ParticleDetectionHowtoseetheinvisible

RobertHarr

WhatIsDetection?• Howcanwe“see”asubatomicparticle?

o Byitsinteractionswithordinarymatter.

• Whatkindofinteractionsdoesasubatomicparticlehavewithordinarymatter?o Ordinarymatteriscomposedofatoms.Interactionscanoccurwiththe:• bulkmaterial,• molecularbonds,• atomicelectrons,and• nuclei.

o Theatomicpictureisveryuseful!13 May 2016 R. Harr 2

AGeneralPurposeDetector

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Detection• Requirescreatingameasurablesignal:

o somethingthatcanbeseenbyeyeo anelectricsignalthatcanberegisteredo achemicalchangetosomethingthatcanbemadevisibleafterachemicalreaction

• 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:γ,W,Z,Ho combinationsofquarks:mesonsandbaryons

o deuterons,alphas,...o newparticles:axions,SUSY,darkmatter,...

• Canwewinnowthislistdown?

WhichParticles?• Whichparticlesaredirectlydetectable?

o Thosethatlivelongenoughtotraversedetectormaterial

• Enumeratethem(it’sasmallnumber):o photonγo electronandpositrone±

o muonandanti-muonµ±

o chargedpions π±

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 K0Sà π+π−

o D+(s) à φ π+ à K+K-π+ (seeplot)

o Z0 à e+e-,µ+µ-,bb• b quarksareseenasjetswithadisplacedvertex• jetsareacollectionofdetectableparticlesinanarrowcone

oW± à e±νe ,µ±νµ• 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

• Iwillnotbeabletocoverthesetoday.o Checkthebackupslidesforsomeinformationonthese.o AgoodintroductioncanbefoundinJackson’sClassicalElectrodynamics.

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ThePhysicsofDetection

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InteractionswithElectrons• Interactionswithatomicelectronsmaybethesinglemostimportantdetectionprocessforparticleexperiments.o Primaryprocessfor“tracking” particleso Alsoimportantforproducinglargenumbersoffreeelectronsforadetectablecurrent.

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ThePhotoelectricEffectFamiliarfromModernPhysicsclass.Einstein’sexplanationrequiresquantizedenergyforphotons.

Energyofliberatedelectronsdependsonthefrequencyoflight(photons).

Rateofelectrons(current)dependsontheintensityofthelight(rateofincidentphotons).

Photomultipliertubesusethephotoelectriceffecttoconvertincomingphotonstoelectrons(singlephotondetection).Silicondetectors(strips,pixels,CCD’s)usethephotoelectriceffecttocreateanelectron-holepair.13 May 2016 R. Harr 15

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ComptonScatteringAsecondtypeofphoton-matterinteraction.Usuallyappearswhendiscussingrelativity.

Importantforintermediateenergyphotons.

Incidentphotonlosesenergywhenejectinganelectronfromanatom.

Processisimportantto“breakdown” ahighenergyphotonintoelectrons.Theenergyofthephotondeterminesthefinalnumberofelectronsproduced.(Seecalorimetry)

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The Bethe-BlochFormula• Semi-classicalcalculationofenergyloss

• Notethecompetitionbetweenln(β2γ2)and–β2o Causesaminimumintheenergylosso MinimumenergylossscalesroughlyasZ/Aofthematerial.

• 4MeVcm2/gforHtoabout1MeVcm2/gforheavyelementso Minimumenergylosswhenβγ ≈3to3.5o Dependsonkinematics,independentofparticletype

• δ termrepresentsdensityeffect13 May 2016 R. Harr 17

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Energy Loss

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DependenceonMaterialEnergylossdependsonthetypeofmaterialandparticle.AtomswithhigherZhavelessenergylossperg/cm2

Buttheirhigherdensity(g/cm3)morethanmakesupforthis.

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ScintillationManymaterialsemitlightwhenatomsormoleculesareexcitedbyachargedparticlepassingnearbyEmittedlightiscalledscintillationScintillatingmediumusuallytransparentLight(onetoa fewphotons)detectedbyaphotomultiplier

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InteractionswithNuclei• Twotypesofinteractionarepossible

o Electromagnetico Strong

• Detailsareinthebackupslides

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(1)Electromagnetic• Mostlysinglephotonexchange

o elasticinteractions

• Whenalightobject(particle)scatterselasticallyfromaheavyobject(nucleus):o littlelossofenergyo canchangedirectionsignificantly

• thinkoftheRutherfordscatteringexperiment• mostscatteringissmallangle• largenumberofsmallanglescatterscanbetreatedstatistically

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PairProductionAthirdtypeofphoton-matterinteraction.Exampleofconversionofenergyintomass.

Importantforhighenergyphotons.

Incidentphotonconvertsintoanelectron-positronpair.

Processisimportantto“breakdown” ahighenergyphotonintoelectrons.Theenergyofthephotondeterminesthefinalnumberofelectronsproduced.(Seecalorimetry.)

Presenceofmatterisrequiredtoconserveenergy-momentum.

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BremstrahlungAGermanwordfor“brakingradiation”.

Importantforhighenergyelectrons.

Incidentelectronemitsaphoton(usuallyx-ray),whenacceleratedintheelectricfieldofanucleus.

Processisimportantto“breakdown” ahighenergyelectronintolowenergyelectrons.Theenergyoftheincidentelectrondeterminesthefinalnumberofelectronsproduced.(Seecalorimetry.)

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SynchrotronRadiationLikeBremstrahlung,butaccelerationduetoa

magneticfield.

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(2)StrongInteractionsImportantformeasuringtheenergyofprotons,neutrons,pions,andkaons(calorimetry).

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ATypicalDetector

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BuildinganExperimentIdeallywewanttoidentifyeverything(particletype,charge,spin,pointoforigin,momentumatpointoforigin)emergingfromacollisioninordertoreconstructexactlywhatoccurred.

Webuildadetectorthatcomesasclosetoachievingthisgoalasmoneyallows.

Interestingcollisionscontainshortlivedparticles(top,bottom,orcharmquarks,WorZbosons,Higgsparticles,oradeconfined quarkstate)thatcannotbedirectlyseen,andmustbeinferredfromtheproductstheyleavebehind.

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CollideParticles

• atveryhighenergiesandconvertenergyintomass(i.e.otherparticles)

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DetectorLayout• Colliderdetectorshaveconvergedonastandardconfiguration.Movingoutwardfromthecollisionwehave:o precisiontrackingo trackingtomeasurecurvatureinasolenoidal magneticfield

o Cerenkovdetector,TOF,transitionradiationdet.(optional)o electromagneticcalorimetero hadronic calorimetero muondetector(alwaysoutermost)

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AGeneralPurposeDetector

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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 cosλ =0.3zBRo whereB isinTesla,R inmeters,z inmultiplesoftheprotoncharge,p inGeV/c.

• Theuncertaintyonp comesfrom2sources:i. resolutionoftrackpositionmeasurementsii. multiplescatteringofmaterialinthedetectoro Thesecondsourceleadsustousethinsilicondetectorsorgaseousdetectors

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Calorimetry• Measuretheenergyinchargedandneutralparticles

• Hadronsinteractdifferentlyfromphotonsandelectrons.EMcalorimeterplacedbeforehadronic calorimeter

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MeasuringEnergy• Converttheenergyofhighenergyparticlesinton lowenergyparticleso n ~E/E0 whereE0 istheaverageenergytoproducealowenergyparticle.

o ThenumberofparticlesisstochasticquantitywithPoissonfluctuationsà 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,visible,orUV.

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MuonDetector• Muons are200timesheavierthanelectrons.• Muons bremstrahl with(200)2 smallerprobability

• Theresultisthattheycanpassthroughmuchmorematerialwithoutinteractingthanotherchargedparticles.

• Muondetectorisbasicallyachargedparticletrackerlocatedbehindlotsofmaterial.

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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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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 interactionswithmaterialcharacterizedbyinteractionlengthλI

o radiationlengths<<interactionlengthso therefore,electromagneticcalorimetersprecedehadroniccalorimetersinadetector

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