Lepton Flavor Violation: Goals and Status of the MEG Experiment at PSI Stefan Ritt Paul Scherrer Institute, Switzerland

26 June '07Particle Colloquium Heidelberg2 Agenda Search for   e  down to Motivation Experimental Method Status and Outlook

Motivation Why should we search for  e  ?

26 June '07Particle Colloquium Heidelberg4 The Standard Model Fermions (Matter) Quarks u up c charm t top d down s strange b bottom Leptons e electron neutrino  muon neutrino  tau neutrino e electron  muon  tau Bosons  photon Force carriers g gluon W W boson Z Z boson Higgs* boson *) Yet to be confirmed Generation I II III

26 June '07Particle Colloquium Heidelberg5 The success of the SM The SM has been proven to be extremely successful since 1970’s Simplicity (6 quarks explain >40 mesons and baryons) Explains all interactions in current accelerator particle physics Predicted many particles (most prominent W, Z ) Limitations of the SM Currently contains 19 (+10) free parameters such as particle (neutrino) masses Does not explain cosmological observation such as Dark Matter and Matter/Antimatter Asymmetry CDF Today’s goal is to look for physics beyond the standard model

26 June '07Particle Colloquium Heidelberg6 Beyond the SM Find New Physics Beyond the SM High Energy Frontier Produce heavy new particles directly Heavy particles need large colliders Complex detectors High Precision Frontier Look for small deviations from SM (g-2) , CKM unitarity Look for forbidden decays Requires high precision at low energy

26 June '07Particle Colloquium Heidelberg7 Neutron beta decay Neutron  decay via intermediate heavy W - boson d d u u d u p n W - e-e- e n  p + + e - + e ~80 MeV ~5 MeV Neutron mean life time: 886 s Neutron mean life time: 886 s  decay discovery: ~1934 W - discovery: 1983  decay discovery: ~1934 W - discovery: 1983

26 June '07Particle Colloquium Heidelberg8 New physics in  decay Can’t we do the same in  decay?   e-e- ? ?  Probe physics at TeV scale with high precision  decay measurement

26 June '07Particle Colloquium Heidelberg9 Discovery: 1936 in cosmic radiation Mass: 105 MeV/c 2 Mean lifetime: 2.2  s The Muon Seth Neddermeyer Carl Anderson e --  W-W- e-e- ≈ 100% < led to Lepton Flavor Conservation as “accidental” symmetry

26 June '07Particle Colloquium Heidelberg10 Lepton Flavor Conservation Absence of processes such as  e  led to concept of lepton flavor conservation Similar to baryon number (proton decay) and lepton number conservation These symmetries are “accidental” because there is no general principle that imposes them – they just “happen” to be in the SM (unlike charge and energy conservation) The discovery of the failure of such a symmetry could shed new light on particle physics

26 June '07Particle Colloquium Heidelberg11 LFV and Neutrino Oscillations Neutrino Oscillations  Neutrino mass   e  possible even in the SM  W-W-   e e-e-  LFV in the charged sector is forbidden in the Standard Model mixing

26 June '07Particle Colloquium Heidelberg12 LFV in SUSY While LFV is forbidden in SM, it is possible in SUSY  W-W-   e e-e-   e-e- ≈ Current experimental limit: BR(   e  ) <

26 June '07Particle Colloquium Heidelberg13 LFV is forbidden in the SM, but possible in SUSY (and many other extensions to the SM) though loop diagrams (  heavy virtual SUSY particles) If   e  is found, new physics beyond the SM is found Current exp. limit is , predictions are around … First goal of MEG: Later maybe push to ($$$) Big experimental challenge Solid angle * efficiency (e,  ) ~ 3-4 % 10 7 – 10 8  /s DC beam needed ~ 2 years measurement time excellent background suppression LFV Summary

26 June '07Particle Colloquium Heidelberg14 History of LFV searches Long history dating back to 1947! Best present limits: 1.2 x (MEGA)  Ti → eTi < 7 x (SINDRUM II)  → eee < 1 x (SINDRUM II) MEG Experiment aims at Improvements linked to advance in technology  → e   → eA  → eee MEG SUSY SU(5) BR(   e  ) =   Ti  eTi = 4x  BR(   eee) = 6x SUSY SU(5) BR(   e  ) =   Ti  eTi = 4x  BR(   eee) = 6x cosmic  stopped   beams stopped 

26 June '07Particle Colloquium Heidelberg15 Current SUSY predictions “Supersymmetric parameterspace accessible by LHC” W. Buchmueller, DESY, priv. comm. current limit MEG goal 1)J. Hisano et al., Phys. Lett. B391 (1997) 341 2)MEGA collaboration, hep-ex/ f t (M)=2.4  >0 M l =50GeV 1) tan 

26 June '07Particle Colloquium Heidelberg16 LFV link to other SUSY proc.   e-e-    e-LFV  g        EDM slepton mixing matrix: In SO(10), eEDM is related to  e  R. Barbieri et al., hep-ph/

Experimental Method How to detect  e  ?

26 June '07Particle Colloquium Heidelberg18 Decay topology  e  e     e  180º  → e  signal very clean E g = E e = 52.8 MeV   e = 180º e and  in time 52.8 MeV E  [MeV] N 52.8 MeV E e [MeV] N 52.8 MeV

26 June '07Particle Colloquium Heidelberg19 Michel Decay (~100%) Three body decay: wide energy spectrum e   e E e [MeV] N 52.8 MeV E e [MeV] N 52.8 MeV Theoretical Convoluted with detector resolution

26 June '07Particle Colloquium Heidelberg20 Radiative Muon Decay (1.4%) e    e  E  [MeV] N 52.8 MeV “Prompt” Background

26 June '07Particle Colloquium Heidelberg21 “Accidental” Background e     e  180º  → e  signal very clean E g = E e = 52.8 MeV   e = 180º e and  in time e    e e    e Annihilation in flight Background Good energy resolution Good spatial resolution Excellent timing resolution Good pile-up rejection

26 June '07Particle Colloquium Heidelberg22 Previous Experiments Exp./ Lab AuthorYear  E e /E e %FWHM  E  /E  %FWHM  t e  (ns)  e  (mrad) Inst. Stop rate (s -1 ) Duty cycle (%) Result SIN (PSI) A. Van der Schaaf (4..6) x < 1.0  TRIUMF P. Depommier x < 3.6  LANL W.W. Kinnison x < 1.7  Crystal Box R.D. Bolton x 10 5 (6..9) < 4.9  MEGAM.L. Brooks x 10 8 (6..7) < 1.2  MEG?????? ~ How can we achieve a quantum step in detector technology?

26 June '07Particle Colloquium Heidelberg23 How to build a good experiment?

26 June '07Particle Colloquium Heidelberg24 Collaboration ~70 People (40 FTEs) from five countries

26 June '07Particle Colloquium Heidelberg25 Paul Scherrer Institute Swiss Light Source Proton Accelerator

26 June '07Particle Colloquium Heidelberg26 PSI Proton Accelerator

26 June '07Particle Colloquium Heidelberg27 Generating muons Carbon Target 590 MeV/c 2 Protons 1.8 mA = p + /s ++ ++ ++ +  + /s

26 June '07Particle Colloquium Heidelberg28 Muon Beam Structure Muon beam structure differs for different accelerators Pulsed muon beam, LANL Duty cycle: Ratio of pulse width over period DC muon beam, PSI Duty cycle: Ratio of pulse width over period Duty cycle: 6 % Duty cycle: 100 % Instantaneous rate much higher in pulsed beam

26 June '07Particle Colloquium Heidelberg29 Muon Beam Line Transport 10 8  + /s to stopping target inside detector with minimal background  + from production target x x xx x x ++ e+e+ - + Lorentz Force vanishes for given v: Wien Filter

26 June '07Particle Colloquium Heidelberg30 Results of beam line optimization ++ R  ~ 1.1x10 8  + /s at experiment  ~ 10.9 mm e+e+ ++

26 June '07Particle Colloquium Heidelberg31 Previous Experiments Exp./ Lab AuthorYear  E e /E e %FWHM  E  /E  %FWHM  t e  (ns)  e  (mrad) Inst. Stop rate (s -1 ) Duty cycle (%) Result SIN (PSI) A. Van der Schaaf (4..6) x < 1.0  TRIUMF P. Depommier x < 3.6  LANL W.W. Kinnison x < 1.7  Crystal Box R.D. Bolton x 10 5 (6..9) < 4.9  MEGAM.L. Brooks x 10 8 (6..7) < 1.2  MEG???? 3 x ~

26 June '07Particle Colloquium Heidelberg32 The MEGA Experiment Detection of  in pair spectrometer Pair production in thin lead foil Good resolution, but low efficiency (few %) Goal was , achieved was 1.2 x Reason for problems: Instantaneous rate 2.5 x 108 m/s Design compromises MHz rate/wire Electronics noise & crosstalk Lessons learned: Minimize inst. rate Avoid pair spectrometer Carefully design electronics Invite MEGA people!

26 June '07Particle Colloquium Heidelberg33 Photon Detectors 50 MeV) Alternatives to Pair Spectrometer: Anorganic crystals: – Good efficiency, good energy resolution, poor position resolution, poor homogeneity – NaI (much light), CsI (Ti,pure) (faster) Liquid Noble Gases: – No crystal boundaries – Good efficiency, resolutions 25 cm CsI  induced shower Density3 g/cm 3 Melting/boiling point161 K / 165 k Radiation length2.77 cm Decay time45 ns Absorption length> 100 cm Refractive index1.57 Light yield75% of NaI (Tl) Liquid Xenon: CsI PMT

26 June '07Particle Colloquium Heidelberg34 Liquid Xenon Calorimeter Calorimeter: Measure  Energy, Position and Time through scintillation light only Liquid Xenon has high Z and homogeneity ~900 l (3t) Xenon with 848 PMTs (quartz window, immersed) Cryogenics required: -120°C … -108° Extremely high purity necessary: 1 ppm H 2 0 absorbs 90% of light Currently largest LXe detector in the world: Lots of pioneering work necessary  

26 June '07Particle Colloquium Heidelberg35 Light is distributed over many PMTs Weighted mean of PMTs on front face  x Broadness of distribution  Dz Position corrected timing  Dt Energy resolution depends on light attenuation in LXe LXe  response x z

26 June '07Particle Colloquium Heidelberg36 Light is distributed over many PMTs Weighted mean of PMTs on front face   x Broadness of distribution   z Position corrected timing   t Energy resolution depends on light attenuation in LXe LXe  response x z

26 June '07Particle Colloquium Heidelberg37 Use GEANT to carefully study detector Optimize placement of PMTs according to MC results

26 June '07Particle Colloquium Heidelberg38 LXe Calorimeter Prototype ¼ of the final calorimeter was build to study performance, purity, etc. 240 PMTs

26 June '07Particle Colloquium Heidelberg39 How to get 50 MeV  ’s?  - p   0 n (Panofsky)  0   LH 2 target Tag one  with NaI & LYSO  - p   0 n (Panofsky)  0   LH 2 target Tag one  with NaI & LYSO

26 June '07Particle Colloquium Heidelberg40 Resolutions NaI tag: 65 MeV < E(NaI) < 95 MeV Energy resolution at 55 MeV: (4.8 ± 0.4) % FWHM LYSO tag for timing calib.: (LYSO) 140 (beam) = 150 ps (FWHM) Position resolution: 9 mm (FWHM) FWHM = 4.8% FWHM = 260 ps To be improved with refined analysis methods

26 June '07Particle Colloquium Heidelberg41 Lessons learned with Prototype Two beam tests, many  -source and cosmic runs in Tsukuba, Japan Light attenuation much too high (~10x) Cause: ~3 ppm of Water in LXe Cleaning with “hot” Xe-gas before filling did not help Water from surfaces is only absorbed in LXe Constant purification necessary Gas filter system (“getter filter”) works, attenuation length can be improved, but very slowly (  ~350 hours) Liquid purification is much faster First studies in 1998, final detector ready in 2007

26 June '07Particle Colloquium Heidelberg42 Xenon storage ~900L in liquid, largest amount of LXe ever liquefied in the world

26 June '07Particle Colloquium Heidelberg43 Final Calorimeter Currently being assembled, will go into operation summer ‘07

26 June '07Particle Colloquium Heidelberg44 Positron Spectrometer Ultra-thin (~3g/cm2) superconducting solenoid with 1.2 T magnetic field Homogeneous Field e + from  +  e +  Gradient Field (COnstant-Bending-RAdius) high p t trackconstant |p| tracks

26 June '07Particle Colloquium Heidelberg45 Drift Chamber Measures position, time and curvature of positron tracks Cathode foil has three segments in a vernier pattern  Signal ratio on vernier strips to determine coordinate along wire

26 June '07Particle Colloquium Heidelberg46 Positron Detection System 16 radial DCs with extremely low mass He:C 2 H 6 gas mixture Test beam measurements and MC simulation:  p/p = 0.8%  = 10 mrad  x vertex = 2.3 mm FWHM

26 June '07Particle Colloquium Heidelberg47 Timing Counter Staves along beam axis for timing measurement Resolution 91 ps FWHM measured at Frascati e - - beam Curved fibers with APD readout for z-position Staves along beam axis for timing measurement Resolution 91 ps FWHM measured at Frascati e - - beam Curved fibers with APD readout for z-position ExperimentSize [cm]ScintillatorPMT att [cm] FWHM[ps] G.D. Agostini3 x 15 x 100NE114XP T. Tanimori3 x 20 x 150SCSN38R T. Sugitate4 x 3.5 x 100SCSN23R R.T. Gile5 x 10 x 280BC408XP TOPAZ4.2 x 13 x 400BC412R R. Stroynowski2 x 3 x 300SCSN38XP BELLE4 x 6 x 255BC408R MEG4 x 4 x 90BC404R

26 June '07Particle Colloquium Heidelberg48 The complete MEG detector

26 June '07Particle Colloquium Heidelberg49 MC Simulation of full detector  e+e+ TC hit “Soft”  s

26 June '07Particle Colloquium Heidelberg50 Beam induced background 10 8  /s produce 10 8 e + /s produce 10 8  /s Cable ducts for Drift Chamber

26 June '07Particle Colloquium Heidelberg51 Detector Performance Prototypes of all detectors have been built and tested Large Prototype Liquid Xenon Detector (1/4) 4 (!) Drift Chambers Single Timing Counter Bar Performance has been carefully optimized Light yield in Xenon has been improved 10x Timing counter 1 ns  100 ps Noise in Drift Chamber reduced 10x Detail Monte Carlo studies were used to optimize material Continuous monitoring necessary to ensure stability!

26 June '07Particle Colloquium Heidelberg52 Sensitivity and Background Rate Aimed experiment parameters: NN 3  10 7 /s T 2  10 7 s (~50 weeks)  /4  0.09 ee 0.90  0.60  sel 0.70 FWHM EeEe 0.8% EE 5%  e  18 mrad tete 180 ps Single event sensitivity (N  T  /4   e    sel ) -1 = 3.6  Prompt Background B pr  Accidental BackgroundB acc   E e  t e  (  E  ) 2 (  e  ) 2  4  % C.L. Sensitivity  1.3  Aimed resolutions:

26 June '07Particle Colloquium Heidelberg53 Current resolution estimates Exp./ Lab AuthorYear  E e /E e %FWH M  E  /E  %FWHM  t e  (ns)  e  (mrad) Inst. Stop rate (s -1 ) Duty cycle (%) Result SIN (PSI) A. Van der Schaaf (4..6) x < 1.0  TRIUMF P. Depommier x < 3.6  LANL W.W. Kinnison x < 1.7  Crystal Box R.D. Bolton x 10 5 (6..9) < 4.9  MEGAM.L. Brooks x 10 8 (6..7) < 1.2  MEG x ~

26 June '07Particle Colloquium Heidelberg54 Current sensitivity estimation Resolutions have been updated constantly to see where we stand Two international reviews per year People are convinced that the final experiment can reach sensitivity

26 June '07Particle Colloquium Heidelberg55 How to address pile-up Pile-up can severely degrade the experiment performance! Traditional electronics cannot detect pile-up TDC Amplifier DiscriminatorMeasure Time Need full waveform digitization to reject pile-up Need full waveform digitization to reject pile-up

26 June '07Particle Colloquium Heidelberg56 Need 500 MHz 12 bit digitization for Drift Chamber system Need 2 GHz 12 bit digitization for Xenon Calorimeter + Timing Counters Need 3000 Channels At affordable price Waveform Digitizing Solution: Develop own “Switched Capacitor Array” Chip

26 June '07Particle Colloquium Heidelberg57 The Domino Principle Shift Register Clock IN Out “Time stretcher” GHz  MHz Waveform stored Inverter “Domino” ring chain ns FADC 33 MHz Keep Domino wave running in a circular fashion and stop by trigger  Domino Ring Sampler (DRS)

26 June '07Particle Colloquium Heidelberg58 The DRS chip 32 channels input DRS chip developed at PSI 5 GHz sampling speed, 12 bits resolution bins on one chip Typical costs ~60 € / channel 3000 Channels installed in MEG Licensing to Industry (CAEN) in progress DRS chip developed at PSI 5 GHz sampling speed, 12 bits resolution bins on one chip Typical costs ~60 € / channel 3000 Channels installed in MEG Licensing to Industry (CAEN) in progress

26 June '07Particle Colloquium Heidelberg59 Waveform examples “virtual oscilloscope” pulse shape discrimination original: first derivation:  t = 15ns Crosstalk removal by subtracting empty channel Quantum Step in Technology!

26 June '07Particle Colloquium Heidelberg60 DAQ System Principle Active Splitter Drift Chamber Liquid Xenon CalorimeterTiming Counter Waveform Digitizing Trigger Event number Event type Busy Rack PC optical link (SIS3100) Rack PC Event Builder Switch GBit Ethernet LVDS parallel bus VME

26 June '07Particle Colloquium Heidelberg61 DAQ System Use waveform digitization (500 MHz/2 GHz) on all channels Waveform pre-analysis directly in online cluster (zero suppression, calibration) using multi-threading MIDAS DAQ Software Data reduction: 900 MB/s  5 MB/s Data amount: 100 TB/year Use waveform digitization (500 MHz/2 GHz) on all channels Waveform pre-analysis directly in online cluster (zero suppression, calibration) using multi-threading MIDAS DAQ Software Data reduction: 900 MB/s  5 MB/s Data amount: 100 TB/year 2000 channels waveform digitizing DAQ cluster

Monitoring How to keep the experiment stable?

26 June '07Particle Colloquium Heidelberg63 Long time stability Especially the calorimeter needs to run stably over years Primary problem: Gain drift of PMT might shift background event into signal region If we find  e , are we sure it’s not an artifact? Need sophisticated continuous calibration! Unfortunately, there is no 52.8 MeV  source available E  [MeV] N 52.8 MeV  e   e 

26 June '07Particle Colloquium Heidelberg64 Planned Calorimeter Calibrations Combine calibration methods different in complexity and energy: MethodEnergyFrequency LED pulser~few MeVContinuously 241 Am source on wire 5.6 MeV  Continuously n capture on Ni 9 MeV  daily p +  7 Li17.6 MeV  daily  0 production on LH 2 54 – 82 MeV  monthly ? LED 100  m gold-plated tungsten wire n  58 Ni 9 MeV

26 June '07Particle Colloquium Heidelberg65 7 Li(p,  ) 8 Be Spectrum 7 Li (p,  ) 8 Be resonant at E p = 440 keV  =14 keV  peak = 5 mb E  0 = 17.6 MeV E  1 = 14.6 MeV 6.1 MeV B peak  0 /(  0+  1 )= Li (p,  ) 8 Be resonant at E p = 440 keV  =14 keV  peak = 5 mb E  0 = 17.6 MeV E  1 = 14.6 MeV 6.1 MeV B peak  0 /(  0+  1 )= 0.72 NaI 12”x12”  spectrum 11 Crystal Ball Data 00

26 June '07Particle Colloquium Heidelberg66 CW Accelerator 1 MeV protons 100  A HV Engineering, Amersfoort, NL 1 MeV protons 100  A HV Engineering, Amersfoort, NL beam spot + -+ - p+p+

26 June '07Particle Colloquium Heidelberg67  0 Calibration NaI target 0000   Tune beam line to  - Use liquid H 2 target  - p    n Tag one  with movable NaI counter Beamline & target change take ~1 day Tune beam line to  - Use liquid H 2 target  - p    n Tag one  with movable NaI counter Beamline & target change take ~1 day 

26 June '07Particle Colloquium Heidelberg68 Midas Slow Control Bus System BTS MagnetLXe purifierLXe storageLXe cryostatNaI mover COBRA DC gas system A/C hut Cooling water VME CratesHV Beamline PC All subsystems controlled by same MSCB system All data on tape Central alarm and history system Also used now at:  SR, SLS, nEDM, TRIUMF All subsystems controlled by same MSCB system All data on tape Central alarm and history system Also used now at:  SR, SLS, nEDM, TRIUMF MSCB HVRSCS-2000

Status and Outlook Where are we, where do we go?

26 June '07Particle Colloquium Heidelberg70 Current Status Goal: Produce “significant” result before LHC R & D phase took longer than anticipated We are currently in the set-up and engineering phase, detector is expected to be completed end of 2007 Data taking will go R&D Engineering Data Taking Set-up

26 June '07Particle Colloquium Heidelberg71 What next? Will we find   e  ? No Improve experiment from to : Denser PMTs Second Calorimeter Improve experiment from to : Denser PMTs Second Calorimeter Yes Carefully check results Be happy Most extensions of the SM (SUSY, Little Higgs, Extra Dimensions) predict   e   More experiments needed Carefully check results Be happy Most extensions of the SM (SUSY, Little Higgs, Extra Dimensions) predict   e   More experiments needed

26 June '07Particle Colloquium Heidelberg72 “Polarized” MEG  are produced already polarized Different target to keep  polarization Angular distribution of decays predicted differently by different theories (compare Wu experiment for Parity Violation) SU(5) SUSY-GUT A = +1 SO(10) SUSY-GUT A  0 MSSM with R A = -1 SU(5) SUSY-GUT A = +1 SO(10) SUSY-GUT A  0 MSSM with R A = -1 Detector acceptance Y.Kuno et al., Phys.Rev.Lett. 77 (1996) 434

26 June '07Particle Colloquium Heidelberg73 Expected Distribution A = +1 B (  +  e +  ) = 1 x x 10 8  + /s 5 x 10 7 s beam time (2 years) P  = 0.97 A = +1 B (  +  e +  ) = 1 x x 10 8  + /s 5 x 10 7 s beam time (2 years) P  = 0.97 S. SUSY 2004, Tsukuba Signal + Background Background

26 June '07Particle Colloquium Heidelberg74 Conclusions The MEG Experiment has good prospectives to improve the current limit for  e  by two orders of magnitude Pushing the detector technologies takes time The experiment is now starting up, so expect exciting results in 2008/2009