Two of the most widely reported stories of the year in particle physics,
- the claim by the CDF experiment that the W boson mass is higher than predicted in the Standard Model (discussed on this blog here and here), and
- the claim by a group of theorists known as the “NNPDF collaboration” that there are (in a sense that I have briefly discussed here) unexpectedly many charm quark/anti-quark pairs “in” (or “intrinsic to”) the proton,
both depend crucially on our understanding of the fine details of the proton, as established to high precision by the NNPDF collaboration itself. This large group of first-rate scientists starts with lots of data, collected over many years and in many experiments, which can give insight into the proton’s contents. Then, with a careful statistical analysis, they try to extract from the data a precision picture of the proton’s internal makeup (encoded in what is known as “Parton Distribution Functions” — that’s the PDF in NNPDF).
NNPDF are by no means the first group to do this; it’s been a scientific task for decades, and without it, data from proton colliders like the Large Hadron Collider couldn’t be interpreted. Crucially, the NNPDF group argues they have the best and most modern methods for the job — NN stands for “neural network”, so it has to be good, right? 😉 — and that they carry it out at higher precision than anyone has ever done before.
But what if they’re wrong? Or at least, what if the uncertainties on their picture of the proton are larger than they say? If the uncertainties were double what NNPDF believes they are, then the claim of excess charm quark/anti-quark pairs in the proton — just barely above detection at 3 standard deviations — would be nullified, at least for now. And even the claim of the W boson mass being different from the theoretical prediction, which was argued to be a 7 standard deviation detection, far above “discovery” level, is in some question. In that mass measurement, the largest single source of systematic uncertainty is from the parton distribution functions. A mere doubling of this uncertainty would reduce the discrepancy to 5 standard deviations, still quite large. But given the thorny difficulty of the W mass measurement, any backing off from the result would certainly make people more nervous about it… and they are already nervous as it stands. (Some related discussion of these worries appeared in print here, with an additional concern here.)
In short, a great deal, both current and future, rides on whether the NNPDF group’s uncertainties are as small as they think they are. How confident can we be?
The problem is that there are very few people who have the technical expertise to check whether NNPDF’s analysis is correct, and the numbers are shrinking. NNPDF is a well-funded European group of more than a dozen people. But in the United States, the efforts to study the proton’s details are poorly funded, and smaller than ever. I don’t agree with Sabine Hossenfelder’s bludgeoning of high-energy physics, much of which seems to arise from a conflation of real problems with imaginary ones — but she’s not wrong when she argues that basic science is under-funded compared to more fancy-sounding stuff. After all, the US has spent a billionish dollars helping to build and run a proton collider. How is it that we can’t spend a couple of million per year to properly support the US-based PDF experts, so that they can help us make full use of this collider’s treasure trove of data? Where are our priorities?
A US-based group which calls itself CTEQ-TEA, which has been around for decades and was long a leader in the field, is disputing NNPDF’s uncertainties, and suggesting they are closer to the uncertainties that CTEQ-TEA itself finds in its own PDFs. (Essentially, if I understand correctly, they are suggesting that NNPDF’s methods fail to account for all possible functional forms [i.e. shapes] of the parton distribution functions, and that this leads the NNPDF group to conclude they know more than they actually do.) I’m in no position, currently, to evaluate this claim; it’s statistically subtle. Nor have I spoken to any NNPDF experts yet to understand their counter-arguments. And of course the CTEQ-TEA group is inevitably at risk of seeming self-serving, since their PDFs have larger uncertainties than those obtained by NNPDF.
But frankly, it doesn’t matter what NNPDF says or how good their arguments are. With such basic questions about nature riding on their uncertainties, we need a second and ideally a third group that has the personnel to carry out a similar analysis, with different assumptions, to see if they all come to the same conclusion. We cannot abide a situation where we depend on one and only one group of scientists to tell us how the proton works at the most precise level; we cannot simply assume that they did it right, no matter how careful their arguments might seem. Mistakes at the forefront of science happen all the time; the forefront is a difficult place, which is why we revere those who achieve something there. We cannot have claims of major discoveries (or lack thereof!) reliant on a single group of people. And so — we need funding for other groups. Otherwise it will be a very long time before we know whether or not the W boson’s mass is actually above the Standard Model prediction, or whether there really are charm quark/anti-quark pairs playing a role in the proton… and meanwhile we won’t be able to answer other questions that depend on precision measurements, such as whether the properties of the Higgs boson exactly agree with the Standard Model.
Prizes worth millions of dollars a year, funded by the ultra-wealthy, are given to famous theoretical physicists whose best work is already in the past. At many well-known universities, the string theory and formal quantum field theory efforts are well-funded, thanks in part to gifts from very rich people. That’s great and all, but progress in science depends not only on the fancy-sounding stuff that makes the headlines, but also on the hard, basic work that makes the headline-generating results possible. Somebody needs to be funding those foundational efforts, or we’ll end up with huge piles of experimental data that we can’t interpret, and huge piles of theory papers that sound exciting but whose relation to nature can’t be established.
I doubt this message will get through to anyone important who can do something about it — it’s a message I’ve been trying to deliver for over 20 years — but in an ideal world I’d like it to be heard by to two groups of people: (1) the funders of particle physics at the National Science Foundation and the Department of Energy, who ought to fund string theory/supersymmetry a little less and proton fundamentals a little more; and (2) Elon Musk, Mark Zuckerberg, Jeff Bezos, Yuri Milner, and other gazillionaires who could solve this problem with a flick of their fingers.