Diffraction 2010, Otranto, Italy - 12/09 2010
Diffraction
Central Exclusive Production
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2

10.6 < | η | < 13.5
| η | > 8.3
5.6 < | η | < 5.9
2.1 < | η | < 3.8
Not yet fully installed
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-
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 remnants
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EARLY DATA
→ WITHOUT PROTON TAGGING
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 S 2 pp → RG + W/Z + RG Info on soft survival S 2 ( γ-exch. dominates for W) pp
→ RG + W Info on soft survival S 2 pp → RG + jj + RG Combined effect of all basic ingredients to CEP
(S 2 , Sudakov suppr., unintegr. f g pp → RG + Y + RG Info on unintegrated f g
, enhanced absorpt)
( 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 p
T
ALFA: σ tot
, d σ el
/dt, σ
SD
(low M) , d 2 σ
SD
/dtd ξ, d 2 σ
DPE
/d ξ
1 d ξ
2
, i.e. d σ/dt
- tests model assumptions,
- governs rates of Pile-up bg
WITH PROTON TAGGING BY ALFA
- Strongly restricts S 2 (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
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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 10 27 cm -2 s -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:
Soft SD trigger:
ALFA.and.(LUCID.
Or.ZDC)
Pythia 45%
Phojet 40%
RP RP
ZDC
LUCID
LUCID
ZDC
RP RP
RP RP
240m
ZDC
140m
LUCID
17m
IP
LUCID
17m
ZDC
140m
RP RP
240m
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First data → Soft diffraction
No proton tagger → try rapidity gaps
Generator level plots - provided by O. Kepka, P. R ůžička, 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) 7

Exclusive:
1) Protons remain intact and can be detected in forward detectors
2) Rapidity gaps between leading protons and central system
X = jj, WW, Higgs, …
Advantages:
= χ b
, χ c
, 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 J
Z
strong suppression of CEP gg→bb background (by (m
- produced central system is 0
= 0, C-even, P-even state:
++ b
/M
X
) 2 )
→ just a few events are enough to determine Higgs quantum numbers.
Find a CEP resonance and you have measured its 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!
Disadvantages:
- Low signal x-section; affected by Pile-up
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BG: Incl. QCD dijets SD dijets DPE dijets
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 R jj distribution and constrain existing models and unintegrated f g
Central system produced in J z
=0, C-even, P-even state → quark jets suppressed by m q
2 /M jj
2
Trigger: Low-Et jet .AND. Veto in MBTS -eff. ~ 65% for CEP wrt jet turn-on; efficiently reduces Incl.QCD bg (by 10 4 )
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) N track
(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 R jj distribution, ,
Other variables: steeper leading jet E
T and more back-to-back leading jets in CEP due to ISR suppression
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- 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
DIJET STUDY STRATEGY:
1) E
T or η spectra of inclusive (ND) QCD dijets
2) Measure and from known (HERA) PDFs get info on F D jj
( β,Q 2 ) and S 2 .
ξ<
0.1
→ 0(1) TeV Pomeron beams;  down to ~ 10 -3 & Q 2 ~10 4 GeV 2
Advantages : - comparatively high rate
σ jj
DPE (E
T
>20 GeV)~10nb
- possibility to separate different effects by studying one process
Strong factorization breaking compared to HERA DPDFs → S 2 ~ 0.1
(usually explained by multiple interactions / absorptions)
4) σ(DPE jj
)/ σ(ND jj
): vary gap size → Sudakov effects and enhanced absorption
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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
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)
See talk by A. Pilkington
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Not corrected for detector effects
1) Veto activity in MBTS on one side of IP
2) N trk
≥ 1 (p
T
> 0.5 GeV, |η| < 2.5)
Calculate R
SS
= N
SS
/(N
SS
+N
DS
); SS = single-sided, DS = double-sided
Ratio
σ SD /
σ DD kept fixed to generator prediction
R
SS sensitive to relative diffractive
X-section σ diff / σ inel
Rate quite well modeled by Pythia 6 and Pythia 8
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Not corrected for detector effects
1) Veto activity in MBTS on one side of IP
2) N trk
≥ 1 (p
T
> 0.5 GeV, |η| < 2.5)
Track properties nicely described by Phojet
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Not corrected for detector effects
In general: Phojet: SD>DD, Pythia6,Pythia8: SD~DD p
T tails: Phojet, Pythia8: SD~DD~ND, Pythia6: only ND (missing hard diffraction)
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Inclusive sample :1) Triggers L1_J5 .or. L1_J10 .or. L1_J15
2) Boundary jets: E
T
> 30 GeV, (E
T1
+ E
T2
)/2>60 GeV
→ get number of events N
INCL
Gap events : 3) Inclusive sample + no jets with Q
0
>30 GeV between the jets in the dijet system
→ get number of events
N
GAP
Selection of boundary jets:
Selection A : 2 jets with highest E
T
Selection B : Most backward and most forward jet
Not corrected for detector effects
Wide-angle radiation
BFKL-like dynamics
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Selection of boundary jets: Selection A : 2 jets with highest E
T
, Selection B : Most backward and most forward jet
Everything here for Selection A). Very similar results for Selection B)
Gap Fraction = N
GAP
/ N
INCL
Corrected for detector effects
- Gap Fraction decreases with E
T and Δη
- Well described by Pythia 6
Next steps: more data
- enlarge Δη range
- lower jet veto cut Q
0
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[FP420 R&D Collab., JINST4 (2009) T10001]
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Diffraction
Photon-induced interactions
Hard SD/DPE (dijets, W/Z, …)
Gap Survival / Underlying event
High precision calibration for the Jet Energy Scale
- Absolute lumi calibration, calibration of FDs
- Factorization breaking in hard diffraction
Central Exclusive Production of dijets:
Evidence for CEP
[Phys.Rev. D77
(2008) 052004]
[arXiv:0908.2020]
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]
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pp
→ p γ (*) γ (*) p
→ p X p, X = e + e , μ + μ , γγ, WW, ZZ, H, tt, SUSY-pairs
Photoproduction
Final state topology similar to CEP:
Rap.gap on the side of intact proton
Diffraction
[arXiv:0908.2020]
- Lepton pair production in γγ interaction : large and well-known QED x-section → use to calibrate absolute LHC lumi and Forward detectors (at 420m).
(p
T
>10 GeV, |η|<2.5, one forward-proton tag: 50μ’s in a 12hrs run at 10 33 cm -2 s -1 )
- Anomalous quartic coupling in pp
→ p γγ p
→ p WW p processes: greatly improved sensitivities compared to LEP results (factors 10 3 at L=10 33 cm -2 s -1 , 10 4 at L=10 34 cm -2 s -1 )
See talk by Ch.Royon
- Diffractive Photoproduction of jets : study the issue of QCD factorization breaking
- Exclusive Photoproduction of Υ
: sensitive to the same skewed unintegrated f g as CEP of H
( σ ~ 1.25pb for Υ → μμ)
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Typical
Higgs
Production
+
=
CEP
Higgs



x-section predicted with uncertainty of 3 or more
(KMR group,
Cudell et al.
Pasechnik, Szczurek) b,W,τ
This process is the core of the physics case of Forward detector upgrade (AFP) gap p gap
H p
1) Protons remain intact and can be detected in forward detectors
2) Rapidity gaps between leading protons and Higgs decay products b,W,τ
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h →bb, mhmax, μ = 200 GeV H →bb, nomix, μ = 200 GeV
Tevatron exclusion region
LEP
Exclusion region
SM: Higgs discovery challenging
MSSM:
1) higher x-sections than in SM in certain scenarios and certain phase-space regions
2) the same BG as in SM
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 21

- 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.
(up to L~10 33 cm -2 s -1 ):
- Elastic and σ tot using ALFA
- Start with ratios X+gaps/X(incl), X=W,Z,jj,
μμ …. Get S 2
- Soft Diffraction using ALFA
- Dijets in SD, DPE and CEP
- Photon-induced processes useful for checks of CEP predictions
(L > 10 33 cm -2 s -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
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B A C K U P S L I D E S
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• Test of soft survival S 2
• Test of absorption effects
• Quark content of Pomeron PDFs
Valery, DIS08
Small spread of predictions
Suitable to extracting S 2 pp  X+ RG+ W+ RG +Y dominates
 photon exchange
Trigger: rapgap.AND.high-pt lepton
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Low luminosity
pp
→ p γγ p
→ p WW p High luminosity
Just two leptons ξ in acc. of FD
N trk
≤ 2 p
T lep1 > 160 GeV, p
T lep2 >10 GeV MET>20 GeV
Improvement of 10 3 to LEP limits!
E. Chapon, O. Kepka, Ch. Royon: arXiv:0909.5237 [hep-ph] arXiv:0908.1061 [hep-ph]
See talk by Ch.Royon
p
T lep2 /p
T lep1 < 0.9
Δφ(lep1,lep2) < 3.1
Improvement of 10 4 to LEP limits!
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