Are you a bo­son bozo? Do quarks leave you quizzi­cal? Do glu­ons get you un­stuck? CATHAL O’CON­NELL has a guide to the zoo of par­ti­cles, known as the Stan­dard Model.

Cosmos - - Particle Physics - CATHAL O’CON­NELL is a sci­ence writer based in Mel­bourne. IMAGES 01 At­las Col­lab­o­ra­tion / CERN 02 Missmj/wiki­me­dia Com­mons

AROUND THE TURN of the 4th cen­tury BC, the Greek philosopher Dem­ocri­tus caught the smell of bak­ing and thought that lit­tle bits of bread must be float­ing through the air and into his nose. He called the lit­tle bits “atoms” (mean­ing “un­cut­table”) and imag­ined them as tiny spher­i­cal balls.

But atoms are not lit­tle solid spheres. They are made of even smaller bits, called par­ti­cles. Sci­en­tists’ best de­scrip­tion of those par­ti­cles and the forces that gov­ern their be­hav­iour is called the Stan­dard Model of par­ti­cle physics, or just “The Stan­dard Model”.

The Stan­dard Model cat­e­gorises all of the par­ti­cles of na­ture, in the same way that the pe­ri­odic ta­ble cat­e­gorises the el­e­ments. The the­ory is called the Stan­dard Model be­cause it is so suc­cess­ful it has be­come “stan­dard”.

And no, there is no Econ­omy Model, nor a Deluxe one. There are, how­ever, still a few kinks to be ironed out (and a cou­ple of whop­ping omis­sions). That’s why it is some­times called the “The­ory of Al­most Every­thing”.


Back in the early 20th cen­tury, sci­en­tists thought there were only three fun­da­men­tal par­ti­cles in na­ture: pro­tons and neu­trons, which make up the nu­cleus of an atom, and elec­trons that whizz around it.

But in the 1950s and 1960s physi­cists started smash­ing these par­ti­cles to­gether and some of them broke. It turned out the pro­tons and neu­trons had even smaller par­ti­cles in­side them.

Many dozens of new par­ti­cles were dis­cov­ered – and for a while no­body could ex­plain them. Physi­cists called it the “par­ti­cle zoo”.

In the 1970s, physi­cists such as Mur­ray Gell-mann found an or­der amongst the chaos. The step they took was sim­i­lar to the one Rus­sian chemist Dmitri Men­deleev took to find an or­der to the chem­i­cal el­e­ments in his pe­ri­odic ta­ble.

The new or­der­ing of the par­ti­cles ex­plained many of the prop­er­ties of the newly dis­cov­ered par­ti­cles, as well as cor­rectly pre­dict­ing some new ones.


The par­ti­cles of the Stan­dard Model make up one big fam­ily. Your first in­tro­duc­tion can be daunt­ing, a bit like at­tend­ing a gath­er­ing with a lot of dis­tant cousins you’ve never heard of. No mat­ter how strange these cousins are, it is im­por­tant to re­mem­ber that they are all re­lated.


Gell-mann and oth­ers placed the par­ti­cles in two main cat­e­gories: fermi­ons and bosons. Fermi­ons make up the stuff we call mat­ter. Fermi­ons are di­vided into two kinds of par­ti­cles, de­pend­ing on the forces they feel: quarks and lep­tons. Bosons trans­mit forces.


Par­ti­cles com­mu­ni­cate with one an­other via four forces: elec­tro­mag­netism, the strong force, the weak force and grav­ity. The Stan­dard Model de­scribes the first three (grav­ity does not fea­ture in the Stan­dard Model, as will be ex­plained be­low).

Dif­fer­ent par­ti­cles com­mu­ni­cate through dif­fer­ent forces, sim­i­lar to the way peo­ple can com­mu­ni­cate in dif­fer­ent lan­guages. For ex­am­ple, only the quarks speak “gluon”. While elec­trons can speak “pho­ton” as well as “W bo­son” and “Z bo­son”.

Elec­tro­mag­netism is the force that holds elec­trons in an atom. It is com­mu­ni­cated by pho­tons.

The strong force keeps the nu­clei of atoms to­gether. With­out it, ev­ery atom in the uni­verse would spon­ta­neously ex­plode. It is com­mu­ni­cated by glu­ons.

The weak force causes ra­dioac­tive de­cay. It is trans­mit­ted by W and Z bosons. QUARKS: (the purple par­ti­cles in the fig­ure) come in six “flavours”, all with weird names. It’s use­ful to see them as com­ing in pairs to make three gen­er­a­tions. These are “up” and “down” (first gen­er­a­tion), “charmed” and “strange” (sec­ond gen­er­a­tion) and “top” and “bot­tom” (third gen­er­a­tion).

Only the up and down quarks are im­por­tant in dayto-day life be­cause they make pro­tons and neu­trons.

The oth­ers make only “exotic” mat­ter, which is too un­sta­ble to form atoms. Physi­cists can cre­ate exotic mat­ter in par­ti­cle ac­cel­er­a­tors, but it usu­ally only lasts a frac­tion of a sec­ond be­fore de­cay­ing.


There are six lep­tons. The best known is the elec­tron, a tiny fun­da­men­tal par­ti­cle with a neg­a­tive charge.

The muon (sec­ond gen­er­a­tion) and tau (third gen­er­a­tion) par­ti­cles are like fat­ter ver­sions of the elec­tron. They also have neg­a­tive elec­tric charge, but they are too un­sta­ble to fea­ture in or­di­nary mat­ter.

And each of these par­ti­cles has a cor­re­spond­ing neu­trino, with no charge.

Neu­tri­nos de­serve a spe­cial men­tion be­cause they are per­haps the least un­der­stood of all the par­ti­cles in the Stan­dard Model.

They are fast but in­ter­act only through the weak force, which means they can eas­ily zip straight through a planet. They are cre­ated in nu­clear re­ac­tions, such as those pow­er­ing the Sun’s core.


Now that we know the fun­da­men­tal par­ti­cles of na­ture, we can be­gin to stack them to­gether in dif­fer­ent ways to make big­ger par­ti­cles.

The most im­por­tant com­pos­ite par­ti­cles are the baryons, made of three quarks. Pro­tons and neu­trons are kinds of baryon. The Euro­pean Or­gan­i­sa­tion for Nu­clear Re­search’s big­gest par­ti­cle col­lider smashes pro­tons to­gether. Be­cause pro­tons are a kind of hadron, it’s called the Large Hadron Col­lider, or LHC


As far as we know, all quarks and lep­tons have twin par­ti­cles of antimatter. Antimatter is like mat­ter ex­cept it has the op­po­site charge. For ex­am­ple, the elec­tron has a coun­ter­part that’s ex­actly the same mass, ex­cept with pos­i­tive charge in­stead of neg­a­tive. When a par­ti­cle of mat­ter meets its antimatter twin, they both an­ni­hi­late in a burst of pure energy.

Antimatter is in­cred­i­bly rare in the Uni­verse, although it does have some im­por­tant roles in tech­nol­ogy. Positron emis­sion to­mog­ra­phy (PET) scan­ners, for in­stance, use the an­ni­hi­la­tion of positrons to see in­side the body.

One of the great mys­ter­ies of physics is why the Uni­verse is made al­most en­tirely of mat­ter. Many par­ti­cle physi­cists are striv­ing to an­swer it.


The bread that Dem­ocri­tus sniffed is made of only the first gen­er­a­tion of fun­da­men­tal par­ti­cles.

Up and down quarks bind to­gether through the strong force to make pro­tons and neu­trons, and the strong force also sticks them to­gether to form the nu­cleus of an atom.


You prob­a­bly no­ticed the loner off to the right side of the Stan­dard Model par­ti­cle ta­ble – the Higgs bo­son. The Higgs is a spe­cial kind of par­ti­cle that gives the other fun­da­men­tal par­ti­cles their mass.

The idea is that there is a field ex­ist­ing everywhere in space. When par­ti­cles move through space, they tend to bump into this field, and this in­ter­ac­tion slows

them down (sim­i­lar to how it’s more dif­fi­cult to move through wa­ter than air). This in­ter­ac­tion is what gives fun­da­men­tal par­ti­cles their mass.

Some par­ti­cles such as pho­tons and glu­ons don’t in­ter­act with the Higgs field, so are mass­less.

Just as pho­tons com­mu­ni­cate the elec­tro­mag­netic force, the Higgs Bo­son com­mu­ni­cates the Higgs Field. The Higgs Bo­son was a the­o­ret­i­cal par­ti­cle un­til 2013 when it was at last dis­cov­ered, us­ing the LHC, although sci­en­tists are still un­cov­er­ing its prop­er­ties.


The big­gest hole in the Stan­dard Model is the lack of grav­ity. The fourth force of na­ture just does not fit into the cur­rent pic­ture.

Grav­ity is also in­cred­i­bly weak com­pared to the other forces (the strong force is 100,000,000,000,000,0 00,000,000,000,000,000,000,000 times stronger than grav­ity, for ex­am­ple).

Some physi­cists think grav­ity is also trans­mit­ted by a kind of par­ti­cle, called a gravi­ton, but so far there is no ev­i­dence that this par­ti­cle ex­ists.


The neu­trino is so tiny com­pared to all the other par­ti­cles that it re­ally begs an ex­pla­na­tion. It’s pos­si­ble that the neu­trino doesn’t get its mass from the Higgs in the same way other par­ti­cles do.


For ob­serv­ing the uni­verse, it looks like a huge por­tion of it is made of Dark Mat­ter – a new kind of stuff that doesn’t in­ter­act with reg­u­lar mat­ter and so is prob­a­bly miss­ing from the Stan­dard Model en­tirely.


Some physi­cists are look­ing for ex­ten­sions to the Stan­dard Model to ex­plain these mys­ter­ies. Su­per­sym­me­try is one ex­ten­sion where ev­ery par­ti­cle has an­other twin with higher mass.

Some of these par­ti­cles would in­ter­act very weakly with or­di­nary stuff and so could be good can­di­dates for Dark Mat­ter.

An over­view of the var­i­ous fam­i­lies of ele­men­tary and com­pos­ite par­ti­cles, and the the­o­ries de­scrib­ing their in­ter­ac­tions. Fermi­ons are on the left, bosons are on the right.

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