Search for R-parity violating Supersymmetric effects in the neutron beta decay
Precise Measurement of the Neutron Beta Decay Parameters a...
Transcript of Precise Measurement of the Neutron Beta Decay Parameters a...
Precise Measurement of the Neutron Beta DecayParameters a and b
Dinko Pocanic, for the Nab Collaboration
University of Virginia
DoE Review of the FnPB/SNS,Oak Ridge, 23 April 2009
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 1 / 24
Outline
Outline
Motivation and Goals
Measurement principlesProton TOF and e-ν correlationSpectrometer designDetection function
Overview of uncertaintiesEvent statistics, rates, running timeSystematic uncertainties
Asymmetric designSpectrometer basics
Summary
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 2 / 24
Basic facts
Goals of the Experiment
I 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
I Measure the Fierz interference term b in neutron decay
with accuracy of ∆b ' 3 × 10−3
current results: none
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 3 / 24
Basic facts
Goals of the Experiment
I 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
I Measure the Fierz interference term b in neutron decay
with accuracy of ∆b ' 3 × 10−3
current results: none
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 3 / 24
Basic facts
Goals of the Experiment
I 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
I Measure the Fierz interference term b in neutron decay
with accuracy of ∆b ' 3 × 10−3
current results: none
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 3 / 24
Basic facts
Goals of the Experiment
I 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
I Measure the Fierz interference term b in neutron decay
with accuracy of ∆b ' 3 × 10−3
current results: none
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 3 / 24
Basic facts
Neutron Decay Parameters (SM)
dw
dEedΩedΩν' keEe(E0 − Ee)
2
×[1 + a
~ke ·~kν
EeEν+ b
m
Ee+ 〈~σn〉 ·
(A
~ke
Ee+ B
~kν
Eν+ D
~ke ×~kν
EeEν
)]with:
a =1 − |λ|2
1 + 3|λ|2A = −2
|λ|2 + Re(λ)
1 + 3|λ|2
B = 2|λ|2 − Re(λ)
1 + 3|λ|2D = 2
Im(λ)
1 + 3|λ|2
λ =GA
GV(with τn ⇒ CKM Vud) (D 6= 0 ⇔ T inv. violation)
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 4 / 24
Basic facts
Neutron Decay Parameters (SM)
dw
dEedΩedΩν' keEe(E0 − Ee)
2
×[1 + a
~ke ·~kν
EeEν+ b
m
Ee+ 〈~σn〉 ·
(A
~ke
Ee+ B
~kν
Eν+ D
~ke ×~kν
EeEν
)]with:
a =1 − |λ|2
1 + 3|λ|2A = −2
|λ|2 + Re(λ)
1 + 3|λ|2
B = 2|λ|2 − Re(λ)
1 + 3|λ|2D = 2
Im(λ)
1 + 3|λ|2
λ =GA
GV(with τn ⇒ CKM Vud) (D 6= 0 ⇔ T inv. violation)
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 4 / 24
Basic facts
Neutron Decay Parameters (SM)
dw
dEedΩedΩν' keEe(E0 − Ee)
2
×[1 + a
~ke ·~kν
EeEν+ b
m
Ee+ 〈~σn〉 ·
(A
~ke
Ee+ B
~kν
Eν+ D
~ke ×~kν
EeEν
)]with:
a =1 − |λ|2
1 + 3|λ|2A = −2
|λ|2 + Re(λ)
1 + 3|λ|2
B = 2|λ|2 − Re(λ)
1 + 3|λ|2D = 2
Im(λ)
1 + 3|λ|2
λ =GA
GV(with τn ⇒ CKM Vud) (D 6= 0 ⇔ T inv. violation)
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 4 / 24
Basic facts
n-decay Correlation Parameters Beyond Vud
I Beta decay parameters constrain L-R symmetric, SUSY extensions tothe SM. [Reviews: Herczeg, Prog. Part. Nucl. Phys. 46, 413 (2001),N. Severijns, M. Beck, O. Naviliat-Cuncic, Rev. Mod. Phys. 78, 991 (2006),
Ramsey-Musolf, Su, Phys. Rep. 456, 1 (2008)]
I Fierz interference term, never measured for the neutron, offers asensitive test of non-(V − A) terms in the weak Lagrangian (S ,T ).[S. Profumo, M. J. Ramsey-Musolf, S. Tulin, PRD 75, 075017 (2007)]
I Measurement of the electron-energy dependence of a and A canseparately confirm CVC and absence of SCC.
[Gardner, Zhang, PRL 86, 5666 (2001), Gardner, hep-ph/0312124]
I A general connections exists between non-SM (e.g., S ,T ) terms ind → ueν and limits on ν masses. [Ito + Prezaeu, PRL 94 (2005)]
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 5 / 24
Measurement principles Proton TOF and e-ν correlation
Nab Measurement principles: Proton phase space
Ee (MeV)
p p2 (M
eV2 /c
2 )
cos θeν = -1
cos θeν = 1
cos θeν = 0
proton phase space
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Note: For a given Ee, cos θeν is a function of p2p only.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 6 / 24
Measurement principles Proton TOF and e-ν correlation
Measurement principles: Proton momentum response
pp2 (MeV2/c2)
Yie
ld (
arb.
uni
ts)
Ee = 0.075 MeV
0.236 MeV
0.450 MeV
0.700 MeV
0
0.1
0.2
0.3
0.4
0.5
0 0.2 0.4 0.6 0.8 1 1.2 1.4
Slope = a
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 7 / 24
Measurement principles Spectrometer design
Measurement principles: Symmetric pectrometer
SegmentedSi detector
NeutronBeam
DecayVolume
TOF regiontransitionregionacceleration
region
Elements of spectrometer to be shared with other planned n decayexperiments, e.g., abBA.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 8 / 24
Measurement principles Spectrometer design
Measurement principles: Spectrometer field profiles
z (m)
B0
B (T)
BTOF
U (104 V)
Nab Spectrometer Field Profiles
0
1
2
3
4
0 0.25 0.5 0.75 1 1.25 1.5 1.75 2
rB =BTOF
B0
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 9 / 24
Measurement principles Detection function
Measurement principles: Detection function (I)
Proton time of flight in B field:
tp =f (cos θp,0)
ppwhere cos θp,0 =
~pp0 · ~Bpp0B
∣∣∣∣∣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)
B0sin2 θp,0
.
To this we add effects of magnetic reflections and, later, of electric fieldacceleration.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 10 / 24
Measurement principles Detection function
Measurement principles: Detection function (II)
The proton momentum distribution within the phase space bounds is givenby
Pp(p2p) = 1 + aβe cos θeν , [recall: cos θeν = f (p2
p)]
while
Pt
(1
t2p
)=
∫Pp(p
2p) Φ
(1
t2p
, p2p
)dp2
p .
Detection function Φ relates the proton momentum and time-of-flightdistributions! To extract a reliably:
I Φ must be as narrow as possible,
I Φ must be understood very precisely.
Two methods (“A” and “B”) pursued to specify Φ.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 11 / 24
Measurement principles Detection function
Measurement principles: Detection function (II)
The proton momentum distribution within the phase space bounds is givenby
Pp(p2p) = 1 + aβe cos θeν , [recall: cos θeν = f (p2
p)]
while
Pt
(1
t2p
)=
∫Pp(p
2p) Φ
(1
t2p
, p2p
)dp2
p .
Detection function Φ relates the proton momentum and time-of-flightdistributions! To extract a reliably:
I Φ must be as narrow as possible,
I Φ must be understood very precisely.
Two methods (“A” and “B”) pursued to specify Φ.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 11 / 24
Measurement principles Detection function
Measurement principles: Detection function (II)
The proton momentum distribution within the phase space bounds is givenby
Pp(p2p) = 1 + aβe cos θeν , [recall: cos θeν = f (p2
p)]
while
Pt
(1
t2p
)=
∫Pp(p
2p) Φ
(1
t2p
, p2p
)dp2
p .
Detection function Φ relates the proton momentum and time-of-flightdistributions! To extract a reliably:
I Φ must be as narrow as possible,
I Φ must be understood very precisely.
Two methods (“A” and “B”) pursued to specify Φ.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 11 / 24
Measurement principles Detection function
Measurement principles: Detection function (II)
The proton momentum distribution within the phase space bounds is givenby
Pp(p2p) = 1 + aβe cos θeν , [recall: cos θeν = f (p2
p)]
while
Pt
(1
t2p
)=
∫Pp(p
2p) Φ
(1
t2p
, p2p
)dp2
p .
Detection function Φ relates the proton momentum and time-of-flightdistributions! To extract a reliably:
I Φ must be as narrow as possible,
I Φ must be understood very precisely.
Two methods (“A” and “B”) pursued to specify Φ.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 11 / 24
Measurement principles Detection function
Measurement principles: Detection function (II)
The proton momentum distribution within the phase space bounds is givenby
Pp(p2p) = 1 + aβe cos θeν , [recall: cos θeν = f (p2
p)]
while
Pt
(1
t2p
)=
∫Pp(p
2p) Φ
(1
t2p
, p2p
)dp2
p .
Detection function Φ relates the proton momentum and time-of-flightdistributions! To extract a reliably:
I Φ must be as narrow as possible,
I Φ must be understood very precisely.
Two methods (“A” and “B”) pursued to specify Φ.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 11 / 24
Measurement principles Detection function
Measurement principles: Detection function (II)
The proton momentum distribution within the phase space bounds is givenby
Pp(p2p) = 1 + aβe cos θeν , [recall: cos θeν = f (p2
p)]
while
Pt
(1
t2p
)=
∫Pp(p
2p) Φ
(1
t2p
, p2p
)dp2
p .
Detection function Φ relates the proton momentum and time-of-flightdistributions! To extract a reliably:
I Φ must be as narrow as possible,
I Φ must be understood very precisely.
Two methods (“A” and “B”) pursued to specify Φ.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 11 / 24
Measurement principles Detection function
Measurement principles: Detection function (II)
The proton momentum distribution within the phase space bounds is givenby
Pp(p2p) = 1 + aβe cos θeν , [recall: cos θeν = f (p2
p)]
while
Pt
(1
t2p
)=
∫Pp(p
2p) Φ
(1
t2p
, p2p
)dp2
p .
Detection function Φ relates the proton momentum and time-of-flightdistributions! To extract a reliably:
I Φ must be as narrow as possible,
I Φ must be understood very precisely.
Two methods (“A” and “B”) pursued to specify Φ.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 11 / 24
Measurement principles Detection function
Measurement principles: Detection function (III)
0.0 0.5 1.0 1.5
pp
2[MeV
2/c
2]
Pp(p
p
2)
Dis
trib
uti
on
Pp(pp
2) ∝ 1+aβ(Ee)cosθeν
2pepνcosθeν = pp
2-pe
2-pν
2
0.00 0.02 0.04 0.06 0.08
1/tp
2[1/µs
2]
10-5
10-4
10-3
10-2
10-1
100
Sp
ectr
om
eter
resp
on
sefu
nct
ion
Φ(⋅
,p
p
2)
pp
2= 0.5 MeV
2/c
2
pp
2= 0.9 MeV
2/c
2
pp
2= 1.3 MeV
2/c
2
Ee = 550 keV
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 12 / 24
Measurement principles Detection function
Measurement principles: Detection function (IV)
0.00 0.02 0.04 0.06 0.08
1/tp
2[1/µs
2]
103
104
105
106
107
Sim
ula
ted
cou
nt
rate
Ee = 300 keV
Ee = 500 keV
Ee = 700 keV
]2sµ [2p1/t
0 0.01 0.02 0.03 0.04 0.05 0.06
Sim
ulat
ed c
ount
rate
1
10
= 300 keVeE
= 500 keVeE
= 700 keVeE
Theoretical calculation Realistic Monte Carlo simulation(method “B”) (1M decays, GEANT4)
Note: 1. central, straight portion sensitive to physics (a),2. edges sensitive to detection function and calibration.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 13 / 24
Measurement principles Detection function
Measurement principles: Detection function (IV)
0.00 0.02 0.04 0.06 0.08
1/tp
2[1/µs
2]
103
104
105
106
107
Sim
ula
ted
cou
nt
rate
Ee = 300 keV
Ee = 500 keV
Ee = 700 keV
]2sµ [2p1/t
0 0.01 0.02 0.03 0.04 0.05 0.06
Sim
ulat
ed c
ount
rate
1
10
= 300 keVeE
= 500 keVeE
= 700 keVeE
Theoretical calculation Realistic Monte Carlo simulation(method “B”) (1M decays, GEANT4)
Note: 1. central, straight portion sensitive to physics (a),2. edges sensitive to detection function and calibration.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 13 / 24
Measurement principles Detection function
Optimized symmetric spectrometer
+
-
-
z
r
dimensions in cm
2.0
6.2
2.011.0
Current density: 3500 A/cm2
+
20
100
0.0036
1.3
11.0 3.0
3.4
3.9
7.0
8.4
2.4
1.0
1.3
The “a-147-beta” Configuration
]-2sµ [-2TOF0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05
-510
-410
-310
-210
-110 0.0002± = 0.0646 µ
RMS
0.008± = 0.381 µ
-410-Qµ
= 0.949 MeV/cpP
0.0002± = 0.0671 µRMS
0.006± = 0.388 µ
-410-Qµ
= 1.14 MeV/cpP
βa147
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 14 / 24
Overview of uncertainties Event statistics, rates, running time
Statistical uncertainties for a and b
Statistical uncertainties for a
Ee,min 0 100 keV 100 keV 300 keVtp,max – – 10 µs 10 µs
σa 2.4/√
N 2.5/√
N 2.6/√
N 3.5/√
N
σa† 2.5/
√N 2.6/
√N – –
† with Ecal and l variable.
Statistical uncertainties for b
Ee,min 0 100 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 Ecal variable.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 15 / 24
Overview of uncertainties Event statistics, rates, running time
Statistical uncertainties for a and b
Statistical uncertainties for a
Ee,min 0 100 keV 100 keV 300 keVtp,max – – 10 µs 10 µs
σa 2.4/√
N 2.5/√
N 2.6/√
N 3.5/√
N
σa† 2.5/
√N 2.6/
√N – –
† with Ecal and l variable.
Statistical uncertainties for b
Ee,min 0 100 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 Ecal variable.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 15 / 24
Overview of uncertainties Event statistics, rates, running time
Statistical uncertainties for a and b
Statistical uncertainties for a
Ee,min 0 100 keV 100 keV 300 keVtp,max – – 10 µs 10 µs
σa 2.4/√
N 2.5/√
N 2.6/√
N 3.5/√
N
σa† 2.5/
√N 2.6/
√N – –
† with Ecal and l variable.
Statistical uncertainties for b
Ee,min 0 100 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 Ecal variable.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 15 / 24
Overview of uncertainties Event statistics, rates, running time
Event rates, statistics and running times
FnPB n decay rate w/nominal 1.4 MW SNS operation: rn ' 19.5/(cm3s) .
Nab fiducial volume is: Vf ' π2 2.42 × 2cm3 ' 18 cm3 .
This gives a rate of about 350 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 several samples of 109 events in several 6-week runs.
Consequently, overall accuracy will not be statistics-limited.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 16 / 24
Overview of uncertainties Event statistics, rates, running time
Event rates, statistics and running times
FnPB n decay rate w/nominal 1.4 MW SNS operation: rn ' 19.5/(cm3s) .
Nab fiducial volume is: Vf ' π2 2.42 × 2cm3 ' 18 cm3 .
This gives a rate of about 350 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 several samples of 109 events in several 6-week runs.
Consequently, overall accuracy will not be statistics-limited.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 16 / 24
Overview of uncertainties Event statistics, rates, running time
Event rates, statistics and running times
FnPB n decay rate w/nominal 1.4 MW SNS operation: rn ' 19.5/(cm3s) .
Nab fiducial volume is: Vf ' π2 2.42 × 2cm3 ' 18 cm3 .
This gives a rate of about 350 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 several samples of 109 events in several 6-week runs.
Consequently, overall accuracy will not be statistics-limited.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 16 / 24
Overview of uncertainties Systematic uncertainties
Systematic uncertainties and checks
I Uncertainties due to spectrometer response
Neutron beam profile: 100 µm shift of beam center induces∆a/a ∼ 0.2 %; cancels when averaging over detectors;measurement of asymmetry pins it down sufficiently;
Magnetic field map:field expansion ratio rB = BTOF/B0;∆a/a ∼ 10−3 ⇒ ∆rB/rB = 10−3, (use calibrated Hall probe);field curvature α, (via proton asymmetry measurement);field bumps ∆B/B must be kept below 2× 10−3 level;
Flight path length: ∆l ≤ 30 µm ⇒ fitting parameter;(∃ consistency check);
Homogeneity of the electric field;
Rest gas: requires vacuum of 10−9 torr or better;
Doppler effect;
Adiabaticity;
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 17 / 24
Overview of uncertainties Systematic uncertainties
Systematic uncertainties and checks (II)
I Uncertainties due to the detector
Detector alignment; Electron energy calibration: requirement 10−4; we’ll use radioactive
sources, other strategies, also as fitting parameter; Trigger hermiticity: affected by impact angle, backscattering, TOF
cutoff (to reduce accid. bgd.); TOF uncertainties; Edge effects;
I Backgrounds
Neutron beam related background; Particle trapping;
I Uncertainties in b: fewer than for a (no proton detection); dominantare energy calibration and electron backgrounds.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 18 / 24
Asymmetric design Spectrometer basics
Asymmetric spectrometer
Four serious challenges can be relieved in an asymmetric spectrometer:
I Achieving a long flight path for protons and, hence, high tp (TOF)resolution,
I Achieving a high degree of proton momentum linearization, and,hence, accuracy of the pp–tp relationship (narrow detection function),
I Greatly reducing the sensitivity to particle trapping in small fieldimperfections in the neutron decay region, and
I Reducing the influence of small nonuniformities in electric potentialfrom ∼ µV level to a more controllable ∼mV level.
Key strategy:
I Move the high-field pinch away from the neutron decay region,
I Have one main, long TOF spectrometer side.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 19 / 24
Asymmetric design Spectrometer basics
Asymmetric spectrometer
Four serious challenges can be relieved in an asymmetric spectrometer:
I Achieving a long flight path for protons and, hence, high tp (TOF)resolution,
I Achieving a high degree of proton momentum linearization, and,hence, accuracy of the pp–tp relationship (narrow detection function),
I Greatly reducing the sensitivity to particle trapping in small fieldimperfections in the neutron decay region, and
I Reducing the influence of small nonuniformities in electric potentialfrom ∼ µV level to a more controllable ∼mV level.
Key strategy:
I Move the high-field pinch away from the neutron decay region,
I Have one main, long TOF spectrometer side.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 19 / 24
Asymmetric design Spectrometer basics
Asymmetric spectrometer
Four serious challenges can be relieved in an asymmetric spectrometer:
I Achieving a long flight path for protons and, hence, high tp (TOF)resolution,
I Achieving a high degree of proton momentum linearization, and,hence, accuracy of the pp–tp relationship (narrow detection function),
I Greatly reducing the sensitivity to particle trapping in small fieldimperfections in the neutron decay region, and
I Reducing the influence of small nonuniformities in electric potentialfrom ∼ µV level to a more controllable ∼mV level.
Key strategy:
I Move the high-field pinch away from the neutron decay region,
I Have one main, long TOF spectrometer side.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 19 / 24
Asymmetric design Spectrometer basics
Asymmetric spectrometer
Four serious challenges can be relieved in an asymmetric spectrometer:
I Achieving a long flight path for protons and, hence, high tp (TOF)resolution,
I Achieving a high degree of proton momentum linearization, and,hence, accuracy of the pp–tp relationship (narrow detection function),
I Greatly reducing the sensitivity to particle trapping in small fieldimperfections in the neutron decay region, and
I Reducing the influence of small nonuniformities in electric potentialfrom ∼ µV level to a more controllable ∼mV level.
Key strategy:
I Move the high-field pinch away from the neutron decay region,
I Have one main, long TOF spectrometer side.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 19 / 24
Asymmetric design Spectrometer basics
Asymmetric spectrometer
Four serious challenges can be relieved in an asymmetric spectrometer:
I Achieving a long flight path for protons and, hence, high tp (TOF)resolution,
I Achieving a high degree of proton momentum linearization, and,hence, accuracy of the pp–tp relationship (narrow detection function),
I Greatly reducing the sensitivity to particle trapping in small fieldimperfections in the neutron decay region, and
I Reducing the influence of small nonuniformities in electric potentialfrom ∼ µV level to a more controllable ∼mV level.
Key strategy:
I Move the high-field pinch away from the neutron decay region,
I Have one main, long TOF spectrometer side.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 19 / 24
Asymmetric design Spectrometer basics
Basic design and features of asymmetric Nab
Segmented
Si detector
decay volume(field rB,DV·B0)
30 kV
0-30 kV
0-31 kV
magnetic filterregion (field B0)
Neutron
beam
TOF region
(field rB·B0)
Stefan Baeßler, March 2009
Features:
I long TOF above n beam,
I displaced magnetic cos θ filter,
I no count rate penalty viz.symmetric Nab.
Height z [m]
0
1
2
3
4
Mag
net
icfi
eld
B[T
]
z1 z2 z3-z2-z1
filter region
decay volume
detector
detector
expansion region
B B z(z)= (1-( ) )0 α2
linear field decay
B r B(z) = B 0
B r B(z) ~
(small gradient,
~10 /cm)
B,DV 0
-3
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 20 / 24
Asymmetric design Spectrometer basics
Asymmetric Nab: expected performance
Simulated detection function and 1/tp distribution:
0.000 0.005 0.010 0.015 0.020 0.025
1/tp
2[1/ms
2]
0
1
2
3
4
5
6x106
det
ecti
onfu
nct
ion
F(1
/tp2
,pp2
)
0.000 0.005 0.010 0.015 0.020 0.025
1/tp2
[1/ms2]
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5x1013
Yie
ld[A
.U.]
Ee = 400 keV
afit
= -0.1041
ideal de tection function
ave/naive = 0.9158
RMS/ave = 0.0653
(using rB = 0.05 and rB,DV = 0.5)
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 21 / 24
Summary
SUMMARY
Nab plans a simultaneous high-statistics measurement of neutron decay
parameters a and b with ∆a/a ' 10−3 and ∆b ' 3 × 10−3 .
I Basic properties of the symmetric Nab spectrometer are wellunderstood and highly optimized.
I The new asymmetric Nab idea looks very promising; details are underextensive analytical and Monte Carlo study.
I Elements of spectrometer may be shared with other neutron decayexperiments, e.g., abBA.
I Development of abBA/Nab Si detectors is ongoing and remains atechnological challenge.
I Experiment received approval in Feb. 2008; could be ready forcommissioning sometime in 2011.
I Crude budget estimate ∼ $2.5M.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 22 / 24
Summary
SUMMARY
Nab plans a simultaneous high-statistics measurement of neutron decay
parameters a and b with ∆a/a ' 10−3 and ∆b ' 3 × 10−3 .
I Basic properties of the symmetric Nab spectrometer are wellunderstood and highly optimized.
I The new asymmetric Nab idea looks very promising; details are underextensive analytical and Monte Carlo study.
I Elements of spectrometer may be shared with other neutron decayexperiments, e.g., abBA.
I Development of abBA/Nab Si detectors is ongoing and remains atechnological challenge.
I Experiment received approval in Feb. 2008; could be ready forcommissioning sometime in 2011.
I Crude budget estimate ∼ $2.5M.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 22 / 24
Summary
SUMMARY
Nab plans a simultaneous high-statistics measurement of neutron decay
parameters a and b with ∆a/a ' 10−3 and ∆b ' 3 × 10−3 .
I Basic properties of the symmetric Nab spectrometer are wellunderstood and highly optimized.
I The new asymmetric Nab idea looks very promising; details are underextensive analytical and Monte Carlo study.
I Elements of spectrometer may be shared with other neutron decayexperiments, e.g., abBA.
I Development of abBA/Nab Si detectors is ongoing and remains atechnological challenge.
I Experiment received approval in Feb. 2008; could be ready forcommissioning sometime in 2011.
I Crude budget estimate ∼ $2.5M.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 22 / 24
Summary
SUMMARY
Nab plans a simultaneous high-statistics measurement of neutron decay
parameters a and b with ∆a/a ' 10−3 and ∆b ' 3 × 10−3 .
I Basic properties of the symmetric Nab spectrometer are wellunderstood and highly optimized.
I The new asymmetric Nab idea looks very promising; details are underextensive analytical and Monte Carlo study.
I Elements of spectrometer may be shared with other neutron decayexperiments, e.g., abBA.
I Development of abBA/Nab Si detectors is ongoing and remains atechnological challenge.
I Experiment received approval in Feb. 2008; could be ready forcommissioning sometime in 2011.
I Crude budget estimate ∼ $2.5M.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 22 / 24
Summary
SUMMARY
Nab plans a simultaneous high-statistics measurement of neutron decay
parameters a and b with ∆a/a ' 10−3 and ∆b ' 3 × 10−3 .
I Basic properties of the symmetric Nab spectrometer are wellunderstood and highly optimized.
I The new asymmetric Nab idea looks very promising; details are underextensive analytical and Monte Carlo study.
I Elements of spectrometer may be shared with other neutron decayexperiments, e.g., abBA.
I Development of abBA/Nab Si detectors is ongoing and remains atechnological challenge.
I Experiment received approval in Feb. 2008; could be ready forcommissioning sometime in 2011.
I Crude budget estimate ∼ $2.5M.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 22 / 24
Summary
SUMMARY
Nab plans a simultaneous high-statistics measurement of neutron decay
parameters a and b with ∆a/a ' 10−3 and ∆b ' 3 × 10−3 .
I Basic properties of the symmetric Nab spectrometer are wellunderstood and highly optimized.
I The new asymmetric Nab idea looks very promising; details are underextensive analytical and Monte Carlo study.
I Elements of spectrometer may be shared with other neutron decayexperiments, e.g., abBA.
I Development of abBA/Nab Si detectors is ongoing and remains atechnological challenge.
I Experiment received approval in Feb. 2008; could be ready forcommissioning sometime in 2011.
I Crude budget estimate ∼ $2.5M.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 22 / 24
Summary
SUMMARY
Nab plans a simultaneous high-statistics measurement of neutron decay
parameters a and b with ∆a/a ' 10−3 and ∆b ' 3 × 10−3 .
I Basic properties of the symmetric Nab spectrometer are wellunderstood and highly optimized.
I The new asymmetric Nab idea looks very promising; details are underextensive analytical and Monte Carlo study.
I Elements of spectrometer may be shared with other neutron decayexperiments, e.g., abBA.
I Development of abBA/Nab Si detectors is ongoing and remains atechnological challenge.
I Experiment received approval in Feb. 2008; could be ready forcommissioning sometime in 2011.
I Crude budget estimate ∼ $2.5M.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 22 / 24
Summary Comparison w/other experiments
Current experiments aiming to measure a
1. Nab: goal is to measure ∆a/a ∼ 10−3
I Best statistical sensitivity,I Challenging but manageable systematics, esp. in asymm. design.
2. abBA: goal is to measure ∆a/a ∼ 10−3
I Similar to Nab, but with a spectrometer optimized for A,B,I Detection function is very broad, syst. uncert. for a very demanding.
3. aCORN: goal is to measure ∆a/a ∼ 0.5 − 2 %I Funded, under construction,I Uses only part of neutron decays.
4. aSPECT: aims to measure ∆a/a ∼ 10−3
I Funded and running; recently overcame trapping problems,I Stat. sensitivity not as good as Nab due to integration; presently
∼ 2 %/day—will likely improve on publ. results, not < 1 % this run,I Easier determination of detection function than in Nab at the present
level of accuracy.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 23 / 24
Summary Comparison w/other experiments
Current experiments aiming to measure a
1. Nab: goal is to measure ∆a/a ∼ 10−3
I Best statistical sensitivity,I Challenging but manageable systematics, esp. in asymm. design.
2. abBA: goal is to measure ∆a/a ∼ 10−3
I Similar to Nab, but with a spectrometer optimized for A,B,I Detection function is very broad, syst. uncert. for a very demanding.
3. aCORN: goal is to measure ∆a/a ∼ 0.5 − 2 %I Funded, under construction,I Uses only part of neutron decays.
4. aSPECT: aims to measure ∆a/a ∼ 10−3
I Funded and running; recently overcame trapping problems,I Stat. sensitivity not as good as Nab due to integration; presently
∼ 2 %/day—will likely improve on publ. results, not < 1 % this run,I Easier determination of detection function than in Nab at the present
level of accuracy.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 23 / 24
Summary Comparison w/other experiments
Current experiments aiming to measure a
1. Nab: goal is to measure ∆a/a ∼ 10−3
I Best statistical sensitivity,I Challenging but manageable systematics, esp. in asymm. design.
2. abBA: goal is to measure ∆a/a ∼ 10−3
I Similar to Nab, but with a spectrometer optimized for A,B,I Detection function is very broad, syst. uncert. for a very demanding.
3. aCORN: goal is to measure ∆a/a ∼ 0.5 − 2 %I Funded, under construction,I Uses only part of neutron decays.
4. aSPECT: aims to measure ∆a/a ∼ 10−3
I Funded and running; recently overcame trapping problems,I Stat. sensitivity not as good as Nab due to integration; presently
∼ 2 %/day—will likely improve on publ. results, not < 1 % this run,I Easier determination of detection function than in Nab at the present
level of accuracy.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 23 / 24
Summary Comparison w/other experiments
Current experiments aiming to measure a
1. Nab: goal is to measure ∆a/a ∼ 10−3
I Best statistical sensitivity,I Challenging but manageable systematics, esp. in asymm. design.
2. abBA: goal is to measure ∆a/a ∼ 10−3
I Similar to Nab, but with a spectrometer optimized for A,B,I Detection function is very broad, syst. uncert. for a very demanding.
3. aCORN: goal is to measure ∆a/a ∼ 0.5 − 2 %I Funded, under construction,I Uses only part of neutron decays.
4. aSPECT: aims to measure ∆a/a ∼ 10−3
I Funded and running; recently overcame trapping problems,I Stat. sensitivity not as good as Nab due to integration; presently
∼ 2 %/day—will likely improve on publ. results, not < 1 % this run,I Easier determination of detection function than in Nab at the present
level of accuracy.
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 23 / 24
Summary Comparison w/other experiments
The Nab collaboration
R. Alarcon1, L.P. Alonzi2, S. Baeßler2∗, S. Balascuta1, J.D. Bowman3†,M.A. Bychkov2, J. Byrne4, J.R. Calarco5, V. Cianciolo3, C. Crawford6,E. Frlez2, M.T. Gericke7, F. Gluck8, G.L. Greene9, R.K. Grzywacz9,V. Gudkov10, F.W. Hersman5, A. Klein11, J. Martin12, S.A. Page6,
A. Palladino2, S.I. Penttila3, D. Pocanic2†, K.P. Rykaczewski3,W.S. Wilburn11, A.R. Young13, G.R. Young3.
1Arizona State University 2University of Virginia3Oak Ridge National Lab 4University of Sussex5Univ. of New Hampshire 6University of Kentucky7University of Manitoba 8Uni. Karlsruhe/RMKI Budapest9University of Tennessee 10University of South Carolina11Los Alamos National Lab 12University of Winnipeg13North Carlolina State Univ.∗Experiment Manager †Co-spokesmen
Home page: http://nab.phys.virginia.edu/
D. Pocanic (UVa) The Nab Experiment/FnPB/SNS 23 Apr ’09 24 / 24