Published on August 18, 2009
The Sun and the particle physics Vladislav Kobychev Insitute for Nuclear Research, Kiev, Ukraine
1. The Sun: basic facts 2. Solar neutrinos 3. Solar axions 4. The Sun and WIMPs
Basic facts on the Sun: •Central temperature: 15 710 000 K 1.4 keV •Surface temperature: 6000 K •Age: 4.6·109 yr. •Distance from Earth: R = 150·106 km (1 Astr. Unit) •Diameter: D = 2R = 1.4·106 km (108 x D ) •Mass: M = 2·1030 kg (300 000 x the Earth's mass) •Average density: 1400 kg/m3 (1.4 x water) (central density: 150 x water). •Composition: 74% H, 24% He, 2% of heavy elements •Solar energy reaching Earth: 1400 W/m2 (solar constant) •Total luminosity: L =1400 W/m2 x 4 x R2 = 4·1026 W The Sun is a very typical star: nothing special. The only special feature (for us) is that it is the closest star.
Production of solar neutrinos pp-chain: pp, pep, 7Be & 8B neutrinos
Production of solar neutrinos Net reaction: p+ + p+ + p+ + p+ + e– + e– 4He (p+ + p+ + n0 + n0) e e
e - what does it mean? The Standard Model of particle physics forbids conversion between different kinds (flavours) of neutrino and considers them massless.
Production of solar neutrinos CNO-chain: 13N, 15O & 17F neutrinos
Production of solar neutrinos Every second, the Sun produces 2·1038 electron neutrinos, and almost all of them escape to the space (very weakly interacting with the matter). On Earth, the solar neutrino flux is ~60 000 000 000 neutrinos/(cm2 s).
Several experiments have been carried out to measure the solar neutrino flux. Homestake: Homestake: radiochem. Cl-Ar experiment. Cl- SAGE and GALLEX/GNO: radiochem. Ga-Ge – GALLEX/GNO: Ga- large fraction of pp neutrinos + 7Be neutrinos. Kamiokande and Super-Kamiokande: water Super-Kamiokande: Cherenkov -- 8B neutrinos. SNO: heavy water -- 8B neutrinos + neutral SNO: current reactions with muon and tau neutrino + elastic scattering of all flavours. Borexino: Borexino: liquid scintillator -- the first experiment to detect low-energy solar neutrinos in real time low-
History of solar neutrino experiments:
Homestake experiment The Homestake solar neutrino experiment was in the Homestake gold mine in South Dakota. The detector was constructed at Brookhaven National Laboratory in the late 1960s by a collaboration led by Dr. Raymond Davis, and has been operated continuously since 1970 (taken over by UPenn in 1984). The detector was a big tank containing 615 tons of liquid perchloroethylene (C2Cl4). Neutrino detection via the reaction: e + 37Cl e - + 37Ar, Eth = 0.814 MeV Collection of radioactive 37Ar (few tens atoms per month!) from all the tank into a small counter.
Homestake Construction (1966)
Homestake Experiment The Solar Neutrino Unit (SNU) = 1 neutrino interaction per second per 1036 atoms But 1036 atoms is ~ 240 million tons of chlorine Homestake could contain ~1030 atoms So, Homestake counted only ~2.5 neutrinos per day (2.55 ± 0.25 SNU) Based on the Standard Solar Model, one should expect 8 ± 1 SNU “Solar neutrino problem”
Homestake experiment The Nobel result* obtained by Davis: The flux of solar neutrinos is ~3 times less then the predicted. predicted. *) The Nobel Prize in Physics, 2002
Solar neutrino problem •All existing experiments on detecting of solar neutrinos gave 1/3 to 1/2 of the predicted neutrino flux (Standard Solar Model + weak interactions theory) •Beside this, different experiments gave different (and disagreeing) experiment-to-theory ratios.
Solar neutrino problem
Solar neutrino problem Possible solutions: 1. We don’t understand the solar interior 2. We don’t understand the behaviour of neutrino
Neutrino oscillations If electron, muon and tau lepton numbers don’t conserve, the neutrino of one flavour can transform to other one. This requires neutrino masses should be non-zero and different mass states non- should have different masses. The neutrino oscillations were predicted by Bruno Pontecorvo in 1957 by analogy with the observed neutral kaon oscillations.
Solar neutrino oscillations Before SNO all the experiments detected only electron Solar neutrinos. SNO found that the most of the flux of solar neutrino are not electron ones. ones. Flux of mu and But the solar core emits tau neutrinos only electron neutrinos, so neutrinos, neutrinos have to flavour oscillate flying from the solar core to the Earth. It is possible only in the Flux of electron case when the flavour neutrinos states of neutrino do not coincide with their mass states, and these mass states have to have (different) masses: disagreement with the Standard Model!
MSW Effect The neutrino oscillations are resonantly gained when the neutrino flux propagates through matter with slowly changing density (Mikheev-Smirnov- (Mikheev-Smirnov- Wolfenstein effect). The effective mass of neutrino depends on the density of electrons in the matter, and in some point where the effective masses of different neutrino flavours crosses, the oscillation enhancement appears. MSW effect explains the observed difference of solar neutrino fluxes in different experiments: Low energy (pp) neutrinos (SAGE, GALLEX) (pp) GALLEX) survive with higher probability: probability: Pee P( e e) = 56%. 56%. High energy (8B) neutrinos survive with lower probability: probability: Pee ~ 32% (theory, SNO) even their passage through Earth can have an observed effect (day-night assimmetry) (day- assimmetry) No good data for intermediate energies.
Laboratori Nazionali del Gran Sasso
Abruzzo, Italy Laboratori 120 km of Rome Nazionali del Gran Sasso External Buildings Asserg (AQ) of the Laboratory Italy ~3500 m w.e. 1 km of depth detector26and supplementary equipment
Steel sphere 278 t PC+PPO (R=6,85 m): (1,5 g/l) - 2212 PMT (8”); - 1350 m3 PC+DMP (5.0 g/l) 2100 m3 water tank: Two 0.125 mm - R=9 m, H=16,9 m; nylon spheres: - 208 PMT in water, - R=4,25 m; looking outwards; - R=5,5 m - shielding from -(Rn-barrier) , &n
Physical program of : Monoenergetic = 862 keV) solar 7Be keV) neutrinos; neutrinos; pep & CNO neutrinos; Antineutrino from reactors and the Sun; Sun; Geoneutrinos; Geoneutrinos; Supernova neutrinos (?) … Rate of (SSM, MSW-LMA), prediced for
Registration of Neutrinos scatter on electrons of liquid organic scintillator (pseudo-cumene): (pseudo-cumene): Low threshold of registration; registration; Good energy resolution; resolution; Good space reconstructions. reconstructions. BUT… No directional sensitivity; sensitivity; No selection between events and other (natural radioactivity). radioactivity). !! HIGHEST REQUIREMENTS TO RADIOPURITY OF SCINTILLATOR AND OTHER MATERIALS !!
RADIOPURITY Uranium Thorium 10-6 g/g in natural substances (human body etc.) 10-17 g/g in Borexino scintillator Potassium 10-4…10-2 g/g in natural substances ~300 g in a 70 kg human body (4000 decays/s of 40K) 10-14 g/g in Borexino scintillator The center of the Borexino detector is possibly the most radiopure place in the Universe.
Data collection The collected data allow to reconstruct for every event: •Total absorbed energy •Position (±10 cm) •Kind of particle (alpha, beta, muons)
The preliminary result of Borexino 49 ± 3stat ± 4syst 7Be / (day · 100 t) (day
Comparison with theor.predictions for : exp.: 49 ± 3stat ± 4syst evt./(day · 100 t) 49 75 ± 4 evt./(day · 100 t) without oscillations evt. 49 ± 4 evt./(day · 100 t) with MSW-LMA evt. MSW- oscillations m122 = 7,92·10–5 2, sin2 12 = 0,314) for the Standard Solar Model BPS07(GS98)
Axions Axion is a hypothetical neutral massive particle, introduced to theory in connection with the problem of strong CP-violation. The QCD includes the so-called -phase, which is experimentally very small (0 or, at least, <10–10), but its smallness is not required by the theory ( -phase can take any value between 0 and ). Peccei and Quinn (1977) proposed a mechanism to make -phase equal 0 by introducing a new symmetry, with being a dynamical variable, of zero value at the minimal energy state. The spontaneous violation of the PQ symmetry creates the Goldstone boson, which was named axion by Frank Wilczek. Axions are considered as one of the best candidates for the Dark Matter particles, because they are massive and their interaction with normal matter should be extremelly small. 23.03.2009 23.03.2009 « » 6
Axion interactions DFSZ-axion hadronic axion Compton effect Primakoff’s effect Bremsstrahlung Mass of axion varies from 10–18 to 1 MeV in different variants of theory. Spin/parity of axion is 0– (pseudoscalar particle). The leptonic axion (DFSZ- axion) interacts with leptons directly, the hadronic axion (KSZV-axion) – only with hadrons.
Laboratory search for solar axions Based on axion-photon conversion in magnetic field. External field Axion Axion
Laboratory search for solar axions The predicted solar axion luminosity for DFSZ-axions: 2 ma3 La 3.6 10 L 1 Their mean energy is predicted to be 4.2 keV. In order to registrate the axions, they are converted to X-ray quanta with strong transversal magnetic field. X-rays are then detected by an appropriate detector. The most sensitive experiment of this kind is the axio- helioscope CAST (CERN), using a huge de- comissioned accelerator magnet.
Laboratory search for solar axions Resonant absorption of solar axions. The thermal excitation of low-energy nuclear levels (of few keV, f.i., 57Fe) can be excited in the solar core (T= 1.4 keV). These levels can (in some conditions) deexcite via emission of an axion which escapes from the Sun almost freely. In Earth, the axion can resonantely excite a nucleus of the same kind which then deexcites by emission of a detectable gamma quantum. Many experiments are based on this scheme. Modification: the level of the nucleus-emitter is populated not by thermal excitation, but in a nuclear reaction (for example, the 478 keV excited level of 7Li is populated by the electron capture of 7Be in the pp- chain with ~10% branching ratio)
Solar axions: continuous spectrum axions: (Primakoff’s effect: photon-to-axion photon-to- conversion in the electric field of a nucleus) 43
57Fe 14.4 keV The monoenergetic lines can also be present in the solar axion spectrum. 44
Solar Core Laboratory 57Fe 57Fe 14.4 keV 14.4 keV gaNN a gaNN 1. Thermal excitation 3. Resonant excitation of a 2. Emission of a target 57Fe nucleus by the axion. monoenergetic axion. 4. Emission of gamma quantum. 5. Detection. The method was proposed: Moriyama [PRL 75(1995)3222]. proposed: Other natural isotopes with low-lying levels, de-excitated via M1-transitions, can be (and are) also used; for 45 example, 83Kr (9.4 keV).
57Fe(‘iron’) solar axions allow to exclude axion mass values between ~14.4 keV and (on today) 0.216 keV [T. Namba, PLB 645 (2007) 398]. Other possibility – non-thermal excitation of source nuclei. non- 7Li is created in the pp- chain (the main energy source of the Sun). 46
Solar Core Laboratory 7Li 7Li 7Be 477.6 keV 477.6 keV gaNN a gaNN 1. Level population via 3. Resonant excitation of a electron capture of 7Be target 7Li nucleus by the axion. 2. Emission of 4. Emission of gamma quantum. a monoenergetic axion. 5. Detection. First exp.: M. Krcmar et al. [PRD 64 (2001) 115016] (ma < 32 keV). keV). Best limit: A.V. Derbin et al. [JETP Lett. 81 (2005) 365] limit: (ma < 16 keV). 47
Our experiment: 1. Lithium fluoride (LiF) was chosen as a target due to: a) its high density of Li nuclei in comparison to other Li compounds; b) chemical passivity; c) non-hygroscopicity. non- 2. Few samples of LiF (powder of 99.99% purity, single crystal) were placed in two HPGe detectors in Laboratori Nazionali del Gran Sasso (3800 m w.e.). 48
If we would observe a gamma peak at 478 keV with area S, mass of axion would be ma = 1.55×1011 × (S tN7)1/4 eV – efficiency of the detector, t – time of measurement, N7 – number of 7Li nuclei in the sample. ma < 13.9 keV (90% C.L.) 49
LiF(W) single crystals Total mass is ~550 g. 50
Of course, the search for mono-energetic axions from the Sun can be performed also without resonant nucleus as a target (the resonant target only allows to decrease its mass by increasing the cross-section). Such the searches were carried out by Borexino and CAST collaborations (both are mentioned above) for 7Li solar axions.
Geophysical search for solar axions Another interesting idea: we have a lot of iron within the Earth core; let us consider it as a target for “iron” solar axions. The resonant absorption of 14.4 keV axions by 57Fe nuclei would heat the Earth core, and the thermal flow through the Earth surface outwards (measured: ~42 TW) would give us the upper limit on probability of such the process. Taking into account that the part of this heat flow is produced by radioactive transitions (U, Th, K) in the Earth crust, we can set the upper limit on the hadronic axion mass of ma<1.6 keV. Danevich et al., Kinematics and Physics of Celestial Bodies, 25(2009)102.
Indirect search for WIMPs by their annihilations within the Sun The Dark Matter is non-barionic matter which dominates in the Universe but is “invisible” – it is still observed only by its gravitaitonal influence to the normal matter. The most common hypothesis is that the DM consists of Weakly Interacting Massive Particles – WIMPs. They are stable and have a mass of tens GeV or more. There are many direct experiments to search for WIMPs, but indirect observations are possible too. One of ways to observe WIMPs indirectly is the search for high-energy neutrinos emitted by annihilating WIMPs captured by the Sun.
Indirect search for WIMPs by their annihilations within the Sun The neutrinos from WIMPs annihilation with energy of GeVs or tens GeV can escape from the Sun and be detected by an appropriate high-energy neutrino (HEN) detector. The collaborations IMB, Kamiokande, AMANDA and MACRO already tried to extract such the information from their observations. For example, AMANDA detector is placed within ice many- km layer on the South pole. A HEN from the Sun propagates through Earth and interacts with rock or ice under the detector. This interaction creates a high-energy muon that keeps the direction of the primary neutrino moving; the PMTs of AMANDA will see an upwarding muon by emission of Cherenkov light in ice. So, surprisingly, this kind of observation of the Sun should be carried out in night, when the Sun is below the horison.
Conclusions I have briefly reviewed the particle physics experiments which are using the Sun as a unique strong source of weakly interacting particles, known or hypothetical (neutrinos, axions and axion-like particles). As it is a very wide area of research, I have focused upon the experiments I participated myself. 1. Observations of solar neutrinos demonstrate that they transforms (oscillates) from electron to muon and tau neutrino and give information on their properties (masses, mixing angles, magnetic moments etc.). By-product: our solar models are correct. 2. Different models of hypothetical axions are checked by using the Sun as a source. 3. Hypothetical WIMPs captured by the Sun and ahhihilating in its core can be detected by observation of high-energy neutrinos – annihilation products.
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