1. Diffraction and Central Exclusive Production
in High Energy pp/ppbar Collisions
Mike Albrow (Fermilab)
LPC Topic of Week Lectures, April 2011
Lecture 2 : Central Exclusive Production:
p + p  p’ + X + p’
where + = rapidity gap Δy > 5 (to be definite), no hadrons
and X = “simple” system fully measured.
p’ = proton (best case),
or low mass state e.g. pππ when it doesn’t matter.
Quantum
Numbers
restricted 1

2. Why do I think CEP is interesting, even important?
These are the cleanest, simplest inelastic pp collisions.
Most collisions  lots of particles, cannot cope:
go to single particle inclusive e.g. pp → W + anything, t + anything, X + anything…
or 2-”particle” inclusive e.g. pp → WW + anything, seeking {H →WW} + …
In contrast, consider WW + nothing (p’s go down pipe, small pT)
H + nothing
Could be HWW.
Know M(WW) if
measure p’s
b-tagged jet
No other tracks
on eμ vertex μ-
No tracks
on vertex
with large pT(rel) or e+
Could be Hbb.
Know M(bb) if
measure p’s
b-tagged jet
Missing ET
Ez too if measure p’s
Quantum numbers constrained: J=0,2, CP=++, M, Γ, Γgg

3. From elastic scattering to exclusive Higgs boson production
= gluon H
IP = {gg} p p p p
About 25% of σ(total)
About of σ(total)
But these are related processes!
Space picture:
2 gluons “turn around” H
Anyway these gluons are
wee and virtual and their
direction is frame-dependent
Color field;
shorted out by
another g exchange 3

4. Central Exclusive Particle Production at High Energy Hadron Colliders
M.G.Albrow, T.D.Coughlin and J.R.Forshaw
Progress in Particle and Nuclear Physics 65 (20l0) l49-84
arXiv:l [hep-ph]
How did we get here (Regge, triple…)

5. In QCD, serious calculations (don’t ask me):H

6. 63 GeV 1960 GeV ~ 7-14,000 GeV (CM) ISR – Tevatron -- LHC ISR Tevatronp p p p G? H? p p p p
ISR
Tevatron
LHC
We are in R&D phase of high precision spectrometers HPS
to add to CMS to measure both protons (Monday’s lecture)

7. W, Z and high-ET jet pairs in SDE at Tevatron. Also heavy Q in jets.
Remember, no absolute distinction experimentally.
Theoretically IP and IR terms, but interference.
Single diffractive
excitation (ISR)
CDF and D0 have observed
W, Z and high-ET jet pairs in
SDE at Tevatron.
Also heavy Q in jets.
W,Z : q/qbar play a role
In constituent models of IP g dominate
(cf p) but q evolve in (of course).
0.95
x > 0.95 M p
M up to about 1.6 GeV at AGS/PS
14 GeV at ISR
440 GeV at Tevatron
3100 GeV at LHC

8. Central Diffractive Excitation
Regge theory: SDE implies
Δy > 3
Δy > 3
x > 0.95
x > 0.95
… both protons coherently scattered
M up to about 3 GeV at ISR – GeV
100 GeV at Tevatron – 1960 GeV
350 GeV at LHC – 7,000 GeV
“Vacuum Excitation”
H,WW
If measure protons:
Missing mass to pp = MCEN

9. Low Mass Central Exclusive Production
Search for “Glueballs”
ISR = 63 GeV p p
+ nothing else
Axial Field Spectrometer (R807)
Added very forward drift chambers
U-Cal
Central drift chamber half
U-Cal

10. Central Exclusive Production (AFS)
G(1710)??
Also
Structures not well understood
beyond f(980). Not studied at higher
αα elastic scattering
on-line dip! Diffraction !
and αα→ α + ππ + α
In CDF/ CMS:

11. Rates p + p  p + X + p, double pomeron exchange
with 2 gaps Δy > 5 (say) are very high…σ many μb.
Require no PU, e-<n>, good for low <n> stores/bunches
Need good trigger: forward gaps (or p’s)
In l0 hours of low-lumi CDF running collected 4 million events,
2 gaps Δy > 4 and central hadrons.
About l0,000 are exclusive h+h- also K0sK0s, hyperons etc
and M(X) up to ~ 80 GeV
Results not yet approved.
Comparison with LHC would be interesting and not very difficult

12. Interesting central states TeV, LHC (apart from multihadrons)* * *
Quark pairs?
Go to e+e- *
* Now measured in CDF *
“Only” CMS can do, in some cases
needing forward p measurement *

13. Not essential to detect protons; can require all forward
CDF Exclusive charmonium: Phys.Rev.Lett 102,242001(2009)
Also in e+e- and e+p Also in e+p Only in hadron-hadron
Not essential to detect protons; can require all forward
detectors to be at noise levels, for ~3 < |η| < ~8.
Quasi-elastic protons inferred.
Implies no pile-up interactions allowed.
Or: Require pT(X) ~ 0 (<~ 1GeV for IP), Δφ ~ π, nass(tracks) = 0
Not sure if p → pππ (e.g.), but maybe don’t care. pT(X) helps

14. CDF
J/ψ
ψ(2S)
γγ → μ+μ-

15. 1 Exclusive Z photoproduction: SM process but small cross section:
Cisek, Schafer, Szczurek arXiv:
u+d+s+c+b
u+d+s
y(Z)
Not including BR
= 7% to ee,μμ
Test of BSM loops, with EW & strong couplings.
Must be done with P-U:
Nass(tracks) = 0, pT(Z) <~ 2 GeV/c
Both p-momenta known, seeing either confirms.
Value of high mass QED μ+μ-
Forward proton momenta are precisely known:
can calibrate momentum scale and resolution of
forward spectrometers for p + p  p + H + p at LHC .
Was advertised as LHC luminosity calibrator, but Van der Meer now 4%

16. J/ψ and Y photoproduction at LHC (14)
A. Szczurek: arXiv:
Simulation pre-data, 100/pb
~ 30 pb
Dashed: Born approximation;
solid: includes absorption corrections
BR (Y → μ+μ-) = 2.5%
30pb x 100/pb x Δy = 5 x = 375 events x acceptance

17. High mass exclusive lepton pairs in CDF
One Z candidate, but very forward BSC
had hits: p dissociation.
Expect ~ in SM, but
“one swallow does not make a Spring”
Keep an eye on CMS Z, nass = 0, small pT
CDF preliminary
High mass 2-photon collisions at LHC

18. Jonathan Hollar,DIS CMS data
Nicolas Schul &
Jonathan Hollar,DIS CMS data
40/pb: σ(pp→p + μμ + p) = 3.33/0.56/0. 14/0. 16 pb
pT(μ) > 4 GeV, |η(μ)| < 2.1, M(μμ) >11.5GeV
Ratio to LPAIR =
Muons are back-to-back
75 GeV
pT of dimuon is small
DiMuon mass distribution

19. Exclusive γγ Production is good test of theory1
Exclusive γγ Production is good test of theory
IP IP → γ γ
gg → γγ via q-loops is the easy part (pQCD)
Need unintegrated gluon pdf g(x1).g(x2) … 4th power
No g-radiation (need at least 2) to make hadrons (Sudakov)
g-Loop integral … proton form factor …
No additional parton-parton interactions to make hadrons
(“gap survival probability” from diffractive cross sections)
or H
Durham Group (KMR) claim factor 3 uncertainty in this calculation.
Others make very different estimates (for IP + IP → H), but leave γγ alone.
CDF published 3 candidates with ET(γ) > 5 GeV
cf KMR: 0.8 (~x 3)
New CDF result coming soon, & search in CMS
(Wenbo Li, Yifei Guo & exclusive group)

20. 0.6 pb for ET > 5 GeV, |η| < 2
KMR predictions:
0.6 pb for ET > 5 GeV, |η| < 2
Probably cannot
be done with P-U

21. Exclusive Di-Jets in CDF
GAP
JET
(p not seen)
JET
IP + IP → J + J
>~ 99% pure gluon jets (unique)
What about 4J (double gg scattering)?
And high ΣET with no jets?
“Almost” exclusive di-jet,
Two jets and nothing else

22. Exclusive Dijets (2 central jets + “nothing”) : CDF
Cross section agrees with ExHuME MC / 3
Durham (inside uncertainty)
DPEMC is IP + IP as “hadron-like”, excluded.
ExHuME: MC with
exclusive di-jets.

23. What is exclusive H cross section?
σ ~ 3 fb (M(H)=125 GeV)
“factor ~ 3 uncertainty”
l00 fb-1  ~ 300 Ae events
(Ae = acceptance, efficiency)
Related, so tests X
disfavored by CDF
Exclusive X X
Durham Gp: Khoze, Martin, Ryskin, Stirling
hep-ph/

24. HPS acceptance in M(H)
with 240m + 420m arms
(Monday)
A more recent calculation with
PDG g(x) dependence

25. Determining Quantum Numbers of Central State (H?)
Is it J = 0, CP = ++?
In gg  X only CP = ++ is allowed.
(a CP –ve A (MSSM) is highly suppressed)
gg  vector (J = 1) forbidden, Landau-Yang theorem.
J = 0, 2 can be distinguished by angular distributions
partial wave analysis. Can even see states hidden in overall M distribution!
Of course this needs many events. W+
“g”
“g” IP IP W-
Moments H(LM) of the cos( ) distributions  M(J=0), M(J=2).
e.g. ISR/R807 glueball search in NPB264 (1986) 154

26. Summary
Any states with vacuum quantum numbers and strong or
electromagnetic couplings can be produced at LHC by
Central Exclusive Production
Cross section pp  p+SMH+p known to factor ~ 3 (~ 1-10 fb)
If protons well measured, can get mass of central state to
~ 2 GeV per event, Quantum numbers (J, CP) and couplings to gg.
In any case it is a glue-rich special strong interaction state.
Lecture #3 Monday 2 pm:
SUSY and other BSM physics, ++ ?
How we can do it by adding High Precision Spectrometers
How YOU (your students) can get into some cool detector
development : l0 ps timing counters, l μrad tracking, etc