130,952 research outputs found
Measurements of the branching fractions of the decays B0s→D∓sK± and B0s→D−sπ+
No full textThe decay mode B0s→D∓sK± allows for one of the theoretically cleanest measurements of the CKM angle γ through the study of time-dependent CP violation. This paper reports a measurement of its branching fraction relative to the Cabibbo-favoured mode B0s→D−sπ+ based on a data sample corresponding to 0.37 fb−1 of proton-proton collisions at s√=7TeV collected in 2011 with the LHCb detector. In addition, the ratio of B meson production fractions f s /f d , determined from semileptonic decays, together with the known branching fraction of the control channel B 0 → D −π+, is used to perform an absolute measurement of the branching fractions: B(B0s→D−sπ+)=(2.95±0.05±0.17+0.18−0.22)×10−3,\hfillB(B0s→D∓sK±)=(1.90±0.12±0.13+0.12−0.14)×10−4,\hfill where the first uncertainty is statistical, the second the experimental systematic uncertainty, and the third the uncertainty due to f s /f d
Precision measurement of D meson mass differences
No full textUsing three- and four-body decays of D mesons produced in semileptonic b-hadron decays, precision measurements of D meson mass differences are made together with a measurement of the D 0 mass. The measurements are based on a dataset corresponding to an integrated luminosity of 1.0 fb−1 collected in pp collisions at 7 TeV. Using the decay D 0 → K + K − K − π +, the D 0 mass is measured to be M(D0)=1864.75±0.15(stat)±0.11(syst)MeV/c2.
The mass differences
M(D+)−M(D0)=4.76±0.12(stat)±0.07(syst)MeV/c2,M(Ds)−M(D+)=98.68±0.03(stat)±0.04(syst)MeV/c2
are measured using the D 0 → K + K − π + π − and D+(s)→K+K−π+ mode
High resolution field imaging with atomic vapor cells
No full textIn this thesis, I report on the development of imaging techniques in atomic vapor cells. This is a relatively unexplored area, despite the ubiquitous use of imaging in experiments with ultracold atoms. Our main focus is in high resolution imaging of microwave near fields, for which there is currently no satisfactory established technique. We detect microwave fields through Rabi oscillations driven by the microwave on atomic hyperfine transitions. The technique can be easily modified to also image dc magnetic fields. In addition, we have developed techniques to image vapor cell processes such as atomic T1 and T2 relaxation. These provide a new window into vapor cell physics, which we have used to obtain spatially resolved information on Rb interactions with the cell walls, and to estimate the Rb relaxation probability in a collision with the cell wall.
As a first application of our imaging techniques, we imaged the dc and microwave magnetic fields inside a state-of-the-art vapor cell atomic clock. This new clock characterisation technique should lead to real improvements in clock performance, and is in the process of being adopted by the atomic clock community.
We have developed a widefield, high resolution imaging setup using a microfabricated vapor cell, which we have used to image microwave and dc magnetic vector fields. With the addition of a 480 nm laser, the setup can be configured to image microwave electric fields. Our camera-based imaging system records 2D images with a 6x6 mm2 field of view at a rate of 10 Hz. It provides up to 50 um spatial resolution, and allows imaging of fields as close as 150 um above structures, through the use of extremely thin external cell walls. This is crucial in allowing us to take practical advantage of the high spatial resolution, as feature sizes in near-fields are on the order of the distance from their source, and represents an order of magnitude improvement in surface-feature resolution compared to previous vapor cell experiments. We demonstrate a microwave magnetic field sensitivity of 1.4 uT/sqrt-Hz per 50x50x140 um3 voxel, at present limited by the speed of our imaging system. Since we image 120x120 voxels in parallel, a single scanned sensor would require a sensitivity of at least 12 nT/sqrt-Hz to produce images with the same sensitivity.
The spatial resolution, distance of approach, and sensitivity of our high resolution setup are sufficient for characterising 6.8 GHz microwave fields above a range of real world devices. However, frequency tunability is essential for wider applications of our imaging technique. Industry is particularly interested in techniques for imaging high frequency microwaves, above 18 GHz, where simulations become increasingly unreliable. I have shown that our technique can be extended to image microwaves of any frequency, in principle from dc to 100s of GHz, by using a large dc magnetic field to Zeeman shift the hyperfine ground state transitions to the desired frequency. I present results from a proof-of-principle setup, where we have used a 0.8 T solenoid to detect and image microwaves from 2.3 GHz to 26.4 GHz
First evidence for the annihilation decay mode B+ → D+sΦ
No full textEvidence for the hadronic annihilation decay mode B+→D+sϕ is found with greater than 3σ significance. The branching fraction and CP asymmetry are measured to be B(B+→D+sϕ)=(1.87+1.25−0.73(stat)±0.19(syst)±0.32(norm))×10−6,\hfillACP(B+→D+sϕ)=−0.01±0.41(stat)±0.03(syst).\hfill The last uncertainty on B(B+→D+sϕ) is from the branching fractions of the B+→D+sD−−0 normalization mode and intermediate resonance decays. Upper limits are also set for the branching fractions of the related decay modes B+(c)→D+(s)K∗0,B+(c)→D+(s)K−−∗0 and B+c→D+sϕ , including the result B(B+→D+K∗0)<1.8×10−6 at the 90% credibility level
Search for CP violation in D + → ϕπ + and D+s→K0Sπ+ decays
No full textA search for CP violation in D + → ϕπ + decays is performed using data collected in 2011 by the LHCb experiment corresponding to an integrated luminosity of 1.0 fb−1 at a centre of mass energy of 7 TeV. The CP -violating asymmetry is measured to be (−0.04 ± 0.14 ± 0.14)% for candidates with K − K + mass within 20 MeV/c 2 of the ϕ meson mass. A search for a CP -violating asymmetry that varies across the ϕ mass region of the D + → K − K + π + Dalitz plot is also performed, and no evidence for CP violation is found. In addition, the CP asymmetry in the D+s→K0Sπ+ decay is measured to be (0.61 ± 0.83 ± 0.14)%
MeSH term explosion and author rank improve expert recommendations
No full textInformation overload is an often-cited phenomenon that reduces the productivity, efficiency and efficacy of scientists. One challenge for scientists is to find appropriate collaborators in their research. The literature describes various solutions to the problem of expertise location, but most current approaches do not appear to be very suitable for expert recommendations in biomedical research. In this study, we present the development and initial evaluation of a vector space model-based algorithm to calculate researcher similarity using four inputs: 1) MeSH terms of publications; 2) MeSH terms and author rank; 3) exploded MeSH terms; and 4) exploded MeSH terms and author rank. We developed and evaluated the algorithm using a data set of 17,525 authors and their 22,542 papers. On average, our algorithms correctly predicted 2.5 of the top 5/10 coauthors of individual scientists. Exploded MeSH and author rank outperformed all other algorithms in accuracy, followed closely by MeSH and author rank. Our results show that the accuracy of MeSH term-based matching can be enhanced with other metadata such as author rank
Study of DJ meson decays to D+π−, D0π+ and D∗+π− final states in pp collisions
No full textA study of D+π−, D0π+ and D∗+π− final states is performed using pp collision data, corresponding to an integrated luminosity of 1.0 fb−1, collected at a centre-of-mass energy of 7 TeV with the LHCb detector. The D1(2420)0 resonance is observed in the D∗+π− final state and the D∗2(2460) resonance is observed in the D+π−, D0π+ and D∗+π− final states. For both resonances, their properties and spin-parity assignments are obtained. In addition, two natural parity and two unnatural parity resonances are observed in the mass region between 2500 and 2800 MeV. Further structures in the region around 3000 MeV are observed in all the D∗+π−, D+π− and D0π+ final states
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
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