Physics News Update, Number 489

Number 489, June 15, 2000, 2000 by Phillip F. Schewe and Ben Stein

RHIC'S FIRST COLLISIONS ARE GOLDEN. In a long-awaited development, Brookhaven's Relativistic Heavy Ion Collider (RHIC) produced its first collisions on June 12, when researchers witnessed gold ions (with an energy of 30 GeV per nucleon) smashing into each other to generate a fireworks display of roughly 1,000 symmetrical particle tracks. (See figure at Physics News Graphics).

Containing four advanced detectors (named BRAHMS, PHENIX, PHOBOS, and STAR), the RHIC facility aims to produce and study quark-gluon plasma (QGP), a hypothetical hot, dense soup of single quarks and gluons last believed to exist naturally in the first millionth of a second after the Big Bang. Subsequently, as the universe expanded and cooled, quarks assembled into extremely-hard-to-tear-apart two- or three-member groups (mesons and baryons, respectively) held together by gluons.

With the eventual goal being 100 GeV per nucleon in each of two heavy-ion beams, the RHIC collisions will produce temperatures and particle densities tens of thousands of times greater than those even at the centers of stars. Besides creating the QGP, other goals include colliding protons at high energies to make what should be the first definitive measurement of the contribution of gluons to the proton's spin.

Also, researchers plan to search for violations of such fundamental physics symmetries as P (parity) and CP (charge-parity) that would come about because of the strong nuclear force; previously the nonconservation of P and CP has only come about because of the weak nuclear force. One of the first major venues for the discussion of prospective RHIC results is expected to be the Quark Matter 2001 Conference in Long Island, NY in January 2001. (Brookhaven release at http://www.pubaf.bnl.gov/pr/bnlpr060800.html; also see Physics Today, October 1999.)

CATCHING TINY LEAKS WITH SOUND. A new "photoacoustic" technique, described at this month's meeting of the Acoustical Society of America in Atlanta, takes just seconds to detect and pinpoint leaks in sealed containers to within a millimeter, even if the leak is so small (less than 10^-5 cm³/s) that it would take a week for it to fill a thimble. According to its developers, no other technique can locate such tiny leaks so quickly.

Automobiles, refrigerators, air conditioners, and other products all include parts that contain liquids or pressurized gases which in some cases may be combustible or hazardous. Finding leaks in these parts could lead to a safer industrial workplace, more reliable consumer products, and lower releases of hazardous gases into the environment. In a technique developed at the University of Michigan, Serdar Yonak and David Dowling (734-936-0423, drd@engin.umich.edu) fill the part being tested for leaks with sulfur hexafluoride, an inert, nontoxic tracer gas. A carbon dioxide laser then scans the sample up to 6000 times per second. When the laser beam passes over a cloud formed by leaking gas, the beam rapidly heats the gas which then expands and generates a sound pulse. This process of producing sound with light is known as "photoacoustics" and it was discovered by Alexander Graham Bell in the 19th century. (It's the opposite of sonoluminescence, the conversion of sound into light.)

However, determining the exact location of the leak can be extremely difficult. To do this, the researchers analyze the photoacoustic sound by employing a state-of-the-art sonar signal processing technique known as matched field processing. An array of sensitive microphones records the sound. Taking into account the microphones' locations, and other parameters such as the speed of sound and the shape of the part being tested, computer processing essentially reconstructs the trajectory of the sound waves backwards in time and converges them to the location of the leak. (Paper 2aPA3 at meeting; background information at http://www.acoustics.org/136th/yonak.htm)