Particle Identification. Ryan Beaty Physics 428 2/7/2013

Transcription

1 Particle Identification Ryan Beaty Physics 428 2/7/2013 1

2 Atlas 2

3 Atlas 3

4 Atlas 4

5 What can we measure for each particle? For charged particles: What is its charge? What is its momentum? Where is its origin? What type of particle? For neutral particles: What is its momentum? What are possible origins? What type of particle? 5

6 What can we measure for each particle? For charged particles: What is its charge? What is its momentum? Where is its origin? What type of particle? For neutral particles: What is its momentum? What are possible origins? What type of particle? 6

7 Particles of interest Calorimeter dealt with identification of neutral particles. We care about charged particles with lifetime long enough to observe them. 7

8 Particle identification detectors Particle ID detectors rely on information from other detectors to find the momentum of the particle (tracker). Momenta can be found by measuring particle in a B field (as seen from the tracker talk). 8

9 Usefulness of particle identification If particle is identified, the noise (useless information) can be cut. Increase in signal to noise ratio. 9

10 Usefulness of particle identification Before Identification After Identification 10

11 Identification methods The tool kit: Time-of-Flight Cherenkov radiation Transition radiation 11

12 Identification methods The tool kit: Time-of-Flight Cherenkov radiation Transition radiation 12

13 Time-of-Flight The basics: - When and why is it useful for particle ID? 13

14 Time-of-Flight The basics: - When and why is it useful for particle ID? - Low momenta. - If the Time-of-Flight of crossing between two points on a particle trajectory is determined, then velocity can be found. - If momenta is known (as it should be from the tracker) then the mass of the particle can be found as well! 14

15 Time-of-Flight The Time-of-Flight is usually measured by scintillators using a photomultiplier to detect the scintillation photons. Other methods include microchannel plates or drift chambers. 15

16 Time-of-Flight Time measurements can differentiate between two different mass particles with the same momentum! 16

17 Time-of-Flight 17

18 Time-of-Flight Question 1: Assuming a spectrometer with the following characteristics: Hint: Same values from the slide above. What time resolution is required to do a particle identification for a pion, kaon, and proton up to 6 GeV/c? 18

19 Time-of-Flight Time slewing. - Times can have a distribution since PMT is not perfect. - A_o is a constant that is dependent on the specific PMT. - ADC is the signal pulse height. 19

20 Time-of-Flight Where TOF is located. 20

21 Identification methods The tool kit: Time-of-Flight Cherenkov radiation Transition radiation 21

22 Cherenkov radiation Radiation by a charged particle as it travels through dielectric. Superluminal speeds. V > phase velocity of light. n = D20 Kamioka Observatory (University of Tokyo) 22

23 Cherenkov radiation 23

24 Cherenkov radiation Conservation of 4 momentum: Invariant Calculating above one gets: Real radiation! In vacuum In media Many different values for beta => many different angles (Cherenkov angle). 24

25 Cherenkov radiation Example: By similar calculation the kinetic energy of muon is 54 MeV. Cherenkov radiation can distinguish between electrons and muons with MeV energy ranges! 25

26 Cherenkov radiation Typical gases are freon, or UV-transparent crystals (CaF_2 or LiF). 26

27 Cherenkov radiation Energy loss per cm due to radiation of photons from particle passing through media(jackson): Number of photons produced per cm: 27

28 Cherenkov radiation Question 2: If the probability of seeing r photons is given by where N is the number of photons ( ), what is the distance necessary to make 1% probability to see 0 photons given that ( )=1, n= (Characteristic of many Freon gases)? 28

29 Cherenkov radiation 29

30 Cherenkov radiation Ring Imaging Cherenkov detector (RICH) 30

31 Cherenkov radiation RICH Pattern recognition is key, but only if one knows what patterns to look for! Should we merge (a) to form a giant ring (b) or three rings (c)? If we know ahead of time which one it is, that is great, often times we only have probabilities to work with. 31

32 Cherenkov radiation Cherenkov photons emitted by a 22 GeV/c pion or kaon. 32

33 Identification methods The tool kit: Time-of-Flight Cherenkov radiation Transition radiation 33

34 Transition radiation 34

35 Transition radiation Transition radiation is a form of EM radiation emitted when a charged particle passes through inhomogeneous media. (Between boundaries with differing refractive indexes). Cherenkov radiation was for homogeneous media (heavy water for example). 35

36 Transition radiation Charged particle crosses boundary of different dielectric constants. Characterize media in terms of n, and plasma frequency. Fields must rearrange, some shake off as transition radiation. 36

37 Transition radiation Transition radiation tracker (TRT). Used for charged-particle tracking and electron/pion separation. 37

38 Transition radiation As noted above, TRT is used for electron/pion separation cannot resolve the rest. Total energy dependent on gamma 38

39 Transition radiation The intensity of the radiation is, means that transition radiation is optimum at highly relativistic speeds, where Cherenkov radiation would not work. Good for high energies! Electron/Hadron discrimination is only possible from momenta from about 1 GeV/c to 100 GeV/c. 39

40 Transition radiation TRT barrel. Straws (drift tubes) 40

41 Transition radiation TRT in atlas is largest in the world! 370,000 straws that are really drift tubes. These are interleaved with polypropylene foils which are working as radiators. 41

42 Transition radiation Increase of intensity by introducing arrangement of foils with some spacing, with X-ray detectors in-between. The reason for X-ray detectors is because the energy of emitted photons is: where the plasma frequency ( ) is dependent on foils used in the radiators. 42

43 Transition radiation The dominate photon emitted in radiation in X-ray (100eV 100keV). Particle ID is obtained by looking at energy deposited in straws at E > 5KeV.. 43

44 Transition radiation Question 3: Consider the detection of 200 GeV pions. The gamma factor is 1428, and the foil that we are interested in is Li. If Li has (plasma energy) what is the energy of the emitted photons? 44

45 Summary Time-of-Flight -Good for low energy. -If momenta is known, this is a good way to calculate velocity Cherenkov radiation -Good for distinguishing between electrons and muons with MeV energy ranges. Transition radiation -If Beta=v/c approaches 1, Cherenkov radiation is no longer good. -Energy is proportional to gamma Good for relativistic particles. 45

46 Extra Slides 46

47 Time-of-Flight Mass error! 47

48 Time-of-Flight Mass error! Note: Short flight lengths ~ 50 cm and dt ~ 0.8 ns, particle ID is only good at sub GeV/c momentum. mass resolution- 0.5 meters Mass error (Gev/c^ e pi K p Momentum (GeV/c) 48

49 Cherenkov radiation The Cherenkov cone of photons can be directly imaged in many applications. A reflecting mirror can be used to focus all parallel rays to a fixed focal point. 49

50 Transition radiation Trailing edge (TE): Independent of particle position as it transits the straw, electrons furthest from the wire, nearest the straw wall. Leading edge (LE): ependent on where the particle transits the straw. Time over Threshold (ToT): Dependent on the particle path length. 50

51 Time-of-Flight Typical value for inner ATLAS detectors. Time separations- 0.5 meters t1-t2 (ns) e-pi pi-k K-p P (GeV/c) 51