Quarks and Leptons

Quarks and Leptons • Announcements • Recitation this weekin lab. BRING QUESTIONS ! • See my by Wed. ifyou have any gradingissues with your exam. • Reading Assignmentsin Particle Adventure(see Schedule link)

p p EM p n Strong Strong 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!

Hadrons Could refer tothese as baryonand anti-baryonif you want Baryons Mesons qqq Hadrons, Baryons and Mesons In 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.

u s d u u u Are there baryons other than protons and neutrons? • The answer is a resounding YES ! • Other quarks can combine to form other baryons. For example: This combination is called a Lambda baryon, or L0 for shortWhat is the charge of this object?) This combination is called a Delta baryon, or D++ for shortWhat’s this one’s charge?

Proton Neutron Note: The neutron differs from a proton only by “d”  “u” quarkreplacement! Let’s make baryons! Quark up down strange Charge Q +2/3 -1/3 -1/3 Mass ~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2] u u u d d d s s s u u d d u d Q = +1M=938 MeV/c2 Q = 0M=940 MeV/c2

Quark up down strange Charge, Q +2/3 -1/3 -1/3 Mass ~5 [MeV/c2] ~10 [MeV/c2] ~200 [MeV/c2] u u u d d d s s s Sigma (S+) Lambda (L) Sigma (S-) Let’s make some more baryons ! u u d d u s s d s Q = -1M=1197 MeV/c2Lifetime~1.5x10-10[s] Q = +1M=1189 MeV/c2Lifetime~0.8x10-10[s] Q = 0M=1116 MeV/c2Lifetime~2.6x10-10[s] These particles have been observed, they really exist, but decay fairlyrapidly. Is S- the antiparticle of S+ ??

d d c s d u 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”). What’s the charge of this particle? What’s the charge of this particle? What’s the charge of this particle? Q= 0, this strangemeson is called a K0 Q= -1, and this charmmeson is called a D- Q=+1, and it’s called a p+ M~500 [MeV/c2]Lifetime~0.8x10-10 [s] M~140 [MeV/c2]Lifetime~2.6x10-8 [s] M~1870 [MeV/c2]Lifetime~1x10-12 [s]

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

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

Charge = -1 e - The Cast of Fundamental Particles + antiquarks +anti-electron(positron) Is nature really like this?

e- m- m=106 MeV/c2 m=0.51 MeV/c2 Muons Recall 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.

1934: To account for the “unseen” momentum in neutron decay: p n e X n  p + e - + X • If this neutrinoin fact existed, one should also observe the reaction: n + p  e+ + n Read as “a neutrino interactswith a proton, producinga positron and a neutron” Neutrino Fermi proposed that the unseen momentum (X) was carried off by a particle dubbed the neutrino (n ). Nobel Laureate: Enrico Fermi

1956: Existence of the neutrino confirmed by puttinga detector near to a prolific source of neutrinos, a nuclear reactor, and observing n+p  e+ + n(Nobel Prize) Neutrino Discovery Photon detectors Fred Reines and Clyde Cowan, 1956 Detector: H2O w/Cadmium Chloride

electron-neutrino muon-neutrino ne nm Neutrinos 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 withelectronsare different particlesfrom the ones associated with muons. Jack Steinberger, Melvin Schwartz and Leon Lederman. 1988 Nobel Prize winners for thediscovery of the “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.

Anti-Lepton Q = +1 Q = 0 e+ ne m+ nm t+ nt Three happy families… • In 1975, researchers at the Stanford Linear Accelerator discovereda third charged lepton, with a mass about 3500 times that of theelectron. It was named the t-lepton. • In 2000, first evidence of the t’s partner, the tau-neutrino (nt)was announced at Fermi National Accelerator Lab. 3 families, just like the quarks… interesting !!!

Anti-lepton (anti-particle) Lepton (particle) positron muon-plus tau-plus electron anti-neutrinomuon anti-neutrinotau anti-neutrino electron muon-minus tau-minus electron neutrinomuon neutrinotau neutrino This all looks Greek to me ?

So here’s the big picture • Quarks and leptons are the most fundamental particles of nature that we know about. • Up & down quarks and electronsare 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.

Quarks and Leptons