This article doesn’t paint the detection as particularly significant. Does anyone know of any way this constraints potential theories of subatomic forces?
“It’s a very interesting measurement, but it’s unclear what we learn from it,” says Juan Rojo at Vrije University Amsterdam in the Netherlands. “There is no rule in quantum chromodynamics which prevents this hadron from existing, but now we’ve measured it exists, we are left not particularly illuminated.”
There are a bunch of higher-mass relatives to the proton and neutron, many of which have already been detected and measured. Another one isn’t a particularly big deal.
In principle, the measurements of this particle’s mass and lifespan provide two more data points to constrain a hypothetical complete theory of the strong nuclear force (or more precisely, the color force of which the strong nuclear force is but one manifestation), but lack of data points has never been the problem in developing such a theory.
Thanks
I remember that in the 1980s, if not before, people were trying to compute the mass of a proton using non-perturbative QCD on a supercomputer. That sort of thing should extend to heavier baryons…
To be sure, we have a complete theory of the strong force (i.e., quantum chromodynamics or QCD) in the same way that we have a complete theory for the other forces within the context of the Standard Model of particle physics. There are numerous ways in which the Standard Model overall is incomplete, but for this post I’ll stick to the Standard Model and the as-far-as-has-been-demonstrated completely-working QCD subpart of it.
Despite being theoretically complete, QCD is notoriously resistant to precise calculations, as it does not permit ever-more-accurate approximations just by adding on terms in a mathematical series. This is more problematic at low energies (e.g., predicting hadron properties) than at high energies (e.g., smashing very high-energy hadrons together in a particle collider).
Calculations from first principles can be done still, but only for certain sorts of questions or physical systems and generally with a few empirical “pivot points” injected to remove as much of the problematic parts of the calculations as desired or tolerated. There is almost always a practical need for the calculation to be relative to some measured value. So, maybe you can’t calculate a or b practically, but you can calculate b-a and you can measure a. If a is “boring” (like the mass of a pion) but b is interesting/unknown, you can calculate b by actually calculating b-a and adding back in the measured value of a.
For the result in the OP: This baryon is the equivalent of taking the proton and changing both “up” quarks with “charm” quarks. Two decades ago, an experiment called SELEX reported evidence of maybe seeing this particle but with a mass very different from expectations derived from QCD and the sorts of shell-game tricks mentioned in the previous paragraph. This was a rather notable discrepancy (evidence of new physics?), yet subsequent QCD-based calculations for the mass of this particle continued to predict the old expected value, not the SELEX value.
Yesterday, LHCb reported discovery of this particle and found it to have the QCD-based predicted mass. This is interesting because (1) it refutes the older SELEX anomalous measurement (as did some other prior measurements, but not with this level of statistical power), thus restoring this corner of QCD to a “yep, it still works” status, (2) it marks the start of the science program of the newly upgraded LHCb experiment, and (3) it is a terrifically difficult measurement to make.
If not for (1), this probably wouldn’t have made it across your desk. There are loads of baryons (easily 100+) that are well-established experimentally and tabulated, and even more mesons. Each one carries some storyline, but they all fit nicely into the QCD picture.
You can explore known (and some speculated) particles and their properties at the Particle Data Group website. The “Baryons” subsection is the most relevant one for this thread.
I would argue that a theory that can’t, in practice, be used to perform calculations of quantities within the scope of the theory is incomplete. And there are lots of things involving the Strong Interaction that we can’t calculate (not even by reference to other, empirically-known quantities). Off the top of my head, for instance, a full understanding of the Strong Force ought to enable determination of the equation of state of neutron star matter, but we don’t know that.
We cannot in practice currently determine whether the game of chess is a theoretical draw, but I would say that we have a complete theory on how the game of chess works (i.e., we know all the rules). That is the sense in which I wanted to emphasize that QCD is complete: we think we know all the rules, and there is plenty that we can practically calculate.
Maybe a better example is the stability of the Kerr black hole, which was proven a few years back for the case of sufficiently slow rotation. This was a question that could not previously be handled by numerical or analytical means. Until it could. But prior to the development of the mathematical tools needed to prove stability in this limited case, I would still have said that general relativity was a complete theory (within its domain of applicability, of course).
You are certainly correct that there are many things that are not practical to calculate in QCD, and some (but not all) of these challenges are much more pernicious than combinatorics. Thus, your semantic position is certainly warranted (and in some contexts, common). But as novel techniques are developed and brought to bear on the problematic systems, the techniques all still sit on the basis that the theory of – the rules of – QCD are correct and complete. This is an important distinction of QCD from, say, a theory of dark matter, which is very much wide open in terms of the particle content, relevant symmetries, or other mathematical structure. We think we have all of that in place for QCD, even if we are still scratching our head over how to deal with it in many physical scenarios.