Spectral Fit of Borexino Phase-III Data for the Detection of CNO Solar
Spectral fit of Borexino Phase-III data for the detection of CNO solar neutrinos Zara Bagdasarian1,2, Davide Basilico3, Giulio Settanta2 for the Borexino collaboration 1University of California Berkeley, 2Forschungszentrum Jülich, 3Università degli Studi di Milano and INFN
Why CNO solar neutrinos? Borexino experiment Signals and Backgrounds Laboratori Nazionali del Gran Sasso Purity: the most radiopure Detection channel: neutrino electron elastic scattering (LNGS) location: 106 fold reduction of Proof of principle of star energy production via liquid scintillator Internal backgrounds : carbon - nitrogen - oxygen (CNO) cycle the cosmogenic background 1 Low-energy threshold After purifications: 232Th and 238U negligible (~10-19 g/g), Important for astrophysics studies of the Sun High light yield 85Kr reduced by factor～4.6; 210Bi reduced by ～2.3 Water tank Good energy and and heavier stars where the process is expected R = 9 m, 2.1 kt water 210Po - the only α-decay background, position resolution key to its parent, 210Bi, constraint 2 to be dominant Shielding 3 Cherenkov muon veto Nylon Outer Vessel Sensitive to Sun core metallicity (Z) problem: R = 5.5 m Cosmogenic background almost 30% difference in predictions from high Stainless Steel Sphere Barrier for 222Rn Calculate for each event the likelihood to (HZ) vs low (LZ) Standard Solar Models [1] R = 6.85 m from steel, PMTs etc. be 11C through the three fold coincidence: PMTs support 12 11 1 Two-fold dependence on Z: indirectly through Buffer + scintillator μ + C → μ + C + n Buffer (250 μs) 2 core temperature and opacity (as for the pp-chain container n + p → D + γ(2.2 MeV) Pseudocumene (PC) 11 11 + (~30 min) 3 solar neutrinos) + directly dependent on the + DMP quencher C → B + e + νe amount of carbon and nitrogen. 208 Outer Detector External backgrounds Solar neutrinos PMTs Nylon Inner Vessel Yesterday’s signal = today’s background: R = 4.25 m, ~ 300 t of γs from 208Tl, 214Bi, 40K - efficient pp-chain neutrinos: (pp), (7Be), and (pep) 2212 Inward-facing liquid scintillator reduction by fiducial volume selection ν ν ν + radial dependence Expected Flux Expected Flux Difference PMTs (PC/PPO solution) Solar � (HZ) (LZ) (HZ-LZ)/HZ m 10 -2 -1 i ν(CNO) pp [10 cm s ] 5.98 (1± 0.006) 6.03 (1± 0.005) -0.8% Z = See Poster #297 Challenges of CNO neutrinos detection ∑ M ν(pep) i>He ∘ Low rate of ν(CNO) pep [108 cm-2 s-1] 1.44 (1± 0.01) 1.46 (1± 0.009) -1.4% Purification I Purification II Thermal insulation 210Bi Shape similar to 210Bi and ν(pep) 7Be [109cm-2 s-1] 4.93 (1± 0.06) 4.50 (1± 0.06) 8.7% Strong correlations → need independent Phase I (2007-2010) Phase II (2012-2016) Phase III (2016-) � constraints: 8B [106 cm-2 s-1] 5.46 (1± 0.12) 4.50 (1± 0.12) 17.6% � Events/(day x 100t keV 210Bi: link with 210Po See Poster #212 First observations of some low- Comprehensive measurement of all Stabilized detector ν(pep): solar luminosity constraint CNO [108 cm-2 s-1] 4.88 (1± 0.11) 3.51(1± 0.11) 28.1% energy solar neutrino components pp-chain solar neutrino components CNO detection
Example from Phase II Data Selection and Creation of PDFs Spectral Fit of Phase-III data Conclusions ℒ (θ)=ℒ (θ)·ℒ (θ)·ℒ (θ) Data selection main steps: A multivariate fit with the likelihood MV dep enr Rad Dataset: July 2016 - February 2020 First detection of CNO neutrinos 11C depleted energy spectrum: 63.6% of exposure with 5.5% of 11C Exposure: 1071,95 days x 71.3 tons WITHOUT systematics: All data Building likelihood profile by all LZ HZ performing multivariate spectral fits: 8 10 Best fit value: CNO = 7.2 cpd/100t 7 Constraints: 10 muon and muon daughter cuts: 210Po (7Be) (CNO) Significance without systematics: 85Kr ν ν (pep) = 2.74 ± 0.04 cpd/100t 106 300 ms after crossing muons ν 210Bi (upper limit): 5.48 σ 105 11.5 ± 1.3 cpd/100t 5σ Significance with systematics: see 11 4 210 C 10 Bi ℒ the talk and [2]
Fiducial volume(FV)cut: ν(pep) n See Talk by G. Ranucci Events l 103 External backgrounds R<2.8 m; -1.8m
Creation of Probability Density Functions (PDFs) : 1σ Counting analysis - compatible results See Poster #93 Applying cuts on PDFs for uniform (MC e-) 11 Radial distribution: Generating all species + uniform electron C enriched spectrum Discrimination between HZ and LZ not and external backgrounds 11 Improve disentanglement of external uniform electrons (flat in sample 36.4% of exposure with 94.5% of C yet possible radial distributions from the internal contaminants energy) in the full Borexino Monte Carlo 102 210Po ν(7Be) References 85Kr Uniform components Applying cuts on all ν(CNO) 11C External backgrounds (pep) [1] A New Generation of Standard Solar Models - Astrophys. J. 835, 2, 202 (2017) ν Best fit Scintillation light generation components 210Bi External background [2] new Borexino arXiv submission PDFs for all neutrinos and 10 and propagation, electronics backgrounds [3] Sensitivity to neutrinos from the solar CNO cycle in Borexino. arXiv:2005.12829 10 response, and energy and energy spectra C [4] Comprehensive measurement of pp-chain solar neutrinos Nature 562, p 496 position reconstruction Normalised with the [5] Simultaneous precision spectroscopy of pp, Be7, and pep solar neutrinos with Borexino fraction of events passing Events/(day x 100t 0.1m) 1 PhaseII. Review D 100 p. 08200 See Poster #181 the cuts to the number of [6] The Monte Carlo simulation of the Borexino detector Astroparticle Physics 97, p136 events generated in FV All set for the spectral fit! 0 0.5 1 1.5 2 2.5 3 R (m)