While I was busy reporting the talks at the “Neutrino Telescope” conference in Venice, LHCb released a startling new result, which I have not much time to describe in much detail this evening (it’s Friday evening here in Italy and I’m going to call the week off), and yet wish to share with you as soon as possible.
The spectroscopy of low- and intermediate-mass hadrons (whatever this means) is a complex topic which either enthuses particle physicists or bores them to death. There are two reasons for this dycothomic behaviour.
On one hand, the fact that new hadron resonances may appear out of the blue if you apply the right selection on collider data, and their true nature can then hit you between the eyes (the signal is then called to “pass the interocular stress test”): no issue of systematics or sculpting of background can then be raised, the signal is genuine and an evaluation of the p-value of the data under the null hypothesis is ridiculous. It can be exhilarating to find such a signal in your data!
On the other hand, the advancement in our knowledge of particle phenomenology we get from discovering new resonances is usually scarce or null. The problem is that we have a very good theory, QCD, but we cannot calculate anything with it at low energy, so we cannot compare it with what we observe and check whether we have found new effects – new physics – or just regular new hadrons belonging to schemes already understood fifty years ago.
Now look at the graph below, courtesy LHCb. Is it not the prettiest you’ve seen in a long time ? In it you can see clear as they can be five distinct sets of events whose nature is clear: they are resonances of well-defined mass and natural width. The analysis details allow you to understand that these all are “Omega_c baryons” – particles containing two strange quarks and one charm quark. Their different masses are explained by different orbital motions of the quarks in the three-body systems, or still more intricate detailed phenomenology.
Now, when I showed this picture from my cellphone to Carlo Rubbia, while we were having lunch at the Neutrino Telescopes conference in Venice today, he shrugged his shoulders: “non serve a niente” – it’s useless, he said.
What the…?!!! I was a bit disturbed by his statement at first. Useless ? It’s an advancement of our knowledge of hadron spectroscopy. And yet, Rubbia has a point. Little or no additional knowledge besides the very quantities that have been measured – five masses and five natural widths – can be extracted from this observation. Or maybe more, and I would be very happy if some QCD phenomenologist stepped in here and explained it to us in the comments thread…
So you get Rubbia’s point: as pretty as these new resonances are, they do not add much to our knowledge. And Rubbia wants to know the neutrino mass hierarchy, and he wants to know whether neutrinos are Dirac or Majorana particles, and he wants to know if there are sterile neutrinos. These are the questions he is working on – as he is still active with Liquid Argon detectors for these studies, with the ICARUS T600 tank being carried at Fermilab soon.
Indeed, one might argue that while the LHC will not discover anything – not at the high-energy frontier with ATLAS and CMS, nor in low-energy spectroscopy with LHCb, nor with heavy ion messy collisions – the real meal to be had is in neutrino physics. Neutrino masses are indeed the very first signal of new physics beyond the standard model, if you spin it in one particular way.
I used to say that the observation that neutrinos have a nonzero mass caused just a rearrangement in our understanding, and not a revolution; much like moving around the furniture in your apartment cannot be called having changed home. But in truth, there seems to be more to be learned in neutrino experiments these days… At least, this is my bit of wisdom after one week of brainwashing at the neutel 2017 conference. If you visit the blog at http://neutel11.wordpress.com, you will find there over 40 new articles I wrote to summarize the talks given there this week, plus 12 posters presented at the conference. If you are interested, do not fail to give a look!