MEG II: Status and Francesco Renga Upgrades INFN Roma for the MEG - - PowerPoint PPT Presentation

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MEG II: Status and Francesco Renga Upgrades INFN Roma for the MEG II Collaboration 1 Francesco Renga - CLFV2019, Fukuoka, 17-19 June 2019 Sensitivity to New Physics A. Crivellin et al. , JHEP 1705 (2017) 117 G. M. Pruna, 2019 PSI User Meeting

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SLIDE 1

MEG II: Status and Upgrades

Francesco Renga INFN Roma for the MEG II Collaboration

1

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SLIDE 2

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

2

  • A. Crivellin et al., JHEP 1705 (2017) 117
  • G. M. Pruna, 2019 PSI User Meeting
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SLIDE 3

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

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)

µ e γ ν ν

Accidental Background

µ e γ µ ν ν

DOMINANT

3

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SLIDE 4

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

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)

µ e γ ν ν

Accidental Background

µ e γ µ ν ν

DOMINANT

4

Γacc ∝ Γ2

µ · εe · εγ · δEe · (δEγ)2 · (δΘeγ)2 · δTeγ

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SLIDE 5

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

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

5

πE5 - up to 108 µ/s 590 MeV, 2.2 mA PSI Ring Cyclotron

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SLIDE 6

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

Ingredients for a search of µ -> e γ

6

Reconstruct the Photon Energy Reconstruct the Relative Time Reconstruct the Relative Angle Reconstruct the Positron Energy

µ+ e+ γ

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SLIDE 7

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

The MEG Experiment

7

Reconstruct the Photon Energy Reconstruct the Relative Angle Reconstruct the Positron Energy

LXe TC DC µ+ e+ γ

LXe calorimeter 16 Drift Chambers in a magnetic field 30 scintillating bars for timing & trigger

Reconstruct the Relative Time

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SLIDE 8

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

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)

8

  • 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
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SLIDE 9

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

The final MEG result (I)

9

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

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SLIDE 10

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

The final MEG result (II)

10

Toy MC sensitivity Median UL = 5.3 x 10-13

DATA

10

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SLIDE 11

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

MEG-II

  • The MEG experiment is undergoing an upgrade that

involves all sub-detectors

11

Larger LXe volume with finer light detector granularity Higher beam intensity Unique-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
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SLIDE 12

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

MEG-II Highlights - The LXe Calorimeter

12

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

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SLIDE 13

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

MEG-II Highlights - The Timing Counters

13

5mm-thick Scintillator Tiles read

  • ut by 3x3 mm2 SiPM

Detector completed, data taken in 2017 and 2018

Calibration with dedicated laser

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SLIDE 14

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

MEG-II Highlights - The Timing Counters

14

σT ~ 35 ps

Already reached the design resolution 5mm-thick Scintillator Tiles read

  • ut by 3x3 mm2 SiPM

Detector completed, data taken in 2017 and 2018

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SLIDE 15

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

MEG-II Highlights - The Drift Chamber

15

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

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SLIDE 16

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

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

16

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

Assembly completed in summer 2018 First data on beam in fall 2018

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SLIDE 17

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

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)

17

Elongation increased in Spring 2019 New HV tests show that all the chamber can now be

  • perated at the proper

working point (1400-1500 V, 5 x 105 gain) with 100 V safety margin

Working point + 100V green = goal reached

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SLIDE 18

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

RDC & Target monitoring

18

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

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SLIDE 19

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

MEG-II Highlights - RDC, DAQ, Trigger

19

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

19

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SLIDE 20

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

MEG-II Highlights - Calibrations

20

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SLIDE 21

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

MEG-II schedule & sensitivity

21

R&D

PROPOSAL

Construction & Commissioning

Engineering Runs

Physics Runs

2013 2014 2015 2016 2017 2018 2019 2020 2021 2023

6 x 10-14

2022

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SLIDE 22

What next?

22

  • G. Cavoto, A. Papa, FR, E. Ripiccini and C. Voena
  • Eur. Phys. J. C (2018) 78: 37
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SLIDE 23

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

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

23

  • A. Crivellin et al., JHEP 1705 (2017) 117
  • G. M. Pruna, 2019 PSI User Meeting
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SLIDE 24

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

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 target

  • S. Cook et al., Phys. Rev. Accel. Beams 20 (2017)

24

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SLIDE 25

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

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 target

  • S. Cook et al., Phys. Rev. Accel. Beams 20 (2017)

25

Possibility for a high intensity DC muon beam under study at PIP-II (FNAL)

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SLIDE 26

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

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

26

BR Exp. 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

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SLIDE 27

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

Photon Reconstruction: Limiting Factors

CALORIMETRY

  • Photon Statistics
  • Scintillator time constant
  • Detector segmentation
  • LaBr3(Ce) looks a very good

candidate:

  • ur 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

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SLIDE 28

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

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)

28

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SLIDE 29

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

  • 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

29

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SLIDE 30

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

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

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SLIDE 31

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

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)

31

FAST SILICON DETECTORS

  • R&D on going for PET application

(TT-PET)

  • M. Benoit et al., JINST 11 (2016) no. 03, P03011
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SLIDE 32

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

Possible Scenarios

32

CALORIMETRY PHOTON CONVERSION

(1 LAYER, 0.05 X0) (70% γ acceptance)

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SLIDE 33

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

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)

33

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

/s] µ [

µ

Γ

8

10

9

10

10

10

  • Exp. 90% C.L. Upper Limit

15 −

10

14 −

10

13 −

10

12 −

10

11 −

10 MEG MEG-II

, no vtx 1 layer, 0.05 X , TPC vtx (cons) 1 layer, 0.05 X , TPC vtx (opt) 1 layer, 0.05 X , no vtx 10 layers, 0.05 X , TPC vtx (cons) 10 layers, 0.05 X , TPC vtx (opt) 10 layers, 0.05 X , Si Tracker 10 layers, 0.05 X

/s] µ [

µ

Γ

8

10

9

10

10

10

  • Exp. 90% C.L. Upper Limit

15 −

10

14 −

10

13 −

10

12 −

10

11 −

10 MEG MEG-II

MEG-II detector calorimetry , TPC vtx (opt)) conversion (1 layer, 0.05 X , TPC vtx (opt)) conversion (10 layers, 0.05 X

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SLIDE 34

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

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

34

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SLIDE 35

Backup

35

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SLIDE 36

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

Positron Reconstruction at High Beam Rate

36

  • 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?

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SLIDE 37

Silicon detector momentum resolution

37

Mu3e momentum resolution (B = 1T)

4x worse than MEG-II

  • A. Kozlinskiy, Mu3e Collaboration, CTD/WIT 2017
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SLIDE 38

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

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

38

µ e+

Optimal target Be, 90 µm

θMS(e+) ~ 2.5 - 3 mrad

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SLIDE 39

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

Multiple Targets?

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

39

  • 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