TORCH: a novel detector
combining TOF and RICH
Roger Forty (CERN)
on behalf of the LHCb RICH group
TORCH (Time Of internally Reflected CHerenkov light)
is a possible solution for low-momentum particle ID
under study for the upgrade of the LHCb experiment
Closely related concept to the TOP of Belle II [Toru Iijima] —
a new generation of PID devices profiting from fast photodetectors
TORCH is at an earlier stage, but is aiming for higher resolution
1.
2.
3.
The LHCb upgrade
TORCH concept
TORCH R&D
Int. Workshop on Probing Strangeness in Hard Processes, Frascati, 18–21 October 2010
1. The LHCb Upgrade
• LHCb is one of the four major experiments at the LHC, dedicated to the
search for new physics in CP violation and rare decays of heavy flavours
• It is a forward spectrometer (10–300 mrad) operating in pp collider mode
Particle identification provided by two RICH detectors [Clara Matteuzzi]
Currently with three radiators: silica aerogel, C4F10 and CF4 gas
RICH-1
Roger Forty
RICH-2
The TORCH detector concept
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LHC luminosity
• LHCb was commissioned ready for the LHC startup in 2008
After some teething trouble the LHC is now performing excellently
• Peak luminosity 1032 cm-2s-1 achieved
Integrated L ~ 20 pb-1 recorded so far
Target for next year: 1 fb-1
• The nominal luminosity for LHCb is
only few  1032 cm-2s-1, to maximize
events with single pp interactions
• First phase of LHCb will run for
next ~ five years, integrating 5–10 fb-1
• Plan to then upgrade the experiment
as doubling time would get too long
Aim to increase luminosity by  10
(will already be available from machine)
Roger Forty
The TORCH detector concept
Exponential increase
 1.5 per week!
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Current performance
• Detector is performing superbly: clean b-hadron signals accumulating rapidly
B → hh signal
Monte Carlo
Applying particle ID cuts with the
RICH system, can select a clean
signal for Bs → K+K(first mass peak for this mode)
→ Excellent K-p separation at high p
B → hh data 3 pb-1
(without PID cut)
Roger Forty
The TORCH detector concept
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Low-momentum PID
• Flavour tagging (distinguishing B from B) is one of the primary
requirements for low-momentum particle ID in LHCb (2–10 GeV)
currently provided by aerogel
• Can now be studied in data using B0–B0 oscillations
• Monte Carlo studies of high-luminosity running indicate that aerogel will
be less effective, due to its low photon yield (< 10 p.e./saturated track)
and the high occupancy environment
1st Phase
Roger Forty
The TORCH detector concept
Upgrade
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Upgrade plan
• Need to prepare for upgrade even though experiment has only just begun to
accumulate data, due to long lead time (R&D + construction + installation)
• Aim for installation of upgrade in 2016, during a planned LHC shutdown
• Main focus is on trigger, which must be upgraded to handle higher luminosity
Current bottleneck is hardware level that reduces 40 MHz bunch crossing rate
to 1 MHz for readout into HLT
→ read out complete experiment at
40 MHz into the CPU farm,
fully software trigger
• RICH system will be kept for PID
with photodetectors replaced
Propose to replace the aerogel
with time-of-flight based detector
• First muon station will be removed
→ space available for new device
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The TORCH detector concept
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2. TORCH concept
• Want positive identification of kaons in region below their threshold for
producing light in the C4F10 gas of RICH-1, i.e. p < 10 GeV
• Difficult to achieve with a RICH system (aerogel was the best choice of
radiator for this region), so possibility of time-of-flight investigated
DTOF (p-K) = 35ps at 10 GeV
over a distance of ~ 10 m
→ aim for 15 ps resolution per track
• Difficult to achieve with scintillator
or other traditional TOF
• Cherenkov light production is prompt
→ use quartz as source of fast signal
• Large-area fast photodetectors under
development by the Picosecond timing
group (http://psec.uchicago.edu/) but unlikely
to be available in time for our application (and we would need ~ 30 m2!)
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The TORCH detector concept
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DIRC-like detector
• Consider instead a first (naïve) design
based on quartz bars, à la DIRC of BaBar:
Cherenkov photons produced in the quartz
transported to the end of the bar by total
internal reflection, where their arrival
would be timed
• 1 cm thickness of quartz is enough to
produce ~ 50 detected photons/track
(assuming a reasonable quantum
efficiency of the photon detector)
3m
Photon arrival time
25 ns
→ ~ 70 ps resolution required
per detected photon
• However, spread of arrival times is much
greater than this, due to different paths
taken by photons in the bar
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The TORCH detector concept
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Planar detector
• Need to measure angles of photons, so their path length can be reconstructed:
~ 1 mrad precision required on the angles in both transverse planes
• This would be prohibitive for a set of quartz bars, but borrow nice idea from
the end-cap DIRC of PANDA [Matthias Hoek]: use a plane of quartz
→ coarse segmentation (~ 1cm) is sufficient for the transverse direction (qx)
Roger Forty
The TORCH detector concept
~ 1 cm
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Focusing system
• To measure the angle in the longitudinal direction (qz) we use a focusing
block, to convert angle of the photon into position on the photodetector
• Event display illustrated for photons from 3 different tracks hitting plane
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The TORCH detector concept
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Photon detection
• Micro-channel plate (MCP) photodetectors are currently the best choice
for fast timing of single photons
~10 mm pores in the MCP
photon
Faceplate
Photocathode
photoelectron
Dual MCP
DV ~ 200V
DV ~ 2000V
Gain ~ 106
DV ~ 200V
Anode
• Anode pad structure can in principle be
adjusted according to need
• Test result from K. Inami et al
[RICH2010] s(t) = 34.2 ± 0.4 ps
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The TORCH detector concept
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Modular design
• For the application in LHCb, transverse dimension of plane to be
instrumented is ~ 5  6 m2 (at z = 10 m)
• Unrealistic to cover with a single quartz plate  evolve to modular layout:
• 18 identical modules
each 250  66  1 cm3
 ~ 300 litres of quartz
in total (less than Babar)
• Reflective lower edge
 photon detectors only
needed on upper edge
18  11 = 198 units
Each with 1024 pads
 200k channels total
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The TORCH detector concept
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Effect of edges
• Reflection off the faces of plate is
not a problem, as the photon angle
in that direction (qz) is measured via
the focusing system
• In the other coordinate (x) position
is measured rather than angle
→ reflection off the sides of the
plate gives ambiguities in the
reconstructed photon path
• Only keep those solutions that give
a physical Cherenkov angle
→ only ~ 2 ambiguities on average
• Effect of the remaining ambiguities
is simply to add a ~ flat background
to reconstructed time distribution
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The TORCH detector concept
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TORCH module
• Focusing block in
quartz or plastic
(should match
refractive index)
• Cylindrical mirror
• Linear array of
photon detectors
• Dimensions have
been chosen to
correspond to
the Planacon MCP
from Photonis
• Plate thickness (~ 1 cm)
to be optimized once p.e. yield known
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The TORCH detector concept
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Photon detector
• Planacon XP85022 comes close
to matching photodetector
requirements for TORCH
Currently available
with 32  32 anode pads
• We require finer granularity in one
direction than other, so assume an
8  128 anode pad layout
In discussion with manufacturers
to secure this development
• Lifetime of MCP may also be an
issue for our application (depends
on gain, and hence electronics)
Following recent development of
longer-lived MCPs [Hamamatsu]
with great interest
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The TORCH detector concept
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Resolution
• Smearing of photon propagation time
due to photodetector granularity ~ 40ps
• Assuming an intrinsic arrival time
measurement resolution per p.e. of 50 ps
the total resolution per detected p.e. is
40  50  70 ps, as required
• For particle ID, need to correct for the
strong chromatic dispersion of quartz
Achieved by measuring the photon angles,
and knowing path of track through quartz
 determine Cherenkov emission angle
cos qC = 1/ bnphase
t – t0 = L ngroup /c
Effectively the wavelength of the photon
is determined by this construction
Roger Forty
The TORCH detector concept
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Performance
• Different time-of-propagation for photons from p or K, but this effect adds
to difference in time-of-flight  increases the sensitivity
• To determine the time-of-flight, we also need a start time t0
This is achieved using the other tracks in the event, from the primary vertex
Most of them are pions, so the reconstruction logic is reversed, and the start
time is determined from their average assuming they are all p
(outliers from other particle types are removed)
• Full algorithm has been studied,
including pattern recognition,
using a simple simulation of the
TORCH detector, interfaced
to the full simulation of LHCb
• Excellent particle ID performance
achieved, up to 10 GeV as required
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The TORCH detector concept
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3. TORCH R&D
•
R&D has been launched on the following aspects:
1.
2.
3.
4.
Photodetector
Performance of existing MCP devices; Development of suitable
anode pad structure; Lifetime; Cost
Readout electronics
Speed; 40 MHz rate; Gain; Noise; Cross-talk
Quartz radiator
Polishing; Required quality for total internal reflection; Cost
Simulation
Detailed simulation of TORCH; tagging performance in upgrade
•
Two 64-channel Planacon MCPs procured from Photonis
Characterisation with laser light source, starting with single channel
electronics, then multichannel readout
•
First results expected in time for Letter of Intent at the end of this year
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The TORCH detector concept
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Lab setup at CERN
Dark box
Single channel
electronics
Laser light
source
Planacon
MCP
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The TORCH detector concept
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Readout electronics
• Under development by Oxford Univ. group
• Starting with 8-channel NINO chips and
HPTDC, developed for the ALICE TOF
• Test-beam studies foreseen for next year
Electronics board support for tests
Ext Clk
Clk Buf
Trigger
HPTDC
JTAG
FPGA
JTAG
HPTDC
MCP Connection
NINO Hits
JTAG
Shared data bus
Spartan 3AN
HPTDC
NINO Hits
SPI Flash
Gigabit Ethernet
PHY
Control bus
SRAM
/ SDRAM
Optional
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The TORCH detector concept
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Conclusions
• TORCH is a novel detector concept
proposed for the upgrade of LHCb
It is intended to complement the
high-momentum particle ID provided
by the RICH system
Isolated tracks
• Based on time-of-flight, determined from
Cherenkov light produced in quartz plate
using photon detectors at the periphery
• Assuming a per-photon resolution of 70 ps
excellent K-p separation achieved up to 10 GeV
• R&D is in progress, starting with the photodetector and readout electronics
Impact of the TORCH on tagging performance in the upgraded experiment
is under study with detailed simulation
• Letter of Intent for the LHCb upgrade will be submitted at end of this year
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The TORCH detector concept
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