n νeνeνeνe p+p+ e-e- The Proton Spectrum in Neutron Beta Decay: Latest Results with the a SPECT Spectrometer The a SPECT collaboration Universität MainzF. Ayala Guardia, M. Borg, W. Heil, R. Muñoz Horta, Y. Sobolev, G. K. Universität KarlsruheF. Glück TU MünchenI. Konorov, H.-F. Wirth ILL GrenobleK. Leung, M. Simson, T. Soldner, O. Zimmer University of VirginiaS. Baeßler Gertrud Konrad International Conference on Particles And Nuclei PANIC08 Eilat, Israel, November 2008 University of Mainz / Germany

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Outline Measurement of the Neutrino-Electron Correlation Coefficient a The Neutron Decay Spectrometer a SPECT Results from the beam times  at FRM-II, Garching (2006) and  at ILL, Grenoble (Spring 2008) Summary and Outlook

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Neutron Decay Parameters (SM) Jackson et al., PR 106, 517 (1957) n νeνeνeνe p+p+ e-e- Neutrino-Electron Correlation Neutron lifetime Beta-Asymmetry Weak coupling constant ratio

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Neutrino-Electron Correlation a and Proton Spectrum in Free Neutron Decay The Correlation Coefficient a Proton kinetic energy is big n νeνeνeνe p+p+ e-e- n νeνeνeνe p+p+ e-e- Proton kinetic energy is small n νeνeνeνe p+p+ e-e- θ Sensitivity of the Proton Spectrum to a Proton spectrum for a = and for a = (4) (PDG 2008) Proton kinetic energy E [eV] Decay rate w(E) Present best experimentsΔa/ a ~ 5% Aim of a SPECT Δa/ a ~ 0.3%

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Setup of a SPECT at PF1b/ ILL Analyzing Plane Electrode Proton detector Neutron decay Protons Magnetic field Anti-magnetic screen

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Spectrometer Sketch and Principle of a Retardation spectrometer Transmission function T U (E) in the adiabatic limit: Neutron Beam +1kV (0V to +800V) -30kV Proton Detector Electrostatic Mirror Decay Volume Analyzing Plane B A ≈ 0.44T B 0 ≈ 2.2T U ExB Proton kinetic energy E [eV] Decay rate w(E) Transmission U = 375V Transmission function T U (E) Adiabatic conversion Proton spectrum for a = and for a = (4) (PDG 2008)

Test run at FRM-II: Background influence on a and possible explanation (Penning traps) Each data point corresponds to an independent measurement run. Neutron Beam -30kV Proton Detector Decay Volume Analyzing Plane ExB Trapped e - Background U A = 780V Strategies to reduce the background New detector with better energy resolution Avoid fluctuating background due to trapped particles:  Reduce field-emission, e.g. reduce acceleration -HV  Improve vacuum Background count rate [s -1 ] Neutrino-Electron Correlation Coefficient a a =-0.103(4) from PDG2008 Measurements w/o visible proton-like peak Measurements with visible proton-like peak S. Baeßler et al., First measurements with the neutron decay spectrometer aSPECT, Eur. Phys. J. A 38, (2008)

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Pulse height spectrum: Beam time at ILL: Principle of extraction of a 470 counts per second at U A = 50 V (one detector pad) Statistical sensitivity on a about 2 % per 24 hours measurement time Background stable Integrated Proton Spectrum: Silicon drift detector at -15kV barrier voltage 50 V 250 V 400 V 500 V 600 V 780 V Count rate [s -1 bin -1 ] Pulse height [ADC channels]

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Systematic effects: The magnetic field ratio Correlation coefficient a exp changes with the magnetic field ratio r B, as described by a slightly different response function T U (E) For 0 A: r B = ; For 50 A: r B ’ = External Helmholtz coils allow us to change magnetic field ratio a =-0.103(4) from PDG2008 F tr (E) with r B = F tr (E) with r B ‘ = F tr (E) with r B = preliminary Helmholtz coils off Helmholtz coils on Uncorrected neutrino electron correlation coefficient a exp Helmholtz coils on Each data point corresponds to about 12 hours of measurement time.

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Summary and Outlook Summary a SPECT extracts a from the measured integrated proton spectrum in free neutron decay Statistical accuracy about 2 % per 24 hours measurement time Studied several systematic effects Total accuracy expected to be well below the present literature value of 5 % Data analysis is currently on the way Outlook Further planned improvements to perform a 0.3 % measurement  Online monitoring of the magnetic field ratio (NMR)  Investigations of the work function of the electrodes (Kelvin probe)

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Cabibbo-Kobayashi-Maskawa-Matrix Unitarity Condition B/D mesons Superallowed Fermi Decays Neutron Decay n → p + e - + ν e Pion Decay Kaon (Ke3) Decays K → e + π + ν e Kaon (Kμ2) Decays K → μ + ν μ Hyperons The Unitarity of the CKM Matrix

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Test run at FRM-II: Proton Spectra Count rate [s -1 bin -1 ] Pulse height [ADC channels] Protons Electronic noise 0 barrier voltage 50 V 400 V 800 V Proton spectrum looks ok, Count rate: ~250 Hz Signal to background ratio: > 10 : 1 Fundamental requirements:  Background stable in time  Background independent of the Analyzing Plane Voltage But … Pulse height spectrum

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Test run at FRM-II: Background Instabilities and Dependency on the AP Voltage Electronic noise not stable in time Proton-like peak leads to systematic changes in the proton count rate ⇒ additional systematic uncertainty; does limit our accuracy Count rate [s -1 bin -1 ] Pulse height [ADC channels] Protons Electronic noise 0 barrier voltage 50 V 400 V 800 V usually 800 V sometimes Proton-like peak Pulse height spectrumMeasurement without neutron beam One component of the background rate does depend on the Analyzing Plane Voltage Analyzing Plane Voltage U A [V] Background count rate [s -1 ]

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 new detector -HV electrodeSAES gettersnew ExB electrode Optimization of the spectrometer New detector with better resolution Avoid fluctuating background due to trapped particles  Re-design of electrodes + better surface conditions  Reduce acceleration -HV  Improve vacuum

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 15 Beam time at ILL: The proton detector Type: Silicon Drift Detector (SDD) Principle: p-doped backside p-doped rings Potential Valley  e - drift to a small anode in the middle P. Lechner et. al., XRS G. Lutz, Semiconductor Radiation Detectors

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 16 Pin-Diode vs. Silicon drift detector PIN-diodeSilicon Drift Detector (SDD) 30 kV (Grenoble Spring 2008) (Munich 2005/ 2006)

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Beam time at ILL: Background Stability Additional neutron shutter allows to monitor background and to study systematic effects  No visible background dependency on the Analyzing Plane voltage Beam time at FRM-II, Garching (2006) Beam time at ILL, Grenoble (Spring 2008) Measurements without neutron beam Analyzing Plane Voltage U A [V] Background count rate [s -1 ]

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Investigations of systematic effects Adiabatic transmission function  Different magnetic field settings  Measurement of the magnetic field at the beam position, before and after the beam time  Different electric field settings Background  Additional neutron shutter  Different trap cleaner and beam shifter ExB settings Edge effect  Different neutron beam profiles  Monte Carlo simulations are currectly on the way Detector efficiency  Different trigger settings  Different post-acceleration voltages  Tests at a proton source + SRIM calculations are currently on the way

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Further on-going improvements On-line monitoring of the magnetic field ratio; NMR Study of surface potential; Kelvin probe In collaboration with Prof. I. Baikie, KP Technologies preliminary ע = γ He-3 B 0 γ He-3 ≈ [MHz/T]

G. Konrad, University of MainzThe a SPECT Experiment, PANIC0810 November ‘08 Material 1Material 2 Kelvin Probe: Tool to measure Work Function _ _ _ V Bias = -V C + Φ1Φ1 E F,1 V Φ2Φ2 E F,2 Φ1Φ1 Φ2Φ2 Φ1Φ1 Φ2Φ2 E F,1 E F,2 E F,1 Material 1 Material 2 Material 1 Material 2 2 Materials with different work functions, isolated 1 st material: to be tested 2 nd material: tip with known work function Electrical Connection: → Charging, until Fermi levels are equal → External electric field → If Material 2 is moved: Capacitance changes, Measurable Current Bias Voltage: → Charge disappears, no external electric field → No current if Material 2 is moved VCVC