Photon performance issues towards 2012 analyses

1. Photon performance issues towards 2012 analyses • Three main aspect relevant for 2012 data analysis : • Impact of pileup on photon reconstruction and new tuning • Impact of pileup on the photon identification a new ID menu optimization • Impact of pileup on photon isolation and new isolation flavours L. Carminati (Universita’ e sezione INFN di Milano) Atlas Italia meeting

2. Impact of pileup on photon : • Dramatic impact of pileup on photon analyses (ie HSG1) with 2011 setup: • tracking level : reconstruction efficiency + conv/unconv classification (increased number of fake conversions) which affects the choice of calibration constants and identification cuts 2011 analysis tools: more fake TRT-TRT conversions Conversion recovery Atlas Italia meeting

3. Impact of pileup on photon reconstruction • Pileup increase impacts on Higgs to gamma gamma analysis at several levels : • calorimeter variables : isolation and shower shapes are distorted by additional interactions. Impact on signal selection efficiency : total efficiency drops from ~50% at m=0 to ~30% for m=40 (mainly due to identification) Atlas Italia meeting

4. Impact of pileup on photon reconstruction • Huge amount of work by the tracking and egamma teams to improve photon reconstruction and reduce the pileup dependence • A few % increase in reconstruction efficiency • Much less fake TRT-TRT double track conversions : fraction of unconverted, single and double track conversions almost flat wrt to NPV (much less migrations) Atlas Italia meeting

5. Impact of pileup on photon reconstruction • Huge amount of work by the tracking and egamma teams to improve photon reconstruction and reduce the pileup dependence • H to gg peak stable wrt NPV and 3(10)% improvement in resolution for m=20(40) Atlas Italia meeting

6. Impact of pileup on photon identification • Pileup affects the shapes of the discriminating variables : • Rhad, Rh and DE turned out to be the most sensitive to pileup, relax cuts • Used for the moment truth conversion flags : samples with the latest tracking changes available, re-tune of the menu ongoing • NPV>20 reco-conv • NPV>20 true-conv • NPV<12 • (NPV>20 dijets) Degradation Migration effect Atlas Italia meeting

7. Impact of pileup on photon identification • New tentative menu (“2012” in PhotonID tool is available) : • Good behavior wrt to in time and out of time pileup wrt to the previous menus Unconverted photons, |h|<0.6 • better efficiency, more stable wrt to pileup • background efficiency also increased • Ongoing work to asses the consequences - solid: 2012 menu - dashed : old menu Atlas Italia meeting

8. Impact of pileup on photon identification • New tentative menu (“2012” in PhotonID tool is available) : • Checked on 2011 data : observed increase background ~ 20-30%. • But 2011 trigger tighter than 2012 menus! So the real increase might be larger. • Increase of signal efficiency using NN and new 2012 menu Atlas Italia meeting

9. Impact of pileup on photon isolation • Pileup on isolation in 2011 data: effects from both in-time and out-of-time • In-time pileup : increase the observed energy in the em calorimeter. Can be cured by: • parameterizing the corrections on some sensible variable (ie NPV,) • using ambient energy density techniques (event by event) : potentially reduce also the RMS. Still some residual dependence on the pileup shown. • Out of time pileup : pileup cancels on average in Lar if no holes within 600 ns window before BC (FBX: number of bunches before current bunch in unit of 50 ns, in window of 600 ns) Atlas Italia meeting

10. Impact of pileup on photon isolation : topo-cluster based isolation Atlas Italia meeting

11. Impact of pileup on photon isolation : topo-cluster based isolation Topocluster based isolation behaves reasonably well as a function of NPV and FBX Atlas Italia meeting

12. Impact of pileup on photon isolation : topo-cluster based isolation Noise suppression in the topocluster based isolation also reduces the width of the isolation distribution Atlas Italia meeting

13. Conclusions • Photon physics is affected by pileup increase in 2012 data : a lot of work done to mitigate the effects and improve the performance : • Impressive progresses from the tracking/egamma side to make the reconstruction efficiency stable wrt to pileup and minimize the conv/unconv migrations • Tentative new 2012 cut based menus made available : the menu is stable wrt to NPV and FBX. Impact is being evaluated on the real analysis. • Optimization for NN id will follow • Improved photon isolation definition : use of topoclusters reduces the dependence on in-time and out of time pileup. • All tools and tunings (hopefully) in place to get rid of the increased pileup level : eager to have the highest possible integrated luminosity as soon as possible Atlas Italia meeting

14. Impact of pileup on photon identification • Out-of-time pileup : decreases the observed calo energy (negative tails in the LAr pulse shape) and should completely cancel the in time pileup contribution (if no trains!) • Clear structure following the LHC beam structure seen • No corrections for this : parameterize as function of the bunch position within a train ? • Interesting progresses shown yesterday • https://indico.cern.ch/getFile.py/access?contribId=6&resId=0&materi • alId=slides&confId=159433 • A lot to do and few people committed : • Re-derive leakage corrections using MC11 samples • Validate the parameterized pileup corrections as a function of NPV (in time pileup) • Optimize ambient energy corrections to reduce the residual dependence • Study 2D corrections for in-time and out-of-time pileup ? • New variables (track isolation?) and/or some mechanism to make the current variables less pileup-sensitive with some noise suppression mechanism (topoclusters, cell level cuts) • The preliminary systematic uncertainty on iso cut is 5% in Hgg analysis (3% before) Atlas Italia meeting