Measurement of azimuthal asymmetries in neutral current deep inelastic scattering at HERA

TL;DR

This study measures azimuthal asymmetries in neutral-current deep inelastic e+p scattering at HERA using the ZEUS detector and an energy-flow method to include neutral and charged hadrons over an extended pseudorapidity range. The azimuthal moments ⟨cosφ⟩, ⟨cos2φ⟩, ⟨sinφ⟩, and ⟨sin2φ⟩ are extracted as functions of hadron η^HCM and min ET^HCM, and are compared to MC generators (Lepto, Ariadne) and to NLO QCD predictions (Disent). The results show that NLO QCD describes the data better than LO MCs for ⟨cosφ⟩ but still underpredicts its magnitude, while ⟨cos2φ⟩ is reasonably described; a nonzero ⟨sinφ⟩ is observed with significance, and ⟨sin2φ⟩ is consistent with zero. The findings indicate that higher-order or resummed contributions may be necessary for a full description of azimuthal correlations in NC DIS.

Abstract

The distribution of the azimuthal angle of charged and neutral hadrons relative to the lepton plane has been studied for neutral current deep inelastic $ep$ scattering using an integrated luminosity of 45 pb-1 taken with the ZEUS detector at HERA. The measurements were made in the hadronic centre-of-mass system. The analysis exploits the energy-flow method, which allows the measurement to be made over a larger range of pseudorapidity compared to previous results. The dependence of the moments of the azimuthal distributions on the pseudorapidity and minimum transverse energy of the final-state hadrons are presented. Although the predictions from next-to-leading-order QCD describe the data better than do the Monte Carlo models incorporating leading-logarithm parton showers, they still fail to describe the magnitude of the asymmetries. This suggests that higher-order calculations may be necessary to describe these data.

Figures (4)

• Figure 1: The definition of the azimuthal angle $\phi$ either in the HCM or the Breit frame. The incoming electron is denoted by $e$, the scattered electron by $e'$, the exchanged virtual photon by $\gamma^*$ and the outgoing hadron or parton by $h$.
• Figure 2: (a) The fraction of BGF (dashed line), QCDC (full line) and QPM (dotted line) processes as a function of pseudorapidity, $\eta^{HCM}$, in the HCM frame for the energy-flow method. (b) For the QCD Compton process, the quark and gluon contributions as a function of $\eta^{HCM}$. These predictions were taken from Lepto 6.5.1 and are shown for the kinematic region 100 $< Q^2 <$ 8000 GeV$^2$, 0.2 $< y <$ 0.8 and $0.01 < x < 0.1$ for hadrons with $p_T>0.15\,GeV$ and $\theta > 8^\circ$.
• Figure 3: The values of $\langle\cos\phi^{HCM}\rangle$, $\langle\cos2\phi^{HCM}\rangle$, $\langle\sin\phi^{HCM}\rangle$ and $\langle\sin2\phi^{HCM}\rangle$, calculated using the energy-flow method as in Eq. (\ref{['eq:method']}), as a function of hadron pseudorapidity, $\eta^{HCM}$. They were obtained in the HCM frame for the kinematic region 100 $< Q^2 <$ 8000 GeV$^2$, $0.01<x<0.1$ and $0.2<y<0.8$ for hadrons with $p_T>0.15\,GeV$ and $\theta > 8^\circ$. The inner error bars are statistical uncertainties, the outer are statistical and systematic uncertainties added in quadrature. The NLO QCD predictions of Disent (solid line), with its associated uncertainty (shaded band), corrected for hadronisation and hadron losses (see text), the predictions of Lepto 6.5.1 (dotted line), and the predictions of Ariadne 4.12 (dashed line) are shown.
• Figure 4: The values of $\langle\cos\phi^{HCM}\rangle$ and $\langle\cos2\phi^{HCM}\rangle$, calculated using the energy-flow method as in Eq. (\ref{['eq:method']}), as a function of hadron minimum transverse energy, $E_{T,min}^{HCM}$. They were obtained in the HCM frame for the pseudorapidity intervals $-5<\eta^{HCM}Ȕ -2.5$, $-2.5<\eta^{HCM}Ȕ -1$ and $-1<\eta^{HCM}Ȕ 0$ in the kinematic region 100 $< Q^2 <$ 8000 GeV$^2$, $0.01<x<0.1$ and $0.2<y<0.8$ for hadrons with $p_T>0.15\,GeV$ and $\theta > 8^\circ$. The inner error bars are statistical uncertainties, the outer are statistical and systematic uncertainties added in quadrature. The predictions of Lepto 6.5.1 (solid line) and of Ariadne 4.12 (dashed line) are shown.