physics particles

7/27/2019 Physics Particles

1/5

It is important to note thatall of the diagrams on amolecular and atomicscale are representationsand not pictures. It isnot possible to seeimages of particles sosmall.

Atoms are often arranged in relatively simple repeated patternsto form crystals and in arrangements which can be verycomplex to form large molecules. Atoms were regarded as thesmallest fundamental particles up to the end of the 19th century.

J J Thompson,Rutherford andMillikan are significantnames in theinvestigation of atomicstructure culminating inthe idea that atomswere constructed from

the fundamentalparticles of protons,neutrons in a very smallcentral nucleus withnegatively chargedelectrons distributed in energy levels around this centre.Although these ideas have been developed further they arestill very important to current chemistry and physics.

In describing the structure of the atoms we use several key definitions which all A level students should know:

The proton number or atomic number is the number of protons in the nucleus. This number absolutely defines the chemical properties of the atom; itdefines the element. The number of electrons in an electrically balanced atom is exactly equal to the number of protons.

The nucleon number or atomic mass gives the total number of protons and neutrons in the nucleus.It follows therefore that the number of neutrons = nucleon number - proton number.

Atoms may have many isotopes. These are versions of the element that have identical chemical properties (the same number of protons) but a differentnumber of neutrons, and therefore a different mass. The number of neutrons is the major determining factor on the stability of the nucleus, i.e. indeciding if the atom is likely to be radioactive.

Atoms and molecules can be ionised. In becoming an ion they either gain or lose one or more electrons. If they gain an electron they becomenegatively charged. If they lose an electron they become positively charged.

Fundamental particles

an introductionThe Fizzics Organisation


2/5

As research progressed over the last half of last century it became clear that some of theseparticles were themselves made of smaller bits.

So far it has not been possible to split up an electron - that is still thought of a one of thefundamental particles.

We currently think that there are two types of fundamental particles - quarks and leptons

The quark seems to be one of the basicconstituents of matter. Six types have beenidentified but two are particularly common. Thesix types are quaintly known as known asflavours: up, down, charm, strange, top, andbottom. (I suppose they had to be calledsomething). The up and down varieties survivein large quantities forming protons andneutrons.

The other four have brief life spans as do theparticles they form.

In addition every quark has an antiparticle - asort of mirror image with the same mass but anopposite charge. In total there are therefore 12quarks.

Three types or flavours of lepton (or six, if youcount the corresponding neutrinos separately ortwelve if you count antiparticles!) have beenidentified: the electron, the muon, the tau lepton or tauEach flavor consists of a pair of particles called aweak doublet. One of this pair is a relativelymassive and charged particle that has the samename as its flavor (such as the electron).

The other is a neutral particle of very tiny masscalled a neutrino (for example the electron neutrino).

And of course there are corresponding antimatterleptons and netrinos.

The antimatter electron has a special name - thepositron.

The electron and the positron are stable (unless they

meet of course) as are all the neutrinos.

u d

t b

sc

Up Down

Charm Strange

Top Bottom

charge charge+ 2/3 - 1/3

Hadrons are all made of of quarks, again split intotwo into two groups:

The baryons are the family of subatomic particlesall of which are made of three quarks. The familynotably includes protons and neutrons, which makeup the atomic nucleus, but many other unstablebaryons exist as well. Mesons, made up of a quark and an antiquark pair.

Note- if a particle meets its antimattermirror image then they mutually

destruct producing energy.

Quarks

and

leptonsThe Fizzics Organisation


3/5

d

u u

d

udProton Neutron

The proton is made from twoup quarks, charge +2/3 and onedown quark,charge -1/3. The total charge is therefore 2/3 + 2/3 -1/3 = +1

The neutron consists of two down and one up quark, the charges canceling toa total of zero.

The quarks themselves have a relatively small mass.

The repulsion of the like charges of the quarks is overcome by particles calledgluons. These glue the quarks together with the strong force whichovercomes the electromagnet force of repulsion between like charges.

One example of a meson is a pion (+), which is made ofan up quark and a down anitiquark. The antiparticle of ameson just has its quark and antiquark switched, so anantipion (-) is made up a down quark and an up antiquark.

Because a meson consists of a particle and an antiparticle, itis very unstable. The kaon (K-) meson lives much longer

than most mesons, which is why it was called "strange" andgave this name to the strange quark, one of its components.

However, all mesons have brief lives measured innanoseconds or less.

ImageArpad Horvorth

Wikipedia

Pion

QuarksThe Fizzics

Organisation


4/5

Beta plus and beta minus decay requires a change in quark character.

Repeating the equation for beta minus decay:

The weak interaction converts a neutron into a proton while emitting an electron and an anti-neutrino.

At the fundamental level (as depicted in the Feynman diagram to the right), this is due to the

conversion of a down quark to an up quark by emission of a W boson; the W boson subsequentlydecays into an electron and an anti-neutrino.

In beta plus decay, energy is used to convert a proton into a neutron, a positron and a neutrino:

energy + p n + e+ + eSo, unlike beta minus decay, beta plus decay cannot occur in isolation because it requires energy,the mass of the neutron being greater than the mass of the proton. Beta plus decay can only happeninside nuclei when the absolute value of the binding energy of the daughter nucleus is higher thanthat of the mother nucleus. The difference between these energies goes into the reaction ofconverting the particles and into the kinetic energy of these particles. Other than that, the sequenceis similar - a mirror image.

Above the Feynman diagram forbeta decay of a neutron into aproton, electron, and electron

antineutrino via an intermediateheavy W- boson.

Below the Feynman diagram forbeta plus decay of a proton intoa neutron.

Within the proton (1) a down quark changes to an up quark(2) creating a force carrier W-which travels outside thenucleus (3).The force carrier W-becomes an electron and an electronneutrino and the neutron has become a proton (4&5)

InteractionsThe Fizzics

Organisation


5/5

Baryon number and conservation

In stable units quarks are always present inthrees, if antiquarks are counted as "negativequarks". Historically, baryon number wasdefined long before the current model ofquarks was established, (e.g. that the total

number of protons and neutrons is conserved)so rather than changing the definition, particlephysicists simply divided by three. Nowadays itmight be more appropriate to speak of theconservation of quark number.

The lepton number is the number of leptons minus thenumber of antileptons.

In equation form,

Analysis of particle interaction relies on lepton number

conservation: the lepton number stays the same throughan interaction. For example, in the beta decay:

Strangeness, denoted as S, is a property of particles, expressed as a quantum number fordescribing decay of particles in strong and electro-magnetic reactions, which occur in a shortperiod of time. The strangeness of a particle is defined as:

where represents the number of strange anti-quarks and represents the number ofstrange quarks.

Interaction RulesOK

The Fizzics

Organisation

physics particles

Documents