Kaon Identification at NA62. Institute of Physics Particle, Astroparticle, and Nuclear Physics groups Conference 2015

Transcription

1 Kaon Identification at NA62 Institute of Physics Particle, Astroparticle, and Nuclear Physics groups Conference 2015 Francis Newson April 2015

2 Kaon Identification at NA62 K πνν NA48 and NA62 K + π + νν NA62 Kaon tagging at NA62 Current status 2

3 K πνν : a theoretically clean environment FCNC loop processes: New physics: large ( Ο(10%) ) deviations from SM possible in many NP models SM predictions: BR(K +! + ) =( BR(K L! 0 ) =( [Brod, Gorbahn, Stamou, Phys. Rev. D 83, (2011)] [G. Buchalla, A.J. Buras, Nucl. Phys. B 412, 106 (1994)] ± 0.29) ± 0.06) error on input parameters pure theory errors Experimental status: BR(K +! + ) =( ) 10 [Phys. Rev. D 77, (2008), Phys. Rev. D 79, (2009)] 3

4 NA48 and NA62 NA62 SPS Previous experiments highlights included: NA48: ε /ε (KL and KS) 02 NA48/1: KS rare decays LHC CERN PS NA48/2 K ± CP violation NA62: Lepton universality (using NA48 apparatus) K + π + νν experiment Proposal Technical run Physics runs Pilot run 4

5 NA62 Experiment Goal: Measure BR(K + π + νν) to 10% precision Requirements: Signal: Ο(100) SM events K + decays, signal acceptance ~10% (since BR(SM) ~ 8x10-11 ) Backgrounds: >10 12 background rejection (<20% background) <10% relative precision background measurement Technique Kaon decay in flight (existing measurements used stopped kaons) Replacing/upgrading the NA48 apparatus 5

6 Analysis strategy PK Pν Pπ Pν Signal: Single charged π + track (positively identified) No other particles Background: - K + decay modes - beam gas and upstream interactions - accidental tracks in time with kaon tracks Background suppression Kinematics Ο( ) Charged particle ID Ο(10 7 ) γ detection Ο(10 8 ) Timing Ο(10 2 ) Analysis requirements Timing and spatial K-π matching Pπ < 35 GeV Fiducial decay region: 5-65m from beginning of the decay volume 6

7 NA62 Detector LAV! RICH! MUV! Large angle photon vetoes! OPAL lead glass! RICH µ/π ID! 1 atm Ne! µ veto Fe/scint! 4 m! KTAG! Differential Cerenkov for K + ID in beam! CHANTI! Charged veto! Beam tracking! Si pixels, 3 stations! Fiducial volume ~60m! 10 6 mbar! 5 MHz K + decays! γ veto! IRC!!! γ veto! SAC! GIGATRACKER! Dipole spectrometer! 4 straw-tracker stations! STRAW!! Forward γ veto! NA48 LKr! LKr! 0! 50! 100! 150! 200! 250 m! 7

8 Kaon tagging requirements Unseparated hadron beam: ~6% K +, 22% p +, 72% π + π + π + π + [scatter] looks like K + ν ν To keep this background below 10-4 we need some combination of: - vacuum ( need < 6 x 10-8 mbar if no kaon tagging ) - kaon tagging ( relaxes vacuum requirements by an order of magnitude ) 8

9 NA62 beam Primary protons from SPS with momentum 400GeV/c Beryllium target Unseparated hadron beam: ~6% K +, 22% p +, 72% π + Central momentum: 75 GeV/c, Δp/p: 1% angular dispersion: 0.07 mrad particle rate: 750 MHz 9

10 Kaon tagging requirements Handle average kaon flux of 50 MHz (in 750 MHz beam) >95% K + ID efficiency < 0.1% mistagging probability < 100ps timing resolution radiation hard Solution: Cherenkov Detector with Achromatic Ring Focus (CEDAR) with Kaon TAGging upgrade (KTAG) 10

11 CEDAR: principal of operation Cherenkov radiation emission angle depends on the particle velocity In a beam with fixed momentum, particle velocity is a function of mass alone Selecting Cherenkov at a fixed emission angle can be used to positively identify particles of a certain mass (kaons) In reality must account for - chromatic dispersion - kaon beam divergence - multiple Coulomb scattering cos = c n gas v particle 11

12 CEDAR PRESSURISED NITROGEN Used successfully since the 1980s Pressurised nitrogen at 1.71bar Focussing optics with chromatic correction 12

13 Kaon - pion separation Wavelength [nm] wavelength (nm) Radius and wavelength and diaphragm Cherenkov photon distribution at diaphragm kaons pions Radius [mm] 1.5mm MC simulation shows clear separation of kaons and pions radius (mm) re 10: < 1x10 Diaphragm -4 pion contamination illumination when requiring forcoincidence N 2,weightedwithquantum of 5 sectors iency. The left hand side distribution is due to kaons. kaon beam divergence is the dominant contribution to spread in radius rms divergence x focal length = 0.07mrad x 3.25 m = 0.23 mm 13 0

14 KTAG design mean photons per kaon 18 KTAG system spread light over 8 sectors of 48 PMTs, so the total rate per PMT is < 5MHz Light distribution achieved with a system of lenses and spherical mirrors Photon positions at the light guide Y [mm] Z (mm) Z [mm] Φ (mm) ure 7: Distribution of optical photons at the entrance plane of 0 14

15 CEDAR KTAG K+, π+, p+ Detector installed in

16 Photon detection KTAG requires (small) PMTs with: sub-nanosecond timing resolution single photo-detection with high quantum efficiency in the wavelength range nm Requirements are met by Hamamatsu PMTs 16mm R9880U-210 peak QE ~ 40% R7400U-03 peak QE ~ 20% 300ps timing resolution With ~18 PMT hits per kaon <100ps resolution on kaon 16

17 PMT installation 8 light guides hold 48 PMTs each, mounted in an aluminium light guide with mylar inserts to maximise collection efficiency PMT signal cables NINO PCB HV cables aluminium light guide mylar inserts 17

18 Front-end and readout system Custom printed circuit board for differential output 1 NINO ASIC per 8 PMTs for signal discrimination and time stretching 4 Time to Digital Converter (TDC) boards each with 4 HPTDC ASICs for analogue to digital conversion 4 custom TEL62 boards (based on TELL1 for LHCb) Radiation tests with 90MeV muons and 1-10MeV neutrons have shown good performance in conditions comparable with NA62 running 18

19 2012 Results Technical run: KTAG was commissioned with 4 out of the 8 sectors operational π + K + fraction of beam particles positively identified proton pressure (bar) 19

20 2014 results Pilot run: full detector installed and commissioned Analysis underway in preparation for data taking this year PMT occupancy all channels firing! 20

21 2014 results Kaons tagged by selecting π + π 0 with downstream detectors All candidates Efficiency Kaon Tagged Preliminary Number of sectors per candidate Number of sectors per candidate Efficiency when requiring 4 sectors is > 95% 21

22 2014 results Timing resolution Kaon timing 2014 data Preliminary Hits / 0.02 ns σ PM t ~280 ps N PM ~18 σ K t < 80 ps Reconstructed hit time candidate time [ns] 22

23 2014 single track data Single tracks reconstructed with straw chamber spectrometer 10 2 muon rejection at trigger level Angle between track and K (rad) K + π + π + π K + μ + ν K + π + π 0 Particle Momentum (GeV/c) Non K decay background analytical contours K + π 0 e + ν K + μ + ν K + π 0 μ + ν K + π + π 0 K + π + π + π K + π + π 0 π 0 KTAG used to distinguish between kaon and non-kaon events Angle between track and K (rad) require KTAG signal Particle Momentum (GeV/c) Angle between track and K (rad) require no KTAG signal Particle Momentum (GeV/c) 23

24 Summary NA62 requires kaon tagging at 50MHz with >95% efficiency and >99.9% purity KTAG, a CEDAR with upgraded light detection system, has been designed to meet these requirements Preliminary results from 2014 show that the detector is already performing as expected The time resolution has been measured as 80ps, surpassing the experimental requirement >95% efficiency has been achieved with 4-fold coincidences We look forward to data taking in July this year 24