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On behalf of the ZEUS and H1 Collaborations

Enrico Tassi Univ. Autonoma de Madrid enrico.tassi@uam.es. Measurements of F 2 and F L at low-Q 2 in e-p interactions at HERA. On behalf of the ZEUS and H1 Collaborations. Outline Motivation

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On behalf of the ZEUS and H1 Collaborations

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  1. Enrico Tassi Univ. Autonoma de Madrid enrico.tassi@uam.es Measurements of F2 and FL at low-Q2 in e-p interactions at HERA On behalf of the ZEUS and H1 Collaborations • Outline • Motivation • Zeus and H1 low-Q2 analyses • New measurements of F2 and FL • Summary and Outlook

  2. Motivation • The soft and hard regimes of QCD: • Regge Phenomenology: successful in describing soft peripheral • processes at high energy - no QCD picture of underlying dynamics yet • -pQCD: extremely successful in describing hard-scattering processes • (down to surprisingly small scales: Q2~1 GeV2) • Can we merge them into a unified QCD picture ? • - HERA has played/is playing an important role towards this goal • → new F2 and FL measurements in the transition region… • (→ this talk) e’ e Photon virtuality allows to smoothly interpolate between non pert. and pert. regions p

  3. Data sets and kinematic coverage Nominal DIS Focus on new low-Q2 analyses: • ZEUS ISR analysis • H1 SVTX 2000 • H1 99 (min. bias) • H1 Compton analysis (covers high-x region) Zeus BPT • Precision H1/Zeus inclusive data cover the transition region between perturbative and non-perturbative domains (Q2 ~ 1GeV2), in a wide range of x and y

  4. ZEUS F2 BPT – (reminder) Phase space: 0.045 < Q2 < 0.65 GeV2 6x10-7 < x < 10-3 F2: - Typical uncert. < 4% - Rise persists down to the smallest Q2 Regge fit gives good description:

  5. ZEUS F2 BPT – (reminder) ~F2/Q2 ?

  6. ZEUS ISR analysis • Emission of ISR γ => reduction of Ee • Lower Ee -> lower Q2 • Measure F2 • Emission of ISR γ => reduction of √s • Access range of y values (for fixed x,Q2) • Needed for measuring FL (see later) -ISR sample well understood -Higher-x reach in some Q2 regions -Statistics at low-x still rather low (but 96 only)

  7. F2 from Compton analysis (H1) • e + p → e + γ + X • kinematics reconstructed with hadrons → detailed final state simulation at low W2 • Luminosity: 9.25 pb-1 • F2 at very high x accessed; domain of fixed target experiments • Complementary measurement to inclusive DIS

  8. Spaghetti Calorimeter Shifted vertex Nominal vertex e+ p ~70cm Backward Silicon Tracker Access to very low Q2 by shifting vertex(H1) • Shifting vertex opens detector acceptance at low Q2 • Precise reconstruction of the scattered positron: ( Δθ = 0.3 mrad, ΔE = 0.3% @ beam energy) • Measure very low positron energy (~3GeV) -> high-y

  9. New low-Q2 F2 (H1)

  10. Rise of F2 towards low x • H1 / ZEUS / NMC data used to fit Q2 dependence for x < 0.01 : • λ(Q2) ~ ln[Q2/Λ2] and c(Q2) ~ const. for Q2 > 3.5 GeV2 • Behaviour is changing at around Q2~ 1 GeV2 • From soft hadronic interactions it is expected that λ→ ~0.08 for Q2→ 0

  11. FL measurements: Glue at low-Q2 From the Zeus NLO-QCD fit For Q2≥ 5 GeV2 gluon density much larger than the sea At lower Q2, xS continues to gently rise at low x whereas the gluon density becomes valence like and tends to be negative Measuring FL around Q2 ~ 1 GeV2 could tell us a lot… Zeus and H1: Two different approaches

  12. First direct determination of FL from ISR (ZEUS) 96+97 sample: ~36 pb-1 1 < Q2 < 30 GeV2 0.11 < y < 0.23 1. Measure F2 2. Determine FL via a 2 param. fit of: Fitting function: N δFL where

  13. New FL determination with ‘shape method’ Idea: Shape of σr at high y is driven by y2/Y+ factor rather than by FL behaviour Fit: with: • Neglects FL variation in a narrow x-range • The whole x-range of measured data is used to fit F2 and FL,

  14. FL determination from low Q2,x H1 data • FL was for the first time extracted at very low Q2 ~ 1 GeV2 • It is clearly non-zero

  15. FL determination from low Q2,x H1 data • Starts to descriminate different predictions • Need to measure the x-dependence of FL

  16. FL determination from H1 data • H1 NLO QCD fit is consistent with data in the DIS region • MRST NLO QCD fit too low at low Q2 • GBW saturation dipole model describes the whole range • BKS model is steeper but still consistent with the data

  17. Summary and Outlook • New H1/Zeus F2 measurements in the important transition region from the non-perturbative to perturbative domain • Rise of F2 : rate is changing at Q2 ~ 1GeV2 (-> gluon at low-Q2) • FL was extracted for the first time with ISR events by ZEUS • H1 FL now cover range 0.75 < Q2< 800 GeV2 • New low-Q2 FL measurem. show that FL>0 and discriminate models • Precision FL measurements (x-dependence!) require low proton energy ep runs

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