LHC limit glimpsed at RHIC

Modern physics has shown that the vacuum is seething with virtual particles springing in and out of existence. In high-energy heavy-ion collisions at accelerators, some of the original beam energy can be consumed by ripping particle-antiparticle pairs out of the vacuum. For example, in the Large Hadron Collider (LHC), soon to turn on at CERN in Geneva, Switzerland, a major source of beam losses for lead-ion collisions is expected to be so-called bound-free pair production (BFPP) in which the countermoving ions don’t interact except to spawn an electron–positron pair; the positron goes off to oblivion while the electron latches onto one of the ions, reducing that ion’s charge. The modified ion’s trajectory will be bent less in the accelerator’s magnets and eventually it will smash into the pipe carrying the beams. A shower of particles will result, and the deposited energy will heat up the pipe and the surrounding magnets. Using 6.3-TeV copper ions (about 100 GeV per nucleon), accelerator physicists have now seen the BFPP effect at an existing machine, the Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory in New York. They spotted a tiny splash of energy amounting to about 200 mW at the predicted location in the pipe. According to CERN scientist John Jowett, BFPP will produce thousands of times more heat at the LHC than it did at RHIC, possibly quenching a few of the LHC’s superconducting magnets and thus limiting the LHC’s peak luminosity for heavy-ion physics. (R. Bruce et al., Phys. Rev. Lett. 99 , 144801, 2007 http://dx.doi.org/10.1103/PhysRevLett.99.144801 . )