This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Dewey, M.S.

Eykens, R.

Gilliam, D.M.

Greene, G.L.

Keith, C.D.

Lamberty, A

Pauwels, J.

Scott, R.D.

Snow, W.M.

English

IUScholarWorks

MEASUREMENT OF THE NEUTRON LIFETIME BY 
COUNTING TRAPPED PROTONS 
M.S. Dewey, D.M. Gilliam, and G.L. Greene 
Nat ional  Ins t i tu te  of Standards a,nd Technology, Gaithersburg, Maryland ,20899 
C.D. Keith and W.M. Snow 
Indiana University, B l o o r n i n g t ~ n ~  Indiana 4 74 0 8  
J. Pauwels, A. Lamberty, and R. Eykens 
Central  Bureau for Nuclear Measurements ,  B-,2440 Geel, Be lg ium 
R.D. Scott 
Scott ish Universities Research and Reactor Center ,  Eas t  Kilbride, 
Glasgow G 7 5  OQ U, United K i n g d o m  
The decay rate of the neutron is an important parameter in low energy weak inter- 
actions. In combination with other measurements of neutron decay parameters, such as 
the electron asymmetry coefficient in polarized neutron decay, it determines the weak po- 
lar vector-axial vector coupling ratio X = a. By comparison with muon decay, the KM 
Sv 
matrix element Vud, one of the parameters of the Standard Model of elementary particle 
interactions, can be determined. It is important to note that present measurements in the 
neutron sector are marginally inconsistent with the unitarity of the Kobayashi-Maskawa 
(KM) the value of Vud inferred from one of the two A measurements of com- 
parable accuracy causes a violation of the unitarity of the KM matrix by 30. Further 
measurements are required to resolve this discrepancy and to search for indications of 
physics beyond the Standard Model, such as the possible existence of right-handed weak 
currents . 
In addition, the neutron decay rate influences theoretical predictions relating to Big 
Bang Nucleosynthesis (BBN) of the primordial helium abundance in the universe and the 
number of different types of light neutrinos Nu (and therefore the number of generations) in 
the Standard Model. The most recent comparison of BBN helium abundance calculations 
to observation constrains Nu 5 3 .04 ,~  consistent with and complementary to the value 
derived from Zo decay, N ,  = 2.985 f 0 . 0 2  Q 0 . 1  .5 However, new measurements6 
of the primordial abundance ratio of deuterium to hydrogen may indirectly change the 
BBN prediction for the helium abundance by modifying the "best" value for the photon to 
baryon ratio 7. For a given value of 7, about 90% of the uncertainty in the BBN prediction 
for the primordial helium abundance comes from the accuracy of the neutron decay rate.7 
Improved neutron decay rate measurements are therefore important for sharpening the 
BBN prediction. 
An experiment is in progress at the NIST Cold Neutron Research Facility to measure 
the decay rate of the neutron? The measurement is performed by confining the protons 
from in-beam neutron decays in a Penning trap and counting the trapped protons while 
simultaneously monitoring the neutron flux passing through the trap. The goal of this 
collaboration is to perform a measurement of the neutron lifetime with an accuracy at 
the 0.1% level. A previous version of the experiment reached an accuracy of 0.5%~' The 
value for the neutron lifetime recommended by the Particle Data Group is 7, = 889.1 f 2.1 
seconds. lo 
The experiment has been running for several months at the NIST Cold Neutron Re- 
search Facility. Approximately eight million decay events have been recorded to date. 
Additional running is planned in late 1995. Data analysis is in progress. Work on the 
absolute calibration of the neutron flux monitor will continue both during and after the 
decay data has been taken. 
1. A.S. Carnoy, et al., Phys. Rev. Lett. 65, 3249 (1990). 
2. J.C. Hardy, et al., Nucl. Phys. A509, 429 (1990). 
3. I.S. Towner, Nucl. Phys. A540, 478 (1992). 
4. P.J. Kernan and L.M. Krauss, Phys. Rev. Lett. 72, 3309 (1994). 
5. ALEPH collaboration, Phys. Lett. B321, 168 (1994); DELPHI collaboration, Phys. 
Lett. B276, 536 (1992); L3 collaboration, 2. Phys. C51,179 (1991); OPAL collabo- 
ration, 2. Phys. C61, 19 (1994). 
6. A. Songaila, et al., Nature 368, 599 (1994). 
7. M.S. Smith, L.H. Kawano, and R.A. Malaney, Astrophys. J. Suppl. 85, 219 (1993). 
8. M.S. Dewey, et al., Proceedings of the Third International Symposium on Weak and 
Electromagnetic Interactions in Nuclei, ed. T.D. Vylov (World Scientific, New York, 
1992), p. 44. 
9. J. Byrne, et al., Phys. Rev. Lett. 65, 289 (1989). 
10. Particle Data Group, Phys. Rev. D 45, S1 (1992). 


Measurement of the Neutron Lifetime by Counting Trapped Protons

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Measurement of the Neutron Lifetime by Counting Trapped Protons

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