T est o f th e e le c tr ic ch arge co n serv a tio n law w ith B o r e x in o d e te c to r
A V is h n e v a 1,29 *, 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, S D a v in i11, A D e r b in 12, L D i N o t o 9, I D r a c h n e v 11, A E t e n k o 13, K F o m e n k o 1, 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 1, 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 13,20,
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 13,20,
I M a c h u lin 13,20, 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 i11, 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 12, 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 12, H S im g e n 22,
M S k o r o k h v a to v 13,20, O S m ir n o v 1A S o tn ik o v 1, S S u k h o t in 13,
Y S u v o r o v 27,13, 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 13, E U n z h a k o v 12, 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 te r 28, M W o jc ik 17, M W u r m 28, Z Y o k le y 7,
0 Z a im id o r o g a 1, 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 Jo in t In stitu te for N uclear Research, 141980 D ubna, R ussia
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 , G enova 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 ’A quila, Italy
12 St. P etersb u rg N uclear Physics In stitu te NRC K urchatov In stitu te , 188350 G atchina, R ussia
13 NRC K urchatov In stitu te , 123182 Moscow, R ussia
29 P resenter. To w hom any correspondence should be addressed.
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
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
21 K epler C enter for A stro and P artic le Physics, Universitaat Tuabingen, 72076 Tuabingen, 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 INFN , 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: *th e2 ch erry 2 o rch ard @ g m ail.co m
A b s t r a c t . T he new lim it on th e electron lifetim e is obtained from d a ta of th e Borexino experim ent. T h e expected signal from th e e ^ yv decay m ode is a 256 keV p h o to n d etected in liquid scintillator. Because of th e extrem ely low radioactive background level in th e Borexino d etec to r it was possible to im prove th e previous m easurem ent by two orders of m agnitude.
1. I n tr o d u c tio n
T he electric charge conservation law is a fundam ental physical principle. T here are no hints for violation of th is law n eith er in th eo ry w ithin th e S ta n d ard m odel nor in any experim ent.
Since th e electric charge non-conservation (CNC) is a d m itte d in exotic theories such as ex tra- dim ensional theories [1], investigation of such processes is an evident way to search for physics beyond th e S ta n d ard model.
T he m ost frequently searched for CNC processes are decays of th e electron into n eu tral particles. Tw o decay m odes are usually accounted for experim entally:
where only effects due to th e electron d isappearance would be observed. However, th e im possibility of occurrence of such processes is presented in [2], w here it is shown th a t such decays would be followed by a huge am ount of low-energy b rem sstrah lu n g photons. For th e process (1) it would m ean th e absence of 256 keV p h oto n while th e electron d isap pearance is m ore m odel-independent and th e corresponding atom ic effects in th e case (2) would rem ain th e same. T hus one can see th a t observing th e 256 keV p hoton from th e electron decay would m ean not only CNC b u t also going beyond th e S tan d ard model.
e ^ y v, ( 1 )
w h e r e a m o n o e n e r g e t i c 2 5 6 k e V p h o t o n i s s e a r c h e d f o r , a n d
e ^ v v v , ( 2 )
2. O v e r v ie w o f e x p e r im e n ts
S tudy of th e electron stab ility has long experim ental history. T he list of th e experim ents in which th e electron decay was being searched for is presented in tab le 1. T here are also plans for
T a b le 1. E x p erim ental te sts for th e electron stability.
year m aterial lim it for e ^ yv lim it for e ^ vvv CL reference
1959 NaI 10iy 10i7 68% [3]
1965 NaI 4 x 1022 2 x 102i 68% [4]
1975 Ge — 5.3 x 102i 68% [5]
1979 NaI 3.5 x 1023 — 68% [6]
1983 Ge 3 x 1023 2 x 1022 68% [7]
1986 Ge 1.5 x 1025 — 68% [8]
1993 Ge 1.63 x 1025 — 68% [9]
1995 Ge 2.1 x 1025 2.6 x 1023 90% [10]
1996 Xe 2 x 1025 1.5 x 1023 68% [11]
1999 NaI — (1.5 - 2.4) x 1023 90% [12]
1999 NaI — 2.4 x 1024 90% [13]
2000 Xe 2 x 1026 — 90% [14]
2002 P X E 4.6 x 1026 — 90% [15]
2007 Ge 1.93 x 1026 — 90% [16]
2012 NaI — 1.2 x 1024 90% [17]
providing analogous studies a t present and fu tu re experim ents [18, 19, 20].
2.1. N a l detectors
E xperim en ts based on N aI d etecto rs were th e first to provide th e lim its on th e electron stab ility [3, 4].The expected signal for th e m ode (2) is a photon w ith m axim al energy of 33.2 keV em itted while filling th e vacancy caused by th e electron disapp earan ce from th e iodine K-shell. T he decay to a photon and a n eu trin o is investigated by searching for th e 256 keV photon. Various coincidence techniques are also applied in such detectors. F irs t was th e search of sim ultaneous 256 keV and 33.2 keV photons occurence [4]. A nother approach based on th e electron cap tu re by a nucleus w ith o ut th e consequent atom ic num ber change was considered recently in [17].
Sim ultaneous observation of th e 33.2 keV photons and th e nucleus deexcitation (417.9 keV) would m ean th e electron disappearance.
2.2. Ge detectors
T he electron stab ility is widely studied using germ anium detectors. T he m ain advantage of such d etecto rs is good energy resolution (ab o u t 1 keV). In ad ditio n, th e background level in th e region of interest is lower th a n th a t in N aI detectors. T h e expected signal is a p h oto n of energy 11.1 keV for th e m ode (2) and a 256 keV ph oton for th e m ode (1), respectively.
2.3. Liquid scintillators
T he strongest lim its on th e electron lifetime w ith respect to th e decay m ode (1) du ring th e last fifteen years have been o b tain ed w ith liquid scintillation detectors. T h eir m ain advantages are large m ass and a possibility of purification from radioactive contam inations. T he first one was
Fit result for the electron decay rate = 1.23 cpd/100 tons
F ig u r e 1. B orexino sp ectrum com position.
D A M A /L X e experim ent [11, 14]. T his d e te c to r contains 6.5 kg ( ~ 2 litres) of liquid xenon. This a p p a ra tu s has ra th e r low energy threshold and is sensitive to b o th electron decay modes.
T he second one is th e p ro to ty p e of th e B orexino d etector, C T F -II [21]. Its m ain goal was to te s t th e purification techniques developed for Borexino. D uring th e tests various scintillators were used. C T F -II was filled w ith 4 tons of P X E (phenylxylylethane) which has less ionization quenching in com parison w ith P C (pseudocum ene) used in Borexino. Large m ass and extrem ely low background level m ade it possible to o b tain a stron ger lim it of 4.6 x 1026 years (90% confidence level) on th e electron lifetime in sh o rter exposure tim e. T his result rem ained th e best until th e sam e stu d y was perform ed in Borexino.
3. T h e B o r e x in o d e t e c t o r
Borexino is a large volum e scintillation d etecto r located deep un derground in th e L ab o rato ri Nazionali del G ran Sasso [22]. Its active m edia contains 278 tons of organic liquid scintillator, namely, pseudocum ene (1,2,4-trim ethylbenzene) w ith ad m ix tu re of P P O (2,5-diphenyloxazole) a t a co n cen tration of 1.5 g /l. B orexino has extrem ely low background level in th e region of interest, nam ely 0.15 d a y - 1 to n - 1 keV- 1 . T he energy threshold is above 50 keV so B orexino is not sensitive to th e d isapp earan ce m ode. By com paring sensitivity of C T F and B orexino th e expected electron lifetim e lim it is estim ated , which exceeds th e previous one at two orders of m agnitude.
4. T h e e le c tr o n d e c a y se a r c h Ą.I. A nalysis approach
T he d a ta set used in th e analysis were acquired from Ja n u a ry 2012 to M ay 2013 (Borexino P h ase 2). This d a ta set was o b tain ed after th e purification cam paign [23] which reduced in p a rticu la r th e co n tam in atio n of 85K r and 210Bi which give a significant co n trib u tio n in th e low-energy region.
T he sam e 408 days d a ta set is successfully used in th e m easurem ent of solar pp -n eu trin o flux [24]. In this analysis th e sam e energy range (150-600 keV) and param eters used in th e fitting procedure are considered. T he only difference is th e ad d itio n of th e 256 keV p hoton line in th e fittin g function. T he sam ple of sp ectral fit is shown on Fig. 4.1. One can see th a t th e sought-for peak (m arked by arrow) is shifted to th e lower energies due to ionization quenching.
4.2. C onstraint on the p p-neutrino event rate
T he 256 keV p h oto n occurrence is strongly correlated w ith pp-n eu trin o event rate. Therefore tre a tin g th e pp-n eu trin o ra te as a free p a ra m ete r in th e fit leads to non-physical values at th e lim it. Indeed, th e 256 keV p h o ton event ra te corresponding to th e 90% confidence level ( ~ 12 c p d /1 0 0 tons) corresponds to zero pp-n eu trin o ra te which is not consistent w ith observations by radiochem ical experim ents [25]. As far as th e la tte r ones are not sensitive to th e electron decay it is reasonable to use th e ir results to co n strain th e pp -n eu trin o event rate. This co n strain t gives th e lim it on th e event ra te of 1.23 c p d /1 0 0 tons. T he lifetime lim it is expressed as t % eNgT/Siim, w here N e is th e to ta l num ber of electrons in th e d etecto r, e is th e fraction of electrons survived after th e fiducial volum e cut, T is th e exposure tim e, and Slim is th e event ra te lim it. It gives th e electron lifetime of t % 7.2 x 1028 years.
4.3. System atic errors study and fin a l results
T he m ain sources of th e system atic errors in th is stu d y are th e following. T h e m ost im p o rta n t is th e precision of th e scintillator light yield m easurem ent ( ~ 1%). It strongly influences th e peak position and therefore affects th e sensitivity. A nother source of system atic errors is th e fiducial m ass m easurem ent precision, which gives negligible effect. Choice of th e energy e stim a to r can also affect th e result. In th e present stu d y two variables are used as energy estim ato rs, namely, num ber of P M T s h it in th e tim e intervals of 230 ns and 400 ns. A fter having accounted for all these effects th e lifetime lim it has becom e weaker and th e final result for th e electron lifetime lim it is Te^ lv % 6.6 x 1028 years a t th e 90% confidence level. This stu d y is described in m ore details in [26].
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).
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