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AN UPDATE ON ABSORPTION LENGTH MEASUREMENT WITH THE OB SYSTEM

AN UPDATE ON ABSORPTION LENGTH MEASUREMENT WITH THE OB SYSTEM. ANTARES Collaboration Meeting Paris (France), September 20th-24th. H Yepes, J Barrios IFIC (CSIC – Universitat de València). OUTLINE.

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AN UPDATE ON ABSORPTION LENGTH MEASUREMENT WITH THE OB SYSTEM

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  1. AN UPDATE ON ABSORPTION LENGTH MEASUREMENT WITH THE OB SYSTEM ANTARES Collaboration Meeting Paris (France), September 20th-24th H Yepes, J Barrios IFIC (CSIC – Universitat de València)

  2. OUTLINE • A BRIEF REMINDER OF THE EXPERIMENTAL • PROCEDURE • DATA TAKING STATUS • DATA ANALYSIS STATUS: (I) Multi-wavelength • analysis and (II) OB systematic effects studies • CONCLUSIONS AND MILESTONES ANTARES Collaboration Meeting Paris, September 20th-24th

  3. F2 THE EXPERIMENTAL PROCEDURE • A BRIEF REMINDER OF THE EXPERIMENTAL PROCEDURE: • 3. Quality cuts applied: • To avoid the electronics dead time (related to Rmin): region where the probability to get more than one photoelectron is negligible (i.e < 1 %). • To avoid noise fluctuations at large distances (related to Rmax): region where the signal will be greater than the noise. • Low efficiency OMs cleaning: from the noise hits projections, only those between (m+3, -3) are considered. • Low and flat level noise along the line is required (<100 kHz). Experimental method: 1. One single top LED of the lowest OB in the line flashes upwards. 2. Signal hits are plotted and fitted (between Rmin, Rmax) by means of an exponential function. Remarks: 1) The efficiencies for the OMs are computed from the normalization of the signal hits to their own noise hits. 2) The total error assigned is computed as the quadratic sum of the statistical and dispersion errors. 3) Transmission length is a lower limit of the absorption length. ANTPLOT-CALI-2010-001 ANTARES Collaboration Meeting Paris, September 20th-24th

  4. DATA TAKING STATUS I • The experience from the analysis has let the optimization of data taking: • Golden runs taken by request, once conditions are met: LOW AND FLAT LEVEL SHAPE OF THE NOISE along the line required. • Different lines/OBs/LEDs/LEDs intensities (L4F2, L4F9, L8F2, L8F9, L2F2 all faces) to study MAINLYsystematic effects and influence of depth on absorption length (L2F9, L8F9). • Runs at three different wavelengths have been taken. GOLDEN RUN Updated until 16/08/2010 ANTARES Collaboration Meeting Paris, September 20th-24th

  5. DATA ANALYSIS STATUS I • CHANGING TO 40K EFFICIENCIES: • Noise based efficiencies are correlated with noise subtraction. • Noise based efficiencies are sensitive to noise fluctuations along the line. • 40K is not affected by variations of the bioluminescence background in time. • The light output of 40K per unit volume is constant over depth . • Our new efficiencies are computed as Dmitry Zaborov has described clearly in the Collaboration Meeting Marseille April 2009, based on 40K coincidences. • TREATMENT OF ERRORS (since the last CM): • Assign one signal intensity per storey computed as the average of the 3 OMs. • Compute error by means of Student’s t. • t follows Student’s distribution: • In order to have 68.27% errors, the one-side tail of the cumulative Student function must be 84.13% and thus t = 1.32 (for n=3) or t = 1.83 (for n=2). • If only one OM in the storey do not use that storey in the fit. ANTARES Collaboration Meeting Paris, September 20th-24th

  6. DATA TAKING STATUS I NEW MEASUREMENTS PERFORMED AT l = 532 nm by means of the laser beacon: • Reference fit criterion (Rmin): Take distances where the probability to get more than one phe is negligible: x = number of signal hits reaching the OM • m = number of signal hits / number of flashes • Laser beacon runs selection: • Standard laser beacon runs at maximum polarizer voltage value. • Low and flat level shape along the line. • Nflashes >= 100k. High intensity at 532 nm ANTARES Collaboration Meeting Paris, September 20th-24th

  7. DATA ANALYSIS STATUS I TRANSMISSION LENGTH RESULTS: • Bad fits? • Increasing rates in time? • Time distributions? Blue UV Green UV Blue Green • One UV run. • Six L2 runs batch. • Some under-over flows not shown. ANTARES Collaboration Meeting Paris, September 20th-24th

  8. DATA ANALYSIS STATUS I ANOMALOUS CASES:  Six L2 runs batch displaced: Not similar effects on error assignment. Not due to noise fluctuations along the line/time. Deepest analysis is being performed: a mystery which has not been solved. UV 47696  Over-underflows values are due to extreme and strange small error assignment in some points. Blue 50370 ANTARES Collaboration Meeting Paris, September 20th-24th

  9. DATA ANALYSIS STATUS I • Some drawbacks have been corrected casually during the below cross-checks performed (i.e F14-F17): OM0 OM1 OM2 EMPTY BINS effect which have a strong dependence on the noise subtraction: a time cut is performed ANTARES Collaboration Meeting Paris, September 20th-24th

  10. DATA ANALYSIS STATUS I If the unexplained runs batch are removed: Blue UV Green UV Blue Green ANTARES Collaboration Meeting Paris, September 20th-24th

  11. DATA ANALYSIS STATUS I Blue UV Green UV Blue Green ANTARES Collaboration Meeting Paris, September 20th-24th

  12. DATA ANALYSIS STATUS I • The mean value of the distribution of the L errors from the fits shows an agreement with the RMS of the transmission length distribution: • UV 1.0 m Vs 1.5 m • Green  1.1 m Vs 1.0 m • Blue  3.0 m Vs 2.9 m. The time stability and the RMS distribution confirms the showed results in the latest Collaboration Meetings, except for the anomalous six runs batch (a deeper analysis is being performed). UV Blue Green ANTARES Collaboration Meeting Paris, September 20th-24th

  13. DATA ANALYSIS STATUS I SUMMARY: * If we assume the fluctuation of the measurements is statistical (not yet clear) the errors for the three wavelengths would be ±0.2m (√entries). • BLUE: • Reasonable fit probabilities • Variability of L ~5% (RMS/L): • RMS of L in agreement with average σfit :3.0 m vs. 2.9 m • Change of L with time not much larger than statistical • Somewhat high probabilities: few entries close to 1. • UV: • Good fit probabilities • Variability of L around 3%(RMS/L): • RMS of L distribution in agreement with average σfit :1.0 m vs. 1.5 m. • Green: • Good fit probabilities • Variability of L around 6% (RMS/L): • RMS of L distribution in agreement with average σfit :1.2 m vs. 1.0 m. • Mean Prob (c2)  should be 0.5 RMS Prob (c2)  should be 1/√12 = 0.29 STABILITY IN TIME IS CONFIRMED FOR DIFFERENT WAVELENGTHS !!! ANTARES Collaboration Meeting Paris, September 20th-24th

  14. DATA ANALYSIS STATUS I • Distribution of relative errors Looking for the optimal assignment of errors: • Histogram entries correspond to those storeys used in fit for each wavelength, for all golden runs selected. Blue UV Green Pathological cases are being studied now  Gaussian distributions suggest an error assignment around 6 %. ANTARES Collaboration Meeting Paris, September 20th-24th

  15. DATA ANALYSIS STATUS I • Pulls distributions  Evidence of BIAS and verification of error coverage Blue UV Green • For Blue and UV pulls distributions, the fitted function parameters are slightly in agreement to the expected center in 0 and width unit gaussian distributions. • For pulls distributions in the green, a deeper analysis is being carried out to determine the origin of BIAS. ANTARES Collaboration Meeting Paris, September 20th-24th

  16. DATA ANALYSIS STATUS I For an error assignment of 6%, we obtain: ANTARES Collaboration Meeting Paris, September 20th-24th

  17. DATA ANALYSIS STATUS I • Most of low c2 probabilities are focused to UV and green runs. • An equal error assignment for all runs should be revisited since student’s t value gives the flatter c2 probability. ANTARES Collaboration Meeting Paris, September 20th-24th

  18. DATA ANALYSIS STATUS I Having in mind that we have to be at photoelectron region, we consider a P(phe>1) ≈ 0.3%. Being consistent with such requirement, we can take one storey before to begin the fit P(phe>1) ≈ 0.5%: Blue UV Green UV Blue Green ANTARES Collaboration Meeting Paris, September 20th-24th

  19. DATA ANALYSIS STATUS I Blue UV Green Blue UV Green ANTARES Collaboration Meeting Paris, September 20th-24th

  20. DATA ANALYSIS STATUS I • Mean Prob (c2)  should be 0.5 RMS Prob (c2)  should be 1/√12 = 0.29 ANTARES Collaboration Meeting Paris, September 20th-24th

  21. DATA ANALYSIS STATUS: SYSTEMATICS • OPTICAL BEACON FACES: LED SYSTEMATICS • There are 6 LEDs placed over the 6 LED Beacon faces. • Optical Beacon chosen for analysis  L2F2. • A batch of six runs, one per LED Beacon face are performed by day for different periods in time to study the influence LED flashing – different OM orientation and light collected, shadowing, etc. Bad runs batch, just for this study ANTARES Collaboration Meeting Paris, September 20th-24th

  22. DATA ANALYSIS STATUS: SYSTEMATICS Amount of light collected by the OMs at different periods of time using all LOB faces: medium light intensity region (F12, used in fit): • For medium light intensity region in the line, a dependence to the LED seems not to be found. • The amount of light percentage collected by one particular OM is higher /lower than the other ones: • OM dependent. • Angle between photon – OM ? Angular acceptance not used. ANTARES Collaboration Meeting Paris, September 20th-24th

  23. DATA ANALYSIS STATUS: SYSTEMATICS Amount of light collected by the OMs at different periods of time using all LOB faces: low light intensity region (F18, used in fit): • For low light intensity region the systematics are not so evident. • At high distances, a correction by angular acceptance could carry out. •  Next step: correction by alignment based on angular acceptance studies. •  The obtained value for the transmission length doesn’t has large changes , without to take into account the angular aceptance, then, should we to perform such analysis? A SECOND ORDER CORRECTION. ANTARES Collaboration Meeting Paris, September 20th-24th

  24. CONCLUSIONS AND MILESTONES Being the transmission length, a lower limit for the absorption length … ANTARES Collaboration Meeting Paris, September 20th-24th

  25. NOISE LEVEL BACKUP NOISE SUBTRACTION: RATE OF CORRELATED COINCIDENCES: Defined as the integral under the coincidence peak (excluding pedestal) normalized to the effective duration of observation period, and properly corrected for dead time of the electronics and data acquisition. Gaussian fit to compute the rate. Average value ~ 14 Hz (R0). R0 may include the loss of glass transparency due to biofouling (if any) and similar effects, so it may be less than for "ideal" Monte Carlo OM. OM angular acceptance can be constrained by the 40K measurements. Fit a constant in the [-1000, -50] ns range (Blevel) and substract the noise contribution (Qnoise, Nnoise): Nsignal = Nhits(tot)– Nnoise = Ntot – Blevel (Tmin - Tmax) = Ntot – <n>Nbins (Tmin - Tmax) ANTARES Collaboration Meeting Paris, September 20th-24th

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