1. Update on Diffractive Dijet Production Search Hardeep Bansil University of Birmingham Birmingham ATLAS Weekly Meeting 13/09/2012

2. Contents ξ and z IP in Herwig++ SD events Cut flows Loss of precision in D3PDs? Next steps 2

3. Reconstructing ξ Proper way to calculate ξ can be done in Pomwig/Herwig++ by looking at proton & pomeron Can calculate M X from invariant mass then convert to ξ (= M X 2 /s) Truth level: all final state particles excluding intact proton (if there is one in event) Reconstructed level: all caloClusters (not just above significance thresholds) Also calculate ξ using E±p z method (C=1 for truth, determine for data) Base choice on position of where forward gap starts using gap algorithm – Gap start at -4.9 use ξ- and gap start at +4.9 use ξ+ 3 Largest rapidity gap between truth particles MYMY MXMX ~ ξX ξX

4. Reconstructing ξ Herwig++ – all gap & jet (R=0.6) cuts applied, Pt > 200 MeV on all objects Only 2000 events per pt range (half for pos η jets, half for neg η jets) Want to calculate C based on resolution of Truth log 10 (ξ) v Recon log 10 (ξ) Generally good correlation being shown - events where disagreement occurs because of the pt cut (not picking correct gap), small stats at reconstructed level 4 Truth ξ v Reconstructed ξ using E+Pz method Truth ξ v Truth ξ using E+Pz method ~~

5. Reconstructing z IP Can determine the actual value of z IP with Pomwig/Herwig++ generator using pomeron and parton emitted by it (= p z[Parton] /p z[Pomeron] ) Notably found events in Pomwig at generator level where z IP was less than 0 (should be 0 < z IP < 1) – now also see them in Herwig++ but not as often Spoke to James Monk about it – suggests that based on how generator must work, it will calculate the 2->2 amplitude in its rest frame, and then boost it according to the PDFs of the incoming proton, Pomeron and the Pomeron flux. – Can boost to a frame with low ξ and z IP compared to x P, so one of the incoming partons was going "the wrong way“. In pomeron-parton rest frame, z IP could then be positive 5 M jj MxMx ξX ξX

6. Reconstructing z IP Change definition of z IP (= p z[Parton] /p z[Pomeron] ) to four vector method Fixes issue with z IP =0 events and showed that most of time, previous definition actually was good 6 ξX ξX (IP) (m) (p)

7. Reconstructing z IP Herwig++ studies - all gap & jet (R=0.6) cuts applied, Pt > 200 MeV on all objects Not really seeing a strong correlation yet Needs more stats and investigation into resolutions 7 Truth z IP v Reconstructed z IP using E+Pz method Truth z IP v Truth z IP using E+Pz method Truth z IP v Reconstructed z IP using E+Pz method, gap and jet cuts relaxed

8. Between Birmingham and Prague groups, have been trying to focus on different topics as part of analysis Birmingham: 2010 Periods A-B, reconstruct diffractive variables, look at cluster significance & other cuts, systematics and background Prague: 2010 Periods A-F, recreate SM2010 analysis, work on trigger strategy to handle periods E-F, filters to select large rapidity gap events in MC Still needed to check that cut flow was consistent between groups using Period B (~ 7 nb -1 ) Had to use D3PD files that contain full information apart from cell significance (cell energy/noise) information which we would normally use for determining gaps Cut flow comparisons 8

9. Used L1Calo stream with (J5 || FJ5) and MinBias stream (MBTS1 && !J5 && !FJ5) Vertex cut used by Prague (PV0 > 4 tracks) inappropriate for our analysis Ignores events with very forward dijets (large gaps) and does not check for pile up vertices either Agree up to medium jet cleaning stage – meant to be doing cleaning manually due to minor bug in D3PD jet cleaning variables Currently forced to use these variables as reading data from branches suggests that fracSamplingMax values are not being read out properly but cannot work out at the moment Losing third of jets and half the dijets Any reason for MBTS1 over MBTS2? Events from Periods B 9 GroupTotal eventsGRL/PV/LAr#Jets Clean + Et20, eta4.4 #Dijet Clean+ 30,20, eta4.4 Dijets After Trigger Dijets with Gap > 3 PRAGUE52119128397306445139732586503504411330 BHAM74973419397306445140005587963505632330

10. Used special DAOD files to show the effect of cell significance (E/noise) Files miss roughly 3M events (16%), without significance cuts should have 330 events with forward gap > 3 Scale down expect 276 events, observe 282 (within stat error) With the cell significance cuts applied get ~25% more events with gaps > 3 and ~50% more events with gaps > 4 Going to check with ESDs to see effect with full sample Events from Periods B 10 Total eventsDijetsClean+30,20, eta4.4TriggerGap>3Gap>4 No sig info14608610873757444422942127528278 With sig info146086108737574444229421275350127

11. Loss of precision in D3PDs? Looking at the cell with the maximum energy significance (E/noise) for each cluster – Topological clusters are meant to be formed based on seed cell with σ > 4.0 Looked at non-TILE clusters with pT > 200 MeV from a small part of run 155112 – Unlike ESDs, observed a discrete distribution with values spread out in units of 1/32. The values do look reasonable. Apparently due to saving space in D3PDs for cluster moments? – Don’t see it for variables such as eta or pT Anyone else encountered this if using D3PDs? 11

12. Next steps Finalise cuts selection Fix issues with reading jet cleaning variables from D3PDs Get MC D3PDs to test truth level code – Request has gone in and Herwig++ SD & Pythia 8 ND dijet events should be coming in couple of weeks, secondary samples longer Fine-tune ξ and z IP calculation for SD & ND samples 12

13. Back up slides