The electron-proton storage ring HERA
The trick is to collide electrons and protons head on with high energy. Had one tried to achieve the same resolution by the old technique as e.g. used at SLAC, one should have needed an electron linear accelerator 6000 km (4000 miles) long.
Equivalent size of a linear accelerator like SLAC which could achieve the same resolution as HERA.
There are four experiments operated by international teams in the HERA research program: ZEUS, H1, HERMES and HERA-B.
Experiments at the high energies of HERA can be understood by a simple intuitive picture: an electron hits a quark inside the proton and ejects it with great force. The quark does not appear directly in this process, because of the very strong gluon forces. It produces instead nuclear particles (mesons and baryons), which stream out of the proton like a jet, marking the direction and energy of the struck quark. At the high HERA energy this process us quite evident. One can even verify the collision dynamics of the electron-quark collision just using a college textbook on elementary (relativistic) mechanics. This is probably the most direct evidence we shall ever have that there are actually quarks inside the proton.
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In an electron-proton collision observed by the H1 detector, an electron of 27.6 GeV energy collides with a quark inside the proton and pushes it out with great force. The struck quark fragments into a jet-like stream of mesons, the electron is reflected with an energy of 236 GeV. It has gained energy in this hard collision.
The H1-detector with a hight of 8 m. |
The international team of the ZEUS detector. |
Now let us take a closer look at the proton. The
gluon-strings between the quarks inside the proton can
break and at the ends quarks and antiquarks appear. One
has therefore to expect a dynamical mixture of quarks,
antiquarks and gluons, in contrast to just three quarks
of the naive proton model.
The measurements at HERA provide such a close up
picture. They show that the number of quark-antiquarks
pairs inside the proton is unexpectedly large. The figure
shows the new measurements together with old measurements
at lower energy. The variable x is a measure of the
energy of the quark inside the proton. The HERA
measurements are a probe of the quarks and antiquarks
inside the proton at small x-values. Their number
rises fast as one goes to small x-values. This was
a great surprise, since the old measurements had not
hinted at such a behavior.
The measurements were carried out by the H1 and ZEUS
detectors, shown in the figures.
The proton, according to HERA measurements: It is densly filled with quarks, antiquarks and gluons.
Number of quarks and antiquarks in the proton, as a function of x, the fractional quark energy inside the proton. The old measurements to the right of the marked x-values do not indicate the strong rise found by HERA experiments. The measurements at different values of the squared momentum transfer Q2 all show the same behavior.
The following figure gives a survey of the measurements made at HERA, together with previous results. It contains in concentrated form our best present knowledge of the structure of the proton, covering a range of four orders of magnitude in the variables x and Q2. It will be a challenge and testing ground for a complete theory of the proton, which is still to be achieved.
The structure function of the proton as a function of x and Q2
The discovery that there is a dense mass of quarks, antiquarks and gluons inside the proton, and the precise measurements of HERA, have met with great interest and triggered a large number of theoretical investigations. An acknowledgment of the importance of this field is the award of the Max-Born-Prize by the German Physical Society to Prof.John Dainton, a former leader of one of the HERA experiments.
Prof. John Dainton from Liverpool University receives the Max-Born-Prize of the German Physical Society. To the right Prof.A.M.Stoneham, Vice President of the Institute of Physics, left DPG President Prof.A.M.Bradshaw
(DPG archive)
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