F L measurement at HERA

1. FL measurement at HERA Kostas Papageorgiou University of the AEGEAN University of the Aegean High Energy Physics Group I. Gialas K. Papageorgiou N. Kontakis K. Theodosiou XXV Workshop on Recent Developments in High Energy Physics & Cosmology 28 March 2007

2. FL MEASUREMENTS AT HERA 1. Motivation 2. Existing measurements 3. Predictions 4. Measuring FL at HERA 5. How well we could do

3. Motivation A major element of the HERA project is related to the understanding of partons distributions at small-x. In this kinematic regime, the dynamics are dominated by gluon pair production These fluctuations are not understood in detail theoretically So far no theory which predicts the x-dependance of cross sections (PDFs). Guided by data FL could be an important ingredient to making progress in the theoretical understanding

4. Motivation In Quark-Parton Model (QPM) hadron is made up of quarks with zero PT σL=0 (Helicity conservation) => FL=0 QCD radiation introduces quark PT , σL# 0 LO pQCD Expected to dominate at small-x FL measurement will be a good test of our understanding of the small-x parton densities ! FL has never been measured at small-x

5. Motivation The unpolarized NC cross section in terms of structure functions What we measure in an experiment is the cross section The structure function xF3 can be extracted by measuring HERA II At small Q2 the xF3, negligible Assumptions currently made to extract F2,FL. OK for F2 for small enough y, since FL contribution suppressed. Need fixed (x,Q2) and different y to separate at high y =>different beam energies.

6. High y cross sections Note the turn over of the cross section with decreasing x at small x in the H1 data The data can be fit consistently with NLO DGLAP by H1 assuming no gluon saturation. The turn over is due to the negative contribution from FL. MRST,CTEQ has trouble fitting the H1 low Q2 data consistently at NLO DGLAP.

7. FL Extraction – H1

8. FL Extraction – H1

9. FL Measurement – ZEUS Initial State Radiation provides a broad band beam - can be used to extract FL For more precision, need to lower the CM energy by changing beam energies

10. Predictions for FL FL predictions from MRST group at different orders in DGLAP, a fit which resums the leading ln(1/x) and β0 terms, and a dipole type model. Very large differences at small Q2 where gluon uncertainty is large

11. Measuring FL

12. Running Conditions

13. Measuring FL

14. Electron finding e

15. Measurement with ZEUS (HES and Presampler)

16. Measurement with ZEUS (Photoproduction background)

17. Measurement with ZEUS (6m tagger acceptance)

18. Measurement with ZEUS

19. Measurement with ZEUS (Reconstructed kinematic variables)

20. Measurement with ZEUS (Uncertainties of FL extraction)

21. Results (Attempt to put information in one number)

22. Conclusions • The measurement of FL is a must for HERA – agreed by both H1 and ZEUS • Basic ingredient in the cross section • Test of perturbative QCD at small x • information on gluon density • The measurement is difficult and requires a long fight with backgrounds and systematics • The detectors are not optimal for the measurement • The accuracy will be moderate but interesting • FL is the prime motivator, but other interesting results expected from low energy run