MATT STRASSLER
Of Particular Significance

Category: Quantum Field Theory

An Extraordinarily Productive Visit to Fermilab

Fermilab’s main building at sunset. [Credit: M. Strassler]

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.

Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON October 20, 2022

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Protons and Charm Quarks: A Lesson From Virtual Particles

There’s been a lot of chatter lately about a claim that charm quarks are found in protons. While the evidence for this claim of “intrinsic charm” (a name that goes back decades) is by no means entirely convincing yet, it might in fact be true… sort of. But the whole idea sounds very confusing. A charm quark has a larger mass than a proton: about 1.2 GeV/c2 vs. 0.938 GeV/c2. On the face of it, suggesting there are charm quarks in protons sounds as crazy as suggesting that a football could have a lead brick inside it without you noticing any difference.

What’s really going on? It’s a long story, and subtle even for experts, so it’s not surprising that most articles about it for lay readers haven’t been entirely clear. At some point I’ll write a comprehensive explanation, but that will require a longer post (or series of posts), and I don’t want to launch into that until my conceptual understanding of important details is complete.

Feynman diagram suggesting a photon is sometimes an electron-positron pair.

But in the meantime, here’s a related question: how can a particle with zero mass (zero rest mass, to be precise) spend part of its time as a combination of objects that have positive mass? For instance, a photon [a particle of light, including both visible and invisible forms of light] has zero rest mass. [Note, however, that it has non-zero gravitational mass]. Meanwhile electrons and positrons [the anti-particles of electrons] both have positive rest mass. So what do people mean when they say “A photon can be an electron-positron pair part of the time”? This statement comes with a fancy “Feynman diagram”, in which the photon is shown as the wavy line, time is running left to right, and the loop represents an electron and a positron created from the photon.

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POSTED BY Matt Strassler

ON September 9, 2022

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The Standard Model More Deeply: Lessons on the Strong Nuclear Force from Quark Electric Charges

For readers who want to go a bit deeper into details (though I suggest you read last week’s posts for general readers first [post 1, post 2]):

Last week, using just addition and subtraction of fractions, we saw that the ratio of production rates

  • R = Rate (e+ e— ⟶ quark anti-quark) / Rate (e+ e— ⟶ muon anti-muon)

(where e— stands for “electron” and e+ for “positron”) can be used to verify the electric charges of the quarks of nature. [In this post I’ll usually drop the word “electric” from “electric charge”.] Specifically, the ratio R, at different energies, is both sensitive to and consistent with the Standard Model of particle physics, not only confirming the quarks’ charges but also the fact that they come in three “colors”. (About colors, you can read recent posts here, here and here.)

To keep the previous posts short, I didn’t give evidence that the data agrees only with the Standard Model; I’ll start today by doing that. But I did point out that the data doesn’t quite match the simple prediction. You can see that in the figure below, repeated from last time; it shows the data (black dots) lies close to the predictions (the solid lines) but generally lies a few percent above them. Why is this? The answer: we neglected a small but noticeable effect from the strong nuclear force. Not only does accounting for this effect fix the problem, it allows us to get a rough measure of the strength of the strong nuclear force. From these considerations we can learn several immensely important facts about nature, as we’ll see today and in the next post.

Figure 1: Data (black dots) showing R as a function of the collision energy 2Ee. Horizontal colored lines show the three predictions for R in the regions where the data is simple and 3, 4 or 5 of the quarks are produced. The minor jumpiness in the data is due to measurement imperfections.
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Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON August 30, 2022

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Celebrating the Standard Model: Checking The Electric Charges of Quarks

A post for general readers who’ve heard of quarks; if you haven’t, you might find this article useful:

Yesterday I showed you that the usual argument that determines the electric charges of the various types of quarks uses circular reasoning and has a big loophole in it. (The up quark, for example, has charge 2/3, but the usual argument would actually allow it to have any charge!) But today I’m going to show you how this loophole can easily be closed — and we’ll need only addition, subtraction and fractions to close it.

Throughout this post I’ll shorten “electric charge” to just “charge”.

A Different Way to Check Quark Charges

Our approach will be to study the process in which an electron and a positron (the electron’s anti-particle) collide, disappear (“annihilate”), and are converted into one or another type of quark and the corresponding anti-quark; see Figure 1. The rate for this process to occur, and the rate of a similar one in which a muon and anti-muon are produced, are all we will need to know.

In an electron-positron collision, many things may happen. Among the possibilities, the electron and positron may be converted into two new particles. The new particles may have much more mass (specifically, rest mass) than the electron and positron do, if the collision is energetic enough. This is why physicists can use collisions of particles with small mass to discover unknown particles with large mass.

Figure 1: (Top) an electron and positron, each carrying energy Ee, collide head-on. (Bottom) from the collision with total energy 2Ee , a quark and anti-quark may emerge, as long as Ee is bigger than the quark’s rest mass M times c2.

In particular, for any quark of mass M, it is possible for an electron-positron collision to produce that quark and a corresponding anti-quark as long as the electron’s energy Ee is greater than the quark’s mass-energy Mc2. As Ee is gradually increased from low values, more and more types of quark/anti-quark pairs can be produced.

This turns out to be a particularly interesting observation in the range where 1 GeV < Ee < 10 GeV, i.e. when the total collision energy (2 Ee) is between 2 and 20 GeV. If Ee is any lower, the effects of the strong nuclear force make the production of quarks extremely complicated (as we’ll see in another post). But when the collision energy is above 2 GeV, things start to settle down, and become both simple and interesting.

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Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON August 26, 2022

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The Standard Model More Deeply: Gluons and the Math of Quark “Color”

For readers who want to dig deeper; this is the second post of two, so you should read the previous one if you haven’t already. (Readers who would rather avoid the math may prefer this post.)

In a recent post I described, for the general reader and without using anything more than elementary fractions, how we know that each type of quark comes in three “colors” — a name which refers not to something that you can see by eye, but rather to the three “versions” of strong nuclear charge. In the post previous to today’s, I went into more detail about how the math of “color” works; you’ll need to read that post first, and since I will sometimes refer to its figures, you may want to keep in handy in another tab.

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Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON August 16, 2022

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The Standard Model More Deeply: The Simplified Math of Quark “Color”

For non-expert readers who want to dig a bit deeper. This is the first post of two, the second of which is here.

In my last post I described, for the general reader and without using anything more than elementary fractions, how we know that each type of quark comes in three “colors” — a name which refers not to something that you can see by eye, but rather to the three “versions” of strong nuclear charge. Strong nuclear charge is important because it determines the behavior of the strong nuclear force between objects, just as electric charge determines the electric forces between objects. For instance, elementary particles with no strong nuclear charge, such as electrons, W bosons and the like, aren’t affected by the strong nuclear force, just as electrically neutral elementary particles, such as neutrinos, are immune to the electric force.

But a big difference is that there’s only one form or “version” of electric charge: in the language of professional physicists, protons have +1 unit of this charge, electrons have -1 unit of it, a nucleus of helium has +2 units of it, etc. By contrast, the strong nuclear charge comes in three versions, which are sometimes referred to as “redness”, “blueness” and “greenness” (because of a vague but highly imprecise analogue with the inner workings of the human eye). These versions of the charge combine in novel ways we don’t see in the electric context, and this plays a major role in the protons and neutrons found in every atom. It’s the math that lies behind this that I want to explain today; we’ll only need a little bit of trigonometry and complex numbers, though we’ll also need some careful reasoning.

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Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON August 11, 2022

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Celebrating the Standard Model: How We Know Quarks Come in Three “Colors”

A post for general readers:

Within the Standard Model, the quarks (and anti-quarks) are my favorite particles, because they are so interesting and so diverse. Physicists often say, in their whimsical jargon, that quarks come in various “flavors” and “colors”.   But don’t take these words seriously! They’re just labels; neither has anything to do with taste or vision. We might just as well have said the quarks come in “gerflacks” and “sharjees”; or better, we might have said “types” and “versions”. 

Today I’ll show you how one can easily see that each of the six flavors of quark comes in three colors (i.e., each gerflack/type of quark comes in three sharjees/versions.)  All we’ll need to do is examine a simple property of the W boson, one of the other particles in the Standard Model.

[Another way to say this is that the Standard Model is often described as having a kind of symmetry named “SU(3)xSU(2)xU(1)”; today we’ll put the “3” in SU(3). ]

Gerflacks and Sharjees of Quarks

We know there are six types/gerflacks/flavors of quarks because each type of quark has its own unique mass and lifetime, a fact that’s relatively easy to confirm experimentally.  Quarks 1 and 2 are called down and up, quarks 3 and 4 are called strange and charm, and quarks 5 and 6 are called bottom and top; again, the whimsical names don’t have any meaning, and we often just label them d, u, s, c, b, t.

But to understand why each type of quark comes in three versions/sharjees/colors is more subtle, because two quarks of the same “flavor” which differ only by their “color” appear the same in experiments (despite our intuition for what the word “color” usually means.)

What, in fact, is a “color”? Each color/sharjee/version is a kind of strong nuclear charge, analogous to electric charge, which we encounter in daily life through static electricity and other phenomena. Electric charge determines which objects attract and repel each other via electrical forces. Electrons have electric charge, and so do quarks; that’s why electrical forces affect them. But quarks, unlike electrons, have strong nuclear charge too, and those charges determine how quarks attract or repel one another via the the strong nuclear force.  

And here’s the interesting point: whereas there is only one version of electric charge (electrons and protons and atomic nuclei have different amounts of it, but it is different amounts of the same thing), there are three different versions/sharjees/colors of strong nuclear charge.  They are often called “red”, “green” and “blue”, or “redness”, “greeness” and “blueness”. (Remember, these are just names for sharjees — for versions of strong nuclear charge. In no sense do they represent actual colors that your eyes would see, any more than the six types/flavors of quarks would taste differently.)

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Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON August 1, 2022

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Celebrating the Standard Model: Why Are Neutrino Masses So Small?

For the general reader interested in particle physics or astronomy:

Most of the Standard Model’s particles have a mass [a rest mass, to be precise], excepting only the photon (the particle of light) and the gluon (found in protons and neutrons.) For reasons not understood at all, these masses stretch out over a range of a trillion or more.

If it weren’t for the three types of neutrinos, the range would be a mere 400,000, from the top quark’s mass (172 GeV/c2) to the electron’s (0.000511 GeV/c2), still puzzling large. But neutrinos make the puzzle extreme! The universe’s properties strongly suggest that the largest mass among the neutrinos can’t be more than 0.0000000001 GeV/c2 , while other experiments tell us it can’t be too much less. The masses of the other two may be similar, or possibly much smaller.

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.

This striking situation is illustrated in Figure 1, in which

  • I’ve used a “logarithmic plot”, which compresses the vertical scale; if I used a regular “linear” plot, you’d see only the heaviest few masses, with the rest crushed to the bottom;
  • For later use, I’ve divided the particles into two classes: “fermions” and “bosons”.
  • Also, though some of these particles have separate anti-particles, I haven’t shown them; it wouldn’t add anything, since the anti-particle of any particle type has exactly the same mass.

As you can see, the neutrinos are way down at the bottom, far from everyone else. What’s up with that? The answer isn’t known; it’s part of ongoing research. But today I’ll tell you why

  • once upon a time it was thought that the Standard Model solved this puzzle;
  • today we know of two simple solutions to it, but don’t know which one is right;
  • each of these requires a minor modification of the Standard Model: in one case a new type of particle, in another case a new phenomenon.
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Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON July 18, 2022

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The Standard Model More Deeply: Masses, Lifetimes and Forces

Today’s post is for readers with a little science/math background:

Last week, I explained, without technicalities, how the various elementary forces of nature can be inferred from the pattern of lifetimes of the known particles.  I did this using an image, repeated below, that organized the particles by their masses and lifetimes.  I’ll add more non-technical posts on the Standard Model in the coming days. But today’s post is a tad more technical, using dimensional analysis (a physicist’s secret weapon) (which I demonstrated here, here and here) to explain key features of the image: the red line, the blue line, and the particles at the upper left, as well as why there is a high-energy and a low-energy version of the weak nuclear force.

Figure 1:  An assortment of the known particles particles clustered into classes according to the “force” that causes them to decay. See this recent post for details.
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Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON July 11, 2022

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Celebrating the Standard Model: The Forces of Nature

A post for general readers:

This is the first of several posts celebrating the hugely successful Standard Model of particle physics, the concepts and equations that describe the basic bricks and mortar of the universe. In these posts, I’ll explain (without assuming readers have a science background) how we know some of its most striking features. We’ll look at simple facts that particle physicists have learned over the decades, and use them to infer basic features of the universe and to recognize deep questions that still trouble the experts.

The Elementary “Forces” of Physics: A Classification of Nature

Perhaps you’ve heard it said that “There are four fundamental forces in nature.” Whether you have or not, today I’ll show you how to verify this yourself. (Actually, there are five forces, though we’ll only see a hint of the fifth today.) The force everybody knows from daily life is gravity; ironically, this force has no measurable impact on particle physics, so it’s the only one we won’t be looking at in this post.

I’d better emphasize, though, that the word “force” is slippery. Normally, in everyday life, a force means something that will push or pull objects around. But when physicists say “force,” they often mean something much more general. Because of that they sometimes use the word “interaction” instead of “force”.

For example, static electricity that holds socks together when they come out of the dryer is an example of an honest electromagnetic force — the socks really are pulled together. So is the force that pulls a magnet to a refrigerator door. But when a light bulb glows, that doesn’t involve a force in the limited sense of a push or pull. Yet it still involves the “electromagnetic interaction”, i.e. the “electromagnetic force” in a generalized sense. That’s because, although it is far from obvious, the emission (or absorption) of light involves the same basic phenomena that govern the force between the socks.

[Physicists use “electromagnetic” rather than “electric” or “magnetic” separately because these two forces are so deeply intertwined that it is often impossible to distinguish them.]

So when physicists say there are “four forces” (or five), they are imposing a classification scheme on the world around us. They mean:

  • All fundamental physical processes in nature can be divided up into five classes.
  • Each class involves one of the following types of interactions:
    1. gravitational (holds the planet together and holds us to the ground),
    2. electromagnetic (creates light, controls chemistry and biology, and dominates daily life),
    3. weak-nuclear (essential in stars and in supernova explosions),
    4. strong-nuclear (forms protons, neutrons, and their agglomerations in atomic nuclei),
    5. Higgs-related (associated with the masses of all known elementary particles).

There are currently no verified exceptions to this classification scheme. And by examining basic facts about the various particles found in nature, we can see these classes (other than gravity) in operation.

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Picture of POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON July 6, 2022

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