1 “ Cross section measurements in DIS ” K. Papageorgiou University of the Aegean On behalf of ZEUS and H1 Collaborations
Dec 31, 2015
1
“ Cross section measurements in DIS ”
K. PapageorgiouUniversity of the Aegean
On behalf of ZEUS and H1 Collaborations
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OUTLINE
Introduction
Cross sections
Power of data sets combination
FL measurement
Motivations & Method
Analysis strategy
H1 & ZEUS results
Summary
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HERA – The Worlds only electron-proton collider
Collides protons with
Ee : 27.5 GeV
Ep : 920 GeV
Centre of mass energy :
GeVs 320
Q2 corresponds to the spatial resolution (wavelength λ) of the probe
(set by s)
2/1~ Q252
max 10~ GeVQ
1000/ ~ 10~ 18min protonRm
e
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Approximately ~ 1 fb-1 of data collected by H1 & ZEUS
In operation since 1992
HERA Running ended 30 June 2007
HERA –I : Until year 2000
• Unpolarized e+ and e- beams
HERA – II : 2002 – Mar 2007
• High luminosity to allow more statistical sensitivity to high Q2
• Longitudinally polarized e+/e- beams to allow direct sensitivity to EW
Low energy Run : March 2007 – June 2007
• A special Run with low proton beam energy (460, 575 GeV) to measure directly FL structure function
HERA – I & II combined integrated Luminosity
~ 0.5 fb-1 per experiment
HERA Luminosity
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• A key tool to “look” inside the proton, allow testing of QCD
p(P) Proton remnant
γ, Ζ0Q2 = -q2=-(k-k΄)2
(xP)
}
Current jet
)(ke
)'(ke
NC
• DIS cross section can be described in terms of kinematic variables
Q2 : Virtuality of the intermediate boson
-- measure of the probing power
x : Bjorken scaling factor
-- fraction of proton’s momentum carried by struck quark
y : Inelasticity
-- energy fraction transferred from lepton in proton rest frame
s : Centre of mass energy of the ep system
qP
Qx
.2
2
kP
qPy
.
.
s x yQ 2
CC
DEEP INELASTIC SCATTERING KINEMATICS
p(P) Proton remnant
W±Q2 = -q2=-(k-k΄)2
(xP)
}
Current jet
)(ke
)'(kv
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The NC DIS cross section expressed in terms of structure functions
F2 -- dominant contribution
to the cross section
)q(q 22 xeF
NC DIS cross section: F2
)],(),(),([2)( 2
32
22
24
2
2
2
QxxFQxFY
yQxFY
xQ
a
dxdQ
pedL
F2 -- SF parameterize target structure, i.e. how far for point-like
If proton is point-like then,
YxQ
a
dxdQ
ped4
2
2
2 2)(
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NC DIS cross section: F2 (H1 & ZEUS combined data)
New HERA-I PDF fit predictions vs H1 & ZEUS combined data NC e+p: (see talk of LI, Gang )
Total uncertainties on the PDF fit predictions included but can barely be resolvedProton scan with high precision NLO fit (DGLAP) – good description of the data Systematic uncertainties are now smaller than statistical uncertainties
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CC e-p cross section (ZEUS)
Chiral structure of weak interactions is directly visible as a function of polarization Linear dependence with lepton polarization
Pe=1 for e- , Pe=-1 for e+
Consistent with SM prediction of No evidence of RH currents. Error band from PDF uncertainty
)0()1()(
epe
CCeepe
CC PPP
0RHCC
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NC e+p cross section: (ZEUS)
Cross sections for NC DIS in e+p collisions with a longitudinal polarized positron beam have been measured using the ZEUS detector Results based on data corresponding to an L=113.3 pb-1 at sqrt(318) GeV are given for both positive and negative values of the longitudinal polarization of positron beam Single differential cross sections are shown in the kinematical region Q2>185GeV and y<0.95Measured cross section consistent with SM prediction
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NC e+p: (H1)
Cross sections for NC DIS in e+p collisions with a longitudinal polarized positron beam have been measured using the H1 detector
Single differential cross section are shown for HERA I and II e+p data
Measured cross section is compared to the prediction of the SM
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xF3 -- is the parity-violating term arising from Z0 exchange
-- experimentally can be extracted by measuring the
-- negligible at small Q2
2
2
2
2 )()(
dxdQ
ped
dxdQ
ped
NC DIS cross section: xF3
)d(2u 3 vv3 x
xF Z
3xF
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FL MEASUREMENT : Motivation & Method
FL -- longitudinal SF, sizeable only at low x, observable at high y
-- FL=0 in QPM (spin ½ partons – Gallan-Gross),
-- propotional to gluon at higher orders, Direct sensitivity to gluon dynamics
)]()1(83
16[
42
2
1
32 zzg
z
xeF
z
dzx
aF
x
sL
To separate F2 , FL w/o theory assumption , one needs to measure the cross section at fixed (x,Q2) and different values of y different beam energies (s)
)],(),([2)( 2
22
24
2
2
2
QxFY
yQxFY
xQ
a
dxdQ
pedL
~ :reduced cross section
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FL MEASUREMENT : Motivation & Method
Separate SF’s at given x ,Q2 using straight line fit and error compensation Intermediate energy for measurement control. At low y and fixed (x,Q2) cross sections need to coincide Renormalization used by H1 and by ZEUS
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DIS events: Identified by scattered electron (Electron Method)
High –y : Low energy electron Low –Q2:
)cos1(2 '2eeemeas EEQ
FL : Analysis strategy
High y / Low s (Ep=460 GeV) Low energy electron in the calorimeter; lot of hadronic activity
Electron finding is difficult at low Q2 large γp background, mostly γ’s and π’s faking an electron needs to be subtracted/controlled
Low y / High s (Ep=920 GeV) High energy well separated electron in the calorimeter; no background
)cos1(2
1'
ee
emeas E
Ey
H1 analysis covers both SpaCal and LAR
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Low-x: visible differences between data sets
High-x: cross sections are consistent
Low-x: the x sections turn over
Reduced cross section measurement (ZEUS)
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Reduced cross section measurement (H1)
H1 preliminary
Ep = 460 GeV
= 575 GeV
= 920 GeV
Combined LAr & SpaCal
ymax ~ 0.9
[@450 & 575 GeV]
xx
21Results consistent with the prediction of H1 PDF fit (obtained using only the H1 high energy x-section data
Overview on all (medium & high Q2 ) FL data (H1)
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Results consistent with H1 fit and with expectations from global parton distribution fits at higher order pertubation theory
DGLAP describes low-x region at HERA
x- averaged FL in the full range (H1)
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Summary• First direct measurement of FL was performed in a new kinematical range
Q2 : (24-110) GeV2 [ZEUS] , Q2 : (12-800) GeV2 [H1]
and Bjorken x : 0.0002 – 0.05
• Data are in good agreement with higher order pQCD predictions
• First data have been published
• HERA experiments provide unique information on proton structure over wide
range of x
an important precision input for physics at LHC
• The combination of the data sets of ZEUS and H1 inclusive cross section is very successful and convincing
greatly improved precision compared to the measurements of each experiment separately
•Analysis is ongoing, thus can be expected to further constrain low-x
• A most remarkable success of HERA!