Transcript 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
Diffraction
Central Exclusive Production
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ATLAS Central Detector
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ATLAS Forward detectors
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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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
remnants
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Diffractive measurements
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 S2
pp → RG + W/Z + RG
pp → RG + W
pp → RG + jj + RG
pp → RG + Y + RG
Info on soft survival S2 (γ-exch. dominates for W)
Info on soft survival S2
Combined effect of all basic ingredients to CEP
(S2, Sudakov suppr., unintegr. fg, enhanced absorpt)
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:
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
P-tagging = info on proton pT, i.e. dσ/dt
WITH PROTON TAGGING BY ALFA
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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
- SD cross section measurement with ~ 15 % syst. uncertainty
- improve model predictions and background estimates for CEP
Expect 1.2-1.8 M events in 100 hrs at 1027cm-2s-1
Very good acceptance for
very low t and ξ.
Global acceptance:
Soft SD trigger:
ALFA.and.(LUCID.
Or.ZDC)
RP
RP
Pythia 45%
Phojet 40%
ZDC
LUCID
ATLAS
LUCID
ZDC
RP
RP
RP
RP
IP
RP
RP
240m
ZDC
140m
LUCID
17m
ATLAS
LUCID
ZDC
17m
140m
240m
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Experimental challenge: define diffractive event
First data → Soft diffraction
No proton tagger → try rapidity gaps
Generator level plots - provided by O. 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)
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Central Exclusive Production
X
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, …
= χb, χc, γγ
See talk by Valery Khoze
Advantages:
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.
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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CEP dijets with early data
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 Rjj distribution and constrain existing models and unintegrated fg
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 Rjj distribution,
,
Other variables: steeper leading jet ET and more back-to-back leading jets in CEP due to ISR suppression
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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
DIJET STUDY STRATEGY:
1) ET or 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
Advantages: - comparatively high rate
σjjDPE(ET>20 GeV)~10nb
- possibility to separate
different effects by studying one process
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
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ATLAS diffractive measurements
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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Diffraction enhanced MB events
Not corrected for detector effects
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
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
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Diffraction enhanced MB events
Not corrected for detector effects
1) Veto activity in MBTS on one side of IP
2) Ntrk ≥ 1 (pT > 0.5 GeV, |η| < 2.5)
Track properties nicely
described by Phojet
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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)
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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
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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
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ATLAS Forward Proton Upgrade for High Lumi
[FP420 R&D Collab., JINST4 (2009) T10001]
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Physics with forward proton tagging at high lumi
Photon-induced interactions
Diffraction
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)
NMSSM h→aa→ττττ for 90 < M < 110 GeV
Triplet scenario [arXiv: 0901.3741]
[JHEP 0710:090,2008]
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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
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).
(pT>10 GeV, |η|<2.5, one forward-proton tag: 50μ’s in a 12hrs run at 1033cm-2s-1)
- Anomalous quartic coupling in pp → p γγ p →p WW p processes: greatly improved sensitivities
compared to LEP results (factors 103 at L=1033cm-2s-1, 104 at L=1034cm-2s-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 fg as CEP of H
(σ ~ 1.25pb for Υ→μμ)
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Central Exclusive Production: Higgs
+”
“
Typical
Higgs
Production
pp  gg  H +x
=”
“
CEP
Higgs
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)
p
b,W,τ
H
This process is the core of the physics case
of Forward detector upgrade (AFP)
gap
p
gap
b,W,τ
1) Protons remain intact and can be
detected in forward detectors
2) Rapidity gaps between leading protons
and Higgs decay products
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MSSM mass scan for CEP Higgs: 5σ-contours
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
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Summary
- 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
Urgent: Definition of rapidity gap
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
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BACKUP
SLIDES
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Diffractive W/Z production
•
•
•
Test of soft survival S2
Test of absorption effects
Quark content of Pomeron PDFs
Small spread
of predictions
Suitable to
extracting S2
Valery, DIS08
pp X+ RG+ W+ RG +Y  photon exchange
dominates
Trigger: rapgap.AND.high-pt lepton
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Anomalous quartic coupling in γγ processes
pp → p γγ p →p WW p
Low luminosity
Ntrk ≤ 2
Just two leptons
pTlep1 > 160 GeV, pTlep2>10 GeV
Improvement of 103 to LEP limits!
E. Chapon, O. Kepka, Ch. Royon:
arXiv:0909.5237 [hep-ph]
arXiv:0908.1061 [hep-ph]
See talk by Ch.Royon
High luminosity
ξ in acc. of FD
MET>20 GeV
pTlep2/pTlep1 < 0.9
Δφ(lep1,lep2) < 3.1
Improvement of 104 to LEP limits!
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