Electron-proton collisions at HERA

16 September 1998

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The Hadron Electron Ring Accelerator (HERA) at the DESY laboratory in Hamburg is operating again with electrons, after working with head-on collisions between protons and antielectrons (positrons) since 1994.While in many respects, electrons and their antiparticles are alike apart from having opposite electric charge, there is one interaction with a proton that occurs more readily with electrons than with positrons. This is when the electron interacts with a quark inside the proton and changes into the elusive particle known as the electron-neutrino. These interactions occur through the weak force - the same force that underlies the initial step in the chain of nuclear reactions that make the Sun and other stars shine.

In this side-view of a recent electron-proton collision in the ZEUS detector at HERA, the electron (unseen) has come along central beam pipe from the left, and the proton (also unseen) from the right. The electron has struck a quark in the proton, and this quark has shot out into the detector, converting instantly into a tight spray or "jet" of particles which leave the tracks pointing to the bottom left-hand corner. The particles then deposit a lot of energy (indicated the yellow blocks) in the uranium calorimeter, which is specially designed to trap particles like this and measure their energy. The electron, meanwhile, has changed into an electron-neutrino, which leaves no tracks. However, the neutrino's presence can be inferred because it disappears with energy and momentum, leaving an apparent imbalance, which is seen more clearly in an end-view of the detector.

Credit: DESY/ZEUS

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