Scientists are trying to understand how long a neutron lives. Why is it so difficult and important?

The mystery of the neutron's life

The neutron is one of the building blocks of matter. The name itself suggests

that this heavy particle is a neutral analogelementary particle, a positively charged proton. Like many other subatomic particles, the neutron is able to stay outside the core for a short time. Within about 15 minutes, it decays into a proton, an electron and a tiny particle - an antineutrino.

But how long it takes a neutron to "fall apart" remains a mystery. And that's why.

One method of measuring measurements gives a result in 887.7 seconds, plus or minus 2.2 seconds.

Another method shows 878.5 seconds, plus or minus 0.8 seconds.

Here they are, those same nine seconds that confuse scientists.

At first it seemed that this difference was due tomeasurement sensitivity. A perfectly valid theory. However, scientists continue to conduct a series of increasingly accurate experiments, and the insidious discrepancy still remains.

This consistency in data differences clearly indicateson some unknown type of physics, scientists are sure. This may indicate a hitherto unknown process of neutron decay. Or such a fundamental discrepancy could indicate a science that goes beyond the standard model that scientists are currently using to explain all of particle physics. After all, there are a number of phenomena that the Standard Model does not fully explain.

In fact, it is not a theory of everything, sincedoes not describe dark matter, dark energy and does not include gravity. However, experimental confirmation of the existence of intermediate vector bosons in the mid-1980s completed the construction of the Standard Model and its acceptance as the main one.

And yet, the need to expand the modelarose in 2002 after the discovery of neutrino oscillations. By the way, it was the confirmation of the existence of the Higgs boson in 2012 that completed the experimental detection of elementary particles predicted by the Standard Model.

To unravel the strange discrepancy inneutron lifetime The Office of Science of the US Department of Energy (DOE) works with other federal agencies, national laboratories, and universities.

Why study the neutron?

Nuclear physicists were the first to study timethe life of the neutron because of its important role in physics. “There are some fundamental quantities in nature that are always important,” explains Jeff Green, a University of Tennessee professor and physicist at Oak Ridge National Laboratory under the Department of Energy. He studied the neutron lifetime for most of his life - about 40 years. "Theories come and go, but the neutron lifetime remains central to many processes."

In fact, the neutron is a useful guide tounderstanding other particles. This is the simplest radioactive particle, therefore it regularly decays into other particles. Thus, it is the neutron that gives scientists a good idea of ​​the weak interaction. About the very force that determines whether neutrons turn into protons or not. Often this process releases energy and causes the nuclei to disintegrate. Weak interactions also play an important role in nuclear fusion, where two protons combine.

The neutron's lifetime can also give scientists an idea of ​​what happened immediately after the Big Bang. Tempting, isn't it?

Neutron star explosion illustration

Just seconds after educationprotons and neutrons, but before they combined into elements, time began. The universe was rapidly cooling down. At some point, it cooled down so much that protons and neutrons combined almost instantly to form helium and hydrogen. If neutrons decayed into protons a little faster or slower, this would have a huge impact on the entire process of the universe. The universe would have a very different balance of elements; it is likely that life itself would not exist at all.

Scientists hope to get a single-digit number fordetermination of the neutron lifetime so that it could be included in various equations describing the Universe. The uncertainty of the neutron lifetime is acceptable, but when it is less than a second. However, getting certainty on just one number proved to be more difficult than physicists expected.

The neutron lifetime is one of the least known fundamental parameters of the Standard Model.

Zhaouen Tang, physicist at the Los Alamos National Laboratory, US Department of Energy (LANL).

Yes, of course, various individualexperiments were able to achieve the required level of accuracy. But the inconsistency between different types of experiments prevents scientists from determining a specific neutron life number.

How the data inconsistency was discovered

Finding out that there is anythe difference in data arose due to the desire of physicists to give answers to all questions about the universe. Using two or more methods to measure the same amount is the best way to ensure accurate measurement. But scientists cannot set timers on neutrons to see how quickly they decay. Instead, they find ways to measure neutrons before and after they decay to calculate their lifetime.

And this is the main problem.

When physicists rip neutrons from atomic nuclei,they put them in a bottle and calculate how many neutrons will remain in it after some time, they assume that neutrons undergo radioactive decay on average after 14 minutes 39 seconds. But when other physicists create beams of neutrons and count the number of protons that appear - particles that are decay products of free neutrons - they get an average life time of about 14 minutes 48 seconds.

The discrepancy between the measurements of the "bottle" and "beam" still persists.

Discrepancies between measurements in the "bottle" andThe "ray" has been around since the days when methods for calculating the neutron lifetime began to yield results in the 1990s. At first, all measurements were so inaccurate that no one was worried about it. But gradually both methods improved, and still differed in estimates.

In beam experiments, machines are usedwhich create fluxes of neutrons. Scientists measure the number of neutrons in a specific volume of the beam. They then direct the flow through the magnetic field into a particle trap formed by the electric and magnetic fields. The neutrons decay in a trap where physicists measure the amount of protons that are left behind.

A proton detector at the National Institute of Standards and Technology used in the beam method.

On the contrary, bottle experiments catchultracold neutrons in a container. Ultracold neutrons travel much slower than normal neutrons - a few meters per second, compared to 10 million meters per second from fission reactions. Scientists measure how many neutrons are in the container first, and then again after a certain period of time. By studying the difference, they can calculate how quickly the neutrons decayed.

Experiment at UNCtau at Los Alamos using the bottle method to measure the neutron lifetime

“The bottle experiment measures the survivors,in the ray experiment, the dead, ”explains Green. “The bottle experiment sounds easy, but it’s actually very difficult. On the other hand, the experiment with rays sounds difficult and in fact it is also difficult to do it.

Green and his collaborators have taken new measurements in2013 at NIST, which helped them even more accurately recalculate a beam experiment they conducted in the same laboratory in 2005. By this time, scientists have completed five bottle experiments and two with beams. Green was convinced that previous beam experiments overlooked one of the biggest sources of uncertainty. Namely, accurate counting of the number of neutrons in the beam. Physicists have improved their measurements of this variable to be five times more accurate. But eight years of hard work left them with nothing. More precisely, with the same gap in results.

Scientists working on experiments withbottles, faced their own difficulties. One of the biggest measurement challenges was to prevent the loss of neutrons when interacting with the material from which the container is made. The leak changes the number of neutrons at the end of the decay and interferes with calculations.

To solve this problem, in the lastthe bottle experiment at LANL, which was supported by the US Department of Science, removed physical walls. Instead, nuclear physicists used magnetic fields and gravity to hold neutrons in place.

“I was convinced that if we did this, wewe can make the neutron live longer and agree with the neutron lifetime as a result of the beam experiments, ”said Chen-Yu Liu, a female professor at Indiana University who led the experiment. "That was my personal bias."

But the difference remains. “It was a big shock for me,” she later admitted, referring to the results, published in 2018. The probability that the difference will be due to coincidence is less than one in 10,000. But this could still be due to experimental error.

Why does the error occur?

Scientists face two typesuncertainties or errors in experiments: statistical or systematic. Statistical errors arise from insufficient data. This prevents us from drawing firm conclusions. Getting big data reduces errors.

Systematic errors are fundamentaluncertainty of the experiment. By the way, they are often not obvious. The two types of neutron lifetime experiments have completely different potential systematic errors. If the results were the same, the experiments would be an excellent test of each other. But that doesn't happen.

"The most difficult thing in measuring the neutron lifetime- it is that it is both too short and too long, ”says Hoogheide. "It turns out that 15 minutes is a really inconvenient time to measure in physics."

Therefore, nuclear scientists continue to work to collect additional data and minimize systematic errors.

Vertical - neutron lifetime in seconds. Red marks the results of experiments with rays, blue - with bottles.

“One of the things that I like the mostin my field, it’s extraordinary attention to detail and how deeply you have to understand every aspect of your experiment to make a reliable measurement, ”said Leah Broussard, nuclear physicist at ORNL.

At NIST, Hoogerheide, Greene and others are conductinga new beam experiment that explores every possible problem as fully as possible. Unfortunately, each setting for each experiment affects the subsequent results, so this work is done according to the system "two steps forward, one step back".

Other work is aimed at finding new ways to measure the neutron lifetime.

Researchers from Johns Hopkins University andDurham University, UK, with the support of the US Department of Energy, figured out how to use NASA data to measure the neutron lifetime. Based on neutrons emanating from Venus and Mercury, they calculated a lifetime of 780 seconds with an error of 130 seconds. But since data collection is not intended for this purpose, the uncertainty is too high to determine a difference in particle life. Zhaouen Tang, a physicist at Los Alamos National Laboratory, sets up an experiment that is a cross between bottle and beam experiments. Instead of measuring protons at the end, it will measure electrons.

A completely new approach. But it looks like all other options have exhausted themselves?

Or not?

New theories. From radical ideas to science fiction

Of course, there is always the possibility that the difference in measurements directly indicates a gap in our knowledge about this fundamental particle - the neutron.

Tang admits he is not going to quitexperiments. "There are so many examples of people who have seen something extraordinary and then just cite a mistake, have not worked hard enough ... And someone else has worked and won the Nobel Prize."

One of the theories of the mystery of neutron life isin that it collapses in a way that scientists simply don't know. It can decay into particles different from the familiar combination of a proton, an electron and an antineutrino. If so, then this explains why neutrons disappear in experiments with bottles, but the corresponding amount protons are not detected in beam experiments.

Other ideas are even more radical.

Some theorists suggest that neutronsdecay into gamma rays and mysterious dark matter. Dark matter makes up 75% of the matter in the universe, but as far as scientists know, it interacts with ordinary matter only through gravity. To test this theory, a group of scientists at LANL ran a version of the bottle experiment in which they measured neutrons and gamma rays. But the proposed gamma rays did not materialize, and scientists have no evidence of the appearance of dark matter from neutrons.

Mirror matter is another possiblea concept that sounds like science fiction. Theoretically, the "missing" neutrons can turn into mirror neutrons, namely into exact copies that exist in the opposite universe. Developing differently than our universe, this mirrored universe would be much colder and helium would predominate in it. While some nuclear scientists like Green think this is "implausible," others are interested in testing this theory, just "just in case."

“This is relatively unexplored territory. This sounds very appealing to me because I have a great neutron source 'in my backyard', said Leah Broussard, referring to the fission neutron source and the high flux isotope reactor, both owned by the Department of Energy Science Office at ORNL.

From left to right: ORNL collaborators Matthew Frost and Leah Broussard are working on a magnetism reflectometer in a fission neutron source that is used to search for specular neutrons. Credit: Genevieve Martin / Oak Ridge National Laboratory, US Department of Energy.

To test this theory, Broussard analyzesdata from a process that mimics a beam lifetime experiment, but is tuned to capture the signature of a potential invisible neutron partner. By launching a beam of neutrons through a specific magnetic field and then stopping it with a material that stops normal neutrons, she and her colleagues may be able to determine if mirror neutrons exist.

Whatever results this experiment brings,work on the elucidation of the neutron lifetime will be continued. “It is very significant that there are so many attempts to accurately measure the neutron lifetime. This speaks of the extremely emotional reaction of scientists to the inconsistency in this area - "I want to finally find out!" "- Broussard concludes. "Every scientist is motivated by a desire to learn, a desire to understand."

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