At the PEP II facility at the Stanford Linear Accelerator Center (SLAC) in California and at the KEKB facility in Japan, electrons with momentum 9.03 GeV/c were made to collide head-on with positrons...

At the PEP II facility at the Stanford Linear Accelerator Center (SLAC) in California and at the KEKB facility in Japan,

electrons with momentum 9.03 GeV/c were made to collide head-on with positrons whose momentum is 3.10 GeV/c (1 GeV

= 109 eV); see Figure 10.42. That is, pc for the electron is 9.03 GeV and pc for the positron is 3.10 GeV. The values of pc

and the corresponding energies are so large with respect to the electron or positron rest energy (0.5 MeV = 0.0005 GeV)

that for the purposes of this analysis you may, if you wish, safely consider the electron and positron to be massless.