1 / 17

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

Download Presentation

On behalf of the ZEUS and H1 Collaborations

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related