R jets measurement
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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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R jets measurement

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Rjets measurement


  • 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…

Motivation for a Rjets measurement

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)

    • 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)

    • 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)

    • 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)

    • Couple of actual plots: big thanks to Claire

    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)

    • 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)

    • 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 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…

    Design of the analysis

    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)

    • 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)

    • 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

    • 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???

    Status of the different components of the analysis

    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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