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,
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
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 )
(J.D. Bjorken, 1992)
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
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
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)
New ZEUS data
prod. at HERA, Zeus
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
MCs should be compared with the CDF data
Governs the rate of the pile-up backgr.
y=-ln , =(1-x)
( also for calculations of the pile-up backgrounds)
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)
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)
LET THE DATA TALK !
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..)
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
+ correlation effects +…
secondary Regge poles can contribute