The Mu3e Ultra-Low-Mass Tracker

The Mu3e ultra-low-mass tracker

Frank Meier Universit¨at Heidelberg for the Mu3e collaboration

25 June 2018

1 / 52 Introduction to Mu3e

Mu3e is an experiment to search for

+ + + µ e e−e → A very rare decay.

We’re in an unusual regime, hence allow for some physics background.

2 / 52 Introduction to Mu3e

µ eee in the standard model. →

+ W γ∗ e−

+ + µ ν¯µ ν¯e e

3 / 52 Introduction to Mu3e

µ eee in the standard model. →

SM: < 1 10 54 × − The suppression comes from the + neutrino masses. e

12 Current best limit: < 1 10− + × W γ∗ (SINDRUM 1988) e−

Alternative models predict BR within 16 reach of Mu3e (< 1 10− ). + + × µ ν¯µ ν¯e e

3 / 52 Introduction to Mu3e –– Signal in rφ-view e– e+

e+ Signal SM: < 1 10 54 × −

4 / 52 Introduction to Mu3e –– Signal in rφ-view e– e+

e+ Signal SM: < 1 10 54 × − P pi = 0

4 / 52 Introduction to Mu3e –– Signal in rφ-view e– e+

e+ Signal SM: < 1 10 54 × − P pi = 0 minv = mµ

4 / 52 Introduction to Mu3e –– Signal in rφ-view e– e+

e+ Signal SM: < 1 10 54 × − P pi = 0 minv = mµ t = t i, j i j ∀

4 / 52 Introduction to Mu3e –– Signal in rφ-view e– e+

e+ Signal SM: < 1 10 54 × − P pi = 0 minv = mµ t = t i, j i j ∀ common vertex

4 / 52 Introduction to Mu3e –– Signal in rφ-view e– ν e– e+ e+

ν + e+ e Signal Radiative decay SM: < 1 10 54 SM: 3.4 10 5 × − × − P p = 0 P p = 0 i i 6 minv = mµ minv < mµ t = t i, j t = t i j ∀ i j common vertex common vertex

4 / 52 Introduction to Mu3e –– Signal in rφ-view – e– ν e– e e+ e+ e+

ν + e+ e e+ Signal Radiative decay Accidental SM: < 1 10 54 SM: 3.4 10 5 background × − × − P p = 0 P p = 0 P p 0 i i 6 i ≈ m = mµ m < mµ m mµ inv inv inv ≈ t = t i, j t = t t t i j ∀ i j i ≈ j common vertex common vertex “bad vertex”

4 / 52 bg = white Expected Sensitivity for µ eee in Phase I

Introduction to Mu3e→ Mu3e Phase I 2 2 10 Full Geant4-based 1015 muon stops at 108 muons/s µ → eee detector simulation at 10-12 10 µ → eeeνν µ → eee at 10-13 Expected SM background 1 µ → eee -14 1 Bhabha + at 10 10− Events per 0.2 MeV/c Michel Prospects for µ eee µ → eee signal at various at 10-15 10−2 branching fractions→ 10−3 96 98 100 102 104 106 108 110 2 mrec [MeV/c ] 5 / 52

A. Perrevoort (PI HD) New Physics in Mu3e DPG 2017 4 / 14 Backgrounds

Irreducible IntroductionBG: radiative to decay Mu3e with internal conversion (IC)

- e ν e+

missing energy from two neutrinos R.M.Djilkibaev, steeply falling! R.V.Konoplich ν e+ PRD79 (2009)

B(μ+ → e+e+e- νν) = 3.4 ·10- 5

very good momentum / 6 / 52 total energy resolution required! André Schöning, Heidelberg (PI) 15 CERN Detector Seminar, March 31, 2017 What governs the detector design?

Hence we need:

I Precise tracking (vertexing and momentum) pixels ⇒ I Good timing (coincidence, event separation) scintillators ⇒ I Minimal material budget design (background suppression, multiple scattering) solutions. . . ⇒ Note: Muons are stopped on a target. No bunch structure.

Rad-hard electronics is not that important.

7 / 52 Mu3e detector concepts

8 / 52 Mu3e detector concepts

Phase-I conﬁguration:

Recurl pixel layers

Scintillator tiles Inner pixel layers

μ Beam Target

Outer pixel layers

9 / 52 Mu3e detector concepts

Phase-I conﬁguration:

Recurl pixel layers

Scintillator tiles Inner pixel layers

μ Beam Target

Outer pixel layers

8 I High rate: 10 muon stops on target per second

I Time resolution (pixels): 20 ns I Vertex resolution: about 200 µm I Momentum resolution: about 0.5 MeV

I All inside a cryogenic 1 T magnet, warm bore I.D. 1 m 9 / 52 Mu3e detector concepts

Let’s focus on the pixels. Monte-Carlo studies led to the following geometry:

100 Layer 4

Layer 3 Modules 50 Layer 2 of 4 ladders

Layer 1 0 mm Target Target Half-shells –50 MuPix chips 4 or 5 ladders

–100

–200 –100 0mm 100 200

(B = 1 T, x/X0 = 0.1% per layer)

10 / 52 Mu3e detector concepts

Identical copies of layers 3/4 will extend the detector in z to extend coverage for recoiling tracks.

100 Layer 4

Layer 3 50 Layer 2

Layer 1 0 mm Target

–50 MuPix chips

–100

–200 –100 0 mm 100 200

11 / 52 Mu3e detector concepts

Ok, we got the geometry. But what about the material budget of the pixel layers?

Let’s put this into perspective:

Experiment Ref. x/X0 per layer [%] ATLAS IBL [1] 1.9 CMS Phase I [2] 1.1 ALICE upgrade [3] 0.3 STAR [4] 0.4 Belle-II IBL [5] 0.2 Mu3e 0.1

12 / 52 Mu3e detector concepts

Coming back to the solutions I will talk about

I Low-material mechanics based on polyimide ﬁlm

I Aluminium ﬂexes to reduce Z

I Interposers to save space

I Stuﬃng all together to make it work

13 / 52 Mu3e Detektor in Rotationskäfig eingebaut

Mu3e mechanics – Mounting structure

Top view of experiment frame, diameter 1 m. ≈ 14 / 52 Mu3e mechanics – Mounting structure It needs to take the load:

Deformation 100 exagerated. Matches measurements on a mock-up well. × 15 / 52 Pixel modules

16 / 52 Mu3e mechanics

N-well E eld

P-substrate Particle

We use a monolithic, depleted pixel sensor, thinned to 50 µm. Active chip size 20 20 mm2, power dissipation 250 mW/cm2 (conﬁrmed by measurements). × ≤ Ivan Peri´c,Nucl.Instrum.Meth. A582 (2007) 876-885 17 / 52 Mu3e mechanics

Central station:

Layer Unit units/layer ladders/unit chips/ladder #chips #links 1 half shell 2 half shell 3 module 4 module Total

18 / 52 Mu3e mechanics

Central station:

Layer Unit units/layer ladders/unit chips/ladder #chips #links 1 half shell 2 2 half shell 2 3 module 6 4 module 7 Total

18 / 52 Mu3e mechanics

Central station:

Layer Unit units/layer ladders/unit chips/ladder #chips #links 1 half shell 2 4 2 half shell 2 5 3 module 6 4 4 module 7 4 Total

18 / 52 Mu3e mechanics

Central station:

Layer Unit units/layer ladders/unit chips/ladder #chips #links 1 half shell 2 4 6 2 half shell 2 5 6 3 module 6 4 17 4 module 7 4 18 Total

18 / 52 Mu3e mechanics

Central station:

Layer Unit units/layer ladders/unit chips/ladder #chips #links 1 half shell 2 4 6 48 2 half shell 2 5 6 60 3 module 6 4 17 408 4 module 7 4 18 504 Total 1020

18 / 52 Mu3e mechanics

Central station:

Layer Unit units/layer ladders/unit chips/ladder #chips #links 1 half shell 2 4 6 48 144 2 half shell 2 5 6 60 180 3 module 6 4 17 408 408 4 module 7 4 18 504 504 Total 1020 1236

18 / 52 Mu3e mechanics

Central station plus two recurl stations:

Layer Unit units/layer ladders/unit chips/ladder #chips #links 1 half shell 2 4 6 48 144 2 half shell 2 5 6 60 180 3 module 6 4 17 1224 1224 4 module 7 4 18 1512 1512 Total 2844 3060

One chip dissipates about 1 W about 3 kW of heat. ⇒

18 / 52 Mu3e mechanics – Layers 1/2 Modules layer 2 design (1 is similar, one facet less)

Inner modules have ladders of 6 chips each. Observe: No V-folds here. 19 / 52 Mu3e mechanics – Layers 1/2 Modules layer 2 design (1 is similar, one facet less)

Exploded view of same part.

19 / 52 Mu3e mechanics – Layers 1/2

More on this: see poster by Simon Muley.

20 / 52 Mu3e mechanics

Shown: One one module per layer inserted. 21 / 52 Mu3e mechanics

22 / 52 Mu3e mechanics

Mupix sensor 50µm SpTAB bonds

HDI ~100µm

4mm Mupix periphery polyimide 15µm

6mm

Radiation length: 0.1% x/X ≈ 0

23 / 52 Mu3e mechanics – V fold gluing Let’s have a look at gluing V-folds on ladders. This is the outcome:

Polyimide Chip

50 µm HDI 0 10 20 mm

24 / 52 Mu3e mechanics – V fold gluing

1. Fill reservoir with 50 µm layer of glue

24 / 52 Mu3e mechanics – V fold gluing

2. Dip stamp into reservoir

24 / 52 Mu3e mechanics – V fold gluing

3. Place bare ladder on jig

24 / 52 Mu3e mechanics – V fold gluing

4. Apply glue

24 / 52 Mu3e mechanics – V fold gluing

5. Place V-folds on jig

24 / 52 Mu3e mechanics – V fold gluing

5. Place V-folds on jig

24 / 52 Mu3e mechanics – V fold gluing

6. Pick V-folds

24 / 52 Mu3e mechanics – V fold gluing

7. Lower V-folds down

24 / 52 Mu3e mechanics – V fold gluing

8. Place curing weight Let glue cure

24 / 52 Mu3e mechanics

Setup for gluing

25 / 52 Mu3e mechanics

Closeup view. Here, dispensing robot was used as an alternative approach.

26 / 52 Mu3e mechanics

Mock-up ladders made of tape heaters

Made of aluminium-polyimide laminate (25 µm thickness each), laser structured meander for heating and temperature measurement.

27 / 52 Mu3e mechanics

Mock-up ladders made of tape heaters

27 / 52 Electrical connectivity

28 / 52 Electrical connectivity

Interposer Samtec Z-Ray

Pitch: 0.8 mm

Model Compressed height ZA8H 0.3 mm ZA8 1 mm

Industry standard component, cost 5–10 e a piece.

Allows use of ﬂexes instead of cables.

29 / 52 Electrical connectivity The nice feature: many connections on a small area.

BIAS GND GND GND GND GND GND GND GND GND GND GND

+ SIN – VDD + D-2 – VDDVDD + D-5 – VDD + D-8 – GND

GND VDD VDD VDD VDD VDD VDD VDD VDD VDD VDD GND

+ CLK – VDD + D-1 – VDDVDD + D-4 – VDD + D-7 – GND

GND VDD VDD VDD VDD VDD VDD VDD VDD VDD VDD GND

+ RES – VDD + D-0 – VDDVDD + D-3 – VDD + D-6 – GND

BIAS GND GND GND GND GND GND GND GND GND GND GND

One stave has 9 data links. Current limit per pin: 0.8 A. This will work. 30 / 52 Electrical connectivity For the HDI, we go for aluminium/polyimide:

Made by LTU (Kharkiv, Ukraine), used in e.g. ALICE pixels for power strips.

31 / 52 Electrical connectivity Process steps: 1. Starts with aluminium foil, thickness of 12.5 µm or 25 µm 2. Create polyimide layer: spinning of primer, drying and polimerisation 3. Photolithographic etching of aluminium traces 4. Etching of polyimide 5. Glueing of layers to form a stack 6. Additional polyimide foil for added dielectric, if needed 7. Tab bonding. Bonds aluminium traces directly, no wire.

via pad connection 150 µm

100 µm chip PIglue pad on chip Al

32 / 52 Electrical connectivity Flex when cut out from panel:

33 / 52 Electrical connectivity Some bond examples under a microscope:

Top: Bond to PCB PCB is visible on the bottom edge of the image

Center: Connecting layers (via) The visible misalignment is a shrinking eﬀect from polymerisation at 350 ◦C. One trace needs to be wider to absorb the tolerances.

Bottom: Vias for a bus

34 / 52 Flexprint Prototypes - Test Structure

Electrical connectivity Bit Error Rate Test Bit error rate tests: data rate [Gb/s] line BER upper limit at 95% CL Rate Line BER 13 1.25 Gb/sGb/s all (95% C.L.)5.5 10  ⇥ 13 2.5 Gb/s1.25 all all ≤ 5.5 × 105.−913 10  ⇥ 13 3.2 Gb/s2.5 all short≤ 5.9 × 104.−113 10  − ⇥ 3.2 short18 ones cm≤ 4.1 × 10 13 fail 4.0 Gb/s18 cm all fail fail 4.0 all fail

35 / 52 Jens Kr¨oger (Uni Heidelberg) Flexprint Designs for Mu3e March 28, 2017 15 / 18 Pixel detector cooling

36 / 52 Pixel detector cooling

100 Endring-PCB (partly flexible)

Distribution-PCB 50 Endring-PCB (flex) to readout board

0 mm

–50 He-pipe (1 circuit) He-pipes (4 circuits)

–100

–200 –100 0mm 100 200

Sketch of supplies: electrical connectivity and cooling circuits.

37 / 52 Pixel detector coolingHe-, Wasser- und Signalwege in Mu3e

He-, Power- and Signal-Supplies in Mu3e

A busy sketch showing all connections Line styles: solid = He, dottedHe supply = water, dashed = power/signal H2O supply Power/signal supply

38 / 52

Mu3e Mockup - Heidelberg - April 2018 - Silvan Streuli Seite 32 Pixel detector cooling

Hose Duct Endring Detector Endring Duct Hose

Hose Duct Endring Detector Endring

Experiment volume

Three helium circuit types ∆p limit to be determined on mock-up, estimated to be O(1 mbar)

39 / 52 Pixel detector cooling

Constraints;

I Very restricted space inside magnet (d = 1 m, l 2.8 m) ≈ I Magnetic ﬁeld of 1 T solenoid valves or motors won’t work inside ⇒ I Helium atmosphere everywhere

I All material must be non-magnetic

I Openings in magnet shield doors limit space for feed-throughs

40 / 52 Pixel detector cooling The V-folds not only give mechanical strength but are used for cooling with gaseous Helium. Simulation results:

41 / 52 P 2 A = 400 mW/cm All pixel layers, no target or SciFi vglobal = 0.5 m/s vgap = 10 m/s vv-fold = 20 m/s Pixel detector cooling –

Required target ﬂows per helium circuit:

Volume He flow speed Cross-section Volume Occurence Volumetric flow m/s cm2 cm3 times m3/min Gap L1/L2 10 12 148 1 0.72 Gap SciFi/L3 10 39 1320 1 2.3 Gap Tile/L3 10 34 1150 2 4.2 V-folds L3 20 3.3 114 3 1.2 Gap L3/L4 10 60 2185 3 10.8 V-folds L4 20 3.9 141 3 1.4 Global flow 0.5 7600 912000 1 23 Total 7750 43

Volumes diﬀer up to factor 20 ⇒ ≈

42 / 52 Cooling simulations

43 / 52 Cooling simulations

I Cooling simulations have been repeated I Two power dissipation cases: 2 I 400 mW/cm (pessimistic) 2 I 250 mW/cm (realistic) 2 I The two cases scale linearly, hence show 400 mW/cm only

I Also pressure drop across ducts and detector volumes have been simulated

44 / 52 Cooling simulations

Simulated ﬂows:

Volume Flow speed (m/s) Direction Gap L1/L2 10 DS → US Gap SciFi/L3 5 US → DS V-folds L3/L4 20 DS → US Gap L3/L4 10 DS → US Global 0.5 US → DS

Following pages show temperature of silicon as heat maps. Other parts removed for clarity.

45 / 52 Cooling simulations

L1/2

46 / 52 Cooling simulations

L3/4

46 / 52 Cooling simulations

L3 only

46 / 52 Cooling simulations Average temperature of ladder translated into thermal expansion of polyimide:

Note: This is a result calling for more studies. Plan is to optimise gas inlets. Interplay with duct layout, hence non-trivial.

47 / 52 Cooling simulations

Expected pressure drops in the circuits from same simulation:

Circuit Duct IN Flange Detector Flange Duct OUT Gap L1/L2 25 7 < 1 9 24 Gap SciFi/L3 6 < 1 3 28 – V-folds L3 25–50 80–90 10–20 50–70 25–25 Gap L3/L4 8 25 < 1 11 – V-folds L4 30–50 60–70 10–20 50–70 20

All pressure values in mbar. If range given: min/max observed per compartment. Some gaps vent to global volume.

This seems manageable and the detector shouldn’t pump up“. ”

48 / 52 Pixel module insertion

49 / 52 Pixel module insertion

50 / 52 Pixel module insertion

50 / 52 Conclusions and outlook

I We have a challenging but exciting detector project:

I Thinned silicon pixel, monolithic, self-triggered pixels I Scintillating detectors for timing (not shown here, sorry. . . ) I Ultra-light weight mechanics

I Gaseous helium cooling

I Readout using Aluminium ﬂexes and micro-twisted pair cables (not shown again. . . ) I Plans:

I Building a full thermo-mechanical mock-up using silicon chip heaters I Implementing module fabrication workﬂows I Further electrical integration

51 / 52 References

[1] ATL-INDET-PROC-2015-001 [2] CERN-LHCC-2012-016, CMS-TDR-11 [3] arXiv:1211.4494v1 [4] G. Contin, talk at PIXEL2016 [5] C. Koﬀmane, talk at PIXEL2016

52 / 52 ENCORE

53 / 52 Assembly procedure

54 / 52 Assembly procedure Mounting Detectors on Beam Pipe

55 / 52 Assembly procedure

56 / 52 Assembly procedure

57 / 52 Assembly procedure

I The following slides show a step-by-step mounting procedure

I Order of sequence has certain freedoms. This is one possible choice.

58 / 52 Assembly procedure

We start with the two beampipes.

Already mounted: copper bars and helium ducts.

Two beampipes still allowed to be mechanically independent for ﬁrst few steps.

59 / 52 Assembly procedure

Outermost endrings

59 / 52 Assembly procedure

Tile detectors

59 / 52 Assembly procedure

Second endrings for recoil stations

59 / 52 Assembly procedure

Endrings for central station

59 / 52 Assembly procedure

Mounting frame for SciFi

59 / 52 Assembly procedure

Endrings for L1/2

Note: Until now, no mechanical connection between US/DS.

59 / 52 Assembly procedure

Two half-shells for L1

Note: From now on everything is in experiment frame and beampipes must be aligned.

59 / 52 Assembly procedure

and same for L2

59 / 52 Assembly procedure

Insert SciFi modules

59 / 52 Assembly procedure

Insert pixel L3 modules

59 / 52 Assembly procedure

and L4

59 / 52 Assembly procedure

L3 of US recoil station

59 / 52 Assembly procedure

L3 of DS recoil station

59 / 52 Assembly procedure

Now detector is in principle ready for insertion into magnet.

59 / 52 Assembly procedure

And ﬁnally: insert target from DS

59 / 52 Multiple scattering

As you know, PDG gives you the following formula:

13.6 MeV p θ = z x/X [1 + 0.038 ln (x/X )] βcp 0 0

Allow me to illustrate that a bit. . .

60 / 52 Multiple scattering

Multiple scattering at LHC energies for an electron:

2.5 ) 0.14 ° - ( e , x/X = 1.0% θ - 0 (mrad) e , x/X = 0.1%

θ 0.12 2 0 0.1 1.5 0.08

1 0.06

0.04 0.5 0.02

0 0 1000 2000 3000 4000 5000 6000 p (MeV)

Thinner detector layers is a nice-to-have, but 1% x/X is ok. 0 61 / 52 How does this looks like at Mu3e energies? Multiple scattering

Multiple scattering at Mu3e energies for an electron:

2.5 ) 0.14 ° - ( e , x/X = 1.0% θ - 0 (mrad) e , x/X = 0.1%

θ 0.12 2 0 0.1 1.5 0.08

1 0.06

0.04 0.5 0.02

0 0 10 15 20 25 30 35 40 45 50 55 p (MeV)

Ooops. Did we loose the curve. . . ? 61 / 52 Multiple scattering

Multiple scattering at Mu3e energies for an electron:

100 ) °

- ( 90 e , x/X = 1.0% θ - 0 5 (mrad) e , x/X = 0.1% θ 80 0 70 4 60 50 3 40 2 30

20 1 10 0 0 10 15 20 25 30 35 40 45 50 55 p (MeV)

40 scale increase. At low energies, matter matters. × 61 / 52 What does that mean for a tracking device? Multiple scattering

e- 50 MeV

50 mm 250 mm

The stage is a simple toy tracker. Particle enters from the left.

62 / 52 Multiple scattering

x/X0 = 1%

e- 50 MeV ~25 mm 50 mm 250 mm

Let’s take pixel layers with x/X0 = 1% each. Observe the substantial scattering at such low momenta.

Note: This sketch is to scale. Per-layer contribution added in quadrature. 62 / 52 Multiple scattering

x/X0 = 0.1%

e- 50 MeV ~6 mm

50 mm 250 mm

Reducing x/X0 to 0.1% helps.

62 / 52 Multiple scattering

To measure the momentum, a B-ﬁeld is present. Hence tracks are helices.

How can we take this to our advantage?

63 / 52 Multiple scattering

Assume a particle in a B-ﬁeld scatters at some detector layer (blue)

64 / 52 Multiple scattering

Let it scatter to the right. . .

64 / 52 Multiple scattering

. . . or to the left. . .

64 / 52 Multiple scattering

Observe the magic point where the scattering eﬀect cancels.

It is after a half turn. 64 / 52 Multiple scattering

Choose radii wisely for best preformance.

64 / 52 I Pixel sensor: MuPix I Mechanics I Readout I Cooling

Multiple scattering

Ok, now you know our basic ingredients to do our job:

I Optimise the radii of the detector I Minimise the material per detector layer

65 / 52 Multiple scattering

Ok, now you know our basic ingredients to do our job:

I Optimise the radii of the detector I Minimise the material per detector layer

I Pixel sensor: MuPix I Mechanics I Readout I Cooling

65 / 52 Silicon heaters

PCB I We’ve prepared single silicon heater assemblies. Bonds Si-heater I Consists of heater (sputtered aluminium on silicon, thinned down to ~40 mm 50 µm) and a ﬂex HDI (2 layers HDI Al/polyimide). Veryclose to ﬁnal design. ~40 mm

I Heater designed to dissipate up to 400 mW/cm2.

I Has a 1000 Ω RTD on it

I Next set of slides: graph paper viewed reﬂected on back of silicon heater

66 / 52 Silicon heaters

67 / 52 Silicon heaters

ϑ = 30 ◦C

68 / 52 Silicon heaters

ϑ = 40 ◦C

68 / 52 Silicon heaters

ϑ = 50 ◦C

68 / 52 Silicon heaters

ϑ = 60 ◦C

68 / 52 Silicon heaters

ϑ = 70 ◦C

68 / 52 Silicon heaters

Before you get too shocked: 0 S e t - u p d e f o r m a t i o n s v i s i b l e I About the magnitude expected from - 1 0 0 )

m CTE mismatch polyimide-silicon. µ (

t - 2 0 0 h g

i I Glue pattern has not been optimised e h

i - 3 0 0 yet.

e g n

a - 4 0 0 I Will calibrate ﬁnite element simulations h C and optimise glue pattern in - 5 0 0 simulation.

- 6 0 0 I If detector is in thermal equilibrium 3 0 4 0 5 0 6 0 7 0 T e m p e r a t u r e ϑ ( ° C ) and stable over time, track-based E r r o r b a r s s h o w s y s t e m a t i c e r r o r alignment can handle this.

I Thermal mass is small, hence equilibrium will be reached fast.

69 / 52 Module assembly procedure

Take endpiece. Glue gas cavity lid.