Solar Neutrinos

Dr Robert Smith Astronomy Centre University of Sussex

Solar Neutrinos 1991 – the last time I gave this talk: “The Solar Neutrino Problem” What was the problem?

Observed flux of neutrinos



one-third of the predicted flux from the Sun alone Conclusion of that talk: new physics may be required – new detectors could resolve the problem in 5-10 years Problem was resolved in 2001 – with a Sussex connection

Solar Neutrinos Outline of talk:

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What is a neutrino?

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Standard model of the Sun and predicted

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neutrino flux “The” Solar Neutrino Experiment

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More recent neutrino detectors Results from SNO (Sudbury Neutrino Observatory)

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Conclusions

Solar Neutrinos – what is a neutrino?

Forces and particles

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Familiar forces: Gravitational * all masses attract each other * long range

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Electromagnetic * all charged particles attract or repel, and are affected by magnetic fields * long range Less familiar: Strong and Weak forces – act only at nuclear scale

Solar Neutrinos – what is a neutrino?

Strong force * binds nuclei * involves heavy particles (“baryons”) such as protons and neutrons * very short range (~nuclear radius) Weak force * causes radioactivity leading to emission of electrons (“beta decay”) * involves light particles (“leptons”) such as electrons

1

* extremely short range (< of

100

a nuclear radius)

Solar Neutrinos – what is a neutrino?

Neutrinos have * no charge * no (or very small) mass * spin * energy and travel at the speed of light. They are produced only by weak interactions , e.g. decay of the neutron: n p + e -



anti-neutrino (all particles have an anti-matter partner, and this is the neutrino’s one; the electron’s anti-particle is the positron , with a positive charge). All neutrinos are associated with a lepton – this one, from neutron decay, is an electron neutrino .

Solar Neutrinos – what is a neutrino?

There are three types of neutrino, each associated with a different lepton: Neutrino

   

e



Associated lepton electron (m e c 2 ~ 0.5 MeV) muon (~200 electron masses) tau (~4000 electron masses ~ 2 proton masses) Neutrinos have a very weak interaction with ordinary matter (typical mean free path ~ 1 light year of water! – depends on energy). Any neutrinos produced at the centre of the Sun escape freely – they are a direct probe of conditions at the centre of the Sun .

Solar Neutrinos – flux from Standard Model Standard model of the Sun Geological record implies: in equilibrium, with age of ~4,550 million years Direct measurements give: mass, radius, luminosity, rotation All other properties come from a stellar model: Force balance - gravity balances internal pressure Energy transport Energy balance - well understood - radiation (centre), convection (outer shell) - more complex, but still well known - luminosity balances nuclear energy source (H fusion, producing He) source of neutrinos: so this is critical

Solar Neutrinos – flux from Standard Model Nuclear reactions in the Sun Most solar energy (98.5%) produced by the pp chain : 4 p 4 He + 2e + + 2ν e + energy This is the net effect of hydrogen fusion – different routes through the chain produce neutrinos of different energies:

Reaction Frequency Neutrino energy (MeV)

p+p 2 H + e + or p+e _ +p 2 H + ν e + ν e Essentially all <0.420

[pep] 4 in 1000 1.442

2 H + p 3 He + photon

Solar Neutrinos – flux from Standard Model

Reaction Freq.

3 He + 3 He 4 He + 2p 85 in 100 or 3 He + p 4 He + e + + ν e [hep] 2 in 10 7 or

ν energy

<18.77

3 He + 4 He 7 Be + photon 15 in 100 7 Be + e _ 7 Li + ν e (90%) 0.861

7 Li + p 2 4 He (10%) 0.383

or 8 7 Be + p 8 B + photon B 8 Be* + e + + ν e 8 Be* 2 4 He 2 in 10,000 <15

Solar Neutrinos – flux from Standard Model Most neutrinos are low energy Only a few have high energy

Solar Neutrinos – measurements “The” Solar Neutrino Experiment First experiment, by Ray Davis, from ~1967: an 85,000 gallon tank of perchloroethylene, C 2 Cl 4 (cleaning fluid) in the 4850-ft deep Homestake Gold Mine in Lead, South Dakota

Solar Neutrinos – measurements Reaction used: ν e + 37 Cl e _ + 37 Ar 24% of all the chlorine atoms are 37 Cl. 37 Ar is a radioactive gas (half-life ~35 days): collect ~once/3 months and count decays (~15 atoms!) 1 solar neutrino unit (SNU)

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1 capture/10 36 atoms /second; 1 37 Ar atom/day

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target 5.35 SNU

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Problems: Background events – cosmic rays and local radioactivity [total: (0.4±0.16) SNU] Reaction needs neutrinos with E > 0.814 MeV

Solar Neutrinos – measurements Threshold means: can’t detect main pp neutrinos, so expect small signal, mainly 8 B neutrinos (but also pep, hep and 7 Be)

Solar Neutrinos - predictions

(from standard solar model ) From 1970, consistent average.

Current best value after 30 years: (7.6+1.3-1.1) SNU (within green band)

Solar Neutrinos - experiment Green band: current best range of prediction (7.6+1.3-1.1) SNU Blue band: experiment range to 2000 (2.56±0.23) SNU

Solar Neutrinos – the first problem

• Conclusions from chlorine experiment:

observed flux too low by factor ~3

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either theory or experiment wrong

• Can’t decide which from a single experiment, but solar models have been tested independently by helioseismology (studies of the interior of the Sun by looking at its vibrations), and the standard solar model has so far passed all the tests – non-standard solar models seem very unlikely.

Solar Neutrinos – later experiments

New experiments (since about 1980) are of three types: • Neutrino scattering in water (Kamiokande, SuperKamiokande) • Radiochemical experiments (like chlorine, but probing different energies) (SAGE, GALLEX) • Heavy water experiment (SNO)

Now look at the principles of each in turn

Solar Neutrinos – scattering experiments 1 Water detectors: Neutrino-electron scattering ν • • e _ e _ • ν • Threshold ~7 MeV – only detect 8 B (and hep) neutrinos. Electrons are accelerated to speeds v > c/n (c = speed of light, n = refractive index of water = 1.344) - “faster than light”. Leads to emission of Cerenkov radiation electrons: e _ in cone about direction of • photons

Solar Neutrinos

The Cerenkov photons are detected by thousands of photomultiplier tubes around the edge of the tank.

Solar Neutrinos – a Cerenkov detection (from SNO)

Solar Neutrinos – gallium experiments

Radiochemical experiments,

ν e

using gallium:

+ 71 Ga e _ + 71 Ge same principle as the chlorine experiment, but a much lower energy threshold of only 0.2332 MeV: can detect pp neutrinos!

Kamiokande threshold

Solar Neutrinos – gallium experiments

Detection similar to the chlorine experiment: 71 Ge is radioactive – extract and count.

But – both Ga and Ge are solids, so details are trickier – dissolve them, and separate solutions.

Two major experiments: GALLEX, Gran Sasso, Italy [Europe/USA/Israel] SAGE, Baksan, Caucasus, Russia [Russia/USA] Both ran in the 1990s; SAGE is still running and has been joined by GNO at Gran Sasso.

Solar Neutrinos – scattering experiments 2 Heavy water experiment: this uses both electron neutrino scattering [ES] ν e and two additional reactions: + d p + p + e _ [“charged current”: CC] which detects only electron neutrinos , and ν + d ν + p + n [“neutral current”: NC] in which deuterium is disintegrated by any of the three neutrino types. The electrons from the scattering and absorption reactions are again detected by Cerenkov radiation ; the neutrons are captured again by deuterium and produce γ-rays which in turn also produce Cerenkov radiation .

Solar Neutrinos – scattering experiments

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Note an important feature of scattering experiments: they detect individual neutrinos , unlike the chlorine and other radiochemical experiments, which only count the total number in a given time. This allows measurement of:

–

arrival times

–

energies

–

direction of source (the chlorine and gallium experiments have no directionality and we simply assume that the Sun is the main source)

Solar Neutrinos – new physics

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Alternative to a non-standard solar model: Models predict about 3 times the flux observed – and there are 3 neutrino types. Is this just a coincidence?

Suppose the electron neutrinos produced in the Sun change into a mixture of all three types between production and detection.

Then expect a reduction by about a factor of 3 in the number of electron neutrinos detected on Earth – can be tested if the other types can be detected.

Solar Neutrinos – new physics

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Neutrino conversion – the MSW effect [Mikhaev, Smirnov & Wolfenstein] Electron neutrinos scattering off electrons inside the Sun can be converted into mu and tau neutrinos if some neutrinos have a mass .

This is impossible in standard theories of the electromagnetic and weak forces (“electro weak theory”) but is possible in new theories that seek to unify the electromagnetic, weak and strong forces (“Grand Unified Theories” or GUTs).

Solar Neutrinos – new physics

• • •

So – the solar neutrino experiments may provide a perfect test for GUTs, and can resolve the question of whether neutrinos have a mass.

The range of masses needed to solve the solar neutrino problem is 10 -6 eV to 10 eV – very small: less than one fifty thousandth of the electron mass.

Test: some scattering experiments (and the heavy water neutral current reaction) can detect all types of neutrino, although they are most sensitive to electron neutrinos.

Solar Neutrinos – new problems

• • • •

Recent experiments have revealed not one solar neutrino problem, but four! No. 2 is: The water scattering experiments (Kamiokande and SuperK) confirm that the observed neutrinos are from the direction of the Sun.

The threshold of ~7 MeV means they detect only 8 B and hep neutrinos, not pep or 7 Be, as seen in the chlorine experiment.

But the observed rate is 3.2±0.45 SNU – greater than the total chlorine rate of 2.55±0.25 SNU.

Solar Neutrinos – new problems

• • • •

The gallium experiments provide a third solar neutrino problem.

They detect the pp neutrinos directly, so they are probing the main reaction in the pp chain.

The standard solar model predicts 73±0.7

SNU from pp and pep neutrinos alone.

Both SAGE and GALLEX/GNO find 71±6 SNU – but they should also detect 7 Be and 8 B neutrinos, amounting to at least another 50 SNU – so there is again a discrepancy.

Solar Neutrinos – results (in SNU) from all experiments but SNO The first three solar neutrino problems: the Chlorine experiment, the Kamioka mine Water experiments and the two Gallium experiments; all rates in SNU.

The fourth problem came from SNO.

Solar Neutrinos – SNO to the rescue The Sudbury Neutrino Observatory (SNO)

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The first heavy-water experiment (1999) 2-km deep nickel mine, Sudbury, Ontario 1000 tonnes of ultra-pure D 2 O Sphere of 9456 light sensors Supporting fluid – ultra-pure H 2 O

Solar Neutrinos – SNO to the rescue

• • • •

SNO results – part 1: June 2001 CC reaction (ν e + d p + p + e _ ) : ν e only Electron scattering (ES): weak sensitivity also to mu and tau neutrinos (flux poorly determined by SNO, but consistent with the very precise measurement from SuperK) The ES flux should be the same as the CC flux if the solar electron neutrinos are unchanged – there should be no other neutrinos But: the ES flux is larger, showing that some electron neutrinos become mu and tau neutrinos on the way from the Sun. This is …….

Solar Neutrinos – results from SuperK and SNO 1 (in SNU) ….. the fourth solar neutrino problem SuperK ES (SuperK) flux is more than SNO CC flux; both are smaller than the Standard Model prediction SNO

Solar Neutrinos – SNO to the rescue

SNO results – part 2: April 2002

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The NC reaction (ν + d ν + p + n) is equally sensitive to all neutrino types

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April 2002 saw the release of the NC results, which for the first time measured the flux of all neutrinos from the Sun – and it agreed with the predicted neutrino flux from the Standard Solar Model !

Solar Neutrinos – results from all experiments (in SNU) SNO: all ν

Solar Neutrinos – SNO to the rescue Other SNO results

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The agreement of the total neutrino flux with the solar prediction proves (with 99.99% certainty) that neutrinos do change type

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If the MSW effect in the Sun causes the type change, there should be a similar smaller effect caused by the Earth, giving a day-night difference in the neutrino flux

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There is a small but definite difference (6% rather than a factor of 3!)

Solar Neutrinos – SNO to the rescue

• • •

Consequences of SNO results

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The type change means that (at least some) neutrinos must have mass The day-night flux difference enables a mass difference to be determined but not yet any individual masses The best-fit mass difference is 7



10 -3 eV This is in line with earlier predictions, but is probably too small to contribute significantly to the mass density of the Universe

Solar Neutrinos

• • •

Conclusions

The four solar neutrino problems showed either that we didn’t understand the centre of the Sun or that non-standard physics was needed: neutrinos must have mass By the 1990s, most people believed neutrinos must have mass – but there was no proof The SNO results proved for the first time that the solar neutrino problem did indeed imply that neutrinos have mass: there is no longer a problem