Use of Coulomb scattering for determining neutrino energies with MACRO

4 Pages·0.07 MB·English

by  M. Sioli

ProceedingsofICRC2001:1 c CopernicusGesellschaft2001 ICRC 2001
(cid:13)
Use of Coulomb scattering for determining neutrino energies with
MACRO
2
0 M.SiolifortheMACROCollaboration
0
Universita`diBolognaandINFN-SezionediBologna,Italy
2

n
a
Abstract. An estimate of the energy of neutrino-induced system(Ahlen,1993),whichprovidesthemuoncoordinates
J
muonsinMACROisprovidedbyamultipleCoulombscat- on a projected view. The other complementaryview of the
9
tering measurement. The MACRO original upward-muon trackingsystem(the“strip”view)cannotbeusedforthispur-

1 datasamplehasbeensubdividedaccordingtothereconstructed posesincethespacialresolutionistoopoor.
v muon energy. Data in each subset are then compared with Theprojecteddisplacementofarelativisticmuonwithen-
6 expectedfluxesfromatmosphericneutrinos. Theresultsare ergyE travellingforadistanceycanbewrittenas:
µ
1
interpretedintermsofneutrinooscillations.
0
1 1 13.6 10−3GeV X
σMS y · (1+0.038ln(X/Xo))(1)
0 x ≃ √3 pβc rXo
2
0 1 Introduction wherepisthemuonmomentum(inGeV/c)andX/Xoisthe
/
x amountofcrossedmaterialintermsofradiationlengths. In
MACRO can be used as a neutrino detector by measuring
e MACRO,amuoncrossingthewholeapparatushas
- neutrinoinducedmuonevents. Fromthestudyoftheupgo-
p X/Xo 25/cosθ and y 480/cosθ cm, giving, on the
ing muon deficit and from the distortion of the relative an- ≃ ≃
e vertical, σMS 10 cm/E(GeV). The muon energy estima-
h gular distribution, MACRO provided evidence for neutrino x ≃
tion can be performed up to a saturation point, occurring
: oscillations (Ahlen, 1995). The tagging of the neutrino in-
v whenσMS iscomparablewiththedetectorspaceresolution.
duced events was performed using a time-of-flight (TOF) x
i The MACRO streamer tube system, with a cross section of
X technique. The results of these studiesare presentedat this
(3 3)cm2,providesaspatialresolutionofσ 3cm/√12
r conference in Montaruli, 2001 for the high energy sample × ≃ ≃
a 1cm. Therefore,themuonenergyestimationthroughMCS
andinSpurio,2001forthelowenergysample.
ispossibleupto 10GeV/√cosθ.
SincetheoscillationprobabilitydependsontheratioLν/Eν, A first energy≃estimation has been presented in Bakari,
where L is the distance travelled by neutrinos inside the
ν 2001,wherethefeasibilityofthisapproachwasshown. The
earthandE istheneutrinoenergy,anestimateofthisratio
ν deflection of the muons inside the detector depends on the
isfundamentalforanyoscillationanalysis. Forhighenergy
muonenergyandwasmeasuredusingthedigitalinformation
muonsL isproperlymeasuredbyMACROusingtherecon-
ν ofthelimitedstreamertubesystem. UsingMonteCarlore-
structedzenithangleofthetrackedmuon. AsfarastheE
ν sults to infer a muon energy from its scattering angle, data
isconcerned,partoftheneutrinoenergyiscarriedoutbythe
were divided into three subsamples with different average
hadroniccomponentproducedintherockbelowthedetector
muonenergyandfivesubsampleswithdifferentaveragevalue
whiletheenergycarriedoutbythemuonisdegradedinthe
of L /E . The measured event rate vs. L /E is in good
propagation up to the detector level. Nevertheless, Monte ν ν ν ν
agreementwiththeexpectations,assumingneutrinooscilla-
Carlo simulations show that the measurement of the muon
tionswithparameters∆m2=2.5 10−3eV2andsin22θ=1.
energyatthedetectorlevelstillpreservesinformationabout ×
Since the interesting energy region for atmospheric neu-
theoriginalneutrinoenergy.
trinooscillationstudiesspansfrom 1GeVuptosometens
Since MACRO is not equipped with a magnet, the only ≃
of GeV, it is important to improve the spatial resolution of
waytoinferthemuonenergyisthroughthemultipleCoulomb
thedetectortopushthesaturationpointashighaspossible.
scattering (MCS) of muons in the 25 radiation lengths
≃ WeimprovedtheMACROspatialresolutionexploitingthe
(Xo)ofdetector. Forthispurpose,weusethestreamertube
TDCs of the MACRO QTP system to operate the limited
Correspondenceto: [email protected] streamertubes in driftmode; first results of this study have
2
beenpresentedinScapparone,2001.TheMACROQTPsys-
temisequippedwitha6.6MHzclockwhichcorrespondstoa
0.16
TDCbinsizeof∆T=150ns.AltoughttheMACROstreamer U)
A. MACRO data
toulubteiso,nwahsegnooopdearsatσedi2n50dµrimftm(Boadttei,stcoanni,r2ea0c0h1)a,isnpaMciAalCrReOs- nts (0.14 (QTP-TDCs)
e
≃ v
the main limitation comes from the TDC bin size. The ex- e
pectedultimateresolutionistherefore er of 0.12 Monte Carlo
σ Vdrift ∆T/√12 2mm, where Vdrift 4 cm/µsis the mb (QTP-TDCs)
≃ × ≃ ≃ u
driftvelocity. N 0.1
SincetheQTPelectronicswasdesignedforslowmonopole MACRO data
analysis,inordertofullyunderstandtheperformanceofthe 0.08 (no QTP-TDC)
QTP TDCs in this context and to measure an absolute en-
ergywe did a calibrationat the CERN PS-T9 test beam. A
0.06
slice of the MACRO detectorwas reproducedin detail: ab-
sorbersmadeofrockexcavatedintheGranSassotunnel,like
0.04
thoseofMACRO,wereused. FollowingtheMACROgeom-
etry,thetrackingwasperformedby14limitedstreamertube
chambers,operatedwiththeMACROgasmixture(He(73%)/n- 0.02
pentane(27%)).InordertostudytheQTP-TDCperformance,
we equippedthe detectorwith standardLecroyTDCs, with 0
-5 -4 -3 -2 -1 0 1 2 3 4 5
a bin size of250ps. Inthisway we wereableto checkthe D x (cm)
linearityoftheQTP-TDCresponseandwecomputedinde-
tailthedriftvelocityinsidethechambersasafunctionofthe
Fig.1.DistributionoftheresidualsforMACROdata(blackpoints)
distancefromthewire. Theexperimentalsetupwasexposed
andforsimulateddata(continuoushistogram).Forcomparison,the
to muons with energy ranging from 1 GeV up to 15 GeV.
distributionofthestreamertuberesolutionusedindigitalmodeis
Each QTP-TDCtime was convertedinto driftcirclesinside
alsoshown(dashedhistogram).
thechambers. We performeddedicatedrunsto estimatethe
spaceresolutionbyremovingtherockabsorbersandwede-
veloped a special tracking to fit the muons along the drift 2 Muonenergyestimation
radii. The distribution of the residuals of the fitted tracks
showed a σ 2 mm, demonstrating the successful use of For the muon energy estimation we followed a neural net-
≃
the QTP-TDCs to operate the streamer tube system in drift work (NN) approach. We chose JETNET 3.0, a standard
mode. package with a multilayer perceptron architecture and with
InordertoimplementthistechniqueintheMACROdata, back-propagation updating. The NN has been configured
we used downgoing muons crossing the whole apparatus, with7inputvariablesand1hiddenlayerandwechoosethe
muonswhose average energyis around250 GeV. Since we Manhattanupgradingfunction.Foreachmuontheinputvari-
were going to consider resolution of the order of few mil- ablesconsideredare:
limetersweusedmorethan15milliondowngoingmuonsto -theaverageoftheresiduals;
alignthewirepositionsusinganiterativesoftwareprocedure. -themaximumoftheresiduals;
Afterthealignment,aresolutionofσ 3mmwasobtained. -thesigmaoftheresiduals;
≃
Thisisafactor3.5betterthanthestandardresolutionofthe -thedifferenceoftheresidualsofthethreefarthesthitsalong
streamertubesystemusedindigitalmode(Fig. 1). Inorder thetrack;
to compare such resolution with Monte Carlo expectations, -theslopeandinterceptofthe“progressivefit”.
we properly inserted inside the simulation code GMACRO As far as the “progressive fit” is concerned, it is defined
thedriftvelocitymeasuredwiththetestbeam. Thedistribu- asthefitoftheabsolutevalueoftheresidualsasa function
tionoftheMACRO downgoingmuonresidualsisshownin ofthenumberoflimited streamertubeplanescrossed. The
Fig. 1 (black circles) together with the GMACRO simula- slopeandtheinterceptofthisfitareverysensitivetolowen-
tion(continuousline). Inthesameplotwesuperimposedthe ergymuons.Whileforhighenergymuonstheabsolutevalue
residualsdistributionobtainedwithstreamertubesindigital oftheresidualsasafunctionofthecrossedplanenumberis
mode(dashedline). roughlyconstant(smallslope), fora low energymuonit in-
Thedifferencebetweentheresolutionobtainedattestbeam creases,asthemuonislosingarelevantfractionofitsinitial
(σ 2mm)withrespecttothatobtainedwithMACROdata energy. The NN was trained using a special set of Monte
≃
(σ 3mm),isfullyunderstoodandcomesfromsystematic Carloeventswithknowninputenergy,crossingthedetector
≃
effects such as gas mixture variations during the runs, the atdifferentzenithangles.
presence of δ rays produced in the rock absorbers causing InFig. 2weshowtheaverageoutputoftheNNasafunc-
earlierstopstoQTP-TDCsandtheresidualmultiplescatter- tionoftheresidualmuonenergyjustbeforeenteringthede-
ingsufferedbythelowenergytailofdowngoingmuons. tector (top) and of the neutrino energy (bottom). The out-
3
ut0.35 ear 6 ear14
Outp 0.3 s/y 5 s/y12
N 0.25 nt nt10
N e 4 e
0.2 v v 8
e 3 e
0.15 6
0.1 2 4
0.05 1 2
0 0
0 -1 -0.9 -0.8 -0.7 -0.6 -0.5 -1 -0.9 -0.8 -0.7 -0.6 -0.5
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
low medium low
Log10 Em(GeV) cosq cosq
ut0.35 ar ar14
utp 0.3 ye 7 ye12
N O0.25 ts/ 6 ts/10
N n 5 n
0.2 e e 8
v 4 v
e e
0.15 6
3
0.1 2 4
0.05 1 2
0 0 0
0 0.5 1 1.5 2 2.5 -1 -0.9 -0.8 -0.7 -0.6 -0.5 -1 -0.9 -0.8 -0.7 -0.6 -0.5
Log10 En(GeV) medium high cosq high cosq
Fig.2.AverageNNoutputasafunctionofthemuonenergy(Monte Fig. 3. Zenith angle distributions for upward going muons in
Carlosimulation). fourenergywindows(blacksquares). Rectangularboxesshowthe
MonteCarloexpectationwiththeno-oscillationhypothesis(statis-
ticalerrorsplus17%systematicscaleuncertaintyontheνµflux).
putoftheNNincreaseswiththemuonresidualenergyupto
parton distributions (Gluck, 1995). The propagation of the
E 40GeV,correspondingtoaneutrinoenergyE 200
µ ν
≃ ≃ muonsfromtheinteractionpointuptothedetectorlevelhas
GeV.
beendoneusingtheFLUKA99package(Fasso`,1995),while
themuonsimulationinsidethedetectorwasperformedwith
GMACRO(theGEANT3.21baseddetectorsimulation).For
3 Dataanalysis
each muon crossing the apparatus, the complete history of
the event was recorded (neutrino energy, interaction point,
Forthisanalysis,weusedthewholesampleofupgoingmuon
muonenergyetc.).
events collected with the upperpart of MACRO (Attico) in
fulloperation,foratotallivetimeof5.5years.Weconsidered Should the upgoing muon deficit and the angular distri-
upgoing muons selected by the TOF system and the muon bution distortion (with respect to the Monte Carlo expecta-
tracksreconstructedwiththestandardMACROtracking.We tion) pointed out by MACRO come from neutrino oscilla-
then selected hits belonging to the track and made from a tionswithparameters∆m2 = (10−3eV2)andsin22θ 1,
O ≃
singlefiredtube,toassociateunambiguouslytheQTP-TDC such deficit and such angular distribution distortion would
timeinformation.Spuriousbackgroundhitshavebeenavoided notmanifestatallneutrinoenergies. Theeffectisexpected
byrequiringatimewindowof2µsaroundthetriggertime. tobestrongeratlowneutrinoenergies(E 10GeV)andto
≤
Finally, we selected events with at least four streamer tube disappearathigherenergies(E 100GeV).WeusedtheNN
≥
planes with valid QTP-TDC data. We fitted the drift cir- toseparatefourdifferentneutrinoenergyregionswhoseme-
clesusingthesametrackingdevelopedtoanalyzetestbeam dianenergyisrespectively12GeV(low),20GeV(medium-
muons.Aftertheselectioncutsquotedabove348eventssur- low),50GeV(mediumhigh)and102GeV(high). InFig. 3
vived,givinganoverallefficiencyofabout50%. weshowthezenithangledistributionsoftheupgoingmuon
We used the informationprovidedby the neuralnetwork events in the four energy regions selected compared to the
toseparatetheupgoingmuonsintodifferentenergyregions expectations of Monte Carlo simulation, assuming the no-
and to study therein the oscillation effects. We studied the oscillationhypothesis.Itisevidentfromtheplotsthatatlow
zenithangledistributionsoftheupgoingmuoneventsinfour energyastrongdisagreementbetweendataandMonteCarlo
regionswithdifferentmuonenergy,selectedaccordingtothe (no-oscillationhypothesis)ispresent,whiletheagreementis
NNoutput.Thesameselectionhasbeenappliedtosimulated restoredwithincreasingneutrinoenergy.
events. The correspondingχ2-probabilitiesfor the no-oscillation
TomakeacomparisonbetweenrealdataandMonteCarlo hypothesisinthesefourwindowsarerespectively1.8%(low),
expectations,weperformedafullsimulationchainbyusing 16.8%(medium-low),26.9%(medium-high)and87.7%(high):
theBartolneutrinoflux(Agrawal,1996)andtheGRV94DIS theχ2/DoFvaluesareclearlyrunningwiththeneutrinoen-
4
1.2
nts Constant 501.8 sc)
of eve600 MSiegamna -0.18 304.6E1-9091 C(no o
ber a/M 1
m500 at
u D
N
0.8
400
0.6
300
0.4
200
100 0.2
0 0
-4 -3 -2 -1 0 1 2 3 4 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5
D Log (L/E)/Log (L/E) Log L(Km)/E(GeV)
10 n n 10 n n 10 n n
Fig.4.Resolutiononthereconstructionoftheratiolog10(Lν/Eν) Fig.5.Data/MC(nooscillation)asafunctionoftheratioLν/Eν.
fromtheNNoutput.
weperformedastudyintermsofL /E . Alsointhiscase,
ν ν
ergy,spanningfrom13.7/5to 1.8/5. Theχ2 hasbeencom- the observed transition from 1 to 0.5 in the ratio of data to
putedusingonlytheangularshape. MonteCarlopredictionistheoneexpectedfromtheneutrino
Finally, we tried to get information on the ratio L /E . oscillation hypothesis with oscillation parameters ∆m2 =
ν ν
TheoutputoftheNNwascalibratedonaneventbyeventba- (10−3eV2)andsin22θ=1.
O
sistohavealinearresponseasafunctionoflog10(Lν/Eν).
Theobtainedresolutionisgivenintermsof∆(log10(Lν/Eν))/
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4 Conclusions
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CERN-Geneva,9-11,October,1995.
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Gluck,M.etal,Z.Phys.C67(1995)433.
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usingmultipleCoulombscatteringtoinfermuonenergy.The
Scapparone, E., Advanced Nato Workshop, Oujda, Morocco,
improvement of the spatial resolution obtained by exploit- (2001).
ingtheTDCscontainedintheQTPelectronicsextendedthe Spurio,M.,atthisConference.
muon residual energyreconstruction up to 40 GeV (cor-
≃
respondingto 200GeVfortheneutrinoenergy). Adedi-
≃
catedrunattheCERNPS-T9testbeamallowedustocheck
theMACROQTP-TDCsandshowedthefeasibilityofoper-
ating the limited streamer tubes in driftmode. The angular
distributionoftheupwardgoingmuonsamplehasbeensub-
dividedintofourenergywindows,showingtheenergytrend
expectedfromtheneutrinooscillationhypothesis.Moreover,