StandardModel

W boson mass too high? Charm quarks in the proton? There’s a (worrisome) link.

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Two of the most widely reported stories of the year in particle physics,

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?

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Celebrating the Standard Model: The Electric Charges of Quarks

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A post for general readers who’ve heard of quarks; if you haven’t, try reading here:

The universe has six types of quarks, some of which are found in protons and neutrons, and thus throughout all ordinary material. For no good reasons, we call them up, down, strange, charm, bottom and top. Today and tomorrow I want to show you how we know their electric charges, even though we can’t measure them directly. The only math we’ll need is addition, subtraction, and fractions.

This also intersects with my most recent post in this series on the Standard Model, which explained how we know that each type of quark comes in three “colors”, or versions — each one a type of strong nuclear charge akin to electric charge.

Today we’ll review the usual lore that you can find in any book or on any website, but we’ll see that there’s a big loophole in the lore that we need to close. Tomorrow we’ll use a clever method to close that loophole and verify the lore is really true.

The Lore for Protons and Neutrons

Physicists usually define electric charge so that

  • the proton has electric charge +1
  • the electron has charge -1,
  • the neutron has charge 0 (i.e. electrically neutral, hence its name).

[Throughout the remainder of this post, I’ll abbreviate “electric charge” as simply “charge“.]

As for the six types of quarks, the lore is that their charges are [using notation that “Qu” means “electric charge of the u quark“]:

  • Up, Charm, Top (u,c,t): Qu = Qc = Qt = 2/3
  • Down, Strange, Bottom (d,s,b): Qd = Qs = Qb = -1/3

But how do we know this?

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The Standard Model More Deeply: The Nature of Neutrinos

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Earlier this week I explained how neutrinos can get their mass within the Standard Model of particle physics, either by engaging with the Higgs field once, the way the other particles do, or by engaging with it twice. In the first case, the neutrinos would be “Dirac fermions”, just like electrons and quarks. In the second, they’d be “Majorana fermions”. Decades ago, in the original Standard Model, neutrinos were thought not to have any mass at all, and were “Weyl fermions.” Although I explained in my last post what these three types of fermions are, today I want go a little deeper, and provide you with a diagrammatic way of understanding the differences among them, as well as a more complete view of the workings of the “see-saw mechanism”, which may well be the cause of the neutrinos’ exceptionally small masses.

[N.B. On this website, mass means “rest mass” except when otherwise indicated.]

The Three Types of Fermions

What’s a fermion? All particles in our world are either fermions or bosons. Bosons are highly social and are happy to all do the same thing, as when huge numbers of photons are all locked in synch to make a laser. Fermions are loners; they refuse to do the same thing, and the “Pauli exclusion principle” that plays a huge role in atomic physics, creating the famous shell structure of atoms, arises from the fact that electrons are fermions. The Standard Model fermions and their masses are shown below.

Figure 1: The masses of the known elementary particles, showing how neutrino masses are much smaller and much more uncertain than those of all the other particles with mass. The horizontal grey bar shows the maximum masses from cosmic measurements; the vertical grey bars give an idea of where the masses might lie based on current knowledge, indicating the still very substantial uncertainty.

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A Big Think Made of Straw: Bad Arguments Against Future Colliders

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Here’s a tip.  If you read an argument either for or against a successor to the Large Hadron Collider (LHC) in which the words “string theory” or “string theorists” form a central part of the argument, then you can conclude that the author (a) doesn’t understand the science of particle physics, and (b) has an absurd caricature in mind concerning the community of high energy physicists.  String theory and string theorists have nothing to do with whether such a collider should or should not be built.

Such an article has appeared on Big Think. It’s written by a certain Thomas Hartsfield.  My impression, from his writing and from what I can find online, is that most of what he knows about particle physics comes from reading people like Ethan Siegel and Sabine Hossenfelder. I think Dr. Hartsfield would have done better to leave the argument to them. 

An Army Made of Straw

Dr. Hartsfield’s article sets up one straw person after another. 

  • The “100 billion” cost is just the first.  (No one is going to propose, much less build, a machine that costs 100 billion in today’s dollars.)  
  • It refers to “string theorists” as though they form the core of high-energy theoretical physics; you’d think that everyone who does theoretical particle physics is a slavish, mindless believer in the string theory god and its demigod assistant, supersymmetry.  (Many theoretical particle physicists don’t work on either one, and very few ever do string theory. Among those who do some supersymmetry research, it’s often just one in a wide variety of topics that they study. Supersymmetry zealots do exist, but they aren’t as central to the field as some would like you to believe.)
  • It makes loud but tired claims, such as “A giant particle collider cannot truly test supersymmetry, which can evolve to fit nearly anything.”  (Is this supposed to be shocking? It’s obvious to any expert. The same is true of dark matter, the origin of neutrino masses, and a whole host of other topics. Its not unusual for an idea to come with a parameter which can be made extremely small. Such an idea can be discovered, or made obsolete by other discoveries, but excluding it may take centuries. In fact this is pretty typical; so deal with it!)
  • “$100 billion could fund (quite literally) 100,000 smaller physics experiments.”  (Aside from the fact that this plays sleight-of-hand, mixing future dollars with present dollars, the argument is crude. When the Superconducting Supercollider was cancelled, did the money that was saved flow into thousands of physics experiments, or other scientific experiments?  No.  Congress sent it all over the place.)  
  • And then it concludes with my favorite, a true laugher: “The only good argument for the [machine] might be employment for smart people. And for string theorists.”  (Honestly, employment for string theorists!?!  What bu… rubbish. It might have been a good idea to do some research into how funding actually works in the field, before saying something so patently silly.)

Meanwhile, the article never once mentions the particle physics experimentalists and accelerator physicists.  Remember them?  The ones who actually build and run these machines, and actually discover things?  The ones without whom the whole enterprise is all just math?

Although they mostly don’t appear in the article, there are strong arguments both for and against building such a machine; see below.  Keep in mind, though, that any decision is still years off, and we may have quite a different perspective by the time we get to that point, depending on whether discoveries are made at the LHC or at other experimental facilities.  No one actually needs to be making this decision at the moment, so I’m not sure why Dr. Hartsfield feels it’s so crucial to take an indefensible position now.

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Fourth Step in the Triplet Model is up.

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Advanced particle physics today: Today we move deeper into the reader-requested explanation of the “triplet model,”  (a classic and simple variation on the Standard Model of particle physics, in which the W boson mass can be raised slightly relative to Standard Model predictions without affecting other current experiments.) The math required is still pre-university level, though … Read more

The Simplest Way to Shift the W Boson Mass?

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Some technical details on particle physics today…

Papers are pouring out of particle theorists’ offices regarding the latest significant challenge to the Standard Model, namely the W boson mass coming in about 0.1% higher than expected in a measurement carried out by the Tevatron experiment CDF. (See here and here for earlier posts on the topic.) Let’s assume today that the measurement is correct, though possibly a little over-stated. Is there any reasonable extension to the Standard Model that could lead to such a shift without coming into conflict with previous experiments? Or does explaining the experiment require convoluted ideas in which various effects have to cancel in order to be acceptable with existing experiments?

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A Few Remarks on the W Boson Mass Measurement

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Based on some questions I received about yesterday’s post, I thought I’d add some additional comments this morning. A natural and persistent question has been: “How likely do you think it is that this W boson mass result is wrong?” Obviously I can’t put a number on it, but I’d say the chance that it’s … Read more

Physics is Broken!!!

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Last Thursday, an experiment reported that the magnetic properties of the muon, the electron’s middleweight cousin, are a tiny bit different from what particle physics equations say they should be. All around the world, the headlines screamed: PHYSICS IS BROKEN!!! And indeed, it’s been pretty shocking to physicists everywhere. For instance, my equations are working erratically; many of the calculations I tried this weekend came out upside-down or backwards. Even worse, my stove froze my coffee instead of heating it, I just barely prevented my car from floating out of my garage into the trees, and my desk clock broke and spilled time all over the floor. What a mess!

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Broken, eh? When we say a coffee machine or a computer is broken, it means it doesn’t work. It’s unavailable until it’s fixed. When a glass is broken, it’s shattered into pieces. We need a new one. I know it’s cute to say that so-and-so’s video “broke the internet.” But aren’t we going a little too far now? Nothing’s broken about physics; it works just as well today as it did a month ago.

More reasonable headlines have suggested that “the laws of physics have been broken”. That’s better; I know what it means to break a law. (Though the metaphor is imperfect, since if I were to break a state law, I’d be punished, whereas if an object were to break a fundamental law of physics, that law would have to be revised!) But as is true in the legal system, not all physics laws, and not all violations of law, are equally significant.

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