A new view inside the proton

21 October 1998

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The Hadron Electron Ring Accelerator (HERA) at the DESY laboratory in Hamburg has recently started recording electron-proton interactions with the highest energy ever. This image shows an extremely violent interaction in the experiment known as H1. An electron from the left, with energy 27.5 GeV, collides head-on with a proton from the right, with energy 920 GeV. (1 GeV, or 1,000,000,000 electron volts is the energy a proton or electron acquires when accelerated by 1,000,000,000 volts.) The fragments from the interaction appear as a jet of particles which leave tracks in the detector (red lines) and also deposit energy (red areas). In this particular interaction, there is a large imbalance between energy going up and energy going down in the detector, which can also be seen in the end view. This imbalance is quantified in terms of Q², the square of the momentum transfer which is missing in the interaction. For this interaction Q² is very large, about 21,500 GeV². The observation of such an imbalance indicates that a neutral particle known as the neutrino has been produced with large energy. The interaction is called a "charged current event". This event is particularly exciting to the H1 physicists because of the large size of Q². Once much more data have been collected, the rate at which such violent electron-proton interactions take place will tell the physicists much about the "deep structure" of the proton, the nucleus of the simplest atom hydrogen.

Credit: DESY/H1

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