LPC meeting summary 07-03-2016 - final
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LPC meeting summary 07-03-2016 - final |
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Main purpose of the meeting: Summary of the LHCC; Presentation of a new strategy to declare Stable Beams in 2016; An overview on the optics used in 2016.
Jamie started the Introduction with a short summary of the LHCC meeting, which took place last week. An important outcome is the request of the LHCC to the experiments to motivate their requests for the heavy ion period with quantitative physics arguments. To this purpose the LPC will organise a joint meeting with the LPCC to prepare the next LHCC.
In addition the LHCC requested a more global planning of the LHC forward physics and heavy ion programme taking into account the longer term future. The LPC will address this by organising common meetings with the LPCC.
When presenting the 2016 update schedule Jamie asked if the scheduled date of the VdM scan would be still adequate for experiments. Witold Kozanecki, who had investigated this question with the experiments, reported that ATLAS and LHCb would not see a need for a change of the date, ALICE had not yet answered, and CMS would be ok with the current date as long as they would have 2 weeks of physics data taking with magnetic field before the scan. Currently the switching on of the magent is foreseen for the 2nd of May which is 2 weeks before the foreseen VdM scan date.
In his talk Jamie pointed out the that experiments should quote the same uncertainty on the beam energy in their physics analysis and currently the recommended uncertainty is based on measurements done with 2013 p-Pb data (0.65% uncertainty on the energy). This recommendation comes from Jörg Wenninger and is documented in a related paper (CERN-ATS-2013-040). Jörg commented that it is possible that the final uncertainty on the beam energy will be lower if the uncertainty is derived from the magnetic model. But currently only private communiation for such an uncertainty exists and a paper documenting this uncertainty needs to be written before this uncertainty can be used by physics papers. Jörg recommended the LPC to underline the high interest of the physics community in this publications in the relevant groups in order to get this works finalised. He could imagine that an uncertainty of 0.2% on the beam energy would be in reach. In the 2016 p-Pb period the measurements of the beam energy will be repeated.
Greg asked how the proton lead beams are related to a beam energy uncertainty. Jörg explained that the masses of Lead nuclei and protons are extremely precisely known. To derive a beam energy measurement one can either fill the same orbig in the machine with protons or with lead ions and measure the difference in the RF, or one can fill both beams into the machine with the same RF frequency and measure the diffrent orbit length for both species. These measurements are free from large systematic effects. However with higher energies the difference in orbits of the two species becomes smaller making the error on the energy proportional to the square of the absolute energy. This effect is expected to be (at least partly) compensated by a set of new more precise BPMs used to measure the orbit.
On a question of Stefano Redaelli it was confirmed that related to AFP the goal would be to have the AFP pots inserted regularly during the intensity ramp up in order to validate their usage similar to the procedure used in 2015 (and foreseen in 2016) for the TOTEM pots.
On a question of Greg Rakness it was remarked that the AFP/ALFA/TOTEM BIS tests foreseen next week for next week do NOT involve handshakes with the experiments. Jörg remarked that these tests will not take place before Thursday.
Matteo presented the proposal for declaration of Stable beams in 2016 and asked if any experiment sees a problem with this proposal. No objections were given from the experiments.
It was remarked by Federico Ronchetti that ALICE is working together with Reyes Alemany Fernandez on a scheme similar to the one used by LHCb, which will optimise the step size to approach the final luminosity with an automatic procedure in a short time. This procedure should be in place in order to avoid possible human errors when operators are trying to optimise the step size by hand to minimize the time of ALICE getting to the target luminosity.
Jörg summarised the optics foreseen for 2016 pp runnging at β* 40cm and for VdM scans. It was mentioned that the optics would not be ideal for CTPPS in view of the potential 750 GeV γγ - peak, since the dispersion with the new optics would be a bit too low and the acceptance of CTPPS would be beyond 800 GeV for inelastically scattered protons. One possibility to correct for this problem would be to distribute the bump to generate the crossing angle in IP5 differently on various corrector dipoles, but care has to be taken here that enough margin for the correctors will be left in order to be still able to do the IP scans for luminosity optimisation. On Wednesday a small meeting of experts is scheduled to discuss the possibilities to increase the dispersion without changing the optics significantly and without effecting luminosity production or availability.
During the discussion of the optics for the VdM scans, Jörg asked ALICE if they prefer a crossing angle of 0 degree or if they would like to keep their crossing angle of 200µrad as in the last year. ALICE needs to come back with an answer to this question before the commissioning with beam starts.