1 Introduction. KOPIO charged-particle vetos. K - RARE Meeting (Frascati) May Purpose of CPV: veto Kl

Transcription

1 Introduction - Purpose of CPV: veto Kl decay modes with a real or apparent π and a pair of charged particles - Examples of background modes: (i) K l π π + π (ii) K l π π ± eν there are always (iii) K l π ± eνγ two charged particles - Required for background estimates: detection efficiencies of CPV + PV for e ±, π ± and µ ± Even with a good veto shield parts of phase space will still be background dominated - Two intrinsic sources of inefficiency: (i) dead layer in front of active medium (π, e + ) (ii) threshold on E signal - Other potential losses: (i) non-relativistic particles (ii) π µ decay with large kink (iii) veto blindness (iv) upstream beam hole /2

2 Simulations have been performed for: - dead layer and threshold effects experimental results for π ± reproduced with GEANT - tails in time distributions, π µ in flight and the upstream hole studied with FastMC - veto blindness pulse shapes of scintillators studied with KOPTICS Open issues: - study requirements to detection efficiency in different regions can we accept a larger inefficiency in forward direction? - position dependent vetoing Do we have to reject all events with a veto signal anywhere in time and space? How does veto enter in trigger? 2/2

3 Requirements: - intrinsic charged particle detection efficiency > 9999% typical value, will depend on momentum and type of particle - solid angle 4π sr with possible exception of upstream beam hole - at least 5 cm distance to the decay region to recognize (n,π ) reaction in CPV - time window <2 ns to avoid accidental vetoing by CPV signals from neighboring beam bunches - veto blindness below intrinsic inefficiency wfd readout, sufficient granularity, thin detectors - reliability redundant readout of each individual detector element 3/2

4 2 kinematic suppression of π + π background with slow pions CPV off, pion and muon decay taken into account m 2 miss/ 2m π E miss minimal energy of charged pions (MeV) largest delay of charged pions (ns) no cut cut at 9 cut at 8 cut at 7 final sample 2 π + π x -2 5 π νν largest delay of charged pions (ns) m 2 miss/ 2m π E miss 4/2

5 contours of constant m2 / 2mπEmiss miss CPV and PV off missing energy (MeV) a) missing energy (MeV) b) c) missing mass (MeV) 3 d) missing mass (MeV) a) K π+π-π b) K π+π-π with one pion below 6 MeV c) K πνν d) K 2π 5/2

6 2 Measurements on the response of plastic scintillator to charged particles at MeV/c cm 2 3 WC WC2 4 5 NaI(Tl) 5 mm 5 mm Experimental setup: 2 mm -5: plastic scintillation detectors WC/2: x-y proportional wire chambers with mm wire spacing The values on the bottom denote the thickness of the corresponding detector The beam extracted from the πm channel at PSI enters from the left 5 mm 5 mm mm mm 3 mm 6/2

7 x4,y4: trajectory coordinates at counter 4 ADC 4: counter 4 signal amplitude of All momenta and particles a) the blue distribution results from the selection by counter 3 b) slice of a) y4-5 mm c) change in response around y4=-2 mm where the scintillator is glued to the light guide d) slices of c y4 (mm) y4 (mm) all events 5< ADC 4 < x4 (mm) ADC 4 a c x4 (mm) all events 5< ADC 4 < 45 ADC 4 < ADC 4 > b d y4 (mm) 7/2

8 Particle identification using tof through secondary beam line 5-36 MeV/c Σ: 23M e - : 52k µ - : 88k : 7k particle identification using time of flight negative electric charge MeV/c Σ: 37M e - : 8k µ - : 62k : 336k 5-29 MeV/c Σ: 35M e - : 5k µ - : 2k : 373k MeV/c Σ: 25M e - : 36k µ - : 298k : 864k 5-22 MeV/c Σ: 5M : : e - : µ - 238k 82k 88k 5-85 MeV/c Σ: 28M e - : 89k µ - : 449k : 452k -2-2 rf phase ( ps/ch) P(e) > 99 P(µ) > 99 P(π) > 99 positive electric charge runs rf phase ( ps/ch) 36 MeV/c Σ: 6M e + : k µ + : 436k π + : 8845k 325 MeV/c Σ: 2M e + : 26k µ + : 69k π + : 93k 29 MeV/c Σ: 29M e + : 7k µ + : 25k π + : 2638k 255 MeV/c Σ: 35M e + : 68k µ + : 57k π + : 2788k 22 MeV/c Σ: 5M e + : 2k µ + : 434k π + : 99k 85 MeV/c Σ: 29M e + : 557k µ + : 246k π + : 967k 8/2

9 log(rate/ch) NaI(Tl) response to e +/- MeV/c e - e + log(rate/ch) 4 3 NaI(Tl) response to pions π energy (MeV) 2 3 energy (MeV) energy (MeV) MeV/c energy (MeV) 9/2

10 5 counter 4 response to 29 MeV/c pions pedestal region 2 pedestal region π ADC value reaching NaI(Tl) disappearing in counter 5 disappearing before counter 5 Pion reactions may result in total loss (pedestal peak) or very broad signal distribution Large difference between the two polarities /2

11 inefficiency inefficiency π detection inefficiencies vetoed by counters 4 and 5 no signal in counter 4 π + measurement full symbols: open symbols: π momentum (MeV/c) signal in 4 below 2 kev π momentum (MeV/c) π detection inefficiencies vetoed by counters 4/5 and WC2 no signal in counter signal in 4 below 2 kev inefficiency inefficiency no signal in 4 π + momentum (MeV/c) π detection inefficiencies vetoed by counters 4/5 and NaI(Tl) measurement measurement full symbols: open symbols: π signal in 4 below 2 kev π momentum (MeV/c) π detection inefficiencies vetoed by counters 4/5, NaI(Tl) and WC2 no signal in signal in 4 below 2 kev Pion detection inefficiency caused by interactions in WC2 and the wrapping of counter 4 (together 8 mg/cm 2 ) and by a 2 kev detection threshold on the signal in counter 4 Left: counter 5 is used as veto counter too Right: NaI(Tl) is used as veto counter too -5-6 π + measurement full symbols: open symbols: π π π + large symbols: measurement small symbols: simulation full symbols: open symbols: π π Bottom: events with more than one hit in the downstream wire chamber plane were rejected momentum (MeV/c) momentum (MeV/c) momentum (MeV/c) momentum (MeV/c) /2

12 Nature of inefficiencies observed in simulation We checked simulated events in which no energy was deposited in counter 4 or where this energy was below 75 kev These events had no energy in NaI(Tl) and only one hit per plane in WC2 π + inefficiency: Out of ten events: - there were six cases in which the pion was scattered into the backward hemisphere without reaching WC2 - there were four cases with a π in the final state without signal in counter 4 In the experiment the decay gammas would be detected in the barrel vetos or calorimeter So most of these events would not contribute to the inefficiency of a hermetic veto system π inefficiency: Out of ten events: - in seven cases the pion disappeared in the wrapping and there were neutrons and maybe a π in the final state - in three cases the pion reached the scintillator In one of them the pion was scattered backward 2/2

13 3 Light collection studies 3/2

14 single photo electrons: fit to exp(ax+bx 2 +cx 3 ) pedestal peak: fit to Gaussian 634 adc ped= adc ped= september 22: run adc2 ped= ADC channel ADC channel ADC channel adc3 ped= adc4 ped= adc5 ped= ADC channel ADC channel 5 2 ADC channel adc6 ped= adc7 ped= ADC channel ADC channel 4/2

15 photo electrons per 2 mm photo electrons per 2 mm photo electrons per 2 mm photo electrons per 2 mm photo electrons per 2 mm PM run 62: top of detector noodle readout, Tyvek wrapping PM 2 PM 3 PM 4 sum PM 2 PM y>5 mm: Kuraray y<5 mm: Bicron 8 4 y x PM 4 PM 3 KOPIO test, πm 9/22 new photo electron calibration 5/2

16 run 6 noodle readout, Tyvek wrapping light absorption by wls fibres photo electrons in 2 mm PM +3 photo electrons in 2 mm PM 2+4 photo electrons about 2% per fibre out of /2=8% geometrical y(mm) y(mm) y(mm) photo electrons in 2 mm sum y(mm) PM 2 PM y>5 mm: Kuraray y<5 mm: Bicron y PM x PM 3 KOPIO test, πm 9/22 new photo electron calibration 6/2

17 photo electrons sum photo electrons photo electrons PM3 PM4 photo electrons teflon black paper photo electrons PM PM2 run 46 direct readout, with/without teflon wrapping PM2 PM 8 4 y PM4 x PM3 KOPIO test, πm 9/22 new photo electron calibration 7/2

18 4 PM PM PM photo electrons PM photo electrons PM PM Windows 22 mm x 6,4 mm PM Housing Coupling with Optical Grease photo electrons PM photo electrons SUM , runs 24 to 26 4, 4 x 5 x 65 scintillator direct readout with 3 pms 3M wrapping optical grease typically 5 photo electrons as for 4 pms PSI test July 23 8/2

19 time wrt beam counter (ns) PM time wrt beam counter (ns) PM time wrt beam counter (ns) earliest (ns) PM time wrt beam counter (ns) earliest runs 24 to 26: 4 x 5 x 65 scintillator direct readout with 3 pms situated at x=5, y=(pm),2(pm2),39(pm4) 3M wrapping and optical grease steps of ns per color about ns for cm beam counter timing corrected for dependence on position and amplitude PSI test July 23 9/2

20 Photoelectron (pe) yields of various light collection schemes Scintillator: BC-42 by BICRON; photomultipliers: Burle 8362E (Ø22 mm) WLS fibers Direct readout Size Thickness Version Wrapping Nr of pe pe [cm 2 ] [mm] PMT s in mm Fiber Geometry 2x25 4x x64 Bicron a straight c Tyvek d 4 9 Kuraray b 3 returning c VM2 e 5 25 Width of windows Grease 2x25 64 mm no black paper 4 f 3 2 4x5 22 mm 3 64 yes Tyvek d g VM2 e 52 8 a BCF-92MC d DuPont trademark f mounted at opposite sides b Y-(2)MS e radiant mirror film produced by 3M g mounted at one side c read from both ends g g The detector modules will be 2 mm thick tiles of plastic scintillator with direct readout 2/2

21 4 Present design of central CPV 2/2