The Experiments H1 and ZEUS at HERA

The electron proton storage ring HERA (fig.8) at the DESY laboratory in Hamburg collides 27.5 GeV electrons or positrons with 920 GeV protons². The storage ring has a circumference of $6.4\,{\rm km}$ and consists of two separate accelerators. The beams are segmented into 180 colliding bunches each, providing a bunch crossing rate of 10 MHz. Four experiments are situated at HERA. The two collider experiments H1 and ZEUS have been in operation since 1992. In 1995 the HERMES experiment started data taking using the polarized electron beam on a fixed polarized gas target [134]. The HERA-B proton-proton fixed target experiment was operated between 1998 and 2003. HERA-B makes use of the proton beam halo using a wire target [135].

Figure 8: The HERA collider with the four experiments H1, ZEUS, HERMES and HERA-B on the left and its pre-accelerators on the right.

The H1 and ZEUS detectors are typical multi-purpose collider experiments. A schematic view of the ZEUS detector is shown in fig.9. The physics programs comprise the full spectrum of QCD studies, measurements of the proton structure functions and exclusive hadronic final states, as well as electroweak physics and searches for new physics phenomena [136]. With an $ep$ center-of-mass energy of 320 GeV the HERA collider experiments H1 and ZEUS are close to the present energy frontier for accelerator based experiments. Only the Tevatron experiments CDF and D0 [137,138] have access to higher center-of-mass energies. Events in deep inelastic $ep$ scattering have been measured down to values of $x$ as low as $\sim 10^{-6}$ and up to values of $Q^2$ of $\sim 50,000$ GeV$^2$. In QCD, measurements of exclusive final states comprise jet physics, heavy flavour production, processes in hard and soft diffraction and hadron spectroscopy.

Figure 9: Schematic view of the ZEUS Detector.

In the years between 1992 and 2000 the collider experiments H1 and ZEUS collected an integrated luminosity of 100 pb$^{-1}$ each. The bulk data were taken in the years 1996 through 2000. In the years 2001/2 a major luminosity upgrade of HERA was put in place accompanied by a number of upgrades of the H1 and ZEUS detectors, as described in section 7. The interaction points were equipped with new focusing magnets which allow for substantially increased specific luminosities. Since 2003/4 the HERA collider is running again and peak luminosities of larger than $4 \cdot 10^{31}$ cm$^{-2}$ s$^{-1}$ have been reached, to be compared with the design luminosity of $\sim 7 \cdot 10^{31}$ cm$^{-2}$ s$^{-1}$. By the year 2007, an integrated luminosity of 500 pb$^{-1}$, useful for physics analyses, is expected to be produced for each of the two collider experiments.

The designs of the H1 and ZEUS detectors were chosen to be somewhat complementary, with emphasis on the reconstruction of the scattered electron in the case of H1 and on the precise calorimetric measurement of the hadronic final states in the case of ZEUS. Both experiments are capable of the triggering and reconstruction of events with heavy quark contents down to very low transverse momenta $p_{t} \gtrsim 1$ GeV. Charmonium is measured using the decays into leptons down to $p_{t} \sim 0$. A heavy quark candidate event in the H1 experiment is displayed in fig.10.

In the following the experiments H1 and ZEUS are described³, emphasizing those components that are most relevant for the triggering and reconstruction of heavy quark events, i.e.the tracking and vertexing detectors and the detectors used for lepton identification.

Figure: Display of a candidate for an event containing beauty quarks in the H1 Detector. The event is a candidate for the beauty production process process $ep \rightarrow e b\bar{b} X$ (as explained in section 6.3). One of the $\bar{b}$ quark decays into a positively charged muon which is detected as a track in the instrumented iron return yoke, the $b$ quark forms a $B$ hadron which decays into a $D^{*+}$ meson. The $D^{*+}$ meson candidate is reconstructed in the decay channel $D^{*+} \rightarrow D^0 \pi ^+ \rightarrow K^-\pi ^+\pi ^+$ from tracks measured in the inner tracking chambers.