Near threshold electroproduction of the $\omega$ meson at $Q^2\approx 0.5{\mathrm{GeV}}^2$

Electroproduction of the $\omega$ meson was investigated in the ${}^{1}H\left(e,e^{\prime }p\right)\omega$ reaction. The measurement was performed at a four-momentum transfer $Q^2\approx 0.5{\mathrm{GeV}}^2$. Angular distributions of the virtual photon-proton center-of-momentum cross sections have been extracted over the full angular range. These distributions exhibit a strong enhancement over $t$-channel parity exchange processes in the backward direction. According to a newly developed electroproduction model, this enhancement provides significant evidence of resonance formation in the $\gamma *p\to \omega p$ reaction channel.

Figure 1

(Color online) Vector meson dominance $t$-channel contributions: (a) diffractive scattering—natural parity exchange, (b) ${\pi }^0$ exchange—unnatural parity exchange.

Figure 2

(Color online) Top view of Hall C. $\mathbf{Q}$ and $\mathbf{D}$ denote quadrupole and dipole magnets, respectively.

Figure 3

(Color online) Total kinematic coverage. Straight lines define the acceptance of the experimental apparatus for all the kinematic settings.

Figure 4

(Color online) Top left: Velocity distribution from the time-of-flight measurement $\left({\beta }_{\mathrm{TOF}}\right)$ for the real coincidence time cut shown in the bottom plot and described in the text. Top right: Distribution of the corrected coincidence time for protons. The estimated random coincidence contribution is overlayed on top of the coincident proton peak. Protons were selected using the TOF cut shown in the bottom plot. Bottom: Typical spectrum of the difference in the velocities as determined by the time-of-flight technique and proton momentum vs the corrected coincidence time.

Figure 5

(Color online) Fixed target ${}^{1}H\left(e,e^{\prime }p\right)X$ scattering process. Here, as well as in the text later, the energy and three-momentum transfer $\nu$ and $\mathit{q}$ are given by $\nu =E-E^{\prime }$ and $\mathit{q}=\mathit{k}-{\mathit{k}}^{\prime }$.

Figure 6

(Color online) Light vector meson electroproduction. The histogram shows events for all accepted momenta for one setting. Note the presence of the pseudoscalar $\eta$ meson signal.

Figure 7

(Color online) Missing mass distribution for ${}^{1}H\left(e,e^{\prime }p\right)X$ showing the decomposition into a peak for $\omega$ and the background.

Figure 8

(Color online) Missing mass $M$ and invariant mass $W$ distributions were broken down into individual contributions for two different momentum settings but the same angular settings. Solid circles with error bars are the data. The shaded histogram is the full Monte Carlo fit. The dotted line histogram corresponds to the resulting phase space yield, the solid line histogram to the $\omega$ yield, and the dashed line histogram to the ${\rho }^0$ yield.

Figure 9

(Color online) Distributions of four-momentum transfers $Q^2$ and $t$ for the same settings as described in the caption of Fig. 8. The cut-off at low values of $-t$ reflects the proton momentum cut applied in the analysis since the proton energy is directly proportional to $t$ in the fixed target regime (see Eq. (3).

Figure 10

(Color online) Fits for two different momentum settings summed over ${\theta }^{*}$ bins for missing mass (top) and invariant mass (bottom). Figure 8 contains the legend explanation.

Figure 11

(Color online) Fits for two different momentum settings summed over ${\theta }^{*}$ bins for four-momentum transfer $t$ (top) and four-momentum transfer $Q^2$ (bottom). Figure 8 contains the legend explanation.

Figure 12

(Color online) The results of signal-background separation, or, equivalently, $\omega$ yield extraction, for two different momentum settings of the hadron arm but the same, ${\theta }_{\gamma \mathrm{p}}=4.33°$, angular setting, Top: central momentum $p_0=1.077\mathrm{GeV}$. Bottom: central momentum $p_0=0.929\mathrm{GeV}$.

Figure 13

(Color online) The angular distributions for different average $W$ and for $\mid {\varphi }^{*}\mid <30°$. Error bars are statistical. The DESY data correspond to the four-momentum transfer $Q^2=0.77{\mathrm{GeV}}^2$, $W=1.82\mathrm{GeV}$, and full ${\varphi }^{*}$ range. The Fraas model shown here was used in the cross section extraction. Both distributions were scaled to $W=1.785\mathrm{GeV}$. The scaling procedure is described in the text.