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Event pictures

Deep Inelastic Scattering

Cross section of the detector

Click on various parts of the detector to show what they do.

The picture above is a computer display of a collision, or "event", in ZEUS.  It enables physicists to tell what particles have been created in the impact.  The display shows a cross sectional view of the detector with the beam pipe shown as a "gap" running horizontally through the middle.  The tracks emerging from the pipe show the paths of charged particles (detected by the experiment) in the central tracking detector and the red section shows the calorimeters which measure the energy of the particles.  The yellow blocks show where energy has been absorbed.  An interesting feature of this particular image is the wave like track running from left to right.  This is a low momentum particle which has had its path curved by the magnetic field in the detector.

In this picture the electron beam enters from the left, and the proton beam from the right.  An electron and a proton collide in the centre. The electron recoils back towards the top left, where it stops in the electromagnetic calorimeter.  A spray of particles shoots down towards the bottom left, stopping in the hadron calorimeter.  This is a "jet" of hadrons  (baryons and mesons) created from the quark in the proton that the electron has hit. (The electron does not actually hit the quark, but rather interacts via a virtual photon force mediator, separating the quark from the other two.)  As quarks are unable to exist alone a hadron jet is produced.  The residue of the proton continues to the left, creating particles that spray out only slightly beyond the beam pipe to deposit energy in the "forward" part of the hadron calorimeter (the yellow blocks o the left).

95_Bcal170_NC3.gif (7182 bytes)

In this image we see the same event, but this time from one end of the detector.  The beam pipe here is the small central disc.  Only information from the central tracking detector and the central "barrel" of the calorimeters are shown in this view.  From this picture we can clearly see conservation of momentum in action.  The path of the electron is shown here as (almost) perpendicular to the beam pipe, thus the hadron shower has to be (almost) perpendicular but in the opposite direction since the initial momentum perpendicular to the pipe was zero.  We can also see from this that the path of the low momentum particle mentioned earlier is in fact spiral (this is shown just to the lower right of the beam pipe).

95_Bcal170_NC2.gif (6538 bytes)

This last image shows a different projection of the same event, in this case what is known as a "Lego plot".  It is a view of the energy deposited in the "barrel" of the uranium calorimeter, rather as if its cylindrical structure had been "unrolled".  The lower edge of the chequered "base" is divided according to the angle (0° - 360°) around the beam pipe.  The right-hand edge is divided according to direction at different angles to the beam pipe (these are not actual angles, hence the values from -3 to +3).  The vertical axis shows the amount of energy deposited.  It is easy to see two main energy deposits.  The one on the right corresponds to the electron, which is absorbed in just one block of the electromagnetic calorimeter.  The other, slightly more spread out deposit corresponds to the hadron jet.

Click any of the pictures for the full event display

  

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