Forward Jet and Particle Production at HERA

TL;DR

The study probes QCD evolution in the forward region of low-$x$ DIS at HERA by measuring forward jets and high-$p_T$ forward hadrons. It compares data to DGLAP, BFKL, and CCFM predictions, including resolved-photon contributions, using multiple MC models and analytic calculations. Results show that models with resolved-photon contributions are needed to describe forward production; pure DGLAP fails, and LO BFKL descriptions carry large uncertainties. The findings support a significant role for non-DGLAP dynamics in forward production, though a full NLO BFKL treatment is required for a robust interpretation.

Abstract

Single particles and jets in deeply inelastic scattering at low x are measured with the H1 detector in the region away from the current jet and towards the proton remnant, known as the forward region. Hadronic final state measurements in this region are expected to be particularly sensitive to QCD evolution effects. Jet cross-sections are presented as a function of Bjorken-x for forward jets produced with a polar angle to the proton direction, theta, in the range 7 < theta < 20 degrees. Azimuthal correlations are studied between the forward jet and the scattered lepton. Charged and neutral single particle production in the forward region are measured as a function of Bjorken-x, in the range 5 < theta < 25 degrees, for particle transverse momenta larger than 1 GeV. QCD based Monte Carlo predictions and analytical calculations based on BFKL, CCFM and DGLAP evolution are compared to the data. Predictions based on the DGLAP approach fail to describe the data, except for those which allow for a resolved photon contribution.

Figures (5)

• Figure 1: Parton evolution in the ladder approximation. The kinematics of forward jets in DIS events are indicated.
• Figure 2: The average transverse energy flow around the forward jet axis over all events is shown for two different minimum $p_{T{\rm jet}}$ values as a function of $\Delta \phi$ (left ), integrated over $|\Delta \eta|< 1$ and as a function of $\Delta\eta$, (right), integrated over $|\Delta \phi| < 1$ rad, as shown by the insets in the top figures. Here $\Delta \eta$ and $\Delta \phi$ are measured with respect to the reconstructed jet axis. Also shown is the DJANGO expectation with full H1 detector simulation.
• Figure 3: The single particle spectra in Bjorken-$x$ are shown for charged particles and $\pi^{0}$-mesons produced in the polar angle range $5^{\circ} < \theta < 25^{\circ}$. In the upper right plot ARIADNE with JETSET was used to calculate the contribution of charged pions to the charged particle measurement which makes possible, when divided by two, a direct consistency check of the two measurements. For comparison, four different Monte Carlo models are overlaid, as well as an analytical calculation labelled BFKL(KLM) based on kwiecinski. $n_{\pi}$ is the number of $\pi^{0}$-mesons and $N$ is the number of DIS events that fall into the specified kinematic range. The full errors are the quadratic sum of the statistical (inner error bars) and systematic uncertainties.
• Figure 4: The forward jet cross-section as a function of Bjorken-$x$ for two $p_{T{\rm jet}}$ cuts: 3.5 GeV and 5.0 GeV. The errors shown are the statistical and systematic uncertainties added in quadrature. In (a) and (c) the curves are model calculations (full line RAPGAP, dashed line LDCMC, dotted line LEPTO and the dashed dotted line ARIADNE). In (b) (d) the full lines are analytic LO BFKL calculations at the parton level (no jet algorithm was applied), the dashed lines $O(\alpha_s^2)$ calculations using DISENT with a cone algorithm applied.
• Figure 5: The azimuthal correlation $\Delta \phi$ (in degrees) between the jet and the scattered electron, for two different $x$ regions. The curves are model calculations (full line RAPGAP, dashed line LDCMC, dotted line LEPTO and the dashed dotted line ARIADNE).