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# HERA

We are continuing to use electron scattering to learn about quarks and the particles they form.  One of the laboratories at the forefront of this research is DESY (Deutsches Elektronen-Synchrotron) in Hamburg, where electrons and protons collide in HERA (Hadron Electron Ring Accelerator).  In HERA, electrons are accelerated in a ring in one direction, the protons in another ring, in the opposite direction.  The two accelerator rings occupy the same tunnel which forms a circle 6.3km in circumference.  The rings intersect at four points around the tunnel, where the electrons and protons collide head on. There are two main experiments for studying deep inelastic scattering, called H1 and ZEUS.

HERA - The proton ring (the large "pipe") with the electron ring below.

Why accelerate the protons as well as the electrons?  In experiments where a beam of particles strikes a stationary target, such as at SLAC, much of the energy of the beam goes into moving the products of the interaction 'forwards' - in the same direction - to conserve momentum.  But if particles collide head on, their momenta are originally in opposite directions, and so the overall momentum is small.  (If the momenta are exactly equal in size, the total is zero!)  This means that a smaller amount of the energy of the particles goes simply into conserving momentum and moving the products.

What do the experiments at HERA find?

The above image represents of the inner structure of a proton as "seen" at HERA.  The purple particles are quarks, the green particles are anti-quarks, and the black spirals are gluons.  There are three more quarks than anti-quarks.  These are the three quarks we would normally refer to when speaking of the proton (two up, one down).  The other pairs of quarks and anti-quarks exist only momentarily; formed from an energetic gluon, they will come back together and annihilate returning once again to a gluon.  As we probe to the smallest current "visibility" we can see up to 100 of these quark/anti-quark pairs at any instant.