April 2016, ScientiﬁcAmerican.com 41
examples of a weak force interaction. To calculate the details of
other, more complex nuclear processes involving the weak force,
we must ﬁrst fully understand how it operates in neutron decay.
Discerning the exact rate of neutron decay would also help
test the big bang theory for the early evolution of the cosmos.
According to the theory, when the universe was about one second
old, it consisted of a hot, dense mixture of particles: protons, neu-
trons, electrons, and others. At this time, the temperature of the
universe was roughly 10 billion degrees—so hot that these parti-
cles were too energetic to bind together into nuclei or atoms.
After about three minutes, the universe expanded and cooled to a
temperature where protons and neutrons could stick together to
make the simplest atomic nucleus, deuterium (the heavy isotope
of hydrogen). From here other simple nuclei were able to form—
deuterium could capture a proton to make an isotope of helium,
two deuterium nuclei could join together to create heavier heli-
um, and small numbers of larger nuclei formed, up to the ele-
ment lithium (all the heavier elements are thought to have been
produced in stars many millions of years later).
This process is known as big bang nucleosynthesis. If, while
the universe was losing heat, neutrons had decayed at a rate that
was much faster than the universe cooled, there would have been
no neutrons left when the universe reached the right tempera-
ture to form nuclei—only the protons would have remained, and
we would have a cosmos made almost entirely of hydrogen. On
the other hand, if the neutron lifetime were much longer than the
time required to cool suciently for big bang nucleosynthesis,
the universe would have an overabundance of helium, which in
turn would have aected the formation of the heavier elements
involved in the evolution of stars and ultimately life. Thus, the
balance between the universal cooling rate and the neutron life-
time was quite critical for the creation of the elements that make
up our planet and everything on it.
From astronomical data we can measure the cosmic ratio of
helium to hydrogen, as well as the amounts of deuterium and other
light elements that exist throughout the universe. We would like to
see if these measurements agree with the numbers predicted by big
bang theory. The theoretical prediction, however, depends on the
precise value of the neutron lifetime. Without a reliable value for it,
our ability to make this comparison is limited. Once the neutron
lifetime is known more precisely, we can compare the observed
ratio from astrophysical experiments with the predicted value
from theory. If they agree, we gain further conﬁdence in our stan-
dard big bang scenario for how the universe evolved. Of course, if
they disagree, this model might have to be altered. For instance,
certain discrepancies might indicate the existence of new exotic
particles in the universe such as an extra type of neutrino, which
could have interfered in the process of nucleosynthesis.
One way to resolve the dierence between the beam and bot-
tle results is to conduct more experiments using methods of com-
parable accuracy that are not prone to the same, potentially con-
founding systematic errors. In addition to continuing the beam
and bottle projects, scientists in several other groups worldwide
are working on alternative methods of measuring the neutron
lifetime. A group at the Japan Proton Accelerator Research Com-
plex (J-PARC) in Tokai is developing a new beam experiment that
will detect the electrons rather than protons produced when neu-
trons decay. In another very exciting development, groups at ILL,
the Petersburg Nuclear Physics Institute in Russia, Los Alamos
National Laboratory, the Technical University of Munich and the
Johannes Gutenberg University Mainz in Germany plan to use
neutron bottles that conﬁne ultracold neutrons with magnetic
ﬁelds rather than material walls. This is possible because the neu-
tron, though electrically neutral, behaves as though it is a small
magnet. The number of neutrons accidentally lost through the
sides of such bottles should be quite dierent from that of previ-
ous measurements and thus should produce quite dierent sys-
tematic uncertainties. We fervently hope that, together, continu-
ing bottle and beam experiments and this next generation of
measurements will ﬁnally solve the neutron lifetime puzzle.
Count the number of decays within the time interval
+ – +
The Beam Method
In ontrast to the ottle ethod, the ea tehniue looks not for neutrons
ut for one of their deay produts, protons. ientists diret a strea
of neutrons through an eletroagneti trap ade of a agneti eld
and ring-shaped high-oltage eletrodes. he neutral neutrons pass right
through, ut if one deays inside the trap, the resulting positiely harged
protons ill get stuk. he researhers kno ho any neutrons ere in
the ea, and they kno ho long they spent passing through the trap,
so by counting the protons in the trap they can measure the number of
neutrons that deayed in that span of tie. his easureent is the deay
rate, hih is the slope of the deay ure at a gien point in tie and
hih allos the sientists to alulate the aerage neutron lifetie.
MORE TO EXPLORE
easureent of the eutron ifetie sing a raitational rap and a o-
eperature olin oating. A. Serebrov et al. in Physics Letters B, Vol. 605,
Nos. 1–2, pages 72–78; January 6, 2005.
he eutron ifetie. FredE.WietfeldtandGeoreyL.GreeneinReviews of Modern
Physics, Vol. 83, No. 4, Article No. 1173; October–December 2011.
Iproed eterination of the eutron ifetie. A. T. Yue et al. in Physical Review
Letters, Vol. 111, No. 22, Article No. 222501; November 27, 2013.
FROM OUR ARCHIVES
ltraold eutrons. R. Golub, W. Mampe, J. M. Pendlebury and P. Ageron; June 1979.
he roton adius role. Jan C. Bernauer and Randolf Pohl; February 2014.
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