1. Update on Diffractive Dijets Analysis Hardeep Bansil Birmingham ATLAS Weekly Meeting 10/04/2014

2. Contents 2 Updates to the analysis Comparisons with Prague Further plans

3. Diffractive dijets Single diffractive events (pp  pX) Rapidity gap from exchange with vacuum quantum numbers “pomeron” Search for hard diffraction with a hard scale set by 2 jets Described by diffractive PDFs + pQCD cross-sections Previous measurements of hard diffractive processes at HERA and Tevatron At Tevatron, ratio of yields of single diffractive to inclusive dijets ≈ 1% Likely to be smaller than this at LHC Measure the ratio of the diffractive to inclusive dijet events Gap Survival Probability – the chance of the gap between the intact proton and diffractive system staying intact due to scattering Tevatron have Gap Survival Probability of 0.1 relative to H1 predictions Khoze, Martin and Ryskin predict LHC to have GSP of ~ 0.03-0.06 3 Rescatter with p ? ξ 3

4. Analysis 4 2010 Period B data with GRL ( L1Calo and MinBias streams ) - ∫L dt = 6.753 nb -1 – Trigger using mixture of L1_MBTS_1 (prescaled) and L1_J5 (unprescaled) – Vertex requirement - 1 PV (5+ associated tracks), no additional vertices (2+ tracks) PYTHIA8 samples of ND, SD and DD events with ATLAS UE Tune AU2-CT10 – Samples produced with jets in different p T ranges, no pile-up – SD+DD reweighted to use super-critical Berger-Streng Pomeron flux POMWIG SD samples generated for alternate diffractive model At least 2 anti- k T jets ( R =0.4 or R =0.6) with p T > 20 GeV, |η| < 4.4 – Require medium quality jet cleaning cuts in data Calculate forward gap (Δη F ) and xi (ξ ± ) in range |η|<4.8 Using “hybrid” p and p T cuts for clusters / stable truth particles p T charged > 200 MeV || p charged > 500 MeV, p neutral > 200 MeV

5. Reweighting Pythia8 SD/DD + Pomwig PYTHIA8 diffractive samples use unconventional model (Schuler-Sjöstrand) in generation Reweight SD based on ξ distribution obtained from inclusive diffractive PYTHIA8 samples Used fit to Berger-Streng model (3 rd order poly) Could also use Donnachie-Landshoff (little difference over measured range) Similar process also for DD using ξ = M X 2 /s POMWIG samples 111112 samples in each dissociation direction for 5 different p T ranges up to 280 GeV Generated over kinematic range 10 -6 < ξ < 0.1 and 10 -6 < |t| < 10 GeV 2 5

6. Differential cross sections Differential cross sections calculated as for given variable X N weighted accounts for trigger efficiency per data event, prescales and unfolding Forward Gap Size (Δη F ) – R=0.4/0.6 jets Cross sections slightly higher for R=0.6 but maintain the same shape For PYTHIA8, ND ~1.3x larger in first bin then SD+DD and ND fairly even for Δη F >2.5 POMWIG ~3x larger csx than data for Δη F >3, slightly higher for R=0.4 6 R=0.4 R=0.6

7. Differential cross sections Differential cross sections calculated as for given variable X N weighted accounts for trigger efficiency per data event, prescales and unfolding Proton Fraction Momentum Loss (ξ ± ) – R=0.4/0.6 jets, no forward gap requirement POMWIG peaks at lower ξ ± than PYTHIA8, partly due to generation choices POMWIG results significantly above data, PYTHIA8 roughly equal or smaller than data csx Use similar process to CMS to estimate rapidity gap survival probability = POMWIG/Data R=0.4: GSP = 0.09±0.03, R=0.6: GSP = 0.21±0.08 7 R=0.4 R=0.6

8. Comparisons with Prague Birmingham internal note now available on CDS and on Soft QCD Twiki Also available on Diffractive Dijets SVN https://cds.cern.ch/record/1670320?ln=en Prague group adding parts to make combined note Unexpected differences in analysis Change to recon hybrid selection - pT clust > 150 MeV for |η clust | > 4.2 Switch from to and use of significance cuts Unfolding – number of iterations Scaling MC to fit data Reweighting Pythia8 Additional scaling by 1.1 for PU Will discuss technicalities in meeting tomorrow 8

9. Comparisons with Prague Currently comparing raw data distributions at recon level Despite differences in analyses, comparisons look promising Hopefully not too much work to work out what to change, why disagreements occur Cut flow comparison also important 9

10. Further plans 10 Complete comparison of Birmingham / Prague analysis Combine documentation from both groups in next week Targeting SM plenary on April 24 with aim for editorial board Currently have 1.5 months left of funding available so will need to push analysis along as quickly as possible