The Solar Neutrino Problem Barbara Sylwester Zakład Fizyki Słońca CBK PAN

One page story - the short story Fusion reactions in the core of the Sun produce a huge flux of neutrinos. They can be detected on Earth using large underground detectors. The measured flux can be compared with theoretical calculations (based upon our understanding of how does the Sun work and the details of the SM of particle physics). The measured flux is too small than expected from theory. The mystery which caused the deficit was called Solar Neutrino Problem. Are the experiments in error? Highly unlikely. We must TRUST the measurements – there are many of them, prepared by different groups, all use diverse detection techniques, calibrated with a variery of sources. Is our model of the solar interior wrong? (We do not understand the Sun well enough.) NO - Reducing the temperature of the Sun by 6% would entirely explain GALLEX data, however the solar seismologists, argue that such a change is not permitted by their results. Is our particle physics wrong? Neutrinos do something unusual beyond the standard theory that accounts for the observed anomaly. YES - there are 3 kinds (called flavors) of neutrinos: electron, muon and tau-neutrinos and their passage through matter can cause one neutrino flavor to „oscilate” into another. Thus the „missing” solar neutrinos could be electron-neutrinos which changed into other types along the way to Earth and therefore escaped detection.

The long story Neutrino – Some facts What is a neutrino? Solar neutrino spectrum Results of past solar neutrino experiments On possible solutions of the problem Additional results (time variations, correlations) Summary New experiments

Neutrino – some facts Neutrino first was postulated in 1930 by Wolfgang Pauli as a solution to a frustrating problem of missing energy in a nuclear reaction called beta decay. He concluded that the products of beta decay must include a third particle which didn’t interact strongly enough for it to be detected. Enrico Fermi called this particle the neutrino which means in Italic „little neutral one”. The neutrino was detected for the first time in The first observation of a neutrino was made by Frederick Reines, who received the 1995 Nobel Prize for this work. The standard unit of neutrino flux is called a "solar neutrino unit" or SNU. Solar Neutrino Unit (SNU) = captures per atom per second As solar neutrinos originate from the nuclear fusion powering the Sun, one can say that the Sun is not producing enough "snus".

What is a neutrino? Neutrino e   Charged Partner electron (e) muon  tau  ) Neutrinos do not carry electric charge. Because they are electrically neutral, they are not affected by the electromagnetic forces and only by a "weak" sub- atomic force of much shorter range than electromagnetism. Therefore they are able to pass through great distances in matter without being affected by it. Three types of neutrinos are known. Each type or "flavor" of neutrino is named after their charged partner (leptons). Hence we have: the electron, muon and tau neutrinos. According to SSMs the Sun produces only electron neutrinos. The Standard Model of particle physics assumes that neutrinos are massless.

Neutrinos -Summary The neutrino is a light (some say massless), neutral (no electrical charge) particle virtually non-interacting with matter. Millions of millions of them are crossing the Earth at each second, but only very few of them would interact with the Earth. In practice you can say - they are invisible. So how can we detect them? Well - you can guess the answer by now - by building a very large detector and waiting long enough.

Solar neutrino spectrum The Sun produces neutrinos with a range of energies  Solar neutrino spectrum predicted by the SSM (Bahcall and Pinsonneault 2004). The spectra from the pp chain are drawn with solid lines; the spectra from reactions with carbon, nitrogen, and oxygen (CNO) isotopes are drawn with dotted lines.  different detectors are sensitive to different energy range. Different green semi-tones denote the thresholds for various targets in the experiments: chlorine (C 2 Cl 4 ): in an old gold mine in Dakota, , R. Davis gallium: GALLEX, Gallium Neutrino Observatory (GNO), Gran Sasso, Italy, the successor project of GALLEX, presently taking data from 1998, RuSsian American Gallium Experiment (SAGE), near Elbrus Mt. (Caucasus), water: SNO (Sudbury Neutrino Observatory), Sudbury, Canada, , Kamiokande, , Super-Kamiokande, finished in 2002, Japan

GALLium EXperiment-GALLEX The target consists of 30.3 tons of gallium, containing 12 tons of 71-gallium, in the form of aqueous gallium chloride solution (101 tons). The target has to be so large because neutrinos only interact very weakly. The determination of the neutrino flux is based on the observation of the interactions between neutrinos and 71-gallium atoms, with the consequent production of 71- germanium atoms. The experiment is sensible to the low energy neutrinos produced in the proton-proton reaction (the principal component of thermonuclear reactions occurring inside the Sun). International collaboration with scientists from France, Germany, Italy, Israel, Poland and the US. Located in San Grasso, Italy.

GALLEX results

Results of GALLEX and its succesor GNO Error bars are ±1s, statistical only.

SAGE; RuSsian American Gallium Experiment To shield the experiment from cosmic rays, it is located deep underground in a specially built facility at the Baksan Neutrino Observatory in the northern Caucasus mountains of Russia (near Mt. Elbrus).

The Super-Kamiokande The Super-Kamiokande is joint Japan-US large underground detector (world's largest underground neutrino observatory ). It is a 50,000 ton tank of water, located approximately 1 km underground in the Kamioka Mine, about 200 km north of Tokyo. The water in the tank acts as both the target for neutrinos, and the detecting medium for the by-products of neutrino interactions. To detect the high-energy particles which result from neutrino interactions, Super-Kamiokande exploits a phenomenon known as Cherenkov radiation. In addition to the light collectors (called "photo-multiplier tubes„) and water, a forest of electronics, computers, calibration devices, and water purification equipment is installed in or near the detector cavity.

Results

What is the solution? Astrophysical Solution (requires a change in the way we think about the Sun) One way to solve the solar neutrino problem is to lower the central temperature of the Sun by a few percent. This will mean fewer high-energy nuclear reactions occurring in the solar core and thus, fewer neutrinos being produced and hence detected. There are a number of ways to lower the central solar temperature. Helioseismology results contradict such solution! Physical Solution (requires a change in the way we think about neutrinos) A current theory in particle physics states that it is possible for neutrinos to transform from one type to another. The Mikheyev-Smirnov-Wolfenstein (MSW) effect claims that electron neutrinos may transform or oscillate into either muon or tauon neutrinos. Therefore, some of the electron neutrinos produced by the Sun are being transformed into the other types that we are not detecting.

What is the solution? The first strong evidence for neutrino oscilation (and so non zero mass) came in 1998 from the Super-Kamiokande collaboration. (Although no tau neutrinos were observed they announced the discovery of evidence for neutrino mass.) More direct evidence came in 2002 from the Sudbury Neutrino Observatory (SNO) in Ontario, Canada. It detected all types of neutrinos coming from the Sun, and was able to distinguish between electron-neutrinos and the other two flavors. The total number of detected neutrinos agrees quite well with the earlier predictions from nuclear physics, based on the fusion reactions inside the Sun. In 2002 Raymond Davis and Masatoshi Koshiba won part of the Nobel Prize in Physics for the work in this direction.

Time variations. Any correlation? Neutrinos and Sunspots: The Homestake experiment has been running for over two solar activity cycles and it has been noticed that the neutrino fluxes are not constant. Many researchers have tried to link solar surface activity with neutrino fluxes and, depending upon whether you believe their statistical arguments, have succeeded. Super-Kamiokande: 1.5 month averages of residual fluxes after subtraction the effect of the Earth’s orbital motion  Neutrino fluxes may vary with about 30- month period, no positive correlation with solar activity. T. Shirai, 2004, Sol. Phys. 222, 199 Day night asymmetry (2000) D-N = ± ± D+N

Summary A mechanism (called MSW, after its authors) has been proposed, by which the neutrinos can change flavor between electron, muon, and tau neutrino types. The MSW phenomenon, also called "neutrino oscillation", requires that the three neutrinos have finite and differing mass, which is still unverified. In 1998 the Super-Kamiokande neutrino detector determined that neutrinos do indeed flavour oscillate, and therefore have mass. The experiment is only sensitive to the difference in the squares of the masses. These differences are known to be very small, less than 0.05 electron volts (Mohapatra, 2005). Combined, these constraints imply that the heaviest neutrino must be at least 0.05 eV, but no more than 0.3 eV.

Summary The best estimate of the difference between the mass eigenstates 1 and 2 was published in 2005 by KamLAND team: Δm 21 2 = eV 2 In 2006, the MINOS experiment measured oscillations from an intense muon neutrino beam, determining the squared mass difference between neutrino mass eigenstates 2 and 3. The initial results indicate Δm 32 2 = eV 2, consistent with previous results from Super-K. MINOS - Main Injector Neutrino Oscillation Search, is an experiment at Fermilab designed to study neutrino oscillations.

Implications For particle physics; the fact that neutrinos do have mass now has to be incorporated into the Standard Model. The cosmological effects of neutrinos with mass (the problem of missing mass or dark matter). If neutrinos have mass - even if it is absolutely minuscule - they could account for a part of the dark matter, or 'missing mass', in the Universe. The neutrino with mass is the serious candidate actually known to exist (there are however many more candidates).

End of the story? The puzzle is not yet completely solved, the research is continuing, more results from new experiment are expected. Further data with better statistics are needed to settle the matter. The mystery appears close to being solved, but the story is not finished yet.

New experiments All mentioned solar neutrino experiments (Chlorine, SUPERKAMIOKANDE, SAGE, and GALLEX) show that the measured solar neutrino flux at the orbit of the Earth is considerably less than predicted by the Standard Solar Model. Because the reduction of the solar neutrino spectrum is most pronounced at intermediate energies (~1 MeV), new detectors that can measure the neutrino radiation from the Sun in this energy regime are especially needed. Several such detectors are in various stages of development and deployment, such as BOREXINO at Gran Sasso, KamLAND in Japan, and the iodine detector at Homestake. MINOS-Main Injector Neutrino Oscillation Search, is a long-baseline experiment designed to study neutrino oscillations (an effect which is related to neutrino mass). It uses two detectors, one located at Fermilab (near Chicago), at the source of the neutrinos, and the other located 450 miles away, in northern Minnesota, at the Soudan Underground Mine State Park in Tower-Soudan.

Neutrino image of the Sun The first solar image in neutrino “light” reconstructed based on the observations made by w Super-Kamiokande. White colour corresponds to the highest number of registered neutrinos and colours from yellow, through red to blue correspond to decreasing intensity of observed neutrinos. Prof. A.K.Wroblewski, Wiedza i Życie, nr 1/1999

The End