Elliptic anisotropy measurement of the f0(980) hadron in proton-lead collisions and evidence for its quark-antiquark composition
Abstract
Abstract Despite the f 0 (980) hadron having been discovered half a century ago, the question about its quark content has not been settled: it might be an ordinary quark-antiquark ( $${{\rm{q}}}\overline{{{\rm{q}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>q</mml:mi> <mml:mover> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> ) meson, a tetraquark ( $${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{q}}}\overline{{{\rm{q}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>q</mml:mi> <mml:mover> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>q</mml:mi> <mml:mover> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> ) exotic state, a kaon-antikaon ( $${{\rm{K}}}\overline{{{\rm{K}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>K</mml:mi> <mml:mover> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> ) molecule, or a quark-antiquark-gluon ( $${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{g}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>q</mml:mi> <mml:mover> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>g</mml:mi> </mml:math> ) hybrid. This paper reports strong evidence that the f 0 (980) state is an ordinary $${{\rm{q}}}\overline{{{\rm{q}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>q</mml:mi> <mml:mover> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> meson, inferred from the scaling of elliptic anisotropies ( v 2 ) with the number of constituent quarks ( n q ), as empirically established using conventional hadrons in relativistic heavy ion collisions. The f 0 (980) state is reconstructed via its dominant decay channel f 0 (980) → π + π − , in proton-lead collisions recorded by the CMS experiment at the LHC, and its v 2 is measured as a function of transverse momentum ( p T ). It is found that the n q = 2 ( $${{\rm{q}}}\overline{{{\rm{q}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>q</mml:mi> <mml:mover> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> state) hypothesis is favored over n q = 4 ( $${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{q}}}\overline{{{\rm{q}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>q</mml:mi> <mml:mover> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>q</mml:mi> <mml:mover> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> or $${{\rm{K}}}\overline{{{\rm{K}}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>K</mml:mi> <mml:mover> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> </mml:math> states) by 7.7, 6.3, or 3.1 standard deviations in the p T < 10, 8, or 6 GeV/ c ranges, respectively, and over n q = 3 ( $${{\rm{q}}}\overline{{{\rm{q}}}}{{\rm{g}}}$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>q</mml:mi> <mml:mover> <mml:mrow> <mml:mi>q</mml:mi> </mml:mrow> <mml:mo>¯</mml:mo> </mml:mover> <mml:mi>g</mml:mi> </mml:math> hybrid state) by 3.5 standard deviations in the p T < 8 GeV/ c range. This result represents the first determination of the quark content of the f 0 (980) state, made possible by using a novel approach, and paves the way for similar studies of other exotic hadron candidates.