C N O an d pep solar n e u tr in o m ea su r em en ts and p e r s p e c tiv e s in B o r e x in o
S D a v in i1,29, M A g o s t in i2, S A p p e l2, G B e llin i3, J B e n z ig e r 4, D B ic k 5, G B o n fin i6, D B r a v o 7, B C a c c ia n ig a 3, F C a la p r ic e 8, A C a m in a ta 9, P C a v a lc a n te 6, A C h e p u r n o v 10, D D ’A n g e lo 3, A D e r b in 11, L D i N o t o 9, I D r a c h n e v 1, A E t e n k o 12, K F o m e n k o 13, D F r a n c o 14, F G a b r ie le 6, C G a lb ia ti8, C G h ia n o 9, M G ia m m a r c h i3, M G o e g e r -N e ff2 , A G o r e t t i8, M G r o m o v 10, C H a g n e r 5,
E H u n g e r fo r d 15, A ld o I a n n i6, A n d r e a Ia n n i8, K J e d r z e jc z a k 17, M K a is e r 5, V K o b y c h e v 18, D K o r a b le v 13, G K o r g a 6, D K r y n 14, M L a u b e n s te in 6, B L e h n e r t 19, E L it v in o v ic h 12,20, F L o m b a r d i6, P L o m b a r d i3, L L u d h o v a 3, G L u k y a n c h e n k o 12,20, I M a c h u lin 12,20, S M a n e c k i7, W M a n e s c h g 22, S M a r c o c c i1, E M e r o n i3, M M e y e r 5, L M ir a m o n ti3, M M is ia s z e k 17,6, M M o n tu s c h i23, P M o s te ir o 8,
V M u r a to v a 11, B N e u m a ir 2, L O b e r a u e r 2, M O b o le n s k y 14, F O r tic a 24, M P a lla v ic in i9, L P a p p 2, L P e r a s s o 9, A P o c a r 26, G R a n u c c i3,
A R a z e t o 6, A R e 3, A R o m a n i24, R R o n c in 6,14, N R o s s i6, S S c h o n e r t2, D S e m e n o v 11, H S im g e n 22, M S k o r o k h v a to v 12,20, O
S m ir n o v 13A S o t n ik o v 13, S S u k h o t in 12, Y S u v o r o v 27,12, R T a r ta g lia 6, G T e s te r a 9, J T h u r n 19, M T o r o p o v a 12, E U n z h a k o v 11, A V is h n e v a 13, R B V o g e la a r 7, F v o n F e ilitz s c h 2, H W a n g 27, S W e in z 28, J W in t e r 28, M W o jc ik 17, M W u r m 28, Z Y o k le y 7, O Z a im id o r o g a 13, S Z a v a ta r e lli9, K Z u b e r 19a n d G Z u z e l17(B o r e x in o C o lla b o r a tio n )
1 G ra n Sasso Science In stitu te (IN FN ), 67100 L ’Aquila, Italy
2 P h ysik-D epartm ent and Excellence C luster Universe, Technische U niversitat M iinchen, 85748 G arching, G erm any
3 D ip artim en to di Fisica, U niversita degli Studi e IN FN , 20133 M ilano, Italy
4 C hem ical E ngineering D ep artm en t, P rin c eto n University, P rinceton, N J 08544, USA
5 In s titu t fur E xperim entalphysik, U niversitat, 22761 H am burg, G erm any
6 IN FN L ab o rato ri N azionali del G ran Sasso, 67010 Assergi (AQ), Italy
7 Physics D ep artm en t, V irginia Polytechnic In stitu te and S tate University, Blacksburg, VA 24061, USA
8 Physics D ep artm en t, P rin c eto n University, P rinceton, N J 08544, USA
9 D ip artim en to di Fisica, U niversita degli Studi e IN FN , Genova 16146, Italy
10 Lomonosov Moscow S tate U niversity Skobeltsyn In s titu te of N uclear Physics, 119234 Moscow, R ussia
11 St. P etersb u rg N uclear Physics In stitu te NRC K urchatov In stitu te , 188350 G atchina, R ussia
12 NRC K urchatov In stitu te , 123182 Moscow, R ussia
13 Jo in t In stitu te for N uclear Research, 141980 D ubna, R ussia
14 A stro P articu le et Cosmologie, U niversite P aris D iderot, C N R S /IN 2P 3, C E A /IR F U , O bservatoire de P aris, Sorbonne P aris C ite, 75205 P aris Cedex 13, France
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15 D ep a rtm en t of Physics, U niversity of H ouston, H ouston, T X 77204, USA
16 I n s titu te for T heoretical and E xperim ental Physics, 117218 Moscow, R ussia
17 M. Smoluchowski In stitu te of Physics, Jagiellonian University, 30059 Krakow, Poland
18 K iev In stitu te for N uclear Research, 06380 Kiev, U kraine
19 D ep a rtm en t of Physics, Technische U niversitat D resden, 01062 D resden, G erm any
20 N ational R esearch N uclear U niversity M E P hI (Moscow Engineering Physics In stitu te ), K ashirskoe highway 31, Moscow 115409, R ussia
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22 M ax -P lan ck -In stitu t fur K ernphysik, 69117 Heidelberg, G erm any
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25 Physics D ep artm en t, Q ueen’s University, K ingston ON K7L 3N6, C anada
26 A m herst C enter for F u n d am en tal Interactio n s and Physics D ep artm en t, U niversity of M assachusetts, A m herst, MA 01003, USA
27 Physics and A stronom y D ep artm en t, U niversity of C alifornia Los Angeles (UCLA), Los Angeles, California 90095, USA
28 I n s titu te of Physics and Excellence C luster PR ISM A , Johannes Gutenberg-Universitaat M ainz, 55099 M ainz, G erm any
E-m ail: s t e f a n o . d a v i n i @ g s s i . i n f n . i t
A b s t r a c t . T he detectio n of neutrinos em itte d in th e CNO reactions in th e Sun is one of th e am bitious goals of B orexino P hase-II. A m easurem ent of CNO neutrinos would be a m ilestone in astrophysics, and would allow to solve serious issues in cu rren t solar models. A precise m easurem ent of th e ra te of neutrinos from th e p ep reactio n would allow to investigate n eutrino oscillations in th e M SW tra n sitio n region. T he p ep and CN O solar neu trin o physics, th e m easurem ent in Borexino P hase-I and th e perspectives for th e new phase are reviewed in this proceeding.
1. I n tr o d u c tio n
B orexino is a real-tim e solar neu trin o d e te c to r th a t is designed to d etect low energy solar n eutrinos [1, 2]. T he m otivating goal of low energy solar n eu trin o detection experim ents is to directly probe th e nuclear reaction processes in th e Sun, and explore n eu trin o oscillations over a broad er range of energies th a n has been done to date.
M ono-energetic (1.44 MeV) pep solar neutrinos, produced in th e pp fusion chain, are an ideal probe to test th e tra n sitio n from vacuum -dom inated oscillation to m atter-en h an ced oscillations predicted by th e M SW -LM A m odel of th e neu trin o oscillations: th e flux of pep predicted by from th e S ta n d ard Solar M odel has a sm all u n certain ty (1.2%) due to th e solar lum inosity con straint.
T he detectio n of neutrinos resulting from th e C N O cycle would have huge im plications in astrophysics: it would be th e first direct evidence of th e nuclear processes th a t are believed to fuel m assive stars (> 1.5 M 0 ). Furtherm o re, a m easurem ent of th e CNO neu trin o flux m ay resolve th e solar m etallicity problem [3]. T he to ta l CNO flux is strongly d epen den t on th e in p u ts to th e solar m odelling, being 40% higher in th e H igh M etallicity (GS98) th a n in th e Low M etallicity (AGSS09) solar m odel [3].
2. T h e B o r e x in o d e t e c t o r
T he features of th e B orexino d e te c to r are described in d etail in [1]. One of th e unique features of th e B orexino d e te c to r is th e very low radioactive background. T he active d e te c to r is 278 tons of tw o-com ponent liquid scintillator com posed of pseudocum ene (PC ) and 2,5-diphenyloxazole (P P O ), a w avelength shifter. T he scintillator is contained in a th in nylon vessel, shielded by
two P C buffers sep arated by a second nylon vessel. T he scintillato r and buffers are contained w ithin a 13.7 m stainless steel sphere th a t is housed in a 16.9 m dom ed w ater ta n k for additional shielding and m uon veto [4].
N eutrinos are d etected by th e ir elastic sc atterin g on electrons in th e liquid scintillator. T he scintillation light is d etected w ith an array of 2 2 0 0 photom ultip lier tu b es m ounted on th e inside surface of th e stainless steel sphere. T he num ber of ph otom ultipliers hit is a m easure of th e energy im p arted to th e electron, b u t has no sensitivity to th e direction of th e neutrino.
B orexino is in d a ta tak in g since M ay 2007. B orexino P h ase-I covers th e period from M ay 2007 to M ay 2010. A fter th e purification of th e scin tillator perform ed betw een M ay 2010 and A ugust 2011, in Novem ber 2011 th e P h ase-II of B orexino sta rte d . Borexino P h ase-II is expected to last until 2016, before th e begining of SOX [5].
B orexino Phase-I solar n eu trin o results, described in d etail in [2], include a high-precision m easurem ent of 7Be neutrinos [6], th e m easurem ent of th e absence of day-night asym m etry of 7Be neutrinos [7], a m easurem ent of 8B solar neutrinos w ith a th resh old recoil electron energy of 3 MeV [8], and th e first tim e m easurem ent of pep solar neutrinos and th e strongest co n strain t up to d a te on CNO solar neutrinos [9]. O th e r results include th e stu d y of solar and o th er unknow n a n ti-n eu trin o fluxes [1 0], observation of geo-neutrinos [1 1, 1 2], m easurem ent of n eu trin o velocity [13], searches for solar axions [14], and experim ental lim its on th e Pauli- forbidden tra n sitio n s in 12C nuclei [15].
T he direct real-tim e m easurem ent of th e solar neutrinos from th e fun dam ental pp reaction [16]
is th e g reatest achievem ent so far of B orexino Phase-II.
3. F ir st e v id e n c e o f pep so la r n e u tr in o s a n d lim it s o n C N O so la r n e u tr in o flu x T he B orexino collaboration rep o rted in 2012 th e first tim e m easurem ent of th e solar pep n eu trino ra te and th e strongest lim its on th e CNO solar neu trin o flux to d a te [9]. T his m easurem ent has been m ade possible by th e com bination of th e low levels of intrinsic background in Borexino-I, and th e im plem entation of novel background discrim ination techniques.
T he detectio n of pep and CNO solar neutrinos is challenging: th e ir expected in teraction rates are a few counts p er day in a 1 0 0 to n ta rg e t, and th e m ain backgrounds, th e cosmogenic P + -em itter 11C and radiogenic 2 1 0Bi, are one order of m agn itud e m ore intense [9].
11C is produced in th e scintillator by cosmic m uon interactio ns w ith 12C nuclei. In 95% of th e cases a t least one free n e u tro n is created in th e 11C sp allation process and th e n c a p tu red in th e scin tillator [4, 17, 18]. T he 11C background can be reduced by applying a space and tim e veto after coincidences betw een signals from th e m uons and th e cosmogenic n eutrons, discarding exposure th a t is m ore likely to contain 11C due to th e correlation betw een th e p aren t muon, th e neu tro n and th e subsequent 11C decay (the Three-Fold-C oincidence, T F C ). T he rejection c riteria were chosen to o b tain th e best com prom ise betw een 11C rejection and preservation of exposure [2, 9].
T he residual 11C surviving th e T F C cu t is still a significant background. T he pulse shape differences betw een e- and e+ were used to discrim inate 11C P+ decays from neu trin o induced e- recoils and P - decays. A slight difference in th e tim e d istrib u tio n of th e scintillation signal arises from th e finite lifetim e of orth o -p o sitro n iu m as well as from th e presence of an n ihilation 7-rays. [19]. A pulse shape p a ra m ete r was co n stru cted using a boosted-decision-tree algorithm [2, 9].
T he analysis is based on a binned likelihood m ultivariate fit perform ed on th e energy, pulse shape, and sp atial d istrib u tio n s of selected scintillation events. T he non-uniform radial d istrib u tio n of th e ex ternal 7-ray background was included in th e m ultiv ariate fit and strongly constrain ed its co n trib u tio n [2, 9].
T he best estim ate for th e in teraction ra te of pep solar neutrinos in B orexino is (3.1 ± 0 .6 (s ta t) ± 0.3 (syst)) co u n ts/(d ay-100ton). If this reduction in th e ap p a re n t flux is due
I s o to p e S p e c s for LS B X P h a s e -I B X P h a s e -I I 238u < 1 0 - 1 6 g /g (5.3 ± 0.5) x 1 0- 1 8 g /g < 8 x1 0 - 2 0 g /g 232Th < 1 0 - 1 6 g /g (3.8 ± 0.8) x 1 0- 1 8 g /g < 9 x 1 0- 1 9 g /g
1 4C / 12C < 1 0 - 1 8 (2.69 ± 0.06) x 1 0- 1 8 unchanged 40k < 1 0 - 1 8 g /g < 0.4 x 1 0- 1 8 g /g unchanged 85K r < 1 c p d /1 0 0t (30 ± 5) c p d /1 0 0 t < 7 c p d /1 0 0 t
39 Ar < 1 c p d /1 0 0t < 85K r < 85K r
210Po not specified ~ 7 0 0 c p d /1 0 0 t (decaying) ~ 8 0 c p d /1 0 0 t (decaying) 210Bi not specified ~ 20 - 7 0 c p d /1 0 0 t (25 ± 2) c p d /1 0 0 t
T a b le 1. R esidual radioactive co n tam in atio n of th e B orexino liquid scintillator before and after th e purification perform ed in 2010-2011. 210P o ra te is a factor 100 less th a n a t th e begin of d a ta taking. 210Bi is a factor 2 less th a n in Phase-I.
to ve oscillation to or vT, we find P ee = 0.62 ± 0.17 a t 1.44 MeV. A ssum ing M SW -LM A solar n eu trin o oscillations, th e B orexino results can be used to m easure th e pep solar n eutrin o flux, corresponding to Tpep = (1.6 ± 0.3) x 108 cm- 2 s- 1 , in agreem ent w ith th e Solar S tan d ard M odel [2, 9].
D ue to th e sim ilarity betw een th e electron-recoil sp ectru m from CNO neutrinos and th e sp ectral shape of 210Bi decay, whose ra te is ~ 10 tim es greater, B orexino Phase-I only provided an upp er lim it on th e CNO n eu trin o in teraction rate . A ssum ing M SW -LM A solar neutrin o oscillations, th e 95% C.L. lim it on th e solar CNO neu trin o flux is 7.7 x 1 08 cm- 2 s- 1 . T he lim it on CNO solar neu trin o flux is 1.5 tim es higher th a n th e flux predicted by th e High M etallicity Solar S tan d ard M odel [2, 9].
4. P r o s p e c t s in B o r e x in o P h a s e -I I
A new m easurem ent of pep and CNO solar neutrinos is foreseen in B orexino P hase-II. T he radioactive background levels in Borexino P hase-II are lower, th an k s to a set of scintillator purifications perform ed after Phase-I. T he residual background co n tam inatio ns are sum m arised in tab le 1. T he co n tam in atio n of 2 1 0Bi, which was th e d om in ant radioactive background, is reduced of a factor ~ 3 . New m ethods for pulse shape d iscrim ination betw een e - and e+, and for constraining th e residual ra te of 2 1 0Bi, are also u nd er study.
5. C o n c lu s io n s a n d o u tlo o k
Borexino has achieved th e necessary sensitivity to provide, for th e first tim e, evidence of th e rare signal from pep solar n eutrinos and to place th e strongest co n strain t on th e CNO solar n eu trin o flux to d ate. This resu lt raises th e prospect for higher precision m easurem ents by Borexino P h ase II, since th e next d o m inant background, 2 1 0Bi, has been reduced by scintillator purification.
A c k n o w le d g m e n ts
R ussian colleagues from M E P h I acknowledge p a rtia l su p p o rt from M E P h I Academ ic Excellence P ro je c t (con tract No. 02.a03.21.0005, 27.08.2013).
R e fe r e n c e s
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[3] B asu S A S P Conference Series 2009 4 1 6 193
[4] Bellini G et al. (Borexino Collaboration) 2011 J I N S T 6 P05005
[5] Bellini G et al. (Borexino Collaboration). 2013 JH E P 1 3 0 8 038
[6] Bellini G et al. (Borexino Collaboration) 2011 Phys. Rev. Lett. 1 0 7 141302 [7] Bellini G et al. (Borexino Collaboration) 2012 Phys. Lett. B 7 0 7 22 [8] Bellini G et al. (Borexino Collaboration) 2010 Phys. Rev. D 82 033006 [9] Bellini G et al. (Borexino Collaboration) 2012 Phys. Rev. L ett 10 8 051302 [10] Bellini G et al. (Borexino Collaboration) 2011 Phys. Lett. B 6 9 6 191 [11] Bellini G et al. (Borexino Collaboration) 2010 Phys. Lett. B 6 8 7 299 [12] A gostini M et al. (Borexino Collaboration) 2015 Phys. Rev. D 9 2 031101 [13] Alvarez Sanchez P et al. (Borexino Collaboration) 2012 Phys. Lett. B 716 401-5 [14] Bellini G et al. (Borexino Collaboration) 2012 Phys. Rev. D 85 092003
[15] Bellini G et al. (Borexino Collaboration) 2010 Phys. Rev. C 81 0343317 [16] Bellini G et al. (Borexino Collaboration) 2014 N ature 5 1 2 383
[17] Bellini G et al. (Borexino Collaboration) 2012 J C A P 05 015 [18] Bellini G et al. (Borexino Collaboration) 2013 J C A P 13 0 8 049 [19] D avini S 2014 Eur. Phys. J. P lus 128 89
