Update on Diffractive Dijets Search Hardeep Bansil University of Birmingham SM Soft QCD meeting 12/12/2011

Diffractive dijets Hard diffraction – Look for single diffractive events (pp  pX) involving a large rapidity gap from pomeron – Then look for dijet system within X Studied at HERA and Tevatron – At Tevatron, ratio of yields of SD to inclusive dijets ≈ 1% – Measure ratio at LHC (expected to be smaller) Gap Survival Probability – The chance of the gap between the intact proton and diffractive system being lost due to scattering (affects measured structure function) – At Tevatron GSF around 10 compared to H1 – Measure this at LHC (Khoze, Martin and Ryskin predict GSF around 30) Study jet and gap properties 2 X (ξ)(ξ) Gap destruction by secondary scattering Rescatter with p?

Gap Finding Algorithm Found out that gap finding algorithm is not fully up to date Detector gap definition is currently missing pT cut on clusters associated with cell 3 Divide calorimeter into 49 rings of 0.2 in η Detector gap definition Calorimeter: no cell above threshold E/σ > S th - probability of noisy cell in ring smaller than (electronic noise only, no pile-up environment) Tracker: no good track above pT > 200 MeV, |η| < 2.5 Truth gap definition No stable particle above pT > 200 MeV Looked at Rapidity Gaps paper for comparison of algorithms – Roughly a 10% difference at small gaps (Δη F < 2.0) and then 5% difference beyond this so will correct for this soon

Analysis Using Athena version AtlasProduction Using MinBias stream data10 period A and B ESDs – Run (period A) excluded due to noise bursts in TILE – Total ∫L dt = 8.71 nb -1 - calculated using online iLumiCalc tool with L1_MBTS_2 ref. trigger Average for selected runs < 0.15  currently ignore pile-up Anti-Kt jets with R=0.6 or R=0.4: – Require >= 2 jets in event – E T Jet1,2 |η| < 4.5 – E T Jet1 > 26 GeV, E T Jet2 > 20 GeV for asymmetric jet E T cuts (NLO), cut values suggested based on work by Radek Zlebcik (Prague) – Jet E T Jet2 limit and η cuts based on jet energy scale systematic – Currently no requirements to ask about jet quality cuts Ask for a forward gap: |η start | = 4.9, Δη F ≥ 2.0 4

Monte Carlo for Analysis Currently using POMWIG LO generator as sample of single diffractive jets – Modifies HERWIG ep photoproduction so e  e+γ becomes p  p+IP – No rapidity gap destruction built in – Generates QCD 2  2 process within diffractive system in different p T ranges (8-17, 17-35, 35-70, 70+ GeV) for SD (system dissociating in ±z direction) – Using MC samples generated by myself (4000 events of each POMWIG sample) Have PYTHIA 6 and PYTHIA 8 Jets samples to use as inclusive jets sample (8-17, 17-35, 35-70, GeV) – PYTHIA 8 J0 sample (8-17 GeV) available only with pile up on Grid – Generated a small sample for temporary use (5000 events) NLO comparison would be ideal – Need someone to work on this (Radek from Prague not working on this anymore) 5 Do Pythia jets samples contain any models of diffraction within them?

Big factor between PYTHIA and data in the first bin – Still have it after many checks (see next slide) – Ratio of MC to Data suggests a GSF of in majority of bins but need to ensure normalisation is correct first Uncorrected Gap Size Distribution 6 MinBias Data Pomwig SD Pythia 8 Jets Pythia 6 Jets POMWIG SD, PYTHIA 6 & 8 Jets weighted relative to luminosity of data runs used and then plotted against MinBias Data AntiKt6 jets, E T Jet 2 > 20 GeV After Forward Gap Cuts

Make MC samples compatible with data - scale them to luminosity of data by applying weighting factor to variables (MC run dependent) L data – luminosity of data N gen – Number of events generated σ gen – Csx of events generated N rec – Number of events reconstructed to run over N gen, N rec,σ gen all taken from AMI, checked multiple times but still get big factor between data and MC Truth level cross sections plotted – Show smooth transitions between different pT range samples – Current approach appears correct? Need to understand and fix this issue – Could it be due to running on this specific data set? – May require checking code line by line! Normalisation Issues 7 AntiKt6 Truth Jets, No Jet E T Cuts, Before Forward Gap Cuts

Tips from CMS Diffractive W/Z search CMS have had difficulty in trying to find diffractive W/Z signals – η = 5 - ∆η F (similar to forward gap) – 300 of W/Z events have gap – Pythia 6/8 tunes (ND) plotted v data – No clear signal above ND – Something to try with dijets? More luck in studying asymmetry of W being in the same hemisphere as the gap – POMPYT (diffractive) incl. with ND – Determined that 50 ± 10% of events with a forward gap > 1.9 are diffractive – Make jet signed η distributions CMS possibly presenting results on diffractive dijets at EDS 2011 – Should be similar to diffractive W/Z 8 ~

Tips from CMS Diffractive W/Z search 9 Signed η distributions Cover a much larger range in η than W/Z search All histograms have similar shape at the moment – Expect Pythia 6/8 Jets to be flatter like CMS?

Noise Study in “Empty” Events Preliminary look at potential effects of noisy cells destroying gaps in empty events by using RNDM stream Define event as empty by having no reconstructed primary vertex (with 5+ associated tracks) + no MBTS counters fired See that not all events have a full gap (∆η F =9.8) – 47556/ events – Less than 1% of events in stream Looking at cells with high energy significance suggests slightly more activity at +η and φ=-π – Which events pass “Empty” cuts but still have activity? – Is there a new noise burst to consider in these runs? Change GRL? – Slightly different point but will OTX problems need to be accounted for? 10 Significant Cells η Significant Cells φ

Ratio of SD to ND Dijets Preliminary study to measure the ratio of the single diffractive to inclusive dijet events based on events passing jet cuts as well as gap cuts Only done on MinBias stream (RNDM stream has very low statistics in comparison) On current results, ratio around 0. 2% Hard to make direct comparison with Tevatron as cuts are not similar + corrections not applied to my results yet CMS W/Z ratio (on earlier slide) around 0.75% 11 MinBias stream Data : 2010 Period A&B ( events total) AntiKt4 Jets (ND)89004AntiKt6 Jets (ND) AntiKt4 Jets + ∆η F >2.0 (SD)173AntiKt6 Jets + ∆η F >2.0 (SD)317 AntiKt4 SD/ND ratio (%)0.194AntiKt6 SD/ND ratio (%)0.165

Comparison with Prague Group in Prague also looking at diffractive dijets Earlier in analysis but already see difference in strategies Complementary strategies Vertex requirement would force ourselves to smaller gap sizes (less likely to see forward jets) Prague agree very well with inclusive jets analysis for 2010 – How do I test this for only Periods A and B? 12 BirminghamPrague Data used & StreamMinBias 2010 A&BL1Calo/JetTauEtmiss - all 2010 VertexNo requirement1(+?) vtx (5+ tracks) TriggerMBTS_2MBTS + Jet Pt dependent Preferred SD MCPomwig SDHerwig++ SD Preferred ND MCPythia 8Herwig++ ND

Comparison with Prague Herwig++ is C++ version of Pomwig but has known “feature”: factor of 3 increase in cross section Issue with compatibility of hadronisation models between Pomwig (cluster model) and Pythia (string model) Herwig++ ND produces events with very large gaps (truth level study) – Unexpected? But we want our samples to go out further in gap size anyway? Will compare HERWIG++ & POMWIG SD samples soon – Generated EVNT samples, trying to implement Fastjet to make direct comparisons Will need to get official Monte Carlo production done soon – Need to settle on stats required / new filter to simulate larger gaps in events with dijets / move from ESD to dAOD with additional cluster info 13 BirminghamPrague Preferred SD MCPomwig SDHerwig++ SD Preferred ND MCPythia 8Herwig++ ND Marek’s plots comparing ND Pythia and Herwig++ Jx samples

Next steps Main aim: Cross section as a function of gap size – similar to the soft diffraction paper, but with a jet requirement – Interpretation of GSF can come later Correct gap algorithm Fix normalisation issues Compare different generators at EVNT level with Fastjet added in Decide on appropriate statistics with Prague group 14