It is always nice to learn that a new hadron is discovered - this broadens our understanding of the extremely complicated fabric of Quantum Chromodynamics (QCD), the theory of strong interactions that govern nuclear matter and are responsible for its stability.
Hadrons are particles formed by quarks and gluons. Thus, they are not "elementary" in the strict sense of the word: that adjective can only be used for quarks and gluons themselves, along with leptons and gauge bosons. Of course, this is only our present understanding of the fundamental building blocks of the universe: it is quite possible that one day we will discover that quarks, too, are composite objects, and so the other score of particles we nowadays call elementary. But for now, we stick to that naming.
A new particle in the news
Today's news, which also percolated to major newspapers like the New York Times, is that the LHCb experiment has found a new baryon called Xi (from the greek letter Ξ), which is composed of two charm quarks and an up quark. The discovered particle has a mass in excess of three proton masses, a bit higher than that of a close relative found in 2002 at Fermilab. This point raises a small controversy, as the LHCb result casts a bit of doubt on the exactness of the previous measurement; but I prefer to direct this article elsewhere today.
First of all, what was seen and how ? The Xi signal was observed by reconstructing its decay into what is called a "fully exclusive final state", i.e. one perfectly defined by its constituents. A Lambda_c baryon, a positively-charged Kaon, and two pions of opposite charge. LHCb first reconstructs the Lambda_c baryon itself, again in its exclusive decay to a proton, a kaon, and a pion.
Then a selection for signal-like events is performed by a multilayer perceptron. They do so by optimizing a figure of merit based on a recipe of Giovanni Punzi which I find a bit outdated, but anyway, it works for them, so I should not bitch about it.
The invariant mass distribution of the selected candidates in the LHCb search is shown below, where you clearly observe the peak resulting from the accumulation in the 3627 MeV whereabouts of all decay products of the Xi particle.
The data (black points with vertical uncertainty bars) are well fit by an almost linear background density and a Breit-Wigner shape for the signal component. The signal passes the "interocular stress test", so there is no sigma-counting needed here: you know you have a signal there if it hits you between the eyes.
Enter Karliner and Rosner
The exact mass of these composite objects has always been a bit of a puzzle for theorists, as in subnuclear physics one plus one plus one is not always three. If you could weigh separately two charm and a up quark by themselves (you cannot, as these particles do not exist as free states -they can only be found inside hadrons containing other quarks) and computed their sum, you would find a result which has little to do with the true mass of the Xi baryon.
The reason is that inertial mass of these composite objects results from the composition of different effects: the mass of the ingredients, and the energy of the interaction that keeps them bound within the system. As we cannot precisely compute the latter, nor measuring the former, we are a bit in the dark in our predictions. Or are we?
Enter Marek Karliner (right) and Jonathan Rosner (below), the first a professor at Tel Aviv University, the second a professor at the University of Chicago. Marek is a long-time friend and visitor of this blog, and I am honored of the fact: he in fact has contributed or helped produce several articles here in the past. He has a keen interest in the physics of these heavy hadronic states, and he understands their physics better than the rest of us. In a series of papers the two theorists produced in the recent past some precise estimates of the mass of these particles, and experimentalists have been confirming their predictions as the relevant data got available. This is especially true of today's new discovery, as the LHCb estimate of the Xi mass is of 3621.4 +- 0.8 MeV, well within one sigma of Karliner's and Rosner's prediction of 3627 MeV (see Phys.Rev. D 90, 94007 (2014))!
I argue it is this kind of research what makes particle physics the solid, foundational field of science it is - not the exciting promise of new exhilarating exotic states of matter which never concretizes, but rather the painstaking collection of confirmations: you think you understand something, put forth a prediction. Months, years, or decades later, finally somebody goes out and measures the system and comes home with a result that matches it. It is thanks to Karliner and Rosner if we may say we understand the world a little bit more today than we did yesterday. Thanks, Marek
UPDATE: Lubos Motl graciously quotes me in a post on this matter, where he disagrees with the excitement of the agreement between theory prediction and experimental estimate. In a nutshell he explains that predicting the masses of heavy hadrons is a kid's game:
"because of some mutual interactions between the quarks, the charm quarks must "feel" whether the third, light quark in the baryon is up or down. And this may change the masses by an amount comparable to the QCD scale, i.e. 0.15 GeV or so."The statement is a bit surprising. First of all, there are incredible intricacies in the phenomenology of low-energy QCD, and it is indeed an exciting field of science, where we have a theory and sort of understand how things work out, but have trouble calculating quantities with it - whenever we can, we should really rejoice.
But the issue is thicker. That the (ccd)-(ccu) isospin splitting could be as large as 150 MeV is a misconception. It is doubly as surprising to hear it, as Lubos starts off with the example of the proton and neutron, where indeed the splitting (uud)-(udd) is of a MeV or so.
Reality is different: the old measurement of the (ccd) bound state was wrong, and the real isospin splitting of the two states cannot be larger than a few MeV, as explained in another paper by Karliner and Rosner, https://arxiv.org/abs/1706.06961.