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 -1.260λ = gA/gV

0.960

0.965

0.970

0.975

0.980V ud

τ n [S

erebro

v05]

τ n [M

AMBO 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 [U

CN

A 2

010]

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 /c

2 ]

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 zB

qq

=

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

l d(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