The MEG Experiment at PSI: a sensitive search for m eg decay Fabrizio Cei INFN and University of Pisa XV Incontri sulla Fisica delle Alte Energie Lecce, 23-25 April 2003
Outline Physics motivations: The e signature: SUSY indications; Connection with neutrino oscillations. The e signature: Signal & Background. The experimental setup: The muon beam; The positron spectrometer; The timing counter; The Liquid Xenon e.m. calorimeter; Trigger & Electronics. Conclusions: Sensitivity; Time profile. Lecce, April 24, 2003 Fabrizio Cei
Physics Motivations In the Standard Model with massive Dirac neutrinos Lepton Flavour Violation processes (as m eg, t eg, m eee, m e) are predicted at immeasurably small levels . However, Super Symmetric Theories predict such processes at much more reasonable rates. Since the SM background is negligible, processes like m eg are clear evidences for Super Symmetry. Problem: are such rates experimentally observable ? Lecce, April 24, 2003 Fabrizio Cei
SUSY Indications Some predictions: LFV processes especially sensitive to SSM grand unified theories (SUSY-GUT). LFV induced by finite slepton mixing through radiative corrections. Some predictions: SUSY SU(5) BR (m eg) 10-14 10-13 SUSY SO(10) BRSO(10) 100 BRSU(5) (R. Barbieri et al., Phys. Lett. B338(1994) 212 R. Barbieri et al., Nucl. Phys. B445(1995) 215) Lecce, April 24, 2003 Fabrizio Cei
Predictions of m eg BR in SU(5) Models Experimental Bound J. Hisano et al., Phys. Lett. B391 (1997) 341 Goal of MEG Experiment Small (< 10) tan b values are highly disfavoured by recent combined LEP data. (ALEPH, DELPHI, L3 & OPAL Collaborations, hep-ex/0107030) Lecce, April 24, 2003 Fabrizio Cei
Connection with n-oscillations Additional contribution to slepton mixing from V21, matrix element responsible for solar neutrino deficit. (J. Hisano & N. Nomura, Phys. Rev. D59 (1999) 116005) Our goal Experimental bound tan(b) = 30O tan(b) = 0O Largely favoured and confirmed by Kamland All solar n experiments combined Lecce, April 24, 2003 Fabrizio Cei
Previous m eg Searches Other LFV searches Lab. Year Upper limit Experiment or Authors PSI 1977 < 1.0 10-9 A. Van der Schaaf et al. TRIUMF < 3.6 10-9 P. Depommier et al. LANL 1979 < 1.7 10-10 W.W. Kinnison et al. 1986 < 4.9 10-11 Crystal Box 1999 < 1.2 10-11 MEGA ~2005 ~ 10-13 MEG The MEG experiment aims to gain two orders of magnitude in the upper limit (not a simple experimental challenge!). MEG Lecce, April 24, 2003 Fabrizio Cei
The MEG Collaboration Lecce, April 24, 2003 Fabrizio Cei INFN & Lecce University G. Cataldi, C. Chiri, P. Creti, F. Grancagnolo, M. Panareo, S. Spagnolo INFN & Pisa University A. Baldini*, C. Bemporad, F.Cei, M.Grassi, F. Morsani, D. Nicolo’, R. Pazzi, F. Raffaelli, F. Sergiampietri, G. Signorelli INFN & Pavia University A.de Bari, P. Cattaneo, G. Cecchet, G. Nardo’, M. Rossella INFN & Genova University S. Dussoni, F. Gatti, D. Pergolesi, R. Valle INFN Roma I D. Zanello ICEPP, University of Tokyo T. Mashimo, S. Mihara, T. Mitsuhashi, T. Mori*, H. Nishiguchi, W. Ootani, K. Ozone, T. Saeki, R. Sawada, S. Yamashita KEK, Tsukuba T. Haruyama, A. Maki, Y. Makida, A. Yamamoto, K. Yoshimura Osaka University Y. Kuno Waseda University T. Doke, J. Kikuchi, H. Okada, S. Suzuki, K. Terasawa, M. Yamashita, T. Yoshimura PSI, Villigen J. Egger, P. Kettle, M. Hildebrandt, S. Ritt Budker Institute, Novosibirsk L.M. Barkov, A.A. Grebenuk, D.G. Grigoriev, B, Khazin, N.M. Ryskulov * Spokepersons Lecce, April 24, 2003 Fabrizio Cei
(muon radiative decay) Signal and Background Background Signal e g “Accidental” e n n e g n n ee g g eZ eZ g “Prompt” m e g n n (muon radiative decay) e+ + g e+ + g n e+ + n qeg = 180° Ee = Eg = 52.8 MeV te = tg g Lecce, April 24, 2003 Fabrizio Cei
Experimental Strategy Use a high-intensity muon beam and m-decay at rest; Measure g time, energy and angle by using a fast & high-resolution e.m. calorimeter; Measure e+ momentum by using a high-resolution spectrometer; Measure e+ time by using fast counters (scintillator bars); Use a trigger scheme based on the e+-g coincidence. Lecce, April 24, 2003 Fabrizio Cei
A simulated event 52.8 MeV photon 52.8 MeV positron Lecce, April 24, 2003 Fabrizio Cei
Required Performances Experimental sensitivity limited by spill-in of background (especially accidental) into signal region high resolution measurements are needed. The accidental BR is To obtain BR (m eg) 10-13 we must have BRacc 3•10-14 and this requires: FWHM Exp./Lab Year DEe/Ee (%) DEg /Eg Dteg (ns) Dqeg (mrad) Stop rate (s-1) Duty cycle (%) 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 2005 0.8 4 0.15 19 2.5 x 107 1 x 10-13 Lecce, April 24, 2003 Fabrizio Cei
Detector Building Switzerland Russia Italy Japan Drift Chambers, Readout & DAQ Russia Manpower, Muons transport solenoid Italy e+ counter (Ge + Pv), Trigger (Pi, Le), Splitter (Le), LXe Calorimeter (Pi, Le) Japan LXe Calorimeter, Superconducting Solenoid Lecce, April 24, 2003 Fabrizio Cei
The Paul Scherrer Institute Experimental Hall The most powerful machine in the world; Proton energy 590 MeV; Nominal operation current 1.8 mA Lecce, April 24, 2003 Fabrizio Cei
The Muon Beam It exists; It provides continuous > 108 +/s (with e+ contamination) on 5x5 mm2; Two separate configurations of the pE5 beam line (U & Z); Muon momentum 29 MeV/c. Primary proton beam Lecce, April 24, 2003 Fabrizio Cei
Beam Line Tests Goal: optimize the beam elements in order to achieve: a) a good m/e separation mass selection device (Wien filter); b) a good coupling between beam and spectrometer solenoidal magnet; c) a high intensity of stopping muons in a spot 5x5 mm2 degrader to reduce the muon momentum before a 150 mm target. Lecce, April 24, 2003 Fabrizio Cei
Results Rm (after filter) m/e separation Rm (total) Rm (after filter) m/e separation Spot size sV 6.5 mm, sH 5.5 mm (U) Lecce, April 24, 2003 Fabrizio Cei
The Positron Spectrometer COnstant Bending RAdius (COBRA) spectrometer Constant bending radius independent of emission angles Gradient field Uniform field High pT positrons quickly swept out Gradient field Uniform field Lecce, April 24, 2003 Fabrizio Cei
Magnetic Field Longitudinal Profile (R = 0) Radial Profile Needed < 50 G: OK) Lecce, April 24, 2003 Fabrizio Cei
Solenoids and Coils Central coil Bc = 1.26 T current = 359 A; Five coils with three different diameters; Compensation coils to suppress the stray field around the Liquid Xenon detector; High-strength aluminium stabilized superconductor thin magnet (1.46 cm Aluminium, 0.2 X0); “Crash” Tests completed; Winding completed @TOSHIBA; Magnet ready to be shipped at PSI within this year. Lecce, April 24, 2003 Fabrizio Cei
(X,Y) ~ 200 mm (drift time); (Z) ~ 300 mm; (t) ~ 10 ns Positron Tracker 17 chamber sectors aligned radially with 10° intervals and 20 wires each; Two staggered arrays of drift cells; Chamber gas: He-C2H6 mixture (50/50) to reduce multiple scattering; Vernier pattern made of 15 mm kapton foils to measure z-position by charge division. (X,Y) ~ 200 mm (drift time); (Z) ~ 300 mm; (t) ~ 10 ns Lecce, April 24, 2003 Fabrizio Cei
Drift Chambers R & D First results with a small-size prototype @ Tokyo university (90Sr source, no magnetic field) Full-size prototype @ PSI; test in November (cosmic muons & 90Sr source, magnetic field); Improved vernier strips structure (uniform resolution); Summary of Drift Chamber simulation. Lecce, April 24, 2003 Fabrizio Cei
Positron Timing Counter BC404 scintillator Two layers of scintillation counters read by PMTs at right angles with each other. Outer: mainly timing measurement Inner: mainly trigger information Goal: timing resolution 100 ps FWHM Lecce, April 24, 2003 Fabrizio Cei
Timing Counter R & D Timing resolution measurement: COsmic Ray TESt Facility (CORTES) 4 small counters + 8 MSGC + 1 scintillator bar (1 cm thick) Measured resolution t improves as ~1/√Npe 2 cm thick to get 100 ps FWHM resolution New design in progress; expected full simulation of geometry and light collection and prototype tests in October/November; final design and building in 2004. Lecce, April 24, 2003 Fabrizio Cei
Liquid Xenon Calorimeter Liq. Xe H.V. Vacuum for thermal insulation Al Honeycomb window PMT Refrigerator Cooling pipe Signals filler Plastic 1.5m 800 l of Liquid Xenon equipped with 800 PMTs; Homogeneous detector; Only scintillation light; Large light yield (~ NaI). Liquid Xenon properties Experimental check Lecce, April 24, 2003 Fabrizio Cei
LXe Calorimeter Performances Full MC simulation of Liquid Xenon behaviour and calorimeter response. Position resolution: corresponding to: angular resolution (practically independent of light absorption length). Z-coordinate resolution (depth of interaction point; important for timing resolution preliminary measurements later): Energy resolution strongly depends on optical properties of Liquid Xenon reconstruction algorithms needed to correct for non homogeneities. Lecce, April 24, 2003 Fabrizio Cei
Energy Reconstruction FWHM(E)/E (%) FWHM 4.1 % Asymptotic value FWHM 2.5 % for labs labs = 100 cm Lecce, April 24, 2003 Fabrizio Cei
Liquid Xenon Calorimeter Prototype The Large Prototype (LP) 40 x 40 x 50 cm3; 228 PMTs, 100 litres Liquid Xenon (the largest in the World). Purposes: Test cryogenic operations on a long term and on a large volume; Measure the Liquid Xenon properties; Check the reconstruction methods; Measure the Energy, Position and Timing resolutions with: a) Cosmic rays; b) -sources; c) 60 MeV eˉ from KSR storage ring; d) 40 MeV from TERAS Compton Backscattering; e) e+ and 50 MeV from p° at PSI. Planned this year Lecce, April 24, 2003 Fabrizio Cei
The LP from “inside” FRONT face LEDs -source FRONT face -sources and LEDs for PMT calibration and monitoring Lecce, April 24, 2003 Fabrizio Cei
Measurement of LXe Optical Properties First tests showed a number of scintillation photons much lower than expected. It improved with Xenon purification: Oxysorb + gas getter + re-circulation (took time). There was a strong absorption due to contaminants (mainly H2O, ~ 3 ppm). March 2002 Now labs> 1 m @ 90% C.L. Lecce, April 24, 2003 Fabrizio Cei
Improvement of labs with time We observed an improvement both in comparison with MC simulation and GXe data No Monte Carlo !! Increase with time Lecce, April 24, 2003 Fabrizio Cei
Radioactive Background in LP -trigger with 5106 gain; Geometrical cuts to exclude -sources; Energy scale: -source 208Tl (2.59±0.06) MeV 40K (1.42 ± 0.06) MeV Other lines ?? uniform on the front face; few 10 min (with non-dedicated trigger); nice calibration for low energy ’s. 40K (1.461 MeV) 208Tl (2.614 MeV) Never seen before ! Lecce, April 24, 2003 Fabrizio Cei
Timing Resolution Measurements our goal Dt = (Dtz2 + Dtsc2)1/2 = (802 + 602)1/2 ps = 100 ps (FWHM) Dtz Time-jitter due to g interaction point (MC: ) Dtsc Scintillation time and photon statistics Measurement of Dtsc2 with 60 MeV electron beam @ Kyoto Sincrotron Ring (N.B. Electron energy spectrum degraded by materials in front of the detector) 52.8 MeV weighted average of the PMT TDCs time-walk corrected; Dtsc vs number of photoelectrons; extrapolation @ 52.8 MeV is ok; new PMT with improved QE: 5 10 % 5 % 10 % Lecce, April 24, 2003 Fabrizio Cei
PMT characterization: Cryogenic Test Facility in Pisa Full design and mechanical drawings completed; Cryostat delivered at the beginning of april; Orders made for dry UHV pumping group, leak detector, UHV components, cryogenic bottle, PMT’s …; some of them already received; Laboratory in preparation. Lecce, April 24, 2003 Fabrizio Cei
Trigger Based on simple quantities: energy (QSUM); 1 board 2 VME 6U 1 VME 9U Type2 LXe inner face (312 PMT) . . . 20 boards 20 x 48 Type1 16 3 2 boards 10 boards 10 x 48 LXe lateral faces (488 PMT: 4:1 fan-in) 12 boards 12 x 48 Timing counters (160 PMT) 2 x 48 4 x 48 Based on simple quantities: energy (QSUM); e+ - coincidence in time and direction; LXe & Timing Counter information (no DC). Built on a FADC-FPGA architecture; More complex algorithms implementable. Lecce, April 24, 2003 Fabrizio Cei
Trigger Performances Trigger Status Beam rate 108 s-1 Fast LXe energy sum > 45 MeV 2103 s-1 g interaction point (PMT of max charge) e+ hit point in timing counter time correlation g – e+ 200 s-1 angular correlation g – e+ 20 s-1 Trigger Status Design and simulation of type1 board completed; Prototype board delivered by late spring. Lecce, April 24, 2003 Fabrizio Cei
Prototypes delivered in autumn Readout Electronics Waveform digitising for all channels; Custom domino sampling chip designed @ PSI; Cost per DSC ~ 1 US$; 2.5 GHz sampling speed @ 40 ps timing resolution; Sampling depth 1024 bins; Readout similar to trigger. Prototypes delivered in autumn Lecce, April 24, 2003 Fabrizio Cei
Sensitivity T = 2.6 • 107 s Rm = 0.3 • 108 m+/s Detector parameters W/4p = 0.09 ee 0.9 esel (0.9)3 = 0.7 eg 0.6 Signal Nsig = BR • T • Rm • W/4p • ee esel eg Single Event Sensitivity SES = 1/(T • Rm • W/4p • ee esel eg ) 4 • 10-14 Correlated Background BRcorr 3 • 10-15 Accidental Background BRacc Rm • DEe • (DEg)2 • (Dqeg)2 • Dteg 3 • 10-14 Upper Limit @ 90 % C.L. BR (m e g) 1 • 10-13 Discovery 4 events (P = 2 • 10-3) correspond to BR = 2 • 10-13 Cuts @ 1.4 FWHM Lecce, April 24, 2003 Fabrizio Cei
Conclusions and Time Schedule The experiment may provide a clean indication of New Physics (SUSY); Measurements and detector simulation make us confident that we can reach a SES of 4 • 10-14 for meg ( BR 10-13); Final prototypes will be ready and measured within November 2003: Large Prototype for photon energy, position and timing resolutions; Full scale Drift Chamber; -Transport and degrader-target. Experiment approved by INFN-CSN1 at beginning of April. Tentative time schedule http://meg.psi.ch http://meg.pi.infn.it http://meg.icepp.s.u-tokyo.ac.jp 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 Planning R & D Assembly Data Taking Now LoI Proposal Revised document Lecce, April 24, 2003 Fabrizio Cei
Lxe Calorimeter Calibration p-p p0n p0 (28 MeV/c) g g 54.9 MeV < E(g) < 82.9 MeV q Eg Requiring q > 170o FWHM = 1.3 MeV Requiring q > 175o FWHM = 0.3 MeV Eg p0 g Eg p- 170o 1.3 MeV for q>170o 0.3 MeV for q>175o 54.9 MeV 82.9 MeV g 175o Am-Be g source 4.43 MeV p-p g n E(g) = 129.4 MeV q Eg Lecce, April 24, 2003 Fabrizio Cei
LXe Calorimeter Calibration (2) m egnn Ee > 0.85; Eg > 0.8; qeg > 120o Crystal box PRD 38 (1988) 2077 te-tg 150 ps 1 5 ~ 6 0.3 ~ 0.4 108 m/s Accidental background R.Tribble’s talk at Univ. of Tokyo Oct. 1999 107 m/s Signal 1/10 Background <1/100 Accidental background Lecce, April 24, 2003 Fabrizio Cei