Particle ID Distinguishing Particles. We have decided now to identify the particle species by a bar code

Transcription

1 Particle ID Distinguishing Particles We have decided now to identify the particle species by a bar code

2 Introduction HEP detector: Measures particle momenta... by means of a spectrometer (tracker and magnetic field) With p, γ, β calculate particle mass m0... Need second observable to identify particle type: p = γm 0 βc Velocity: Time-of flight Cherenkov angle Transition radiation τ 1/β cos θ =1/βn γ 1000 γ,β Energy loss: Total energy: Bethe-Bloch Calorimeter de dx z2 β 2 ln(aβγ) E = γm 0 c 2

3 Introduction Special signatures for neutrals: Muons: Photons : Neutrons : Total energy deposited in electromagnetic shower; use energy measurement, shower shape and information on neutrality (e.g. no track)... Energy in calorimeter or scintillator (Li, B, 3 He) and information on neutrality (e.g. no track)... K0, Λ,... : Reconstruction of invariant masses... Neutrinos : Identify products of charged and neutral current interactions... Minimum ionizing particles; penetrates thick absorbers; measure signal behind complete detector...

4 Introduction Particle ID [CMS Detector Slice]

5 2 +2*'$D5(:'D$*2$D&30(2:40&*3D Time-of-Flight Method Basic idea: Measure signal time difference between two detectors with good time resolution [start and stop counter; also: beam-timing & stop counter] Typical detectors: Scintillation counter Resistive Plate Chamber (RPC) Coincidence setup or TDC measurement with common start/stop from interaction time particle Scintillator I PMT Scintillator II PMT Start Discriminators multichannel analyzer TDC Stop

6 Time-of-Flight Method Distinguishing particles with ToF: [particles have same momentum p] 1 t = L 1 = L v 1 v 2 c t = Relativistic particles, E pc m i c 2 : t L pc 2 (E 1 E 2 )= L pc 2 Example: Pion/Kaon separation... [mk 500 MeV, mπ 140 MeV] L pc β 1 β 2 p 2 c 2 + m 2 1 c4 (pc + m2 1c 4 2pc ) (pc + m2 2c 4 2pc ) t = Lc m 2 2p 2 1 m 2 2 Assume: p = 1 GeV, L = 2 m... t 800 ps Particle 1 : velocity v1, β1; mass m1, energy E1 Particle 2 : velocity v2, β2; mass m2, energy E2 Distance L : p 2 c 2 + m 22 c4 For L = 2 m: distance between ToF counters Requiring Δt 4σt K/π separation possible up to p = 1 GeV if σt 200 ps... Cherenkov counter, RPC : σt 40 ps... Scintillator counter : σt 80 ps... 2m c 2(1000) 2 MeV 2 /c MeV 2 /c 4

7 Time-of-Flight Method σt σt σt 4σt Difference in time-of-flight in σt... [L = 2 m]

8 Time-of-Flight Method Mass resolution... p = βγm 1 τ m 2 = p 2 β 2 1 = p 2 2 L 2 1 τ δ(m 2 2 )=2p δp L τδτ p2 m 2 /p 2 a use * Use: L 2 2 δl L 3 p2 τ 2 β = L/τ γ =(1 β 2 ) 1 [ c =1] * p 2 τ 2 L 2 = m 2 + p 2 = E 2 σ(m 2 )=2 =2m 2 δp p m 4 σp p +2E2 δτ τ 2E2 δl L 2 + E 4 σ τ τ 2 + E 4 σ L L 2 1 /2 Usually: δl L δp p δτ τ σ(m 2 )=2E 2 σ τ τ Uncertainty in time measurement dominates...

9 Resistive Plate Chambers 1.3. LARGE AREA PARTICLE DETECTORS 23 Readout Strips (X) Basic idea: Use parallel plate chamber with high field... Electrons of ionization clusters start to produce an avalanche immediately... Induced signal = sum of all simultaneously produced avalanches... Insulator Graphite Coating Insulator High Resistivity Electrode Gas Gap High Resistivity Electrode Readout Strips (Y) HV GND Signal: immediate... in contrast to e.g. wire chambers where avalanche only generated in vicinity of wire... But: Electron avalanche develops according to Townsend [see above]: n = n 0 e αx particle Figure 1.5: Schematic image of an RPC geometry as in [36, 37] cathode Schematic + + view sensitive region of avalanche process E o [the smaller the better] a) b) anode Schematic image of typical RPC geometry Gap size matters! G = n n 0 = e αx α: Townsend coefficient x : traversed path length G : amplification (gain) Raether limit: G 10 8 ; αx = 20; then sparking sets in Thus: only avalanches traversing full gas gap produce detectable signal, i.e. limited signal region close to cathode c) d) As maximum gain < 10 8 ; sensitive region limited to 25% of gap... Figure 1.6: A schematic Time jitter: image ~ time of the to development cross sensitive of an region avalanche... in an RPC and the electric field deformations caused by the avalanche charges at large gain. E 0 is

10 Resistive Plate Chambers Pestov chamber [1970] [First example of resistive plate chamber] Glass electrode (Pestov glass) + metal electrode particle cathode RPC 100 μm gap Operated at very high gas pressure: 12 atm [For large density of primary ionization i.e. good detection efficiency] anode Gas gap of 100 μm; time resolution: 50 ps Disadvantages: Pestov glass [Resistivity: Ωcm] Mechanical constraints high pressure Non-commercial glass (high resistivity) Limited sensitive volume Long tails of late events MRPC particle electrode Multi-gap RPC [Developed for ALICE particle ID] avalanche Idea: very high gas gain for immediate avalanche production, but mechanism to stop avalanche growth before sparking Solution: add boundary layers invisible to fast induced signal; external electrodes sensitive to any of the initiated avalanches electrode

11 Resistive Plate Chambers Multi-gap Resistive Plate Chamber Pick-up electrode particle HV (10 kv) Stack of equally spaced resistive plates with voltage applied to external surfaces... 8 kv 6 kv glass plates Internal plates electrically floating... Electrodes on external surfaces... [Resistive plates transparent to induced signal] Internal plates take correct voltage... [Feedback due to electron/ion flow] 4 kv 2 kv 0 kv gas gap: [250 μm] carbon layer Feedback principle: Mylar Pick-up electrode 8 kv 8 kv Flow of electrons Flow of positive ions A 6 kv 4 kv 2 kv 0 kv B 6.5 kv 4 kv 2 kv 0 kv A: Same 2 kv across each gap; same gain, i.e. same charge flow... B: Flow to layer with 6.5 kv not symmetric; flow decreased for electrons and increased for ions... System will go back to symmetric state with 2kV for all gaps...

12 Resistive Plate Chambers ALICE MRPC [Time-of-Flight System] 130 mm active area 70 mm Double stack; each stack with 5 gaps [i.e. 10 gaps in total] 250 micron gaps separated by standard fishing lines Resistive plates from soda lime glass [commercially available] 400 micron internal glass 550 micron external glass Area: 160 m 2 Channels: 160 k [size: 2.5 x 3.5 cm 2 ] Flat cable connector Differential signal sent from strip to interface card M5 nylon screw to hold fishing-line spacer honeycomb panel (10 mm thick) PCB with cathode pickup pads external glass plates 0.55 mm thick Honeycomb panel (10 mm thick) internal glass plates (0.4 mm thick) PCB with anode pickup pads Mylar film (250 micron thick) 5 gas gaps of 250 micron PCB with cathode pickup pads connection to bring cathode signal to central read-out PCB Silicon sealing compound

13 Resistive Plate Chambers ALICE MRPC [Time-of-Flight System] Particle ID in high multiplicity environment... Needs: ToF with very high granularity... and coverage of full ALICE barrel... Gas detector is only choice! Alice Pb+Pb Event [Simulation]

14 Resistive Plate Chambers Cross Section of the ALICE detector ToF TRD ToF array arranged as barrel with radius 3.7 m [Divided into 18 sectors]

15 Resistive Plate Chambers ALICE MRPC [Time-of-Flight System] Entries/50 ps! 1000! 800! 600! Strip10; H.V. ± 6 kv! Uncorrected time spectrum σt 60 ps 400! 200! 0! 1000! 1500! 2000! 2500! 3000! 3500! Time with respect to timing scintillators [ps]! 1200! Entries/50 ps! 1000! 800! 600! Strip 10; H.V. ± 6 kv! Time spectrum corrected for slewing σt 45 ps 400! Time Resolution... is indeed < 50 ps 200! 0! -1000! -500! 0! 500! 1000! Time with respect to timing scintillators [ps]!

16 Specific Energy Loss Use relativistic rise of de/dx for particle identification... Key problem: Landau fluctuations Average energy loss in a 1 cm layer of argon-methane μ/π separation impossible, but π/κ/p generally be achievable Need to make many de/dx measurements and truncate large energy-loss values... [determination of 'truncated mean'] Probability K π p = 50 GeV normalized de/dx µ e K π Energy loss distribution; 50 GeV pions and kaons p [1 cm layer Ar/Methane] Energy loss [kev] momentum p [GeV]

17 Specific Energy Loss TPC Signal [a.u.] Measured energy loss [ALICE TPC, 2009] Momentum [GeV] Bethe-Bloch Remember: de/dx depends on β!

18 Specific Energy Loss Truncated energy loss distributions for various momenta... [ALPEH TPC] Counts π e K P = [GeV/c] p Counts π P = [GeV/c] K p e de /dx de /dx Counts 10 3 π P = [GeV/c] Counts 10 2 K π P = [GeV/c] 10 2 e 10 e de /dx de /dx

19 Cherenkov Radiation Cherenkov The contribution radiation of Cherenkov amounts to radiation less than to the 1% en o compared minimum-ionising to that from particles. ionisation Forand lightexcitation, gases (He, Eq H minimum-ionising to about 5% [21, particles. 22]. For gases with Z 7 t Cherenkov radiation amounts to less than 1% of the minimum-ionising particles. For light gases (He, H) thi to about 5% [21, 22]. See: Lecture 3 Reminder: Polarization effect... Cherenkov photons emitted if v > c/n... Cherenkov angle: cos θ = 1 c nβ Simple Geometric derivation: AB = βc t AC = c/n t fast particle c/n t A light cos θ = AC / AB = c/n t/(βc t) = 1/nβ θ C βc t wavefront B A v < n c c/n B v > c n parti Fig v Illustration < of the Cherenkov n c v > n c c/n effect [1 determination of the Cherenkov angle. Fig A : v Illustration < c/n of the Cherenkov effect [140, determination of the Cherenkov angle. Induced dipoles symmetrically arranged around particle path; no net dipole moment; no Cherenkov radiation B : v > c/n Symmetry is broken as particle faster the electromagnetic waves; non-vanishing dipole moment; radiation of Cherenkov photons

20 Cherenkov Radiation Application See: Lecture 3 Threshold detection: Observation of Cherenkov radiation β > βthr π, K, p C1 C2 [ momentum p ] n1 n2 > n1 Choose n1, n2 in such a way that for: n2 : n1 : βπ, βk > 1/n2 and βp < 1/n2 βπ > 1/n1 and βk, βp < 1/n1 Light in C1 and C2 Light in C2 and not in C1 Light neither in C1 and C2 identified pion identified kaon identified proton

21 Cherenkov Radiation Application Differential Cherenkov detectors: Selection of narrow velocity interval for actual measurement... Threshold velocity: [cos θ = 1] β min = 1 n Radiator θ Al-Mirror particle track Maximum velocity: [θ = θmax = θt] Cherenkov angle limited by total reflection air light guide sin θ t =1/n cos θ max = 1 sin 2 θ t =1/nβ max β max = 1 n2 1 PMT Example: Diamond, n = 2.42 βmin = 0.413, βmax = 0.454, i.e. velocity window of Δβ = Suitable optic allows Δβ/β 10-7 Working principle of a differential Cherenkov counter

22 Cherenkov Radiation Application Ring Imaging Cherenkov Counter Optics such that photons emitted under certain angle form ring... Typical: RD = Rs/2 Particle 1 Focal length of spherical mirror: f = Rs/2... Cherenkov light emitted under angle: θc... Radius of Cherenkov ring: r = f θc = Rs/2 θc... β = 1 n cos(2r/r s ) Determination of β from r... Photon detection: Photomultiplier, MWPC Parallel plate avalanche counter... Gas detectors filled with photosensitive gas... [e.g. vapor addition or TMAE (C5H12N2)] Interaction point Detector surface RD r Radiator Particle 2 Spherical mirror with radius Rs Working principle of a Ring Imaging Cherenkov Counter (RICH)

23 Cherenkov Radiation Application See: Lecture 3 Measurement of Cherenkov angle: Use medium with known refractive index n β Principle of: RICH (Ring Imaging Cherenkov Counter) DIRC (Detection of Internally Reflected Cherenkov Light) DISC (special DIRC; e.g. Panda) LHCb RICH Event [December 2009] LHCb RICH

24 Transition Radiation See: Lecture 3 Transition radiation occurs if a relativist particle (large γ) passes the F&'1$34,+,($@4/($ +B$γ$'8,C/$'8,5+$9DDD$/.*+$E boundary between two media with different refraction indices... 8'8*7*+1$,&-/&$9G$ +&*6$6,()+'(+$*(+,$'(,+4/&=$3&, [predicted by Ginzburg and Frank 1946; experimental confirmation 70ies] Number of Events Effect can be explained by rearrangement of electric field... transition radiation n1 n2 Rearrangement of electric field yields transition radiation Energy loss distribution for 15 GeV pions and electrons in a TRD... Energy deposit [kev]

25 .3 Übergangsstrahlendetektoren in Übergangsstrahlendetektor besteht aus zwei Komponenten: ) Radiator Nachweisgerät Dünne Folien aus Materialien mit kleinem Z Proportionalkammern Transition Radiation Application Detection Principle: [Electron-ID] Radiator TR! 0 + TR Anode γ-absorption + See: Lecture 3 Electron e Radiator foils de/dx Wires ) Detector Signal: δ-electron TR!!Elektronen TR [Transition Radiation] de/dx Q>Q S Drift time

26 Super Module Integration ALICE TRD Install electronics, assembles into one super module 10 Introduction Electron ID Tracking Triggering 2 Design objectives and mechanical struc Gas System Summary References Bac Readout Boards TRAP Chip Figure 2.3: View of the TRD detectors cut in!-direction the space frame and detector boundaries of subsequent detectors (eg. HMPID). This was done in o to minimize shadowing and dead areas. A front view of one sector of detector is shown in Fig. 2.3 including the supermodule casing and rails that facilitate the installation of the supermodule as one piece. Many MCMs connected to signal cables from a chamber. One chamber equipped with readout electronics TRD supermodule

27 ALICE TRD cathode pads pion electron anode wires amplification region cathode wires drift region primary clusters Drift Chamber entrance window x Radiator z pion TR photon electron Transition Radiation [TR] for charged Particles with γ > 1000

28 ALICE TRD cathode pads pion electron anode wires amplification region cathode wires drift region primary clusters Drift Chamber entrance window x Radiator Avalanche near anode wires [high field] TR-Signal Gas: Xenon [High γ-absorption] z pion TR photon electron Transition Radiation [TR] for charged Particles with γ > 1000

29 Transition Radiation ATLAS as Example Straw Tube Tracker with interspace filled with foam See: Lecture 3 Barrel assembly Tracking & transition radiation End-cap assembly

30 Transition Radiation ATLAS as Example See: Lecture 3 High threshold probability ATLAS preliminary ATLAS Preliminary Electron candidates Generic tracks Fit to data Mainly Pions Electron candidates Generic tracks Electrons (MC) Generic tracks (MC) Pion momentum (GeV) factor 3 10 Electrons from Conversions TRT TRT endcap Electron momentum (GeV)

31 Particle ID Comparison π/k Separation [Comparison of different PID methods Threshold Cherenkov Counter RICH Cherenkov Time-of-Flight DISC de/dx Multiple de/dx Transition Radiation Momentum p [GeV]