Particle Detection - RHIGrhig.physics.wayne.edu/REU/new_talks/Detectors2017.pdf · 2a. Pair...

ParticleDetectionHowtoseetheinvisible

RobertHarr

Outline• Definitionofparticledetection

o Theparticlesofconcerno Theparticleinteractions

• ThePhysicsofdetectiono Breakdownthemostcommoninteractions

14 April 2017 R. Harr 2

WhatIsDetection?• Howcanwe“see”asubatomicparticle?

o Byitsinteractionswithordinarymatter.

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

o Theatomicpictureisveryuseful!14 April 2017 R. Harr 3

AGeneralPurposeDetector


Detection• Requirescreationofameasurablesignal:

o somethingthatcanbeseenbyeyeo achemicalchangetosomethingthatcanbemadevisibleafterachemicalreaction

o anelectricsignalthatcanberegistered

• Wenormallyconsiderthedetectormaterialtobeatresto incidentparticleonfixedtargetgeometry


WheretoBegin?• Somanypossibledetectormaterials:o gas,liquid,solido atomicormolecularo HtoU

• Andsomanypossiblesignals:o heat,light,electric

• Canwesimplifythis?


• 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


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.


WhatCanBeMeasured?• Firstistodetectthataparticleispresent.

o Forlowenergyphotons,darkmatter,and[lowenergy]neutrinos,thatisaboutallthatcanbemeasured

• Forotherparticleswewouldliketoknow:o chargeo typeofparticle(mass)o energyandmomentumo spin(?!)

• Thesearedeterminedfromthewayparticlesinteractwithamacroscopicdetector(matter).


TheInteractions• Thebasicinteractionscanbefurthersubdividedintoasurprisinglyshortlist.

• Iwillnotdiscussinteractionsthateffectmolecularbonds.o Thesearenotusedinelectronicdetectorssincechemicalprocessingisneeded(uptonow).

o Thesecomeintoplaywithphotographicemulsionsandsometypesofplasticdetectors.

o Otherfamousexample:RayDavis’solarneutrinoexpt.o Consultreferencesifyouneedmoreinformation.


Interactions(cont’d)1. Interactionswith(orrequiring)atomicnuclei-

- maybefamiliarfrommodernphysicscourses:

a) Pairproductionb) Bremstrahlungc) Nuclear(hadronic)interaction

• Thefirstappliestophotons,thesecondtoelectrons(mostly),andthelasttohadrons.


Interactions(cont’d)2. Interactionswithatomicelectronsarewidely

used:a) ComptonScatteringb) Photoelectriceffectc) Ionizationofatomsd) Excitationofatoms

• Thefirsttwoapplyonlytophotons,thelasttwoapplytoallparticles.


Interactions(cont’d)3. Interactionswithbulkmaterialmaybethe

leastfamiliar:a) Cerenkovradiationb) Transitionradiation

• Iwillnotbeabletocoverallofthesetoday.o Iwilltalkaboutthosemostrelevantto(some)REUprojects.

o Checkthebackupslidesforsomeinformationonthese.o AgoodintroductioncanbefoundinJackson’sClassicalElectrodynamics.


ThePhysicsofDetection


15

1.InteractionswithElectrons

Interactionswithatomicelectronsisthemostimportantdetectionprocess.Primaryprocessfor“tracking” particlesAlsoimportantforproducinglargenumbersoffreeelectronsforadetectablecurrent.


16

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

17

1b.ComptonScatteringAsecondtypeofphoton-matterinteraction.Usuallyappearswhendiscussingrelativity.

Importantforintermediateenergyphotons.

Incidentphotonlosesenergywhenejectinganelectronfromanatom.

Processisimportantto“breakdown” ahighenergyphotonintoelectrons.Theenergyofthephotondeterminesthefinalnumberofelectronsproduced.(Seecalorimetry)


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

�dE

dx

= Kz

2

Z

A

1

b2

1

2

ln

✓2m

e

c

2b2g2

T

max

I

2

◆�b2� d

2

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19

Energy Loss


DependenceonMaterialEnergylossdependsonthetypeofmaterialandparticle.AtomswithhigherZhavelessenergylossperg/cm2

Buttheirhigherdensity(g/cm3)morethanmakesupforthis.


1d.ScintillationManymaterialsemitlightwhenatomsormoleculesareexcitedbyachargedparticlepassingnearbyEmittedlightiscalledscintillationScintillatingmediumusuallytransparentLight(onetoafewphotons)detectedbyaphotomultiplier


2.InteractionswithNuclei

Twotypesofinteractionarepossible:Electromagnetic

Strong

Detailsareinthebackupslides


Electromagnetic• Mostlysinglephotonexchange

o elasticinteractions

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

• thinkoftheRutherfordscatteringexperiment• mostscatteringissmallangle• largenumberofsmallanglescatterscanbetreatedstatistically


24

2a.PairProductionAthirdtypeofphoton-matterinteraction.Exampleofconversionofenergyintomass.

Importantforhighenergyphotons.

Incidentphotonconvertsintoanelectron-positronpair.

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

Presenceofmatterisrequiredtoconserveenergy-momentum.


25

2b.BremstrahlungAGermanwordfor“brakingradiation”.

Importantforhighenergyelectrons.

Incidentelectronemitsaphoton(usuallyx-ray),whenacceleratedintheelectricfieldofanucleus.

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


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


ATypicalDetector


CollideParticles

• atveryhighenergiesandconvertenergyintomass(i.e.otherparticles)


Build anExperimentIdeallywewanttoidentifyeverything(particletype,charge,spin,pointoforigin,momentumatpointoforigin)emergingfromacollisioninordertoreconstructexactlywhatoccurred.

Webuildadetectorthatcomesasclosetoachievingthisgoalasmoneyallows.

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


DetectorLayout• Colliderdetectorshaveconvergedonastandardconfiguration.Movingoutwardfromthecollisionwehave:1. precisiontracking2. trackingtomeasurecurvatureinasolenoidal magnet3. Cerenkovdetector,TOF,transitionradiationdet.

(optional)4. electromagneticcalorimeter5. hadronic calorimeter6. muondetector(alwaysoutermost)


AGeneralPurposeDetector


1,2.Tracking• measuretheorigin(vertex)ofchargedtracks• measurethemomentumofchargedtracks• requires:

o amagneticfieldofsufficientstrengthandextent(tomeasureradiusofcurvature)

o largevolumetracking(drivenbycost)o highprecisionmeasurementsnearthecollisionpt.o possiblysomeparticleID(dE/dx,TOF,Cerenkov,TRD)


MeasuringMomentum• Inauniformmagneticfield

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

• Theuncertaintyonp comesfrom2sources:i. resolutionoftrackpositionmeasurementsii. multiplescatteringofmaterialinthedetectoro Thesecondsourceleadsustousethinsilicondetectorsorgaseousdetectors


4,5.Calorimetry• Measuretheenergyinchargedandneutralparticles

• Hadronsinteractdifferentlyfromphotonsandelectrons.EMcalorimeterplacedbeforehadronic calorimeter


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

o ThenumberofparticlesisastochasticquantitywithPoissonfluctuationsà wewillmeasuren ± √n particles

o ThemeasuredenergyisE ± √E,sotherelativemeasurementuncertaintyis1/√E


PhotonDetection• Detectingphotonsisimportantinmanysituations:astronomy,calorimetry,TOF,...

• Commondetectorsareo photomultipliers

• fastsignals,highsensitivity,lowgranularityo CCDs

• highgranularity,timeintegrating


CCD• Thesensorsfordigitalcameras• Semiconductordevices,usuallysilicon• CanbecustomizedforsensitivityinIR,vis.,UV.• Importantdevelopmentfortelescopes.


6.MuonDetector• Muons are200timesheavierthanelectrons.• Theprobabilityforbremstrahlung is(200)2 timessmallerformuonsthanelectrons(forsamep)

• Theresultisthattheycanpassthroughmuchmorematerialwithoutinteractingthanotherchargedparticles.

• Muondetectorisbasicallyachargedparticletrackerlocatedbehindlotsofmaterial.



Summary• Thephysicsofparticlesinteractionswithmatterisattheheartofparticledetection.

• Advancesindetectortechnologyenableimprovementsinexperimentsovertime.

• Modernexperiments(STAR,ALICE,Belle,CMS)exploittheinteractionsofparticleswithmattertomeasuretheparticlesemergingfromcollisionsinthemostusefulwaypossible.


Backup


(1)Electromagnetic(cont’d)• MultipleCoulombscattering(MCS)

o yieldsaroughlyGaussiandistributionofscatteringangles

o X0 iscalledtheradiationlengthofthematerial• characterizeselectromagneticinteractionswithnucleiinamaterial

o Scatteringindependentofparticletype(excepte-)


P (q)dq =1p

2pq0eq2/2q2

0dq

q0 =13.6MeV

bpc

z

rx

X0

1+0.038ln

✓x

X0

◆�

43

TransitionRadiationRadiationproducedwhenahighlyrelativisticelectronpassesthroughmaterialswithdifferentindicesofrefraction.


CerenkovRadiationEmittedwhenaparticleexceedsthespeedoflightinamediumLiketheshockwavefromasupersonicplaneConeangleisrelatedtoparticlevelocity


45

SynchrotronRadiationLikeBremstrahlung,butaccelerationduetoa

magneticfield.


ElectromagneticShowers• Tomeasuretheenergyofanincomingparticle

o Convertasinglehigh-energyparticleintomanylow-energyparticles,andcountthenumberofparticles.

o Theconversioniscalledashower.o pairproduction:1photonà 2electronso Comptonscattering:1photonà 1photon+1electrono bremstrahlung:1electronà 1electron+1photon

• AftertraversingdistanceXofamedium,theaverageenergyofanelectron/photonis:


hEi = E0e�X/X0

Hadronic Showers• Sameideaasforelectromagneticshowers

o incomingparticleisahadrono interactionswithmaterialcharacterizedbyinteractionlengthlI

o radiationlengths<<interactionlengthso therefore,electromagneticcalorimetersprecedehadroniccalorimetersinadetector


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