29,843 research outputs found
Nonlinear double Compton scattering in the full quantum regime
A detailed analysis of the process of two photon emission by an electron
scattered from a high-intensity laser pulse is presented. The calculations are
performed in the framework of strong-field QED and include exactly the presence
of the laser field, described as a plane wave. We investigate the full quantum
regime of interaction, where photon recoil plays an essential role in the
emission process, and substantially alters the emitted photon spectra as
compared to those in previously-studied regimes. We provide a semiclassical
explanation for such differences, based on the possibility of assigning a
trajectory to the electron in the laser field before and after each quantum
photon emission. Our numerical results indicate the feasibility of
investigating experimentally the full quantum regime of nonlinear double
Compton scattering with already available plasma-based electron accelerator and
laser technology.Comment: 5 pages, 3 figure
Dark energy models toward observational tests and data
A huge amount of good quality astrophysical data converges towards the
picture of a spatially flat universe undergoing the today observed phase of
accelerated expansion. This new observational trend is commonly addressed as
Precision Cosmology. Despite of the quality of astrophysical surveys, the
nature of dark energy dominating the matter-energy content of the universe is
still unknown and a lot of different scenarios are viable candidates to explain
cosmic acceleration. Methods to test these cosmological models are based on
distance measurements and lookback time toward astronomical objects used as
standard candles. I discuss the characterizing parameters and constraints of
three different classes of dark energy models pointing out the related
degeneracy problem which is the signal that more data at low (z= 0- 1), medium
(1<z<10) and high (10 <z< 1000) redshift are needed to definitively select
realistic models.Comment: 17 pages, 9 figures, Lectures for 42nd Karpacz Winter School of
Theoretical Physics: Current Mathematical Topics in Gravitation and
Cosmology, Ladek, Poland, 6-11 Feb 200
Relativistic three-body recombination with the QED vacuum
Electron-positron pair annihilation into a single photon is studied when a
second free electron is present. Focussing on the relativistic regime, we show
that the photon emitted in the three-lepton interaction may exhibit distinct
angular distributions and polarization properties. Moreover, the process can
dominate over two-photon annihilation in relativistic electron-positron plasmas
of few-MeV temperature. An analogy with three-body recombination of electrons
with ions is drawn.Comment: 5 pages, 4 figure
Do Spinors Frame-Drag?
We investigate the effect of the intrinsic spin of a fundamental spinor field
on the surrounding spacetime geometry. We show that despite the lack of a
rotating stress-energy source (and despite claims to the contrary) the
intrinsic spin of a spin-half fermion gives rise to a frame-dragging effect
analogous to that of orbital angular momentum, even in Einstein-Hilbert gravity
where torsion is constrained to be zero. This resolves a paradox regarding the
counter-force needed to restore Newton's third law in the well known spin-orbit
interaction. In addition, the frame-dragging effect gives rise to a {\it
long-range} gravitationally mediated spin-spin dipole interaction coupling the
{\it internal} spins of two sources. We argue that despite the weakness of the
interaction, the spin-spin interaction will dominate over the ordinary inverse
square Newtonian interaction in any process of sufficiently high-energy for
quantum field theoretical effects to be non-negligible.Comment: V2: published version, mostly minor clarifications from V
Spectroscopy of B_c Mesons in the Relativized Quark Model
We calculate the spectrum of the charm-beauty mesons using the relativized
quark model. Using the wavefunctions from this model we compute the radiative
widths of excited c\bar{b} states. The hadronic transition rates between
c\bar{b} states are estimated using the Kuang-Yan approach and are combined
with the radiative widths to give estimates of the relative branching ratios.
These results are combined with production rates at the Tevatron and the LHC to
suggest promising signals for excited B_c states. Our results are compared with
other models to gauge the reliability of the predictions and point out
differences.Comment: 15 pages, 1 fig. uses revtex4. References adde
<i>‘What retention’ means to me</i>: the position of the adult learner in student retention
Studies of student retention and progression overwhelmingly appear adopt definitions that place the institution, rather than the student, at the centre. Retention is most often conceived in terms of linear and continuous progress between institutionally identified start and end points.
This paper reports on research that considered data from 38 in-depth interviews conducted with individuals who had characteristics often associated with non-traditional engagement in higher education who between 2006 and 2010 had studied an ‘Introduction to HE’ module at one distance higher education institution, some of whom had progressed to further study at that institution, some of whom had not. The research deployed a life histories approach to seek a finer grained understanding of how individuals conceptualise their own learning journey and experience, in order to reflect on institutional conceptions of student retention.
The findings highlight potential anomalies hidden within institutional retention rates – large proportions of the interview participants who were not ‘retained’ by the institution reported successful progression to and in other learning institutions and environments, both formal and informal. Nearly all described positive perspectives on lifelong learning which were either engendered or improved by the learning undertaken. This attests to the complexity of individuals’ lives and provides clear evidence that institution-centric definitions of retention and progression are insufficient to create truly meaningful understanding of successful individual learning journeys and experiences. It is argued that only through careful consideration of the lived experience of students and a re-conception of measures of retention, will we be able to offer real insight into improving student retention
The Semiclassical Coulomb Interaction
The semiclassical Coulomb excitation interaction is at times expressed in the
Lorentz gauge in terms of the electromagnetic fields and a contribution from
the scalar electric potential. We point out that the potential term can make
spurious contributions to excitation cross sections, especially when the the
decay of excited states is taken into account. We show that, through an
appropriate gauge transformation, the excitation interaction can be expressed
in terms of the electromagnetic fields alone.Comment: 12 pages. Phys. Rev. C, Rapid Communication, in pres
Theory of Interfacial Plasmon-Phonon Scattering in Supported Graphene
One of the factors limiting electron mobility in supported graphene is remote
phonon scattering. We formulate the theory of the coupling between graphene
plasmon and substrate surface polar phonon (SPP) modes, and find that it leads
to the formation of interfacial plasmon-phonon (IPP) modes, from which the
phenomena of dynamic anti-screening and screening of remote phonons emerge. The
remote phonon-limited mobilities for SiO, HfO, h-BN and
AlO substrates are computed using our theory. We find that h-BN
yields the highest peak mobility, but in the practically useful high-density
range the mobility in HfO-supported graphene is high, despite the fact
that HfO is a high- dielectric with low-frequency modes. Our
theory predicts that the strong temperature dependence of the total mobility
effectively vanishes at very high carrier concentrations. The effects of
polycrystallinity on IPP scattering are also discussed.Comment: 33 pages, 7 figure
"Quantum Interference with Slits" Revisited
Marcella [arXiv:quant-ph/0703126] has presented a straightforward technique
employing the Dirac formalism to calculate single- and double-slit interference
patterns. He claims that no reference is made to classical optics or scattering
theory and that his method therefore provides a purely quantum mechanical
description of these experiments. He also presents his calculation as if no
approximations are employed. We show that he implicitly makes the same
approximations found in classical treatments of interference and that no new
physics has been introduced. At the same time, some of the quantum mechanical
arguments Marcella gives are, at best, misleading.Comment: 11 pages, 3 figure
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