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Expanding the Particle Zoo at BESIII and Other Stories


The last decade has been marked by discoveries of unexpected particles in meson spectra. It started with the X(3872) and Y(4260) by BaBar and Belle. These particles, as well as other
XYZ objects reported in the mean time, do not fit into a simple quark-antiquark description
of mesons. Note, this quark model has been used so far to describe most of the meson spectra remarkably well, particularly in the charmonium (c¯c) region. Now, the story of surprises continues with the Z_c(3900) and Z_c(4020) states at BESIII, popping out unexpectedly as candidates for a new form of matter: four-quark matter. The BESIII is a spectrometer in Beijing at an electron-positron collider facility (BEPCII), suitable for studying charm and charmonium production, and doing XYZ physics. However, it can be also used to study the tau-lepton, in direct collisions at the threshold for tau tau production. The tau lepton is the heaviest lepton measured with the least precision, important for constraining the Standard Model. A review of recent exciting results from this tau-charm factory will be presented.