Strangeness In Standard Model

Before we try to understand the origin of strange matter and strangeness in physics, let us get familiar with physics’s standard model. The standard model of elementary particles and fundamental forces, developed throughout the 20th century, describes the behavior and composition of particles through a classification of three of the four fundamental forces known (the strong, the weak, and the electromagnetic force) as well as the smallest particles known, arranged into two main categories: the fermions and the bosons.

Fermions are 12 elementary particles (purple and green in the diagram above) with a spin of a ½, each with a corresponding antiparticle. They are classified by the charges they carry and how they interact. Within the family of fermions, there are 6 quarks and 6 leptons. Quarks were recognized and theorized by Murray Gell-Mann, classifying quarks with flavor and strangeness.

These particles, specifically the strange quark, which will be focussed on in this article, interacts via the strong force and form larger composite particles known as either mesons or baryons. Mesons are identified as being made up of a quark and an antiquark, while baryons are typically made up of three quarks. The most popular baryons are also known as protons and neutrons.

The second large family of particles, the bosons (Red and Yellow), contain the gauge and scalar bosons known as force carriers and mediate the fundamental interactions between elementary particles. In the case of the strong force discussed earlier, mediating the interaction between quarks, the gluon is the one responsible for it.

Quantum Numbers in Particle Physics

The standard model also associates a charge number, a baryon number, a lepton number, and a spin number to each elementary particle. These quantum numbers must be conserved in a particle interaction. I’ll give you a simple example to explain this. Consider the lepton number of a particle. It’s a quantum number assigned to each lepton in the standard model. All the six leptons (electron, muon, tau, electron-neutrino, muon-neutrino, and tau-neutrino) have a lepton number L = 1. The antimatter counterparts of these leptons have L = -1.

Now consider the following particle interaction:

γ → e+ e+

This interaction is known as pair production. In pair production, a gamma-ray photon (γ) disintegrates into an electron (e) and a positron (e+), which is the antimatter counterpart of an electron. Such an interaction is possible because of the conservation of certain quantum numbers, before and after the interaction.. I’ll explain two of them which are quite evident.

1. Electric charge: Look at the above reaction closely. On the left-hand side, we have a photon, which is a neutral particle. So the electric charge on the left is zero. On the right-hand side, we have an electron (e) having a charge of 1e (e is the elementary charge), and a positron, having charge -1e (antimatter has an opposite charge). So the total charge on the right is -1 + 1 = 0. Hence, in pair production, electric charge is conserved.
2. Lepton Number: Another quantum number that is conserved in this reaction is the lepton number. Again, let’s start with the left-hand side. A photon is a boson. Hence there is no lepton number assigned to it in the standard model. We have L = 0 on the left. On the right, L = 1 for an electron and -1 for a positron, making a total of 0, which is equal to the right-hand side. Hence, the lepton number is conserved.

Strangeness Number

Just like the leptons are assigned a lepton number, quarks are assigned a strangeness number. It’s not the only quantum number associated with the quarks, but we will stick to it in this article. All quarks have a strangeness number of 0, apart from the strange quark (of symbol s), the third lightest quark out of six, which has a strangeness number of 1. Thus, when a particle or matter is referred to as strange, it simply means that its composition contains a strange quark.

Typical subatomic particles containing strange quarks are kaons, strange D mesons, or Sigma baryons. Amongst these particles, the kaon was the first one to be discovered in 1947. The usage of the term ‘strange’ predates the concept of strange quark. Physicists noted that these particles were easily created during collisions but decayed much more slowly than expected. This means that the strong interaction was responsible for their formation, but the weak interaction governed their decay. Because of this ‘strange’ behavior, these particles were dubbed as strange particles.

However, only in 1964 did Murray Gell-Mann and George Zweig actually classified the composing quarks in kaons as “strange.” Strangeness is also a fundamental property of nuclear reactions as for a reaction to take place. A required condition is the conservation of strangeness (just like the conservation of mass or charge). Thus, the strangeness number of reactants must equal the strangeness number of products.

But, the above line only holds for electromagnetic and strong interactions. In weak interactions, strangeness isn’t always conserved. Let me give you an examples to show the same:

Consider the decay of a positive kaon (K+) into two pions (π+ and π) as shown below

A positive kaon is a strange particle with strangeness S = -1. Pions do not contain any strange quark; hence they have S = 0. In this interaction, strangeness is violated. Strangeness isn’t violated in all weak interactions. All in all, the amount of strangeness can change in a weak interaction reaction by +1, 0, or -1 (depending on the reaction).

Strange Matter in the Universe

In nature, stranger matter is said to occur in the core of neutron stars and are also theorized to appear in the form of isolated droplets known as strangelets. Strange particles, or strangelets, would play an important role in the decay of quark matter. Quark matter, as opposed to nuclear matter, is more stable and solely contains quarks (no leptons, unlike atoms, which contain electrons). According to the “strange matter hypothesis,” droplets of nuclear matter (like the atoms we see around us) are metastable and would decay into strangelets in a great enough timespan.

Another property of strange matter that helps explain where the strange matter is to be found is its stability. Strange particles or strange matter are only said to be stable at high pressure, thus explaining why it occurs in the core of neutron stars, where the pressure is elevated. The stars meeting these pressure conditions are also known as “strange stars” and are, in theory, several kilometers across, coated with a thin crust of nuclear matter.

It is more possible that the strangelets, the products of nuclear matter decay, would also be the type of strange matter to be found at the collision or formation of these strange neutron stars. Thus, although the existence of strange quarks has been confirmed experimentally and that strange matter is heavily hypothesized as occurring in the core of neutron stars, the isolated existence of strangelets remains more speculative.

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