w/z production at the lhc and the parton distributions c.s. kim (w/ y.s. jeong, f. halzen)
TRANSCRIPT
W/Z Production at the LHC and the Parton Distributions
C.S. Kim (w/ Y.S. Jeong, F. Halzen)
Outline
• Introduction• Sea quark contributions • W charge asymmetry, A(Y)• B(Y) and B(ε) for the charm quark.• Summary
W and Z cross section
X = W or Z
To test the standard model (SM)
To test pQCD current prediction is known up to NNLO.
To determine the parton distribution functions (PDF) of the proton
Drell-Yan mechanism can produce W/Z bosons.
• The PDF of proton can be investigated using the relation of the differential cross sections.
zz
WW
dYd
dYddYdYB
/
//)(
dYddYd
dYddYdYA
WW
WW
//
//)(
Tevatron vs. LHC Initial status of collision (p-pbar vs. p-p)
– different pattern of the rapidity distributions
Tevatron vs. LHC
A. Kusina et al. Phys. Rev. D 85, 094028 (2012)
High energy of the LHC– large event rate– lower momentum fractions of the par-tons, where sea quarks dominate.
yZWBA e
s
Mx /
/
Valence and Sea quark contributionsto the Cross Sections for W/Z production
As energy increases, the rela-tive contributions of the sea quarks become more impor-tant.
Valence and Sea quark contributionsto the Differential Cross Section on Y
• The sea-sea contributions domi-nate at the central rapidity region while the valence-sea contributions are bigger in the forward and backward regions.
• The sea-sea contributions to W+ and W- are the same.
Valence and Sea quark contributionsto the Differential Cross Section on Y
At higher energies, the relative contributions of sea-sea inter-actions are also increased.e.g.) W+: 75% at 7 TeV → 86% at 14 TeV
W Charge Asymmetry, A(Y)
dYddYd
dYddYdYA
WW
WW
//
//)(
)()()()()()()()(23
2)( 2121
2
2121
2
21 xuxsxsxuVxuxdxdxuVxxG
KXWppdY
dusud
FW
)()()()()()()()( 2121
2
2121
2xcxdxdxcVxcxsxsxcV cdcs
)()()()()()()()( 2121
2
2121
2xcxbxbxcVxuxbxbxuV cbub
)()()()()()()()(23
2)( 2121
2
2121
2
21 xuxsxsxuVxuxdxdxuVxxG
KXWppdY
dusud
FW
)()()()()()()()( 2121
2
2121
2xcxdxdxcVxcxsxsxcV cdcs
)()()()()()()()( 2121
2
2121
2xcxbxbxcVxuxbxbxuV cbub
Vud2 = Vcs
2 ~ 0.95, Vus2 = Vcd
2 ~ 0.05, Vub2 ~ 10-5, Vcb
2 ~ 10-3
⇒ The W charge asymmetry is mainly from u-d quark interactions.
),()( xcxc )()( xbxb ),()( xsxs
)()(
)()(
)()(
)()()(
22
222
11
111 xdxu
xdxu
xdxu
xdxuYA
)()()()(
)()()()(
)()()()(
22
22
11
11
22
22
1
xdxuxdxu
xdxuxdxu
xdxuxdxu
)()()()(
)()()()(
)()()()(
22
22
11
11
11
11
2
xdxuxdxu
xdxuxdxu
xdxuxdxu
W Charge Asymmetry, A(Y)
The Reconstructed Rapidity
Y
TLll
YTLll
eppE
eppEY
l
l
,,
,,ln2
1
])2(1ln[ lYY
TTl
TW
lpp
MM
,,
22
2
2,,
2
,,2
TTlTTlT llppppM
LZWZW
LZWZW
pE
pEY
,//
,//ln2
1
K. Hagiwara et al. Phys. Rev. D 41, 1471 (1990)
Definition of the rapidity
The rapidity that can be experimentally recon-structed
W Charge Asymmetry with the Reconstructed Rapidity
• Fully calculated with all flavors. • A(Y)=A(-Y) while A(Y)=-A(-Y) for p-pbar collisions.• The results for the NLO and NNLO PDF sets are almost de-
generated.
Charm Quark Distributions
Charm-quark contributions at the LHC
• The shaded area indicates the charm contributions.
• Like the sea contribution case, the charm contribution increases with the collision energy.
At 7 TeV, (29%, 33%, 8%) for (W+, W-, Z) → (36%, 38%, 11%) at 14 TeV. (at Y=0)
zz
WW
dYd
dYddYdYB
/
//)(
Charm Quark Distributions
B(Y) in terms of ε
))()((
)(2
xdxu
xc
ε of CTEQ6.6 and MSTW2008 PDF is between 0.5 – 0.75.
iYzz
WW
dYd
dYddYdB
/
//)(
The quantity ε for CTEQ6.6 and MSTW2008 PDF sets - about 0.6 for 7 TeV, and 0.7 for 14 TeV.
Summary
• At the LHC energies, the sea quark contributions are considerable.
• We investigated the ways to explore the parton distributions; W charge asymmetry and B quanti-ties.
• The charge asymmetry is little affected by the heavy quarks. Therefore, it is proper to investigate the u(v)(x) and d(v)(x).
• The quantities B(Y) and B(ε) can be used to deter-mine the charm distribution with .
))()(/()(2 xdxuxc