Neutrino Oscillation Search at MiniBooNE

0.18 MB·English

by  Z. Djurcic

Neutrino Oscillation Search at MiniBooNE
7
0 Z. Djurcic∗ for the MiniBooNE Collaboration a†
0
2 aColumbia University, New York, NY 10027, USA

n
a This article reports the status of a νµ → νe oscillation search in MiniBooNE (Booster Neutrino Experiment)
J experiment. If an appearance signal is observed, it will imply Physics Beyond the Standard Model such as the

0 existenceof light sterile neutrino.
1

1
v
1. INTRODUCTION 2. THE EXPERIMENT
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1
In recent years, the solar-neutrino [1], reactor- MiniBooNE is a fixed target experiment cur-
0
1 neutrino [2], atmospheric-neutrino [3], and rently taking data at Fermi National Accelera-
0 accelerator-neutrino [4] experiments have con- tor Laboratory. The neutrino beam is produced
7 firmed the existence of neutrino oscillations. from 8.89 GeV/c protons impinging on a 71 cm
0
These results implied the existence of two inde- long and 1 cm diameter beryllium target. The
x/ pendent ∆m2 regions, with ∆m2 ∼8×10−5eV2 target is located inside a magnetic focusing horn
e in the solar, and with ∆m2 ∼ 3 × 10−3eV2 in that increases the neutrino flux at the detector
p- the atmospheric sector. The Standard Model in- by a factor of ∼5, and can operate in both neg-
e corporates two separate oscillation regions with ative and positive polarities for ν and ν¯ running.
h three known neutrino flavors: ν , ν , and ν . MiniBooNEcollectedapproximately6×1020pro-
e µ τ
:
v However, an unconfirmed evidence for neutrino tons on target (POT) in neutrino mode. This
i oscillations came from the LSND [5] experiment data sample is currently used in the neutrino os-
X
with ∆m2 at ∼ 1eV2 value. The discovery of cillation analysis. Mesons produced in the target
r
a nonzeroneutrinomassesthroughtheneutrinoos- decay-in-flight in a 50 m long decay pipe. The
cillations has raised a number of very interesting neutrinobeamiscomposedofν fromK+/π+ →
µ
questions about the neutrinos and their connec- µ+ +ν decays. The neutrino beam propagates
µ
tions to other areas of physics and astrophysics. througha450mofadirtabsorberbeforeentering
One question is whether there are sterile neutri- thedetector. Thereisasmallcontaminationfrom
nos that do not participate in the standard weak ν . The processesthat contribute to the intrinsic
e
interactions. This question is primarily being ν inthebeamareµ+ →e+ν ν¯ ,K+ →π0e+ν ,
e e µ e
addressed by the MiniBooNE experiment. The and K0 → π±e±ν . Early in 2006, MiniBooNE
L e
MiniBooNEexperimentwillconfirmorrefutethe switched the polarity of the horn to select nega-
LSND result with higher statistics and different tive sign mesons.
sourcesofsystematicerror. IftheLSNDneutrino The MiniBooNE detector is a 12.2 m diame-
oscillationevidence is confirmed, it will, together terspherefilledwith800tonsofpuremineraloil.
with solar, reactor, atmospheric and accelerator ThecenteroftheMiniBooNEneutrinodetectoris
oscillation data, imply Physics Beyond the Stan- positioned L=541m fromthe front ofthe beryl-
dard Model such as the existence of light sterile lium target. The vessel consists of two optically
neutrino [6]. While LSND observed an excess of isolated regions divided by a support structure
ν¯ events in a ν¯ beam, MiniBooNE is a ν →ν located at 5.5 m radius. The inner volume is
e µ µ e
search. the neutrino target region, while the outer vol-
ume forms the veto region. There are 1280 8-
∗Email address: [email protected]
inchphoto-tubes (PMTs)pointedinwardprovid-
†MiniBooNEExperimentissupportedbyNSFandDOE.
1
2
ing 10% coverage, and 292 outward-faced PMTs trino induced events are identified by requiring
in the veto region. Data analysis require a fidu- that the event is observed during the beam spill,
cial volume cut at 500 cm from the center of the have fewer than 6 veto PMT hits, and greater
tank to ensure good event reconstruction, result- than200tankPMThits. Thesesimplecutsyield
ing in a 445 ton target region. The outer volume a cosmic ray rejection that is better than 1000:1.
serves as a veto shield for identifying particles The MiniBooNE detector is calibrated using a
both entering and leaving the detector. The rate laser system, cosmic rays and neutrino interac-
ofneutrinocandidatesperprotondeliveredtothe tions. The calibration procedures cover the full
target was constant (1.089×10−15 ν/POT) over energy range from 50 to 1000 MeV using differ-
the period of data acquisition, as demonstrated ent event types. Stopping cosmic ray muons are
inFigure1. ProtonsdeliveredtotheMiniBooNE used to calibrate the energy scale for muon-type
eventsandmeasurethepositionandanglerecon-
struction resolution, when their path length can
be identified. This is accomplishedwithascintil-
lator hodoscope on the top of the detector, com-
binedwithscintillationcubesatvariouspositions
within the detector volume. Analysis of the cos-
mic muons indicates that the energy and angle
resolutionofmuonsis10%and6.4o,respectively.
Observed Michel electrons from muon decay are
usedtocalibratetheenergyscaleofelectron-type
events at the 52.8 MeV Michel endpoint, with
12% resolution. Reconstructedπ0 events provide
anotherelectron-likecalibrationsource. Thepho-
tons thatareemitted inπ0 decaysspana consid-
erablerangetoover1000MeV. Theπ0 massde-
rived from reconstructed energies and directions
oftwoγ-rayshasapeakat136.3±0.8MeV. The
resolution on the reconstructed π0 mass peak is
20 MeV. This is in an excellent agreement with
the135.0MeV expectation,providingacheckon
the energy scale and the reconstruction over the
Figure 1. Calibratedneutrino rate per proton on
fullenergyrangeofinterestfortheν appearance
e
target.
analysis.
3. NEUTRINO FLUX, CROSS SEC-
targetaremonitoredbytwotoroidslocatedinthe
TION,ANDDETECTORMODELING
beam line. The main MiniBooNE trigger is an
accelerator signal indicating a beam spill. Every ThefluxmodelingusesaGeant4-basedsimula-
beam trigger opens a 19.2 µs window in which tionofbeamlinegeometry. Hadronproductionin
all events are recorded. Other triggers include thetargetisbasedonSanford-Wangparametriza-
a supernova trigger, a random strobe trigger for tion of p−Be cross-section,with parameters de-
beam-off measurements, a laser calibration trig- terminedbyaglobalfittop−Beparticleproduc-
ger,cosmicmuontriggers,andatriggertorecord tion data. Simulated neutrino flux has an energy
neutrino events from the NuMI beam line. distribution with a peak at ∼ 0.7GeV. There-
Theinteractionpoint,eventtime,energies,and fore, the average L/E ratio is ∼ 0.8km/GeV
ν
the particle tracks are recorded from the times compared to LSND’s L/E ∼1km/GeV.
ν
andchargesofthePMThitsinthedetector. Neu- The NUANCEv3 [7] event-generator simulates
3
interactions in the detector. The cross section
model describes the various neutrino interaction 2 2500
processes on CH2, which include the Llewellyn- /c
Smith free nucleon quasi-elastic cross section, V Monte Carlo Simulation
e
the Rein-Seghal resonant and coherent pion pro- M2000 NC p0

duction cross section, a Smith-Moniz Fermi gas 5 Data
/
model, and final state interactions based on π- s
t
Carbon scattering data. n1500
e
The MiniBooNE detector is modeled using an v
E
extended GEANT3-based simulation. An ex-
tended light propagation model of the detector 1000
describes the emission of optical and near-UV
photons via Cerenkov radiationand scintillation.
500
Each photon is individually tracked, undergoing
scattering, fluorescence, and reflection, until it
is absorbed. The response of the electronics to
0
thephotoelectronsresultingfromphotonshitting 0 50 100 150 200 250 300 350 400 450 500
PMT’s is simulated, with the final output be- Reconstructed Mass (MeV/c2)
ingdigitizedwaveformssimulatingthechargeand
timechannelsoftheelectronicsinaformidentical
to that used for data.
Figure 2. (Preliminary) Mass distribution of π0
4. THE BACKGROUNDS IN THE AP- candidates obtained using a two ring likelihood
PEARANCE SIGNAL fit. The data are the black points, the black line
is Monte Carlo simulated neutrino interactions,
The signature of an oscillation event is the
while the red line is the subset of these events
ν + n → e + p reaction. The backgrounds in
e which have at least one pi0 in the final state.
the oscillation analysis are divided in two main
categories: intrinsic ν events in the beam, and
e
ν events that are mis-identified as ν events.
µ e
Thebeamthatarrivesatthedetectorisalmost
pure ν with a small (0.6%) contamination of ν the decay of π0 can appear much like primary
µ e
coming from muon and kaon decays in the de- electron emerging from a ν charged current in-
e
cay pipe. The ν from µ-decay are directly tied teraction if one of the gammas from the decay
e
to the observed ν interactions. Taking into ac- overlaps the other, or is too low in energy to be
µ
count a small solid angle subtended by the Mini- detected. Over 99% of the NC π0 are rejected in
BooNEdetector,thepionenergydistributioncan the appearance analysis. In addition to its pri-
bedeterminedfromtheenergyoftheobservedν mary decay ∆ → π N, the ∆ resonance has a
µ
events. Thepionenergyspectrumisthenusedto branchingfractionof0.56%totheγ N finalstate.
predict the ν from µ-decay. A second source of Theγ-raymaymimicanelectronfromν interac-
e e
νe originates from Ke3-decay. This component tion. Therateof∆productioninneutralcurrent
is constrained using high energy ν charged cur- interactions can be estimated from the data, us-
µ
rent quasi-elastic (CCQE) events, that originate ing the sample of reconstructed π0 decays. Most
primarily from kaon decays. ν events can be easily identified by their pene-
µ
Mis-identified ν events are π0, ∆-decays, and tration into the veto region when exiting muons
µ
ν CCQE events. Most π0 are identified by the fire the veto, or by muons stopping in the inner
µ
reconstructionoftwoCerenkovringsproducedby detector and producing a Michel electron after a
twodecayγ-rays,asshowninFigure2. However, few microseconds.
4
5. SIGNAL SEPARATION FROM THE electrons following the primary interaction, and
BACKGROUND requiringthevetotohavelessthansixhitstoen-
sure there is no cosmic muon contamination and
Particle identification (PID) is performed by
the tank to have greater than 200 hits to sup-
different algorithms that use the difference in
press decay electrons from cosmic muons. The
characteristics of the Cerenkov and scintillation
events are then fit under the single electron and
lightassociatedwithelectrons,muons,protonsor
muon ring hypotheses and a ratio is formed with
π0’s, as shown in Figure 3. These algorithms in-
the resulting likelihoods. Events with likelihood
ratios favoring the muon hypothesis are rejected.
The eventis thenfitwithtworingfits,bothwith
the mass free and fixed to the nominal π0 mass.
In the case from Figure 3, separationbetween π0
1
02000 Monte Carlo Simulation
s/0.1800 NC p0
nt1600 Data
e
v
E1400
1200
1000
800
600
400
200
0
-0.5 -0.4 -0.3 -0.2 -0.1 0
log(L/Lp)
e
Figure4. (Preliminary)Thelogarithmofthelike-
Figure 3. Cartoon showing how particle identifi- lihood ratio formed from fitting neutral current
cation acquire an input from the Cerenkov rings. π0candidatesunderasingleelectronringhypoth-
From the top to the bottom, the rings are: an esis and a two ring hypothesis. The data are the
ideal particle, an electron, a stopping muon, and black points, the black line is Monte Carlo sim-
π0. ulated neutrino interactions, while the red line is
the subsetoftheseeventswhichhaveatleastone
π0 in the final state.
cludeamaximumlikelihoodmethodandboosted
decision trees [8]. Figure 4 shows the logarithm
of the likelihood ratio formed from fitting neu- and ν candidates is achieved. Similar PID sep-
e
tralcurrentπ0 candidatesunderasingleelectron arationis performedbetweenelectronsand other
ring hypothesis and a two ring hypothesis where typesofdetectedevents. Anexampleofaboosted
the kinematics are fixed to give the nominal π0 decisiontreewherethemuon/electronseparation
mass. Electron-likeeventsshouldhavelargerpos- wasmeasuredwith cosmicraymuons andassoci-
itive values, while π0’s should have negative val- ated electrons is given in [9]. The PID removes
ues. Theeventsareselectedbyrequiringnodecay ∼ 99.9% of ν CCQE interactions, ∼ 99% ef-
µ
5
fective π0 producing interactions, and preserves Table 1
a high efficiency for ν interactions. An opti- The MiniBooNE analysis is verified by different
e
experimentalcross-checksforeacheventclassrel-
mized PID is expected to allow a small contami-
nation of an oscillation signal with mis-identified evant to νe appearance search.
π0 (∼ 83%), ∆-decays (∼ 7%) , and ν CCQE K+ HARP [10], External Data
µ
events (∼10%). MiniBooNE Data
K0 E910 [11], External Data
MiniBooNE Data
µ MiniBooNE Data
π0 NuMI, MiniBooNE Data
Other (∆, etc) NuMI, MiniBooNE Data
6. BLIND ANALYSIS STRATEGY AND
CROSS-CHECKS
7. REMAINING STEPS IN THE OSCIL-
MiniBooNE is conducting a blind analysis in
LATION SEARCH
order to complete an unbiased oscillation search.
That means that the regionwhere the oscillation Whenthefinalsetofthe analysiscutsisdeter-
νe candidates are expected is closed for the anal- mined and associated systematics evaluated, the
ysis. In the example given in Figure 4, the mass data sample that potentially contains the oscilla-
from the fit is required to be greater than 50 tion candidates will be un-blinded. The compo-
MeV/c2. The likelihood ratio of the single elec- sition ofthe finalsample will be predicted by the
tronfitandthefixedmasstworingfitisrequired MiniBooNEMonteCarlosimulation,withMonte
to favor the π0 hypothesis. The latter two re- CarlosamplefilteredthroughthesamesetofPID
quirementsareforblindness, inorderto keepthe cuts. IfMiniBooNEconfirmstheLSNDresult,an
regionwithpotentialoscillationνecandidatesout excess of events in the data distributions when
oftheanalysisreachuntilthefinalsetofselection compared to Monte Carlo will be observed. The
cutsisformedandthePIDisoptimized. Inprac- final event sample will be evaluated using a χ2
tice, the MiniBooNE analysis cannot use its own function
datatoverifyPIDalgorithmswithanoscillation-
likeν datasample. However,animportantcross- χ2 =X(Oi−Pi)(Cij)−1(Oj −Pj), (1)
e
check of electron event reconstruction and par- i=1
ticle identification comes from NuMI events ob- applied to the data and Monte Carlo energy dis-
served in the MiniBooNE detector. Neutrinos tribution of the oscillation candidates. O is the
i
produced by the decay of mesons moving along number of observedevents in an energy bin i. P
i
the NuMI beamline and in the vicinity of the is the Monte Carlo prediction that takes into ac-
NuMI target may reach the MiniBooNE detec- count oscillation parameters (sin22θ,∆m2). The
tor. NuMI events consist of ν , ν , π±, π0 and covariance matrix C account for the statistical
e µ ij
∆ over the range of energies relevant to the ap- and systematic uncertainties in the energy bins.
pearanceanalysis. Akaonandnon-kaonfractions Systematic errors are associated with neutrino
in NuMI Monte Carlo are extracted from a fit to flux, neutrino cross sections, and the detector
the data. Such Monte Carlo is then compared model.
to our Monte Carlo prediction. The NuMI sam- The flux prediction has the uncertainties cor-
ple therefore allows an independent check of the responding to the production of π, K, and K
L
MiniBooNE PID algorithms performance in iso- particles in the MiniBooNE target. These un-
lating ν . Other important checks are performed certaintiesarequantifiedbyafittoexternaldata
e
witheitherMiniBooNEorexternaldata. Acom- setsfrompreviousexperimentsonmesonproduc-
plete list of cross-checksis given in Table 1. tion. The cross section uncertainties are eval-
6
uated by continuously varying underlying cross Ahmed et al., Phys. Rev. Lett. 89, (2002)
sectionmodelparametersintheMonteCarlocon- 011301.; S.N. Ahmed et al., Phys. Rev. Lett.
strained by MiniBooNE data. Uncertainties on 92, (2004) 181301.
theparametersmodelingtheopticalpropertiesof 2. K. Eguchi et al., Phys. Rev. Lett. 90, (2003)
theoilintheMiniBooNEdetectorareconstrained 021802.;K.Eguchietal.,Phys.Rev.Lett.94,
by a fit to the calibration sample of Michel elec- (2005) 081801.
trons. Theuncertaintiesarecurrentlybeingeval- 3. S.H. Hirata et al., Phys. Lett. B 280 (1992)
uated. 146.; Y. Fukuda et al., Phys. Lett. B 335
(1994)237.;S.Fukudaetal.,Phys.Rev.Lett.
81 (1998) 1562.
8. CONCLUSION
4. M.H. Ahn et al., Phys. Rev. Lett. 90 (2003)
TheMiniBooNEexperimentwillconfirmorre- 041801.
fute the LSND oscillation signal with approxi- 5. C. Athanassopoulos et al., Phys. Rev. Lett.
mately 6× 1020 protons on target collected for 75 (1995) 2650.; C. Athanassopoulos et al.,
the analysis. The oscillation appearance analysis Phys. Rev. D 64 (2001) 112007.
isunderway,withcurrentworkonthe systematic 6. M. Sorel, J.M. Conrad, and M. Shaevitz,
errorevaluationthatcombinestheerrorsfromthe Phys. Rev. D 70 (2004) 073004.
ν flux, ν cross-sections, and the detector model- 7. D. Casper, Nucl. Phys. Proc. Suppl. 112
ing. (2002) 161.
The MiniBooNE Collaboration: A.A. Aguilar- 8. B.P. Roe et al., Nucl. Inst. Meth. A 543,
Arevalo, A.O. Bazarko, S. J. Brice, B.C. Brown, 577 (2005).;H-J. Yang,B.P. Roe and J.Zhu,
L. Bugel, J. Cao, L. Coney, J.M. Conrad, Nucl. Inst. Meth. A 555, 370 (2005).
D.C. Cox, A. Curioni, Z. Djurcic, D.A. Finley, 9. Z. Djurcic, AIP Conf. Proc. 842 (2006) 828,
B.T. Fleming, R. Ford, F. Garcia, G.T. Garvey, arXiv:hep-ex/0601014.
A.Green,C.Green,T.L.Hart,E.Hawker,R.Im- 10. The HARP Experiment at CERN,
lay,R.A.Johnson,P.Kasper,T.Katori,T.Kobi- http://harp.web.cern.ch/harp/.
larcik, I. Kourbanis, S. Koutsoliotas, J.M. Link, 11. The E910 Experiment at BNL,
Y. Liu, Y. Liu, W.C. Louis, K.B.M. Mahn, http://www.phy.bnl.gov∼e910/html/home.html.
W. Marsh, P. Martin, G. McGregor, W. Met-
calf, P.D. Meyers, F. Mills, G.B. Mills J. Mon-
roe, C.D. Moore, R.H. Nelson, P. Nienaber,
S. Ouedraogo, R.B. Patterson, D. Perevalov,
C.C. Polly, E. Prebys, J.L. Raaf, H. Ray,
B.P. Roe, A.D. Russell, V. Sandberg, R. Schi-
rato, D. Schmitz, M.H. Shaevitz, F.C. Shoe-
maker, D. Smith, M. Sorel, P. Spentzouris,
I. Stancu, R.J. Stefanski, M. Sung, H.A. Tanaka,
R. Tayloe, M. Tzanov, M.O. Wascko, R. Van
de Water D.H. White, M.J. Wilking, H.J. Yang,
G.P. Zeller, E.D. Zimmerman.
REFERENCES
1. B.T. Cleveland et al., Astrophysics. J. 496
(1998) 505.; J.N. Abdurashitov et al., Phys.
Rev.C.,60(1999)055801.;W.Hampelet al.,
Phys.Lett.B447(1999)127.;Q.R.Ahmedet
al., Phys.Rev.Lett.87,(2001)071301.;Q.R.