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

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1 Exclusive Processes at HERA Robert Ciesielski (Rockefeller University) Saturaon 1000+, Vth Workshop on QCD and Diffracon at the LHC 5–7 December 2016, Krakow, Poland for the H1 and ZEUS Collaboraons

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  • 1

    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 – 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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    Ψ(2S)/J/Ψ(1S) ratio in exclusive DIS

    R. Ciesielski, Exclusive Processes at HERA

    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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    Ψ(2S)/J/Ψ(1S) ratio in exclusive DIS

    R. Ciesielski, Exclusive Processes at HERA

    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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    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, 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

    R. Ciesielski, Exclusive Processes at HERA

    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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    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 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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    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 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

    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 : 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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    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) +0.47-0.40

    (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 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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    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● 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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    Exclusive ρ0 photoproduction with a LN

    R. Ciesielski, Exclusive Processes at HERA

    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