1 Methods of Experimental Particle Physics Alexei Safonov Lecture #11

SEAN YEAGER Research Topic Assignment 2

Multiple Scattering Charged particle passing through a material Coulomb scattering Hadronic projectiles will also scatter strongly Distribution described by Moliere We use θ 0 as a measure of how big each deflection is

Defining θ 0 β is the particle's velocity p is the particle's momentum z is the particle's charge x/X 0 is the thickness of the material in terms of scattering lengths Roughly Gaussian for small θ 0 Rutherford scattering for large θ 0 (bigger tails correspond to backscattering)

Visual Representation θ is the exit angle ψ is the line from entry to exit y ~ xψ s is perpendicular to ψ

ALEXX PERLOFF Research Topic Assignment 6

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MAIN LECTURE STARTS HERE 11

Today We mostly finished basics of particle interactions So far we cared about what happens with the particle (energy losses, stopping power etc.) This time: Talk about effects on the media from passing particle Cherenkov radiation Scintillation Transitional radiation Reminders of basics: Measurement of the momentum for a particle in magnetic field Next time we will start talking about actual detectors Types, characteristics etc 12

Charged Particle in Magnetic Field A charged particle with momentum p moves in a helix is the part of p transverse to B B is in Tesla R is in meters z is +1 or -1 charge If you can map particle’s trajectory, you can solve for p It’s not velocity To measure velocity, one needs to know the mass of the particle 13

Charged Particle Tracking If you have a detector that can find positions of the particle’s trajectory moving in magnetic field at several points: “Reconstruct” R and use B to get momentum p Often more convenient: curvature k=1/R Resolution if you For p=1 TeV: k=0.3B/10 3 To have dp=10%: dk=0.1k=0.3x10 -4 B For L~1m, N=10,  =1mm: 0.3x10 -4 B = /1 x 7 Need B=10 x7 /0.3= 200 T 200 T is insane! Then one need either much larger L or much better  Typical resolutions are some tens to hundreds of microns (need only 2T for 10 micron resolution) 14

Scintillation When a charged particle passes through media, it excites molecules Some materials will emit a small fraction of this energy as optical photons Various plastic scintillators (polystyrene) are frequently used in particle physics If the media is optically transparent, you get light propagating inside the material What if you can detect the light? Then you can tell there was a particle passing through it But photons can get re-absorbed Attenuation length – how much a photon will travel before it is reabsorbed Want to pick materials which don’t have absorption wave lengths close to that of the emitted photons 15

Scintillation The problem is that most good scintillators are not “transparent enough” Small attenuation lengths Solution: add “waveshifters” into the mix Scintillators are usually composites Waveshifter is material that absorbs primary photons and re-emits photons Often more than one waveshifter Goal: get lots of light that can propagate far Larger detectors are cheaper than small ones Mix wave-shifters to make a composite material to optimize how much light you get (you want more), photons wave length (depends on how you detect it) and the attenuation length (want large enough so it propagates far enough to get collected) 16

Scintillators Scintillator crystals (inorganic scintillators): They are more dense which can come handy Have dopants, e.g. NaI crystal with thallium dopant (Tl) A simple detector to detect charged particles 17

Scintillator Detectors Single charged particle detector Coincidence To reduce noise Or even “sandwiches”: Calorimeter 18

Photomultipliers (PMT) One of the oldest detectors of photons: Something you would need if you wanted to collect the light from a scintillator Details matter (material for the window: quartz, glass, what kind, amplification, operating voltage) Now solid state PMTs are becoming more and more used Silicon based (SPMT) 19

Cherenkov Radiation A particle moving in media with the speed higher than the speed of light in the media Media: electrically polarizable dielectric Similar to the supersonic “boom” As the charged particle passes, it polarizes media It gets back by emitting a photon Usually collective effects are random and no light But if the wave of emissions happens faster than the speed of light, can get a collective effect of coherent constructive interference 20

Cherenkov Radiation The emitted spectrum: Mostly in UV part Visible part appears to be blue Important as e.g. glass window of a PMT will kill most of the signal Reacts to velocity (not momentum like tracking) Can be used as a single threshold detector 21

Imaging Detectors As Cherenkov light comes in cones, can arrange a detector in which you will see rings Ring Imaging Cherenkov (RICH) Detector Angle depends on the velocity of the particle Can distinguish particles and measure their velocities LHCb detector has RICH 22

Transition Radiation When a relativistic charged particle crosses a boundary of two media with different refractive indices Number of photons when a particle crosses a border of vacuum and media: The number is typically not very large (but grows with gamma) Solution is to use multiple surfaces Many hundreds of very thin foils e.g. polypropylene to not reabsorb photons X-rays can be detected and will signify that there was indeed a particle 23

What Else? A number of technologies exploring what we have already learnt about ionization Semiconductor detectors – essentially a diod which you use as an ionization chamber Ionization creates currents Gazeous detectors – chambers with little material where ionization electrons/ions are made to form avalanches and create currents Neutrons/protons detected via nuclear interactions (create showers including charged particles which we know how to register) Talked about it last time A number of early detectors relying on ionizatoin no longer used Cloud/bubble etc. chambers - slow and difficult to maintain and operate 24