The MEG Experiment at PSI: a sensitive search for

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1 The MEG Experiment at PSI: a sensitive search for µ eγ decay Fabrizio Cei INFN and University of Pisa XV Incontri sulla Fisica delle Alte Energie Lecce,, April 2003

2 Outline Physics motivations: SUSY indications; Connection with neutrino oscillations. The µ eγ 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. Fabrizio Cei 2

3 Physics Motivations In the Standard Model with massive Dirac neutrinos Lepton Flavour Violation processes (as µ eγ, τ eγ, µ eee, µ 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 µ eγ are clear evidences for Super Symmetry. Problem: are such rates experimentally observable? ( ~ 50 ) Fabrizio Cei 3

4 SUSY Indications LFV processes especially sensitive to SSM grand unified theories (SUSY-GUT) GUT). LFV induced by finite slepton mixing through radiative corrections. Some predictions: SUSY SU(5) BR (µ eγ) SUSY SO(S ) BR SO() 0 BR BR SO() (R. Barbieri et al., Phys. Lett. B338(1994) ) 212 R. Barbieri et al., Nucl.. Phys. B445(1995) 215) BR SU(5) Fabrizio Cei 4

5 Predictions of µ eγ BR in SU(5) Models Experimental Bound Goal of MEG Experiment J. Hisano et al., Phys. Lett. B391 (1997) 341 Small (< ) tan β values are highly disfavoured by recent combined LEP data. (ALEPH, DELPHI, L3 & OPAL Collaborations, hep-ex/07030) ex/07030) Fabrizio Cei 5

6 Connection with ν-oscillations Additional contribution to slepton mixing from V 21, matrix element responsible for solar neutrino deficit. (J. Hisano & N. Nomura, Phys. Rev. D59 (1999) ) log( m /ev ) (b) m 2 (ev 2 ) 90% CL 95% CL Largely favoured -6-7 and confirmed by Kamland -7 99% CL % CL -9 MSW small angle LMA LOW log(tan θ) All solar -11 solar ν experiments -3-2 sin 2 2 MSW large angle MSW large angle small mass -1 Just so experiments combined 1 ) µ e γ Br( Experimental bound tan(β) ) = 30 O Experimental bound MSW LMA Our goal MSW LOW tan(β) ) = 0 O Vacuum θfabrizio Cei M ν R2 (GeV) 6

Previous µ eγ Searches Lab. Year Upper limit Experiment or Authors Other LFV searches PSI 1977 < 1.0-9 A. Van der Schaaf et al. TRIUMF 1977 < 3.6-9 P. Depommier et al. LANL 1979 < 1.7 - W.

7 Previous µ eγ Searches Lab. Year Upper limit Experiment or Authors Other LFV searches PSI 1977 < A. Van der Schaaf et al. TRIUMF 1977 < P. Depommier et al. LANL 1979 < W.W. Kinnison et al. LANL 1986 < Crystal Box LANL 1999 < MEGA PSI ~2005 ~ -13 MEG The MEG experiment aims to gain two orders of magnitude in the upper limit (not a simple experimental challenge!). MEG Fabrizio Cei 7

8 The MEG Collaboration 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 & PaviaP 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 Fabrizio Cei 8

9 Signal and Background e + µ + γ θ eγ = 180 E e = E γ = 52.8 MeV t e = t γ Signal µ e γ Prompt µ e γ ν ν (muon radiative decay) e + µ + γ ν Background ν Accidental µ e ν ν µ e γ ν ν ee γ γ ez ez γ ν e + µ + ν γ Fabrizio Cei 9

10 Experimental Strategy Use a high-intensity intensity muon beam and µ-decay at rest; Measure γ 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 + -γ coincidence. Thin Superconducting Coil Muon Beam Drift Chamber γ e + Stopping Target Liq. Xe Scintillation Detector Timing Counter 1m γ Liq. Xe Scintillation Detector e + Drift Chamber Fabrizio Cei

11 A simulated event 52.8 MeV photon 52.8 MeV positron Fabrizio Cei 11

12 Required Performances Experimental sensitivity limited by spill-in in of background (especially accidental) into signal region high resolution measurements are needed.. The accidental BR is BR R E E θ acc µ e To obtain BR (µ eγ) -13 we must have BR acc 3 2 γ FWHM 2 eγ t eγ BR acc and this requires: Exp./Lab Year E e /E e (%) E γ /E γ (%) t eγ (ns) θ eγ (mrad) Stop rate (s -1 ) Duty cycle (%) BR (90% CL) SIN x x -9 TRIUMF x x -9 LANL x x - Crystal Box x 5 (6..9) 4.9 x -11 MEGA x 8 (6..7) 1.2 x -11 MEG x x -13 Fabrizio Cei 12

13 Detector Building Switzerland 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 Fabrizio Cei 13

14 The Paul Scherrer Institute Experimental Hall The most powerful machine in the world; Proton energy 590 MeV; Nominal operation current 1.8 ma Fabrizio Cei 14

15 It exists; The Muon Beam It provides continuous > 8 µ + /s (with e + contamination) ) on 5x5 mm 2 ; Two separate configurations of the πe5 beam line (U & Z); Muon momentum 29 MeV/c /c. Primary proton beam Fabrizio Cei 15

Beam Line Tests Goal: optimize the beam elements in order to achieve: a) a good µ/e separation mass selection device (Wien( filter); b) a good coupling between beam

16 Beam Line Tests Goal: optimize the beam elements in order to achieve: a) a good µ/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 mm 2 degrader to reduce the muon momentum before a 150 µm target. Fabrizio Cei 16

17 R µ (total) R µ (after filter) µ/e separation Results (U branch) 9.5 (Z branch) / s Spot size σ V 6.5 mm, σ H 5.5 mm (U) µ + /s 11σ (U branch) 7σ (Z branch) µ Fabrizio Cei 17

18 The Positron Spectrometer COnstant Bending RAdius (COBRA)) spectrometer Constant bending radius independent of emission angles Gradient field Uniform field High p T positrons quickly swept out Gradient field Uniform field Fabrizio Cei 18

19 Magnetic Field Longitudinal Profile (R = 0) Radial Profile Needed < 50 G: G OK) Fabrizio Cei 19

20 Solenoids and Coils Central coil B c = 1.26 T current = 359 A; 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 X 0 ); Crash Tests completed; Winding TOSHIBA; Magnet ready to be shipped at PSI within this year. Fabrizio Cei 20

Positron Tracker 17 chamber sectors aligned radially with intervals and 20 wires each; Two staggered arrays of drift cells; Chamber gas: He-C 2 H 6 mixture (50/50) to reduce

21 Positron Tracker 17 chamber sectors aligned radially with intervals and 20 wires each; Two staggered arrays of drift cells; Chamber gas: He-C 2 H 6 mixture (50/50) to reduce multiple scattering; Vernier pattern made of 15 µm kapton foils to measure z-positionz by charge division. σ(x,y) ~ 200 µm (drift time); σ(z) ~ 300 µm; σ(t) ~ ns Fabrizio Cei 21

Drift Chambers R & D First results with a small-size size prototype @ Tokyo university ( 90 Sr source, no magnetic field) σ σ R Z = = 93 ± µ m 425 ± 7µ m Full-size prototype @ PSI; ; test in November

22 Drift Chambers R & D First results with a small-size size Tokyo university ( 90 Sr source, no magnetic field) σ σ R Z = = 93 ± µ m 425 ± 7µ m Full-size PSI; ; test in November (cosmic muons & 90 Sr source, magnetic field); Improved vernier strips structure (uniform resolution); Summary of Drift Chamber simulation. P e + θ e x + orig / P e + = % = 9 12 mrad = mm Fabrizio Cei 22

23 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 0 ps FWHM Fabrizio Cei 23

24 Timing Counter R & D Timing resolution measurement: COsmic Ray TESt Facility (CORTES) 4 small counters + 8 MSGC + 1 scintillator bar (11 cm thick) Measured resolution σ 60 ps t FWHM 140 ps σ t improves as ~1/ ~ 1/ N pe 2 cm thick to get 0 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 Fabrizio Cei 24

25 Liquid Xenon Calorimeter Cooling pipe Refrigerator H.V. Signals Vacuum for thermal insulation 800 l of Liquid Xenon equipped with 800 PMTs; Homogeneous detector; Only scintillation light; Large light yield (~ NaI). Liq. Xe PMT Plasticfiller Al Honeycomb window 1.5m Liquid Xenon properties Density Boiling and melting points Energy per scintillation photon Radiation length Experimental Decay time check Scintillation light wave length Scintillation light absorption length 2.95 g/cm3 165 K, 161 K 24 ev 2.77 cm 4.2 ns, 22 ns, 45 ns 175 nm > 0 cm Attenuation length (Rayleigh scattering) 30 cm Refractive index 1.74 Fabrizio Cei 25

26 LXe Calorimeter Performances Full MC simulation of Liquid Xenon behaviour and calorimeter response. Position resolution: corresponding to: σ σ σ x ϑ y σ 6. 5 ϕ 5 mm 7 mrad angular resolution (practically independent of light absorption length). Z-coordinate resolution (depth of interaction point; ; important for timing resolution preliminary measurements later): Z Energy resolution strongly depends on optical properties of Liquid Xenon σ 5 mm reconstruction algorithms needed to correct for non homogeneities. Fabrizio Cei 26

27 Energy Reconstruction FWHM(E)/E (%) FWHM 4.1 % λ abs = 0 cm Asymptotic value FWHM 2.5 % for λ abs Fabrizio Cei 27

28 Liquid Xenon Calorimeter Prototype 40 x 40 x 50 cm 3 ; 228 PMTs, 0 litresl 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) α-sources; c) 60 MeV e from KSR storage ring; d) 40 MeV γ from TERAS Compton Backscattering; e) e + and 50 MeV γ from π at PSI. trigger counters 2 inch PMT Liquid Xenon The Large Prototype (LP) PMT holder (x 228) trigger counters Fabrizio Cei Planned this year 3 3 1

29 The LP from inside LEDs α-source α-sources and LEDs for PMT calibration and monitoring Fabrizio Cei 29

30 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 H 2 O, ~ 3 ppm). March 2002 Now λ abs > 1 90% C.L. R = exp( a + bx ) Fabrizio Cei 30

31 Improvement of λ abs with time We observed an improvement both in comparison with MC simulation and GXe data Fabrizio Cei 31

32 Radioactive Background in LP α-trigger with 5 6 gain; Geometrical cuts to exclude α-sources; Energy scale: α-source 208 Tl (2.59±0.06) MeV 40 K (1.42 ± 0.06) MeV Other lines?? uniform on the front face; few min (with non- dedicated trigger); nice calibration for low energy γ s. 40 K (1.461 MeV) 208 Tl (2.614 MeV) Never seen before! Fabrizio Cei 32

33 Timing Resolution Measurements t = ( t( z2 + t sc2 ) 1/2 = ( ) 1/2 ps = 0 ps (FWHM) t z Time-jitter due to γ interaction point (MC: σ z 5 mm t z 70 ps ) t sc Scintillation time and photon statistics Measurement of t sc2 (N.B. Electron energy spectrum degraded by materials in front of the detector) our goal with 60 MeV electron Kyoto Sincrotron Ring 52.8 MeV weighted average of the PMT TDCs time-walk corrected; t sc vs number of photoelectrons; 52.8 MeV is ok; new PMT with improved QE: 5 % 5 % % Fabrizio Cei 33

PMT characterization: Cryogenic Test Facility in Pisa Full design and mechanical drawings completed; Cryostat delivered at the beginning of april; Orders

34 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. Fabrizio Cei 34

35 Trigger Based on simple quantities: γ energy (QSUM); e + - γ coincidence in time and direction; LXe & Timing Counter information (no DC). Built on a FADC-FPGA FPGA architecture; More complex algorithms implementable. LXe inner face (312 PMT) LXe lateral faces (488 PMT: 4:1 fan-in) Timing boards Type1 Type1 Type1 Type1 Type1 Type1 12 boards 2 boards 20 x 48 Type2 Type2 boards 1 board x 48 Type2 2 boards Type1 counters Type1 3 Type1 (160 PMT) 12 x 48 Type2 Type2 2 x 48 4 x 48 2 x 48 1 board Type2 2 VME 6U 1 VME 9U Fabrizio Cei 35

36 Trigger Performances Beam rate 8 s -1 Fast LXe energy sum > 45 MeV 2 3 s -1 γ interaction point (PMT of max charge) e + hit point in timing counter time correlation γ e s -1 angular correlation γ e + Trigger Status 200 s -1 Design and simulation of type1 board completed; Prototype board delivered by late spring. Fabrizio Cei 36

Readout Electronics Waveform digitising for all channels; Custom domino sampling chip designed @ PSI; Cost per DSC ~ 1 US$; 2.

37 Readout Electronics Waveform digitising for all channels; Custom domino sampling chip PSI; Cost per DSC ~ 1 US$; 2.5 GHz sampling 40 ps timing resolution; Sampling depth 24 bins; Readout similar to trigger. Prototypes delivered in autumn Fabrizio Cei 37

38 Sensitivity Detector parameters Signal Single Event Sensitivity Correlated Background Accidental Background Upper 90 % C.L. Discovery T = s R µ = µ + /s Ω/4π = 0.09 ε e 0.9 ε sel (0.9) 3 = 0.7 ε γ FWHM N sig = BR T R µ Ω/4π ε e ε sel ε γ SES = 1/(T R µ Ω/4π ε e ε sel ε γ ) 4-14 BR corr 3-15 BR acc R µ E e ( E γ ) 2 ( θ εγ ) 2 t εγ 3 BR (µ e γ) 1-13 Fabrizio Cei events (P P = 2-3 ) correspond to BR = 2-13

39 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 4-14 for µ eγ ( BR -13 ); Final prototypes will be ready and measured within November 2003: Large Prototype for photon energy, position and timing resolutions; Full scale Drift Chamber; µ-transport and degrader-target target. Experiment approved by INFN-CSN1 at beginning of April. Tentative time schedule LoI Proposal Revised document Now tokyo.ac.jp Planning R & D Assembly Data Taking Fabrizio Cei 39

40 Lxe Calorimeter Calibration 1. π - p π 0 n π 0 (28 MeV/c /c) γ γ 54.9 MeV < E(γ) < 82.9 MeV γ E γ θ π 0 E γ Requiring θ > 170 ο FWHM = 1.3 MeV Requiring θ > 175 o FWHM = 0.3 MeV π - γ Entries/0.1MeV MeV for θ>170 o 0.3 MeV for θ>175 o E γ Energy (MeV) o 54.9 MeV 82.9 MeV 175 o 1. Am-Be γ source 4.43 MeV π - p γ n E(γ) = MeV Energy (MeV) E γ opening angle (deg) θ Fabrizio Cei 40

41 LXe Calorimeter Calibration (2) µ eγνν E e > 0.85; E γ > 0.8; θ eγ > 120 o Crystal box PRD 38 (1988) µ/s 1 5 ~ ps Accidental background 7 µ/s Signal 1/ Background <1/ ~ ps t e -t γ Accidental background t e -t γ R.Tribble Tribble s talk at Univ. of Tokyo Oct Fabrizio Cei 41

The MEG experiment at PSI

Donato Nicolo`, Universita` di Pisa & INFN 1 NOON 2003, Kanazawa February, 10-14 2003 The MEG experiment at PSI Status and Prospects Donato Nicolo` Dipartimento di Fisica dell Universita`di Pisa and Istituto