The H1 Experiment

The design of the 2800 ton H1 detector [139], schematically shown in fig.10, emphasizes charged particle tracking in the central region as well as high calorimetric resolution for electromagnetic energy depositions.

The primary components of the H1 tracking system are two coaxial cylindrical jet-type drift chambers (CJC) covering the polar angle region between 15$^\circ$ and 165$^\circ$. The two chambers consist of 30 (60) drift cells respectively with 24 (32) sense wires each strung parallel to the beam axis. The sense wires are read out at both ends, and the $z$-coordinate is measured by charge division with a mean $z$-resolution of $\sigma_z=55\,{\rm mm}$. The spatial resolution of the CJC in the ${r \varphi}$ plane is $\sigma_{r \varphi}=130\,\mu{\rm m}$. The momentum resolution in the plane transverse to the 1.2 Tesla solenoidal field is $\sigma(p_t)/p_t \, = \, 0.006 \, p_t [{\rm GeV}] \, \oplus \, 0.015$. The magnetic field is produced by a 5 m long superconducting solenoid of 5.8 m in diameter which encloses the calorimeter. Two further inner drift chambers and two multiwire proportional chambers (MWPC) serve to measure the longitudinal track coordinates and to provide trigger information.

A Central Silicon Track detector (CST) [140] is situated around the beam pipe, consisting of two 36 cm long concentric cylindrical layers of double-sided silicon strip detectors, at radii of $57.5$ mm and $97$ mm from the beam axis. The CST covers a pseudo-rapidity range of $30^{\circ} < \theta < 150^{\circ}$ for tracks passing through both layers. The double-sided silicon detectors provide resolutions of 12 $\mu$m in $r$-$\phi$ and 25 $\mu$m in $z$. Average hit efficiencies are 97% (92%) in $r$-$\phi$ ($z$). For a central track with CST $r$-$\phi$ hits in both layers, the transverse distance of closest approach $dca$ of the track to the nominal vertex in $x$-$y$ can be measured with a resolution of $\sigma_{dca} \approx 33\;\mu$m$\oplus 90 \;\mu$m$/p_t [$GeV$]$, where the first term represents the intrinsic resolution (including alignment uncertainties) and the second term is the contribution from multiple scattering in the beam pipe and the CST; $p_t$ is the transverse momentum of the track.

The Forward Tracking Detectors cover a polar angular range between $5^\circ$ and $30^\circ$. The system consists of three supermodules composed of three planar drift chambers, a multiwire proportional chamber, a transition radiator and a radial drift chamber. The MWPCs serve for trigger purposes and complement the polar angular coverage of the central proportional chambers. The H1 main calorimeter employs a fine-grain liquid argon (LAr) sandwich structure in the barrel and forward (proton-beam) region (with angular range from 4$^\circ$ to 155$^\circ$ in polar angle). In the backward region (with angular range from 155$^\circ$ to 177.5$^\circ$) a lead/scintillating-fiber calorimeter [141] provides an excellent energy resolution of $\sigma(E)/E \, = \, 0.07/\sqrt{E {\rm [GeV]}} \, \oplus \, 0.01$, and a time resolution better than 1 ns. The electromagnetic section of the liquid argon calorimeter uses lead plates as absorber material. In the hadronic section (which provides a depth of $\sim 5$ nuclear interaction lengths) steel plates are used. In total there are 31,000 electromagnetic and 14,000 hadronic readout channels, segmented longitudinally and tranverse to the shower direction. The electromagnetic LAr calorimeter achieves an energy resolution of $\sigma(E)/E \, = \, 0.12/\sqrt{E {\rm [GeV]}} \, \oplus \, 0.01$. The high degree of segmentation allows for a distinction between hadronic and electromagnetic energy depositions in the offline reconstruction, resulting in a hadronic energy resolution of $\sigma(E)/E \, = \, 0.55/\sqrt{E {\rm [GeV]}} \, \oplus \, 0.01$.

Muons are identified as minimum ionizing particles in both the calorimeters and in the iron magnetic field return yoke surrounding the magnetic coil. The iron system is instrumented with 16 layers of limited-streamer tubes of 1 cm$^2$ cell size. Altogether the muon system consists of $\sim 100,000$ channels. Up to five out of 16 layers are used for triggering. In order to provide a two-dimensional track measurement five of the 16 layers are equipped in addition with strip electrodes glued perpendicular to the sense wire direction.

The H1 trigger and readout system consists of four levels of hardware and software filtering. Three of these layers, the first (L1) and second (L2) level trigger and the asynchronous online filtering (L4) - were operated in HERA-I. The third level is prepared to be used by the H1 Fast Track Trigger system (described in section 7.1.5). The L1 system is phase-locked to the HERA accelerator clock signal of 10.4 MHz and provides a trigger decision for each bunch crossing after 2.3 $\mu$s. The subdetector systems feed data into front-end pipelines and generate fast information (trigger elements) about general properties of the event. The trigger elements are sent to the central trigger logic which makes decisions on the basis of 128 logical combinations of these trigger elements. The L1 decisions are then validated by the second level trigger allowing 20 $\mu$s for the decision. The L2 trigger system implements conditions on topological properties of the events. Neural nets are used to combine information from several detector components. The subdetector data are read out asynchronously by the central data acquisition electronics and fed into the software filter (L4). The reading of events from the front-end buffers takes about 1.2 ms, during which no new events can be recorded. This dead-time is inherent to the architecture of the read-out electronics. At a typical L4-input rate of 50 Hz the dead-time is about $\sim 8\%$. In the L4 software filter the events are fully reconstructed and classified in different physics categories and monitoring channels. The reconstruction of a physics event typically requires 200 ms. Events classified as physics as well as monitor events are permanently stored at a typical rate of 5 to 10 events per second.