More Soup (part 3) – The Bosons

Ok so here we are again.
We’ve done the leptons…….
We’ve looked at the hadrons ……..
So what’s next ….?

Meet the Bosons !

So the first question that springs straight away to most peoples lips is the same as it was for the other ‘Soup’ posts concerning leptons and hadrons, namely ….. “what the hell is a boson?”

Great questions says I and unfortunately slightly more difficult to answer clearly to those without at least a modicum of science in their background.
The simplest answer I could come up with is that bosons are fundamental particles that are concerned with ‘force’ unlike the leptons and hadrons (which are collectively called the fermions) which are particles of ‘matter’.

Anticipating the blank stares of some readers ……. How can you have a particle that is concerned with a force ?????
A better way of thinking about it would be to imagine it as a particle that ‘carries’ or ‘mediates’ a force rather than actually is the force itself.

Let kick straight off with some names and descriptions.
The bosons are categorised into six types. There are four ‘gauge’ bosons – the photon (y), the gluon (g), the W boson (W±) and the Z boson (Z0) – all of which have been proved and observed experimentally.
In addition to these primary four, there are two other, much stranger, bosons. These are the Higgs boson (H0) and the graviton (G).

Proton-proton collision at the LHC – the search for the Higgs particle.

Each of these bosons is the force carrier for one of the fundamental forces in the universe.
The photon is the carrier of the electromagnetic force, the W and Z bosons mediate the weak nuclear force and the gluon mediates the strong nuclear force.
The graviton, as it’s name suggests, is theorised to be responsible for the force of gravity but what about the Higgs?
The Higgs boson is postulated to be the fundamental force carrying particle that is responsible for giving mass to all matter. Thats the easy way to put it.
The slightly (and by that I mean a lot!) more complex way of referring to this phenomenon is to say that under the standard model of particle physics, something called the “Higgs Field” gives mass to some fundamental particles via spontaneous symmetry breaking using the Higgs mechanism. Thats pretty mind blowing stuff so I’ll leave that thread right there for the time being. The next post might be my attempt at explaining it a little more clearly – you never know 😉

These forces ……..? What exactly are they? and what do they do?
Good questions.
We might as well start with the ‘biggie’ and the one most people will be familiar with.

Electromagnetism was originally thought to be two separate forces, electricity and magnetism, but is now unified into the single force. It is the force responsible for just about everything in the world around us. It gives shape to all matter through the intermolecular forces between individual molecules. It binds electrons to the atomic nucleus in various ‘shells or orbits’ (not the best description, chemists will know why, but good enough for here) to form atoms which are in turn used to build molecules. This electron binding (and subsequent interacting and releasing) is the basis for all chemistry. Electromagnetism manifests as both electric fields and magnetic fields. Both of these phenomena are simply different aspects of electromagnetism. A changing electric field generates a magnetic field and conversely a changing magnetic field generates an electric field.

What’s up next …..?

Strong nuclear force.
The strong nuclear force, sometimes called the strong interaction, is the force responsible for keeping the nucleus of an atom ‘stable’ for want of a better start point. It is present in two forms. The force that keeps the protons and neutrons bound together in an atomic nucleus and also the force that binds the quarks together to form these two nucleons and other hadrons. The strong nuclear force is about 100x stronger, at an atomic level, than electromagnetism.

Lastly ……

Weak nuclear force.
Weak nuclear force is very short ranged and its bosons (W & Z) primarily do not transmit or mediate a force. Their primary function is to transmutate particles. By exchanging a particle of weak nuclear force (the aforementioned W and Z bosons) electrons go to neutrinos, quarks mix types and a neutron changes into a proton emitting an electron in the process. This last interaction is called Beta Decay, a type of radiation, and is the most commonly used example of the weak nuclear force.
At extremely high energy levels, the weak nuclear force and electromagnetism begin to act the same, and this is called electroweak unification.

Confused ……? Yep, me too. Here are some real world examples that might help.

We have particles that make up matter (stuff like your laptop, the air you’re breathing, the chair you’re sitting on and you yourself) – the protons, neutrons and electrons.

We also have the particles that “cause” (in the loosest sense) forces to work – eg the “photons” of light coming from your screen, the “gluons” present in the nucleus of all matter that prevents its building blocks from flying apart at crazy speeds and the “W and Z bosons” without whose transforming power we would have no radioactivity.

Bang! – thats your bosons done!


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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