Diffraction at the LHC: a theoretical review
Alan Martin (Durham), Physics at the LHC, Split, Sept-October 2008
Soft diffraction
Hard diffraction
Predictions for ,…
especially pp p + A + p with A = H(bbbar)
survival of rapidity gapsdepends on soft rescattering
Optical theorems
High mass diffractive dissociation
at high energyuse Regge
triple-Pomeron diag
but screening important g3P
but screening important so total suppressed
gN2
so (g3P)bare increased
M2
elastic unitarity
S2el = e- is the probability of no inelastic interaction
diagonal in b
from model fitsto elastic data
directly related to elastic data
LHCTevatron
Low-mass diffractive dissociation
include high-mass diffractive dissociation
Elastic amp. Tel(s,b) bare amp.
introduce diffve estates i, k (combns of p,p*,..) which only undergo “elastic” scattering (Good-Walker)
multichannel eikonal
(20%)
(SD 80%)
(40%)
PPPPPR
PPR
P
RRP
RRR
RRP
P
PPP
triple-Regge analysis of d/dtdincluding screening
fit: 2 = 171 / 206 d.o.f.
Luna+KMR;Poghosyan,Kaidalov
TevatronCERN-ISR
(includes compilation of SD data by Goulianos and Montanha)
g3P= gN ~0.2g3P large, need to includemulti-Pomeron effects
New analysis of soft data
3-channel eikonal, i with i=1,3
include multi-Pomeron diagrams
attempt to mimic BFKL diffusion in log qt by including three components to approximate qt distribution –possibility of seeing “soft hard” Pomeron transition
KMR
model:
Use four exchanges in the t channel
a = Plarge, Pintermediate, Psmall, R
3 to mimic BFKL diffusion in ln qt
soft pQCD
average qt1~0.5, qt2~1.5, qt3~5 GeVVRP1 ~ gPPR,gRRP
VPiPj ~ BFKL
solve for aik(y,b)
by iteration
sec. Reggeon
bare pole absorptive effectsevolve up from y=0
evolve down from y’=Y-y=0
Parameters
All soft data well describedg3P=gN with=0.25 (compared to =0.2 in Luna et al.)
Pi = 0.3 (close to the BFKL NLL resummed value)’P1 = 0.05 GeV-2
These values of the bare Pomeron trajectory yield, afterscreening, the expected soft Pomeron behaviour ---“soft-hard” matching (since P1 heavily screened,….P3~bare)
R = -0.4 (as expected for secondary Reggeon)
Results
multi-Pomeron coupling from dSD/ddt data ( ~0.01)
diffractive eigenstates from SD(low M)=2mb at sqrt(s)=31 GeV, -- equi-spread in R2, and t dep. from del/dt
= (0) - 1
KMR 3-ch eikonal, multi-Regge analysis of available “soft” data
predict at LHC:total = 90.5 mb
sqrt(s)
Other fits with absorptiveeffects predict total~90 mb
Sapeta, Golec-Biernat;Gotsman, Levin, Maor
Predictions for LHCtotal (mb)
pppXparton multiplicity
All Pom. comptshave bare=0.3
“soft”, screened,little growth,partons saturated
“hard” ~ no screeningmuch growth, s0.3
total = 90.5 mbel = 20.8 mbSD = 14.8 mb
“large”
“small”
~ g, sea
more valence
LHC (x0.1)
gap
electron
outgoingproton
X
diffractive DIS: epeX+p (*pX+p)
DIS: epeX (*pX)
HERA finds that about 10% of these events are
DIS
Diffractive DIS
If then assume, Regge factorization:
gap
same
diffractive partons gD, qD can be used to predict diffractive processes with hard scale? Yes, but…
Diffve partonsfrom HERA data
direct+resolvedPomeron(cf. photon)
HERA
CDF
softrescatt.
CDF diffr. dijet data gap
gap
HERA prediction
S2() ~ 0.1
Photoproduction of leading n
ZEUS data
S2 ~ 0.48
D’Alesio, Pirner;Nikolaev,Speth,Zakharov;Kaidalov,Khoze,M,Ryskin;Kopeliovich,Potashnikova,Schmidt,Soffer.
Advantages of pp p + (Hbb) + p
-- accurate determination of MH
using tagged protons, MH=Mmissing
-- MH=Mdecay must match MH=Mmissing
-- bbbar QCD background suppressed by Jz=0 selection rule
-- S/B ~ O(1) for SM 120 GeV Higgs (…but ~ few fb)
-- x 10 for some SUSY Higgs scenarios Kaidalov+KMRHeinemeyer,Khoze et alCox,Loebinger,Pilkington
-- can determine JPC. Selection rule favours 0++ production
e.g. MA > 140 GeV: then h hSM
H, A decouple from gauge bosonsH, A bbbar, enhanced by tan
Survival Probability of gaps for pp p + H +p
average over diff. estates i,k
over bsurvival factor w.r.t. soft i-k interaction
hard m.e. i k H
prob. of proton to bein diffractive estate i
S2 ~ 0.02 for 120 GeV Higgs at the LHC
-- irreducible QCD ggPPbbbar events-- gluons mimicing b jets-- Jz=2 contribution
New results:NLO calculation of ggPPbbbar reduces irreducible backgroundby factor of 2 or more Shuvaev et al
Also, experimentally, there has been a reduction in the chance that gluons mimic b jets.
bbbar background to pp p + (Hbbbar) + p signal
Experimental checks of calculation of (pp p + A + p)
KMR cross section predictions are consistent with the recent observed rates of three exclusive processes at the Tevatron:
Early LHC runs can give detailed checks of all of theingredients of the calculation of (pp p + A + p),even without proton taggers
ppbar p + + pbar
ppbar p + dijet + pbar
ppbar p + c + pbar (68 c0 J/ + events)
CDF
CDF
3 events observed (one due to 0)
(excl )measured ~ 0.09pb
(excl )predicted ~ 0.04pb(= 10 fbfor ET
>14 GeV at LHC
CDF exclusive dijet
ET
ET
exclusive cross section v ET
bbbar prod.suppressedin exclusiveregion -- asexpected
exclusive region
Early LHC checks of pp p + A + p ?
Possible checks of:
(i) survival factor S2: W+gaps, Z+gaps
(ii) generalised gluon fg : p p
(iii) Sudakov factor T : 3 central jets
(iv) soft-hard factorisation #(A+gap) evts (broken by enhanced #(inclusive A) evts absorptive effects) with A = W, dijet, …
gap
gap
KMR Sen
Evidence is that S2en ~ 1
for pp p + H + p
Sen
-- explicit calc. using soft model
-- kinematic suppression, need y > 2.3 to establish Pomeron exchange
-- HERA leading neutron data, no energy dep. in n yield
-- after including S2eik we are left with b > 0.6 fm, where
Q2saturation < 0.3 GeV2 (Watt et al), so S2
en ~ 1
Early LHC probe of S2en
Seik
There is controversy about its size.
S2en = gap survival to rescattering
on intermediate partons
inclusive diffractive
A = dijet or W or ….
known from HERA
pp diffve dijetpp inclve dijet
rough estimates of enhanced absorption S2en
Possibility for LHC to probe S2enhanced
Exclusive production as probe of odderon and fg
x 0.025 (br for )
exch odderon exch
comparable ?
If |y|<2.5, then sample fg(x1,x2) with xi in (10-4, 10-2)
Bzdak, Motyka,Szymanowski,Cudell
can separate by pt of upper proton if it is tagged
For small pt exch dominates For pt > 1 GeV
odderon should show up
Conclusions – soft diffraction
-- screening/unitarity/absorptive corrections are vital-- Triple-Regge analysis with screening g3P increased by ~3
importance of multi-Pomeron diagrams-- Latest analysis of all available “soft” data: multi-ch eikonal + multi-Regge + compts of Pom. to mimic BFKL (showed some LHC predictions ….. total ~ 90 mb)
-- LHC can explore multigap events probe multi-Pomeron structure
soft-hard Pomeron transition emerges “soft” compt. --- heavily screened --- little growth with s “intermediate” compt. --- some screening “hard” compt. --- little screening --- large growth (~pQCD)
SD DPE
LHC is a powerfulprobe of modelsof soft processes
soft analysis allows rapidity gap survival factors to be calculated for any hard diffractive process
Exclusive central diffractive production, ppp+H+p, at LHC hasgreat advantages, S/B~O(1), but ~ few fb for SM Higgs. However, some SUSY-Higgs have signal enhanced by 10 or more.Very exciting possibility, if proton taggers installed at 420 m
Formalism consistent with CDF data for pp(bar) p + A + p(bar) with A = dijet and A = andA c
More checks with higher MA valuable.
Processes which can probe all features of the formalism used tocalculate (ppp+A+p), may be observed in the early LHC runs, even without proton taggers
Conclusions – hard diffraction
