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The average neutron lifetime value $\tau_n$ is an open problem in p...
[Weak force](https://en.wikipedia.org/wiki/Weak_interaction) is one...
A free neutron decay reaction can be written as: $$n^0 \rightar... You can learn more about the experimental setup used to trap the co... Beta decay is a type of radioactive decay that is well understood b... In order to reduce systematic errors, scientists use two different ... One of the techniques that scientists use to measure the neutron li... Interestingly different bottle experiments give similar results and... The other method that physicists use to measure the average neutron... Having a precise idea of the average neutron lifetime \tau_n will... 38 Scientiﬁc American, April 2016 L UCKILY FOR LIFE ON EARTH, MOST MATTER IS NOT RADIOACTIVE. WE TAKE THIS FACT FOR granted, but it is actually somewhat surprising because the neutron, one of the two components of atomic nuclei (along with the proton), is prone to radioac- tive decay. Inside an atomic nucleus, a typical neutron can survive for a very long time and may never decay, but on its own, it will transform into other par- ticles within 15minutes, more or less. The words “more or less” cover a disturb- ing gap in physicists’ understanding of this particle. Try as we might, we have not been able to accurately measure the neutron lifetime. This “neutron lifetime puzzle” is not just embarrassing for us experimentalists; resolving it is vital for understanding the na- ture of the universe. The neutron decay process is one of the sim- plest examples of the nuclear “weakinteraction—one of natures four fundamental forces. To truly understand the weak force, we must know how long neutrons live. Furthermore, the survival time of the neutron determined how the lightest chemical ele- ments ﬁrst formed after the big bang. Cosmologists would like to calculate the expected abundances of the elements and compare them with astrophysical measurements: agreement would con- ﬁrm our theoretical picture, and discrepancy could indicate that undiscovered phenomena aected the process. To make such a comparison, however, we need to know the neutron lifetime. More than 10 years ago two experimental groups, one a Rus- sian-led team in France and the other a team in the U.S., attempt- ed separately to precisely measure the lifetime. One of us (Gelten- bort) was a member of the ﬁrst team, and the other (Greene) was a member of the second. Along with our colleagues, we were sur- prised and somewhat disturbed to ﬁnd that our results disagreed considerably. Some theoreticians suggested that the dierence arose from exotic physics—that some neutrons in the experi- ments might have transformed into particles never before detect- ed, which would have aected the dierent experiments in diver- gent ways. We, however, suspected a more mundane reason—per- haps one of our groups, or even both, had simply made a mistake or, more likely, had overestimated the accuracy of its experiment. The U.S. team recently completed a long, painstaking project to study the most dominant source of uncertainty in its experiment in hopes of resolving the discrepancy. Rather than clearing up the situation, that eort conﬁrmed our earlier result. Similarly, other re searchers later conﬁrmed the ﬁndings of Geltenbort’s team. This discrepancy has left us even more perplexed. But we are not giving up—both groups and others continue to seek answers. TIMING NEUTRONS IN THEOR Y, measuring the neutron lifetime should be straightfor- ward. The physics of nuclear decay are well understood, and we have sophisticated techniques for studying the process. We know, for instance, that if a particle has the possibility of transforming into a lower-mass particle or particles while conserving such char- acteristics as charge and spin angular momentum, it will. Free neutrons display this instability. In a process called beta decay, a neutron breaks up into a proton, an electron and an antineutrino (the antimatter counterpart of the neutrino), which collectively sum to a slightly lower mass but the same total charge, spin angu- lar momentum and other conserved properties. These conserved properties include “mass-energy,” meaning that the daughter particles carry the dierence in mass in the form of kinetic ener- gy, the energy of motion. We cannot predict exactly when a particular neutron will de - cay because the process is a fundamentally random quantum phe- nomenon—we can say only how long neutrons live on average. Thus, we must measure the average neutron lifetime by studying the decay of many neutrons. Investigators have employed two experimental methods—one called the bottle” technique and the other the “beam” ap proach. Bottle experiments conﬁne neutrons in a container and count how many are left after a given time. The beam method, in con- trast, looks not for the disappearance of neutrons but rather for the appearance of the particles into which they decay. The bottle approach is particularly challenging because neu- trons can pass easily through matter and thus through the walls of most containers. Following a suggestion ﬁrst explicitly made by Russian physicist Yuri Zel’dovich, experimentalists who use the bottle approach—as Geltenbort and his colleagues in France do— get around the problem by trapping extremely cold neutrons (that is, those with a very low kinetic energy) within a container of very smooth walls [see box on page 40]. If the neutrons are slow enough and the bottle smooth enough, they reﬂect from the walls and hence remain in the bottle. To achieve this eect, the neu- trons must move at speeds on the order of just a few meters per second, as opposed to the roughly 10 million meters per second neutrons travel when emitted during nuclear ﬁssion, for instance. These “ultracold” neutrons are so slow that you could “outrun Peter Geltenbort is a sta scientist at the Institut Laue-Langevin in renoble, rance, where he uses one of the most intense neutron sources in the world to research the fundamental nature of this particle. eorey . reene is a professor of physics at the University of Tennessee, with a oint appointment at the Oak Ridge National Laboratorys Spallation Neutron Source. He has been studying the properties of the neutron for more than 40 years. © 2016 Scientific American April 2016, ScientiﬁcAmerican.com 39Graphics by Nigel Hawtin them. The most accurate bottle experi- ment to date took place at the Institut Laue-Langevin (ILL) in Grenoble, France. Unfortunately, no bottle is ever perfect. If neutrons occasionally leak out of the bottle, we will attribute this loss to beta decay and will get the wrong lifetime. We must therefore be sure to correct our cal- culations so as to count only those parti- cles that actually undergo beta decay. To make that correction, we use a clev- er technique. The number of neutrons lost through the walls of the bottle de pends on the rate at which neutrons bounce against the walls. If the neutrons are slower or the bottle is bigger, the bounce rate, and thus the loss rate, will go down. By varying both the size of the bottle and the energy (veloc- ity) of the neutrons in successive trials, we can ex trapolate to a hypothetical bottle in which there are no collisions and thus no wall losses. Of course, this extrapolation is not perfect, but we do our best to account for any error this calculation introduces. In the beam method—used by Greene and others at the National Institute of Standards and Technology (NIST) Center for Neutron Research—we send a stream of cold neutrons through a magnetic ﬁeld and a ring of high-voltage electrodes that traps positively charged particles [ see box on page 41 ]. Because neutrons are electrically neutral, they pass right through the trap. If, however, a neutron decays within the trap, the resulting positively charged proton gets “stuck.” Periodically we “openthe trap and expel and count the protons. In princi- ple, the proton trapping and detection are nearly perfect, and we must make only very small corrections for the possibility that we missed decays. WHERE COULD WE GO WRONG? TO BE USEFUL, a measurement must be accompanied by a reliable estimate of its accuracy. A measurement of a persons height that has an uncertainty of one meter, for example, is much less meaningful than a measurement that has an uncertainty of one millimeter. For this reason, when we make precision measure- ments we always report an experimental uncertainty; an uncer- tainty of one second, for instance, would mean our measure- ment had a high probability of being no more than a second shorter or a second longer than the true value. Any measurement has, in general, two sources of uncertainty. Statistical error arises because an experiment can measure only a ﬁnite sample—in our case, a ﬁnite number of particle decays. The larger the sample, the more reliable the measurement and the lower the statistical error. The second source of uncertainty—systematic error—is much more dicult to estimate because it arises through im perfections in the measurement process. These ﬂaws may be something sim- ple, like a poorly calibrated meter stick used to measure a persons height. Or they can be more subtle, like a sampling bias—in a tele- phone poll, for example, one might overly rely on calls to land lines rather than to cell phones and thus fail to capture a truly representative population sample. Experimentalists go to great lengths to reduce these systematic errors, but they are im possible to eradicate completely. The best we can do is carry out a detailed study of all imaginable sources of error and then estimate the lin- gering eect each might have on the ﬁnal result. We then add this systematic error to the statistical error to give a best estimate of the overall reliability of the measurement. In other words, we put great eort into estimating the known unknowns. Of course, our great fear is that we have overlooked an “un - known unknown”—a systematic eect that we do not even know we do not know—hidden within the experimental procedure. While we go to extreme pains to explore all possible uncertainties, the only way to overcome this type of additional error with real conﬁdence is to perform another, completely independent mea- surement using a totally dierent experimental method that does not share the same systematic eects. If two such measurements agree within their quoted uncertainties, we have conﬁdence in the results. If, on the other hand, they disagree, we have a problem. For the measurement of the neutron lifetime we have two such independent methods: the beam and the bottle. The most recent result from the beam experiment at NIST gave a value for the neutron lifetime of 887.7 seconds. We determined the statisti- cal uncertainty in our estimate to be 1.2seconds and the system- atic uncertainty 1.9seconds. Combining those errors statistically gives a total uncertainty of 2.2 seconds, which means that we believe the true value of the neutron lifetime has a 68 percent probability of being within 2.2seconds of the measured value. The bottle experiment at ILL, on the other hand, measured a neutron lifetime of 878.5 seconds with a statistical uncertainty of 0.7second, a systematic uncertainty of 0.3second and a total uncertainty of 0.8second. These are the two most precise neutron lifetime experiments of each type in the world, and their measurements dier by approximately nine seconds. Such a time span may not sound like a lot, but it is signiﬁcantly larger than the calculated uncer- tainties for both experiments—the probability of obtaining a How Neutrons Decay Despite decades of trying, scientists hae not een ale to denitiel measre ho long netrons lie otside o atomic ncleithe est eeriments in the orld rodce conicting reslts lthogh the length othe netron lietime is ndetermined the case o netron deca is ell knon hrogh a rocess called eta decaa netron transorms into a roton and releases an electron and an antinetrino the antimatter conterart to the netrino article he deca ensres that the nal articles charge and sin anglar momentm tall to eal those o the original article BASICS Neutron Proton Electron Antineutrino Charge = –1 Spin angular momentum = + ½ Charge = +1 Spin angular momentum = + ½ Charge = 0 Spin angular momentum = + ½ Charge = 0 Spin angular momentum = + ½ Charge = 0 Spin angular momentum = -½ { { © 2016 Scientific American dierence of this size by chance alone is less than one part in 10,000. We must therefore seriously consider the possibility that the discrepancy results from an unknown unknown—we have missed something important. EXOTIC PHYSICS AN EXCITING explanation for the dierence could be that it actually re ﬂects some exotic physical phenomenon not yet discovered. A reason to think such a phenomenon might exist is that although the bottle and beam methods disagree, other beam studies show good agreement among them selves, as do other bottle studies. Imagine, for example, that in addition to the regular beta de - cay, neutrons decayed via some previously unknown process that does not create the protons sought in beam experiments. The bot- tle experiments, which count the total number of lost” neutrons, would count both the neutrons that disappeared via beta decay as well as those that underwent this second process. We would therefore conclude that the neutron lifetime was shorter than that from “normal” beta decay alone. Meanwhile the beam exper- iments would dutifully record only beta decays that produce pro- tons and would thus result in a larger value for the lifetime. So far, as we have seen, the beam experiments do measure a slightly longer lifetime than the bottles. A few theorists have taken this notion seriously. Zurab Berezhi- ani of the University of LAquila in Italy and his colleagues have suggested such a secondary process: a free neutron, they propose, might sometimes transform into a hypothesized “mirror neutron that no longer interacts with normal matter and would thus seem to disappear. Such mirror matter could contribute to the total amount of dark matter in the universe. Although this idea is quite stimulating, it remains highly speculative. More deﬁnitive con- ﬁrmation of the divergence between the bottle and beam meth- ods of measuring the neutron lifetime is necessary before most physicists would accept a concept as radical as mirror matter. Much more likely, we think, is that one (or perhaps even both) of the experiments has underestimated or overlooked a systemat- ic eect. Such a possibility is always present when working with delicate and sensitive experimental setups. WHY THE NEUTRON LIFETIME MATTERS FIGURING OUT WHAT WE MISSED will of course give us experimental- ists peace of mind. But even more important, if we can get to the bottom of this puzzle and precisely measure the neutron lifetime, we may be able to tackle a number of long-standing, fundamen- tal questions about our universe. First of all, an accurate assessment of the timescale of neutron decay will teach us about how the weak force works on other parti- cles. The weak force is responsible for nearly all radioactive de cays and is the reason, for instance, that nuclear fusion occurs within the sun. Neutron beta decay is one of the simplest and most pure Fill with neutrons Count #1 #1 #2 #3 Time Number of neutrons observed Count #2 Count #3 Dierent Techniques, Dierent Results Scientists have tried to main technies to measre the aerage netron lietime the ottle and the eam methods he arios ottle measrements oer the ears tend to agree ith one an other ithin their calclated error ars as do the eam measre ments he reslts rom the to technies hoeerconict he discreanc aot eight seconds eteen the ottle and eam aeragesma not seem like mch t it is signicantl larger than the measrementsncertainthich means the diergence rere sents a real rolem ither the researchers hae nderestimated the ncertaint o their reslts ormore eciting the dierence arises rom some nknon hsical henomenon EXPERIMENTS The Bottle Method ne ay to easure ho long neutrons lie is to ll a ontainer ith neutrons and empty it after various time intervals under the same con- ditions to see ho any reain. hese tests ll in points along a ure that represents neutron deay oer tie. ro this ure, sientists use a siple forula to alulate the aerage neutron lifetie. eause neutrons oa- sionally esape through the alls of the ottle, sientists ary the sie of the ottle as ell as the energy of the neutronsoth of hih aet ho many particles will escape from the bottle—to extrapolate to a hypothetical ottle that ontains neutrons perfetly ith no losses. See a video about neutron beta decay at Scienticmerican.comapr2016neutron-lifetime SCIENTIFIC AMERICAN ONLINE Neutron Lifetime (seconds) Year of Experiment Neutron Lifetime Measurements Beam method Bottle method Beam method average* (blue zone): 888.0 + 2.1 seconds 1990 900 895 890 885 880 875 870 1995 2000 2005 2010 2015 Bottle method average (green zone): 879.6 + 0.6 seconds Uncertainty Disagreement *The beam method average does not include the 2005 measurement, which was superseded by the 2013 beam study. © 2016 Scientific American 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. Time Number of neutrons going through trap Measured slope Neutron beam (known intensity) passes through Count the number of decays within the time interval Trap + + ProtonElectrodes The Beam Method In ontrast to the ottle ethod, the ea tehniue looks not for neutrons ut for one of their deay produts, protons. ientists diret a strea of neutrons through an eletroagneti trap ade of a agneti eld and ring-shaped high-oltage eletrodes. he neutral neutrons pass right through, ut if one deays inside the trap, the resulting positiely harged protons ill get stuk. he researhers 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 deayed in that span of tie. his easureent is the deay rate, hih is the slope of the deay ure at a gien point in tie and hih allos the sientists to alulate the aerage neutron lifetie. MORE TO EXPLORE easureent of the eutron ifetie sing a raitational rap and a o- eperature olin oating. A. Serebrov et al. in Physics Letters B, Vol. 605, Nos. 1–2, pages 72–78; January 6, 2005. he eutron ifetie. FredE.WietfeldtandGeoreyL.GreeneinReviews of Modern Physics, Vol. 83, No. 4, Article No. 1173; October–December 2011. Iproed eterination of the eutron ifetie. A. T. Yue et al. in Physical Review Letters, Vol. 111, No. 22, Article No. 222501; November 27, 2013. FROM OUR ARCHIVES ltraold eutrons. R. Golub, W. Mampe, J. M. Pendlebury and P. Ageron; June 1979. he roton adius role. Jan C. Bernauer and Randolf Pohl; February 2014. scientificamerican.com/magazine/sa © 2016 Scientific American Having a precise idea of the average neutron lifetime \tau_n will help us: - have a better understanding of the weak force - test our big bang nucleosynthesis models against astronomical data One of the techniques that scientists use to measure the neutron lifetime is called **Bottle Method**. It consists of trapping a given number of neutrons in a container and open it a different times and measure how many neutrons are still left in the container (the ones that did not decay). We can construct the graph that you see in the article and infer the value of the neutron average lifetime \tau_n. Interestingly different bottle experiments give similar results and the same is true for different beam experiments. This could indicate that there is something more fundamental about the discrepancy of these results. Neutrons could have other types of decays, that are not being detected by the beam experiments (they only detect protons). Beta decay is a type of radioactive decay that is well understood by scientists. In this image we see a neutron decaying into a proton, an electron and an antineutrino (\beta^-  decay). Neutrons and protons are not elementary particles, they are called **hadrons and are composed of 3 quarks.** A beta decay can be described by:$$ n \rightarrow p + e^- + \bar{\nu_e}$$and at the subatomic level:$$ d \rightarrow u + e^- + \bar{\nu_e}$$Here is a Feynman diagram depicting the \beta^-  decay: ![beta dacay](http://i.imgur.com/ICjVoAD.png) a down quark **d** decays into an up quark **u** an electron **e** and an antineutrino **\bar{\nu}**. You can learn more about it here: [Beta Decay](https://en.wikipedia.org/wiki/Beta_decay) The other method that physicists use to measure the average neutron lifetime is the **Beam method**. A beam of neutrons (with very well known density) is produced from a beam of protons. The beam is directed to an electromagnetic trap. Neutrons are not affected by the magnetic field, but the protons that they decay into can be captured. Knowing the number of neutrons that decay and the time it takes the beam to go through the trap one can infer the average lifetime of the neutron \tau_n. Here is a scheme of the neutron beam trap: ![beam method](http://i.imgur.com/JzTW8oX.png) You can learn more here about the experimental setup here: [How SNS Works](http://neutrons.ornl.gov/content/how-sns-works) The average neutron lifetime value \tau_n is an open problem in physics. There are two main types of experiments currently underway to try and determine the value of \tau_n: - **bottle traps**: that count the number of neutrons that survive after various intervals - **beam experiments**: that look for the particles into which neutrons decay. These experiments give different results for the value of \tau_n. Resolving the discrepancy is vital to answering a number of fundamental questions about the universe like: - How does the weak force really work? - Can we theoretically describe particle abundance from astronomical data in order to gain a better understanding of nucleosynthesis? This paper presents the results obtained by the two types of experiments and discusses the possible sources of discrepancies. A free neutron decay reaction can be written as:$$ n^0 \rightarrow p^+ + e^- + \bar{\nu_e} where **n** is the neutron, **p** the proton, **e** the electron and , **$\bar{\nu}$** the anti-neutrino. The Feynman diagram that describes this reaction at the subatomic level is as follows: ![feynamdiagram](http://i.imgur.com/U5rkVFY.png) You can learn more about the experimental setup used to trap the cold neutrons at ILL here: [Institut Laue-Langevin, Ultracold Neutron Facility](https://www.ill.eu/instruments-support/instruments-groups/instruments/pf2/description/instrument-layout/) [Weak force](https://en.wikipedia.org/wiki/Weak_interaction) is one of the four fundamental forces of nature (the others are strong force, electromagnetism, and gravitation). The weak force is responsible for radioactive decay, which plays an essential role in neutron decay, also know as $\beta$ decay. The mediators of the weak force are the [$W^\pm$ and $Z^0$ bosons](https://en.wikipedia.org/wiki/W_and_Z_bosons). In order to reduce systematic errors, scientists use two different methods and different detectors to measure the average lifetime of the neutron. The most recent results of the two experiments are: * **NIST: ** The neutron lifetime at NIST, $\tau_n = (887.7 ± 2.2)$ s. You can read their paper here: [Improved Determination of the Neutron Lifetime](https://arxiv.org/pdf/1309.2623.pdf) * **ILL: ** The neutron lifetime at ILL, $\tau_n = (878.5 ± 0.8)$ s. You can read their paper here: [Measurement of the neutron lifetime using a gravitational trap and a low-temperature Fomblin coating](http://nvlpubs.nist.gov/nistpubs/jres/110/4/j110-4ser1.pdf) The difference between the two results is approximately **9 seconds**. This is a very significative difference, which may indicate that their might be additional systematic errors that are not being take into account.