Discovery of the Neutrino Mass

Discovery of the Neutrino MassDiscovery of the Neutrino MassDiscovery of the Neutrino MassDiscovery of the Neutrino Mass

P1X* Frontiers of Physics Lectures http://ppewww.ph.gla.ac.uk/~psoler/

P1X_neutrino.ppt21-22 October 2003

Dr Paul SolerUniversity of Glasgow

Discovery of the Neutrino Mass 2

OutlineOutline1. Introduction: the structure of matter2. Neutrinos:

2.1 Neutrino interactions2.2 Neutrino discovery and questions2.3 Neutrino oscillations

3. Atmospheric neutrinos3.1 Superkamiokande experiment3.2 Discovery of neutrino mass 3.3 Long-baaseline neutrino experiments

4. The Solar Neutrino Puzzle:4.1 Solar model and the Homestake experiment4.2 Kamiokande and Superkamiokande experiments4.3 Gallium experiments4.4 Sudbury experiment: the solution of the puzzle

5. The future: a neutrino factory?


MotivationMotivationMotivation for the Frontiers of Physics lectures:

1. Bring to your attention some of the most exciting fields of physics research at a level that can be easily understood

2. Help you to understand the link between undergraduate physics and front-line research.

3. Use some of the concepts learned in these lectures to improve understanding of undergraduate physics

Neutrino physics: exciting recent discoveries have shown that neutrinos have mass, Nobel prizes for R. Davis and M. Koshiba.

Motivation for the Discovery of the Neutrino Mass lectures:


ReferencesReferences

1. “Detecting Massive Neutrinos”, E. Kearns, T. Kajita, Y. Totsuka, Scientific American, August 1999.

2. “Solving the Solar Neutrino Problem”, A.B. McDonald, J.R. Klein, D.L. Wark, Scientific American, April 2003.

Reading for the “Discovery of Neutrino Mass” lectures

Web references:2002 Nobel Prize in Physics:

http://www.nobel.se/physics/laureates/2002/Super-Kamiokande and K2K web-sites:

http://www.phys.washington.edu/~superk/http://www.ps.uci.edu/~superk/http://neutrino.kek.jp/

Sudbury web-site:http://www.sno.phy.queensu.ca/

More on neutrinos:http://wwwlapp.in2p3.fr/neutrinos/anhistory.html


1. The structure of matter1. The structure of matter How do we find out about the smallest constituents of matter?

– Build more powerful “microscopes”:

Wavelength of probe

becomes smaller as

energy (momentum)

of probe becomes

larger.

p

h

For sub-atomic

particles, we use

powerful accelerators

(e.g. CERN, Fermilab).

sJh 34106.6


1. The structure of matter 1. The structure of matter (cont.)(cont.)

Two types of particles:– Fermions (half-integer spin particles): make up the known matter and occupy

“space” because of Pauli exclusion principle.Examples: quarks, protons, neutrons, electrons, muons, neutrinos, ...

– Bosons (integer spin particles): carriers of the forces between fermionsExamples: photons for electromagnetic interactions, W and Z bosons for weak interactions, gluons for strong interactions

Fermions come in three families (why?, we don’t know) and have antiparticles as well. One neutrino for every electron, muon and tau.

Electrons take

up space

Muons unstable

(cosmic rays)ee

Taus very unstable

Quarks give

most of mass

Protons: uud

Neutrons: ddu

Exotic quarks:

rare and unstable


1. The structure of matter 1. The structure of matter (cont.)(cont.)

Forces:– Gravity: very weak, long interaction, mediated by graviton (never observed!).– Electromagnetic: keeps atoms together, mediated by photon – Strong: keeps nuclei and nucleons (ie. protons, neutrons) together, mediated

by gluons. Very short range interaction– Weak: responsible for some radioactive decays (ie. beta decay), mediated by

W+, W- and Z0 massive gauge bosons. Relatively short range and weak due to mass of the bosons.


2. Neutrinos:2. Neutrinos: Neutrinos:

– Originally suggested by Pauli in 1930 as a “desperate remedy to overcome law of conservation of energy” in beta decay:

– The neutrino was originally postulated as a massless, chargeless and very weakly interacting particle: practically indetectable!

eepn

1.05KE (MeV)

Radium-E spectrum(Bi-210)

Number beta rays

Why is the electron spectrum continuous?

A third particle (neutrino) is taking away part

of the energy


2.1 Neutrino interactions2.1 Neutrino interactions Neutrino interactions:

– One of the ways neutrinos interact is through inverse beta decay:

– Cross-section (average area of neutrino in collision) is very small: on average a neutrino would travel 1600 light-years of water before interacting!

epne

1600105.11 21 cmn

A

N

volume

protonsfreenumn A2

.

light-years

enpeor

Mean free path:

In water: 32223

107.618

1062

cmn

24410 cm

p

e


2.2 Neutrino discovery 2.2 Neutrino discovery Reines and Cowan observed neutrinos for the first time in 1953

(Nobel prize for Reines in 1995) They used 400 l of a mixture of water and cadmium chloride (Cd) An antineutrino from a nuclear reactor (6 x1020 s-1) very rarely

interacted with the protons in the target (2.8 hr -1):

– The positron (e+) produces two photons, followed about 20 ms later by the neutron interacting with a Cd nucleus that produced another spray of photons

enpe

en

Scintillator

Scintillator

H2O + CdCl2


2.2 Neutrino questions 2.2 Neutrino questions Neutrinos are all around us:

– Produced by nuclear reactions in radioactive rocks (trace uranium thorium in granite, etc.), in the sun (solar neutrinos) and from cosmic rays hitting the atmosphere (atmospheric neutrinos).

– Very difficult to detect because they are so weakly interacting.– Produced in copious quantities inside nuclear reactors.– Generated by high energy accelerators

Two main problems:– Solar neutrino problem: nuclear reactions in the sun produce

electron neutrinos e (energies up to 14 MeV). The number detected on earth by experiments is between 30%-50% of what is expected.

– Atmospheric neutrino problem: high energy particles (cosmic rays) hitting the upper part of the atmosphere. There should be twice as many muon neutrinos as electron neutrinos e (energies up to 10 GeV). Experiments detect approximately equal numbers.

Both can be resolved through neutrino oscillations


2.3 Neutrino oscillations 2.3 Neutrino oscillations If neutrinos have mass, theoretically, a neutrino of one species could

change into another species:– For example: a muon neutrino changes into a tau neutrino

– Probability that a of energy E converts to a after travelling a distance L is:

Notice that the probability of oscillations is zero if the mass of the neutrinos are zero!

L=length of neutrino path (in m) E=energy neutrino (in MeV)

mmass of(in eV) m

mass of (in eV)

mixing angle between two neutrinos

(eV= electronVolt=energy of one electron accelerated by 1 Volt=1.6x10-19 J)

E

Lmm vv22

2227.1

sin)2(sin)Pr(


3. Atmospheric Neutrinos 3. Atmospheric Neutrinos Cosmic rays provide an abundant source of neutrinos. Protons hit upper part of atmosphere producing cascade of particles

including pions that decay (on average) into 2 muon neutrinos for each electron neutrino produced in an interaction

ee


3.1 Super-Kamiokande 3.1 Super-Kamiokande experiment experiment

Kamiokande experiment: started 1987, 5000 tons water, 1000 photomultipliers Super-kamiokande experiment: started 1997 (M. Koshiba leader experiment)

50,000 tons of water, surrounded by 11,000 phototubes to detect flashes of light in the water.

Super-Kamiokande

experiment is underground

Inside a mine in Japan to

shield it from the very

large number of cosmic

rays.


3.1 Super-Kamiokande 3.1 Super-Kamiokande experiment experiment

Super-Kamiokande detects faint flashes of Cherenkov light inside huge tank of 50,000 tons of water.

Electron neutrinos make a recoil electron and muon neutrinos make a recoil muon.

Rings of Cherenkov light are formed from the electron or the muon. The detector can distinguish between electrons (fuzzy rings) and muons (clean edge on ring).

XeNe XN


3.2 Discovery of neutrino mass 3.2 Discovery of neutrino mass Results from Super-Kamiokande:

– There are less muon neutrinos than expected. The number of muon neutrinos disappearing depends on the angle of the neutrino (ie. It depends on whether the neutrino was produced in the atmosphere above or on the other side of the earth). First evidence for neutrino oscillations in 1998 !!!!


3.2 Discovery of neutrino mass 3.2 Discovery of neutrino mass As the distance from production increases then more muon neutrinos disappear.

E

Lmm vv22

2227.1

sin)2(sin)Pr(

1)2(sin45 2

eVmm vv 020.0057.02/122

16.0)2.23(sin)41.0(sin)Pr(/10/.. 222 radGeVkmELge

2322 102.3 eVmm vv

Therefore 84% of survive journey!


3.2 Discovery of neutrino mass 3.2 Discovery of neutrino mass Consequences of discovery:

– Neutrino oscillations responsible for atmospheric muon neutrino deficit.

– Since electron neutrino spectrum well predicted, it must be muon neutrinos changing into tau neutrinos .

– Since then neutrinos have mass!!– Mass of the neutrinos have to be greater than 0.05+-0.02 eV.

If either the or is much smaller than the other, then m~0.05 eV.

Both or could have a mass much larger than m~0.05 eV as long as the difference of the mass squared is 3.2x10-3 eV2.

Since there were so many neutrinos produced soon after the big-bang, if they have a mass, it could provide a large portion of the missing mass of the universe (up to 20%).

2322 102.3 eVmm vv


3.3 Long-baseline experiments 3.3 Long-baseline experiments Long-baseline experiments with accelerators will verify that oscillations are

really taking place in Super-Kamiokande.– K2K (from the KEK accelerator in Japan to Super-Kamiokande): 250 km

baseline of neutrinos. So far they observe 56 events when they expected 80 events, consistent with 3x10-3 eV2 mass-squared difference.

– MINOS: neutrino beam from Fermilab in Chicago to a mine in Minnesotta (750 km), will start taking data in 2005. Another beam from CERN to Gran Sasso (CNGS) laboratory in Italy (also 750 km) to start in 2006.


4. Solar Neutrinos4. Solar Neutrinos


4. The Solar Neutrino Puzzle4. The Solar Neutrino Puzzle Ray Davis (Brookhaven National Laboratory) proposed an experiment in the

1960s to measure neutrinos from the sun. Why are neutrinos emitted from the sun? Nuclear fusion powers sun:

Energy of sun is due to burning hydrogen into helium. The measured photon luminosity is: 3.9x1026 J s-1.Energy per neutrino = 26.7x106x1.6x10-19= 4.3x10-12 J/neutrinoNumber of neutrinos = 3.9x1026/4.3x10-12 = 9.1x1037 neutrino s-1

Distance from sun to earth =R= 1.5x1013 cm.Therefore:

(64 billion neutrinos per second through your finger nail of 1 cm2 !!!!)

MeVeHep e 7.26224 4

2110213

37

2104.6

)105.1(4

101.92

4

cmsR

NumberneutrinosofFlux e


4. The Solar Model4. The Solar Model In reality, chain of reactions needed to burn 4 hydrogen nuclei into helium nucleus. There are two main cycles: the pp cycle (98.5% of the total sun’s power comes from

these reactions) and the CNO cycle catalysed by carbon, nitrogen and oxygen (not very important in the sun with only 1.5% of power output).

Most abundant neutrinos are low energy (<0.42 MeV) pp reaction with flux 6.0x1010 cm-2 s-1. Most important for detection are 8B neutrinos because they have high energy (<14 MeV) but only consist of 10-4 of all solar neutrinos.

PP cycle:


4.1 Homestake experiment4.1 Homestake experiment Ray Davis’ Chlorine experiment inside Homestake mine in Lead, South Dakota:

100,000 gallons (615 tons) of

cleaning fluid (C2Cl4) eArCle

3737Expect about 1.5 Ar atoms/day


4.1 Homestake and Solar Model4.1 Homestake and Solar Model Results from the Ray Davis chlorine experiment: sensitive to 8B and 7Be neutrinos

(0.814 MeV threshold).Measured 2.56+-0.23 SNU (0.48 atoms/day), Solar Model Expectation = 7.7+-1.3 SNU (1.5 atoms/day)Observation about 1/3 the expected number of solar neutrinos

1 SNU = 1 interaction per 1036 target atomsper second

Is there something wrongwith experiment,

somethingwrong with solar model or something wrong with theneutrinos?


4.2 Super-Kamiokande 4.2 Super-Kamiokande experimentexperiment

Results Super-Kamiokande experiment: can also measure solar neutrinos– Proof that neutrinos come from sun: angular correlation– Neutrino flux is 46.5% that expected from the solar model

Confirmation Solar NeutrinoPuzzle!


4.3 Gallium experiments4.3 Gallium experiments Similar experiments to chlorine but with gallium:

– Lower threshold (0.233 MeV) so sensitive to the lower end of the pp chain– Further evidence of missing solar neutrinos (55% of expectation)

Expectation: 129+-8 SNU

Observed: 70.8+-6 SNU


4.4 Sudbury Neutrino 4.4 Sudbury Neutrino ObservatoryObservatory

Heavy water (D2O) experiment in Canada:

1000 tonnes D2O

Acrylic Vessel

104 8” PMTs

Phototube SupportStructure (PSUP)

6500 tonnes H2O

Surface: 2 km(D=deuterium=proton+neutron)



Acryllic vessel with photomultiplier tubes:

All components Made out of very low radioactivity materials



Faint flashes of Cherenkov light recorded by photomultipliers:



Results:– Charged current (CC):– Elastic scattering (ES):– Neutral current (NC):

CC 1967.7 +61.9 +60.9

+26.4 +25.6ES 263.6 +49.5 +48.9NC 576.5#E

VE

NT

S

eppde ee xx

xx pnd

About 35% electron neutrinosmake it to earth (from CC) but flux of all neutrino species (fromNC and ES) as expected:Neutrinos change species in flight:

(35% SSM)

(100% SSM)

Neutrino Oscillations!


4.4 Solar neutrino puzzle solution4.4 Solar neutrino puzzle solution Sudbury Neutrino Observatory has confirmed neutrino

oscillations from solar neutrinos and has confirmed the solar model of fusion in the sun.

Experiments only sensitive to electron neutrinos (e) see a deficit but Sudbury experiment that is sensitive to all neutrino flavours sees the expected total number of neutrinos.

Electron neutrinos (e) oscillated into muon neutrinos () in trajectory from the sun to the earth. However, the evidence shows that the transition happened inside the sun, due to an enhancement of the oscillations because of the high matter density of the sun.

Parameters: 4.25.32 e

eVmm vv32/122 104.8

2522 101.7 eVmm

evv


4.4 Solar neutrino puzzle solution4.4 Solar neutrino puzzle solution More confirmation: KamLAND experiment in Kamioka mine in Japan

shows that reactor (~2 MeV) disappearing in flight (~180 km).8.32e

2522 105.5 eVmmevv

e


4.5 The future: a neutrino 4.5 The future: a neutrino factory?factory? Future directions for neutrino physics: build a neutrino factory

to fire very intense beams of neutrinos at 700 km to one experiment and around 7000 km to other side of the world.

Main aim: why is the universe made of matter rather than antimatter? CP violation of neutrino oscillations might be explanation and a neutrino factory could measure this effect.


ConclusionsConclusions Neutrinos are very misterious particles and we are only starting to

undertand their nature. Super-Kamiokande experiment discovered neutrino oscillations ( to

from the deficit of muon neutrinos from cosmic rays hitting the upper layers of the atmosphere. This implies that neutrinos have mass with mass-squared difference of 3x10-3 eV2 (largest mass greater than 0.05 eV).

A number of experiments looking at solar neutrinos have also seen a deficit in the number of electron neutrinos from the sun. Confirmation of neutrino oscillations in solar neutrinos came from the Sudbury experiment that showed that it is only the electron neutrinos that are missing while the total flux of neutrinos is as expected. Hence electron neutrinos e are changing to another species (probably with a mass-squared difference of 7.1x10-5 eV2.

The field of neutrino physics still has a bright future, with open questions: – What is the absolute mass of neutrinos?– Are neutrinos their own antiparticle?– And the most ambitious question of all: are neutrinos responsible for the

matter-antimatter asymmetry of the universe?

Discovery of the Neutrino Mass

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