The top quark is the heaviest known matter corpuscle we consider elementary. 
Elementary is an overloaded word in English, so I need to explain what it means in the context of subatomic particles. If we grab a dictionary we get several possibilities, like e.g.- elementary: pertanining to or dealing with elements, rudiments, or first principles

- elementary: of the nature of an ultimate constituent; uncompounded
- elementary: not decomposable into elements or other primary constituents
- elementary: simple

In the case of the top quark, we could well say it's simple, but we would be lying to ourselves - how can a particle we consider point-like weigh as much as a full tungsten atom ? We have not understood that yet, and we are indeed far from giving even a tentative answer to this riddle. In fact, the six quarks we have discovered, which make up nuclear matter, have so varied masses (the heaviest one, top, being approximately 60,000 times heavier than the lightest one, up) that one wonders what mass really is. 

Is mass of matter corpuscles caused by inertia in the Higgs field or by binding energy of constituents?  In the first case we have a down-to-earth "guessplanation" of the phenomenon: as quarks move about in the Higgs field, the heavier of them are slowed down more by a more intense interaction with the Higgs field. Of course note that this explanation, which we know is correct but might not be the full story, is only moving the problem to another department rather than solving it, for the different interactions of the Higgs boson with matter particles are still unknown and free parameters in our model, just as masses are. 

In the second case, as Jorge Cham and Daniel Whiteson would put it (see the cover of their aptly named book, right), we have no idea. If quarks are composite objects, then we know what can cause their mass to be large of small - pretty much the same thing happens with the proton itself, which has a mass much larger than the sum of its constituents, as most of it is due to its internal dynamics. Yet we have no idea of what quarks may be composed of, so we are clueless anyway.

You now realize how studying the top quark is not an idle occupation for particle physicists. After its first spotting by the CDF experiment in 1994, the top quark has inspired hundreds of dedicated studies, measurements of its properties, its production rates in different settings, and its precise mass determination of course. The Tevatron experiments did this for over a decade, and then passed the baton to the ATLAS and CMS experiments, where the larger center-of-mass energy of LHC proton-proton collisions guarantees very high production rates, and consequently larger datasets with smaller backgrounds.

Until recently, we knew how often a top quark or a pair of top quarks is produced when you smash together a proton and an antiproton (at the Tevatron), as well as when the projectiles are two protons (at the LHC). But nobody had seen top quarks emerge from a proton-nucleus collision yet. When a proton hits a nucleus, the "hard" interaction is still between a quark or gluon in the proton and a similar counterpart in either a proton or a neutron, but the much messier environment makes multiple parton collisions much more frequent, and the resulting hot environment makes the physics considerably more complex and still largely uncharted. Would top quarks be produced with the rates one can compute with perturbative Quantum Chromodynamical calculations in that setting?

This question has an answer now, as CMS has finally produced the first observation of top quarks emitted in proton-lead collisions. The breakthrough happened last year, when a significant integrated luminosity of proton-lead collision (174 inverse nanobarns) was made available by the LHC, at a center-of-mass energy per nucleon pair (8.16 TeV) sufficiently large to make the heavy top quark a possible product. Those data were analyzed during the past few months, and a week ago the result came out.

The top quark events were sought in a way not too different from the way they are spotted in proton-proton collisions: the signature is that of "single lepton" top pair decays, where one top yields three jets (one of which comes from the hadronization of a b-quark), while the other yields a b-quark jet and an electron-neutrino or muon-neutrino pair. B-tagging is still the powerful weapon that can flag the top pair production, but in proton-lead collisions the background from W+jets events or "multijet QCD production" events is larger and harder to size up with standard means.

The CMS analysis categorizes events based on the number of b-tags that are observed in the event, and sure enough, two-jet combinations of untagged jets show the W-->jj signal quite clearly when two b-jets are present in the event, as shown in the picture above. The number of b-jets in the data (black points with vertical uncertainty bars) is 0, 1, or >=2 from left to right. The "correctly paired" jets from W decay are estimated by the red component. The yellow template shows real top contributions where the two jets have been paired incorrectly, while backgrounds are in blue.

By pairing these untagged jets to a b-jet a three-jet combination is formed, which also evidences the top signal as shown below (where data and predicted components are shown by the same colour convention).

In the end, a combined fit to electron and muon events allows to derive a cross section of 45+-8 nb for the process cross section. This is well compatible with theoretical calculations. The significance of the obtained signal is well above the 5-sigma mark, and authors correctly avoid quoting a number which has very little relevance at that point. For one should not forget that the so-called "Z value" (a number of sigma units off the background-only prediction) is just a proxy for the p-value of the background-only hypothesis. This makes Z-values above 5 or 6 quite meaningless to quote, as this implies insisting that one can estimate the probability of the observation under the background-only hypothesis to precisions that are completely unreliable, as even tiny non-Gaussian tails in the systematic uncertainties would then play a dominant role.

In conclusion, I am very happy for this new CMS result and I congratulate with the joint team of nuclear and subnuclear physicists who pulled it off. Now we have yet another probe to test the complex physics of hot quark gluon plasma in nuclear collisions of high energy.


Tommaso Dorigo is an experimental particle physicist, who works for the INFN at the University of Padova, and collaborates with the CMS experiment at the CERN LHC. He coordinates the European network AMVA4NewPhysics as well as research in accelerator-based physics for INFN-Padova, and is an editor of the journal Reviews in Physics. In 2016 Dorigo published the book “Anomaly! Collider physics and the quest for new phenomena at Fermilab”. You can purchase a copy of the book by clicking on the book cover in the column on the right.