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14 September 2007

14 September 2007. Chiara Casella. Precise measurement of the muon lifetime with the FAST experiment: First results. Journée de réflexion du DPNC. OUTLINE. Goal of the experiment & Theoretical motivations The FAST experiment general experimental concept

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14 September 2007

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  1. 14 September 2007 Chiara Casella Precise measurement of the muon lifetime with the FAST experiment: First results Journée de réflexion du DPNC

  2. OUTLINE • Goal of the experiment & Theoretical motivations • The FAST experiment • general experimental concept • description of the setup elements (beam; target; readout; DAQ; LV2 trigger) • Muon LifetimeAnalysis (from RUN 2006 sample) • data sample (run 06) • analysis procedure (from raw data to histograms) • fit procedure (i.e. muon lifetime measurement) • Study of the systematic uncertainty • Results & Future plans

  3. GOAL OF THE EXPERIMENT & MOTIVATIONS FAST goal : precision measurement of the muon lifetime dtm/tm~ 2 ppm [~ 4ps] Past muon lifetime measurements [PDG 06] ULTIMATE FAST GOAL One order of magnitude improvement on the current world average PRESENT ANALYSIS (2006 data sample): world average competetitive muon lifetime measurement - as single experiment - 1974 1984 2 ppm 18 ppm 1973 1984

  4. At present (after 1999, Ritbergen & Stuart, Phys.Rev.Lett. 82, 488 1999) the limit on the accuracy on GF (9ppm) is totally dominated by the experimental accuracy on the muon lifetime (18ppm) FAST: Precise measurement of GF It will make accessible the effects of MW on GF calculations high energy exp, performed at low scale GOAL OF THE EXPERIMENT & MOTIVATIONS FAST goal : precision measurement of the muon lifetime dtm/tm~ 2 ppm [~ 4ps] Past muon lifetime measurements [PDG 06] ULTIMATE FAST GOAL One order of magnitude improvement on the current world average PRESENT ANALYSIS (2006 data sample): world average competetitive muon lifetime measurement - as single experiment - 1974 1984 2 ppm 18 ppm 1973 1984

  5. THE FAST EXPERIMENT Size of the statistical sample  max achievable precision Readout & DAQ Chain PMT muon source: DC p+ beam stopped in the target p+ p = 170 MeV/c TARGET [Active scintillator target]: • stopping material for p+ and m+ • detector for e+ PARALLELISATION needed i.e. treat more pme events at the same time inside the target: • High granularity of the target • Tracking capabilities • Ability to disantangle overlapping events • High beam rate • Huge data rate throughput • Systematics under control SIGNATURE FOR AN EVENT [ pme ]: Signature: • p+ & m+ close in time & space [high pulses] • e+ track [mip’s]

  6. Paul Scherrer Institute (PSI) p e m MUON SOURCE (i.e. DC p+ BEAM) • source =pdecay at rest in the target  isotropic & unpolarizedmsource Sm Sn nm m+ p+ Sp= 0 - pM1 at PSI • momentum - RF frequency - size - intensity - purity :p+DCbeam : 170 MeV/c (±3%) : 50.633 MHz (T=19.75 ns) : variable (~ 10 x 10 cm2) : variable (~ 500 kHz) : ~ 50% Brugg (AG) Typical landscape, summertime, Middle of NoWhere (AG)

  7. TARGET • Active target : • stopping material for p+/m+ • detector for the particles Solid plastic scintillator (Bicron BC400) 20 cm 12.8 cm • high granularity • tracking capabilities • fast imaging detector • identical replicated mini-detectors • vertical arrangement 32 x 48 19.2 cm • 1536 pixels (8 x 12 [4x4]-bundles) 1 pixel = scintillator bar (4 x 4 x 200 mm3) with 2 WLSF • 4x4x200mm3 bars • two grooves machined • bundle of 4x4 pixels [96 bundles] • fibers housed in a special mask BC400 solid plastic scintillator plates 2 WLSF (BCF-92) inserted and glued into the grooves diffusive reflective paint (BC-620) 20cm 4 mm

  8. FAST READOUT & DAQ CHAIN X 96 • Double THR discriminators: LOW (LL) & HIGH (HL) LL  mip’s (e+ tracks) HL  p +/m + (stop points) • remotely adjustable thr (50 mV – 2 V) • custom made units (PSI) • Position Sensitive PhotoMultipliers (PSPM’s) • Hamamatsu H6568-10 - multianode (4x4) X 96 - Preamplifiers - shaper & preamp (x5) - custom made units (PSI) X 96 TARGET • CAEN v767 128-channs TDCs X 16 • time stamping of the raw pulses from discri • TDC time window: [-10,+20]ms w.r.t. Trigger • trigger : p in the target (entry/stop moment) • output of the TDCs: into the DAQ PC’s beam TRIGGER (LV1) Second Level TRIGGER (LV2) - Hardware trigger for the TDCs - Selective trigger (definition of ROI) - Event quality selection - DATA REDUCTION SYSTEM - Custom made FPGA based electronics (CIEMAT)

  9. DAQ ARCHITECTURE - 16 TDC’s in 4 VME crates • VME-PCI interface: PVIC link (20 MB/s/node) 80 MB/sec (~ 7 TB/day): max allowed data rate

  10. DAQ ARCHITECTURE - 16 TDC’s in 4 VME crates • VME-PCI interface: PVIC link (20 MB/s/node) 80 MB/sec (~ 7 TB/day): max allowed data rate This has to be ONLINE The full set of data cannot be stored on disk Only histograms are saved as the output of the analysis process Histo: control, lifetime, syst HISTOGRAMS: (~ 1200 histos) • control • lifetime • for systematics EVENTS Analysis RAW DATA Ev.Builder

  11. RUN 2006: DATA SAMPLE • 3 weeks data taking (Nov – Dec 2006) • first physics data taking run for FAST • running conditions very close to the final one except for the reduced adopted rate : LV2 trigger rate ~ 30 kHz (pion rate ~80 kHz) • total statistics : 1.073 x 1010 events  precision comparable with world average Muon lifetime histogram ‘’THE’’ PLOT te distribution i.e. te-tp [in TDC tickmarks]

  12. MUON LIFETIME HISTOGRAM Finest time resolution: 1 TDC tickmark = 1.0416667 ns • negative times (pure bkg events)description of the background (totally decoupled from the signal) from the negative times region & positive times (signal+bkg) use the background description also for the positive times region • background: flat component no time dependent + RF periodicity structure (TRF) beam induced background 1 bin = 1 TDC tick t=0 : p in the target tm measurement = fit this distribution: • Use this fine binning distribution to understand all the structures in the data • Rebin (with RF periodicity) the histogram to minimize the effects of the periodic background on the lifetime • Define a proper fitting function • Binned maximum likelihood fit (TMinuit-ROOT)

  13. STEPS FOR THE FIT: 1. understand the background (t<0) TRFis not an exact multiple of 1 TDC tick every peak: slightly shifted sampling of the bkg shape 1 PERIOD SAMPLE

  14. STEPS FOR THE FIT: 1. understand the background (t<0) convolution (TRF) TRFis not an exact multiple of 1 TDC tick every peak: slightly shifted sampling of the bkg shape 1 PERIOD SAMPLE

  15. STEPS FOR THE FIT: 1. understand the background (t<0) convolution (TRF) TRFis not an exact multiple of 1 TDC tick beam TOF p e every peak: slightly shifted sampling of the bkg shape m 1. Periodic background shape: • exactly reproduces the beam content • dominated by electrons (e) 1 PERIOD SAMPLE 2. Extraction of the exact RF period from the fit of the negative background: • T_RF = (18.960051 +/- 0.000003) ticks  0.2 ppm

  16. FFT frequency spectrum Residuals [600.6000] ticks T=32 ticksT=16 ticks STEPS FOR THE FIT: 2. periodic structures in the data • Positive times region fit • Fast Fourier Transform of its residual

  17. FFT frequency spectrum Residuals [600.6000] ticks - Convolution applied on the residual (T = 32) mod(t,32) T=32 ticksT=16 ticks Fitting function: TDC non linearity effects per mille level effect confirmed also in lab tests [ TDC CLK = 32 ticks ] STEPS FOR THE FIT: 2. periodic structures in the data • Positive times region fit • Fast Fourier Transform of its residual

  18. STEPS FOR THE FIT: 3. rebin the histogram Rebin the histogram using the measured beam period to minimize the influence of the periodic background on tm measurement TRF 1 new bin = 18.960051 original bins “SMART REBIN” i. e. rebin + bin sharing procedure : original bin new bin separator • different algorithms tried (uniform, linear, quadratic, exp) for the sharing of evts inside the original bin • no appreciable differences found (see systematics) • default: uniform distribution inside the bin information loss only concerns details of the background structure, not muon lifetime

  19. (p1,m1,e1) (p2,m2,e2) (p2,m2,e2) (p1,m1,e1) TDC time window TDC time window next window previous window STEPS FOR THE FIT: 4. boundary effects in the lifetime distr. Already forseen in the systematic study with MC simulations behaviour e+t/t Increase of muon decay events at the boundary of the TDC window Due to overlapping events in the TDC window [t_min,t_max] (p1,m1,e1) (p2,m2,e2) (p1,m1,X) X=beam pcl  flat acc. X=p2  (p1,m1,p2)  peaked at t_max X=e2 (p1,m1,e2)  peaked at t_min

  20. THE FIT Signal TDC non linearity Accidental bkg Time dependent background • Fit interval: [600,20000] ticks Largest region of stable lifetime start stop • any precise description of the periodic beam induced background is required (because of the rebin) • high quality fit

  21. FIT QUALITY FFT of the residuals s=[1.003 +/- 0.022] X=[-0.0005 +/- 0.0314] residuals

  22. FIT QUALITY FFT of the residuals s=[1.003 +/- 0.022] X=[-0.0005 +/- 0.0314] residuals

  23. SYSTEMATICS STUDY • Evaluated with several dedicated histograms (produced online) • Different classes of systematic errors studied, with different sets of specific lifetime histograms • General recipe: - Any deviation inconsistent with the statistical fluctuations is considered to be of systematic origin • A PRIORI consistency criteria = 3 sigma’s 1. Fit histograms corresponding to the sub-samples & compute the average 2. Look if there are statistically incompatible points (i.e. deviation from the average Dtm > 3s) 3. How much the average changes when those points are excluded 4. Quote this variation as (signed) systematic shift SUMMARY TABLE OF SYSTEMATICS SUMMARY TABLE OF SYSTEMATICS  At present, the determination of the systematic uncertainty is limited by the statistics  There is no evidence of large systematic effects * * * * Examples described here

  24. SYSTEMATICS: HOMOGENEITY OF THE TARGET 2 examples for no evidence of a systematic effect beyond the expected statistical fluctuations lifetime vs position of the pion in the target (x coord) lifetime vs position of the pion inside the PSPM p

  25. B • B • B • B • B SYSTEMATICS: HOMOGENEITY OF THE TARGET Lifetime VS position of the tube in the target: • evidence for a systematic effect – due to 2 tubes SYSTEMATICS -- GEOMETRY : • lifetime vs pion position (x,y) • lifetime vs position inside the PSPM • lifetime vs detection efficiency • lifetime vs position of PSMP in the target • lifetime vs position of TDC chip in the target • lifetime vs position of TDC in the target Systematics associated to the target (dis)homogeneity

  26. SYSTEMATICS : TIME STABILITY & RATE DEPENDENCE TIME STABILITY Data set divided in 89 subsets of similar size (~ 1.2 108 evts) similar duration (~ 4 hours) Nominal fit applied to every subset: GOOD TIME STABILITY WITHIN STATISTICAL UNCERTAINTY of 15 ppm RATE DEPENDENCE • Trigger rate (LV2) ~ 30 kHz • Profit of the small spread in range to study lifetime VS rate NO EFFECT OF SYSTEMATICS RELATED TO THE TRIGGER RATE Limited rate interval available small sample higher rate

  27. RESULTS & CONCLUSIONS arXiv hep-ex_0707.3904 & submitted to PLB • first precise measurement of the positive muon lifetime with the FAST experiment: • run 2006 [3 weeks data taking, Nov-Dec06] • data sample : 1.073 1010m+ decay events • precision comparable to PDG, value compatible with PDG • uncertainty dominated by the statistics • FAST measurement of the Fermi coupling constant GF • not yet in the regime where the effects of MWstarts to be visible: • GF : Fermi model + O(a2) 1.16635319 x 10-5 GeV-2 • GF : Fermi model + O(a2) + MW 1.16635258 x 10-5 GeV-2 0.000009 x 10-5 GeV-2 present uncertainty

  28. OUR MEASUREMENT IN THE WORLD SCENARIO  after more than 20 years two new experiments (MuLan & FAST) are taking the lead 18 ppm 8.2 ppm GF accuracy: 9 ppm  4 ppm ppm accuracy on GF expected soon!

  29. FUTURE PLANS FOR FAST • Run 2006 largely proved the reliability & the feasibility of the measurement, but a few more steps are needed to achieve the final FAST goal • Need to increase the working rate 30 kHz (LV2)  100 – 120 kHz (LV2)  solve some malfunctioning on the TDCs (strong evidence of need to replace CAEN TDCs with their updated version: CAEN v767  CAEN v1190A)  upgrade of the DAQ hardware i.e. double number of PVIC nodes max bandwidth: 80 MB/sec  160 MB/sec  consider a new mode of reading the TDCs (continuous mode VS trigger matching mode) • Analysis: - (Obviously) higher statistics  New study of the systematics - Improve understanding of background - Extended LV1 tagging (wide coinc.: all types of pcls) - Pulsed structure is expected to be highly reduced • Look for a new PhD student ! • 2007 (Sept – Dec) : get FAST ready for high rate running conditions • 2008 (full accelerator time) : extensive (… final ! ) data taking “FAST apartment” close to Villigen, PSI ?

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