"But why," you might be curious to ask - "are not nuclei and quarks both governed by the strong interaction, QCD? What makes you sapient on the latter and ignorant of the former?" Well, nuclei are very complex, multi-body systems that we really cannot describe by working out the solutions of equations of motion from a Lagrangian density as we do with top quarks, e.g. (the top quark was the topic of my laurea thesis, so that is as good an example as any, in my case). Over the past century, nuclear physicists have been very ingenuous in concocting models that in some way could explain the dynamics of atomic nuclei. Their focus on collective behavior of many nucleons is very different from the focus of particle physicists on the basic two-to-two particle reactions. Sure, the fundamental physical laws of both systems are the same, but the mathematics, the methods of analysis, and even the jargon has parted us over time, especially since the 1970s.
A small diatribe during question time
That said, I can still enjoy when I see nuclear physicists debate on a nuclear physics topic if I happen to be in the room, and such was the case at the 13th edition of the ICNFP conference in Crete, which I am just flying off of as I write this note. At the conference Sara Pucillo (a PhD student from INFN-Torino who represented the ALICE collaboration) showed several interesting results, and after her presentation a lattice QCD physicist from Graz University, Leonid Grozman, hijacked the question time by making a statement (not a question) along the lines that ALICE has not demonstrated QGP formation with the studies shown, and that the temperature of the plasma that produce that very special state of matter is much higher (IIRC he mentioned an energy of 300 MeV) than what ALICE is probing.
His comment, however, was a bit aggressive in the way it was expressed - or at least, that is what I read between the lines. In fact, it forced the PhD student to explain that she was only reporting the results of ALICE and that she would propagate the commenter's statement to her collaboration. What else could she do after all - getting in an argument with a senior physicist on a topic he was clearly very opinionated about? That would not have been very wise. So, while the whole thing was not a big deal, and afterwards the ALICE speaker did not look as if she had been bothered by the episode, it did leave a bad aftertaste in my mouth, and I am going to explain why below.
Grozman is undoubtedly a knowledgeable, acknowledged scientist: this much I can see. He obviously disagrees with the ALICE results and their implication on the possibility that a phase of quark-gluon plasma is being formed in the highest-centrality collisions (those where two nuclei smash completely head-on, creating the densest environments). Whether he is right or wrong is not the point for us to discuss here; it is of course legitimate for a scientist to bring up such a question during a conference, and indeed the very purpose of conferences is to discuss open question and disagreements, with a view to understanding different viewpoints and finding consensual views when possible. However, when I asked him, after the talk, what he wished to accomplish with his aggressive comment, other than creating embarrassment in a PhD student, he said he did not care about that, and he was only concerned
with the science.
I explained to him that in my opinion, patronizing students should not be, in a Machiavellian sense, a means justified by the higher goal of doing Science, and that given his status he should pick his battles more wisely and find interlocutors at his seniority level for that business. He replied that the student was representing the collaboration and therefore he was fully justified. We discussed the matter for a while, but neither of us changed opinion.
What do you think? Am I perhaps overly sensitive to the way some academics believe that Science (with a capital S) allows them to use bad manners? Should we foster heated discussions between researchers of different status (and obviously, different stakes) in our conferences, or rather try to avoid them and protect our young researchers, avoiding the possibly resulting toxic climate? Mind you, I repeat it here - the above incident was really a very mild one, and I am rather using it as a pretext for writing about the general issue here.
Ok, back to the physics...
To finish this piece with some more science, I will go back to ALICE and their measurements, as I believe I have left several things to be explained
for the benefit of those of you who are still not introduced to nuclear physics and yet courageous enough to venture this far down the page.
Over the past decade or so, ALICE has produced a wealth of studies of nuclear collisions to probe the dynamics of the hot fireball created when the LHC smashes beams of heavy nuclei together. Sara's talk focused on a subset of those measurements - in particular analyses devoted to measure a possible enhancement of strange particles production at high energy densities, an effect that was long ago indicated as one of the signatures of the formation of a quark-gluon plasma. You can find her presentation here if you are curious to know more.
Anyway, strange particles are composite objects made up by one or more strange quarks in addition to ordinary quarks. In a static model of hadrons (hadrons are particles subjected to the strong force, the glue that holds together protons and neutrons, and in fact the nuclei they make up) the strange quark is not a constituent of neutrons and protons, the default ingredients of nuclei of atoms: these are made up of the lightest quark duo, up and down quarks.
In the 1950ies many hadrons were discovered at accelerators which exhibited the property of being produced copiously, while they decayed very slowly. If the same interaction were responsible of the production and decay of these particles (as one would indeed expect), one would not be able to explain how the interaction did its job effectively in creating these particles with high probability, but not in allowing them to disintegrate into lighter particles fast (i.e., with high probability). The strength of an interaction is indeed proportional to the probability of the phenomena it mediates: that is what did not add up with strange hadrons.
Hadrons of the specified kind were thus dubbed "strange". A whole set was found - the kappa mesons, the lambda baryons, the sigma, the xi, and the omega baryons: these are all relatively slowly decaying; then there are other strange hadrons that decay very quickly into the ones I mentioned - without any change in the number of strange quarks, though. Only later it was hypothesized, and then proven, that the interaction responsible for production and decay were different ones: the strong interaction can easily produce pairs of strange-antistrange quarks, which end up constituting strange hadrons in various combinations, but is unable to create single strange quarks; and then these strange hadrons linger around, not decaying very quickly because the strong interaction may not transmute the strange quarks into lighter ones. This is a trick that only the weak interaction can pull off, but with longer time scales. The enigma of strangeness was thus solved.
The presentation by Sara Pucillo was very nice and thick with interesting results, which I cannot pay justice to here. She did show how ALICE is observing enhancements of strange particles production in events of higher energy density. Maybe that can be interpreted as an evidence of the deconfined state called "quark-gluon plasma", a phase of matter where the quarks and gluons, which are normally confined within protons and neutrons (hadrons), become free to move independently in a hot, dense medium. Or so I understood.
The four panels above illustrate a measurement of the abundance of events where multiple copies of particles with strangeness of different kind are identified, thanks to ALICE's excellent particle ID capabilities: K, Lambda, Xi and Omegas. These are respectively systems containing a strange quark and a lighter antiquark, or systems of three quarks including one, two, or three strange quarks. The data are shown by the points with vertical error bars and wider boxes describing the effect of systematic uncertainties, while the lines show fits to the distribution of probability to observe N particles of the given kind in ALICE collisions. The different colours indicate collisions that have different total particle multiplicities. This observable - the total number of tracklets they can reconstruct in their forward detectors - correlates with the centrality of the collisions, and thus with how hot and messy was the created quark-gluon blurb.
While the above effects, and many others I have no time (nor the intent) of explaining here, can be explained by the properties of quark-gluon plasma, correlation is not causation, and this one I ventured to show today is but a small piece of the large puzzle that is being assembled. So, I do not know if ALICE is putting in evidence the characteristics of quark-gluon plasma yet. You can also comment on this bit in the thread below, if you do not want to miss a chance of patronizing me on the matter...
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