Determination of the beam-spin asymmetry of deuteron photodisintegration in the energy region $E_{\gamma }=1.1$–2.3 GeV

The beam-spin asymmetry, $\Sigma$, for the reaction $\gamma d\to pn$ has been measured using the CEBAF Large Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility (JLab) for six photon-energy bins, between 1.1 and 2.3 GeV, and proton angles in the center-of-mass frame, ${\theta }_{\mathrm{c}.\mathrm{m}.}$, between ${25}^{\circ }$ and ${160}^{\circ }$. These are the first measurements of beam-spin asymmetries at ${\theta }_{\mathrm{c}.\mathrm{m}.}={90}^{\circ }$ for photon-beam energies above 1.6 GeV, and the first measurements for angles other than ${\theta }_{\mathrm{c}.\mathrm{m}.}={90}^{\circ }$. The angular and energy dependence of $\Sigma$ is expected to aid in the development of QCD-based models to understand the mechanisms of deuteron photodisintegration in the transition region between hadronic and partonic degrees of freedom, where both effective field theories and perturbative QCD cannot make reliable predictions.

Figure 1

The $s^{10}$ scaled differential cross section of elastic $pn$ scattering as a function of $s$ and $E_{\gamma }$. Data are from Refs. [47, 48] and the curves correspond to the HRM model fits used to calculate predictions for $\Sigma$.

Figure 2

A three-dimensional view of CLAS showing the torus magnet, the three regions of drift chambers (R1–R3), the Čerenkov counters (CC), the time-of-flight detector (TOF), and the electromagnetic calorimeters (EC). The CLAS reference frame, also indicated in the here, was defined with the $z$ axis along the beam line and the $y$ axis perpendicular to the horizontal. Figure taken from Ref. [56].

Figure 3

$\Delta \beta$ as a function of $p$. The lines show the $\pm 3\sigma$ cut from the mean applied to identify protons.

Figure 4

Tagger hit multiplicity showing on average 14 photons as possible candidates for the true photon that initiated the event.

Figure 5

Coincidence time between all reconstructed photons and identified protons. The 2-ns bunch structure of the incident beam is evident. The solid vertical lines indicate the $\pm 1$-ns cuts applied to identify the photon that initiated the reaction.

Figure 6

Angular distribution of protons as detected in the six sectors of CLAS. The black curves indicate the fiducial cuts applied to remove events that fell in regions where the acceptance changes rapidly.

Figure 7

$z$ component of the event vertex. The solid vertical lines indicate the cuts applied to select events that originated within the target.

Figure 8

Missing-mass-squared distribution of the reaction $\gamma d\to pX$. The peak corresponds to reactions where the missing particle is a neutron. The dashed vertical line indicates the nominal mass squared of the neutron, and the solid lines show the $\pm 3\sigma$ missing-mass-squared cuts applied to selected deuteron photodisintegration events.

Figure 9

Missing-mass-squared distribution of the reaction $\gamma d\to pX$ of Para events in the 1.5–1.7 GeV photon-energy bin. The yellow (light gray, filled) histogram indicates background-subtracted deuteron photodisintegration events determined using the probabilistic event-weighting method [68, 69]. The red (gray, hatched) histogram indicates background events.

Figure 10

(a) Enhancement distribution and (b) calculated degree of photon polarization for events with the coherent-edge position at 1.5 GeV. The enhancement distribution is fitted with the analytic bremsstrahlung calculation [70] (solid red [gray] line) and the degree of photon polarization is calculated (dashed red [gray] curve). The degree of photon polarization is then corrected for differences between the fit and the enhancement, as well as statistical fluctuations, using information from data with adjacent coherent-edge positions (solid blue [dark gray] line). More information on the procedure is found in Refs. [62, 71].

Figure 11

Ratio $R\left(\varphi \right)$ for a specific kinematic bin $(E_{\gamma }=1.3\text{–}1.5$ GeV and ${\theta }_{\mathrm{c}.\mathrm{m}.}=120.0\text{–}132.5^{\circ })$ fitted to the function $F\left(\varphi \right)$ (see text) to determine the beam-spin asymmetry for that bin.

Figure 12

Beam-spin asymmetries determined from fitting generated data as a function of the true value ${\Sigma }_{\mathrm{gen}}$ for 36 variable $\varphi$ bins (equal to an average bin width $\Delta \varphi =13.{50}^{\circ })$. The correction factor due to a variable $\varphi$-bin width is determined by fitting ${\Sigma }_{\det }$ vs ${\Sigma }_{\mathrm{gen}}$ to a first-order polynomial (see text for details). The statistical uncertainties of the points are smaller than the symbol size.

Figure 13

Beam-spin asymmetry as a function of the incident photon energy for ${\theta }_{\mathrm{c}.\mathrm{m}.}={90}^{\circ }$. The red (gray, triangle) and cyan (light gray, circle) points show the results from Yerevan [11, 12], whereas the blue (dark gray, square) points are the results from the present work. The solid and dash-dotted lines are the QGSM [53] and HRM [54, 55] predictions, respectively. The blue band (gray) indicates the systematic uncertainties of the present CLAS measurements.

Figure 14

Beam-spin asymmetry as a function of photon energy for four different proton angles in the center-of-mass frame: (a) ${\theta }_{\mathrm{c}.\mathrm{m}.}={30}^{\circ }$, (b) ${\theta }_{\mathrm{c}.\mathrm{m}.}={55}^{\circ }$, (c) ${\theta }_{\mathrm{c}.\mathrm{m}.}={90}^{\circ }$, and (d) ${\theta }_{\mathrm{c}.\mathrm{m}.}={135}^{\circ }$. The angular bin size varies with photon energy. The blue (gray) bands indicate the systematic uncertainties of the present CLAS measurements.

Figure 15

Beam-spin asymmetry as a function of the proton center-of-mass angle, ${\theta }_{\mathrm{c}.\mathrm{m}.}$. Each panel (a)–(f) indicates the results for the different photon-energy bins between 1.1 and 2.3 GeV. The solid and dash-dotted lines are the QGSM [53] and HRM [54, 55] predictions, respectively. The blue (gray) bands indicate the systematic uncertainties of the present CLAS measurements.