CLICdp Overview: Overview of physics potential at CLIC

2 MB·English

by  Aharon Levy

TheJournal’snamewillbesetbythepublisher
DOI:willbesetbythepublisher
(cid:13)c Ownedbytheauthors,publishedbyEDPSciences,2015
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CLICdp Overview
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n
Overview of physics potential at CLIC
a
J
AharonLevy1,ab
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1 OnbehalfoftheCLICdpCollaboration

1TelAvivUniversity,TelAviv,Israel
]
x
e
- Abstract. CLICdp,theCLICdetectorandphysicsstudy,isaninternationalcollabora-
p
tionpresentlycomposedof23institutions. Thecollaborationisaddressingdetectorand
e
physics issues for the future Compact Linear Collider (CLIC), a high-energy electron-
h
positronacceleratorwhichisoneoftheoptionsforthenextcollidertobebuiltatCERN.
[
Precisionphysicsunderchallengingbeamandbackgroundconditionsisthekeytheme

1 for the CLIC detector studies. This leads to a number of cutting-edge R&D activities
v withinCLICdp.ThetalkincludesabriefintroductiontoCLIC,acceleratoranddetectors,
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hardwareR&DaswellasphysicsstudiesatCLIC.
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. 1 Introduction
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The Compact Linear Collider detector and physics (CLICdp) collaboration [1] is a detector and
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physicsR&Dinternationalcollaborationwhichincludes23institutesfromalargegeographicarea.
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: An overview of the activities of CLICdp [2] is presented in this talk. It first describes shortly
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the Compact Linear Collider (CLIC) [3] with an overview of the physics scope which would drive
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X the energy staging of the collider. This is followed by a description of the detector requirements
r andexperimentalconditionsdictatedbythestructureofthebeamandbythephysicsprogram. The
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CLIC detector concept, the subdetectors and the respective R&D work will be discussed. Next, a
descriptionisgivenoftheCLICphysicscapabilities,includingexamplesofbenchmarkstudiesbased
on full detector simulations with overlay of beam-induced background processes. The examples of
physicscapabilitiescoverthefollowingphysicstopics: Higgs,TopandNewPhysics.
2 The CLIC accelerator
Currently there are two TeV-scale linear e+e− colliders under development, CLIC [3] and ILC [4].
Both machines are designed for high luminosities, of the order of 1034 cm−2sec−1. The ILC uses
superconducting RF cavities, while CLIC usesa two-beam accelerator scheme at room temperature
(Fig1). Ahighcurrent(∼100A)low-energydrive-beamservesasanRFsourceforthelow-current
(∼1A)high-energymainbeam.
ae-mail:[email protected]
bThisworkwaspartiallysupportedbytheEUAIDAprojectandbytheGerman-IsraelFoundation(GIF)
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Figure1. SuperconductingRFcavitiesfortheILC(a)andnormalconductingaccelerationstructuresforCLIC
(b).
TheILChasagradientof32MV/mwhileCLIChasagradientof100MV/mandthereforecanbe
ashortermachineforthesamecentre-of-mass(cms)energy. TheILCisdesignedforupto500GeV,
upgradableto1TeV.CLICisdesignedforupto3TeVandwillbestaged. Thestagingwouldbein
threesteps,forexample500GeV,1.4TeVand3TeV.Inoneofthescenarios,basedonaccelerating
unitsof100MeV/m,itwillstartasa11.4kmlongcolliderat500GeV,increaseitsenergyto1.5TeV,
resultingina27.2kmmachine,andfinallyextendtoalengthof48.3kmforacmsenergyof3TeV
(see Fig. 2). More details about the CLIC parameters for the three scenarios are listed in Table 1.
drive beam
detector
main beam
BDS
L=1.87 km
accelerator 100 MV/m
L=2.75 km
L=2.75 km
unused arcs

Figure2.SimplifiedupgradeschemeforaCLICstagingscenario.
CLICisclearlyanambitiousprojectwhichrequireshighpowerconsumptionbut,aswillbediscussed
later,ithasthepotentialofproducingexcitingphysicsresults. Apossibleimplementationofsucha
machinenearCERNisshowninFig3.
ThefirststagewillallowforguaranteedprecisionHiggsandtopphysics,sotheenergyofthefirst
stage will be chosen near the tt¯threshold. The total integrated luminosity of ∼500 fb−1 required at
this energy will require to run for four years. Four more years will be needed to collect ∼1.5 ab−1
of integrated luminosity at the 1.4 TeV stage, for more precision Higgs physics, and, hopefully fol-
lowing LHC discoveries, precision measurements of new physics phenomena Beyond the Standard
Model(BSM)orindirectsearchesofBSMthroughhigh-precisionobservables. The3TeVstagewill
allowforadecentmeasurementoftheelusiveHiggsself-coupling,andforadditionalaccurateBSM
measurementsandsearches.
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Table1.ParametersfortheCLICenergystages.
Parameter Symbol Unit Stage1 Stage2 Stage3
√
Centre-of-massenergy s GeV 500 1500 3000
Repetitionfrequency f Hz 50 50 50
rep
Numberofbunchespertrain n 312 312 312
b
Bunchseparation ∆ ns 0.5 0.5 0.5
t
Acceleratinggradient G MV/m 100 100 100
Totalluminosity L 1034cm−2s−1 1.3 3.7 5.9
√
Luminosityabove99%of s L 1034cm−2s−1 0.7 1.4 2
0.01
Maintunnellength km 11.4 27.2 48.2
Chargeperbunch N 109 3.7 3.7 3.7
Bunchlength σ µm 44 44 44
z
IPbeamsize σ /σ nm 110/2.6 ≈60/1.5 ≈40/1
x y
Normalisedemittance(endoflinac) (cid:15) /(cid:15) nm - 660/20 660/20
x y
Normalisedemittance (cid:15) /(cid:15) nm 660/25 - -
x y
Estimatedpowerconsumption P MW 235 364 589
wall
Legend
Lake Geneva
CERN existing LHC
Potential underground siting :
CLIC 500 Gev
CLIC 1.5 TeV
CLIC 3 TeV
Jura Mountains IP
Geneva
Figure3.CLICfootprintsnearCERN,showingvariousimplementationstages.
Onepossibletime-linefortheCLICmachinewouldbetohavea(hopefully)positivedecisionin
2018-19, followed by a 4-5 year preparation phase and a start of construction in 2024-25 alongside
withdetectorsconstructionandstartofoperationin2032-33.
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3 CLIC experimental environment
3.1 Thebeamstructure
Theexpectedcrosssectionsofthemostinterestingprocessesatthethreestagesareoftheorderoffb,
sohighluminositiesareneeded. Inordertoachievesuchhighluminositiesatalinearcollider, very
smallbeamsandahighbeamrepetitionrateareneeded. Itwillhave50bunchtrainspersecond,each
trainwillbe156nslong,andwillinclude312buncheseachseparatedby0.5ns(seeFig4). Thiswill
giveaverylowdutycyclewhichcanbeusedtopowerpulsethedetectorcomponents(seelater). This
reduces power dissipation and significantly reduces on-detector cooling infrastructure. Beam sizes
Figure4.ThebeamstructureofCLIC.Thereare312bunchesinatrain,thebunchesbeing0.5nsapart,thetrain
lengthis156nsandthetrainsareseparatedby20ms.
willbeverysmall. Withsuchabeam,detailedinTable2,weexpectonaveragelessthanonephysics
eventperbunchtrain. Thismeansthatnotriggerisneededandalleventscanberecorded.
Table2.Someparametersofa3TeVCLICbeam.BXstandsforbunchcrossing.
CLICat3TeV
L(cm−2s−1) 5.9×1034
BXseparation 0.5ns
#BX/train 312
trainduration 156ns
repetitionrate 50Hz
dutycycle 0.00078%
σ /σ 45/1nm
x y
σ 44µm
z
3.2 Beam-inducedbackground
ThesmallbunchsizesrequiredtoachievehighluminosityatCLICgiverisetostrongelectromagnetic
radiation, so-called beamstrahlung, from the electron and positron bunches in the high field of the
oppositebeam.ThisprocessisdepictedinFig.5andgivesrisetothecreationofnumerousincoherent
NewFrontiersinPhysics2014
electron-positron pairs, predominantly produced in very forward direction and with low transverse
momenta. The large majority of particles from incoherent pairs leave the detector in the forward
Figure5.PairproductionthroughtheBeamstrahlungprocess.
region, where some of them give rise to back-scattering off the most forward detector elements. A
smallfractionoftheparticlesfromincoherentpairsreachesthecentraldetectorregiondirectly.
Theaverageenergyloss,δE,ofabeamparticleduetobeamstrahlungis,infirstapproximation,
N2γ
δE ∝ L , (1)
(σ +σ )2σ
x y z
whereNisthenumberofbunchparticles,γ istheparticleLorentzfactorandσ ,σ ,σ arethebeam
L x y z
dimensions in x,y and z coordinates. This equation shows that the energy loss, and therefore the
numberofradiatedphotons,dependsonthemachineparametersandincreaseswithdecreasingbeam
dimensions.
Anothersourceofbackgroundistheγγ→hadronsprocess,throughthediagramshowninFig.6,
whichfillsupthedetectorwithsome19TeVofenergyperbunchtrainat3TeV,soagoodselection
isneededtobeabletoremovethisbackgroundfromtherecordeddata. At3TeV,theexpectedrateof
Figure6.Diagramdescribingthetwo-photontohadronsprocess.
thistypeofbackgroundis3.2γγ →hadronseventsperbunchcrossingwithinvariantmass M >2
γγ
GeV.
3.3 Theluminosityspectrum
Thebeamstrahlungandinitialstateradiationimplythatnotallthee+e− collisionsatCLICwilltake
place at the nominal cms energy, as the beam particles may radiate photons before their collision.
√
Therefore,theeffectivecmsenergyofthecollision, s(cid:48),willbelessthanthenominalcmsenergyof
√ √
the machine, s. Thus the luminosity spectrum will have a peak at s corresponding to collisions
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E
d0.02
/
N
d
0.015
0.01
0.005
0
0 1000 2000 3000
s' [GeV]
√ √
Figure7. Theluminosityspectrumasafunctionoftheeffectivecms, s(cid:48),foranominalCLICcmsenergy s
of3TeV.
withno radiationprior tothe e+e− interactionand along tailtowards lowerenergies. The expected
luminosity spectrum for the 3 TeV CLIC stage is shown in Fig. 7. At this energy, about 1/3 of the
luminosityisinthemostenergetic1%fractionofthespectrum,andincreasestoabove1/2at500GeV
(L inTable1).
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4 CLIC detector concept
4.1 ImpactofCLICconditionsonthedetector
The presence of beam-induced backgrounds impacts on the detector design. The e+e− pairs from
incoherentpairproductiongiverisetohighoccupanciesincertainregionsofthedetector,inparticular
theforwardcalorimetry,thevertexdetectorandtheinnertrackingregions. Inthevertexandtracking
detectors, high occupancies can be mitigated by appropriate shielding for back-scattered particles
from the very forward regions and by the use of very small cell sizes, sometimes even smaller than
strictlyneededfordetectorresolutionpurposes. Backgroundparticlesfromγγ→hadroneventslead
to significant energy depositions in the calorimeter systems, at the level of 20 TeV per bunch train
at3TeVcentre-of-massenergy. Aschemehasbeendevelopedtosuppresstheseenergydepositions
in the data. The scheme relies on precise hit timing, at the level of 1ns in the calorimetry and 10
ns in the tracking, combined with fine-grained calorimetry. This allows to reconstruct all particles,
whetherfrombackgroundorfromthephysicseventunderstudy,usingparticleflowanalysis. Inthis
waythetimeofcreationofeachparticlecanbecalculatedveryaccurately,allowingfortherejection
ofout-of-timeparticlesthatdonotmatchthetimeofthephysicsevent.
TheeffectivenessoftheaboveprocedureisillustratedinFig.8,showinganeventfromthereaction
e+e− → H+H− → tb¯bt¯ → 8jets at 3 TeV before and after beam-induced background suppression
accordingtothisprocedure.
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Figure8.An8-jeteventoverlayedwithbackground(a)andafterfiltering(b).
4.2 Detectorneedsfromphysicsaims
4.2.1 Momentumresolution
OneofthephysicssubjectstobediscussedinSection5isHigsstrahlung,e+e− → HZ. Thisprocess,
in which the Higgs is reconstructed as the particle recoiling off the Z in the final state, is unique in
allowing a model-independent determination of Higgs mass and coupling without any assumptions
abouttheinvisibledecayoftheHiggs. TheZ wouldbereconstructedfromitsmuondecayandthus
one needs a very good momentum resolution. Another physics process needing high momentum
resolutionwouldbethesearchforasmuonthroughthemuonmomentumendpoint. Thesetwoand
othersimilarstudiesrequireamomentumresolutionof
σ /p2 ∼2×10−5GeV−1. (2)
pT T
4.2.2 Jet-energyresolution
In order to reconstruct W and Z from di-jets and be able to separate them, it is important to have
excellentjet-energyresolution. Forinstance,fora2.5%resolutiononthemass,a3.5%resolutionon
thejetenergyisneeded. Thereforetherequirementforhighenergyjetswouldbe
σ /E ≤3.5% (3)
E
forjetswithE >100GeV(≤5%at50GeV).
4.2.3 Impactparameterresolution
In order to measure the Higgs branching ratios, it is important to be able to tag jets from c and b
quarks. This calls for a good ability to reconstruct secondary vertices. This leads to the following
requirementontheimpactparameterresolution:
σrφ =5⊕15/(p[GeV]sin32θ)µm. (4)
4.2.4 Angularcoverage
Formanyofthephysicsstudies,theabilitytotagforward-goingelectronsdowntoverysmallpolar
angleisimportant. Thisisdiscussedintheforwardcalorimetrysection.
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4.3 ApossibleCLICdetector
The CLIC detector concept is derived from the ILC detector concepts, ILD [5] and SiD [6], and is
adapted to the conditions at CLIC. A schematic view of the CLIC detector is shown in Fig. 9. It
is composed of barrel and endcap sections. Going outwards from the interaction point, the detector
comprises: a low-mass vertex detector, a silicon-based main tracker, a fine-grained electromagnetic
calorimeter(ECAL),afine-grainedhadroncalorimeter(HCAL),alarge4to5Teslasuperconducting
solenoid and an instrumented return yoke with muon identification layers. In the forward direction,
near the incoming and outgoing beams, compact fine-grained forward calorimeters, LumiCal and
BeamCal,arelocated. Thefinalfocusingquadrupolesfortheincomingbeamsarealsoplacedinside
thedetectorvolume. Theoveralllengthofthedetectorisapproximately13mforaheightof14m. In
complex forward
return yoke with
region with final
Instrumentation
beam focusing
for muon ID
e-
strong solenoid
4 T – 4.5 T
fine grained (PFA) e+
calorimetry, 1 + 7.5 Λ
i 6.5 m
main silicon-based ultra low-mass
tracker (large pixels vertex detector
and strips) with ~25 µm pixels
Figure9. ApossibleCLICdetector,derivedfromthetwoILCdetectorconcepts. Thetextintheframedboxes
givessomeexplanationonthedifferentcomponentsofthedetector.
thefollowing,afewpointsonsomeimportantitemsoftheCLICdetector.
4.3.1 Vertexdetector
Thevertexdetectorisanessentialpartofthetrackingsystem. Moreover,thevertexdetectorprovides
essential information on flavour tagging through the measurement of displaced vertices. The vertex
detectorcomprisesthreedouble-layersofpixeldetectorsinthebarrelregionaswellasintheforward
discregion.Asingle-pointresolutionforeachindividuallayerof3µmisrequired,andtime-stamping
forindividualhitsatthelevelof10ns.Occupanciesfrombeam-inducedbackgroundswillbehighand
willlimitthemaximumallowedpixelsurface. Overallavertexdetectorof2Gpixelsforanindividual
pixel surface of 25×25 µm2 is foreseen. The maximum amount of material amounts to 0.2%X per
0
layer. Theradiationlevelswillbesignificantlyless(factor10−4)thanatLHC.
Tomeettheserequirements,verythinhybriddetectorscomposedof 50µmthinresistivesilicon
sensorsand50µmASICsareunderdevelopment. Tofurthermeetthelow-massrequirement,power
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pulsing at the 50 Hz beam train rate is foreseen. This will keep the heat dissipation at the level of
50 mW/cm2 and will allow for air cooling. A vertex detector having these qualities could look like
the one shown in Fig. 10. The CLIC vertex detector is the subject of a very active detector R&D
Figure10. Apossiblevertexdetectorlayout.ThetwoinnersiliconbarrelstripslayersareSIT1andSIT2,and
thevertexbarrelisVXB.
project[7].
4.3.2 Tracker
ThemaintrackeratCLICwillbebasedonsilicontechnology. Itwillcompriselargepixelelements
aswellassiliconmicrostrips,adaptedtolocaloccupancylevels. Singlepointresolutionsnear7µm
arerequiredaswellastime-stampingforindividualhitsatthelevelof10ns. Thedesignofasilicon
trackerconceptandtheassessmentofthecorrespondinghardwareimplementationoptionsforCLIC
arestillunderstudy.
4.3.3 CalorimetryandPFA
Thejetenergyresolutionandthebeam-inducedbackgroundrejectiondrivetheoveralldetectordesign.
As already mentioned above, the good jet energy resolution and background rejection are achieved
throughfine-grainedcalorimetryandbyaparticle-flowanalysis(PFA)[8]. Figure11isanillustration
ofhowonerecognisesindividualparticlesinjetsusingthePFAalgorithm. Typically60%ofajetis
composedofchargedparticles,30%ofphotonsand10%oflong-livedneutralhadrons.Chargedparti-
clesprofitfromtheexcellentmomentumresolutioninthetracker,whilephotonsarewellmeasuredin
theECAL.NeutronsrelyonmeasurementsintheHCAL,whichislessgoodduetohadronicshower
fluctuations. Byreconstructingindividualparticlesinjets,thebestinformationcanbeusedforeach
ofthemandagoodoveralljetenergyresolutioncanbeobtained. Thecombinationofhardwareand
softwareisprovidingthewantedresult.
4.4 CLICforwardcalorimetry
Twosmallcompactcalorimetersareplannedintheforwardregion(Fig.12,thetworedcircles),with
theaimtomeasuretheluminosity,toprovideelectronvetocapabilitiesatsmallanglesaswellasafast
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Figure11. Simulationofa250GeVjetproducedintheILDdetector,consistingofdifferenttypesofparticles
reconstructedusingparticle-flowanalysis.Theinsertatthetopleftshowstheseparationofthetypesofparticles.
Figure12. (a)PartofaCLICdetectorshowingthetwoforwardcalorimeters,LumiCalandBeamCalinsidethe
redcircles.(b)LumiCal,acompactluminositycalorimeter,composedofTungstenlayersandsiliconsensors.
beammonitoringsystemandtocomplementthehermeticityofthefulldetector.Theluminositydetec-
tor,LumiCal,coveringtheangularregionof38-110mrad,willperformprecisionmeasurements[9]
oftheBhabhaeventrateandtherebymeasuretheluminosity. TheveryforwardcoverageofBeam-
Cal, 10-40mrad, enhancesthehermiticityofthedetector. TheBeamCalissubjecttohighdosesof
radiation, eventhoughbyfourordersofmagnitudelessthanthoseintheforwardregionofLHC.It
requirestheBeamCaltouseradiation-hardsensors;GaAssensorsarepresentlyproposed.