How Many Types of Particle Are There?

A lot more than this...

Crying your pardon
    Firstly, an apology.  I’ve been completely inactive on my website for the past month.  This is partly because I was preparing for the Institute of Physics annual public Science lecture (which I delivered on November 22nd to a gracious and patient audience). 
    Last year I was able to transcribe and summarise the lecture in a couple of blogs but this year I’m afraid that wouldn’t be possible.  The main topic covered was the Standard Model of Particle Physics and that's not easy to describe in an essay.  Personally, I foam at the mouth with excitement when the whole topic of particle behaviour is discussed, but apparently some plebs don’t share my excitement.  As a result, there was a lot of stuff I had to edit out of my lecture...stuff I’m now going to subject you to.
    Full disclosure: this will be a self-indulgent blog that will bore many of my readers.  I’m doing it anyway because it’s my website and I freaking love this stuff.  How dost thou like them apples?

Jerry likes them just fine

Seventeen is a Magic Number...Apparently
     I once recorded a barbershop-quartet song I wrote about the standard model of particle physics (cos I’m just that awesome) but if you’ve not come across it before, the standard model is usually depicted like the grid above or sometimes in a wheel like this:

    A particle is something which holds itself together.  Technically, this means you are a particle because your body doesn't fall apart spontaneously.  If you want to separate a human into pieces then it can be done obviously, it just requires effort (energy) to do so.  This means you aren't a "fundamental" particle because you have a structure i.e. you're made of smaller particles.
    A fundamental particle is something which holds itself together but has no internal architecture; they are the tiniest nuggets of stuff and aren't made of smaller bits.  As strange as it sounds, you can’t chop these particles in half because there literally is no half for them to be chopped into. 
    What’s more, we’re fairly confident this really is the bottom rung of the ladder.  We have lots of reasons to suspect these particles are the true building blocks of the Universe, which means every object or process you can think of is the result of interactions between the particles listed above.  With the exception of gravity (which doesn’t play nicely) what you see is the alphabet of reality.  Well...almost. 
    In truth, the seventeen particles of the standard model are not the whole story.  Nature is rarely so considerate or simple.  In fact, she seems to have a complete disregard for what humans will find intuitive and tends to prefer intricate complexity wherever possible.  It’s almost like our brains evolved for the purposes of hunting and breeding rather than conceptualising the quantum-mechanical nature of reality.

"Let me get this straight, some of them STILL favour the Copenhagen interpretation??"

Divide and Describe
     Asking how many types of particle there are is like asking how many types of human there are.  If you speak to a contact-lens designer they might say five: brown-eyed, blue-eyed, grey-eyed, green-eyed and hazel-eyed humans.  That’s not wrong, but it would be useless information to a hemotologist.  They might say there are eight types of human based on the blood groups A+, A-, B+, B-, AB+, AB-, O+ and O-. 
     To give a full picture that includes both properties, we might therefore say there are really forty types of human: blue-eyed people for each of the eight blood groups, brown-eyed people for each of the eight blood groups and so on.  But we could always subdivide again based on something like hair colour for instance – blonde, brunette and ginger – to yield 120 types of human. 
    We could categorise and cross-categorise the human population according to gender, sex, sexuality, skin-colour, language, dietary habits or whatever else we felt like.  The sheer number of possible “human particles” is staggering because there are so many different properties available.  A similar complication arises when we want to describe particles of the Universe...which is a much better way to spend the time - after all, putting humans into categories is sort of frowned upon these days.

Can't think why

Let’s Just Say There Are Five
     For the sake of clarity I’m going to say there are five main properties/characteristics a particle can have.  This isn’t the whole story, but a lot of particle properties aren't independent of each other so we can count them as one thing.  For example, a tiger has the property of orange stripes and also the property of black stripes.  Those are distinct things but logically they must occur together.  We can group both properties into one and say tigers have the property of being stripey. 
    Also (for the quantum physicists among you) I'm going to totally ignore superpositions and list only particle types whose eigenstate has been measured.  If you don’t like it, see above comment about apples. 

Here are the five main properties I'm going to consider:
 
Mass: This property has several different meanings but I'm going to take the simplest one.  Particle mass means roughly the same thing it does in everyday life: a measure of how heavy a particle is or how reluctant it is to change trajectory.  For fundamental particles it can take any value from 0 up to 0.02 milligrams (anything above that is physically impossible).

Charge: This property means how willing a particle is to be around other particles with the same property.  It comes in two varieties called positive and negative.  Particles with opposite charges will attract while particles with identical charges repel.  Particles that have zero charge are unaffected by particles with the other charges. 
 
Colour: This one is a little harder to visualise because it doesn’t compare to anything in our everyday world.  The name is also misleading because it doesn’t refer to the appearance of a particle (it’s just a word we use to describe it) it actually refers to whether a particle can be separated from other particles with corresponding properties.  Particles with zero colour are able to move around on their own but particles with colour must clump together in specific arrangements.  The interactions and types of colour available are quite complicated so I won’t go into detail here, although my recent Instagram post (@timjamesScience) explains the basics if you're curious.  The important fact is that unlike charge which comes in two varieties, colour comes in combinations of three: red, green and blue.

Spin: Like colour, spin is a misleading name because it describes a property we don't have a way of visualising yet.  It was originally believed that small particles were literally spinning as they moved through space.  All particles had a spin and, if the particles also had charge, the two properties combined to form magnetism (non-charged particles still had spin, but weren't magnetic).  It was then discovered that particles are not literally balls rotating in space, but the word had stuck. 
    Spin values can either be whole numbers or half-numbers (technically whole multiples or half multiples of a specific number called h but we won't worry about that).  Particles with whole-number spin are able to occupy the same physical location as each other without interacting - think of two beams of light overlapping - and we call these particles bosons.  Those with half-number spin will stack against each other, like your body and the chair you are sitting on, and we call these particles fermions.  As well as having a numerical valule, spin also comes in two varieties called up and down (like positive and negative charges) as well as sometimes, for bosons, zero.

Chirality: Chirality is yet another property we can't easily visualise.  It's also difficult to quickly describe what it does.  Charge is all about repulsions and attractions, colour is all about whether particles can be independent or go around in groups and so on.  Chirality's main feature takes some serious explaining.  I'm going to make the decision to skip over it therefore, because this  blog is already too chunky.  The key feature of chirality is that every particle has it, and it comes in two varieties called left-handed and right-handed.  E-mail me if you want to know more. 


    At this point, many particle physicists will be mashing their teeth in anger at how glibly I'm summarising the various properties, ignoring a lot of the subtlety.  For example, chirality and mass are closely related, I've brushed over things like isospins and hypercharges.  And then there's a whole other property called helicity which (confusingly) also has left and right-handed versions.  This property relates to the spin but changes depending on how you view the particle.  My five properties thing is a fudge but if you don't like it, see aforementioned apples comment. 
    What I'm going to say is that from these five properties we can describe all the known particles available in the Universe.  We might not be able to visualise what the particles are actually doing to give them these properties but at least there are only five things to take into consideration.  While human particles are easier to visualise, there's so much variety it becomes impossible to explain their behaviour. 

Bosons (Spin is a whole number)

Photons - The simplest particles of all.  They have no mass, no charge and no colour.  They have two different spin possibilities (up and down) as well as two chiralities (left and right) giving us four types of photon in total: Up Left, Up Right, Down Left, Down Right.
 
Z’s – Z particles have no colour and no charge but they do have mass.  They can also have one of the three spin values (Up, Down, None) and both chiralities, giving us six: Up L, Up R, None L, None R, Down L, Down R.
 
W’s – W’s are similar to Z’s.  They have mass, three spins and two chiralities, but they also come in two different charges, positive and negative, meaning there are twelve W particles: UpLPositive, UpLNegative, UpRPositive, UpRNegative, NoneLPositive, NoneLNegative, NoneRPositive, NoneRNegative, DownLPositive, DownLNegative, DownRPositive, DownRNegative.
 
Gluons – Gluons have no mass or charge but they do have colour in eight versions (see my Instagram post) two spins and two chiralities, giving us a total of thirty-two gluons, which I'm not going to write out.
 
Higgs – The Higgs boson particle has mass but no colour or charge.  It has a spin of 0 which makes the count a little simpler, but both chiralities, giving us only two types: Left-handed Higgs and Right-Handed Higgs.

Fermions – (Spin is a half)
 
Quarks – These particles have all five properties.  There are six different quark masses available, each with a different name: up, down, charm, strange, top and bottom (NB: the quark names of "up" and "down" are unrelated to their spin i.e. you can have an Up-quark with a down spin.  Yeah...I know).  Up, charm and top quarks have a charge of +2/3 whereas down, strange and bottom quarks have a charge of -1/3.  
    Quarks also possess one of three colours (red, green or blue) giving 18 types so far.  Quarks can also come in charge-reversed versions called anti-quarks.  Anti-up, anti-charm and anti-top quarks have charges of -2/3 while anti-down, anti-strange and anti-bottom have charges of +1/3 (the reverse of the “ordinary” quarks).  This gives us 36. Then we have either up or down spin, giving us 72, and then the left and right handed chiralities, giving us a grand total of 144 possible quark types.

Leptons – These fermions possess no colour.  There are three "flavours" with mass, called the electron, the muon and the tau each with a charge of either -1 or +1 (six so far).  Then there are the up and down spins (giving us twelve) and then the left and right chiralities (twenty four).
    The remaining leptons have no charge (possibly no mass either) and are called neutrinos.  Neutrinos come in three flavours: electron-neutrinos, muon-neutrinos and tau-neutrinos.  There are also anti-neutrinos for each, but because neutrinos have a charge of zero you can almost think of anti-neutrinos as having a charge of anti-zero.  I mean, you probably shouldn't think of it like that...I'm just saying you could.  Then of course you factor in the up and down spins, giving us twelve neutrino types so far.
    What’s really strange (apart from the fact that they sort of  have mass and sort of have anti-zero charge) is that all neutrinos are right-handed and all anti-neutrinos are left-handed.  This means we don't have to double our neutrino number because when we counted the anti-neutrinos, we already took chirality into account by accident...nature's weird.  So that's twelve neutrinos.

The Grand Total

Photons x 4
Z's x 6
W's x 12
Gluons x 32
Higgs's x 2
Quarks x 144
Electrons x 24
Neutrinos x 12

236 different types of particle
 
Is That It?
    In all honesty we don’t know.  These are the 236 types of particle which definitely exist and which can’t be broken down, but there is absolutely the possibility, in fact the likelihood, of there being more.  For instance, left-handed neutrinos and right-handed anti-neutrinos have never been discovered but it seems reasonable to suggest they exist somewhere.
   There are lots of other hypothesised particles which many physicists believe may be real but haven’t been discovered.  Things like the graviton (the particle responsible for causing gravity), the inflaton (the particle which played a key role in the early expansion of the Universe) or the Majorana fermion (which does things).
    Then there are the so called quasi-particles, some of which don’t exist independently but merge with already existent particles (like Goldstone bosons) some of which aren’t  self-contained units but act as if they are (like phonons) and some of which don’t exist at all but we act as if they do to make the equations neater (like Popov-ghosts).  In short, the Universe is awfully big and awfully complicated.  The standard model we have now is probably only a glimpse of what nature has in store.

Image credits
Standard Model Grid: businessinsider
Jerry Smith: pinimg
Standard Model Wheel: Symmetrymagazine
Ceasar: comicvine
Nick Griffin: guim
Insane Clown Posse: wennermedia