Neutrinos from the sun and from radioactive sources

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Nuclear Physics B (Proc. Suppl.) 237–238 (2013) 77–81 www.elsevier.com/locate/npbps

Neutrinos from the sun and from radioactive sources A. Ianni5,speaker , G. Bellini2 , J. Benziger3 , D. Bick4 , G. Bonfini5 , D. Bravo6 , M. Buizza Avanzini2 , B. Caccianiga2 , L. Cadonati7 , F. Calaprice8 , C. Carraro9 , P. Cavalcante5 , A. Chavarria8 , D. D’Angelo2 , S. Davini9,10 , A. Derbin11 , A. Etenko12 , D. Franco1 , K. Fomenko5,13 , C. Galbiati8 , S. Gazzana5 , C. Ghiano1,5 , M. Giammarchi2 , ¨ M. GOger-Neff14 , A. Goretti8 , L. Grandi8 , E. Guardincerri9 , S. Hardy6 , Andrea Ianni8 , A. Kayunov11 ,V. Kobychev17 , D. Korablev13 , G. Korga10 , Y. Koshio5 , D. Kryn1 , M. Laubenstein5 , T. Lewke14 , E. Litvinovich12 , L. Ludhova,2 ,B. Loer8 , F. Lombardi5 , P. Lombardi2 , I. Machulin12 , S. Manecki6 , W. Maneschg15 , G. Manuzio9 , Q. Meindl14 , E. Meroni2 , L. Miramonti2 , M. Misiaszek16,5 , D. Montanari8,5 , P. Mosteiro8 , V. Muratova11 , L. Oberauer14 , M. Obolensky1 , F. Ortica18 , K. Otis7 , M. Pallavicini9 , L. Papp5,6 , L. Perasso9 , S. Perasso9 , A. Pocar7 , R.S. Raghavan6 , G. Ranucci2 , A. Razeto5 , A. Re2 , P.A. Romani18 , A. Sabelnikov12 , R. Saldanha8 , C. Salvo9 , S. Sch¨oenert14 , H. Simgen15 , M. Skorokhvatov12 , O. Smirnov13 , A. Sotnikov13 , S. Sukhotin12 , Y. Suvorov5 , R. Tartaglia5 , G. Testera9 , D. Vignaud1 , R.B. Vogelaar6 , F. Von Feilitzsch14 , J. Winter14 , M. Wojcik16 , A. Wright8 , M. Wurm4 , J. Xu8 , O. Zaimidoroga13 , S. Zavatarelli9 , G. Zuzel16 1 APC,

Laboratoire AstroParticule et Cosmologie, 75231 Paris cedex 13, France di Fisica, Universit´a degli Studi e INFN, Milano 20133, Italy 3 Chemical Engineering Department, Princeton University, Princeton, NJ 08544, USA 4 Institut f¨ ur Experimentalphysik, Universit¨at Hamburg, Germany 5 INFN Laboratori Nazionali del Gran Sasso, Assergi 67010, Italy 6 Physics Department, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA 7 Physics Department, University of Massachusetts, Amherst 01003, USA 8 Physics Department, Princeton University, Princeton, NJ 08544, USA 9 Dipartimento di Fisica, Universit´ a e INFN, Genova 16146, Italy 10 Department of Physics, University of Houston, Houston, TX 77204, USA 11 St. Petersburg Nuclear Physics Institute, Gatchina 188350, Russia 12 NRC Kurchatov Institute, Moscow 123182, Russia 13 Joint Institute for Nuclear Research, Dubna 141980, Russia 14 Physik Department, Technische Universit¨ at M¨unchen, Garching 85747, Germany 15 Max-Plank-Institut f¨ ur Kernphysik, Heidelberg 69029, Germany 16 M. Smoluchowski Institute of Physics, Jagiellonian University, Krakow, 30059, Poland 17 Kiev Institute for Nuclear Research, Kiev 06380, Ukraine 18 Dipartimento di Chimica, Universit´ a e INFN, Perugia 06123, Italy 2 Dipartimento

Abstract A brief review of the solar neutrino observations is given. Future solar neutrino measurements are discussed. The use of an artificial neutrino source to be used with low threshold solar neutrino detectors is presented. At present the neutrino source is mainly planned for short baseline neutrino studies. Keywords: solar neutrinos, solar standard model, neutrino source, neutrino oscillations, sterile neutrino

1. Solar neutrinos The sun is an intense source of electron neutrinos. As many as 65 × 109 cm−2 s−1 neutrinos are hitting the earth 0920-5632/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nuclphysbps.2013.04.061

due to nuclear fusion reactions in the core of the sun. Solar neutrinos are a unique tool to probe the physics at the core of the sun and neutrino propagation in a dense matter (ρ (0) ∼ 100 g/cm3 ). Solar neutrinos are pro-

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φ pp ∝

+0.14 +0.03 −0.06 −0.02 +0.73 −0.07 S 11 S 33 S 34 S 1,14 L τ ·

1013

7

109

13

N14

10

15

O14

105

17

Be7 pep 1.2

7 8

10

B 14

F17 hep 30

1000 0.1

0.2

0.5

1.0

2.0

5.0

10.0

20.0

Neutrino Energy in MeV

(1)

Figure 1: Solar neutrino spectrum [2].

Ratio to SSM-GS98

(Z/X)−0.08 Op+0.14  where L is the present luminosity of the sun (MeV/s), τ the present age, Op the opacity, Z/X the heavy elements to hydrogen ratio abundance and S i j are known as astrophysical factors and are related to the interaction cross-sections of processes taking place in the pp and CNO chains. Eq. (1) shows how uncertainties on these input parameters will affect the overall uncertainty on the predicted neutrino fluxes. In Fig. 1 the spectrum of solar neutrinos at earth is shown according to [2]. From this plot we conclude that the sun is an intense source of sub-MeV electron neutrinos with mean energy of about 0.53 MeV. We notice that the SSM is a framework which improves with time together with a more accurate determination of its input parameters and the details of the physics description included in the model. In 2005 new opacity calculations and a revised determination of solar surface heavy-element abundances have produced a significant disagreement between the SSM and helioseismology measurements [3, 4]. As a consequence, we distinguish two SSM outputs named high (SSM-GS98) [5] and low (SSM-AGSS09) [6] metallicity, respectively. In Fig. 1 we show the SSM-GS98 output. Solar neutrino observations have been taking place since 1970 by a number of experiments: Homestake [7], Kamiokande [8], Super-Kamiokande [9], Gallex/GNO [10], SAGE [11], SNO [12] and Borexino [13]. A summary of measurements is shown in Fig. 2. This figure shows the so-called Solar Neutrino Problem that is the reduction of observed against predicted electron solar neutrino flux as measured by charged-currect capture and elastic scattering interactions. The deficit of solar neutrinos is changing with energy and it is of the order of 40% or larger. On the contrary, when solar neutrinos are observed through neutral-current interactions (SNO-NC) there is a good agreement between predictions and observations. The Solar Neutrino Problem

pp 0.6

1011 Flux cm2 s1 

duced by two main reactions chains which turn hydrogen into helium. These chains are known as pp-cycle and CNO-cycle. A detailed description of these reactions can be given by the Solar Standard Model (SSM) [1, 2]. The SSM is a framework from which we make predictions on the production of solar neutrinos. In particular, the most abundant source of neutrinos comes from the reaction: p + p → d + e+ + νe , Eνe < 0.42 MeV. The flux of these pp neutrinos, taken as an example, is determined to depend on a number of input parameters as follows:

1.2 1 0.8 0.6 0.4 0.2 0 10-1

1

10 Energy [MeV]

Figure 2: Solar neutrino predictions and observations normalized to the SSM. From left to right the lower (black) points show observations from Gallex/GNO and SAGE, Borexino, Homestake, SuperKamiokande, SNO-NC (upper point).

is solved. All together these measurements can be explained in the framework of matter-enhanced neutrino oscillations (MSW-LMA) [14, 15, 16, 17]. In Fig. 3 the survival probability of solar electron neutrinos as predicted by the oscillation scenario and as determined by observations. The most interesting feature of the survival probability is the so-called upturn at 2-3 MeV. Future measurements of pep and 8 B solar neutrinos could improve our determination of this effect predicted by the matter to vacuum transition of neutrino oscillations inside the sun, while moving from high to low energy. Besides the upturn of the survival probability future measurements should try to solve the metallicity problem by observing CNO neutrinos. At present only SAGE, Super-Kamiokande and Borexino are in operation for solar neutrino measurements. Table 1 reports the present predictions of the SSM with different metal abundances in comparison with observations from all solar neutrino experiments. In the

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pp - all solar B - SNO B - SK + SNO NC Homestake + 8.B 7. Be - Borexino pep - Borexino 8. B - Borexino MSW Prediction 8.

Pee:
e survival probability

0.8

8.

0.7 0.6 0.5 0.4 0.3 0.2 0.1 10-1

1 E
[MeV]

10

Figure 3: Solar neutrino survival probability as predicted by the MSW-LMA scenario and as determined by present observations.

Figure 5: Predicted spectrum in Borexino with the 51 Cr source.

2. Neutrino Source in Borexino

Figure 4: Solar neutrino spectrum in Borexino phase II.

coming years Super-Kamiokande aims to operate with a 4 MeV detection threshold [18] and Borexino will collect data for the phase II with a lower background after a new purification of the liquid scintillator. In Fig. 4 the expected solar neutrino spectrum in Borexino for the phase II is shown. This spectrum includes subtraction of 11 C cosmogenic background according to the method reported in [20]. The solid black line shows the 210 Bi background at the level of 5 cpd/100tons. The dashed line which goes up to 5 MeV shows the CNO neutrino contribution. The goal of Borexino for the phase II is the observation of CNO neutrinos. At present the bound reported in Table 1 comes from data collected in Borexino before new purifications of the liquid scintillator. A possible new low threshold experiment is expected to start taking data in 2014, SNO+ [19], which is replacing the SNO experiment with a k-ton scale liquid scintillator detector. The deep location of SNO+ will allow a better determination of the pep and CNO neutrinos provided the radiopurity will be at the level of Borexino.

Recently, a re-evaluation of the reactor antineutrino flux found a 3% increase with respect to previous calculations. This fact in combination with a significant smaller neutron lifetime, which affects the inverse βdecay cross section, produces a 6% deficit of the experimentaly measured reactor antineutrino flux [21]. As a consequence, this apparent deficit has triggered new interest in the possibility to explain short baseline reactor antineutrino measurements by means of sterile neutrinos in the eV scale. This idea has been to a large extent discussed in a white paper [22]. In the white paper the possibility to search for sterile neutrinos with an artificial neutrino source is discussed as well. A PBq electron neutrino source has been developed for the GALLEX experiment at the Gran Sasso Laboratory [23] and by the SAGE experiment at Baksan [24] to determine the extraction efficiency of 71 Ge for solar neutrino measurements. An artificial neutrino source to be used for short baseline oscillations, neutrino magnetic moment and other rare processes at MeV scale has been introduced in a number of works [23, 27, 28, 29, 30]. In this framework a low threshold solar neutrino detector it is an ideal instrument for searching short baseline neutrino oscillations. As a matter of fact, a 144Ce −144 Pr source has been proposed to be used in KamLAND [25] or in a k-ton scale detector for sterile neutrino search. This project is given the name of Ce-LAND [25]. In Borexino the plan is to make use of a high intensity (370 PBq) 51 Cr source located outside the water tank at some 8.5 m away from the center of the detector. The 51 Cr source does not imply any change in the detector set-up and schedule for solar neutrino measurements in the next three years. In Fig. 5 we show the expected spectrum in Borexino with the 51 Cr source. The con-

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Source

pp pep 7 Be 8 B 13 N 15 O 17 F CNO

Flux [cm2 s−1 ] SSM-GS98 5.98(1 ± 0.006) × 1010 1.44(1 ± 0.012) × 108 5.00(1 ± 0.07) × 109 5.58(1 ± 0.13) × 106 2.96(1 ± 0.15) × 108 2.23(1 ± 0.16) × 108 5.52(1 ± 0.18) × 106 5.24 × 108

Flux [cm2 s−1 ] SSM-AGSS09 6.03(1 ± 0.006) × 1010 1.47(1 ± 0.012) × 108 4.56(1 ± 0.07) × 109 4.59(1 ± 0.13) × 106 3.76(1 ± 0.15) × 108 1.56(1 ± 0.16) × 108 3.40(1 ± 0.16) × 106 3.76 × 108

Flux [cm2 s−1 ] Data 10 6.06(1+0.003 −0.01 ) × 10 1.60(1 ± 0.19) × 108 4.48(1 ± 0.05) × 109 5.40(1 ± 0.03) × 106 < 6.7 × 108 < 3.2 × 108 < 59 × 106 < 7.7 × 108

Δm214

Table 1: SSM predictions for solar neutrino fluxes and present observations. The CNO limit is at 2σ.

elastic scattering at 1 MeV scale for non-standard processes such as a neutrino magnetic moment and new super-weak interactions couplings. As an example the sensitivity to the Weinberg angle at 1 MeV will be at the level of 2.6% with the 51 Cr. A neutrino magnetic moment at the level of 3 × 10−11 μB at 3σ can be measured.

10

1 RA: 95% C.L. RA: 99% C.L. 51

Cr: 95% C.L.

3. Acknowledgement

51

10-1

Cr: 99% C.L.

Solar+KL: 95% C.L. Solar+KL: 99% C.L.

-2

10

-1

10

1

sin2(2θ14)

Figure 6: Predicted sensitivity for sterile neutrino oscillations in Borexino with the 51 Cr source.

tribution of monoenergetic neutrinos from the source is clearly visible: solid thick line. In Fig. 6 we report the expected sensitivity for a 370 PBq 51 Cr source for a rate and shape analysis including systematic effects. In particular, we have assumed a 1% uncertainty on the source activity. In the plot we have shown in green the excluded are from solar neutrino measurements [31]. The Reactor Anomaly contours are taken from [32]. At the end of the solar neutrino program it might be possible to make changes in the detector to allow the deployment of a 1.85 PBq 144Ce −144 Pr source as described in [25] at the center of liquid scintillator volume. This possibility will allow to reach a sensitivity to totally cover the reactor anomaly. The antineutrino source could also be placed inside the water tank at some 7.5 m away from the center of the detector. The fiducial mass for antineutrinos can be made larger with the use of PPO in the inner buffer of the present detector configuration. Besides sterile neutrinos the 51 Cr and 144Ce sources in Borexino allows to search in the neutrino-electron

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