1. Diffractive Physics at the CERN Large Hadron Collider Jean-Pierre Revol CERN 6th International Conference on Physics and Astrophysics of Quark Gluon Plasma December 6-10, 2010 Goa, India

2. INTRODUCTION Diffraction is an old field [Leonardo Da Vinci (~1500); F. Grimaldi (1665)) that moved from electromagnetic waves to particle physics in the 1950’s [R. Serber; R.J. Glauber; E.L. Feinberg and I. Pomeranchuk (1956); M.L. Good & W.D. Walker (1960); Donnachie & Landshoff (1998); etc.]. Both elastic and inelastic diffraction processes are being studied at LHC. Why do we (all LHC experiments) care about diffraction at LHC? – “Because it’s there!”: ≥ 25% of σ inel – Diffraction can provide information on the proton structure and interaction mechanism – Access to non-perturbative QCD processes* – In practice, one cannot ignore diffraction when normalizing data to specific event classes! 2 * Even though there is also hard diffraction which may belong to the perturbative regime

3. Diffractive processes Cross-sections and Kinematic are the issues Indication that M fragments as inelastic system with √s = M 3 M E.G. Boos et al., Z. Phys. C Particles and Fields 17, (1983) √s = 22.4 GeV/c σ at 7 TeV relative to σ tot (QGSM)  26% 12.5% 6.5% ≤1% ≤0.1% Gap M1M1 M2M2 +UA4, Phys. Lett. B116 (1986) 459

4. pp elastic scattering Diffraction pattern analogous to Fraunhofer diffraction of light – exponential slope B at low |t| increases with √s – minimum (dip) moves to lower |t| as 1/σ tot – interaction region grows (as also seen from rising σ tot ) – depth of minimum changes (shape of proton profile changes) – depth of minimum differs between pp and pp different mix of processes? (Donnachie & Landshoff, introduced 3-gluon exchange  no dip in pp) 4 M.M. Islam et al. proton as “condensate enclosed chiral bag?” Small |t| Diffraction Intermediate|t| ω exchange High|t| quark scattering – M. Bozzo / LHCC/ Nov. 17, 2010 – Quark-antiquark condensate Shell of baryonic charge density Valence quarks (confined in a core)

5. Elastic diffraction at LHC Predictions exist for LHC as well as a very first measurement by TOTEM: (0.003 < |t| < 10 GeV 2 )] – later on TOTEM and ATLAS ALFA detector (0.0006 < |t| < 0.1 GeV 2 ) should add a lot more to this issue. 5 M.M. Islam, Jan Kaspar, R. J. Luddy, A. V. Prokudin Very Preliminary measurement from TOTEM Courtesy K. Eggert √s = 7 TeV Diffraction qq ω exchange –

6. The advent of the Pomeron V. Gribov introduced, within the Regge theory, a vacuum pole (Pomeron with α(0) = 1) in order to have a constant total cross section. When it was found (ISR) that the total cross section was increasing with √s, a supercritical Pomeron (α(0) > 1) was required. 6 Regge trajectory at t = 0 Exchange of colour singlet object  rapidity gap  Diffraction Ordinary Reggeons Colour singlet object with C=P=+1 K. Goulianos

7. Wihin Regge theory, very successful fit of total cross section by Donnachie – Landshoff with power law: ;  = α P (0) -1 = 0.08 Problem: this violates unitarity Froissard-Martin bound: Restauration of unitarity: Regge-poles (single Pomeron exchange) are not the only singularities of the amplitude (e.g. branch points corresponding to the exchange of several Reggeons)  Multipomeron exchange Total cross section 7 A.Donnachie and P.V. Landshoff Phys.Lett. B296 (1992) 227-232 K.A. Ter-Martirosyan, Sov. J. Nucl. Phys. 10 (1969) R = ordinary Reggeon or Pomeron  =0.12 But no high-M diffraction!

8. Modeling particle production Analogous to the optical theorem, Muller– Kancheli’s theorem relates the inclusive cross-section for the reaction: h 1 +h 2 →h 1 +X to the forward scattering amplitude of the three-body hadronic process: h 1 +h 2 +h 1 →h 1 +h 2 +h 1. 8 Example of single diffraction Elastic amplitude T = –i(S-I) + unitarity of S (SS + =I); σ tot =Im(T) – – interacting Reggeons SDDD High mass 

9. Interacting Pomerons Enhanced graphs result in Pomeron intercept ( Δ) renormalization 9 P ~ 0.2 ~ 0.01 GeV -2 Kaidalov et al., Sov. J. N.P. 44 (in high s regime) Exchange of any number of Pomeron Any number of Pomerons transformed into any number of Pomerons Δ 0 = intercept of bare Pomeron Δ = 0.2 with interacting Pomerons (all graphs) Δ = 0.12 (Graph I) Δ = 0.2 (Graph I) Higher orders In this approach, the scenario with interacting Pomerons deviates from the scenario without interacting Pomerons above LHC energy G PPP IIIIII Eikonal approximation Δ from fit to σ tot, σ el. and σ Diff. elastic Total P P P

10. Particle production at LHC 10 At high energy, simple Mueller-Kancheli graph should dominate Alice 0.9 & 2.36 TeV   = (α P -1) = 0.2 (instead of 0.12!).  Nature of Pomeron needs to be understood better! LHC clearly entered the realm of non-perturbative QCD and of Pomerons. 10 Martin Poghosyan Predicted correctly the CMS point at 7 TeV pp p p p + p  c+ X C C – – NSD INEL CMS ALICE

11. Modelling particle production The triple-Reggeon model is in good agreement with FNAL and ISR data for soft diffraction dissociation, but: – Higher-energy data from SPS and Tevatron do not show the fast increase of cross-section with energy expected from fits to lower energy data. At Tevatron energy, the predicted value for the single diffraction cross-section is larger than the measured total cross-section! – AND the Triple-Pomeron diagram violates unitarity! 11 SD DD  Needs higher orders again!

12. Modelling particle production After adding ordinary Reggeons (Dressed triple-Reggeon and loop diagrams), and a lot of hard work … (A. Kaidalov and M. Poghosyan), the result is amazing! 12 Alexei Kaidalov 1940–2010 “Immorally succesful model” A. De Rujula, Evian, 1992

13. Fit to existing cross section data Elastic scattering and total cross-section data 13 Details in: M. Poghosyan EDS Blois(2009)

14. Single diffraction dissociation Fermilab SD data: Schamberger et al., Phys. Rev. D17 14 Details in: M. Poghosyan EDS Blois(2009)

15. Single diffraction dissociation ISR data: Armitage et al. NP B194 15

16. Single diffraction dissociation 16 Details in: M. Poghosyan EDS Blois(2009) ISR data: Armitage et al. NP B194

17. Single diffraction dissociation Fermilab data at fixed t 17 Details in: M. Poghosyan EDS Blois(2009)

18. SD and DD cross sections Resulting predictions for integrated SD and DD cross-sections (not a fit!) 18 Details in: M. Poghosyan EDS Blois(2009) UA5 Tevatron SD: slow increase with √s DD: Little increase with √s √s (TeV) σ tot (mb) σ elastic (mb) σ inelastic (mb) σ* SD (mb) σ** DD (mb) 0.966.814.652.28.25.7 796.424.871.6126.2 1410829.578.5136.4 * SD: M 2 /s < 0.05 ** DD: Δη > 3

19. Particle production 19 Incorporate QGSM into model to fragment strings to describe particle production (Kaidalov & Poghosyan) UA1, UA4, UA5, and TEVATRON NSD data remarkably well reproduced (Not a fit!) Details in: M. Poghosyan EDS Blois(2009) UA4 √s = 540 GeV

20. SD and NSD well described by QGSM, but not Inelastic! 20 SD NSD Inelastic Experimental cross sec. Theoretical cross sec. Inelastic 1SD + NSD The UA5 puzzle

21. A practical issue for experiments NSD and INEL event classes defined to compare data between experiments. Corrections are largest contribution to systematic uncertainty in multiplicity measurements. 21 NSD INEL SD DD SD DD To reduce uncertainties (cross- sections and kinematics): measure SD and DD processes at LHC

22. ATLAS observation of diffraction Observing SD and DD, using Minimum Bias Trigger Scintillators (MBTS) selection: require activity only on one side of the MBTS (2.09 < |η| < 3.84 and at least one track p T ≥ 0.5 GeV/c in |η| < 2.5. No correction for detector effects 22 ATLAS Note ATLAS-CONF-2010-048 Discrepancies between generators imply a high systematic uncertainty ≥ 40%!

23. ATLAS observation of diffraction 23 Track distributions for the single-sided MBTS requirement PseudorapidityMultiplicity pTpT

24. ATLAS observation of diffraction Indirect rapidity gap study via η distribution study (with respect to edge of MBTS which has no hit) – Δη=|η MBTS -η track | – In all cases PYTHIA 6 not doing as well as PHOJET. PYTHIA 8 underpredicting at small Δη (sensitive to kinematics). 24 Δη

25. ATLAS observation of diffraction Even though PYTHIA 8 and PHOJET are globally similar, they differ in the contribution mix! 25

26. ATLAS future diffraction programme Diffractive measurements using future ALFA (4 Roman Pots on each side partially installed – complete in January) – Prime motivation is precise measurement of the luminosity (optical theorem) (t ~ 6x10 −4 GeV 2 ) interestingly small! – Contribution to diffraction in elastic scattering of protons 6.3 < E proton < 7 TeV SD for ξ < 0.01; ND proton for 0.01 < ξ < 0.1. Diffractive measurements at high luminosity – AFP project under discussion in the ATLAS Collaboration to detect protons with additional proton taggers at 220 mand 420 m from the interaction point. 3-D Si detector (10 μm) and TOF (5-10 ps) X = H  bb; H  ττ; triplet Higgs, SUSY CP‐X (i.e. CP viola Ang) Higgs, etc. 26 X H0H0

27. CMS diffractive di-jet candidate events 27

28. CMS observation of diffraction Enriching sample in SD events – Trigger with Beam Scintillator Counters (BSC) (|η|3.23-4.65): events with signal on one side only 28 Note! q

29. CMS observation of diffraction 29 cc √s = 0.9 TeV √s = 2.36 TeV √s = 0.9 TeV; E HF+ <8 GeV The presence of diffractive processes is needed to account for the data quantity related to the energy of the diffracting system

30. LHCb Diffractive-Enriched Samples Demanding no track in the backward planes of LHCb VELO detector enriches the MB data sample with SD and DD events: – pseudorapidity coverage with PID and momentum measurement 2 < η < 5 – momentum acceptance starting at p = 2 GeV/c – VELO angular coverage –4 < η < +1.5 and 1.5 < η < 5 “Future study of possible 'exclusive' onia events, which could occur through double Pomeron exchange (p+p  p+p+X) as well as central exclusive production” Perhaps, unique acceptance at LHC because of LHCb tracking performance in forward region! 30 Michael Schmelling, May 31st, 2010

31. ALICE Exclusive J/Psi candidates in the muon arm: – No pixel trigger; No V0A trigger (opposite to muon arm) – V0C trigger with ≤ 2 hits in the two inner rings (overlap with the muon spectrometer) and no hits in 2 outer rings (-2.7 < η <-1.7). – No FMD hits in 1.7< η < 5.0 and -2.4 < η <-1.7. Allow hits in -3.4 < η< -2.4 – No ZDC hit; No TPC tracks (if TPC is available in the run). 31 J/Ψ candidate recorded August,29 at 00:44:48. Mass (μ + μ – ) = 3.0968 GeV/c 2 ; p T (μ + μ – ) = 0.371 GeV/c ; y = -3.44 μ + : p T = 1.579 GeV/c μ – : p T = 1.444 GeV/c

32. Measuring SD and DD with ALICE Strategy in ALICE: measure gap distributions (SPD, V0 and FMD) over 8 units in η. Delicate because of secondaries. As in ATLAS and CMS one gets sensitivity to Diffraction, however PYHTIA and PHOJET differ (data in between the two MC). Next step, minimize sensitivity to details of generator. 32 Gaps width distribution in units of η ALICE Work in progress ALICE Work in progress ALICE Work in progress ALICE Work in progress ALICE Work in progress ALICE Work in progress

33. Single and double gap events ALICE is measuring properties of events with a gap selection (V0A, V0C gaps, no Gap, double gap) Trigger is logical OR of V0A, Pixel, V0C 33 On-going analysis, no trigger and detector corrections, Qualitative only Track multiplicity f2f2 f0f0 ρ K0K0 Enhancement of J PC = 1 ++ states as expected in central diffraction

34. ALICE upgrade Discussions in the collabortion of possible addition of counters to extend the pseudorapidity coverage 34 55m 18m 20m V0A V0C ADD1 ADD2 ADA1 ADA2 G. Herrera Corral – Diffraction 2010 ADA 5.5 ≤ η ≤ 7.5 ADD - 7.5 ≤ η ≤ - 5.5 Would double the pseudorapidity coverage

35. Monte Carlo Kinematics PYTHIA6 and PHOJET 35 Sparsh Navin Pythia8 SD PYTHIA 8 globally similar to PHOJET, but not in details (%)SD/InelDD/Inel PYTHIA 6 19.113.0 PHOJET14.05.1 QGSM16.88.6 pseudorapidity pTpT pTpT pTpT

36. Example of Single Diffraction Experiments measure only part of the diffracted mass range 36 Coherence: De Broglie wave length of exchanged particle ≥ proton size M 2 /s ≤ 0.05 (soft diffraction) (above = Hard diffraction?) Kinematic limit M = m p + m π Example: UA5 at √s = 900 GeV σ SD = 7.8 ± 1.2 mb (UA5); Measured for M > 2.5 GeV, and extrapolated down to M = m p + m π UA5 experimental range ( 7.7x10 -6 < M 2 /s < 0.05) PYTHIA and PHOJET generate SD in different kinematic ranges: mp 2 /s < M 2 /s < 0.4 (PYTHIA-6) mp 2 /s < M 2 /s < 0.5 (PHOJET) Coherence limit (soft Diffraction) Normalize MC generators to proper experimental Mass range! dN/dξ ξ Model extrapolation

37. Conclusion and outlook Diffraction is a challenging phenomenon, including many processes. It is a significant part of LHC physics. It has been observed qualitatively at LHC by ATLAS, CMS and ALICE. The next challenge is to measure both cross sections and kinematic properties of diffractive processes at LHC energies. This will result in much higher precision in measurements of global event properties. Will have to be done without measuring the outgoing proton nor the diffracted system until ATLAS (with ALFA detector) and CMS/TOTEM do it. LHC will contribute to a better understanding the non- perturbative domain of QCD (bulk of particle production and diffraction). 37

38. Cross section predictions at LHC QGSM predictions 38 √s (TeV) σ tot (mb) σ elastic (mb) σ inelastic (mb) σ* SD (mb) σ** DD (mb) 0.966.814.652.28.25.7 796.424.871.6126.2 1410829.578.5136.4 * SD: M 2 /s < 0.05 ** DD: Δη > 3 26.6% 25.4% 24.7% √s = 7 TeVσ inelastic (mb) σ SD (mb) σ DD (mb) (SD+DD)/I NEL (%) PYTHIA71.513.79.332.2 PHOJET76.210.73.919.2 QGSM71.6126.225.4 (SD+DD)/INEL (%) How can one make sense of such comparison if definitions are different? Details in: M. Poghosyan EDS Blois(2009)

39. SD PYTHIA vs PHOJET Different kinematic ranges (√s = 900 GeV) 39 Marek Bombara

40. Comparison with data 40 Multiplicity distributions: UA5 NSD data Details in: M. Poghosyan EDS Blois(2009)

41. Single diffraction dissociation 41 Details in: M. Poghosyan EDS Blois(2009)

42. Single diffraction dissociation 42 Details in: M. Poghosyan EDS Blois(2009)

43. Heavy Ion diffraction? Observed in pA  pX by Helios (T. Akesson et al., Z. Phys. C. Particles and Fields 49, 355-366 (991) 43 Diffraction in heavy ion collisions at LHC is a challenge as there is a huge background from electromagnetic interactions. One would have to find a strong motivation