arXiv subjects

J. Anderson

Publications and source records attributed to J. Anderson.

At least 253 records · Page 14Linked to original sources

arXiv2015

Observation of two new $Ξ_b^-$ baryon resonances

Two structures are observed close to the kinematic threshold in the $Ξ_b^0 π^-$ mass spectrum in a sample of proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb$^{-1}$ recorded by the LHCb experiment. In the quark model, two baryonic resonances with quark content $bds$ are expected in this mass region: the spin-parity $J^P = \frac{1}{2}^+$ and $J^P=\frac{3}{2}^+$ states, denoted $Ξ_b^{\prime -}$ and $Ξ_b^{*-}$. Interpreting the structures as these resonances, we measure the mass differences and the width of the heavier state to be $m(Ξ_b^{\prime -}) - m(Ξ_b^0) - m(π^{-}) = 3.653 \pm 0.018 \pm 0.006$ MeV$/c^2$, $m(Ξ_b^{*-}) - m(Ξ_b^0) - m(π^{-}) = 23.96 \pm 0.12 \pm 0.06$ MeV$/c^2$, $Γ(Ξ_b^{*-}) = 1.65 \pm 0.31 \pm 0.10$ MeV, where the first and second uncertainties are statistical and systematic, respectively. The width of the lighter state is consistent with zero, and we place an upper limit of $Γ(Ξ_b^{\prime -}) < 0.08$ MeV at 95% confidence level. Relative production rates of these states are also reported.

hep-ex↗

arXiv2015

Measurement of the Z+b-jet cross-section in pp collisions at $\sqrt{s}=7{\mathrm{\,Te\kern -0.1em V}}$ 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 $7{\mathrm{\,Te\kern -0.1em V}}$. The Z bosons are reconstructed in the ${\text{Z}/γ^*}\!\rightarrow{μ^{+}μ^{-}}$ final state from muons with a transverse momentum larger than $20{\mathrm{\,Ge\kern -0.1em V}}$, while two transverse momentum thresholds are considered for jets ($10{\mathrm{\,Ge\kern -0.1em V}}$ and $20{\mathrm{\,Ge\kern -0.1em V}}$). Both muons and jets are reconstructed in the pseudorapidity range $2.0 < η< 4.5$. The results are based on data corresponding to $1.0{\,\mbox{fb}^{-1}}$ 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 \begin{equation*} σ(\text{$\text{Z}/γ^*(μ^{+}μ^{-})$+b-jet}) = 295 \pm 60~(\text{stat}) \pm 51~(\text{syst}) \pm 10~(\text{lumi}) {\,\mbox{fb}} \end{equation*} for ${p_{\rm T}}$(jet)$>10{\mathrm{\,Ge\kern -0.1em V}}$, and \begin{equation*} σ(\text{$\text{Z}/γ^*(μ^{+}μ^{-})$+b-jet}) = 128 \pm 36~(\text{stat}) \pm 22~(\text{syst}) \pm 5~(\text{lumi}) {\,\mbox{fb}} \end{equation*} for ${p_{\rm T}}$(jet)$>20{\mathrm{\,Ge\kern -0.1em V}}$.

hep-ex↗

arXiv2015

Measurement of the track reconstruction efficiency at LHCb

The determination of track reconstruction efficiencies at LHCb using $J/ψ\rightarrowμ^{+}μ^{-}$ decays is presented. Efficiencies above $95\%$ are found for the data taking periods in 2010, 2011, and 2012. The ratio of the track reconstruction efficiency of muons in data and simulation is compatible with unity and measured with an uncertainty of $0.8\,\%$ for data taking in 2010, and at a precision of $0.4\,\%$ for data taking in 2011 and 2012. For hadrons an additional $1.4\,\%$ uncertainty due to material interactions is assumed. This result is crucial for accurate cross section and branching fraction measurements in LHCb.

hep-ex↗

arXiv2015

Making FORS2 fit for exoplanet observations (again)

For about three years, it was known that precision spectrophotometry with FORS2 suffered from systematic errors that made quantitative observations of planetary transits impossible. We identified the Longitudinal Atmospheric Dispersion Compensator (LADC) as the most likely culprit, and therefore engaged in a project to exchange the LADC prisms with the uncoated ones from FORS1. This led to a significant improvement in the depth of FORS2 zero points, a reduction in the systematic noise, and should make FORS2 again competitive for transmission spectroscopy of exoplanets.

astro-ph.SR↗

arXiv2015

Measurement of the semileptonic $CP$ asymmetry in $B^0-\overline{B}{}^0$ mixing

The semileptonic $CP$ asymmetry in $B^0-\overline{B}{}^0$ mixing, $a_{\rm sl}^d$, is measured in proton-proton collision data, corresponding to an integrated luminosity of 3.0 fb$^{-1}$, recorded by the LHCb experiment. Semileptonic $B^0$ decays are reconstructed in the inclusive final states $D^-μ^+$ and $D^{*-}μ^+$, where the $D^-$ meson decays into the $K^+π^-π^-$ final state, and the $D^{*-}$ meson into the $\overline{D}{}^0(\rightarrow K^+π^-)π^-$ final state. The asymmetry between the numbers of $D^{(*)-}μ^+$ and $D^{(*)+}μ^-$ decays is measured as a function of the decay time of the $B^0$ mesons. The $CP$ asymmetry is measured to be $a_{\rm sl}^d = (-0.02 \pm 0.19 \pm 0.30)\%$, where the first uncertainty is statistical and the second systematic. This is the most precise measurement of $a_{\rm sl}^d$ to date and is consistent with the prediction from the Standard Model.

hep-ex↗

arXiv2015

Hubble Space Telescope observations of the Kepler-field cluster NGC 6819. I. The bottom of the white dwarf cooling sequence

We use Hubble Space Telescope (HST) to reach the end of the white dwarf (WD) cooling sequence (CS) in the solar-metallicity open cluster NGC 6819. Our photometry and completeness tests show a sharp drop in the number of WDs along the CS at magnitudes fainter than mF606W = 26.050+/- 0.075. This implies an age of 2.25+/-0.20 Gyr, consistent with the age of 2.25+/-0.30 Gyr obtained from fits to the main-sequence turn-off. The use of different WD cooling models and initial-final-mass relations have a minor impact the WD age estimate, at the level of ~0.1 Gyr. As an important by-product of this investigation we also release, in electronic format, both the catalogue of all the detected sources and the atlases of the region (in two filters). Indeed, this patch of sky studied by HST (of size ~70 arcmin sq.) is entirely within the main Kepler-mission field, so the high-resolution images and deep catalogues will be particularly useful.

astro-ph.SR↗

arXiv2014

Determination of the branching fractions of $B_{s}^{0} \to D_{s}^{\mp} K^{\pm}$ and $B^{0} \to D_{s}^{-} K^{+}$

Measurements are presented of the branching fractions of the decays $B_{s}^{0} \to D_{s}^{\mp} K^{\pm}$ and $B^{0} \to D_{s}^{-} K^{+}$ relative to the decays $B_{s}^{0} \to D_{s}^{-} π^{+}$ and $B^{0} \to D^{-} π^{+}$, respectively. The data used correspond to an integrated luminosity of 3.0 fb$^{-1}$ of proton-proton collisions. The ratios of branching fractions are $\dfrac{\mathcal{B}(B_{s}^{0} \to D_{s}^{\mp} K^{\pm})}{\mathcal{B}(B_{s}^{0} \to D_{s}^{-} π^{+})} = 0.0752 \pm 0.0015 \pm 0.0019$ and $\dfrac{\mathcal{B}(B^{0} \to D_{s}^{-} K^{+})}{\mathcal{B}(B^{0} \to D^{-} π^{+})} = 0.0129 \pm 0.0005 \pm 0.0008,$ where the uncertainties are statistical and systematic, respectively.

hep-ex↗

arXiv2014

Determination of gamma and -2beta_s from charmless two-body decays of beauty mesons

Using the latest LHCb measurements of time-dependent CP violation in the B^0_s -> K^+K^- decay, a U-spin relation between the decay amplitudes of B^0_s -> K^+K^- and B^0 -> π^+π^- decay processes allows constraints to be placed on the angle gamma of the unitarity triangle and on the B^0_s mixing phase -2β_s. Results from an extended approach, which uses additional inputs on B^0 -> π^0π^0 and B^+ -> π^+π^0 decays from other experiments and exploits isospin symmetry, are also presented. The dependence of the results on the maximum allowed amount of U-spin breaking is studied. At 68% probability, the value γ= ( 63.5 +7.2 -6.7 ) degrees modulo 180 degrees is determined. In an alternative analysis, the value -2β_s = -0.12 +0.14 -0.16 rad is found. In both measurements, the uncertainties due to U-spin breaking effects up to 50% are included.

hep-ex↗

arXiv2014

Measurement of the CP-violating phase $β$ in $B^0\rightarrow J/ψπ^+π^-$ decays and limits on penguin effects

Time-dependent CP violation is measured in the $B^0\rightarrow J/ψπ^+π^-$ channel for each $π^+π^-$ resonant final state using data collected with an integrated luminosity of 3.0 fb$^{-1}$ in $pp$ collisions using the LHCb detector. The final state with the largest rate, $J/ψρ^0(770)$, is used to measure the CP-violating angle $2β^{\rm eff}$ to be $(41.7\pm 9.6_{-6.3}^{+2.8})^{\circ}$. This result can be used to limit the size of penguin amplitude contributions to CP violation measurements in, for example, $B_s^0\rightarrow J/ψϕ$ decays. Assuming approximate SU(3) flavour symmetry and neglecting higher order diagrams, the shift in the CP-violating phase $ϕ_s$ is limited to be within the interval [$-1.05^\circ$, +$1.18^\circ$] at 95% confidence level. Changes to the limit due to SU(3) symmetry breaking effects are also discussed.

hep-ex↗

arXiv2014

Measurement of the forward $W$ boson cross-section in $pp$ collisions at $\sqrt{s} = 7 {\rm \, TeV}$

A measurement of the inclusive $W \to μν$ production cross-section using data from $pp$ collisions at a centre-of-mass energy of $\sqrt{s} = 7 {\rm \, TeV}$ is presented. The analysis is based on an integrated luminosity of about $1.0 {\rm \, fb}^{-1}$ recorded with the LHCb detector. Results are reported for muons with a transverse momentum greater than $20 {\rm \, Ge\kern -0.1em V\!/}c$ and pseudorapidity between 2.0 and 4.5. The $W^{+}$ and $W^{-}$ production cross-sections are measured to be $$σ_{W^{+} \to μ^{+}ν} = 861.0 \pm 2.0 \pm 11.2 \pm 14.7 {\rm \, pb},$$ $$σ_{W^{-} \to μ^{-}\overlineν} = 675.8 \pm 1.9 \pm 8.8 \pm 11.6 {\rm \, pb},$$ where the first uncertainty is statistical, the second is systematic and the third is due to the luminosity determination. Cross-section ratios and differential distributions as functions of the muon pseudorapidity are also presented. The ratio of $W^{+}$ to $W^{-}$ cross-sections in the same fiducial kinematic region is determined to be $$\frac{σ_{W^{+} \to μ^{+}ν}}{σ_{W^{-} \to μ^{-}\overlineν}} = 1.274 \pm 0.005 \pm 0.009,$$ where the uncertainties are statistical and systematic, respectively. Results are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics.

hep-ex↗

arXiv2014

Statistical analysis and modeling of intermittent transport events in the tokamak SOL

The turbulence observed in the scrape-off-layer of a tokamak is often characterized by intermittent events of bursty nature, a feature which raises concerns about the prediction of heat loads on the physical boundaries of the device. It appears thus necessary to delve into the statistical properties of turbulent physical fields such as density, electrostatic potential and temperature, focusing on the mathematical expression of tails of the probability distribution functions. The method followed here is to generate statistical information from time-traces of the plasma density stemming from Braginskii-type fluid simulations, and check this against a first-principles theoretical model. The analysis of the numerical simulations indicates that the probability distribution function of the intermittent process contains strong exponential tails, as predicted by the analytical theory.

physics.plasm-ph↗

arXiv2014

Measurements of $C\!P$ violation in the three-body phase space of charmless $B^{\pm}$ decays

The charmless three-body decay modes $B^{\pm} \rightarrow K^{\pm} π^{+} π^{-}$, $B^{\pm} \rightarrow K^{\pm} K^{+} K^{-}$, $B^{\pm} \rightarrow π^{\pm} K^{+} K^{-}$ and $B^{\pm} \rightarrow π^{\pm} π^{+} π^{-}$ are reconstructed using data, corresponding to an integrated luminosity of 3.0\;$\mbox{\,fb}^{-1}$, collected by the LHCb detector. The inclusive $C\!P$ asymmetries of these modes are measured to be $$ A_{C\!P}(B^{\pm} \rightarrow K^{\pm} π^{+} π^{-})= +0.025 \pm 0.004 \pm 0.004 \pm 0.007 \\ A_{C\!P}(B^{\pm} \rightarrow K^{\pm} K^{+} K^{-}) = -0.036 \pm 0.004 \pm 0.002 \pm 0.007 \\ A_{C\!P}(B^{\pm} \rightarrow π^{\pm} π^{+} π^{-})= +0.058 \pm 0.008 \pm 0.009 \pm 0.007 \\ A_{C\!P}(B^{\pm} \rightarrow π^{\pm} K^{+} K^{-})= -0.123 \pm 0.017 \pm 0.012 \pm 0.007 $$ %% where the first uncertainty is statistical, the second systematic, and the third is due to the $C\!P$ asymmetry of the $B^{\pm} \rightarrow J/ψK^{\pm}$ reference mode. The distributions of these asymmetries are also studied as functions of position in the Dalitz plot and suggest contributions from rescattering and resonance interference processes.

hep-ex↗

arXiv2014

Search for CP violation in $D^0\toπ^-π^+π^0$ decays with the energy test

A search for time-integrated CP violation in the Cabibbo-suppressed decay $D^0\toπ^-π^+π^0$ is performed using for the first time an unbinned model-independent technique known as the energy test. Using proton-proton collision data, corresponding to an integrated luminosity of 2.0 fb$^{-1}$ collected by the LHCb detector at a centre-of-mass energy of $\sqrt{s}$ = 8 TeV, the world's best sensitivity to CP violation in this decay is obtained. The data are found to be consistent with the hypothesis of CP symmetry with a p-value of (2.6 +/- 0.5)%.

hep-ex↗

arXiv2014

Precision measurement of the mass and lifetime of the Xi_b^- baryon

We report on measurements of the mass and lifetime of the Xi_b^- baryon using about 1800 Xi_b^- decays reconstructed in a proton-proton collision data set corresponding to an integrated luminosity of 3.0 fb^{-1} collected by the LHCb experiment. The decays are reconstructed in the Xi_b^- --> Xi_c^0 pi^-, Xi_c^0 --> pK-K-pi+ channel and the mass and lifetime are measured using the Lambda_b^0 --> Lambda_c^+ pi- mode as a reference. We measure M(Xi_b^-) - M(Lambda_b^0) = 178.36 +- 0.46 +- 0.16 MeV/c^2 and tau(Xi_b^-)/tau(Lambda_b^0) = 1.089 +- 0.026 +- 0.011, where the uncertainties are statistical and systematic, respectively. These results lead to a factor of two better precision on the Xi_b^- mass and lifetime compared to previous best measurements, and are consistent with theoretical expectations.

hep-ex↗

arXiv2014

Measurement of the inelastic $pp$ cross-section at a centre-of-mass energy of $\sqrt{s}=7$ TeV

The cross-section for inelastic proton-proton collisions, with at least one prompt long-lived charged particle of transverse momentum $p_{\rm T}>0.2$ GeV/$c$ in the pseudorapidity range $2.0<η<4.5$, is measured by the LHCb experiment at a centre-of-mass energy of $\sqrt{s}=7$ TeV. The cross-section in this kinematic range is determined to be $σ_{\rm inel}^{\rm acc} = 55.0 \pm 2.4$ mb within the spectrometer acceptance with an experimental uncertainty that is dominated by systematic contributions. Extrapolation to the full phase space, using PYTHIA 6, yields $σ_{\rm inel} = 66.9 \pm 2.9 \pm 4.4$ mb, where the first uncertainty is experimental and the second is due to the extrapolation.

hep-ex↗

arXiv2014

Precision luminosity measurements at LHCb

Measuring cross-sections at the LHC requires the luminosity to be determined accurately at each centre-of-mass energy $\sqrt{s}$. In this paper results are reported from the luminosity calibrations carried out at the LHC interaction point 8 with the LHCb detector for $\sqrt{s}$ = 2.76, 7 and 8 TeV (proton-proton collisions) and for $\sqrt{s_{NN}}$ = 5 TeV (proton-lead collisions). Both the "van der Meer scan" and "beam-gas imaging" luminosity calibration methods were employed. It is observed that the beam density profile cannot always be described by a function that is factorizable in the two transverse coordinates. The introduction of a two-dimensional description of the beams improves significantly the consistency of the results. For proton-proton interactions at $\sqrt{s}$ = 8 TeV a relative precision of the luminosity calibration of 1.47% is obtained using van der Meer scans and 1.43% using beam-gas imaging, resulting in a combined precision of 1.12%. Applying the calibration to the full data set determines the luminosity with a precision of 1.16%. This represents the most precise luminosity measurement achieved so far at a bunched-beam hadron collider.

hep-ex↗

arXiv2014

Measurement of CP violation parameters in B0->D K*0 decays

An analysis of B0->D K*0 decays is presented, where D represents an admixture of D0 and D0b mesons reconstructed in four separate final states: K-pi+, pi-K+, K+K- and pi+pi-. The data sample corresponds to 3.0fb-1 of proton-proton collision, collected by the LHCb experiment. Measurements of several observables are performed, including CP asymmetries. The most precise determination is presented of rB(DK*0), the magnitude of the ratio of the amplitudes of the decay B0->D K+ pi- with a b->u or a b->c transition, in a K pi mass region of +/-50 MeV/c2 around the K*(892) mass and for an absolute value of the cosine of the K*0 helicity angle larger than 0.4.

hep-ex↗

arXiv2014

Observation of $B^0_s \to K^{\pm}K^\mp$ and evidence for $B^0_s \to K^{-}π^+$ decays

Measurements of the branching fractions of $B^0_s \to K^{*\pm}K^\mp$ and $B^0_s \to K^{*\pm}π^\mp$ decays are performed using a data sample corresponding to $1.0 \ {\rm fb}^{-1}$ of proton-proton collision data collected with the LHCb detector at a centre-of-mass energy of $7\mathrm{\,TeV}$, where the $K^{*\pm}$ mesons are reconstructed in the $K^0_{\rm S}π^\pm$ final state. The first observation of the $B^0_s \to K^{*\pm}K^\mp$ decay and the first evidence for the $B^0_s \to K^{*-}π^+$ decay are reported with branching fractions \begin{eqnarray} {\cal B}(B^0_s \to K^{*\pm}K^\mp) & = & (12.7\pm1.9\pm1.9) \times 10^{-6}, \\ {\cal B}(B^0_s \to K^{*-}π^+) & = & ~(3.3\pm1.1\pm0.5) \times 10^{-6}, \end{eqnarray} where the first uncertainties are statistical and the second are systematic. In addition, an upper limit of ${\cal B}(B^0 \to K^{*\pm}K^\mp) < 0.4 \ (0.5) \times 10^{-6}$ is set at $90\,% \ (95\,%)$ confidence level.

hep-ex↗