Status of the TORCH time-of-flight project
Neville Harnew, Srishti Bhasin, Thomas Blake, Nicholas Brook, Tom Conneely, David Cussans, Maarten van Dijk, Roger Forty, Christoph Frei, Emmy Gabriel, Rui Gao, Timothy Gershon, Thierry Gys, Tom Hadavizadeh, Thomas Hancock, Michel Kreps, James Milnes, Didier Piedigrossi, Jonas Rademacker
SStatus of the TORCH time-of-flight project
N. Harnew a, ∗ , S. Bhasin b,c , T. Blake f , N.H. Brook b , T. Conneely g ,D. Cussans c , M. van Dijk d , R. Forty d , C. Frei d , E.P.M. Gabriel e , R. Gao a ,T.J. Gershon f , T. Gys d , T. Hadavizadeh a , T.H. Hancock a , M. Kreps f ,J. Milnes g , D. Piedigrossi d , J. Rademacker c a Denys Wilkinson Laboratory, University of Oxford, Keble Road, Oxford OX1 3RH, UnitedKingdom b University of Bath, Claverton Down, Bath BA2 7AY, United Kingdom. c H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL,United Kingdom d European Organisation for Nuclear Research (CERN), CH-1211 Geneva 23, Switzerland e School of Physics and Astronomy, University of Edinburgh, James Clerk MaxwellBuilding, Edinburgh EH9 3FD, United Kingdom f Department of Physics, University of Warwick, Coventry, CV4 7AL, United Kingdom g Photek Ltd., 26 Castleham Road, St Leonards on Sea, East Sussex, TN38 9NS, UnitedKingdom
Abstract
TORCH is a time-of-flight detector, designed to provide charged π/K parti-cle identification up to a momentum of 10 GeV/ c for a 10 m flight path. Toachieve this level of performance, a time resolution of 15 ps per incident particleis required. TORCH uses a plane of quartz of 1 cm thickness as a source ofCherenkov photons, which are then focussed onto square Micro-Channel PlatePhotomultipliers (MCP-PMTs) of active area 53 ×
53 mm , segmented into 8 ×
128 pixels equivalent. A small-scale TORCH demonstrator with a customisedMCP-PMT and associated readout electronics has been successfully operated ina 5 GeV/ c mixed pion/proton beam at the CERN PS facility. Preliminary re-sults indicate that a single-photon resolution better than 100 ps can be achieved.The expected performance of a full-scale TORCH detector for the Upgrade IIof the LHCb experiment is also discussed. Keywords:
Time-of-flight, particle identification, Cherenkov radiation,micro-channel plate photomultipliers, LHCb upgrade
PACS: ∗ Corresponding author
Email address:
[email protected] (N. Harnew)
Preprint submitted to Elsevier December 27, 2018 a r X i v : . [ phy s i c s . i n s - d e t ] D ec . Introduction The TORCH (Time Of internally Reflected CHerenkov light) detector willmeasure the time-of-flight (ToF) of charged particles over large areas, with theaim to provide Particle IDentification (PID) of pions, kaons and protons up to10 GeV/ c momentum and beyond [1]. The difference in ToF between pions andkaons over a ∼
10 m flight path at 10 GeV/ c is 35 ps, hence to achieve positiveidentification of kaons, TORCH aims for a time resolution of ∼ in front of the current RICH 2detector [2]. The proposed experimental arrangement is shown in Fig. 1.TORCH combines timing measurements with DIRC-style reconstruction, atechnique pioneered by the BaBar DIRC [3] and Belle II TOP [4] collaborations.The production of Cherenkov light is prompt, hence TORCH uses planes of1 cm thick quartz as a source of fast signal, which also facilitates a modulardesign. Cherenkov photons travel to the periphery of the quartz plates by totalinternal reflection where they are reflected by a cylindrical mirror surface of aquartz block. This focuses the photons onto a plane of pixellated Micro-ChannelPlate Photomultipliers (MCP-PMTs) where their positions and arrival timesare measured. The expectation is that typically 30 photons will be detected percharged track, hence the required ToF resolution dictates the timing of singlephotons to a precision of around 70 ps.A schematic of the TORCH geometry in the longitudinal and transverseplanes is shown in Fig. 2, showing the focussing block and the LHCb modulararrangement. For every photon hit in the MCP-PMTs, the Cherenkov angle θ c Figure 1: A schematic of the LHCb experiment, showing TORCH located directly upstreamof the RICH 2 detector. igure 2: Schematics of a TORCH module showing possible reflection paths: (a) the focussingblock and MCP-PMT plane, (b) a single LHCb module. and the photon path length of propogation through the quartz L is measured.From knowledge of the dispersion relation within the quartz, a correction forchromatic dispersion is then made to the photon time of propogation. Fromsimulation, a ∼
2. MCP-PMT development
In order to achieve the desired 1 mrad angular precision, TORCH requires aphoton detector with a fine granularity in the focussing direction and a coarsegranularity in the non-focussing direction. The pixel structure of MCP-PMTscan in principle be adjusted to the resolution required provided the charge foot-print is small enough. An anode granulatity of 128 × ×
53 mm active area on a 60 mm pitch. MCP-PMTs are well known for fasttiming of single photon signals, ∼
30 ps, however tube lifetime has been an issuein the past.A major focus of the TORCH project has been on the development of theMCP-PMTs, which has been conducted in collaboration with an industrial part-ner, Photek (UK). A three-phase R&D programme was defined. Phase 1 sawthe development of a single-channel MCP-PMT with extended lifetime, accom-plished with an atomic-layer deposition (ALD) coating of the MCPs [5], andwhere an excellent timing resolution of better than ∼
35 ps was also achieved [6].The extended lifetime is required for the harsh environment of the LHC, where3 igure 3: The quantum efficiency of a Photek Phase 1 MCP-PMT as a function of wavelengthand collected integrated charge, as measured on the tube axis. integrated anode charges of at least 5 Ccm − are expected. Lifetime measure-ments have since been conducted over a period of 2.5 years using single-photonillumination from a blue LED, and are now reaching an integrated charge of6.16 Ccm − . Figure 3 demonstrates the results. Some loss of quantum efficiencyis seen above 3 Ccm − , with a factor 2 loss at 5 Ccm − . A gain drop is alsomeasured, but which can be recovered by an increase of MCP high voltage [7].The MCP-PMT lifetime is close to the required performance, and it is expectedthat the Phase 3 tubes will improve on this.The Phase 2 MCP-PMTs are circular in construction with a 40 mm diameterand a square 26.5 × active area containing 4 ×
32 pixels, a quarter sizeof the final geometry. Beam tests on these tubes were successfully completed in2015/16 and are reported elsewhere [8].The final TORCH devices, the Phase 3 tubes, have high active area ( > ×
53 mm activearea with 64 ×
64 pixels. The effective resolution of 128 × × × . Beam tests with a small-scale TORCH demonstrator Several test-beam campaigns have been conducted between 2015–2018 at theCERN PS T9 beamline with a 5 GeV/ c mixed pion/proton beam. The small-scale TORCH demonstrator [8] consists of a 12 × × quartz radiatorplate with a matching focusing block, both manufactured by Schott, Germany.The radiator plate is mounted in an almost vertical position, tilted backwardsby 5 ◦ with respect to the horizontal incidence of the beam.We report here preliminary results from the November 2017 campaign, wherethe demonstrator is read out with a single Phase 3 MCP-PMT in the 64 × ∼
11 m flight path provide a time reference.The pattern of measured MCP-PMT hits for 5 GeV/ c pions is shown inFig.4 (a). Here clustering has been applied over simultaneous MCP-PMT col-umn hits to obtain the centroid position of each photon. The beam impingesapproximately 14 cm below the plate centre-line and close to the plate side, a po-sition which has been chosen to give a cleanly resolved pattern. The Cherenkovcones which are internally reflected in the quartz radiator result in hyperbola-likepatterns at the MCP-PMT plane; reflections off module sides result in a foldingof this pattern. Chromatic dispersion spreads the lines into bands. Since thequartz is read out by only a single MCP-PMT, the full pattern is only sampled,which accounts for the observed discontinuities.For the timing measurement, a simultaneous correction is made for time-walkand integral non-linearities of the NINO/HPTDC electronics using a data-drivenmethod [8]. For each single 4-wide pixel row, the measured MCP time-stampfor each cluster is plotted relative to the downstream borosilicate station (T2)versus the measured 64-wide column position. An example data distribution is Figure 4: The patterns of hits measured in the TORCH demonstrator with 5 GeV/ c pions.(a) 64 × × igure 5: (a) The time-of-arrival of single Cherenkov photons from a 5 GeV/ c pion beam,relative to the T2 beam time-reference station, as a function of detected 64-wide column pixelnumber. The overlaid lines represent the simulated patterns for light reflected only off thefront and back faces of the radiator plate (purple), light undergoing one (red), two (orange)and three (yellow) reflections off the side faces. The top left distributions correspond tomultiple reflections from the bottom horizontal face. (b) The residuals between observed andsimulated Cherenkov photon arrival times for pixel row 4 (November 2017 data). shown in Fig. 5 (a) showing good agreement when compared to simulation. Thedistribution of residuals between the measured and simulated times of arrivalis shown in Fig. 5 (b). Core distributions have resolutions (sigmas) of approx-imately 100 - 125 ps (which is photon energy and MCP-PMT row dependent).The tails are due to imperfect calibration and backscattering from the MCP topsurface. The timing resolution of the timing reference is ∼
50 ps and, subtractedin quadrature, gives ∼
85 - 115 ps time resolution of the TORCH demonstrator,approaching the target resolution of 70 ps per photon. Future improvements arepossible such as incorporating charge to width calibrations of the front-end elec-tronics and reducing the current limitation imposed by the 100 ps time binningof the HPTDC.
4. Development of half-length TORCH prototype
A prototype of a half-length TORCH module, 125 × × (length,width and thickness), is currently under construction. The module will beequipped with ten MCP-PMTs ( ∼ × c pion beam inJune 2018. The pattern of measured MCP-PMT hits is shown in Fig. 4 (b). Re-sults are currently being analysed, and calibrations and timing measurementsare in progress. The full-scale module is planned for test-beam running in Oc-tober/November 2018. 6 . TORCH for the LHCb Upgrade II The RICH system currently provides PID for the LHCb experiment [13],where discrimination of pions, kaons and protons is essential for CP violationmeasurements, exotic spectroscopy and particle tagging. However, LHCb has nopositive kaon or proton identification below ∼
10 GeV/ c . Therefore the proposalis to install TORCH immediately upstream of the RICH 2 detector, where itwould be located ∼ × , divided into 18 modules, each66 cm wide and 2.5 m high with 11 MCP-PMTs per module.Studies are underway to evaluate the performance of TORCH in the LHCbexperiment. A luminosity of 2 × cm − s − has been simulated ( × π/K/p hypotheses are compared to the MCP-PMT photon spatial hits andarrival times, and log-likelihoods are then computed.Figure 6 shows the efficiency of TORCH to positively identify kaons and pro-tons as a function of momentum and the probability of misidentification. Goodseparation between π /K/p in the 2 −
10 GeV/ c range and beyond is observed.Studies have also started on key physics channels and tagging performance,and these will form the basis of a Technical Proposal to construct a full-scaleTORCH detector for the start-up of LHC Run 4, with installation in the LongShutdown 3 (LS3) in 2024. Figure 6: The efficiency of TORCH in LHCb to positively identify (a) kaons and (b) protonsas a function of momentum and the probability that they are misidentied. The curves arefor two different delta-log-likelihood cuts and for a luminosity of 2 × cm − s − . Thesimulated sample is for heavy flavour decays in pp collisions, including pile-up. . Summary The performance of a small-scale TORCH demonstrator in a 5 GeV/ c mixedpion/proton beam has been reported. A customised 64 × × Acknowledgments
The support is acknowledged of the Science and Technology Research Coun-cil, UK, and of the European Research Council through an FP7 Advanced Grant(ERC-2011-AdG 299175-TORCH).
References [1] M. Charles and R. Forty: TORCH: Time of flight identification withCherenkov radiation, Nucl. Instrum. Meth. A639 (2011) 173 – 176.[2] The LHCb Collaboration, Expression of Interest for a Phase-II LHCb Up-grade, CERN-LHCC-2017-003 (2017).Physics case for an LHCb Upgrade II, LHCB-PUB-2018-009, CERN-LHCC-2018-027 (2018).[3] I. Adam et al., The DIRC Particle Identification System for the BABARExperiment. Nucl. Instrum. Meth. A538 (2005) 281 – 357.[4] T. Abe et al., Belle II Technical Design Report’. arXiv:1011.0352, (2010).U. Tamponi (on behalf of the BelleII Collaboration), The TOP counter ofBelle II: status and first results. These proceedings.[5] T. M. Conneely, J. S. Milnes, J. Howorth (Photek Ltd), Characterisationand lifetime measurements of ALD coated microchannel plates in a sealedphotomultiplier tube, Nucl. Instrum. Meth. A732 (2013) 388 – 391.[6] T. Gys et al., Performance and lifetime of micro-channel plate tubes forthe TORCH detector, Nucl. Instrum. Meth. A766 (2014) 171 – 172.[7] T. Gys et al., The TORCH detector R&D: Status and perspectives, Nucl.Instrum. Meth. A876 (2017) 156 – 159.[8] N. Brook et al., Testbeam studies of a TORCH prototype detector, Nucl.Instrum. Meth. A908 (2018) 256 – 268.89] L. Castillo Garcia et al.et al.