Photodisintegration of ${}^4\mathrm{He}$ into $p+t$

The two-body photodisintegration of ${}^4\mathrm{He}$ into a proton and a triton has been studied using the CEBAF Large-Acceptance Spectrometer (CLAS) at the Thomas Jefferson National Accelerator Facility. Real photons produced with the Hall-B bremsstrahlung-tagging system in the energy range from $0.35$ to $1.55$ GeV were incident on a liquid ${}^4\mathrm{He}$ target. This is the first measurement of the photodisintegration of ${}^4\mathrm{He}$ above $0.4$ GeV. The differential cross sections for the $\gamma$${}^4\mathrm{He}$$\to \mathit{pt}$ reaction were measured as a function of photon-beam energy and proton-scattering angle and are compared with the latest model calculations by J.-M. Laget. At $0.6\text{−}1.2$ GeV, our data are in good agreement only with the calculations that include three-body mechanisms, thus confirming their importance. These results reinforce the conclusion of our previous study of the three-body breakup of ${}^3\mathrm{He}$ that demonstrated the great importance of three-body mechanisms in the energy region $0.5\text{−}0.8$ GeV.

Figure 1

Diagram of the $\gamma$${}^4\mathrm{He}$$\to \mathit{pt}$ reaction in the center-of-mass frame. The angle between the beam direction and the emitted proton in the center-of-mass frame is denoted as ${\theta }_p$.

Figure 2

One-body mechanisms included in the Laget model showing proton (left) and triton (right) exchange. This figure is from Ref. [7].

Figure 3

Two-body mechanisms. The top diagrams show π and ρ exchange. The bottom diagrams show the nucleon-nucleon rescattering final-state interactions. The diagrams on the right come from the antisymmetry of the two active nucleons. This figure is from Ref. [7].

Figure 4

Three-body mechanisms showing meson double scattering. The diagrams on the bottom correspond to the antisymmetry of the two nucleons in the pair that absorbs the intermediate pion. This figure is from Ref. [7].

Figure 5

Previous world data for the photodisintegration of ${}^4\mathrm{He}$ into $p+t$, plotted as a function of the incident photon energy (open triangles [1], closed squares [2], closed circles [3], open circles [4], and closed diamonds [5]) compared with Laget's model calculations, including one-body diagrams only (dashed lines), two-body diagrams (dot-dashed lines), and three-body diagrams (solid lines). This figure is from Ref. [7].

Figure 6

CLAS is a nearly 4π-sr detector system used to observe multiparticle final states. Three-dimensional representation of CLAS, with a portion of the system cut away to highlight elements of the detector system as described in the text.

Figure 7

Difference between tagger and the start-counter (ST) times (solid histogram). The tagger and ST coincidence time for the selected events is required to be within 1 ns (shaded histogram). Secondary peaks corresponding to nearby beam buckets are also visible.

Figure 8

Hadron mass calculated from the TOF and tracking information. The solid histogram shows the mass distribution for all the positively charged hadrons, the dashed histogram is the selected sample of protons and tritons that are detected in coincidence, and the shaded histogram shows the same distribution after applying all the kinematic cuts to remove the background (see Sec. 3c for details).

Figure 9

Distributions of the proton (left) and triton (right) missing-mass squared before (top) and after (bottom) the energy-loss corrections. Gaussian fits have been performed to determine the mean value and the width of each distribution (see Table 1).

Figure 10

$\mathit{pt}$ two-body final-state kinematics require the missing energy (upper left), missing-mass squared (upper right), and missing momentum (lower left) to be zero, and the $\mathit{pt}$ opening angle in the center-of-mass frame (lower right) to be ${180}^{°}$. The peaks correspond to the real $\mathit{pt}$ events from the two-body breakup of ${}^4\mathrm{He}$. The shaded areas correspond to the nearly background-free sample of $\mathit{pt}$ events after the kinematic cuts described in this section were applied. The upper left panel shows the cut applied to the missing energy for $\mathit{pt}$ events; the other three panels show the derived event distributions.

Figure 11

Cuts were applied on various kinematic variables to remove the background. Upper left: The difference between the measured and calculated proton scattering angles. Upper right: The difference between the magnitude of proton and triton momenta. Lower left: The difference between the proton and triton azimuthal angles. Lower right: The sum of the cosines of the proton and triton scattering angles. All quantities are shown in the center-of-mass frame.

Figure 12

Calculated values for the missing-mass squared for the detected proton (left) and the detected triton (right), before (solid histogram) and after (shaded histogram) applying the kinematic cuts. The background is completely removed by the kinematic cuts (see Sec. 3c for details).

Figure 13

Simulated TOF masses for Monte Carlo generated events, plotted with both logarithmic (top) and linear (bottom) scales, before (solid histogram) and after (shaded areas) applying all the cuts.

Figure 14

Acceptance as a function of photon energy $E_{\gamma }$ for various proton-scattering-angle bins.

Figure 15

Total angle-integrated cross section as a function of $E_{\gamma }$ in linear (top) and logarithmic (bottom) scales. Error bars indicate statistical uncertainties. The logarithmic plot was fitted with an ${\mathit{Ae}}^{-{\mathit{BE}}_{\gamma }}$ functional form with $A=1.35\hspace{0.5em}\mu \mathrm{b}$, and $b=7.8\hspace{0.5em}{\mathrm{GeV}}^{-1}$.

Figure 16

Measured differential cross sections as a function of $E_{\gamma }$ for ${\theta }_p^{\mathrm{c}.\mathrm{m}.}={40}^{°}$, ${50}^{°}$, ${60}^{°}$, ${70}^{°}$, ${80}^{°}$, ${90}^{°}$, ${100}^{°}$, ${110}^{°}$, ${120}^{°}$, and ${130}^{°}$. Error bars indicate statistical uncertainties.

Figure 17

Measured differential cross sections as a function of ${\theta }_p^{\mathrm{c}.\mathrm{m}.}$ for $E_{\gamma }=0.4$, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, and 1.1 GeV. Error bars indicate statistical uncertainties.

Figure 18

Measured differential cross sections compared with the Laget calculations [8]. The results are shown as a function of ${\theta }_p^{\mathrm{c}.\mathrm{m}.}$ for four photon-energy bins. The curves are calculations based on the Laget model [7] including one-body (dashed line), two-body (dash-dotted line), and three-body (solid red line) mechanisms. Error bars indicate statistical uncertainties.

Figure 19

Measured differential cross sections (closed circles) compared with the data from Ref. [2] (closed squares), Ref. [4] (open blue circles), and the Laget calculations [8] (curves), as a function of photon energy for three proton c.m. angles. The curves are as described in Fig. 18. Error bars indicate statistical uncertainties.