The CMS collaboration have released this image of a collision showing more than 100 charged particles from a single collision event. It could be the result of a quark-gluon plasma previously seen only in Heavy-Ion collisions. It also makes a good desktop wallpaper. Click to get the hi-res version.
The CMS analysis of these events looks at the correlations between particles flying off at different angles. The resulting plots look like this
In this plot Δη is a measure of how far apart the particles are in terms of the polar angle away from the beam axis. Δφ is the difference in the azimuthal angle in radians. In this plot the peak which has been cut-off at the front shows particles on a similar trajectory and is an expected observation. The ridge along the back is at a separation of 180 degrees in azimuthal angle. It extends over a range of the polar angle and is not predicted in the standard Monte-Carlo simulations based on QCD. The smaller ridge at zero azimuthal separation is not expected either. The effect intensifies for events with larger numbers of particles.
The interpretation offered by CMS is that some kind of clusters are formed, which then radiate particles isotropically. Such clusters could be droplets of quark-gluon plasma, but other explanations might be possible. Similar results have been observed at RHIC when Copper ions were collided but it is a surprise to see this with single protons.
The results presented are based on the only first inverse picobarn (1/pb) of data taken. So far 3.6/pb have been collected and much more will be generated in the next five weeks as the luminosity is pushed up by another factor of 10. Following that, the LHC will also turn to heavy-ion collisions using lead nuclei. The ALICE experiment is optimised for exploring the effects of these events, but CMS and ATLAS will also be used. Today’s report from CMS shows that interesting results could come from unexpected places.