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R jets measurement. Outline. Motivation for a R jets measurement What is this measurement? Why is it interesting? Other R jets measurement within ATLAS Design of the analysis Level of precision aimed Some propositions Analysis design questions that need to be addressed

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Presentation Transcript
  • Motivation for a Rjets measurement
    • What is this measurement?
    • Why is it interesting?
    • Other Rjets measurement within ATLAS
  • Design of the analysis
    • Level of precision aimed
    • Some propositions
    • Analysis design questions that need to be addressed
  • Status of the different components of the analysis
    • Efficiencies, acceptances
    • Backgrounds
    • Others…
measurement to be made i
Measurement to be made (I)
  • Measuring Rjets means measuring:
  • Produce a Rjets measurement for different jets multiplicity
    • Sensitivity to different physics
    • Different selections, efficiency values and backgrounds
  • Show the evolution of Rjets in terms of jets/gauge bosons kinematics

 Probe Rjets in different phase space volume/regions

      • Not necessarily a ratio of differential cross sections…
  • Compare with some theoretical predictions
measurement to be made ii
Measurement to be made (II)
  • We aim to produce something similar to:

Jet kinematic variable

Theoretical prediction


*This is just an illustration of what the final measurement

must look like.

-Data points are pure fiction

-Theoretical prediction made from ppbar at √s=1.96TeV

why is it interesting i
Why is it interesting? (I)
  • Comparison of the measured Rjets “distribution” with SM expectations allows to look for new physics simultaneously in many different channels, in a model-independent way.
    • Any new physics contribution to any of the final states will be a “background” not considered in the measurement.

 Excess of W+jets or of Z+jets candidate events

  • Jets multiplicity and direction of a deviation with respect to theory provide information on the type of new physics
  • Many of the systematic uncertainty cancel in the ratio
    • keep the uncertainty at the level of the stats error (under control)
    • Measurement can be done despite a poorer understanding of the detector (early data)
    • “Conspiracious” cancellation might happens too, but that’s life…
why is it interesting ii
Why is it interesting? (II)
  • Such a result would provide a discovery
  • We might get something like:


  • Statistically limited, but still a good indication for new physics
why is it interesting iii
Why is it interesting? (III)
  • Couple of actual plots: big thanks to Claire
why is it interesting iv
Why is it interesting? (IV)
  • Strong evidence for new physics (discovery…) if:
    • Deviation consistently increasing in an unprobed kinematic region
    • Good agreement with theory in the low kinematic region
    • Good agreement with theory for the 1-jets measurement
      • Not aware of a good 1-jet+met+l or 1-jet+2-l exotic/susy signature
  • W(l)+jets and Z(ll)+jets events can be used to make data-driven estimate of the dominant background to jets+met events
    • Globally good agreement with the theory would give confidence that such estimate are good and that no loss of sensitivity will results from exotic signal contamination
      • One of the big concern for data-driven background estimate in susy
why is it interesting v
Why is it interesting? (V)
  • Can “calibrate” our measurements tools and methods
    • Deviations displayed in slide 7, but starting at a small value of the kinematic variable indicate an under-estimate of SM backgrounds
    • Shift of the distribution indicates a bad estimate of the efficiencies

 Good agreement indicate a good understanding of the

more fundamental featured of analysis, preparing for

more complex studies

  • It is important for the ATLAS collaboration to understand lepton energy scale and resolution as well as efficiencies to reconstruct, identify and trigger on them, before understanding similar quantities for jets
    • Balance of leptons will be used for jets calibration
why is it interesting vi
Why is it interesting? (VI)
  • Other ratios (W+n+1-jets/W+n-jets, etc) are interesting measurements to do too, but don’t benefit of the above advantages
    • Jets systematics don’t cancel in the ratio (not same selections)
    • Sensitivity to new physics might be reduced
      • Same signal can affect both the numerator and denominator
    • Cannot be used to support background prediction to Jets+met physics for which ATLAS group as strong interests
    • Less interesting calibration measurement
      • Lepton efficiency and some backgrounds will cancel in the ratio…
      • That might be interesting at some point, but Oxford group developped

expertise in the estimate of such quantities, so don’t want to by-pass

chance of testing it…

Oxford group interest converged in a Rjets measurements

other r jets atlas analyses
Other Rjets ATLAS analyses
  • Other groups also manifested interests in a Rjets measurement
    • That’s natural, given the relatively small number of analyses that could be made with early data!
  • ATLAS policy is to make sure that the important SM analyses are made by at least two independent groups
    • Analyses must provide parallel insight of the same quantities…
  • The most mature ATLAS “competition” come from Wuppertal
    • Analysis driven toward background estimate to top
    • Backgrounds estimated from migration matrix and MC
    • Bin the ratio in terms of inclusive jets multiplicity and not in terms of the kinematic of a given jets multiplicity

 Low sensitivity to new physics

  • We are significantly orthogonal to them but our analysis need to gain visibility soon within the collaboration…
level of uncertainty aimed
Level of uncertainty aimed
  • First questions to answer as this decides the amount of work that need to be done on the different components
    • Ex: no need to work on a 1% bias regarding some efficiencies if the overall uncertainty is expected to be 10%
  • Of course uncertainties evolve with the analysis and can’t be decide once for all initially
  • Uncertainties we need to estimate:
    • Statistical component in a low kinematic region (calibration region) and in a sample of high region (nes physics region)
    • Uncertainties due to jets selections and energy scales/resolution
    • Theoretical uncertainty on R.
    • Others???
  • Still need to be done: I’ll take care of getting this info
analysis design propositions i
Analysis design: propositions (I)
  • Exclusive jets counting
      • Ex: One jets above 30 GeV, no events with 2nd jet above 20 GeV
    • Decouple 1-jets from 2-jets
    • Reduce backgrounds from top and taus
    • More information on the type of new physics signal if any
  • Kinematic variables to plots Rjets against:
    • Vector Boson PT:
      • distinct reconstruction for W and Z, but same for 1-jet and 2-jets bins
      • Less dependent of jets energy scale (just from Met)
    • Vectorial sum of Jets ET:
      • Small dependence on jets energy scale (not flat)
      • Same quantity for Z and W
    • Scalar sum of jets ET:
      • Less dependence on JES, but less sensitivity to phase space variation
    • Leading Jet ET:

 

analysis design propositions ii
Analysis design: propositions (II)
  • For lepton selections: use the standard ATLAS isEM
    • But not forced to since we will computed efficiencies
      • Do we want to re-optimised for fake rejection???
      • Do we need to change few cuts for an easier QCD bkg estimate???
  • It will be better to we all use the OxfordAnalysis package
    • Analysis has to be made in version 14, which is what we have ntuplized
    • It will simplify validation
    • Provide homogeneity and simply gathering of different pieces
    • Few peoples (James and I …) will work for everybody 
  • Use NLO calculation from MCFM for the theoretical prediction
    • To be discussed with Giulia
design questions
Design questions
  • Should we veto on extra leptons to remove backgrounds?
    • Ex: no isolated track of a given quality except the reconstructed leptons
  • Need to choose jets selections
    • No standard available
    • The choice should not be really important (cancel in the ratio)

 I would propose to choose jets selections of the jets+met analysis

  • Unfortunately some of the selections will change when will have data
    • Ex: need to remove non-collision fake source of Met
  • Other questions that worth now to be mentioned???
different components
Different components
  • Our analysis essentially consists in a background, lepton efficiencies and acceptance calculation

 A lot of work has been made in this group on that!!!

  • Inputs on:
    • Lepton efficiencies and acceptance  Ellie
    • QCD bkg to We+jets  Kristin, Hugo, Alessandro, Maria
    • Top bkg to We+jets  Maria
  • What is missing:
    • Background from taus
      • Can we use data-driven techniques???
    • Background to Zee+jets
      • QCD: use same sign leptons???
  • Hope to combine with same analysis on muons by Matthias