Leading Baryons at HERA with the ZEUS Forward Detectors.

1. Leading Baryons at HERAwith the ZEUSForward Detectors. Alberto Garfagnini. Universita’ della Calabria and INFN on behalf of the ZEUS Collaboration Brussels, DIS 98, 4-8 April 1998

2. Outline • Physics motivations; • ZEUS forward detectors: LPS and FNC; • General properties of Leading Baryons (LB) in DIS: energy spectra, uncorrected ratios, rapidity gaps, ...; • dN/dxL for LP; • |t| slopes for LP and LN; • Conclusions A.Garfagnini

3. Leading Baryon Kinematics Scattered electron measured in CAL (Q2 > 4 GeV2) or BPC (Q2 ~ 0.1-0.8 GeV2) Leading Baryon (LB) detected in LPS (p) or FNC (n) • The LB kinematics can be described by: • pt’ transverse momentum of the LB; • pz’ longitudinal momentum of the LB. Additional variables: Fraction of beam momentum carried by the LB Four-momentum squared transferred at the proton vertex A.Garfagnini

4. LB Production Pictures • ‘REGGE’ approach: • neutral exchange: LPS • diffraction (Pomeron) • p exchange • charged exchange: FNC • O.P.E. • p structure function ? • ‘Fragmentation’ approach: e’ LB from recombination effect of the remnant jet fragmentation e p p , n A.Garfagnini

5. ZEUS Forward Detectors Forward Neutron Calorimeter: lead-scintillator calorimeter (10 interaction lengths) Towers of 5cm vertical dim. Leading Proton Spectrometer: 6 stations of mstrip detectors along beam line. • 6 plans of detectors per station: • three different strip orientation 0o; +45o; -45o; • (strip pitch 115 mm (0o), 81 mm (45o). • detector cut-out to follow the 10s beam profile. A.Garfagnini

6. Forward Detectors Acceptance FNC acceptance: determined by the beam-pipe apertures Measure: - En; Energy scale ~ 1% Energy resolution: s(En) ~ 0.65 En LPS acceptance (S4-S6 - 1994 configuration): determined by the beam-pipe apertures and magnet strength. Measure: - pt - xL = |p’|/|pbeam| Resolution: - Dpz ~ 0.3% - Dpt ~ 3.0% A.Garfagnini

7. LP Momentum Spectrum (xL vs Q2) LP xL spectrum measured for low and high photon virtualities. Arbitrary normalization to the same area for xL < 0.91. No strong dependence on Q2 for LP momentum spectrum for 0.6 < xL < 0.9 A.Garfagnini

8. LP Momentum Spectrum (xL vs W2) A.Garfagnini

9. LN Energy Spectrum No dependence of the xL spectrum of LP and LN on W and Q2 for 0.6 < xL < 0.9 A.Garfagnini

10. Properties of Events with a LN Rate of DIS events with a neutron in bins of Q2 and xBJ . The plot is uncorrected for FNC acceptance and integrated over neutron energy 0.2 < xL < 1.0 A.Garfagnini

11. Properties of Events with a LN No strong dependence of the ratio on Q2, in xBJ slices, for different neutron energy. A.Garfagnini

12. Properties of Events with a LN Results are compared with a numerical PDF integration. or Some xBJ dependence for xL > 0.8 neutrons. The p exchange model better than Pomeron parametrisation Not just a kinematical effect; we are sensitive to F2p. A.Garfagnini

13. Properties of Events with a LP Same ratio for LPs with 0.6 < xL < 0.9. Within the limit of our statistics, the LP rate is independent of xBJ and Q2. A.Garfagnini

14. dN/dxL for LPs • The acceptance corrected 1/N dN/dxL distribution has been measured for LP at low and high Q2. • The integral of the distribution for 0.60 < xL < 0.91 gives: • LP/NTOT (Q2 > 4 GeV2) = 12.7 ± 0.3 (stat.) ± 0.9 (sys.)%; • LP/NTOT (0.1 < Q2 < 0.8 GeV2) = 13.0 ± 0.5 (stat.)+0.7-0.8 (sys.)%; A.Garfagnini

15. dN/dxL MC Comparison The measured dN/dxL spectrum for LP production is compared with the predictions of Lepto 6.5 and Ariadne MCs. • The integral of the distribution for 0.60 < xL < 0.91 gives: • LEPTO SCI = 8.2 %; • ARIADNE = 4 %; The soft-colour interaction mechanism of Lepto gives a qualitative description of the LP spectra and is able to predict the strong rise of the diffractive peak. A.Garfagnini

16. single dissociation 2.5 units Measured activity • double dissociation Possible activity 1.5 units NO activity GAPCUT selection ZEUS Leading Baryon pseudorapidity +4 +2 0 -2 -4 An event is accepted if: A.Garfagnini

17. LB and GAPCUT At xL ~ 1 almost all the LP tagged events show a rapidity gap, while for 0.6 < xL < 0.9 only a small fraction does. The LN have a almost flat behaviour over the whole xL range, and in the 0.2 < xL < 0.9 interval the majority of LB is generated by a mechanism that is not diffractive. Lepto SCI reproduces, partly, only the LP spectrum. Neither Lepto SCI nor Ariadne reproduce the LN behaviour. A.Garfagnini

18. LB Slopes Parameter ‘b’ vs xL Systematic uncertainty: LPS ~ 10-20 % on ‘b’ slopes, FNC overall error of 2 GeV-2. Good agreement for LPs and LNs with xL < 0.9 Similar production mechanism. A.Garfagnini

19. Conclusions • The xL spectrum of LBs is independent of Q2 and W. • The rate of LNs has been measured in the xBJ and Q2 plane and found to be largely independent of these variables. At high xL , high xBJ, there is a depletion of neutrons, consistent with the expectations of a OPE calculation. • The dN/dxL distribution for LP has been measured. The fraction of LP in the 0.60 < xL < 0.91 interval is: 12.7 ± 0.3 (stat.) ± 0.9 (sys.)% for Q2 > 4 GeV2 and • 13.0 ± 0.5 (stat.)+0.7-0.8 (syst.)% for 0.1 < Q2 < 0.8 GeV2. • Standard fragmentation model in DIS (ARIADNE) does not describe LB production. Additional soft mechanisms (as in LEPTO) describe some features of the data (e.g. diffractive peak for LPs) but do not fully reproduce LB data. A.Garfagnini