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