While Reviews in Physics covers the whole of Physics research, three of the four papers on the finish line focus on experimental high-energy physics. One of them appeared in the Arxiv just a few days ago in its final, reviewed form. It is a short review of the status and prospects of our investigation of the production of single top quarks in LHC proton-proton collisions.
Why do we care about single top quark production, when we can study top quark pair production (in quark-antiquark pairs) more easily ? Indeed, the single top quark production is about three times less frequent in LHC collisions, as the production mechanism involves the electroweak interaction, while top pairs can be readily produced by strong interaction processes.
The reason why the "weaker" single production is just three times less frequent and not a hundred times less so, as one would otherwise expect given the difference in the two forces, is that the top quark is heavy, so producing two at a time demands much higher-energy collisions, which are harder to get.
If you're puzzling over the above, reasoning that the LHC always provides 13 TeV collisions between the protons in the beams, I need to explain that the collisions that matter for new particle creation actually take place between two partons, one in each proton: these are the proton constituents, quarks or gluons. Each parton carries a variable fraction of its parent's momentum, and typically this fraction is small. Hence the small rate of collisions yielding enough energy to produce two 173-GeV-heavy top quarks.
Andrea did a very careful job in his review, explaining in detail what we have been able to measure with the many analyses performed by ATLAS and CMS, and what we have learned from those measurements.
Of course, measuring the properties of processes which are accurately predicted by the standard model may be considered unexciting - but one never knows how new physics will eventually show up: of course it would be nice if we found a new resonance at 4 TeV in the new data, but it might just happen that the first hint of new physics instead appears in a departure of a measured property of standard model processes from the theory predictions. That hint can then be used as a compass to make way toward a deeper understanding of Nature.
One such avenue toward new physics - a possible one, that is - is the observation of a flavour-changing neutral current decay of the top quark. Here one can use both top quark pairs and single top quark events, but the latter have recently proven to offer more sensitivity, as shown in the graph below.
The CMS measurements shown by vertical and horizontal lines in this "limit graph" are the most stringent, and are obtained by searches involving the production of a single top quark and final state topologies involving photons or Z bosons. Each line is an "upper limit" to the corresponding branching fraction, the probability of the stated decay. So far we have not found any evidence of these decays, which "violate" a rule of the standard model whereby all quark decays take place by charged-current weak interaction, i.e. the emission of a W boson and the change of one unit of the quark charge.
Flavour-changin neutral currents are ones mediated by photons or Z bosons, or more complex combinations. In the standard model they should happen, but only through rare loop processes that have ridiculously small probability. Hence observing a signal of these processes would be a giant step forward in our understanding of electroweak interactions.
I look forward to announcing here the first issue of Reviews in Physics, which I think should feature Giammanco's article.
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