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ForwardPhysics–anewwindowonhighenergyinteractions
toberecordedintheinnertrackerascloseaspossibletotheinteractionpointtoallowforthe
reconstructionofτ’sandb-jets.Thisrequirementisalsoofagreatimportanceforadetectionof
(7,eandπ)shouldbewellidentifiedandmeasuredwithagoodenergyresolution.Itaffectsespecially
diphotonanddielectronmassmeasurement–therequiredresolutionisatthelevelof1%atmassof
100GeV.Moreoveragoodmeasurementofthedirectionofparticlesfromelectromagneticcomponent
isaprerequisiteforacorrectreconstructionofthelocalizationofaprimaryinteractionpoint(or
pointsincaseofthepile-up).Finally,ahadroncomponenthastobepreciselymeasured.Good
coverageforbothcomponents,electromagneticandhadronic,allowstostudyjetsandtomeasure
missingtransverseenergyortransversemass.Inadditiontotheserequirementsfortheconstruction
oftheCMSdetector,onemoreshouldbeaddedtoallowtoexploitallthephysicsopportunities
providedbytheLHCenergies.Thedetectorshouldbeashermeticaspossible.Thistranslatesinto
averygoodkinematicalcoverageathighpseudorapidities,thatisintheforwardregion.
Aprecisemeasurementofthemomentaofchargedparticles,especiallymuons,requiresastrong
magneticbendingpower.Itisprovidedbyalargesuperconductingsolenoidwhichisamaincom-
ponentoftheCMSaroundwhichotherdetectorsubsystemsarebuilt.Thesolenoidis12.9mlong
andhasaradiusof5.9m.Itisabletoproduceauniformmagneticfieldof3.8Twhichallows
todeterminesignsofmuonsuptoatransversemomentaof1TeV.Thetrackerdetectorsandthe
calorimetersareplacedinsidethemagnetcoil.Themuondetectorsinterleavedwitha12-sidediron
shieldssurroundthemagnetcoilclosingandguidingthemagneticfield.Alldetectorsaremountto
rigidsupportstructureswhichpreventtheirmovementswhenthemagneticfieldisramped-up.
TheCMSdetectorisdividedintofourparts:barrel,endcaps,hadronicforwardcalorimeter
regionandveryforwarddetectors.Thebarrelcovers|n|<0.9,whiletheendcaps1.5<|n|<3.0.
Inbetween,0.9<|n|<1.5,isatransitionregionwherethesubsystemsfromboth,barreland
endcaps,arepresent.Thehadronicforwardcalorimetercovers3.0<|n|<5.2,whilethehigher
pseudorapiditiesareaddressedbyasetofveryforwarddetectors.
2.2.1.Barrelandendcapsdetectors
Thebarrelandtheendcapsdetectorsformafewlayersaroundthenominalinteractionpoint.These
are:innertrackingdetectors,electromagneticcalorimeter,hadroniccalorimeter,muonchambers.
Betweenthelasttwothesolenoidislocated.
Theinnertrackingsystemiscomposedoftwokindsofdetectors:
•Closesttotheinteractionpointpixeldetectorsareplaced.Thefirstlayerofthepixeldetectors
islocatedinthebarreljust4.4cmfromthebeamaxisandthelastone10.2cm.Thebarrel
layershavelengthof53cmandaresupplementedbyendcappixeldetectorsdisks.Thesizeof
asinglepixelisjust100×150pm2.Highgranularityisanecessityforhighpile-upeventsin
whichafewhundredchargedparticlescanbeproduced.Spatialresolutionofpixeltrackeris
10pmintherφplaneand20pminthezdirection.
•Inthebarrelbetweenr120and55cm,siliconmicrostripdetectorswithahighgranularity
areplaced.TheyformtheTrackerInnerBarrel(TIB)systemmadeof4layers.Furtherfrom
thebeamaxis6layersofsiliconmicrostripswithlargerthicknessarelocated.Theyarecalled
theTrackerOuterBarrel(TOB)system.TheTIBlayersextendto|z|<65cm,whileTOBto
|z|<110cm.TheendcapssystemcomprisesoftheTrackerEndCap(TEC)andtheTracker
InnerDisks(TID).TheTECisformedby9disksateachsideoftheinteractionpoint.Theyare
locatedintheregion120cm<|z|<280cm.TheTIDsystem,3smallerdisks,isclosertothe
interactionpointandplacedinsideTOB.Toensurestableoperationthemicrostripdetectors
arecooleddowntoatemperaturearound−200C.
