Download - In situ MET performance in early data
28 January 2009 David - CAT SUSY / MET Meeting 1
In situ MET performance in early data
• Overview of ATLAS studies presented in the CSC era• To bring everybody up-to-date so that we can discuss which
area might be of interest for us
28 January 2009 David - CAT SUSY / MET Meeting 2
Minimum bias data
• First thing to do, look at data supposedly without real missing Et
• Need to get rid of non-collision events (beam-gas, cosmics, beam-halo) first
• Mean missing Et from minbias simulations: 3-4 GeV• Plot MET Ex, MET direction phi, Sigma_MET vs. SumEt
Just like Moritz has already shown:
28 January 2009 David - CAT SUSY / MET Meeting 3
Minimum bias data
• Next logical steps would then be to extend MET resolution vs SumEt to higher SumEt values with QCD dijets
• Consistency check between 2 very different event topologies
N.Kanaya, http://indico.cern.ch/conferenceDisplay.py?confId=41367
• Moritz started looking at this minbias stuff
• We thought there is more to do and contribute in the area of MET performance in the presence of pile-up (much related to minbias of course)
28 January 2009 David - CAT SUSY / MET Meeting 4
MET in Z->ee, mumu
• Production cross section ..• Select Z->ll events, assume no real missing
Et, investigate MET performance as a function of hadronic activity
• Take hadronic recoil into account, plot missing Et projected onto the l+l- bisector and the orthogonal axis
• Obtain measure for MET resolution, check for possible bias as function of recoil pt
• 16k selected events for 10/pb expected (cf. CSC note)
E.Dobson
Bias from underestimation of hadronic recoil
28 January 2009 David - CAT SUSY / MET Meeting 5
MET in Z->tautau
And after their cuts: 210 signal over 16 background events for 100/pb in 66 to 116 GeV inv. mass range…
Not for this year!
28 January 2009 David - CAT SUSY / MET Meeting 6
MET in Z->tautau
Reconstruct Z invariant mass assuming m_tau=0, directions of both neutrino systems (measured by MET) are coincident with the measured tau decay products
Background subtracted mass peak
• Assume Z mass peak position is only a function of MET scale
• For 0.8 GeV precision on the Z mass, MET scale can be determined to 8% (assuming 3sigma precision)
28 January 2009 David - CAT SUSY / MET Meeting 7
MET in W->lnu
• Assume neutrino pt equals MET
• Get transverse mass templates from simulations, convolved with MET resolution and scale, in bins of SumEt
• Fit templates to measured transverse mass distributions to get MET resolution and scale from best fit
28 January 2009 David - CAT SUSY / MET Meeting 8
MET in W->lnu
• W->munu events, for 30/pb
• No SumEt binning done, background not considered
28 January 2009 David - CAT SUSY / MET Meeting 9
MET in semi-leptonic ttbar
• Possible to investigate MET in W transverse mass, and difference in pt between leptonic and hadronic top (after full kinematic fit of final state)
• They claim 7000 selected events in 200/pb• Even longer term, many other influences (e.g. jet energy scale)
28 January 2009 David - CAT SUSY / MET Meeting 10
MET in QCD dijet events
• Not done for the note• After minbias the first sample we’ll look at to learn about MET• Also assume ideally zero MET in such events• Cannot probe if we reonstruct real MET correctly, but can go
up high in SumEt• Need to select events which you believe have 2 jets, back-to-
back, and hence should have 0 MET– E.g. exactly 2 jets, no 3rd jet above x GeV > 160 deg
• Such events are also the first approach to get the jet response flat in eta, i.e. get the relative jet-energy scale right (back-to-back jets have to be balanced in energy), so we cannot use exactly the same events to test the MET scale, only the resolution