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V.A. Khoze (IPPP, Durham)

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

Selected Topics on Central Exclusive Production

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

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..)

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refer. purposesExHume tuning

revisited

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This is what matters for the CEP rates ! KMRS-04

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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 to CEP, (BBKM-06) (agreement with KMR at the TPF-level) • New papers/talks by GLM-07 ( Strikman et al, 06-07 )

or

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Absorptive effects and soft survival factors

(J.D. Bjorken, 1992)

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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….)

15 years on

2 2( , ) /S s b b

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PDF’s DEMOCRACY

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

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KMR-00(07): use 2(3)-channel eikonal+ ‘soft’ enhanced contributions

(A. Martin’s talk)

Bartels,Bondarenko,Kutak,Motyka-06

used pert.thy.corrn could be large and H(excl) modified ? KMR-06 arguments for small effect

enhanced absorption,discussed first KKMR-01

in the diffractive dijet context

“enhanced”correctionto H(excl)?

Semi-enhanced hard rescattering and soft-hard factorization

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Leading neutronprod. at HERA, Zeus

eikonal enhanced

gap due to exchange ~ exclusive Higgs

yi > 2 – 3

correction prop. to rap. interval prop. to energy(negative)

Prob. to observe leading neutronmust decrease with energyBut expt. flat small enhanced correction

KKMR ‘06

New ZEUS data

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

On top of KMR theoretical arguments

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EXPERIMENTAL CHECKS(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 normalization and the shape of the distribution)

•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).

• CDP of γγ (….,) , PRL paper (in line with the KMRS calculations)

Leading neutrons at HERA

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

Governs the rate of the pile-up backgr. MCs should be compared with the CDF data

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y=-ln , =(1-x)

( also for calculations of the pile-up backgrounds)

Higher sensitivity to the parameters of models for Soft Diffraction

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Exclusive dijet monitor & Interferometer

Dijet rate- combined effect of all basic ingredients (Surviv, Sudakov, pdfs, Enhanc. Absp) ( ET > 10 GeV)

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

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. distribtsPractically 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)

Advantages• 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

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d

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KMR-02 Correlations

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KMR-02

High ET central jets are not required (in principle)

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weak dependence on (t) – integrated effect

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rich diffractive structures

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Myths

For the channel bgds are well known and incorporated in the MCs:

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

Reality

The complete background calculations are still in progress (uncomfortably & unusually large 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!

bb

bb

On top of MC studies(Andy, Marek et al. )

Uncertainties in the non-PU background calculation

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(+ n soft gluons)

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

More MC studies needed

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Conclusion

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

LET THE DATA TALK !

(J.D. Bjorken 1992)

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BACKUP

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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 –spaceFor 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 factor in theeffective PP –luminosity (even for the integrated over proton transverse momenta quantities) (KMR-hep-ph/0111078)

In practice, there is no factorized form, and should be viewed as the softsurvival factor appropriately averaged over the diffractive eigenstates (2,3 channel eikonals..)

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In the topical case of production

the introduction of the and angular correlations raise effective from 0.02 to 0.026 ( KKMR hep-ph/0307064)

There are other assumptions. for example, factorization of the unintegrated distributions

still schematically

+ correlation effects +…

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model B(2) as compared to the elastic data

secondary Regge poles can contribute

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2 ( 0.024TPFS BBKM (LL)

Difference on the level of only a factor of 2 is still a miracle !

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