G e o -n e u tr in o r esu lts w ith B o r e x in o
R R o n c in 1,2,29, M A g o stin i3, S A p p e l3, G B e llin i4, J B en zig er5, D B ick 6, G B o n fin i1, D B ra v o 7, B C a ccian iga4, F C alap rice8, A C a m in a ta 9, P C a v a lca n te1, A C h ep u rn o v 10, D D ’A n g e lo 4, S D a v in i11, A D e r b in 11, L D i N o to 9, I D r a c h n e v 11, A E te n k o 13, K F om en k o14, D F ranco2, F G a b riele1, C G a lb ia ti8, C G h ia n o 9, M G iam m arch i4, M G o eg er-N eff3, A G o r e tti8, M G ro m o v 10, C H a g n er6, E H u n g erford 15, A ld o Ia n n i1, A n d rea Ian n i8,
K J ed rzejcza k 17, M K a iser6, V K o b y ch ev 18, D K o r a b lev 14, G K o rg a 1, D K ry n 2, M L a u b en ste in 1, B L eh n ert19, E L itv in o v ich 13,20,
F L om b ard i1, P L om bardi4, L L udhova4, G L u k yan ch en k o13,20,
I M a ch u lin 13,20, S M an eck i7, W M a n esch g 22, S M a rc o cci11, E M ero n i4, M M ey er6, L M ira m o n ti4, M M isia sz ek 17,1, M M o n tu sc h i23,
P M o steir o 8, V M u ra to v a 11, B N eu m a ir3, L O b erau er3,
M O b o le n sk y 2, F O rtica 24, M P a lla v ic in i9, L P a p p 3, L P er a sso 9, A P o c a r 26, G R an u cci4, A R a z e to 1, A R e 4, A R o m a n i24, N R o ss i1, S S ch o n ert3, D S em en o v 11, H S im g en 22, M S k orok h vatov13,20, O S m irn o v 14A S o tn ik o v 14, S S u k h o tin 13, Y S u vorov27,13, R T a rta glia 1, G T estera 9, J T h u rn 19, M T orop ova13, E U n zh a k o v 11, A V ish n e v a 14, R B V og ela ar7, F von F eilitzsch 3, H W an g27, S W ein z28, J W in te r 28, M W o jcik 17, M W u rm 28, Z Y o k ley 7, O Z aim id o ro ga 14, S Z avatarelli9, K Z u b er19 and G Z u zel17 (B o rex in o C o lla b o ratio n )
1 IN FN L ab o rato ri Nazionali del G ran Sasso, 67010 Assergi (AQ), Italy
2 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
3 P h ysik-D epartm ent and Excellence C luster Universe, Technische U niversitat M unchen, 85748 G arching, G erm any
4 D ip artim en to di Fisica, U niversita degli Studi e IN FN , 20133 M ilano, Italy
5 C hem ical E ngineering D ep artm en t, P rin c eto n University, P rinceton, N J 08544, USA 6 In s titu t fur E xperim entalphysik, U niversitat, 22761 H am burg, G erm any
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 I n s titu te of N uclear Physics, 119234 Moscow, R ussia
11 G ra n Sasso Science In stitu te (IN FN ), 67100 L ’Aquila, Italy
12 St. P etersb u rg N uclear Physics In stitu te N RC K urchatov In stitu te , 188350 G atchina, R ussia
13 NRC K urchatov In stitu te , 123182 Moscow, R ussia
14 Jo in t In stitu te for N uclear Research, 141980 D ubna, R ussia
15 D ep a rtm en t of Physics, U niversity of H ouston, H ouston, T X 77204, USA 16 In s titu te for T heoretical and E xperim ental Physics, 117218 Moscow, R ussia 29 P resenter. To w hom any correspondence should be addressed.
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
21 K epler C enter for A stro and P artic le Physics, U niversitat T ubingen, 72076 Tubingen, G erm any
22 M ax -P lan ck -In stitu t fur K ernphysik, 69117 Heidelberg, G erm any
23 D ip artim en to di Fisica e Scienze della T erra U niversita degli Studi di F erra ra e IN FN , Via S aragat 1-44122, F errara, Italy
24 D ip artim en to di Chim ica, U niversita e INFN , 06123 P erugia, Italy 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 un 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: r o m a i n .r o n c in @ ln g s .in f n .it
A b s t r a c t . Borexino is a liquid scintillator d etec to r p rim ary designed to observe solar neutrinos. Due to its low background level as well as its position in a nuclear free country, Italy, Borexino is also sensitive to geo-neutrinos. Borexino is leading th is interdisciplinary field of n eutrino geoscience by studying electron an tin eu trin o s which are em itted from th e decay of radioactive isotopes present in th e cru st and th e m antle of th e E arth . W ith 2056 days of d a ta ta k en betw een D ecem ber 2007 and M arch 2015, Borexino observed 77 an tin eu trin o candidates.
If we assum e a chondritic T h /U m ass ratio of 3.9, th e num ber of geo-neutrino events is found to be 23.7+5 7 (stat) +0'6 (syst). W ith th is m easurem ent, Borexino alone is able to reject th e null geo-neutrino signal a t 5.9
a,
to claim a geo-neutrino signal from th e m antle a t 98% C.L. and to re stric t th e radiogenic h eat pro d u ctio n for U and T h betw een 23 and 36 T W .1. I n tr o d u c t io n
Geo-neutrinos are electron antineutrinos which are produced by the decay of radioactive isotopes present in the crust and the mantle of our planet. Since the chemical composition of the E arth is not yet perfectly known, having a new source of information will help to b etter understand our planet. The idea of using geo-neutrinos as direct messengers was suggested in 1965 by G.
Eder [1] and in 1968 by G. M arx [2] before being reviewed by L.M. Krauss, S.L. Glashow and D.N. Schramm in 1984 [3]. So far, only the KamLAND experiment in Japan [4, 5] and the Borexino experiment in Italy [6, 7, 8] have reported geo-neutrino measurements.
2. G e o - n e u tr in o a n a ly s is a n d r e s u lts
In Borexino, the detection of geo-neutrinos relies on the signature of the inverse P decay (IBD) reaction ve + p ^ e+ + n where the positron, the “prom pt” signal, is followed in time by the neutron capture on hydrogen, the “delayed” signal. The prom pt and the delayed signals are correlated in space and time, allowing to accurately identify electron antineutrino signal. W ith
Q Q O O Q O
an IBD threshold of 1.806 MeV, only geo-neutrinos coming from the decay of and Th chains can be detected.
Despite Italy is a nuclear free country, the dom inant background remains electron antineutrinos em itted by abroad nuclear reactors. It is nonetheless possible to estim ate the
expected number of nuclear reactors events, —rea ct, as follows:
R M .
4 f—re
act
— ^ ^ ^ P ™ * d Epe ^ ) a ( E,
e)Pee
( E,
e, Lr
), (1)r
= 1 m = 1 ri=
1where r runs over the number of nuclear reactors R considered, m runs over the number of months M considered, nm stands for the exposure in m onth m and includes detector efficiency, Lr is the detector-reactor distance, Prm is the effective therm al power of reactor r in m onth m, i runs over the spectral components of 235U, 238U, 239P u and 241Pu, f
i
is the power fraction of component i, Ei the average energy released per fission of component i, 0i
( Ei
?e) the antineutrino energy spectrum per fission of component i, a ( Epe) the IBD cross section and Pe e(Epe, Lr
) the survival probability of the em itted antineutrinos of energy Epe created at distance Lr.T able 1. Estim ated background components in term s of number of events taken from [8]. The combined upper limit is obtained by M onte Carlo.
9Li-8He 0.194-0:029
Accidental coincidences 0.221 ± 0.004
Time correlated 0.035—0:028
( a ,n ) in scintillator 0.165 ± 0.010 ( a ,n ) in buffer < 0.51 Fast n ’s — in W T) < 0.01 Fast n ’s — in rock) < 0.43 Untagged muons 0.12 ± 0.01 Fission in PM Ts 0.032 ± 0.003
214B i-214po 0.009 ± 0.013
Total 0.78—.'10
< 0.65 (combined)
O ther backgrounds can mimick an IBD reaction in Borexino, like (a, n) background, accidental coincidences and cosmogenic background such as 9Li-8He. In Borexino, the overall background rate is estim ated to be a factor 100 lower th an the antineutrino one. The estim ated background for each components is reported in table 1.
In order to measure the number of geo-neutrinos and antineutrinos from nuclear reactors, we implement an unbinned maximum likelihood fit of the prom pt energy spectrum of our antineutrino candidates. We define the log-likelihood function as follows:
ln L (N geo, —reacO —ac^ —LiHe, —an) = —exp(—geo, —reacO —ac^ —LiHe, —an)
— 1 / — (— ) \ 2
+ ln (—e (Ei , Ngeo, —react) + —g ^ ^ —ac^ —LiHe, —an)) — (---g ——\—g ) , 2 g g v (^bg)est ;
(2) with:
—e (Ei , —geo, —react) = /g e o ^ ^ —geo) + —eac-A^ —react) (3) /bg(Ei, —acc, —LiHe, —an) = /acc(Ei, —acc) + /LiHe(Ei, —LiHe) + /an( E i, —an) (4) where —exp corresponds to the expected to tal number of events and i runs over the — = 77 antineutrino candidates. /geo, /react, /acc, /LiHe and /an are the individual spectra of the geo-neutrinos, the antineutrinos from nuclear reactors, the accidental coincidences, the 9Li-8He
F ig u r e 1. —2 A i n L profiles for N geo (a) and Nreact (b).
constrains the background components reported in table 1.
If we assume a chondritic T h /U mass ratio of 3.9, our best fit values are N geo = 23.7+5 5 (stat) +q 9 (syst) and Nreact = 52.7+f'7 (stat) +°'9 (syst) events, which is equivalent to 43.51J0'8 (stat) (syst) and 96.6+I5'5 (stat) +4 0 (syst) TN U 30 respectively. This result allows to reject the null geo-neutrino signal at 5.9a. Figure 1 shows the —2 A l n L profiles for N geo and N react.
A signal from the mantle can then be assessed by retrieving the crust signal (investigated in [9]
and [10]) to the total signal measured in Borexino. Using the geo-neutrino log-likelihood profile and assuming a Gaussian approxim ation for the crust contribution, one can extract a signal from the mantle equal to 20.9+10 ' 3, leading to a 98% C.L. geo-neutrino signal from the mantle.
Finally, a fit where both U and T h spectra are left as free param eters has also been performed, restricting the radiogenic heat production from these isotopes between 23 and 36 TW .
3. I n v e s tig a tio n o n a p o s sib le g e o r e a c to r
In addition to the standard geo-neutrino analysis, we report an investigation on a possible natural nuclear reactor, called georeactor, standing inside the E arth. We assume this reactor to release a constant power for the whole d ata taking period. The Monte Carlo spectrum is built such th a t 235u/ 238U has been set to 0.75/0.25 while the P u contribution is set to 0. The
F ig u r e 2. —2 A l n L profile for Ngeoreact.
fit has been done in the energy range above 1510 p.e. in order to get rid of the geo-neutrino spectrum . The background components have been normalized to the [1510, 5000 p.e.] energy
30 O ne TN U corresponds to one event d etected over one year exposure of 1032 ta rg e t protons a t 100% efficiency.
range of interest and the reactor component has been constrained to the theoretical value and error of 56 ± 2 (30 ± 1 in the [1510, 5000 p.e.] energy range of interest).
Figure 2 shows the —2 A ln L profile for Ngeoreact. The best fit value is 0 and the upper limit in term s of number of events is 8.4 (10.5) at 90% C .L. (95% C .L.). This limit is usually expressed in term s of TW . On the whole energy range, 1 TW is found to be equal to 4.4 events with an exposure of 5.5 x 1031 proton x year, oscillation through core and mantle taken into account.
It corresponds to 2.5 events in the [1510, 5000 p.e.] energy range of interest, which leads to an upper limit of 3.4 TW (4.2 TW ) at 90% C.L. (95% C.L.).
4. C o n c lu sio n
From 2056 days of d ata taking, Borexino alone is able to reject the null geo-neutrino signal at 5.9 a, to claim a geo-neutrino signal from the mantle at 98% C.L. and to restrict the radiogenic heat production for U and T h between 23 and 36 TW . W ith a signal-to-background ratio of the order of 100, Borexino provides a real tim e spectroscopy of geo-neutrinos. Finally, we have investigated the hypothesis of a georeactor and we have set an upper limit for a 3.4 TW georeactor (4.2 TW ) at 90% C.L. (95% C.L.).
A c k n o w le d g m e n ts
Russian colleagues from M EPhI acknowledge partial support from M EPhI Academic Excellence P roject (contract No. 02.a03.21.0005, 27.08.2013).
R e fe re n c e s
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