ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 1

Vladimir Chekelian (MPI for Physics, Munich)

ep 27.5 GeV 920 GeV s = 318 GeV

• DIS & NC & Polarisation• Unpolarised DIS: HERA II• xF3

• Polarised NC

Neutral Current DIS with polarised e+/e- at HERA

HERA II: longitudinal polarisation of the lepton beam

(for CC see talk of Alex Tapper)

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 2

Deep Inelastic Scattering at HERA

ˆ ( )DIS pdf x ^

perturbative QCD cross section pdf universal parton distribution functions

DIS kinematics: Q2 = -q2 = -(k-k’)2 virtuality of *, Z0 x = Q2/2(Pq) Bjorken x y = (Pq)/(Pk) inelasticity Q2 = sxy s=(k+P)2

probe proton with the spatial resolution of ~ 1/Q probe the EW sector of the Standard Model

Neutral Current (NC) DIS : e p e X

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 3

NC cross section2 2

2 23

2 22 4 2

2( , ) ( , ) ( , )

eNC

L

pd Y x Q y x Q Y x Qd

Fxd xQ

F FQ

x

21 (1 )Y y

Reduced cross section:

F2 dominant contribution

xF3 important at high Q2;

difference in the sign for e FL important only at high y;

expected to be negligible at high Q2& x; in QPM FL = F2 – 2xF1 = 0

e+

e-

4 22

2 2 2 3

1

2

e pNC

LNC

xF F x

Q yd YYdx Y

FdQ Y

L

L

R

R

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 4

Polarised NC Structure Functions

in QPM:

22 22 2

2 2 2 22 2 2 2

22 22 2

3 3 32 2 2 2

2

2 ( )

e eZ Z

e e e e e eZ Z

Z Ze e e e e ee

Z Ze

Q QF F v a F v a v a F

Q M Q M

Q QxF a v xF v a v a xF

Q M

P

P PQ

P

M

2 2 22 2 2

3 3

, , , 2 , ( )

, 2 , ( )

Z Zq q q q q

q

Z Zq q q q

q

F F F x e e v v a q q

xF xF x e a v a q q

, R Le

R L

N NP

N N

2 21

2 24 1W W

Z Z

M M

M M

NR(NL)- number of right (left) handed leptons in the beam

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 5

HERA II performance

- longitudinally polarised lepton beams

natural transverse polarisation (Sokolov-Ternov effect) + spin rotators

typically Pe = 30-40% build-up time ~30min

HERA II: - detectors and luminosity upgrade

data taking with positrons till mid 2007: double the sofar collected statistics low proton energy run for direct FL measurement

e-

e+

e+

e+ e-

Lumi (Pe) e+p (2003-04) e-p (2004-05)

H1 27 pb-1 (+34%) 30 pb-1 (+37%) 21 pb-1 ( -40%) 69 pb-1 ( -27%)

ZEUS 12 pb-1 (+32%) 43 pb-1 (+33%) 12 pb-1 ( -41%) 79 pb-1 ( -27%)

HERA I 100 pb-1 15 pb-1

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 6

NC&CC DIS at HERA II

Neutral Current

e±p

Charged Current

Unpolarised results: combine e+(e-) data with different polarisations and correct for small residual polarisation

Probe proton: quarks are pointlike down to 1/1000 of the proton radius r < 10-18 m

EW component of SM: NC CC at Q2 MZ

2,MW2

electro-weak unification

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 7

Unpolarised NC: d2/dxdQ2 2

2 3NC L

y YF F xFY Y

e-p lumi at HERA II: 6-8 fold of HERA I

determine xF3

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 8

Structure Function xF3

Precision of xF3 is improved by ~ 20% due to larger e-p statistics at HERA II mostly due to Z interference

32

NC NCYxFY

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 9

2 2 23 3 Z e(Q +M ) /(a Q )ZxF xF

Little dependence on Q2 transform to one Q2 value of 1500 GeV2

and average (all Q2 & H1 & ZEUS)

Z3 u u d dx = 2x e a (U U)+e a (D D)F

1 1

30 0

1 5(2 )

3 3Z

v v

dxxF u d dx

x

consistent with H1 PDF 2000 fit: 1.12 ± 0.02 ZEUS+Jets fit: 1.06 ± 0.02

0.65

30.02 1.21 0.09(sta) 0.08(sys)ZF dx

Combined H1 & ZEUS results:

0.65

0.02 0.09 0.09(sta) 0.08(sys)dx

consistent with zero

Sum rule (counting of the valence quarks):

Z3 v v

xx = 2 u +d +

3F

sea sea=2(u u+c c)+(d d+s s)

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 10

Polarised NC: d2/dxdQ2

small differences between cross sections at highest Q2 due to polarisation

polarised double differential cross sections are used in the combined PDF & EW fit see talk of Yong Dok Ri

to emphasize an effect of polarisation:

single diff. cross sections

e-p

Pe=-0.27

Pe=+0.33

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 11

Polarised NC: d/dQ2

Each experiment: 2 lepton beam polarities and 2 polarisation periods

H1

e-p

e-p

e+p

e+p

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 12

NC: polarisation ratios (Pe>0)/(Pe<0 )

Polarisation effects in NC at HERA are established

2 2 3e p Z ZNC e Z e ZeF a F xP a F

e+p e+p

e-pe-p

2

2 2ZZ

Q

Q M

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 13

Polarisation asymmetryPolarisation asymmetry (H1, ZEUS, H1 & ZEUS):

( ) ( )2

( ) ( )NC R NC L

R L NC R NC L

P PA

P P P P

2 22

2 2 2 22

1 /

4 /

Ze v v

Z Z v v

a Q d uF QA

Q M F Q M d u

A+and A- are of opposite sign

A = A+-A- 0 at low Q2 and significantly >0 at high Q2

for A with respect to zero (Q2 > 5000 GeV2): /dof = 4.0 probability of 3.1 10-3 for A to be zero

0

0R

L

P

P

a direct measure of parity violation in NC

A+

A-

ICHEP'06, 27.07.2006 V. Chekelian, NC DIS at HERA 14

Summary - The NC cross sections for ep scattering with longitudinally polarised lepton beams are measured at HERA for the first time.

- The structure functions xF3, xF3Z directly sensitive to

valence quark distributions are determined.

- Clear evidence of parity violation in the NC interactions at high Q2 is observed.

- The results are presented individually for H1, ZEUS and, for the first time, as combined HERA results.

- The Standard Model is able to provide an excellent description of the data.