The Quark Structure of Hadrons:
An Introduction to the Phenomenology and Spectroscopy

by Claude Amsler
Springer, 2018, 277 pages
reviewed by T. Nelson

ince the creation of SARS-CoV-2 in 2019, it has become clear that biology has surpassed physics as the most dangerous branch of science. But maybe some new discoveries in particle physics can help correct that injustice.

This book describes some new dangerous-sounding particles, including glueballs, multiquark mesons, heavy tetraquarks, and other ‘exotic’ states of matter. The first few chapters, where the author tells us the background, are a bit challenging to get through, not because they're so difficult but because the author keeps forgetting to mention what he's talking about. In Chapter 5, Quark-Antiquark Nonets, for example, I was halfway through the section on nonet mixing angle before I realized he was talking about electroweak interactions of quarks.

It's an important topic, and not without controversy. A cynic might even say that Kobayashi and Maskawa each got 2/3 of a Nobel prize for discovering it, while Cabibbo, who made the original discovery, got −1/3 of a Nobel prize.* But Cabibbo got his revenge by being named first in the CKM matrix that describes how quarks change into other quarks via weak interactions. It's nearly impossible to understand most of this from Amsler's lightning eleven-page exposition; it seems to be included not to teach but to remind readers who already know QFT and gauge theory inside out. If not, a good reference is the 81-page Chapter 7 (“Electroweak Interactions of Quarks”) in Gauge Theories of the Strong, Weak, and Electro­magnetic interactions by Chris Quigg.

Some of these quarks are quite massive. For instance, the top quark weighs about the same as an atom of tungsten, while the up quark “weighs” only about 1.7–3.3 AU, or about the same as a deuterium atom. However, this is misleading. Mass and energy are interconvertible, and quark mass vanishes due to chiral symmetry. Indeed, most of the mass of a nucleon isn't due to quarks or Higgs bosons at all, but to gluons. Without the Higgs, Amsler says, we and the matter in our environment would only be one percent lighter. This also means that mesons (which consist of two quarks) don't just sit around; since most of their mass is in the gluons, they can be modeled by a gluon tube spinning at nearly the speed of light. There aren't just a few mesons, either, but 190 of them, which are listed along with the 180 known baryons (which consist of three quarks, the same number as a proton), in a gigantic table on pages 4–5.

Left: Feynman diagram from page 88 showing J / Ψ meson created from an electron-positron collision and subsequently decaying. Right: creation of a J/Ψ from hadron collisions. Springs represent gluons, dashed lines are virtual photons, and a heavy line represents three gluons. The meson can decay into a number of products (electron-positron pairs, muons, or quarks). The branching ratios of the different decay pathways are accurately predicted by the theory

Finally in Chapter 8 the author starts discussing specific particles and the experimental evidence. The more interesting mesons [J/Ψ meson (cc̅, containing a charmed + anticharmed quark) and the Upsilon (1S) meson (bb̅, containing a bottom + antibottom quark)] are used to explain the evidence for the bigger quarks such as charmed, top, and bottom. This is followed by chapters on quarkonium, glueballs, multiquark states, and heavy baryons.

The decay chains of these particles are quite compli­cated. So much so that to a beginner it may seem as if anything can decay into anything else, but this is decidedly not the case. SU(2) and SU(3) symmetries in QCD (quantum chromodynamics) nicely predict the decay products, decay rate, and branching ratios of the various pathways. SU stands for special unitary group, and it's a way of understanding particles by way of their symme­tries. QCD's ability to make such specific and accurate predictions suggests that symmetry is not just an abstract mathematical diversion but reflects a profound truth in nature.

The strength of this book is the emphasis on experiments. There are numerous color graphs and diagrams illustrating the spectra, experimental setup, and decay pathways. It also covers topics like Young tableaux, which aren't covered in most particle physics books but are described more thoroughly in Cheng and Li's 1984/1988 book Gauge Theory of Elementary Particle Physics.

The chapters are quite short, which brings us to the main weakness of this book: each chapter has to be re-read a few times because of the author's terse writing style and his penchant for leaving out critical details. Readers with no prior background will find themselves spending as much time on the Internet searching for more detailed explanations as reading the book.

Glueballs, or particles composed entirely of gluons with no quarks, have not yet been convincingly observed, mainly because they mix with mesons. Claims have been made for ground state glueballs f₀(1370), f₀(1500) and f₀(1710), where the number indicates the mass in MeV. You will never hear Captain Picard ordering his ship to fire glueballs at anybody. If they existed and were stable they would act somewhat like gamma rays. But they sound cute—hardly dangerous at all. It's almost as if physicists aren't even trying.

* I tried this joke at a party one time, and it bombed.

sep 24, 2023

Antimatter: What it is and why it's important in physics and everyday life

by Beatriz Gato-Rivera
Springer, 2021, 296 pages
reviewed by T. Nelson

ell, there's not much you can say about antimatter. It's matter, it has opposite electrical charge, and, um, it's married to its uncle matter. Any more than that, and things start getting technical.

That makes this book perfect for non-scientists who don't want to spend a year studying QM, then QFT, and then gauge theory. There's some light, easy-to-understand physics, along with nice color pictures of the accelerators and some of the scientists and PR people from various labs.

The most interesting chapters are on the history of how these particles were discovered. Carl Anderson knew his experiments were detecting positrons, but he was prevented from saying so by his supervisor Robert Millikan, who insisted they were protons. Through a series of lucky breaks where a large number of his competitors misinterpreted their own cloud chamber results, Anderson finally defied Millikan and published by himself. He was suitably rewarded, though he lost years of effort.

Most of the remainder of the book discusses cosmology, some speculation about dark matter, production of positrons from ⁴⁰K in bananas, and the fact that the inner Van Allen radiation belt is the most abundant source of antiprotons near the Earth, far exceeding the amount from cosmic rays.

In Chapter 8, the author spends several pages on the question of whether antimatter has negative mass. This might have made it fall up instead of down. Last week we learned the answer: it doesn't, which makes a fitting end to this book. It's non-technical, suitable for anyone who had high school physics, and with few technical errors. No index.

oct 02, 2023