The X17 particle (X17 boson) is a hypothetical

subatomic particle In physics, a subatomic particle is a particle smaller than an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a baryon, lik ...

proposed by Attila Krasznahorkay and his colleagues to explain certain anomalous measurement results; these anomalous measurements are known as ATOMKI anomaly or beryllium (⁸Be) anomaly or X17 anomaly. 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 has a half-life on the order of 8.19 seconds, decaying into two alpha particles. This has importa ...

nuclei and in

helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

nuclei. The X17 particle could be the

force carrier In quantum field theory, a force carrier is a type of particle that gives rise to forces between other particles. They serve as the quanta of a particular kind of physical field. Force carriers are also known as messenger particles, intermedia ...

for a postulated

fifth force In physics, a fifth force refers to a hypothetical fundamental interaction (also known as fundamental force) beyond the four known interactions in nature: gravitational, electromagnetic, strong nuclear, and weak nuclear forces. Some speculativ ...

, possibly connected with

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

, and has been described as a protophobic (i.e., ignoring

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

vector boson In particle physics, a vector boson is a boson whose spin equals one. Vector bosons that are also elementary particles are gauge bosons, the force carriers of fundamental interactions. Some composite particles are vector bosons, for instance any ...

with a mass near .

History

In 2015, Krasznahorkay and his colleagues at ATOMKI, 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 half odd-intege ...

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

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

). In an effort to find a dark photon, the team fired

s at thin targets of

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

, which created beryllium-8 nuclei in an excited state, which then decayed to the ground state and produced pairs of

s and

positron The positron or antielectron is the particle with an electric charge of +1''elementary charge, e'', a Spin (physics), spin of 1/2 (the same as the electron), and the same Electron rest mass, mass as an electron. It is the antiparticle (antimatt ...

s. 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 the excited 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 , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

s and electrons at

femtometre The femtometre (American spelling femtometer), symbol fm, (derived from the Danish and Norwegian word 'fifteen', ) is a unit of length in the International System of Units (SI) equal to 10−15 metres, which means a quadrillionth of one metre. ...

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 In academic publishing, a preprint is a version of a scholarly or scientific paper that precedes formal peer review and publication in a peer-reviewed scholarly or scientific journal. The preprint may be available, often as a non-typeset versi ...

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 Science journalism conveys reporting about science to the public. The field typically involves interactions between scientists, journalists and the public. Origins Modern science journalism originated in weather and other natural history obs ...

, 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. In 2021 the workshop "Shedding light on X17" was held at Centro Enrico Fermi in Rome, Italy. The workshop discussed the ATOMKI anomaly and its theoretical interpretation and future experiments to confirm and explain it. One of the experiments that plans to repeat the original ATOMKI lithium–beryllium experiment is MEG II at PSI institute; the measurement was planned (in 2021) to be completed in 2022. Also Universite de Montreal's 6 MV (6 megavolt) Tandem Van de Graaff Facility in Montreal has an experiment that attempts to reproduce the ATOMKI measurement; data taking should take place in early 2023. In 2022, another preprint was published by Krasznahorkay et al. supporting the X17 particle hypothesis.

CERN The European Organization for Nuclear Research, known as CERN (; ; ), is an intergovernmental organization that operates the largest particle physics laboratory in the world. Established in 1954, it is based in Meyrin, western suburb of Gene ...

's NA64 experiment and NA62 experiment have reported in 2021 and 2023 respectively results of conducted searches that have put stringent limits for the existence of the X17 particle. In early 2023 the MEG II experiment performed its replication of the ATOMKI lithium–beryllium experiment; as of January 2024 the results have not yet been published (although the measurements were made in early 2023). As of September 2024, the analysis of measurement results has been done but an article has not been published.

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 review ...

and should therefore be considered preliminary. In late 2019, a follow-up paper was published in '' Acta Physica Polonica B''. Efforts by

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 Cherry picking, suppressing evidence, or the fallacy of incomplete evidence is the act of pointing to individual cases or data that seem to confirm a particular position while ignoring a significant portion of related and similar cases or data th ...

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 X17 particle is not consistent with the

Standard Model The Standard Model of particle physics is the Scientific theory, theory describing three of the four known fundamental forces (electromagnetism, electromagnetic, weak interaction, weak and strong interactions – excluding gravity) in the unive ...

, 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