Editing Fifth force (section)

===Claims of new particles===
{{main|X17 particle}}
In 2015, Attila Krasznahorkay at [[ATOMKI]], the Hungarian Academy of Sciences's Institute for Nuclear Research in [[Debrecen]], Hungary, and his colleagues posited the existence of [[X17 particle|a new, light boson]] only 34&nbsp;times heavier than the electron (17&nbsp;MeV).<ref name="Cartlidge2016">{{cite journal |last=Cartlidge |first=Edwin |year=2016 |title=Has a Hungarian physics lab found a fifth force of nature? |journal=Nature |doi=10.1038/nature.2016.19957 |s2cid=124347962 }}</ref> In an effort to find a [[dark photon]], the Hungarian team fired protons at thin targets of [[lithium-7]], which created unstable [[beryllium-8]] nuclei that then decayed and ejected pairs of electrons and positrons. Excess decays were observed at an opening angle of 140° between the {{math|e}}{{sup|+}} and {{math|e}}{{sup|−}}, and a combined energy of 17&nbsp;MeV, which indicated that a small fraction of beryllium-8 will shed excess energy in the form of a new particle.

In November&nbsp;2019, Krasznahorkay announced 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 [[X17 particle|X17]] particle's existence.<ref>{{cite news |title=Scientists may have discovered fifth force of nature, laboratory announces |newspaper=[[The Independent]] |place=London, UK |url=https://www.independent.co.uk/news/science/dark-matter-particle-hungary-atomki-nuclear-research-force-nature-a9210741.html |access-date=2019-11-26 |df=dmy-all}}</ref>

Feng ''et al''. (2016)<ref name=Feng-etal-2016/> proposed that a protophobic (i.e. "proton-ignoring") X-boson with a mass of 16.7&nbsp;MeV with suppressed couplings to protons relative to neutrons and electrons and [[femtometer]] range could explain the data.<ref>{{cite magazine |title=New boson claim faces scrutiny |magazine=Quanta Magazine |url=https://www.quantamagazine.org/new-boson-claim-faces-scrutiny-20160607/ |access-date=2019-11-24 |df=dmy-all}}</ref> The force may explain the [[Anomalous magnetic dipole moment#Muon|muon {{nowrap|{{mvar|g}} − 2}} anomaly]] and provide a dark matter candidate. Several research experiments are underway to attempt to validate or refute these results.<ref name="Cartlidge2016"/><ref name=Feng-etal-2016>{{cite journal |last1=Feng |first1=Jonathan L. |last2=Fornal |first2=Bartosz |last3=Galon |first3=Iftah |last4=Gardner |first4=Susan |last5=Smolinsky |first5=Jordan |last6=Tait |first6=Tim M. P. |last7=Tanedo |first7=Philip |title=Protophobic Fifth-Force Interpretation of the Observed Anomaly in {{sup|8}}Be Nuclear Transitions |journal=Physical Review Letters |date=11 August 2016 |volume=117 |issue=7 |page=071803 |doi=10.1103/PhysRevLett.117.071803 |arxiv=1604.07411 |pmid=27563952 |bibcode=2016PhRvL.117g1803F |s2cid=206279817 }}</ref>