1. Ricardo GoncaloZEUS Week - London June 20021 Neutral Current 99/00 e + p Analysis Status Ken Long Matthias Moritz Henning Scnurbusch Ricardo Gonçalo Thanks to Matthias for most plots in the talk ZEUS Collaboration Meeting London - June 26 th 2002 Outline Analysis overview Sinistra/EM comparison Comparison of analyses Our own y bump Conclusions and outlook

2. Ricardo GoncaloZEUS Week - London June 20022 The 1999/00 y bump – Matthias, Ricardo Yes, we have a y-bump … but it’s different from the y-bump in 1996/97: 1996/97: low y, medium Q 2 1999/00: Q 2 >5000 GeV 2, y>0.65 @ Q 2 >5000 GeV 2, y>0.65: 206 events in data / 158.4 in MC

3. Ricardo GoncaloZEUS Week - London June 20023 The 1999/00 y bump First seen by Xiang (EXO) and confirmed by all NC analyses  Seen with both EM and Sinistra  Seen with DA, JB and electron methods  Seen with both Corandcut and cells  No time (run) or  dependence  Events were scanned and look OK  No clear spike in a distribution that could explain the bump  Migration? Q 2 distribution OK but x and y shifted. There is a deficit at lower y… Q 2 >5000GeV2 x>0.65

4. Ricardo GoncaloZEUS Week - London June 20024 The 1999/00 y bump Q 2 >5000GeV 2 ; y>0.65 In bump region positrons are detected both in the forward BCAL and in the FCAL Many events in the bump have a well measured positron track Q 2 >5000GeV 2 X>0.65 e + track not in CTD / in CTD acceptance

5. Ricardo GoncaloZEUS Week - London June 20025 The 1999/00 y bump Monte Carlo Data Ee/E DA shows a shift in the FCAL: this may just be caused by energy loss in dead material – not a problem with DA method On the side of the hadronic final state: bump events seem to have  H ~90deg  kinematics or detector effect?

6. Ricardo GoncaloZEUS Week - London June 20026 The 1999/00 y bump 1 2 3 GeV/ev We tried looking at the hadronic energy flow in Q 2 >5000GeV 2 events Migration of  H to the centre of BCAL (  H ~90deg) ? But P T had /P T DA versus  H looks OK and bump still there with electron method  cannot be the only cause

7. Ricardo GoncaloZEUS Week - London June 20027 The 1999/00 y bump B-F supercrack Tried looking at the fraction of MC positrons in accepted events as a function of Z at the exit radius of CTD and as a function of FCAL radius. No obvious dip in efficiency except for the supercrack  MC looks OK

8. Ricardo GoncaloZEUS Week - London June 20028 The 1999/00 y bump In summary: Before anything else, we must see if the bump is a detector effect There must be more than one effect: both the scattered positron and the hadronic final state are affected But it may also be something else… Can this be caused by the PDFs in the Monte Carlo?  Must check HERACLES  Reweight to ZEUS-only PDFs? Many possibilities have been tried, but the work is still in progress …

9. Ricardo GoncaloZEUS Week - London June 20029 Analysis overview Since last Collaboration Meeting: Systematic uncertainties were fixed (in connection with 98/99 NC and CC papers): Hadronic and electron energy scales Hadronisation (using MEPS) Photoproduction normalisation & systematic uncertainty Systematic variation of selection cuts New cuts were added (in connection with 98/99 NC paper): MC validity: y JB.(1-x DA ) 2 20GeV cut) Projection of hadronic system on FCAL (calculated with  H and Zvtx) must be at a radius R > 20 cm – “  cut ”– excludes low y region affected by cross-talk from FPC

10. Ricardo GoncaloZEUS Week - London June 200210 Positron identification:  Sinistra (option 5): P>0.9  EM: P>0.001 (P>0.01 outside fwd CTD acceptance)  Electron energy: E e > 10GeV  Isolation: E cone < 5GeV If positron is in CTD acceptance:  Matched positron track with P trk >5GeV  Track to CAL cluster DCA < 10cm Outside of forward CTD acceptance:  Transverse momentum: P t,e = E e sin  e > 30GeV Event selection and reconstruction 45cm Vertex-dependent CTD acceptance

11. Ricardo GoncaloZEUS Week - London June 200211 Background rejection:  38 (44 in fwd) GeV < E-P Z < 65 GeV  y e < 0.95  Balance of transverse momentum: P T /  E T < 4 GeV 1/2 Acceptance cuts:  Positron Z pos. in BCAL (super crack): -98.5 cm < Z < 164 cm  Electron in RCAL: R < 175 cm  Chimney in RCAL: | x | 90cm  FCAL projection of hadronic system (  H,Zvtx): R > 20 cm (“gamma cut”) Event selection and reconstruction Other cuts:  Zvxt close to IP: -50 cm < Zvtx < 50 cm  MC validity: y JB.(1-x DA ) 2 < 0.004 Kinematic variables reconstruction: Double Angle method Hadronic angle reconstruction: Corandcut New cuts

12. Ricardo GoncaloZEUS Week - London June 200212 Analysis results Rejected due to cut on hadronic system projection on FCAL (gamma cut) d  /dxdQ 2

13. Ricardo GoncaloZEUS Week - London June 200213 Analysis results: d  /dxdQ 2

14. Ricardo GoncaloZEUS Week - London June 200214 Systematic errors for d  /dQ 2  2%

15. Ricardo GoncaloZEUS Week - London June 200215 Systematic errors for d  /dxdQ 2 - Henning Not exactly our final systematic uncertainties yet: no gamma max or FCAL island splitting, different hadronic energy scale, no gamma cut

16. Ricardo GoncaloZEUS Week - London June 200216 Comparison: Matthias, Henning,Ricardo d  /dQ 2 Much work has been done in comparing the analyses, and the agreement is already very reasonable Some differences are still there, but the work is advancing fast Cuts & corrections have evolved fast, recently.  Different HAC scale corrections for MM (A.Tapper), than for RG & HS (C.Cormack)  CTD acceptance cuts still different in points shown Latest results will be compared soon

17. Ricardo GoncaloZEUS Week - London June 200217 Sinistra/EM comparison - Matthias We intend to use EM for final event selection

18. Ricardo GoncaloZEUS Week - London June 200218 Sinistra/EM comparison Photoproduction EM better suited for high-Q 2 / Sinistra is more efficient, but takes more PhP For most events the same candidate is picked by EM and Sinistra For some events the electron cell list or the candidate are different  migrations

19. Ricardo GoncaloZEUS Week - London June 200219 Sinistra/EM comparison Cuts failed by EM candidates candidate failed only this cut Several cuts rejected event Cuts failed by Sinistra candidates

20. Ricardo GoncaloZEUS Week - London June 200220 Conclusions and outlook  Much effort done in comparing the three analyses. They are in good agreement and still converging.  The EM / Sinistra comparison is in a very good shape.  A lot of work has been done in understanding systematics. This has been useful also for other analyses.  Y-bump should be our last major problem and must be understood. Much work has been going into this and we expect to solve this one soon.