11 research outputs found
Effective Field Theory of Broken Spatial Diffeomorphisms
We study the low energy effective theory describing gravity with broken
spatial diffeomorphism invariance. In the unitary gauge, the Goldstone bosons
associated with broken diffeomorphisms are eaten and the graviton becomes a
massive spin-2 particle with 5 well-behaved degrees of freedom. In this gauge,
the most general theory is built with the lowest dimension operators invariant
under only temporal diffeomorphisms. Imposing the additional shift and SO(3)
internal symmetries, we analyze the perturbations on a FRW background. At
linear perturbation level, the observables of this theory are characterized by
five parameters, including the usual cosmological parameters and one additional
coupling constant for the symmetry-breaking scalars. In the de Sitter and
Minkowski limit, the three Goldstone bosons are supermassive and can be
integrated out, leaving two massive tensor modes as the only propagating
degrees of freedom. We discuss several examples relevant to theories of massive
gravity.Comment: 26 pages, V2 more references, several remarks and a new subsection
are added, V3 a major revision, with two new subsections added, as well as
several new discussions on the construction of our EF
Luminosity for laser-electron colliders
High intensity laser facilities are expanding their scope from laser and
particle-acceleration test beds to user facilities and nuclear physics
experiments. A basic goal is to confirm long-standing predictions of
strong-field quantum electrodynamics, but with the advent of high-repetition
rate laser experiments producing GeV-scale electrons and photons, novel
searches for new high-energy particle physics also become possible. The common
figure of merit for these facilities is the invariant describing the electric field strength in
the electron rest frame relative to the ``critical'' field strength of quantum
electrodynamics where the vacuum decays into electron-positron pairs. However,
simply achieving large is insufficient; discovery or validation requires
statistics to distinguish physics from fluctuations. The number of events
delivered by the facility is therefore equally important. In high-energy
physics, luminosity quantifies the rate at which colliders provide events and
data. We adapt the definition of luminosity to high-intensity laser-electron
collisions to quantify and thus optimize the rate at which laser facilities can
deliver strong-field QED and potentially new physics events. Modeling the
pulsed laser field and electron bunch, we find that luminosity is maximized for
laser focal spot size equal or slightly larger than the diameter of the dense
core of the electron bunch. Several examples show that luminosity can be
maximized for parameters different from those maximizing the peak value of
in the collision. The definition of luminosity for electron-laser
collisions is straightforwardly extended to photon-laser collisions and lepton
beam-beam collisions
High-charge 10 GeV electron acceleration in a 10 cm nanoparticle-assisted hybrid wakefield accelerator
In an electron wakefield accelerator, an intense laser pulse or charged
particle beam excites plasma waves. Under proper conditions, electrons from the
background plasma are trapped in the plasma wave and accelerated to
ultra-relativistic velocities. We present recent results from a
proof-of-principle wakefield acceleration experiment that reveal a unique
synergy between a laser-driven and particle-driven accelerator: a high-charge
laser-wakefield accelerated electron bunch can drive its own wakefield while
simultaneously drawing energy from the laser pulse via direct laser
acceleration. This process continues to accelerate electrons beyond the usual
decelerating phase of the wakefield, thus reaching much higher energies. We
find that the 10-centimeter-long nanoparticle-assisted wakefield accelerator
can generate 340 pC, 10.4+-0.6 GeV electron bunches with 3.4 GeV RMS convolved
energy spread and 0.9 mrad RMS divergence. It can also produce bunches with
lower energy, a few percent energy spread, and a higher charge. This
synergistic mechanism and the simplicity of the experimental setup represent a
step closer to compact tabletop particle accelerators suitable for applications
requiring high charge at high energies, such as free electron lasers or
radiation sources producing muon beams
Revisiting Experimental Signatures of the Ponderomotive Force
The classical theory of single-electron dynamics in focused laser pulses is the foundation of both the relativistic ponderomotive force (RPF), which underlies models of laser-collective-plasma dynamics, and the discovery of novel strong-field radiation dynamics. Despite this bedrock importance, consensus eludes the community as to whether acceleration of single electrons in vacuum has been observed in experimental conditions. We analyze an early experiment on the RPF with respect to several features that were neglected in modeling and that can restore consistency between theory predictions and experimental data. The right or wrong pulse profile function, laser parameters, or initial electron distribution can each make or break the agreement between predictions and data. The laser phase at which the electron’s interaction with the pulse begins has a large effect, explaining why much larger energies are achieved by electrons liberated in the focal region by photoionization from high-Z atoms and by electrons ejected from a plasma mirror. Finally, we compute the difference in a typical electron spectrum arising from fluctuating focal spot size in state-of-the-art ultra-relativistic laser facilities. Our results emphasize the importance of thoroughly characterizing laser parameters in order to achieve quantitatively accurate predictions and the precision required for discovery science
Creating QED photon jets with present-day lasers
© 2021 authors.Large-scale, relativistic particle-in-cell simulations with quantum electrodynamics (QED) models show that high-energy (1<Eγ75 MeV) QED photon jets with a flux of 1012sr-1 can be created with present-day lasers and planar, unstructured targets. This process involves a self-forming channel in the target in response to a laser pulse focused tightly (f number unity) onto the target surface. We show the self-formation of a channel to be robust to experimentally motivated variations in preplasma, angle of incidence, and laser stability, and present in simulations using historical shot data from the Texas Petawatt. We estimate that a detectable photon flux in the tens of MeV range will require about 60 J in a 150 fs pulse.11Nscopu
The acceleration of a high-charge electron bunch to 10 GeV in a 10-cm nanoparticle-assisted wakefield accelerator
An intense laser pulse focused onto a plasma can excite nonlinear plasma waves. Under appropriate conditions, electrons from the background plasma are trapped in the plasma wave and accelerated to ultra-relativistic velocities. This scheme is called a laser wakefield accelerator. In this work, we present results from a laser wakefield acceleration experiment using a petawatt-class laser to excite the wakefields as well as nanoparticles to assist the injection of electrons into the accelerating phase of the wakefields. We find that a 10-cm-long, nanoparticle-assisted laser wakefield accelerator can generate 340 pC, 10 ± 1.86 GeV electron bunches with a 3.4 GeV rms convolved energy spread and a 0.9 mrad rms divergence. It can also produce bunches with lower energies in the 4–6 GeV range