Final result of the MEG experiment and prospects on e searches - PowerPoint PPT Presentation

Final result of the MEG experiment and prospects on µ → e γ searches Cecilia Voena INFN Roma on behalf of the MEG collaboration 2 nd International Conference on Charged Lepton Flavor Violation Charlottesville, June 20-22 2016 1

Outline • Brief history of µ → e γ searches • The MEG experiment • Analysis method • Final MEG result • The upgrade: MEG-II • Prospects 2

Why µ → e γ 4 ⎛ ⎞ ∝ m ν • Standard Model prediction for BR: < 10 − 55 ⎜ ⎟ m W ⎝ ⎠ • Current experimental limit close to predictions in many New Physics models • Would be clear sign of New Physics • Intense muon beam available • Clean experimental signature Simultaneous back-to-back e + and γ with E γ =E e+ =52.8MeV ∝ 3

A long quest µ → e γ BR limit (90% C.L.) The sensitivity greatly First experiment: improved every time that a Hinks&PonteCorvo more intense muon “source” was available => more muons With a given muon “source” improvements are obtained with detectors improvements => lower background Final MEG result Year 4

The location: PSI lab The Paul Scherrer Institute Continuous muon beam up to 2x10 8 µ + /s Multi-disciplinary lab: - fundamental research, cancer therapy, muon and neutron sources - protons from cyclotron (D=15m, E proton =590MeV I=2.2mA) 5

1.4MW Proton Cyclotron at PSI 1.4MW Proton Cyclotron at PSI The Unique Facility for μ → e ữ Search Provides world’s most powerful DC muon beam > 10 /sec 6

The MEG experiment for µ → e γ search 7

Detector concept: search for µ → e γ • I µ ≈ 3 · 10 7 µ/s stopped in a thin plastic target • Drift Chambers in highly-gradient B-field: - 16 drift chamber modules - very light Measure: - gradient magnetic field - Positron energy E e+ to sweep out Michel positrons - Positron vertex - Positron track Gradient Magnetic Field ometer) 8

Detector concept: search for µ → e γ • Liquid Xe Calorimeter • Timing Counter - 900l liquid Xe - 15 scintillating bars for two sectors - read out by PMTs - read out by PMTs Measures: - Photon energy E γ Measures: - Photon time and - Positron time at vertex at conversion point impact on TC quantum efficiencies using LEDs and esolution om the target to the 9 using a dedicated CW accelerator to

Backgrounds • Accidental background - Accidental coincidence ∝ of e + and γ : - Proportional to I 2 µ while signal proportional to I µ - Compromise between high intensity and low background • Radiative muon decay ∝ background - Proportional to I µ - Note: e + and γ simultaneous as for signal 10

Final dataset • Published results - 2008 dataset: BR<2.8x10 -11 @90% CL NPB 834 (2010),1 - 2009-2010 dataset: BR<2.4x10 -12 @90% CL PRL,107 171801 (2011) - 2009-2011 dataset: BR<5.7x10 -13 @90% CL PRL 110, 201801 (2013) - 2009-2013 data set: 7.5x10 14 stopped µ + this result 11

Detector resolutions quantum efficiencies using LEDs and • Photon energy • Positron energy σ E γ ~1.9% σ Ee+ ~300 keV using a dedicated CW accelerator to Shallow and deep events Positron efficiency 12

Detector resolutions • Relative time σ Te γ ~130ps T e γ = T XEC − L γ ⎡ ⎤ c − T TC − L e + ⎢ ⎥ ⎣ c ⎦ • Relative angle σ θ e γ ~15mrad, σ φγ ~ 9mrad - Decay point of the muon from the quantum efficiencies using LEDs and esolution intersection of the DC track with om the target to the the target plane using a dedicated CW accelerator to - Relative angle from the combination of track direction+ decay point + photon conversion point - No physical process to accurately calibrate the angle - We have to rely on careful geometrical alignment and separate calorimeter and drift chamber resolutions 13

Analysis strategy • Blind-box likelihood analysis strategy • Observables: E e+, E γ , θ e γ , φ e γ ,T e γ 60 (MeV) 58 γ E 56 Blinding Box 54 Negative Positive Timing Timing 52 Side-band Analysis Side-band Window 50 48 Energy Side-band 46 3 2 1 0 1 2 3 − − − t (ns) + e γ 14

Analysis strategy � � • Likelihood function N sig , N RMD , N ACC , t L = e − N y N obs ! C ( N RMD , N ACC , t ) × d N obs � � � N sig S ( x i , t ) + N RMD R ( x i ) + N ACC A ( x i ) . , i = 1 s • Accidental pdfs fully defined from data sidebands: n- - very solid determination of the dominant background • Signal and radiative decay pdfs by combining results of calibration • Correlations between kinematic variables taken into account • Normalization from Michel & RD decays 15

Improvements in the analysis vs last publication Comparison 2009-2011 • Non planar, non negligible target vs last publication ok deformation observed - taken into account in the likelihhod analysis - 13% worse sensitivity • Photons from e+ annihilations inside DC were identified & removed - background rejection~2% - signal inefficiency~1% • Revised the algorithm to recover missing first turn of positron in the DC - Signal efficiency improved by 4% 16

γ Sensitivity from toy Monte Carlo • Average 90% CL upper limit on branching ratio with − γ null-signal hypotesis − • Checked with data sideband-fit • Sensitivity = 5.3x10 -13 − − 140 − − − − − − − 120 − 100 − 80 60 − 40 − 20 − − − − 13 − 10 × Θ 0 0 5 10 15 20 γ Upper limit − − − − − − − − 17

Unblinding the full data set: likelihood fit The best fitted likelihood function (projection) is shown "Signal" is magnified for illustrative purposes 800 (a) (b) (c) 450 sum 3 10 700 400 350 600 E e E γ accidental 300 500 2 10 Total 250 400 Accidental t e γ 200 300 Radiative 150 10 200 radiative 
 Signal 100 decay 100 50 1 − 0.6 − 0.4 − 0.2 0 0.2 0.4 0.6 0.05 0.051 0.052 0.053 0.054 0.055 0.056 0.048 0.05 0.052 0.054 0.056 0.058 t (ns) E (GeV) E (GeV) e γ e γ 400 (d) 400 (e) (f) NO SIGNAL 350 350 300 300 R sig N acc = 7684 ± 103 2 10 250 250 N RD = 663± 59 θ e γ φ e γ 200 200 signal 150 150 10 100 100 50 50 1 − 0.04 − 0.02 0 0.02 0.04 − 0.06 − 0.04 − 0.02 0 0.02 0.04 0.06 − 10 − 8 − 6 − 4 − 2 0 2 4 (rad) θ (rad) φ R e γ e γ sig 18

2D likelihood projection and event distribution Requiring Requiring 1 σ , 1.64 σ , 2 σ contours are shown 19

BR(µ → e γ ) limit result BR (µ → e γ ) < 4.2x 10 -13 at 90% C.L. submitted to EPJC DATA timing sideband Note: Upper limit from frequentistic procedure a la Feldman-Cousins 20

Next: MEG upgrade: MEG-II Extending the search of µ → e γ is complementary to New • Physics searches at the high energy frontier optimized to enhance sensitivity (accidental background prop. to I 2 µ ) 21 � 15

MEG-II detector highlights: Liquid Xenon Liquid Xenon Calorimeter with higher granularity in inner face: => better resolution, better pile-up rejection Large UV-ext SiPM • Developed UV sensitive MPPC (vacuum UV 12x12mm 2 SiPM) • Detector under commissioning (calibrations by end of 2016) 22 17

MEG-II detector highlights: Drift Chamber • Single volume drift chamber with 2 π coverage - 2m long Gradient 
 - 1200 sense wires Magnetic - stereo angle (8°) DC Field - low mass Old - high trasparency to TC TC (double signal efficiency) y! Wiring in progress, to be • DC completed by end of 2016) New TC 23

MEG-II detector highlights: Timing Counter • Scintillator tiles read by SiPM - 1/4 of the detector installed and tested on beam with Michel decays last December • To be completed and tested by the end of 2016 w TC ec !) 
 24

MEG-II detector highlights: Radiative Decay Counter 50% of the background photons comes from radiative muon decay with • positron along the beam line Can be vetoed by detecting the positron in coincidence with the γ • 25

New Electronics • New version of DRS custom digitization board integrating both digitization, triggering and some HV (four times more channels than before) About 1000 channels ready to be tested for the end of the • year Final production expected in spring 2017 • 26

MEG-II goals • Beam rate ~7x10 7 µ/s • Final sensitivity: 4x10 -14 27


 
 
 
 MEG-II schedule • Successfull pre-engineering run in late 2015 • Engineering run foreseen at end of 2016 with several parts of the MEG-II detector Expect full detector ready and run in 2017 • Note: this schedule assume exclusive use of PiE5 beam line by MEG-II 28

Conclusion • New constraint on the µ → e γ decay set by the MEG experiment with its final dataset: 7.5x10 14 stopped µ + BR (µ → e γ ) < 4.2x 10 -13 at 90% C.L. submitted to EPJC • MEG-II detector is in the construction phase - same design of MEG but better resolution • By the end of a decade sensitivity pushed to ~4x10 -14 • Ultimate µ + → e + γ ? - PSI HiMB Project: ~1.3x10 10 µ/s seems possible.. - Need to fight accidental background (photon conversion?) 29

Backup 30

Examples 31

Calibrations 32