Nab, a new precise study of neutron beta decay at SNS Dinko Po cani - - PowerPoint PPT Presentation

Nab, a new precise study of neutron beta decay at SNS

Dinko Poˇ cani´ c

University of Virginia

Jefferson Lab Newport News, VA, 4 May 2012

Outline

Goals and motivation CKM matrix: Vud (messy neutron results) Non-V − A interaction terms; Second class currents Measurement principles Electron-neutrino correlation a; detection function Apparatus Spectrometer and its optimization Si Detectors; Electrode and vacuum systems Overview of uncertainties Event rates, statistical uncertainties Systematic uncertainties Polarized program: abBA/PANDA Measurement principle Rates and uncertainties Summary

• D. Poˇ

cani´ c (UVa) Nab experiment: Outline 4 May ’12 2 / 43

Neutron Decay Parameters (SM)

dw dEedΩedΩν ≃ peEe(E0 − Ee)2 ×

• 1 + a

pe · pν EeEν + b m Ee + σn ·

• A

pe Ee + B pν Eν

• + . . .
• where:

a = 1 − |λ|2 1 + 3|λ|2 A = −2|λ|2 + Re(λ) 1 + 3|λ|2 B = 2|λ|2 − Re(λ) 1 + 3|λ|2 λ = GA GV (with τn ⇒ CKM Vud) also: C = κ(A + B) where κ ≃ 0.275 .

• D. Poˇ

cani´ c (UVa) Nab experiment: Goals and motivation 4 May ’12 3 / 43

Goals of the Nab experiment

◮ Measure the electron-neutrino parameter a in neutron decay

with accuracy of ∆a a ≃ 10−3 current results: −0.1054 ± 0.0055 Byrne et al ’02 −0.1017 ± 0.0051 Stratowa et al ’78 −0.091 ± 0.039 Grigorev et al ’68

◮ Measure the Fierz interference term b in neutron decay

with accuracy of ∆b ≃ 3 × 10−3 current results: none (in n decay)

• D. Poˇ

cani´ c (UVa) Nab experiment: Goals and motivation 4 May ’12 4 / 43

Quark-lepton (Cabibbo) universality

Basic weak-interaction V-A form (e.g., µ decay): M ∝ e|lα|νe → ¯ ueγα(1 − γ5)uν is replicated in hadronic weak decays: M ∝ p|hα|n → ¯ upγα(GV − GAγ5)un with GV,A ≃ 1 . Departure from GV = 1 (CVC) comes from weak quark (Cabibbo) mixing: GV = Gµ cos θC(= GµVud) cos θC ≃ 0.97 3 q generations lead to the Cabibbo-Kobayashi-Maskawa (CKM) matrix (1973):   Vud Vus Vub Vcd Vcs Vcb Vtd Vts Vtb   CKM unitarity cond.: ∆V2 = 1 − (|Vud|2 + |Vus|2 + |Vub|2)

?

= 0, stringently tests the SM.

• D. Poˇ

cani´ c (UVa) Nab experiment: Goals and motivation 4 May ’12 5 / 43

SM parameters determining Vud

1.290 1.280 1.270 1.260

λ = gA/gV

0.960 0.965 0.970 0.975 0.980

Vud

τn [Serebrov05] τn [MAMBO II] τn [PDG 2010] ft(0+→0+) [Hardy09] ft(0+→0+) [Liang09 – PKO1] ft(0+→0+) [Liang09 – DDME2] PIBETA [Pocanic04] λ [PDG 2010] A [UCNA 2010] A [PERKEO II, prel.] Kaons +Unitarity [PDG 2010]

τ −1

n

= |Vud|2|gV |2(1 + 3|λ|2)

• D. Poˇ

cani´ c (UVa) Nab experiment: Goals and motivation 4 May ’12 6 / 43

Status of A and λ in n decay

• 0.125
• 0.120
• 0.115
• 0.110

PERKEO II, prelim. Δ A/A = 0.1% (abBA goal)

Average:

• 0.1187(8)

UCNA, 2010 PERKEO II, 2002 Liaud, 1997 PERKEO,1986 Yerozolimskii, 1997 Beta Asymmetry A

Uncertainty of the average scaled up by factor 2.3× Global fit χ2/dof = 27/5 ! Statistical probability for this χ2 is 6 × 10−5.

• D. Poˇ

cani´ c (UVa) Nab experiment: Goals and motivation 4 May ’12 7 / 43

Status of A and λ in n decay (cont’d)

Δλ/ = 0.03% (Nab/abBA goals) λ PERKEO II, prelim. Mostovoi, 2001

Average:

• 1.2733(20)

UCNA, 2010 PERKEO II, 2002 Liaud, 1997 PERKEO,1986 λ = / g g

A V

• 1.28
• 1.26
• 1.25
• 1.27

Yerozolimskii, 1997 Goals for ∆a, ∆A: ⇒ ∆λ ≃ 3.5×10−4 i.e., an order of magn. improvement.

∆λ λ ≃ 0.27∆a a ≃ 0.24∆A A

• D. Poˇ

cani´ c (UVa) Nab experiment: Goals and motivation 4 May ’12 8 / 43

n-decay correlation parameters beyond Vud

◮ Beta decay parameters constrain L-R symmetric, SUSY extensions to

the SM. [Reviews: Herczeg, Prog. Part. Nucl. Phys. 46, 413 (2001),

• N. Severijns, M. Beck, O. Naviliat-ˇ

Cunˇ ci´ c, Rev. Mod. Phys. 78, 991 (2006), Ramsey-Musolf, Su, Phys. Rep. 456, 1 (2008)]

◮ Fierz int. term, never measured for the n, along with B, offers a

sensitive test of non-(V − A) terms in the weak Lagrangian (S, T). [S. Profumo, M. J. Ramsey-Musolf, S. Tulin, PRD 75, 075017 (2007)]

◮ Measurement of the electron-energy dependence of a and A can

separately confirm CVC and absence of SCC. [Gardner, Zhang, PRL 86, 5666 (2001), Gardner, hep-ph/0312124]

◮ A connection exists between non-SM (e.g., S, T) terms in d → ue¯

ν and limits on ν masses. [Ito + Pr´

ezaeu, PRL 94 (2005)]

• D. Poˇ

cani´ c (UVa) Nab experiment: Goals and motivation 4 May ’12 9 / 43

Updated limits for RH S and T currents n decay

• 0.15
• 0.10
• 0.05

0.00 0.05 0.10 0.15

• 0.15
• 0.10
• 0.05

0.00 0.05 0.10 0.15

RS/LV

RT/LA

neutronandnucleardecays (survey,95%C.L.) Δχ

2

C.L. 2.30 68.3% 90% 95.4% 4.61 6.17 neutrino mass (68%C.L.) neutrinomass (68%C.L.) muondecay “90%C.L.”

Present limits (n decay data) (SM values at origin of plot.) [τn = 881.8(13) s ]

S V

• 0.15
• 0.10
• 0.05

0.00 0.05 0.10 0.15

• 0.15
• 0.10
• 0.05

0.00 0.05 0.10 0.15

R /L

RT/LA

neutron and nucleardecays (survey,95%C.L.) Δχ

2

C.L. 2.30 68.3% 90% 95.4% 4.61 6.17 neutrinomass (68%C.L.) neutrinomass (68%C.L.) muondecay “90%C.L.”

Projected limits with: τn, a = −0.10588(10),

b ≡ 0, A = −0.1186(1), B = 0.9807(30), C = −0.23875(24).

[After: G. Konrad, W. Heil, S. Baeßler, D. Poˇ cani´ c, F. Gl¨ uck, arXiv 1007.3027.]

• D. Poˇ

cani´ c (UVa) Nab experiment: Goals and motivation 4 May ’12 10 / 43

Limits for LH S and T currents n decay

LS/LV

• 0.3
• 0.2
• 0.1

0.0 0.1 0.2 0.3

• 0.3
• 0.2
• 0.1

0.0 0.1 0.2 0.3

LT/LA

neutronand nucleardecays (survey,68%C.L.) superallowed 0 →0 decays (68%C.L.)

+ +

“presentlimits” (68%C.L.) muondecay “90%C.L.” nucleardecays ( (In),90%C.L.) P

107

Δχ

2

C.L. 2.30 68.3% 90% 95.4% 4.61 6.17

Present limits (n decay data) (SM values at origin of plot.) [τn = 881.8(13) s ]

• 0.04
• 0.02

0.00 0.02 0.04

• 0.04
• 0.02

0.00 0.02 0.04

LS/LV

LT/LA

Δχ

2

C.L. 2.30 68.3% 90% 95.4% 4.61 6.17 “futurelimits” (68%C.L.) superallowed 0 →0 decays (68%C.L.)

++

neutronand nucleardecays (survey,68%C.L.) nucleardecays ( (In),90%C.L.) P

107

Projected limits with: τn, a = −0.10588(10),

b = 0 ± 0.003, A = −0.1186(1), B = 0.9807(30), C = −0.23875(24).

[After: G. Konrad, W. Heil, S. Baeßler, D. Poˇ cani´ c, F. Gl¨ uck, arXiv 1007.3027.]

• D. Poˇ

cani´ c (UVa) Nab experiment: Goals and motivation 4 May ’12 11 / 43

Correlation parameters with recoil correction

[Gardner, Zhang, PRL 86, 5666 (2001), Gardner, hep-ph/0312124]

Most general form of hardonic weak current consistent with (V-A): p(pp)|Jµ|n(pn, P) = ¯ up(pp)

• f1(q2)γµ − if2(q2)

Mn qµ + f3(q2) Mn qµ + g1(q2)γµγ5 − ig2(q2) Mn σµνγ5qν + g3(q2) Mn γ5qµ

• un(pn, P)

a, A, B ⇒ λ = g1 f1 while τn ∝ (f1)2 + 3(g1)2 However, f2 (weak magnetism) and SCC’s (g2,g3), remain unresolved in beta decays (best tested in A=12 system). With recoil corrections, Gardner and Zhang find: a(Ee) = func(f2) while A(Ee) = func(f2, g2)

• D. Poˇ

cani´ c (UVa) Nab experiment: Goals and motivation 4 May ’12 12 / 43

Current and planned experiments aiming to measure a

• 1. Nab: goal is to measure ∆a/a ≃ 10−3
• Discussed in this talk.
• 2. aCORN: goal is to measure ∆a/a < 1 %; (with 0.5 %syst)
• Funded, under way at NIST,
• Uses only part of neutron decay phase space.
• 3. aSPECT: aims to measure ∆a/a ≃ 3 × 10−3 (∼ 1 % short-term)
• Funded and running;
• Singles measurement!
• will become part of the PERC program with improvements.
• D. Poˇ

cani´ c (UVa) Nab experiment: Goals and motivation 4 May ’12 13 / 43

How to accomplish the goals of Nab?

Measure: ∆a a ≃ 10−3 and ∆b ≃ 3 × 10−3. Basic approach: (n → p + e− + ¯ νe)

◮ Detect electrons directly, in Si detectors, ◮ Measure electron energy in Si detectors, ◮ Detect protons, after acceleration, in Si detectors, ◮ Determine proton momentum from TOF over a long

flightpath (electron provides start pulse). A complex magneto-electrostatic apparatus is required to guide particles (nearly) adiabatically to detectors. Location: FnPB at SNS (backup NG-C at NIST).

• D. Poˇ

cani´ c (UVa) Nab experiment: Measurement principles 4 May ’12 14 / 43

Electron–neutrino angle from Ee and Ep

− ν

ν

e θ e n p

Conservation of momentum in n beta decay,

• pp +

pe + pν = 0 , yields p2

p = p2 e + 2pepν cos θeν + p2 ν .

Neglecting proton recoil energy, Ee + Eν = E0, so that pν = E0 − Ee. Therefore: cos θeν is uniquely determined by mea- suring Ee and Ep (or pp ⇒ TOFp).

• D. Poˇ

cani´ c (UVa) Nab experiment: Measurement principles 4 May ’12 15 / 43

Nab measurement principles: proton phase space

e (MeV) p 2 (MeV2/c2)

E p cos θeν = -1 cos θeν = 1 cos θeν = 0 proton phase space Yield (arb. units) Ee = 100 keV 300 keV 500 keV 700 keV

0.5 1 1.5 0.2 0.4 0.6 0.8

NB: For a given Ee, cos θeν is a function of p2

p only.

Slope ∝ a

❆ ❆ ❆ ❆ ❆ ❆ ❆ ❑ ❆ ❆ ❆ ❆ ❑ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❑ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❆ ❑

Numerous consistency checks are built-in!

• D. Poˇ

cani´ c (UVa) Nab experiment: Measurement principles 4 May ’12 16 / 43

But wait: protons fly in all directions!

(not just straight to a small detector) How, then, do we relate TOF to pp? Answer: adiabatic longitudinalization!

• B

r v⊥

• B
• pp

θ mv2

⊥

r = ev⊥B

• r

r = mv⊥ eB . Conservation of L and E yields: L = mv⊥r = m2v2 sin2 θ eB = const.,

• r

sin θpB ∝ √ B .

Proton Trajectory Magnetic Field Adiabatic conversion p^ p^ p‖ p‖

• D. Poˇ

cani´ c (UVa) Nab experiment: Measurement principles 4 May ’12 17 / 43

Nab apparatus in FnPB

Apparatus extends:

• ∼6 m above beam height,
• ∼1.5 m below beam height

(existing pit).

• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 18 / 43

◮ Collect and detect

both electron and proton from neutron beta decay.

◮ Measure Ee and TOFp

and reconstruct decay kinematics Key requirements:

◮ Magnetic field shape, ◮ Electrode system, ◮ Hermeticity, ◮ Ultra-high vacuum ◮ Silicon detectors, ◮ No particle trapping.

P ✐ ✛ P P ✐ ❆ ❑ ❈ ❈ ❖ ❈ ❈ ❖ ❈ ❈ ❖ ❈ ❈ ❖ ❈ ❈ ❖ ✻

Nab principles of operation

• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 19 / 43

Measurement principles: detection function

Proton time of flight in B field: tp = f (cos θp,0) pp where cos θp,0 = pp0 · B pp0B

• decay pt.

. For an adiabatically expanding field prior to acceleration, f (cos θp,0) = l

z0

mp dz cos θp(z) = l

z0

mp dz

• 1 − B(z)

B0 sin2 θp,0

. To this we add effects of magnetic reflections and, also, of electric field acceleration.

• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 20 / 43

Key requirements on the detection function

The proton momentum distribution for Ee = const. within the phase space bounds is given by Pp(p2

p) = κ1 + κ2ap2 p ,

[recall: cos θeν = f (p2

p)]

while Pt 1 t2

p

• =
• Pp(p2

p) Φ

1 t2

p

, p2

p

• dp2

p .

Detection function Φ relates the proton momentum and time-of-flight distributions! To extract a reliably:

◮ Φ must be as narrow as possible, ◮ Φ must be understood precisely.

⇒ (near-)adiabaticity in spectrometer design.

• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 21 / 43

Spectrometer optimization: detection function

kine- matic input

pp2 (MeV2/c2) Yield (arb. units)

Ee = 500 keV

0.1 0.2 0.3 0.4 0.2 0.4 0.6 0.8 1 1.2 1.4

mean:0.00394 s RMS:0.00015 μ

• 2

μs

• 2

0.000 0.001 0.002 0.003 0.004

1/tp

2 [1/µs2]

101 102 103 104

Spectrometer response function Φ(⋅ , pp

2)

mean

Ep = 500 eV analyt. calcul’n

0.00 0.02 0.04 0.06 0.08

1/tp

2 [1/µs2]

103 104 105 106 107

Simulated count rate

Ee = 300 keV Ee = 500 keV Ee = 700 keV

1/tp

2 [1/µs2]

Simulatedcounts[A.U.] 0.002 0.004 0.006 Ee =300keV Ee =500keV Ee =700keV

MC GEANT simul’n

• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 22 / 43

Ee measurement optimization (backscattering)

∆t ≤ 20 ns required ✟ ✟ ✟ ✟ ✟ ✙

• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 23 / 43

Analysis strategy

1/tp

2 [1/µs2]

Simulatedcounts[A.U.] 0.002 0.004 0.006 Ee =300keV Ee =500keV Ee =700keV ◮ Use edges to determine and

verify shape of detection function Φ(pp, 1/tp);

◮ Use central part of Pt(1/t2 p)

(∼ 70%) to extract a.

• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 24 / 43

Spectrometer Coil design and B field profile

466.25 0.03 5.00 25.28 43.81 37.50 0.50 481.25 14.75 25.90 67.09 0.47 41.66 14.77 25.90 4.34 4.34 10.52 20.58 38.16 3.13 3.13 29.94 16.41 30.24 3.19 3.28 4.93 12.92 8.00 16.77 c1i z r c6i c4i c5i c3i c2i c1o c6o c4o c5o c3o c2o Magneticfield [T] B z [m] z [cm] 1 20

• 1

2

• 20

3 10 4

• 10

5

• 30

1 2 3 4 5 Bz (on axis) Bz (on axis) Bz (off axis)

Magneticfield [T] B

1 2 3 4 5 Decay volume Decay volume Si detector Filter 4 mflightpath isomitted here

• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 25 / 43

Key components of the Nab apparatus:

Beam shutter (part of WBS 1.07) Beam stop Spectrometer magnet (WBS 1.02) Passive magnetic shield (WBS 1.03) Neutron guide (WBS 1.08) (include spin flipper – WBS 1.13) Beam pipe (WBS 1.08)

200cm 200cm 300cm

Top view: Side view:

Magnet Pit Detector DAQ in its Faraday cage (WBS 1.10) Collimators (part of WBS 1.07) Detector, Preamps, and detector mount (WBS 1.09) Biological shielding (part of WBS 1.06)

Not shown in figure:

• Main electrode system
• Vacuum system
• HV system
• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 26 / 43

Si detector prototypes (15 cm diameter)

Front Back

✲

n beam LANL group has full-size prototypes from Micron Corp. Full thickness t = 2 mm; dead layer thickness td ≤ 100 nm. Key properties:

◮ hermeticity preserved with Si detectors, ◮ beam imaged (p–e correlated in ≤ 7 pixels), ◮ detect protons down to ∼15 keV.

Further detailed testing currently under way at LANL.

• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 27 / 43

Nab electrode and vacuum system

Gate valve

Main UHV volume Detector UHV TP TP TP GP

Getter pumps Turbo pump (+ev. 2nd turbo pump, + roughing pump, not shown)

Detector UHV ◮ Must be well integrated with rest of

spectrometer,

◮ Pres.gas < 10−8 Torr, to avoid scattering,

HV discharges,

◮ Three sets of pumps: external turbos,

cold bore, internal getter,

◮ Electrode coatings — sensitive issue

(more below)

• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 28 / 43

Electrode, detector and readout package

• D. Poˇ

cani´ c (UVa) Nab experiment: Apparatus 4 May ’12 29 / 43

Statistical uncertainties for a and b

Statistical uncertainties for a

Ee,min 100 keV 100 keV 300 keV tp,max – – 100 µs 40 µs σa 2.4/ √ N 2.5/ √ N 2.5/ √ N 2.5/ √ N σa† 2.5/ √ N 2.6/ √ N 2.6/ √ N 2.7/ √ N σa§ 4.1/ √ N 4.1/ √ N 4.1/ √ N 4.1/ √ N

† with Ecalib and LTOF variable; § using inner 70% of p2 p data.

Statistical uncertainties for b

Ee,min 100 keV 200 keV 300 keV σb 7.5/ √ N 10.1/ √ N 15.6/ √ N 26.3/ √ N σb†† 7.7/ √ N 10.3/ √ N 16.3/ √ N 27.7/ √ N

†† with Ecalib variable.

• D. Poˇ

cani´ c (UVa) Nab experiment: Overview of uncertainties 4 May ’12 30 / 43

Nab event rates, statistics and running times

Nab expects data rates of about 600 evts./s. In a typical ∼ 10-day run of 7 × 105 s of net beam time we would achieve σa a ≃ 2 × 10−3 and σb ≃ 6 × 10−4 We plan to collect samples of 1 − 2 × 109 events in several 6–8-week runs. Overall accuracy will not be statistics-limited. Analysis methods to be used:

• A. parametrize edges and width of Φ(pp, 1/tp) by fitting; use central part
• f Φ (∼ 70%) to extract a in a multiparameter fit, and
• B. specify all possible parameters of Φ by direct measurement; ⇒

treat a, µ = 1/t2

p(pp), and Ndecays as free parameters in a two-step

fitting procedure,

◮ as well as a hybrid of the two methods.

• D. Poˇ

cani´ c (UVa) Nab experiment: Overview of uncertainties 4 May ’12 31 / 43

Nab systematic uncertainties: Method B

Experimental parameter (∆a/a)SYST Magnetic field: curvature at pinch 5 × 10−4 ratio rB = BTOF/B0 2.5 × 10−4 ratio rB,DV = BDV/B0 3 × 10−4 LTOF, length of TOF region (*) U inhomogeneity: in decay / filter region 5 × 10−4 in TOF region 1 × 10−4 Neutron Beam: position 4 × 10−4 width 2.5 × 10−4 Doppler effect small unwanted beam polarization small Adiabaticity of proton motion 1 × 10−4 Detector effects: Ee calibration (*) Ee resolution 5 × 10−4 Proton trigger efficiency 2.5 × 10−4 Accidental coincidences small Residual gas small Background small Sum 1 × 10−3 (*) Free fit parameter

• D. Poˇ

cani´ c (UVa) Nab experiment: Overview of uncertainties 4 May ’12 32 / 43

Experimental parameter (∆a/a)SYST Magnetic field: . . . curvature at pinch 5 × 10−4 . . . ratio rB = BTOF/B0 2.5 × 10−4 . . . ratio rB,DV = BDV/B0 3 × 10−4 Sum 1 × 10−3

Steps:

• 1. Measure field map (relative

measurement).

• 2. Determine position of electron and

proton flux tubes in field map. Magnetic field B [T] z [m]

1

• 1

2 3 4 5 1 2 3 4 5 B (on axis)

z

Decay volume Si detector

z [cm]

20

• 20

10

• 10
• 30

B (on axis)

z

B (off axis)

z

Magnetic field B [T]

1 2 3 4 5 Decay volume Filter

Systematic uncertainty budget: B field

• D. Poˇ

cani´ c (UVa) Nab experiment: Overview of uncertainties 4 May ’12 33 / 43

Systematic uncertainty budget: Electrostatic potential

Experimental parameter (∆a/a)SYST

• El. pot. inhomogeneity:

. . . in decay vol./filter reg. 5 × 10−4 . . . in TOF region 1 × 10−4 Sum 1 × 10−3

Key specification: Electrostatic potential fluctuations in decay volume and filter region ∆U < 10 mV .

Different metals: ∆U ∼ 1 V. Different crystal orient.: ∆U ∼ 300 mV.

• D. Poˇ

cani´ c (UVa) Nab experiment: Overview of uncertainties 4 May ’12 34 / 43

Systematic uncertainty budget: Adiabaticity

Experimental parameter (∆a/a)SYST Adiabaticity of proton motion 1 × 10−4 Sum 1 × 10−3 Adiabatic approximation fails for lower overall

• B, with these consequences:

◮ proton TOF is changed, ◮ proton passage through filter

field not according to expectations, i.e.,

◮ detection function Φ not as

well described analytically.

Effects of B field scaling:

B scale factor 0.2 0.3 0.5 0.7 2 ∆(1/t2

p)

• 0.40%
• 0.10%

−1.6 · 10−4 −4 · 10−5 8 · 10−5 Protons lost 0.70% 0.40% 0.15% 6 · 10−4 −5 · 10−4 ∆a/a

• 4.7%
• 1.0%
• 0.2%

−5 · 10−4 4 · 10−4 ∆cos θ0 5 · 10−4 2 · 10−4 6 · 10−4 negligible negligible

⇒ Considerable flexibility in scaling of B remains!

• D. Poˇ

cani´ c (UVa) Nab experiment: Overview of uncertainties 4 May ’12 35 / 43

Systematic uncertainty budget: Ee resolution

Experimental parameter (∆a/a)SYST Electron energy resolution 1 × 10−4 Sum 1 × 10−3

Uncertainty on a is based on a 1% determination

• f

electron energy response.

10 100 1000 10000 100000 0.005 0.01 0.015 0.02 Yield 1/tp

2 [1/µs2]

Ee = 75 keV Ee = 75 keV, Ee response Ee = 225 keV Ee = 225 keV, Ee response Ee = 375 keV Ee = 375 keV, Ee response Ee = 525 keV Ee = 525 keV, Ee response Ee = 675 keV Ee = 675 keV, Ee response

GEANT4—preliminary!

• D. Poˇ

cani´ c (UVa) Nab experiment: Overview of uncertainties 4 May ’12 36 / 43

Systematic uncertainty budget: Proton trigger efficiency

Experimental parameter (∆a/a)SYST Proton trigger efficiency 2.5 × 10−4 Sum 1 × 10−3

Uncertainty in a is based on a 10 keV threshold and measurement

• f efficiency slope of 50%.

Yield

10

1

10

2

10

3

10

4

10

5

detected Ep [keV] (w/o electronic noise)

5 10 15 20 25 average energy loss: 11 keV

Threshold?

• D. Poˇ

cani´ c (UVa) Nab experiment: Overview of uncertainties 4 May ’12 37 / 43

Remarks on the Polarized Program

• D. Poˇ

cani´ c (UVa) Nab experiment: Polarized program: abBA/PANDA 4 May ’12 38 / 43

abBA/PANDA configuration:

◮ A: detect electrons

in upper, protons in lower detector;

◮ B/C: detect

protons in upper, electrons in lower detector;

Segmented Sidetector TOFregion (field ∙ ) r B

B

Uup (upperHV) Udown (lowerHV) magneticfilter region(field ) B0 decayvolume (field ∙ ) r B

B,DV

Polarizerwith spin-reversal

• D. Poˇ

cani´ c (UVa) Nab experiment: Polarized program: abBA/PANDA 4 May ’12 39 / 43

abBA/PANDA rates and statistical uncertainties

Additions to Nab apparatus: (supermirror) polarizers Event rates: decays in DV: nd = dNd dt ≃ 250 s−1 , and e’s in UD: neU = dNeU dt ≃ 30 s−1 . (He-3 polarizers may give higher rates.)

Ee lower cutoff (keV) none 100 200 250 σA (symm., 2 det’s) 2.7/√Nd 2.9/√Nd 4.8/√Nd 7.4/√Nd σA (asymm., 1 det.) 4.3/√Nd 4.8/√Nd 7.8/√Nd 11.9/√Nd

To reach ∆A/A = 1 × 10−3 we need Nd = 1.7 × 109 or 75 live days.

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abBA/PANDA systematic uncertainties Experimental parameter (∆A/A)SYST Neutron Beam: position not relevant profile & edge effect small Doppler effect small polarization ≤ 1 × 10−3 U inhomogeneity: small Detector effects: Ee calibration 2 × 10−4 Trigger efficiency small Accidental coincidences small Residual gas small Background small Sum under study

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Key points about Nab

◮ Nab offers an alternative way to access λ = gA/gV with

competitive precision,

◮ makes full use of phase space information available, ◮ coincident measurement technique provides high level of

background suppression,

◮ not statistics–limited, ◮ polarized program (abBA/PANDA) is a natural and highly

competitive continuation,

◮ can run at both FnPB/SNS and NG-C/NIST. ◮ funded!

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The collaboration:

• R. Alarcon1, L.P. Alonzi2§, S. Baeßler2∗, S. Balascuta1§, J.D. Bowman3†,

M.A. Bychkov2, J. Byrne4, J.R. Calarco5, T.E. Chupp13, V. Cianciolo3,

• C. Crawford6, E. Frleˇ

z2, M.T. Gericke7, F. Gl¨ uck8, G.L. Greene3,9, R.K. Grzywacz9, V. Gudkov10, F.W. Hersman5, A. Klein11, M. Lehman2§,

• J. Martin12, S.A. Page6, A. Palladino2§, S.I. Penttil¨

a3‡, D. Poˇ cani´ c2†, K.P. Rykaczewski3, W.S. Wilburn11, A.R. Young14.

1Arizona State University 2University of Virginia 3Oak Ridge National Lab 4University of Sussex

• 5Univ. of New Hampshire

6University of Kentucky 7University of Manitoba

• 8Uni. Karlsruhe/RMKI Budapest

9University of Tennessee 10University of South Carolina 11Los Alamos National Lab 12University of Winnipeg

• 13Univ. of Michigan

14North Carolina State Univ. †Co-spokesmen ∗Experiment Manager ‡On-site Manager §Graduate Students

Home page: http://nab.phys.virginia.edu/

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