L. Jensen

Four-forms and the vanishing of the cosmological constant

Abstract We show that the dynamics (and statics) of a four-form are more subtle than previously appreciated, especially when used in conjuction with the Baum-Hawking-Coleman mechanism. We investigate the properties of the four-forms on closed euclidean, open euclidean, and lorentzian manifolds. We find that only under special conditions is their behaviour such that the cosmological constant vanishes.

AWAKE, The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN

The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wakefield generation and electron acceleration driven by proton bunches. It is a proof-of-principle R&D experiment at CERN and the world׳s first proton driven plasma wakefield acceleration experiment. The AWAKE experiment will be installed in the former CNGS facility and uses the 400 GeV/c proton beam bunches from the SPS. The first experiments will focus on the self-modulation instability of the long (rms ~12 cm) proton bunch in the plasma. These experiments are planned for the end of 2016. Later, in 2017/2018, low energy (~15 MeV) electrons will be externally injected into the sample wakefields and be accelerated beyond 1 GeV. The main goals of the experiment will be summarized. A summary of the AWAKE design and construction status will be presented.

Bubble Nucleation for Flat Potential Barriers

Abstract We have studied false vacuum decay for effective potentials in which the false vacuum is separated from the true vacuum by a “flat” potential barrier. By flat, we mean that, near the top of the barrier, the potential varies quartically, rather than quadratically with the field (to leading order). We have discovered several new types of bubble solutions. One type reduces in the flat space-time limit to the mathematical solution introduced by Lee and Weinberg to describe “tunneling without barriers.” Based on our analysis, though, we propose a significantly different interpretation of the curved space solution. We numerically study these solutions (plus the Hawking-Moss solution) for a toy model to examine how the dominant tunneling mode may change as a function of parameters. We propose a variant of the new inflationary scenario based on these results.

Electron cloud studies and analyses at SPS for LHC-type beams

A summary of the main results obtained so far from the electron cloud studies using strip detectors, pick-ups, COLDEX and a 100 MHz coaxial resonator will be presented. The spatial and energy distributions of the electrons in the cloud measured by the strip detectors will be detailed and compared to the results obtained with a conventional retarding field detector. The evidence of the scrubbing effect and of the NEG coatings as remedies to reduce the electron cloud activity will also be shown. In a second part, the improved hardware of the experiments will be presented together with the program of measurements foreseen for the 2003 SPS run.

Acceleration of electrons in the plasma wakefield of a proton bunch

High-energy particle accelerators have been crucial in providing a deeper understanding of fundamental particles and the forces that govern their interactions. To increase the energy of the particles or to reduce the size of the accelerator, new acceleration schemes need to be developed. Plasma wakefield acceleration1–5, in which the electrons in a plasma are excited, leading to strong electric fields (so called ‘wakefields’), is one such promising acceleration technique. Experiments have shown that an intense laser pulse6–9 or electron bunch10,11 traversing a plasma can drive electric fields of tens of gigavolts per metre and above—well beyond those achieved in conventional radio-frequency accelerators (about 0.1 gigavolt per metre). However, the low stored energy of laser pulses and electron bunches means that multiple acceleration stages are needed to reach very high particle energies5,12. The use of proton bunches is compelling because they have the potential to drive wakefields and to accelerate electrons to high energy in a single acceleration stage13. Long, thin proton bunches can be used because they undergo a process called self-modulation14–16, a particle–plasma interaction that splits the bunch longitudinally into a series of high-density microbunches, which then act resonantly to create large wakefields. The Advanced Wakefield (AWAKE) experiment at CERN17–19 uses high-intensity proton bunches—in which each proton has an energy of 400 gigaelectronvolts, resulting in a total bunch energy of 19 kilojoules—to drive a wakefield in a ten-metre-long plasma. Electron bunches are then injected into this wakefield. Here we present measurements of electrons accelerated up to two gigaelectronvolts at the AWAKE experiment, in a demonstration of proton-driven plasma wakefield acceleration. Measurements were conducted under various plasma conditions and the acceleration was found to be consistent and reliable. The potential for this scheme to produce very high-energy electron bunches in a single accelerating stage20 means that our results are an important step towards the development of future high-energy particle accelerators21,22.Electron acceleration to very high energies is achieved in a single step by injecting electrons into a ‘wake’ of charge created in a 10-metre-long plasma by speeding long proton bunches.

Bubble formation in anisotropic cosmologies

Abstract We discuss the decay of the false vacuum in homogeneous cosmologies, both isotropic and anisotropic. If the false vacuum is isotropic, it is expected that the decay rate is dominated by spherically symmetric bubbles. However, if the universe during its past evolution was trapped in an anisotropic false vacuum one expects this anisotropy to be present in the decay products (bubbles) of that vacuum. In this paper we show the existence of two new, anisotropic types of bubble solution of Kantowski-Sachs type for a general double well potential. The first represents the decay of four-dimensional de Sitter space, and the second the decay of the product of the two-sphere and two-dimensional de Sitter space. We also derive qualitative features of the classical space-times emerging after tunneling.

Non-chaotic Kaluza-Klein cosmology

Abstract We investigate the evolution of higher-dimensional, homogeneous, anisotropic cosmologies. The natural extension to higher dimensions of the three-dimensional mixmaster cosmologies are analysed, and it is found — contrary to the three-dimensional case — that the higher-dimensional cosmologies exhibit no chaotic behaviour.

Is the universe euclidean

Abstract We argue that a lorentzian approach to the quantum behaviour of gravity is healthier than a euclidean approach. In a lorentzian formulation we show how it is possible to obtain a convergent kernel describing the quantum evolution of the universe from a path integral. We also present two straightforward examples which illustrate the advantages of the lorentzian formalism. Its implications for wormhole dynamics are discussed.

Beam commissioning of the SPS LSS6 extraction and TT60 for LHC

The new fast extraction system in LSS6 of the SPS and the first 100 m of transfer line TT60 was commissioned with low intensity beam in late 2006. The layout and functionality of the main elements are briefly explained, including the various hardware subsystems and the control system. The systems safety procedures, test objectives and measurements performed during the beam commissioning are described.

The quantum cosmology of an anisotropic universe

Abstract Surveys of the microwave background indicate that the universe is isotropic to more than one part in 10 5 . Due to the arbitrariness of the initial conditions of the universe at the big bang singularity one cannot predict this; it is usually put in by hand. We therefore construct the quantum cosmology of an anistropic universe according to the “no-boundary” prescription of Hartle and Hawking. Such a model has a well-defined behavior at the classical singularity. We then show it also implies that a large universe, such as ours, is isotropic.