Experimental Observation Of Lepton Pairs
Of Invariant Mass Around 95 GeV/c²
At The CERN SPS Collider
不変質量95 GeV/c²近傍のレプトン対の実験的観測
G. Arnison et al. (UA1 Collaboration).
Contents
Phys. Lett. B126, 398-410, 1983.
1. Introduction
2. Experimental Method
3. Data Analysis
4. Result
Yoribayashi Yuta
5. Summary
Jinnouchi / Shibata Lab
2009/7/7
1
1. Introduction
― Intermediate Vector Boson Z0 ―
Z0 is the Intermediate Vector
Boson predicted by the
electroweak theory as the
mediator of weak neutral
currents.

Electroweak theory is a
unified theory of
electromagnetic and weak
interactions.


Z0

There are charged
intermediate vector bosons
W± and a neutral Z0.

e－
e－
Example of weak neutral current:
ν-e scattering
expressd in Feynman diagram.
2
2. Experimental Method
― Proton-Antiproton Collider ―
In quark picture
In hadron picture
u u  Z0
d  d  Z0
p p Z  X
0
e－
proton
270 GeV
u

u
d

anti-proton
~45GeV
Z0
~45GeV
e+
u

u
d

270 GeV
e or μ
Proton antiproton collider is suitable for this experiment.
3
― Invariant Mass ―
Before decay
M inv
2
Z0
2
E p
2
After decay

E, p
θ
  2
2
2
M inv  E1  E2    p1  p2 
 E1  E2 
2

 2  2
 
 p1  p2  2 p1 p2 cos 
e+

e－

E2 , p2 

E1 , p1 
4
― UA1 Experiment at CERN ―
CERN SPS(Super
Proton Synchrotron)
collider is located in
Geneva, Switzerland.

In this experiment, a
centre-of-mass energy is
p
UA1

E p  E p  270GeV
s  540GeV
SPS
anti-p
The integrated
luminosity is

1
Ldt

55
nb

5
― Detector ―

Central tracking chamber
B = 0.7 T

Electromagnetic calorimeter

Hadron calorimeter

Muon chamber
beam
6
― Event Display ―
All tracks from a
collision are displayed.
 All calorimeter hits are
displayed.

beam axis
Thresholds are raised to
PT > 2 GeV/c
for charged tracks and
ET > 2 GeV
for calorimeter hits.
beam axis
+
－
 Then only a e e
pair
survives these cuts.
－

e
e+
7
3. Data Analysis
4 e+e－ pairs are observed.
 This figure is one of them.
 The other events are similar
to this.
 This is electromagnetic
energy deposition at angles
>5° with respect to the beam
direction.

φ: azimuthal angle
η: pseudorapidity
e－
φ
beam axis
θ
e+
   
   ln  tan  
  2 
8
4. Result
This figure shows invariant
mass of observed lepton pairs.

From these observations, we
deduce the mass for the Z0
particle,
m Z = (95.2 ± 2.5) GeV/c2

0
Invariant Mass of Lepton pair [Gev/c2]
A,B,C,D are e+e－ pair events.
9
5. Summary

Proton-antiproton collider at CERN were used to produce Z0.

Quark and antiquark annihilate to Z0 and decay to a lepton pair.

UA1 observed this lepton pair.

These events fit well the hypothesis that they are produced by the process
p + p → Z0 + X
e+ + eμ+ + μ-
This paper reports the observation of four e+e- pairs which have the
signature of a two-body decay of a particle of mass
mZ = (95.2 ± 2.5) GeV/c2 .

0

With this discovery, electroweak theory was established.

W±
10
bosons had been observed by UA1 5 monthes earlier in the same year.
fin
11
― Central Tracking Chamber ―
This gas-filled central detector consist of
six main modules over 6000 sense wires
for detecting electrical signals.

The wire are all arranged in planes
separated by 20 cm and all wires are
parallel to the magnetic field.

The electrons produced by the ionization
and the passage of these particles can be
reconstructed from the electrical signals.

The momentum is

p  RqB
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Weak Boson
• There are W and Z boson in weak boson.
• W decay to one lepton and one neutrino pair.
• Mass of W is
mW= 80.398(25) GeV
• Mass of Z is
mZ= 91.1876(21) GeV
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Data Selection
• Electron trigger
ET > 10 GeV
• Muon trigger
|η| ≤ 1.3 ( ~ 30.5º ≤ θ )
• Jet trigger
ET > 20 GeV
in a localized calorimeter.
• ET trigger
ET > 50 GeV (|η| ≤ 1.4 )
2009/7/7
14
― Background ―
Two isolated high ET electrons
ET > 25 GeV ( PT > 7 GeV/c )
 E
hadron <0.8 GeV


jets → 1 isolated track ( PT > 25 GeV/c )
1 track observed
fake muon probability ~ 2×10-3
fake e± probability ~ 6×10-3
↓
negligible
heavy flavour jets ( bb, cc )
2 events with an isolated μ ( PT > 15 GeV/c )
1 event with an isolated e ( PT > 25 GeV/c )
other jets fake leptons → 10-4 events

Onium decay from a new quark
σ(QQ) : negligibly small

15
Data Confidence
Magnetic deflection in 1/p
units compared to the inverse
of the energy deposited in the
electromagnetic calorimeters.

Ideally, all electrons should
lie on the 1/E = 1/p line.

16
Compare with Prediction
－ Angle of Weinberg －
• From their preliminary result,
sin 2 W mW   0.236  0.030
• They parametrize the Z0 mass with the well-known
fomula
MW
  cos W
MZ
• They find
  0.94  0.06
• This is in excellent agreement with the prediction of
the minimal model.
17
Compare with Prediction
－ Angle of Weinberg －
18
Weinberg角
• |γ＞ と|Z0＞は|W0＞と|B0＞を基底に取ったとき以下の
ように記述される。
    cos W
 0 
 Z    sin W


0
sin W  B

cos W  W 0





• θWはWeinberg角と呼ばれ、実験的に求められる。
• Zボソンの質量は理論的に以下の関係を満たすように固定
されている。M
W
MZ
 cos W
• 本論文ではこの関係式より実験結果の整合性を確かめて
いる。
19