High-$E_T$ dijet photoproduction at HERA

The cross section for high-$E_T$ dijet production in photoproduction has been measured with the ZEUS detector at HERA using an integrated luminosity of $81.8{\mathrm{pb}}^{-1}$. The events were required to have a virtuality of the incoming photon, $Q^2$, of less than $1{\mathrm{GeV}}^2$ and a photon-proton center-of-mass energy in the range $142<W_{\gamma p}<293\mathrm{GeV}$. Events were selected if at least two jets satisfied the transverse-energy requirements of $E_T^{\mathrm{jet}1}>20\mathrm{GeV}$ and $E_T^{\mathrm{jet}2}>15\mathrm{GeV}$ and pseudorapidity (with respect to the proton beam direction) requirements of $-1<{\eta }^{\mathrm{jet}1,2}<3$, with at least one of the jets satisfying $-1<{\eta }^{\mathrm{jet}}<2.5$. The measurements show sensitivity to the parton distributions in the photon and proton and to effects beyond next-to-leading order in QCD. Hence these data can be used to constrain further the parton densities in the proton and photon.

Figure 1

Examples of (a) direct and (b) resolved dijet photoproduction diagrams in positron-proton, $ep$, collisions in LO QCD. This direct-photon process is the collision of a photon, $\gamma$, and gluon, $g$ from the proton. This resolved-photon process is a collision of a parton from the photon and a gluon, $g$, from the proton.

Figure 2

The theoretical uncertainties (see Sec. 5) for sample distributions: (a) $x_{\gamma }^{\mathrm{obs}}$, (b) ${\overline{E}}_T$ for $x_{\gamma }^{\mathrm{obs}}\le 0.75$, (c) “Low-$x_{\gamma }^{\mathrm{obs}}$ 3,” and (d) “High-$x_{\gamma }^{\mathrm{obs}}$ 2,” which are defined in Table 1. The uncertainties are the total (outer shaded band), that from varying ${\mu }_R$ (inner shaded band), the experimental uncertainties of data propagated in the ZEUS-JETS fit (solid lines), and using the most different photon PDF, CJK (dashed line) instead of AFG04.

Figure 3

Predictions of the fraction of the cross section initiated by gluons for sample distributions: (a) $x_{\gamma }^{\mathrm{obs}}$, (b) ${\overline{E}}_T$, for $x_{\gamma }^{\mathrm{obs}}\le 0.75$, (c) “Low-$x_{\gamma }^{\mathrm{obs}}$ 3,” and (d) “High-$x_{\gamma }^{\mathrm{obs}}$ 2,” which are defined in Table 1. The gluon fractions are from the proton using the CTEQ5M1 PDF (long-dashed line), and from the photon using the AFG04 (solid line) and CJK PDFs (short-dashed line).

Figure 4

Measured cross section $d\sigma /d{\overline{E}}_T$ for (a) $x_{\gamma }^{\mathrm{obs}}>0.75$ and (b) $x_{\gamma }^{\mathrm{obs}}\le 0.75$ compared with NLO QCD predictions using the AFG04 (solid line) and CJK (dashed line) photon PDFs. The data (dots) are shown with statistical (inner bars) and statistical and systematic uncertainties added in quadrature (outer bars) along with the jet energy-scale (Jet ES) uncertainty (shaded band). The NLO QCD predictions are shown (NLO QCD $\otimes$ HAD) multiplied by the hadronization corrections, $C_{\mathrm{had}}$, discussed in Sec. 5. The predictions using AFG04 are also shown with their associated uncertainties (shaded histogram) as discussed in Sec. 5. The ratios to the prediction using the AFG04 photon PDF are shown at the bottom of the figure.

Figure 5

Measured cross section $d\sigma /d{E}_T^{\mathrm{jet}1}$ for (a) $x_{\gamma }^{\mathrm{obs}}>0.75$ and (b) $x_{\gamma }^{\mathrm{obs}}\le 0.75$. For further details, see the caption to Fig. 4.

Figure 6

Measured cross section $d\sigma /d\overline{\eta }$ for (a) $x_{\gamma }^{\mathrm{obs}}>0.75$ and (b) $x_{\gamma }^{\mathrm{obs}}\le 0.75$. For further details, see the caption to Fig. 4.

Figure 7

Measured cross section $d\sigma /d{x}_p^{\mathrm{obs}}$ for (a) $x_{\gamma }^{\mathrm{obs}}>0.75$ and (b) $x_{\gamma }^{\mathrm{obs}}\le 0.75$. For further details, see the caption to Fig. 4.

Figure 8

Measured cross section $d\sigma /d\Delta |{\varphi }^{\mathrm{jj}}|$ for (a) $x_{\gamma }^{\mathrm{obs}}>0.75$ and (b) $x_{\gamma }^{\mathrm{obs}}\le 0.75$ compared with NLO QCD predictions using the AFG04 (solid line) photon PDF. Predictions from the MC programs Herwig (dot-dashed) and Pythia (dashed), area normalized to the data by the factors given, are also shown. The data (dots) are shown with statistical (inner bars) and statistical and systematic uncertainties added in quadrature (outer bars) along with the jet energy-scale (Jet ES) uncertainty (shaded band). The NLO QCD predictions are shown (NLO QCD $\otimes$ HAD) multiplied by the hadronization corrections, $C_{\mathrm{had}}$, discussed in Sec. 5. The predictions using AFG04 are also shown with their associated uncertainties (shaded histogram) as discussed in Sec. 5.

Figure 9

Optimized cross sections $d\sigma /d{x}_p^{\mathrm{obs}}$ for $x_{\gamma }^{\mathrm{obs}}>0.75$ in the kinematic regions defined in Table 1. For further details, see the caption to Fig. 4.

Figure 10

Optimized cross sections $d\sigma /d{x}_p^{\mathrm{obs}}$ for $x_{\gamma }^{\mathrm{obs}}\le 0.75$ in the kinematic regions defined in Table 1. For further details, see the caption to Fig. 4.

Figure 11

Measured cross section for $d\sigma /d{x}_{\gamma }^{\mathrm{obs}}$ compared with NLO QCD predictions using the AFG04 (solid line), CJK (dashed line), AFG (dotted line), GRV (dashed and double-dotted line), and SAL (dashed and single-dotted line) photon PDFs. The data (dots) are shown with statistical (inner bars) and statistical and systematic uncertainties added in quadrature (outer bars) along with the jet energy-scale (Jet ES) uncertainty (shaded band). The NLO QCD predictions are shown (NLO QCD $\otimes$ HAD) multiplied by the hadronization corrections, $C_{\mathrm{had}}$, discussed in Sec. 5. The predictions using AFG04 are also shown with their associated uncertainties (shaded histogram) as discussed in Sec. 5. The ratios to the prediction using the AFG04 photon PDF are shown at the bottom of the figure.

Figure 12

Measured cross section for $d\sigma /d{\overline{E}}_T$ for $x_{\gamma }^{\mathrm{obs}}\le 0.75$. For further details, see the caption to Fig. 11.

Figure 13

Measured cross section for (a) $d\sigma /d{x}_p^{\mathrm{obs}}$ and (b) $d\sigma /d\overline{\eta }$ both for $x_{\gamma }^{\mathrm{obs}}\le 0.75$. For further details, see the caption to Fig. 11.