Massive Neutrino – a search over 50 years

April 14th, 2003 at Palm desert

Gang Li, IGPP, UCR

Science News of the year

                                                               

Raymond Davis Jr.

Masatoshi Koshiba

Riccardo Giacconi

"for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos"

"for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources"

The 2002 Nobel Prize Laureates in Physics

What is the world made of

•Different things in this world share the same characteristics.

•Nature is made from a few fundamental building blocks.

•The Greek thinker Empedocles (not the commonly acknowledged Aristotle!) first classified the fundamental elements as fire, air, earth, and water.

The ancient Chinese believed that the five basic components of the physical universe were earth, wood, metal, fire, and water. And in India, the Samkhya-karikas by Ishvarakrsna (c. 3rd century AD) proclaims the five gross elements to be space, air, fire, water, and earth.

How small can we go, atoms?In 1990, people believe that atom is the fundamental component of everything.

Soon however, it is realized that atoms can be categorize into groups that shared similar chemical properties. ====> atoms were made up of simpler building blocks.

Atoms are made of nuclei and electrons.

Can the division go further? What are electron and nucleus made of?

Electrons and quarks•Electron shows no sign of being able to be divided further. Nucleus, with the study of nuclear decay, we know they consist of protons and neutrons.

•Protons and neutrons are much heavier than electrons. They are not fundamental particles. Instead, we believe they consist of quarks, which, according to our current theory, are as fundamental as electrons.

•Unlike electron, “seen” regularly in modern experiments, free quarks can not be “seen”---- they are always bounded in nuclei and we can only infer their existence from experiments.

Scales of an atom – a modern view

•It is possible that our current theory is not complete and quarks and electrons are not fundamental after all, and will turn out to be made up of other, more fundamental particles.

•How small can a quark or electron be? We don't know exactly the answers; but they are definitely smaller than 10-18 meters,

The Standard Model•6 quarks.

•6 leptons.

All matter can be “cooked” by quarks and leptons.

• Force carrier particles

Fermions

BosonsAll interactions (strong, weak, EM), except gravity are carried through these intermediate bosons.

Quarks

The idea of quark is proposed by Murray Gell-Mann and George Zweig in 1964. After 18 year, in 1995, the top quark (the last one) is confirmed experimentally at Fermi-Lab.

Leptons

As the case for quarks, there are six leptons: e, , and corresponding neutrinos e , and .

The need for neutrinos

Nuclear decays. Missing energy of nuclei decays: the energy of radiated electrons is not single valued, rather, the spectrum is continuous.

Maybe a sign of violation of energy conservation?

Wolfgang Pauli proposed in 1930 that there must be a second particles alongside with the emitted electron. This particle must be very light and neutral, so that it is very hard to detect.

Later, Enrico Fermi named it neutrino when he developed the nuclear decay theory.

Today we know that neutrinos are very light and they only participate in Weak Interaction and Gravitation.

Beta decay spectrum

                                                   

G. J. Neary, Proc. Phys. Soc. (London), A175, 71 (1940). The solid line is the prediction from Fermi’s theory.

This beta decay spectrum shows a continuous distribution for the emitted electrons. The y-axis is the intensity which reflects the probability of finding electrons at the corresponding kinetic energy. The ending point at the far right represents the total radiated energy. To insure energy conservation, there must be another undetected particle. This leaded Pauli to propose neutrinos.

The four interactions

It seems that all phenomena in nature can be well described by these four interactions.

Force as exchange of particles

Summary of the four fundamental interactions

                                                                                                                                     

Neutrinos, only “feel” weak interaction and gravitation.

The discovery of neutrinosWolfgang Pauli: Considered by Albert Einstein an exceptional theoretic physicist, he was awarded the Nobel Prize in 1945 for the discovery of the Exclusion Principle, also known as the Pauli Principle. In 1930, he postulated the existence of neutrino as a remedy for avoiding violation of energy conservation in nuclear beta decays.In 1953, Frederick Reines and Clyde

Cowan did the first neutrino experiment. Their targets are the anti-neutrinos from the nuclear plant at Hanford, Washington. However, they did not obtain convincing signals until 1956, when they redid the experiment. This time, at Savannah River, South Carolina.

The first detection of neutrino – schematics

positron and a neutron. The positron then annihilate with an electron and gives two photons simultaneously. The neutron, after 15s, will be captured by a cadmium nucleus, generating another photon. Detecting all these photons, with a 15s interval, signals the presence of (anti) neutrinos.

Experiment principles: a target of 400 liters of water mixed with cadmium chloride is used. Neutrinos from nearby nuclear plant interacts with a proton of the target, releasing a

Sun-- a super nuclear reactor

Sun as seen by SOHO Sun as seen by a neutrino telescope

Star burning process— CNO vs pp chainStars represent a delicate balance between gravity and gas pressure due to the burning of the core. The net effect of the burning is to convert 4 protons to a helium atom, plus an excess of energy. Two major cycles are in operation depending on the mass of the star. In the case of our sun, the pp chain is the dominant. During the burning process, plenty neutrinos are produced.

The pp chain

The pp chain– steps 1-3

The pp chain– steps 4-7

The pp chain– steps 8–11

Solar neutrinos and their production location

Solar-neutrino spectrum

Detecting neutrinos from Sun

Ray Davis began to look for solar neutrinos, start in 1969, at Homestake mine. The site is 3km underground. 600 tons of chlorine solvent are used. Neutrinos above 1 MeV is detected.

Detection principles:

a) Reverse of beta decay, detecting the radioactive product. Homestake and GALLAX.

b) Rely on Cherenkov radiation. Super K and SNO.

Solar neutrino experiments (1) -- HOMESTAKE

The first solar neutrino detector ever built, the target is a 615 tons of tetrachloroethylene. When bombarded by an anti-neutrino, the 37Cl becomes 37Ar, which is radioactive with a half-life of 35 days. The 37Ar is then isolated and detected. The number of Argon atoms detected gives the number of neutrino. The site is 3km underground, located in Homestake golden mine, South Dakota.

Solar neutrino experiment (2)-- the GALLAX

Located in Abruzzo, Italy, the detector consists of 12.2 tons of watered Gallium. The experiment principle is similar to that of HOMESTAKE: when hit by a neutrino, 71Ga becomes 71Ge, a radioactive isotope with a half-life of 11.43 days. The whole set of 71Ga and the eventual 71Ge atoms are filtered through a chemical system allowing to isolate with great efficiency and great purity the Germanium atoms, which are then counted to give the number of neutrino interactions.

Solar neutrino experiments (3)-- the Super Kamiokande

• The Super-Kamiokande detector is a 50,000 ton tank of water, located approximately 1 km underground. The water in the tank acts as both the target for neutrinos, and the detecting medium for the by-products of neutrino interactions.

• The inside surface of the tank is lined with 11,146 50-cm diameter light collectors called "photo-multiplier tubes". In addition to the inner detector, which is used for physics studies, an additional layer of water called the outer detector is also instrumented light sensors to detect any charged particles entering the central volume, and to shield it by absorbing any neutrons produced in the nearby rock.

Detecting principle behind Super K

Super Kamiokande exploits a different detection principle: the Cherenkov radiation. • Each PMT is sensitive to illumination by a

single photon of light. It measures the total amount of light reaching it, as well as the time of arrival; allowing one to reconstruct the energy and starting position, respectively, of any particles passing through the water.

•The collective effect of the over 11,000 PMTs measurement allows one to reconstruct the distinctive ring pattern, revealing the direction of the incident particle.

•Finally, from the ring pattern - most notably whether it has the sharp edges characteristic of a muon, or the fuzzy, blurred edges characteristic of an electron, one can reliably distinguish muon-neutrino and electron-neutrino interactions. 

As shown above, when charged particles traverse the water with a velocity greater than 75% of the speed of light, a radiated cone of light centering the trajectory of the particle is observed.

Solar neutrino experiment (4) -- the SNO The Sudbury Neutrino Observatory (SNO) is a detector built 6800 feet under ground, in INCO's Creighton mine near Sudbury, Ontario. Similar to SuperK, SNO contains an array of 9600 photomultiplier tubes and is also a Cherenkov detector. Unlike SuperK, the target of SNO is heavy-water. It uses 1000 tons of heavy water, contained in a 12 meter diameter acrylic vessel. The detector is immersed in light (normal) water within a 30 meter barrel-shaped cavity (the size of a 10 story building!) excavated from Norite rock.

With the heavy water, the SNO can detect all three flavors of neutrinos. So the SNO detector will be able to observe separately the number of electron neutrinos and the number of all neutrinos. This allows a determination of the probability for these flavor oscillations to occur.

Uniqueness of SNO

Conflicts between experiments and theory

Neutrino oscillation – a “solution”

Neutrinos produced at the sun is solely e.

On their way to earth, however, they “oscillate” to or .

Oscillation from SNO

Physical Review Letters, vol. 89, p. 011301 (2002)

Using all three reactions: charged current, neutral current and elastic scattering reactions, SNO collaboration is able to confirm that flavor transformation between e and

() DO happen as neutrinos propagate from Sun to Earth.

Atmospheric ’s– the source

Atmospheric ’s are produced when cosmic rays (mostly protons) collide with atmospheric nuclei, giving secondary cosmic rays of and K. These mesons will then decay, giving two types of neutrinos: and e

Reactions producing and e :

1) u +

2) u e + + e

The number of should be

twice of that of e .

                  Experimentally, people look for deviation of R from 1, where R is

and e as seen in SuperK

A typical e event.A typical event

Electron neutrino induced events have a fuzzy edged ring because electrons can generate more electromagnetic shower through brehmstrahlung and pair production. These electron-positron pairs will then generate many small overlapping rings.

Atmospheric ’s – the geometry

Neutrinos can come from all directions:

1) L = 10-30 km for down-going events.

2) L = 1.2 104 km for up-going events.

If there is neutrino oscillation, since the survival possibility depends on L, observation should show a dependence.

Missing !

Data of e agree with expectation. No more, No less. NO (e ) oscillate to/from e ( ).

come from below shows a decrease of 50%. oscillates to (or sterile neutrino).

Oscillations between and

P , t

21 sin2 2 sin2 1.27

m2c4

eV2GeV

cp

ct

km

Why do we care if neutrinos are massive?

• Candidate for dark matter : Scientists believe that the hadronic mater only makes up a small portion ~ 5% of the universe. Invisible material, so called dark matter, is inferred through gravitation effect. If neutrinos were massive, they would be good candidates to this dark matter, since they are copious~330 neutrinos/cm3.

• Candidate for High Energy cosmic rays: Ultra high energy cosmic ray (>= 1020 eV) is detected at earth, with its origin and propagation mechanism still remains a mystery. A massive neutrino could be a very good candidate.

References and further readings

Books: • Neutrino astrophysics, by John Bahcall• Neutrino Physics by K Zuber

Websites:

1) Super K homepage http://www-sk.icrr.u-tokyo.ac.jp/doc/sk /

2) SNO homepage http://www.sno.phy.queensu.ca/

3) Science magzine: http://www.sciencemag.org/

4) Scientific american: http://www.sciam.com/

5) John Bahcall’s homepage http://www.sns.ias.edu/~jnb/

Sun as a neutrino factory

Some mathematics

, t 1 cos e im12t / 4 p 2 sin e im2

2t / 4p

P , t

21 sin2 2 sin2 1.27

m2c4

eV2GeV

cp

ct

km

2) After a period of time t of propagation, the 1 and 2 behave differently and the evolves to,

1) At t= 0, assuming a is produced. This can be expressed as a combination of 1 and 2 through,

3) The survival probability that this state is still a is

Another view of SNO Detector

About units and constants

In this course, we will discuss materials from the infinitesimal elementary particles to the gigantic universe itself. This corresponds to an enormous range of the length scales.

• Length units: 1 fm = 10-15m, 1 A = 10-10 m,

1AU =1.5* 1011m, 1 ly = 9.46*1015m, 1pc = 3.26 lr.

• Velocity units: speed of light, c = 3*108m/s.

• Energy: eV, 1 eV = 1.6*10-19 J. 1 MeV = 106 eV

• Einstein’s E-m relation: E = mc2

• Natural unit: h = 1 and c = 1.

• Mp = 938 MeV, Me =0.511 MeV, M= 0.