# What is a GeV/beam? What is 45.6GeV/beam for 1 electron only

## Main Question or Discussion Point

I'm doing an essay on L3 experiment and the LEP collider and the first energy the electrons and positrons had were "45.6GeV/beam". I want to convert this to the amount of energy for one electron/positron and see how much mass a single electron/positron gains.

I know 1GeV = 10^9 * 1.6 x 10^-19 J

and I know E = mc^2 +(pc)^2

and electron mass 9.11 x 10^-31kg but I need to know what the "per beam" bit means?!!

Thank you

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jtbell
Mentor
I need to know what the "per beam" bit means?!!
I think what they mean is that both beams (the beam of electrons and the beam of positrons) contain particles with energy 45.6 GeV.

Oh? So one electron has 45.6GeV of energy!?

mfb
Mentor
Right.

(the alternative number is "per collision", 2*45.6 GeV)

ChrisVer
Gold Member
The change of the mass won't exceed the +0.4% of $m_e=511 keV$
It should be approximately $m_e^{1-loop} (46.5~GeV) \approx 512.6 ~keV$

vanhees71
LEP was built to study W and Z physics in high precision. To create a Z boson in an $e^+ e^-$ collision you need at least the $Z$ mass of about 91 GeV. That's why at first they had a total collision energy around this value. So the electron and the positron each have a total energy (including rest energy) around half this value. Later it was upgraded to make even two W bosons (for which you need $2 m_W \simeq 160 \mathrm{GeV}$). At the very end of its time in the year 2000 they pushed the cm. energy even up to around 210 GeV if I remember right, because there was some hint of a possible Higgs signal. They let LEP even run longer than planned for a while. Then Maiani had to make the difficult decision to stop it, because otherwise the construction of the LHC, using the LEP tunnnel, would have been delayed too much, and such an experiment to be delayed is very expensive!