arXiv subjects

S. Baker

Publications and source records attributed to S. Baker.

At least 181 records · Page 10Linked to original sources

arXiv2018

Studies of the resonance structure in $D^{0} \to K^\mp π^\pm π^\pm π^\mp$ decays

Amplitude models are constructed to describe the resonance structure of ${D^{0}\to K^{-}π^{+}π^{+}π^{-}}$ and ${D^{0} \to K^{+}π^{-}π^{-}π^{+}}$ decays using $pp$ collision data collected at centre-of-mass energies of 7 and 8 TeV with the LHCb experiment, corresponding to an integrated luminosity of $3.0\mathrm{fb}^{-1}$. The largest contributions to both decay amplitudes are found to come from axial resonances, with decay modes $D^{0} \to a_1(1260)^{+} K^{-}$ and $D^{0} \to K_1(1270/1400)^{+} π^{-}$ being prominent in ${D^{0}\to K^{-}π^{+}π^{+}π^{-}}$ and $D^{0}\to K^{+}π^{-}π^{-}π^{+}$, respectively. Precise measurements of the lineshape parameters and couplings of the $a_1(1260)^{+}$, $K_1(1270)^{-}$ and $K(1460)^{-}$ resonances are made, and a quasi model-independent study of the $K(1460)^{-}$ resonance is performed. The coherence factor of the decays is calculated from the amplitude models to be $R_{K3π} = 0.459\pm 0.010\,(\mathrm{stat}) \pm 0.012\,(\mathrm{syst}) \pm 0.020\,(\mathrm{model})$, which is consistent with direct measurements. These models will be useful in future measurements of the unitary-triangle angle $γ$ and studies of charm mixing and $C\!P$ violation.

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arXiv2018

Evidence for the rare decay $Σ^+ \to p μ^+ μ^-$

A search for the rare decay $Σ^+ \to p μ^+ μ^-$ is performed using $pp$ collision data recorded by the LHCb experiment at centre-of-mass energies $\sqrt{s} = 7$ and $8$ TeV, corresponding to an integrated luminosity of $3 fb^{-1}$. An excess of events is observed with respect to the background expectation, with a signal significance of 4.1 standard deviations. No significant structure is observed in the dimuon invariant mass distribution, in contrast with a previous result from the HyperCP experiment. The measured $Σ^+ \to p μ^+ μ^-$ branching fraction is $(2.2\,^{+\,1.8}_{-\,1.3})\times 10^{-8}$, where statistical and systematic uncertainties are included, which is consistent with the Standard Model prediction.

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arXiv2018

Search for massive long-lived particles decaying semileptonically in the LHCb detector

A search is presented for massive long-lived particles decaying into a muon and two quarks. The dataset consists of proton-proton interactions at centre-of-mass energies of 7 and 8 TeV, corresponding to integrated luminosities of 1 and 2 1/fb, respectively. The analysis is performed assuming a set of production mechanisms with simple topologies, including the production of a Higgs-like particle decaying into two long-lived particles. The mass range from 20 to 80 GeV and lifetimes from 5 to 100 ps are explored. Results are also interpreted in terms of neutralino production in different supersymmetric models, with masses in the 23-198 GeV range. No excess above the background expectation is observed and upper limits are set on the production cross-section for various points in the parameter space of theoretical models.

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arXiv2018

Observation of the $Λ_b^0\toΛϕ$ decay

The $Λ_b^0\toΛϕ$ decay is observed using data corresponding to an integrated luminosity of 3.0fb$^{-1}$ recorded by the LHCb experiment. The decay proceeds at leading order via a $b\to s\bar{s}s$ loop transition and is therefore sensitive to the possible presence of particles beyond the Standard Model. A first observation is reported with a significance of $5.9$ standard deviations. The value of the branching fraction is measured to be $(5.18\pm1.04\pm0.35\,^{+0.67}_{-0.62})\times10^{-6}$, where the first uncertainty is statistical, the second is systematic, and the third is related to external inputs. Triple-product asymmetries are measured to be consistent with zero.

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arXiv2018

Search for the rare decay $Λ_{c}^{+} \to pμ^+μ^-$

A search for the flavor-changing neutral-current decay $Λ_{c}^{+} \to pμ^+μ^-$ is reported using a data set corresponding to an integrated luminosity of $3.0\rm fb^{-1}$ collected by the LHCb collaboration. No significant signal is observed outside of the dimuon mass regions around the $ϕ$ and $ω$ resonances and an upper limit is placed on the branching fraction of $\mathcal{B} (Λ_{c}^{+} \to pμ^+μ^-) < 7.7~(9.6)\times 10^{-8}~{\rm at}~90\%~(95\%)$ confidence level. A significant signal is observed in the $ω$ dimuon mass region for the first time.

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arXiv2018

Test of Lepton Flavor Universality by the measurement of the $B^0 \to D^{*-} τ^+ ν_τ$ branching fraction using three-prong $τ$ decays

The ratio of branching fractions ${\cal{R}}(D^{*-})\equiv {\cal{B}}(B^0 \to D^{*-} τ^+ ν_τ)/{\cal{B}}(B^0 \to D^{*-} μ^+ν_μ)$ is measured using a data sample of proton-proton collisions collected with the LHCb detector at center-of-mass energies of 7 and 8 TeV, corresponding to an integrated luminosity of 3$~$fb$^{-1}$. The $τ$ lepton is reconstructed with three charged pions in the final state. A novel method is used that exploits the different vertex topologies of signal and backgrounds to isolate samples of semitauonic decays of $b$ hadrons with high purity. Using the $B^0 \to D^{*-}π^+π^-π^+$ decay as the normalization channel, the ratio ${\cal{B}}(B^0 \to D^{*-} τ^+ ν_τ)/{\cal{B}}(B^0 \to D^{*-}π^+π^-π^+)$ is measured to be $1.97 \pm 0.13 \pm 0.18$, where the first uncertainty is statistical and the second systematic. An average of branching fraction measurements for the normalization channel is used to derive ${\cal{B}}(B^0 \to D^{*-} τ^+ ν_τ) = (1.42 \pm 0.094 \pm 0.129 \pm 0.054) \%$, where the third uncertainty is due to the limited knowledge of ${\cal{B}}(B^0\to D^{*-}π^+π^-π^+)$. A test of lepton flavor universality is performed using the well-measured branching fraction ${\cal{B}}(B^0 \to D^{*-} μ^+ν_μ)$ to compute ${\cal{R}}(D^{*-}) = 0.291 \pm 0.019 \pm 0.026 \pm 0.013$, where the third uncertainty originates from the uncertainties on ${\cal{B}}(B^0 \to D^{*-}π^+π^-π^+)$ and ${\cal{B}}(B^0 \to D^{*-} μ^+ν_μ)$. This measurement is in agreement with the Standard Model prediction and with previous measurements.

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arXiv2018

Measurement of the ratio of the $B^0 \to D^{-} τ^+ ν_τ$ and $B^0 \to D^{-} μ^+ ν_μ$ branching fractions using three-prong $τ$-lepton decays

The ratio of branching fractions ${\cal{R}}(D^{*-})\equiv {\cal{B}}(B^0 \to D^{*-} τ^+ ν_τ)/{\cal{B}}(B^0 \to D^{*-} μ^+ν_μ)$ is measured using a data sample of proton-proton collisions collected with the LHCb detector at center-of-mass energies of 7 and 8 Tev, corresponding to an integrated luminosity of 3$~$fb$^{-1}$. For the first time ${\cal{R}}(D^{*-})$ is determined using the $τ$ lepton decays with three charged pions in the final state. The $B^0 \to D^{*-} τ^+ν_τ$ yield is normalized to that of the $B^0\to D^{*-} π^+π^-π^+$ mode, providing a measurement of ${\cal{B}}(B^0\to D^{*-}τ^+ν_τ)/{\cal{B}}(B^0\to D^{*-}π^+π^-π^+) = 1.97 \pm 0.13 \pm 0.18$, where the first uncertainty is statistical and the second systematic. The value of ${\cal{B}}(B^0 \to D^{*-} τ^+ ν_τ) = (1.42 \pm 0.094 \pm 0.129 \pm 0.054)\% $ is obtained, where the third uncertainty is due to the limited knowledge of the branching fraction of the normalization mode. Using the well-measured branching fraction of the $B^0 \to D^{*-} μ^+ν_μ$ decay, a value of ${\cal{R}}(D^{*-}) = 0.291 \pm 0.019 \pm 0.026 \pm 0.013$ is established, where the third uncertainty is due to the limited knowledge of the branching fractions of the normalization and $B^0\to D^{*-}μ^+ν_μ$ modes. This measurement is in agreement with the Standard Model prediction and with previous results.

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arXiv2018

Measurements of the branching fractions of $Λ_{c}^{+} \rightarrow p π^{-} π^{+}$, $Λ_{c}^{+} \rightarrow p K^{-} K^{+}$, and $Λ_{c}^{+} \rightarrow p π^{-} K^{+}$

The ratios of the branching fractions of the decays $Λ_{c}^{+} \rightarrow p π^{-} π^{+}$, $Λ_{c}^{+} \rightarrow p K^{-} K^{+}$, and $Λ_{c}^{+} \rightarrow p π^{-} K^{+}$ with respect to the Cabibbo-favoured $Λ_{c}^{+} \rightarrow p K^{-} π^{+}$ decay are measured using proton-proton collision data collected with the LHCb experiment at a 7 TeV centre-of-mass energy and corresponding to an integrated luminosity of 1.0 fb$^{-1}$: \begin{align*} \frac{\mathcal{B}(Λ_{c}^{+} \rightarrow p π^{-} π^{+})}{\mathcal{B}(Λ_{c}^{+} \rightarrow p K^{-} π^{+})} & = (7.44 \pm 0.08 \pm 0.18)\,\%, \frac{\mathcal{B}(Λ_{c}^{+} \rightarrow p K^{-} K^{+})}{\mathcal{B}(Λ_{c}^{+} \rightarrow p K^{-} π^{+})} &= (1.70 \pm 0.03 \pm 0.03)\,\%, \frac{\mathcal{B}(Λ_{c}^{+} \rightarrow p π^{-} K^{+})}{\mathcal{B}(Λ_{c}^{+} \rightarrow p K^{-} π^{+})} & = (0.165 \pm 0.015 \pm 0.005 )\,\%, \end{align*} where the uncertainties are statistical and systematic, respectively. These results are the most precise measurements of these quantities to date. When multiplied by the world-average value for $\mathcal{B}(Λ_{c}^{+} \rightarrow p K^{-} π^{+})$, the corresponding branching fractions are \begin{align*} \mathcal{B}(Λ_{c}^{+} \rightarrow p π^{-} π^{+}) &= (4.72 \pm 0.05 \pm 0.11 \pm 0.25) \times 10^{-3}, \mathcal{B}(Λ_{c}^{+} \rightarrow p K^{-} K^{+}) &= (1.08 \pm 0.02 \pm 0.02 \pm 0.06) \times 10^{-3}, \mathcal{B}(Λ_{c}^{+} \rightarrow p π^{-} K^{+}) &= (1.04 \pm 0.09 \pm 0.03 \pm 0.05) \times 10^{-4}, \end{align*} where the final uncertainty is due to $\mathcal{B}(Λ_{c}^{+} \rightarrow p K^{-} π^{+})$.

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arXiv2018

Gaia Data Release 2. Calibration and mitigation of electronic offset effects in the data

The European Space Agency Gaia satellite was launched into orbit around L2 in December 2013. This ambitious mission has strict requirements on residual systematic errors resulting from instrumental corrections in order to meet a design goal of sub-10 microarcsecond astrometry. During the design and build phase of the science instruments, various critical calibrations were studied in detail to ensure that this goal could be met in orbit. In particular, it was determined that the video-chain offsets on the analogue side of the analogue-to-digital conversion electronics exhibited instabilities that could not be mitigated fully by modifications to the flight hardware. We provide a detailed description of the behaviour of the electronic offset levels on microsecond timescales, identifying various systematic effects that are known collectively as offset non-uniformities. The effects manifest themselves as transient perturbations on the gross zero-point electronic offset level that is routinely monitored as part of the overall calibration process. Using in-orbit special calibration sequences along with simple parametric models, we show how the effects can be calibrated, and how these calibrations are applied to the science data. While the calibration part of the process is relatively straightforward, the application of the calibrations during science data processing requires a detailed on-ground reconstruction of the readout timing of each charge-coupled device (CCD) sample on each device in order to predict correctly the highly time-dependent nature of the corrections. We demonstrate the effectiveness of our offset non-uniformity models in mitigating the effects in Gaia data. We demonstrate for all CCDs and operating instrument and modes on board Gaia that the video-chain noise-limited performance is recovered in the vast majority of science samples.

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arXiv2018

Gaia Radial Velocity Spectrometer

This paper presents the specification, design, and development of the Radial Velocity Spectrometer (RVS) on the European Space Agency's Gaia mission. Starting with the rationale for the full six dimensions of phase space in the dynamical modelling of the Galaxy, the scientific goals and derived top-level instrument requirements are discussed, leading to a brief description of the initial concepts for the instrument. The main part of the paper is a description of the flight RVS, considering the optical design, the focal plane, the detection and acquisition chain, and the as-built performance drivers and critical technical areas. After presenting the pre-launch performance predictions, the paper concludes with the post-launch developments and mitigation strategies, together with a summary of the in-flight performance at the end of commissioning.

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arXiv2018

Gaia Data Release 2: Processing the spectroscopic data

The Gaia Data Release 2 contains the 1st release of radial velocities complementing the kinematic data of a sample of about 7 million relatively bright, late-type stars. Aims: This paper provides a detailed description of the Gaia spectroscopic data processing pipeline, and of the approach adopted to derive the radial velocities presented in DR2. Methods: The pipeline must perform four main tasks: (i) clean and reduce the spectra observed with the Radial Velocity Spectrometer (RVS); (ii) calibrate the RVS instrument, including wavelength, straylight, line-spread function, bias non-uniformity, and photometric zeropoint; (iii) extract the radial velocities; and (iv) verify the accuracy and precision of the results. The radial velocity of a star is obtained through a fit of the RVS spectrum relative to an appropriate synthetic template spectrum. An additional task of the spectroscopic pipeline was to provide 1st-order estimates of the stellar atmospheric parameters required to select such template spectra. We describe the pipeline features and present the detailed calibration algorithms and software solutions we used to produce the radial velocities published in DR2. Results: The spectroscopic processing pipeline produced median radial velocities for Gaia stars with narrow-band near-IR magnitude Grvs < 12 (i.e. brighter than V~13). Stars identified as double-lined spectroscopic binaries were removed from the pipeline, while variable stars, single-lined, and non-detected double-lined spectroscopic binaries were treated as single stars. The scatter in radial velocity among different observations of a same star, also published in DR2, provides information about radial velocity variability. For the hottest (Teff > 7000 K) and coolest (Teff < 3500 K) stars, the accuracy and precision of the stellar parameter estimates are not sufficient to allow selection of appropriate templates. [Abridged]

astro-ph.GA↗

arXiv2018

Gaia Data Release 2: Properties and validation of the radial velocities

For Gaia DR2 (GDR2), 280 million spectra, collected by the RVS instrument on-board Gaia, were processed and median radial velocities were derived for 9.8 million sources brighter than Grvs = 12 mag. This paper describes the validation and properties of the median radial velocities published in GDR2. Quality tests and filters are applied to select, from the 9.8 million radial velocities, those with the quality to be published in GDR2. The accuracy of the selected sample is assessed with respect to ground-based catalogues. Its precision is estimated using both ground-based catalogues and the distribution of the Gaia radial velocity uncertainties. GDR2 contains median radial velocities for 7 224 631 stars, with Teff in the range [3550, 6900] K, which passed succesfully the quality tests. The published median radial velocities provide a full sky-coverage and have a completness with respect to the astrometric data of 77.2\% (for $G \leq 12.5$ mag). The median radial velocity residuals with respect to the ground-based surveys vary from one catalogue to another, but do not exceed a few 100s m/s. In addition, the Gaia radial velocities show a positive trend as a function of magnitude, which starts around Grvs $\sim 9$ mag and reaches about $+500$ m/s at Grvs $= 11.75$ mag. The overall precision, estimated from the median of the Gaia radial velocity uncertainties, is 1.05 km/s. The radial velocity precision is function of many parameters, in particular the magnitude and effective temperature. For bright stars, Grvs in [4, 8] mag, the precision is in the range 200-350 m/s, which is about 3 to 5 times more precise than the pre-launch specification of 1 km/s. At the faint end, Grvs = 11.75 mag, the precisions for Teff = 5000 K and 6500 K are respectively 1.4 km/s and 3.7 km/s.

astro-ph.IM↗

arXiv2018

A measurement of the $CP$ asymmetry difference in $\varLambda_{c}^{+} \to pK^{-}K^{+}$ and $pπ^{-}π^{+}$ decays

The difference between the $CP$ asymmetries in the decays $\varLambda_{c}^{+} \to pK^{-}K^{+}$ and $\varLambda_{c}^{+} \to pπ^{-}π^{+}$ is presented. Proton-proton collision data taken at centre-of-mass energies of $7$ and $8\,\mathrm{TeV}$ collected by the LHCb detector in 2011 and 2012 are used, corresponding to an integrated luminosity of $3\,\mathrm{fb}^{-1}$. The $\varLambda_{c}^{+}$ candidates are reconstructed as part of the $\varLambda_{b}^{0} \to \varLambda_{c}^{+}μ^{-}X$ decay chain. In order to maximize the cancellation of production and detection asymmetries in the difference, the final-state kinematic distributions of the two samples are aligned by applying phase-space-dependent weights to the $\varLambda_{c}^{+} \to pπ^{-}π^{+}$ sample. This alters the definition of the integrated $CP$ asymmetry to $A_{CP}^{\text{wgt}}(pπ^{-}π^{+})$. Both samples are corrected for reconstruction and selection efficiencies across the five-dimensional $\varLambda_{c}^{+}$ decay phase space. The difference in $CP$ asymmetries is found to be \begin{align*} Δ{A^{\text{wgt}}_{CP}} &= A_{CP}(pK^{-}K^{+}) - A_{CP}^{\text{wgt}}(pπ^{-}π^{+})\\ &= (0.30 \pm 0.91 \pm 0.61)\,\%, \end{align*} where the first uncertainty is statistical and the second is systematic.

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arXiv2018

Measurement of the $b$-quark production cross-section in 7 and 13 TeV $pp$ collisions

Measurements of the cross-section for producing \bquark quarks in the reaction $pp\to b\bar{b} X$ are reported in 7 and 13 TeV collisions at the LHC as a function of the pseudorapidity $η$ in the range $2<η<5$ covered by the acceptance of the LHCb experiment. The measurements are done using semileptonic decays of $b$-flavored hadrons decaying into a ground-state charmed hadron in association with a muon. The cross-sections in the covered $η$ range are $72.0\pm 0.3\pm6.8~μ$b and $144\pm 1\pm 21~μ$b for 7 and 13 TeV. The ratio is $2.00\pm0.02\pm0.26$, where the quoted uncertainties are statistical and systematic, respectively. The agreement with theoretical expectation is good at 7 TeV, but differs somewhat at 13 TeV. The measured ratio of cross-sections is larger at lower $η$ than the model prediction.

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arXiv2018

Observation of the decay $Λ^0_b \to p K^- μ^+ μ^-$ and a search for $C\!P$ violation

A search for $C\!P$ violation in the decay $Λ^0_b \to p K^- μ^+ μ^-$ is presented. This decay is mediated by flavour-changing neutral-current transitions in the Standard Model and is potentially sensitive to new sources of $C\!P$ violation. The study is based on a data sample of proton-proton collisions recorded with the LHCb experiment, corresponding to an integrated luminosity of $3$ $\mathrm{fb}^{-1}$. The $Λ^0_b \to p K^- μ^+ μ^-$ decay is observed for the first time, and two observables that are sensitive to different manifestations of $C\!P$ violation are measured, $Δ\mathcal{A}_{C\!P} \equiv \mathcal{A}_{C\!P}(Λ^0_b \to p K^- μ^+ μ^-)-\mathcal{A}_{C\!P}(Λ^0_b\to pK^- J/ψ)$ and $a_{C\!P}^{\widehat{T}-odd}$, where the latter is based on asymmetries in the angle between the $μ^+μ^-$ and $p K^-$ decay planes. These are measured to be $Δ\mathcal{A}_{C\!P} = (-3.5 \pm 5.0(stat) \pm 0.2(syst))\times 10^{-2}$, $a_{C\!P}^{\widehat{T}-odd} = (1.2 \pm 5.0(stat) \pm 0.7(syst))\times 10^{-2}$, and no evidence for $C\!P$ violation is found.

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arXiv2018

Measurement of $CP$ observables in $B^{\pm} \rightarrow D K^{*\pm}$ decays using two- and four-body $D$ final states

Measurements of $CP$ observables in $B^{\pm} \rightarrow D K^{*\pm}$ decays are presented, where $D$ denotes a superposition of $D^0$ and $\overline{D^0}$ meson states. Decays of the $D$ meson to $K^{-}π^{+}$, $K^{-}K^{+}$, $π^{-}π^{+}$, $K^{-}π^{+}π^{-}π^{+}$ and $π^{-}π^{+}π^{-}π^{+}$ are used and the $K^{*\pm}$ meson is reconstructed in the $K^{0}_{\mathrm{S}}π^{\pm}$ final state. This analysis uses a data sample of $pp$ collisions collected with the LHCb experiment, corresponding to integrated luminosities of 1 $\mathrm{fb}^{-1}$, 2 $\mathrm{fb}^{-1}$ and 1.8 $\mathrm{fb}^{-1}$ at centre-of-mass energies $\sqrt{s} = $ 7 TeV, 8 TeV and 13 TeV, respectively. The sensitivity of the results to the CKM angle $γ$ is discussed.

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arXiv2018

First measurement of the $CP$-violating phase $ϕ_s^{d\overline{d}}$ in $B_s^0\to(K^+π^-)(K^-π^+)$ decays

A flavour-tagged decay-time-dependent amplitude analysis of $B_s^0\to(K^+π^-)(K^-π^+)$ decays is presented in the $K^{\pm}π^{\mp}$ mass range from 750 to 1600 MeV$/c^2$. The analysis uses $pp$ collision data collected with the LHCb detector at centre-of-mass energies of $7$ and $8$ TeV, corresponding to an integrated luminosity of $3.0$ fb$^{-1}$. Several quasi-two-body decay modes are considered, corresponding to $K^{\pm}π^{\mp}$ combinations with spin 0, 1 and 2, which are dominated by the $K_0^*(800)^0$ and $K_0^*(1430)^0$, the $K^*(892)^0$ and the $K_2^*(1430)^0$ resonances, respectively. The longitudinal polarisation fraction for the $B_s^0\to K^*(892)^0\overline{K}^*(892)^0$ decay is measured as $f_L=0.208 \pm 0.032 \pm 0.046$, where the first uncertainty is statistical and the second is systematic. The first measurement of the mixing-induced $CP$-violating phase, $ϕ_s^{d\overline{d}}$, in $b\to d\overline{d}s$ transitions is performed, yielding a value of $ϕ_s^{d\overline{d}}=-0.10$ $\pm$ $0.13$ (stat) $\pm$ $0.14$ (syst) rad.

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arXiv2018

Search for the lepton-flavour violating decays $B^0_{(s)} \rightarrow e^\pm μ^\mp$

A search for the lepton-flavour violating decays $B^0_s \rightarrow e^\pm μ^\mp$ and $B^0 \rightarrow e^\pm μ^\mp$ is performed based on a sample of proton-proton collision data corresponding to an integrated luminosity of 3 fb$^{-1}$, collected with the LHCb experiment at centre-of-mass energies of 7 and 8 TeV. The observed yields are consistent with the background-only hypothesis. Upper limits on the branching fractions are determined to be $\mathcal{B}(B^0_s \rightarrow e^\pm μ^\mp) < 5.4\,(6.3) \times 10^{-9}$ and $\mathcal{B}(B^0 \rightarrow e^\pm μ^\mp) < 1.0\,(1.3) \times 10^{-9}$ at $90\%\,(95\%)$ confidence level, which are the strongest limits on these decays to date.

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