Measurement of Azimuthal Asymmetries in Inclusive Production of Hadron Pairs in e+e- Annihilation at Belle

TL;DR

Evidence of statistically significant azimuthal asymmetries for charged pion pairs in e(+)e(-) annihilation at a center-of-mass energy of 10.52 GeV is found, which can be attributed to a transverse polarization of the primordial quarks.

Abstract

The Collins effect connects transverse quark spin with a measurable azimuthal dependence in the yield of hadronic fragments around the quark's momentum vector. Using two different reconstruction methods we observe statistically significant azimuthal asymmetries for charged pion pairs in e+e- annihilation at a center-of-mass energy of 10.52 GeV, which can be attributed to a transverse polarization of the primordial quarks. The measurement was performed using a sample of 79 million hadronic events collected with the Belle detector.

Figures (4)

• Figure 1: Definition of the azimuthal angles of the two hadrons. In each case, $\phi_i$ is the angle between the plane spanned by the lepton momenta and the thrust axis $\hat{n}$, and the plane spanned by $\hat{n}$ and the hadron transverse momentum $P_{h i \perp}$.
• Figure 2: Top: Unlike(U)-sign and like(L)-sign pion pair normalized rate $R_0$ vs. $2\phi_0$ in the bin $z_1 (z_2) \in [0.5,0.7]$, $z_2 (z_1) \in [0.3,0.5]$. Bottom: Pion pair double ratio $R_0^U/R_0^L$ vs. $2\phi_0$ in the same bin. The solid and slashed lines show the results of the fit described in the text.
• Figure 3: Values of $A_0$ and $A_{12}$ as functions of z, corrected for the contribution of charm events. The lower scales show the boundaries of the bins in $z_1$ and $z_2$; see the text. The shaded band shows the size of the systematic errors.
• Figure 4: Values of $A_0$ and $A_{12}$ as a function of $Q_T$ (not corrected for charm background) for the data samples with $T>0.8$ (triangles) and $T<0.8$ (squares).