SOLAR NEUTRINO PROBLEM SOLVED

Fakulteta za matematiko in fiziko Oddelek za fiziko Jadranska 19 1000 Ljubljana UROŠ BOROVŠAK SOLAR NEUTRINO PROBLEM SOLVED ADVISOR dr. TOMAŽ PODOBNIK Ljubljana, April 2, 2003

Abstract Since the end of the sixties, physics has been occupied with the so-called Solar Neutrino Problem. Since the first experiment in Homestake, all experiments measuring the solar neutrino flux on the Earth have been showing a deficit of the neutrinos when compared to the theoretical predictions of the Standard Solar Model. New results from the SNO experiment served some strong evidence that the discrepancy can be explained by the neutrino flavour transformation. Povzetek Fizika se že od konca šestdesetih let ukvarja s tako imenovanim problemov sončevih nevtrinov. Vse meritve, ki so jih opravili od prvega eksperimenta v Homestaku naprej, kažejo na primanjkljaj sončevih nevtrinov na Zemlji v primerjavi s teoretičnimi napovedmi standardnega sončevega modela. Najnovejši rezultati pridobljeni z eksperimentom SNO pa so postregli z razlago primanjkljaja sončevih nevtrinov s pomočjo nevtrinskih oscilacij. 1

Table of contents Abstract... 1 Introduction... 3 Solar Neutrinos production... 3 Solar Neutrino Experiments... 3 Homestake experiment... 3 Kamiokande and SuperKamiokande experiments... 5 Neutrino oscillations... 8 The SNO experiment...9 Electron Scattering...9 Charged Current Reaction (CC)... 10 Neutral Current Reaction (NC)... 10 Conclusions... 12 Bibliography... 12 2

Introduction Neutrinos are neutral elementary particles which interact with matter through the weak interactions. Due to the weakness of the interaction the matter is almost transparent for neutrinos. In the Sun, neutrinos are copiously produced in the nuclear fusion and decay processes within the core. Since they rarely interact, these neutrinos pass through the Sun and the Earth (and you) unhindered. For example, the sun produces 1.8 x 10 38 neutrinos every second. Billions of neutrinos stream through your body every second, yet only one or two of the higher energy neutrinos will scatter from you in your lifetime [1]. Solar Neutrinos production The Standard Solar Model (SSM) describes the Sun and is based on observed parameters of surface luminosity, surface temperature, solar mass and solar radius. The energy is produced mainly by the p-p chain reaction set (Figure 1). The light and the neutrinos are by-products of the chain [2]. The net result of this chain is a fusion of four protons into a single helium nucleus 4 He [3]. Within the Standard Solar Model a spectrum of solar neutrinos can be calculated (Figure 2). Spectrum can be further used for predicting rates of solar neutrinos detected by different experiments on the Earth. The predictions of SSM in are usually quoted in terms of Solar Neutrino Units (SNU). SNU is a product of calculated solar neutrino flux on Earth and neutrino absorption cross section in target atom, in units of 10-36 absorption per target atom per second [2]. Solar Neutrino Experiments Homestake experiment The first solar neutrinos were detected by the chlorine detector in the Homestake mine. The detector has been operating since the beginning of the seventies. It was built in an old mine, approximately 1500m below the surface; layers of material above the detector substantially diminishes background from cosmic-ray muons. Muons are disturbing measurements with their interaction with detector material, producing same products as neutrinos. Detector is filled by 615 tons of C 2 Cl 4 (Figure 3). 3

Figure 1: The p-p chain reaction set, most important reactions are marked [4]. Figure 2: Solar Neutrinos spectra for reactions in the p-p chain reaction. The thresholds for different types of experiment are also shown [5]. Figure 3: The chlorine detector in Homestake, a picture of the tank under construction (left), a look at the tank (right) [6]. 4

In Homestake, neutrinos from the Sun are detected via reaction [2]: ν e + 37 Cl 37 Ar + e -. (1) The cross-section for reaction is 4.26 x 10-42 cm 2. The product 37 Ar is extracted by radiochemical method. The chemical differences between chlorine and argon are enabling the researchers to extract even single argon atoms from the fluid. A small known amount of pure 36 Ar or 38 Ar is added as a carrier gas to the fluid at the beginning of each run, and this argon is removed (along with the 37 Ar "signal") by bubbling a helium carrier gas through the tank and passing this gas through a charcoal filter. The fraction of argon recovered (about 95%) is a direct measure of recovery efficiency, and the number of radioactive 37 Ar atoms is determined using proportional counters. The 37 Ar decays with a half-life of 35 days by capturing an electron from K (~ 90%) and L (~ 10%) shells. The K capture produces Auger electrons with energy of 2.82 KeV. The threshold energy for detection is 0.814 MeV. In average, only one third of predicted flux was measured (Table 1); approximately 46 detected decays of 37 Ar per harvest (every 2 months) were expected, but only 14 were observed. This discrepancy between experimental measurements and theoretical predictions is called the solar neutrino problem (SNP); either the Homestake experiment or the predictions of the neutrino's flux within the SSM were wrong. Detector Predicted event rates Measured event rates Homestake 1. 2 7.7 1.0 2.56 ± 0.16 ± 0.16 Table 1: Event rates for Homestake Experiment (The values are given in SNU (10-36 absorption per target atom per second)) [2]. Kamiokande and SuperKamiokande experiments The SuperKamiokande detector was put in operation in 1996, following a similar but smaller Kamiokande experiment. The SK detector is located 2700 meters of water equivalent below the surface in the Kamioka Mozumi mine in Japan. SuperKamiokande consists of a huge cylinder (2r = 40m, h = 40m), filled with 50.000 tons of water. The walls of the tank (side, top and bottom) are covered by about 13000 photomultiplier tubes (PMT's) (Figure 4). 5

Figure 4: SuperKamiokande detector, a schema of the detector (left), a picture of interior of detector (right) [7] [4]. Solar neutrinos in Super-Kamiokande are measured through the following reaction [2]: ν x + e - ν x + e -. (2) This is the so-called electron scattering reaction (ES) (Figure 5). Although the reaction is sensitive to all neutrino flavours (the neutrino in reaction is labelled with x that means all flavours), the electron neutrino dominates by a factor of six. When the speed of the recoiled electron exceeds the speed of light in water, it emits the Cherenkov light, which is detected by the photomultiplier tubes (PMTs) ( Figure 6); the amount of light is proportional to the incident neutrino energy [8]. Figure 5: Electron scattering (ES) [8]. 6

Figure 6: A coin of the Cherenkov light emitted by a recoiled electron from reaction (2), projected on the wall of the SK detector [4]. The recoiled electrons are approximately collinear with the incident neutrinos ( Figure 7), which allows for an efficient background rejection. The direction of recoiled electron coincides with the symmetry axis of its Cherenkov light coin. Figure 7: Angular distribution of events in SuperKamiokande detector with respect to cosθ sun, where cosθ sun represents the angle between the Sun and the direction of candidates for recoiled electron in (2) [9]. The measured fluxes (2) with observed Cherenkov ring in the detector SK (Table 2) confirm the deficit of the measured solar neutrinos flux detected on the Earth when compared to predictions based on the SSM. 7

Detector Predicted 8 B neutrinos flux Measured neutrinos flux SuperKamiokande + 1. 01 5.05 + 0.08 2.32 ± 0.03 0.81 Table 2: Results from SuperKamiokande (Results are given in 10 6 events cm -2 s -1 ) [9]. Neutrino oscillations The most elegant solution of the SNP that would explain the observed solar neutrino rates on the Earth involves the so-called neutrinos oscillations ( Figure 8). The solution assumes that a fraction of electron neutrinos produced in the Sun's core changes it flavour before hitting the Earth. The neutrinos with changed flavour (muon neutrinos or tau neutrinos) cannot produce 37 Ar in the Homestake chlorine experiment while their cross-section for the ES in the SK detector is for about a factor of six smaller than the corresponding cross-section of electron neutrinos. The oscillations can occur if neutrinos are massive. If so, the neutrinos with well defined lepton numbers (ν e and ν µ ) do not necessarily coincide with the states with well defined masses (ν 1 and ν 2 ). The bases can be rotated with respect to each other for a relative angle Θ. 0.07 The oscillations then take place in the case of the non-zero mass differences m 2 = m 2 2 1 m2 for the states ν1 and ν 2, and for the non-trivial mixing angles Θ 0, Π/2, Π,. Figure 8: Neutrinos oscillating [4]. Figure 9: Two neutrino bases [4]. 8

The SNO experiment The idea of the oscillation of solar neutrinos was thoroughly tested by the SNO (The Sudbury Neutrino Observatory) experiment in Canada. SNO cavity is located 2070m (6010m of water equivalent) below the surface [8]. SNO detector contains 1000 tons of heavy water D 2 O (impurities must be less than 10-14 g/g of heavy water), and surrounded with 7000 tons of light water (with impurity levels maintained at less than 10-13 g/g of water). The Cherenkov light is detected with 10.000 PMTs. Figure 10: A schema of the SNO detector (left) and a picture of the acrylic vessel for heavy water (right) [10][8]. Because of the heavy water neutrinos can be detected via three different reactions: Electron Scattering This reaction is not unique to heavy water and is also exploited for neutrinos detection in the light water detectors. We have already described it when we were discussing about SuperKamiokande experiment (Figure 5). 9

Charged Current Reaction (CC) ν e + d p + p + e -. (3) After an interaction of an electron neutrino with a neutron in a deuteron, the neutron transforms into a proton, and the neutrino into an electron ( Figure 11). When the electron exceeds the speed of light in the medium (heavy water) it emits Cherenkov light, detected by the photomultiplier tubes. Figure 12 shows a ring of such Cherenkov light in the SNO detector. This reaction is specific for electron neutrinos and we can only detect them by it. Figure 11: Charged Current Reaction (CC) [8]. Figure 12: Computer reconstructed ring of the Cherenkov light in the SNO detector [11]. Neutral Current Reaction (NC) ν x + d p + n + ν x. (4) In this reaction neutrino reacts with the deuterium and the deuterium nucleus breaks apart; the liberated neutron is then thermalized in the heavy water as it scatters around. The reaction can eventually be observed due to gamma rays which are emitted when the neutron is finally captured by another nucleus ( Figure 13). The gamma rays scatter electrons which 10

produce detectable light through the Cherenkov process discussed above. The neutral current reaction is equally sensitive to all three neutrino types. Figure 13: Neutral Current Reaction (NC) [8]. The idea behind the experiment is the following. If the neutrinos flavour oscillations are indeed responsible for the deficit of the flux of electron solar neutrinos on the Earth, the rate of each of the three neutrino interactions in the SNO detector will be affected in a different way. The rate of the NC is completely insensitive to the neutrino flavour oscillations since the cross-section for the reaction is the same for any of the three neutrinos flavours (electron, muon and tau). On the other hand the effect of the oscillations would be most pronounced in case of the CC interaction, which can only be realised by the electron neutrinos with the unmixed flavour. Oscillations would also substantially diminish the rate of the electron scatterings since the cross-section for the neutrinos with the changed flavour (muon and tau) is for a factor of six smaller then the cross-section of the electron neutrinos. Reaction Predicted 8 B neutrinos flux Measured neutrinos flux CC + 1. 01 + 0.06 0.09 5.05 0.81 1.76 0.05( stat ) + 0.09( syst) ES + 1. 01 + 0.24 0.12 5.05 0.81 2.39 0.23( stat ) 0.12( syst) NC + 1. 01 + 0.44 0.46 5.05 0.81 5.09 0.43( stat ) 0.43( syst) Table 3: Results from the SNO (Results are given in 10 6 events cm -2 s -1 ) [12]. The measured flux of the three types of neutrino interactions in the SNO detector are displayed in Table 3. The measured fluxes are compared to the expected flux calculated within the SSM and with no neutrinos flavour oscillations assumed. The comparison reveals the deficit for measured CC and ES interactions while there is no such deficit for the NC interaction. The results therefore strongly support the SSM and the neutrinos oscillation scenario. 11

Experiments showed for mixing parameters next results: 2 m = 5 x 10-5 ev 2 and sin2θ = 0.87 [13]. The SNP problem finally seems to be solved. As a consistency test, the rates of ES interactions measured by the SK and the SNO experiments can be compared (Tables 2 and 3). The results nicely coincide within the expected statistical fluctuations. Conclusions The results of the solar neutrino experiments imply the following: The SSM description of the Sun is accurate. The solar neutrino fluxes on the Earth can be described by the neutrino flavour oscillation. Contrary to the massless neutrinos within the Standard Model of elementary particles and interaction, neutrino flavour oscillations imply massive neutrinos. Massive neutrinos increase the overall density of matter in the Universe and could seriously influence the cosmology. Bibliography [1] The SNO Collaboration, The Neutrino and the SNP, Available at: http://owl.phy.queensu.ca/sno/neutrino.html [2] C. W. Kim, A. Pevsner, Neutrinos in Physics and Astrophyisics, Harwood Academic Publishers, 1993 [3] J. N. Bachall, SLAC Beam Line 31N1 (2001) 2-12, Available at: http://lanl.arxiv.org/abs/astro-ph/0009259 [4] T. Podobnik, Nobelova nagrada za fiziko 2002, Kolokvij Oddelka za fiziko 16. December 2002 [5] John Bahcall Home Page, Available at: http://www.sns.ias.edu/~jnb/index.html [6] Raymond Davis Home Page, Available at : http://www.bnl.gov/bnlweb/raydavis/pictures.htm 12

[7] SuperKamiokande Home Page, Detector, Available at: http://www-sk.icrr.u-tokyo.ac.jp/sk/ykphd/chap3-3.html [8] The SNO Collaboration, The SNO Detector, Available at: http://www.sno.phy.queensu.ca/sno/sno2.html [9] M. B. Smy, Solar Neutrino Results from Super-Kamiokande, Available at: http://lanl.arxiv.org/abs/hep-ex/0106064 [10] The solar neutrino problem, Available at: http://astrosun.tn.cornell.edu/courses/astro201/sun_neutrino.htm [11] Some interesting events edited by Chris Kyba, Available at: http://www.sno.phy.queensu.ca/sno/events/chris/ [12] Q. R. Ahmad et al., The SNO Collaboration, Phys. Rev. Lett. 89 (2002) 011301, Available at: http://www.sno.phy.queensu.ca/sno/results_04_02/nc/sno_nc_results.pdf [13] Q. R. Ahmad et al., The SNO Collaboration, Phys. Rev. Lett. 89 (2002) 011302, Available at: http://www.sno.phy.queensu.ca/sno/results_04_02/daynight/sno_daynight_results.pdf 13