If you want to know more
This event is similar to a standard deep inelastic scattering event except that this time there is an additonal energy deposit in the bottom electromagnetic calorimeter with no track leading to it. This is caused by a photon (a gamma ray) which has been radiated by the electron. This can also be seen in the Lego Plot below.
Again this is similar to the deep inelastic scattering event except in this example two hadron jets are visible, not just one. The additonal jet is probably caused by a gluon radiated by a quark. Like quarks, gluons do not exist on their own and so another shower of particles is created.
Here we see an example of deep inelastic scattering where a muon has been produced. The muon passes through all the inner detectors and calorimeters and is revealed in the out-lying muon chamber (inthe bottom left of the image).
In the above images we can clearly see a jet of particles with significant vertical momentum downwards, but there appears to be nothing vertically upwards to counter-balance this. As the original proton and electron beams had no vertical momentum it would appear that momentum has not been conserved. So what has happened?
This is an example of what is known as a charged current event. In this, the electron interacts with the quark via a charged W boson instead of a photon, which is neutral. In order to conserve charge the electron changes into a neutral particle - an electron neutrino. This particle is not observed in the detector since neutrinos tend not to interact with other particles. This neutrino has conserved the momentum by moving off in a vertically upwards manner. A hadron jet is still formed and this can be clearly seen. This event is also shown on the Lego Plot below.
The difference between charged current and neutral current (which is the type of event we have been talking about before) is illustrated clearly in the following diagrams.
illustration shows the interaction in a neutral current
event. We can follow the electron through and see
it interact with the highest quark via a gamma ray (the
diagram also gives the possibility of a Z boson). This changes the
electron's course but it remains an electron. We
can also see the quarks creating hadron jets.
This illustration shows the interaction in a charged current event. Here the electron interacts with a quark but this time via a W boson (with a negative charge). At this point we observe the electron changing into an electron neutrino. Again we can see the quarks forming hadron jets.
Once again we find a situation where in order to conserve momentum you would expect a particle (or more than one) to be travelling in a downwards direction. This time the electron has remained an electron and it is the hadron jet which is missing. What has gone wrong?
Nothing, this 'missing momentum' is not shown as this is an example of W boson production. This exchange particle then decays into neutrinos which are not detected (they would be detected in the outer section of the detector, not shown). This has similarities to beta-decay, which the W boson mediates, and the 'missing' energy in this decay appears as a neutrino.