1. Mike Albrow Diffraction in High Energy Collisions CERN June09 1 Diffraction in High Energy Collisions Mike Albrow (Fermilab) FNAL-CERN School June 2009

2. Mike Albrow Diffraction in High Energy Collisions CERN June09 2 Diffraction in High Energy Collisions (a.k.a Vacuum Exchange, Vacuum Excitation) Mike Albrow, Fermilab Landscape: from elastic scattering to p + H + p Basics: Elastic and Total x-section, diffractive excitation Double Pomeron Exchange = Vacuum Excitation Central Exclusive Production at Tevatron Central Exclusive Production at LHC :

3. Mike Albrow Diffraction in High Energy Collisions CERN June09 3 From elastic scattering to exclusive Higgs boson production H p ppp p p p p = gluon But these are related processes! We will get to that. About 25% of σ(total) About of σ(total) Standard Model: Fundamental matter particles are fermions: Spin ½ leptons and quarks u,d,s,c,b,t and force particles are bosons: Spin 1 γ, g, W, Z (& spin 0 H) (In principle also Spin 2 graviton, but negligible so far, …) Exchanges Elastic: Q = 0, colour = 0, J >=1 @ H.E.

4. Mike Albrow Diffraction in High Energy Collisions CERN June09 4 In ancient times, pre-QCD (1960’s), Theory of strong interactions being developed: “Regge Theory” Pre-quarks, pre-gluons, pre-deep inelastic scattering. Based on scattering amplitudes “S” (Square  cross sections) S(s,t) S(s,t) required to be: Analytic (no singularities) Unitary (no probabilities > 1) Crossing symmetric (s t) pn s t t-channel exchange dominated by virtual: All good things! This still gives the best description of low-E reactions e.g. But: Effective angular momentum / spin of exchange α(t) and complex QCD became dominant, Regge theory almost left behind.

5. Mike Albrow Diffraction in High Energy Collisions CERN June09 5 Total cross section Elastic scattering Diffractive excitation Much of strong interaction physics is here. Not (yet) described by/understood in QCD When Q^2 small α(Q^2) large, “non-perturbative” COMPETE Collbn fits LHC Expect σT ~ 110 nb How to measure? Total rate  inelastic Need Luminosity Elastic scattering Dedicated expt: TOTEM Strong coupling not Regge spin!

6. Mike Albrow Diffraction in High Energy Collisions CERN June09 6 i j = i jj i 2 = α(t=0) α(t) ii jj j j i i 2 Total cross section and elastic scattering closely related: Elastic scattering described by amplitude: Total cross section by Imaginary part of forward scattering amplitude. Regge theory: Optical theorem Couplings at vertices and propagator (~ Feynman diagrams) k k

7. Mike Albrow Diffraction in High Energy Collisions CERN June09 7 Relation between σT and dσ/dt exploited at LHC by TOTEM CMS Measure small angle elastic and total rate simultaneously: Can derive both σT and machine luminosity!  Calibrate luminosity monitors. Coulomb scattering: known cross section but extremely hard at LHC. Another QED process may be better for luminosity calibration: (later)

8. Mike Albrow Diffraction in High Energy Collisions CERN June09 8 Total and elastic cross sections: fall then rise (universal) ln s R P R(t), P(t) R (Reggeon) = sum of all allowed meson exchanges P (Pomeron) = (?) sum of all allowed non-meson (gg etc?) exchanges. Glueballs s t

9. Mike Albrow Diffraction in High Energy Collisions CERN June09 9 If α(0) (~ J) > 1 total and elastic cross sections rise with s < 1 fall At low energy ρ-exchange dominates, but At high energy (rising cross sections) another strongly interacting exchange with α(0) > 1 (1+ε) dominates. POMERON IP longitudinal rapidity y p p p p Other exchanges with J >= 1: Photon γ and Z (between q, p would break up) IP

10. Mike Albrow Diffraction in High Energy Collisions CERN June09 10 y For small values speed and rapidity y are identical Rapidity Rapidity Gap Relativistic speed addition Hyperbolic tanh addition

11. Mike Albrow Diffraction in High Energy Collisions CERN June09 11 pp elastic scattering at CERN ISR. Note “diffraction pattern” Small t, exponential, ~ 1/”size of p” Large t ~ << 1 fermi, structure VERY small t, γ-exchange Large distance ( several fm) Coulomb beats strong Coulomb scattering Coulomb cross section is ~ known (QED) … so Lumi calibrator

12. Mike Albrow Diffraction in High Energy Collisions CERN June09 12 At LHC large angle elastic scattering uncertain! Fig. from TOTEM (Latino) p p g g g 0.1 fm Proton structure intermediate between “point-like” and parton (q+g) beam. If 2 q scattered through “large” angle, 3 rd must also or p breaks up. A handle on IP in q/g terms.

13. Mike Albrow Diffraction in High Energy Collisions CERN June09 13 Rapidity gaps, with (almost) no Regge theory The mother of all rapidity gaps Δy …. Elastic scattering: Gap = no particles: = 24.4 LEP e+e- = 8.4 ISR pp = 15.3 TeV pp = 18.6 LHC pp t Rap-gap cross sections go like: Thus: over large gaps exchange has J >= 1, Q = 0, color singlet. Only 2 possibilities: Photon γ (in ee and pp and ep) Gluon (ep and pp) … ? color !!? Cancel it with >= 1 other gluon. Call it pomeron (gg) α(0)=1+ε Pomeron has C = +1 (2 gluons OK). C = -1 OK with (ggg) : odderon Z obeys the above rules, OK in e+e- but p inevitably break up.

14. Mike Albrow Diffraction in High Energy Collisions CERN June09 14 Large ( >~4) rapidity gaps only possible by (t) exchange of 4-momentum with: No color or charge, and effective spin at t ~ 0 >= 1. J = 1, α(0) >=1. But (a) we have such large gaps in strong interactions (b) QCD is THE theory of strong interactions. Unlike QED, there is no elementary (q,g) object with these properties. (c) In QCD, with Regge theory to describe exchanges of states in the t-channel, only >= 2 g exchange can work. gg (C = +1)  Pomeranchukon  Pomeron IP ggg (C = -1)  Odderon O (not yet detected. α < 1 ?) Isaak Pomeranchuk 1913 - 1966 Tullio Regge

15. Mike Albrow Diffraction in High Energy Collisions CERN June09 15 Single Diffractive Dissociation : at low energies At ISR energies 7.4  22-63 GeV found scaling high x_F peak. Diffractive excitation of high mass “states” M ts

16. Mike Albrow Diffraction in High Energy Collisions CERN June09 16 PS 7.4 --- 1.6 resonances ISR 63 --- 14 Tevatron 1960 --- 430 Jets/W/Z LHC1 10,000 --- 2200 top ?? Diffractive excitation range (“rule of thumb”) (GeV) p IP p p-IP total cross section optical theorem : p-IP elastic scattering IP.. at high M IP

17. Mike Albrow Diffraction in High Energy Collisions CERN June09 17 Ingelman-Schlein description of high mass diffraction SDE = Single Diffractive Excitation Suppose IP has constituent (q,g) structure F(Q2,β=x) Go into frame of X, and look for jets. Kinematics tells parton momentum fractions β. Jets in SDE observed at CERN SppS Collider (not v. high ET) ET > 20 GeV jets in CDF, D0 at Tevatron. X J J p IP

18. Mike Albrow Diffraction in High Energy Collisions CERN June09 18 Diffractive Di-Jets and Diffractive Structure Functions Seen in Roman Pots Curves normalized here At this Luminosity, pile-up dominates pJJ trigger. Require (effectively rap-gap on pbar side) SDE of hard states, eg t-tbar requires no pile-up to associate fwd p with t-tbar event.

19. Mike Albrow Diffraction in High Energy Collisions CERN June09 19 Single Diffractive Di-Jets p-JJ cont. 4-vectors t-distribution ~ independent of t Ratio of diffractive : non-diffractive structure functions

20. Mike Albrow Diffraction in High Energy Collisions CERN June09 20 HERA (ep) Deep Inelastic Scattering & Diffractive DIS The normal structure function conditional on leading proton (or gap) Defined independently of notion of the exchange (“pomeron”) Measured in detail by H1 and ZEUS Interpretable as measuring the structure of the pomeron CDF: measured with jets Rapidity gaps suppressed in pp compared with ep. Gaps don’t survive additional interactions. “Rapidity Gap Survival Probability”

21. Mike Albrow Diffraction in High Energy Collisions CERN June09 21 Diffractive (SDE) production of W & Z (CDF) About 1% of all W & Z at Tevatron produced diffractively  q / qbar in IP at Q^2 ~ M(W)^2 W p stays intact 0.03 < xi < 0.10 |t| < 1 GeV2

22. Mike Albrow Diffraction in High Energy Collisions CERN June09 22 Diffractive production of Z (SDE) at Tevatron Diffractively produced Z have small pT and y opposite p

23. Mike Albrow Diffraction in High Energy Collisions CERN June09 23 Rapidity Gaps Between Jets : JGJ y JJ Predicted by Bjorken. Observed by D0 and CDF in Run 1 (1995) Charged track multiplicity in central 2.5 – 4.0 units of rapidity between jets ~ 65 GeV. 0.85% +/- ~ 0.24% have gaps. [D0 < 1.1% @ 95% CL] Is high-Q2 q/g scatter by color-singlet? BFKL Pomeron (mostly)? J J J J...or by single g, with soft color exchange across event? GAP Early study at LHC?

24. Mike Albrow Diffraction in High Energy Collisions CERN June09 24 BFKL and Mueller-Navelet Jets Color singlet (IP) exchange between quarks Enhancement over 1g exchange – multiRegge gluon ladder Jets with large y separation …Δy >~ 5 n minijets in between (inelastic case) Large gap in between (elastic case) Fundamental empirical probe of new regime: non-perturbative QCD at short distances. q q gg

25. Mike Albrow Diffraction in High Energy Collisions CERN June09 25 Central Diffractive Production or Double Pomeron Exchange Remember: pp pp IP Double-triple-Regge or Quintuple Regge: Must happen & rate calculable in RT IP + IP  IP + IP elastic Optical theorem  IP + IP  X total cross section p p X (better > 4) rapidity gaps = no hadrons Full range: At LHC(10) Δy(p-p) = 18.5. 18.5-6=12.5

26. Mike Albrow Diffraction in High Energy Collisions CERN June09 26 Vacuum Excitation (Lab frame) p p p p p pp G J J 1) 2) 3A) 3B) 1536 TeV vacuum Soft recoil, no excitation, no forward pion production,...

27. Mike Albrow Diffraction in High Energy Collisions CERN June09 27 Mass range of central diffraction (DPE) scales like √s, ~ 0.05 √s At ISR, 3 GeV / 63 GeV … resonances, glueball search At Tevatron, 100 GeV / 1960 GeV … jets At LHC, 500 GeV/10TeV  700 GeV/14 TeV … WW, ZZ, H …

28. Mike Albrow Diffraction in High Energy Collisions CERN June09 28 Central Exclusive Production (AFS at ISR) Structures not well understood beyond f(980). Not studied at higher All σ cut by f0? G(1500)?? Coherent scattering α stay intact. In LHC Au + Au (e.g. ALICE): can be IP or photon (more likely) Au Au may also fragment

29. Mike Albrow Diffraction in High Energy Collisions CERN June09 29 Central diffractive production or Double IP Exchange at Tevatron M(max) ~ 100 GeV : Jets measured in Roman pots Rapidity gap >~ 4 units Q: Is IP just a soft mush of q and g, or sometimes leading g + colour bleaching? Di-Jet mass fraction needs a “hard” component, with IP ~ 1 gluon + soft g/gg/…

30. Mike Albrow Diffraction in High Energy Collisions CERN June09 30 Component of pomeron that is leading gluon  g + g  J + J g + g  g + g  g + g  H c-loop b-loop t-loop p + p  p + H + p @ LHC & nothing else produced! p + H + p should happen at a detectable rate: Measure p very precisely  mass of central state (e.g. H) σ ~ 2 GeV Central state must have C = +1, P = +1, Even spin (0, 2 distinguishable) Width can be measured if > ~ 3 GeV. Close states (e.g. h, H in SUSY) can be separated. Need precision measurements of protons, together with central H-like event. Even g + g  γ + γ 50%

31. Mike Albrow Diffraction in High Energy Collisions CERN June09 31 Central Exclusive Production pp  p + X + p where X is a simple system completely measured pp At CERN ISR Glueball Search At Tevatron & LHC through q-loops (box) + color bleaching (g) W W

32. Mike Albrow Diffraction in High Energy Collisions CERN June09 32 Exclusive Di-Jets “Almost” exclusive di-jet, Two jets and nothing else JET Observed in CDF, QCD tests & related to p+H+p Interesting QCD: gap survival, Sudakov factor Nearly all jets should be gg …. qq suppressed by M(q)/M(JJ) (Jz=0 rule) Gluon jet physics. J J GAP

33. Mike Albrow Diffraction in High Energy Collisions CERN June09 33 c J/ψ γ μ+μ+ μ-μ- & nothing else in all CDF -7.4 < |η| < + 7.4 Added to CDF: Beam Shower Counters BSC: Scintillator paddles tightly wrapped around beam pipes. Detect showers produced in beam pipes if p or p dissociate. e.g. 8 + 10 counters If these are all empty, p and p did not dissociate (or BSC inefficient, could estimate from data) but went down beam pipe with small (<~ 1 GeV/c) transverse momentum. CDF central BSC (size greatly exaggerated!) - 50 m CDF measured exclusive  c  J/ψ +  → μ+μ- 

34. Mike Albrow Diffraction in High Energy Collisions CERN June09 34 402 events

35. Mike Albrow Diffraction in High Energy Collisions CERN June09 35 Now allow photons: EmEt spectrum with J/ψ mass cut: Empirical functional form MC also estimates only few % of under the cut 65 events above 80 MeV cut. 3 events below (estimated from fit)  4% background under J/psi  # = 65 +/- 8

36. Mike Albrow Diffraction in High Energy Collisions CERN June09 36 Dimuons: Upsilon Region Invariant Mass 0 associated tracks pT(μμ) < 1.5 GeV/c CDF Run II Preliminary Trigger: μ+μ- |η| 4 GeV/c Inclusive Search for/measurement of photoproduction of Y, Y’ (not before seen in hadron-hadron) Status: Candidates: analysis in progress. QED continuum check Y : cf HERA (we resolve states) Can we see ? Y(1S) Y(3S) Y(2S)

37. Mike Albrow Diffraction in High Energy Collisions CERN June09 37 Categories of Diffraction at LHC Can have a 4-unit gap with 2 TeV SD Can have (VF)J – G6 – (VF)J Can have 2 x 3-unit gaps with 700 GeV DP Can have p-G3-X-G3-X-G3-p with M(X) ~ 12 GeV

38. Mike Albrow Diffraction in High Energy Collisions CERN June09 38 Exciting the vacuum with photons γ Doesn’t work! E-p conservation forbids it; except for v. short times (evanescent) γ γ Does work! E-p conservation allows it; Energy injection promotes loop to reality. Heirarchy: γ γ How do e,μ,τ,q… know what mass to have?

39. Mike Albrow Diffraction in High Energy Collisions CERN June09 39 Photon “beams” radiated from electrons and protons LEP etc: e+e- (~ background free) HERA: e p (more background, little done) pp/ ppbar: Very high b/g … Seen in CDF γ e,p Phys.Rev.Lett 98,112001(2007) PRL 102, 242001 (2009) PRL May 2009 Tevatron as a  collider!

40. Mike Albrow Diffraction in High Energy Collisions CERN June09 40 E not ET! CLC BSC M(ee) = 49.3 GeV/c2 |Δφ-π| = 6 mrad = 0.34 deg, pT(ee) = 210 MeV M reach Tevatron >~ HERA, LEP ! M reach LHC  ~ 300 GeV –ish Includes γγ → WW (~ 50 fb) High mass γγ→e+e- event in CDF

41. Mike Albrow Diffraction in High Energy Collisions CERN June09 41 All our measurements agree with QED: So what? 1)It shows we know how to select rare exclusive events in hadron-hadron environment 2)No other h-h cross section is so well known theoretically except Coulomb elastic (inaccessible). Probably best possible Luminosity calibration at LHC e.g. 3)Outgoing p-momenta extremely well-known (limited by beam spread). Calibrate forward proton spectrometers. 4)Practice for other γγ collisions at LHC: Luminosity calibration at LHC

42. Mike Albrow Diffraction in High Energy Collisions CERN June09 42 Khoze, Martin and Ryskin, hep-ph/0111078, Eur.Phys.J. C23: 311 (2002) KMR+Stirling hep-ph/0409037 36 fb Exclusive 2-Photon Production Tevatron Claim factor ~ 3 uncertainty ; Correlated to p+H+p H Phys.Rev.Lett. 99,242002 (2007) 3 candidates, 2 golden

43. Mike Albrow Diffraction in High Energy Collisions CERN June09 43 Exclusive Z production : CDF Search Allowed in SM (like V) but ~ 0.3 fb (Motyka+Watt) Could be enhanced by BSM loops 2.2/fb : 318K M > 40 GeV; 183K in Z window 82-98 GeV Require no other interaction, no additional tracks, all calorimeters in noise (E) 8 with BSC empty Interesting?!  -IP-Z eff.coupling. ZOOM IN to see how! ~ record E(  )

44. Mike Albrow Diffraction in High Energy Collisions CERN June09 44 Central Exclusive Production of Higgs ? Higgs has vacuum quantum numbers, vacuum has Higgs field. So pp  p+H+p is possible in principle. Allowed states: Process is gg  H through t-loop as usual with another g-exchange to cancel color and even leave p’s in ground state. If we measure p’s (4-vectors): H ! Aim: to be limited by incoming beam momentum spread Realistic; ~ 2 GeV J >= 2 strongly suppressed at small |t| t

45. Mike Albrow Diffraction in High Energy Collisions CERN June09 45 Features of pp  p + H + p at LHC 1) S:B can be high, > 1, even for 2) Mass resolution ~ 2 GeV/event, for any final state 3) Quantum numbers determined, e.g. 4) If width >~ 3 GeV, directly measure width. 5) In MSSM can have close h,A,H. Then A excluded, h-H resolved. 6) If

46. Mike Albrow Diffraction in High Energy Collisions CERN June09 46 Central Exclusive H Production gg fusion: main channel for H production. Another g-exchange can cancel color, even leave p intact. p p  p + H + p Theoretical uncertainties in cross section, involving skewed gluon distributions, gluon k_T, gluon radiation, Sudakov ff etc. Nothing else on emu vertex! Price ~ 1/2000 – 1/10000 σ (excl) ~ 1 – 10 fb cf ~ 20 pb

47. Mike Albrow Diffraction in High Energy Collisions CERN June09 47 FP420 : Forward Protons 420m & 240m from CMS & ATLAS CMS CMS: Inner Vacuum Tank insertion 420 & 240m240 & 420m ||| ATLAS

48. Mike Albrow Diffraction in High Energy Collisions CERN June09 48 ~ 8 m p BEAM BPM QUARTIC ~ 8 layers 10um x-y pixels 3 mm Resolution Rad hardness Edgelessness Speed, S/N Availability Enthusiasts! 6mm(y) x 24mm (x) covers distribution GASTOF MCP QUARTIC FP420 = Forward Protons 420m from x … also 240m under study Best ever spectrometers! 420 m vacuum, 120m 8T dipoles, ~10μm origin (x,y), 1 μrad track 3D- silicon ~ 8 μm over 8m. Fast timing P-U reduction factor ~ 25 Normal low-β, design for 10^34 Detector area 6mm x 24 mm 420m too far for L1 trigger latency. 240m not, but > M. inefficient, other channels OK

49. Mike Albrow Diffraction in High Energy Collisions CERN June09 49 What is exclusive H cross section? Calculation involves: gg  H (perturbative, standard, NLO) Unintegrated gluon densities Prob.(no other parton interaction) (“Gap survival”) Proton form factor Prob.(no gluon radiation  no hadrons) Sudakov Suppression Durham Gp: Khoze, Martin, Ryskin, Stirling hep-ph/0505240 ++ σ ~ 3 fb (M(H)=125 GeV) “factor ~ 3 uncertainty” 100 fb^-1  ~ 100-1000 Ae events (Ae = acceptance, efficiency) Other estimates differed by “large” amounts! But exclusive  c etc is a check. Exclusive

50. Mike Albrow Diffraction in High Energy Collisions CERN June09 50 Cross section for p+p  p + SMH + p at LHC, x branching fractions: Small (~ fb) but S:B can be high. ExHuMe “verified” by 2-photon, & JJ < 140 GeV : bbar, > 140 GeV : WW(*) FP420 Acceptance fn. Mass: (a) 420+420 (b) 420+240 (a)(b)

51. Mike Albrow Diffraction in High Energy Collisions CERN June09 51 Simulations of SMH  b-bbar signals & background Cox, Loebinger and Pilkington arXiv:0709.3035 (JHEP) (a)(b) (a)300/fb = 3 years at 10^34, 420+420, L1 trigger on jets, muons, 25 kHz (b)Same with no pile-up background – very high resolution p-timing... and if 420+420 in L1 trigger future upgrade in latency?

52. Mike Albrow Diffraction in High Energy Collisions CERN June09 52 l JJ pp WW* Can use ~ 50% of WW (all but JJJJ) SMH(135-200)  WW(*) … Various missing masses !! Unfortunately very few events (SM) Durham Gp: Khoze, Martin, Ryskin, Stirling hep-ph/0505240 Also (50fb)

53. Mike Albrow Diffraction in High Energy Collisions CERN June09 53 Non-SM cases : no Higgs? MSSM Higgses? 1)No SMH? Can we exclude? Suppose measure 100 exclusive in CMS. (M(  ) > 10 GeV)  predict p+SMH+p to ~ 20% (trigger study in progress) Suppose expect (say) 100 pHp events in 30 fb^-1, see < 40. Conclusion? 2) No SMH or MSSM-Hs? WW physics becomes very interesting! W fsi 3) In case of SUSY, Forward p-tagging can be crucial! Cross section can be much higher than SMH. Decays to enhanced. A(CP –ve) highly suppressed. Kaidalov Khoze Martin Ryskin hep-ph/0307064 Preview of ILC/CLIC physics

54. Mike Albrow Diffraction in High Energy Collisions CERN June09 54 Can have {h, A, H} close together in mass (few GeV) Hard to resolve by inclusive production. Exclusive advantages: higher production than SM, A highly suppressed Excellent mass resolution could separate h and H (unique) Excellent mass resolution could even measure H widths (if ~ few GeV) J.Ellis, J.S.Lee and A.Pilaftsis, PRD71:075007, hep-ph/0502251 Durham Group (KMRS) MSSM H h A

55. Mike Albrow Diffraction in High Energy Collisions CERN June09 55 Summary Strong Interaction well understood at large Q2 (QCD) but most interactions difficult to describe: Total cross sections, elastic scattering, diffraction dissociation… Regge theory has some validity but connection to QCD obscure. Hard interactions : Jets, W, Z … provide a tool (probing pomeron) Pomeron has a hard component: g (most momentum) + (soft) g/gg This allows ~ “tagged gluon beams” at LHC, & γ-beams Measuring p + p  p + X + p new window on SM & BSM physics FP420/240 project in CMS & ATLAS Warm-up: Measure diffractive JJ, W, Z, γγ at LHC

56. Mike Albrow Diffraction in High Energy Collisions CERN June09 56 Thank you Back-ups

57. Mike Albrow Diffraction in High Energy Collisions CERN June09 57 e.g. Schafer and Szczurek: arXiv:0705.2887 [hep-ph] Some predictions for J/psi photoproduction: Machado,Goncalves 3.0 nb Motyka and Watt: 3.4 +- 0.4 nb Schafer & Szczurek ~ 2.8 nb Nystrand 2.7+0.6-0.2 nb Our result: 3.92 +- 0.62 nb Take 3.0 +- 0.3 Y is much lower. Allow Pile-Up (x 10) More data (x 3) More Δy (x >4) Our limits on O-exchange are close to, and constrain, theoretical predictions

58. Mike Albrow Diffraction in High Energy Collisions CERN June09 58 Summary of Results M = 3-4 GeV/c2 No strong evidence for odderon