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Diffraction and Central Exclusive Production at ATLAS. Marek Taševský Institute of Physics, Academy of Sciences, Prague On behalf of the ATLAS collaboration Diffraction 2010, Otranto, Italy - 12/09 2010. Diffraction Central Exclusive Production. ATLAS Central Detector.

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diffraction and central exclusive production at atlas

Diffraction and Central Exclusive Production at ATLAS

Marek Taševský

Institute of Physics, Academy of Sciences, Prague

On behalf of the ATLAS collaboration

Diffraction 2010, Otranto, Italy - 12/09 2010


Central Exclusive Production

atlas forward detectors
ATLAS Forward detectors

10.6 < | η | < 13.5

| η | > 8.3

5.6 < | η | < 5.9

2.1 < | η | < 3.8

Not yet fully


diffraction at lhc
Diffraction at LHC:

- Forward proton tagging in special runs with ALFA

- Combined tag of proton in ALFA on one

side and remnants of dissociated proton in

LUCID on the other side

- Central rapidity gap in EM/HAD calorimeters

(|η|<3.2) and inner detector (|η|<2.5)

- Rapidity gaps on both sides of IP:

Double Pomeron Exchange: parton from

Pomeron brings a fraction β out of ξ into the hard

subprocess → Pomeron remnants spoil the gaps

Central exclusive production: β = 1 → no Pomeron


diffractive measurements
L1 trigger: Rapidity gap (veto in MBTS/Calo/LUCID/ZDC) .AND. Low_Et

Start with ratios X+gaps/X(incl.), X=W,Z,jj,μμ -> get information on S2

pp → RG + W/Z + RG Info on soft survival S2 (γ-exch. dominates for W)

pp → RG + W Info on soft survival S2

pp → RG + jj + RG Combined effect of all basic ingredients to CEP

(S2, Sudakovsuppr., unintegr. fg, enhanced absorpt)

pp → RG + Y + RG Info on unintegrated fg (γ- or Odderon exchange)

Hard SD, Hard DD

L1 trigger: ALFA (one side) .AND. MBTS/Calo/LUCId/ZDC (the other side)

High rate soft diffraction: P-tagging = info on proton pT, i.e. dσ/dt

ALFA: σtot, dσel/dt, σSD(low M), d2σSD/dtdξ, d2σDPE/dξ1dξ2

- tests model assumptions,

- governs rates of Pile-up bg

- Strongly restricts S2 (info on enhanced absorption), not

sensitive to higher-order (Sudakov) effects

pp → p + jj + p:

Advantages: rel. high rate

separate different effects in one process

High rate γp and γγ processes

Diffractive measurements



soft sd measurement with alfa

Introduction – physics case

Soft SD measurement with ALFA

Soft SD can be measured during a special elastic calibration run provided that ALFA can be

combined with LUCID/ZDC [ATLAS-COM-PHYS-2007-056]

- measurement of cross section and t-, ξ-distributions Expect 1.2-1.8 M events in 100 hrs at 1027cm-2s-1

- SD cross section measurement with ~ 15 % syst. uncertainty

- improve model predictions and background estimates for CEP

Very good acceptance for

very low t and ξ.

Global acceptance:

Pythia 45%

Phojet 40%

Soft SD trigger:




























experimental challenge define diffractive event
Experimental challenge: define diffractive event

First data → Soft diffraction

No proton tagger → try rapidity gaps

Generator levelplots - provided byO. Kepka, P. Růžička, Prague

Calorimeter method:

1) Divide Calorimeter into rings in rapidity.

2) If a ring has no cell with significance E/σ > X

(σ of cell Gaussian noise dist)→ consider this ring to be empty

3) Find largest continuous gaps of empty rings

4) Get SD, DD and ND contributions by fitting gap

distribution in data using MC function

Gap size

Start of gap from calo edge (-4.8; 4.8)

central exclusive production
Central Exclusive Production


1) Protons remain intact and can be

detected in forward detectors

2) Rapidity gaps between leading protons

and central system


X = jj, WW, Higgs, …

= χb, χc, γγ

See talk by Valery Khoze


I) Outgoing protons not detected in the main ATLAS detector. If installed, very forward proton detectors would give much better mass resolution than the central detector (see project AFP later)

II) Central system produced in a JZ= 0, C-even, P-even state:

- strong suppression of CEP gg→bb background (by (mb/MX)2)

- produced central system is 0++ → just a few events are enough to determine Higgs quantum numbers.

Standard searches need high stat. (φ-angle correlation of jets in VBF of Higgs) and coupling to Vector Bos.

III) Access to main Higgs decay modes in one (CED) process: bb, WW, ττ: information about Yukawa coupling Hbb!


- Low signal x-section; affected by Pile-up

Find a CEP resonance and you have measured its quantum numbers!!

cep dijets with early data
CEP dijets with early data

BG: Incl. QCD dijets SD dijets DPE dijets

  • Central system produced in Jz =0, C-even, P-even state → quark jets suppressed by mq2/Mjj2

Trigger: Low-Et jet .AND. Veto in MBTS -eff. ~ 65% for CEP wrt jet turn-on; efficiently reduces Incl.QCD bg (by 104)

Exclusivity cuts:

1) MBTS Veto corresponds to cutting on - reduces Incl. QCD bg (has large ξ, protons broken up)

2) rap.gap at least on one side – use rapgaps that are reproducible by theory! – see e.g. S.Marzani’s work

3) Ntrack (outside dijet) < X – reduces Incl. QCD bg

4) single vertex – reduces overlap (Pile-up) bg

5) Look for excess of events over predicted bg in Rjjdistribution, ,

Other variables: steeper leading jet ET and more back-to-back leading jets in CEP due to ISR suppression

Observed by CDF: Phys.Rev. D77 (2008) 052004

In good agreement with KMR but still big uncertainties

Motivation: reduce the factor three of uncertainty in calculations of production

x-section at LHC (KMR)

Measure Rjj distribution and constrain existing models and unintegrated fg

dijets in sd and dpe using rapidity gaps
Dijets in SD and DPE using rapidity gaps

- Gap defined by LUCID/ZDC + FCAL

- Look for hard scatter events with

SD: gap on one side of detector; DPE: two gaps on each side of detector


1) ETor η spectra of inclusive (ND) QCD dijets

2) Measure and from known (HERA) PDFs get

info on FDjj(β,Q2) and S2.

ξ<0.1 → 0(1) TeV Pomeron beams;  down to ~ 10-3& Q2 ~104 GeV2

Strong factorization breaking compared to HERA DPDFs → S2 ~ 0.1

(usually explained by multiple interactions / absorptions)

4) σ(DPEjj)/σ(NDjj): vary gap size → Sudakov effects and enhanced absorption

Advantages: - comparatively high rate

σjjDPE(ET>20 GeV)~10nb

- possibility to separate

different effects by studying one process

atlas diffractive measurements
ATLAS diffractive measurements

1) Diffractive enhanced MB events at √s = 7 TeV (ATLAS-CONF-2010-031)

2) Dijet production with a jet veto at √s = 7 TeV (ATLAS-CONF-2010-085)

ATLAS philosophy for the early data:

  • Do not extrapolate to full coverage with some MC model
  • Do not correct data for diffractive/non-diffractive background

First: understand well the detector and define the diffractive event

See talk by A. Pilkington

diffraction enhanced mb events
Diffraction enhanced MB events

1) Veto activity in MBTS on one side of IP

2) Ntrk≥ 1 (pT > 0.5 GeV, |η| < 2.5)

Calculate RSS = NSS/(NSS+NDS); SS = single-sided, DS = double-sided

Not corrected for detector effects

Ratio σSD/σDD kept fixed

to generator prediction

RSS sensitive to relative diffractive

X-section σdiff/σinel

Rate quite well modeled

by Pythia 6 and Pythia 8

diffraction enhanced mb events1
Diffraction enhanced MB events

1) Veto activity in MBTS on one side of IP

2) Ntrk≥ 1 (pT > 0.5 GeV, |η| < 2.5)

Not corrected for detector effects

Track properties nicely

described by Phojet

diffraction enhanced mb events2
Diffraction enhanced MB events

Not corrected for detector effects

In general: Phojet: SD>DD, Pythia6,Pythia8: SD~DD

pT tails: Phojet, Pythia8: SD~DD~ND, Pythia6: only ND (missing hard diffraction)

gaps between jets
Gaps between jets

Inclusive sample:1) Triggers L1_J5 .or. L1_J10 .or. L1_J15

2) Boundary jets: ET > 30 GeV, (ET1 + ET2)/2>60 GeV → get number of events NINCL

Gap events: 3) Inclusive sample + no jets with Q0>30 GeV between the jets in the dijet system

→ get number of events NGAP

Selection of boundary jets:

Selection A: 2 jets with highest ET

Selection B: Most backward and most forward jet

Not corrected for detector effects

Wide-angle radiation

BFKL-like dynamics

gaps between jets1
Gaps between jets

Selection of boundary jets: Selection A: 2 jets with highest ET, Selection B: Most backward and most forward jet

Everything here for Selection A). Very similar results for Selection B)

Gap Fraction = NGAP / NINCL

Corrected for detector effects

- Gap Fraction decreases

with ET and Δη

- Well described by Pythia 6

Next steps: more data

- enlarge Δη range

- lower jet veto cut Q0

atlas forward proton upgrade for high lumi
ATLAS Forward Proton Upgrade for High Lumi

[FP420 R&D Collab., JINST4 (2009) T10001]

physics with forward proton tagging at high lumi
Physics with forward proton tagging at high lumi

Photon-induced interactions


- Absolute lumi calibration, calibration of FDs

- Factorization breaking in hard diffraction

Hard SD/DPE (dijets, W/Z, …)

Gap Survival / Underlying event

High precision calibration for the Jet Energy Scale

Central Exclusive Production of dijets:


for CEP


[Phys.Rev. D77

(2008) 052004]

CDF: Observation of Exclusive Charmonium Prod. and

γγ→μμ in pp collisions at 1.96 TeV [arXiv:0902.1271]

Central Exclusive Production of Higgs

- Higgs mass, quantum numbers, discovery in MSSM

SM h→WW*, 140 < M < 180 GeV [EPJC 45 (2006) 401]

MSSM h→bb, h→ττ, 90 < M < 140 GeV

MSSM H→bb (90 < M < 300), H→ ττ (90 < M <160 GeV) [JHEP 0710:090,2008]

NMSSM h→aa→ττττ for 90 < M < 110 GeV

Triplet scenario[arXiv: 0901.3741]

rich p and physics via forward proton tagging
Rich γp and γγ physics via forward proton tagging

pp → p γ(*)γ(*) p →p X p, X = e+e-, μ+μ-, γγ, WW, ZZ, H, tt, SUSY-pairs

  • Lepton pair production in γγinteraction: large and well-known QED x-section → use to calibrate absolute LHC lumi and Forward detectors (at 420m).

(pT>10 GeV, |η|<2.5, one forward-proton tag: 50μ’s in a 12hrs run at 1033cm-2s-1)

- Anomalous quartic coupling inpp → p γγp →p WW pprocesses: greatly improved sensitivities

compared to LEP results (factors 103at L=1033cm-2s-1, 104 at L=1034cm-2s-1)

  • Diffractive Photoproduction of jets: study the issue of QCD factorization breaking
  • Exclusive Photoproduction of Υ: sensitive to the same skewed unintegrated fgas CEP of H

(σ ~ 1.25pb for Υ→μμ)


Final state topology

similar to CEP:

Rap.gap on the side

of intact proton



See talk by Ch.Royon

central exclusive production higgs
Central Exclusive Production: Higgs








pp  gg  H +x

pp p+H+p

Extra screening gluon conserves color, keeps proton intact (and reduces your σ)

x-section predicted

with uncertainty

of 3 or more

(KMR group,

Cudell et al.

Pasechnik, Szczurek)


This process is the core of the physics case

of Forward detector upgrade (AFP)

1) Protons remain intact and can be

detected in forward detectors

2) Rapidity gaps between leading protons

and Higgs decay products







mssm mass scan for cep higgs 5 contours
MSSM mass scan for CEP Higgs: 5σ-contours

h→bb, mhmax, μ = 200 GeV

H→bb, nomix, μ = 200 GeV







EPJC 53 (2008) 231: using proposed Forward detectors

Experimental efficiencies taken from

CERN/LHC 2006-039/G-124

Four luminosity scenarios (ATLAS+CMS):

1) 60 fb-1 – low lumi (no pile-up)

2) 60 fb-1 x 2 – low lumi (no pile-up) but improved signal efficiency

3) 600 fb-1 - high lumi (pile-up suppressed)

4) 600 fb-1 x2 – high lumi (pile-up suppressed) but improved signal efficiency

SM: Higgs discovery challenging


1) higher x-sections than in SM in certain scenarios and certain phase-space regions

2) the same BG as in SM


- Two new diffractive analyses with first ATLAS data presented:

1) Studies of diffractive enhanced Minimum Bias events in ATLAS

2) Measurement of dijet production with a jet veto in pp collisions at 7 TeV using ATLAS det.

And much more can be studied:

Low Luminosity (up to L~1033cm-2s-1):

- Elastic and σtot using ALFA

- Start with ratios X+gaps/X(incl), X=W,Z,jj,μμ …. Get S2

- Soft Diffraction using ALFA

- Dijets in SD, DPE and CEP

- Photon-induced processes useful for checks of CEP predictions

High Luminosity Upgrade (L > 1033cm-2s-1)

Possible upgrade (AFP) to install forward proton taggers at 220 and 420 m from IP

- Provides a good mass measurement of new physics

- pp→p+(γγ→μμ)+p as excellent tool for absolute calibration of AFP420

Urgent: Definition of rapidity gap

diffractive w z production
Diffractive W/Z production

Valery, DIS08

  • Test of soft survival S2
  • Test of absorption effects
  • Quark content of Pomeron PDFs

Small spread

of predictions

Suitable to

extracting S2

pp X+ RG+ W+ RG +Y  photon exchange dominates

Trigger: rapgap.AND.high-pt lepton

anomalous quartic coupling in processes
Anomalous quartic coupling in γγ processes

Low luminositypp → p γγp →p WW p High luminosity

Just two leptons ξ in acc. of FD

Ntrk ≤ 2 pTlep1 > 160 GeV, pTlep2>10 GeV MET>20 GeV

pTlep2/pTlep1 < 0.9

Δφ(lep1,lep2) < 3.1

E. Chapon, O. Kepka, Ch. Royon:

arXiv:0909.5237 [hep-ph]

arXiv:0908.1061 [hep-ph]

Improvement of 103 to LEP limits!

Improvement of 104 to LEP limits!

See talk by Ch.Royon