Reference data for PbPb - pp at 1.38 TeV
To fully exploit the heavy-ion data recorded during the first Pb-Pb run of LHC, at the end of last year, proton-proton reference data at the equivalent centre of mass energy are needed. An LHC run at this energy (sqrt(s) = 2.76 TeV) was scheduled early 2011, to have the results available for Quark Matter in May.
After the winter shutdown the access to the experiments was closed at the end of February when the accelerator started the commissioning with beam.
Also for ALICE this meant to recommission the detectors and to bring the newly installed EMCAL and TRD modules to operation. In a tight schedule all the necessary preparations for data taking were made, including the test of the trigger and DAQ configurations to be used during this very short, few fills, but essential run.
ALICE was ready for physics by mid of March and started taking data at sqrt(s) = 7 TeV, waiting for the long aspired energy.
A technical stop at the end of March and the scrubbing campaign1 scheduled right afterwards resulted in a significant break in physics operation of the LHC. And until the very last moment it was unclear whether this quite important run at intermediate energy could take place before the technical stop.
Ready to start ALICE waited patiently – and finally collisions were observed early in the morning of Friday 27 March. Unfortunately, the beams were dumped after not even 3 hours.
A second fill in the evening did not last much longer and serious doubts about the possibility to complete the run before the start of the technical stop started rising.
Thanks, however, to the commitment of the LHC team the instabilities responsible for the dump of the beam were solved and an exceptionally long fill, more than 21 hours!, was delivered followed by a ‘normal' 8 hours fill.
By Monday morning at 5:30 am a total of more than 35 hours of stable beams was delivered, as requested and planned. And the technical stop could happily start at 6 am as scheduled. ALICE fully profited by the performance of the machine, recording more than 90 Mio of events, all trigger classes included, with an efficiency of 94%.
Many thanks to the LHC team and congratulations to ALICE for the excellent performance!
The synchrotron radiation, emitted by the circulating bunches, induces electron emission from the beam pipe walls. These photoelectrons are accelerated towards the opposite wall by the positive charge of the bunched beams, leading to further electron emission.
This electron cloud gives rise to a multipacting process, and therefore also beam instabilities and an undesired increment of both pressure and temperature. The pressure rises due to the electron stimulated gas desorption.
The process of scrubbing consists of stimulating this phenomenon and cleaning the beam pipe from trapped gas - such that next time the photoelectrons are excited, there are no major impacts on the beam.