Experimental observation of lepton pairs
of invariant mass around 95 GeV/c2
at the CERN SPS Collider
G. Arnison et al., UA1 Collaboration
Phys. Lett. B126 (1983) 398-410
Contents
1. Introduction
2. Experimental methods:
ppCollider and UA1 Detector
3. Data Analysis
4. Results
5. Summary
Shibata Lab.
13_05204
Atsushi Kurihara
1
1. Introduction
1.1 Z0 as an intermediate vector boson
• Electroweak theory is a part of
standard model of particle
physics.
u (d)
• Z0, a gauge boson, is the
intermediate vector boson in
electroweak interaction.
• The Z0 mass is not predicted
directly by electroweak theory.
The Z0 mass is predicted to be
mZ = 94 ± 2.5 GeV/c2 from
analysis of experimental data of
neutral current.
+
e
Z
u (d)
+
m
( )
0
e- (m )
-
2
1.2 Production and decay of Z0
• pcontains anti-quarks as valence quarks.
• Therefore, Z0 and W ±are expected to be produced at
pp
relatively low beam energies in
collision:
u+ u ® Z 0 d + d ® Z 0
u+ d ® W+ u + d ® W-
p + p ® Z0 + X
Z 0 ® e+ + e- or m + + m -
e-
p
Z
p
0
e+
3
2. Experimental methods:
pp Collider and UA1 Detector
2.1 SPS Collider
• CERN SPS (Super
Proton Synchrotron)
collider is located in
Geneva, Switzerland.
•
p’s and p
’s are
accelerated to
270
GeV in SPS.
√s = 540 GeV.
p
p
SPS
4
2.2 Production of
SPS
p (26 GeV/c)
SPS
p (26 GeV/c)
p’s
p Accumulator
p: 0 ® 26 GeV/c
p: 3.5 ® 26 GeV/c
target
• Proton beam of 26 GeV/c hits a nuclear target and ’s are
p
produced.
• The p’s of 3.5 GeV/c are collected in Antiproton Accumulator
(AA).
• The p’s are then extracted from AA and anti-clockwise
accelerated in Proton Synchrotron (PS) and injected to SPS.
5
2.3 Stochastic cooling in AA
• The phase space of the pbunch is
reduced by Stochastic cooling in
Anti-proton Accumulator (AA) in
order to increase luminosity of
pp collisions in SPS.
• The phase space of the pbunch is
six-dimensional:
, dx dz
, dy dz
, Dp
.
Dx, Dy, Dz
zis the direction of the beam axis.
are the sizes,
Dx, Dy, Dz
Antiproton Accumulator
A feed-back signal is
sent before the beam
comes around
dx dz, dy dz
are the slopes, and
Dpis the momentum width of the
p
bunch.
6
2.4
UA1 Detector
6m
Calorimeter
p
p
Central tracking detector
Magnetic field is 0.7 T.
It is perpendicular to this page.
Muon detector
7
3. Data Analysis
• The figure shows all tracks of
charged particles and calorimeter
hits from an pp
collision.
• Then, thresholds are raised to pT >
2 GeV/c for charged tracks and ET
> 2 GeV for calorimeter hits.
Only one positron-electron pair
survives these mild cuts.
+
e
q
e+
ET º Esinq
-
e
e-
8
• This figure shows
electromagnetic
energy depositions.
+270.0
• The dominant feature
is two very prominent
electromagnetic
energy depositions.
j
-90.0
+
e
j
q
-
e
-3.0
h
+3.0
j : azimuthal angle
h : pseudo-rapidity
æ qö
h º - ln ç tan ÷
è 2ø
5° £ q £ 175°
-3.13 £ h £ 3.13 9
4. Results
• 4 e+e-pairs and 1
m +pair
m - from Z0 decay are observed.
• This table shows the invariant mass of the lepton pairs.
• From this observation, UA1 deduced a mass value of Z0 to
be mZ = 95.2 ± 2.5 GeV/c2.
Event
Mass (GeV/c2)
A
91 ± 5
B
97 ± 5
C
98 ± 5
D
95 ± 5
μ+μ-
95 ± 8
Mean
95.2 ± 2.5
(A, B, C, D are
e+epair
events. )
Invariant mass
(GeV/c2)
95.2 GeV/c2
10
5. Summary
• Electroweak theory is a part of standard model of particle
physics.
• Z0 is the intermediate vector boson in electroweak interaction.
• An
collider was proposed and constructed to search for Z
andpp
W boson.
• Z0 can be generated from
pair:
•
collision and decay to
pair or
pp 0
e+e+
+
+ p+
p
®
Z
®
e
+
e
or
m
+
m
m mfor these lepton pairs.
UA1 looked
• In this experiment, 4
are observed.
pairs and 1
e+e-
• UA1 deduced a mass value of Z0 to be
95.2 ± 2.5 GeV/c2.
pair from Z0 decay
m +m -
mZ =
• With this discovery, together with discovery of W in the same
±
year (1983), electroweak theory was established.
11
補足
12
Mass of Z and W boson
• Z0 mass is predicted from Nucl. Phys. B 167, 397 (1980) and
Rev. Mod. Phys. 53, 211 (1981).
• Mass of Z0 is 91.1876 ± 0.0021 GeV/c2.
±
• Mass of W is 80.385 ± 0.015 GeV/c2.
13
• The central tracking detector is selfsupporting cylinder having a diameter of
2.2 m and length of about 6 m.
• This cylinder is split into six half-moon
section.
• In case of failure, it can be removed and
replaced by other standard elements.
• The gas in the chamber is a mixture of
40% argon and 60% ethane at
atmospheric pressure.
• All wires run parallel to the magnetic field,
while the wires in the forward chambers
are organised in horizontal planes and
the wire in the central chambers in
vertical planes.
14
Trigger
i. Electron trigger
ET ≥ 10 GeV
ii. Muon trigger
|η| ≤ 1.3
iii. Jet trigger
ET ≥ 20 GeV
In a localized calorimeter cluster
iv. A global ET trigger
ΣET > 50 GeV (for all calorimeter)
|η| ≤ 1.4
15
Event selection
i. Single, isolated electromagnetic cluster with ET >
15 GeV and missing energy events with Emiss > 15
GeV, in order to extract
events.
W± ® e± + n
ii. Two or more isolated electromagnetic clusters with
ET > 25 GeV/c2 for
candidates.
0
+
Z
®
e
+
e
iii. Muon pair selection to find
events.
0
+
Z
®
m
+
m
iv. Events with a track reconstructed in central detector,
of transverse momentum within one standard
deviation, pT > 25 GeV/c, in order to evaluate some
of the background contributions.
16
Data confidence
E = (mc ) + (pc)
2 2


2
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.
17