V.A. Khoze (IPPP, Durham)

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Selected Topics on Central Exclusive Production
V.A. Khoze (IPPP, Durham)
with a bit of personal flavour
(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, Risto, Michele...)
Higgs sector study- one of the central targets
of FP420 physics menu
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revisited
refer. purposes ExHume tuning
t
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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)

or
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 )
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Absorptive effects and soft survival factors
(J.D. Bjorken, 1992)
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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.)
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PDFs 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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Semi-enhanced hard rescattering and soft-hard
factorization
enhanced correction to sH(excl)?
enhanced absorption, discussed first KKMR-01 in
the diffractive dijet context
Bartels,Bondarenko,Kutak,Motyka-06 ?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. Martins talk)
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On top of KMR theoretical arguments
New ZEUS data
Leading neutron prod. at HERA, Zeus
g
g
g
KKMR 06
gap due to p exchange exclusive Higgs
eikonal
yi gt 2 3 correction prop. to rap. interval
prop. to g energy (negative) Prob
. to observe leading neutron must decrease with g
energy But expt. ? flat ? small
enhanced correction
enhanced
g
g
g
g
?SD may change (flat) behaviour at the LHC if
enh . contr. is large
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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)mingt50 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).

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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.
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Higher sensitivity to the parameters of models
for Soft Diffraction
y-ln ?, ?(1-x)
( also for calculations of the pile-up
backgrounds)
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Exclusive dijet monitor Interferometer
CEP of diphotons (rate permitting) would provide
an excellent combined test at Mgt10-20 GeV (better
accuracy!)
Dijet rate- combined effect of all basic
ingredients (Surviv, Sudakov, pdfs, Enhanc.
Absp) ( ET gt 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)
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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Correlations
KMR-02
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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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Uncertainties in the non-PU background calculation
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 Jz2 production.
? NLO radiative contributions (hard blob
and screened gluons) ? NNLO one-loop box
diagram (mass- unsuppressed, cut-non-reconstructib
le) ? 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. )
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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 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)
LET THE DATA TALK !
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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 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 factor in
the effective 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 soft survival
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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