Track-Based Alignment of the Inner Detector of ATLAS

5.1 MB·English

by  Ana Ovcharova

Track-Based Alignment of the Inner Detector of ATLAS
AnaOvcharovaaonbehalfoftheATLASCollaboration
LawrenceBerkeleyNationalLaboratory(LBNL),USA
Abstract. ATLASisamultipurposeexperimentattheLHC.ThetrackingsystemofATLAS,embeddedina
2Tsolenoidalfield,iscomposedofdifferenttechnologies:siliconplanarsensors(pixelandmicrostrips)anddrift-
2
tubes.TheprocedureusedtoaligntheATLAStrackerandtheresultsofthealignmentusingdatarecordedduring
1
2010and2011usingLHCproton-protoncollisionrunsat7TeVarepresented.Validationofthealignmentis
0
2 performedbymeasuringthealignmentobservablesaswellasmanyotherphysicsobservables,notablyresonance
invariantmassesinawidemassrange(KS, J/Ψ andZ).TheE/pdistributionsforelectronsfromZ → eeand
n
W → eν are also extensively used. The results indicate that, after the alignment with real data, the attained
a
precisionofthealignmentconstantsisapproximately5µm.Thesystematicerrorsduetothealignmentthatmay
J
affectphysicsresultsareunderstudy.
4
2

1 Introduction imizedisgivenby:
]
t
e (cid:88)
d χ2 = r(τ,a)TV−1r(τ,a),
TheATLASInnerDetector(ID),showninFig.1,iscom-
s- posedofthePixel,theSemiconductorTracker(SCT)and Trks
n theTransitionRadiationTracker(TRT).TheIDisdesigned whereV isthehitcovariancematrixandr(τ,a)isthevec-
.i toachievethemomentum andvertexresolutionsrequired toroftrackresiduals,whichareafunctionofboththetrack
s
c forhigh-precisionmeasurements[1].Toensurethatthere- parameters, τ, and the alignment constants, a. ID align-
i sulting requirements on track reconstruction are met, the ment implements two flavors of χ2-based algorithms: the
s
y position and orientation of each active detector element Global χ2 and the Local χ2. In the Global χ2 approach,
h mustbeknownwithsufficientaccuracysuchthattrackpa- a simultaneous minimization with respect to all track pa-
p rameterresolutionisdegradedbylessthan20%ofthede- rameters and alignment constants is done. This approach
[ sign values. The following is an outline of the procedure, ensures that full correlation between alignable objects in-

resultsandsomeofthechallengesofthealignmentofthe tersected by a common track is retained. In the Local χ2
1
v ID. minimization, module correlations are discarded, render-
1 ing alignment less computationally intensive. However, it
6 isnecessarytoperformmultipleiterationstoreachconver-
9 gence. The alignment uses isolated high-p tracks to re-
2 Alignment strategy T
4 ducetheimpactofpatternrecognitionambiguitiesandof
.
1 multiple scattering. Both collision and cosmic ray tracks
0 The alignment is derived by minimizing track residuals are used to maximize long-distance correlations between
2 which are defined as the difference between the expected detectorelements.IDalignmentisstagedatseverallevels
1 andthemeasuredhitpositions.Theχ2functiontobemin- of granularity, corresponding to the hierarchy of its me-
v: chanicalstructure.Table1showsthesubstructuresandal-
i a e-mail:[email protected] gorithmsusedatdifferentlevelsintheAutumn2010align-
X
ment.ThenumbersintheDoFcolumnrepresenttheprod-
r uctofthenumberofsubstructuresandthealloweddegrees
a
of freedom for each. For example, at Level 2, the Pixel
halfshellswereallowedallthreerotationsandthreetrans-
lations, while the endcaps only two translations and one
rotation [2]. In the latest alignment, discussed in Sec. 4,
the Global χ2 was used at all levels but the wire-by-wire
alignment of the TRT (approximately 700,000 DoF). The
latterusedtheLocalχ2approachduetocomputationalre-
strictions.
3 Alignment performance
TheAutumn2010alignmentwasthefirsttouse7TeVcol-
Fig.1.TheATLASInnerDetector. lision data in addition to pixel module wafer deformation
EPJWebofConferences
Table1.OverviewofthealignmentlevelsfortheAutumn2010 ×103
m
alignmentasdescribedin[2]. µ Autumn 2010 Alignment ATLAS Preliminary
Level Structures #DoF Method ks / 12 100 FSFWWprHiHnMMg //2220..331550== A11l21ig48n µµmmment TsR =T 7b aTrereVl
PIX:whole n trac 80 Track pT > 15 GeV
Level1 SCT:barrel+2endcaps 41 Globalχ2 s o 60
TRT:barrel+2endcaps Hit
40
PIX:halfshells+disks
Level2 SCT:layers+disks 852 Globalχ2
20
TRT:modules+wheels
PIX:modules -1 -0.5 0 0.5 1
Level3 SCT:modules 722104 Localχ2 Residual [mm]
TRT:wires
Fig. 4. Improvement in resolution in the direction transverse to
theTRTwiresaftertheAutumn2010alignment.
×103
m
µ Autumn 2010 Alignment ATLAS Preliminary
Hits on tracks / 2 345000 FSFWWprHHinMMg //2220..331550== A91l 6iµg µmnmment PTsrixa =ec kl7 b pTaTer r>Ve l15 GeV [GeV]M +µµ 999468 ZSS pu→rmin mµgµ e2 rM0 21C011 a1l iaglnigmnemnetnt DaAtaT L∫2A 0L1S d1t,P =rs e0 l=i.m7 70i n Tfaber V1y
20 92
10 90
88
-0.15 -0.1 -0.05 0 0.05 0.1 0.15 1.05 < η < 2.5
Local x residual [mm] 86
3 2 1 0 1 2 3
Fig.2.Improvementinresolutioninthedirectionofhighestgran- Positive muon φ
ularityofthePixelmodulesaftertheAutumn2010alignment.
Fig. 5. An indication of a weak mode misalignment present in
theendcapAregion(black)andtheeffectofusingconstrained
m ×103 alignmenttoremoveit(red).
µ 90 Autumn 2010 Alignment ATLAS Preliminary
n tracks / 4 678000 SFFWWprHHinMMg //2220..331550== A22l58ig µµnmmment TSsrCa =Tc k 7b paTTrer >Ve l15 GeV helicalformoftracksattheexpenseofbiasingthetrackpa-
o rameters.Theyarecommonlyreferredtoas“weakmodes”
Hits 50 andcanbeidentifiedbyexaminingthekinematicsofreso-
40 nancedecayssuchasZ → µµ, J/Ψ → µµand K → ππ.
S
30 The bias introduced in the track momenta by such mis-
20 alignments violates the symmetries inherent in these de-
10 caysandtherebyresultsinunexpecteddependencesofthe
reconstructedinvariantmassonvariouskinematicobserv-
-0.2 -0.1 0 0.1 0.2
ables.AstrikingexampleisthedependenceoftheZinvari-
Local x residual [mm]
antmassontheφtrackparameterofthepositivemuon,see
Fig.3.Improvementinresolutioninthedirectionofhighestgran- Fig. 5. The approach to correct such misalignments is to
ularityoftheSCTmodulesaftertheAutumn2010alignment. constrain some parameters during the alignment, thereby,
minimizingthepossibilityofretainingbiases.Someexam-
ples of useful constraints are: momentum measurements
inputfromtheproductionsurvey.Itwasalsothefirstwire- bytheMuonSpectrometer,vertexpositionconstraintand
by-wireTRTalignment.Theimpactoftheseimprovements thecalorimeterderivedconstraint.Thecalorimeterderived
isevidentinthereducedwidthoftheresidualdistributions constraint,orE/pconstraint,usesthefactthatthecalorime-
in the barrel sections of all sub-detectors, the Pixel, SCT terresponseforpositronsandelectronsshouldbethesame.
and TRT, shown in Figs. 2, 3 and 4, respectively. Simi- Differencesbetweentheratioofenergytomomentummea-
lar trends are observed in the endcap regions, where the surement between electrons and positrons in Z → ee or
large track statistics used in this alignment were particu- W → eν decays can then be attributed to mismeasure-
larlyadvantageous.[2] ment of the momentum in the tracker and used to obtain
corrections to the reconstructed track momenta in bins of
azimuthalangleandpseudorapidity.Duringalignment,the
4 Weak modes and constrained alignment trackmomentaarethenconstrainedtothecorrectedvalue.
TheE/pcorrectionhasresultedinthelatestsignificantim-
There exist systematic detector deformations that cannot provementinthealignmentasevidencedbytheincreasein
bedetectedusingtheoutlinedapproachastheyretainthe theZinvariantmassresolutionshowninFig.6.
HadronColliderPhysicsSymposium,2011,PosterSession
References
V
Ge 3000 Spring 2011 alignment ATLAS Preliminary
ates / 1 2500 ZS u→m mµµe rM 2C011 alignment Data∫ 2 L0 1d1t ,= s0 .=7 07 fTbe 1V 1.CAETRLNALSarCgoelHlaabdorroatnioCno,lTlihdeerA,JTILNASSTE3xSp0er8i0m0e3n(t2a0t08th)e.
did 2000 ID tracks 2. ATLAS Collaboration, Alignment of the ATLAS In-
n
Z ca 1500 11..0055 << ηηµµ+ << 22..55 nperortoDnetceocltloisrioTnrsacaktin√gsS=ys7temTeVw,itAhT2L0A1S0-CLHOCNFp-r2o0to1n1--
1000 012,https://cdsweb.cern.ch/record/1334582(2011).
500
0
60 70 80 90 100 110 120
Mµ+µ [GeV]
Fig.6.ImprovementintheZmassresolutionduetousingcon-
strained alignment to remove weak mode misalignment in the
endcapAregion
LLeevveell 11 aalliiggnnmmeenntt
m]
X translation [µ 105 PSSSTTTRRRiCCCxTTTeTTT l BEEBEEannannrddrddrr eeCCCCllaaaapppp ACAC AApTril L- MAaSy 20p1r1eliminary
Global 0
-5
-10 Cfaoiolulirneg Pocwuter Tesctohpnic. Coooflfing Traomropid.
179711079721579801479931879931980141980151380161480401080481180611480631680661480711082281482371282421482481682511682511982721682741782781783001383021183045
Run number
Fig.7.Changesinthealignmentconstantsonarun-by-runbasis.
Notetheimpactofincidentssuchasmagnetrampingandcooling
interruptions.
5 Run-by-run alignment monitoring
Significantchangesofthedetectoralignmentoccurdueto
externalfactors.Someoftheidentifiedcausesincludetem-
perature changes and magnet ramping. The time-ordered
globalshiftsofselectedsubstructuresinthedirectiontrans-
verse to the beam pipe are shown in Fig. 7. The largest
changesobservedarelessthan10µm.Tomonitorandbet-
ter understand this behavior, the Level 1 alignment con-
stantsarenowrecomputedonarun-by-runbasis.
Additionally, as resonances have been shown to be a
powerful probe in uncovering weak-mode misalignments
and,thereby,momentumbiases,plotsofthereconstructed
mass as a function of various kinematic variables and the
massitself(asinFigs.5and6)arealsoproducedautomat-
ically for every run as a part of the ATLAS data quality
monitoring.
6 Conclusion and outlook
The current implementation of the alignment procedure
hasbeenshowntobeeffectiveandwellsuitedforthechal-
lengesposedbythealignmentoftheATLASID.Thenext
stepistoevaluatethesystematicscausedbyresidualmis-
alignments. It has already been seen that resonances are
a powerful handle for tackling this problem and ongoing
studieswillsoonprovidequantitativemeasuresofanyre-
mainingbiases.