Observation of an exotic narrow doubly charmed tetraquark

Abstract: Conventional hadronic matter consists of baryons and mesons made of three quarks and quark-antiquark pairs, respectively. The observation of a new type of hadronic state, a doubly charmed tetraquark containing two charm quarks, an anti-$u$ and an anti-$d$ quark, is reported using data collected by the LHCb experiment at the Large Hadron Collider. This exotic state with a mass of about 3875 MeV$/c^2$ manifests itself as a narrow peak in the mass spectrum of $D^0D^0\pi^+$ mesons just below the $D^{*+}D^0$ mass threshold. The near threshold mass together with a strikingly narrow width reveals the resonance nature of the state.

• The paper identifies a narrow resonance near the D*+ D0 threshold, confirming the existence of an exotic doubly charmed tetraquark state.
• It employs precise vertex reconstruction and maximum-likelihood fitting on proton-proton collision data at 7, 8, and 13 TeV to isolate the signal.
• The study reports a Tcc mass around 3875 MeV and a width of about 410 keV with a 22-sigma significance, providing crucial insights into QCD and multi-quark interactions.

Observation of a Doubly Charmed Tetraquark at LHCb

The research presented in this document details the detection and analysis of an exotic hadronic structure, a doubly charmed tetraquark, utilizing data from proton-proton collisions recorded by the LHCb experiment at the CERN Large Hadron Collider. The tetraquark discovered is comprised of two charm quarks and two light antiquarks ($\cquark\cquark\uquarkbar\dquarkbar$) and is observed as a narrow resonance in the $\Dz\Dz\pip$ mass spectrum just below the $\Dstarp\Dz$ mass threshold. This observation is detailed in Nature Physics under the publication number LHCb-PAPER-2021-031.

Experimental Context and Methodology

In exploring quantum chromodynamics (QCD) through hadron spectroscopy, exotically clustered quarks provide a unique probe into the non-perturbative region of the theory that describes strong interactions. Such exotic states potentially include tetraquarks ($\quark_1\quark_2\quarkbar_3\quarkbar_4$) and pentaquarks — configurations not initially accounted for in the conventional quark model focusing on baryons and mesons alone. The data analyzed spans collision energies of 7, 8, and 13 TeV with a substantial integrated luminosity of 9 fb$^{-1}$ covering a large pseudorapidity range.

The LHCb detector facilitates precise measurements of the kinematics of charged particles and efficiently reconstructs decay vertices, which is crucial for isolating occurrences of such rare phenomena amidst significant hadronic backgrounds. Specific reconstruction techniques were employed, including statistical subtraction of non-$\Dz$ backgrounds. This involved a maximum-likelihood fit to the $\Dz\Dz\pip$ mass spectrum using a model comprising signal and background components.

Observational Details

The observed narrow peak is ascribed to a tantalizingly new state, denoted as $\Tcc$, with a mass of approximately 3875 MeV and a width of about 410 keV. The notable proximity of this signal to the $\Dstarp\Dz$ mass threshold coupled with its prompt production supports its classification as a genuine resonance rather than a molecular state, offering crucial insights into the potential binding mechanisms and dynamic behaviors of multi-quark configurations.

The statistical significance of the $\Tcc$ signal reaches 22 standard deviations, underlining a robust confirmation of its existence. Importantly, the mass position, calculated with respect to the $\Dstarp\Dz$ threshold, is evaluated as $-273\pm61$ keV, asserting its probable coupling to the $\Dstarp\Dz$ channel primarily through a P-wave interaction, given the presumed quantum numbers $J^{P}=1^{+}$.

Scientific Implications

The successful identification of $\Tcc$ provides critical empirical grounding for theories predicting stable multi-quark states, enriching the landscape of exotic hadrons. Not only does it extend the phenomenology around theories concerning hadronic molecules and tightly-bound diquark structures, but it also offers potential insights into doubly heavy tetraquarks and theoretical frameworks predicting their stability and interactions.

With the precision available from the LHCb setup, these findings invite further inquiry into the bound states of heavy quark configurations and the unconventional interactions therein. This research prods at the boundaries of our understanding of QCD, suggesting a reconsideration or extension of existing models to integrate these structures within the complex tapestry of known particle physics.

As theoretical and experimental frameworks advance, it's anticipated that further data could shed light on the nature and formation mechanisms of such exotic states, potentially paving paths toward undiscovered phenomena in both the standard model and its extensions.