Charmonium Production

A special case of charm fragmentation is charmonium production where the two charm quarks combine to form a colorless hadron. Like open charm production, charmonium production at HERA occurs dominantly through photon-gluon fusion: a photon emitted from the incoming electron or positron interacts with a gluon from the proton to produce a $c\bar{c}$ pair that subsequently forms a charmonium state. Comprehensive reports on the physics of charmonium production are available [165,166].

In this section the measurements of the production rates and polarization states of $J/\psi$ and $\psi(2S)$ mesons from H1 and ZEUS (see table 5) are described.

Table 5: Overview of the measurements of inelastic charmonium production at H1 and ZEUS.

	Analysis	Data	Kinematic Region
			$Q^2$	$W_{\gamma p}$	$p^2_{t,\psi}$	$z_{J/\psi}$	$Y_{\psi,lab}$
			[GeV$^2$]		[GeV]		(Rapidity)
H1
[27]	$J/\psi$ in $\gamma p$	1994	$<4$	$50...150$		0.45...0.9
[28]	$J/\psi$ in DIS	95-97	2...80	40...180		0.2...1.0
[29]	$J/\psi$ in $\gamma p$	96-00	$<1$	60...260	1...60	0.05...0.9
[30]	$J/\psi$ in DIS	97-00	2...100	50...225	$>1$	0.3...0.9
ZEUS
[31]	$J/\psi$ in $\gamma p$	1994	$<4$	$50-180$	$>1$	$0.4...0.9$
[32]	$J/\psi$ and $\psi(2S)$ in $\gamma p$	96-97	$<1$	$50...180$		$0.1...0.9$
[33]	$J/\psi$ in DIS	96-00	$2...80$	50...250		0.2...0.9	$-1.6...1.3$
[34]	$J/\psi$ in $\gamma p$ polarization	96-00	$<1$	50...180	$>1$	0.1...0.9