MEG II: Status and · Francesco Renga - CLFV2019, Fukuoka, 17-19 June 2019 Sensitivity to New...

MEG II: Status and Upgrades Francesco Renga

INFN Roma for the MEG II Collaboration

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Francesco Renga - CLFV2019, Fukuoka, 17-19 June 2019

Sensitivity to New Physics• Operators in EFT mix at the loop level:

• the naive view (i.e. µ -> e γ only sensitive to dipole operators) has to be abandoned

- µ -> e γ also sensitive to 4-fermion operators and can give the strongest bounds in some scenario

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A. Crivellin et al., JHEP 1705 (2017) 117 G. M. Pruna, 2019 PSI User Meeting


µ -> e γ searches

µ+

e+

γ

Positron and photon are monochromatic (52.8 MeV),

back-to-back and produced at the same time;

Radiative Muon Decay (RMD)

µe

γ

νν

Accidental Background

µ

e

γ

µν

ν

DOMINANT

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µ -> e γ searches

µ+

e+

γ

Positron and photon are monochromatic (52.8 MeV),

back-to-back and produced at the same time;

Radiative Muon Decay (RMD)

µe

γ

νν

Accidental Background

µ

e

γ

µν

ν

DOMINANT

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�acc / �2µ · "e · "� · �Ee · (�E�)

2 · (�⇥e�)2 · �Te�


PSI Aerial View

The most intense DC muon beam in the world

• The ring cyclotron at PSI (Villigen, CH) serves the most intense DC muon beam lines in the world

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πE5 - up to 108 µ/s590 MeV, 2.2 mA

PSI Ring Cyclotron


Ingredients for a search of µ -> e γ

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Reconstruct the Photon Energy

Reconstruct the Relative Time

Reconstruct the Relative Angle

Reconstruct the Positron Energy

µ+

e+

γ


The MEG Experiment

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Reconstruct the Photon Energy

Reconstruct the Relative Angle

Reconstruct the Positron Energy

LXe

TCDCµ+

e+

γ

LXe calorimeter

16 Drift Chambers in a magnetic field

30 scintillating bars for timing & trigger

Reconstruct the Relative Time


Likelihood Analysis

• Likelihood analysis of 5 discriminating variables (Ee,Eγ, θeγ,φeγ,Teγ): - year-by-year and event-by-event PDFs - careful treatment of correlations (from

well understood geometrical effects)

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• Accidental Background PDFs are fully defined from data sidebands: • very solid determination of the (largely) dominant background

• Signal and radiative decay PDFs by combining the results of the calibration procedures

• Normalization from the observed number of µ -> e ν ν and RMD


The final MEG result (I)

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NACC = 7684 ± 103 NRMD = 663 ± 59 NSIG (best fit) = -2.2

Magnified signal (BR = 4 x 10-11)

BR < 4.2 x 10-13 @ 90% C.L.

Eur. Phys. J. C76 (2016) no. 8, 434

7.5 x 1014 µ on target


The final MEG result (II)

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Toy MC sensitivity Median UL = 5.3 x 10-13

DATA

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MEG-II

• The MEG experiment is undergoing an upgrade that involves all sub-detectors

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Larger LXe volume with finer light

detector granularity

Higher beam intensityUnique-volume Drift Chamber

Scintillator Tile TC

RMD Veto

First physics run expected in 2020

UL ~ 6 x 10-14 in 3 years of run

Eur. Phys. J. C78 (2018) no.5, 380


MEG-II Highlights - The LXe Calorimeter

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We developed large-area (12x12 mm2), UV-sensitive MPPCs to cover the inner

face of the LXe calorimeter Better Resolution, better pile-up rejection

σE ~ 1%, σposition ~ 2/5 mm (x,y/z)

MEG MEG-II

First events/spectra from 2017 data


MEG-II Highlights - The Timing Counters

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5mm-thick Scintillator Tiles read out by 3x3 mm2 SiPM

Detector completed, data taken in 2017 and 2018

Calibration with dedicated laser


MEG-II Highlights - The Timing Counters

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σT ~ 35 ps

Already reached the design resolution

5mm-thick Scintillator Tiles read out by 3x3 mm2 SiPM

Detector completed, data taken in 2017 and 2018


MEG-II Highlights - The Drift Chamber

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The challenge: minimal material budget (to reduce MS of 50 MeV e+) and very high granularity (to cope with the high rate) —> small cells

(down to < 6 mm) + extremely thin wires (20 µm W(Au) + 40-50 µm Al(Ag)) Innovative wiring technique (no feedthroughs)

Severe problems of wire fragility in presence of contaminants + humidity

σE ~ 130 keV, σangles ~ 5 mrad, 2x larger positron efficiency


The challenge: minimal material budget (to reduce MS of 50 MeV e+) and very high granularity (to cope with the high rate) —> small cells

(down to < 6 mm) + extremely thin wires (20 µm W(Au) + 40-50 µm Al(Ag)) Innovative wiring technique (no feedthroughs)

Severe problems of wire fragility in presence of contaminants + humidity

MEG-II Highlights - The Drift Chamber

16σE ~ 130 keV, σangles ~ 5 mrad, 2x larger positron efficiency

Assembly completed in summer 2018

First data on beam in fall 2018


MEG-II Highlights - The Drift Chamber• Low wire elongation in 2018 (50% of elastic limit) to limit the impact of

wire fragility —> electrostatic stability problems (inner layers could not reach the working point)

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Elongation increased in Spring 2019

New HV tests show that all the chamber can now be operated at the proper

working point (1400-1500 V, 5 x 105 gain)

with 100 V safety margin

Working point + 100V green = goal reached


RDC & Target monitoring

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50% of acc. background photons come from RMD w/ positron along the beam line

Can be vetoed by detecting the positron in coincidence with the photon

A new detector (LYSO + plastic scint.) built and tested in 2017 -> 16% better sensitivity

The target position in MEG-II has to be known with an accuracy ~ 100 µm to not

compromise the angular resolution

A system of photo cameras has been installed to monitor the target position

<< 100 µm resolution reached

RMD Veto


MEG-II Highlights - RDC, DAQ, Trigger

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Trigger and DAQ will be integrated in a single, compact system

(WaveDAQ)

Also provides power and amplification for SiPM/MPPC

Had to face severe common-noise problems — now fixed —

The design and test of the DAQ electronics is going to be finalized in the next few months, mass production will start immediately after

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MEG-II Highlights - Calibrations

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MEG-II schedule & sensitivity

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R&DPROPOSAL

Construction & Commissioning

Engineering Runs

Physics Runs

2013 2014 2015 2016 2017 2018 2019 2020 2021 2023

6 x 10-14

2022

What next?

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G. Cavoto, A. Papa, FR, E. Ripiccini and C. Voena Eur. Phys. J. C (2018) 78: 37


Sensitivity to New Physics

• Even just a factor 10 in µ -> e γ can improve its New Physics sensitivity beyond the reach of the current µ -> e conversion experiments

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A. Crivellin et al., JHEP 1705 (2017) 117 G. M. Pruna, 2019 PSI User Meeting


The next generation of high intensity muon beams

HiMB Project @ PSI

x4 µ capture eff. x6 µ transport eff.

1.3 x 1010 µ/s A. Knecht, SWHEPPS2016

MuSIC Project @ RCNP

Thick production target

π capture solenoid

4 x 108 µ/s at the production targetS. Cook et al., Phys. Rev. Accel. Beams 20 (2017)

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The next generation of high intensity muon beams

HiMB Project @ PSI

x4 µ capture eff. x6 µ transport eff.

1.3 x 1010 µ/s A. Knecht, SWHEPPS2016

MuSIC Project @ RCNP

Thick production target

π capture solenoid

4 x 108 µ/s at the production targetS. Cook et al., Phys. Rev. Accel. Beams 20 (2017)

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Possibility for a high intensity DC muon beam under study

at PIP-II (FNAL)


Toward the next generation of µ -> e γ searches: Photon Reconstruction

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BR E

xp. UL

Beam Rate

Photon Conversion Calorimetry

Improved calorimetry

Reminder: Acc. Bkg. ~ Rµ2

Sens. ~ S/√B ~ const. if non-zero background

Photon Conversion

Low efficiency (~ %) Extreme resolutions

+ eγ Vertex

MEGA/Mu3e

Calorimetry

High efficiency Good resolutions

MEG: LXe calorimeter 10% acceptance


Photon Reconstruction: Limiting Factors

CALORIMETRY • Photon Statistics • Scintillator time constant • Detector segmentation

• LaBr3(Ce) looks a very good candidate: - our simulations & tests indicate

that ~ 800 keV resolution can be reached

- extreme time resolution (~ 30 ps) - large acceptance - very expensive 27

PHOTON CONVERSION • Interactions in the converter (conversion

probability, e+e- energy loss and MS)

SIGNAL BKG

VERTEXING


Toward the next generation of µ -> e γ searches: Positron Reconstruction

• Tracking detectors in a magnetic field are the golden candidates: - high efficiency - better resolutions w.r.t. calorimetry (σ(Ee) down to 0.2% vs. > 1%)

• Need a very light detector in order to minimize the multiple scattering at Ee ~ 52.8 MeV - e.g. MEG drift chambers gave ~ 2 x 10-3 X0 over the whole

positron trajectory (200 µm silicon equivalent)

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• Positron Reconstruction is ultimately limited by Multiple Scattering - MS in the target & tracker-> angular resolutions - MS in the tracker -> momentum resolution

• Silicon trackers are not competitive with gaseous detectors in terms of resolutions (C-h. Cheng et al. arXiv: 1309.7679)

- e.g. worse momentum resolution by a factor ~ 2 - …but maybe unique solution at high beam rate.

Limiting Factors: Positron Reconstruction

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Photon and Positron timing

• Timing plays a crucial role in µ -> e γ searches (accidental coincidences!!!): - need a very good positron and photon timing - σ(Teγ) ~ 80 ps in MEG-II

• LiBr3(Ce) calorimeters + positron scintillating counters like in MEG can give the required performances

• For photon conversion, need to detect e+ or e- in a fast detector

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What about stacking multiple layers?

converter

scintillators


An active conversion layer

• Good photon timing in a detector with multiple conversion layers implies active material in the conversion layer: - thin, to not deteriorate the energy resolution

• Scintillating fibers have poor “timing to thickness” figures (~ 1 ns for 250 µm fibers)

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FAST SILICON DETECTORS • R&D on going for PET application

(TT-PET)

M. Benoit et al., JINST 11 (2016) no. 03, P03011


Possible Scenarios

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CALORIMETRY

PHOTON CONVERSION

(1 LAYER, 0.05 X0)

(70% γ acceptance)


Expected Sensitivity

A few 10-15 seems to be within reach for a 3-year run at ~ 108 µ/s with calorimetry (expensive) or ~ 109 µ/s with conversion (cheap)

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Fully exploiting 1010 µ/s and breaking the 10-15 wall seem to require a novel experimental concept

/s]µ [µΓ

810 910 1010

Exp.

90%

C.L

. Upp

er L

imit

15−10

14−10

13−10

12−10

11−10

MEG

MEG-II

, no vtx0

1 layer, 0.05 X, TPC vtx (cons)

01 layer, 0.05 X

, TPC vtx (opt)0

1 layer, 0.05 X, no vtx

010 layers, 0.05 X

, TPC vtx (cons)0

10 layers, 0.05 X, TPC vtx (opt)

010 layers, 0.05 X

, Si Tracker0

10 layers, 0.05 X

/s]µ [µΓ

810 910 1010

Exp.

90%

C.L

. Upp

er L

imit

15−10

14−10

13−10

12−10

11−10

MEG

MEG-II

MEG-II detectorcalorimetry

, TPC vtx (opt))0

conversion (1 layer, 0.05 X, TPC vtx (opt))

0conversion (10 layers, 0.05 X


Conclusions

• MEG currently gives the best limits on muon cLFV

• MEG-II will improve the sensitivity by a factor ~ 10, down to 6 x 10-14

- Calorimeter & Timing counter tested in 2017 and 2018

- Drift chamber construction completed and first data collected on beam in 2018 — recommissioning in Spring 2019 allowed to bring the full detector to running conditions

• Engineering run in 2019 with the full detector under running conditions - First physics data expected in 2020

• Options for a future experiment under study:

• A further factor 10 seems possible in a medium term with incremental improvements

• Going below 10-15 seems to require a novel experimental concept

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Backup

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Positron Reconstruction at High Beam Rate

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A. Baldini et al., MEG Upgrade Proposal, arXiv:1301:7225

Expected aging (gain loss) in the

MEG-II Drift Chamber

Would a gaseous detector be able to cope with the very high occupancy at > 109 µ/s?

Silicon detector momentum resolution

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Mu3e momentum resolution (B = 1T) 4x worse than MEG-II

A. Kozlinskiy, Mu3e Collaboration, CTD/WIT 2017


Muon Stopping Target

• The target plays a crucial role in determining the positron angular resolution, due to the Multiple Coulomb Scattering:

- target must be as thin as possible

• In order to stop a significative fraction of muons, it must be at the Bragg peak:

- muons not stopped by the target are stopped in the gas right after, giving background without contributing to the signal

➡ enough thickness to stop ~ all muons

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µ

e+

Optimal target Be, 90 µm

θMS(e+) ~ 2.5 - 3 mrad


Multiple Targets?

• Does it make sense to use multiple thinner targets in sequence?

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- probably not: many muons would decay in the gas between the two targets (background, efficiency loss,…)

• Does it make sense to use multiple staggered targets?

- probably yes: with photon direction from conversion, it could reduce the acc. bkg. by a factor of 2

µ

µ

e+

e+

γ

MEG II: Status and · Francesco Renga - CLFV2019, Fukuoka, 17-19 June 2019 Sensitivity to New...

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