Diffraction and Central Exclusive Production at ATLAS Marek Taševský Institute of Physics, Academy of Sciences, Prague On behalf of the ATLAS collaboration Diffraction 2010, Otranto, Italy - 12/09 2010 Diffraction Central Exclusive Production

ATLAS Central Detector

ATLAS Forward detectors 10.6 < | η | < 13.5 | η | > 8.3 5.6 < | η | < 5.9 2.1 < | η | < 3.8 Not yet fully installed

Diffraction at LHC: - Forward proton tagging in special runs with ALFA - Combined tag of proton in ALFA on one side and remnants of dissociated proton in LUCID on the other side - Central rapidity gap in EM/HAD calorimeters (|η|<3.2) and inner detector (|η|<2.5) - Rapidity gaps on both sides of IP: Double Pomeron Exchange: parton from Pomeron brings a fraction β out of ξ into the hard subprocess → Pomeron remnants spoil the gaps Central exclusive production: β = 1 → no Pomeron remnants

Diffractive measurements EARLY DATA → WITHOUT PROTON TAGGING L1 trigger: Rapidity gap (veto in MBTS/Calo/LUCID/ZDC) .AND. Low_Et Start with ratios X+gaps/X(incl.), X=W,Z,jj,μμ -> get information on S2 pp → RG + W/Z + RG Info on soft survival S2 (γ-exch. dominates for W) pp → RG + W Info on soft survival S2 pp → RG + jj + RG Combined effect of all basic ingredients to CEP (S2, Sudakov suppr., unintegr. fg, enhanced absorpt) pp → RG + Y + RG Info on unintegrated fg (γ- or Odderon exchange) Hard SD, Hard DD L1 trigger: ALFA (one side) .AND. MBTS/Calo/LUCId/ZDC (the other side) High rate soft diffraction: P-tagging = info on proton pT, i.e. dσ/dt ALFA: σtot, dσel/dt, σSD(low M), d2σSD/dtdξ, d2σDPE/dξ1dξ2 - tests model assumptions, - governs rates of Pile-up bg - Strongly restricts S2 (info on enhanced absorption), not sensitive to higher-order (Sudakov) effects pp → p + jj + p: Advantages: rel. high rate separate different effects in one process High rate γp and γγ processes WITH PROTON TAGGING BY ALFA

Soft SD measurement with ALFA Introduction – physics case Soft SD measurement with ALFA Soft SD can be measured during a special elastic calibration run provided that ALFA can be combined with LUCID/ZDC [ATLAS-COM-PHYS-2007-056] - measurement of cross section and t-, ξ-distributions Expect 1.2-1.8 M events in 100 hrs at 1027cm-2s-1 - SD cross section measurement with ~ 15 % syst. uncertainty - improve model predictions and background estimates for CEP Very good acceptance for very low t and ξ. Global acceptance: Pythia 45% Phojet 40% Soft SD trigger: ALFA.and.(LUCID. Or.ZDC) ATLAS RP RP LUCID LUCID RP RP ZDC ZDC IP RP RP LUCID ATLAS LUCID ZDC RP RP ZDC 240m 140m 17m 17m 140m 240m

Experimental challenge: define diffractive event First data → Soft diffraction No proton tagger → try rapidity gaps Generator level plots - provided by O. Kepka, P. Růžička, Prague Calorimeter method: 1) Divide Calorimeter into rings in rapidity. 2) If a ring has no cell with significance E/σ > X (σ of cell Gaussian noise dist) → consider this ring to be empty 3) Find largest continuous gaps of empty rings 4) Get SD, DD and ND contributions by fitting gap distribution in data using MC function Gap size Start of gap from calo edge (-4.8; 4.8)

Central Exclusive Production 1) Protons remain intact and can be detected in forward detectors 2) Rapidity gaps between leading protons and central system X X = jj, WW, Higgs, … = χb, χc, γγ See talk by Valery Khoze Advantages: I) Outgoing protons not detected in the main ATLAS detector. If installed, very forward proton detectors would give much better mass resolution than the central detector (see project AFP later) II) Central system produced in a JZ = 0, C-even, P-even state: - strong suppression of CEP gg→bb background (by (mb/MX)2) - produced central system is 0++ → just a few events are enough to determine Higgs quantum numbers. Standard searches need high stat. (φ-angle correlation of jets in VBF of Higgs) and coupling to Vector Bos. III) Access to main Higgs decay modes in one (CED) process: bb, WW, ττ : information about Yukawa coupling Hbb! Disadvantages: - Low signal x-section; affected by Pile-up Find a CEP resonance and you have measured its quantum numbers!!

CEP dijets with early data Central system produced in Jz =0, C-even, P-even state → quark jets suppressed by mq2/Mjj2 Trigger: Low-Et jet .AND. Veto in MBTS -eff. ~ 65% for CEP wrt jet turn-on; efficiently reduces Incl.QCD bg (by 104) Exclusivity cuts: 1) MBTS Veto corresponds to cutting on - reduces Incl. QCD bg (has large ξ, protons broken up) 2) rap.gap at least on one side – use rapgaps that are reproducible by theory! – see e.g. S.Marzani’s work 3) Ntrack (outside dijet) < X – reduces Incl. QCD bg 4) single vertex – reduces overlap (Pile-up) bg 5) Look for excess of events over predicted bg in Rjj distribution, , Other variables: steeper leading jet ET and more back-to-back leading jets in CEP due to ISR suppression BG: Incl. QCD dijets SD dijets DPE dijets Observed by CDF: Phys.Rev. D77 (2008) 052004 In good agreement with KMR but still big uncertainties Motivation: reduce the factor three of uncertainty in calculations of production x-section at LHC (KMR) Measure Rjj distribution and constrain existing models and unintegrated fg

Dijets in SD and DPE using rapidity gaps - Gap defined by LUCID/ZDC + FCAL - Look for hard scatter events with SD: gap on one side of detector; DPE: two gaps on each side of detector DIJET STUDY STRATEGY: 1) ET or η spectra of inclusive (ND) QCD dijets 2) Measure and from known (HERA) PDFs get info on FDjj (β,Q2) and S2. ξ<0.1 → 0(1) TeV Pomeron beams;  down to ~ 10-3 & Q2 ~104 GeV2 Strong factorization breaking compared to HERA DPDFs → S2 ~ 0.1 (usually explained by multiple interactions / absorptions) 4) σ(DPEjj)/σ(NDjj): vary gap size → Sudakov effects and enhanced absorption Advantages: - comparatively high rate σjjDPE(ET>20 GeV)~10nb - possibility to separate different effects by studying one process

ATLAS diffractive measurements 1) Diffractive enhanced MB events at √s = 7 TeV (ATLAS-CONF-2010-031) 2) Dijet production with a jet veto at √s = 7 TeV (ATLAS-CONF-2010-085) ATLAS philosophy for the early data: Do not extrapolate to full coverage with some MC model Do not correct data for diffractive/non-diffractive background First: understand well the detector and define the diffractive event See talk by A. Pilkington

Diffraction enhanced MB events 1) Veto activity in MBTS on one side of IP 2) Ntrk ≥ 1 (pT > 0.5 GeV, |η| < 2.5) Calculate RSS = NSS/(NSS+NDS); SS = single-sided, DS = double-sided Not corrected for detector effects Ratio σSD/σDD kept fixed to generator prediction RSS sensitive to relative diffractive X-section σdiff/σinel Rate quite well modeled by Pythia 6 and Pythia 8

Diffraction enhanced MB events 1) Veto activity in MBTS on one side of IP 2) Ntrk ≥ 1 (pT > 0.5 GeV, |η| < 2.5) Not corrected for detector effects Track properties nicely described by Phojet

Diffraction enhanced MB events Not corrected for detector effects In general: Phojet: SD>DD, Pythia6,Pythia8: SD~DD pT tails: Phojet, Pythia8: SD~DD~ND, Pythia6: only ND (missing hard diffraction)

Gaps between jets Inclusive sample:1) Triggers L1_J5 .or. L1_J10 .or. L1_J15 2) Boundary jets: ET > 30 GeV, (ET1 + ET2)/2>60 GeV → get number of events NINCL Gap events: 3) Inclusive sample + no jets with Q0>30 GeV between the jets in the dijet system → get number of events NGAP Selection of boundary jets: Selection A: 2 jets with highest ET Selection B: Most backward and most forward jet Not corrected for detector effects Wide-angle radiation BFKL-like dynamics

Gaps between jets Selection of boundary jets: Selection A: 2 jets with highest ET, Selection B: Most backward and most forward jet Everything here for Selection A). Very similar results for Selection B) Gap Fraction = NGAP / NINCL Corrected for detector effects - Gap Fraction decreases with ET and Δη - Well described by Pythia 6 Next steps: more data - enlarge Δη range - lower jet veto cut Q0

ATLAS Forward Proton Upgrade for High Lumi [FP420 R&D Collab., JINST4 (2009) T10001]

Physics with forward proton tagging at high lumi Photon-induced interactions Diffraction - Absolute lumi calibration, calibration of FDs - Factorization breaking in hard diffraction Hard SD/DPE (dijets, W/Z, …) Gap Survival / Underlying event High precision calibration for the Jet Energy Scale Central Exclusive Production of dijets: Evidence for CEP [arXiv:0908.2020] [Phys.Rev. D77 (2008) 052004] CDF: Observation of Exclusive Charmonium Prod. and γγ→μμ in pp collisions at 1.96 TeV [arXiv:0902.1271] Central Exclusive Production of Higgs - Higgs mass, quantum numbers, discovery in MSSM SM h→WW*, 140 < M < 180 GeV [EPJC 45 (2006) 401] MSSM h→bb, h→ττ, 90 < M < 140 GeV MSSM H→bb (90 < M < 300), H→ ττ (90 < M <160 GeV) [JHEP 0710:090,2008] NMSSM h→aa→ττττ for 90 < M < 110 GeV Triplet scenario [arXiv: 0901.3741]

Rich γp and γγ physics via forward proton tagging pp → p γ(*)γ(*) p →p X p, X = e+e-, μ+μ-, γγ, WW, ZZ, H, tt, SUSY-pairs Lepton pair production in γγ interaction: large and well-known QED x-section → use to calibrate absolute LHC lumi and Forward detectors (at 420m). (pT>10 GeV, |η|<2.5, one forward-proton tag: 50μ’s in a 12hrs run at 1033cm-2s-1) - Anomalous quartic coupling in pp → p γγ p →p WW p processes: greatly improved sensitivities compared to LEP results (factors 103 at L=1033cm-2s-1, 104 at L=1034cm-2s-1) Diffractive Photoproduction of jets: study the issue of QCD factorization breaking Exclusive Photoproduction of Υ: sensitive to the same skewed unintegrated fg as CEP of H (σ ~ 1.25pb for Υ→μμ) Photoproduction Final state topology similar to CEP: Rap.gap on the side of intact proton Diffraction [arXiv:0908.2020] See talk by Ch.Royon

Central Exclusive Production: Higgs “+” “=” Typical Higgs Production CEP Higgs pp  gg  H +x pp p+H+p Extra screening gluon conserves color, keeps proton intact (and reduces your σ) x-section predicted with uncertainty of 3 or more (KMR group, Cudell et al. Pasechnik, Szczurek) b,W,τ This process is the core of the physics case of Forward detector upgrade (AFP) 1) Protons remain intact and can be detected in forward detectors 2) Rapidity gaps between leading protons and Higgs decay products H gap p gap p b,W,τ

MSSM mass scan for CEP Higgs: 5σ-contours h→bb, mhmax, μ = 200 GeV H→bb, nomix, μ = 200 GeV Tevatron exclusion region LEP Exclusion region EPJC 53 (2008) 231: using proposed Forward detectors Experimental efficiencies taken from CERN/LHC 2006-039/G-124 Four luminosity scenarios (ATLAS+CMS): 1) 60 fb-1 – low lumi (no pile-up) 2) 60 fb-1 x 2 – low lumi (no pile-up) but improved signal efficiency 3) 600 fb-1 - high lumi (pile-up suppressed) 4) 600 fb-1 x2 – high lumi (pile-up suppressed) but improved signal efficiency SM: Higgs discovery challenging MSSM: 1) higher x-sections than in SM in certain scenarios and certain phase-space regions 2) the same BG as in SM

Summary And much more can be studied: - Two new diffractive analyses with first ATLAS data presented: 1) Studies of diffractive enhanced Minimum Bias events in ATLAS 2) Measurement of dijet production with a jet veto in pp collisions at 7 TeV using ATLAS det. And much more can be studied: Low Luminosity (up to L~1033cm-2s-1): - Elastic and σtot using ALFA - Start with ratios X+gaps/X(incl), X=W,Z,jj,μμ …. Get S2 - Soft Diffraction using ALFA - Dijets in SD, DPE and CEP - Photon-induced processes useful for checks of CEP predictions High Luminosity Upgrade (L > 1033cm-2s-1) Possible upgrade (AFP) to install forward proton taggers at 220 and 420 m from IP - Provides a good mass measurement of new physics - pp→p+(γγ→μμ)+p as excellent tool for absolute calibration of AFP420 Urgent: Definition of rapidity gap

B A C K U P S L I D E S

Diffractive W/Z production Test of soft survival S2 Test of absorption effects Quark content of Pomeron PDFs Valery, DIS08 Small spread of predictions Suitable to extracting S2 pp X+ RG+ W+ RG +Y  photon exchange dominates Trigger: rapgap.AND.high-pt lepton

Anomalous quartic coupling in γγ processes Low luminosity pp → p γγ p →p WW p High luminosity Just two leptons ξ in acc. of FD Ntrk ≤ 2 pTlep1 > 160 GeV, pTlep2>10 GeV MET>20 GeV pTlep2/pTlep1 < 0.9 Δφ(lep1,lep2) < 3.1 E. Chapon, O. Kepka, Ch. Royon: arXiv:0909.5237 [hep-ph] arXiv:0908.1061 [hep-ph] Improvement of 103 to LEP limits! Improvement of 104 to LEP limits! See talk by Ch.Royon