1. Update on Diffractive Dijet Production Search Hardeep Bansil University of Birmingham Soft QCD WG Meeting 29/04/2013

2. Diffractive dijets Single diffractive events (pp  pX) Rapidity gap from colourless 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 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 Tevatron have Gap Survival Probability of 0.1 relative to H1 predictions Khoze, Martin and Ryskin predict LHC to have GSP of ~ 0.03 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 2 Rescatter with p? ξ

3. Analysis Using L1Calo stream data10 period A and B ESDs (GRL applied) Herwig++ and Pythia 8 for SD & ND samples No rapidity gap destruction built in Pythia 8 DD samples Look at DD samples if possible to see if can put limit on SD + DD (M Y < 7 GeV) Event selection: GRL, PV0 with n tracks >4, No PU vtx (having n tracks >1) Anti-k t jets with R=0.6 or R=0.4: Require >= 2 jets in event passing medium jet cleaning cuts E T Jet1,2 |η| 30 GeV, E T Jet2 > 20 GeV Ask for a forward gap: |η start | = 4.9, Δη F ≥ 3.0 Biggest region in η from edge of detector absent of particles Defined w.r.t. tracks and noise suppressed EM clusters with p T >200 MeV Stable truth particles with p T >200 MeV 3

4. Basic Distributions With significance cuts, currently seeing: 793 events passing trigger & Ak6 selection cuts with Gap > 3 209 events passing trigger & Ak6 selection cuts with Gap > 4 Gaps in 2010 Period A&B go up to 6.5 at most, Monte Carlo goes up to 7.0 Will need to get cut flow to agree between Bham/Prague 4 L1Calo data Herwig++ SD Herwig++ ND Pythia 8 SD Pythia 8 ND Pythia 8 DD

5. Gap Correlations Comparison of raw events with Forward Truth Gap to Reconstructed Gap Correlation mostly good Get band of events with large truth gap but small reconstructed gap 5 Herwig++ SD+ND Gap Weighted Sample Pythia 8 SD+DD+ND Gap weighted Sample

6. Update on Noise Clusters 6 Difference from Soft Diffractive analysis in way treatment of clusters for gap algorithm Previously checked that clusters not affected by noise but currently no way to check this with D3PDs (until new moment added in) In mean time look at RNDM stream with EMPTY triggers to see if clusters pass significance cuts in empty events – look at 2010 Period B specifically for runs in GRL – as some Period A already studied GRL, pass LAr cut, pass EMPTY trigger – look at hadronically and electromagnetically calibrated clusters Modified code so only cells with positive significances contribute to cluster maps Eta-phi maps use 128 bins in phi, 196 bins in eta

7. Noise Clusters (EM) 7 Run 153565 – 843427 events, 101629 passing all cuts, 602 EM clusters above cuts Busiest run across Period B in terms of events and cluster activity in EMPTY events Across Period B noise levels are low 0.4% to 0.8% probability of a noise cluster per event (even lower than the target 1.4%) Noise bursts unlikely to be an issue Hadronic clusters have more events above cuts (TILE noise has double gaussian shape, leading to more noise above significance cuts) but still small probabilities EM cluster calibration favoured for analysis

8. Floating gaps Like forward gap algorithm but now look for largest gap across any part of the detector – particularly in DD events Floating gaps help to reduce number of very small gaps, effect only noticeable up to gap sizes of 3 Beyond this point, largest gap in event most likely to be forward Next thing to study in DD events is size of both Mx and My 8 Forward Gap Floating Gap Forward Gap Floating Gap Pythia 8 DD Gap Weighted Sample Pythia 8 DD Unweighted Sample

9. Gap Start on Intact Proton side Wanted to verify results from Vlasta’s talk about gap start (from forward gap algorithm on truth level) being on same side as intact proton in SD events Consistent results between gap weighted and unweighted samples Confirms behaviour Pythia 8 getting good agreement between gap side and intact proton whereas Herwig++ SD only matches half of the time Gap size is in relation to forward gap algorithm Test hadron level largest gap, how big Mx is in these events 9 Gap Size0.0-0.50.5-1.01.0-2.02.0-3.03.0-4.04.0-5.05.0-6.06.0-7.07.0-8.0 Pythia 864%83%92%97%98% 100% Herwig++53%57%56%55%54%53% 51%50% Actual gap p η=0 Actual gap p η=0 Observed Expected Unexpected

10. Reconstructing ξ Proper way to calculate ξ can be done in SD MC 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) Now 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+ 10 Largest rapidity gap between truth particles MYMY MXMX ~ ~~

11. Reconstructing ξ Pythia 8 comparison – Before & after gap & jet cuts In general correlation in log10 between Truth ξ and Recon ξ good Truth ξ not shown (needs fix) so not attempting calculation of C yet Interesting to note that after gap and jet cuts majority of events have log10(ξ) < 2.0 11 Truth ξ v Reconstructed ξ using E+Pz method After Cuts Truth ξ v Reconstructed ξ using E+Pz method Before Cuts ~ ~

12. Reconstructing ξ Herwig ++ comparison – Before & after gap & jet cuts In general correlation in log10 between Truth ξ and Recon ξ good Begin to observe additional structure in selection Interesting to note that after gap and jet cuts majority of events have log10(ξ) < 2.0 12 Truth ξ v Reconstructed ξ using E+Pz method After Cuts Truth ξ v Reconstructed ξ using E+Pz method Before Cuts ~

13. Next steps Update Xi calculation for E+/-pz in truth Investigate Mx and My distribution in DD events Sort out cut flows between Birmingham/Prague 13