Update on Diffractive Dijet Production Search

Hardeep Bansil University of Birmingham

Birmingham ATLAS Weekly Meeting13/09/2012

Contents• ξ and zIP in Herwig++ SD events• Cut flows• Loss of precision in D3PDs?• Next steps

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Reconstructing ξ • Proper way to calculate ξ can be done in Pomwig/Herwig++ by looking at proton &

pomeron

• Can calculate MX from invariant mass then convert to ξ (= MX2 /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±pz 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+

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Largest rapidity gap between truth particles

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ξX

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 log10(ξ) v Recon log10(ξ)

• Generally good correlation being shown - events where disagreement occurs because of the pt cut (not picking correct gap), small stats at reconstructed level

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Truth ξ v Reconstructed ξ using E+Pz methodTruth ξ v Truth ξ using E+Pz method

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Reconstructing zIP • Can determine the actual value of zIP with Pomwig/Herwig++ generator using

pomeron and parton emitted by it (= pz[Parton]/pz[Pomeron])

• Notably found events in Pomwig at generator level where zIP was less than 0 (should be 0 < zIP < 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 zIP compared to xP, so one of the incoming partons

was going "the wrong way“. In pomeron-parton rest frame, zIP could then be positive

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Mjj Mx

ξX

Reconstructing zIP • Change definition of zIP (= pz[Parton]/pz[Pomeron]) to four vector method

• Fixes issue with zIP=0 events and showed that most of time, previous definition actually was good

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ξX(IP)

(m)

(p)

PIIP

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mpz

Reconstructing zIP • 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

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Truth zIP v Reconstructed zIP using E+Pz methodTruth zIP v Truth zIP using E+Pz method

Truth zIP v Reconstructed zIP using E+Pz method, gap and jet cuts relaxed

• 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

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• 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 readingdata from branches suggeststhat fracSamplingMaxvalues are not being readout properly but cannotwork out at the moment

• Losing third of jets and half the dijets

• Any reason for MBTS1over MBTS2?

Events from Periods B

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Group Total events GRL/PV/LAr #Jets Clean + Et20, eta4.4

#Dijet Clean+ 30,20, eta4.4

Dijets After Trigger

Dijets with Gap > 3

PRAGUE 52119128 39730644 5139732 586503 504411 330

BHAM 74973419 39730644 5140005 587963 505632 330

• 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

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Total events Dijets Clean+30,20, eta4.4

Trigger Gap>3 Gap>4

No sig info 14608610 8737574

444229 421275 282 78

With sig info 14608610 8737574

444229 421275 350 127

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?

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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 zIP calculation for SD & ND samples

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Back up slides