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Is it possible to improve our knowledge of  13 by oscillation experiments at reactors?

Is it possible to improve our knowledge of  13 by oscillation experiments at reactors?. Donato Nicolo` Pisa University & INFN,Pisa NUFACT Working Group, Geneva December 11 2002. Summary. Current status Physics motivations The Chooz result What next?

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Is it possible to improve our knowledge of  13 by oscillation experiments at reactors?

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  1. Is it possible to improve our knowledge of 13 by oscillation experiments at reactors? Donato Nicolo` Pisa University & INFN,Pisa NUFACT Working Group, Geneva December 11 2002

  2. Summary • Current status • Physics motivations • The Chooz result • What next? • Improvement of statistics and systematics • Potentialities of two-distance tests • The Krasnoyarsk project (Kr2Det) • An alternative experimental frame • Signal and Background • Expected sensitivity • Rate test • Shape test • Outlook and conclusions Donato NICOLO`

  3. Physics motivations • noscillation hypothesis is dominant for atmospheric ’s • but nemmixingcould still be present at a subdominant level • (sin²(213  0.1 at • (limited by Chooz results, also SK at a minor extent) • Implications for measurement ofdCP at LBL accelerator experiments SK data (also confirmed by MACRO) • SK + SNOresults (energy spectra, CC rate, day/night effect) • strongly favour the LMA MSW solution to solar neutrino deficit • The effect might be seen by KamLAND as: • areduction of integral ratevs predictions; • a modulation of the detected energy spectrumforDmsol2 2·10-4eV2 •  a medium baseline reactor experiment is needed • (S. Schönert et al., hep-ex/020313) Donato NICOLO`

  4. The Chooz result R = 1.01  2.8%(stat)  2.7%(syst) M.Apollonio et al., Phys. Lett. B 466 (1999) 415 sin²(213  0.11  |Ue3|  0.03 confirmed by Palo Verde R = 1.01  2.4%(stat)  5.3%(syst) F. Boehm et al., PR D64 (2001) 112001 Sensitivity could be lowered to sin²(213  10-3atneutrino factories (construction to be started in ~2010) Is it possible to reach an intermediate (sin²(213 ~ 10-2) step by reactor experiments? factor 5 improvement  errors must be ~ 0.5% Donato NICOLO`

  5. Possible improvements • systematics • Comparison of neutrino fluxes and spectra at two distances • L1 ~ 100 m, L2 ~ 1 km •  independent of absolute normalisation (spectra, cross section, power, burn-up) • which contributed at 2% to the overall systematic uncertainty at Chooz • Identical detectors (detection technique, geometrical shape)or as • similar as possible minimise differences in efficiencies and calibrations. • statistics • Large volume detectors (target ~50 t)  signal larger by ~10 • to be located underground (300 m.w.e) neutron background suppression • Detector design suited to shield the target from external radioactivity •  accidental background suppression Donato NICOLO`

  6. A possible detector Event signature prompt positron signal (mimicked byn+p n+p) delayed neutronsignal (t = 180 ms) neutron efficiency lower (~50%) than Chooz Mini version of KamLAND (or Borexino) 50 t liquid scintillation target(no Gd-doping) (l ~ 10 m, stable) Mineral oil buffer (~ 1m thick) shield against PMT radioactivity Active cosmic veto Donato NICOLO`

  7. The Kr2Det project • Based on L.V.Mikaelyan’s idea • to be operated by Krasnoyarsk nuclear plant (Russia) • one (over three) reactor still operating,W = 1.6 GWth • L1 = 250 m, L2 = 1100 m • twin detectors (with the possibility to swap their position)  ssyst ~ 0.5% • thick rock overburden (600 m.w.e.) • (neutron flux suppressed by a factor ~3 w.r.t. Chooz) • possibility to measure the background during the reactor stop for refueling • sstat = 0.5 % could be reached in ~ 3y data taking but... plant operating is not guaranteed Russian authorities are strongly demanded to stop reactor operations for safety reasons Donato NICOLO`

  8. A possible alternative • Chooz2 • Far detector located in the underground site (L2~1000 m, 300 m.w.e.) hosting the previous experiment  S/N ~ 10 • Near detector placed in a new shallow site (L1~100 m, 40 m.w.e.), to be built on purpose • m vertical intensity reduced to only 30% of the surface intensity •  near detector must be smaller (else it would be overwhelmed by cosmic rays!) • systematic error could not be better than 1% • 5t scintillator target size looks like a reasonable compromise • signal: • background: • Rm ~ 2 KHz(2 orders of magnitudes larger than at the underground site) • Nn ~ 3times as low as underground F.Boehm et al. PRD 62 (2000) 092005 • neutron flux n ~ 30 timesas large as underground at full reactor power Donato NICOLO`

  9. Statistical uncertainties reactor 1 • A realistic plant operation frame • reactors operating at full power, • 90% duty cycle • alternated stop (~1month) for refueling • During 3 years data taking one collects: • 800 d with both reactors ON (“ON” period) • 200 d with one reactors OFF (“OFF” period) • R1= (625 ± 2) ev/d • R2 = (63 ± 0.7) ev/d •  sstat = 1.1% • limited by background subtraction reactor 2 time Far detector Donato NICOLO`

  10. A more optimistic scenario • Background measurement could be improved if reactors: • ramp up/down slowly • are both OFF for a while • (both conditions were fulfilled at Chooz) • With 30 d reactor shutdown • R2= (125 ± 0.6) ev/d •  sstat = 0.5% OK • How much does it cost? • 0.05$/kWh  8.5GW 0.3  24 h = 3 M$ • in 1 day •  ~ 100 M$ in 1 month Chooz data Donato NICOLO`

  11. Rate test Based on the comparison of integral rates at two distances = test variable (normalised to 1 in absence of oscillations) statistics systematics Use ² instead of likelihood ratio for statistic test (OK in aGaussian regime) ² = ² - ²min errors added in quadrature • Apply Feldman-Cousins procedure to determine 90% confidence intervals in(sin²(2,dm²)for: • Kr2Det • Chooz2 Donato NICOLO`

  12. Rate sensitivity plot • At dm² = 3x10 ³ eV² • Kr2Det • sin²(2  0.02 • Chooz2 • sin²(2  0.04 • not much encouraging... • Chooz • sin²(2  0.1 Donato NICOLO`

  13. Spectrum test Based on the comparison of positron spectra at two distances Improved sensitivity in the “lobe” regions  more powerful test r 1 month reactors OFF  stat = 0.5% Spectra arranged in 16 bins(DE=0.4 MeV) Build statistic as threshold effects energy scale calibration spectra normalisation (relative efficiencies, target volumes) e+ energy (MeV) FC procedure Donato NICOLO`

  14. Spectrum test sensitivity 1 month reactors OFF 90% sensitivity contours as functions of sa (sgneglected) • At dm² = 3x10 ³ eV² • sa = 0.5% • sin²(2  0.015 • sa = 1.0 % • sin²(2  0.025 • sa = infinite (shape test) • sin²(2  0.04 • Chooz • sin²(2  0.1 Donato NICOLO`

  15. Spectrum test sensitivity (more realistic) reactors ON-OFF 90% sensitivity contours as functions of sa (sgneglected) • At dm² = 3x10 ³ eV² • sa = 0.5% • sin²(2  0.03 • sa = 1.0 % • sin²(2  0.04 • sa = infinite (shape test) • sin²(2  0.1 • Chooz • sin²(2  0.1 Donato NICOLO`

  16. nenx mixing present at subdominant level in the atmospheric n range could influence the solution to the solar ndeficit to be investigated at future neutrino factories down to sin²(2  10 ³ Sensitivity at reactor experiments could be pushed tosin²(2  0.02 if: both detectors were located undergroundsyst = 0.5% background could be measured with reactors OFF stat = 0.5% one reactor plants are preferred, underground if possible Do they exist? Else it is difficult to obtain better than 1% accuracy, both statistics and systematics a factor 23 improvement with respect to Chooz Outlook and conclusions Donato NICOLO`

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