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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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


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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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 – Bethe-Heither 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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Mass spectra and backgroud subtraction, 2-track sample:

J/ψ → μ+μ-ψ(2S) → μ+μ-

Non-resonant di-muon 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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Mass spectra and backgroud subtraction, 4-track 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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Ratios vs Q2, W and t:

● Increasing with Q2● Independent of W and t

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Comparison with theory predictions:

H1 measurements:Eur. Phys. J. C10 (1999) 373

HIKT, Hüfner et al.: dipole model, dipole-proton constrained by inclusive DIS dataAR, Armesto and Rezaeian: impact parameter dependent CGC and IP-Sat modelKMW, Kowalski Motyka Wat: QCD description and universality of quarkonia productionFFJS, Fazio et al.: two component Pomeron modelKNNPZZ, Nemchik et al.: color-dipole cross section derived from BFKL generalised eq.LM, Lappi and Mäntysaari : dipole picture in IP-Sat 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


Resolved Pomeron model Two-Gluon Exchange model

Motivation:

Resolved pomeron model in DDIS describes well various event observables in dijet productionTwo-gluon 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 Two-Gluon Exchange model

J.Bartels et al., PLB 386 (1996) 389

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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 Two-Gluon 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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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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Comparison to theory predictions:

Data vs Resolved Pomeron and Two-Gluon 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), Two-Gluon Exchange successful for β>0.3

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Exclusive ρ0 photoproduction with a LN


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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Signal and background processes:

σ(γp → ρ0nπ+) = |Aa + Ab + Ac |2 :: interference effects

For large s and t’ → 0 : pion exchange dominates

Signal : Drell-Hiida-Deck diagrams Background

pion exchange neutron exchange “direct pole” proton dissociation

γp → ρ0nπ+ γp → ρ0nYπ+

fbg

= 0.34 ± 0.05

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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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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) +0.47-0.40

(model) μb

(θn = 24 GeV

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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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Differential cross section dσγp

/dt' of ρ 0 meson:

ρ0 slope strongly changing from low-t’ to high-t’ region, as expected for a double-peripheral 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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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● Two-gluon-exchange 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 t-dependence of the γp → ρ0nπ+ cross section shows a behaviour typical

for exclusive double peripheral exchange processes● The elastic photon-pion cross section, σ(γπ+ → ρ0π+) at < W

γπ > = 24 GeV

extracted in the one-pion-exchange 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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All properties of the selected π+π- are consistent with ρ0 photoproduction

Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23

Exclusive Processes at HERA · 2018. 11. 20. · exclusive ep scattering at HERA Production of...

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