The conference is a periodic event where particle physics and cosmology are discussed with an attention to interdisciplinarity. It takes place in the city of Blois, in central France, a nice town on the river Loire. There, a sizable number of interesting talks have been taking place in the last few days. But one in particular has stirred the interest of particle physicists worldwide.
The talk in question was given yesterday by Sheldon Stone, a physicist from Syracuse University who works in the LHCb experiment. LHCb is one of the four experiment that analyze the collisions produced by the Large Hadron Collider, CERN's world record particle accelerator. Unlike ATLAS and CMS, that are poised to study the highest-energy collisions between protons, and instrument the respective collision regions with symmetric and concentric layers of detection elements, LHCb looks only in one direction, concentrating its sight on collisions whereby a very energetic quark or gluon in one of the colliding protons hits a less energetic counterpart in the other proton.
The collisions studied by LHCb are ones where only a small amount of kinetic energy is converted into new particles. This is useful because this way the experiment may study the production of B hadrons, particles containing a bottom quark in their interior. The physics of B hadrons is so interesting that there are even other dedicated experiments around the world -the most notable is Belle 2 in Japan, which however looks at B hadrons produced by electron-positron collisions. LHCb rather benefits from the higher production rate of the studied particles, although in a considerably messier setting.
Stone's talk focused on the search for exotic hadrons. Hadrons, as maybe you already know, are particles composed of quarks: the ones fully described by the "static quark model" first enunciated in the early sixties by Murray Gell-Mann and George Zweig. The two physicists hypothesized the existence of quarks in order to categorize the scores of hadrons until then discovered in particle collisions into a set of multiplets of particles endowed with similar properties.
Irreducible Representations ? Watch Your Language!
In the original quark model the "multiplets" contain irreducible representations of the symmetry group SU(3). This is a group describing the symmetry of composite quark systems under the exchange of one kind of quark with another. Since hadrons are bound by the strong force, which is insensitive to the flavor of quarks, one finds that there exist sets of hadrons which behave similarly under the action of the strong force. For instance, you can liken the negative pion to a negative kaon - the first is composed of a down quark and an anti-up quark; the latter is produced if you substitute the down quark with a strange quark.
Gell-Mann and Zweig's quark model could explain all observed hadrons in the sixties. Because of that, physicists investigated with momentum the possibility that there existed hadrons not fitting in the tidy "multiplet" scheme of the symmetry group SU(3). Finding a hadron that would not fit in that description would mean disproving the whole model, so it was a very attractive field of research.
Of course one should also bear in mind that in the late sixties quarks were not believed, by the majority of particle physicists, to be real entities, but rather just a mathematical concoction that was successful at explaining what had already been found until then. Quarks would be universally accepted as real particles only in 1974, with the discovery of the J/ψ meson.
The way to look for new particles in the sixties was by using detectors called "bubble chambers". These were vessels filled with a supersaturated liquid which, invested by particles created in the collision of a hadron beam with a fixed target - or with the liquid of the tank itself - released tiny bubbles when the ionization was deposited by charged particles crossing the liquid. One could then reconstruct the trajectory of these charged particles, which would curve under the action of a magnetic field. One could thus reconstruct the momentum of the particles, and combine pairs or triplets of them to form potential decay systems of hypothetical new particles created in the collision.
The large number of collisions studied, and the unprincipled way the research was carried out, with physicists plotting all possible particle combinations in search for bumps in the resulting mass spectra, caused a problem. In fact, there were dozens of claims for discovery of "structures" in mass spectra, potential new particles which attracted the attention of theorists and which took more time to be disproven as statistical fluctuations.
The issue was that of the "look-elsewhere effect", the multiplication of probability to discover something that arises when you look for something in many possible ways. At one point Arthur Rosenfeld, a Berkeley physicist, demonstrated that the large number of studied reactions and mass spectra was producing a number of claims per year that was in line with the number of spurious fluctuations one should have expected given the multiplicity of observed spectra. He thus proposed that one should reach a statistical significance of "five sigma" - five standard deviations - before one could claim to have observed a genuine new particle. It took more than 20 years after Rosenfeld's proposal for his threshold to be adopted in particle physics searches.
But let's return to today's exotic hadrons - in the sixties and seventies nobody was able to show their existence, but maybe today we are precisely doing that. Exotic hadrons are ones that do not fit in the quark model, which had to be enlarged significantly after the discovery of the three heaviest quarks. We should be facing a SU(6) group rather than a SU(3) one, but the top quark does not actually contribute (it is too heavy to form stable systems). Still, the complications of the strong interaction at low energy (that at which reactions involving light quarks occur) make this a field still entirely to be explored.
The quark model allows for quark-antiquark states called "mesons" and three-quark states (with their antiparticles, i.e. three-antiquark states) called "baryons". Whatever is held together by the strong force and is not composed by the above combinations of quarks is exotic. So it is tetraquarks, pentaquarks, glueballs. Or other even funnier states.
Pentaquarks and Would-Be Tetraquarks
Stone's presentation concentrated on pentaquarks and tetraquarks. LHCb has collected a large statistics of proton-proton collisions now, and results based on 3-inverse-femtobarns of integrated luminosity were presented. He started with pentaquark candidates. These were announced some time ago by the collaboration, as structures in the J/ψ plus proton mass spectrum, when the two particles are taken from the decay products of the Λ_b baryon, a state composed of a up, a down, and a bottom quark. The lambda_b decays to those two particles plus a kaon, and the hypothesis is that the decay actually proceeds as the creation of a pentaquark state (uudcc) and a kaon.
LHCb showed ample evidence for the production of two distinct states, one of mass of 4380 MeV, with a width of 200 MeV, and another with a narrower width and a higher mass of 4450 MeV and a 40 MeV width. One of the striking plots shown in the presentation is in the slide shown below.
In the four graphs you can see the mass of J/ψ plus proton (called "Pc") for subsets of the lambda_b decay data taken at different intervals of mass of kaon and proton. The signal is expected to not be visible in the upper left plot - which it isn't - and then gradually appear in higher ranges of kaon-proton mass. The two resonances are now well established, as a very detailed angular analysis proves.
The second part of Stone's presentation discussed a topic on which I had the occasion to write here in the past. The hadrons sought are ones that may be produced in the decay of B hadrons. When the B (say, a up-antibottom combination) decays to a J/ψ φ K final state, one might try to plot the mass of J/ψ φ combinations, by assuming that the decay really proceeded through the creation of a state X, i.e. B->XK, with then the decay of the X to the said meson pair.
The search was first done by CDF a while ago, and it returned the evidence for at least two resonant states, called tentatively Y(4140) and Y'. After the CDF observation, which took a considerable time to be published due to some reluctance of the experiment to produce the claim of a new resonance, there came a confirmation by the CMS experiment and then some conflicting results, notably with a LHCb analysis which initially appeared to deny the presence of those states.
Now LHCb redeemed itself by analyzing a much larger dataset, where the presence of not just two, but four distinct resonant states appear to be borne well by the evidence in the data. The money plot is shown below, where on top of the potato-shaped "phase space" distribution you can count the four peaks well separated from one another.
The jury is still out on the real nature of the four states - and it will remain there for a while, I reckon. Indeed, Stone remarked in the conclusions that "lattice QCD" calculations of the predicted masses that states such as the ones observed should have are direly needed. In any case, I think these results are remarkable and that LHCb deserves due congratulations!
In Summary
In summary, LHCb claims the observation of two pentaquark states Pc produced in the decay of the Lambda_b baryon, plus four new states X in the decay of B hadrons. These finds are really very interesting and they further clarify that the spectroscopy of hadronic states is an exciting new field of investigation, one on which we should focus with more attention in the future. As exotic physics appears to belong there, rather than in the tails of mass distributions of very high-energy collisions that are the primary objective of ATLAS and CMS studies.
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