More Soup (part 2) – The Hadrons

We have already looked at the leptons in part 1, so now we’re moving on to the hadrons. Please don’t forget that I’m keeping this purposefully simple and hopefully jargon-free(ish) to give the greatest degree of accessibility, to as wide an audience as possible.

A beginners guide to the Hadrons.

So ….what the hell is a hadron?
This a question many people ask but few find a satisfactorily simple answer to.
Simply put, a hadron is a composite subatomic particles made up of quarks and gluons. We’ll look at quarks in a little more detail further on in this post but for now we can just think of them as little building blocks. As for gluons, we’ll be looking at them in a future post (probably ‘More Soup – part 3’) but temporarily, just think of them as the force that binds the quarks together. This a somewhat simplistic view but suits our purposes for now.

Hadrons can be grouped into two main types. The mesons, that are made up of a quark and an antiquark, and the baryons, that are composed of three quarks.
To put this into a little bit of context lets use some examples. Some of the best and most commonly known baryons (a type of hadron) are the proton (p) and neutron (n) found in the atomic nuclei.


Both of these particles are stable when found in the nucleus. The neutron is unstable when outside of the nucleus (called a free neutron) and decays into a proton via the emission of an electron and an antineutrino.

The mesons, however, are a different kettle of fish!
Tending to be very short lived, with durations of 10-8 secs and shorter, the most common of the mesons are the Pion (pi-meson) and the Kaon (k-meson). There are many other varieties of meson, including the rho, B zero and eta-c meson but unlike the pion and kaon, these three, and most other mesons, are only detectable by the products of their decay.
What does this mean in real terms? Well ….. they are so short lived (10-23 secs in the case of eta-c) that the only way we can “see” them is to look for the ‘things’ they break down into when they decay into more stable products. It the case of eta-c (the charmed eta meson or ηc) the products could be a pair of photons or a trio of pions.

Pion decay in an early bubble chamber at CERN.

Like the leptons that we have looked at before, both the baryons and mesons have antiparticles that correspond to each positive particle. The proton (p) has an antiproton ( p ) and the positive pion (π+) has the negative pion (π) as an antiparticle. However, there are some mesons, such as the neutral pion (π0), that are there own antiparticle!
Quite unlike the leptons mentioned previously, all hadrons are affected by the strong interaction as well as the weak interaction, electromagnetism and gravitation. This is an important difference as it is this strong interaction that holds the nucleus together. Were it not so, why would the two protons in a helium nucleus stay together? The force of electrostatic repulsion would surely push them apart at a high velocity! We shall return to these binding forces in later posts.

So now we know what a hadron is, well …. we have a better idea anyway.
They are two or three of these little quark ‘building blocks’ stuck together. Great.
Quarks however …..erm ….. ok, off we go again.

Realistically, the quarks really deserve a whole “More Soup” post all to themselves, but what the hell, lets keep it simple and give you a quick guide now.

A quick guide to quarks.

As we’ve seen, these little guys are the very very simplest things that matter can be comprised of. They are held together in threes (baryons) or twos (mesons) and are a fundamental particle i.e. they’re not made of anything simpler – well thats what physicists currently think anyway.

Just like the leptons, they are categorised into six types or ‘flavours’, arranged into three generations.

In the first generation is the up (u) and down (d) quark. [both discovered SLAC 1968]
In the second generation are strange (s) and the charm (c) quark. [discovered SLAC 1968 and SLAC & Brookhaven 1974]
Lastly, in the third generation are the bottom (b) and top (t) quarks [discovered Fermilab 1977 and Fermilab 1995]
As before, each of these quarks has its own antiparticle – an antiquark e.g. the anti top quark (t) or the anti strange quark (s).
Note – All anti particles are designated with a little bar above the letter.

An example of some hadrons with their component quarks are listed below.
Proton (p) – up, up, down. (abbreviated to ‘uud’)
Neutron (n) – up, down, down. (udd)
Sigma minus (Σ) – down, down, strange (dds)
Charmed eta meson (ηc) – charm, anticharm (cc)
Pi minus meson (π) – down, antiup (du)

Before we leave this article, there are plenty of physicists out there who will be yelling about various qualities such as spin (isospin), parity (both C and G), strangeness, charm, charge, baryonic number and colour that can also apply to hadrons.
For you guys and gals – don’t worry. We’re taking this subatomic soup stuff a little at a time don’t forget. All those attributes will come up again later, in the future ‘More Soup (part 5) – Everything Else!’ post (probably).

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More Soup (part 1) – The Leptons

Time for another post so I thought I’d go back to the particle physics.

Wow there’s a lot of subatomic particles.
The more I stir this quantum soup, the more I need to get to grips with my mesons, baryons, quarks, antineutrinos and lambda particles. Not forgetting, of course, the gluons, photons and bosons!
Ok …. so lets start explaining what’s what. I want to keep it really simple for now, so it’s as accessible as possible.
Most particles already get grouped together to form families, so I’m thinking some sort of list or glossary might be in order!

Evidence of neutrino interactions in a bubble chamber.

Let’s start with……..

A beginners guide to the Leptons.

Leptons are fundamental particles. By fundamental, we mean that they are composed of nothing smaller (by current thinking). They are divided into two main groups: the charged leptons (ie electrons) and the neutral leptons (ie. neutrinos).
These leptons are further classified into six distinct types, also known as flavours, occurring in what is known as three generations. Lots of sub atomic particles have favours and generations as we’ll see in subsequent posts but lets not worry about this now.

In the first generation is the electron (e) and the electron neutrino (Ve).

In the second generation are muon (μ) and the muon neutrino (Vμ).

Lastly, in the third and final generation are the tau (τ) and tau neutrino (Vτ)

This last lepton, the tau neutrino, was only discovered eleven years ago in 2000 at Fermilab in the US and indicates just how recently new discoveries are being made in the field of particle physics.

Each of these six fundamental particles also has a corresponding anti particle. Its exact opposite, if you like. An example of this would be an electron (e) and the corresponding antiparticle, an anti-electron , often called a positron (e+). They  differ from each other only in that some of their properties have equal magnitude but opposite sign.

All leptons have some intrinsic properties like mass, charge and spin (that’s quantum spin by the way). Unlike hadrons, they are not affected by the strong interaction, only by weak interaction, electromagnetism (except neutrinos which are electrically neutral) and gravitation.

Ok … so now we know what they are and how we classify them but ….. what exactly do they do and where can we find some ??
I guess most people know about electrons. Go back to some fairly standard secondary school science and we are told that they spin round the nucleus of an atom and are divided up into “energy shells”. Sort of like this —>

However that’s not really the case. We need to really think more along the line of an electron cloud – a little like an electron probability map. This diagram (left) shows what I mean. The purple spot in the centre representing the nucleus and the haze around the outside representing the probability of finding an electron in that location. The darker the haze the greater the chance that an electron could be found there.
Its all to do with quantum mechanics and a gentleman called Heisenberg. He came up with a principle – the aptly named Heisenberg Uncertainty Principle – that stated that you could not know a particles exact speed and position with 100% certainty. This is better stated as – the more precisely one of these qualities is known, the less precisely the other one can be determined.

But, again… I digress, so …. back to the leptons then.

Electrons (wherever they are!) control most of the chemical properties of the elements and govern how elements react with each other. They are very light weight, having a very small mass and are very stable.
Muons are heavier, with taus being the real heavyweights. Both muons and taus are very very unstable and short lived with lifespans ranging from 2.1×10-6 in the case of muons, to 2.9×10−13 for taus. Both muons and taus can only be created by high energy collisions such as those created by cosmic rays or in particle accelerators. Both particles break down readily, via particle decay, explaining their short lifespans. Muons decay into electrons and various neutrinos. Taus however, are much more complicated and are the only leptons that can decay to various hadrons as well as other leptons.

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