Major changes are under way at Hamburg’s DESY laboratory. Both the HERA electron-proton collider and its experiments are being upgraded following successful runs in 1999 and 2000 in which each experiment accumulated more than 100 inverse picobarns of data.
HERA has achieved a peak luminosity (collision rate) exceeding 2 x 1031/cm2/sec, well beyond its design specification. Nevertheless, a long shutdown began last September to upgrade the luminosity by a factor of around five and to install spin rotators to provide polarized beams for the collider’s two general-purpose detectors, H1 and ZEUS.
Polarized electron or positron beams will open up the precision exploration of the helicity structure of the electroweak current at unprecedented momentum transfer.
HERA’s increased luminosity will be achieved by introducing new superconducting magnets well inside the H1 and ZEUS detectors, as well as rebuilding 200 m of the accelerator around the interaction points. Both collaborations are refurbishing their current detectors and introducing completely new capabilities to exploit the full potential of the upgraded HERA. They are paying particular attention to the forward direction – the direction of the 920 GeV proton beam – and to vertex and luminosity measurement.
At the largest momentum transfers, both hadrons and the scattered lepton tend to go forward. To deal with the higher track density in this region, ZEUS is adding two modules filled with straw tube chambers; H1 has rebuilt its forward detector to host five additional planar drift chambers; and both are complementing their forward detectors with additional wheels of silicon detectors – eight for H1, four for ZEUS – positioned around new elliptical beryllium-aluminium beam pipes.
The ZEUS silicon detector is the most challenging single upgrade project currently underway at DESY. Its central barrel consists of 30 “ladders”, each of which contains five modules of four single-sided silicon microstrip detectors arranged in pairs with orthogonal strip directions. The elliptical shape of the beam pipe is necessary to avoid the intense synchrotron radiation generated by the new superconducting quadrupoles. This shape implies a complex geometry in which ladders are placed such that most emerging charged particles intersect three detector layers.
H1, which has used silicon detectors for several years, is extending the number of layers in the backward direction and adapting its silicon detector arrangement to the new beam-pipe shape. A challenge when introducing the new silicon detectors from the far end of the H1 detector is that more than 1000 electrical contacts are neither visible nor accessible during installation. A precision docking mechanism – nicknamed “The MIR Solution” because of resemblances to the space programme – is activated remotely to establish the necessary connections. Moreover, H1 will replace its central proportional chamber with a five-layer chamber surrounding the vertex detector and providing sufficient redundancy for triggering.
Further detector modifications are also needed to cope with the higher luminosity. Multiple photons from the Bethe-Heitler process in a single bunch, accompanied by much increased synchrotron radiation, make it necessary for both experiments to rebuild their luminosity monitors. H1 has developed a new tungsten-fibre calorimeter with high-rate sampling of Cerenkov light. The new ZEUS monitor consists of two main elements: a lead scintillator sandwich calorimeter fronted by “active filters” – two carbon absorbers separated by aerogel counters – and a spectrometer that detects electron-positron pairs from photons converting in a thin window in the beam pipe. Other upgrades will improve the trigger selectivity and the data handling.
The HERA shutdown ends in June, with the first dedicated high luminosity run planned before the end of 2001. Polarization tuning will start next year