The strong CP problem: We're just lost

While we like to brag about how nature works the same way for stars as it does inside our stomachs, there is something that makes it biased.

December 14, 2012 02:12 pm | Updated November 10, 2021 10:02 am IST

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Unsolved problems in particle physics are just mind-boggling. They usually concern nature at either the smallest or the largest scales, and the smaller the particle whose properties you're trying to decipher, the closer you are to nature’s most fundamental principles, principles that, in their multitudes, father civilisations, galaxies, and all other kinds of things.

One of the most intriguing such problems is called the ‘strong CP problem’. It has to do with the strong force, one of nature’s four fundamental forces, and what's called the CP-violation phenomenon.

The strong force is responsible for most of the mass of the human body, most of the mass of the chair you’re sitting on, even most of the mass of our Sun and the moon.

Yes, the Higgs mechanism is the mass-giving mechanism, but it gives mass only to the fundamental particles, and if we were to be weighed by that alone, we’d weigh orders of magnitude lesser. More than 90 per cent of our mass actually comes from the strong nuclear force.

The relationship between the strong nuclear force and our mass is unclear (this isn’t the problem I’m talking about). It’s the force that holds together quarks, a brand of fundamental particles, to form protons and neutrons. As with all other forces in particle physics, its push-and-pull is understood in terms of a force-carrier particle – a messenger of the force’s will, as it were.

This messenger is called a gluon, and the behaviour of all gluons is governed by a set of laws that fall under the subject of quantum chromodynamics (QCD).

- Dr. Murray Gell-Mann is an American scientist who contributed significantly to the development of theories of fundamental particles, including QCD

According to QCD, the farther two gluons get away from each other, the stronger the force between them will get. This is counterintuitive to those who’ve grown up working with Newton’s inverse-square laws, etc. An extension of this principle is that gluons can emit gluons, which is also counter-intuitive and sort of like the weird >Banach-Tarski paradox .

Protons and neutrons belong to a category called hadrons, which are basically heavy particles that are made up of three quarks. When, instead, a quark and an antiquark are held together, another type of hadron called the meson comes into existence. You’d think the particle and its antiparticle would immediately annihilate each other. However, it doesn't happen so quickly if the quark and antiquark are of different types (also called flavours).

One kind of meson is the kaon. A kaon comprises one strange quark (or antiquark) and one up antiquark (or quark). Among kaons, there are two kinds, K-short and K-long, whose properties were studied by Orreste Piccioni in 1964. They’re called so because K-long lasts longer than K-short before it decays into a shower of lighter particles, as shown:

Strange antiquark -->up antiquark + W-plus boson (1)

W-plus boson -->down antiquark + up quark

Up quark --> gluon + down quark + down antiquark (2)

The original other up quark remains as an up quark.

Whenever a decay results in the formation of a W-plus / W-minus / Z boson, the weak force is said to be involved. Whenever a gluon is seen mediating, the strong nuclear force is said to be involved.

In the decay shown above, there is one weak-decay (1) and one strong-decay (2). And whenever a weak-decay happens, a strange attitude of nature is revealed: bias.

- Handed spin (the up-down arrows indicate the particle's momentum)

The universe may not have a top or a bottom, but it definitely has a left and a right. At the smallest level, these directions are characterised by spinning particles. If a particle is spinning one way, then another particle with the same properties but spinning the other way is said to be the original’s mirror-image. This way, a right and a left orientation are chosen.

As a conglomeration of such spinning particles, some toward the right and some toward the left, comes together to birth stuff, the stuff will also acquire a handedness with respect to the rest of the universe.

And where the weak-decay is involved, left and right become swapped; parity gets violated.

Consider the K-long decay depicted above (1). Because of the energy conservation law, there must be a way to account for all the properties going into and coming out of the decay. This means if something went in left-handed, it must come out left-handed, too. However, the strange antiquark emerges as an up antiquark with its spin mirrored.

- Physicists Tsung-Dao Lee and Chen Ning Yang (Image from the University of Chicago archive)

As Chen Nin Yang and Tsung-Dao Lee investigated in the 1950s, they found that the weak-decay results in particles whose summed up properties were exactly the same as that of the decaying particle, but in a universe in which left and right had been swapped! In addition, the weak-decay also forced any intervening quarks to change their flavour.

- Even in the Feynman diagram shown above, a neutron decays into a proton because a down quark is turned into an up quark (The mediating W-minus decays into an electron and an electron antineutrino).

This is curious behaviour, especially for a force that is considered fundamental, an innate attribute of nature itself. Whatever happened to symmetry, why couldn’t nature maintain the order of things without putting in a twist? Sure, we’re now able to explain how the weak-interaction swaps orientations, but there’s no clue about why it has to happen like that. I mean… why?!

And now, we come to the strong CP problem(!): The laws governing the weak-interaction, brought under electroweak theory (EWT), are very, very similar to QCD. Why then doesn’t the strong nuclear force violate parity?

This is also fascinating because of the similarities it bears to nature’s increasing degree of prejudices. Why an asymmetric force like the weak-interaction was born in an otherwise symmetric universe, no one knows, and why only the weak-interaction gets to violate parity, no one knows. Pfft.

More so, even on the road leading up to this problem, we chanced upon three other problems, and altogether, this provides a good idea of how much humans are lost when it comes to particle physics. It’s evident that we’re only playing catching up, building simulations and then comparing the results to real-life occurrences to prove ourselves right. And just when you ask “Why?”, we’re lost for words.


Even the Large Hadron Collider (LHC), a multi-billion dollar particle sledgehammer in France-Switzerland, is mostly a "How" machine. It smashes together billions of particles and then, using seven detectors positioned along its length, analyses the debris spewn out.

- An indicative diagram of the layout of detectors on the LHC

Incidentally, one of the detectors, the LHCb, sifts through the particulate mess to find out how really the weak-interaction affects particle-decay. Specifically, it studies the properties of the B-meson, a kind of meson that has a bottom quark/antiquark ( b -quark) as one of its two constituents.

The b -quark has a tendency to weak-decay into its antiparticle, the b* -quark, in the process getting its left and right switched. Moreover, it has been observed the b* -quark is more likely to decay into the b -quark than it is for the b -quark to decay into the b* -quark. This phenomenon, involved in a process called baryogenesis, was responsible for today's universe being composed of matter and not antimatter, and the LHCb is tasked with finding out... well, why?

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