Standard Model of Particles (IMAGE) DOE/Fermi National Accelerator Laboratory Caption The Standard Model of elementary particles and forces includes six quarks, which bound together to form composite particles. Physicists have an excellent understanding of how three quarks cluster together to form protons, neutrons and heavier baryons, and how a quark and anti-quark bind together to create pions, kaons and other mesons. Experiments have revealed unexpected examples of composite quark structures -- named X and Y particles -- that are not the usual mesons and baryons. Credit Fermilab Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.