ParticleDetectionHowtoseetheinvisible
RobertHarr
Outline• Definitionofparticledetection
o Theparticlesofconcerno Theparticleinteractions
• ThePhysicsofdetectiono Breakdownthemostcommoninteractions
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WhatIsDetection?• Howcanwe“see”asubatomicparticle?
o Byitsinteractionswithordinarymatter.
• Whatkindofinteractionsdoesasubatomicparticlehavewithordinarymatter?o Ordinarymatteriscomposedofatoms.Interactionscanoccurwiththe:• bulkmaterial,• molecularbonds,• atomicelectrons,and• nuclei.
o Theatomicpictureisveryuseful!14 April 2017 R. Harr 3
AGeneralPurposeDetector
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Detection• Requirescreationofameasurablesignal:
o somethingthatcanbeseenbyeyeo achemicalchangetosomethingthatcanbemadevisibleafterachemicalreaction
o anelectricsignalthatcanberegistered
• Wenormallyconsiderthedetectormaterialtobeatresto incidentparticleonfixedtargetgeometry
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WheretoBegin?• Somanypossibledetectormaterials:o gas,liquid,solido atomicormolecularo HtoU
• Andsomanypossiblesignals:o heat,light,electric
• Canwesimplifythis?
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• Therearehundredsofparticles:o leptons,quarkso bosons:g,W,Z,Ho combinationsofquarks:mesonsandbaryons
o deuterons,alphas,...o newparticles:axions,SUSY,darkmatter,...
• Canwewinnowthislistdown?
WhichParticles?• Whichparticlesaredirectlydetectable?
o Thosethatlivelongenoughtotraversedetectormaterial• Enumeratethem(it’sasmallnumber):
o photongo electronandpositrone±
o muonandanti-muonµ±
o chargedpions p±o chargedkaons K±
o protonandanti-protonp±
o neutronsn andanti-neutronsn,andneutralkaon K0
o afewofthestrangebaryons,inspecialcaseso somespeculativeparticleslikedarkmatterormag.monopoles
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hadrons
WhatAboutOtherParticles?• Detectionofallotherparticlesisviatheirdecayproducts,forexample:o K0S à p+p-
o D+(s) à f p+ à K+K-p+ (seeplot)
o Z0 à e+e-,µ+µ-,bb• b quarksareseenasjetswithadisplacedvertex• jetsareacollectionofdetectableparticlesinanarrowcone
oW± à e±ne ,µ±nµ• neutrinosaren’tseensomuchasinferredfromanimbalanceofmomentumperpendiculartothecollisionaxisreferredtoasmissingtransverseenergyorMET.
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WhatCanBeMeasured?• Firstistodetectthataparticleispresent.
o Forlowenergyphotons,darkmatter,and[lowenergy]neutrinos,thatisaboutallthatcanbemeasured
• Forotherparticleswewouldliketoknow:o chargeo typeofparticle(mass)o energyandmomentumo spin(?!)
• Thesearedeterminedfromthewayparticlesinteractwithamacroscopicdetector(matter).
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TheInteractions• Thebasicinteractionscanbefurthersubdividedintoasurprisinglyshortlist.
• Iwillnotdiscussinteractionsthateffectmolecularbonds.o Thesearenotusedinelectronicdetectorssincechemicalprocessingisneeded(uptonow).
o Thesecomeintoplaywithphotographicemulsionsandsometypesofplasticdetectors.
o Otherfamousexample:RayDavis’solarneutrinoexpt.o Consultreferencesifyouneedmoreinformation.
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Interactions(cont’d)1. Interactionswith(orrequiring)atomicnuclei-
- maybefamiliarfrommodernphysicscourses:
a) Pairproductionb) Bremstrahlungc) Nuclear(hadronic)interaction
• Thefirstappliestophotons,thesecondtoelectrons(mostly),andthelasttohadrons.
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Interactions(cont’d)2. Interactionswithatomicelectronsarewidely
used:a) ComptonScatteringb) Photoelectriceffectc) Ionizationofatomsd) Excitationofatoms
• Thefirsttwoapplyonlytophotons,thelasttwoapplytoallparticles.
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Interactions(cont’d)3. Interactionswithbulkmaterialmaybethe
leastfamiliar:a) Cerenkovradiationb) Transitionradiation
• Iwillnotbeabletocoverallofthesetoday.o Iwilltalkaboutthosemostrelevantto(some)REUprojects.
o Checkthebackupslidesforsomeinformationonthese.o AgoodintroductioncanbefoundinJackson’sClassicalElectrodynamics.
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ThePhysicsofDetection
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1.InteractionswithElectrons
Interactionswithatomicelectronsisthemostimportantdetectionprocess.Primaryprocessfor“tracking” particlesAlsoimportantforproducinglargenumbersoffreeelectronsforadetectablecurrent.
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1a.ThePhotoelectricEffectFamiliarfromModernPhysicsclass.Einstein’sexplanationrequiresquantizedenergyforphotons.
Energyofliberatedelectronsdependsonthefrequencyoflight(photons).
Rateofelectrons(current)dependsontheintensityofthelight(rateofincidentphotons).
Photomultipliertubesusethephotoelectriceffecttoconvertincomingphotonstoelectrons(singlephotondetection).Silicondetectors(strips,pixels,CCD’s)usethephotoelectriceffecttocreateanelectron-holepair.14 April 2017 R. Harr 16
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1b.ComptonScatteringAsecondtypeofphoton-matterinteraction.Usuallyappearswhendiscussingrelativity.
Importantforintermediateenergyphotons.
Incidentphotonlosesenergywhenejectinganelectronfromanatom.
Processisimportantto“breakdown” ahighenergyphotonintoelectrons.Theenergyofthephotondeterminesthefinalnumberofelectronsproduced.(Seecalorimetry)
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1c.The Bethe-BlochFormula• Semi-classicalcalculationofenergyloss
• Notethecompetitionbetweenln(b2g2)and–b2o CausesaminimumintheenergylossoMinimumenergylossscalesroughlyasZ/Aofthematerial.
• 4MeVcm2/gforHtoabout1MeVcm2/gforheavyelementsoMinimumenergylosswhenbg ≈3to3.5o Dependsonkinematics,independentofparticletype
• d termrepresentsdensityeffect14 April 2017 R. Harr 18
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Energy Loss
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DependenceonMaterialEnergylossdependsonthetypeofmaterialandparticle.AtomswithhigherZhavelessenergylossperg/cm2
Buttheirhigherdensity(g/cm3)morethanmakesupforthis.
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1d.ScintillationManymaterialsemitlightwhenatomsormoleculesareexcitedbyachargedparticlepassingnearbyEmittedlightiscalledscintillationScintillatingmediumusuallytransparentLight(onetoafewphotons)detectedbyaphotomultiplier
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2.InteractionswithNuclei
Twotypesofinteractionarepossible:Electromagnetic
Strong
Detailsareinthebackupslides
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Electromagnetic• Mostlysinglephotonexchange
o elasticinteractions
• Whenalightobject(particle)scatterselasticallyfromaheavyobject(nucleus):o littlelossofenergyo canchangedirectionsignificantly
• thinkoftheRutherfordscatteringexperiment• mostscatteringissmallangle• largenumberofsmallanglescatterscanbetreatedstatistically
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2a.PairProductionAthirdtypeofphoton-matterinteraction.Exampleofconversionofenergyintomass.
Importantforhighenergyphotons.
Incidentphotonconvertsintoanelectron-positronpair.
Processisimportantto“breakdown” ahighenergyphotonintoelectrons.Theenergyofthephotondeterminesthefinalnumberofelectronsproduced.(Seecalorimetry.)
Presenceofmatterisrequiredtoconserveenergy-momentum.
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2b.BremstrahlungAGermanwordfor“brakingradiation”.
Importantforhighenergyelectrons.
Incidentelectronemitsaphoton(usuallyx-ray),whenacceleratedintheelectricfieldofanucleus.
Processisimportantto“breakdown” ahighenergyelectronintolowenergyelectrons.Theenergyoftheincidentelectrondeterminesthefinalnumberofelectronsproduced.(Seecalorimetry.)
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2c. StrongInteractionsImportantformeasuringtheenergyofprotons,neutrons,pions,andkaons(calorimetry).
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ATypicalDetector
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CollideParticles
• atveryhighenergiesandconvertenergyintomass(i.e.otherparticles)
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Build anExperimentIdeallywewanttoidentifyeverything(particletype,charge,spin,pointoforigin,momentumatpointoforigin)emergingfromacollisioninordertoreconstructexactlywhatoccurred.
Webuildadetectorthatcomesasclosetoachievingthisgoalasmoneyallows.
Interestingcollisionscontainshortlivedparticles(top,bottom,orcharmquarks,WorZbosons,Higgsparticles,oradeconfined quarkstate)thatcannotbedirectlyseen,andmustbeinferredfromtheproductstheyleavebehind.
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DetectorLayout• Colliderdetectorshaveconvergedonastandardconfiguration.Movingoutwardfromthecollisionwehave:1. precisiontracking2. trackingtomeasurecurvatureinasolenoidal magnet3. Cerenkovdetector,TOF,transitionradiationdet.
(optional)4. electromagneticcalorimeter5. hadronic calorimeter6. muondetector(alwaysoutermost)
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AGeneralPurposeDetector
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1,2.Tracking• measuretheorigin(vertex)ofchargedtracks• measurethemomentumofchargedtracks• requires:
o amagneticfieldofsufficientstrengthandextent(tomeasureradiusofcurvature)
o largevolumetracking(drivenbycost)o highprecisionmeasurementsnearthecollisionpt.o possiblysomeparticleID(dE/dx,TOF,Cerenkov,TRD)
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MeasuringMomentum• Inauniformmagneticfield
o p cosl =0.3zBRo whereB isinTesla,R inmeters,z inmultiplesoftheprotoncharge,p inGeV/c.
• Theuncertaintyonp comesfrom2sources:i. resolutionoftrackpositionmeasurementsii. multiplescatteringofmaterialinthedetectoro Thesecondsourceleadsustousethinsilicondetectorsorgaseousdetectors
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4,5.Calorimetry• Measuretheenergyinchargedandneutralparticles
• Hadronsinteractdifferentlyfromphotonsandelectrons.EMcalorimeterplacedbeforehadronic calorimeter
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MeasuringEnergy• Converttheenergyofhighenergyparticlesinton lowenergyparticleso n ~E/E0 whereE0 istheaverageenergytoproducealowenergyparticle.
o ThenumberofparticlesisastochasticquantitywithPoissonfluctuationsà wewillmeasuren ± √n particles
o ThemeasuredenergyisE ± √E,sotherelativemeasurementuncertaintyis1/√E
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PhotonDetection• Detectingphotonsisimportantinmanysituations:astronomy,calorimetry,TOF,...
• Commondetectorsareo photomultipliers
• fastsignals,highsensitivity,lowgranularityo CCDs
• highgranularity,timeintegrating
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CCD• Thesensorsfordigitalcameras• Semiconductordevices,usuallysilicon• CanbecustomizedforsensitivityinIR,vis.,UV.• Importantdevelopmentfortelescopes.
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6.MuonDetector• Muons are200timesheavierthanelectrons.• Theprobabilityforbremstrahlung is(200)2 timessmallerformuonsthanelectrons(forsamep)
• Theresultisthattheycanpassthroughmuchmorematerialwithoutinteractingthanotherchargedparticles.
• Muondetectorisbasicallyachargedparticletrackerlocatedbehindlotsofmaterial.
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Summary• Thephysicsofparticlesinteractionswithmatterisattheheartofparticledetection.
• Advancesindetectortechnologyenableimprovementsinexperimentsovertime.
• Modernexperiments(STAR,ALICE,Belle,CMS)exploittheinteractionsofparticleswithmattertomeasuretheparticlesemergingfromcollisionsinthemostusefulwaypossible.
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Backup
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(1)Electromagnetic(cont’d)• MultipleCoulombscattering(MCS)
o yieldsaroughlyGaussiandistributionofscatteringangles
o X0 iscalledtheradiationlengthofthematerial• characterizeselectromagneticinteractionswithnucleiinamaterial
o Scatteringindependentofparticletype(excepte-)
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TransitionRadiationRadiationproducedwhenahighlyrelativisticelectronpassesthroughmaterialswithdifferentindicesofrefraction.
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CerenkovRadiationEmittedwhenaparticleexceedsthespeedoflightinamediumLiketheshockwavefromasupersonicplaneConeangleisrelatedtoparticlevelocity
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SynchrotronRadiationLikeBremstrahlung,butaccelerationduetoa
magneticfield.
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ElectromagneticShowers• Tomeasuretheenergyofanincomingparticle
o Convertasinglehigh-energyparticleintomanylow-energyparticles,andcountthenumberofparticles.
o Theconversioniscalledashower.o pairproduction:1photonà 2electronso Comptonscattering:1photonà 1photon+1electrono bremstrahlung:1electronà 1electron+1photon
• AftertraversingdistanceXofamedium,theaverageenergyofanelectron/photonis:
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hEi = E0e�X/X0
Hadronic Showers• Sameideaasforelectromagneticshowers
o incomingparticleisahadrono interactionswithmaterialcharacterizedbyinteractionlengthlI
o radiationlengths<<interactionlengthso therefore,electromagneticcalorimetersprecedehadroniccalorimetersinadetector
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