63,188 research outputs found
Measurement of the ratio of prompt χ c to J / ψ production in pp collisions at √s = 7 TeV
No full textThe prompt production of charmonium χ c and J / ψ states is studied in proton-proton collisions at a centre-of-mass energy of √s = 7 TeV at the Large Hadron Collider. The χ c and J / ψ mesons are identified through their decays χ c → J / ψ γ and J / ψ → 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 → J / ψ γ) / σ (J / ψ), is determined as a function of the J / ψ transverse momentum in the range 2 < p 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 T J / ψ region
J. Champion [picture].
No full textInscription: title below photograph.; In the album: Mrs W.A.S. Dunlop (Mary Paule Cunningham of Lanyon) [photograph album, c. 1904-1914].; Also available in an electronic version via the Internet at: http://nla.gov.au/nla.pic-an25101977
High-temperature magnetic stability of small magnetite particles
No full textThe stability of magnetic domain structures of small grains of magnetite were examined between room temperature and the Curie temperature using a high-resolution three-dimensional micromagnetic algorithm. At all times the minimum resolution used was determined by calculating the exchange length. Using an unconstrained model, the single domain (SD) to multidomain (MD) threshold grain size d0 was found to be nearly independent of temperature up to ∼450°C. Above this temperature, d0 was observed to rise sharply. Energy barriers between metastable domain states trapped in local energy minimums (LEM) were determined using a constrained algorithm. Three types of domain structure were considered: SD, vortex, and double vortex (effectively three domain), in a range of grain sizes with side length between 30 and 300 nm. In addition, the effect of varying shape was also considered by examining asymmetric grains with aspect ratios up to 1.4. From the numerical solutions energy barriers between LEM states were determined. It was found that MD grains 300 nm in size display higher stability than smaller SD grains (∼50 nm). Double vortex states were found to be less stable than single vortex states at nearly all temperatures. Blocking temperatures as function of grain size for both symmetric and asymmetric grains were determined and agree well with experimental results. Transdomain thermoremanence analysis indicated that there are a limited number of grain sizes and shapes which will nucleate domain wall-type structures during cooling. Such nucleation events would cause the total measured remanence to decrease with cooling in conflict with Néel's analytical theory for remanence cooling behavior but in agreement with experimental observations
Evidence for the decay B0→J/ψω and measurement of the relative branching fractions of meson decays to J/ψη and J/ψη′
No full textFirst evidence of the B 0 → J / ψ ω decay is found and the B s 0 → J / ψ η and B s 0 → J / ψ η ′ decays are studied using a dataset corresponding to an integrated luminosity of 1.0 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 B 0 → J / ψ ρ 0 decay:frac(B (B 0 → J / ψ ω), B (B 0 → J / ψ ρ 0)) = 0.89 ± 0.19 (stat) - 0.13 + 0.07 (syst),frac(B (B s 0 → J / ψ η), B (B 0 → J / ψ ρ 0)) = 14.0 ± 1.2 (stat) - 1.5 + 1.1 (syst) - 1.0 + 1.1 (frac(f d, f s)),frac(B (B s 0 → J / ψ η ′), B (B 0 → J / ψ ρ 0)) = 12.7 ± 1.1 (stat) - 1.3 + 0.5 (syst) - 0.9 + 1.0 (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 s 0 mesons. The ratio of the branching fractions of B s 0 → J / ψ η ′ and B s 0 → J / ψ η decays is measured to befrac(B (B s 0 → J / ψ η ′), B (B s 0 → J / ψ η)) = 0.90 ± 0.09 (stat) - 0.02 + 0.06 (syst)
Keyword based categorisation of diary entries to support personal Internet content pre-caching on mobile devices
No full textThis paper presents a study into the effectiveness of our algorithm for automatic categorisation of real users' diary entries, as a first step towards personal Internet content pre-caching on mobile devices. The study reports an experiment comparing trial subjects allocations of 99 diary entries to those predicted by a keyword-based algorithm. While leaving considerable grounds for improvement, results are positive and show pave the way for supporting mobile services based on categorising users' diary entries
Measurement of the time-dependent CP asymmetry in B0 -> J/ψ KS0 decays
No full textThis Letter reports a measurement of the CP violation observables SJ/ψK0S and CJ/ψK0S in the decay channel B0→J/ψK0S performed with 1.0 fb−1 of pp collisions at s√=7 TeV collected by the LHCb experiment. The fit to the data yields SJ/ψK0S=0.73±0.07(stat)±0.04(syst) and CJ/ψK0S=0.03±0.09(stat)±0.01(syst). Both values are consistent with the current world averages and within
expectations from the Standard Model
Studying the growth of galaxies with JWST
No full textThe James Webb Space Telescope (JWST or Webb) is a large (6.5m diameter
primary mirror) infrared (0.6 < λ < 30μm) space telescope. It impressive size and
wavelength coverage will revolutionise the field of galaxy formation and evolution,
enabling the community to push back the observational frontier to within a few
100 million years of the Big Bang. With JWST expected to launch in 2021,
a detailed understanding of its capabilities is essential to guarantee the success
of the mission. In this thesis, I present a new analysis of how best JWST can
be utilised to study the evolution and formation of galaxies across cosmic time.
Primarily this work uses simulated JWST and ancillary HST observations in
an attempt to assess JWST's capabilities, specifically its ability to study high-mass,
high-redshift, dusty star-forming galaxies in the Universe. Additionally, I
use currently available data from the UV/optical to sub-mm to perform a new
study of the prevalence of high-mass, high-redshift, dusty star-forming galaxies
accessible before the launch of JWST.
Firstly, I introduce a piece of software that I have built to simulate observations
that will be made using the Mid-infrared Instrument (MIRI) onboard JWST.
This piece of software is capable of simulating all of MIRI's available observing
modes: MIRI Imager, Low-Resolution Spectrograph (LRS), Medium Resolution
Spectrometer (MRS). I then use this software to show the potential for measurements
of the Hα emission line in high-redshift star-forming galaxies using the
MRS IFU. At redshifts z > 6.7, MIRI is the only instrument onboard JWST
that can directly observe Hα, a sensitive star-formation indicator, tracing very
recent star-formation. I show that with approximately ≃ 6-hour integration time
with the MIRI MRS, the Hα emission line in the brightest known z ≃ 7 galaxies
can be detected to a SNR of approximately ≃ 11. Therefore, I conclude that
the MIRI MRS could be an impressive tool for determining the star-formation
rates of high-redshift galaxies in the epoch of reionisation. Secondly, I present an
overview of the Public Release Imaging for Extragalactic Research (PRIMER)
proposal submitted as part of the Director's Discretionary Early Release Science
(DD-ERS) programme for Cycle-1 of JWST. PRIMER is a large (52hr), deep,
fully-sampled NIRCam and MIRI imaging programme, designed to efficiently
(≈ 75% observing efficiency) observe the faintest galaxies in the best-studied
available (Non-GTO Covered) equatorial HST CANDELS field, COSMOS. As
well as detailing the science goals and observational design of this programme,
I place it in the context of other planned JWST programmes (ERS & GTO)
focussed on observations of high-redshift galaxies.
Thirdly, I present a new analysis of the potential power of deep, near-infrared,
imaging surveys with the JWST to improve our knowledge of galaxy evolution.
In this work, I properly simulate what can be achieved with realistic survey
strategies, and utilise rigorous signal:noise calculations to calculate the resulting
posterior constraints on the physical properties of galaxies. I explore a broad
range of assumed input galaxy types (> 20,000 models, including extremely
dusty objects) across a wide redshift range (out to z ≃ 12), while at the
same time considering a realistic mix of galaxy properties based on our current
knowledge of the evolving population (as quantified through the Empirical Galaxy
Generator: EGG). While our main focus is on imaging surveys with NIRCam,
spanning λobs = 0:8 - 5:0 μm, an important goal of this work is to quantify
the impact/added-value of: i) parallel imaging observations with MIRI at longer
wavelengths and ii) deeper supporting optical/UV imaging with HST (potentially
prior to JWST launch) in maximising the power and robustness of a major
extragalactic NIRCam survey. I show that MIRI parallel 7.7-μm imaging is
of most value for better constraining the redshifts and stellar masses of the
dustiest (AV > 3) galaxies, while deep B-band imaging (reaching mAB ' 28:5mag)
with ACS on HST is vital for determining the redshifts of the large numbers of
faint/low-mass, z < 5 galaxies that will be detected in a deep JWST NIRCam
survey.
Finally, I attempt to assess the prevalence of the dusty subset of galaxies that
MIRI will observe, utilising the current deepest UV/optical/near-IR and far-
IR/sub-mm observations. The data collected from numerous telescopes covers a
total of 1.8 deg2 in the UDS and COSMOS fields. Combining the UV and IR data,
I assess the total number density as a function of cosmic time and compare the
methods of detecting/selecting high-redshift, high mass dusty galaxies. I calculate
the total star-formation rate (SFR) as a function of redshift and compare the
contribution from sub-mm galaxies and UV/optically-selected dusty galaxies. I
also calculate the star-formation rate density (SFRD) across cosmic history in
an attempt to extend the Madau & Dickinson (2014) plot to higher redshifts
for obscured star-formation. I show that both the UV/optical and (sub)-mm
approach to detecting high-redshift dusty galaxies both produce a consistent
estimation of their evolving comoving number density at high redshift. I find
clear evidence of a rapid decline in the total comoving number density of these
objects beyond z > 3, which results in a similarly steep decline in the SFRD
of the Universe contributed by obscured star-formation. Comparing to recent
work through in the literature, my results strengthen the existing evidence for a
transition between unobscured & obscured star formation as the dominant mode
of star-formation activity at z ≃ 2-3
Letter from J. E. Gavin to Louis C. Cramton regarding Sale of Bright Angel Trail
No full textLetter from J. E. Gavin to Louis C. Cramton regarding the Bright Angel Trail controversy, including newspaper clipping
Regional and cellular distribution of ephrin-B1 in adult mouse brain
No full textThe membrane-bound proteins ephrins and their receptors, Eph receptor tyrosine kinases, are
known for their key role during development of the central nervous system (CNS). Ligand/
receptor interactions as a result of cell–cell contacts activate intracellular signalling pathways
which mediate specific cellular responses. Activation can occur bidirectionally in both the
receptor and the ligand-bearing cells. Eph receptor and ephrin families have been implicated
in synaptic plasticity in themature brain: effects include long-term potentiation/depression of
excitatory transmission (LTP/LTD) and an action on the structure and number of synaptic
contacts. However, due to the redundancy of binding between receptors and ligands, the role
of individual proteins has not yet been completely elucidated. Ephrin-B1 has been suggested
to play a role in synaptic plasticity in the hippocampus, but its expression and localization at
pre- or post-synaptic sites has been poorly documented, most likely due to the apparent low
activity of the corresponding gene in mature brain. Here we present immunohistochemical
data demonstrating a broad but highly regulated cellular distribution of ephrin-B1 in the
mature mouse brain. We show that ephrin-B1 is expressed post-synaptically on dendritic
spines in the cortex, supporting a role in synaptic plasticity in this region. However, the
prevalent extra-synaptic distribution in regions such as the hippocampus and cerebellum
suggests an additional structural role, perhaps at the neuron/glia interface
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