Cherenkov radiation

1888 predicted by O. Heaviside Deformation of the electromagnetic field of a charged, moving particle 1901 predicted by Kelvin 1904 predicted by Sommerfeld

Cherenkov: 1934 experimentally observed

Frank & Tamm 1937 theoretical explanation

1958 Nobel price

Katharina Müller autumn 14 1 Cherenkov radiation

2 2 2 −dE Z 1 1 2 me c T max  C =K z2 [ ln −2− − ] dX A 2 2 I2 2 Z

Reminder: Bethe Bloch δ(β): density correction due to polarisation of the material logarithmic rise gets attenuated

Dipoles

charged particles polarise material time dependent dipole field → dipole radiation vc/n asymmetric → Cherenkov radiation

Energy loss due to Cherenkov radiation small 1-5% of dE/dX(MIP)

c/n light velocity in material n: refraction index (Brechungsindex)

Katharina Müller autumn 14 2 Cherenkov Radiation

Angle between Cherenkov photons and track of particle → velocity of particle

particle l = t β c part photons l = t c/n c 1 light cos  = = c n c n 

correct for recoil: 1 ℏ k 1 cos  =  1−  c n  2p n2

p: momentum of particle, ℏk momentum of photon (k = 2 π/λ) ℏk<

● Threshold: Cherenkov emission only for β>1/n

● βs=1/n emission forward direction θc = 0

● maximum for β=1 , Cherenkov angle θc = arccos 1/n → Cherenkov radiation occurs only in media and for frequencies with n(β) >1 E s 1 1 ● Threshold energy  = = = s m c2 2 2 0 1− s 1−1/ n

Measurement of γs allows mass determination if energy is known

Katharina Müller autumn 14 3 Refraction index: Dispersion

Refraction index depends on velocity : Sellmeier parametrisation: 2 2 2 2 2 2 2 n =1B1  / −C 1 B2  / −C 2B3  / −C 3

Katharina Müller autumn 14 4 Cherenkov-angle vs 

Liquids, solids Gases e l g n a n o i s s i m e

velocity β velocity β

→ high relativistic particles: small emission angle → large refraction index: large scattering angle

Katharina Müller autumn 14 5 Photon yield Intensity of the Cherenkov radiation: # γ per unit length of particle path and per unit of wave length depends on charge and velocity of particle → Photon yield

 2 dN 2 1 d  2 2  2− 1 2 −1 =2   z ∫1−  ≈ 2  z sin c = 490sin c [cm ] 400 - 700 nm dx 22 2   1 n 2 1 1150sin 2 [cm−1] incl. UV 200-700nm blue! c

dN/dλ

Example: characteristic glow from reactor

Katharina Müller autumn 14 6 Photon yield vs 

Energy loss very small (1% dE/dX(MIP)) Number of emitted photons ~1/λ2: detect visible and UV light

Liquids, solids gases

Yield increased by factor 2-3velocity if UV light β detected velocity β

→ if any light is emitted, then the particle β exceeds 1/n Yield increased by factor 2-3 if UV light detected

Katharina Müller autumn 14 7 Types of Cherenkov detektors

Cherenkov detectors are mainly used for particle ID

● Threshold – Cherenkov detector

Only particles with β>βc emit Cherenkov light simple construction: radiator and light detector (Photomultiplier)

● Differential Cherenkov detector (DISC-Zähler) Use Θ(β) allows to determine Θ-interval

● Ring-Imaging-Cherenkov-Detectors (RICH) Measure Θ(β) spherical mirror used to focus light onto photon detector centre of ring: direction of particle

radius → Θc → velocity β

Katharina Müller autumn 14 8 Threshold-Cherenkov-Counter

Allows separation of particles with same momenta but different masses

Assume separation of two particles p1=p2 m2>m1 Threshold: choose radiator such that β1>1/n and β2<1/n 2 1 1  require: particle 2 does not radiate:  = , rsp.  =  n2= 2 2 n 2 1−1/ n2 2−1  2 Diffraction index needs to be adjusted, stabilised with high accuracy!

2 Length of radiator: lighter particle emits 490 sin Θc photons per cm 2 c 2 2 −1 particle 1: # photons: N =490 m1−m2[cm ]L q p2 L: length of radiator, q: quantum efficiency, depends on energy, thickness and type of material

Katharina Müller autumn 14 9 Threshold-Cherenkov-Counter

Combination of several threshold Cherenkov counters

x x x  x x K x p

Aerogel Neopentan Ar­Ne n=1.025 1.0017 1.000135

K-p-π Separation up to roughly 100 GeV in beam with fixed momenta

Katharina Müller autumn 14 10 Radiators transparent material: solids, liquids, gases

material n-1 βc θc Photons/cm (max) solid natrium 3.22 0.24 76.3 462 Lead sulfite 2.91 0.26 75.2 457 Diamond 1.42 0.41 65.6 406 Zinc sulfite 1.37 0.42 65.0 402 silver chloride 1.07 0.48 61.1 376 Flint glass 0.92 0.52 58.6 357 Lead crystal 0.67 0.60 53.2 314 Plexiglass 0.48 0.66 47.5 261 Water 0.33 0.75 41.2 213 Silica Aerogel Aerogel 0.025-0.075 0.93-0.98 12.6-21.5 24-66 Pentan 1.7 10-3 0.9983 6.7 7 Air 2.9 10-4 0.9997 1.38 0.3 He 3.3 10-5 0.999971 0.46 0.03

Diffraction index of gas radiators may be modified with pressure

(n-1) = (n0-1)p/p0

Katharina Müller autumn 14 11 Cherenkov-Counter

→ tune refraction index by setting pressure

Example: CO2- pressure for Cherenkov radiation vs p

radiation

Mazziotta, GLAST no radiation (2005)

Katharina Müller autumn 14 12 Differential Cherenkov Counters

Measure Cherenkov angle → direct measurement of β → select particles in velocity interval

Minimum: Cherenkov requirement 1  = min n Maximum: total refraction 1 1 1 sin = , cos=  max= quartz radiator n n   n2−1

Example: diamond n=2.42 βmin= 0.413 βmax = 0.454

● Used for particle id in beam with fixed momentum ● Particles need to be parallel to axis velocity resolution up to Δβ/β= 10-5 Pion/K separation up to 100 GeV very precise timing signal

Discovery of anti-proton in 1955 by Chamberlain, Segre et. al. at Berkeley. Nobel Prize in 1959, Physical Review Letters , Nov. 1, 1955

Katharina Müller autumn 14 13 Ring Imaging Cherenkov Counter (RICH)

project cone of Cherenkov light onto matrix of photon detectors (Multiwire proportional chambers, photomultipliers, TPCs )

signals on matrix form a ring incoming particle Matrix of photon detectors centre: direction radius: velocity

measured photons

radiator with diffraction index n

Geometrical focussing: radiator thickness small wrt. distance to photon detectors

Katharina Müller autumn 14 14 Ring Imaging Cherenkov Counter (RICH)

● Spherical mirror (Radius Rs ) with centre at interaction vertex Focal width R /2 s Detector R Mirror R ● Cherenkov light is reflected onto photon detector D s

at radius RD

● radiator between Rs and RD = Rs/2

● measured radius r depends on β c rc Rs 1 r = f θ = θ cos θ = →β=1/(ncos(2r / R )) c c 2 c c nβ c s

● momentum known → particle ID 2 p√1−β p=γ m β c → m= Radiator 0 cβ

● particle known → measure momentum Δ p Δβ Δ γ p=γ m β c → =γ2 ≃ 0 p β γ

First used by DELPHI (LEP) W. Adam et al Nucl. Instr. And Meth in Phys. Res A 343 (1994) 68

Katharina Müller autumn 14 15 RICH, Example DELPHI (LEP) (1989-2000)

Improved identification: different radiators Photon Detectors: Multiwire proportional chambers, TPC, photomultiplier single event accumulated Optimised for K/π/p separation up to 30 GeV

Liquid radiator

Gas radiator

Ring is defined by few photons only Dominant error: Chromatic error due to energy dependence of refraction index n(E)

Katharina Müller autumn 14 16 RICH, Example DELPHI (LEP)

B → K*0  K*0 →K

No identification with dEdX but with RICH

Katharina Müller autumn 14 17 Delphi RICH: Efficiency of identification

Pi Efficiency Kaon Mis­id p Mis­id

Pi Mis­id Kaon Efficiency p Mis­id

Pi Mis­id Kaon Misid p Efficiency

Inefficiencies

Katharina Müller autumn 14 18 Detection of Cherenkov light

Energy of Cherenkov photons: eV → photoeffect dominates, → strong Z and E dependence Photoeffect: Photon is absorbed, transmits energy onto electron.

Photoemission threshold Wph of various materials Ultra Visible Infra

Violet Red GaAs TMAE,CsI (UV) Bialkali (IR) Multialkali TEA 12.3 4.9 3.1 2.24 1.76 1.45 E [eV]

100 250 400 550 700 850 λ [nm]

Ideal photo cathode: absorbs all γ, emits all electrons

Katharina Müller autumn 14 19 Detection of UV-photons

Admixture of organic vapour: quantum efficiency vs wave length TMAE high efficiency for small λ

BUT: radiation damage

Energy[eV] 9,0 8,5 8,0 7,5 7,0 6,5 6,0 5,5

0,6 transparency cutoff 0,6 of fused silica

0,5 0,5 TMAE TEA y

c 0,4 0,4 n e i c i f f 0,3 0,3 e

m u t

n 0,2 0,2 a u Q 0,1 0,1

0,0 0,0

140 150 160 170 180 190 200 210 220 230  2 Wavelength [nm] dN 2 1 d  Reminder: # Photons: =2  z ∫1−  → important to detect photons at small λ dx 2 2 2 1 n  

Katharina Müller autumn 14 20 Detection of Cherenkov light II

Photo cathodes: thin layer of metal or semi-conductor with low work function( Austrittsarbeit)

eV photons typical material, CSI: Threshold 6 eV =210 nm ● high QE ● stable in air ● cathode should not charge up

0.4 PC32 (@STAR) 0.35 PC33

E PC34

Q 0.3

e PC35 d

o 0.25 PC37, PC39 h t PC38

a Alice CSI cathode:

c 0.2 o

t average quantum efficiency 15% (155-210 nm) o

h 0.15 p

I

s Production: stability, reproducibility difficult

C 0.1 for a long time 0.05 today: reproducible quality 0 5.5 6 6.5 7 7.5 8 photon energy [eV]

Katharina Müller autumn 14 21 Detection of Cherenkov light

Position sensitive gas detectors (MWPC, TPC) or photomultiplier

Delphi Admixture of TMAE Sensitivity in UV region Disadvantage: slow! long drift time μs Ageing problems due to TMAE

Alice

ALICE MWPC: multiwire proportional chamber with photocathode fast signal < 100 ns

Katharina Müller autumn 14 22 Detection of Cherenkov light: photomultiplier

For example: SuperKamiokande, Ice Cube

Medicine: Silicon photomultipliers (SiPM): avalanche photodiode array on common Si 20-100 μm.

Katharina Müller autumn 14 23 LHCb RICH

RICH 2

RICH 1

http://lhcb­public.web.cern.ch/lhcb­public/en/Detector/RICH­en.html

Katharina Müller autumn 14 24 LHCb RICH: 2 detectors, 3 radiators RICH 1 Acceptance 25-300 mrad RICH 2 15-120 mrad p<70 GeV p<150 GeV,

2 radiators: Aerogel 2-11 GeV, C4F10 10 – 70 GeV 1 Radiator CF4 17-150 GeV

note the scale difference!

http://lhcb­public.web.cern.ch/lhcb­public/en/Detector/RICH­en.html

Katharina Müller autumn 14 25 LHCb RICH

RICH1 RICH 2

B with and without RICH B → 

red: Signal, yellow Background (B →K etc)

http://lhcb­public.web.cern.ch/lhcb­public/en/Detector/RICH­en.html

Katharina Müller autumn 14 26 LHCb RICH

Katharina Müller autumn 14 27 Characteristics of LHCb RICH

Radiator

emission point of photon chromatic aberration(λ)

photon detector granularity uncertainty of track

Photon yield

Fundamental limits for velocity resolution: Chromatic aberration Δn /n of diffraction index n(λ) geometrical uncertainty in position measurement of photons

Katharina Müller autumn 14 28 ALICE: HMPID HMPID

HMPID: High momentum particle Identification Detector 5% acceptance in central region http://aliceinfo.cern.ch/Public/en/Chapter2/Chap2_HMPID­en.html

Katharina Müller autumn 14 29 ALICE HMPID

Katharina Müller autumn 14 30 ALICE HMPID

Pb-Pb collisions, dN/dy=6000: 50 particles/m2 (pad occupancy 13%) MIP

TOF & HMPID Correlation

Track reconstruction → Extrapolation, match with MIP Signal in HMPID MWPC Cone reconstruction → β (β=1: ca 20 photons) p (Hough transformation) K π π/K separation up to 3 GeV p/K separation up to 5 GeV

Katharina Müller autumn 14 31 DIRC Detector (BABAR) 1999-2008

DIRC: Detector for Internally Reflected Cherenkov light

BABAR: Requirements low material budget measurement in calorimeter down to 20 MeV! radiation hard (10 krad in 10 years) π/K separation up to 4 GeV

Principle: measure light outside of main detector light transmission through total reflection in quartz beams (200 reflections!) coupling to PMs via water ϴ information preserved but left-right and bottom-top ambiguities

Total reflection for ϴ>1/sin(n) = 42.7o (Quartz)

http://www.slac.stanford.edu/BFROOT/www/Detector/DIRC/PID.html

Katharina Müller autumn 14 32 DIRC Detektor (BABAR)

DIRC: Detector for Internally Reflected Cherenkov light - Use solid radiator which reflects light → Total reflection instead of mirrors main advantage: small amount of material in detector Readout: 13000 photomultipliers

principle: light transmission via total reflection in quart beams (200 reflections!) 144 quartz beams: 5m long, 1.7 cm thick coupling to PMs with water SLAC-PUB-6047 Katharina Müller autumn 14 33 DIRC detector (BABAR)

Needs additional background suppression: time window (+-8 ns) 1-2 background hits per event and sector

reconstruct segments of circles http://www.slac.stanford.edu/BFROOT/www/Detector/DIRC/PID.html

Katharina Müller autumn 14 34 DIRC detector (BABAR): performance Kaon efficiency DIRC- identified pions

Pion-Kaon separation DIRC- identified kaons

Katharina Müller autumn 14 35 Reconstruction of Cherenkov rings/segments

(a) (b) (c)

Challenges (example LHCb) ● non circular rings: optical distortions by tilts in mirror system ● fuzzy rings: finite spatial resolution, imperfections in focusing, chromatic dispersion ● partial information: rings shared between two halves of detector, inefficiencies ● background ● various radii ● large range in number of hits on ring ● time constraints: use information on trigger level

1) Likelihood method: needs track information, fast, robust 2) Hough transformation: no track information needed, slow, memory intensive 3) Markov chain 4) Clustering

Katharina Müller autumn 14 36 RICH: Likelihood method use track parameters and knowledge of RICH ● predict position of photons for particle hypothesis (kaon, pion, etc) ● compare prediction with observed photon distribution ● calculate log-likelihood ● choose solution with maximum likelihood predictions from tracks C4F10 aerogel

compare expectation from tracks to data ● robust ● fast (Trigger) ● efficient ● BUT: cannot find rings without a track

used by HERA-B, STAR, AMS, BaBar, CLEO, DELPHI …

Katharina Müller autumn 14 37 RICH: Hough transformation

Hough transformation: technique is to find imperfect instances of objects within a certain class of shapes ● construct for each point all parameters in parameter space ● look for maxima Simple example: transformation of line input image transformation into parameter space x cos(θ) + y sin(θ)=r e l g n a

distance from centre circles: transform into parameter space: centre point (x,y) and radius pro: independent of tracking, accommodates for missing hits, fuzzy patterns, ring overlap con: CPU time, memory modification: use track info for centre (ALICE) used by ALICE, STAR, HADES, Superkamiokande

Katharina Müller autumn 14 38 RICH, Example SLD (SLAC) 1992-98 (Additional material)

Two radiators (liquid and gas fluorocarbon), one photon detector → K/π/p separation up to 30 GeV e/π separation up to 6 GeV Cherenkov angle vs momentum

SLD Detector http://www.slac.stanford.edu Inefficiency!

Katharina Müller autumn 14 39 RICH, Example SLD (SLAC)

→  n Example D* analyses D* K o i  t e/ 3 Tracks Particle ID a r a p e S

/K

K/p

Vertex Vertex+Particle ID

Katharina Müller autumn 14 40