274 research outputs found
Coherent and incoherent radiation from a channel-guided laser wakefield accelerator
Coherent and incoherent electromagnetic radiation emitted from a\ud
channel-guided laser wakefield accelerator (LWFA) is calculated based on the\ud
Lienard–Wiechert potentials. It is found that at wavelengths longer than the\ud
bunch length, the radiation is coherent. The coherent radiation, which typically\ud
lies in the infrared range, shows features that reveal details of the acceleration process and properties of the electron bunch, such as its duration, charge, energy, and offset with respect to the wakefield axis. It is found that the LWFA emits energy predominantly in the coherent range of frequencies. The incoherent range of the spectrum, which extends to the x-ray frequency range, consists of rather broad peaks caused by the acceleration. The radiated energy, power and the pulse duration are estimated
Charged particle interaction with a chirped electromagnetic pulse
It is found that a charged particle can get a net energy gain from the interaction with an electromagnetic chirped pulse. Theoretically, the energy gain increases with the pulse amplitude and with the relative frequency variation in the pulse
Generation of ultra-short relativistic-electron-bunch by a laser wakefield
The possibility of the generation of an ultra-short (about one micron long) relativistic (up to a few GeVs) electron-bunch in a moderately nonlinear laser wakefield excited in an underdense plasma by an intense laser pulse is investigated. The ultra-short bunch is formed by trapping, effective compression (both in longitudinal and in transverse directions) and acceleration of an initially nonrelativistic (with kinetic energy of a few hundreds keVs) e-bunch that is injected in front of the laser pulse. The initial bunch may be of poor quality, may have a duration in the order of the laser pulse length or longer and can be generated by a laser-driven photo-cathode RF gun. Our 1D and 3D calculations predict that the accelerated ultra-short bunch will show a low energy spread of less than one percent and a low transverse emittance in the order of a nanometer. An energy gain in the GeV-range is feasible at an accelerating distance of a few centimetres. The total number of accelerated electrons is restricted by the beam loading effect only and can reach a value of 10/sup 8/-10/sup 9/
Femtosecond electron-bunch dynamics in laser wakefields and vacuum
Recent advances in laser wakefield acceleration demonstrated the generation of extremely short (with a duration of a few femtoseconds) relativistic electron bunches with relatively low (of the order of couple of percent) energy spread. In this article we study the dynamics of such bunches in drift space (vacuum) and in channel-guided laser wakefields. Analytical solutions were found for the transverse coordinate of an electron and for the bunch envelope in the wakefield in the case of arbitrary change in the energy. Our results show strong bunch dynamics already on a millimeter scale propagation distance both in plasma and in vacuum. When the bunch propagates in vacuum, its transverse sizes grow considerably; the same is observed for the normalized bunch emittance that worsens the focusability of the bunch. A scheme of two-stage laser wakefield accelerator with small drift space between the stages is proposed. It is found that fast longitudinal betatron phase mixing occurs in a femtosecond bunch when it propagates along the wakefield axis. When bunch propagates off axis, strong bunch decoherence and fast emittance degradation due to the finite bunch length was observed
Electron bunch injection at an angle into a laser wakefield
External injection of electron bunches longer than the plasma wavelength in a
laser wakefield accelerator can lead to the generation of femtosecond
ultrarelativistic bunches with a couple of percent energy spread. Extensive
study has been done on external electron bunch (e.g. one generated by a
photo-cathode rf linac) injection in a laser wakefield for different
configurations. In this paper we investigate a new way of external injection
where the electron bunch is injected at a small angle into the wakefield. This
way one can avoid the ponderomotive scattering as well as the vacuum-plasma
transition region, which tend to destroy the injected bunch. In our
simulations, the effect of the laser pulse dynamics is also taken into account.
It is shown that injection at an angle can provide compressed and accelerated
electron bunches with less than 2% energy spread. Another advantage of this
scheme is that it has less stringent requirements in terms of the size of the
injected bunch and there is the potential to trap more charge
Biomechanical Basis Of External Fixation In Patients With Femur Fractures
Advantages of osteosynthesis of femur fractures by means of rod type external fixation devices have been under the study. Computer modelling on the basis of methods of deformable firm body mechanics for pin and rod devices for external fixation has been performe
Thermodynamic Derivation of the Tsallis and R\'enyi Entropy Formulas and the Temperature of Quark-Gluon Plasma
We derive Tsallis entropy, Sq, from universal thermostat independence and
obtain the functional form of the corresponding generalized entropy-probability
relation. Our result for finite thermostats interprets thermodynamically the
subsystem temperature, T1, and the index q in terms of the temperature, T,
entropy, S, and heat capacity, C of the reservoir as T1 = T exp(-S/C) and q = 1
- 1/C. In the infinite C limit, irrespective to the value of S, the
Boltzmann-Gibbs approach is fully recovered. We apply this framework for the
experimental determination of the original temperature of a finite thermostat,
T, from the analysis of hadron spectra produced in high energy collisions, by
analyzing frequently considered simple models of the quark-gluon plasma.Comment: 4 pages 1 Figure PRL style, revised presentatio
Measurement of charm production at central rapidity in proton-proton collisions at TeV
The -differential production cross sections of the prompt (B
feed-down subtracted) charmed mesons D, D, and D in the rapidity
range , and for transverse momentum GeV/, were
measured in proton-proton collisions at TeV with the ALICE
detector at the Large Hadron Collider. The analysis exploited the hadronic
decays DK, DK, DD, and their charge conjugates, and was performed on a
nb event sample collected in 2011 with a
minimum-bias trigger. The total charm production cross section at TeV and at 7 TeV was evaluated by extrapolating to the full phase space
the -differential production cross sections at TeV
and our previous measurements at TeV. The results were compared
to existing measurements and to perturbative-QCD calculations. The fraction of
cdbar D mesons produced in a vector state was also determined.Comment: 20 pages, 5 captioned figures, 4 tables, authors from page 15,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/307
Flavor and LHC Searches for New Physics
Uncovering the physics of electroweak symmetry breaking (EWSB) is the
raison-d'etre of the LHC. Flavor questions, it would seem, are of minor
relevance for this quest, apart from their role in constraining the possible
structure of EWSB physics. In this short review article, we outline, using
flavor-dependent sleptons as an example, how flavor can affect both searches
for supersymmetry, and future measurements aimed at understanding the nature of
any new discoveries. If the production cross-sections for supersymmetry are
relatively low, as indicated by the fact that it has not revealed itself yet in
standard searches, the usual assumptions about the superpartner spectra need
re-thinking. Furthermore, one must consider more intricate searches, such as
lepton-based searches, which could be susceptible to flavor effects. We start
by reviewing the flavor structure of existing frameworks for mediating
supersymmetry breaking, emphasizing flavor-dependent models proposed recently.
We use the kinematic endpoints of invariant mass distributions to demonstrate
how flavor dependence can impact both searches for supersymmetry and the
Inverse Problem. We also discuss methods for measuring small-mass splittings
and mixings at the LHC, both in models with a neutralino LSP and in models with
a charged slepton (N)LSP.Comment: 11 pages, 1 figure. Invited Review for EPJ
J/psi Production as a Function of Charged Particle Multiplicity in pp Collisions at sqrt{s} = 7 TeV
The ALICE collaboration reports the measurement of the inclusive J/psi yield
as a function of charged particle pseudorapidity density dN_{ch}/deta in pp
collisions at sqrt{s} = 7 TeV at the LHC. J/psi particles are detected for p_t
> 0, in the rapidity interval |y| < 0.9 via decay into e+e-, and in the
interval 2.5 < y < 4.0 via decay into mu+mu- pairs. An approximately linear
increase of the J/psi yields normalized to their event average
(dN_{J/psi}/dy)/ with (dN_{ch}/deta)/ is observed
in both rapidity ranges, where dN_{ch}/deta is measured within |eta| < 1 and
p_t > 0. In the highest multiplicity interval with = 24.1,
corresponding to four times the minimum bias multiplicity density, an
enhancement relative to the minimum bias J/psi yield by a factor of about 5 at
2.5 < y < 4 (8 at |y| < 0.9) is observed.Comment: Submitted to Phys. Lett.
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