In hindsight, physicists had several reasons to suspect that this simple measurement would probe the limits of our understanding of nuclear forces.<\/p>\n

First, this system is particularly persnickety. The energy needed to produce the transiently inflated helium nucleus \u2014 the state researchers want to study \u2014 lies just above the energy needed to expel a proton and just below that same threshold for a neutron. That makes everything hard to calculate.<\/p>\n

The second reason has to do with Weinberg\u2019s effective field theory. It worked because it allowed physicists to ignore the less important parts of the equations. Van Kolck contends that some of the parts deemed less important and routinely ignored are in fact very important. The microscope provided by this particular helium measurement, he said, is illuminating that basic error.<\/p>\n

\u201cI cannot be too critical because these calculations are very difficult,\u201d he added. \u201cThey\u2019re doing the best they can.\u201d<\/p>\n

Several groups, including van Kolck\u2019s, plan to repeat Bacca\u2019s calculations and find out what went wrong. It\u2019s possible that simply including more terms in the approximation of the nuclear force might be the answer. On the other hand, it\u2019s also possible that these ballooning helium nuclei have exposed a fatal flaw in our understanding of the nuclear force.<\/p>\n

\u201cWe exposed the puzzle, but unfortunately we have not solved the puzzle,\u201d Bacca said. \u201cNot yet.\u201d<\/p>\n<\/div>\n

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