Baryon number (B) conservation means that the number of quarks minus the number of anti-quarks in any interaction remains constant. Lepton number (L) conservation means that the number of leptons (electrons, muons, tau leptons, and neutrinos) minus the number of anti-leptons in any interaction remains constant.
The Standard Model separately conserves B and L in all interactions except sphaleron interactions, which have been never observed and are theoretically confined to extremely high energy scales and mass-energy densities, which the Large Hadron Collider (LHC) (the most powerful particle collider of all time), cannot reach.
The conservation of baryon number and lepton number is established remarkably robustly in experiments.
Some of the main experimental searches that have not detected B and L non-conservation are the searches for neutrinoless double beta decay, the search for tree-level flavor changing neutral currents, and the search for proton decay. These non-detections have ruled out or tightly constrained many theories in physics including Majorana neutrino mass and most of the simpler grand unified theories (GUTs), such as SU(5).
Baryon number (B) conservation underlies the apparent stability of ordinary matter by forbidding the decay of nucleons, while lepton number (L) conservation plays a central role in the structure of lepton interactions and the possible origin of neutrino mass.
In the Standard Model, B and L are accidental global symmetries rather than imposed fundamental principles. However, they are expected to be violated in many extensions of the theory, including frameworks of unification and processes in the early Universe.
This review summarizes the status of experimental tests of B and L conservation and discusses them within a unified framework for interpreting current and future searches across different processes and experimental approaches, outlining historical and theoretical motivation, key physical processes, as well as their broader connections and complementarity to other searches.
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The X17 with Chiral Couplings
Max H. Fieg ,1, 2, ∗ Toni M¨akel¨a ,1, † Tim M.P. Tait ,1, ‡ and Miˇsa Toman 1, §
1Department of Physics and Astronomy, University of California, Irvine, CA 92697 USA
2Fermi National Accelerator Laboratory, Batavia, IL 60510 USA
recently, the topic has been reinvigorated [9, 10] by the
observation of the PADME positron annihilation experi-
ment, entirely dissimilar to the nuclear experiments, yet
hinting at an excess compatible with the X17 [11]
Regardless, new experimental observations of
the transition and/or improved nuclear modeling could
dramatically clarify the situation.
Further investigations for the origin of the ATOMKI
excesses are clearly needed. The X must couple to elec-
trons, and future PADME measurements with greater in-
tegrated luminosity can either confirm or refute the new
particle interpretation. More detailed studies on the ma-
trix elements for helium and carbon would help place the
mapping from nucleon-level couplings to the correspond-
ing nuclear transition rates on firmer footing. Should fu-
ture measurements firmly establish a self-consistent pic-
ture for the X17 that includes chiral couplings, a ded-
icated model-building effort would be warranted to address the challenges of realizing self-consistent UV theory
consistent with all experimental constraints
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