Post on 28-Dec-2015
Gauge Boson-Pair Production in pp Collision at 14 TeV
Gauge Boson-Pair Production in pp Collision at 14 TeV
Using ATLAS at the LHC
Erez Reinherz-Aronis
Supervisors:Prof. Gideon Alexander
Prof. Erez Etzion
Gauge Boson-Pair Production in pp Collision at 14 TeV
Gauge Boson-Pair Production in pp Collision at 14 TeV
Using ATLAS at the LHC
Outline:
LHC (reminder)
Physics background
Experimental procedure
Large Hadron ColliderLarge Hadron Collider• pp collider• Tunnel length: 26.7 Km • The beams energy, up to 7 TeV ( = 14 TeV)• Particles per bunch: ~1011
• Bunch crossing every 25 ns • Luminosity: Low and High phases
( )S pp
A reminder
0 05 0 1. .
LL
GoalsGoals
• In the gauge boson pair production we focus on the ZZ channel in their decay to lepton pair
• Measurement of the at 14 TeV• Measurement of and vs. • Z polarization with the emphasis on the longitudinal
part using the SDM method• Angular correlation between the two decay
planes
( )tot pp ZZ X
Z l l
l l
( )S ZZ( )
Z
d qq ZZ
d cos
( )tot qq ZZ
Physics backgrondPhysics backgrond
• LEP2 measured • agrees with the SM expectation
( )tot e e ZZ
• Tevatron ( at ~2 TeV) D0(CDF) reported 1(3) ZZ candidates
pp
Previous knowledgePrevious knowledge
Features of the ZZ channelFeatures of the ZZ channel
• Expected ( )tot pp ZZ X
Features of the channelFeatures of the channel
• Expected differential cross section
Zcos
( )tot uu ZZ
( )
Z
d uu ZZ
d cos
• Expected total cross section
200 GeV
250 GeV
300 GeV
Demonstrated by the uū→ZZ channel
qq ZZqq ZZ
Determination of the Z polarization Determination of the Z polarization
• Spin Density Matrix, :Angle definitions
0, ,
Spin Density MatrixSpin Density Matrix
• 3x3 matrix
• Were the Z SM coupling constants are:
2 2 20
2 2
1 3 3 3(1 ) 2 (1 ) (1 ) 2 =
8 4 8
3 3(1 ) (1 );
8 4
Z
Z
zf cos
T L f
dx Rx x x Rx
d cos
x x x cos
2 22 2 2 2
2 2
1
2R L
R w L wR L
g gg sin g sin R
g g
0 1T L
The SM expectationThe SM expectation• for light quarks
( ) [GeV]S ZZ
0 ( )qq ZZ
0 ( )L
200 GeV250 GeV
uū0.050.02
• Planes correlation
Average value for A( ) 1 (2 )F Acos
( )F
Experimental procedureExperimental procedure
• ZZ→4μ (golden channel)
• Expected number of events for ℒ=100 fb-1
ZZ→4μZZ→4l (e, μ)
Branching ratio (Br)0.0010.004
Produced No. of events14005600
Identification efficiency 25%
No. events350
Identification of the ZZ channelIdentification of the ZZ channel
Monte Carlo studyMonte Carlo study
• Pythia 6.221 with full simulation
• Identification efficiency
• ΓATLAS(Z)=4.8 GeVΓPDG(Z)=2.5 GeV
Eve
nts
Main BackgroundsMain Backgrounds
• Gluon fusion (gg→ZZ) on average 20-25%
• From MC study
Background (ZZ→4μ ) 6%8%
4tt 4Zbb
Gauge Boson-Pair Production in pp Collision at 14 TeV
Gauge Boson-Pair Production in pp Collision at 14 TeV
Using ATLAS at the LHC
Erez Reinherz-Aronis
Supervisors:Prof. Gideon Alexander
Prof. Erez Etzion
Measurement of the Z PolarizationMeasurement of the Z Polarization
• 3x3 matrix
• Were the Z SM coupling constants are:
2 2 20
2 2
1 3 3 3(1 ) 2 (1 ) (1 ) 2 =
8 4 8
3 3(1 ) (1 );
8 4
Z
Z
zf cos
T L f
dx Rx x x Rx
d cos
x x x cos
2 22 2 2 2
2 2
1
2R L
R w L wR L
g gg sin g sin R
g g
0 1T L
A reminder
Measurement of the Z PolarizationMeasurement of the Z Polarization
• Alternative method - proposed by Renard et al.*• Fit is not needed (provides results for low statistic)• No need for bins in • One can define:
2
20
5 1
2 5
T f
L f
cos
cos
1
1( ) ( )
N
i Z i f kk
cos cosN
f
• Lower Order SM diagrams for ZZ pair production in pp collision
• event in ATLASZZ e e X
From: Cern Courier - May -2007,
“The LHC: illuminating the high-energy frontier”
Taken from LEP EW WGhep-ph/0701261
hep-ph/0212396
From the PDG
Nucl. Phys. B321 (1989) 561
Phys. Rev D36 (1987) 1570
• Z boson couplings to fermions
gLgRgVgA
e, , 1/201/4-1/4
e, , - 1/2+sin2Wsin2W- 1/4+sin2W1/4
u, c, t1/2-2/3sin2W-2/3sin2W1/4-2/3sin2W- 1/4
d, s, b- 1/2+1/3sin2W1/3sin2W- 1/4+1/3sin2W
1/4