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Performance of the NOMAD-STAR detector
G. Barichello, A. Cervera-Villanueva, D C. Daniels, M. Ellis, D. Ferrere, M.
Gouanere, J J. Gomez-Cadenas, C. Gossling, J A. Hernando, W. Huta, et al.
To cite this version:
G. Barichello, A. Cervera-Villanueva, D C. Daniels, M. Ellis, D. Ferrere, et al.. Performance of the NOMAD-STAR detector. Nuclear Instruments and Methods in Physics Research Section A:
Accelerators, Spectrometers, Detectors and Associated Equipment, Elsevier, 2003, 506, pp.217-237.
�in2p3-00020245�
CERN-EP/2003-017
March 31,2003
Performance of the NOMAD-STAR detector.
G. Barichello a
,A. Cervera{Villanueva b;c
,D.C.Daniels d
,E. doCoutoe Silva b;1
,M. Ellis e; 2
,
D.Ferrere b;3
,J.J. Gomez{Cadenas b;c
,C. Gossling f
,M. Gouanere g
,J.A.Hernando c;4
,W. Huta b
,
J.Kokkonen b;5
,V.E.Kuznetsov b;h;6
,L.Linssen b
,B.Lisowski f;7
,J.Long b
,A.Lupi i;8
,
O.Runolfsson b
,
B. Schmidt f;9
,F.J.P.Soler b;e;10
D. Steele j
,M. Stipcevic k
,G. Vidal-Sitjes b
,M. Veltri i
.
Abstract
TheNOMAD-STARdetectorisasiliconvertexdetectorinstalledintheNOMADspectrometer
attheCERNSPSneutrinobeam. Itconsists offourlayersofapassiveboroncarbidetargetwith
atotalmassof45kgandvelayersof600singlesidedsiliconmicrostripdetectorscoveringatotal
areaof 1.14m 2
. About11,500
charged currentinteractionswerereconstructed in the ducial
volume of NOMAD-STAR from theneutrino run in 1998. Thepotentialuse of silicon detectors
for
(
e )$
oscillationsdepends on theobservation of the candidates by the experimental
signature of a largeimpact parameter, in the caseof theone prongdecay of the , ora double
vertex, in the caseofthe three prongdecay. The main aimof NOMAD-STAR isto measure the
impact parameter and vertex distributions of charged current interactions, which constitute the
mainbackgroundsfortheoscillationsignals,tounderstandthesignicanceofapotentialsignalin
afuture experiment. The present paperdescribesthe experience gained in the operation of this
siliconvertexdetector,andtheperformanceachievedwithit.
PACS:13.15.+g;14.60.Lm;14.60.Pq;29.40.Gx;29.40.Wk
KeywordsNeutrinophysics;Siliconmicrostripdetectors.
a. Universityof Padova,Padova,Italy.
b. CERN,Geneva,Switzerland.
c. IFIC,Universidad de Valencia, Spain.
d. HarvardUniversity,Cambridge, MA,USA.
e. Universityof Sydney,Sydney,Australia.
f. DortmundUniversity,Dortmund, Germany.
g. LAPP,Annecy,France.
h. Joint InstituteforNuclearResearch, Dubna,Russia.
i. Universityof Urbino,Urbino, and INFN,Florence, Italy.
j. Universityof Lausanne, Lausanne,Switzerland.
k. RudjerBoskovic Institute, Zagreb,Croatia.
Submitted to Nuclear Instrumentation and Methods.
1
NowatStanfordLinearAcceleratorCenter,USA.
2
NowatRutherfordAppletonLaboratory,UK.
3
NowattheUniversitedeGeneve,Geneva,Switzerland.
4
NowatUniversidaddeSantiagodeCompostela,Spain.
5
NowatHelsinkiInstituteofPhysics,Finland
6
NowatFermiNationalLaboratory,USA.
7
OnleaveofabsencefromInstituteofElectronTechnology,Warsaw,Poland.
8
NowatUniversityofGlasgow,Glasgow,UK.
9
NowatCERN,Geneva,Switzerland.
10
NowattheUniversityofGlasgowandRutherfordAppletonLaboratory.
There now exists strong evidence for neutrino oscillations from atmospheric neutrinos [1, 2 , 3 , 4],
which canbeinterpretedas
$
oscillations,andfrom solarneutrinos[5 ,6 ,7 ,8 ,9,10 ,11 ], with
the
e
$
oscillationhypothesis asthemostplausible[12 ]. Theseresultsare tobeveriedbylong
baselineaccelerator and reactor experiments[13 , 14 , 15 ,16 , 17 ]. Inaddition,twoexperimentsat the
shortbaselineCERNSPSbeam,CHORUSandNOMAD[18 ,19 ],havebeensearchingfortheexclusive
(
e ) $
mode and have found no evidence for oscillations, setting limitsin the cosmologically
relevanthighm 2
regionofphase-space[20,21 ]. CHORUSdistinguishesthe
from thebackground
of
interactions by identifying the kink from the short-lived decay inside an emulsion target (a
topological search), whileNOMAD identies the decay by its kinematical signature inside a light
driftchambertarget. For arecent and comprehensivereview ofneutrino oscillations,see[22 ].
Proposals have been made for a future
(
e ) $
appearance experiment containing a large
surface silicon tracker with a passive target [23 ] or with an emulsion target [24, 25 ]. The signature
of a potential
interaction needed to separate it from the large background of
charged current
(CC) events relies on the relatively long lifetime of the candidate (c = 86:93 m), which results
inan impact parameter distribution (see Fig 1) that is wider on average than theimpact parameter
distributionof
events. Theimpactparameter(d) isdenedastheprojected signeddistance ofthe
closest approach of the from a
CC interaction (or the decay track from the one-prong decay
of a inthe case of a
CCinteraction) to the vertex producedby the remaining hadronic jet. In
the case of a three prong decay of the (for example, a decay to three pions), the most signicant
signature for the selection of the candidates is the observation of a doublevertex consistent with
the lifetime.
ν
Background
d
CC
ντ
~
µ
µ τ
νµ
µ
d~0
τ CC ν Signal
62 m
µ
ν
µ
µ τ
ν
~ν
Search
Figure 1: Denitionof theimpactparameter.
Thefullyelectronicsignalfromthesilicondetectorisparticularlywellsuitedforaneutrinofactory
environment [26 , 27 , 28 ] because of thehigh event rate. In addition,other short-lived particles,such
ascharm particlesfrom neutrinointeractions,couldbe identied.
To understand the capabilities of a silicon detector in a neutrino experiment, a prototype of an
instrumentedSilicon TARget (NOMAD{STAR) was installed in NOMAD at the CERN SPS at the
beginning of 1997 and it was operational until the end of the NOMAD 1998 run. NOMAD{STAR
was installedupstreamof therstNOMAD driftchamber [19 ,29 ] (see Fig.2).
Beam Neutrino
Calorimeter Hadronic Veto Planes
Muon Chambers
B = 0.4 T Front
Calorimeter Modules Preshower
TRD Dipole Magnet
Trigger Planes Electromagnetic Calorimeter Drift Chambers
z y Boron carbide-silicon target
1 meter
Figure2: The NOMADdetector withtheSiliconTARget(NOMAD{STAR).
NOMAD{STAR consists of four layers of boron carbide (B
4
C) with a total massof 45 kg, inter-
leavedwithvelayersofsingle{sidedsiliconmicrostripdetectors 11
(seeFig.3). Boroncarbideprovides
thebestcompromisebetweenhighdensity(=2:49g/cm 3
)andlongradiationlength(X
0
=21:7cm)
forlow Zmaterials.
The ve layers of silicon detectors have an active surface of 1.14 m 2
. Each layer consists of 10
overlappingladders,with12siliconmicrostripdetectorsperladder,readoutbylow{noiseVA1chips 12
.
Each of the ladders are 72 cm long, and, to our knowledge, are the longest silicon ladders built to
date. There are 5 chipsto a ladderand there aretwo ladders for each repeater board connected to
the mother-board. The detectors are single{sided stripdetectors withthickness 300 m, dimensions
33.5mm59.9mm,andwithstripandreadoutpitchesof25 mand 50m,respectively. Thereare
641possiblereadoutstripsbutonly640ofthemareactuallyreadout. Thatconstitutes6,400channels
perlayer, which is a total of 32,000 readout channels forthe whole detector. The readout stripsare
parallel to the NOMAD magnetic eld (x axis) and provide informationon the y projection of the
track. The detectors are AC coupled, FOXFET biased [31 ] and passivated with silicon oxide. The
performanceof the siliconladders hasbeen described in [32 ], where a beam of pions with momenta
higherthan100 GeV/c wasusedto determine thatthepointresolution ofa ladderof 12 detectors is
about5m. AdetaileddescriptionoftheNOMAD{STARdetectoranditsconstructioncan befound
inreference [33 ].
The present paper describes the experience gained in the operation of NOMAD-STAR and the
measuredperformanceof thedetector. Section 2describesthedatacollected duringtheoperation of
NOMAD-STAR,section3describesthepedestalandnoiseperformanceofthesiliconladders,section4
shows the optimisation of thehit-ndingeÆciency foreach of the NOMAD-STAR ladders, section 5
11
ManufacturedbyHamamatsuPhotonics,Japan.
12
AcommercialversionoftheVikingchip[30 ]
(comprising ten staggered ladders each), along with the veto and trigger scintillators. The neutrino
beamenters fromtheleft. Each siliconladderisrepresentedbythesiliconsensorandthefourcarbon
brerodsthat comprisethesupportingbackbone, allenclosed inthealuminiumcasing.
describes the track reconstruction procedures and the results obtained, while sections 6 and 7 will
show thevertexresolution and nalimpact parameterdistributionachieved withNOMAD-STAR.
2 Data
2.1 Monte Carlo
2.1.1 Geometry
The detectorcomponents ofNOMAD{STAR fallinginthe ducialvolumeare implementedindetail
inthe GEANT3.21 [34 ] descriptionof thedetector. SeeFig. 3fora diagram,with a fulldescription
ofthedetectorinRef.[33 ]. The600 silicondetectorsarepositionedindividuallyinthevolume,asare
the carbon bre pieces and insulating kapton layer (on which the silicon detectors rest) comprising
the laddersupport. Aluminium covers, placedin front and behind each siliconlayer, have also been
implementedand sohavethe scintillatorsforthevetoand trigger system.
The individual elements are all placed in their nominal positions relative to each other, except
for the silicon detectors, whose positions are smeared according to distributions obtained from the
detector alignment procedure [35 , 36 ]. The positionof NOMAD{STARwith respectto theNOMAD
frame-of-referencehasbeendeterminedbyan opticalsurveyaftertheinstallation ofNOMAD{STAR
inNOMAD[33 ].
2.1.2 Hit reconstruction and charge sharing model
Onlythose hitspassingthroughtheactive areas ofthesilicondetectors arerecorded,afterwhichthe
detector response issimulatedin a process called the\digitisation". In thecase of NOMAD{STAR,
isexpressed inADCunits, withone ADCunitcorresponding to approximately250 e .
Thedigitisationalgorithmisbasedondataobtainedfromtest-beamexperimentsofsiliconmodule
prototypes [32 ] and from muonspassing throughNOMAD{STAR duringthe data-taking. The total
charge deposited by a charged particle traversing the detector follows a Landau distribution with a
peak-value of approximately 100 ADCcounts above pedestal. The charge is sharedamongst several
readoutstrips,takingintoaccounttheinterstripand backplanecapacitancesaccordingto\Algorithm
C" of Ref. [32 ]. Instead of a strip retaining all the charge deposited on it, it was found that 17% of
thecharge is passedonto each neighbouringstrip with 5%of thischarge being lostto thebackplane
throughcapacitivecoupling 13
. It isassumedthatforthereadoutstripsthecapacitive couplingtothe
ampliersismuchlargerthanthattothebackplaneandthattheoatingstripsretainnochargeinthe
end. Inaddition, the stripsare smeared with thenoise, which hasa Gaussian distribution,centered
at zero and assumed to have a standard deviation of 6 ADC counts. The common-mode noise (see
sub-section 3.2) isnotsimulated.
In the rst step of the charge sharing model, the total deposited charge is shared between the
nearest readoutand oatingstrips, thecharge split inproportion to thedistance of the hitfrom the
strips. In thenext step,70 %of thecharge onthereadout stripisread out, while15 %goesto each
of the neighbouringoating strips (in addition to the charge retained by the oating strip). Of the
charge on a oatingstrip,5 %of the charge passes to the backplaneand is lost, withthe rest of the
charge distributedevenlybetween thetwo neighbouringreadoutstrips(47.5% each strip). This step
isrepeateduntilthecontributiontothereadoutchargeisnegligible,inpracticeeighttimes. Thetotal
chargewhichis readoutis thesumof thereadoutchargesforthat stripfortheeight steps.
2.1.3 Beam Monte Carlo
The NOMAD neutrino beam simulation package NUBEAM [37]wasused to determine the uxand
averageenergiesof thefourspeciesofneutrinospassingthroughNOMAD{STAR. Theseare
,
,
e and
e
,withanegligiblecontributionfrom
[38,39 ]. The beam simulationis basedon GEANT
3.21 [34 ] with the hadronic component reweighted by the stand-alone implementation of FLUKA
[40 ] corrected byresults from the SPYexperiment [41 ]. The detector was placeddeliberatelybelow
the centre point of the NOMAD driftchambers so that the beam would pass through the centre of
NOMAD{STAR (the neutrino beamformsan angle of 42 mrad with respect to thehorizontalz-axis
of NOMAD). The concentration and average energyof theneutrinospeciesareshown intable 1and
thepredictedspectra inNOMAD{STAR are showninFig. 4.
Neutrino Average Energy (GeV) Concentration (%)
30.60 94.12
19.83 5.02
e
42.18 0.69
e
31.11 0.17
Table 1: NUBEAM predictionsfor theNOMAD{STAR target.
2.1.4 Monte Carlo Statistics
TheNUBEAMprogramdescribedaboveisusedtodeterminethebeamspectraandcompositiontobe
usedasaninputforNEGLIB,theNOMADeventgeneratorbasedonLEPTO6.1[42 ]andJETSET7.4
13
InRef.[32 ]thereisanerrorandandbshouldbereversed.Itshouldread=0.172andb=0.052
e e
[43 ] thatsimulatestheneutrinoeventsintheNOMAD{STARvolume. TheNOMAD-STAR GEANT
simulation is included in the overall GENOM program that is the GEANT descriptionof the whole
NOMADdetector. A totalof69,000
chargedcurrent(CC) eventsweregeneratedandthese willbe
usedforcomparisonsto data throughoutthispaper.
2.2 Data acquired
2.2.1 Running conditions
NOMAD-STAR tookdata continuously duringthe 1997 and 1998 SPSneutrino runs at CERN. The
trigger was formed by the logic condition V
8
V
S
T
S
T
1
, where V
S
is the logical OR of signals
from two scintillators in front of the target in anticoincidence, T
S
is the OR from two scintillators
downstreamof thetarget, V
8
is thesignalfrom thecentralNOMADveto alsoinanticoincidenceand
T
1
is the signal from the rst of the two NOMAD trigger scintillator planes [33 , 44 ]. A total of 13
CAEN VME V550 ash ADC modules, with 10 bit resolution, operating at 1 MHz clockrate, were
usedto read outthe32,000 strips. Eachof the25 ADCchannels (2 channelspermodule)usedserial
multiplexingto read out its corresponding 1280 strips. The sensors had a 60 V reverse-bias voltage
and these were read out by the VA1 chipswith a shaping time of 3 s. The ADC integration time
was1 s, soeach ADCreadoutall thechannelsina timeof 1.3 ms. Thetimingsignalsof theV550
moduleswere controlledbya CAENV551B sequencer. The pedestalswerecalculatedonlinepriorto
adatarunandsubsequentlysubtractedinsidetheADCmoduleswiththezeros suppressed. Thedata
transfer fromthe13 VME V550modulesto acentralCPU tookapproximately10 ms.
There were twoneutrino spillsineach 14.4s SPScycle. Eachspillwasapproximately5 ms wide,
separated by a 2.4 s interval to allow for the 2.0 s spill used for the SPS test-beams (known as the
\at-top"). SincetheNOMAD-STARreadoutwassolong,aspecialbusylogicformedinadedicated
the normal neutrino runningperiod,the trigger rate was measuredto be (0:330:07)=10 13
protons
on target (pot). The livetime is dened as thefraction of timethat a NOMAD-STAR trigger is not
inhibitedbythebusy signalswhen anactualV
8 V
S T
S T
1
triggerres. Thiswasmeasured with
dedicatedscalerstobe(595)%. Duringthe1998run,thereweretwoperiodswhentheSPSranwith
negative particle focusing in the West Area Neutrino Facility (WANF), causing a predominantly
beam. The rateduringthese runningperiodswas(0:170:07)=10 13
potwithalivetimeof (755)%.
A timing problemof the trigger logic in 1997 also reducedthe trigger rate forthe standard positive
focused
beam down to (0:170:07)=10 13
pot during the data taking of that year. An additional
trigger made of the coincidence V
8
V
S
T
S
T
1
was also used to select muons passing through
NOMAD{STAR duringtheSPSat-top.
2.2.2 Event lter
Asmentionedpreviously,a timingproblemof the trigger logic in1997 signicantlyreducedthe rate
ofgoodeventsinthe1997 run. Therelativetimingof thecoincidencesignalsfortriggeringwasoset
so there was only a small eective area of the trigger scintillators that remained active. Out of a
total number of 98,858 NOMAD{STAR events triggered, only 998 events were good events in the
NOMAD{STARducialvolume[45 ]. Inaddition,sincethetriggerwasbiasedtowardsoneedgeofthe
ducial volume it was diÆcult to obtain proper eÆciency measurements. For this reason, this data
sample willnot be usedanyfurther.
The main data sample comes from the 1998 run. A total of 423,249 NOMAD{STAR triggers
were recorded throughout the year. The great majority of these triggers are caused by interactions
in the vicinityof the NOMAD{STAR volume (for example, the coil of the NOMAD magnet and in
twoadditionaltargetsmadeofaluminiumandcarbonsituatedbeneathNOMAD{STAR).Alterwas
developed to pre-select validNOMAD{STAR events. Thesewere subdividedinto three categories:
NOMAD{STAR vertex events, wherea primaryvertexis reconstructed intheNOMAD{STAR
volumefrom siliconhits(11,528 events);
NOMAD{STAR track events, which are events that do not pass the NOMAD{STAR vertex
criteriabuthave at leastone reconstructed NOMAD{STAR track(29,442 events);
DCvertex events, whichareeventsthatdo notpass thetwopreviouscriteriabuthave arecon-
structedvertexintheNOMAD{STAR ducialvolumefromthereconstructionofdriftchamber
(DC)tracks(4,600events). TheseverticesareduetotracksthathavenottriggeredtheNOMAD-
STAR triggerscintillators(eitherescapingontheedgesorduetoscintillatorineÆciencies)orto
misreconstructed verticesfromthe DCinto NOMAD{STAR.
From the Monte Carlo samples, it was shown that 100% of neutrino interactions in NOMAD{
STARthatproduceavalidNOMAD{STAR triggerwereacceptedbythesethreecriteria. Inpractice,
onlythe11,528 events intherst category (NOMAD{STAR vertexevents) areused forany analysis
sincesiliconhitsareneeded to measure theperformanceoftheNOMAD{STAR detector.
3 Pedestals and Noise
3.1 Pedestals
The numbering scheme for the NOMAD{STAR silicon layers adhered to in this paper will be the
following. The layers are numbered 1-5 with layer 1 the furthest upstream and layer 5 next to the
driftchambers. Each siliconlayer contains10 ladders,ladder1 being thebottom-most ladder, and 5
