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

R.Goncalo IoP 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

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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Deep Inelastic Scattering at HERA

),(),(),(2

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)( 2223

224

2

2

2

QxFyQxxFYQxFYxQdQdx

ped NCL

NCNCNCBorn

Leading order cross section:

2)1(1 yY

Kinematic variables:

Q² four momentum transfer

x Bjorken scaling variable

y inelasticity

Only 2 independent variables:

Q2 = x.y.s

(s is the c.m. energy)

with:

e (k)

p (P)X (P’)

e (k’)

,Z0 (Q2=-q2)

FL negligible at high Q2

Our aim: to determine the proton structure through the measurement of the proton structure functions

xF3 parity violating

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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Depleted uranium compensating calorimeter:

99.7% solid angle coverage

Timing resolution < 1ns (E>4.5 GeV)

Energy resolution (test beam)

HERA and the ZEUS detector

%18%35 .. EE

Eem

Ehad

Central Tracking Detector (CTD)

Angular acceptance 15o<<164o

r- resolution 230 m

Magnetic field of 1.43 T achieved with thin superconducting solenoid

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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Neutral current cross section measurementOffline selection cuts:

Find the electron

•Vertex within: -50 < Zvtx < 50 cm

•Scattered electron cuts:

•Electron energy: Ee > 10 GeV

•Electron isolation

•Within CTD acceptance:

Electron track momentum > 5 GeV

Track and cluster matching

•Forward electrons: PT,e > 30 GeV

•Total transverse momentum should be small: PT,tot < 4ET,tot

•Background cut: 38 < < 65 GeV,

with: i

izi

ii pEE )()cos(

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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Neutral current cross section measurement

Low Q2:

exchange

similar cross sections for e-p and e+p

High Q2:

and Z0 exchange plus interference term

cross sections greater for

e-p (d/dQ2 xF3) than for

e+p (d/dQ2 -xF3)

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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F2 sensitive to valence and

sea quarks

All flavors contribute to F2

Why measure F2?

Gluon density can be extracted from d F2 /dlnQ2

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)( 2223

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ped NCL

NCNCNCBorn

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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How to measure F2em ?

)),(),((

)),(),((222

2

222

QxqQxqexF

QxqQxqAxF

qem

qNC

•Start by measuring Neutral Current reduced cross section

•Cross section has contributions from and Z0 exchange and their interference (at high Q2, reduced cross section falls for e+p data and rises for e-p data)

•Apply corrections to get F2 from R

)1)(1)(,(~32

22 radFxF

ewF

em

LQxF

Reduced cross section:

2

21

4

2 12~dxdQ

d

YxQ

NC

peNC

pe

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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xF3 sensitive to valence

quarks only

Sea quarks created in pairs

=> (quark-antiquark)=0

))()((3 xqxqBxxF qNC

32

32

~

~

xFY

YF

xFY

YF

NC

pe

NC

pe

And How?

Why measure xF3?

),(),(),(2

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)( 2223

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ped NCL

NCNCNCBorn

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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),(),(),(2

.

)( 2223

224

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2

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First xF3 measurement from ZEUS

•First direct look at valence quarks at high Q2

•Yellow band is the calculated FL 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

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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Conclusions and outlook Present Neutral Current results from ZEUS are systematics limited up to Q2 800 GeV but there is room for improvement for higher Q2 using the currently available data.

Neutral current cross sections will be measured at high Q2 using data from 1998/99 (e-p) and 1999/00 (e+p) with s = 318 GeV.

Measurement of F2 and xF3 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.

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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My neutral current cross section measurement

•Only d/dQ2 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...

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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Luminosity upgrade and polarisation

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1.

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2

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Neutral current: 4 different x-sections will allow measurement of couplings au, vu, ad, and vd (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

R.Goncalo IoP HEPP Conference, Southampton, 3-5 April 2001

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Luminosity upgrade and polarisation

xF3 measurement from H1: different binning

With both lepton flavors and polarizations 4 different cross sections

xF3 from H1