HCP: Particle Physics Module, Lecture 4 Plan for today: Neutrinos mass (?) dark matter problem in the universe solar neutrino problem oscillations! Possible future of particle physics  superstring theory

New chart: Old chart: Why the change?

Neutrinos Neutrinos are zero (or nearly zero) mass particles that only interact through the weak interaction. Originally proposed by Pauli (1933) to explain the apparent “missing” energy and momentum in the beta decay process: n  p + e- + e  First directly detected in late 1950’s using neutrinos from a nuclear reactor with the process: e + p  n + e+ Very difficult experiment because neutrinos hardly interact at all: Example: Probability of a neutrino interacting as it passes through the Earth is 1 part in 1015 = 10-15 !

Neutrino Mass Why do we say that the neutrino mass is “nearly zero”? From experiments we have “upper limits” on the mass of the neutrinos: e : mass < 3 eV/c2  : mass < 0.2 MeV/c2  : mass < 20 MeV/c2 Ways to directly measure neutrino mass: Search for “missing energy” in nuclear beta decays Look at neutrinos from a supernova! “Direct search” for neutrino mass using tritium beta decay: Search for “missing energy” near end point; the highest energy decay electrons would have less energy if the neutrinos have mass.

Or Look at Electron Neutrinos from a Supernova (exploding star) Supernova 1987A: observed in optical telescopes and in two large underground neutrino detectors. Huge burst of neutrinos (of varying energies) emitted during the explosion Measure: 1. neutrino energy in the detector 2. neutrino arrival time: if neutrinos have mass, the lower energy ones would arrive later 12 neutrinos observed This analysis led to an upper limit on the electron neutrino mass of 20 eV/c2 Neutrino arrival time (seconds)

Why is Neutrino Mass of Interest? Possible explanation of the “dark matter problem” in the universe Solar neutrino problem Neutrino oscillations (if neutrino oscillations are observed, then neutrinos definitely have mass) 4. The indirect evidence of neutrino mass from the recent observations of neutrino oscillations is the first evidence of physics “beyond” the Standard Model (the neutrinos are assumed to be massless in the Standard Model)

Dark Matter Problem Observation of the motion of stars in our galaxy indicates that there is not enough visible matter to account for how fast the stars are moving. Conclusion: 90% of the mass of the galaxy is in some “invisible” (not giving off light) form Could it be neutrinos? There are ~ 100-1000 neutrinos in every cubic inch of space (the “relic” neutrinos from the Big Bang). If they had a mass of around 2 – 20 eV/c2 they could explain the problem. Observed Velocity of stars in our galaxy  Expected if all mass is “visible” of the galaxy

How Does the Sun Shine? The Sun’s energy comes from nuclear fusion reactions: proton – proton “chain” of nuclear fusion reactions Basically, p + p + p + p  He4 + “stuff” + 26 MeV 4 protons fuse released energy Part of the released energy comes off as visible sunlight, BUT some of it is carried off by neutrinos that reach us here at Earth.  Observation of these neutrinos here on Earth is some of the best evidence that nuclear fusion reactions power the Sun.

Solar Neutrino Detection The neutrinos coming from the sun are all electron type neutrinos (e): They come in many different energies (from 0.1 – 16 MeV). We can detect them here on Earth with: LARGE (due to low neutrino interaction rate) UNDERGROUND (to reduce cosmic ray background (mostly +,-)) DETECTORS

The Chlorine Experiment (The Original Solar Neutrino Experiment) Large tank full of 100,000 gallons of cleaning fluid located 1 mile underground in the Homestake Gold Mine in South Dakota.

The Chlorine Experiment (The Original Solar Neutrino Experiment) Large tank filled with 615 tons of cleaning fluid (C2Cl4). Located 1 mile underground in the Homestake Gold Mine in South Dakota. Search for products of the reaction: e + 37Cl  e+ + 37Ar   from the Sun in the cleaning fluid Only one 37Ar atom is created every 2 days! They chemically “sweep” the tank every two months to collect about 30 37Ar atoms.

The Chlorine Experiment Kamiokande and Super-Kamiokande

Solar Neutrino Problem As of 2001, four different solar neutrino experiments lead to the same conclusion  The amount of neutrinos coming from the Sun is much less than the predictions of the Standard Solar Model Theory  Experiment  Results from four different experiments on the number of electron neutrinos coming from the Sun.

Solar Neutrino Problem As of the year 2001, the question was: What is causing the apparent deficit of e coming from the Sun? Error in the Standard Solar Model of the Sun OR b) New neutrino physics?  The e neutrinos are oscillating to other flavors ( or ) that we do not detect. As we will see now, experiments since 2001 have confirmed that the answer is b).

Neutrino Oscillations If neutrinos have non-zero mass, they can oscillate. Oscillation: The neutrino can change from one type to another as it travels along: e    e This type of effect turns out to be responsible for the solar neutrino problem  new neutrino physics!

Super-Kamiokande in Japan: 50,000 tons of water + 11,000 phototubes

Example of Cerenkov light cone in the Super-K detector

but sometimes accidents happen …

Sudbury Neutrino Detector in Ontario,Canada

Acrylic vessel containing “heavy” water surrounded by 10,000 phototubes on a geodesic dome structure

Excavation of the cavity

Acrylic sphere with phototube dome partially done

Geodesic dome containing 10,000 phototubes

First Observation of Neutrino Oscillations Neutrino oscillations have in fact been observed. The first clear evidence for neutrino oscillations came in 1998 Super-Kamiokande in Japan: underground detector with 50,000 tons of water and 11,200 photomultiplier tubes Process: Pions are generated from cosmic ray events Pions decay to neutrinos in the upper atmosphere +  + +  e+ +  + e A detector on Earth should detect two muon neutrinos for every one electron neutrino. Super-Kamiokande observes a ratio of 1:1 NOT 2:1 as expected. Explanation: The muon neutrinos () are oscillating to another neutrino type (probably tau neutrinos ()) which is not detected by Super-K

Solution of Solar Neutrino Problem – SNO 2001 SNO: (Sudbury Neutrino Observatory, Canda) : large, underground tank of “heavy” water (D2O) viewed by 10,000 photomultiplier tubes Previous solar neutrino experiments could only detect electron type neutrinos (e ), while the SNO detector can detect all three types (e   ) They agree with all the other experiments that they are seeing only about 1/3 of the expected number of electron type neutrinos (e) BUT When they sum all three types they observe as many solar neutrinos as expected! So, the “missing” electron type neutrinos are oscillating to another type (probably muon type neutrinos, but SNO can’t distinguish between those and the tau type neutrinos)

Solution of Solar Neutrino Problem – SNO 2001 They really are coming from the Sun’s direction!

Further Evidence that Electron Neutrinos Oscillate - 2002 KAMLAND: Large, underground tank in Japan with 1000 tons of scintillator and 2000 photomultiplier tubes One of the radioactive decay products from nuclear reactors is electron type anti-neutrinos (e); KAMLAND detects these from nuclear reactors all over Japan and Korea They also observe less electron-type neutrinos than expected! Interpretation: The electron type neutrinos are oscillating to another type – probably muon neutrinos

Summary of Three Pieces of Experimental Evidence for Neutrino Oscillations 1. Super-Kamiokande (1998) in Japan 50 kton underground tank of water viewed by 11,200 phototubes sensitive to "atmospheric" neutrinos from Observe less muon neutrinos than expected Interpretation: The muon neutrinos are OSCILLATING to tau neutrinos 2. SNO (Sudbury Neutrino Observatory) (2001) in Canada Large, underground tank of "heavy" water (D2O) viewed by 10,000 phototubes Observing solar neutrinos; sensitive to all three types They observe as many solar neutrinos as expected Solar neutrino problem solved! "Missing" electron type neutrinos are oscillating to another type. 3. KAMLAND (2002) in Japan Large, underground tank with 1 kton of scintillator and 2000 phototubes Observing electron type neutrinos from nuclear reactors throughout Japan They observe less electron type neutrinos than they expect Interpretation: the electron type neutrinos are oscillating to muon type neutrinos

Neutrino Mass Spectrum The observation of neutrino oscillations implies that at least two of the known neutrinos have mass:

Where Are We Headed?  “Theory of Everything” What does that theory need to be able to do? Unify gravity with the three other forces Until recently this seemed very difficult because there was no successful way to make a quantum theory of gravity. Possible solution: “Superstring theory” = Supersymmetry + string theory Allows for gravity to be quantized 2. Gives unification of all 4 forces into a single unified force

Why Is It Difficult to Make a Quantum Theory of Gravity? At distances as short as the Planck length (10-35 m) the energy fluctuations implied by the Heisenberg uncertainty principle become very large  In fact, the energy fluctuations are so large that virtual black holes form. Space-time looks highly curved and “foamy” on these length scales. It is not the smooth collection of spacetime points required by quantum field theories like QED = quantum electrodynamics QCD = quantum chromodynamics

String Theory String theory: Instead of being pointlike, the fundamental particles of nature (quarks and leptons) are described by strings. R ~ 10-35 m (Planck length) The fundamental particles correspond to the different vibrational modes of the string.

Supersymmetry A symmetry where every known particle has a supersymmetric partner If this symmetry exists, it is clearly badly broken in our low-energy world since we don’t seem to see the supersymmetric partners. The supersymmetric particles will be searched for at the new high energy accelerators (like LHC)

Superstring Theory Superstring theory is the “marriage” of these two concepts: Superstring theory = string theory + supersymmetry Probem with superstring theory: It is currently only understood how it works at the Planck energy (1019 GeV). There are no predictions that can be checked with current accelerators which have the highest energies ~ 104 GeV.  Something to look forward to!