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Physics News Update
Number 841, October 3, 2007 by Phil Schewe

The Vacuum Strikes Back

Modern physics has shown that the vacuum, previously thought of as a state of total nothingness, is really a seething background of virtual particles springing in and out of existence until they can seize enough energy to materialize as “real” particles. In high energy collisions at accelerator labs, some of the original beam energy can be consumed by ripping particle-antiparticle pairs out of the vacuum. Sometimes this process is the very reason for doing the experiment, but sometimes it is only a detriment.

For example, in the Large Hadron Collider (LHC), under construction at the CERN lab in Geneva, a major source of beam losses (particles exiting from the usable beam) for heavy-ion collisions is expected to be a class of event in which the counter-moving ions pass each other and don’t interact except to spawn a pair of particles---an electron and positron---one of which (the positron) goes off to oblivion while the other (the electron) latches onto one of the ions.

This ion, bearing an extra electric charge, will now behave slightly differently as it races through the chain of powerful magnets that normally steer the particles around the accelerator. Going a certain distance, the modified ion will leave its fellows and smash into the beam pipe carrying the beams, thus heating up the pipe and surrounding magnet coils.

Fearing these future beam losses, accelerator physicists have sought to observe this effect at an existing machine, the Relativistic Heavy Ion Collider (RHIC) at the Brookhaven Lab on Long Island. And they found what they were looking for, a tiny splash of energy amounting to about .0002 watts, or about what a firefly puts out. The RHIC beam for these tests consisted of copper ions each carrying 6.3 TeV of energy (about 100 GeV per nucleon). According to CERN scientist John Jowett (john.jowett@cern.ch, 41-22-7676-643) this troublesome class of events, referred to as bound-free-pair production (or BFPP, the bound referring to the electron and the free to the positron), will be much more formidable at LHC than at RHIC.

First of all, the pair production scales as the atomic number of the nucleus (or the charge of the nucleus, denoted by the letter Z) raised to the seventh power. The LHC heavy-ion collisions will use beams composed of lead ions. The more highly charged nucleus and the larger energies (574 TeV per lead nucleus) mean the BFPP process should be some 100,000 times more prominent than in the test at RHIC.

This would amount to about 25 watts, the equivalent of a reading lamp. That doesn't sound like much but, when deposited in the ultra-cold (1.9 K) magnets of the LHC, it could bring them to the brink of "quenching" out of their superconducting state, interrupting the
operation of the huge machine. (Bruce et al., Physical Review Letters, 5 October 2007)

Gamma Rays From Thunderclouds

Have been observed by ground-based detectors, providing new insights into mechanisms for accelerating electrons to high energies, as high as 10 MeV, in the atmosphere. Ground observations of thundercloud gammas has been made before as part of monitoring regular nuclear plant operations. The new measurements, however, represent the first time that such gamma studies were made with detailed scientific objectives in mind, including determinations of particle species, arrival direction, and energy spectrum.

On the night of 6 January 2007 two powerful low-pressure air masses collided over the Sea of Japan. A nearby array of gamma detectors provided information on the energy and the timing of the gammas, which are the highest-category of electromagnetic radiation. The array is operated by the University of Tokyo and the Cosmic Radiation Laboratory of RIKEN in Japan. The gamma production, the researchers believe, works like this: an energetic seed electron, perhaps liberated from an atom by an intruding cosmic ray, ionizes many air molecules, which in turn are accelerated by the high electric fields present in the thunderclouds.

This flock of fast electrons can then emit gamma radiation (bremsstrahlung, or "braking radiation") as they are slowed by surrounding air. The gamma production actually occurs before the eventual lightning strike, says Teruaki Enoto of the University of Tokyo (enoto@amalthea.phys.s.u-tokyo.ac.jp, 81-3-5841-4173), and the reason for this is not entirely known.

Previous thundercloud-related gammas were studied by satellite and only measured very brief bursts, with durations of msec. By contrast, the Tokyo-RIKEN work indicates bursting behavior that could last for minutes, testifying to the quasi-static nature of the acceleration mechanism at work in the clouds. The electrostatic potential in the clouds might be as high as 10 million volts. (Tsuchiya et al., Physical Review Letters, upcoming article)

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