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Phone conference
Phone conference
Description
Connect by phone to the conference "LHC luminosity",
calling the +41 22 767 7000
WG members:
ALICE: F. Antinori (+K. Safarik)
ATLAS: M.Shapiro (+ E.Nurse, B.Heinemann)
CMS: D. Marlow (+V.Halyo)
LHCb: J.Panman
LHC programme coordinator: M. Ferro-Luzzi
LPCC: M.Mangano
1st mtg of the LHC luminosity task force LLTF (see bottom of the minutes for the new WG name)
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Present (possibly via phone): everyone
Agenda (see also http://indico.cern.ch/conferenceDisplay.py?confId=84183 ):
Massi/Helmut: LPC/LSB views
Jaap: LHCb views
Dan: CMS views
Marjorie: ATLAS views
Federico: ALICE views
Massi presented the work ongoing within the LHC on luminosity (e.g. vdM scans in the preXmas run, plans for upcoming run), and explained the mechanism by which the collaborations are providing feedback to the machine on the planning of "direct" luminosity measurements. The LHC backgrounds and beam-conditions study group, in cooperation with the LHC programme coordination, provide the forum to deal with these aspects. He suggested that this channel be maintained as the primary forum for such discussions, and that the LLTF focus on "indirect" luminosity determinations. He suggests to change the name to something like "cross-section" or "rate normalization" task force.
LHCb intends to rely on the direct measurement given by the LHC, until the luminosity is large enough to start using Z->mumu and pp->pp mu+mu-. In between there will be no particular interest in using other indirect probes, like MB triggers etc.
CMS will ultimately rely on the direct measurements, but intends to cover the period before these are sufficiently accurate with the indirect rate determination ("online" and "offline" methods), also as a way to cross-check the luminosity reported by the LHC. The proposed techniques will initially rely on the online pattern of ET deposits in the fwd hadronic calorimeter, offline fwd/bwd coincidences, vertex counting, and the overall consistency of these inputs. Major limitation in this technique is the MC-dependent acceptance.
ATLAS: in addition to the ultimate-accuracy vdM scans, ATLAS will rely for the absolute measurement on forthcoming RPs, as well as on EM/EW processes. The relative measurements, to be used in the interim period, will be based on the overall consistency of a set of measurements from the LUCID, BCM, ZDC, MB triggers, fwd cals, vtx counting, and high-rates procs like J/psi. The correlations between these tools will help assessing the overall systematics, and the most accurate one will be posted on DIP. Marjorie showed studies ofthe MC-dependence of the extracted cross section (phojet vs pythia), which ranges between 10 and 15% depending on the beam energy. MC acceptances can vary by up to 25%. Proposed goals for this group include:
- agreement on a baseline value for cross sections, and on criteria to post luminosity on LHC page
- agreement on generators to use, and path to tuning
- definition of interactions with LBS and LPC
ALICE: relative luminosity based on event counting. For the online measurements, counting the number of events satisfying trigger conditions involving Level-0 trigger signals, in particular from the Silicon Pixel Detector (|eta| < 2), the V0 scintillator array (-3.7 < eta < -1.7 and 2.8 < eta < 5.1), and the muon arm (-4 < eta < -2.4). Additional conditions are included in the offline selections, depending on the particular analysis. For the absolute normalisation, two methods will be used to evaluate the triggered cross-section:
i) Monte Carlo estimate, assuming a given value of the inelastic cross
section
ii) direct measurement via luminosity scan
Stress on the need of the LLTF to be engaged in the planning of the luminosity scans and related measurements.
Share the need for the LLTF to address the systematics coming from the Monte Carlo. Call for a commonly agreed operational definition of SD and DD.
Discussion:
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- the separation of tasks proposed by Massi is accepted by everyone. The LPC will be ex-officio member of the LLTF and will liaise between LPC/LBS and the LLTF, inviting joint mts when useful. Possible trivial actions include to fwd the minutes or agendas of the joint LPC/LBS mtgs to the LLTF mailing list.
- expressed concerns about further theoretical developments in the area of diffractive MCs (e.g. phojet is not being supported any longer)
Proposed actions and open issues
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(this part includes my own elaboration of what was discussed, and in the process I raise a few questions, so please comment)
1. A session of the forthcoming meeting of the MB&UE WG (March 1-2) will discuss the modeling of diffraction; we shall try to include a discussion specific to the issues raised above. The discussion should include the following elements:
- definition of the models under consideration
- operational definition of SD and DD (possibly at the particle level, so that the classification of SD and DD is common to all experiments)
- overview of the inputs available for the tuning
- prospects for further theoretical developments
Can you suggest the names of people who are knowledgable about the luminosity issues and can be present to contribute to or drive this discussion? Marjorie, who are the people who did the studies of MC-dependence of the cross section extraction? Emily? Anyone from CMS?
One more point: I think it would useful to this WG to have a clear picture of the MC-systematics of the separate channels used by the various experiments. E.g., taking ATLAS as an example, what are the relative sensitivities to the SD/DD and non-diffractive components of the LUCID, BCM, ZDC, MB triggers, fwd cals, vtx counting determinations of rate? Furthermore: what role can these measurements play in the tuning of the MC tools, e/g/ in the determination of the relative SD/DD/nD fractions and in the validation of their kinematical properties? Can we define observables that enhance the sensitivity to the separation between SD/DD/nD, to drive the tunings?
We may not be able to explore these issues during the MB&UE mtg. Do we want to plan a focussed discussion at a later date?
2. Determination of a common reference cross section: how do we do this? We could review the parameterizations of the extrapolations of sigma_{tot,SD,DD} to high energy, and agree on a reference central value. Is this enough? I am not an expert, but we can organize a small mtg with experts from theory to prepare a proposal. Suggestions?
COMMENTS FOLLOWING THE MEETING:
- We agree on a new name for the WG, namely "Rate normalization WG"
- The first discussion of the systematics related to MC modeling will take place during the March 1-2 meeting of the MB&UE WG
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