12 October 2017
For most of each operational year, the Large Hadron Collider (LHC) typically smashes protons together to generate vast amounts of data for physicists to analyse. These proton-proton (pp) collisions were responsible, for example, for the discovery of the Higgs boson by CMS in 2012. But the LHC also accelerates and collides other species of particles: the nuclei of heavy atoms. These so-called “heavy-ion” collisions help physicists probe the properties and related phenomena of a particle soup known as the “quark-gluon plasma” or QGP that existed during the very first fractions of a second after the big bang. The collisions of these heavy nuclei can produce the QGP-like medium in a controlled environment, allowing us to travel back in time nearly 14 billion years and observe conditions of the early Universe.
So far, lead nuclei have been the heavy ions involved in collisions at the LHC, being smashed either into one another (lead-lead or PbPb collisions) or into protons (proton-lead or pPb collisions). The pPb collisions as well as the conventional pp collisions can serve as reference for non-QGP effects; however, following the observation of QGP-like signatures in these “small systems”, questions about the conditions for the onset of QGP production have become more important. Therefore, the LHC saw a new species of heavy ions travel through its tunnels today en route to collisions inside CMS: xenon nuclei!
One of the first-ever xenon-xenon collision events recorded by CMS during the LHC’s one-day-only heavy-ion run with xenon nuclei. The large number of tracks emerging from the centre of the detector show the many simultaneous nucleon-nucleon interactions that take place when two xenon nuclei, each with 54 protons and 75 neutrons, collide inside CMS.
Xenon (Xe) is a noble gas with an atomic number of 54 and its nucleus is around 40% lighter than that of lead. Xenon ions were introduced into the CERN accelerator complex for the first time this year to deliver data to the North Area experimental facilities at CERN for a special data-collection run. Although not part of the original plan for the year, it was later decided to collide these ions at a centre-of-mass energy of 5.44 TeV in the LHC, albeit for one day only, to provide yet another set of data for studying the QGP.
The QGP-like medium created in xenon-xenon (XeXe) collisions is expected to be a bit cooler and shorter lived when compared to the one created in PbPb collisions. Any differences observed in the measurements performed on data from the collisions of these two species should provide valuable information on the production and nature of the QGP. CMS looks forward to recording and analysing these novel data provided by the LHC under the unique running conditions of the next few hours.
More images of the above collision event can be found below and in high resolution at https://cds.cern.ch/record/2289035.