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Selected Topics on Central Exclusive Production. V.A. K hoze (IPPP, Durham). (based on works with A. K aidalov, M. R yskin, A.D. M artin amd W.J. S tirling). aims : to list & expose the main uncertainties in the theoretical expectations for CEP rates,

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Selected Topics on Central Exclusive Production

V.A. Khoze (IPPP, Durham)

(based on works with A. Kaidalov, M. Ryskin, A.D. Martin amd W.J. Stirling)

aims: to list & expose the main uncertainties in the theoretical expectations for CEP rates,

to propose the measurements which will allow to restrict the predictions

By popular demand (ADR, Brian..)

Higgs sector study- one of the central targets of FP420 physics menu



refer. purposes

ExHume tuning


 How reliable are the calculations ?

 Are they well tested experimentally ?

● How well we understand/model soft physics ?

● How well we understand hard diffraction ?

●Is ‘hard-soft factorization’ well justified ?

 What else could/should be done in order to

improve the accuracy of the calculations ?

So far the Tevatron diffractive data have been Durham-friendly)


clouds on the horizon ?

Theory side - •Hard rescattering corrections toCEP, (BBKM-06)

(agreementwith KMR at the TPF-level)

•New papers/talks by GLM-07 ( Strikman et al, 06-07 )


15 years on

Available data on soft diffraction at high energies are still rather fragmentary.

Theoretical models contain various assumptions and many parameters.

Durham models are tuned to describe available ‘soft’ diffractive data at

high energies and predict the total, elastic, SD and DD dissociation cross sections

which can be tested at the LHC.

Durham models allowed to make predictions for the CEP jj and diphotons

at the Tevatron which are broadly confirmed by the data , more tests to come.

A way to compare the models : with the same exponential slope b in ME

(an agreement within a factor of 2 is still a miracle! )

MC model predictions should be confronted with the CDF data ( e.g. proton spectra in SD)

At the moment- no need to revise the Exhume default numbers,

but we have to be opened-eyed.

( note, on the theory side –downward tendency (stronger absorption effects), but

CDF data rather favour upward )

Recall: in reality survival factor is not universal (depends on the nature of the hard process, kinematics, selection criteria, acceptances, pt- spread….)



KKMR -04 – factor of 1.5-2 difference between CTEQ6M and MRST02

 very recently, CLP-07, factor of 4 difference between CTEQ6L1 and MRST2002NLO,

CTEQL1 7.38 fb for SM Higgs.

Here we are on the conservative side, but further studies and tests are needed

Higher-Order QCD effects

  • Uncomfortably large higher-order QCD effects in the case of exclusive
  • processes, exemplified by the Sudakov effect.
  • Seen now in the dijet exclusive data. (Thanks to CDF )

Further serious theoretical studies needed,NNLO Sudakov ?

Self-consistent combined treatment of higher order effects in unintegrated

struct. functs and in the hard cross-section – requires detailed studies


Semi-enhanced hard rescattering and soft-hard factorization



to sH(excl)?

enhanced absorption,

discussed first KKMR-01

in the diffractive dijet context


used pert.thy.corrn could be

large and s H(excl) modified ?

KMR-06 arguments for small effect

KMR-00(07): use 2(3)-channel eikonal

+ ‘soft’ enhanced contributions

(A. Martin’s talk)


On top of KMR theoretical arguments

New ZEUS data

Leading neutron

prod. at HERA, Zeus




KKMR ‘06

gap due to p exchange

~ exclusive Higgs


yi > 2 – 3

correction prop. to rap. interval

prop. to g energy


Prob. to observe leading neutron

must decrease with g energy

But expt.  flat

 small enhanced correction






SD may change (flat) behaviour at the LHC if enh . contr. is large



  • (Yesterday and Today)
  • Up to now the diffractive production data are consistent with K(KMR)S results
  • Still more work to be done to constrain the uncertainties
  • CED high-Et dijets
  • (CDF: Run I, Run II) data up to (Et)min>50 GeV
  • ‘Factorization breaking’ between the effective diffractive structure functions measured at the Tevatron and HERA.
  • (KKMR-01 ,a quantitative description of the results, both in normalizationand the shapeof thedistribution)
  • The ratio of high Et dijets in production with one and two rapidity gaps
  • Preliminary CDF results on exclusive charmonium CEDP.
  • Energy dependence of the RG survival (D0, CDF).
  • CDPofγγ (….,) ,PRL paper
  • (in line with the KMRS calculations)
  • Leading neutrons at HERA

Available CDF data on proton spectra are well described by KMR model

MCs should be compared with the CDF data

Governs the rate of the pile-up backgr.


Higher sensitivity to the parameters of models for Soft Diffraction

y=-ln , =(1-x)

( also for calculations of the pile-up backgrounds)


Exclusive dijet monitor & Interferometer

CEP of diphotons (rate permitting) would provide an excellent combined test at M>10-20 GeV (better accuracy!)

Dijet rate- combined effect of all basic ingredients (Surviv, Sudakov, pdfs, Enhanc.Absp)

( ET > 10 GeV)

ET-dependence -dominantly Sudakov (+anom dimens), weaker dependence on Surviv.

At low ET- higher sensitivity to the Enhanced Absorption

Correlations between proton transverse momenta, azim. distribts

Practically insensitive to pdfs and Sudakov effects.

High sensitivity to soft model parameters.

Proton opacity scanner (KMR-02, also Kupco et al-05, Petrov et al -05)


• Comparatively high rate (3 orders of magnitude higher than for the Higgs at the same ET).

• Possibility to separate different effects and to restrict different uncertainties by

studying the same process



High ET central jets are not required (in principle)


Uncertainties in the non-PU background calculation


For the channelbgds are well known and incorporated in the MCs:

Exclusive LO - production (mass-suppressed) + gg misident+ soft & hard PP collisions.


The complete background calculations are still in progress

(uncomfortably & unusuallylarge high-order QCD and b-quark mass effects).

About a dozen various sources (studied by Durham group)

 admixture of |Jz|=2 production.

 NLO radiative contributions (hard blob and screened gluons)

 NNLO one-loop box diagram (mass- unsuppressed, cut-non-reconstructible)’

 ‘Central inelastic’ backgrounds

 b-quark mass effects in dijet events – still incomplete

potentially, the largest source of theoretical uncertainties!

On top of MC studies

(Andy, Marek et al. )


Should be (strongly) reduced by the existing cuts : mass matching, azimuth. correlations etc

More MC studies needed



We are now at the qualitatively new stage when the theoretical predictions

for the CEP cross sections have reached the level of a factor of 3 accuracy.

So far Durham group has been able to describe/predict the diffractive data.

Essential improvement of the accuracy will require a lot of work and may not happen

until the LHC experiments come FORWARD and produce the data (already) in the early runs.

This will not be easy. It is not like a walk in the park

(J.D. Bjorken 1992)



In reality KMR –calculational procedure is (much) more complicated

To account for the effects of the screening corrections the calculations are performed

in impact parameter bt –space

For illustration in a single-channel eikonal approx. hor the process

is the Fourier transform to impact parameter space

The soft rescattering effects are denoted only symbolically by a factorin the

effective PP –luminosity (even for the integrated over proton transverse momenta quantities)


In practice, there is no factorized form, and should be viewed as the soft

survival factor appropriately averaged over the diffractive eigenstates (2,3 channel eikonals..)


In the topical case of production

the introduction of the and angular correlations raise effective

from 0.02 to 0.026 ( KKMRhep-ph/0307064)

There are other assumptions.

for example, factorization of the unintegrated distributions

still schematically

+ correlation effects +…


model B(2) as compared to the elastic data

secondary Regge poles can contribute