Search for displaced vertexes arising from decays of new, heavy particles in 7 TeV pp collisions in ATLAS

0.92 MB·English

by  F. M. Brochu

Search for displaced vertexes arising from decays of new, heavy particles
in 7 TeV pp collisions in ATLAS.
FredericM.Brochu,,a
H.E.Pgroup,UniversityofCambridge,J.J.ThomsonAvenue,CAMBRIDGECB30HE,UnitedKingdom.
OnbehalfoftheATLAScollaboration.
Abstract. Wepresenttheresultsofasearchforneutralinosdecayingatasignificantdistancefromtheirproduc-
2 tionpointintochargedhadronsandahighmomentummuon,formingdisplacedvertexes.Theanaly√siswasper-
1 formedwith33pb−1ofppcollisiondatacollectedbytheATLASexperimentattheLHCin2010at s=7TeV.
0 Theposterwillshowsomehighlightsoftheanalysis.
2

n
1 Introduction
a
J

9

]
InsupersymmetricscenarioswithR-parityviolation(RPV)
x
e [1],thelightestsupersymmetricparticle(LSP),oftentaken
- tobetheneutralinoχ˜0,isnolongerstable.Decayproducts
p 1
andLSPlifetimedependontheinvolvedR-paritycoupling
e
h typeandamplitude.
[ Ifthecouplingstrengthissmallenough,theLSPlifetime

becomeslargeenoughtoallowittodecayawayfromthe
1
Interaction Point (IP), leading to displaced vertex topolo-
v
gies.
2
Wepresenttheresultsofasearch[2]forneutralinosdecays
4
7 into a muon and two hadronic jets via the RPV coupling
1 λ(cid:48) in 33 pb−1 of pp collision data collected by the AT- Fig.2.DetailsofATLAS’InnerDetector.
2ij
. LASexperimentattheLHCin2010withprotonbeamsof
1
0 3.5TeVeach. 2 Reconstruction of Displaced Vertexes
2 TheneutralinodecaychainisshowninFigure1.
1
2.1 TheATLASdetector
:
v
i
X TheATLASdetector[3]ismadeofacompositeinnertrack-
ing system, called the Inner Detector (ID), a calorimeter
r
a q system and an extensive muon spectrometer (MS). The
j
~ IDoperatesina2Tmagneticfieldandprovidestracking
χχ~~0 µ λλ‘‘2ij qqii aranpdidvietryterxaningfeor|ηm|a<tion2.f5o,rwchhaerrgeeηdp≡art−icllnestainn(tθh/e2)p,seaunddoθ-
is the polar angle, defined with respect to the cylindrical
symmetryaxis(thezaxis)ofthedetector.
µ FromtheIPoutwards,thefollowingcomponentsoftheIn-
nerDetectorarefound:
Fig.1.LightestNeutralinodecaychainwithanon-zeroRPVcou- – thePixeldetector,madeofhighresolutionSiliconPixel
plingλ(cid:48) . stripsformingthreebarrellayersandthreeforwarddisks
2ij
oneachside.
– theSiliconMicrostripTracker(SCT)madeof4barrel
layersand9forwarddisksoneachside.
Theseneutralinosarepair-producedinthereaction – the Transition Radiation Tracker (TRT) composed of
pp→q˜q˜ →qχ˜0qχ˜0. straw-tube elements interleaved with transition radia-
1 1
tionmaterialforelectronidentification.
ThesesubdetectorsareshowninFigure2.
a e-mail:[email protected]
EPJWebofConferences
V 103
e data 2010
G
1 Dijet MC
s / 0. 102 Wttb,aZr MMCC (cid:242) ATLAS
ce Ldt = 33 pb-1
erti 10
V
1
10-1
0 1 2 3 4 5 6 7 8 9 10
Vertex mass [GeV]
Fig.5.Exampleofaselecteddisplacedvertexfromdetectorma-
terialinteractions(here,inthefirstPixellayer).
Fig. 3. Distribution of the reconstructed displaced vertex mass
aftermuonandeventselectioncuts.
ATLAS simulation
m]180 0.25
ertices dDaijteat 2M0C10 [mrDV114600 0.2
v102 W,Z MC
er of ttbar MC 120 0.15
b 100
m
10
Nu ATLAS 80 0.1
60
(cid:242)
1 Ldt = 33 pb-1 40 0.05
20
10-1 0 0
0 50 100 150 200 250 300
2 4 6 8 10 12 |z | [mm]
DV
Number of tracks in vertex
Fig.6.VertexReconstructionefficiencyasafunctionofthever-
texpositionintheR−zplane,afterapplicationofthedetector
Fig.4.Distributionofthenumberoftracksassociatedtothere-
materialmapveto.
constructeddisplacedvertexaftermuonandeventselectioncuts.
Some of the reconstructed vertexes at this stage are com-
Inthisanalysis,wearelookingforLSPdecaysinsidethe
ingfrominteractionswiththedetectormaterial,asshown
Pixel detector, so we make an extensive use of the Inner
inFigure5.
Detector data, combined with the MS for the reconstruc-
Hadronic interactions are selected and used to build de-
tionofthemuon.
tector material maps[4] . These maps are used to veto re-
constructedvertexes,asonecanseeontheefficiencymap
reproducedinFigure6.
Afterreconstruction,themuoncandidatemustpassa p
T
2.2 EventSelectionandReconstruction.
cuttightenedto45GeVandhavebothMSandIDdata.
Thiscutrepresentthemuonselectionlevel.
EventsarerequiredtopasstheHighLevelmuonTrigger: The reconstruction efficiency as a function of the recon-
at least one reconstructed muon with transverse momen- structeddisplacedvertexradiusR isshowninFigure7
DV
tum pT > 40GeV.Selectedeventsmusthaveatleastone afterapplyingthedifferentcutselectionlevelsmentioned
good primary vertex, with |z| < 200 mm and at least 5 above.Thedominantsystematicserrorswerefoundtocome
trackspointingtowardsit. fromthefollowingsources:
Non-pointing tracks reconstructed by the Inner Detector
trackingalgorithmsareusedforthereconstructionofdis- – muon trigger efficiency, 4.3 %, evaluated with a Z →
placedvertexes.Weuseonlytrackswithtransverseimpact µµsample.
parameter|d |>2mmand p >1GeV. – evolution of the muon reconstruction efficiency as a
0 T
Thesecutsformtheeventselectionlevel. function of d0, from 3.5 % to 8 % depending on the
Atthispoint,thedominantbackgroundsareeventswithW signalinvestigated,evaluatedfromcosmicsmuons.
andZdecayingtomuons.DistributionsofDisplacedVer- – vertex reconstruction efficiency, 3 to 4.3 % evaluated
tex quantities with these backgrounds overlaid are shown fromKs0controlsamples.
inFigures3and4.
Asecondsetofcutsonvertexreconstructionobservables 3 Results and Limits.
areapplied:vertexfitqualityχ2 <5∗n.d.o.f,vertexmass
M > 10GeVandatleast4tracksassociatedtothever- Thevertexcutsdefineasignalregionwhichisrepresented
vtx
tex.Thissetofcutsdefinesthevertexreconstructionlevel. inFigure8.Nodataeventpassesthefinalselection,sowe
Presentedatthe2011HadronColliderPhysicssymposium(HCP-2011),Paris,France,November14-182011.
ciency 00..891 ATLASsimulation EVevretnetx s seelelecctitoionn F. [pb]103 700 GeV ~q, 494 GeV c~0
Effi 00..67 700 GeV q~, 494 GeV c~01 Muon selection n x B.102 71105.500 T GGeVeeVV ~q ,~~qq 4,, 911400 88G GGeVeeVV c~ 01c~c~1010
000...345 sectio 10 PPRROOSSPPIINNOO:: ss((mm ~~qq == 1750100 GGeeVV))
-
0.2 s
s
0.1 o
r
0 C 1
Efficiency 000...7891 ATLAS70s0i mGeuVla q~ti,o 1n08 GeV c~01 EVMevureotnentx ss seeelleelreccDcttiiVtooio nn[nmm] 10-1 (cid:242) LAdtT =L 3A3 pSb-1
0.6
0.5
0.4 10-2
0.3 1 10 102 103
0.2 ct [mm]
0.1
00 20 40 60 80 100 120 140 160 180
r [mm] Fig.9.Limitsontheproductσ(pp→qχ˜0qχ˜0)×B.R(χ˜0→µqq)
0 20 40 60 80 100 120 140 D1V60 180 asafunctionoftheneutralinolifetimecτ1 1fordifferen1tbench-
Fig. 7. Reconstruction efficiency as a function of the displaced χ˜01
vertexradiusfordifferent,cumulativesetofcuts. markpointsandeventgenerators.
· 10-3
V]
Ge 1.6
s [
mas102 1.4
x
erte 1.2
V
1
Signal region
10 0.8
ATLAS
(cid:242) 0.6
Ldt = 33 pb-1
0.4
Data 2010
1
Signal MC 0.2
0
2 3 4 5 678910 20 30 40
Number of tracks in vertex
Fig.8.Remainingeventsindata(blue,proportionalellipses)and
definitionofthesignalregionfromsimulation(greylevelboxes).
set limits on the product of the production cross-section
and the branching ratio of the neutralino to the selected
decay mode. Limits are established as a function of the
lifetimeoftheneutralinoinFigure9.
References
1. B.CAllanachetal.,Phys.Rev.D75,(2007)035002.
2. TheATLASColl.,CERN-PH-EP-2011-131,submitted
toPhysicsLettersB.http://arxiv.org/pdf/1109.2242
3. TheATLASColl.,JINST3,(2008),S08003.
4. The ATLAS Coll., arXiv:1110.6191.
http://arxiv.org/pdf/1110.6191