarks in the quenched approximation. We study the scaling of this quantity and check the consistency between mesons decaying from rest and from a state with a nonzero spatial momentum. The cancellation of power law contributions that arise in the NRQCD formulation of heavy-light currents is discussed. On the coarsest lattice the D_s meson decay constant f_{D_s} is calculated. Our best values for the decay constants are given by f_{B_s}=187(4)(4)(11)(2)(7)(6) MeV and f_{D_s}=223(6)(31)(38)(23)(9)(_-1⁺³) MeV.

Glueball spectrum from an anisotropic lattice study

Colin Morningstar et al. — Fri, 01 Feb 2013 13:48:17 PST

The spectrum of glueballs below 4 GeV in the SU(3) pure-gauge theory is investigated using Monte Carlo simulations of gluons on several anisotropic lattices with spatial grid separations ranging from 0.1 to 0.4 fm. Systematic errors from discretization and finite volume are studied, and the continuum spin quantum numbers are identified. Care is taken to distinguish single glueball states from two-glueball and torelon-pair states. Our determination of the spectrum significantly improves upon previous Wilson action calculations.

Glueball spectrum and matrix elements on anisotropic lattices

Y. Chen et al. — Thu, 31 Jan 2013 13:54:31 PST

The glueball-to-vacuum matrix elements of local gluonic operators in scalar, tensor, and pseudoscalar channels are investigated numerically on several anisotropic lattices with the spatial lattice spacing ranging from 0.1–0.2 fm. These matrix elements are needed to predict the glueball branching ratios in J/ψ radiative decays which will help identify the glueball states in experiments. Two types of improved local gluonic operators are constructed for a self-consistent check and the finite-volume effects are studied. We find that lattice spacing dependence of our results is very weak and the continuum limits are reliably extrapolated, as a result of improvement of the lattice gauge action and local operators. We also give updated glueball masses with various quantum numbers.

Group-theoretical construction of extended baryon operators in lattice QCD

Subhasish Basak et al. — Thu, 31 Jan 2013 13:54:30 PST

The design and implementation of large sets of spatially extended, gauge-invariant operators for use in determining the spectrum of baryons in lattice QCD computations are described. Group-theoretical projections onto the irreducible representations of the symmetry group of a cubic spatial lattice are used in all isospin channels. The operators are constructed to maximize overlaps with the low-lying states of interest, while minimizing the number of sources needed in computing the required quark propagators. Issues related to the identification of the spin quantum numbers of the states in the continuum limit are addressed.

Clebsch-Gordan construction of lattice interpolated fields for excited baryons

Subhasish Basak et al. — Thu, 31 Jan 2013 13:54:28 PST

Large sets of baryon interpolating field operators are developed for use in lattice QCD studies of baryons with zero momentum. Operators are classified according to the double-valued irreducible representations of the octahedral group. At first, three-quark smeared, local operators are constructed for each isospin and strangeness and they are classified according to their symmetry with respect to exchange of Dirac indices. Nonlocal baryon operators are formulated in a second step as direct products of the spinor structures of smeared, local operators together with gauge-covariant lattice displacements of one or more of the smeared quark fields. Linear combinations of direct products of spinorial and spatial irreducible representations are then formed with appropriate Clebsch-Gordan coefficients of the octahedral group. The construction attempts to maintain maximal overlap with the continuum SU(2) group in order to provide a physically interpretable basis. Nonlocal operators provide direct couplings to states that have nonzero orbital angular momentum.

Differential cross sections and recoil polarizations for the reaction γp→K⁺Σ⁰

B. Dey et al. — Wed, 30 Jan 2013 11:44:44 PST

High-statistics measurements of differential cross sections and recoil polarizations for the reaction γp→K⁺Σ⁰ have been obtained using the CLAS detector at Jefferson Lab. We cover center-of-mass energies (√s) from 1.69 to 2.84 GeV, with an extensive coverage in the K⁺ production angle. Independent measurements were made using the K⁺pπ^-(γ) and K⁺p(π^-,γ) final-state topologies, and were found to exhibit good agreement. Our differential cross sections show good agreement with earlier CLAS, SAPHIR, and LEPS results, while offering better statistical precision and a 300-MeV increase in √s coverage. Above √s≈2.5 GeV, t- and u-channel Regge scaling behavior can be seen at forward and backward angles, respectively. Our recoil polarization (P_Σ) measurements represent a substantial increase in kinematic coverage and enhanced precision over previous world data. At forward angles, we find that P_Σ is of the same order of magnitude but opposite sign as P_Λ, in agreement with the static SU(6) quark model prediction of P_Σ≈-P_Λ. This expectation is violated in some mid- and backward-angle kinematic regimes, where P_Σ and P_Λ are of similar magnitudes but also have the same signs. In conjunction with several other meson photoproduction results recently published by CLAS, the present data will help constrain the partial-wave analyses being performed to search for missing baryon resonances.

Differential cross sections and spin density matrix elements for the reaction γp→pω

M. Williams et al. — Wed, 30 Jan 2013 11:44:43 PST

High-statistics differential cross sections and spin-density matrix elements for the reaction γp→pω have been measured using the CEBAF large acceptance spectrometer (CLAS) at Jefferson Lab for center-of-mass (c.m.) energies from threshold up to 2.84 GeV. Results are reported in 112 10-MeV wide c.m. energy bins, each subdivided into cosθ_c.m.^ω bins of width 0.1. These are the most precise and extensive ω photoproduction measurements to date. A number of prominent structures are clearly present in the data. Many of these have not previously been observed due to limited statistics in earlier measurements.

Partial wave analysis of the reaction γp→pω and the search for nucleon resonances

M. Williams et al. — Wed, 30 Jan 2013 11:44:41 PST

An event-based partial wave analysis (PWA) of the reaction γp→pω has been performed on a high-statistics dataset obtained using the CLAS at Jefferson Lab for center-of-mass energies from threshold up to 2.4 GeV. This analysis benefits from access to the world’s first high-precision spin-density matrix element measurements, available to the event-based PWA through the decay distribution of ω→π⁺π^-π⁰. The data confirm the dominance of the t-channel π⁰ exchange amplitude in the forward direction. The dominant resonance contributions are consistent with the previously identified states F₁₅(1680) and D₁₃(1700) near threshold, as well as the G₁₇(2190) at higher energies. Suggestive evidence for the presence of a J^P=5/2⁺ state around 2 GeV, a “missing” state, has also been found. Evidence for other states is inconclusive.

Differential cross sections for the reactions γp→pη and γp→pη'

M. Williams et al. — Wed, 30 Jan 2013 11:44:39 PST

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.

Improved stochastic estimation of quark propagation with Laplacian Heaviside smearing in lattice QCD

Colin Morningstar et al. — Wed, 30 Jan 2013 11:44:37 PST

A new method of stochastically estimating the low-lying effects of quark propagation is proposed which allows accurate determinations of temporal correlations of single-hadron and multihadron operators in lattice QCD. The method is well suited for calculations in large volumes. Contributions involving quark propagation connecting hadron sink operators at the same final time can be handled in a straightforward manner, even for a large number of final time slices. The method exploits Laplacian Heaviside (LapH) smearing. Z_N noise is introduced in a novel way, and variance reduction is achieved using judiciously-chosen noise-dilution projectors. The method is tested using isoscalar mesons in the scalar, pseudoscalar, and vector channels, and using the two-pion system of total isospin I=0, 1, 2 on large anisotropic 24³×128 lattices with spatial spacing a_s∼0.12 fm and temporal spacing a_t∼0.034 fm for pion masses m_π≈390 and 240 MeV.

Nucleon, Δ, and Ω excited state spectra in Nf=2+1 lattice QCD

J. Bulava et al. — Wed, 30 Jan 2013 11:44:35 PST

The energies of the excited states of the nucleon, Δ, and Ω are computed in lattice QCD, using two light quarks and one strange quark on anisotropic lattices. The calculation is performed at three values of the light quark mass, corresponding to pion masses m_π=392(4), 438(3), and 521(3) MeV. We employ the variational method with a large basis of interpolating operators enabling six energies in each irreducible representation of the lattice to be distinguished clearly. We compare our calculation with the low-lying experimental spectrum, with which we find reasonable agreement in the pattern of states. The need to include operators that couple to the expected multihadron states in the spectrum is clearly identified.

Novel quark-field creation operator construction for hadronic physics in lattice QCD

Michael Peardon et al. — Wed, 30 Jan 2013 11:44:33 PST

A new quark-field smearing algorithm is defined which enables efficient calculations of a broad range of hadron correlation functions. The technique applies a low-rank operator to define smooth fields that are to be used in hadron creation operators. The resulting space of smooth fields is small enough that all elements of the reduced quark propagator can be computed exactly at reasonable computational cost. Correlations between arbitrary sources, including multihadron operators can be computed a posteriori without requiring new lattice Dirac operator inversions. The method is tested on realistic lattice sizes with light dynamical quarks.

Excited state nucleon spectrum with two flavors of dynamical fermions

John Bulava et al. — Wed, 30 Jan 2013 11:44:31 PST

Highly excited states for isospin 1/2 baryons are calculated for the first time using lattice QCD with two flavors of dynamical quarks. Anisotropic lattices are used with two pion masses, m_π=416(36) MeV and 578(29) MeV. The lowest four energies are reported in each of the six irreducible representations of the octahedral group at each pion mass. The lattices used have dimensions 24³×64, spatial lattice spacing a_s≈0.11 fm, and temporal lattice spacing a_t=1/3a_s. Clear evidence is found for a 5^-/2 state in the pattern of negative-parity excited states. This agrees with the pattern of physical states and spin 5/2 has been realized for the first time on the lattice.

First results from 2+1 dynamical quark flavors on an anisotropic lattice: Light-hadron spectroscopy and setting the strange-quark mass

Huey-Wen Lin et al. — Wed, 30 Jan 2013 11:44:30 PST

We present the first light-hadron spectroscopy on a set of N_f=2+1 dynamical, anisotropic lattices. A convenient set of coordinates that parameterize the two-dimensional plane of light and strange-quark masses is introduced. These coordinates are used to extrapolate data obtained at the simulated values of the quark masses to the physical light and strange-quark point. A measurement of the Sommer scale on these ensembles is made, and the performance of the hybrid Monte Carlo algorithm used for generating the ensembles is estimated.

Lattice QCD determination of patterns of excited baryon states

Subhasish Basak et al. — Wed, 30 Jan 2013 11:44:28 PST

Energies for excited isospin I=1/2 and I=3/2 states that include the nucleon and Δ families of baryons are computed using quenched, anisotropic lattices. Baryon interpolating field operators that are used include nonlocal operators that provide G₂ irreducible representations of the octahedral group. The decomposition of spin 5/2 or higher spin states is realized for the first time in a lattice QCD calculation. We observe patterns of degenerate energies in the irreducible representations of the octahedral group that correspond to the subduction of the continuum spin 5/2 or higher. The overall pattern of low-lying excited states corresponds well to the pattern of physical states subduced to the irreducible representations of the octahedral group.