Quarks and Leptons

Announcements

1. Recitation this weekin lab. BRING QUESTIONS !

2. See my by Wed. ifyou have any gradingissues with your exam.

3. Reading Assignmentsin Particle Adventure(see Schedule link)

Hadrons Hadrons are particles which interact via the strong interaction.(“hadro” is a Greek root for “strong”)

Protons and neutrons bind together in the nucleus because of thestrong interaction. It can’t be electrical force, because protonsrepel each other, and the neutron is electrically neutral.

Clearly, the strong force must be stronger than the EM force, since the EM force tries to push the protons apart, but yet the nucleus stays intact!

np

Strongp

pEM

Strong

Hadrons, Baryons and MesonsIn nature, we find that all particles which contain quarks interact via the Strong Interaction.

This is why protons and neutrons are hadrons; because they contain quarks !

So, all particles which contain quarks (or antiquarks) interact via the strong interaction.

There are two classes of particles which we know about that contain quarks and/or antiquarks.

Hadrons

Baryons Mesons

qqq qqq qq

Could refer tothese as baryonand anti-baryonif you want

Are there baryons other than protons and neutrons?

The answer is a resounding YES !

Other quarks can combine to form other baryons. For example:

us d

This combination is called a Lambda baryon, or 0 for shortWhat is the charge of this object?)

This combination is called a Delta baryon, or ++ for shortWhat’s this one’s charge?

u

uu

Flavor Q/e

u +2/3

d -1/3

s -1/3

Note: The neutron differs from a proton only by “d” “u” quarkreplacement!

Let’s make baryons!

u d s

Charge Q

Mass

+2/3 -1/3 -1/3

~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2]

Quark up down strange

u u d d ss

uu

d

ProtonQ = +1

M=938 MeV/c2

du

d

NeutronQ = 0

M=940 MeV/c2

Let’s make some more baryons !

su

d

Lambda ()

Q = 0M=1116 MeV/c2

Lifetime~2.6x10-10[s]

su

u

Sigma ()

Q = +1M=1189 MeV/c2

Lifetime~0.8x10-10[s]

sd

d

Sigma ()Q = -1

M=1197 MeV/c2

Lifetime~1.5x10-10[s]

u d s

Charge, Q

Mass

+2/3 -1/3 -1/3

Quark up down strange

u u d d ss

~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2]

Is - the antiparticle of + ??

These particles have been observed, they really exist, but decay fairlyrapidly.

Mesons Mesons are the 2nd member of the hadron family.

They are formed when a quark and an anti-quark “bind” together. (We’ll talk more later about what we mean by “bind”).

ud

What’s the charge of this particle?

cd

What’s the charge of this particle?

Q=+1, and it’s called a +Q= -1, and this charmmeson is called a D-

sd

What’s the charge of this particle?

Q= 0, this strangemeson is called a K0

M~140 [MeV/c2]Lifetime~2.6x10-8 [s]

M~1870 [MeV/c2]Lifetime~1x10-12 [s]

M~500 [MeV/c2]Lifetime~0.8x10-10 [s]

Building hadrons

Generations

I II III

Charge = -1/3

d(down)

s(strange)

b (bottom)

Charge = +2/3

u(up)

c (charm)

t(top)

The top quarkdecays before ithas time to forma baryon or meson.

So, one can build many, many possible baryons by combining any of the 3 quarks (5 x 5 x 5 = 125)

One can build many mesons by forming qq combinations: 5x5 = 25

Back to the Particle ZooSo, many of the particles discovered at accelerator experiments aresimply different types of baryons and mesons ( qqq or qq )

The Cast of Fundamental Particles

Generations

I II III

Charge = -1/3

d(down)

s(strange)

b (bottom)

Charge = +2/3

u(up)

c (charm)

t(top)

+ antiquarks

+anti-electron(positron)

Is nature really like this?

e -Charge = -1

MuonsRecall that we discussed a particle called the muon. It was discoveredin cosmic ray experiments (1937).

It was also used in the experimentaltest of time dilation.

We find that a muon behaves almost identical to an electron,except its mass is about 200 timesmore than the electron’s mass.

e

m=0.51 MeV/c2 m=106 MeV/c2

Neutrino

Fermi proposed that the unseen momentum (X) was carried off by a particle dubbed the neutrino ( ).

Nobel Laureate: Enrico Fermi

If this neutrino in fact existed, one should also observe the reaction:

+ p e+ + nRead as “a neutrino interactswith a proton, producinga positron and a neutron”

1934: To account for the “unseen” momentum in neutron decay:

np

eX

n p + e - + X

Neutrino Discovery

Detector: H2O w/Cadmium Chloride

Fred Reines and Clyde Cowan, 1956

Photon detectors

1956: Existence of the neutrino confirmed by puttinga detector near to a prolific source of neutrinos, a nuclear reactor, and observing +p e+ + n (Nobel Prize)

Neutrinos

Jack Steinberger, Melvin Schwartz and Leon Lederman. 1988 Nobel Prize winners for thediscovery of the “muon-neutrino”

In 1962, an experiment was conducted at BrookhavenNational Lab (Long Island).

The researchers wanted to knowif there is more than one type ofneutrino, or are there more?

They found in fact that theneutrinos associated withelectrons are different particlesfrom the ones associated with muons.

e

electron-neutrino

muon-neutrino

Leptons

The electron, the muon and their neutrinos, like the quarks, appear to be fundamental. That is, so far, we are unable to findthat they are made up of anything smaller.

However, they behave very differently than the quarks. They have integral charge (0 or ±1). They do not “bind” to form hadrons. They do not participate in the strong interaction.

The electron, muon and neutrino belong to a general classof particles called LEPTONS.

Three happy families… In 1975, researchers at the Stanford Linear Accelerator discovered

a third charged lepton, with a mass about 3500 times that of theelectron. It was named the -lepton.

In 2000, first evidence of the ’s partner, the tau-neutrino () was announced at Fermi National Accelerator Lab.

Family Leptons

Q = -1 Q = 0

1 e- e

3 families, just like the quarks… interesting !!!

Q = +1

e+

Q = 0

e

Anti-Lepton

This all looks Greek to me ?

e

e

electron

muon-minus

tau-minus

electron neutrino

muon neutrino

tau neutrino

Lepton (particle)

e

e

positron

muon-plus

tau-plus

electron anti-neutrino

muon anti-neutrino

tau anti-neutrino

Anti-lepton (anti-particle)

So here’s the big picture Quarks and leptons are the most fundamental particles of nature that we know about.

Up & down quarks and electrons are the constituents of ordinary matter.

The other quarks and leptons can be produced in cosmic ray showers or in high energy particle accelerators.

Each particle has a correspondingantiparticle.