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

1. Dec 15, 2014

### applestrudle

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

2. Dec 15, 2014

### Staff: Mentor

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.

3. Dec 15, 2014

### applestrudle

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

4. Dec 15, 2014

### Staff: Mentor

Right.

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

5. Dec 15, 2014

### ChrisVer

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

6. Dec 22, 2014

### 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!

7. Dec 22, 2014

### Staff: Mentor

They would have needed roughly 125+90=215 GeV (electron+positron->Z->Z+H) and missed it by about 10 GeV (the exclusion limit went up to 115 GeV).