Exclusive ρ 0 Meson Photoproduction with a Leading Neutron at HERA Lidia Goerlich Institute of Nuclear Physics PAS, Kraków on behalf of the H1 collaboration ● Introduction ● Selection of events ● Drell-Hiida-Deck diagrams and diffractive background ● Extraction of the ρ 0 signal ● γ p and γπ cross sections ● Summary [ Eur. Phys. J. C76 (2016) 41 ] Meson2016 2-7 June, 2016 Kraków, Poland 1
HERA E p = 920 - 460 GeV E e± = 27.6 GeV ● HERA – the world’s only ep collider HERA (1992 – 2007) ● two colliding beam experiments : H1 and ZEUS γ * p → hadrons (Q 2 ) Q 2 – photon virtuality Q 2 >> 1 GeV 2 deep inelastic scattering (DIS) Q 2 ~ 0 GeV 2 photoproduction 2
Physics motivation e’ e + + p → e + + ρ 0 + n + π + , ρ 0 → π + π - γ Kinematics: e ● √ s ep centre-of-mass energy ( √ s = 319 GeV) ● Q 2 photon virtuality ( photoproduction ) W γ p ● y inelasticity γ p centre-of-mass energy ● W γ p ● x L fraction of the proton energy carried by the leading neutron p ● t |4-momentum transfer| 2 at the proton vertex |4-momentum transfer| 2 at the γρ vertex ● t’ A virtual photon emitted from the electron interacts with a virtual pion from the proton cloud ● exclusive ρ 0 photoproduction on virtual pion : first extraction of elastic photon-pion cross section σ ( γπ + → ρ 0 π + ) ● sensitivity to pion flux models ● importance of absorption effects in leading baryon production at HERA 3
e + + p → e + + ρ 0 + n + π + , ρ 0 → π + π - Forward Neutron Calorimeter ( FNC ) H1 FNC @ H1 ● Lead-scintillator sandwich calorimeter at 106 m from IP ● Detection of n and γ / π 0 ● Preshower: 60 X 0 Main Calo : 8.9 λ <A( θ < 0.8 mrad ) > ≈ 30% ● Selection of exclusive events in untagged ( scatterd e + not detected) photoproduction: ● 2 oppositely charged tracks in Central Tracker (low multiplicity Fast Track Trigger) ● leading neutron in FNC ( forward π + from the proton vertex not measured ) ● no additional signals above noise in the main H1 calorimeters and forward detectors 4
Phase space of measurements γ * + p → ρ 0 n π + , ρ 0 → π + π - ● Photoproduction : Q 2 < 2 GeV 2 , <Q 2 > = 0.04 GeV 2 ● Low p T of ρ 0 : |t’| < 1 GeV 2 , < |t’| > = 0.20 GeV 2 ● Small ρ 0 mass : 0.3 < M ππ < 1.5 GeV ● π + , π - in 20 < W γ p < 100 GeV, Central Tracker : < W γ p > = 45 GeV ● Leading neutron: E n > 120 GeV, θ n < 0.75 mrad Data sample : L = 1.16 pb -1 , ~ 7000 events Soft process without hard scale present → application of Regge formalism Unique measurement of Double Peripheral Process mediated by exchange of two, pion and Pomeron, Regge trajectories Pomeron – phenomenological object with vacuum quantum numbers 5
Drell-Hiida-Deck diagrams and diffractive backgrounds Pion exchange Neutron exchange Direct pole Proton dissociation (a) (b) (c) (d) DiffVM MC, Regge theory + Vector Pompyt MC, signal (a) = pion flux + elastic Dominance Model, elastic, single & scaterring of pion on photon double-dissociation processes; Estimation of background from ω (782), φ (1020) and ρ ’(1450-1700) The Drell-Hiida-Deck model (contributions of graphs a, b, c): ● At large s and t → 0 A b ≈ -A c and π -exchange contribution dominates ● Interference effects are important to explain data σ ( γ p → ρ 0 n π + ) = |A a + A b + A c | 2 ● Slope of t’ distribution dependent on M n π + → importance of interference effects 6
Extraction of the ρ -meson signal ● Distortion of the ρ 0 mass shape due to the interference between the resonant and non-resonant π + π - production is characterised by the Ross&Stodolsky skewing parameter n RS ● contribution for the reflection from ω (782) → π + π - π 0 added Analysis region (0.6 < M ππ < 1.1 GeV) extrapolated using BW to the full mass range: 2m π < M ππ < M ρ + 5 Γ ρ 2 of the π + π - system ● skewing parameter n RS vs. p T ● H1 and ZEUS values of n RS ( from γ p → ρ 0 p ) 7 are in agreement
Extraction of the ρ -meson signal Polar angle distribution of the π + 04 r in the helicity frame : Spin-density matrix element 00 probability that ρ 0 has helicity 0 ─ Empirical fit: 04 in diffractive ρ 0 photo- and r 00 electro-production at HERA 8
Signal and background decomposition ● Different shapes of leading neutron energy for signal and background ● background mostly due to proton dissociation ● shape of signal and background modelled by POMPYT and DIFFVM MC Background fraction fit to the data : F bg = B / ( S+B ) = 0.34 ± 0.05 Shape comparison control plots 9
σ γ p and σ γπ cross sections One-pion-exchange approximation σ γ p = σ ep / Φ γ σ γπ = σ γ p / Γ π photon flux : pion flux : Φ γ = ∫ f γ /e ( y, Q 2 ) dydQ 2 Γ π = ∫ f π /p ( x L ,t ) dx L dt Effective photon flux from the Vector Dominance Model converts the ep cross section into a real γ p cross section at Q 2 = 0 Non-Reggeized pion flux in the light-cone representation ( Holtmann et al . ) R π n – radius of the pion-neutron Fock state 10
σ γ p and σ γπ cross sections γ p cross section N bgr – diffractive dissociation background from MC L – integrated luminosity A · ε – correction for detector acceptance and efficiency F – photon flux integrated over kinematic region 20 < W γ p < 100 GeV, Q 2 < 2 GeV – numerical factor extrapolating from the measured π + π - mass range C ρ to full BW resonance γ p cross section in the range 0.35 < x L < 0.95 and averaged over 20 < W γ p < 100 GeV, precision: δ stat = 2%, δ sys = 14.6% σ ( γ p → ρ 0 n π + ) = (310 ± 6 stat ± 45 sys ) nb, for θ n < 0.75 mrad (full acceptance of FNC) σ ( γ p → ρ 0 n π + ) = (130 ± 3 stat ± 19 sys ) nb, for p T, n < 0.2 GeV (OPE dominated range) cross section of elastic photoproduction of ρ 0 on a pion target OPE σ el ( γπ + → ρ 0 π + ) = (2.33 ± 0.34(exp) ± 0.47 0.40 (model)) µ b, for <W γπ > = 24 GeV 11
Energy dependence of σ ( γ p → ρ 0 n π + ) ● Regge motivated power law fit σ ≈ W δ : δ = -0.26 ± 0.06 stat ± 0.07 sys ● POMPYT MC prediction : only Pomeron exchange 12 12
d σ γ p /dx L : constraining pion flux d σ γ p / dx L compared in shape to predictions based on different models for the pion flux Pion flux models Shape of the x L distribution disfavoured by the data reproduced by most of the pion flux models 13
d 2 σ γ p / (dx L dp T,n 2 ) : constraining pion flux x L dependence of the p T slope Fit by exponential function 2 ] of the leading neutron exp[-b n (x L )p T,n is not described by models in each x L bin → importance of absorptive corrections ? 14
Estimation of absorption corrections p r el = σ el ( γπ → ρ 0 π ) / σ el ( γ p → ρ 0 p) = 0.25 ± 0.06 (experiment) r el = 0.57 ± 0.03 (theory : Optical theorem + eikonal approach + data ) Absorption factor K abs = 0.44 ± 0.11 15
Differential cross section d σ γ p /dt’ of ρ 0 Strongly changing slope between the low-t’ and high-t’ b 1 = 25.7 ± 3.2 GeV -2 , b 2 = 3.62 ± 0.32 GeV -2 ● Geometric picture: <r 2 > = 2b 1 · ( ħ c) 2 ≈ 2fm 2 ≈ (1.6R p ) 2 → ρ 0 produced at large impact parameter (ultraperipheral process) ● Double Peripheral Process interpretation: slope of t’ distribution dependent on the invariant mass of the ( n π + ) system , low M(n π + ) → large slope, high M(n π + ) → less steep slope 16
Summary ● The photoproduction cross section for exclusive ρ 0 production associated with a leading neutron is measured for the first time at HERA ● Differential cross sections for the reaction γ p → ρ 0 n π + show behaviour typical for exclusive double peripheral process ● The elastic photon-pion cross section, σ ( γπ + → ρ 0 π + ), is extracted in the one-pion-approximation ● The differential cross sections for the leading neutron are sensitive to the pion flux models ● The estimated cross section ratio r el = σ el ( γπ → ρ 0 π ) / σ el ( γ p → ρ 0 p) = 0.25 ± 0.06 suggests large absorption corrections, of the order of 60%, suppressing the rate of the studied reaction γ p → ρ 0 n π + 17 17 17
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