The case for a new quark with a mass of 1.68 GeV

Some CKM matrix element measurements differ from Standard Model predictions by 3$\sigma$. Also, high energy collisions consistently produce twice the number of charm quarks predicted by Standard Model event generators. Both of these discrepancies can be resolved by a fourth down-type quark whose primary decay is to a right-chiral charm quark and a virtual W boson. Surprisingly, neither direct searches nor precision electroweak experiments rule out such a quark, as long as its mass is just slightly larger than that of the charm quark. If such a quark exists and has a mass of 1.68 GeV, some of the exotic charmonium-like hadron resonances discovered in the last few years could be re-interpreted as normal mesons involving the new quark.Comment: 5 pages. arXiv admin note: text overlap with arXiv:2112.0446