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R jets measurement

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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R jets measurement

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  1. Rjets measurement

  2. Outline • 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…

  3. Motivation for a Rjets measurement

  4. 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

  5. Measurement to be made (II) • We aim to produce something similar to: Jet kinematic variable Theoretical prediction Measurement *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

  6. 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…

  7. Why is it interesting? (II) • Such a result would provide a discovery • We might get something like: or • Statistically limited, but still a good indication for new physics

  8. Why is it interesting? (III) • Couple of actual plots: big thanks to Claire

  9. 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

  10. 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

  11. 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

  12. 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…

  13. Design of the analysis

  14. 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

  15. 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:       

  16. 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

  17. 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???

  18. Status of the different components of the analysis

  19. 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

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