Investigating the nature of the K$^*_0(700)$ state with $π^\pm$K$^0_{\rm S}$ correlations at the LHC

The first measurements of femtoscopic correlations with the particle pair combinations $\pi^\pm$K$^0_{\rm S}$ in pp collisions at $\sqrt{s}=13$ TeV at the Large Hadron Collider (LHC) are reported by the ALICE experiment. Using the femtoscopic approach, it is shown that it is possible to study the elusive K$^*_0(700)$ particle that has been considered a tetraquark candidate for over forty years. Boson source parameters and final-state interaction parameters are extracted by fitting a model assuming a Gaussian source to the experimentally measured two-particle correlation functions. The final-state interaction is modeled through a resonant scattering amplitude, defined in terms of a mass and a coupling parameter, decaying into a $\pi^\pm$K$^0_{\rm S}$ pair. The extracted mass and Breit-Wigner width, derived from the coupling parameter, of the final-state interaction are found to be consistent with previous measurements of the K$^*_0(700)$. The small value and increasing behavior of the correlation strength with increasing source size support the hypothesis that the K$^*_0(700)$ is a four-quark state, i.e. a tetraquark state. This latter trend is also confirmed via a simple geometric model that assumes a tetraquark structure of the K$^*_0(700)$ resonance.


Submitted to: PLB
e-Print: arXiv:2312.12830 | PDF | inSPIRE
Figure group

Figure 1

Top row: $\pi^\pm$K$^0_{\rm S}$ correlation functions experimentally measured (blue dots) compared with PYTHIA8+GEANT3 simulations (red squares) obtained in pp collisions at $\sqrt{s}=13$ TeV for $0-100\%$ multiplicity class and $k_T>0$ (left), $0-100\%$ multiplicity class and $k_{\rm T}<0.5$ GeV/$c$ (center), and 0--5$\%$ multiplicity class and $k_{\rm T}<0.5$ GeV/$c$ (right). The PYTHIA8+GEANT3 correlation function is normalized tothe data at $k^*=0.5$ GeV/c. Bottom row: Ratio of Data to PYTHIA8+GEANT3 simulations for the three studied cases Statistical uncertainties are represented by bars.

Figure 2

Example fit of Eq. \ref{eq:fit1} to the corrected correlation functions after Eq. \ref{eq:fit5} has been used to remove the PYTHIA8+GEANT3 overcompensation of theK$^*(892)$, for $\pi^\pm$K$^0_{\rm S}$ from $\sqrt{s}=13$ TeV pp collisionsfor $0-100\%$ multiplicity class and $k_T>0$ (left), $0-100\%$ multiplicity class and $k_{\rm T}<0.5$ GeV/$c$ (center), and 0--5$\%$ multiplicity class and $k_{\rm T}<0.5$ GeV/$c$ (right). Statistical uncertainties are represented as bars.

Figure 3

The extracted Breit--Wigner parameters from the $\pi^\pm$K$^0_{\rm S}$ femtoscopic correlation inpp collisions at $\sqrt{s}=13$ TeV compared with those for K$^*_0(700)$ from the BES  and the E791  experiments. The horizontal and vertical bars represent the total uncertainties The ``ALICE average'' value is the weighted average of the three ALICE points.

Figure 4

The $\lambda$ parameter as a function of source size $R$ extracted from the $\pi^\pm$K$^0_{\rm S}$ femtoscopy measurement in pp collisions at $\sqrt{s}=13$ TeV. Results are compared with the previous ALICE measurements, obtained from K$^0_{\rm S}$K$^0_{\rm S}$  and $\pi\pi$  femtoscopy studies in pp and Pb–Pb collisions and the calculations from a toy geometric model (see text). The modelcalculations for the tetraquark and diquark hypotheses for the K$^*_0(700)$ are shown as black and light green dashed lines, respectively,the short dashed lines representing the Gaussian $\rho(r)$ and the long dashed lines representing the exponential $\rho(r)$.