Status and prospects of Lepton Flavor Violation searches Marco Grassi INFN Sezione di Pisa DISCRETE 2010 Rome, 6-11 December

Outline Physics motivation for cLFV searches The muons The taus m  eg (MEG) m  e e e (SINDRUM) mA  eA (Mu2e, COMET, PRISM/PRIME) The taus t  mg, eg (BABAR, BELLE) t  lll (BABAR, BELLE) Conclusions Rome - 10 December 2010

Physics motivations The Standard Model is believed to be a low-energy approximation a of a more fundamental theory DM, number of parameters, unexplained symmetries, flavor structure, … All models beyond the SM contains new particles that could either be directly discovered (high energy frontier) or seen through their contributions in loops (high precision frontier) Rome - 10 December 2010

Physics motivation 1) The cLFV decay is undetectably small in the extended Standard Model, which takes into account the neutrino masses and mixings Example m  e g decay BR ~ 10-54 2) New Physics scenarios usually enhance the rate of cLFV decay by 30-40 orders of magnitude, through loops of new particles cLFV decays are clean, no SM contaminated, evidence of new physics The expected rates are close to the experimental upper bound and within the capabilities of present and near future experiments Rome - 10 December 2010

Physics connections →γ EDM μ→eγ g - 2 μZ→eZ G.Isidori et al. Phys. Rev. D75 (2007) 115019 Example In a specific model MSSM /GUT extension with large tan →γ μ→eγ EDM g - 2 μZ→eZ This is still a rare process, as proton decay or dark matter search or neutrino oscillation. Also here one searches for the violation of one of the SM accidental symmetries If the E821 anomaly is real (3.2 ) It should be Rome - 10 December 2010

Physics connections g-2 , EDM Within a given NP scenario different processes are linked together with a specific pattern We could even have signal on one channel and nothing on others μ→eγ , μZ→eZ , →eγ , →γ , →lll , →h g-2 , EDM or in other words The structure and the couplings of NP beyond the SM could be constrained by multiple precision measurements unfortunately No clean and direct evidence of New Physics has been established so far Rome - 10 December 2010

The muons Muons are excellent probes for high precision measurements intense continuous and pulsed beams can be obtained; long lived; simple final states at low energy : small detectors; Rome - 10 December 2010

Historical perspective Hinks & Pontecorvo ? ! Crystal Box MEGA Each improvement linked to the technology either in the beam or in the detector Always a trade-off between various sub-detectors to achieve the best “sensitivity”

μ→eγ : MEG experiment Picture ~60 physicists, 12 Institutions, 5 Countries Italy, Japan, Russia, Switzerland, USA

Easy signal selection with μ+ decay at rest Experimental method Detector outline • Stopped beam of >107 μ /sec in a 175 μm target • g detection Liquid Xenon calorimeter based on the scintillation light - fast: 4 / 22 / 45 ns - high LY: ~ 0.8 * NaI - short X0: 2.77 cm • e+ detection magnetic spectrometer composed by solenoidal magnet and drift chambers for momentum scintillation counters for timing Easy signal selection with μ+ decay at rest e+ + g Ee = Eg = 52.8 MeV qeg = 180° Rome - 10 December 2010

Signal and Background qeg = 180° FWHM m+ e+ g n m+ e+ g n m+ e+ g Signal Prompt Accidental m+ e+ g n m+ e+ g n m+ e+ g qeg = 180° Ee = Eg = 52.8 MeV Te = Tg at 3x107 m-stop/s The accidental background is dominant and it is determined by the experimental resolutions FWHM Exp./Lab Year ΔEe/Ee (%) ΔEγ /Eγ Δteγ (ns) Δθeγ (mrad) Stop rate (s-1) Duty cyc.(%) BR (90% CL) SIN 1977 8.7 9.3 1.4 - 5 x 105 100 3.6 x 10-9 TRIUMF 10 6.7 2 x 105 1 x 10-9 LANL 1979 8.8 8 1.9 37 2.4 x 105 6.4 1.7 x 10-10 Crystal Box 1986 1.3 87 4 x 105 (6..9) 4.9 x 10-11 MEGA 1999 1.2 4.5 1.6 17 2.5 x 108 (6..7) 1.2 x 10-11 MEG 2008 - x 1 0.15 19 3 x 107 23 x 10-13

Electrostatic separator μ beam Electrostatic separator COBRA Transport solenoid Target Quadrupoles Intensity’ (μ-stop/s) Low 2.5 x 106 Normal 3.2 x 107 Max 2 x 108 characteristics P = 27.7 MeV/c ΔP = 0.3 MeV/c σX = 9.5 mm σY = 10. mm MEG target Material CH2 q = 22.5o thick = 205 mm Size = 15x25 cmxcm Rome - 10 December 2010

The positron spectrometer 16 low-mass Drift Chambers in a Helium atmosphere with a graded magnetic field : very low total material budget (< 0.005 X0); fast expulsion of tracks from the spectrometer even at large polar angles. Graded Field Design Resolutions Momentum: 200 keV/c Direction: 4.5 mrad Rome - 10 December 2010

The timing counter 2 detectors (upstream & downstream) for precise positron timing and trigger; 15 plastic scintillating bars per detector read by PMTs: timing phi position trigger 1 layer of scintillating fibers per detector, read by APDs: zposition Design Resolution Timing : 45 ps

The LXe calorimeter The largest LXe calorimeter in the world: 800 liters; Fast response: t = 4ns / 22ns / 45ns; Good light yield: ~ 75% ofNaI(Tl); Light collected by 846 PMTs. Design Resolutions Energy: 2.4 MeV (FWHM) Conversion Point: 4 mm Time: 65 ps Hamamatsu R9288 Rome - 10 December 2010

MEG key elements MEG is a precision experiment and demands for unprecedented detector resolutions accurate determination of detector response with time The detector is complemented with an impressive set of calibration tools However the limiting factor is not the beam intensity but the resolutions there is space for improvements the resolutions are not yet the design one, but are constantly improving Rome - 10 December 2010

Calibration tools m radiative decay Laser LED e m n p0 gg alpha PMT Gain Higher V with light att. Can be repeated frequently alpha PMT QE & Att. L Cold GXe LXe Laser (rough) relative timing calib. < 2~3 nsec Nickel g Generator 9 MeV Nickel γ-line NaI Polyethylene 0.25 cm Nickel plate 3 cm 20 cm quelle on off Illuminate Xe from the back Source (Cf) transferred by comp air  on/off Proton Acc Li(p,)Be LiF target at COBRA center 17.6MeV g ~daily calib. + B target  (4.4, 11.7, 16.1 MeV) lines – Energy + Timing K Bi Tl F Li(p, 0) at 17.6 MeV Li(p, 1) at 14.6 MeV m radiative decay p0 gg p- + p  p0 + n p0  gg (55MeV, 83MeV) p- + p  g + n (129MeV) 10 days to scan all volume precisely (faster scan possible with less points) LH2 target g e+ e- e n m Lower beam intensity < 107 Is necessary to reduce pile-ups Better st, makes it possible to take data with higher beam intensity A few days ~ 1 week to get enough statistics MEG Detector Standard Calibrations Rome - 10 December 2010

Resolutions πº Eg Ee+ Teg µ→eγ runs RMD Michel g bck Double gaussian momentum resolution s(p) = 390 KeV (0.74%) (core) Positron angle resolution measures using multi-loop tracks s(f) = 7.1 mrad (core) s(q) = 11.2 mrad Overall angular resolution combining XEC+DCH+target s(f) = 12.7 mrad (core) s(q) = 14.7 mrad πº µ→eγ runs RMD 40 MeV < Eg < 48 MeV Resolution corrected for a small energy-dependence s(t) = (142  15) ps Stability along the run < 15 ps Michel g bck 48 50 52 54 56 58 Average upper tail for deep conversions sR = (2.1 0.15) % Systematic uncertainty on energy scale < 0.6 % Rome - 10 December 2010

Uniformity Calorimeter uniformity Timing uniformity and stability 55 MeV g (from p0 decay) Eg resolution vs conversion point Close to design value Timing uniformity and stability Events from raditive m decay (RD) Requires LXe+TC+track Close to design value (if rescaled for the different Eg energy) Rome - 10 December 2010

Analysis principle 2009 data sample A µ→eγ event is described by 5 kinematical variables We decided to adopt a likelihood analysis strategy on a wide blind box The blinding variables are Hidden until analysis is fixed Use of the sidebands our main background comes from accidental coincidences RMD can be studied in the low Eγ sideband Left Sideband Right Sideband Blind Box E Sideband Rome - 10 December 2010

Analysis principle Likelihood function is built in terms of Signal, radiative Michel decay RMD and accidental background BG number of events and their probability density function PDFs Un-binned likelihood fit of over the entire blind box Three different analysis for cross check PDF Approach (freq. or Bayes) Rome - 10 December 2010

Probability Density Functions Signal - calibration data (p0, Michel edge, CW, XEC single events ...) for photon/positron energy and relative angle - RD data on sideband for timing (corrected for energy dependence) RMD - 3-D theoretical distribution folded with detector response to take into account kinematical constraints - direct measurement for timing BG - Everything measured on sidebands Important: PDF for BG are measured ! Rome - 10 December 2010

Systematic uncertainties The effect of systematics is taken into account in the calculation of the confidence region by fluctuating the pdfs according to the uncertainty values overall effect of systematics: ΔNsig ~ 1 Uncertainty Normalization 8% Pe+ εγ εTRG Eγ scale 0.4% Light yield stability, gain shift Eγ resolution 7% Ee scale 50 keV from Michel edge Ee resolution 15% teγ center 15 ps teγ resolution 10% RMD peak Angle 7.5 mrad Tracking + LXe position Angular resolution Ee–φe correlation 50% MC evaluation

Normalization B.R. = Nsig x (1.01 ± 0.08) × 10-12 The normalization factor is obtained from the number of observed m decay positrons taken simultaneously (prescaled) with the m→eg trigger Cancel at first order Absolute e+ efficiency and DCH instability Instantaneous beam rate variations O(1) ~18k 107 theory resolution acceptance B.R. = Nsig x (1.01 ± 0.08) × 10-12

Control samples MEGPreliminary MEGPreliminary MEGPreliminary Likelihood analysis performed on the Left and Right teγ sidebands gives Nsig and NRMD = 0 BR < (4 ÷ 6) x 10-12 Experiment sensitivity is computed as the average 90% upper limit on toy experiments no signal in MC generation 6.1 x 10-12 MEGPreliminary MEGPreliminary MEGPreliminary Blue lines are 1(39.3 % included inside the region w.r.t. analysis window), 1.64(74.2%) and 2(86.5%) sigma regions. For each plot, cut on other variables for roughly 90% window is applied. MEGPreliminary

Likelihood analysis MEGPreliminary Signal RADIATIVE DECAY ACCIDENTAL TOTAL (dashed: 90% U.L) Best fit: Nsig = 3.0, NRMD = 35+24-22 (expected , NRMD = 32 ± 2) Nsig < 14.5 @ 90% C.L. (Feldman-Cousins)

Result From the analysis of 2009 data the MEG Upper Limit is BR(m→eg ) @90% CL < 1.5 x 10-11 Notes The three analyses are consistent The Nsig = 0 hypothesis has a probability of 20-60% The MEG published result on 2008 data was (Nucl.Phys. B834(2010)1-12) BR(m→eg ) @90% CL < 2.3 x 10-11 The best Upper Limit from MEGA is (Phys.Rev. D65 (2002) 112002) BR(m→eg ) @90% CL < 1.2 x 10-11 MEGPreliminary Rome - 10 December 2010

Blind box content MEGPreliminary MEGPreliminary MEGPreliminary

Blind box content MEGPreliminary MEGPreliminary MEGPreliminary

MEG Event display Events in the signal region were checked carefully An event in the signal region

MEG 2010 2010 run prematurely ended by a quench in the Beam Transport Solenoid 2009 statistics only doubled Found burned feed-through, if nothing else, the 2011 run shall start after the PSI winter shutdown The collaboration is considering the publication of combined 2009-2010 data in this spring Rome - 10 December 2010

MEG perspectives - Seen detector improvements Not only statistics MC description of the LXe to implement different Alignment more dedicated XEC–DCH coincidence Matrix of lead dices to improve Lxe position reconstruction Mott scattering target to calibrate the tracker with monochromatic variable energy positron tracking improvement better treatment of rapidly varying magnetic field cross talk between adjacent Vernier pads shadow effect from the anode wires on the Vernier pads - Seen detector improvements - Two full year run 2011 / 12 - Working hard to push the sensitivity down to 10-13 - New results soon ! ! ! Analysis Inclusion of information from the sidebands in the likelihood variations of proton current

+e+e+e- background signal   eee accidental   e n n e+e- e+e- correlated   e e e n n e+ + n e- e+ e+ + n Coplanarity Vertexing Ee = m Te+ = Te+ = Te- e+ n e+ n e- e- + Rome - 10 December 2010

+e+e+e- : SINDRUM I Present limit from SINDRUM : B(m3e ) < 1x10-12 U.Bellgardt et al. Nucl.Phys. B299(1988)1 Generalities This channel has only charged particles in the final state The experiment needs only a tracking system but Sustain the entire Michel decay rate Down to low momentum Large solid angle coverage SINDRUM I parameters beam intensity 6x106 m+/s m+ momentum 25 MeV/c magnetic field 0.33T acceptance 24% momentum res. 10% FWHM vertex res.  2 mm2 FWHM timing res.  ns target length 220 mm target density 11 mg/cm2 Is there any future? Expression of interest from Heidelberg for a search aiming at 10-15 based on ultra thin Si tracker with MAPS readout Comment: background scales  quadratically with beam SINDRUM sensitivity 10-12 sensitivity 10-15 beam 6x106 beam 6x108 background 10-13 background 10-10

RPC (radiative pion capture) MIO (muon decay in orbit) -e- conversion Background Signal coherent LFV decay  (A,Z)  e (A,Z) RPC (radiative pion capture)  (A,Z)   (A,Z-1) e- - g (A,Z) - g (A,Z) MIO (muon decay in orbit)  (A,Z)  e n n (A,Z) Ee = mm -EB -ER e- - n (A,Z) Rome - 10 December 2010

-e- physics Effects from almost all NP scenarios These do not contribute to eg Rome - 10 December 2010

-e- : SINDRUM II result SINDRUM II parameters beam intensity 3x107 m-/s m- momentum 53 MeV/c magnetic field 0.33T acceptance 7% momentum res. 2% FWHM S.E.S 3.3x10-13 counts/channel momentum (MeV/c) Present limit from SINDRUM II : B(me:Au ) < 7x10-13 E. Bertl et al. Eur.phys.J. C47 (206) 337 Rome - 10 December 2010

-e- step forward 1 particle in the final state: no accidental Background chance to explore very low BR Event selection based on e- detection only m lifetime ~.86 ms on Al .35 ms on Ti e energy ~105 MeV on Al 104.3 MeV on Ti Key element: beam ! new and expensive beam line and m channels Ultra high intensity m beam ~ 1011 m/s Pulsed P beam Short beam-on (dt ~ 10ns) Long beam-off ( Dt ~ 1 ms ) Huge P extinction factor (10-9 or FFAG) Low momentum m (~ 68 MeV/c) Narrow momentum spread (<2 % with FFAG) Rome - 10 December 2010

Full discussion by F. Cervelli this afternoon Mu2e at Fermilab -e- : Al Sign selection and antiprotons rejection Graded magnetic field to select electrons with P>90 MeV/c and recover backwards going electrons Expected tracker resolution 900 keV/c FWHM @100 MeV 8 GeV, 100 ns width bunches Full discussion by F. Cervelli this afternoon Rome - 10 December 2010

Mu2e summary Goal: explore region down to 10-16 in 5 years expected 40 evts for Rme =10-15 Critical element: proton beam Background – free measurement: requires p-extinction of 10-9 Detector magnet; straight tracker far from the bulk of m decay Detector: again resolutions on momentum and energy 900 KeV/c FWHM @100 MeV/c Status: got CD-0 approval Time scale: several years D. Glezinsky, NuFact 09 Rome - 10 December 2010

COMET at J-PARC -e- : Al Y. Kuno, Nufact 10 Rome - 10 December 2010

COMET summary Similar to Mu2e Goal: explore region down to 6x10-17 in 2 years expected 40 evts for Rme =10-15 Critical element: beam Background – free measurement: requires p-extinction of 10-9 Detector magnet; curved to protect the detector from the bulk of m decay Detector: high resolutions on momentum and energy 400 KeV/c FWHM @100 MeV/c Status: Stage 1 approval in 2009 Timescale: (very optimistic) construction 2012 running 2017 Rome - 10 December 2010

PRISM/PRIME : 2nd stage A. Sato, NP08 PRIsm Muon Electron conversion experiment Phase Rotated Intense Slow Muon source A. Sato, NP08 Rome - 10 December 2010

PRISM: motivation Phase rotation - Energy spread of m reduced to 3 % FWHM by means of RF in the FFAG ring - Well defined m momentum 68 MeV/c - Large aperture, high intensity  1011÷12 m/s - Long m flight length 10-23 p suppression - Kickers in the FFAG : p extinction not crucial Phase rotation Small energy spread allows very thin targets Momentum resolution  350 keV (FWHM) could be possible Background reduced at 0.1 events Experiment sensitive down to BR  10-18 Experimental demonstration of phase rotation in FFAG in progress at Osaka Far in the future … Rome - 10 December 2010

The taus Taus are excellent probes for high precision measurement with differences with respect to muons Massive initial state, many channels Massive initial state, effects enhanced by 35 orders of mag Intensity could be an issue 10+10 t/year vs 10+8 m/s Short lived Complex detectors Rome - 10 December 2010

LFV in t decays In NP models limits are linked: μ→eγ at 10-12 implies →γ at >10-9 impressively close to experimental bounds Rome - 10 December 2010

Detection strategy b - factories are also excellent t - factories Search strategy Identify tt couples with one of the possible t – tags on one side Search LFV on the other side Background Generally larger for mg and eg channels, smaller for the other modes Tails of the tt distributions, mm, uds, Bhabha, cc Rome - 10 December 2010

t  mg/eg BABAR Data sample 468 fb-1 @(4S) 90 fb-1 @(nS) (963  7) x 106 t decays Total sample collected by BABAR BABAR Collaboration hep-ex/0908.2381v2 BABAR Collaboration PRL 104 (2010) 021802 Full discussion by A. Cervelli this afternoon See also E. Guido on Tuesday Rome - 10 December 2010

t  mg/eg BABAR Recipe Look for one single m (or e) plus one  in the signal side Reconstruct (it should be = mt) Reconstruct (it should be zero) Blind analysis to evaluate the background Open a 2s region and count eg Green ellipse (2 s’s): events observed 2 (m) 0 (e) eff. (6.1  0.5) % (m) events expected 3.6 (m) 1.6 (e) (3.9  0.3) % (e) Upper Limit @ 90% C.L.: BR( t  mg ) < 4.4 x 10-8 BR( t  eg ) < 3.3 x 10-8 mg Rome - 10 December 2010

See talk of K. Hayasaka on Tuesday t  mg/eg BELLE Data sample Integrated luminosity 535 fb-1  4.77 x 108 t pairs Similar search strategy: look for two opposite charge tracks, accompanied in “signal-side” by one or more photons reduce other background by cuts on missing quantities; examine surviving events in the plane (DE, Mmg); BELLE Collaboration (K. Abe et al.), PL B666 (2008) 16-22 See talk of K. Hayasaka on Tuesday Rome - 10 December 2010

t  mg/eg BELLE 3 s ellipse mg 2 s ellipse eg Data Signal MC Maximum likelihood fit to signal & bck. UL: BR( tmg ) < 4.5 x 10-8 BR( teg ) < 1.2 x 10-7

t  lll BABAR Standard 1-prong tag BABAR Coll. PhysRevD81, 111101 (2010) Standard 1-prong tag Three tracks of opposite tot charge on the signal Data sample 468 fb-1 Search based on invariant mass and DE Very low background on these channels no excess observed U.L. Range: (1.8 ÷ 3.3) x 10-8 (90% C.L.) Rome - 10 December 2010

t  lll BELLE Data sample 782 fb-1 Very low background as for BABAR. BELLE Coll. PL B687 (2010) 139-143 Data sample 782 fb-1 Very low background as for BABAR. U.L. Range: (1.5 ÷ 2.7) x 10-8 (90% C.L.) t3l search as no irreducible background (no photons no problems with initial state radiation) Rome - 10 December 2010

Future: super B factories Projects of Super-B factories in Japan (KEKB upgrade) approved start 2014 5 years Italy (Frascati) approval requested Expected luminosities: 1035 cm-2 s-1 (SuperKEKB), 1036 cm-2 s-1 (SuperB) Both projects would reach an integrated luminosity L = 75 ab-1 SuperB could exploit the beam polarization to reduce the background on t  lg Detector upgrades are foreseen Expected sensitivities - BR(t  lg) < 2 x 10-9 - BR(t  3l) < 2 x 10-10 - BR(t  lh or lhh) < (2 ÷ 6) x 10-10 Specific talks this afternoon A. Cervelli SuperB S. Korpar SuperKEKB Rome - 10 December 2010

Conclusions General arguments suggest that New Physics should be close to the experimental reach, both in particle searches and effects through loops The structure and the couplings of NP could be constrained by multiple precision measurements MEG is close to publish new data and it’s ready to explore an interesting region down to a few 10-13 Challenging μ→e conversion searches are expected to start in 2016 Data on t LFV decays are not yet fully analyzed, and new powerful machines are under construction or planned Rome - 10 December 2010

University of Virginia Graphically Super-B factories MEG Mu2e, COMET E. Craig Dukes University of Virginia

Bibliography Barbieri et al,, Nucl. Phys B445 (1995) 225 Hisano et al., Phys. Lett. B391 (1997) 341 Masiero et al., Nucl. Phys. B649 (2003) 189 Bettoni et al., Phys. Rep. 434 (2006) 74 Calibbi et al., Phys. Rev. D74 (2006) 116002 Isidori et al., Phys. Rev. D75 (2007) 115019 Raidal et al., Eur. Phys. J. C57 (2008) 13 Rome - 10 December 2010

Spare

The Paul Sherrer Institute (PSI) The most powerful continuous machine (proton cyclotron) in the world; Proton energy 590 MeV; Power 1.2 MW; Nominal operational current 2.2 mA. Rome - 10 December 2010

Expected performance Rome - 10 December 2010

-e- : sensitivity R.Kitano et al Phys.Rev.D66(2002)096002 Rome - 10 December 2010