Spontaneous self-sustained currents on the cathodes of multiwire proportional chambers pose a problem for detectors in experiments at the Large Hadron Collider with prolonged exposure to radiation. The nature of spontaneous currents is studied using samples of a chamber cathode on which such currents occurred. A set of atomic force microscopy procedures for detecting and studying point emission centers is developed.

The mechanism of occurrence of spontaneous self-sustained currents in a multiwire proportional chamber from the experiment at the Large Hadron Collider has been considered. Atomic force microscopy, Rutherford backscattering, and Raman spectroscopy on the copper foil of the chamber cathode revealed the formation of nanocarbon structures and their fluorinated compounds, which are well known as low-threshold sources of field emission of electrons.

A comprehensive study is performed of the cathode of a multiwire proportional chamber subjected to prolonged irradiation in an experiment at the LHC. The structural-phase state of the copper foil on the cathode is analyzed via nuclear scanning microscopy, atomic scanning microscopy, Raman spectroscopy, and X-ray phase analysis. Nanostructural formations of carbon are found in the zone of spontaneous electron emission on the electrode.

Atomic-force microscopy methods were used to study specimens of cathodes of multiwire proportional chambers subjected to a long-term exposure to a β-source 90Sr in the course of laboratory longevity tests. The changes in the surface morphology of the copper foil at the detector cathode that have occurred as a result of action of an electron beam have been described. The quantitative estimate and analysis of the evolution of radiation defects depending on the irradiation dose are presented. The similarity of the radiation-induced defects in the laboratory prototypes and in the full-scale proportional chambers after their use for 10 years at the Large Hadron Collider is demonstrated.

Using Atomic Force Microscopy Methods, we studied the samples of the cathodes of multiwire proportional chambers after long-term irradiation with a beta-source 90Sr at the longevity test. The changes in the morphology of the copper foil surface at the cathode of the detector as a result of the influence of the electron flow are described. It is presented a quantitative assessment and analysis of the evolution of the resulting radiation defects depending on the irradiation conditions. It is shown the similarity of the radiation defects in the laboratory prototypes and in full scale proportional chambers that have been operated at the LHC for almost 10 years.

A method is presented for restoring the performance of gas discharge detectors wherein a spontaneous self-sustaining current, i.e., Malter effect, occurs. A successful practical implementation of the method is demonstrated by the example of recovery of operability for multiwire proportional chambers used in the muon detector of the LHCb experiment carried out at the Large Hadron Collider. Four proportional chambers wherein Malter currents regularly occur during the experiment were subjected to high-voltage discharge training in the working gas mixture of 40% Ar + 55% CO2 + 5% CF4 with 2% of oxygen added. It is shown that, with addition of oxygen, the recovery of the proportional chambers occurs tens of times faster in compare to the training in the working gas mixture. The reconstructed chambers were installed in the LHCb muon detector and have been working in a collider beam experiment for more than two years already.

The production fraction of the Bc-meson with respect to the sum of B- A nd B0 mesons is measured in both 7 and 13 TeV center-of-mass (c.m.) energy pp collisions produced by the Large Hadron Collider (LHC), using the LHCb detector. The rate, approximately 3.7 per mille, does not change with energy, but shows a transverse momentum dependence. The Bc-Bc+ production asymmetry is also measured and is consistent with zero within the determined statistical and systematic uncertainties of a few percent.

R. Aaij et al. (LHCb Collaboration) (Received 25 June 2016; published 11 January 2017) The first full amplitude analysis of Bþ → J=ψφKþ with J=ψ → μþμ−, φ → KþK− decays is performed with a data sample of 3 fb−1 of pp collision data collected at ffiffi s p 1⁄4 7 and 8 TeV with the LHCb detector. The data cannot be described by a model that contains only excited kaon states decaying into φKþ, and four J=ψφ structures are observed, each with significance over 5 standard deviations. The quantum numbers of these structures are determined with significance of at least 4 standard deviations. The lightest has mass consistent with, but width much larger than, previous measurements of the claimed Xð4140Þ state. The model includes significant contributions from a number of expected kaon excitations, including the first observation of the K ð1680Þþ → φKþ transition.

Article history: Received 9 January 2017 Received in revised form 6 April 2017 Accepted 4 May 2017 Available online 12 May 2017 Editor: L. Rolandi A search for CP violation in D± → η′π± and D±s → η′π± decays is performed using proton–proton collision data, corresponding to an integrated luminosity of 3 fb−1, recorded by the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The measured CP-violating charge asymmetries are ACP(D± → η′π±) = (−0.61 ± 0.72 ± 0.53 ± 0.12)% and ACP(D±s → η′π±) = (−0.82 ± 0.36 ± 0.22 ± 0.27)%, where the first uncertainties are statistical, the second systematic, and the third are the uncertainties on the ACP(D± → K 0 S π±) and ACP(D±s → φπ±) measurements used for calibration. The results represent the most precise measurements of these asymmetries to date. © 2017 The Author. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.

Background: The rate lambda(pp mu) characterizes the formation of pp mu molecules in collisions of muonic p mu atoms with hydrogen. In measurements of the basic weak muon capture reaction on the proton to determine the pseudoscalar coupling g(P), capture occurs from both atomic and molecular states. Thus knowledge of lambda(pp mu) is required for a correct interpretation of these experiments.Purpose: Recently the MuCap experiment has measured the capture rate Lambda(S) from the singlet p mu atom, employing a low-density active target to suppress pp mu formation [V. Andreev et al. (MuCap Collaboration), Phys. Rev. Lett. 110, 012504 (2013)]. Nevertheless, given the unprecedented precision of this experiment, the existing experimental knowledge in lambda(pp mu) had to be improved.Method: The MuCap experiment derived the weak capture rate from the muon disappearance rate in ultrapure hydrogen. By doping the hydrogen with 20 ppm of argon, a competing process to pp mu formation was introduced, which allowed the extraction of pp mu from the observed time distribution of decay electrons.Results: The pp mu formation rate was measured as lambda(pp mu) = (2.01 +/- 0.06(stat) +/- 0.03(sys)) x 10(6) s(-1). This result updates the pp mu value used in the abovementioned MuCap publication.Conclusions: The 2.5x higher precision compared to earlier experiments, and the fact that the measurement was performed under nearly identical conditions as the main data taking, reduces the uncertainty induced by lambda(pp mu) to a minor contribution to the overall uncertainty of Lambda(S) and g(P), as determined in the MuCap experiment. Our final value for lambda(pp mu) shifts Lambda(S) and g(P) by less than one-tenth of their respective uncertainties compared to our results published earlier.

. The MuCap experiment at the Paul Scherrer Institute performed a high-precision measurement of the rate of the basic electroweak process of nuclear muon capture by the proton, μ^-+p→ n+ν_μ . The experimental approach was based on the use of a time projection chamber (TPC) that operated in pure hydrogen gas at a pressure of 10bar and functioned as an active muon stopping target. The TPC detected the tracks of individual muon arrivals in three dimensions, while the trajectories of outgoing decay (Michel) electrons were measured by two surrounding wire chambers and a plastic scintillation hodoscope. The muon and electron detectors together enabled a precise measurement of the μ p atom’s lifetime, from which the nuclear muon capture rate was deduced. The TPC was also used to monitor the purity of the hydrogen gas by detecting the nuclear recoils that follow muon capture by elemental impurities. This paper describes the TPC design and performance in detail.

The MuCap experiment at the Paul Scherrer Institute has measured the rate Λ(S) of muon capture from the singlet state of the muonic hydrogen atom to a precision of 1%. A muon beam was stopped in a time projection chamber filled with 10-bar, ultrapure hydrogen gas. Cylindrical wire chambers and a segmented scintillator barrel detected electrons from muon decay. Λ(S) is determined from the difference between the μ(-) disappearance rate in hydrogen and the free muon decay rate. The result is based on the analysis of 1.2 × 10(10) μ(-) decays, from which we extract the capture rate Λ(S) = (714.9 ± 5.4(stat) ± 5.1(syst)) s(-1) and derive the proton's pseudoscalar coupling g(P)(q(0)(2) = -0.88 m(μ)(2)) = 8.06 ± 0.55.

The production of χb(1P ) mesons in pp collisions at a centre-of-mass energy of 7 TeV is studied using 32 pb−1 of data collected with the LHCb detector. The χb(1P ) mesons are reconstructed in the decay mode χ b (1P ) → Υ (1S)γ → μ + μ −γ. The fraction of Υ (1S) originating from χb(1P ) decays in the Υ (1S) transverse momentum range 6 < pT Υ (1S) < 15 GeV/c and rapidity range 2.0 < y Υ (1S) < 4.5 is measured to be \( \left( {20.7\pm 5.7\pm 2.1_{-5.4}^{+2.7 }} \right)\% \), where the first uncertainty is statistical, the second is systematic and the last gives the range of the result due to the unknown Υ (1S) and χb(1P) polarizations.

The ratio of branching fractions of the radiative B decays B0 -> K*0 gamma and Bs0 -> phi gamma has been measured using 0.37 fb-1 of pp collisions at a centre of mass energy of sqrt(s) = 7 TeV, collected by the LHCb experiment. The value obtained is BR(B0 -> K*0 gamma)/BR(Bs0 -> phi gamma) = 1.12 +/- 0.08 ^+0.06_-0.04 ^+0.09_-0.08, where the first uncertainty is statistical, the second systematic and the third is associated to the ratio of fragmentation fractions fs/fd. Using the world average for BR(B0 -> K*0 gamma) = (4.33 +/- 0.15) x 10^-5, the branching fraction BR(Bs0 -> phi gamma) is measured to be (3.9 +/- 0.5) x 10^-5, which is the most precise measurement to date.

The first observation of the decay B-0 -> (D) over bar (0) K+ K- is reported from an analysis of 0.62 fb(-1) of pp collision data collected with the LHCb detector. Its branching fraction is measured relative to that of the topologically similar decay B-0 -> (D) over bar (0) pi(+) pi(-) to be B(B-0 -> (D) over bar (0) K+ K-)/B(B-0 -> (D) over bar (0) pi(+) pi(-)) = 0.056 +/- 0.011 +/- 0.007; where the first uncertainty is statistical and the second is systematic. The significance of the signal is 5.8 sigma. Evidence, with 3.8 sigma significance, for B-s(0) -> (D) over bar (0) K+ K- decays is also presented. The relative branching fraction is measured to be B(B-s(0) -> (D) over bar (0) K+ K-)/B(B-0 -> (D) over bar (0) K+ K-) = 0.90 +/- 0.27 +/- 0.20. These channels are of interest to study the mechanisms behind hadronic B decays, and open new possibilities for CP violation analyses with larger data sets.

The B-s(0) -> J/psi K-s(0) branching fraction is measured in a data sample corresponding to 0.41 fb(-1) of integrated luminosity collected with the LHCb detector at the LHC. This channel is sensitive to the penguin contributions affecting the sin 2 beta measurement from B-0 -> J/psi K-s(0)). The time-integrated branching fraction is measured to be B(B-s(0) -> J/psi K-s(0)) = (1.83 +/- 0.28) x 10(-5). This is the most precise measurement to date. (C) 2012 CERN. Published by Elsevier B.V. All rights reserved.

The first observation of the decay (B) over bar (0)(S) -> (DK)-K-0*(0) using pp data collected by the LHCb detector at a centre-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 36 pb(-1), is reported. A signal of 34.4 +/- 6.8 events is obtained and the absence of signal is rejected with a statistical significance of more than nine standard deviations. The (B) over bar (0)(S) -> (DK)-K-0*(0) branching fraction is measured relative to that of (B) over bar (0) -> D-0 rho(0): B((B) over bar (0)(S)->(DK)-K-0*(0))/B((B) over bar (0)-> D-0 rho(0)) = 1.48+/-0.34+/-0.15+/-0.12, where the first uncertainty is statistical, the second systematic and the third is due to the uncertainty on the ratio of the B-0 and B-s(0) hadronisation fractions. (C) 2011 CERN. Published by Elsevier B.V. All rights reserved.