The X17 particle is a hypothetical subatomic particle proposed by Attila Krasznahorkay and his colleagues to explain certain anomalous measurement results. The particle has been proposed to explain wide angles observed in the trajectory paths of particles produced during a nuclear transition of

beryllium-8 Beryllium-8 (8Be, Be-8) is a radionuclide with 4 neutrons and 4 protons. It is an unbound resonance and nominally an isotope of beryllium. It decays into two alpha particles with a half-life on the order of 8.19 seconds. This has important r ...

atoms and in stable

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

atoms. The X17 particle could be the

force carrier In quantum field theory, a force carrier, also known as messenger particle or intermediate particle, is a type of particle that gives rise to forces between other particles. These particles serve as the quanta of a particular kind of physical fi ...

for a postulated fifth force, possibly connected with

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...

, and has been described as a protophobic (i.e., ignoring protons) vector boson with a mass near . The NA64 experiment at CERN looks for the proposed X17 particle by striking the

electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...

beams from the Super Proton Synchrotron on fixed target nuclei.

History

In 2015, Krasznahorkay and his colleagues at

ATOMKI ATOMKI is the Institute for Nuclear Research, Hungarian Academy of Sciences. The institute is located in Debrecen and was established in 1954 by Sándor Szalay, the founding director. ATOMKI became independent from the Institute of Experimental ...

, the Hungarian Institute for Nuclear Research, posited the existence of a new, light

boson In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0,1,2 ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have odd half-integer spi ...

with a mass of about (i.e., 34 times heavier than the

). In an effort to find a dark photon, the team fired protons at thin targets of

lithium-7 Naturally occurring lithium (3Li) is composed of two stable isotopes, lithium-6 and lithium-7, with the latter being far more abundant on Earth. Both of the natural isotopes have an unexpectedly low nuclear binding energy per nucleon ( for lit ...

, which created unstable beryllium-8 nuclei that then decayed and produced pairs of

s and positrons. Excess decays were observed at an opening angle of 140° between the and particles and a combined energy of approximately . This indicated that a small fraction of beryllium-8 might shed its excess energy in the form of a new particle. The result was successfully repeated by the team. Feng et al. (2016) proposed that a "protophobic" X boson, with a mass of , suppressed couplings to protons relative to

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

s and electrons at

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range, could explain the data. The force may explain the − 2 muon anomaly and provide a dark matter candidate. , several research experiments are underway to attempt to validate or refute these results. Krasznahorkay (2019) posted a preprint announcing that he and his team at ATOMKI had successfully observed the same anomalies in the decay of stable helium atoms as had been observed in beryllium-8, strengthening the case for the existence of the X17 particle. This was covered in science journalism, focusing largely on the implications that the existence of the X17 particle and a corresponding fifth force would have in the search for dark matter.

Skepticism

, the ATOMKI paper describing the particle has not been

peer reviewed Peer review is the evaluation of work by one or more people with similar competencies as the producers of the work ( peers). It functions as a form of self-regulation by qualified members of a profession within the relevant field. Peer revie ...

and should therefore be considered preliminary. In late 2019, a follow-up paper was published in '' Acta Physica Polonica B''. Efforts by CERN and other groups to independently detect the particle have been unsuccessful so far. The ATOMKI group had claimed to find various other new particles earlier in 2016 but abandoned these claims later, without an explanation of what caused the spurious signals. The group has also been accused of cherry-picking results that support new particles while discarding

null result In science, a null result is a result without the expected content: that is, the proposed result is absent. It is an experimental outcome which does not show an otherwise expected effect. This does not imply a result of zero or nothing, simply a res ...

s. The X‑17 particle is not consistent with the Standard Model, so its existence would need to be explained by another theory.

References

{{portal bar, Chemistry, Physics Bosons Dark matter Hypothetical elementary particles Force carriers

History

Skepticism

See also

References