Searching for the neutrino magnetic moment at the Kuo-Sheng nuclear power plant Outline 1. Introduction & Overview 2. The experiment result 3. Future H. B. Li 李浩斌 Department of Physics, Academia Sinica.

Neutrino Radiative Decay (Gn): scattering mn n with mass  possible niL njR + g vertices Experimental Probe:  Study ne + e-  nX + e-  Focus on low recoil energy ※ sm  T–1 ※ decouples SM “background” [ ……… LE reactor f(ne) not accurately known]  Look for Excess in Reactor ON/OFF Neutrino Radiative Decay (Gn):  sm & Gn related:  same vertices – real g for Gn Electron Recoil Spectra

PDG Limit : 1.5 X 10-10 mB (from SK spectral shape) an allowed solution for the solar neutrino data (before KamLAND)

Kuo-Sheng Nuclear Power Plant KS NPS-II : 2 cores X 2.9 GW KS n Lab: 28 m from core#1

ULB-HPGe + Anti-Comptons KS Expt. : Period I Configuration Period I : June 01 – April 02 (60 days OFF) Shielding & Veto [one side] ULB-HPGe + Anti-Comptons CsI(Tl) Array (46 kg)

KS Expt. : Period I Detector ULB-HPGe [1 kg] CsI(Tl) [46 kg] FADC Readout [16 ch., 20 MHz, 8 bit] Multi-Disks Array [600 Gb]

HPGe data Data Volume: HPGe Performance: Analysis:  Total Live Time 196/52 days ON/OFF HPGe Performance:  1.06 kg mass  0.4 keV RMS at 10 keV  5 keV threshold [Unique Data]  Range of O(1 cpd) [counts kg-1 day-1 keV-1] background c/f Dark Matter expt. Analysis:  Cosmic Veto (5 ms gate)  Anti-Compton (Well+Base detectors)  Pulse Shape Disc. (veto elect. noise, accidentals)

Background and spectrum Low background detector 20/keV day +veto 1/keV day On-off subtraction O(0.01) /keV day 10.37 keV peak decaying

spectrum Uncertainty(E>3MeV) : 5% Uncertainty(E<3MeV) : 30% Solid curve : fission, flux = Dot curve : fission + neutron capture, flux =

Efficiency and systematic error Event Selection Suppression Efficiency Raw Data 1.0 1.0 Anti-Compton 0.06 0.99 Cosmic-Ray Veto 0.96 0.95 Pulse Shape Analysis 0.86 1.0 Combined Efficiency 0.05 0.94 Source Uncertainties DAQ Live Time ON/OFF <0. 2% <0.30 Efficiencies for magnetic scattering <0.2% <0.01 Rates for magnetic scattering 24% 0.23 SM background subtraction 23% 0.03 Combined Systematic Error - <0.4

Spectra & Residual Based on 196/52 days of Reactor ON/OFF data No anomalous structures or residuals

Result Fit OFF spectra to fOFF Fit ON spectra to E/keV fOFF c2/dof 12-61 p4 80/96 Fit ON spectra to fOFF + fSM + k2 fMM [10-10 mB] [using best-fit values of (fOFF;dfOFF)] Fit Results (@ c2/dof = 48/49 ): k2 = -0.5  1.4 (stat.)  0.4 (sys.) Limits derived : mn < 1.3 (1.0) X 10-10 mB @ 90(68)% C.L. Improvement in Period 2:  4p anti-Compton coverage  improved inner shielding  Aim of analysis threshold: ~ 5 keV  study “ultra-low” energy Ge prototype [threshold : 200-300 eV] Residual Plot

Reactor Sensitivities

Summary Best published limit. Only experiment at 10 keV range. Background level O(1 cpd) [counts kg-1 day-1 keV-1] Comparable to underground CMD experiment. Flexible design. Upgrade to 186 kg of CsI(Tl). Improve anti-Compton detector. Improve trigger system and electronic. Improve the measurement of cosmic veto time.

“New Limits on Neutrino Magnetic Moment from the Kuo-Sheng Reactor Neutrino Experiment”, hep-ex/0212003, To be published in Phys. Rev. Lett.