324 research outputs found
Efimov universality with Coulomb interaction
The universal properties of charged particles are modified by the presence of
a long-range Coulomb interaction. We investigate the modification of Efimov
universality as a function of the Coulomb strength using the Gaussian expansion
method. The resonant short-range interaction is described by Gaussian
potentials to which a Coulomb potential is added. We calculate binding energies
and root mean square radii for the three- and four-body systems of charged
particles and present our results in a generalised Efimov plot. We find that
universal features can still be discerned for weak Coulomb interaction, but
break down for strong Coulomb interaction. The root-mean-square radius plateaus
at increasingly smaller values for strong Coulomb interaction and the
probablity distributions of the states become more concentrated inside the
Coulomb barrier. As an example, we apply our universal model to nuclei with an
alpha-cluster substructure. Our results point to strong non-universal
contributions in that sector.Comment: 18 pages, 9 figures, final version (with small orthographical
corrections
Colliders
Modern particle physics relies on high energy particle accelerators to
provide collisions of various types of elementary particles in order to deduce
fundamental laws of physics or properties of individual particles. The only way
to generate particle collisions at extremely high energies is to collide
particles of counter-rotating beams...called "particle-colliders". This
write-up gives a short briefing on the physics motivation of various particle
colliders ( colliders, colliders, ...), a summary of the
historical evolution and a mathematical treatment to describe collider
performance.Comment: 25 pages, 18 figure
Universal physics of bound states of a few charged particles
We study few-body bound states of charged particles subject to attractive
zero-range/short-range plus repulsive Coulomb interparticle forces. The
characteristic length scales of the system at zero energy are set by the
Coulomb length scale and the Coulomb-modified effective range
. We study shallow bound states of charged particles with
and show that these systems obey universal scaling laws
different from neutral particles. An accurate description of these states
requires both the Coulomb-modified scattering length and the effective range
unless the Coulomb interaction is very weak (). Our findings are
relevant for bound states whose spatial extent is significantly larger than the
range of the attractive potential. These states enjoy universality -- their
character is independent of the shape of the short-range potential.Comment: 8 pages, 6 figures, extended discussion, results unchanged, to appear
in Phys. Lett.
The influence of residual vertical dispersion on LEP performance
During LEP Operation for Luminosity production, the closed orbit is systematically corrected towards a reference orbit which has been empirically found to produce high luminosities. Machine studies have been undertaken to try and understand the mechanism by which the vertical closed orbit affects the luminosity. The dominant parameter has been found to be the residual vertical dispersion, in particular the residual vertical dispersion at the interaction points. This paper reports on studies to investigate how the quality of the closed orbit affects the residual vertical dispersion and especially the residual vertical dispersion at the interaction points
Multi-bunch Feedback Systems
Coupled-bunch instabilities excited by the interaction of the particle beam
with its surroundings can seriously limit the performance of circular particle
accelerators. These instabilities can be cured by the use of active feedback
systems based on sensors capable of detecting the unwanted beam motion and
actuators that apply the feedback correction to the beam. Advances in
electronic technology now allow the implementation of feedback loops using
programmable digital systems. Besides important advantages in terms of
flexibility and reproducibility, digital systems open the way to the use of
novel diagnostic tools and additional features. We first introduce
coupled-bunch instabilities, analysing the equation of motion of charged
particles and the different modes of oscillation of a multi-bunch beam, showing
how they can be observed and measured. Different types of feedback systems will
then be presented as examples of real implementations that belong to the
history of multi-bunch feedback systems. The main components of a feedback
system and the related issues will also be analysed. Finally, we shall focus on
digital feedback systems, their characteristics, and features, as well as on
how they can be concretely exploited for both the optimization of feedback
performance and for beam dynamics studies.Comment: 44 pages, contribution to the CAS - CERN Accelerator School: Advanced
Accelerator Physics Course, Trondheim, Norway, 18-29 Aug 201
Nanoscale electrochemistry of sp2 carbon materials: from graphite and graphene to carbon nanotubes
Carbon materials have a long history of use as electrodes in electrochemistry, from (bio)electroanalysis to applications in energy technologies, such as batteries and fuel cells. With the advent of new forms of nanocarbon, particularly, carbon nanotubes and graphene, carbon electrode materials have taken on even greater significance for electrochemical studies, both in their own right and as components and supports in an array of functional composites.
With the increasing prominence of carbon nanomaterials in electrochemistry comes a need to critically evaluate the experimental framework from which a microscopic understanding of electrochemical processes is best developed. This Account advocates the use of emerging electrochemical imaging techniques and confined electrochemical cell formats that have considerable potential to reveal major new perspectives on the intrinsic electrochemical activity of carbon materials, with unprecedented detail and spatial resolution. These techniques allow particular features on a surface to be targeted and models of structure–activity to be developed and tested on a wide range of length scales and time scales.
When high resolution electrochemical imaging data are combined with information from other microscopy and spectroscopy techniques applied to the same area of an electrode surface, in a correlative-electrochemical microscopy approach, highly resolved and unambiguous pictures of electrode activity are revealed that provide new views of the electrochemical properties of carbon materials. With a focus on major sp2 carbon materials, graphite, graphene, and single walled carbon nanotubes (SWNTs), this Account summarizes recent advances that have changed understanding of interfacial electrochemistry at carbon electrodes including: (i) Unequivocal evidence for the high activity of the basal surface of highly oriented pyrolytic graphite (HOPG), which is at least as active as noble metal electrodes (e.g., platinum) for outer-sphere redox processes. (ii) Demonstration of the high activity of basal plane HOPG toward other reactions, with no requirement for catalysis by step edges or defects, as exemplified by studies of proton-coupled electron transfer, redox transformations of adsorbed molecules, surface functionalization via diazonium electrochemistry, and metal electrodeposition. (iii) Rationalization of the complex interplay of different factors that determine electrochemistry at graphene, including the source (mechanical exfoliation from graphite vs chemical vapor deposition), number of graphene layers, edges, electronic structure, redox couple, and electrode history effects. (iv) New methodologies that allow nanoscale electrochemistry of 1D materials (SWNTs) to be related to their electronic characteristics (metallic vs semiconductor SWNTs), size, and quality, with high resolution imaging revealing the high activity of SWNT sidewalls and the importance of defects for some electrocatalytic reactions (e.g., the oxygen reduction reaction). The experimental approaches highlighted for carbon electrodes are generally applicable to other electrode materials and set a new framework and course for the study of electrochemical and interfacial processes
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