1. Diffractive Dijet Production Issues with Analysis Hardeep Bansil Birmingham Weekly ATLAS Meeting 15/09/2011

2. Contents Theory / Motivation Current issues Next steps 2

3. Diffractive dijets Look for single diffractive events (pp  pX) – Involve a rapidity gap due to colourless exchange with vacuum quantum numbers “pomeron” Then look for dijet system within X – Hard diffraction Sensitive to the diffractive structure function (dPDF) of the proton Studied at HERA and Tevatron – At Tevatron, ratio of yields of SD to inclusive dijets ~ 1% – Likely to be smaller than this at LHC 3

4. Motivation Understand the structure of the diffractive exchange by comparison with predictions from electron-proton data and be able to get a measure of F D jj Measure the ratio of the single diffractive to inclusive dijet events Gap Survival Probability – the chance of the gap between the intact proton and diffractive system being lost due to scattering (affects measured structure function e.g. Tevatron results a factor of 10 smaller than H1 predictions) 4 Rescatter with p? (ξ)(ξ)

5. Current issues No proper plots today …... for various reasons 5

6. Analysis Bottleneck Running over 2010 periods A&B (∫L dt = 8.75 nb -1, 75mil events) Grid running fast enough (only some slow sites) Major bottleneck in running locally over ntuples  now taking over 2 days – FAR TOO LONG – ISSUES WITH NTUPLE? – ISSUES WITH READING DISK? Tried selecting only a few branches to load per event Looking into skimming events (1 jet with > 10 GeV no big difference – trying with 20 GeV) Possibly try multithreading (PROOF)? 6 Running jobs on grid  get ntuples (1/1.5 days) Running over ntuples on batch server to get useful histograms (2+ days) Run over histos to get final plots (5 minutes)

7. Monte Carlo Generators Currently using Pomwig LO generator Generated QCD 2  2 process within diffractive system in different p T ranges (8-17, 17-35, 35-70, 70+ GeV) – Single Diffractive “SD” (system dissociating in ±z direction) – Double Pomeron Exchange “DPE” won’t be needed Central Diffractive, not Double Diffractive – scale SD Will request to get official Monte Carlo production done – Pomwig already validated so no need to directly compare with another generator Rapgap possible – Working with version modified to output in HepMC format – Currently getting help from Rivet authors to get this running Herwig++ samples available soon – Prague group really pushing for this generator Issues with storing ESDs on Grid – Current ESD samples only safe until end of year 7

8. AntiKt η confusion Analysis uses both AntiKt R=0.4 and R=0.6 jets Using MinBias data sample, ask for forward gap (∆η F >2.0) with jet 1 > 26 GeV, jet 2 > 20 GeV, jets not within gap AntiKt6 – before and after gap cuts, see a (approximately) symmetric η distribution for jet 1, similar for jet 2 8 Before forward gap cuts After forward gap cuts

9. AntiKt η confusion Analysis uses both AntiKt R=0.4 and R=0.6 jets Using MinBias data sample, ask for forward gap (∆η F >2.0) with jet 1 > 26 GeV, jet 2 > 20 GeV, jets not within gap AntiKt4 – fewer events passing cuts with smaller jets See a big spike between 0<η<1 in both cases, after gap cuts gets even worse  investigate further but not from allocating variable incorrectly 9 Before forward gap cuts After forward gap cuts

10. Next steps Fix issues Make plots 10