“ Cross section measurements in DIS ” K. Papageorgiou University of the Aegean On behalf of ZEUS and H1 Collaborations.
Download ReportTranscript “ Cross section measurements in DIS ” K. Papageorgiou University of the Aegean On behalf of ZEUS and H1 Collaborations.
“
Cross section measurements in DIS
”
K. Papageorgiou
University of the Aegean
On behalf of ZEUS and H1 Collaborations
1
OUTLINE Introduction Cross sections Power of data sets combination F L measurement Motivations & Method Analysis strategy H1 & ZEUS results Summary 2
HERA – The Worlds only electron-proton collider
Collides protons with e
Ee : 27.5 GeV
Ep : 920 GeV
Centre of mass energy : s
320
GeV
Q 2 corresponds to the spatial resolution (wavelength λ) of the probe
~ 1 /
Q
2
Q
2 max ~ 10 5
GeV
2
(set by s)
min ~ 10 18
m
~
R proton
/ 1000 3
HERA Luminosity
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 Q 2
•
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 F L structure function
HERA – I & II combined integrated Luminosity ~ 0.5 fb -1 per experiment
4
DEEP INELASTIC SCATTERING KINEMATICS •
A key tool to “look” inside the proton, allow testing of QCD
NC
e
(
k
' )
CC
e
(
k
) γ, Ζ 0
e
(
k
) (xP) Q 2 = -q 2 =-(k-k΄) 2 (xP)
v
(
k
' ) Q 2 = -q 2 =-(k-k΄) 2 W ± p(P) } Proton remnant p(P) } Proton remnant Current jet Current jet •
DIS cross section can be described in terms of kinematic variables
Q 2 : Virtuality of the intermediate boson
Q
2
s x y
-- measure of the probing power x : Bjorken scaling factor
x
Q
2 2
P
.
q
-- 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
y
P
.
q P
.
k
5
NC DIS cross section: F 2
The NC DIS cross section expressed in terms of structure functions d
2 (
e
dxdQ
2
p
) 2
a xQ
4 2
Y
[
F
2 (
x
,
Q
)
y Y
2
F L
(
x
,
Q
2 )
xF
3 (
x
,
Q
2 )]
F 2 - dominant contribution F
2
to the cross section
q e q
2
x
(q q )
F 2 -- SF parameterize target structure, i.e. how far for point-like If proton is point-like then, d
2 (
e
p
)
dxdQ
2 2
a
2
xQ
4
Y
6
NC DIS cross section: F 2 (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 resolved
Proton scan with high precision
NLO fit (DGLAP) – good
description of the data Systematic uncertainties are now smaller than statistical uncertainties
7
CC e p cross section (ZEUS)
Chiral structure of weak interactions is directly visible as a function of polarization
Linear dependence with lepton polarization
e
p CC
(
P e
) ( 1
P e
)
e
p CC
(
P e
0 )
Pe=1 for e , Pe=-1 for e +
Consistent with SM prediction
RH of No evidence of RH CC currents. Error band from PDF uncertainty
8
CC e p cross section (H1 & ZEUS) 9
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 Q 2 >185GeV and y<0.95
Measured cross section consistent with SM prediction
10
NC e + p: (ZEUS) 11
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
12
NC DIS cross section: xF 3
xF 3 -- is the parity-violating term arising from Z 0 exchange -- experimentally can be extracted by measuring the d
2
dxdQ
2
- negligible at small Q 2 xF
3 (
e
p
)
d
2 (
e
p
)
dxdQ
2
xF
3
Z
x
3 (2u v d v ) 13
F L MEASUREMENT : Motivation & Method
F L -- longitudinal SF, sizeable only at low x , observable at high y -- F L =0 in QPM (spin ½ partons – Gallan-Gross), -- propotional to gluon at higher orders, Direct sensitivity to gluon dynamics F L
4
a
s x
2 1
x
dz z
3 [ 16 3
F
2 8
q e q
2 ( 1
x
)
zg z
(
z
)]
:reduced cross section d
2 (
e
dxdQ
2
p
) 2
a xQ
4 2
Y
[
F
2 (
x
,
Q
2 )
y Y
2
F L
(
x
,
Q
2 )]
To separate F 2 , F L w/o theory assumption
, one needs to measure the cross section at
fixed (x,Q 2 ) and different values of y
different beam energies (s)
14
F L MEASUREMENT : Motivation & Method
Separate SF’s at given x ,Q 2 using straight line fit and error compensation Intermediate energy for measurement control.
At low y and fixed (x,Q 2 ) cross sections need to coincide Renormalization used by H1 and by ZEUS
15
F L : Analysis strategy DIS events: Identified by scattered electron (Electron Method) High
meas
1 2
E e
'
E e
( 1 Low energy electron Low –Q 2 :
Q
2
meas
2
E e E e
' ( 1 cos
e
)
H1 analysis covers both SpaCal and LAR Low y / High s (Ep=920 GeV)
High energy well separated electron in the calorimeter;
no background
High y / Low s (Ep=460 GeV)
Low energy electron in the calorimeter;
lot of hadronic activity
Electron finding is difficult at low Q 2 large γp background , mostly γ’s and π’s faking an electron
16
needs to be subtracted/controlled
Reduced cross section measurement (ZEUS)
Low-x: visible differences between data sets
High-x: cross sections are consistent
Low-x: the x sections turn over
17
Reduced cross section measurement (H1) x
H1 preliminary Ep = 460 GeV = 575 GeV = 920 GeV
Combined LAr & SpaCal ymax ~ 0.9
[@450 & 575 GeV]
18
F L measurement (ZEUS)
Results are consistent with F L from ZEUS JETS PDF’s
19
F L measurement (H1 published results)
Results are consistent with the prediction of H1 PDF
20
Overview on all (medium & high Q 2 ) F L data (H1)
Results consistent with the prediction of H1 PDF fit (obtained using only the H1 high energy x-section data
21
x- averaged F L in the full range (H1)
Results consistent with H1 fit and with expectations from global parton distribution fits at higher order pertubation theory
22
DGLAP describes low-x region at HERA
Summary •
First direct measurement of F L was performed in a new kinematical range Q 2 : (24-110) GeV 2 [ZEUS] , Q 2 : (12-800) GeV 2 [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!
23
Back-up slides 24
Comparison of FL data with pQCD 25
Comparison of FL data with Dipole Model 26
NC e+p: (ZEUS) 27
NC e+p: (ZEUS) 28
NC e+/-p: (H1) 29