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pp- neutrino analysis

pp- neutrino analysis. Oleg Smirnov On behalf of the “pp” working group. Borexino General Meeting, Milano, December 16, 2013. Main topics of the talk. “dt” energy variables and “win” variables for random background sampling

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pp- neutrino analysis

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  1. pp- neutrino analysis Oleg Smirnov On behalf of the “pp” working group Borexino General Meeting, Milano, December 16, 2013

  2. Main topics of the talk • “dt” energy variables and “win” variables for random background sampling • 14C fit with low threshold (providing independent measurement of 14C activity and L.Y.) • pile-up modeling in our analysis • energy scale, energy resolution (improved) • Scintillation line shape description (improved) • “Standard” fit and results • Few comments

  3. Spectrum in energy scale Spectral Components: 1)14C free (+independent measurement) 2)Pile-up constrained from independent measurement 3)7Be neutrino fixed at measurement 3)210Po free (2 pars) 4)210Bi free 5)85Kr fixed at 0 6)CNO+other neutrino species fixed at SSM/LMA(HM) 14C+pileup 210Po pp 7Be 210Bi CNO pep+8B 85Kr

  4. Energy variables Two of them: npmts_dt1 and npmts_d2 (both in non-normalized to 2000 PMTs version) Based on the number of triggered PMTs in the fixed-length energy window (230 ns for dt1 and 400 ns for dt2), easier to model the pile-up. Pile-up is scaled as dT while other components remains unchanged - possibility for pile-up cross-checks. Clustering efficiency 50% @ ΔT≈230 ns

  5. win1 and win2 variables Other two new variables: npmts_win1 and npmts_win2 are created for random noise sampling (win1 has 230 ns frames and win2 has 400 ns frames within 16 us of tt64 events). Are used in one of the methods for pile-up construction (“convolution” method, the result can be seen in the tail of 14C in the plot).

  6. The second cluster data Events in the second cluster (black line) have an advantage of lower (software) threshold. The first cluster events are shown in the same scale in green. Example of fit of the second cluster events in 40-80 npmts range

  7. 14C from the second cluster Less sensitive to the details of the variance and line shape description (because of lower threshold and lower statistics involved). Results obtained with the second cluster can be used to set constraints on 14C count rate and/or the light yield at low energy. Part of systematics is the same for the “standard” fit and for the second cluster fit: 14C shape factor, uncertainty of quenching etc. 14C rate estimated with periods 9+10+11+12 data (2.69±0.07)·10-18 g/g 14C/12C

  8. Methods to construct pile-up • Synthetic pile-up Data+Data (sample from the end of the DAQ gate is superimposed on the data) processed with Echidna in a standard way • Synthetic pile-up D+D has been produced for each period and for the sum of periods applying the same cuts as for the data • Convolution with “win” (randomly sampled data) variables. • trivial once “win” variables are provided • Analytical convolution of two 14C spectra (used to fit and replace the D+D with a smooth line). Two factors influencing the energy scale (and hence the shape) are identified: energy shift due to the LY position dependence double quenching in convolution. In standard approach the spectrum was calculated in the unquenched energy scale and only then quenched Combined scale factor is 0.91 (and geometric only applied on the double quenched spectrum is 0.935).

  9. Pile-up rate estimation • From our analysis estimated count of 14C in IV is 108 Bq for 270 t • In one day we got (108)2 ·8.64·104·4·10-7 =403 cpd of 14C pile-up in ΔT=400 ns gates in IV or 150 cpd in 100 tones; • For ΔT=230 ns the pile-up rate reduces to 232±12 cpd in IV or: • We need the shape and the counting rate/both the shape and the counting rate could be influenced by the reconstruction alrorithm 86±5 cpd/100 tones Comparison of pp and analytical pileup shape before we identified the energy scaling Now pileup end-point has shifted here breaking the degeneracy even more

  10. Synthetic pile-up spectrum fit with analytical convolution 163±8 cpd/100 tones Compared to 232±12 cpd in IV or 86±5 cpd/100 tones

  11. Energy scale Parameters of the energy scale (only 1 free): • L.Y. – free (verified vs independent measurement with 14C of the second cluster) • kB – fixed at the value kB=0.0109 cm/MeV found from the calibration data • <NLivePmts> - fixed at calculated value. Average number of live PMTs • gc=0.122 – geometric correction parameter, fixed at value found with MC (no sensitivity in the low energy part) The same as in 7Be analysis

  12. Energy resolution The same as in 7Be analysis New! Not in MC yet Intrinsic line width • Parameters of the energy resolution (2 free): • vT – free. Takes into account spatial non-uniformity of the light collection (basically 210Po parameter because it has negligible contribution in the low energy part of the spectrum). The same for Po and 14C • σint - free. Intrinsic line width (extra width compared to sqrt(Nphotons)) for β-particles (absent or negligible for α). • vf - fixedat calculated value (variance of the number of live Pmts). • v1=0.17 - fixedat value found with MC (no sensitivity in the low energy part). Intrinsic resolution measured for a EJ301 liquid scintillator. Various colours represent the data obtained with different radiation sources placed at various angles. (figure from 2012 JINST 7 P06011)

  13. Improvement of energy resolution calculation “weighted” NLivePmts distribution for 210Po peak: Fill(1,1)Fill(1,e-(t-t0)/T) If quenching factor, for 5.3 MeV α is known, the decay of Po and decrease of NLivePmt in time lead to the higher “effective” LY for Po and lower “effective” variance of the Po line We’ve tried also response function weighted over NLivePmts . The improvement of χ2 is of the order of 2-3 sigmas with all the best parameters of the fit unchanged, and with a drawback of heavy calculations. Decided to keep “unweighted” response function.

  14. Improvement of Line shape description for high statistics Line shape – the shape of the detector’s response for uniformly distributed monoenergetic particle. Generalized Gamma Function - originally developed for Charge variable and applied in 7Be analysis (matters only for 210Po peak precise fit). The statistics involved was much lower, at the level of 105 events. In “pp” analysis : up to 107 events in a bin so the limits of applicability have to be verified with this statistics. Significant deviations from MC distribution are found in the tails of GGF. New approximation has been tested that approximates better the base binomial distribution: the scaled Poisson distribution. The choice is based on the quality of the fit of the very simple basic response. GGF χ2=288.3/70 107 MC events In center SP χ2=44.4/70 The mean values and variances of both shapes are the same

  15. Scaled Poisson vs GGF Fraction of events in tail for 3 base functions (<N>=49.9, source at center)

  16. Data subdivision for analysis Phase II

  17. Data selection • Discussed in detail in Livia’s talk • Data cuts – the same of 7Be excluding shape sensitive cuts (i.e. soft α/β and some others) • FV cut – the same of 7Be (R<3.021 && |Z|<1.67 corresponding to 75.5 tones of LS)

  18. Improvements of official fitter since last GM • Included σint (beta_resolution_1) in signal variance calculation (there is an option to switch it off for α’s) • Added v1=0.16 value for resolution (see pp analysis report for details). • Scaled Poisson included as a default choice for response shape • Analytical pile-up energy scale • “Double quenching” for 14C • “pp” spectrum shape adjusted • Spectral shapes of 210Bi,85Kr and 14C are verified • Use of time-weighted NLivePmts values for 210Po

  19. “Standard” fit • Variable: npmts_dt1 • Range: [60-220] • Response function shape: Scaled Poisson • Free spectral components: 14C,210Bi,210Po,pp • Fixed spectral components: 7Be (paper central value), pep+CNO+8B (SSM/LMA(HM)),85Kr=0 • Energy scale variables: LY free, kB fixed, feq calculated • Energy resolution vars: σint and vT free; vf calculated • Synthetic pile-up is our default option (constrained) • Free: LY+2 resolution vars+Po position+4 spectral components +constrained pup • 8 free parameters and 1 (pileup) constrained

  20. Results of “standard” fit for ~542.5 days of data (Periods 9+10+11+12)

  21. We have relatively high χ2 • χ2=197.3/151 is high (p value = 0.0068) • Content of bin 60 for combined statistics is 5·105 demanding 0.14% model precision (and we can’t guarantee at this level the flatness of FV(E) related to reconstruction at very low E) • Solution we are checking : go to higher low limit constraining 14C rate at independent measurement

  22. 87Rb? Of course the elements are earth, water, fire and air. But what about rubidium? Surely You can’t ignore rubidium.

  23. 87Rb – should we be paranoic? Second forbidden β-decay with E0=283.3 keV end-point Δχ2=+1 @ 100 cpd of 87Rb/100t with corresponding pp count decrease of 40 cpd/100 t 87Rb at 100 cpd/100 t Correlation: 1 cpd/100t of 87Rb simulates 0.4 cpd/100t of pp

  24. 87Rb vs 40K • though the Rb is quite abundant in nature (the twenty-third most abundant element in the Earth's crust) the typical abundance of Rb is factor (2-4 )·103 lower than that of K (which is one of most abundant and contributes 1.5% of the crust weight). • Taking into account the abundance of radioactive isotopes: 0.278 of 87Rb against 1.17·10-4 of 40K, and the ratio of live-times 47.2 vs 1.28 billions yr, we get the ratio for the typical activity of 87Rb: • The chemistry of the elements is the same, Rb substitutes K, so we can expect the same purification factor if started from the natural ratio of Rb/K. • 40K limit from the pep-analysis is <0.11 cpd/100 t (68% C.L.)* *published value is <0.4 cpd/100 t corresponding to 95% C.L. (<1.7·10-15 Knat [g/g Sc] ) <4.6·10-16 Knat [g/g Sc]

  25. below 17% precision measurement pp = 146 ± 10 (stat) ± 22 (syst*) cpd/100 t *details on systematics in Pablo’s talk

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