G. Arnison et al., UA1 Collaboration 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: Collider and UA1 Detector 3. Data Analysis 4. Results 5. Summary Shibata Lab. 13_05204 Atsushi Kurihara

1.1 Z0 as an intermediate vector boson 1. Introduction 1.1　Z0 as an intermediate vector boson Electroweak theory is a part of standard model of particle physics. 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.

1.2 Production and decay of Z0 contains anti-quarks as valence quarks. Therefore, Z0 and W are expected to be produced at relatively low beam energies in collision: ± or Quark from p and anti-quark from p annihilate and Z0 is produced.

2. Experimental methods: Collider and UA1 Detector 2.1 SPS Collider CERN SPS (Super Proton Synchrotron) collider is located in Geneva, Switzerland. ’s and ’s are accelerated to 270 GeV in SPS. SPS √s = 540 GeV.

2.2 Production of ’s SPS SPS : Accumulator : target Proton beam of 26 GeV/c hits a nuclear target and ’s are produced. The ’s of 3.5 GeV/c are collected in Antiproton Accumulator (AA). The ’s are then extracted from AA and anti-clockwise accelerated in Proton Synchrotron (PS) and injected to SPS.

2.3 Stochastic cooling in AA The phase space of the bunch is reduced by Stochastic cooling in Anti-proton Accumulator (AA) in order to increase luminosity of The phase space of the bunch is six-dimensional:　　　　 Antiproton Accumulator collisions in SPS. , , , , , . A feed-back signal is sent before the beam comes around is the direction of the beam axis. are the sizes, are the slopes, and is the momentum width of the bunch.

2.4 UA1 Detector Calorimeter Muon detector Central tracking detector Tracks are recorded by the central detector. Magnet yoke is used as hadronic calorimeter. The momenta of charged tracks are determined by deflection in the central dipole magnet generating a field of 0.7 T over a volume of 7 × 3.5 × 3.5 m3. ビームが曲がらないか 磁場はワイヤーと並行 Magnetic field is 0.7 T. It is perpendicular to this page.

3. Data Analysis The figure shows all tracks of charged particles and calorimeter hits from an 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.

This figure shows electromagnetic energy depositions. The dominant feature is two very prominent electromagnetic energy depositions. : azimuthal angle : pseudo-rapidity 4 e+e- pairs and 1 μ+μ- pair from Z0 decay are observed.

4. Results 4 pairs and 1 pair 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 In the present work, a mass value of Z0 is mZ=91.1876±0.0021 GeV/c2 Invariant mass (GeV/c2) 95.2 GeV/c2 (A, B, C, D are pair events. )

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 and W boson. Z0 can be generated from collision and decay to pair or pair: UA1 looked for these lepton pairs. In this experiment, 4 pairs and 1 pair from Z0 decay are observed. UA1 deduced a mass value of Z0 to be mZ = 95.2 ± 2.5 GeV/c2. With this discovery, together with discovery of W in the same year (1983), electroweak theory was established. The Z0 mass is predicted to be mZ=94±2.5 GeV/c2 ±

補足

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. Nucl. Phys. B 167, 397 (1980) and Rev. Mod. Phys. 53, 211 (1981) ±

The central tracking detector is self-supporting 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.

Trigger Electron trigger Muon trigger ET ≥ 10 GeV Jet trigger A global ET trigger ET ≥ 10 GeV |η| ≤ 1.3 ET ≥ 20 GeV In a localized calorimeter cluster ΣET > 50 GeV (for all calorimeter) |η| ≤ 1.4

Event selection Single, isolated electromagnetic cluster with ET > 15 GeV and missing energy events with Emiss > 15 GeV, in order to extract events. Two or more isolated electromagnetic clusters with ET > 25 GeV/c2 for candidates. Muon pair selection to find events. 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.

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.