KMI Topics
Development of the TOP counterfor the Belle II experiment
Kodai Matsuoka (KMI, Nagoya Univ.)

2The Belle II experiment• Elucidate a physics beyond the Standard Modelthrough precise measurement of B and t decays at
Super-KEKB.
SuperKEKB

Belle II detector• Luminosity: x40 of KEKB
 8 x 1035 cm–2s–1• Statistics: x50 of Belle 50 ab–1• Upgrade the Belle detector
 Belle II

Dampingring for e+e+ source

RF-gun3

The Belle II detectorp efficiency > 95%
K fake rate < 5%

p efficiency > 89%
K fake rate < 12%TOPACC

4Contents• Focus on the detector R&D–Design and concept of the TOP counter–Components of the TOP counter–Assembly of a prototype TOP counter–Test of the prototype with the positron
beam at LEPS

5

Design and conceptof the TOP counter

6TOP (Time Of Propagation) counter• A novel RICH (Ring Imaging Cherenkov) detector
K or pCompact
Nonmassive

Cherenkov photons
generated in the quartz bar
travel in the bar as they aretotally reflected on thequartz/air boundaries.

MirrorTOP

TOF (~1 m)
e+2 cmAir (n=1)pCKQuartz(n=1.47)@ 400 nme–1TOP  cos C n

TOP depends on C or .
 Measure (TOF + TOP) to identify K/p.

Air (n=1)

7Key of the TOP counter• Hit timing difference between 3 GeV/c K and p– DTOF ~ 50 ps/m
– DTOP ~ 75 ps/m
p (3 GeV/c)
K (3 GeV/c)

Quartz

PDF for K (3 GeV/c)

51015time (ns)MCP-PMTTo distinguish K/p, the ‘ring’ image
has to propagate undistorted along
the bar and measured with goodtiming resolution (~50 ps).

8PID with the TOP counter• Measure the hit timing of ~20 Cherenkov photons.
– In addition, background ~3 hits/50 ns• Calculate likelihood for K and p using the PDFs.
– Inputs for the PDF:
• Momentum and incident position of the track measured
by the inner detectors.
(3 GeV/c, –20 cm, 200 cm, π/2, π/2)
πK (PDF)

Datap (PDF)TOP counter is achallenging detector.

9Expected performance• From a MC simulation
54
32
1
–0.55

0.15

100
98
96
94
92
90
88
86
0.85
cos

K fake ratep (GeV/c)p (GeV/c)p efficiency

514121086424321–0.55

0.150.85cos

Benchmark channels
e–e+Efficiency

Fake rateB  pp~95%~6%B  rg~98%~3%

Need to verify this expected performance with a beam data.

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Example of the impact of the PID improvement• Separation of K and p is a critical issue for thestudy of B  r g.
5 ab–1B  rgB  rgB  K*gB  K*g

Belle (Phys. Rev. Lett. 101 (2008) 111801• 86% p efficiency• 8.3% K fake rate

Belle II LOI• 90% p efficiency• 3% K fake rate

11Components of the TOP counter12Quartz bar• The quality of Cherenkov ring image has to be
maintained after O(100) reflections on the quartzsurface.
Requirements (for the largest surfaces)
Length1250 ± 0.50 mm

Width

450 ± 0.15 mm

Thickness

20 ± 0.10 mm

Flatness< 6.3 mm

Perpendicularity< 20 arcsec

Parallelism< 4 arcsec

Roughness< 5 Å (RMS)

20 mm

Polished by companies to meet the stringent requirements.

13Quartz reflectance and transmittance• Requirement– Internal surface reflectance > 99.90%– Bulk transmittance > 98.5%/m(2 GeV/c e, 0 cm, 135 cm, π/2, π/2)(2 GeV/c e, 0 cm, 135 cm, π/2, π/2)

When roughness = 20 Å (RMS),
Transmittance = 98.5%/mreflectance ~ 99.82%
 10% loss after 7 m travel 16% (30%) loss after 100 (200) reflections
cf. 10% (18%) loss for 99.90% reflectance14

Quartz bar box• Made of aluminum honeycomb panels.
 Low mass
• Support the quartz with PEEK buttons.
 Enable the total reflection on the quartz surfaces.
Quartz bar on the honeycomb panel

PEEK button
Al honeycomb

AirQuartz
Al honeycombAir15Focusing mirror• The photon velocity in the quartz varies with the wavelength(chromatic dispersion)  worsen the time resolution.

Incident angle to the mirror varies with .
 Light of different  is focused on
different points on the focal plane.
 Correction of the chromatic error.

1n ( )  

Spherical
mirror(r=5 m)cos C 

16Wavelength cut filter• To reduce the chromatic dispersion further, cut
light below 340 nm with a filter in front of the PMT.

transmittance

IHU340 (ISUZU Glass)(2 GeV/c e, 0 cm, 135 cm, π/2, π/2)

10.8
0.6
0.40.20200

300

400500600

wavelength (nm)70017MCP-PMT (Micro Channel Plate PMT)
• Developed at Nagoya in collaboration withHAMAMATSU Photonics K.K. for the TOP counter.
MCP-PMT

PhotonMCPse–(Cross-section)

Micro channel
13°10 mm5.275 mm
Short distance to multiply
the photoelectron Small transit time spread

Use 32 PMTs per TOP module, 512 PMTs in total.

e–~1 kV / 400 mm

Square shape tomaximize thephoto-coverage

4 x 4 anodes

23 mm

Photocathode~3.5 kV

400 mmMicro Channel Plate (MCP)18

Specification of the MCP-PMT• Square shape, small dead region (active area: 69.4%)• NaKSbCs photocathode; QE ≥ 24% (28% on average) at 380 nm• Collection efficiency: 50~55% (≈ MCP aperture ratio)• 2 x 106 gain at ~3.4 kV  Capable of detecting single photon.
• Transit Time Spread (TTS): ~40 psec
• Dark noise rate < 100 kHz• Work in 1.5 T

s = 40.0 ps

19Mass production of the MCP-PMTs• The mass production started in 2011.
– The performance of every PMT is checked at Nagoya.
Setup to check the gain and TTSSetup to measure the QE

MCP-PMT on themoving stage

Pico second
pulse laser

Xe lamp
MCP-PMT on themoving stage• More than 300 PMTs in hand.

• All the 550 PMTs will be delivered by March 2014.

20Angle/polarization dependence of QE• QE depends on the photon incident angle andpolarization.
– TOP counter detects photons at various angles.
– Cherenkov light is linearly polarized.
(2 GeV/c e, 0 cm, 135 cm, π/2, π/2)s-wavep-wave

QE for p/s-wave at any angle can be calculated from Fresnel equations
with n, k and d of the photocathode measured by ellipsometry.

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Readout electronics• Based on a waveform-sampling ASIC (IRS**)
being developed at Hawaii Univ.
– Chip intrinsic time resolution is <25 ps.
(Cross-section)

Front-end readout with 8 PMTs

22

Assembly ofthe prototype TOP counter

23

Alignment of the quartz bars
• Need to glue the two quartz bars w/ D < 0.2 mrad and Dx,y < 100 mm.
Adjustment of the relative position

Plunger

Laser sensor

Polyacetal heads Dx(y)Adjustable to O(10 mm)
Measurement precision: ±5 mmPolyacetal
QuartzVinyl chloride

Micrometerhead
Aluminum

AluminumRail

Adjustment of the relative angle
Flat mirror

D

Autocollimator

Micrometer
head

Adjustable to O(0.01 mrad)
Measurement precision: ±0.01 mrad

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Gluing of the quartz bars
• Two quartz bars, a short bar with the mirror and a prism wereglued together with an optical adhesive, which was then cured
by ultraviolet light.
The first full-size prototype

Put the glue

Wait ~hour for the glue togo down~100 mm gap

Succeeded gluing all the quartz
components

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Mounting the MCP-PMTs and electronics• Because the IRS readout was premature, we prepared abackup readout based on the CFD (constant fraction discriminator)
as well as the IRS.
CFD front-end with 2 PMTs

16 outputs of each PMT weremerged into 4 at the PMT socket
to reduce the number of channels(little impact on the PID performance)

Full 32 PMTs and the CFD mounted on the quartz

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June 2013

Test of the prototype
with the positron beamat LEPS27

LEPS (Laser Electron Photon beamline at SPring-8)• Evaluate the performance of the TOP counter withthe 2 GeV/c positron beam at LEPS

8 GeV ee’Back Compton
scatteringg (< 2.4 GeV)
0.3-1 MHze–Dipole magnet(0.7 T)TOP counter

Trigger (40×40 mm2)×2
Trigger (5×5 mm2)×2~140TOPTOF

SciFi tracker x/y×2

DC2 DC3 （f2 mm fiber）
Floore+Timing
counter

TOF counters

Laser (351 nm)

Pb target(1.5 mm)

Drift chambers (DC)• Coincidence of the 4 triggers
 Beam fluctuation ~1.5 mrad
• Trigger rate ~10 Hz• e+ momentum measured by
the LEPS spectrometer• EM shower cut by requiring
NTOF = 1

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29

Data set•Got the same data set both with the IRS andCFD readouts

1. cos ~ 0, x = 0 cm

Only this data taken with
the CFD is shown today

2. cos ~ 0.4, x = 0 cm
x

3. cos ~ 0.4, x = 20 cm
54
32
1–0.55



14
12
10
8
64 Because the distance between the beam
2 incident position and PMTs is longer forcos ~ 0 in this beam test than in Belle II,0.85
the K fake rate should be worse a little.
cos

PMTp (GeV/c)

K fake rate in Belle II~3%0.15

30Timing resolution
Time difference between the beamtiming estimated from the accelerator
RF clock and the timing counter

PMTLaser
Timing resolution measured by the lasers < 65 ps
Laser fluctuation
is dominants = 37.2 ps
Laser lighthardly reach

RF resolution = 37.2/√2 = 26.3 ps(TDC resolution of 25 ps is dominant) Use the RF clock as the beam timing

The timing resolutionof the system wasproved to be good.

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TOP distribution for the beam
Histogram of the photon detection timing relative to the beam timing(sum of 16,750 events)
1 reflection
on the sidech88 (center of the quartz)

2 reflections
QuartzReflected at the mirror

MCP-PMTDirectphotons

Cherenkov“ring”
image
Beam data(cos ~ 0)World-first complete Cherenkov image
measured by the TOP counter!

Cherenkov“ring”image
PDF(expectation)

Good agreement with the data

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Incident particle•• Incident position and angle

Quartz bar• Dimensions• Index()• Reflectance• Transmittance

Mirrored Detection plane

PDF calculation
MCP-PMTs• QE(ch, )• Index and thickness of thephotocathode• CE(ch)• TTS(ch)• Charge sharing rate (~10%)• Cross-talk rate (~3%)• Light reflection on thephotocathode and edge
• PMT alignment

Cherenkov light• C()• Number of photons produced• Polarization• Phase shift at reflections

Readout electronics• Jitter• Efficiency• Cross-talk

There are many things which we have to understand.

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Comparison between the data and PDF
Achieved this level of our understanding
ch88 (center of the quartz)

DataPDF

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 reconstruction• Calculate likelihood with PDF() event-by-eventand get  at the maximum likelihood
 at the maximum likelihood
Preliminary

 resolution as a function of NhitPreliminary

Due to large
backgroundmean = 1.001s = 0.0043

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Pseudo PID test
• Calculate likelihood for 2 GeV/c e+ (=1) and for
3 GeV/c K (=0.987) event-by-event
Likelihood ratio
Preliminary

K-likeLikelihood ratio as a function of Nhit
Preliminary

e-likeFake rate = 6.4%
(comparable to a MC result)

Preliminary but promising result

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Summary and plan
• The TOP counter is a novel RICH detector from
Nagoya developed for the Belle II experiment.
• After tremendous studies for more than a decade,the prototype TOP counter with almost the same
design as the final one was successfully fabricated.
• The prototype was tested with the 2 GeV/c e+ beamat LEPS.
– This is the first data for the full size TOP counter.
– Deepened our understanding of the TOP counter.
– Will prove the principle of TOP with this data.
• Fabricate 16 TOP counters next year and installthem on March 2015.

• Belle II physics run starts in autumn of 2016.