Absence of day-night asymmetry of 862 keV Be-7 solar neutrino rate in Borexino and MSW oscillation parameters

0.49 MB·English

by  The Borexino Collaboration

Absence of a day–night asymmetry in the 7Be solar neutrino rate in Borexino
G.Bellinih,J.Benzigerl,D.Bickr,S.Bonettih,G.Bonfinie,M.BuizzaAvanzinih,B.Caccianigah,L.Cadonatip,
F.Calapricel,C.Carraroc,P.Cavalcanteh,A.Chavarrial,D.D’Angeloh,S.Davinic,A.Derbinm,A.Etenkog,
F.vonFeilitzschn,K.Fomenkob,D.Francoa,C.Galbiatil,S.Gazzanae,C.Ghianoe,M.Giammarchih,
M.Go¨ger-Neffn,A.Gorettil,e,L.Grandil,E.Guardincerric,S.Hardyq,AldoIannie,AndreaIannil,V.Kobychevf,
D.Korablevb,G.Korgae,Y.Koshioe,D.Kryna,M.Laubensteine,T.Lewken,E.Litvinovichg,B.Loerl,P.Lombardih,
F.Lombardie,L.Ludhovah,I.Machuling,S.Maneckiq,W.Maneschgi,G.Manuzioc,Q.Meindln,E.Meroni1,
L.Miramonti1,M.Misiaszekd,D.Montanarie,l,P.Mosteirol,V.Muratovam,L.Oberauern,M.Obolenskya,F.Orticaj,
M.Pallavicinic,L.Pappq,C.Pen˜a-Garayo,L.Perassoh,S.Perassoc,A.Pocarp,R.S.Raghavanq,G.Ranuccih,
A.Razetoe,A.Reh,A.Romanij,A.Sabelnikovg,R.Saldanhal,C.Salvoc,S.Scho¨nerti,H.Simgeni,
M.Skorokhvatovg,O.Smirnovb,A.Sotnikovb,S.Sukhoting,Y.Suvorove,g,R.Tartagliae,G.Testerac,D.Vignauda,
2 R.B.Vogelaarq,J.Wintern,M.Wojcikd,A.Wrightl,M.Wurmn,J.Xul,O.Zaimidorogab,S.Zavatarellic,G.Zuzeld
1
0 aAPC,LaboratoireAstroParticuleetCosmologie,75231Pariscedex13,France
2 bJointInstituteforNuclearResearch,Dubna141980,Russia
cDipartimentodiFisica,Universita´eINFN,Genova16146,Italy
n
dM.SmoluchowskiInstituteofPhysics,JagiellonianUniversity,Krakow,30059,Poland
a eINFNLaboratoriNazionalidelGranSasso,Assergi67010,Italy
J
fKievInstituteforNuclearResearch,Kiev06380,Ukraine

5 gNRCKurchatovInstitute,Moscow123182,Russia
hDipartimentodiFisica,Universita´degliStudieINFN,Milano20133,Italy

] iMax-Plank-Institutfu¨rKernphysik,Heidelberg69029,Germany
x jDipartimentodiChimica,Universita´eINFN,Perugia06123,Italy
e kChemicalEngineeringDepartment,PrincetonUniversity,Princeton,NJ08544,USA
- lPhysicsDepartment,PrincetonUniversity,Princeton,NJ08544,USA
p
mSt.PetersburgNuclearPhysicsInstitute,Gatchina188350,Russia
e
nPhysikDepartment,TechnischeUniversita¨tMu¨nchen,Garching85747,Germany
h
oInstitutodeF`ısicaCorpuscular,CSIC-UVEG,ValenciaE-46071,Espan˜a
[
pPhysicsDepartment,UniversityofMassachusetts,Amherst01003,USA
2 qPhysicsDepartment,VirginiaPolytechnicInstituteandStateUniversity,Blacksburg,VA24061,USA
v rInstitutfu¨rExperimentalphysik,Universita¨tHamburg,Germany
0
5
1
2
4. Abstract
0
Wereporttheresultofasearchforaday–nightasymmetryinthe7BesolarneutrinointeractionrateintheBorexino
1
1 detectorattheLaboratoriNazionalidelGranSasso(LNGS)inItaly.ThemeasuredasymmetryisAdn=0.001±0.012
: (stat) ± 0.007 (syst), in agreement with the prediction of MSW-LMA solution for neutrino oscillations. This result
v
disfavorsMSWoscillationswithmixingparametersintheLOWregionatmorethan8.5σ. Thisregionis,forthefirst
i
X time,stronglydisfavoredwithouttheuseofreactoranti-neutrinodataandthereforetheassumptionofCPTsymmetry.
r Theresultcanalsobeusedtoconstrainsomeneutrinooscillationscenariosinvolvingnewphysics.
a
Keywords:
solarneutrinos,day–nighteffect,CPTviolation,neutrinooscillations
Inthelasttwodecadessolarneutrino[1,2,3]andre- oscillations[5]withLargeMixingAngle(LMA)oscil-
1 7
actor anti-neutrino [4] experiments have demonstrated lationparameters.Agenericfeatureofmatter-enhanced
2 8
that solar electron neutrinos undergo flavor conversion neutrinooscillationsisthepotentialforthecoherentre-
3 9
along their trip from the Sun’s core to the Earth. The generationoftheν flavoreigenstatewhensolarneutri-
4 10 e
conversioniswelldescribedbytheso-calledMikheyev- nos propagate through the Earth [6], as they do during
5 11
Smirnov-Wolfenstein(MSW)matter-enhancedneutrino thenight.Thus,thereisthepotentialforthosesolarneu-
6 12
PreprintsubmittedtoElsevier January19,2013
trinoexperimentsthatareprincipally,orentirely,sensi-
13
tive to ν to detect different solar neutrino interaction
14 rates dureing the day and during the night. Solar neu- eg 20
15 d Ideal exposure function (3 years of live time)
16 trino day-night asymmetry measurements are sensitive ays/ 18
17 tobothνeappearanceanddisappearance. d 16 Experimental exposure function
Themagnitudeofthisday-nighteffectisexpectedto
18 14
depend on both neutrino energy and the neutrino os-
19
12
cillation parameters. Previous experiments [7, 8] have
20
shown that for high energy (∼5-15 MeV) solar neutri- 10
21
22 nos, the day–night asymmetry is less than a few per- 8
cent, in agreement with the MSW-LMA prediction.
23 6
At lower neutrino energies (around 1 MeV), the pre-
24
4
dicted day–night asymmetry for MSW-LMA is also
25
small (<0.1%) [9]; however, other scenarios including 2
26
different MSW solutions and neutrino mixing involv-
27 -180 -160 -140 -120 -100 -80 -60 -40 -20 0
28 ing new physics [10] predict much larger day–night q z (deg)
effects. For example, in the so-called LOW region
29
(10−8 eV2<δm2<10−6 eV2) of MSW parameter space, Figure1:Theexperimentalexposurefunction(blackcontinuousline)
30
whichiscurrentlystronglydisfavoredonlybytheKam- andtheidealexposurefunction(reddottedline). Theintervalfrom
31 −180◦to−90◦correspondstodaytimeandtheonefrom−90◦to0◦
LAND anti-neutrino measurement under the assump-
32 tonighttime. WerecallthatatLNGSlatitudetheSunisneveratthe
33 tionofCPTsymmetry,theday-nightasymmetrywould zenith.
range between about 10% and 80% for neutrino ener-
34
giesnear1MeV.Wepresentherethefirstmeasurements
35
sensitivetotheday–nightasymmetryforsolarneutrinos
36
new physics scenarios that cannot be rejected with the
below1MeV.Thisresultisanessentiallynewandinde- 65
37
currentlyavailabledata.
pendentwaytoprobetheMSW-LMApredictionandis 66
38
39 potentiallysensitivetonewphysicsaffectingthepropa- 67 The Borexino detector [12, 13] is located in Hall C
40 gationoflowenergyelectronneutrinoinmatter. Partic- 68 of the Laboratori Nazionali del Gran Sasso (latitude
41 ularly, this result is independent from the KamLAND 69 42.4275◦N)inItalyandhastakendatasinceMay2007.
42 measurement, which probes anti-neutrino interactions 70 Thesensitivedetectorconsistsof∼278tonsofverypure
43 athigherenergies(>1.8MeV). 71 organic liquid scintillator contained in a 4.25 m radius
44 The Borexino experiment at LNGS detects low en- 72 nylonvessel. Thescintillatorisviewedby2212photo-
45 ergysolarneutrinosbymeansoftheirelasticscattering 73 multipliersandisshieldedagainstexternalneutronsand
46 onelectronsinalargevolumeliquidscintillatordetec- 74 γradiation[14]. Theenergyofeachcandidateeventis
47 tor. Real-timedetection(with≈1µsabsolutetimereso- 75 measured by the total amount of collected light, while
48 lution)ofalleventsismadebycollectingthescintilla- 76 thepositionoftheeventisreconstructedusingthetime-
49 tionlightwithalargesetofphotomultipliers. Thevery 77 of-flightofthelighttothephotomultipliers.
low intrinsic radioactivity of the scintillator and of the Thedatausedinthisanalysiswerecollectedbetween
50 78
materialssurroundingitallowsacleanspectralsepara- May 16th, 2007 and May 8th, 2010 and correspond to
51 79
tionbetweentheneutrinosignalsandtheresidualback- 740.88 live days after applying the data selection cuts.
52 80
ground.Astheneutrino-electronelasticscatteringcross Wedefine“day”and“night”usingθ ,theanglebetween
53 81 z
sectionisdifferentforν andν -ν ,Borexinocanmea- theverticalz-axisofthedetector(positiveupward)and
54 e µ τ 82
suretheelectronneutrinosurvivalprobabilityandis,as thevectorpointingtothedetectorfromtheSun,follow-
55 83
aresult,sensitivetotheday-nighteffect. ing[2]. Notethat,withthisdefinition,cosθ isnegative
56 84 z
We recently released a precise measurement of the during the day and positive during the night. The dis-
57 85
7Be neutrino interaction rate in Borexino with a total tance that the neutrinos propagate within the Earth is
58 86
uncertaintylessthan5%[11]. InthisLetter,wepresent small for negative cosθ (the ∼1.4 km LNGS overbur-
59 87 z
astudyoftheday-nightasymmetryinthesame7Beso- den)andrangesupto12049kmforpositivecosθ . Our
60 88 z
larneutrinorate,placingastringentlimitonthesizeof day and night livetimes were 360.25 and 380.63 days,
61 89
the possible effect. We show that this limit improves respectively. Thedistributionofθ correspondingtothe
62 90 z
theconstraintonthesolarneutrinooscillationparame- live time (experimental exposure function) is shown in
63 91
tersfromsolarneutrinoexperimentsaloneandexcludes Fig.1anditsasymmetrywithrespectto-90◦ ismainly
64 92
2
duetomaintenanceandcalibrationactivitieswhichare
93
normallycarriedoutduringtheday.
9945 As discussed in [11], scintillation events due to 7Be eV 400
k
96 solar neutrinos cannot be distinguished from back- 5 104 300
97 gevroeunnt-dbye-veevnetnst(bcaossism.oTgheneicsisgnaanldarnaddiboaaccktigvriotyu)ndoncoann- s / 200
98 t
n
99 tributionsarethereforedeterminedusingaspectralfitto e 100
100 theenergyspectrumoftheeventsreconstructedwithina ev103
suitablefiducialvolume(86.01m3in[11]),andpassing 0.55 0.6 0.65 0.7 0.75 0.8
101
aseriesofcutswhicheliminatemuonsandshort–lived
102
cosmogenicevents,timecorrelatedbackgroundevents,
103 102 Day spectrum
andspuriousnoiseevents(detailsofthiseventselection
104
will be published in [15]). The experimental signature Night spectrum
105
ofthemono–energetic862keV7Besolarneutrinosisa
106 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6
107 Compton-likeelectronscattering“shoulder”atapprox- MeV
imately660keV.
108
Intheanalysisreportedhereweuseasphericalfidu-
109 Figure2: Theenergyspectrumofeventsduringday(red)andnight
cial volume significantly larger than the one used in
110 (black)normalizedtothedaylive–timeintheenlargedFV.Theinsert
[11]inordertoincreasethesizeofthedatasample.This showsthe7Beneutrinoenergywindow. See[11]fordetailsonthis
111
choice is justified by the fact that the additional exter- spectralshape.
112
nalbackgroundthatentersthislargerfiducialvolumeis
113
duetogammaradioactivityemittedbythematerialssur-
114
roundingthescintillatorvolume. Asthisbackgroundis
115
count rate measured in [11], a correction has been ap-
expectedtobethesameduringdayandnight,itshould 135
116
notaffecttheday–nightasymmetry1. 136 pliedtotheexposurefunctiontoaccountfortheannual
117
modulationoftheneutrinofluxduetotheseasonalvari-
We determined that our sensitivity to the day– 137
118
night effect is maximized by a 3.3m fiducial ra- 138 ation of the Earth-Sun distance. Before correction, the
119
asymmetric distribution of our day and night livetime
dius, which gives a 132.5ton fiducial mass containing 139
120
4.978×1031 e−. With this choice of fiducial mass, the 140 throughouttheyearisexpectedtoincreasethemeasured
121
signal–to–background (S/B) ratio in the “7Be neutrino 141 7Be neutrino count rate by 0.37% during the night and
122
energywindow”(550to800keV)is0.70±0.04. This 142 decreaseitby0.39%duringtheday. Thedayandnight
123
spectrainFig 2arestatisticallyidentical,asprovedby
valueissmallerthantheonein[11]duetotheincrease 143
124
the fit to the data shown in Fig. 3. Indeed, by fitting
in spatially non-uniform backgrounds produced by ex- 144
125
ternalgammaraysand222Rnevents. 145 withaconstantdistributionthedatainFig.3weobtain
126 Theday–nightasymmetry,Adn,ofthe7Becountrate 146 a χ2 probability = 0.44. Any deviation from a straight
linewouldbeasignatureofday–nightmodulation. For
isdefinedas: 147
illustration,weincludeinFig.3theexpectedshapefor
Adn =2 RRNN +−RRDD = R(cid:104)Rdi(cid:105)ff (1) 114489 t=he0L.9O55W).soFliutttiionng(t∆hem21d2i=st1ri.b0u·ti1o0n−7weitVh2aanfldattasntr2a(iθg1h2)t
150
line yields χ2/ndf = 141.1/139, showing that the data
whereR andR arethe7Beneutrinointeractionrates 151
127 N D areconsistentwiththenoday–nighteffecthypothesis.
128 during the night and the day, respectively, Rdiff is their 152
129 difference,and(cid:104)R(cid:105)istheirmean. 153 One way to quantitatively constrain Adn is to deter-
130 Fig. 2showsthedayandnightenergyspectrasuper- 154 mine RD and RN separately by independently fitting
131 imposedandnormalizedtothesamelive–time(theday 155 the day and night spectra using the same spectral fit-
132 one),whileFig.3showstheθzdistributionoftheevents 156 ting technique used in determining the total 7Be flux
133 in the 7Be neutrino energy window normalized by the 157 in [11] and then comparing the results using Eq. 1.
134 experimentalexposurefunction. Byusingthetotal7Be 158 Note that because these neutrinos are mono–energetic,
we expect the shape of the 7Be electron recoil spec-
159
trum to be identical during day and night. This yields
160
1Asexplainedlateron,notallbackgroundsrelevantforthisanal- A = 0.007±0.073. Thismethodhasthevirtueofal-
ysisarethesameduringdayandnight. Particularly,thebackground 161 dn
inducedby210Poαsisnotthesamebecauseofthelong210Polifetime 162 lowingforthepossibilityofdifferentbackgroundrates
andofthedifferentlengthofdaysandnightsinsummerorwinter. 163 duringdayandnight.However,thisanalysisislesssen-
3
ingelectronrecoilspectruminducedbytheinteraction
194
of7BeneutrinosistoosmalltobeshowninFig.4. In
) 195
g
de 30 196 order to see its spectral shape we plot the recoil spec-
y * 197 trum with an amplitude corresponding to the expected
s / (da 25 119989 dfraoyTm–hnetihgRehdditffaayrseyasmnudlmtniesitgrcyhotnfsofiprremtcheterdaLbuOysWirnegmsaoolvpuiutnilogsena.slphhapaeevaennatls-
t 200
n 20 ysis based statistical subtraction technique [11] before
e 201
v creatingthedifferencespectrum. Inthiscase,noresid-
e 202
15 ual210Popeakisexpectedorobservedinthedifference
203
spectrum. Fitting the data between 0.25 and 0.8 MeV
204
10 using only the 7Be recoil shape yields a result consis-
205
tentwiththepreviousone. Thedifferenceinthecentral
206
-160 -140 -120 -100 -80 -60 -40 -20207 valuesisincludedinthesystematicuncertainty.
q (deg) Using(cid:104)R(cid:105)=46±1.5(stat)+1.6 (syst) cpd/100t[11]
z −1.5
weobtainA =0.001±0.012(stat)±0.007(syst)from
dn
Eq. 1. ThestatisticalerrorinA isgivenby
dn
Figure3:Normalizedθz–angledistributionoftheeventsintheFVin
toLhrOebWi7tBhseaoslnubeteuioetrnninr(oe∆mmeno21ve2re=gd1y..0Tw·hin1ed0bo−lw7ue.eVlTin2heeaniesdffttehacent2eo(xθfp1te2hc)et=eEd0ae.r9ffth5e’5cs)t.ewlliitphtitchael σAdn = R(cid:104)dRif(cid:105)f (cid:118)(cid:116)σR22diff + σ<2(<RR>2>)(cid:39) σ(R(cid:104)Rdi(cid:105)ff)
diff
because the total relative experimental error associated
sitive than the one described below and is not used for 208
164 with(cid:104)R(cid:105)isnegligiblewithrespectto σ(Rdiff).
165 thefinalresult. 209 Rdiff
ThemainsystematicerrorsarelistedinTable1. The
A stronger constraint on A is obtained by mak- 210
166 dn dominant uncertainties are associated with the differ-
ingtheveryreasonableassumptionthatthemainback- 211
167
groundsthatlimitthesensitivityin[11](85Krand210Bi) 212 encebetweentheRdiff centralvaluesobtainedwithand
168
without statistical subtraction of the α events, and the
are the same during day and night. With this assump- 213
169 tion, A is obtained by subtracting the day and night 214 maximum effect on Rdiff from potential small changes
170 dn in the 210Bi background in the detector. These uncer-
spectra (normalized to the day live time) following the 215
171
taintieswillbedetailedin[15].
second term in Eq. (1) and then searching for a resid- 216
172 This new tight constraint on the day-night effect in
ual component having the shape of the electron recoil 217
173 spectrumdueto7Beneutrinos. IfA =0andtheback- 218 7Be solar neutrinos has interesting implications on our
174 dn
understanding of neutrino oscillations. To investigate
groundcountrateswereconstantintimethesubtracted 219
175
this, we calculated the expected day–night asymme-
spectrumwouldbeflat. 220
176 try for 862keV neutrinos under different combinations
ThesubtractedspectrumisshowninFig.4,wherethe 221
177
ofmixingparametersintheMSWoscillationscenario.
lowerplotisazoomoftheupperoneintheenergyre- 222
178
The comparison of these predictions with our experi-
gionbetween0.55and0.8MeV.Theresultisaflatspec- 223
179
mentalnumberisdisplayedontherightpanelofFig.5.
trum, consistent with zero, except for a clear negative 224
180
Theredregionisexcludedat99.73%c.l. (2d.o.f.). In
210Popeakvisibleinthelowenergyregion. Thisnega- 225
181
particular,theminimumday–nightasymmetryexpected
tivepeakarisesbecausethe210Pobackgroundcountrate 226
182 in Borexino is decaying in time (τ = 138.38 days), 227 in the LOW region (10−8 eV2 (cid:47) ∆m2 (cid:47) 10−6 eV2) is
183 1/2
andthedayandnightlivetimearenotevenlydistributed
184
185 overthe3yearsofdatataking.The210Pocountratewas Sourceoferror ErroronAdn
186 highestatthetimeoftheinitialfillinginMay2007,and Live–time <5·10−4
187 has since decayed. Therefore, the 210Po count rate has Cutefficiencies 0.001
188 beenhigheronaverageduringthesummers(whendays Variationof210Biwithtime ±0.005
189 are longer), leading to a noticeable effect in the sub- Fitprocedure ±0.005
tracted spectrum. This effect is taken into account by
190 Totalsystematicerror 0.007
includingboththe210Poand7Bespectralshapesinthe
191
192 fit. Fittingbetween0.25and0.8MeV,weobtainRdiff =
193 0.04±0.57(stat)cpd/100t. Theamplitudeoftheresult- Table1:ListofsystematicerrorsonAdn.
4
V
e
k
5 0
s / 210Po = -21.8 – 0.9 cpd/100t
nt-500
e 7Be = 0.04 – 0.57 cpd/100t
v
e 7Be = 12 cpd/100t
-1000
Night - Day spectrum
-1500
0.4 0.6 0.8 1 1.2 1.4 1.6
MeV
V
e
k 150
5
s / 100
nt 50
e
v
e 0
-50
Figure5: Neutrinooscillationsparameterestimationinthreesolar
0.55 0.6 0.65 0.7 0.75 neutrinodataanalyses(with2d.o.f.):1)99.73%c.l.excludedregion
MeV bytheBorexino7Beday–nightdata(hatchedredregionintheright
panel);2)68.27%,95.45%,and99.73%c.l. allowedregionsbythe
solarneutrinodatawithoutBorexinodata(leftpanel); 3)Samec.l.
Figure 4: Difference of night and day spectra in the FV. The fit is allowedregionsbyallsolarneutrinodataincludingBorexino(filled
performedintheenergyregionbetween0.25and0.8MeVwiththe contoursinrightpanel). Thebestfitpointintheleft(right)panel
residual210Pospectrumandtheelectronrecoilspectrumduetothe is∆m2 =(5.2+1.6)·10−5,tan2θ=0.47+0.04 (0.46+0.04). TheLOW
7Besolarneutrinointeraction.Thefitresultsareincpd/100t.Thetop regionisstrong−l0y.9excludedbythe7Be−d0a.0y3–night−d0a.t0a3whiletheal-
panelshowsanextendedenergyrangeincludingtheregiondominated
lowedLMAparameterregiondoesnotchangesignificantlywiththe
bythe11Cbackgroundwhilethebottompanelisazoomofthe7Be
inclusionofthenewdata.
energywindowbetween0.55and0.8MeV.Thebluecurveshowsthe
shape ofelectron recoilspectrum thatwouldbe seenassuming the
LOWsolutionasinFig.3.
99.73%c.l. neutrinomixingparameterregionsallowed
249
byallsolarneutrinodatawithoutBorexino. Thebest-fit
250
228 0.117, which is more than 8.5σ away from our mea- 251 pointisintheLMAregion(∆m2 =(5.2+−10..69)·10−5 eV2
229 surement,assuminggaussianerrorsforAdn. 252 and tan2θ =0.47+−00..0043) and a small portion of the LOW
230 Thiseffectcanalsobeseeninaglobalanalysisofall 253 regionisstillallowedat∆χ2 =11.83.
solarneutrinodata. Wehavecarriedoutsuchananaly- The right panel of Fig. 5 shows the regions of al-
231 254
sis,assumingtwoneutrinooscillations(i.e. θ =0,we lowed parameter space after adding the Borexino data
232 13 255
havecheckedthattheinclusionofthethirdfamilydoes (the 7Be total count rate [11], the day–night asymme-
233 256
notchangeanyoftheconclusionsandwillbepublished try reported in this paper, and the 8B total count rate
234 257
in[15]),includingtheradiochemicaldata[1],theSuper- above3MeV(0.22±0.04(stat)±0.01(syst))cpd/100
235 258
KamiokandephaseIandphaseIIIdata[2],andtheSNO tandspectralshape(5binsfrom3to13MeV)[18])to
236 259
LETA data and phase III rates [3]. The analysis takes theanalysis. TheLMAregionisonlyslightlymodified
237 260
intoaccounttheexperimentalerrors(thesystematicand (thenewbestfitpointis∆m2 =(5.2+1.6)10−5 eV2 and
238 261 −0.9
statisticalerrorssummedinquadrature)andthetheoret- tan2θ = 0.46+0.04), but the LOW region is strongly ex-
239 262 −0.03
icalerrorsinthetotalcountrates,includingthecorrela- cluded at ∆χ2 > 190. Therefore, after the inclusion of
240 263
tionofthe7Beand8Btheoreticalfluxes [16]. Weuse the Borexino day–night data, solar neutrino data alone
241 264
fluxpredictionsfromarecenthighmetallicitystandard can single out the LMA solution with very high confi-
242 265
solar model [17] and we include the bin-to-bin corre- dence, without the inclusion of anti-neutrino data and
243 266
lationsintheuncertaintiesinthepredicted8Bneutrino thereforewithoutinvokingCPTsymmetry.
244 267
245 recoil spectrum resulting from the uncertainties in the 268 Thisresultisanessentiallynewandindependentway
246 predictedneutrinospectrum,andfromenergythreshold 269 to probe the MSW-LMA prediction and is potentially
247 uncertaintiesandenergyresolutionintheexperiments. 270 sensitive to new physics affecting low energy electron
The left panel of Fig. 5 shows the 68.27, 95.45 and neutrinointeractions. Asanexample, wenotethatour
248 271
5
272 day-nightasymmetrymeasurementisverypowerfulin 329 [12] G.Alimontietal.(BorexinoColl.), Nucl.Instrum.Methods
273 testingmassvaryingneutrinoflavorconversionscenar- 330 Phys.Res.A600(2009),568.
ios. We find, for example, that our A data excludes 331 [13] G.Alimontietal.(BorexinoColl.), Nucl.Instrum.Methods
274 dn 332 Phys.Res.A609(2009),58.
the set of MaVaN parameters chosen in [10] to fit all
275 333 [14] G. Bellini et al. (Borexino Coll.) JINST 6 (2011) P05005;
276 neutrinodataatmorethan10σ. 334 arXiv:1101.3101v2[physics.ins-det].
277 asyImnmcoentrcyluisniotnh,e wineterhaacvtieonseraartcehoefd8f6o2r kaeVda7yB–neigsoh-t 333356 [15] Gta.ilBsealblionuite7tBale.(dBaotareaxnianloysCiso,lld.)a,ya–rntiicglhetinseparrecpharaantdiosnowlairthndeue--
278 lar neutrinos in Borexino. The result is A = 0.001 ± 337 trinooscillationsanalysis.
279 dn 338 [16] J.N.Bahcall,Phys.Rev.Lett.12(1964),300;C.Pen˜a-Garay
280 0.012(stat)±0.007(syst),consistentbothwithzeroand 339 andA.SerenelliarXiv:0811.2424[astro-ph](2008).
withthepredictionoftheLMA-MSWneutrinooscilla- 340 [17] A.Serenelli,W.Haxton,andC.Pen˜a-Garay,arXiv:1104.1639
281
tionscenario.Withthisresult,theLOWregionofMSW 341 [astro-ph].
282 342 [18] G. Bellini et al. (Borexino Coll.) Phys. Rev. D 82 (2010),
283 parameterspaceis,forthefirsttime,stronglydisfavored 343 033006;arXiv:0808.2868v3(astro-ph).
bysolarneutrinodataalone. Theresultconstrainscer-
284
tainflavorchangescenariosinvolvingnewphysics.
285
ThisworkwasfundedbyINFNandMIURPRIN
286
2007(Italy),NSF(USA),BMBF,DFG,andMPG(Ger-
287
many),NRCKurchatovInstitute(Russia),andMNiSW
288
(Poland). Wegratefullyacknowledgethegeneroussup-
289
portoftheLaboratoriNazionalidelGranSasso.
290
This work is dedicated to the memory of our friend
291
Raju Raghavan, the father of this experiment, and a
292
greatscientist.
293
References
294
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