Exclusive Processes at HERA · 2018. 11. 20. · exclusive ep scattering at HERA Production of...
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Exclusive Processes at HERA
Robert Ciesielski(Rockefeller University)
Saturation 1000+, Vth Workshop on QCD and Diffraction at the LHC5–7 December 2016, Krakow, Poland
for the H1 and ZEUS Collaborations

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● Measurement of the cross section ratio σ(Ψ(2S))/σ(J/Ψ(1S)) in deep inelasticexclusive ep scattering at HERA
● Production of exclusive dijets in diffractive deep inelastic scattering at HERA
● Exclusive ρ0 meson photoproduction with a leading neutron at HERA
R. Ciesielski, Exclusive Processes at HERA
Recent H1 and ZEUS results on exclusive production:
Nucl. Phys. B 909 (2016) 934
Eur. Phys. J. C76 (2016) 1, 41
Eur. Phys. J. C76 (2016) 1, 16
Outlook

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R. Ciesielski, Exclusive Processes at HERA
Inclusive and Exclusive Diffraction
Inclusive Exclusive
N=proton – elastic processN= dissociated proton system – pdiss process

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Ψ(2S)/J/Ψ(1S) ratio in exclusive DIS
R. Ciesielski, Exclusive Processes at HERA
Ratio sensitive to radial wave function of charmonium
ψ(2S) wave function different from J/ψ wave function:
● Has a node at ~0.35 fm● ≈ 2
pQCD calculations predict: R ~ 0.17 (PhP) and rise of R with Q2 (DIS)
Motivation:

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Ψ(2S)/J/Ψ(1S) ratio in exclusive DIS
R. Ciesielski, Exclusive Processes at HERA
Data: Full statistics of HERA I and HERA II (1996 – 2007) Integrated luminosity 468/pb
Samples and event selection:
Decay channels: J/ψ → μ+μ ψ(2S) → μ+μ ψ(2S) → J/ψ π+π , J/ψ → μ+μ
Monte Carlo samples:● Signal – exclusive VM production with DIFFVM● Background – BetheHeither μ+μ production with GRAPE
Event selection:● Scattered electron with E > 10 GeV measured in CAL● Two reconstructed tracks identified as muons ● for ψ(2S) → J/ψ π+π additionally two pion tracks from μμ vertex● Nothing else in the detector (above noise)
Scattered proton undetected, contribution from proton dissociative events with MN < 4 GeV. Assuming
cross section ratio does not vary with MN
– results not affected by proton dissociation background.
Kinematic region:
● 30 < W < 210 GeV● 2 < Q2 < 80 GeV2● t < 1 GeV2

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Ψ(2S)/J/Ψ(1S) ratio in exclusive DIS
R. Ciesielski, Exclusive Processes at HERA
Mass spectra and backgroud subtraction, 2track sample:
J/ψ → μ+μψ(2S) → μ+μ
Nonresonant dimuon background fitted with a straight line in the sideband regions: 2.99 < Mμ+μ < 2.62 GeV 4.05 < Mμ+μ < 5.00 GeV
then subtracted from the signal regions: 3.59 < Mμ+μ < 3.79 GeV 3.02 < Mμ+μ < 3.17 GeV

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Ψ(2S)/J/Ψ(1S) ratio in exclusive DIS
R. Ciesielski, Exclusive Processes at HERA
Mass spectra and backgroud subtraction, 4track sample:
ψ(2S) → J/ψ π+π J/ψ → μ+μ
ΔM = Mμμππ  Mμμ
No background events in the ψ(2S) signal region: 3.02 < Mμμ < 3.17 GeV 0.5 < ΔM < 0.7 GeV
(upper limit of 3 events at 90% C.L. estimated)
After cut on Mμμ

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Ψ(2S)/J/Ψ(1S) ratio in exclusive DIS
R. Ciesielski, Exclusive Processes at HERA
Ratios vs Q2, W and t:
● Increasing with Q2● Independent of W and t

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Ψ(2S)/J/Ψ(1S) ratio in exclusive DIS
R. Ciesielski, Exclusive Processes at HERA
Comparison with theory predictions:
H1 measurements:Eur. Phys. J. C10 (1999) 373
HIKT, Hüfner et al.: dipole model, dipoleproton constrained by inclusive DIS dataAR, Armesto and Rezaeian: impact parameter dependent CGC and IPSat modelKMW, Kowalski Motyka Wat: QCD description and universality of quarkonia productionFFJS, Fazio et al.: two component Pomeron modelKNNPZZ, Nemchik et al.: colordipole cross section derived from BFKL generalised eq.LM, Lappi and Mäntysaari : dipole picture in IPSat model
All models describe the increase of the σ(ψ(2S))/σ(J/ψ) ratio with Q2

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Exclusive dijet production in DIS
R. Ciesielski, Exclusive Processes at HERA
Resolved Pomeron model TwoGluon Exchange model
Motivation:
Resolved pomeron model in DDIS describes well various event observables in dijet productionTwogluon exchange well suited for exclusive dijets
Distribution of angle φ between the lepton and dijet production planes allows to distinguish between both theoretical approaches.
A > 0 in Resolved Pomeron modelA < 0 in TwoGluon Exchange model
J.Bartels et al., PLB 386 (1996) 389

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Exclusive dijet production in DIS
R. Ciesielski, Exclusive Processes at HERA
Data: Full statistics of HERA II (2003 – 2007), Integrated luminosity 372/pb
Data sample and event selection:
Monte Carlo:● SATRAP for diffractive sample and detector level corrections ● ARIADNE and PYTHIA6.2 for nondiffractive and PhP backgrounds● RAPGAP for model predictions at hadron level: Resolved Pomeron (G. Ingeman
and P. Schlein) and TwoGluon Exchange (J. Bartels and H. Jung)
Jet reconstruction: Kinematic region: ● Durham jet algorithm in γ*IP rest frame
in exclusive mode – all objects in jets● Resolution parameter  ymin=0.15
Proton dissociation background f
pdiss = 45% ±4% (stat) ± 15% (sys)

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Exclusive dijet production in DIS
R. Ciesielski, Exclusive Processes at HERA
Cross sections dσ/dβ and dσ/dφ in bins of β:β = x / x
IP – Pomeron momentum fraction
entering hard subprocess
dσ/dφ cross section fitted with:
→ β dependence of A parameter

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Exclusive dijet production in DIS
R. Ciesielski, Exclusive Processes at HERA
Comparison to theory predictions:
Data vs Resolved Pomeron and TwoGluon Exchange models:
● None of the models predicts correctly the magnitute of cross section.Factor of 2 difference → NLO ?
● Resolved Pomeron model does not reproduce A(β), TwoGluon Exchange successful for β>0.3

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Exclusive ρ0 photoproduction with a LN
R. Ciesielski, Exclusive Processes at HERA
Motivation:First measurement of ρ0 photoproduction with a leading neutron at HERA
measure γπ+ → ρ0π+ cross section
Mean W ~ 24 GeV is low → soft regime, Regge framework most appropriate:
exchange of two Regge trajectoriesin a Double Peripheral Process (DPP)
Constraints to pion flux models
Study of absorption effects in leading baryon production
A presence of the neutron carrying a large fraction of the proton beam momentum x
L is accompanied
by the pion exchange process
→

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Exclusive ρ0 photoproduction with a LN
R. Ciesielski, Exclusive Processes at HERA
Signal and background processes:
σ(γp → ρ0nπ+) = Aa + Ab + Ac 2 :: interference effects
For large s and t’ → 0 : pion exchange dominates
Signal : DrellHiidaDeck diagrams Background
pion exchange neutron exchange “direct pole” proton dissociation
γp → ρ0nπ+ γp → ρ0nYπ+
fbg
= 0.34 ± 0.05

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Exclusive ρ0 photoproduction with a LN
R. Ciesielski, Exclusive Processes at HERA
Data: 2006 – 2007 e+p data, integrated luminosity 1.16/pb
Samples and event selection:
LN detection: Forward Neutron Calorimeter (FNC) located at 106 m from the H1 interaction point limited acceptance : ~ 30% p
T,n < xL ·0.69 GeV (θ
n = 0.04 GeV2 – photoproduction events● Only two oppositely charged pion candidates
in the central tracker with 0.3 < Mππ < 1.5 GeV● A hadronic cluster in the FNC with E
n > 120 GeV
● Nothing else in the detector (above noise level)
OPE dominant range : pT,n
< 0.2 GeV
All properties of the selected π+π are consistent with ρ0 photoproduction

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Exclusive ρ0 photoproduction with a LN
R. Ciesielski, Exclusive Processes at HERA
Cross section measurement:
Kinematic region:
● 20 < Wγp
< 100 GeV● 2m
π < Mρ < Mρ + 5Γρ
● t’ < 1 GeV2● 0.35 < x
L < 0.95
σ(γp → ρ0nπ+) = 310 ± 6(stat) ± 45(syst) nb
σ(γp → ρ0nπ+) = 130 ± 3(stat) ± 19(syst) nb
σ(γπ+ → ρ0π+) = 2.33 ± 0.34 (exp) +0.470.40
(model) μb
(θn = 24 GeV

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Exclusive ρ0 photoproduction with a LN
R. Ciesielski, Exclusive Processes at HERA
Contraining pion flux:
● xL distribution of the leading neutron generally well described
● FMS and NSSS pion fluxes disfavoured
● None of the models can reproduce the t dependence of the leading neutron – effect of absorptive corrections ?
Exponential bn slope from fits to
neutron pT
2 spectra.

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Exclusive ρ0 photoproduction with a LN
R. Ciesielski, Exclusive Processes at HERA
Differential cross section dσγp
/dt' of ρ 0 meson:
ρ0 slope strongly changing from lowt’ to hight’ region, as expected for a doubleperipheral process – slope dependence on the invariant mass of the nπ+ system
Geometric picture :
Photon finds pion in a cloud which extends far beyond the proton radius
< r2 > = 2b1·(hc)2 ~ 2 fm2 ~ (1.6R
p)2

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Exclusive ρ0 photoproduction with a LN
R. Ciesielski, Exclusive Processes at HERA
γπ+ → ρ0π+ cross section:
Compare γπ+ → ρ0π+ and γp → ρ0p cross sections :
in agreement with a previous ZEUS measurement (NPB 637 (2002) 3).Significantly lower than expected → large absorption corrections : k
abs = 0.44 ± 0.11
σ(γπ+)/σ(γp) = 0.25 ± 0.06 at = 24 GeV

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Summary● Cross section ratio ψ(2S)/J/ψ(1S) measured by ZEUS
● The ratio grows with Q2 and is constant with W and t● Comparison with model prodictions: all predictions are consistent with the
data, some discrimination between the models possible
● First measurement of exclusive dijets in diffractive DIS by ZEUS● Cross section significantly larger than predicted by models● Twogluonexchange model more successful in describing the data at high β
than resolved Pomeron model
● Exclusive ρ0 photoproduction associated with a leading neutron measured for the first time at HERA by H1● The tdependence of the γp → ρ0nπ+ cross section shows a behaviour typical
for exclusive double peripheral exchange processes● The elastic photonpion cross section, σ(γπ+ → ρ0π+) at < W
γπ > = 24 GeV
extracted in the onepionexchange approximation● The cross section ratio for elastic photoproduction of the ρ0 meson on the
pion and on the proton suggest large absorption corrections.
Thank you for your attention!

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Backup slides

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Exclusive ρ0 photoproduction with a LN
R. Ciesielski, Exclusive Processes at HERA
All properties of the selected π+π are consistent with ρ0 photoproduction
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