Neutrino Physics

Neutrino Physics Joe Grange UPS c/o 2006 Univ. of Florida PhD candidate

Today • What are neutrinos? • Neutrino history • New physics! The oscillating neutrino • Modern neutrino detectors • What can neutrinos tell us about our universe?

n History: • Early 20th century physicists thought the things that make up our macroscopic world (nuclei, electrons, photons) were the only things that make up the universe • If that’s the case, then the common radioactive “beta (electron) decay” involves only a proton and an electron n p e Assuming energy + momentum conservation, electron would have the same energy in every decay

Intersection of data and theory Data • Hypothesis was wrong! So either: • the decay involves more than two particles or • energy is not conserved • Latter option might sound drastic, but, e.g. Niels Bohr (right) was ready to abandon E conservation • Wolfgang Pauli (left) chose the former

“A desperate remedy” • 1930, Pauli proposed a light, electrically neutral particle carried away the missing energy • “I dare not publish this idea…but only those who wager can win…” • “I have done a terrible thing. I have postulated a particle that cannot be detected” • A few years later, Enrico Fermi further incorporated this particle into present theory, named it • “Neutr-”: neutral • “-ino”: little one n

Neutr- “ino”: how small? • Not totally sure! Active area to find the absolute mass, but from indirect measurements we know it’s ~0.1 eV (electron-volts) Analogy: if a n weighs as much as a penny… An electron would weigh as much as a car, a proton would weigh as much as a space shuttle, and a human would weigh 20x Jupiter!

Why “cannot be detected”? • Can be detected, it just turned out to be very hard • hadn’t seen it before, must have extremely low probability to interact • Of the four fundamental forces (gravitation, strong nuclear, electricity & magnetism, weak), neutrinos only interact weakly. Aptly named! • at the dentist’s office, a vest with a few mm of lead stops x-rays, meanwhile you’d need ~1 light year(10 trillion miles) of lead to stop a n!

First proposed expt: “El Monstro” • First proposed n experiment used an atomic bomb as the source of n’s! • different times! • To shield the detector from the blast, simultaneously drop it with the explosion • Ultimately went with a more responsible source of nuclear reactor

Finding n’s • So folks thought n’s carried away the missing energy in beta decay • Just like any other physical reaction, ought to be able to “reverse” the reaction n n e- n p W n e+ W n n p n n p p e- e+

First n detection: 1956 • Discovery of n ~25 years after proposed by W. Pauli • Detection: • e+ annihilates with e- in medium, detect g rays • neutron captures in medium, releases g’s • Basic rule of n detection: • nothing comes in • outgoing particles consistent with n interaction

n “flavors” • That the neutrinos from nuclear reactors produce electrons important! • Three “flavors” of neutrinos (+antineutrinos) that dictate what they can produce • “electron-type” ne • “muon-type” nm • “tau-type” nt • Lepton flavor conservation • that is, only a nt can produce a t, etc.

How n’s interact with matter Charged-partner conversion Scattering nm nm nm ne t- m- N N N N’ N N’ No way!

1960s: let’s look for more n’s! • The sun predicted to be a very hot source of ne’s (and only ne’s) • Ray Davis stuck a giant (600t) vat of cleaning fluid (C2Cl4) inside a mine • ne’s from the sun should interact with Cl, turn it to Ar • Count the Ar atoms! • 2/3 are missing? • Prediction: 5.7 ± 0.9 Ar/day • Data: 1.9 ± 0.2 Ar/day

1990s: data confirmed • Using modern n detection techniques (more later), the ne deficit confirmed What’s happening?! Three options: Are independent experiments wrong? Do we not understand the physics of the sun? Is something happening to the n’s?

A new way of testing n’s • 2001: An experiment with unique sensitivity to ne but also all n flavors (ne, nm, nt) releases data sensitive to ne’s exclusively (tests previous expts) look at all n flavors (tests total n flux)

A new way of testing n’s • 2001: An experiment with unique sensitivity to ne but also all n flavors (ne, nm, nt) releases data • only way of “looking” at nm, nt from the sun! • energy too low (~10 MeV) to create m, t particles • Me = 0.5 MeV • Mm = 105 MeV • Mt = 1800 MeV sensitive to ne’s exclusively (tests previous expts) look at all n flavors (tests total n flux)

Solution! • Data confirms both theory and previous experiments! • We know the n’s have flavor ne when created in the sun, yet 2/3 have flavor either nmor nt on earth! • This was the first definitive evidence that n’s oscillate! • that is, they can be “born” one flavor, and detected as another

n oscillation • What does this mean?! (quantum mechanics ahead!) • The npropagation state (i.e. Hamiltonian or mass eigenstate) is NOT the nflavor state (i.e. interaction eigenstate) • Rather, the propagation states (n1, n2, n3) are quantum mechanical admixtures of the three flavor (ne, nm, nt) states • n’s are constantly in an entangled state of ne, nm, nt! Immediate implications: • n’s have mass! Previously thought to have m = 0 • lepton flavor not always conserved!

n oscillation • Just like the famous example of Schrödinger's cat: if the cat has an equal probability of being alive or dead • Then, quantum mechanically, it exists in superposition of both states. In some sense, simultaneously alive and dead! • Any n is simultaneously ne, nm, and nt! P = ½ ( ) + ½( )

Going a little deeper • For simplicity, let’s pretend only 2 flavor and propagation states • Simplifying assumptions: • n’s travel near speed of light • n mass is small compared to it’s energy • Then, after travelling distance L, a ne of energy E has q = arbitrary mixing angle Turns out to be a good assumption! Dm2 = (mn1 - mn2)2 Probability to be detected nm: Probability to be detected ne:

Going a little deeper • ne “disappearance”: • nm “appearance”: simple harmonic oscillator!

Going a little deeper • ne “disappearance”: • nm “appearance”: simple harmonic oscillator! Nature: q oscillation amplitude; Dm2 oscillation frequency Experiment: E n energy, L distance n travels

Going a little deeper n created as ne Prob. detected ne ~q, osc. amplitude Prob. detected nm ~Dm2, osc. frequency

Going a little deeper n created as ne Prob. detected ne Experimental game: nature has chosen oscillation parameters q, Dm2, we choose n E, propagation distance L, look for appearance or disappearance! ~q, osc. amplitude Prob. detected nm ~Dm2, osc. frequency

Proton-atmosphere collisions Where do neutrinos come from? The sun Supernova • Roughly 100 trillion neutrinos from the sun pass through your body every second! • on average, only 2 will interact in your lifetime Nuclear Fusion Everything we know about n’s made possible by these sources • Neutrinos created in neutron decay as a by-product of nuclear fission Nuclear reactors Particle accelerators • Incredible when you consider how “bright” they are from the ~ 1% of energy that goes to photons • Around 99% of thermal energy released in core-collapse supernovae carried away by n! • n beam created by smashing protons into nuclear material • safe to stand in! • High-energy cosmic particles (from supernova?) collide with atmospheric atoms to produce particle showers, including those that decay to n

Other n sources • Bananas • a single banana emits about 1 million n’s per day from naturally-occurring radioactive potassium • The big bang! • Still around! Big bang so strong and n’s so weakly interacting that ~50 big bang n’s pass through your thumb per second!

Available n energies • Luckily, even the natural n sources span a wide energy range! • Gives us the E in the quantity L/E we use to probe n oscillations. Next up, L but first…

Modern n detection • All about collecting the tiniest signals of light (photons) • “Photomultiplier” tube exploits Einstein’s photoelectric effect • e.g., Super Kamiokande, a gigantic detector houses ~11,000 tubes in 50,000 tons of ultra pure H20! incoming photons ejected electrons (signal) light can behave like a particle and a wave!

How do n’s lead to photons? • Light travels slower in detector medium (water, oil, lead, ice) than in vacuum • vacuum: light travels at c; medium: light travels at c/n, nthe index of refraction • Particles can have v > c/n! Just like a sonic boom from a jet: when traveling faster than speed of sound, a sonic shock wave follows. n detectors see “light booms”! “Cherenkov light” creates ring of light

Modern n detection • n flavor determined by charged partner production difference in ring due to mass: mm ~200 * me

n detectors around the world ANTARES - at the bottom of the Mediterranean sea! 350m MiniBooNE observes accelerator n’s SNO - buried in a Canadian mine

n detectors around the world • The biggest detector in the world: “IceCube” at south pole • Use 1 cubic km of ice as a neutrino detector!

Last experimental issue:n travel distance L • Naturaln sources: • Artificial sources: • n travel distance limited by diameter of Earth! atmospheric ~40 km or 13,000 km solar MINOS experiment: shoots n’s from IL to MN

n oscillation parameters found! • Fortunately, both natural n sources (solar, atmospheric) provide n oscillations we’re sensitive to on Earth! • Both oscillation modes confirmed by man-made n sources with different n E, travel distance L, but same ratio L/E

n oscillation parameters found! • Fortunately, both natural n sources (solar, atmospheric) provide n oscillations we’re sensitive to on Earth! • Both oscillation modes confirmed by man-made n sources with different n E, travel distance L, but same ratio L/E KamLAND nuclear reactor expt confirms solar osc. parameters MINOS accelerator expt confirms atmospheric n oscillations (confirmed by other experiments too!)

n oscillation parameters found! • oscillation parameters: Confirmation with Super-K, K2K and MINOS data Confirmation with SNO, Kamland data

So what do we know? • From only these two sets of oscillation parameters, we know almost the entire mixing matrix U c = cos s= sin • Only remaining pieces: q13, d • q13: “how much flavor state ne is in the mass state n3?” • d: do neutrinos and anti-neutrinos oscillate differently?

Exciting times for n’s! • March 7: Daya Bay (China) reactor experiment announces discovery of q13! • April 3 (two days ago!!): RENO (Korea) reactor experiment announces independent confirmation! • “During preparation of this paper, Daya Bay reportedobservation of a non-zero value for q13” • d only meaningful if q13 is non-zero • sin(0) = 0 s13 = sin q13

d: are n’s responsible forour universe? • Our universe clearly dominated by matter, yet we’ve no idea why • In the lab, if we create/destroy matter we always create/destroy antimatter in equal quantities - should be true for big bang too • Where did all the antimatter go?? • n’scould be responsible. Because n’s so prolific in early universe, even a small difference between neutrino, anti-neutrino oscillations could’ve tipped the balance in favor of matter domination

Other outstanding issues • What is the absolute n mass? • we only know square of mass state differences • KATRIN experiment hopes to measure it • Are there more than three n mass states? • many hints, nothing conclusive yet • What’s the ordering of the mass states? • Are neutrinos their own antiparticle?

Summary • n physics is a young field, but incredible progress is being made • n oscillations one of the very few concrete results not predicted by the hallowed standard model • Along the way, we are constructing the biggest particle detectors in history • In the near future (decades?) we may know everything about n’s, including whether or not they’re responsible for the formation of our universe

Thanks for listening! jgrange@alum.ups.edu

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Going a little deeper • Explicitly, • Say the sun created a ne: • Multiply by QM time propagator: • Simplifying assumptions: • n’s travel near speed of light • n mass is small compared to it’s energy • only two propagation states contribute to oscillations

30cm it only takes ~1/10 A to stop a heart… we run 174 kA through the horn, around 106 times more! Beryllium “slugs” - our target! 70cm

I B protons (Ampere’s Law) 5 × 1012 protons, 5 times a second! For current flowing along a long, straight wire,

protons

However, focusing is NOT perfect. • Not all get defocused, mostly due to low angle production and higher energies • opposite charged particles will not get swept away if they • don’t “notice” the magnetic field protons • This leads to beam, hence data, contamination • Contamination varies based on energy of incoming protons, • current, horn/target geometry, and horn polarity

Neutrino Physics