charged current cross sections with polarised lepton beam at zeus
DESCRIPTION
DIS2006, 20/April/2006-24/April/2006 at Congress Center EPOCHAL Tsukuba. Charged Current Cross Sections with polarised lepton beam at ZEUS. Hiroshi Kaji (KEK) on behalf of the ZEUS collaboration. Contents. Introduction Cross section measurement Single differential cross section - PowerPoint PPT PresentationTRANSCRIPT
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Charged Current Cross Sections with polarised lepton beam at ZEUS
Hiroshi Kaji (KEK)on behalf of the ZEUS collaboration
DIS2006, 20/April/2006-24/April/2006at Congress Center EPOCHAL Tsukuba
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Contents
• Introduction
• Cross section measurement– Single differential cross section– Polarisation vs. total cross section– Limit on RHCC cross section– Double differential cross section
• Summary
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Introduction
CC DIS process
NC DIS process Deep inelastic scattering (DIS) can beclassified into two processes– Neutral Current (NC) process
exchanging neutral gauge boson ( or Z)
– Charged Current (CC) process exchanging charged gauge boson (W) -- A scattered neutrino is in the final state. -- Topic of this talk
kp
qpy
qp2
Qx
)'kk(qQ2
222
: virtuality of exchanged boson
: Bjorken x
: inelasiticity
Kinematic variables
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Unpolarised CC DIS cross section
• e+p data
• e–p data
• ui (di) denotes densities of the u-type (d-type) quark and upper bar indicates anti-quark.
)x,Q(d)y1()x,Q(uQM
M
2
G
dxdQ
d 2i
22i
2
22W
2WF
2
peunpolCC
)x,Q(d)y1()x,Q(uQM
M
2
G
dxdQ
d 2i
22i
2
22W
2WF
2
peunpolCC
sensitive to density of d quark
Coupling Propagator Parton Distribution Function (PDF)
sensitive to density of u quark
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Polarised CC cross section• Polarised electron beam at HERA.
– Definition of polarisation, Pe, is asymmetry of No. of right-handed and left-handed electrons in the beam.
• Since CC DIS is pure weak interaction, polarised CC cross section is as a function of Pe
• If cross section is non zero at P=–1(+1) for e+p (e–p) data,
– Evidence of right-handed component, RHCC, which does not exist in the SM.
)x,Q(2
P1)x,Q( 2pe
LHCCe2pe
polCC
Can be tested by HERA-II polarised data
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ZEUS CC analysis
Pe>0 Pe<0
e+p data
L=12.3 pb–1
<Pe>=+0.32±0.01L=11.5 pb–1
<Pe>=–0.41±0.01
e–p data
L=42.7 pb–1
<Pe>=+0.33±0.02L=78.8 pb–1
<Pe>=–0.27±0.01
Since EPS2005-- e+p cross section has been published.
(hep-ex 0602026, submitted to PLB)
-- e–p analysis (new ZEUS preliminary)Data with integrated luminosity of 122 pb–1 were analyised.
HERA-I (16.4 pb–1)Last summer (41.8 pb–1)
Pe/Pe=2% (5%) for e+p (e–p) data
e–p data
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Event characteristics
CC DIS has large missing transverse momentum, PT,miss, which is interpreted as transverse momentum of scattered neutrino.
Scattered neutrino escapes detection.
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Kinematic reconstruction
• Jacquet-Blondel method is used.
• Kinematic variables are reconstructed with PT,miss and of hadronic system.
• where Ee is beam energy of electronand s is center-of-mass energy squared.
22
,
i
iY
i
iXmissT PPP
Jacquet-Blondel method
JB
JBJB
JB
missTJB
eJB
ys
Qx
y
PQ
Ey
2
2,2
1
2
i
iZ
i PE All variables described here are measured by calorimeter.
For hadron, (E)/E=0.35/√E, in GeV
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Event selection
0
We use different event selection in two 0 region.
=> See next slide.
22
miss,T
22miss,T
0P
Pcos
Efficiency of vertex finding changes by a value of 0, which can be naively interpreted as scattering angle of quark.
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Event selection• Selection at 0>0.4 rad
– PT,miss>12 GeV– PT,miss(–1ir)>10 GeV– NC rejection– PhP rejection– vertex cut– tracking cut
• Selection at 0<0.4 rad– PT,miss>14 GeV– PT,miss(–1ir)>12 GeV– Selection based on forward C
AL information• Kinematic range
– Q2>200GeV2
– y<0.9
Tracking information is used for rejection of non-ep background at high-0 region.
Relatively higher threshold is used at low-0 region.CAL shower shapes are used in stead of tracking information.This threshold is related to threshold of PT,miss.
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Event distribution : e–p data
• No. of events selected ~5000 events
• Both Pe>0 and Pe<0 samples are plotted together. (122 pb–1)
• Histograms are DJANGOH+ARIADNE with CTEQ5D PDF.
• Contamination of background events from other processes
<1%• Data were well described by M
C.can be used for estimating– efficiency of selection– detector effects
ZEUS preliminary
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Single differential cross sectionse+p CC cross section (hep-ex 0602026) e–p CC cross section (ZEUS preliminary)
Both for e+p and e–p, measured cross sections from different polarisations have the same shape.Only their scales are different.Consistent with SM prediction =>
)x,Q(2
P1)x,Q( 2
LHCCe2pe
pol
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Pe vs. Cross section
pb.)sys(.)stat(3.17.46 5.13.1
peCC
pb.)sys(.)stat(3.14.87 6.25.2
peCC
<Pe>=–0.27±0.01
<Pe>=+0.33±0.02
ZEUS preliminary
Cross sections clearly change with polarisation.Consistent with SM prediction.
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Pe vs. Cross section
ZEUS preliminary
Cross sections were meausured as a function of Pe.
Linear relation between cross section and polarisation is clearly shown.
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Right-handed CC DIS cross section
pbsysstatPGeVQ epe .)(0.5.)(1.38.0)1,200( 22
pb.)sys(2.1.)stat(9.34.7peRHCC
ZEUS preliminary
Right-handed CC DIS cross section is extrapolated with new preliminary e–p and HERA-I result.
Uncertainty of polarisation is included in the systematic error.
e+p result (hep-ex 0602026)
Rimit on RHCC,RHCC<10.6 pb (95% C.L.), corresponds to mass of WR boson, MWR>180 GeV (95% C.L.)
with an assumption that RHCC and LHCC have same coupling.
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Reduced cross sectionsZEUS preliminary
<Pe>=–0.268
-- Measurements were done with much larger statistics than before.
-- The results will bring better understanding of PDFs.
i
ii
i
W
W
F
dyux
dxdQ
d
M
QM
G
x
2
2
22
2
22
2
)1(
2~
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Reduced cross section
i
ii
i
W
W
F
dyux
dxdQ
d
M
QM
G
x
2
2
22
2
22
2
)1(
2~
ZEUS preliminary
<Pe>=+0.329
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Summary• CC DIS was measured with HERA-II polarised d
ata.• e+p result was published. (hep-ex 0602026)• e–p result has been updated with integrated lumi
nosity of 122 pb–1.– Linear relation between cross section and polarisation
is clearly shown.– e–p CC DIS cross section at P=+1 extrapolated to be
consistent with SM– Reduced cross sections were measured with large lu
minosity of HERA-II.
pbsysstatPGeVQ epe .)(0.5.)(1.38.0)1,200( 22
(ZEUS preliminary)
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Back up
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Event distribution : e+p data
• Both P>0 and P<0 samples are plotted together. (23.8 pb–1)
• No. of events selected– 469 events
• Contamination of background events is suppressed well.– typically, <1%– 5% at low Q2 region
hep-ex 0602026
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ZEUS detector
• Both Pe>0 and Pe<0 polarization samples were collected.
ZEUS detector
-- General detector for collider experiment.
-- Mainly, calorimeter and tracking system were used this analysis.