First Observation of a Doubly Charged Tetraquark and Its Neutral Partner

Area of Science:

• Particle Physics
• Quantum Chromodynamics (QCD)
• Hadron Spectroscopy

Background:

• Isospin symmetry relates different charge states of particles, providing insights into fundamental interactions.
• The search for exotic hadrons, such as tetraquarks, expands our understanding of QCD beyond the quark model.

Purpose of the Study:

• To perform a combined amplitude analysis of B0 and B+ decays into D-D_s-pi final states.
• To search for and characterize new resonant states within these decay channels.
• To investigate the nature and quantum properties of observed exotic states.

Main Methods:

• Utilized a large dataset from proton-proton collisions at 7, 8, and 13 TeV recorded by the LHCb detector.
• Performed a combined amplitude analysis of B^{0}→D[over ¯]^{0}D_{s}^{+}π^{-} and B^{+}→D^{-}D_{s}^{+}π^{+} decays.
• Determined the masses, widths, and spin-parity of resonant states using sophisticated data analysis techniques.

Main Results:

• Observed two new resonant states with masses around 2.908 GeV and widths around 0.136 GeV.
• Identified the first doubly charged open-charm tetraquark state [cs[over ¯]ud[over ¯]] and a neutral tetraquark [cs[over ¯]u[over ¯]d].
• Both states exhibit spin-parity 0+, and their consistent parameters suggest they form an isospin triplet.

Conclusions:

• The discovery of these two tetraquarks provides strong evidence for exotic hadron states.
• The observed isospin triplet nature of these tetraquarks offers a unique laboratory to study quark interactions.
• These findings contribute significantly to the ongoing exploration of the complex hadron spectrum predicted by QCD.