Last month the LHCb experiment, at CERN’s Large Hadron Collider (LHC), reported the discovery of a new particle. While this received a reasonable amount of attention, it didn’t really cause as much excitement as, say, last year’s unconfirmed hints of a new particle from the ATLAS and CMS experiments (also at the LHC), which turned out in the end to be just a statistical glitch.
Those hints evaporated when more data came in, as such glitches do. The new particle at LHCb has passed a much higher statistical threshold, and seems to be here to stay. The animation here shows how the signal developed in the data over time as the LHCb experiment recorded and analysed more data. The peak indicating the presence of the new particle, call a Ξcc++ (pronounced Ksi c c plus plus) is pretty convincing by eye, an impression confirmed by solid statistical analysis. So why isn’t there (even) more excitement about this particle? There are good reasons, and they are worth looking into.
The main reason is that the new particle is not what a particle physicist would call ‘fundamental’. In this context that means that it is made up of smaller particles bound together. Those smaller particles are quarks in this case, meaning that the Ξcc++ is a hadron – the generic name for particles made of quarks. The most common hadrons are the proton and the neutron, which are the constituent parts of atomic nuclei. There are known to be many more hadrons too – all of them made of quarks and/or antiquarks.
The current “Standard Model” of particle physics allows for no new fundamental particles, but it does allow for new hadrons – as long as they are made up of the quarks we already know. This means that a new fundamental particle would change our view of physics in a way that a new hadron does not.
Furthermore, there are some outstanding problems with the Standard Model which give good reasons to hope that additional fundamental particles exist; the discovery of such a particle could be a breakthrough in addressing these questions. The usual ‘top three’ would probably be what is Dark Matter, why is there so much more matter than antimatter around, and how does gravity fit in? Generally, physicists don’t expect additional hadrons to help much with any of that.
For example, in the Standard Model the strong force, which is responsible for binding quarks together to form hadrons, treats matter and antimatter equally. But this equal treatment is imposed rather arbitrarily. It would be very easy to allow some asymmetry here which could help explain the observed preponderance of matter over antimatter in the universe; from some points of view, this would in fact be more natural. It just hasn’t been observed so far.
We don’t expect to observe the strong force playing favourites in this new hadron, but expectations can be confounded. And this is a new form of hadron – the first seen containing two charm quarks. Maybe some subtle differences in behaviour could be revealed by studying it.
While we’re speculating, it is worth pointing out that some physicists have proposed that Dark Matter may well be some exotic form of hadronic material.
Neither of these possibilities is really seen as a good bet by most theorists, but it is the job of experiment to make measurements and observations of new phenomena. This hadron is certainly new.
The strong force is responsible for most of the mass of atoms and molecules, it dominates the interactions between particles at the high energies seen in stars, supernovae and cosmic rays. I have a feeling that understanding the strong force, and the behaviours that emerge from it, is a somewhat underrated endeavour in physics.
The trouble is that is a very difficult problem. We have a lot of data on hadrons already, and it is not clear how discovering more of them will help much. Making predictions using the strong force is very challenging, even when only a few quarks are involved. For more measurements to lead to a leap in understanding, we will probably need a parallel leap in theoretical inventiveness or precision.
In the meantime, perhaps we set our expectations too high. LHCb is collecting data on a region of nature to which we have never before had access. Discovering a new planet orbiting a distant star doesn’t necessarily revolutionise our understanding of astrophysics – but is it still exciting and important. Maybe that’s the best way to look at the Ξcc++ . Guy Wilkinson of Oxford University, who led LHCb while these data were being taken, said of the new discovery that, in contrast to other hadrons containing three quarks in which the three quarks perform an elaborate dance around each other,
… a doubly heavy baryon [such as Ξcc++] is expected to act like a planetary system, where the two heavy quarks play the role of heavy stars orbiting one around the other, with the lighter quark orbiting around this binary system
It is right that we take delight in finding such an object.