84 research outputs found
Multi-messenger observations of a binary neutron star merger
On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta
Measurement of D s <sup>±</sup> production asymmetry in pp collisions at √s=7 and 8 TeV
The inclusive production asymmetry is measured in collisions
collected by the LHCb experiment at centre-of-mass energies of
and 8 TeV. Promptly produced mesons are used, which decay as
, with . The measurement is
performed in bins of transverse momentum, , and rapidity, ,
covering the range GeV and . No kinematic
dependence is observed. Evidence of nonzero production asymmetry is
found with a significance of 3.3 standard deviations.Comment: All figures and tables, along with any supplementary material and
additional information, are available at
https://cern.ch/lhcbproject/Publications/p/LHCb-PAPER-2018-010.htm
Search for CP violation in Λb0→pK− and Λb0→pπ− decays
A search for CP violation in Λb0→pK− and Λb0→pπ− decays is presented using a sample of pp collisions collected with the LHCb detector and corresponding to an integrated luminosity of 3.0fb−1. The CP -violating asymmetries are measured to be ACPpK−=−0.020±0.013±0.019 and ACPpπ−=−0.035±0.017±0.020, and their difference ACPpK−−ACPpπ−=0.014±0.022±0.010, where the first uncertainties are statistical and the second systematic. These are the most precise measurements of such asymmetries to date
Search for dark photons produced in 13 TeV collisions
Searches are performed for both promptlike and long-lived dark photons,
A
0
, produced in proton-proton
collisions at a center-of-mass energy of 13 TeV, using
A
0
→
μ
þ
μ
−
decays and a data sample corresponding
to an integrated luminosity of
1
.
6
fb
−
1
collected with the LHCb detector. The promptlike
A
0
search covers
the mass range from near the dimuon threshold up to 70 GeV, while the long-lived
A
0
search is restricted to
the low-mass region
214
<m
ð
A
0
Þ
<
350
MeV. No evidence for a signal is found, and 90% confidence
level exclusion limits are placed on the
γ
–
A
0
kinetic-mixing strength. The constraints placed on promptlike
dark photons are the most stringent to date for the mass range
10
.
6
<m
ð
A
0
Þ
<
70
GeV, and are
comparable to the best existing limits for
m
ð
A
0
Þ
<
0
.
5
GeV. The search for long-lived dark photons is the
first to achieve sensitivity using a displaced-vertex signature
Measurement of asymmetry in decays
We report the measurements of the -violating parameters in decays observed in collisions, using a data set corresponding to an integrated luminosity of recorded with the LHCb detector. We measure , , , , , where the uncertainties are statistical and systematic, respectively. These parameters are used together with the world-average value of the mixing phase, , to obtain a measurement of the CKM angle from decays, yielding \gamma = (128\,_{-22}^{+17})^\circ modulo , where the uncertainty contains both statistical and systematic contributions. This corresponds to evidence for violation in the interference between decay and decay after mixing.We report the measurements of the CP -violating parameters in B → D K decays observed in pp collisions, using a data set corresponding to an integrated luminosity of 3.0 fb recorded with the LHCb detector. We measure C = 0.73 ± 0.14 ± 0.05, A = 0.39 ± 0.28 ± 0.15, , S = −0.52 ± 0.20 ± 0.07, , where the uncertainties are statistical and systematic, respectively. These parameters are used together with the world-average value of the B mixing phase, −2β , to obtain a measurement of the CKM angle γ from B → D K decays, yielding γ = (128 )° modulo 180°, where the uncertainty contains both statistical and systematic contributions. This corresponds to 3.8 σ evidence for CP violation in the interference between decay and decay after mixing
Measurement of the electron reconstruction efficiency at LHCb
The single electron track-reconstruction efficiency is calibrated using a sample corresponding to 1.3 fb−1 of pp collision data recorded with the LHCb detector in 2017. This measurement exploits B+→ J/ψ(e+e−)K+ decays, where one of the electrons is fully reconstructed and paired with the kaon, while the other electron is reconstructed using only the information of the vertex detector. Despite this partial reconstruction, kinematic and geometric constraints allow the B meson mass to be reconstructed and the signal to be well separated from backgrounds. This in turn allows the electron reconstruction efficiency to be measured by matching the partial track segment found in the vertex detector to tracks found by LHCb's regular reconstruction algorithms. The agreement between data and simulation is evaluated, and corrections are derived for simulated electrons in bins of kinematics. These correction factors allow LHCb to measure branching fractions involving single electrons with a systematic uncertainty below 1%
A Fourier approach to fields and electron optical phase-shifts calculations
The Fourier method is applied to calculate fields and electron optical phase shifts in specimens having long-range electromagnetic fields, like reverse biased p-n junctions or stripe magnetic domains. It is shown that this approach not only allows to take into account rather easily the effect of the fringing fields protruding in the space around the specimen, but also to obtain solutions to interesting models in analytical form. (C) 2002 Elsevier Science B.V. All rights reserved
Influence of charged oxide layers on TEM imaging of reverse-biased p-n junctions
Experimental observations of reverse-biased p-n junctions by means of the out-of-focus method display features which cannot be interpreted within the standard theory of an abrupt p-n junction. In order to reconcile theory and experiment it is necessary to introduce an active role of the specimen surfaces. In particular it is shown how the introduction of a suitable surface density charge at the interface between the silicon and oxide created after the thinning process allows us to explain the main features of the experimental results. Moreover, some questions left unanswered by previous observations made by Lorentz and holographic methods will be clarified. The results point out that oxide charging cannot be overlooked and should be properly taken into account whenever semiconductor devices are observed by transmission electron microscopy techniques, especially when these methods are employed for the analysis of dopant diffusion in submicron devices
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