A new preliminary result by the CMS collaboration [CMS-PAS-HIN-19-001, cds.cern.ch/record/2699428] demonstrates for the first time that top quarks can be produced in nucleus-nucleus collisions.
To provide the context for this breakthrough, let us start by recalling that the flagship accelerator of CERN is named Large Hadron Collider (LHC) and not Large Proton Collider, in spite of being most famous for how its record-energy proton-proton collisions brought the discovery of the Higgs boson and a vast program of searches for new physics. In a typical year of LHC operations, in fact, while six months are spent for proton-proton collisions at the highest achievable centre-of-mass energy, one is devoted to heavy ion beams. The two kinds of runs are for very different kinds of physics: proton-proton collisions are, for example, how we precisely pinned down the properties of top quark, while heavy ion collisions are used to investigate dense nuclear matter at extreme energy. In particular, lead-lead collisions at the LHC have already clarified several properties of the quark-gluon plasma (QGP), an extreme state of matter that filled the Universe in its first microseconds of life, but also opened new questions about it.
The top quark, the heaviest elementary particle known, is a unique and potentially very powerful probe of the inner content of nuclear matter. Crucially, it is the only quark whose decay lifetime of the order of a yoctosecond (10-24 s) is faster than the timescales expected for the expansion and cooling down of the QGP.
Heavy ion collisions produce a humunguous number of particles, putting all the algorithms of CMS under stress, with some of them needing a dedicated re-optimization in order to work at all.
The figure shows an actual example of how a top quark event appears, in the busy environment created by a heads-on lead-lead collision inside CMS. Although the number of background particles is overwhelming, some features of the top quark signal are recognizable: a very energetic electron leaving a large deposit in the electromagnetic calorimeter, a very energetic muon reaching the end of the dedicated muon detectors, and two very energetic hadronic jets containing signs (not visible in this picture) of originating from the hadronization of b quarks (hence passing a so called “b tagging” algorithm, which had to be retuned specifically for this lead-lead run in order not to be almost blinded by this large particle multiplicity). We interpret this event as originating from the chain gg→tt→WbWb→eνbμνb, with the two neutrinos gone undetected.
Two complementary analyses were performed: one based entirely on the features of the two leptons observed in the event, intentionally ignoring any information related to the hadronic jets in order to stay away from any potential bias induced by the QGP; and another analysis where the presence of b-tagged jets is exploited, as they are a tell-tale sign of the presence of top quarks in the events, with extra uncertainties added to account for potential biases. The two analyses yield results consistent with each other and with extrapolations from previous measurements of top-antitop cross section in proton-proton collisions at the same centre-of-mass energy per nucleon.