Observation of Aubry transition in finite atom chains via friction

The highly nonlinear many-body physics of a chain of mutually interacting atoms in contact with a periodic substrate gives rise to complex static and dynamical phenomena, such as structural phase transitions and friction. In the limit of an infinite chain incommensurate with the substrate, Aubry predicted a structural transition with increasing substrate potential, from the chain's intrinsic arrangement free to slide on the substrate, to a pinned arrangement favoring the substrate pattern. To date, the Aubry transition has not been observed. Here, using a chain of cold ions subject to a periodic optical potential we qualitatively and quantitatively establish a close relation between Aubry's sliding-to-pinned transition and superlubricity breaking in stick-slip friction. Using friction measurements with high spatial resolution and individual ion detection, we experimentally observe the Aubry transition and the onset of its hallmark fractal atomic arrangement. Notably, the observed critical lattice depth for a finite chain agrees well with the Aubry prediction for an infinite chain. Our results elucidate the connection between competing ordering patterns and superlubricity in nanocontacts - the elementary building blocks of friction.Comment: 5 pages, 4 figure

Multislip Friction with a Single Ion

A trapped ion transported along a periodic potential is studied as a paradigmatic nanocontact frictional interface. The combination of the periodic corrugation potential and a harmonic trapping potential creates a one-dimensional energy landscape with multiple local minima, corresponding to multistable stick-slip friction. We measure the probabilities of slipping to the various minima for various corrugations and transport velocities. The observed probabilities show that the multislip regime can be reached dynamically at smaller corrugations than would be possible statically, and can be described by an equilibrium Boltzmann model. While a clear microscopic signature of multislip behavior is observed for the ion motion, the frictional force and dissipation are only weakly affected by the transition to multistable potentials.Comment: 8 pages, 7 figure

Measurement of ψ(2S) polarisation in pp collisions at √s = 7 TeV

The polarisation of prompt ψ(2S) mesons is measured by performing an angular analysis of ψ(2S) → μ[superscript +]μ[superscript −] decays using proton-proton collision data, corresponding to an integrated luminosity of 1.0 fb[superscript −1], collected by the LHCb detector at a centre-of-mass energy of 7 TeV. The polarisation is measured in bins of transverse momentum p[subscript T] and rapidity y in the kinematic region 3.5 < p[subscript T] < 15 GeV/c and \(2.0 , and is compared to theoretical models. No significant polarisation is observed.National Science Foundation (U.S.

Search for long-lived particles decaying to jet pairs

A search is presented for long-lived particles with a mass between 25 and 50 GeV/c[superscript 2] and a lifetime between 1 and 200 ps in a sample of proton–proton collisions at a centre-of-mass energy of √s = 7 TeV, corresponding to an integrated luminosity of 0.62 fb[superscript −1], collected by the LHCb detector. The particles are assumed to be pair-produced by the decay of a standard model-like Higgs boson. The experimental signature of the long-lived particle is a displaced vertex with two associated jets. No excess above the background is observed and limits are set on the production cross-section as a function of the long-lived particle mass and lifetime.National Science Foundation (U.S.

Measurement of the resonant and CP components in B¯0→J/ψπ+π− decays

The resonant structure of the reaction B¯0→J/ψπ+π− is studied using data from 3  fb−1 of integrated luminosity collected by the LHCb experiment, one third at 7 TeV center-of-mass energy and the remainder at 8 TeV. The invariant mass of the π+π− pair and three decay angular distributions are used to determine the fractions of the resonant and nonresonant components. Six interfering π+π− states, ρ(770), f0(500), f2(1270), ρ(1450), ω(782) and ρ(1700), are required to give a good description of invariant mass spectra and decay angular distributions. The positive and negative charge parity fractions of each of the resonant final states are determined. The f0(980) meson is not seen and the upper limit on its presence, compared with the observed f0(500) rate, is inconsistent with a model where these scalar mesons are formed from two quarks and two antiquarks (tetraquarks) at the eight standard deviation level. In the qq¯ model, the absolute value of the mixing angle between the f0(980) and the f0(500) scalar mesons is limited to be less than 17° at 90% confidence level.National Science Foundation (U.S.)United States. Dept. of Energ

Measurement of Υ production in pp collisions at √s = 2.76 TeV

The production of Υ(1S), Υ(2S) and Υ(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[superscript −1] collected in proton–proton collisions at a centre-of-mass energy of √s = 2.76 TeV. The differential production cross-sections times dimuon branching fractions are measured as functions of the Υ transverse momentum and rapidity, over the ranges p[subscript 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 σ(pp → Υ(1S)X) × B(Υ(1S) → μ[superscript +]μ[superscript −]) = 1.111 ± 0.043 ± 0.044 nb, σ(pp → Υ(2S)X) × B(Υ(2S) → μ[superscript +]μ[superscript −]) = 0.264 ± 0.023 ± 0.011 nb, σ(pp → Υ(3S)X) × B(Υ(3S) → μ[superscript +]μ[superscript −]) = 0.159 ± 0.020 ± 0.007 nb, where the first uncertainty is statistical and the second systematic.National Science Foundation (U.S.

Measurement of the Z+b-jet cross-section in pp collisions at √s = 7 TeV in the forward region

The associated production of a Z boson or an off-shell photon γ * with a bottom quark in the forward region is studied using proton-proton collisions at a centre-of-mass energy of 7 TeV. The Z bosons are reconstructed in the [Z over γ] * → μ [superscript +] μ [superscript −] final state from muons with a transverse momentum larger than 20 GeV, while two transverse momentum thresholds are considered for jets (10 GeV and 20 GeV). Both muons and jets are reconstructed in the pseudorapidity range 2.0 < η < 4.5. The results are based on data corresponding to 1.0 fb [superscript −1] recorded in 2011 with the LHCb detector. The measurement of the Z+b-jet cross-section is normalized to the Z+jet cross-section. The measured cross-sections are 0.1 σ( [Z over γ] *(μ[superscript+] μ[superscript −]) + b-jet) = 295 ± 60 (stat) ± 51 (syst) ± 10 (lumi) fb for p [subscript T] (jet) > 10 GeV, and 0.2 σ( [Z over γ]*(μ[superscript +] μ[superscript −]) + b-jet)=128±36 (stat) ±22 (syst) ±5 (lumi) fb for p [subscript T](jet) > 20 GeV.National Science Foundation (U.S.

Measurement of charged particle multiplicities and densities in pp collisions at √s = 7 TeV in the forward region

Charged particle multiplicities are studied in proton–proton collisions in the forward region at a centre-of-mass energy of √s = 7 TeV with data collected by the LHCb detector. The forward spectrometer allows access to a kinematic range of 2.0 < η < 4.8 in pseudorapidity, momenta greater than 2 GeV/ c and transverse momenta greater than 0.2 GeV/ c. The measurements are performed using events with at least one charged particle in the kinematic acceptance. The results are presented as functions of pseudorapidity and transverse momentum and are compared to predictions from several Monte Carlo event generators.National Science Foundation (U.S.

Measurement of the inelastic pp cross-section at a centre-of-mass energy of √s = 7 TeV

The cross-section for inelastic proton-proton collisions, with at least one prompt long-lived charged particle of transverse momentum p [subscript T] > 0.2GeV/c in the pseudorapidity range 2.0 < η < 4.5, is measured by the LHCb experiment at a centre-of-mass energy of √s = 7 TeV. The cross-section in this kinematic range is determined to be σ [acc over inel] = 55.0 ± 2.4 mb with an experimental uncertainty that is dominated by systematic contributions. Extrapolation to the full phase space, using Pythia 6, yields σ [subscript inel] = 66.9 ± 2.9 ± 4.4 mb, where the first uncertainty is experimental and the second is due to the extrapolation.National Science Foundation (U.S.

First Measurement of the Charge Asymmetry in Beauty-Quark Pair Production

The difference in the angular distributions between beauty quarks and antiquarks, referred to as the charge asymmetry, is measured for the first time in b[bar over b] pair production at a hadron collider. The data used correspond to an integrated luminosity of 1.0  fb[superscript −1] collected at 7 TeV center-of-mass energy in proton-proton collisions with the LHCb detector. The measurement is performed in three regions of the invariant mass of the b[bar over b] system. The results obtained are A[b[bar over b] over C](40 105  GeV/c[superscript 2]) = 1.6 ± 1.7 ± 0.6%, where A[b[bar over b] over C] is defined as the asymmetry in the difference in rapidity between jets formed from the beauty quark and antiquark, where in each case the first uncertainty is statistical and the second systematic. The beauty jets are required to satisfy 2 20  GeV, and have an opening angle in the transverse plane Δϕ > 2.6  rad. These measurements are consistent with the predictions of the standard model.National Science Foundation (U.S.