Email: ph116@u.washington.edu

Physics 116Session 41NeutrinosDec 8, 2011

Nobel Prize in Physics 1995Awarded to Fred Reines "for pioneering experimental contributions to lepton physics"

Reines & Cowan at work, 1956

Announcements • Final exam: Monday 12/12, 2:30-4:20 pm• Same length/format as previous exams (but you can have 2 hrs)• Kyle Armour is away this week; see TAs in study center• JW will have extra office hours Thu-Fri this week:

• 12:45-1:15pm before class, • 2:30-3pm after class (my office B303 PAB, or B305 conf room next door)

• Practice questions and formula pages posted, review tomorrow

Announcements PHYS 248: A new general-education physics course you might be interested in…

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Lecture Schedule (to end of term)

Today

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The “Standard Model” of Particle Physics

Basic ingredients of matter are the fundamental particles: quarks and leptons

6 quarks6 leptons

+ their antiparticles(Symmetry!)

These types of particles are called 'fermions'

(from http://www.fnal.gov)

Fundamental forces are mediated by photons, gluons, Z’s and W’sThese types of particles are called 'bosons'

(after Enrico Fermi)

(after Satrendyanath Bose)Le

pton

s

Last time…

Now let’s look at those leptons…

I work on 2 projects in Japan, studying physics of neutrinos:

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• Super-Kamiokande (since 1995)• Multiple physics goals:

– Study interactions of high energy neutrinos from earth’s atmosphere– Watch for evidence of proton decay (>1033 yr half-life !)

• But Super-K contains 1033 protons…

– Watch for neutrinos from a supernova– Neutrino astrophysics

• Look for distant galaxies emitting beams of neutrinos

– Far detector for T2K…

• T2K (since 2006)• neutrino “oscillations” studies

– Generate a beam of muon neutrinos with particle accelerator– Sample the beam to check its properties (“near detector”)– Send it through the Earth 300 km (takes about 0.001 sec)– See if particles come out still muon-flavored

• Count how many change flavors (using the “far detector”)

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Q: What are neutrinos?

• Neutrinos = subatomic particles with:– no electric charge– (almost) no mass– only “weak force” interactions with matter

That doesn't sound very interesting! But…– neutrinos are made in (almost) every radioactive decay– neutrinos are as abundant as photons in the Universe

• Several hundred per cm3 everywhere in the Universe– even though they are nearly massless, they make up a significant

proportion of the mass in the Universe!

• You are emitting ~ 40,000 neutrinos/sec right now (40K decays)• Neutrinos can penetrate the entire Earth (or Sun) without blinking

– maybe we can study earth's core with neutrinos?– astronomical window into places we can't see with light

Symbol: ν(Greek letter nu)

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Q: Where do neutrinos come from?

• Radioactive decays = 'weak nuclear force' in action– Example: beta decay of neutron

• 'beta ray' = old term for electron

– another example: muon decay

neutron (lepton number = 0) proton (lepton number = 0)

electron (lepton number = +1)

anti-ν (lepton number = -1)(must be anti to conserve lepton #)

µ- (lepton number = +1)

ν (lepton number = +1)

electron (lepton number = +1)

anti-ν (lepton number = -1)

lepton number = conserved physical property (new kind of 'charge') that only leptons have

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Q: Who said we need them?

• Wolfgang Pauli, 1930~33

– If β-decay of nuclei produces only 2 particles (electron and daughternucleus), it does not seem to conserve momentum!

• Emitted electrons can have any energy up to maximum allowed by conservation of energy (EMAX = [parent mass - daughter mass]*c2)

• Pauli: There must be a neutral, ~ massless 3rd particle emitted– Fermi suggested the name 'neutrino' = little neutral one

Pauli (with Heisenberg and Fermi)

Electron energies observed. Energy released is 18 keV. But usually electron carries away much less!!

"I've done a terrible thing - I've invented a particle that can't be detected!" - Pauli

beta decays of tritium (H3)

…but he was wrong!

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Q: How were they first ‘seen’?

• Fred Reines and Clyde Cowan, 1956– ν source: initially, nuclear reactor in Hanford, WA (later they

moved to more powerful Savannah River reactor in South Carolina)

– Detector: water with CdCl2– inverse beta decay:

p n eν ++ → +Observed light flashes from e+ annihilationfollowed by decay of neutron

Nobel Prize in Physics 1995Awarded to Fred Reines "for pioneering experimental contributions to lepton physics"

Super-Kamiokande and T2K, in Japan

Super-Kamiokande Underground Neutrino Observatory• In Mozumi mine of Kamioka Mining Co, near Toyama City• Detects natural (solar, atmospheric) and beam (T2K) neutrinos

T2K (Tokai to Kamiokande) long baseline experiment• Neutrino beam is generated and sampled at Tokai (particle physics

lab, near Tokyo)• Beam goes through the earth to Super-K, 300 km away

ToyamaToyama

SKSKSK TokaiTokai

Super-Kamiokande

• US-Japan collaboration • (~100 physicists)• 1000 m of rock overhead to

block cosmic ray particles• 50,000 ton ring-imaging

water Cherenkov detector• Inner Detector: 11,146

phototubes, 20” diameter • Outer Detector: 1,885

phototubes, 8” diameter

Control Room

InnerDetectorOuter

DetectorMt. Ikeno

Entrance 2 km

Water System

Tank

Linac cave

Electronics Huts

• 50,000 cubic meters of ultra-pure water• Neutrino interactions make charged particles in water• Began operation in April, 1996• Published first evidence for neutrino mass in June, 1998• Typically records about 15 neutrino events per second

40m tall

Just how big is Super-K?

• Checking photomultiplier tubes by boat as the tank fills (1996)

View into Super-K from tank top: an application of the photoelectric effect

• Each photomultiplier tube is 20 inches in diameter!

Cherenkov light in water: applying ph116 optics

• Neutrino interacts in a nucleus in the water (oxygen or hydrogen)

• Produces a charged muon or electron, which carries an electromagnetic field

• Muon travels at v ~ c, but light travels at v=c/n ~ ¾ c in water

• Muon is going faster than its fields can travel in water: "shock wave" builds up

• Cherenkov light is emitted, in characteristic 42o rings around the particle direction

• Cherenkov 'rings' are fuzzy for electrons and sharp for muons– electrons scatter in the water– heavier muons travel in straight paths

until stopped

ν µ

light rays (v=0.75c)

v ≈ c

water (n=1.33)

light waves

Neutrino “events”: νe and νµ

Electron Neutrino Event

Inner Detector

Outer Detector

MUONNeutrino Event

Electrons scatter in water and produce fuzzy Cherenkov rings; Muons travel in straight lines and produce sharp rings

Map of phototubes: imagine a soup can, cut open and unfolded to show the inside:

June 5, 1998: Press clippings…

Super‐K: underground neutrino observatory

Tokyo

Kamioka

TokaiN

18J. Wilkes, UW Physics

Q: How do you make a neutrino beam?

(180m of earth)

JPARC 30 GeV proton accelerator

GPS

100m decay pipeSuper-Kamiokande

(300 km of earth)

proton beam

beam monitorstarget,

magnets

beam monitors

beam monitors

Near Detectors

GPS

T2K (Tokai to Kamioka)Started data-taking 2010

GPS provides time synchronization accurate to ~50 nanoseconds

pions

T2K beamNeutrino beamline

At the J-PARC lab (in Tokai):

• 30 GeV high intensity proton accelerator

• proton beam aimed at SK, makes neutrino beam

• “Near detectors”sample the beam Fukushima 75 kmSuper-K 295 km

Pacific ocean

Tokyo 100 km

20J. Wilkes, UW Physics

Near detectors

Godzilla waded ashore here in Godzilla 2000 !!

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What are we looking for? Interference effects!

• Neutrino oscillations = quantum wave effects visible on a macroscopic scale

• Neutrinos have 2 sets of properties: flavor (electron, muon or tau) and mass (m1, m2 or m3)

• Each flavor state is a mixture of mass states, and vice-versa

• Mass states have different wavelengths(rest energy, momentum ~ λ)– So the different mass states making up

a muon neutrino interfere!

– We may observe a few electron neutrinos, after some time/distance

• Plan: Generate beam of muon neutrinos, count how many e neutrinos appear after travelling 300 km (t ~ 1 microsecond, by our clock)

Super-K discovered this spacing: first proof that nu’s have mass. We still don’t know if order of mass states is “normal” or “inverted”.

each mass is a mixture of flavors

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What about “faster than light” neutrinos in the news?

• OPERA experiment at CERN is very similar to T2K: neutrino beam goes to a highway tunnel in Italy (Gran Sasso): same physics goals

• In November, they reported neutrinos arriving a tiny bit sooner than expected - see for example

http://www.nytimes.com/2011/11/19/science/space/neutrino-finding-is-confirmed-in-second-experiment-opera-scientists-say.html

• Claim neutrinos arrive 60 nanoseconds early - travel time at speed c should be 2.4 millisec, so difference is about 1 part in 100,000

• Few physicists believe this is really due to violation of Einstein’s special relativity: too many other measurements to explain away!– Most likely: OPERA missed some item in their calculation of time delays

between when particles actually pass through detectors, and when electronic signal is recorded by data system

• T2K is doing a very careful study to respond (UW students lead this)!– We’ve already spotted many tiny inconsistencies in our logbooks…

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Next: Is there even further substructure?

• Do quarks and leptons have smaller things inside them?

• Much current debate on this topic!

• Could all the particles be different “states” of a more basic entity? "String theory" suggests so. – Universe is actually 11-dimensional (!?)

• All but 3 space dimensions are folded up inside “strings”…– Particles correspond to different vibrational modes

– The Fabric of the Cosmos (now on PBS) by Brian Greene, describes this view

• One difficulty: totally inaccessible for experimental tests!– “Planck Scale”, 10-35 meters, requires solar-system sized accelerator!

• we need new ideas...

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What’s the Nature of the Vacuum?

• What’s going on when there is nothing there?• Quantum Mechanics tells us the vacuum must be a turmoil of

continuous production and annihilation of particle-antiparticle pairs:E=mc2 in action– Uncertainty also says you can violate energy conservation temporarily:

[“borrowed” energy] x [time of “loan”] ~ Planck's constant (very tiny number)

electron

antielectron (positron)

This happens all around us, all the time, in “empty” space.What impact does this sea of “virtual particles” have on the expansion of the Universe? Is this related to Dark Energy?

“borrowed” energy “returned” energy

pair creation annihilation

Vacuum!

∆E ∆t ~ h

Quiz question

• Neutrinos are fundamental particles that

A. Are massless

B. Are composed of 3 quarks

C. Interact only via the weak nuclear force, the force responsible for radioactive decays

D. Are theoretically predicted but have never been observed

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