particle physics

Emerging From the Writing Cave

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I have just submitted a book, entitled Waves in an Impossible Sea, to my publisher, Basic Books (a trade press well known for Gödel, Escher, Bach, as well as numerous other science and math titles.) If all goes smoothly, it will go on sale in March 2024 or thereabouts. Now that it is mostly done, … Read more

In Brief: Unfortunate News from the Moon

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Sadly, the LunaH-MAP mini-satellite (or “CubeSat”) that I wrote about a couple of days ago, describing how it would use particle physics to map out the water-ice in lunar soil, has had a serious setback and may not be able to carry out its mission. A stuck valve is the most likely reason that its … Read more

The Artemis Rocket Launch and Particle Physics

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A post for general readers:

The recent launch of NASA’s new moon mission, Artemis 1, is mostly intended to demonstrate that NASA’s incredibly expensive new rocket system will actually work and be safe for humans to travel in. But along the way, a little science will be done. The Orion spacecraft at the top of the giant rocket, which will actually make the trip to the Moon and back and will carry astronauts in future missions, has a few scientific instruments of its own. Not surprisingly, though, most are aimed at monitoring the environment that future astronauts will encounter. But meanwhile the mission is delivering ten shoe-box-sized satellites (“CubeSats“) which will carry out various other scientific and/or technological investigations. A number of these involve physics, and a few directly employ particle physics.

The use of particle physics detectors for the purpose of studying the not-so-empty space around the Moon and Earth is no surprise. Near any star like the Sun, what we think of as the vacuum of space (and biologically speaking, it is vacuum: no air and hardly any atoms, making it unsurvivable as well as silent) is actually swarming with subatomic particles. Well, perhaps “swarming” is an overstatement. But nevertheless, if you want to understand the challenges to humans and equipment in the areas beyond the Earth, you’ll inevitably be doing particle physics. That’s what a couple of the CubeSats will be studying, entirely or in part.

What’s more of a surprise is that one of the best ways to find water on the Moon without actually landing on it involves putting particle physics to use. Although the technique is not new, it’s not so obvious or widely known, so I thought I’d draw your attention to it.

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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: Checking The Electric Charges of Quarks

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

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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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The Standard Model More Deeply: Masses, Lifetimes and Forces

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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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Celebrating the Standard Model: The Forces of Nature

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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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