New luminosity record
: HERA delivers 87 pb-1 polarized positron-proton interactions in 2004
When on Monday August 16th at 7 a.m. the electron-proton storage ring HERA at DESY in Hamburg began its two months 2004 summer shutdown, HERA had broken a number of records. It has:
-delivered a luminosity of 3,8x10+31 cm-2s-1 exceeding its previous record of 2,0x10+31 cm-2 s-1 ,
-delivered an integrated luminosity of 87 pb-1 , beating the record year 2000, and become the first storage ring providing longitudinally polarized high energy positrons in colliding beam mode.
It has been a long and hard struggle to get HERA back into successful operation after a challenging upgrade in the years 2000 and 2001. Unexpectedly severe backgrounds prevented the two collider experiments H1 and ZEUS from taking data when HERA restarted in 2001. The main causes were found to be the strong heating of the beam pipe due to the short positron bunches and the intense synchrotron radiation from the positrons close to the experiments. These resulted in a degradation of the vacuum � the spray of particles from the interaction of the proton beam with the residual gas produced the unacceptable backgrounds.
Close collaboration between the HERA crew and the experiments, aided by external and internal advisory committees, allowed one problem after the other to be identified, understood and solved. Major changes to the beam collimation system, to the vacuum system and to the detectors were required. Finally, early in 2004, victory could be claimed: H1 and ZEUS would be able to take data at the nominal HERA beam currents (100 mA of protons and 50 mA of positrons). From then on, the HERA machine crew were able to concentrate on steadily increasing the HERA currents and the experimenters on taking data efficiently. In parallel, the positron polarization was improved steadily: values in excess of 50% were reached. Work still remains to be done to reliably achieve high polarization at high luminosities.
All three HERA experiments, the positron-proton collider experiments H1 and ZEUS, as well as the HERMES experiment in which the polarized positrons interact with a polarized gas target, have successfully taken data in 2004 and first interesting results from all three experiments will already be presented at ICHEP-04, the International Conference on High Energy Physics, which takes place in Beijing from August 16th to 21st: Examples are the first and long awaited textbook measurement of the polarization dependence of the weak interaction cross section by H1 and ZEUS, and the world�s first determination of the structure of the proton by measuring the scattered positron and the hadronic final state using a target transversally polarized to the direction of the positron beam by the HERMES experiment. These results are interesting, but also demonstrate that about ten times more data, taken with both electrons and positrons are required to exploit the scientific potential of the upgraded HERA collider. This is the ambitious aim for future HERA running.
In the two months of the 2004 summer shutdown, the HERA crew will continue to improve the vacuum system, exchange components which have caused inefficiencies in running and carry out legally required regular safety checks. When HERA comes back into operation in October 2004, the challenge will be to demonstrate that HERA and its experiments are also able to run and efficiently take data with electrons.
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