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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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R jets measurement

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 r jets measurement

Motivation for a Rjets measurement

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…

    Design of the analysis

    Design of the analysis

    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???

    Status of the different components of the analysis

    Status of the different components of the analysis

    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

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