Kaon Identification at NA62 Institute of Physics | Particle, Astroparticle, and Nuclear Physics groups Conference 2015
Francis Newson April 2015 Kaon Identification at NA62
• K→πνν
• NA48 and NA62
• K+→π+νν NA62
• Kaon tagging at NA62
• Current status
2 The � → ��� decays: a theoretical clean environment K → πνν : a theoretically clean environment FCNC loop FCNCprocesses: loop s d processes:coupling and highest CKM suppression New physics: large ( �(10%) ) deviations from SM possible in many NP models
Very clean theoreticallySM predictions: + + +0.80 11 Short distanceBR (contributionK ⇡ ⌫⌫¯)=(7.81 0.71 0.29) 10 error on input ‘pure theory’ ! ± ⇥ parameters errors No hadronic uncertainties0 +0.40 11 BR(KL ⇡ ⌫⌫¯)=(2.43 0.37 0.06) 10 ! ± ⇥
SM predictions[Brod, [Brod Gorbahn,, Gorbahn Stamou,, Stamou Phys. Rev., Phys. D 83, Rev.034030 D (2011)] 83, 034030 (2011)] [G. Buchalla, A.J. Buras, Nucl. Phys. B 412, 106 (1994)] [G. Buchalla, A.J. Buras, Nucl. Phys. B 412, 106 (1994)]
0 -11 ° BR(KL pExperimentalnn) = (2.43 ± 0.39 status:± 0.06 )×10 1 error: uncertainty from input parameters + + + + +1.-1511 10 BR(K p nnBR) =( K(7.81 ±⇡0.75⌫⌫¯ ±)=(10.29. )73×101.05) 10 2° error: pure theoretical uncertainty ! ⇥ [Phys. Rev. D 77, 052003 (2008), Phys. Rev. D 79, 092004 (2009)]
3 10/03/2015 Giuseppe Ruggiero 6 NA48 and NA62
Previous experiments NA62 highlights included: SPS ’97-’01 NA48: ε’/ε (KL and KS)
’02 NA48/1: KS rare decays
’03-’04 NA48/2 K± CP violation LHC ’07-’08 NA62: Lepton universality PS CERN (using NA48 apparatus) K+ → π+νν experiment Proposal Technical run Physics runs
2005 2010 2015 2020 Pilot run 4 NA62 Experiment Goal: Measure BR(K+→π+νν) to 10% precision
Requirements: Signal: �(100) SM events ⇒ 1013 K+ decays, signal acceptance ~10% (since BR(SM) ~ 8x10-11) Backgrounds: >1012 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 Analysis strategy Signal: Pπ Single charged π+ track PK (positively identified) Pν No other particles Pν Background: + - K decay modes - beam gas and - accidental tracks in upstream interactions time with kaon tracks
Background suppression Analysis requirements Kinematics �(104 - 105) Timing and spatial K-π matching 7 Charged particle ID �(10 ) Pπ < 35 GeV 8 γ detection �(10 ) Fiducial decay region: 5-65m from Timing �(102) beginning of the decay volume 6 NA62The NA62 Detector experiment at the SPS!
LAV! RICH! MUV! Large angle photon vetoes! RICH µ/π ID! µ veto OPAL lead glass! 1 atm Ne! Fe/scint!
CHANTI! KTAG! Charged Differential Cerenkov veto! + for K ID in beam! γ veto ! Fiducial volume ~60m! ! ! 10−6 mbar! IRC!
4 m
! 5 MHz K+ decays! γ veto ! Beam tracking! SAC! Si pixels, 3 stations!
GIGATRACKER!
!
Dipole spectrometer! Forward γ veto! 4 straw-tracker stations! NA48 LKr! STRAW! LKr! 0! 50! 100! 150! 200! 250 m! Searches for rare and forbidden decays with NA62 – M. Moulson (Frascati) – DPF 2013 – Santa Cruz – 16 August 2013! 11! 7 Kaon tagging requirements
• Unseparated hadron beam: ~6% K+, 22% p+, 72% π+ π+ π+ looks like K+ π+ [scatter] 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 NA62 beam
• Primary protons from SPS with momentum 400GeV/c
• Beryllium target
• Unseparated hadron beam: ~6%The K+ Beam, 22% p+, 72% π+ Proton beam line before target • Central momentum:commissioned 75 GeV/c,up to 20% nominal Δp/p: intensity 1% Secondary beam line fully commissioned.The Beam Proton beam line before target commissioned up to 20% nominal intensity Secondary beam line fully commissioned. angular dispersion: 0.07 mrad
particle rate: 750 MHz
9
10/03/2015 10/03/2015 GiuseppeGiuseppe Ruggiero Ruggiero 18 18 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 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 c certain mass (kaons) cos ✓ = ngasvparticle • In reality must account for - chromatic dispersion - kaon beam divergence - multiple Coulomb scattering 11 CEDAR
PRESSURISED NITROGEN
• Used successfully since the 1980s
• Pressurised nitrogen at 1.71bar
• Focussing optics with chromatic correction
12 126 introduce up to several % ine ciency on some PMs, de- 1800 700 127 pending on their actual gain, a new voltage divider with 1600 1400 600 128 di↵erential output was designed, with the aim of feeding Wavelength [nm] 1200 500 129 the PM signal directly into the NINO input, without any 1000
800 130 active circuit in between. This approach has several in- 400 600
300 131 trinsic advantages: avoiding the introduction of any fur- 400
200 132 ther noise source, partially compensating for the lack of a 200 0 97 98 99 100 101 102 103 104 105 133 preamplifier with a gain factor of 2 because of the di↵er- Radius [mm] ⇡ 134 ential output, and reducing sensitivity to common mode 135 noise. Figure 11: Diaphragm illumination for H2,weightedwithquantum e ciency. The left hand side distribution is due to kaons. 136 Data collected during the test beam allowed to refine 137 the Monte Carlo simulation to the point of allowing to esti- conf R[mm]
143 3.1. Monte Carlo simulation Table 1: Monte Carlo estimated performance for several configura- 144 A full simulation of the CEDAR has been integrated tions: N2 and H2 options with di↵erent spherical mirror radii R and di↵erent PM models. The average number of hits
158 Both N2 and H2 are considered as Cherenkov radiator,176 The structure of the KTAG is arranged in octants, each 159 the former giving better optical performance (the CEDAR177 functionally replacing one of the original PMs, all within 160 was designed for N2) and the latter introducing less mate-178 a 1.6 m 1.6 m 40 cm enclosure. The enclosure provides ⇥ ⇥ 161 rial on the NA62 beam line; the KTAG has been designed179 structural support, light tightness and thermal stability, 162 to be compatible with both solutions. While with N2 the180 complying with safety rules in case of use H2 as Cherenkov 163 distributions of light at the diaphragm for kaons and pions181 radiator. Each octant consists in a spherical convex mirror 164 are clearly separated (fig. 10), this is not true for H2 (fig.182 and a photodetection module (lightbox, LB). Cherenkov 165 11), but it is still possible to choose a diaphragm aperture183 light exits the CEDAR’s quartz window almost parallel to 4 166 that fulfills the rejection requirement of 10 pion contam-184 the beam, but in the opposite direction. It is reflected, 167 ination, sacrificing a fraction ( 40%) of the photoelectrons185 after about 15 cm, on the spherical mirror in a direction ⇡ 168 from kaons, without losing e ciency. Table 1 summarizes186 roughly orthogonal to the beam (outwards) and it is col- Kaon - pion separation 187 lected, after about 22 cm, by Winston’s cones in the corre- 188 CherenkovRadius photonand wavelength distribution and diaphragm at diaphragm sponding LB. Each LB is a mechanical enclosure housing 1800 189 700 an array of 64 Winston’s cones, arranged in a honeycomb kaons pions 1600
1400 190 pattern on a spherical surface (fig. 12), up to 64 PMs, the 600 Wavelength [nm] 1200 191 front-end electronics, a heat sink and a HV distribution 500 1000 192 board. 800 400 600 193 Spherical mirrors are installed on fixed supports on a
wavelength (nm) 300 400 194 cylinder surrounding the beam pipe, while LBs are at- 200 200 195 0 tached to an octagonal structure housing the water based 97 98 99 100 101 102 103 104 105 Radius [mm] 1.5mm radius (mm) 196 cooling system. It is necessary to remove the heat pro- • MC simulation shows clear separation of kaons and pions 197 duced by the electronics to avoid it to be transferred to the -4 Figure• 10:< 1x10 Diaphragm pion contamination illumination when requiring for coincidenceN2,weightedwithquantum of 5 sectors e ciency. The left hand side distribution is due to kaons. 198 radiator gas inside the CEDAR, thus changing the refrac- • 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 4 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 100 100 104 80 80 1400 Y [mm] 60 60 1200 40 103 40 20 1000 0 20 102 800 Hit Time - Trigger [ns] -20 0
-40 -20 600 10 -60 -40
-80 Z (mm) 400 -60 -100 1 0 10 20 30 40 50 60 70 80 90 100 200 Time Width [ns] -80
-100 0 -100 -80 -60 -40 -20 0 20 40 60 80 100 Figure 5: Time performance of the radiation hard preamplifier: there Z [mm] is much more uncorrelated noise with respect to fig. 6, and the overall Φ (mm) 14 response suggests a distortion of the signal. Figure 7: Distribution of optical photons at the entrance plane of the Winston’s cones. The array of PMs is shown; the dashed ones are not installed. 91 formance (fig. 5), while the other, already characterized 92 for the NA62 RICH detector, was used as a comparison 116 light distribution for comparision with MonteCarlo simu- 93 reference and to test new PMs in a second stage. 117 lation (figs. 9). 94 The read-out was based on HPTDC ASIC [11] and 95 TELL1 [12]; this was part of a development stage of the
96 final common NA62 read-out system, composed of custom118 3. Finalizing the design 97 HPTDC based daugther boards and TEL62 [13]. 119 FLUKA simulation showed that radiation hardness is a
98 2.3. New photomultipliers 120 requirement for the front-end electronics (up to 0.4 Gy/year), 121 but the radiation hard preamplifier did not perform as ex- 99 As already mentioned, the original light collection and 122 pected during the test beam. Figs. 5 and 6 show the time 100 detection system is not suitable for the intensity of NA62 2 123 and time over threshold distributions for the two pream- 101 beam; which will produce few MHz/mm photons at the 124 plifiers. The radiation hard preamplifier proved to be too 102 CEDAR exit windows. The basic components of the new 125 noisyTHIRD ANGLE for PROJECTION this application. Although it was foreseen it could 103 design are new PMs, namely Hamamatsu metal package 8±0.1 3 CONES 24 POLISH INTERNAL CONICAL EQUI SPACED 3 SURFACES TO 4nm 0.2 104 photomultipliers R7400-U03, and an array of Winston’s R1 A 105 cones to maximize the sensitive area (fig. 7), on which the 20.05 REF
11.75° 11.75° 106 light is spread by a convex spherical mirror. The array was
77 73 M74 X 1 107 designed to be arranged on a portion of a spherical surface, 37°
22 108 to minimize the correlation between the incident angle of
R100 109 an optical photon and its impinging position on the array. PMT RADIUS
2 HOLES M4 X 10 DEEP 18 8 110 A small prototype of such an array (3 PMs and their cor- A 103 REF SECTION A-A
16 111 responding cones) was built to replace one of the original 60.77 REF 112 PMs of the CEDAR, with the possibility to vary its dis-
THE UNIVERSITY OF LIVERPOOL 113 tance from the quartz exit window and to rotate around its DEPARTMENT OF PHYSICS PROJECT NA62/CEDAR
METRIC SCREW THREADS DRAWING PRACTICE AND TOLERANCE UNLESS OTHERWISE SPECIFIED:- WELDING SYMBOLS TO BS 499 TITLE TO BS3643 CLASS 6H/6g INTERPRETATION TO BS308 ALL DIMENSIONS IN MILLIMETRES PMT HOLDER - TEST BEAM TOLERANCES MATERIAL DRAWN UNLESS OTHERWISE SPECIFIED ALUM P.SUTCLIFFE 114 SURFACE TEXTURE VALUES IN MICROMETRES Figure 8: Schematic drawing of the 3 PMs prototype used in the axis (fig. 8). It allowed to test the new PMs, the construc- SPEC DATE ARRGT. DRAWING DRAWING NUMBER 19-Jul-11 P.SUTCLIFFE FIRST ISSUE MACHINED 0.3 DIMS.± HE30 19-Jul-11 A SURFACE TEXTURE 3.2 TO BE NP53-01-27 TREATMENT CHECKED NP53-01-29 ISSUE DATE NAME DESCRIPTION MACHINE ALL OVER OR WHERE MARKED ANGULAR 0.5 ± ° NONE PS
FINISH APPROVED 115 tion technique and to have reference points concerning the 2011 testMODIFICATIONS beam. REMOVE BURRS AND BREAK SHARP EDGES DO NOT SCALE NATURAL PS SHEET 1 OF 1 SHEETS SCALE:- 2:1 A2
30
100 120 y [mm] 25 4 80 10 20 100 60 15 40 103 10 80 20
0 5 102 60
Hit Time - Trigger [ns] -20 0
-40 -5 40 10 -60 -10 -80 20 -15 -100 1 0 10 20 30 40 50 60 70 80 90 100 -20 0 Time Width [ns] -30 -20 -10 0 10 20 30 x [mm]
Figure 6: Time performance of the RICH preamplifier: secondary peaks along the vertical axis (> 0) are due to signal reflections in- Figure 9: Simulation of the illumination of the 3 PMs prototype, in troduced by the attenuators. aspecificconfiguration,withconesprojectionoverlaid.
3 KTAG CEDAR
K+, π+, p+
Detector installed in 2014 15 collection systems are under optimization with a Geant4-based study including most of the modifications concerning the CEDAR detector. The photon detector technology was tested at CERN: the dark count, the response in charge and the time resolution for CEDAR PMTs are discussed in Sec. 5.5. The number of PMTs has been decided with a Monte-Carlo simulation of the CEDAR photon detector, explained in Sec. 5.6. Cross checks and validations will follow from the Geant4 simulation of the whole apparatus. New electronics is needed to cope with a few MHz rate on single device (i.e. per readout channel). The proposed readout system design, introduced in Sec. 5.4.2, was tested in a test beam described in the next chapter.
5.4 The CEDAR PMT
The photodetector choice must accommodate all CEDAR working requirements. Photomultiplier Tubes, or PMTs, are electron tube devices which convert light (pho- tons) into a measurable electric current (electrons)5. PMTs have a high resistance to the damage by radiation and a low ( few Hz) dark-count rate. In addition, PMTs with quartz input windows are sensitive to UV and near-UV wavelengths.5.4. The re- Photo-multiplier Tubes
5The reader can refer to [94] for details on the main characteristics of a PMT and its operation mode. Photon detection
• KTAG requires (small) PMTs with:
• sub-nanosecond timing resolution
Figure 5.6.: Left: Simulation• singleFigure 5.5: Simulation ofphoto-detection path of Cherenkov taken photons by Chrenkov (left-panel) with with the photons rayhigh tracing from fromquantum the CEDAR efficiency quartz in the the CEDAR quartz windows (blue), to the external mirrors (red) and to the new PMT windowswavelength (blue),planes (green). to Mechanical the new designrange PMT of the newplanes 250-500nm CEDAR (green), photon detector via (right-panel) the spherical mirrors (red). Right: Thewith mechanical the main structure framework positioned after the forquartz KTAG. windows, surrounding the nose and housing external mirrors, light collection cones and PMTs. PMT socket •side (=looking inwards) HV mapping 2014 Requirements are met124 by Hamamatsu PMTs The same HV mapping 16mm scheme is used for all octants
The numbers indicated on the HV patch panel and light guide R9880U-210 R7400U-03 sketches indicate the cone IDs peak QE ~ 40% peak QE ~ 20%
(but the green numbers on the patch panel sketch refer to the HV socket IDs) • (a) Arrangement of300ps PMTs in a singletiming light guide.resolution(b) The two PMTs: R9880U-210 on the left 0 22 32 41 51 52 61 72 82 27 38 48 58 59 68 78 87 15 Figure 5.7.: PMTs• used in KTAG. The 16 R7400U-03 PMTs are coloured red. The 32 R9880U- 16 23 24 33 34 35 42 43 44 15 45 46 47 36With 37 25 26~18 31 PMT hits per kaon → <100ps resolution on kaon 210 PMTs are coloured yellow. 16
32 53 54 62 63 64 73 74 75 76 77 65 66 67 55 56 57 47 reflecting off a spherical mirror, travelling15 out radially. The spherical mirror spreads the light out over a ‘light-guide’, a piece of machined aluminium, indented with Mylar coated ‘light-cones’ which guide the light towards PMTs at their narrower ends. Figure 5.6 shows the path taken by Cherenkov light. The light-guides are built to house up to 64 PMTs but simulation shows that KTAG can meet the required specifications with 48 PMTs per octant, making 384 channels in total. With this set-up the rate per PMT is 5 MHz. ⇠
5.4. Photo-multiplier Tubes
The quantities used to describe PMT properties are defined in appendix A. KTAG uses two kinds of PMTs with properties suitable for single photon counting in a high rate environment, the Hamamatsu R7400U-03 and R9880U-210 modules. The former are reused from the
43 PMT installation
• 8 light guides hold 48 PMTs each, mounted in an aluminium light guide with mylar inserts to maximise collection efficiency
PMT signal NINO PCB cables
aluminium HV cables light guide mylar inserts
17 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 2012 Results
• Technical run: KTAG was commissioned with 4 out of the 8 sectors operational
+ π+ K
fraction of beam proton particles positively identified
pressure (bar) 19 Kaon ID and timing: KTAG
CEDAR optics (radiator N2) Cerenkov light split in 8 spots TDC readout (48 x 8 PMs) 2014 results< 100 ps time resolution > 95% K ID efficiency (> 99.9% purity) • Pilot run: fullRate detector at full installed intensity and 50 commissioned MHz • Analysis underwayCommissioned in preparation in 2014 for data taking this year
PM occupancyPMT screenshot occupancy from 2014 data KTAG K/p/p separation π+ K+ 2012 data
proton Entries / Triggers Entries all channels firing! 20 Pressure [bar] 10/03/2015 Giuseppe Ruggiero 19 2014 results Kaon ID Kaons tagged by selecting a �+�0 decay topology in the detectors downstream • Kaons tagged by selectingKaon π+πID0 with downstream detectors Kaons tagged by selecting a �+�0 decay topology in the detectors2014 downstream data All candidates Kaon EfficiencyEfficiency 2014 data All candidates Kaon Efficiency
Kaon Tagged Candidates / CandidatesSector
Kaon Tagged Preliminary Candidates / CandidatesSector Number ofN -sectorsfold coincidence per candidate Preliminary Sectors • 10/03/2015 Giuseppe RuggieroEfficiencyN-fold whencoincidence requiring20 ≥ 4 sectors is > 95% Number of sectors per candidateSectors 21 10/03/2015 Giuseppe Ruggiero 20 2014 results
• Timing resolution Kaon timing 2014 data
Preliminary
��� � ~280 �� ��� ~18
Hits / 0.02 ns 0.02/ Hits �� � < 80 ��
Reconstructed hit time – candidate time [ns]
22
10/03/2015 Giuseppe Ruggiero 21 First Look at 2014 Data Quality 2014 single trackEvents withdata only 1 track in the spectrometer reconstructed (40 ns time window) 102 muon rejection at trigger level. analytical contours + + + − � → � � � �+ → ���+� Single tracks �+ → �+� �+ → ���+� reconstructed with �+ → �+� straw chamber �+ → �+�� �+ → �+�+�− spectrometer �+ → �+�0 �+ → �+����
102 muon rejection at trigger level First Look at 2014 Data Quality
Angle between track and K (rad)and K track between Angle Apply KTAG for K ID Non K decay background Particle Momentum (GeV/c) K ID from KTAG in time with the track No K ID from KTAG 10/03/2015 Giuseppe Ruggiero 50
require require KTAG no KTAG KTAG used to signal signal distinguish between kaon and non-kaon events Angle betweentrack and K (rad) Angle between track and K (rad) and K between track Angle Particle Momentum (GeV/c) Particle Momentum (GeV/c) 23
10/03/2015 Giuseppe Ruggiero 51 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