arXiv subjects

J. Anderson

Publications and source records attributed to J. Anderson.

At least 451 records · Page 25Linked to original sources

arXiv2013

Initial deep LOFAR observations of Epoch of Reionization windows: I. The North Celestial Pole

The aim of the LOFAR Epoch of Reionization (EoR) project is to detect the spectral fluctuations of the redshifted HI 21cm signal. This signal is weaker by several orders of magnitude than the astrophysical foreground signals and hence, in order to achieve this, very long integrations, accurate calibration for stations and ionosphere and reliable foreground removal are essential. One of the prospective observing windows for the LOFAR EoR project will be centered at the North Celestial Pole (NCP). We present results from observations of the NCP window using the LOFAR highband antenna (HBA) array in the frequency range 115 MHz to 163 MHz. The data were obtained in April 2011 during the commissioning phase of LOFAR. We used baselines up to about 30 km. With about 3 nights, of 6 hours each, effective integration we have achieved a noise level of about 100 microJy/PSF in the NCP window. Close to the NCP, the noise level increases to about 180 microJy/PSF, mainly due to additional contamination from unsubtracted nearby sources. We estimate that in our best night, we have reached a noise level only a factor of 1.4 above the thermal limit set by the noise from our Galaxy and the receivers. Our continuum images are several times deeper than have been achieved previously using the WSRT and GMRT arrays. We derive an analytical explanation for the excess noise that we believe to be mainly due to sources at large angular separation from the NCP.

astro-ph.IM↗

arXiv2013

First observation of the decay $B^{+} \rightarrow π^{+} μ^{+} μ^{-}$

A discovery of the rare decay $B^{+} \rightarrow π^{+} μ^{+} μ^{-}$ is presented. This decay is observed for the first time, with 5.2 $σ$ significance. The observation is made using $pp$ collision data, corresponding to an integrated luminosity of 1.0 fb$^{-1}$, collected with the LHCb detector. The measured branching fraction is (2.3 $\pm$ 0.6 (stat.) $\pm$ 0.1 (syst.))$\times 10^{-8}$, and the ratio of the $B^{+} \rightarrow π^{+} μ^{+} μ^{-}$ and $B^{+} \rightarrow \K^{+} μ^{+} μ^{-}$ branching fractions is measured to be 0.053 $\pm$ 0.014 (stat.) $\pm$ 0.001 (syst.).

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arXiv2013

First evidence for the annihilation decay mode $B^{+} \to D_{s}^{+} ϕ$

Evidence for the hadronic annihilation decay mode $B^{+} \to D_s^{+}ϕ$ is found with greater than $3σ$ significance. The branching fraction and \CP asymmetry are measured to be \mathcal{B}(B^{+} \to D_s^{+}ϕ) &=& (1.87^{+1.25}_{-0.73}({\rm stat}) \pm 0.19 ({\rm syst}) \pm 0.32 ({\rm norm})) \times 10^{-6}, \mathcal{A}_{CP}(B^{+} \to D_s^{+}ϕ) &=& -0.01 \pm 0.41 ({\rm stat}) \pm 0.03 ({\rm syst}). The last uncertainty on $\mathcal{B}(B^{+} \to D_s^{+}ϕ)$ is from the branching fractions of the $B^+ \to D_s^+ \kern 0.2em\bar{\kern -0.2em D}{}^0$ normalization mode and intermediate resonance decays. Upper limits are also set for the branching fractions of the related decay modes $B^{+}_{(c)} \to D^{+}_{(s)} K^{*0}$, $B^{+}_{(c)} \to D^{+}_{(s)} \kern 0.2em\bar{\kern -0.2em K}{}^{*0}$ and ${B_c^{+} \to D^{+}_{s}ϕ}$, including the result ${\mathcal{B}(B^+ \to D^+ K^{*0})} < 1.8 \times 10^{-6}$ at the 90% credibility level.

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arXiv2012

First observation of the decays $\bar{B}^0_{(s)}\to D_s^+K^-π^+π^-$ and $\bar{B}^0_s\to D_{s1}(2536)^+π^-$

The first observation of the decays $\bar{B}^0_{s}\to D_s^+K^-π^+π^-$ and $\bar{B}^0\to D_s^+K^-π^+π^-$ are reported using an integrated luminosity of 1.0 fb$^{-1}$ recorded by the LHCb experiment. The branching fractions, normalized with respect to $\bar{B}^0_{s}\to D_s^+π^-π^+π^-$ and $\bar{B}^0_{s}\to D_s^+K^-π^+π^-$, respectively, are measured to be {\br(\bar{B}^0_{s}\to D_s^+K^-π^+π^-)\over\br(\bar{B}^0_{s}\to D_s^+π^-π^+π^-)} &= (5.2\pm0.5\pm0.3)\times10^{-2}, {\br(\bar{B}^0\to D_s^+K^-π^+π^-)\over\br(\bar{B}^0_{s}\to D_s^+K^-^π^+π^-)} &= 0.54\pm0.07\pm0.07, where the first uncertainty is statistical and the second is systematic. The $\bar{B}^0_{s}\to D_s^+K^-π^+π^-$ decay is of particular interest as it can be used to measure the weak phase $γ$. First observation of the $\bar{B}^0_s\to D_{s1}(2536)^+π^-, D_{s1}^+\to D_s^+π^-π^+$ decay is also presented, and its branching fraction relative to $\bar{B}^0_{s}\to D_s^+π^-π^+π^-$ is found to be {\br(\bar{B}^0_s\to D_{s1}(2536)^+π^-, D_{s1}^+\to D_s^+π^-π^+)\over\br(\bar{B}^0_{s}\to D_s^+π^-π^+π^-)} &= (4.0\pm1.0\pm0.4)\times10^{-3}.

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arXiv2012

Measurement of the D+/- production asymmetry in 7 TeV pp collisions

The asymmetry in the production cross-section σof D+/- mesons, A_P = (σ(D+) - σ(D-))/(σ(D+) + σ(D-)), is measured in bins of pseudorapidity ηand transverse momentum p_T within the acceptance of the LHCb detector. The result is obtained with a sample of D+ -> K_S pi+ decays corresponding to an integrated luminosity of 1.0 fb^-1, collected in pp collisions at a centre of mass energy of 7 TeV at the Large Hadron Collider. When integrated over the kinematic range 2.0 < p_T < 18.0 GeV/c and 2.20 < η< 4.75, the production asymmetry is A_P = (-0.96+/-0.26+/-0.18)%. The uncertainties quoted are statistical and systematic, respectively. The result assumes that any direct CP violation in the D+ -> K_S pi+ decay is negligible. No significant dependence on ηor p_T is observed.

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arXiv2012

Measurement of the cross-section ratio $σ(χ_{c2})/σ(χ_{c1})$ for prompt $χ_c$ production at $\sqrt{s}=7$ TeV

The prompt production of the charmonium $χ_{c1}$ and $χ_{c2}$ mesons has been studied in proton-proton collisions at the Large Hadron Collider at a centre-of-mass energy of $\sqrt{s}=7$ TeV. The $χ_c$ mesons are identified through their decays $χ_c\to J/ψ\,γ$ with $J/ψ\to μ^+ μ^-$ using 36 $\mathrm{pb^{-1}}$ of data collected by the LHCb detector in 2010. The ratio of the prompt production cross-sections for the two $χ_c$ spin states, $σ(χ_{c2})/σ(χ_{c1})$, has been determined as a function of the $J/ψ$ transverse momentum, $p_{\mathrm{T}}^{J/ψ}$, in the range from 2 to 15 GeV/$c$. The results are in agreement with the next-to-leading order non-relativistic QCD model at high $p_{\mathrm{T}}^{J/ψ}$ and lie consistently above the pure leading-order colour singlet prediction.

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arXiv2012

Measurement of the ratio of prompt $χ_{c}$ to $J/ψ$ production in $pp$ collisions at $\sqrt{s}=7$ TeV

The prompt production of charmonium $χ_{c}$ and $J/ψ$ states is studied in proton-proton collisions at a centre-of-mass energy of $\sqrt{s}=7$ TeV at the Large Hadron Collider. The $χ_{c}$ and $J/ψ$ mesons are identified through their decays $χ_{c}\rightarrow J/ψγ$ and $J/ψ\rightarrow μ^+μ^-$ using 36 pb$^{-1}$ of data collected by the LHCb detector in 2010. The ratio of the prompt production cross-sections for $χ_{c}$ and $J/ψ$, $σ(χ_{c}\rightarrow J/ψγ)/ σ(J/ψ)$, is determined as a function of the $J/ψ$ transverse momentum in the range $2 < p_{\mathrm T}^{J/ψ} < 15$ GeV/$c$. The results are in excellent agreement with next-to-leading order non-relativistic expectations and show a significant discrepancy compared with the colour singlet model prediction at leading order, especially in the low $p_{\mathrm T}^{J/ψ}$ region.

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arXiv2012

First evidence for the decay Bs -> mu+ mu-

A search for the rare decays Bs->mu+mu- and B0->mu+mu- is performed using data collected in 2011 and 2012 with the LHCb experiment at the Large Hadron Collider. The data samples comprise 1.1 fb^-1 of proton-proton collisions at sqrt{s} = 8 TeV and 1.0 fb^-1 at sqrt{s}=7 TeV. We observe an excess of Bs -> mu+ mu- candidates with respect to the background expectation. The probability that the background could produce such an excess or larger is 5.3 x 10^-4 corresponding to a signal significance of 3.5 standard deviations. A maximum-likelihood fit gives a branching fraction of BR(Bs -> mu+ mu-) = (3.2^{+1.5}_{-1.2}) x 10^-9, where the statistical uncertainty is 95% of the total uncertainty. This result is in agreement with the Standard Model expectation. The observed number of B0 -> mu+ mu- candidates is consistent with the background expectation, giving an upper limit of BR(B0 -> mu+ mu-) < 9.4 x 10^-10 at 95% confidence level.

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arXiv2012

Observation of excited Lambda_b0 baryons

Using pp collision data corresponding to 1.0 fb-1 integrated luminosity collected by the LHCb detector, two narrow states are observed in the Lambda_b0pi+pi- spectrum with masses 5911.97 +- 0.12(stat) +- 0.02(syst) +- 0.66(Lambda_b0 mass) MeV/c^2 and 5919.77 +- 0.08(stat) +- 0.02(syst) +- 0.66(Lambda_b0 mass) MeV/c^2. The significances of the observations are 5.2 and 10.2 standard deviations, respectively. These states are interpreted as the orbitally-excited Lambda_b0 baryons, Lambda_b*0(5912) and Lambda_b*0(5920).

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arXiv2012

Measurement of the $B^0_s\rightarrow J/ψK^{*0}$ branching fraction and angular amplitudes

A search for the decay $B^0_s\rightarrow J/ψK^{*0}$ with $K^{*0} \rightarrow K^-π^+$ is performed with 0.37 fb$^{-1}$ of $pp$ collisions at $\sqrt{s}$ = 7 TeV collected by the LHCb experiment, finding a $\Bs \to JψK^-π^+$ peak of $114 \pm 11$ signal events. The $K^-π^+$ mass spectrum of the candidates in the $B^0_s$ peak is dominated by the $K^{*0}$ contribution. Subtracting the non-resonant $K^-π^+$ component, the branching fraction of \BsJpsiKst is $(4.4_{-0.4}^{+0.5} \pm 0.8) \times 10^{-5}$, where the first uncertainty is statistical and the second systematic. A fit to the angular distribution of the decay products yields the \Kst polarization fractions $f_L = 0.50 \pm 0.08 \pm 0.02$ and $f_{||} = 0.19^{+0.10}_{-0.08} \pm 0.02$.

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arXiv2012

A study of the $Z$ production cross-section in $pp$ collisions at $\sqrt{s} = 7$ TeV using tau final states

A measurement of the inclusive $Z \to ττ$ cross-section in $pp$ collisions at $\sqrt{s} = 7$ TeV is presented based on a dataset of 1.0 fb$^{-1}$ collected by the LHCb detector. Candidates for $Z \to ττ$ decays are identified through reconstructed final states with two muons, a muon and an electron, a muon and a hadron, or an electron and a hadron. The production cross-section for $Z$ bosons, with invariant mass between 60 and 120 GeV/$c^2$, which decay to $τ$ leptons with transverse momenta greater than 20 GeV/$c$ and pseudorapidities between 2.0 and 4.5, is measured to be $σ_{pp \to Z \to ττ} = 71.4 \pm 3.5 \pm 2.8 \pm 2.5$ pb; the first uncertainty is statistical, the second uncertainty is systematic, and the third is due to the uncertainty on the integrated luminosity. The ratio of the cross-sections for $Z \to ττ$ to $Z \to μμ$ is determined to be $0.93 \pm 0.09$, where the uncertainty is the combination of statistical, systematic, and luminosity uncertainties of the two measurements.

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arXiv2012

Measurement of $b$-hadron branching fractions for two-body decays into charmless charged hadrons

Based on data corresponding to an integrated luminosity of 0.37 fb^-1 collected by the LHCb experiment in 2011, the following ratios of branching fractions are measured: B(B0 -> pi+ pi-) / B(B0 -> K+pi-) = 0.262 +/- 0.009 +/- 0.017, (fs/fd) * B(Bs -> K+K-) / B(B^0 -> K+pi-) = 0.316 +/- 0.009 +/- 0.019, (fs/fd) * B(Bs ->pi+ K-) / B(B0 -> K+pi-) = 0.074 +/- 0.006 +/- 0.006, (fd/fs) * B(B0 -> K+K-) / B(Bs -> K+K-) = 0.018 {+ 0.008 - 0.007} +/- 0.009, (fs/fd) * B(Bs -> pi+pi-) / B(B0 -> pi+pi-) = 0.050 {+ 0.011 - 0.009} +/- 0.004, B(Lambda_b -> p pi-) / B(Lambda_b -> p K-) = 0.86 +/- 0.08 +/- 0.05, where the first uncertainties are statistical and the second systematic. Using the current world average of B(B0 -> K+pi-) and the ratio of the strange to light neutral B meson production fs/fd measured by LHCb, we obtain: B(B0 -> pi+pi-) = (5.08 +/- 0.17 +/- 0.37) x 10^-6, B(Bs -> K+K-) = (23.0 +/- 0.7 +/- 2.3) x 10^-6, B(Bs -> pi+K-) = (5.4 +/- 0.4 +/- 0.6) x 10^-6, B(B0 -> K+K-) = (0.11 {+ 0.05 - 0.04} +/- 0.06) x 10^-6, B(Bs -> pi+pi-) = (0.95 {+ 0.21 - 0.17} +/- 0.13) x 10^-6. The measurements of B(Bs -> K+K-), B(Bs -> pi+ K-) and B(B0 -> K+K-) are the most precise to date. The decay mode Bs -> pi+pi- is observed for the first time with a significance of more than 5 sigma.

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arXiv2012

Measurement of the ratio of branching fractions BR(B0 -> K0 gamma)/BR(Bs0 -> phi gamma) and the direct CP asymmetry in B0 -> K0 gamma

The ratio of branching fractions of the radiative B decays B0 -> K*0 gamma and Bs0 phi gamma has been measured using an integrated luminosity of 1.0 fb-1 of pp collision data collected by the LHCb experiment at a centre-of-mass energy of sqrt(s)=7 TeV. The value obtained is BR(B0 -> K*0 gamma)/BR(Bs0 -> phi gamma) = 1.23 +/- 0.06(stat.) +/- 0.04(syst.) +/- 0.10(fs/fd), where the first uncertainty is statistical, the second is the experimental systematic uncertainty and the third is associated with the ratio of fragmentation fractions fs/fd. Using the world average value for BR(B0 -> K*0 gamma), the branching fraction BR(Bs0 -> phi gamma) is measured to be (3.5 +/- 0.4) x 10^{-5}. The direct CP asymmetry in B0 -> K*0 gamma decays has also been measured with the same data and found to be A(CP)(B0 -> K*0 gamma) = (0.8 +/- 1.7(stat.) +/- 0.9(syst.))%. Both measurements are the most precise to date and are in agreement with the previous experimental results and theoretical expectations.

hep-ex↗

arXiv2012

Measurement of the Bs effective lifetime in the J/psi f0(980) final state

The effective lifetime of the Bs meson in the decay mode Bs->J/ψf0(980) is measured using 1.0/fb of data collected in pp collisions at sqrt(s) = 7 TeV with the LHCb detector. The result is 1.700 +/- 0.040 +/- 0.026 ps where the first uncertainty is statistical and the second systematic. As the final state is CP-odd, and CP violation in this mode is measured to be small, the lifetime measurement can be translated into a measurement of the decay width of the heavy Bs mass eigenstate, Γ_H = (0.588 +/- 0.014 +/- 0.009)/ps.

hep-ex↗

arXiv2012

Evidence for the decay $B^0\to J/ψω$ and measurement of the relative branching fractions of $B^0_s$ meson decays to $J/ψη$ and $J/ψη^{'}$

First evidence of the $B^0\to J/ψω$ decay is found and the $B_{s}^0\to J/ψη$ and $B_{s}^0\to J/ψη^{\prime}$ decays are studied using a dataset corresponding to an integrated luminosity of 1.0 $\mathrm{fb}^{-1}$ collected by the LHCb experiment in proton-proton collisions at a centre-of-mass energy of $\sqrt{s} = 7$ TeV. The branching fractions of these decays are measured relative to that of the $\mathrm{B}^0\to J/ψρ^0$ decay: $$\frac{Br(B^0\to J/ψω)}{Br(B^0\to J/ψρ^0)} = 0.89 \pm0.19(stat)^{+0.07}_{-0.13}(syst)$$ $$\frac{Br(B^0_s\to J/ψη)}{Br(B^0\to J/ψρ^0)} = 14.0 \pm 1.2(stat)^{+1.1}_{-1.5}(syst)^{+1.1}_{-1.0}(\frac{f_d}{f_s}),$$ $$\frac{Br(B^0_s\to J/ψη^{\prime})}{Br(B^0\to J/ψρ^0)} = 12.7\pm1.1(stat)^{+0.5}_{-1.3}(syst)^{+1.0}_{-0.9}(\frac{f_d}{f_s}),$$ where the last uncertainty is due to the knowledge of $f_d/f_s$, the ratio of b-quark hadronization factors that accounts for the different production rate of $B^0$ and $B^0_s$ mesons. The ratio of the branching fractions of $B^0_s\to J/ψη^{\prime}$ and $B^0_s\to J/ψη$ decays is measured to be $$ \frac{Br(B^0_s\to J/ψη^{\prime})}{Br(B^0_s\to J/ψη)} = 0.90\pm0.09(stat)^{+0.06}_{-0.02}(syst).$$

hep-ex↗

arXiv2012

M87 at metre wavelengths: the LOFAR picture

M87 is a giant elliptical galaxy located in the centre of the Virgo cluster, which harbours a supermassive black hole of mass 6.4x10^9 M_sun, whose activity is responsible for the extended (80 kpc) radio lobes that surround the galaxy. The energy generated by matter falling onto the central black hole is ejected and transferred to the intra-cluster medium via a relativistic jet and morphologically complex systems of buoyant bubbles, which rise towards the edges of the extended halo. Here we present the first observations made with the new Low-Frequency Array (LOFAR) of M87 at frequencies down to 20 MHz. Images of M87 were produced at low radio frequencies never explored before at these high spatial resolution and dynamic range. To disentangle different synchrotron models and place constraints on source magnetic field, age and energetics, we also performed a detailed spectral analysis of M87 extended radio-halo using these observations together with archival data. We do not find any sign of new extended emissions; on the contrary the source appears well confined by the high pressure of the intra-cluster medium. A continuous injection of relativistic electrons is the model that best fits our data, and provides a scenario in which the lobes are still supplied by fresh relativistic particles from the active galactic nuclei. We suggest that the discrepancy between the low-frequency radio-spectral slope in the core and in the halo implies a strong adiabatic expansion of the plasma as soon as it leaves the core area. The extended halo has an equipartition magnetic field strength of ~10 uG, which increases to ~13 uG in the zones where the particle flows are more active. The continuous injection model for synchrotron ageing provides an age for the halo of ~40 Myr, which in turn provides a jet kinetic power of 6-10x10^44 erg/s.

astro-ph.GA↗

arXiv2012

Deep HST Imaging in NGC 6397: Stellar Dynamics

Multi-epoch observations with ACS on HST provide a unique and comprehensive probe of stellar dynamics within NGC 6397. We are able to confront analytic models of the globular cluster with the observed stellar proper motions. The measured proper motions probe well along the main sequence from 0.8 to below 0.1 M$_\odot$ as well as white dwarfs younger than one gigayear. The observed field lies just beyond the half-light radius where standard models of globular cluster dynamics (e.g. based on a lowered Maxwellian phase-space distribution) make very robust predictions for the stellar proper motions as a function of mass. The observed proper motions show no evidence for anisotropy in the velocity distribution; furthermore, the observations agree in detail with a straightforward model of the stellar distribution function. We do not find any evidence that the young white dwarfs have received a natal kick in contradiction with earlier results. Using the observed proper motions of the main-sequence stars, we obtain a kinematic estimate of the distance to NGC 6397 of $2.2^{+0.5}_{-0.7}$ kpc and a mass of the cluster of $1.1 \pm 0.1 \times 10^5 \mathrm{M}_\odot$ at the photometric distance of 2.53 kpc. One of the main-sequence stars appears to travel on a trajectory that will escape the cluster, yielding an estimate of the evaporation timescale, over which the number of stars in the cluster decreases by a factor of e, of about 3 Gyr. The proper motions of the youngest white dwarfs appear to resemble those of the most massive main-sequence stars, providing the first direct constraint on the relaxation time of the stars in a globular cluster of greater than or about 0.7 Gyr.

astro-ph.SR↗

arXiv2012

A model-independent Dalitz plot analysis of $B^\pm \to D K^\pm$ with $D \to K^0_{\rm S} h^+h^-$ ($h=π, K$) decays and constraints on the CKM angle $γ$

A binned Dalitz plot analysis of $B^\pm \to D K^\pm$ decays, with $D \to K^0_{\rm S} π^+π^-$ and $D \to K^0_{\rm S} K^+ K^-$, is performed to measure the CP-violating observables $x_{\pm}$ and $y_{\pm}$ which are sensitive to the CKM angle $γ$. The analysis exploits 1.0 $\rm fb^{-1}$ of data collected by the LHCb experiment. The study makes no model-based assumption on the variation of the strong phase of the D decay amplitude over the Dalitz plot, but uses measurements of this quantity from CLEO-c as input. The values of the parameters are found to be $x_- = (0.0 \pm 4.3 \pm 1.5 \pm 0.6) \times 10^{-2}$, $y_- = (2.7 \pm 5.2 \pm 0.8 \pm 2.3) \times 10^{-2}$, $x_+ = (-10.3 \pm 4.5 \pm 1.8 \pm 1.4)\times 10^{-2}$ and $y_+ = (-0.9 \pm 3.7 \pm 0.8 \pm 3.0)\times 10^{-2}$. The first, second, and third uncertainties are the statistical, the experimental systematic, and the error associated with the precision of the strong-phase parameters measured at CLEO-c, respectively. These results correspond to $γ= (44^{\,+43}_{\,-38})^\circ$, with a second solution at $γ\to γ+ 180^\circ$, and $r_B = 0.07 \pm 0.04$, where $r_B$ is the ratio between the suppressed and favoured B decay amplitudes.

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