Fermi Lecture 2
Barry C Barish 17-October-2019
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Frontiers in Physics and AstrophysicsIntroduction to Elementary Particles
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Enrico Fermi
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Enrico Fermi Lectures 2019-2020Frontiers of Physics and Astrophysics
• Explore frontiers of Physics and Astrophysics from an Experimental Viewpoint
• Some History and Background for Each Frontier • Emphasis on Large Facilities and Major Recent
Discoveries • Discuss Future Directions and Initiatives ---------------------------------------------------------------------- • Thursdays 4-6 pm • Oct 10,17,24,one week break, Nov 7 • Nov 28, Dec 5,12,19 • Jan 9,16,23 • Feb 27, March 5,12,19 3
Fermi Lecture 2
Fermi Lectures 2019-2020 - Barry C Barish
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• Course Title: Large Scale Facilities and the Frontiers of Physics • The Course will consist of 15 Lectures, which will be held from
16:00 to 18:00 in aula Amaldi, Marconi building, according to the following schedule:
• 10 October 2019 - Introduction to Physics of the Universe 17 October 2019 - Elementary Particles 24 October 2019 - Particle Accelerators 7 November 2019 - The Higgs Boson 28 November 2019 - The Future of Particle Physics 5 December 2019 - Neutrinos12 December 2019 - Neutrino Oscillations19 December 2019 - Dark Matter and Gravitational Waves (1)9 January 2020 - Gravitational Waves (2) 16 January 2020 - Gravitational Waves (3) 23 January 2020 - Gravitational Waves (4) 27 February 2020 - Topics in Astrophysics and large-scale surveys5 March 2020 - An Introduction to Cosmology and the Early Universe 12 March 2020 - Dark Energy19 March 2020 - The Future
• All Lectures and the supporting teaching materials will be published by the Physics Department.
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Frontiers 1What we know: Constituents of the Standard Model
Discovery announced July 2010
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Frontiers 1Particle Physics: What we Know
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In the last lecture, I discussed the main questions physicists were grappling with at the turn of this century. Some have been answered and some have not, and I will explore many of those during the course.
But, first, let’s look a little at “What We Know!!”
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Frontiers 2What we know: Evolution of the Universe
The universe today is the product of a long chain of events, as shown in this artists conception of cosmological evolution beginning with the big bang. We study both the chronology and the causal connections.
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Frontiers 2What we know: The Vacuum
According to the rules of quantum field theory, the vacuum is not empty but it is actively populated by particle-antiparticle pairs that appear, annihilate, and disappear, existing fo only brief instants.
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Frontiers 2What we know: The Elementary Constituents and Forces
The fundamental particles include both the fermions, the matter particles, and bosons, the force carriers. Masses of all particles are given in GeV/c2, a unit in which the mass of the proton is approximately 0.94, electric charge is listed in units of the electron charge. The Higgs particle discovered after this chart (e.g. 2012)! Is added to the bosons. Also, neutrino mass must be added.
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New Technologies Enable New DiscoveriesFrontiers 2
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Pion Discovery
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DONUT (Direct Observation of NU Tau) July, 2000
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Donald Glazer (1952)
Bubbles form at nucleation sites in regions of higher electric fields
⇒ ionization tracks
Bubble Chamber
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1953
Donald Glaser with his Bubble
Chamber
“Gargamelle” An Event – First Evidence for the Weak Neutral Current
Principle of a Bubble
Chamber
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Liquid superheated by sudden expansion
Bubbles allowed to grow over ∼ 10ms
then collapsed during compression stroke
hydrogen, deuterium, propane Freon
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High beam intensities swamp film
Acts as both target & detector
Slow repetition rate
Track digitization cumbersome
Difficult to trigger
Mechanically Complex
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Discovery of the Omega MinusFrontiers 2
Electric field imposed to prevent recombination
Medium must be chemically inactive
(so as not to gobble-up drifting electrons)and have a low ionization threshold
(noble gases often work pretty well)23
Ionization DetectorsFrontiers 2
signal smaller than initially produced pairs
signal reflects total amount of ionization
initially free electrons accelerated and further ionize mediumsuch that signal is amplified proportional to initial ionization
acceleration causes avalanch of pairsleads to discharge where signal size is independent of initial ionization
minimum ionizing particle
heavily ionizing particle
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t = Lc/β
1/β = ( 1 − 1/γ2 )−1/2
β2 = 1 − 1/γ2
≃ 1 − 1/(2γ2)
Δt ≃ Lc/2 (1/γ22 − 1/γ1
2)
Time Of Flight (TOF): An Application of Prompt Timing
(used to discriminate particle masses)
Δt = Lc (1/β1 − 1/β2)
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Advances in Technology: ComputingGreat Discoveries in Physics
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High Energy Particle Detectors :
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π π±+ → + + + 028 ?p p p p
• New kinds of particles are made out of the kinetic energy: mesons (pions) with mass of 140 MeV each.
• These secondary particles are made prolifically, by the Strong Interactions. Clearly they are not nucleons
• But they are hadrons!
π γ γ→ +0
-16And then the with lifetime of ~10 sec
Frontiers 2Collision of 300 GeV protons on stationary nucleon
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+
0
-
3
Three states:
3 states, so 1 (2 1) 3
Since 0 for pions, then we should have
I I
BQ I
π
π
π
⎛ ⎞⎜ ⎟
= + =⎟⎜ ⎟⎝ ⎠
=
⎜
=
Frontiers 2Can we use Isospin for Pions? YES
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Particles decay when they can ! (That is, the decay is not forbidden!)
The lifetime gets shorter (or decay rate gets larger) with increase of either • energy release • strength of the responsible force
2 /mc τΔ =!
The uncertainty principle requires that the mass of the particle be uncertain by
Frontiers 2Lifetimes are Important
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112 2 10 Mev-cmcmc
c cτ τ τ
−×Δ = = =
! !
proton neutron
π± π0
???????
Mass(MeV)
DecayForce
Lifetmτ (sec)
cτ(cm)
MassUncert.
p 938 None ∞ ∞ 0 MeVn 940 Weak 890 2.6 1013 7.7 10-25
π± 140 Weak 2.6 10-8 780 2.6 10-14
π0 135 EM 8.4 10-17 2.5 10-6 8.0 10-6
.33 10-23 1.0 10-13 200 MeVStrong
Frontiers 2How Uncertain are Particle Lifetimes
π= + − = + +2 2 2 2 2( ) 2cm p p pE E m p m m m E
Experiment colliding beam π and stationary p E , p = pion total energy, momentum
When Ecm~ MΔ , interaction rate increases dramatically
Ecmp
Frontiers 2Experiment Colliding a beam of p on a stationary
target of protons
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The First Resonance Discovered!
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Energy width, Γ, gives lifetime
Kinetic Energy peak, E0-mπ
πΔ = + +2 202p pM m m m E
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"First" Nucleon Resonancehas 4 charge states: Q=+2,+1,0,-1
3/2 3/21232 MeV =120 MeV
=.5 10 sec
J IM
τ
= =
= Γ
×
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Frontiers 2My Thesis – Inelastic Production of the First πN Resonance
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Frontiers 2My Thesis – Inelastic Production of the First πN Resonance
Experimental Setup at the Berkeley 184-inch Cyclotron
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Frontiers 2My PhD Thesis
Inelastic Production of the First πN Resonance
Inelastic Pion Scattering. The data showed climbing the peak, early evidence for producing the Δ(1238)
resonance.
Good enough for a PhD thesis!!
These are just the tip of the iceberg: At least twenty πN states with mass< 2200 MeV. All decay with short lifetimes because they decay by Strong Interactions
Frontiers 2Many Pion + Proton Resonate States
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2 2 2( ) (e.g. ., .
) .p p
M E E p pππ
π π π π+ +
+ −
−
− +
+
= + − +
+ → + + + +! !
ρ-mesonω-meson
About 40 such strongly decaying mesons with mass less than 2500 MeV now observed
Frontiers 2Events with Many Pions, Many Resonances
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3 cm Pb plate
kink
3 cm Pb plate
gaps
First observed (1947) by Rochester and Butler in Cloud Chamber of “vee” events ... (kinks)
Early on there were other “Strange” eventsFrontiers 2
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0 0
0
0
p Kp
K
π
π
π π
−
−
+ −
+ →Λ +
Λ → +
→ +
Force
Strong weak weak
Frontiers 2Bubble Chamber Event Illustrating Strange Events
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0 0Decays
weak int.-1 0 0 +1
0 0 S changes
p Kπ π π− + −Λ → + → +
0 0Production
strong interaction0 0 -1 +1 Strangeness
conser d
ve
p Kπ − + → Λ +
singlet: one charge state doublet: two
charge states: K0 , K+
Frontiers 2Description – Strangeness Quantum Number (S)
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Particle Symbol Charge Strange- ness
Mass(GeV/c2)
Nucleon (p,n) 0, +1 0 0.938Lambda (Λ0) 0 -1 1.115Sigma (Σ+,Σ0,Σ-) -1,0,+1 -1 1.190Xi (Ξ0,Ξ-) -1,0,+1 -2 1.320
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Note that the Isospin (from multiplicity) requires
hypercharge defined
1 ( )
2
Q I B S
Y B S
− = +
≡ +
Frontiers 2Strange Baryon and Meson States (Lowest Mass)
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Frontiers 2History of Particle Discoveries (1930 – 1965)
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weakly decaying
strong decays
Existence of these excited states (or levels) indicates compositeness (like atoms and nuclei)
Frontiers 2There are many Higher Mass (Stongly Decaying) Strange States
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These were exciting times, discovering all of the elementary particles! BUT, so many particles and rules could almost get boring!
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We needed good ideas to bring
order to the field!!!
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Gell-Mann’s “Eightfold Way”Frontiers 2
Murray Gell-Mann proposed that nature had an organizing scheme for the rapidly growing set of elementary particles based on the “eight fold way” For example, the lightest baryons are arranged into a hexagonal array shown below
Murray Gell-Mann 52
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