Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay $\mu\rightarrow eee$. In order to reduce background by up to 16 orders of magnitude, decay vertex position, decay time and particle momenta have to be measured precisely. A pixel tracker based on 50mm thin high voltage monolithic active pixel sensors (HV-MAPS) in a magnetic field will deliver precise vertex and momentum information. Test beam results like an excellent efficiency of > 99:5% and a time resolution of better than 16.6 ns obtained with the MuPix HV-MAPS chip developed for the Mu3e pixel tracker are presented

Augustin, H.

Berger, N.

Bravar, S.

Corrodi, S.

Damyanova, A.

Förster, F.

Gredig, R.

Herkert, A.

Huang, Q.

Huth, L.

Kiehn, M.

Kozlinskiy, A.

Maldaner, S.

Perić, I.

Philipp, R.

Robmann, P.

Schöning, A.

Shrestha, S.

Vom Bruch, D.

Weber, T.

Wiedner, D.

English

Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay μ → eee. In order to reduce background by up to 16 orders of magnitude, decay vertex position, decay time and particle momenta have to be measured precisely. A pixel tracker based on 50 μm thin high voltage monolithic active pixel sensors (HV-MAPS) in a magnetic field will deliver precise vertex and momentum information. Test beam results like an excellent efficiency of >99.5% and a time resolution of better than 16.6 ns obtained with the MuPix HV-MAPS chip developed for the Mu3e pixel tracker are presented.ISSN:1748-022

Augustin, Heiko

Berger, Niklaus

Bravar, Sandro

Corrodi, Simon

Damyanova, Antoaneta

Gredig, Roman

Herkert, Adrian

Huang, Qinhua

Huth, Lennart

Kiehn, Moritz

Kozlinskiy, Alexandr

Maldaner, Stephan

Perić, Ivan

Philipp, Raphael

Robmann, Peter

Schöning, André

Shrestha, Shruti

vom Bruch, Dorothea

Wiedner, Dirk

Repository for Publications and Research Data

The MuPix high voltage monolithic active pixel sensor for the Mu3e experiment

KITopen

2015 JINST 10 C03044PUBLISHED BY IOP PUBLISHING FOR SISSA MEDIALABRECEIVED: November 1, 2014ACCEPTED: January 5, 2015PUBLISHED: March 25, 2015TOPICAL WORKSHOP ON ELECTRONICS FOR PARTICLE PHYSICS 2014,22–26 SEPTEMBER 2014,AIX EN PROVENCE, FRANCEThe MuPix high voltage monolithic active pixelsensor for the Mu3e experimentH. Augustin,a N. Berger,b S. Bravar,c S. Corrodi,d A. Damyanova,c F. Förster,aR. Gredig,e A. Herkert,a Q. Huang,a L. Huth,a M. Kiehn,a A. Kozlinskiy,aS. Maldaner, f I. Perić,g R. Philipp,a P. Robmann,e A. Schöning,a S. Shrestha,aD. vom Bruch,a T. Weber f and D. Wiednera,h,1aPhysikalisches Institut der Universität Heidelberg,INF 226, 69120 Heidelberg, GermanybInstitut für Kernphysik, Johannes Gutenberg-Universität Mainz,55128 Mainz, GermanycDépartement de Physique Nucléaire et Corpusculaire à l’Université de Genève,24, Quai Ernest-Ansermet, CH-1211 Genève 4, SwitzerlanddInstitut für Teilchenphysik ETH Zürich,HPK E 25, Schafmattstrasse 20, 8093 Zürich, SwitzerlandePhysik-Institut Universität Zürich,Winterthurerstrasse 190, 8057 Zürich, Switzerlandf Helmholtz Instiute Mainz,55099 Mainz, GermanygInstitut für Prozessdatenverarbeitung und Elektronik, KIT,Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, GermanyhCERN,Route de Meyrin 385, CH-1211 Genève 23, SwitzerlandE-mail: dwiedner@cern.chABSTRACT: Mu3e is a novel experiment searching for charged lepton flavor violation in the raredecay µ → eee. In order to reduce background by up to 16 orders of magnitude, decay vertexposition, decay time and particle momenta have to be measured precisely. A pixel tracker basedon 50 µm thin high voltage monolithic active pixel sensors (HV-MAPS) in a magnetic field willdeliver precise vertex and momentum information. Test beam results like an excellent efficiencyof > 99.5% and a time resolution of better than 16.6 ns obtained with the MuPix HV-MAPS chipdeveloped for the Mu3e pixel tracker are presented.KEYWORDS: Electronic detector readout concepts (solid-state); Particle tracking detectors (Solid-state detectors)1Corresponding author.c© CERN 2015, published under the terms of the Creative Commons Attribution 3.0License by IOP Publishing Ltd and Sissa Medialab srl. Any further distribution of thiswork must maintain attribution to the author(s) and the published article’s title, journal citation and DOI.doi:10.1088/1748-0221/10/03/C030442015 JINST 10 C03044Contents1 The Mu3e experiment 12 Detector concept 13 High Voltage Monolithic Active Pixel Sensors 24 The MuPix HV-MAPS chip 35 Test beams 36 MuPix performance 47 Summary and outlook 61 The Mu3e experimentThe Mu3e experiment [1, 2] aims at measuring the rare charged lepton flavor violating decayµ+→ e+e−e+, which is highly suppressed in the standard model to a branching ratio (BR) < 10−54.This means that an observation of the decay µ+ → e+e−e+ is a clear indication for new physicsbeyond the standard model. The goal of the Mu3e experiment is to search for µ→ eee events witha branching fraction sensitivity of 10−16 while the most successful previous experiment establishedan exclusion limit of BR < 10−12 (90% CL) [3]. In order to achieve this precision in a few years ofrunning, the decay of muons at rest has to be studied at rates of 109 per second or more. Backgroundcomes from two main sources, which are combinatorial background and the radiative muon decaywith internal conversion µ → eeeνν . In order to discriminate between combinatorial backgroundand the µ → eee signal, precise vertex and timing information is required. Background from thedecay µ→ eeeνν can be suppressed by measuring the momentum sum of the decay products, sincepart of the momentum is carried away by the neutrinos. As the maximum momentum of positronsand electrons from the muon decay at rest is 53MeV/c, the vertex position and the momentummeasurement precision is dominated by multiple Coulomb scattering [4].2 Detector conceptThe Mu3e detector is designed as a cylindrical tube around a hollow double cone shaped muonstopping target, see figure 1. Around the target there are two layers of pixel vertex detectors, whichhelp to measure the vertex positions [5]. Two more pixel tracker layers are further outside andallow for momentum measurement in the 1T magnetic field. The outer two layers of the pixeldetector are extended both upstream and downstream of the target. All pixel sensors have a pixelsize of 80× 80µm2 and are arranged in sensor ladders. The full tracking detector has about 275– 1 –2015 JINST 10 C03044TargetInner pixel layersScintillating bresOuter pixel layersRecurl pixel layersScintillator tilesμ BeamFigure 1. Mu3e detector with signal event: the muon beam hits a fixed target in the center and a stoppedpositive muon decays into two positrons (red) and one electron (blue). The vertex is determined by two innerpixel tracker layers, which are surrounded by a fiber hodoscope for time and two further pixel tracker layersfor momentum measurement. In the forward and backward region the momentum and time of the re-curlingelectrons and positrons are more precisely determined with two more tracking stations and scintillating tiledetectors.million pixels. Since the pixel chips are thinned to 50µm, one layer of the tracking detector has only0.1% of a radiation length [6]. The thin pixelated tracking detectors allow for precise measurementof the vertex position and momenta of the charged particles, with a vertex resolution better than200µm [7, 8]. The momentum resolution is enhanced by measuring the charged particle trackswith the help of the outer two double layers of the pixel detector before and after curling in themagnetic field. In case of an exact half turn between exiting and entering the two outer doublelayers multiple Coulomb scattering effects cancel to first order, leading to a momentum resolutionof well below 0.5MeV/c. In addition to the pixel detector, two timing systems help to rejectcombinatorial background. The first timing detector is a fiber tracker which is positioned justinside the third layer of the central tracking station. With three layers of 250µm thick fibers, it canmeasure the time of particle tracks with an accuracy of better than 1ns [9, 10]. The second timingsystem is based on scintillating plastic tiles of 8.5×7.5×5mm3 size [11–13]. This tile detector isplaced inside the up- and downstream extensions of the pixel detector. Charged particles traversingthe tile detector generate in the order of 1000 photons, which are used for precise time measurementof better than 100ps accuracy.The Mu3e detector has a trigger-less 100GB/s readout to an event filter farm [14–18]. Fastonline track finding and reconstruction on the graphical process units (GPUs) of the event filterfarm allows data reduction to around 100MB/s for online storage and analysis.3 High Voltage Monolithic Active Pixel SensorsThe High Voltage Monolithic Active Pixel Sensors (HV-MAPS) [19–21] combine the advantagesof fast hybrid pixel detectors and thin monolithic active pixel sensors (MAPS). The HV-CMOStechnology in which the HV-MAPS are fabricated allow to integrate analogue and digital readoutelectronics in the same die as the silicon sensor. Figure 2 shows four pixel cells of a HV-MAPS– 2 –2015 JINST 10 C03044Figure 2. Working principle of a high voltage monolithic active pixel sensor (HV-MAPS).sensor. A deep N-well on a P-substrate accommodates both P- and N-type transistors. Applyinga reverse bias voltage of 60V results in a depleted region of around 10µm thickness. The chargeproduced by passing ionization in this active layer is collected via drift in less than 1ns. Since thefirst amplification stage is on top of the active region inside the pixel, the input capacitance and withit the noise can be kept low. The HV-MAPS chips can be thinned to 50µm without compromisingthe performance and thus make ultra low material budget tracking detectors possible.4 The MuPix HV-MAPS chipThe MuPix chips are high voltage monolithic active pixel sensors for the Mu3e pixelatedtracker [22, 23]. While the single pixel size is 80×80µm2, the entire sensor chip measures around1× 2cm2 for the inner pixel layers and 2× 2cm2 for the outer pixel layers. To keep multipleCoulomb scattering low, the sensors are thinned to 50µm. Analogue and digital readout electronicsare integrated on the MuPix chips, see figure 3. Every hit is converted on-chip into a pixel addressand an 8-bit time stamp. This zero suppressed data is serialized and output via 1 to 4 LVDS linksat rates of 0.8 to 1.25GB/s each.The chip prototype MuPix4 has 32×40 pixels of 80×92µm2 size. Inside the pixel cells thereis a single stage amplifier. MuPix4 has a digital zero suppressed readout electronic supplying pixeladdress and time stamp on parallel outputs, which runs stably at hit rates in excess of 1MHz. Thereadout state machine of MuPix4 is running on an external FPGA. A second amplification stageis implemented from MuPix6 prototype on, in order to enlarge the efficiency plateau. The latestversion of the MuPix chip, MuPix7, with the complete system on a chip (SoC), including on-chipreadout control and a fast data serializer has been submitted recently.5 Test beamsTest beam campaigns carried out to characterize the MuPix chip prototypes were performed at a170GeV π beam at CERN SPS, a 5GeV electron test beam at DESY and a 250MeV π beam atPSI [24]. The test setup used for the DESY test beam is based on the MuPix4 prototype, togetherwith a FPGA based readout card directly plugged onto the MuPix4 test board, see figure 4 left side.The MuPix chip was characterized in the device under test position inside the Aconite beam tele-scope, using three upstream and three downstream layers of MIMOSA26 pixel sensors as referenceplanes. The MIMOSA26 consists of 576 by 1152 pixels with a pitch of 18.5µm thinned to 50µm.The resulting pointing resolution makes sub-pixel details of the MuPix prototypes accessible.– 3 –2015 JINST 10 C03044Figure 3. MuPix4 HV-MAPS prototype, the pixel matrix is marked in green, the digital part is markedin blue.Figure 4. Test beam setups for the MuPix chip. Left side: MuPix4 test setup used in the DESY test beamOctober 2013. Right side: MuPix-telescope consisting of 4 planes equipped with MuPix6 prototypes.As a next step a four layer beam telescope purely based on the MuPix chip prototypes has beendeveloped and successfully tested [25], see figure 4 right side.6 MuPix performanceThe signal to noise ratio has been studied for various temperatures and high voltage settings inthe laboratory. The signal for this study was injected with the help of a pulse generator, the pulseheight was calibrated with a 55Fe source. A threshold scan for the MuPix4 chip with these pulseswas carried out at a high-voltage of −70V and a temperature of 70 ◦C, see figure 5 left side. Byfitting an s-curve to this scan, taking the 50% point and subtracting the baseline voltage of 0.780V,the voltage corresponding to the 55Fe signal can be determined. A second threshold scan without– 4 –2015 JINST 10 C030440.78 0.80 0.82 0.84 0.86 0.88 0.90020406080100Ratio [%]Threshold [V]Temperature: 70°CHV: -70VBL: 0.780Vσ: 0.002Vxc: 0.843VSNR: 31.500.77 0.78 0.79 0.80 0.81020406080100 Temperature: 70°CHV: -70Vxc: 0.780Vσ: 0.002VRatio [%]Threshold [V]Figure 5. Signal to noise ratio of the MuPix4 prototype. Left side: threshold scan with injected pulses, signalis defined by the 50% point of the s-curve. Right side: threshold scan without injection, noise is defined bythe 50% point of the s-curve.Figure 6. Performance of the MuPix4 prototype for 5 GeV electron tracks. Left side: detection efficiency.Right side: pixel cell track residuals in units of the pixel pitch.injection was fitted with an s-curve and the 50% point minus the baseline is used to determine thenoise, see figure 5 right side. The signal to noise ratio from this measurement was 31.5.Efficiency, spatial resolution and time resolution have been investigated with the help of the5GeV electron beam at DESY and the Aconite beam telescope as a reference tracking system.After careful alignment to sub-micron precision with the help of the EUDET telescope software, thedetection efficiency for electron tracks has been determined. Figure 6 left side shows the efficiencyof the MuPix4 prototype at a depletion voltage of −70V. The average efficiency of the MuPix4chip is 99.672 (−0.068 +0.060)%. For the same data, the spatial resolution of the MuPix4 pixel,which are 80× 92µm2 in size, has been determined, see figure 6 right side. The resolution isdominated by the pixel size as expected. The pixel time stamp resolution was studied in the DESYelectron test beam by taking the difference between trigger time stamp and MuPix4 pixel timestamp, see figure 7, right side. The 8-bit MuPix4 time stamp is generated with the help of a Graycounter running at 100MHz, the trigger time stamp stems from the Trigger Logic Unit (TLU) ofthe Aconite beam telescope. The distribution of the time difference between the pixel time stampsof the MuPix4 chip and the TLU time stamps has a width of σ = 16.6ns, setting an upper limit to– 5 –2015 JINST 10 C03044 -400 -200 0 40050010001500200025003000200Difference between trigger and timestamp [ns]σ = 16.6 nsHits per 10 ns bin Timestamp frequency 100 MHzFigure 7. Time difference between MuPix4 time stamps and the Trigger Logic Unit (TLU) for 5 GeVelectron tracks averaged over all pixels.the actual single pixel time stamp resolution. While this time resolution measurement is based onan average over time stamps of all pixels of the MuPix4 chip, the resolution for a single pixel isexpected to be better. For both efficiency and time resolution further improvements are expectedby applying per pixel threshold tuning.7 Summary and outlookThe Mu3e experiment is aimed at measuring the branching ratio of the charged lepton flavor vio-lating decay µ → eee with a sensitivity of BR < 10−16. In order to suppress background, precisetiming, good vertex and momentum resolution are required. The detector is based on two tim-ing systems and an ultra light pixelated silicon tracker around a muon stopping target in a 1Tsolenoidal field. In total the Mu3e detector has 275 million pixels of 80× 80µm2 size. The pixelsensors for Mu3e, MuPix, are implemented as high-voltage monolithic active pixel sensors (HV-MAPS), which combine fast charge collection via drift, in-pixel analogue electronics and on-chipdigital readout electronics with a total thickness of 50µm. The MuPix4 prototype has been exten-sively tested in the laboratory and at test beam campaigns. A signal to noise ratio at −70V and70 ◦C of 31.5 has been measured in the laboratory. Test beam studies with the 5GeV electron beamat DESY have resulted in an average pixel efficiency of 99.672 (−0.068 +0.060)%, a hit resolutiongiven by the cell size of 80×92µm2 and an upper limit of the pixel time resolution of σ = 16.6ns.A telescope based on four planes of MuPix chips has been developed and successfully testedand a first MuPix chip with the complete system on a chip has been submitted. The production ofa large 1×2cm2 sized MuPix chip is planned for next year.AcknowledgmentsN. Berger, Q. Huang, A. Kozlinskiy, S. Shrestha and D. vom Bruch would like to thank theDeutsche Forschungsgemeinschaft for supporting them and the Mu3e project through an EmmyNoether grant. M. Kiehn acknowledges support by the IMPRS-PTFS. Our thanks go to the LHCb– 6 –2015 JINST 10 C03044test beam co-ordination and the Timepix experts for making our first test beams at SPS possible.We would like to thank DESY for the valuable beam time and the DESY test beam co-ordinationteam for their tireless effort. We thank PSI for extensive and valuable beam time and especially theteam of the piM1 area for permanently improving already excellent conditions.References[1] A. Blondel et al., Research proposal for an experiment to search for the decay µ → eee (2012),available at: http://www.psi.ch/mu3e/DocumentsEN/ResearchProposal.pdf.[2] MU3E collaboration, N. Berger, The Mu3e experiment,Nucl. Phys. Proc. Suppl. B 248-250 (2014) 35.[3] SINDRUM collaboration, U. Bellgardt et al., Search for the decay µ+→ e+e+e−,Nucl. Phys. B 299 (1988) 1.[4] N. Berger et al., Multiple Coulomb scattering in thin silicon, 2014 JINST 9 P07007.[5] N. Berger et al., A tracker for the Mu3e experiment based on high-voltage monolithic active pixelsensors, Nucl. Instrum. Meth. A 732 (2013) 61 [arXiv:1309.7896].[6] N. Berger et al., The ultra-lightweight support structure and gaseous helium cooling for the Mu3esilicon pixel tracker, 2014 JINST 9 C08023.[7] M. Kiehn, Track fitting with broken lines for the Mu3e experiment, Diploma Thesis,Heidelberg University (2012).[8] S. Schenk, A vertex fit for low momentum particles in a solenoidal magnetic field with multiplescattering, Bachelor Thesis, Heidelberg University (2013).[9] A. Damyanova, Development of a scintillating fibre tracker/time-of-flight detector with SiPM readoutfor the Mu3e experiment at PSI, Master Thesis, Geneva University (2013).[10] M. Baumgartner, Comparison of measured and simulated light yield in plastic scintillators,Bachelor Thesis, Zurich University (2013).[11] W. Shen, Development of high performance readout ASICs for silicon photomultipliers (SiPMs),Ph.D. Thesis, Heidelberg University (2012).[12] T. Hartwig, Messung der Zeitauflösung eines Szintillator-SiPM-Systems für den Kacheldetektor desMu3e-Experiments, Bachelor Thesis, Heidelberg University (2013).[13] C. Licciulli, Präzise Zeitmessung für das Mu3e-Experiment, Master Thesis, Heidelberg University(2013).[14] S. Bachmann et al., The proposed trigger-less TBit/s readout for the Mu3e experiment,2014 JINST 9 C01011.[15] C. Grzesik, Fast optical readout for the Mu3e experiment, Bachelor Thesis, Heidelberg University(2014).[16] S. Corrodi, Fast optical readout of the Mu3e pixel detector, Master Thesis, ETH Zürich (2014).[17] F. Förster, HV-MAPS readout and direct memory access for the Mu3e experiment, Master Thesis,Heidelberg University (2014).[18] J. Kröger, Data transmission at high rates via kapton flexprints for the Mu3e experiment,Bachelor Thesis, Heidelberg University (2014).– 7 –2015 JINST 10 C03044[19] I. Perić, A novel monolithic pixelated particle detector implemented in high-voltage CMOStechnology, Nucl. Instrum. Meth. A 582 (2007) 876.[20] ATLAS R&D collaboration, I. Perić et al., Strip technology and HVMAPS, PoS(Vertex 2012)021.[21] I. Perić et al., High-voltage pixel detectors in commercial CMOS technologies for ATLAS, CLIC andMu3e experiments, Nucl. Instrum. Meth. A 731 (2013) 131.[22] A.-K. Perrevoort, Characterization of high voltage monolithic active pixel sensors for the Mu3eexperiment, Master Thesis, Heidelberg University (2012).[23] H. Augustin, Charakterisierung von HV-MAPS, Bachelor Thesis, Heidelberg University (2012).[24] R. Philipp, Characterization of high voltage monolithic active pixel sensors for the Mu3e experiment,Master Thesis, Heidelberg University (2014).[25] L. Huth, Development of a tracking telescope for low momentum particles and high rates consistingof HV-MAPS, Master Thesis, Heidelberg University (2014).– 8 –

Mu3e is a novel experiment searching for charged lepton flavor violation in the rare decay μ → eee. In order to reduce background by up to 16 orders of magnitude, decay vertex position, decay time and particle momenta have to be measured precisely. A pixel tracker based on 50 μm thin high voltage monolithic active pixel sensors (HV-MAPS) in a magnetic field will deliver precise vertex and momentum information. Test beam results like an excellent efficiency of >99.5% and a time resolution of better than 16.6 ns obtained with the MuPix HV-MAPS chip developed for the Mu3e pixel tracker are presented

Augustin, H

Berger, N

Bravar, S

Corrodi, S

Damyanova, A

Förster, F

Gredig, R

Herkert, A

Huang, Q

Huth, L

Kiehn, M

Kozlinskiy, A

Maldaner, S

Perić, I

Philipp, R

Robmann, P

Schöning, A

Shrestha, S

Bruch, D vom

Weber, T

Wiedner, D

CERN Document Server

Peric, I.

Schoening, Andre

DESY

DESY Publication Database

H. Augustin

N. Berger

S. Bravar

S. Corrodi

A. Damyanova

F. Förster

R. Gredig

A. Herkert

Q. Huang

L. Huth

M. Kiehn

A. Kozlinskiy

S. Maldaner

I. Perić

R. Philipp

P. Robmann

A. Schöning

S. Shrestha

D. vom Bruch

T. Weber

D. Wiedner

Crossref

Journal of Instrumentation

https://publikationen.bibliothek.kit.edu/1000055078/3850050

The MuPix high voltage monolithic active pixel sensor for the Mu3e experiment

Abstract

Similar works

Full text