charged current dis cross sections with a ... - · xviii international workshop on deep-inelastic...

Motivation Selection CC Cross Sections Summary

Charged Current DIS Cross Sections with aLongitudinally Polarised Positron Beam at ZEUS

Katie OliverUniversity of Oxford

On behalf of the ZEUS collaboration

XVIII International Workshop on Deep-Inelastic ScatteringJoint Session: Electroweak and Parton Densities

21st April 2010

Katie Oliver (University of Oxford) ZEUS CC DIS cross sections with longitudinally polarised e+ 21st April 2010 1 / 15


Charged Current Interaction: Motivation

Extraction of MW

d2σ(e+p)

dxdQ2 = (1 + P)×G2

F M4W

2π(Q2 + M2W )2

[u + c + (1− y)2 (d + s)

]

Test of the chiral nature ofthe Standard Modelσ(Pe) = (1 + Pe)σ(Pe = 0)

Sensitivity to thed quark PDF



Data2006/2007 e+p HERA data is used

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 10

2

4

6

8

10

12

14

eP

)-1

Lum

inos

ity

(pb )-1p (132 pb+ZEUS CC e

ZEUS

Data set L(pb−1) Pe (%)Total 132 +3.4Right-handed 75.8 +33Left-handed 56.0 −36

• Luminosity uncertainty: 2.6%• Runs with Pe < 15% were

rejected to allow reliablepolarisation measurement

• Polarisation uncertainty:4.0% (LH), 3.7% (RH)



Charged Current Event Selection I

Large missing transverse momentum (pT ) due to neutrino

p Xp

qq’

W+

e+ ν–

e



Charged Current Event Selection II

Kinematic reconstruction from hadrons (Jacquet-Blondel)

δ =∑

i(E i − pi

z)

p2T =

(∑i pi

x)2

+(∑

i piy)2

yJB = δ2Ee

xJB =p2

TsyJB(1−yJB) Q2

JB = sxJByJB

Kinematic Cuts• Q2

JB > 200 GeV2

• yJB < 0.9

CC Selection• PT > 12 GeV

Background Rejection• Tracking cuts (vertex and quality)• Cosmic µ rejection (CAL energy fractions)• γP & beam gas rejection (jet shape cuts)• NC rejection (electron cuts)



Charged Current BackgroundsPhotoproduction

p Xp

q–

q

e+ e+

0.6%

Single-W

p Xp

qW+

l+

νl

q’

q’

e+ e+

0.3%

Di-leptons

p Xp

l-

l+

e+ e+

0.2%

Neutral current

p Xp

qq

γ, Z0

e+ e+

<0.02%



Control Plots

• Total data sample is shown(≈ 3000 events, 132 pb−1)

• Signal histograms:DJANGOH + ARIADNEwith CTEQ5D PDFs.

• ep backgroundcontamination: < 1.5%

• Data are well described byMC⇒ MC is used toestimate efficiencies ofselection and detectoreffects

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9002 > 200 GeV

JB2Q

)-1p (132 pb+e

SM MC

BG MC

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nts

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nts

(GeV)T,missP )2 (GeVJB2Q

JBxJB

y

P / VAPV (cm)vtxZ

(a) (b)

(c) (d)

(e) (f)

ZEUS



Single Differential Cross Sections I

Single differential crosssections as a function ofQ2, x and y :

• Cross sectionsmeasured at differentpolarisations differ inmagnitude but not shape

• Consistent with SM

310 410

-610

-510

-410

-310

-210

-110= +0.33)

e, P-1p (76 pb+ZEUS CC e

= -0.36)e

, P-1p (56 pb+ZEUS CC e

2 > 200 GeV2Q

SM (ZEUS-JETS)= +0.33

ep P+e

= -0.36e

p P+e

310 410

0.5

1

1.5

2

2.5

310 410

0.5

1

1.5

2

2.5 SM (ZEUS-JETS)SM (HERAPDF1.0)SM (CTEQ6.6)SM (MSTW08)

ZEUS(a)

(b)

)2 (GeV2Q

)2 (GeV2Q

)2 (

pb/G

eV2

/dQ

σdD

AT

A/ S

M(Z

EU

S-JE

TS)



Single Differential Cross Sections II

-210 -110-110

1

10

210

310

= +0.33)e

, P-1p (76 pb+ZEUS CC e= -0.36)


2 > 200 GeV2Q

SM (ZEUS-JETS)= +0.33

ep P+e

= -0.36e

p P+e

-210 -1100

0.2

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1

1.2

1.4

1.6

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1.6

SM (ZEUS-JETS)SM (HERAPDF1.0)SM (CTEQ6.6)SM (MSTW08)

ZEUS(a)

(b)

x

x

/dx

(pb)

σdD

AT

A/ S

M(Z

EU

S-JE

TS)

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90

20

40

60

80

100

120

140 = +0.33)e

, P-1p (76 pb+ZEUS CC e= -0.36)


SM (ZEUS-JETS)

2 > 200 GeV2Q

= +0.33e

p P+e= -0.36

ep P+e

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90.4

0.6

0.8

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1.6


ZEUS(a)

(b)

y

y

/dy

(pb)

σdD

AT

A/ S

M(Z

EU

S-JE

TS)



Reduced Cross Section σ̃e+pCC I

σ̃ = (1 + Pe)[u + c + (1− y)2 (d + s)

]

• Positron data is sensitive to dvalence quark

• Precise data can furtherconstrain PDFs

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0.2 p+ZEUS CC e

= +0.33e, P-176 pb= - 0.36e, P-156 pb

-210 -1100

0.1

0.2 SM (ZEUS-JETS)= +0.33eP= - 0.36eP

2 = 280 GeV2Q 2 = 530 GeV2Q 2 = 950 GeV2Q

2 = 1700 GeV2Q 2 = 3000 GeV2Q 2 = 5300 GeV2Q

2 = 9500 GeV2Q 2 = 17000 GeV2Q 2 = 30000 GeV2Q

x

σ~

ZEUS



Reduced Cross Section σ̃e+pCC II

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=0)e

, P-1(132 pb

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0.1

0.2SM (ZEUS-JETS)

SM (HERAPDF1.0)SM (CTEQ6.6)SM (MSTW08)

2 = 280 GeV2Q 2 = 530 GeV2Q 2 = 950 GeV2Q

2 = 1700 GeV2Q 2 = 3000 GeV2Q 2 = 5300 GeV2Q

2 = 9500 GeV2Q 2 = 17000 GeV2Q 2 = 30000 GeV2Q

x

σ~

ZEUSZEUS

x(d+s)2(1-y))c+ux(

• Unpolarised reduced crosssection in bins of Q2 as afunction of x

• Contributions are shown fromquarks and antiquarks:

• x (u + c) term dominates atlow x

• (1− y)2 x (d + s) termdominates at high x(valence region)



Reduced Cross Section σ̃e+pCC III

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p+ZEUS CC e=0)e, P-1(132 pb

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SM (ZEUS-JETS)

SM (HERAPDF1.0)SM (CTEQ6.6)SM (MSTW08)

)c+ux(

x = 0.015 x = 0.032

x = 0.068 x = 0.13

x = 0.24 x = 0.42

2 < 60 000 GeV2200 < Q

2(1-y)

σ~

ZEUS

1

• Unpolarised reduced crosssection in bins of x as afunction of (1− y)2

→ helicity structure of CCinteractions

• At leading order in QCD:• Intercept gives (u + c)

contribution• Slope gives the (d + s)

contribution.



Total Cross Sections IσCC(Pe = −0.36) = 22.9± 0.82(stat.)± 0.60(lumi.)± 0.40(syst.) pbσCC(Pe = +0.33) = 48.0± 1.01(stat.)± 1.25(lumi.)± 0.77(syst.) pb

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100

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140)-1p (132 pb+ZEUS CC e)-1p (23.8 pb+ZEUS CC e

)-1=0) (60.9 pbe

p (P+ZEUS CC e)-1p (175 pb-ZEUS CC e

)-1=0) (16.4 pbe

p (P-ZEUS CC e


eP

) (p

b)2

> 2

00 G

eV2

(Q

CC

σ

ZEUS

σe±pCC (Pe) = (1± Pe) · σe±p

CC (Pe = 0)

• Test chiral nature of the SM• Search for a non-zero cross

section at Pe = −1 due tothe existence of W bosonscoupling to RH particles(WR)



Total Cross Sections II

• Measure cross section in 8bins of polarisation

• Do not constrain linear fit tozero at Pe = -1

• Derive upper limit onσe+p

CC (Pe = −1)⇒ lowerlimit on mass of WRassuming:

• gL = gR• Light νR -1 -0.5 0 0.5 1

0

10

20

30

40

50

60

70

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0

10

20

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40

50

60

70)-1p (132 pb+ZEUS CC e

= 0)e

, P-1p (60.9 pb+ZEUS CC e

)e(1+P⋅=0)e(Pσ=-1) + e(Pσ) = e(PσFit:

1.43 (syst.) pb± 1.78 (stat.) ±=-1) = -0.87 e(Pσ

eP

) (p

b)2

> 2

00 G

eV2

(Q

CC

σ

σe+pCC (Pe = −1) = −0.87± 1.78 (stat.)± 1.43 (syst.) pb

MWR > 198 GeV (95%CL)



Summary

• Total, single-differential and reduced charged current DIS crosssections are presented for 2006/2007 e+p HERA data (132 pb−1)with polarised positron beams

• The Standard Model describes the data well• The results of this analysis should help to constrain the d-quark

and sea PDF→ important for PDFs at LHC


HERA and ZEUS NC & CC W masses

Back-Up Slides



HERA and ZEUS

HERA collided protons with longitudinally polarized e±

• ZEUS was a multi-purpose collider detector at HERA• 0.5 fb−1 luminosity taken by ZEUS

Days of running

H1

Inte

grat

ed L

umin

osit

y / p

b-1

Status: 1-July-2007

0 500 1000 15000

100

200

300

400electronspositronslow E

HERA-1

HERA-2 HERA Energiesp beam: 920 GeV

e± beam: 27.5 GeV

Centre of mass energy: 318 GeV



HERA: Longitudinally Polarised Positrons

• Positron beam becomes transversely polarized by emission ofsynchrotron radiation

• Spin rotators provide longitudinally polarized positrons at both IPs• Mean polarization: 30 - 40%

BeamDirection

Polarimeter

TransversePolarimeter

Spin Rotator

Spin Rotator

pe

Spin RotatorSpin Rotator

Spin Rotator

Spin Rotator

Longitudinal

��

��

��

��

��

��

��

��

��

��

��

��

Pe =NR − NL

NR + NL



Kinematics & Deep Inelastic Scattering

Neutral currentexchange of γ or Z ◦

Charged currentexchange of W±

Probing power of the lepton:Q2 = −q2 = −(k − k ′)2

= sxy

Bjorken scaling variable, the fraction of theproton’s momentum carried by the struck quarkx = Q2

2p.q

Inelasticity, the energy fraction transferredfrom the lepton in the proton’s rest framey = p.q

p.k

Centre of mass energy squared:s = (p + k)2



Sokolov-Ternov Effect

• Naturally-occurring transverse polarisation of lepton beams instorage rings

• When electrons are injected, spins are randomly oriented• As they accelerate round the ring, vertical B-field is created• The electrons emit synchrotron radiation.• When an electron emits a photon there is a probability that the

projection of the electron’s spin onto the vertical axis may flipdirection

• The probability per unit time that the electrons spin will flip from upto down is greater than the probability per unit time that theelectrons spin will flip from down to up

• → Over time, lepton beams become transversely polarised



HERA Polarimeters

• HERA polarimeters use Compton scattering of circularly polarisedlaser beams off the lepton beam to measure the polarisation ofthe lepton beam.

• The polarimeters determine transvere or longitudinal componentsof lepton beam polarisation by measuring asymmetries betweenthe scattered photons from circularly polarised laser beams withpositive and negative helicity.



HERA and LHC Kinematic Regions



ZEUS

• Hermetic - 4π detector• Tracking:

• Central Tracking Detector(CTD), a cylindrical driftchamber

• Silicon Microvertex detector(MVD) (HERA-II)

• Compensating uraniumscintillator calorimeter (UCAL)

• µ chambers

Optimised for precision measurement of the hadronic final state



Charged Current Cross Sections

Charged current reduced cross section

σ̃(e−p)CC = (1− Pe) [u + c + (1− y)2(d + s)]

σ̃(e+p)CC = (1 + Pe) [u + c + (1− y)2(d + s)]

• Linear polarisation dependence• Electron (positron) data is sensitive to u (d) valence quark• Probes flavour structure of the proton



Charged Current & Neutral Current Events

CC Event: Large missingtransverse momentum (pT )due to neutrino

NC Event: High pT isolatedscattered electron/positronand balanced total pT

Final selection: ∼ 103 events persample

Final selection: ∼ 105 events persample



High Q2 NC and CC Cross Sections

• NC and CC high Q2 crosssections are measured veryprecisely over many orders ofmagnitude in Q2



WR Mass Limits

• Assuming MνR > MWR :• UA2 excluded 100-251 GeV in WR →dijets channel• D0 (CDF) excluded MWR < 739 (MWR < 800) GeV in search for tb

final state• Assuming MνR < MWR :

• CDF excluded 1.00 TeV

• Cosmological limits also exist



Extraction of W Mass

• The fall in the cross section dσ/dQ2 with increasing Q2 dependson M4

W/(Q2 + M2

W )2.• Fitting dσ/dQ2 with GF fixed at the PDG value of 1.16639 · 10−5

GeV−2, using the ZEUS-S fit PDFs and MW treated as a freeparameter, gives:

MW = 78.9± 2.0(stat.)± 1.8(syst.)+2.0−1.8 (PDF) GeV

• This measurement, in the space-like region, is in good agreementwith the more precise measurements of W -boson mass in thetime-like region


charged current dis cross sections with a ... - · xviii international workshop on deep-inelastic...

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