The Liquid Xenon Calorimeter of the MEG Experiment

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The Liquid Xenon Calorimeter of the MEG Experiment Fabrizio Cei INFN and Universita’ di

The Liquid Xenon Calorimeter of the MEG Experiment Fabrizio Cei INFN and Universita’ di Pisa Incontri di Fisica delle Alte Energie – IFAE 2006 Pavia, 19 -21 April 2006 19 April 2006 Fabrizio Cei 1

Outline • • • Brief overview of the MEG experiment; The Liquid Xenon scintillation

Outline • • • Brief overview of the MEG experiment; The Liquid Xenon scintillation calorimeter; The Calorimeter (Large) Prototype; Measured and expected performances; Status of calorimeter preparation; Conclusions. 19 April 2006 Fabrizio Cei 2

Overview of the MEG experiment 19 April 2006 Fabrizio Cei 3

Overview of the MEG experiment 19 April 2006 Fabrizio Cei 3

The MEG Experiment at PSI Search for Lepton Flavour Violating decay e The Paul

The MEG Experiment at PSI Search for Lepton Flavour Violating decay e The Paul Scherrer Institute v The most powerful continuous machine in the world; v Proton energy 590 Me. V; v Power 1. 1 MW; v Nominal operation current 1. 8 m. A. 19 April 2006 Fabrizio Cei 4

The MEG Collaboration ~ 40 FTEs 19 April 2006 Fabrizio Cei 5

The MEG Collaboration ~ 40 FTEs 19 April 2006 Fabrizio Cei 5

The e decay – 1) • Forbidden in the Standard Model of electroweak interactions

The e decay – 1) • Forbidden in the Standard Model of electroweak interactions because of the conservation of lepton family numbers. • With massive neutrinos (we know that mn > 0 !) and mixing, e is allowed but at a negligible level (relative probability ~ 10 -55) 19 April 2006 Fabrizio Cei 6

The e decay – 2) • All SM extensions enhance the rate through mixing

The e decay – 2) • All SM extensions enhance the rate through mixing in the high energy sector. Predictions in the range 10 -12 10 -15 SUSY ≈ 10 -12 SM Background negligible clear evidence for physics beyond the standard model 19 April 2006 Fabrizio Cei 7

Historical perspective BR < 0. 1 Present limit 1. 2 x 10 -11 Improvements

Historical perspective BR < 0. 1 Present limit 1. 2 x 10 -11 Improvements in physics linked with improvements in technology. 19 April 2006 Fabrizio Cei 8

 e : Signal and Background Signal e e+ + physical e nn n

e : Signal and Background Signal e e+ + physical e nn n qe = 180° Ee = E = 52. 8 Me. V Te = T 19 April 2006 e+ + n Accidental background dominant in the signal region Fabrizio Cei accidental enn ee e. Z n e+ + n 9

 e : required performances The sensitivity is limited by the accidental background BR

e : required performances The sensitivity is limited by the accidental background BR ( e ) 10 -13 allowed, but needed FWHM Some of them already fulfilled ! Exp. /Lab Year DEe/Ee (%) DEg /Eg (%) Dteg (ns) Dqeg (mrad) Stop rate (muons 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 1977 10 8. 7 6. 7 - 2 x 105 100 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 8 8 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 2007 0. 8 4 0. 15 19 2. 5 x 107 100 1 x 10 -13 19 April 2006 Fabrizio Cei Need of a DC beam 10

MEG Experiment Layout • Muon beam stopped in a 150 m target. • Liquid

MEG Experiment Layout • Muon beam stopped in a 150 m target. • Liquid Xenon calorimeter (800 l, 850 PMTs) for photon detection using scintillation light: fast response (~ 20 ns) and high light yield (~ 0. 8 Na. I). • (Thin wall) solenoidal spectrometer & drift chambers for positron momentum measurement. qeg = 180° e + + Ee = Eg = 52. 8 Me. V 19 April 2006 • Scintillation counters for positron timing. g Easy signal selection for + decaying at rest Fabrizio Cei 11

Positron track One MC event Energy release in LXe Hits on TC 19 April

Positron track One MC event Energy release in LXe Hits on TC 19 April 2006 Fabrizio Cei 12

The Liquid Xenon Calorimeter 19 April 2006 Fabrizio Cei 13

The Liquid Xenon Calorimeter 19 April 2006 Fabrizio Cei 13

The Calorimeter – 1) Refrigerator H. V. Signals Measurement of energy, direction and timing

The Calorimeter – 1) Refrigerator H. V. Signals Measurement of energy, direction and timing Cooling pipe Vacuum for thermal insulation Liquid Xenon properties Liq. Xe Al Honeycomb window PMT Experimental check Plasticfiller 1. 5 m 19 April 2006 Fabrizio Cei 14

The Calorimeter – 2) v Homogeneous 0. 8 m 3 volume of Liquid Xenon

The Calorimeter – 2) v Homogeneous 0. 8 m 3 volume of Liquid Xenon - pulse tube refrigerator - 67 cm < r < 108 cm - |cos(q)| < 0. 35; |f| < 60 o DW 10 % v Only scintillation light; v Read by 846 PMTs (Hamamatsu): - 2 inches diameter; - Maximum photocathodic coverage on the photon entrance face 43 %; - Immersed in Liquid Xenon; - Low temperature (165 o. K); - Quartz window for matching with scintillation light wavelength (178 nm); v Thin entrance wall; v Waveform digitizing @ 2 GHz for pile-up rejection. 19 April 2006 Fabrizio Cei 15

PMT R & D history - 1) The MEG calorimeter will work in an

PMT R & D history - 1) The MEG calorimeter will work in an intense background environment. Photons will be produced by several sources (muon radiative decay, bremsstrahlung, positron annihilation, neutron capture in Xenon and materials surrounding the detector …). We estimated that the light level due to the background would correspond to a few A anodic current for a PMT gain G = 106. At so high rates one expects undesired behaviours because of Ø increasing of photocathode resistivity at low temperatures; Ø changes in amplification when the average anodic current becomes comparable with the PMT base current (50 A). 19 April 2006 Fabrizio Cei 16

PMT R & D history – 2) 1 st generation: R 6041 Q Photocathode:

PMT R & D history – 2) 1 st generation: R 6041 Q Photocathode: Rb-Cs-Sb Material to reduce surface resistivity: Mn layer Q. E. @ 165 o. K: ~ 5 % ON OFF 2 nd generation: R 9288 TB K-Cs-Sb Al strips ~ 15 % ON Overlinearity (hidden in the previous plot by Q. E. drop) OFF Photon high rate simulated by high frequency (tens of k. Hz) LED pulsing 19 April 2006 Fabrizio Cei 17

PMT R & D history – 3) 3 rd generation: R 9869 Q Doubling

PMT R & D history – 3) 3 rd generation: R 9869 Q Doubling the Al strips produces a better stabilization of resistivity at low temperatures. Photocathode: K-Cs-Sb Material to reduce surface resistivity: Al strips (doubled) Q. E. @ 165 o. K: ~ 15 % 19 April 2006 Fabrizio Cei Insertion of Zener diodes in the last two stages of base amplification chain removes overlinearity 18

Xenon purity – 1) Energy resolution strongly depends on scintillation light absorption: - reduced

Xenon purity – 1) Energy resolution strongly depends on scintillation light absorption: - reduced number of photoelectrons; - loss of uniformity (combined with Rayleigh scattering). Xenon almost transparent to its own scintillation light, but possible contaminants can be very opaque … 19 April 2006 Fabrizio Cei 19

Xenon purity – 2) We developed a purification system to reduce impurities below ppb.

Xenon purity – 2) We developed a purification system to reduce impurities below ppb. Xenon is circulated in liquid phase (100 l/hour by means of a Barber-Nicols cryogenic fluid pump) and water vapor is removed by a purifier cartridge filled with molecular sieves. Purification performances (old system, new one is much faster) Purification system tested and improved by means of the Calorimeter Prototype. 19 April 2006 Fabrizio Cei 20

The Calorimeter Prototype (Large Prototype) 19 April 2006 Fabrizio Cei 21

The Calorimeter Prototype (Large Prototype) 19 April 2006 Fabrizio Cei 21

The Large Prototype - 1) Ø Presently the largest Liquid Xenon calorimeter in the

The Large Prototype - 1) Ø Presently the largest Liquid Xenon calorimeter in the world: 40 x 50 cm 3; ~ 70 liters of Liquid Xenon Ø 228 PMTs (types R 6041 & R 9288, not the newest ones); Ø Measurements: - cryogenic and long term operation; - absorption length; - energy, timing and position resolution. Ø Operating conditions similar to that of final detector. 19 April 2006 Fabrizio Cei 22

The Large Prototype – 2) Seen from inside -sources and LEDs for PMT calibration

The Large Prototype – 2) Seen from inside -sources and LEDs for PMT calibration and LEDs 19 April 2006 -source Fabrizio Cei monitoring 23

The Large Prototype - 3) q Home-made Polonium alpha –sources mounted on 50 micron

The Large Prototype - 3) q Home-made Polonium alpha –sources mounted on 50 micron tungsten wires (to be replaced by commercial Am sources, specifically developed by SORAD Ltd. ); q ~ 50 Bq for each source; q First application of this type of sources; preprint submitted to NIM. q Already used for Q. E. determination. 19 April 2006 Fabrizio Cei 24

Measured Performances of the Calorimeter 19 April 2006 Fabrizio Cei 25

Measured Performances of the Calorimeter 19 April 2006 Fabrizio Cei 25

Measurement of absorption length By using alpha sources (on walls and on wires) is

Measurement of absorption length By using alpha sources (on walls and on wires) is possible to give a lower limit of the Xenon absorption length labs and an estimate of the light yield. v labs > 95 cm (95 % C. L. ) v Light Yield ~ 37500 scintillation photons/Me. V (0. 9 Na. I) 19 April 2006 Fabrizio Cei 26

Measurement of energy resolution Charge exchange reaction p -p p 0 n v Liquid

Measurement of energy resolution Charge exchange reaction p -p p 0 n v Liquid Hydrogen target to maximize photon flux; v p 0 Frame: g monochromatic spectrum LAB Frame: g flat spectrum; v Back-to-back configuration: E = 55, 83 Me. V; v Even a modest collimation ( 5 o) guarantees a sufficient monochromaticity (DE 0. 3 Me. V); v Need of an opposite side detector (a Na. I array with LYSO preshower). 19 April 2006 Fabrizio Cei 27

Experimental setup H 2 target LYSO Eff 14 % LP Na. I S 1

Experimental setup H 2 target LYSO Eff 14 % LP Na. I S 1 Eff (S 1&& LP) 88 % beam 19 April 2006 Fabrizio Cei 28

Energy spectra in Na. I & LP 83 Me. V 55 Me. V correlation

Energy spectra in Na. I & LP 83 Me. V 55 Me. V correlation 129 Me. V line from p-p n (LXe sensitive to 9 Me. V neutrons) 19 April 2006 Fabrizio Cei 29

Energy resolution @ 55 Me. V Event selection: PMTs Ø LP && S 1

Energy resolution @ 55 Me. V Event selection: PMTs Ø LP && S 1 && (Na. I + LYSO); Ø 83 Me. V line in Na. I + LYSO 65 Me. V < ENa. I + LYSO < 95 Me. V; Ø No saturated PMTs; Ø Collimator: r < 4 cm. Collimator FWHM: DE/E = (4. 9 0. 4) % 19 April 2006 Fabrizio Cei 30

Measurement of timing resolution LXe – LYSO timing difference @ 55 Me. V high

Measurement of timing resolution LXe – LYSO timing difference @ 55 Me. V high gain normal gain 110 psec s. D t LYSO Beam Normal gain 110 Ɵ 64 Ɵ 61 = 65 High gain 103 Ɵ 64 Ɵ 61 = 53 19 April 2006 Fabrizio Cei 103 psec FWHM = 153 ps 125 ps sz 1 2 cm 31

Status of Calorimeter Preparation 19 April 2006 Fabrizio Cei 32

Status of Calorimeter Preparation 19 April 2006 Fabrizio Cei 32

 • PMTs: 850 PMTs under testing in PSI and in Pisa LXe facility

• PMTs: 850 PMTs under testing in PSI and in Pisa LXe facility (3 4/day); • Cryostat under construction; delivery at PSI in spring 2006; • Gas System: almost ready in p. E 5 area at PSI. 19 April 2006 Fabrizio Cei 33

PMT mounting 19 April 2006 Fabrizio Cei 34

PMT mounting 19 April 2006 Fabrizio Cei 34

Conclusions 19 April 2006 Fabrizio Cei 35

Conclusions 19 April 2006 Fabrizio Cei 35

v The MEG experiment is expected to start engineering runs in 2006; v Experimental

v The MEG experiment is expected to start engineering runs in 2006; v Experimental tests with sub-detectors showed that many of the needed resolutions were already fulfilled ; v For LXe calorimeter, we obtained an absorption length labs > 100 cm, an energy resolution DE/E < 5 % @ 55 Me. V and a timing resolution of ~ 150 ps FWHM; v We successfully used alpha sources mounted on wires for calibration and monitoring of the detector (the first application of these sources); v The calorimeter building and the PMT testing and calibration are in advanced state. 19 April 2006 Fabrizio Cei 36

The MEG web page Please, visit it ! 19 April 2006 Fabrizio Cei 37

The MEG web page Please, visit it ! 19 April 2006 Fabrizio Cei 37

Backup slides 19 April 2006 Fabrizio Cei 38

Backup slides 19 April 2006 Fabrizio Cei 38

Comparison with LHC 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009

Comparison with LHC 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 “Supersymmetric parameter space accessible by LHC” MEGA R&D Engineering Data Plans • Data taking from 2007 on to reach 10 -13 sensitivity (90% CL) G E M • Obtain a “significant” result before the LHC era • Eventually reach 10 -14 during LHC era 19 April 2006 Fabrizio Cei W. Buchmueller, DESY, priv. comm. 39

Connection with n-oscillations – 1) Additional contribution to slepton mixing from V 21, matrix

Connection with n-oscillations – 1) Additional contribution to slepton mixing from V 21, matrix element responsible for solar neutrino deficit. (J. Hisano & N. Nomura, Phys. Rev. D 59 (1999) 116005) tan(b) = 30 O tan(b) = 0 O Experimental bound Largely favoured and confirmed by Kamland MEG After Kamland All solar n experiments combined 19 April 2006 Fabrizio Cei 40

Connection with n-oscillations – 2) Correlation between BR ( e ) & sin 2(q

Connection with n-oscillations – 2) Correlation between BR ( e ) & sin 2(q 13) (still unknown !) 10 -11 In these models, BR ( e ) is one of the most sensitive tool to measure sin 2(q 13). 10 -14 A. Masiero et al. , hep-ph/0407325 19 April 2006 Fabrizio Cei Sensitivity of future long-baseline experiments 41

MC Simulation q Geometry: full simulation of calorimeter structure, internal and external vessels, PMT

MC Simulation q Geometry: full simulation of calorimeter structure, internal and external vessels, PMT holders and honeycomb; q Scintillation light tracking: - decay curve and wavelength spectrum of LXe scintillation; - absorption and Rayleigh scattering in Liquid Xenon; - Fresnel and total reflection on PMT quartz window and PMT holders; - PMT quartz window transmittance; q Outputs: - Energy deposit, position and timing in Liquid Xenon; - Waveform output: hit timing of scintillation photons for each PMT with digitizer binning. (~ 8 x 104 photoelectrons @ 50 Me. V). 19 April 2006 Fabrizio Cei 42

Calibration techniques - 1) The required performances of the detector demand multiple and complementary

Calibration techniques - 1) The required performances of the detector demand multiple and complementary calibration and monitoring methods. v Alpha sources: Q. E. determination, LXe optical properties, energy scale stability, permanently installed within the calorimeter. But: no good energy reference, useless for timing; v -lines from neutron capture in Ni (Eg = 9 Me. V): absolute energy scale, light yield stability, usable very frequently. But: useless for timing & Q. E. , low sensitivity to optical properties; v Charge exchange reaction: p-p p 0 n (followed by p 0 decay in two gamma’s): energy scale determination, absolute timing and position calibration, simultaneous calibration of the whole apparatus. But: difficult to use frequently, hardware modifications needed, useless for Q. E. , low sensitivity to optical properties; 19 April 2006 Fabrizio Cei 43

Calibration techniques - 2) v Cockroft-Walton Proton accelerator based on - Resonant cross section

Calibration techniques - 2) v Cockroft-Walton Proton accelerator based on - Resonant cross section at Ep = 440 ke. V (speak = 6 mbarn, G 15 ke. V); - Main method: simultaneous calibration of the whole apparatus, useful for absolute energy scale and monitoring, frequently usable. 19 April 2006 Fabrizio Cei 44

Alpha sources in LXe Simulated Measured 50 m thick gold plate clipped around the

Alpha sources in LXe Simulated Measured 50 m thick gold plate clipped around the wire 19 April 2006 Fabrizio Cei 100 m thick tungsten wire 45

Radioactive Background in LP q -trigger with 5 106 gain; q Geometrical cuts to

Radioactive Background in LP q -trigger with 5 106 gain; q Geometrical cuts to exclude -sources; q Energy scale: -source ü 208 Tl (2. 59± 0. 06) Me. V ü 40 K (1. 42± 0. 06) Me. V ü Other lines ? ? q uniform on the front face; q few 10 min (with nondedicated trigger); q nice calibration for low energy ’s. 40 K (1. 461 Me. V) 208 Tl (2. 614 Me. V) Never seen before ! 19 April 2006 Fabrizio Cei 46

Measurement of position resolution Reconstruction by (localized) weighted average method (40 Me. V gamma

Measurement of position resolution Reconstruction by (localized) weighted average method (40 Me. V gamma beam with 1 mm collimator) 19 April 2006 Fabrizio Cei 47

Sensitivity T = 2. 6 • 107 s Detector parameters R = 0. 3

Sensitivity T = 2. 6 • 107 s Detector parameters R = 0. 3 • 108 +/s W/4 p = 0. 09 ee 0. 9 esel (0. 9)3 = 0. 7 Cuts @ 1. 4 FWHM e 0. 6 Signal Nsig = BR • T • Rm • W/4 p • ee esel e Single Event Sensitivity SES = 1/(T • R • W/4 p • ee esel e ) 4 • 10 -14 Correlated Background BRcorr 3 • 10 -15 Accidental Background BRacc R • DEe • (DE )2 • (Dqe )2 • Dte 3 • 10 -14 Upper Limit @ 90 % C. L. BR ( e ) 1 • 10 -13 Discovery 4 events (P = 2 • 10 -3) correspond to BR = 2 • 10 -13 19 April 2006 Fabrizio Cei 48