Ask Ethan: Is it true that proof of the existence of a new, fifth interaction is obtained?

Original author: Ethan Siegel
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Model of an accelerator used to bombard lithium in a key experiment. Located at the entrance to the Institute for Nuclear Research at the Hungarian Academy of Sciences.

The standard model of elementary particle physics - particles and their interactions, describing everything that we have ever created or collided in the laboratory - surprisingly well cope with the prediction of everything that is visible in our experiments. From matter to antimatter, from synthesis to fission, from massless to the heaviest particles - these fundamental rules have passed all experimental tests. But perhaps an unexpected phenomenon is hiding in the traces of radioactive decay. Our reader from Hungary wants to know:
News on the opening of the fifth interaction in Hungary is very widely covered. I would be interested to know your point of view on this matter. Do you think this is true, or are you skeptical?

If you have come across reports of the discovery of the fifth interaction, then the experiment in question is based on an extremely unstable isotope: beryllium-8.

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If we talk about the constituent matter, the most important part of the puzzle is likely to be this isotope. Our Sun, and almost all stars, receive energy by synthesizing helium from hydrogen, in particular helium-4, with two protons and two neutrons. In the later stages of life, the core of the Sun, filled with helium, will shrink and warm up, and try to create even heavier elements. If you combine two nuclei of helium-4, you can get a nucleus with four protons and four neutrons: beryllium-8. The only problem is the extreme instability of beryllium-8, which after 10 -17s decays back into two helium-4. Only in the nuclei of the red giants is the density of matter high enough so that it is possible to adjust the third nucleus of helium-4 in time and create carbon-12, and successfully build increasingly heavier elements.

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Otherwise, as in all laboratory experiments, beryllium-8 simply decomposes back into two helium nuclei. But our experimental technologies are very sophisticated, and even in the short moments of his life, we can not only create beryllium-8 in another way (bombarding lithium-7 with protons), but also create it in an excited state in which, before decay, it will emit a high-energy photon. This photon will have enough energy to be able to decay into an electron / positron pair - which happens with all photons of sufficiently high energies. By measuring the relative angle between the electron and the positron, you expect it to be the smaller, the greater the photon energy. This follows from the laws of conservation of energy and momentum, mixed with small random variables depending on the orientation of the decay.

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But the Hungarian team led by Atilla Krasnakhorkai did not find this at all. With increasing angle, the fraction of electrons and positrons should decrease. But scientists have found an unexpected relative increase at an angle of 140 degrees, which can mean a lot. For example:

• An error in the experiment when it is measured not a signal, but something else.
• An analysis error when applying the wrong slice (you decide what data is worth leaving and what information will be useless polluting noise that you need to get rid of).
• If the result is reliable, this may indicate the existence of a new particle: either a composite particle consisting of particles of a standard model, or, more interestingly, a completely new, fundamental one.

The data seems pretty good. Of course, the same Hungarian team announced the discovery of “irregularities” in the decays of excited beryllium-8, but not with such a degree of significance - 1 chance out of 10 11 that this is a statistical randomness (6.8-σ) - and not with such number of events: hundreds of events in many channels over the background. Only a massive unstable particle would decay with a different scattering angle than the massless particles (photons) expected in this experiment - and this is still the main explanation for the “roughness” of the graph at an angle of 140º. If that turns out to be true. Krasnakhorkai expresses great confidence in his result, measured using equipment thoroughly updated compared to their previous experiments.

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The result may not be justified; it may not be possible to reproduce; this may be an experiment error. This is the best part, but also the burden of scientific work: even the most reliable and breakthrough results must be independently confirmed. But if it is a new particle, it can change everything. The rest energy of a particle - 17 MeV / c 2 - is very interesting. Its spin is 1, which indicates that it is a boson (or something similar). She moves a long enough distance to measure her lifetime, 10-14seconds - which tells us that this is a weak, and not electromagnetic, decay - that is, it is an unrelated state of leptons. It cannot be a combination of two quarks, because it is too light - otherwise it would have to be 10 times heavier. If it is a real particle, it is most likely some kind of completely new type of particles that is not part of the Standard Model.

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Such an explanation applies to everything:

• It would lead to the appearance of just such an angle of spread (140º) of decay products, due to the ratio of its rest mass to the masses of the electron and positron into which it decays.
• It would give us the first exit beyond the Standard Model, which, in our opinion, should exist, and which we still have not found.
• In potential, it could even explain the anomalous value of the magnetic moment of the muon, a heavier relative of the electron.

But this is only if the particle really exists. A result of 6.8-σ would be exciting in the case of blind analysis, but a team of scientists specifically looked for a particle of this type. In science, there is a history of discoveries of exactly what scientists were looking for, even when in fact this did not exist. Fokke de Boer - who conducted these experiments before Krasnakhorkai - discovered such particles, but could not confirm and reproduce his results.

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We know that outside the Standard Model there must be new fundamental physics, new particles and new interactions, and perhaps the first hint of this was discovered in this experiment. But, answering the reader’s question, I am skeptical of the results at the same time, and I can imagine that they are real. The discovery of a neutrino moving faster than light on OPERA and the discovery of the Higgs boson in CMS / ATLAS experiments were of the same quality. Only time and additional studies will determine what type this new result will turn out to be, potentially capable of being a particle of dark matter.

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