Quarks and Leptons

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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)

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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
  cant 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!

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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.
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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?)
Flavor Q/e
u 2/3
d -1/3
s -1/3
This combination is called a Delta baryon, or
D for shortWhats this ones charge?
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Lets 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
d
s
u
u
d
d
s
s
u
u
d
d
u
d
Q 1M938 MeV/c2
Q 0M940 MeV/c2
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Lets make some more baryons !
u
u
d
d
u
s
s
d
s
Q -1M1197 MeV/c2Lifetime1.5x10-10s
Q 1M1189 MeV/c2Lifetime0.8x10-10s
Q 0M1116 MeV/c2Lifetime2.6x10-10s
These particles have been observed, they really
exist, but decay fairlyrapidly.
Is S- the antiparticle of S ??
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Mesons

• Mesons are the 2nd member of the hadron family.
• They are formed when a quark and an anti-quark
  bind together. (Well talk more later
  about what we mean by bind).

Whats the charge of this particle?
Whats the charge of this particle?
Whats the charge of this particle?
Q -1, and this charmmeson is called a D-
Q 0, this strangemeson is called a K0
Q1, and its called a p
M500 MeV/c2Lifetime0.8x10-10 s
M140 MeV/c2Lifetime2.6x10-8 s
M1870 MeV/c2Lifetime1x10-12 s
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Building hadrons
Generations Generations 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.
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Back to the Particle Zoo
So, many of the particles discovered at
accelerator experiments aresimply different
types of baryons and mesons ( qqq or qq )
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The Cast of Fundamental Particles
Generations Generations 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?
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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 electrons mass.
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Neutrino
Fermi proposed that the unseen momentum (X) was
carried off by a particle dubbed the neutrino
(n ).
Nobel Laureate Enrico Fermi
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Neutrino Discovery
Photon detectors
Fred Reines and Clyde Cowan, 1956
Detector H2O w/Cadmium Chloride
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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
withelectrons are different particlesfrom the
ones associated with muons.
Jack Steinberger, Melvin Schwartz and Leon
Lederman. 1988 Nobel Prize winners for
thediscovery of the muon-neutrino
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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.

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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 ts partner, the
  tau-neutrino (nt) was announced at Fermi
  National Accelerator Lab.

Family Leptons Leptons
Q -1 Q 0
1 e- ne
2 m- nm
3 t- nt
3 families, just like the quarks interesting !!!
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This all looks Greek to me ?
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So heres 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.