ATLAS Upgrade for the HL-LHC: meeting the challenges of a five-fold increase in collision rate

2.3 MB·English

by  Peter Vankov

ATLAS Upgrade for the HL-LHC: meeting the challenges of a five-fold
increase in collision rate
PeterVankov1,a
DeutschesElektronenSynchrotron,DESY,Notkestrasse85,22607Hamburg,Germany
OnbehalfoftheATLAScollaboration
2
1
0 Abstract. With the LHC successfully collecting data at 7 TeV, plans are actively advancing for a series of
2
upgrades leading eventually to about five times the LHC design-luminosity some 10-years from now in the

n High-Luminosity LHC (HL-LHC) project. Coping with the high instantaneous and integrated luminosity will
a requiremanychangestotheATLASdetector.Thedesignsaredevelopingrapidlyforanall-newinner-tracker,
J significantchangesinthecalorimeterandmuonsystems,aswellasimprovedtriggers.Thisarticlesummarizes

6 theenvironmentexpectedattheHL-LHCandthestatusofvariousimprovementstotheATLASdetector.
2

] 1 Introduction providingafinalaveragetrigger-rateofafewhundredHz.
t
e Theoveralldimensionsof44minlengthand25mindi-
d
ATLAS[1],attheCERNLargeHadronCollider(LHC)[2], ameter,makeATLASthelargestdetectorincolliderexper-
-
s is a general-purpose experiment designed to explore the iments.
n proton-proton (pp) coll√isions at the LHC with center-of- √ATLAShasbeensuccessfullytakingppcollisiondata
.i massenergiesofupto s = 14TeVatamaximumlumi- at s = 7 TeV (3.5 TeV per beam) since March 2010.
s
c nosityofLpeak =1034cm−2s−1.Thehighcollisionenergy As a result of the excellent performance and operation of
i alongwiththehighluminosityattheLHCwouldeventu- the experiment, by the end of October 2011, ATLAS has
s recorded an integrated luminosity of 5 fb−1 with stable
y allyallowobservationofnewphysicsattheTeVscale.AT-
h LASisconstructedtofullyexploitthephysicspotentialof beams, corresponding to an overall data-taking efficiency
p the LHC, which includes the discovery of the Higgs par- of94%.
[ ticle, as well as searches for effects beyond the Standard In the next years, LHC will undergo a series of up-
grades leading ultimately to five times increase of the in-
1 Model(SM).
stantaneousluminosityintheHigh-LuminosityLHC(HL-
v As illustrated in Fig. 1, ATLAS comprises three ba-
9 sicsubsystems:theInnerDetector,ID(Pixel,SCT,TRT), LHC)project.Thegoalistoextendthedatasetfromabout
6 housed in a solenoid creating a magnetic field of 2 T, the 300fb−1,expectedtobecollectedbytheendoftheLHC
4 run (in ∼ 2020), to 3000 fb−1 by ∼ 2030. The foreseen
Calorimetrysystem(LiquidArgonandTile)andtheMuon
5 higher luminosity at the HL-LHC is a great challenge for
Spectrometer(MS)withitsassociatedsuperconductingto-
1. roidalmagnetssupplyingamagneticfieldof0.5T.Athree- ATLAS. Meeting it will require significant detector opti-
0 mizations, changes and improvements, which are subject
2 oftheseproceedings.
1
:
v
i 2 HL-LHC and the ATLAS Upgrade Plans
X
r The main motivation for the HL-LHC is to extend and to
a
improvetheLHCphysicsprogram[3].Dependingonthe
results from the LHC data, some of the physics problems
that could be addressed at the HL-LHC are: measuring
of the Higgs rare decays and Higgs self-couplings; per-
formingacompletesupersymmetryspectroscopy;search-
ing (extending limits) for new gauge bosons (W(cid:48),Z(cid:48)); se-
archingforaquarkandleptonsubstructure.
The harsher radiation environment and higher detec-
tor occupancies at the HL-LHC imply major changes to
Fig.1.TheATLASexperiment.
most of the ATLAS systems, specially those at low radii
andlargepseudorapidity,η.Ageneralguidelineforthese
leveltriggersystemisusedtoselecttheeventsofinterest, changes is maintaining the same (or better) level of de-
tector performance as at the LHC. The ID, forward ca-
a e-mail:[email protected] lorimeter and forward muon wheels will be affected the
EPJWebofConferences
most by the higher particle fluxes and radiation damage,
requiringreplacementorsignificantupgrade,whereasthe
barrelcalorimetersandmuonchambersareexpectedtobe
capable of handling the conditions and will not be mod-
ified. New, radiation-hard tracking detectors with higher
granularityandhigherbandwidth,aswellasradiation-hard
front-end (FE) electronics are foreseen. The higher event
ratesandeventsizeswillbeachallengeforthetriggerand
dataacquisition(DAQ)systems,whichwillrequireasig-
nificantexpansionoftheircapacity.
TheATLASupgradeisplannedinthreephases,which
correspond to the three long, technical shutdowns of the
LHC towards the HL-LHC. Phase-0 (∼ 24 months) will
Fig.2.Cross-sectionviewofthecurrentPixelB-layer,thenew
take place in 2013 and 2014, the Phase-I (∼ 12 months)
beampipeandtheIBL.
willbeduring2018,andfinally,thePhase-II(∼24months)
isscheduledfor2022-2023.
4 ATLAS Upgrade: Phase-I
3 ATLAS Upgrade: Phase 0
In 2018, the LHC will be stopped for an upgrade of the
injectorsandthecollimators.UpgradeoftheLINAC2and
Themainobjectiveofthe2013-2014shutdownoftheLHC
increaseoftheProtonSynchrotronBoosteroutputenergy
istoperforminterventionswhichwillpermitthemachine
areplanned.Thedata-takingwillberesumedafteroneyear
tooperateatitsdesignparameters:center-of-massenergy
√ shutdownwithluminosityof2×1034cm−2s−1.Duringthe
of s=14TeVandluminosityof1×1034cm−2s−1.
shutdown,ATLASintendstoaccomplishthesecondstage
ATLASwillusethistwoyearsperiodfordetectorcon-
ofitsupgradeprogram,thePhase-I.
solidation works, including a new ID cooling system, a
InPhase-I,installationofnewMuonSmallWheelsand
new neutron shielding of the MS, and a new beam pipe.
introducing of new trigger schemes (Fast TracKer, topo-
The current beam pipe in the forward region is made of
logicaltriggers,improvedL1Calogranularity)areproposed
stainless steel which is a source of high backgrounds for
tohandleluminositieswellbeyondthenominalvalues.
the MS. The new beam pipe will be of aluminum, thus,
reducingthebackgroundsby10-20%.
ThecentralATLASupgradeactivityinPhase-0isthe
4.1 NewMuonSmallWheels
installationofanewbarrellayerinthepresentPixeldetec-
tor,thesocalledIBLproject.
AreplacementofthefirstendcapstationoftheMuonSpec-
trometer, the Muon Small Wheel (MSW), built of Mon-
3.1 InsertableB-layer itored Drift Tubes (MDT) and Cathode Strip Chambers
(CSC), is proposed. The concern is that for luminosities
The Insertable B-layer (IBL) is an additional, 4th pixel L > Lpeak, in addition to the higher number of pile-up
layer which will be inserted between the innermost pixel events per bunch-crossing, large amounts of cavern back-
layer(theB-layer)andthebeampipe,asshowninFig.2, ground will be induced, affecting a large |η| region of the
duringthePhase-0upgrade.Tomaketheinstallationofthe MSW.Thecurrentsysteminthisregionwillstrugglebadly
IBLpossible,anewbeampipeinthecentralregion,with tocopewiththisandthereforeareplacementisrequired.
reducedby4mmradius(r=29mm→r=25mm),builtof ThenewMuonSmallWheelsmustensureefficienttrack-
Beryllium,isenvisaged. ing at high particle rate (up to L = 5 × 1034 cm−2s−1 )
AsdemonstratedinRef[4],itisexpectedthattheIBL and large |η|, with position resolution of < 100 µm. Fur-
willimprovethevertexresolution,secondaryvertexfind- thermore,thenewMSWwillbeintegratedintotheLevel-
ingandb-tagging,henceextendingthereachofthephysics 1 trigger [9]. Several detector technologies are under in-
analysis. It will compensate for defects (irreparable fail- vestigationatthemoment:smalldiameterMDT’s(sMDT)
uresofmodules)intheexistingB-layer,assuringtracking complementedwithfasttriggerchambers-ResistivePlate
robustness.Moreover,IBLwillhelptopreservethetrack- Chambers(RPC)orThinGapChambers(TGC);Finestrip
ing performance at the luminosity beyond Lpeak, e.g. in TGC’s;Micro-MEshGAseousStructures(MicroMEGAs);
Phase-1,whentheB-layerwillsufferfromradiationdam- orsomeothercombinationsofthese.
ageandhighpile-upoccupancies.
ThebaselineconceptoftheIBLconsistsof14staves,
mounteddirectlyonthebeampipewithatiltangleof14◦. 4.2 NewTriggerSchemes
Oneachstavethereare16to32modulesdependingonthe
sensor type. Currently, two silicon sensor types are under At Phase-I, more sophisticated triggers will be required.
consideration: planar and 3D. The IBL modules will be For this, the Fast TracKer (FTK) trigger project has been
equipped with a new readout chip, FE-I4 [5], which has initiated [6]. At the FTK, the track finding and fitting are
been specially developed to function at high data transfer conducted at a hardware level, which makes it extremely
rates (∼ 160 Mb/s). The FE-I4 design allows an increase fast. At the current ATLAS, this task is performed by the
oftheIBLsegmentationbydecreasingthepixelsizefrom triggerLevel-2softwarefarm.FTKwillprovidethetrack
50µm×400µmto50µm×250µm. parametersatthebeginningoftheLevel-2processing.This
2011HadronColliderPhysicssymposium(HCP-2011)
way,theloadonLevel-2willbediminishedandextrare-
sources will be available for more advanced selection al-
gorithms, which ultimately could improve the b-tagging,
leptonidentification,etc.
Suggestionsarealsoinplaceforcombiningtriggerob-
jects at Level-1 (topological triggers) and for implement- Fig.3.ThebaselinelayoutofthenewInnerDetector,traversedby
ing full granularity readout of the calorimeter. The latter simulated23pile-upevents(left)and230pile-upevents(right).
will strongly improve the triggering capabilities for elec-
tronsandphotonsatLevel-1.
5.2 Calorimeterandtriggerupgrades
The HL-LHC conditions will have a major impact on the
5 ATLAS Upgrade: Phase-II Calorimetry system. To ensure an adequate performance,
a replacement of the cold electronics inside the LAr Ha-
The ATLAS Phase-II upgrade is scheduled for 2022 and dronicendcap,aswellas,areplacementofallon-detector
2023. During this time, LHC will be out of operation for readoutelectronicsforallcalorimetersmayneedtobean-
furnishing with new inner triplets and crab cavities. As a ticipated. Also, the operation of the Forward Calorimeter
result, an instantaneous luminosity of 5 × 1034 cm−2s−1 (FCal)couldbecompromised.TomaintaintheFCalfunc-
should be achieved. The goal is to accumulate 3000 fb−1 tioningattheHL-LHC,twopossiblesolutionsareconsid-
ofdataby∼2030. ered[7]:first,completereplacementoftheFCal,andsec-
ond, installation of a small warm calorimeter, Mini-FCal,
ATLASPhase-IIpreparationsincludeanewInnerDe-
infrontoftheFCal.TheMini-Fcalwouldreducetheion-
tectorandfurthertriggerandcalorimeterupgrades.
izationandheatloadsoftheFCaltoacceptablelevels.
The planned trigger upgrades for Phase-II, are con-
nectedwithimplementingaTrackTriggeratLevel-1/Level-
5.1 NewInnerDetector
2, applying full granularity of calorimeter at Level-1 and
improvingthemuontriggercoverage.
RunningatnominalLpeakfortheLHC,willbring,onav-
erage,∼28primaryinteractions(pile-upevents)perbunch
crossing, every 25 ns. The number of pile-up events at 6 Conclusions
5×1034cm−2s−1isthereforeexpectedtobe∼140.(Should
luminosity levelling not be fully effective or some other ATLAScollaborationhasdevisedadetailedprogramtore-
schemeadopted,7×1034cm−2s−1shouldatleastbeaccom- flectthechangesintheLHCconditionstowardstheHigh-
modated.) This will result in 5 to 10 times higher detec- LuminosityLHC,characterizedbyhightrackmultiplicity
tor occupancies, which is beyond the TRT design param- and extreme fluences. At each of the 3 phases of the up-
eters. Furthermore, by 2022, the Pixel and the SCT sub- gradeprogram, actionswillbe undertakento reassurethe
systems, would seriously degrade their performance due stableandefficientperformanceoftheATLASdetector.
to the radiation damage of their sensors and FE electron-
ics.Becauseofallthesefactors,ATLAShasdecidedtore-
placetheentireInnerDetectorwithanew,all-siliconInner
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