X17: Status and Perspectives

Recently, a group directed by A. J. Krasznahorkay observed an anomaly in the emission of electron–positron pairs in three different nuclear reactions, namely, the <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mn>3</mn></msup></semantics></math></inline-formula>H(p,e<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mo>−</mo></msup></semantics></math></inline-formula>e<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mo>+</mo></msup></semantics></math></inline-formula>)<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mn>4</mn></msup></semantics></math></inline-formula>He, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mn>7</mn></msup></semantics></math></inline-formula>Li(p,e<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mo>−</mo></msup></semantics></math></inline-formula>e<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mo>+</mo></msup></semantics></math></inline-formula>)<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mn>8</mn></msup></semantics></math></inline-formula>Be, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mn>11</mn></msup></semantics></math></inline-formula>B(p,e<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mo>−</mo></msup></semantics></math></inline-formula>e<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mo>+</mo></msup></semantics></math></inline-formula>)<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mn>12</mn></msup></semantics></math></inline-formula>C processes. Kinematics indicate that this anomaly might be due to the de-excitation of <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mn>4</mn></msup></semantics></math></inline-formula>He, <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mn>8</mn></msup></semantics></math></inline-formula>Be, and <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mn>12</mn></msup></semantics></math></inline-formula>C nuclei with the emission of a boson with a mass of about 17 MeV, rapidly decaying into e<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mo>−</mo></msup></semantics></math></inline-formula>e<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mspace width="-2.pt"></mspace><mo> </mo></mrow><mo>+</mo></msup></semantics></math></inline-formula> pairs. The result of the experiments performed with the singletron accelerator of ATOMKI is reviewed, and the consequences of the so-called X17 boson in particle physics and in cosmology are discussed. Forthcoming experiments designed to shed light on the possible existence of the X17 boson are also reported.