This has been an exceptional few days, and I’ve had no time to breathe, much less blog. In pre-covid days, visits to the laboratories at CERN or Fermilab were always jam-packed with meetings, both planned and spontaneous, and with professional talks by experts visiting the labs. But many things changed during the pandemic. The vitality of labs like Fermilab and CERN depends on their many visitors, and so it is going to take time for them to recover from the isolation and work-from-home culture that covid-19 imposed on them.
My visit, organized by the LHC Physics Center [LPC], the US organizing center for the CMS experiment at the Large Hadron Collider [LHC], is my first trip to Fermilab since before 2020. I feared finding a skeleton crew, with many people working from home, and far fewer people traveling to Fermilab from other institutions. There is some truth in it; the place is a quieter than it was pre-2020. But nevertheless, the quality of the Fermilab staff and the visitors passing through has not declined. It is fair to say that in every meeting I’ve had and every presentation I have attended — and yesterday I started at 7:30 and ended at 4 without a single break — I have learned something new and important.
Today I’ll just give you a flavor of what I’ve learned; each one of these topics deserves a blog post all its own.
- One Fermilab postdoc explained a new and very powerful technique for looking for long-lived particles at CMS, using parts of the CMS detector in a novel, creative way. Because it’s possible that the Higgs boson (or top quark, Z boson, W boson, bottom quark, or some unknown particle) can sometimes decay to a long-lived particle, which travels a macroscopic distance before decaying to a spray of other particles, this is an important scientific target. It’s one the existing LHC experiments weren’t really designed to study, but with a wide range of creative developments, they’ve developed an impressive range of techniques for doing so.
- Another has a strategy for looking for certain decays of the Higgs boson that would be extremely difficult to find using standard techniques. Specifically, decays in which only hadrons are produced are very difficult to observe; hadrons are so abundant in collisions at the LHC that this is a signal drowned in background. But there is a possible way around this if the Higgs boson is kicked hard enough sideways in the collision.
- A third is digging very deep into the challenging subject of low-energy muons and electrons. Particles with energy below 5 GeV become increasingly difficult to observe, for a whole host of reasons. But again, there can be decays of the Higgs boson (or other known particles) which would predominantly show themselves in these low-energy, difficult-to-identify muons or electrons. So this is a frontier where new ground needs to be broken.
- A visiting expert taught me more about the technical meaning of “intrinsic charm”, which was widely over-reported as meaning that “there are charm quarks in the proton”. Understanding precisely what this means is quite subtle, even for a theoretical physics expert, and I’m still not in a position where I can explain it to you properly — though I did discuss it a closely related issue carefully. Moreover, he questions whether the story is actually correct — it depends on a claim of statistical errors being small enough, but he has doubts, and some evidence to support his doubts. (The same doubts, incidentally, potentially affect whether the difference of the W boson from the Standard Model prediction is really as significant as has been claimed.) In my opinion, it is not yet certain that there really is “intrinsic” charm in the proton. You can definitely expect another blog post about this!
- Another visiting expert pointed out that in some limited but interesting cases, there could be very slowly-moving particles captured not only in the core of the Earth but also floating near its surface, a possible target for underground experiments that are sensitive to extremely low energy collisions of unknown particles with atoms.
- Then there are the applications of machine learning in particle physics, which are increasingly being used in the complex environment of the LHC to make certain basic techniques of particle physics much more efficient. I heard about several very different examples, at least one of which (involving the identification of jets from bottom quarks) has already proven particularly successful.
- A visiting CMS experimentalist pointed out to me that in a search through LHC data that she’d been involved in for many years, there are two surprising collisions observed with an extraordinary amount of energy, and very unusual (but similar) characteristics. It’s hard to quantify how unusual they are, but hopefully we will soon hear about a similar search at ATLAS, which could add or subtract weight from this observation. In any case, upcoming data from Run 3 will give us enough information, within a year or two, to see if this hint is actually of something real.
- If these events aren’t a fluke and represent something real and new, then one of the local theorists at Fermilab is the fellow to talk to; back in 2018, when only one of these events had been observed, he and a couple of others thought through what the options are to explain where it might have come from. The options are unusual and would certainly be surprising to most theorists, but he convinced me that they’re not inconsistent with theoretical reasoning or with other data, so we should keep an open mind.
- Yet another visiting theorist taught me about the possibility of non-linearities in quantum physics. Steven Weinberg tried to consider this possibility some time ago, but it turned out his approach violated causality; but now, inspired by old ideas of Joe Polchinski, there’s a new proposal to try this in another way. I’m grateful for that 45 minute conversation, at the end of which I felt pretty confident that I understood how the idea works. Now I can go off and think about it. When I understand its implications in some very simple settings (the only way I ever deeply understand anything), I’ll explain it to you.
- Oh, and on top of this, I gave a talk on Tuesday, about powerful and sweeping strategies for searches in LHC data that haven’t yet been done, but ought to be, in my opinion. My ideas about this are 10-15 years old, but I have stronger arguments now that rely on Open Data. That of course led to a variety of follow-up conversations.
The visit’s not over; I’ve got one more day to try to drink from this fire-hose.
13 Responses
Hey Matt, I was looking back and you seemed to have been interested in making a post discussing weak charges (https://profmattstrassler.com/articles-and-posts/particle-physics-basics/the-known-forces-of-nature/the-strength-of-the-known-forces/)
I have been struggling with understanding weak isospin, weak hypercharge, weak charge, and how all those quantities relate to the oberved behaviour of particle interactions. I guess what I’m asking is how can we use these quantities to predict how particles will interact with the weak force, like how knowing the electric charges of particles allows me to predict their interactions with the electromagnetic force.
For instance, how would I predict that neutrinos would be repelled by electrons via elastic scattering mediated by the Z boson?
I’d also appreciate if you could go more in depth by what you were thinking particularly when you mentioned the weak force can push and pull, and perhaps how charges relate to those pushes and pulls.
Thanks in advance
The weak interaction is indeed more complicated than electromagnetism, not because its foundation is complicated but because the Higgs field makes the Z boson a mixture of the hypercharge boson and one of the isosopin boson. The Z’s interactions can be found on page 7 of https://www2.ph.ed.ac.uk/~playfer/PPlect16.pdf ; the interactions with anti-particles come with a minus sign. If two particles interact via the Z, you multiply the two couplings together to get the overall force strength; identical particles will repel, anti-particles attract, and for other pairs you have to look at the product of the couplings to see what they do. Remember, though, that the force falls off like Exp[- m_Z r (c / hbar)] , and so you have to slam these particles together at energy of order mZ c^2 in order to see a substantial effect, in which case a standard non-relativistic force isn’t what you observe. You could potentially observe a familiar nonrelativistic force if there were as-yet-unknown heavy particles with mass of order 1 TeV/c^2 that have interactions with the Z boson.
The link “A new proposal” points to the intrinsic charm post rather than non-linear QM.
Hertz discovered the photoelectric effect from his EM experiments, with this quote from Michael Fowler’s page at virginia.edu:
“I occasionally enclosed the spark B in a dark case so as to more easily make the observations; and in so doing I observed that the maximum spark-length became decidedly smaller in the case than it was before. On removing in succession the various parts of the case, it was seen that the only portion of it which exercised this prejudicial effect was that which screened the spark B from the spark A. The partition on that side exhibited this effect, not only when it was in the immediate neighborhood of the spark B, but also when it was interposed at greater distances from B between A and B. A phenomenon so remarkable called for closer investigation.”
And here we are 135 years later with the 27 km LHC in the hands of sociable, groups of collaborators from around the world that would probably make Hertz’s head spin in disbelief. I’m confident that like Hertz, they’ll come across something unexpected that will profoundly affect theoretical physics within the next decade or two.
Matt, are you aware of any past or future proposals to run the LHC over winter to compared data taken then to summer six months later?
Comparisons of LHC data over various six month periods, or 12 hour periods, are not hard to do. I think there is data as early as March and as late as November, from certain years, so unless you have a good reason to think the effect you’re looking for is so tiny that you can only see it by comparing mid-January to mid-July, you should still be able to check that there’s no measurable variation, even without a winter run. More challenging is that run conditions change from day to day and week to week, so you can only safely compare things that are clear and easy to measure, such as the mass of the Z boson or J/Psi. The ratio of those masses could easily be monitored over any time period you want. Whether anyone has done it as carefully as possible I don’t know, but that’s something even a theorist could do.
I suspect this lies more in the area of data science rather than theoretical physics in looking for any changes in the LHC data correlated with: (a) changes in the position of the sun; (b) tilt of the LHC wrt the ecliptic; (c) position of the Earth in its orbit around the sun etc. Eight years ago, professional theoretical and experimental physicists were beaten by data scientists/amateurs in the Higgs Boson Machine Learning Challenge: https://www.kaggle.com/competitions/higgs-boson/leaderboard. I’m neither a physicist or data scientist and so I’d expect this obvious type of analysis was routinely done very early on in the running of the Tevatron and LHC.
It’s actually not so easy if there is an effect that only affects some relatively rare processes. But the real problem is to think of a way, from a theoretical (i.e. math) point of view, that you could have an observable effect on the LHC without having some other kind of effect that would have been seen already. This is not easy; long-range forces that are large enough to affect microscopic particle physics tend to have lots of other effects on astronomy and cosmology that have already been excluded.
Thank you for sharing this. I grew up near Fermilab in Downers Grove, but I never really visited the facility until about a decade ago, when I sat in on a talk by Brian Greene. I have since retired from teaching, but I find your discussions illuminating and stimulating. My research and industry work has always been at much lower energies, under 1 GeV.
Regarding possible non-linearities in quantum mechanics, the Weinberg paper to which Matt referred is most likely:
S. Weinberg, Precision Tests of Quantum Mechanics, Physical Review Letters 62, 5 (1989).
http://personalpages.to.infn.it/~mussod/PhysRevLett.62.485.pdf
Matt also mentioned the brilliant theorist Joseph Polchinski, who sadly died at only 63 years of age, as inspiring a new testing approach. Polchinski is best known for his work on superstring theory and D-branes. Alas, from that hyper-dimensional, hyper-energetics starting point I have no idea what aspect of Polchinski’s work might might have been used to build on Weinberg’s far more experimentally focused proposals. Superstrings and D-branes are, um, not noted for inspiring specific, testable experimental ideas in quantum mechanics. 🙂
Polchinksi did not merely work on superstring theory and D-branes, and should be famous for some other things. On whatever field he touched — quantum mechanics, quantum field theory, condensed matter physics, and cosmic strings — he left a mark. My work with him gave new insight into the notion of the Pomeron, a basic concept in strongly interacting gauge theories. None of us who knew him are surprised that there were hidden gems in his quantum mechanics work.
Pomerons and Regge trajectories! Now *that’s* some [not-super] string theory with solid and experimentally meaningful meat on it! Thanks!
Matt, you made a deeply intriguing comment on “… the possibility of non-linearities in quantum physics … Steven Weinberg[‘s] approach violated causality; but now, inspired by old ideas of Joe Polchinski, there’s a *new proposal* ” The link goes to your discussion of protons and charm, but for me and likely others, it’s a bit tricky figuring out which part of that discussion is the quantum non-linearity part. Could you perhaps specify the connection briefly? Or is does the link go to the wrong location?
I’m delighted to hear you are feeling better! Your comments and insights have been missed.
I’m neither student nor scientist, but I still very much appreciate the effort you put into these posts. Your enthusiasm and clarity are contagious and fun! Thank you