Poster on the W+charm measurement for the ATLAS collaboration UK workshop 2014.

1. Measurement
of
the
produc1on
of
a
W
boson
in
associa1on
with
a
charm
quark
in
pp
collisions
at
√s
=
7
TeV
with
the
ATLAS
detector
ATLAS
UK
2014
Giacomo
Snidero
(Queen
Mary
University
of
London)
The
producFon
of
a
W
boson
in
associaFon
with
a
single
charm
quark
(W+charm)
is
studied
using
4.6
pb−1
of
pp
collision
data
at
√s
=
7
TeV
collected
with
the
ATLAS
detector
at
the
Large
Hadron
Collider
(LHC)
[1].
In
events
in
which
a
W
boson
decays
to
an
electron
or
muon,
the
charm
quark
is
tagged
either
by
its
semileptonic
decay
to
muons
(W+c-­‐jet)
or
by
the
presence
of
a
charmed
meson
(W+D(*)).
Cross
secFons
integrated
over
a
fiducial
kinemaFc
range
and
differenFal
as
a
funcFon
of
the
pseudorapidity
of
the
lepton
from
the
W
boson
decay
are
reported.
Results
are
compared
to
the
predicFons
of
next-­‐to-­‐leading
order
QCD
calculaFons
obtained
from
different
parton
distribuFon
funcFon
(PDF)
sets.
The
measured
cross
secFons
support
the
hypothesis
of
an
SU(3)
symmetric
composiFon
of
the
light
quark
sea
in
the
proton.
Measurement
moFvaFon
&
strategy
• W+charm
is
produced
at
LO
by
the
scauering
of
a
gluon
with
a
down-­‐type
quark
(d,
s,
b).
The
contribuFon
of
each
quark
flavour
is
determined
by
CKM
matrix.
At
LHC
energy,
the
strange-­‐quarks
iniFated
processes
account
for
about
90%
of
the
total.
• W+charm
is
thus
sensiFve
to
the
strange
PDF,
which
is
loosely
constrained
by
neutrino-­‐nucleon
deep
inelasFc
scauering
data
[2].
Some
PDF
analyses
suggest
s-­‐quark
sea
is
suppressed
with
respect
to
the
d-­‐quark
sea;
others,
like
an
ATLAS
analysis
using
W/Z
cross
secFons
data
[3],
support
a
SU(3)
flavour
symmetric
sea.
• The
W
boson
is
selected
via
its
leptonic
decay
into
muon
or
electron
(pTl
>20
GeV,
pTν
>25
GeV,
mTW
>40
GeV).
• Two
independent
analyses,
differing
in
the
the
c-­‐quark
tagging
method,
are
performed:
W+c-­‐jet
and
W+D(*).
• The
W
boson
and
c-­‐quark
charges
have
a
full
correlaFon
→
signal
has
"opposite
sign"
events.
Backgrounds
are
(moistly)
charge
symmetric
and
thus
reduced
by
evaluaFng
the
signal
yields
as
the
difference
between
opposite
and
same
charge
(OS-­‐SS)
events.
(W+cc/bb
backgrounds
cancel
out).
W+c-­‐jet
analysis
• Select
events
sample
with
a
charm-­‐jet,
idenFfied
by
a
semileptonic
decay
into
a
muon
within
the
jet
(soJ
muon
tagging,
c-­‐jet:
pT
>25
GeV,
|η|
<2.5).
• Cut-­‐and-­‐count
events
to
obtain
signal
yield.
• Major
backgrounds
2.5
(data-­‐driven
esFmated):
W+2
light-­‐jets,
mulF-­‐jets,
1.5
Z+jets
(muon
chan.
only).
Other
backgrounds
(Monte
Carlo
esFmated):
t-­‐quark,
di-­‐bosons.
• Leading
systemaFc
uncertainFes:
c-­‐quark
decay
modelling,
jet
energy
scale.
W+D(*)
2.5
2
1.5
1
0.5
s/d PDF
raFo
-5 10 -4 10 -3 10 -2 10 -1 10
ATLAS Internal
∫ -1
Ldt = 4.6 fb s = 7 TeV (2011)
analysis
25
• Select
events
sample
with
D(*)
meson
hadronic
decays,
reconstructed
from
tracks
with
the
correct
charge
combinaFon.
4
decay
channels:
D→
Kππ
,
D*
→
D0π
with
D0
→
Kπ/Kππ0/Kπππ
(D(*):
pT
>8
GeV,
|η|
<2.2).
• Signal
yield
measured
from
fits
to
the
mass
distribuFons
in
the
different
decay
channels.
2000 ATLAS Internal
∫ Ldt = 4.6 fb s = 7 TeV
m(D)
∫ Ldt = 4.6 fb
1.75 1.8 1.85 1.9 1.95 2 2.05 2.1 2.15 2.2
1500
2.4
1000
2.2
2
500
1.8
1.6
1.4
1.2
900 ATLAS Internal
1
800
700
0.8
600
∫ Ldt = 4.6 fb 500
s = 7 TeV (2011)
0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4
400
300
200
100
∫ Ldt = 4.6 fb s = 7 TeV
0
(Data)/σOS-SS fid
aMC@NLO
CT10
MSTW2008
NNPDF2.3
HERAPDF1.5
ATLAS-epWZ12
135 140 145 150 155 160 165 170 175 180 185
5 10 15 20 25 30
aMC@NLO
CT10
MSTW2008
NNPDF2.3
HERAPDF1.5
ATLAS-epWZ12
∫ Ldt = 4.6 fb s = 7 TeV (2011)
c NNPDF2.3coll + W
10 20 30 40 50 60 70
0.96 Soft muon p
+0.16 +0.21
0.18 0.24
30 40 50 60 70 80 90 100 110 120
[GeV]
1
0.5
∫ Ldt = 4.6 fb s = 7 TeV (2011)
References
σOS-SS
Data
17.8 ± 1.9 ± 0.8 [pb]
Stat
Stat+syst
[1]
ATLAS
CollaboraFon,
ATL-­‐COM-­‐PHYS-­‐2013-­‐1354;
[2]
NuTeV
CollaboraFon,
Phys.Rev.
D64
(2001)
112006;
[3]
ATLAS
CollaboraFon,
Phys.Rev.Leu.
109
(2012)
012001.
predictions 22
∫ Ldt = 4.6 fb σ(W-­‐+c-­‐jet) σ(W++D*-­‐)
c NNPDF2.3coll - W
10 20 30 40 50 60 70
σOS-SS
FIG. 10. Measured fiducial cross sections compared to di↵erent PDF shows the central value of the measurement, the inner error band corresponds band to the quadratic sum of the statistical and systematic uncertainties. inner error bars on the theoretical predictions show the 68% confidence with each PDF set, while the outer error bar represents the total theoretical fragmentation and scale uncertainties).
Data
22.4 ± 1.8 ± 1.0 [pb]
Stat
Stat+aMC@syst
NLO
CT10
MSTW2008
NNPDF2.3
HERAPDF1.5
ATLAS-epWZ12
∫ Ldt = 4.6 fb s = 7 TeV (2011)
∫ Ldt = 4.6 fb s = 7 TeV (2011)
σ(W+)/σ(W-­‐)
W+c-­‐jet rs
Wc NNPDF2.3coll
1611 can be written as
-
W
0.4 0.6 0.8 1 1.2 1.4
5 10 15 20 25 30 35
5 10 15 20 25 30 35
Giacomo
Snidero
for
W+charm
analysis
team
(G.
Aad,
H.
Arnold,
L.
Caminada,
L.
Cerrito,
K.
Lohwasser,
M.
Shapiro,
G.
Snidero,
M.
Vanadia,
C.
Weiser)
1600
1400
∫ Ldt = 4.6 fb s = 7 TeV (2011)
1200
Data
33.6 ± 0.9 ± 1.8 [pb]
Stat
Stat+syst
aMC@NLO
CT10
MSTW2008
NNPDF2.3
HERAPDF1.5
ATLAS-epWZ12
c NNPDF2.3coll + W
10 20 30 40 50 60 70
∫ Ldt = 4.6 fb s = 7 TeV
ATLAS Internal
0
σOS-SS
[pb] fid
Data
17.8 ± 1.9 ± 0.8 [pb]
Stat
Stat+syst
-1
• Major
backgrounds
(data-­‐driven
esFmated):
W+light-­‐jets,
mulF-­‐jets.
Other
backgrounds:
t-­‐quark.
• Leading
systemaFc
uncertainFes:
tracking
efficiency,
signal
modelling.
Abstract
T
SMT jet p
2.2
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
• W+charm
cross
secFon
measured
with
a
total
uncertainty
of
~
5-­‐7%.
• W+charm
favours
PDF
sets
with
enhanced
s-­‐quark
contribuFon
(ATLAS-­‐
epWZ12,
NNPDF2.3coll),
supporFng
symmetric
light
quark
sea.
PDF
sets
with
suppressed
s-­‐quark
sea
(NNPDF2.3,
MSTW2008)
are
disfavoured.
• Consistent
picture
from
the
W+c-­‐jet
and
the
W+D(*)
analyses.
• Measured
strange-­‐to-­‐down
PDF
raFo:
• W++c/W-­‐+c
cross
secFon
raFos,
which
provide
sensiFvity
to
strange/
anF-­‐strange
PDF
difference,
indicate
a
symmetry
to
within
~
3%.
Results
m(D) [GeV]
OS-SS Events/12 MeV
0
-1
WD
π±π± ± D±→ K
Data
Fit
Signal
Background
1000
800
600
400
200
2.6
2.4
2.2
2
1.8
∫ Ldt = 4.6 fb
-
D + W
aMC@NLO
CT10
MSTW2008
NNPDF2.3
HERAPDF1.5
ATLAS-epWZ12
NNPDF2.3coll
135 140 145 150 155 160 165 170 175 180 185
) [MeV] 0 Δ m = m(D*)-m(D
OS-SS Events/MeV
0
ATLAS Internal
-1
WD*
π±)π± ± π±→ (K
D*±→ D
Data
Fit
Signal
Background
) [MeV] 0 Δ m = m(D*)-m(D
OS-SS Events/MeV
0
-1
WD*
π±π0)π± ± π±→ (K
D*±→ D
Data
Fit
Signal
Background
1.6
5 10 15 20 25 30 35
1.4
1.2
1600 ATLAS Internal
1400
1200
∫ Ldt = 4.6 fb s = 7 TeV (2011)
1000
800
600
400
200
∫ Ldt = 4.6 fb s = 7 TeV
0
OS-SS [pb]
fid σ
Data
21.2 ± 0.9 ± 1.0 [pb]
Stat
Stat+syst
ATLAS Internal
-1
0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6
(Data)/σOS-SS fid
1
aMC@NLO
CT10
MSTW2008
NNPDF2.3
HERAPDF1.5
ATLAS-epWZ12
NNPDF2.3coll
135 140 145 150 155 160 165 170 175 180 185
) [MeV] 0 Δ m = m(D*)-m(D
OS-SS Events/MeV
0
-1
WD*
± π ) ± π± π ± π± π±→ (K
D*±→ D
Data
Fit
Signal
Background
-
D* + W
Figure 17. Results of the fits to the distributions of m(D) and m = m(D⇤) − m(D0) in
OS-SS WD(⇤) events. The fit results are shown in the inclusive sample defined by pDT
5 10 15 20 25 30 35
8 GeV
OS-SS [pb]
fid σ
and |⌘D| 2.2: D± ! K⌥⇡±⇡± (top left), D⇤± ! D0⇡± ! (K⌥⇡±)⇡± (top right),
D⇤± ! D0⇡± ! (K⌥⇡±⇡0)⇡± (bottom left) and 1.5
D⇤± ! D0⇡± ! (K⌥⇡±⇡⌥⇡±)⇡± (bottom
right). The data distributions are shown by the filled markers, where the error bars show the sta-tistical
∫ 1.4
Ldt = 4.6 fb 1.3
s = 7 TeV (2011)
Data
Q
2 = m 0.92 ± 0.05 ± 0.01
Stat
Stat+syst
ATLAS internal W 2
uncertainty. The fit result is shown by the solid line. 1.2
The filled histogram represents the
signal template normalised according to the fit result, while the contribution of the combinatorial
background is shown by the dotted line.
!
. (12)
Equation 9.
correlated
predictions and
j on the
parameters btheo
j
and rep-1630
uncertainty
minimized
section mea-1633
Section
for asym-1636
asymmetric
functions
(13)
Equation 11.
the val-1640
parameter
value +S+
i,j
T 25GeV
• mWT
uncertain-1643
coefficients are
(14)
(15)
in the
The cross-section
The central value fs = 0.31 at Q2 = 1.9 GeV2
1670 is cho-1671
sen to be consistent with determinations of this fraction
1672 using the neutrino-nucleon scattering data with an un-1673
certainty spanning the range from 0.23 to 0.38. This
1674 model uncertainty is parameterized as an eigenvector in
1675 the 2-minimization.
1676 The 2-minimization procedure not only gives infor-1677
mation about the overall compatibility of the predictions
1678 with the data, but also allows constraints on the PDF
1679 eigenvectors to be obtained. HERAPDF1.5 is the only
1680 publicly available PDF set where the e↵ect of varying the
1681 strange density is parameterized by one eigenvector (fs).
1682 The 2-minimization procedure discussed above can be
1683 used as follows to calculate a value for fs based solely
1684 on the measurements discussed here while ignoring all
1685 previous measured or assumed values of fs. The 2-
1686 minimization is repeated for the HERAPDF1.5 PDF set
1687 after artificially increasing the uncertainty of the strange
1688 fraction fs. This procedure corresponds to a free fit of the
1689 eigenvector representing fs while all other eigenvectors
1690 are constrained to the values determined for the HERA-1691
PDF1.5 PDF. A value of
rs ⌘ 0.5(s + s)/d = fs/(1 − fs) = 0.96 +0.16
−0.18
+0.21
−0.24
is determined at Q2 = 1.9 GeV2
1692 and is independent of
1693 x as implemented in the HERAPDF1.5 PDF. The first
1694 uncertainty represents the experimental and theoretical
1695 uncertainties and the second uncertainty corresponds to
1696 the scale uncertainty of the W +c calculation. Since the
1697 scale uncertainty is the dominant uncertainty, its e↵ect
1698 is assessed separately by repeating the fit under the as-1699
sumption of perfect knowledge of the scale. The resulting
1700 strange fraction is shown in Figure 14 as a function of x
at Q2 = m2
∫ Ldt = 4.6 fb Data
1701 W. For the HERAPDF1.5 PDF the s-quark
OS-SS Events / 5 GeV
0
3 ×10
Data
W+c
W+light
Z+jets
Multijet
Top+Diboson
ATLAS Internal
-1 s = 7 TeV, ∫Ldt = 4.6 fb
W→μν 1,2 jets
[GeV]
T
OS-SS Events / 2 GeV
0
3 ×10
Data
W+c
W+light
Z+jets
Multijet
Top+Diboson
ATLAS Internal
-1 s = 7 TeV, ∫Ldt = 4.6 fb
W→μν 1,2 jets
Figure 9. Distribution of the SMT jet pT (left) and soft muon pT (right) in OS-SS events of
W+1,2 jets sample for the muon channel. The normalisations of the W+light and Z/!⇤+jets
backgrounds and the shape and normalisation of the multijet background are obtained with data-driven
methods. All other backgrounds are estimated with MC simulations and normalised to their
theoretical cross sections. The signal contribution is normalised to the measured yields.
Cross-section determination
7.1 Definition of the fiducial phase space
The cross sections OS−SS
fid (WD(⇤)) and OS−SS
fid (Wc) are measured in a common fiducial
region defined in terms of the W-boson kinematics as follows:
• p`
T 20 GeV and |⌘`701 | 2.5
• p⌫
40GeV
where `, ⌫ are the lepton and the neutrino from the decay W ! `⌫. The leptons are
defined at the Born level, i.e. before QED FSR radiation. As discussed in the following,
the measured raw yields are corrected for detector effects to obtain the cross sections in the
fiducial region of the measurement. The charm quark is identified either by a D(⇤) meson,
and the corresponding cross section measures the production of events with pD(⇤)
T 8GeV
and |⌘D| 2.2, or through a muon from the semileptonic decay of a charmed hadron
embedded in a jet of particles with pjet
T 25 GeV and |⌘jet| 2.5. In the latter case, the
cross section measures the production of events with exactly one c-jet and any additional
∫ ∫ CT10 MSTW2008 HERAPDF1.5 W+c (partial !2/ndof) 3.8/11 6.1/11 3.5/11 W−c (partial !2/ndof) 9.0/11 10.3/11 8.3/11 W+D− (partial !2/ndof) 3.6/4 3.7/4 3.7/4 W−D+ (partial !2/ndof) 3.7/4 4.6/4 3.3/4 W+D⇤− (partial !2/ndof) 2.9/4 6.0/4 2.2/4 W−D⇤+ (partial !2/ndof) 3.0/4 4.4/4 2.4/4 Nexp 114 114 114 Ntheo 28 22 16 Correlated !2 (exp) 0.8 1.8 0.9 ∫ Correlated !2 (theo) 6.8 4.4 3.7 Correlated !2 (scale) 0.6 2.5 1.1 Total !2/ndof 33.6/38 41.3/38 28.0/38 Probability 67.4% 32.9% 88.4% TABLE IX. Quantitative comparison of fiducial cross sections to di↵erent for the di↵erent cross-section measurements, the number of nuisance uncertainties (Nexp), the number of nuisance parameters for the uncertainties !2 corresponding to the experimental uncertainties, the uncertainties the total !2/ndof and corresponding probability are given.
1.1
1
0.9
0.8
0.7
0.6
-3 10 -2 10 -1 10
0.4 0.6 0.8 1 1.2 1.4
x
rs = 0.5 (s+ s)/d
0.5
data (*) HERAPDF1.5 + ATLAS Wc/WD
ATLAS-epWZ12
HERAPDF1.5
Ass =
s(x,Q2) − ¯s(x,Q2)
s(x,Q2)
(10)
uncertainties.
The |⌘`1617 1618 FIG. 14. Ratio of strange-to-down sea quark distributions
rs = 0.5(s + s)/d as a function of x as assumed in HERA-PDF1.5
⇡ R±c (CT10) − R±c (Data),
PDF compared to the ratio obtained from the fit
including the ATLAS Wc/WD(⇤) data and the ratio obtained
from ATLAS-epWZ12. The ratio rs is shown at Q2 = m2
W.
TABLE OS−fid and 1730 1731 son 1733 number 1734 with 1735 for contains 1736 1737 averaged shown (aMC@NLO,CT10) fid
σOS-SS
(aMC@NLO,CT10) fid
(Data)/σOS-SS fid
σOS-SS
0.6
-
D + vs W
-
D* + W
c + vs W
-
D + W
c + vs W
-
D* + W
ATLAS Internal
-1
s = 7 TeV (2011)
68% CL ellipse area
Hashed: meas. uncert.
Open: tot. uncert.
y=x
(aMC@NLO,CT10) fid
σOS-SS
(aMC@NLO,CT10) fid
(Data)/σOS-SS fid
σOS-SS
0.8
+ D
-
vs W + D*
-
W
c
-
vs W + D
-
W
c
-
vs W + D*
-
W
ATLAS Internal
-1
s = 7 TeV (2011)
68% CL ellipse area
Hashed: meas. uncert.
Open: tot. uncert.
y=x
FIG. 11. 68% C.L. contours of the measured cross sections normalized to the theoretical prediction obtained from aMC@NLO simulation using the CT10 PDF. The filled ellipses show the experimental uncertainties, while the open show the total uncertainties, including the uncertainties of the prediction. The left figure shows the correlations among W+D⇤−, W+D− and W+c cross sections, while the right figure is for W−D⇤+, W−D+ and W−c.
c) - (W fid
c)/σOS-SS + (W fid
σOS-SS
ATLAS Internal
-1
Data
0.90 ± 0.03 ± 0.02
Stat
Stat+syst
) (*)+ OS-SS(W-D
fid (*)-)/σ D + OS-SS(W
fid σ
(*) WD
aMC@NLO
CT10
MSTW2008
NNPDF2.3
HERAPDF1.5
ATLAS-epWZ12
NNPDF2.3coll
ATLAS Internal
-1
FIG. 12. Measured ratios !OS−SS
fid (W+c)/!OS−SS
fid (W−c) (left) and !OS−SS
fid (W+D(⇤)−)/!OS−SS
fid (W−D(⇤)+) (right) resulting
from the averaging procedure compared to di↵1612 erent where PDF the predictions s and s based distributions on aMC@are NLO. averaged The blue over vertical the
lines show central values of the measurements, the inner error bands show the statistical uncertainties and the outer error bands the experimental uncertainties. The PDF predictions are shown by the black markers. The error bars on the predictions correspond
1613 W tained ses. 1616 [pb] fid
ATLAS Internal
-1
Data
33.6 ± 0.9 ± 1.8 [pb]
Stat
Stat+syst
[pb] fid
ATLAS Internal
-1
Data
37.3 ± 0.8 ± 1.9 [pb]
Stat
Stat+syst
OS-SS [pb]
fid σ
-
D + W
aMC@NLO
CT10
MSTW2008
NNPDF2.3
HERAPDF1.5
ATLAS-epWZ12
NNPDF2.3coll
ATLAS Internal
-1
OS-SS [pb]
fid σ
+ D
aMC@NLO
CT10
MSTW2008
NNPDF2.3
HERAPDF1.5
ATLAS-epWZ12
NNPDF2.3coll
ATLAS Internal
-1
Data
21.2 ± 0.9 ± 1.0 [pb]
ATLAS Internal
-1
22.1 ± 0.8 ± 1.0 [pb]
ATLAS Internal
-1
Not reviewed, for internal circulation only
November 15, 2013 – 11 : 44 DRAFT 13
parton momentum fraction x
fraction of anti-strange to anti-down quarks (s/d)
0
NNPDF2.3 collider only
CT10
NNPDF2.3
epWZ
MSTW2008
HERA1.5
2W
Figure 2: Depicted is the ratio of the anti-strange to the anti-down quark PDF distribution for di↵erent
PDFs evaluated at the scale Q2 = M= (80.385GeV)2. This is a measure of di↵erences in the parton
distributions for strange and down sea quarks. The range in x relevant for the measurement presented
in here is from 101 to 103. If no error bands are present, the PDF set in question fixes this fraction
without assigning an uncertainty.
c-­‐jet
pT
(meas.+theo.)
(scale)