Structure of the proton
Protons are produced to collide in the LHC. At high enough energies, protons do not react as a whole, so that only their constituent parts interact with each other. This gives you the possibility to draw a conclusion about the inner structure of the proton by analysing the products of the collisions. The measurement task is to find and count all events in which a W particle was produced. The W particle decays into a lepton (electron or positron, or muon or antimuon) and the accompanying neutrino already within the proton. We call these events signal events. In addition you'll see a lot of background processes.

Task 1

Discover the structure of the proton!

Select all the signal events (events producing a W particle) from the 50 events of your data package. For these, you determine the electric charge of the W particle. After combining the results you determine the ratio of the number of positively charged W particles to the number of negatively charged W particles. We call this Rą.


The following outline gives you a better overview of all possible signal and background events:



In order to identify an event as a signal event you should notice the following criteria. It is realy a signal event, if



  • there is EXACTLY ONE Lepton (either an electron or a positron or a muon or an anti-muon),
  • which appear isolated (meaning it DOES NOT appear inside a JET) and
  • has a transverse momentum (Pt) greater than 20 GeV. Furthermore
  • a missing transverse momentum (MET) of AT LEAST 25 GeV is required in the event and
  • the angle between the charged lepton and the MET line in transversal plane must be between 160° und 200°.


Only if an event fulfils all these criteria a W particle was created in this event and thus we call it a signal event.