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The Nab Experiment
2012-10-26 Christopher CrawfordDNP Fall Meeting, Newport Beach, CA University of Kentucky
DNP Fall Meeting, Newport Beach, CA 2
Correlations in Neutron DecayParity violation implies a rich phenomenology in neutron decay.
V-A implies that all experimental quantities can be related to
the axial and vector coupling constants gA and gV.
Neutron beta decay measurements give:• Test of CKM unitarity• Test of CVC / search for SCC• Test of tensor or scalar couplings
Jackson et al., PR 106, 517 (1957)
2012-10-26
Goals: Nab: δa/a = 0.1%, δb = 0.1% abBA: δA/A = 0.1%, δB/B=0.1%
DNP Fall Meeting, Newport Beach, CA 3
Determination of Vud matrix element
2012-10-26
-1.290 -1.280 -1.270 -1.260
λ = gA/gV
0.960
0.965
0.970
0.975
0.980V
ud
τ n [S
ereb
rov0
5]τ n
[MAM
BO II]
τ n [P
DG 2010
]
ft(0+→0+) [Hardy09]
ft(0+→0+) [Liang09 – PKO1]
ft(0+→0+) [Liang09 – DD-ME2]
PIBETA [Pocanic04]
λ [P
DG
201
0]
A [
UC
NA
201
0]
A [PERKEO II, prel.]
Kaons+Unitarity [PDG 2010]
courtesy S. Baessler
DNP Fall Meeting, Newport Beach, CA 4
Present limits (n decay data)SM: (0,0)
Future limits,assuming a = -0.1059(1), A = -0.11860(3),B = 0.987(1), τn = 882.2(8) s
Sensitivity to scalar/tensor couplings
G. Konrad, S.B. et al., ArXiv:1007.3027; courtesy S. Baessler
Left-handed currents Right-handed currents
2012-10-26
DNP Fall Meeting, Newport Beach, CA 5
Ee [MeV]
p p2 [M
eV2 /
c2 ]
cos θeν = 1
Proton phase space (Dalitz plot) Probability (arb. units)
0
0.25
0.5
0.75
1
1.25
1.5
0 0.2 0.4 0.6 0.8
cos θeν = 0cos θeν = -1
Ee =
75 keV
236 keV
450 keV
700 keV
Nab cos(θ) spectrometer
courtesy S. Baessler 2012-10-26
DNP Fall Meeting, Newport Beach, CA 6
SegmentedSi detector
decay volume (field rB,DV∙B0)
0 kV
0 kV
30 kV
magnetic filterregion (field B0)
Neutronbeam
TOF region (field rB∙B0)
Proton Trajectory
Magnetic Field
Adi
abat
ic
conv
ersi
on
p
||p
p
||p
Proton TOF measurement• Magnetic filter and longitudinalization of proton momentum
z [m]0 0 1-1 2 3 4 5
1
2
3
4
5
B z (on axis)
Decayvolume
Si detector
courtesy S. Baessler
2012-10-26
DNP Fall Meeting, Newport Beach, CA 7
Sim
ulat
ed c
ount
s [A
.U.]
0.002 0.004 0.0060
Ee = 300 keV
Ee = 500 keV
Ee = 700 keV
1/tp2 [µs-2]
2p
2p,0 0
sin ( ) ( )
sin
z B z
B
q
q=
Detector response function• TOF vs. pp assuming
adiabatic proton spin transport
• Edges confirm calibration
• Slope of central part used to extract correlation ‘a’
courtesy S. Baessler 2012-10-26
DNP Fall Meeting, Newport Beach, CA 8
• Systematic uncertainties– Electron energy resolution
– backgroundGoal: Δb ~ 3×10-3
2 ee
e
1 3 1m
dw E bE
0 200 400 600 800Ee,kin (keV)
Yie
ld(a
rb.u
nits
)
b = +0.1SM
2% of events in tail(deadlayer,external bremsstrahlung)
Yie
ld1
101
102
103
104
105
detected Ee [keV]
0 50 100 150 200 250 300
Detector response to decayelectron with Ee = 300 keV
Fierz interference term ‘b’• From the shape of the
electron energy spectrum
courtesy S. Baessler 2012-10-26
PLANNED systematic uncertainty budget:
About 2×109 events can be detected in 6 weeks (Decay volume V = 246 cm3, decay density nd = 20 cm-3, 12.7 % of decay protons go to upper detector, 80% duty factor)
→ (Δa/a)stat < 1×10-3 can be reachedCompare to Δa/a = 5 % of existing experimental results
lower Ee cutoff none 100 keV
100 keV
300 keV
upper tp cutoff none none 40 μs 40 μs
Δa2.4/√N 2.5/√N 2.5/√N 2.6/√N
Δa (Ecal, l variable)
2.5/√N 2.6/√N 2.7/√N 2.7/√N
Δa (Ecal, l variable, inner 70% of data)
4.1/√N 4.1/√N 4.1/√N 4.1/√N
Experimental parameter Systematic uncertainty Δa/a
Magnetic field ... curvature at pinch 5·10-4
… ratio rB = BTOF/B0 2.5·10-4
… ratio rB,DV = BDV/B0 3·10-4
Length of the TOF region (*)Electrical potential inhomogeneity: … in decay volume / filter region 5·10-4
… in TOF region 1·10-4
Neutron Beam: … position 4·10-4
… profile (including edge effect) 2.5·10-4
… Doppler effect smallUnwanted beam polarization can be made smallAdiabaticity of proton motion 1·10-4
Detector effects: … Electron energy calibration (*)… Electron energy resolution 5·10-4
… Proton trigger efficiency 2.5·10-4
Residual gas smallBackground smallAccidental coincidences smallSum 1·10-3
Uncertainty BudgetPLANNED statistical uncertainty budget:
DNP Fall Meeting, Newport Beach, CA 10
Spectrometer magnet
Si detectors
Neutron beam
Nab Setup at the SNS• Fundamental Neutron Physics Beamline
2012-10-26
DNP Fall Meeting, Newport Beach, CA 11
6” ion-implanted silicon detectors• 2 mm thick, 127 pixels• 70-100 nm dead layer
2012-10-26
front
back
fiducial volume
background
dark noisepedestal
proton,32 keV0.5 mm
Dead-layer thickness measured at NCSU
DNP Fall Meeting, Newport Beach, CA 12
DAQ – Digital Signal Processing• Readout waveform (energy,
time)– offline pulse shape analysis
• Form e-p coincidences– 50 – 750 keV prompt electron– 30 keV proton 12 – 40 μs TOF
• Energy and Timing Resolution– 2 keV electron (energy sum)– 10 ns proton TOF resolution
• Resolve backscattered events
• HV Optical Isolation– 30 kV potential between
detectors
• Event / Data rate– 5000/s electrons; 600/s protons – Accidentals <1% in 40 us window– 256 channels, 100 MHz, 12 bit
ADC– 11 MB/s data rate -> 12 TB
• Trigger levels– 1) DIGITIZER threshold,
2) FPGA readout, 3) CPU storage– Energy sum trigger, adjacent pixels– Read out 7 nearest neighbor pixels
in each detector
.
2012-10-26
main FGPA
hits
trigger bus lines
digitizerFPGA
fiberoptics
readout
rear I/Omodule
DNP Fall Meeting, Newport Beach, CA 13
R. Alarcona, L.P. Alonzib , S. Baessler.b,c (Project Manager), S. Balascutaa, L. Barrón-Palos n, J.D. Bowmanc (Co-Spokesmen), M.A. Bychkovb, J. Byrned, J.R. Calarcoe, T.V. Ciancioloc, C. Crawfordf, E. Frležb, M.T. Gerickeg, F. Glückh, G.L. Greenec,i, R.K. Grzywaczi, V. Gudkovj, F.W. Hersmane, T. Itok, A. Kleink, M. Makelak, J. Martinl, S. McGovernb, S. Pageg, A. Palladinob, S.I. Penttiläc (On-site Manager), D. Počanićc (Co-Spokesmen), K.P. Rykaczewskic, A. Salas-Baccib, W.S. Wilburnk, A.Youngm
a Department of Physics, Arizona State University, Tempe, AZ 85287-1504b Department of Physics, University of Virginia, Charlottesville, VA 22904-4714c Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831d Department of Physics and Astronomy, University of Sussex, Brighton BN19RH, UKe Department of Physics, University of New Hampshire, Durham, NH 03824f Department of Physics and Astronomy, University of Kentucky, Lexington, KY 40506g Department of Physics, University of Manitoba, Winnipeg, Manitoba, R3T 2N2, Canadah IEKP, Universität Karlsruhe (TH), Kaiserstraße 12, 76131 Karlsruhe, Germanyi Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996j Department of Physics and Astronomy, University of South Carolina, Columbia, SC 29208k Los Alamos National Laboratory, Los Alamos, NM 87545l Department of Physics, University of Winnipeg, Winnipeg, Manitoba R3B2E9, Canadam Department of Physics, North Carolina State University, Raleigh, NC 27695-8202n Universidad Nacional Autónoma de México, México, D.F. 04510, México
Nab Collaboration
2012-10-26