Double Pomeron # Exchange from the ISR* (via the Tevatron) to the LHC*

1. Double Pomeron# Exchange from the ISR* (via the Tevatron) to the LHC* Michael Albrow Fermilab # “the P-word” p IP X IP p frame dependent Note: X is independent of nature of colliding hadrons (?) Same for π+π+ scattering and Ω-Ξ0 scattering (?) Fundamental in S.I. • * Intersecting Storage Rings (1971-1984) • ** Large Hadron Collider (2009 - )

2. Exclusive Central Production:p + p  p + X + p y ISR p p LHC Δy p p G = glueball (SI) H = Higgs (WI) Both: JPC = 0++ No charge No color Δy W+ π+ π- H G W- Δy p p Δy MGA, Coughlan, Forshaw: Review: CEP in hh-collisions arXiv:1006.1289, PPNP p p t-channel exchange over Δy >~ (3-)4 : only γ or pomeron IP (or odderon O)

3. Regge Theory demands double pomeron exchange DPE SDE Triple Regge / Trippple pomeron Double-Triple Regge / Quintuple pomeron

4. Both beam particles coherent, xF > 0.95, Δy > (3) 4, IP+IP  X σ(γγ→ X) much smaller, & very low t IGJPC(X) = 0+0++, 2++ Q = 0, S = 0 etc. Low-Nussinov IP = {gg} (+…) Exchange must be Q = 0, Color = 0, J/ α(t=0) >= 1: Pair of gluons: pomeron (Pomeranchukon) or photon γ Couplings β,g not << 1 so non-perturbative, but Regge calculus “works” to some extent. Do not think IP emitted like a free particle! Spacelike only. So double pomeron exchange is a good “laboratory” for meson spectroscopy and especially for hunting glue-rich states or “glueballs” In string model of hadrons: Meson = open string Glueball = closed string

5. ISR Phase I (Search for DPE) No particle ID but π+π- assumed. M(π+π-) <~ 1 GeV Before R807: Axial Field Spectrometer |yπ| < 1.5 |yπ| < 1.0 2 gapsΔy > 3 Later Axial Field Spectrometer: π/K/p ID, better resolution and high statistics.

6. ISR: Axial Field Spectrometer (R807) First sophisticated high-pT spectrometer in pp. Forerunner of p-pbar collider experiments. ISR (1971-84) Intersecting Storage Rings pp, ppbar, dd, pd αα,.. DC ribbon beams > 50 amps !! Uranium-scintillator full-azimuth calorimeter 37%/sqrt(E) hadron showers Axial Field Magnet (~Helmholtz coils) Jets! When ΣET = 35-40/63 GeV (& UA2, UA1) Central drift chamber half

7. Low Mass Central Exclusive Production pp  p X p X fully measured Search for “Glueballs” ISR = 63 GeV p p + nothing else Axial Field Spectrometer (R807) Added very forward drift chambers for p U-Cal π π U-Cal

8. Central Exclusive Production (AFS) 1500 Structures not well understood Only now being studied at higher (RHIC, Tevatron … LHC) f0(980) and f0(1500) seen as dips. One claim (Ochs and Minkowski): σ = f0(600) is very broad 400-1700 MeV scalar glueball, “cut” at 980 and 1500 MeV by f0’s in destructive interference. observed, same M-spectrum, can only be coherent. O&M: hep-ph/9811518

9. High statistics  Moments of angular distributions, project out: S-wave (J = 0) amplitude2 ~ All S-wave up to 1650 MeV 600 1 GeV Small (~6%) D-wave (J=2) at f2(1270) More D-wave ~ 1900-2200 MeV D-wave (J = 2) amplitude2 40 J=2 glueball expected ~ 2.1 GeV 1 GeV Gluey, but is σ(400-1700) the “Lightest Scalar Glueball”?

10. σ/f0(600) is a very broad scalar resonance in ππ. Has been called “the Higgs boson of the strong interaction” Γ ~ 1 GeV  Lifetime τ ~ ħc/Γ ~ 1/5 fm It “decays” before exiting the hadronic interaction. NOT like a π or any isolatable meson. NOT really a bound state. Strong S-wave ππ scattering NOT narrow (as was supposed, from Zweig’s rule) …but αS >~ 1 NB: σ/f0(600) is NOT on the IP trajectory. First state on IP has J = 2 expected at M(G2) ~ 2.1 GeV/c2 Glueball situation still controversial, but never an isolated “hadron” (unless M(G) < 2 m(π) and strong decay forbidden) Note: The IP does not get “emitted” from the proton either, do not interpret Regge diagrams that way!

11. Double pomeron exchange at RHIC (STAR) (Wlodek Guryn’s talk) p + p  p + X + p with p detected, X = low mass hadrons for identified hadrons Work going on to isolate exclusive channels, perhaps more data. DPE in UA8 (Schlein et al) But limited stats/resln

12. Moving along: ISR – Tevatron -- LHC CM energy: 63 1960 ~ 14,000 GeV (eventually) p p p p G? H? p p p p D IP E ISR Tevatron LHC FP420: R&D project; proposing extensions to ATLAS & CMS. In CMS: High Precision Spectrometers, HPS In ATLAS: ATLAS Forward Protons, AFP

13. Central Exclusive Production of Higgs Higgs has vacuum quantum numbers, vacuum has Higgs field. So pp  p+H+p is possible. 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 measure p’s: J >= 2 strongly suppressed at small p angle (t) t H 4-vectors ! + suppresion of backgrounds http://www.fp420.com

14. Exclusive Higgs, not via γγ (possible, but σ much too small) * gg fusion is the main H production process in hadron-hadron collisions. * If H  JJ it’s swamped by QCD background. * Let a 2nd gluon cancel the color, allowing (requiring) both p to stay intact. Possible, but one pays a big price! How much? t H ~10-5 – 10-4 Input: gg  H (standard) Unintegrated g(x,x’) gg in color singlet C.S. No gluon radiation  hadrons No spectator parton-parton ints. Large uncertainties at best Color Singlet {gg} exchange = pomeron IP

15. Q1: Is it really possible to produce a Higgs without any other particles? Can we benefit from p + H + p exclusivity? Q2: If so, is the cross section in reach at LHC? There are similar reactions that we can measure at the Tevatron, and we have (in CDF). J/ψ γ c χc0 q γ γ t  all q No final state S.I. Very small x-sn (10-12σinel) χc0 has Q.N.(H) t  c Lower Q2 CANDIDATES π0π0 b/g ? OBSERVED

16. Q1: Is it really possible to produce a Higgs without any other particles? Can we benefit from p + H + p exclusivity? Q2: If so, is the cross section in reach at LHC? Q3: Will the (non-PileUp) background swamp any signal? Q4: Can the PU background be “killed”, and how? Q5: Can suitable detectors (and mechanics) be built? Q6: Can the forward proton spectrometers be precisely calibrated? YES Probably, it depends! NO Largely, timing & kinematics On paper, yes YES, if stable YES : HPS = High Precision Spectrometers (for CMS) AFP = Atlas Forward Protons (for ATLAS)

17. CDF Forward Detectors: MINIPLUG BSC 1 BSC 2,3,4 CLC Cherenkov Luminosity Counters BSC important as rap gap detectors. FSC for CMS?

18. 37 cit. PHOTOPRODUCTION J/ψ QED ψ(2S) γγ→μ+μ- Ask for γ … find with J/ψ, from χc → J/ψ+γ

19. QED and photoproduction observed as expected. χc exclusive production observed  p+H+p should happen, at rate similar to Durham group prediction, ~ 1-10 fb (SMH) at LHC >>> All χc0? Want to see χc in other channels (~ 7% hadronic & “clean”)

20. Exclusive Di-Jets in CDF: IP + IP  Jet + Jet JET JET GAP JET (p not seen) Detected (Roman pots) JET Tests mechanisms, but is B/G to exclusive H → b+bbar (so must b-tag!) “Almost” exclusive di-jet, Two jets and nothing else

21. Exclusive Dijets (2 central jets + “nothing”) : CDF J p p inferred (GAP) detected J Cross section agrees with ExHuME MC / 3 (inside uncertainty) based on Durham Gp calculations. ExHuME: MC with exclusive di-jets.

22. New Study of DPE in CDF Without (unfortunately) detecting forward p’s, study central hadron production between two large rap-gaps. Difficult to trigger on low mass (<~ 1 GeV/c2) central hadrons. New CDF data set: All CDF detectors “empty” for 1.1 < |η| < 5.9 both sides. (Δy > 4.8) In |η| < 1.1 : 2 Electromagnetic showers ET > 0.5 GeV 2 EM+Hadronic towers ET > 0.5 GeV 1 or 2 tracks pT > 1.5 GeV 1 “jet” ET > 3 GeV in “OR” Note: this integrates overt1, t2, φ12and allows low mass dissociation eg p  pππ. Rate is high even at low-Lumi: ~ 2M events in 4 hours

23. Some (selected) DPE studies of interest in CDF and CMS ~ all glue  High mass characteristics Low mass exclusives dσ/dM(ηmax) nch vs M Event shapes: Thrust T(M), Circularity Jets: 2-jets (gluon jets)…xg and DPS (2x 22) or 24 Drell-Yan & q-qbar Bose-Einstein correlations (size of π-emission region) Extremes of n(+-)/n(0) ? Anything in hadron-hadron except the very rare, & …

24. Proposal to add Forward Shower Counters FSC to CMS What: We propose to install a set of scintillation counters around both outgoing beam pipes at CMS, ~ 60m – 120 m Why: (a) As veto in Level 1 diff. triggers to reduce useless pile-up events. (b) To detect rapidity gaps in diffractive events (p or no-p). (c) Measure “low” mass diffraction and double pomeron exchange. (d) Measure σINEL (if luminosity known, e.g. by Van der Meer) (e) Help establish exclusivity in central exclusive channels (f) To monitor beam conditions on incoming and outgoing beams. (g) To test forward flux simulations (MARS etc.) (h) Additional Luminosity monitor. ! Also: They may provide valuable tests of radiation environment to be expected for HPS = High Precision Spectrometers

25. 2009 JINST 4 P10001 FSC Accessible warm beam pipe between BMX magnets Can put scintillators at several z-locations FSC = Forward Shower Counters Do not see primary particles, but showers in pipe and other material.

26. Big discussions: What is a gap? A: I know one when I see one. No teeth ---- no hadrons Can I be 100% sure?Maybe not, that’s a background or inefficiency

27. Noise levels in calorimeters (How gappy is a gap?) Central CDF EM Cal: -0.66 < η < +0.66 Signal-Noise separation in calorimeters: Hottest PMT in regions: 0-bias with tracks 0-bias with no tracks 80 MeV (or no beams or 1 beam) CMS (Nicola Schul) Log(Max ET) (hottest tower/event) Peak at 32 MeV, Cut at 80 MeV E in HF, no track events: (Single diffraction e.g.) 99% of 0-bias notracks have no channel > 4.2 GeV HF = Hadron Forward 4 < η < 5.2

28. What about total inelastic cross section σINEL? And total σTOT if you know σEL ? Not done at Tevatron! Can measure rate of totally empty events, P(0) = exp(-<ninel>) But this misses all the low mass diffraction that give hits only with |η| >~ 6, or M <~ 5 GeV/c2 This is many mb! Nobody can measure σINEL directly, only σTOT - σEL (?) With FSC, P(0) only faked by events with all particles in cracks (can study with fake cracks) or inefficient regions (small); and inefficient because of noise (can study with data). FSC fills 2 huge cracks: 5(6) < |η| < 8(9…) Control data: No beams, 1 beam Bunch Lumi dependence Exclusive l+l-, etc. η poor variable here!

29. Efficiency for detecting forward particles, GEANT(Lamsa, Orava): Low- : > 20% for > 9.0, > 60% for 9.5 < < 11.5 FSC efficiencies vs η at pT = 0.5 GeV/c (p & n dominate)

30. Efficiency SDE vs Mass: FSC & others FSC alone ZDC alone 10 GeV Generated diffractive mass (PYTHIA/PHOJET) as log(MX), MX in GeV/c2, cf to calculated from rapidity gap edge: (a) full η coverage (b) η < 4.7 (no FSC) >4 hits in FSC or > 1 track in T1/HF or T2/Castor or ZDC(min)

31. Low mass DPE in CMS Central events (0-bias trigger) with forward rap-gaps (FSC, ZDC, CASTOR, HF) studied for generic Double Pomeron Exchange processes (~ 0.1 mb) Low mass DPE: HF VETO HF VETO FSC hits FSC hits y Even without seeing quasi-elastic protons, gaps >~ 4 units select D IP E CENTRALSTATE In β*=90m TOTEM running: p + X + p  should be CMS priority! CMS-side trigger = ZDC-FSC-HF veto; TOTEM side = pp (not-elastic)

32. Change of Subject, Machine, Experiment! Some pictures of exclusive μ+μ- events in CMS

33. CMS Exclusive dimuon candidates

34. CMS Exclusive dimuon candidates

35. CMS Exclusive dimuon candidates

36. Summary, Conclusions • Double Pomeron Exchange DPE is a special kind of interaction: • Strong but ~pure glue, constrained quantum numbers, clean. • Exclusive states and hadron (glueball) spectroscopy: High mass multiparticle states: pomeron structure, jets, … At LHC reaching central masses ~ 200-500 GeV :fn(ηmax) >> Want maximal forward coverage for gaps : ZDC, FSC + >> Want gaps in Level 1 triggers.

37. Thank you