1. Neutral Current Deep Inelastic Scattering in ZEUS The HERA collider NC Deep Inelastic Scattering at HERA The ZEUS detector Neutral current cross section measurement Some ZEUS structure functions results Conclusions and outlook Ricardo Gonçalo Imperial College

2. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 2 HERA and the ZEUS detector 1996-1997 running:  820 GeV protons  27.5 GeV positrons  Integrated luminosity 38.6 pb-1 1999-2000 running:  920 GeV protons  27.5 GeV positrons  Integrated luminosity 66.0 pb-1 Bunch crossings every 96ns Centre of mass energy:  s = 318 GeV (98/00) 300 GeV (92/97) 1998-1999 running:  920 GeV protons  27.5 GeV electrons  Integrated luminosity 16.6 pb -1

3. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 3 Deep Inelastic Scattering at HERA Leading order cross section: Kinematic variables: Q²  four momentum transfer x  Bjorken scaling variable y  inelasticity Only 2 independent variables: Q 2 = x.y.s (  s is the c.m. energy) with: e  (k) p (P) X (P’) e  (k’) ,Z 0 (Q 2 =-q 2 ) F L negligible at high Q 2 Our aim: to determine the proton structure through the measurement of the proton structure functions xF 3 parity violating

4. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 4 Depleted uranium compensating calorimeter:  99.7% solid angle coverage  Timing resolution 4.5 GeV)  Energy resolution (test beam) HERA and the ZEUS detector Central Tracking Detector (CTD)  Angular acceptance 15 o <  <164 o  r-  resolution  230  m Magnetic field of 1.43 T achieved with thin superconducting solenoid

5. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 5 Neutral current cross section measurement Offline selection cuts: Find the electron Vertex within: -50 < Z vtx < 50 cm Scattered electron cuts: Electron energy: E e > 10 GeV Electron isolation Within CTD acceptance:  Electron track momentum > 5 GeV  Track and cluster matching Forward electrons: P T,e > 30 GeV Total transverse momentum should be small: P T,tot < 4  E T,tot Background cut: 38 <  < 65 GeV, with:

6. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 6 Neutral current cross section measurement Low Q 2 :  exchange  similar cross sections for e - p and e + p High Q 2 :  and Z 0 exchange plus interference term  cross sections greater for e - p (d  /dQ 2  xF 3 ) than for e + p (d  /dQ 2  -xF 3 )

7. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 7  F 2 sensitive to valence and sea quarks  All flavors contribute to F 2 Why measure F 2 ?  Gluon density can be extracted from d F 2 /dlnQ 2

8. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 8 How to measure F 2 em ? Start by measuring Neutral Current reduced cross section Cross section has contributions from  and Z 0 exchange and their interference (at high Q 2, reduced cross section falls for e + p data and rises for e - p data) Apply corrections to get F 2 from  R Reduced cross section:

9. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 9 xF 3 sensitive to valence quarks only Sea quarks created in pairs => (quark-antiquark)=0 And How? Why measure xF 3 ?

10. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 10 First xF 3 measurement from ZEUS First direct look at valence quarks at high Q 2 Yellow band is the calculated F L multiplied by 10 Measurement used e + p data from 1996-97 (c.m. energy of 300 GeV) and e - p data from 1998-99 (c.m. energy of 318 GeV) => corrections necessary

11. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 11 Conclusions and outlook  Present Neutral Current results from ZEUS are systematics limited up to Q 2  800 GeV but there is room for improvement for higher Q 2 using the currently available data.  Neutral current cross sections will be measured at high Q 2 using data from 1998/99 (e - p) and 1999/00 (e + p) with  s = 318 GeV.  Measurement of F 2 and xF 3 can be made with available data. Outlook Lepton beam polarization after the 2000/2001 shutdown and high luminosity expected (1 fb -1 until 2006). Improved tracking (new vertex detector and forward tracking) This will allow:  Setting limits for the mass of W - R, W + L, using Charged Current measurements,  The direct measurement of weak couplings to quarks u and d.  Possibility of lower systematic errors for lepton tagging in fwd region.  Charm tagging.

12. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 12 My neutral current cross section measurement Only d  /dQ 2 so far Statistical errors only Analysis of 1998/99 e-p data and 1999/00 e+p data going in parallel Still a lot of work to be done...

13. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 13 Luminosity upgrade and polarisation Neutral current: 4 different x-sections will allow measurement of couplings a u, v u, a d, and v d (HERA sensitive to u & d; LEP sensitive to c & b) Charged Current: cross section linear on polarisation. Limits or measurement of mass of W - R, W + L

14. R.GoncaloIoP HEPP Conference, Southampton, 3-5 April 2001 14 Luminosity upgrade and polarisation xF 3 measurement from H1: different binning With both lepton flavors and polarizations  4 different cross sections xF 3 from H1