“Experimental Observation of Isolated Large Transverse Energy Electrons with Associated Missing Energy at = 540 GeV” Okamura Yusuke Shibata lab. G. Arnison et al., UA1 Collaboration Phys. Lett. 122B (1983) Contents: 1. Introduction 2. Experimental Method 3. Analysis 4. Summary Physics Colloquium July 7th, 2008 s

2 1. Introduction Weak Interaction Fermi made a theory of β-decay in 1930's. The interaction was a contact interaction. ( no intermediate particle ) Weinberg and Salam made a theory for ElectroWeak Interaction in 1960's. The ElectroWeak Interaction is a combined framework for Electromagnetic Interaction and Weak Interaction. The intermediate particles of Weak Interaction are W and Z. The mass of W and Z are large. The range of interaction is short. Experimental discovery of W and Z is important to establish ElectroWeak Theory. n p ν e － e ‐ n p ν e － e ‐ Fermi’s Model ± W ‐ Weinberg-Salam’s Model p ν e Z p ν e ± charged current neutral current β-decay

3 - p + p → W + X e + ν (－)(－) ± ± We look for the following event ； p p - u u d u - d - u - W + ν e e + collision two-body decay 2. Experimental Method CERN SPS Proton-Antiproton Collider Accelerator : proton and antiproton collisions at = 540 GeV pp － p E E p － = 270 GeV s

4 ① ③ ④ ⑤ ② ⑥ ⑦ In cross section The UA1 detector ◎ Hadronic Calorimeter ・ energy measurement of hadrons ◎ Electromagnetic (EM) Calorimeter ( consists of two parts ) ・ energy measurement of e and ◎ Drift Chamber ( in magnetic field ) ・ measurement of charged tracks and momenta 155° 25° 0° beam axis beam crossing point ± The name of experimental group is UA1 Detector

5 Event Selections This experiment was carried in a 30-day period. ◎ Recorded events ◎ Candidate events of W ： 5 events ： conditions: ・ large transverse energy of electron ・ large missing transverse energy (neutrino) ・ no hadron jet Electron Search for W → e + ν ± ± (－)(－) ﾆｭｰﾄﾘﾉはこうやって測定した ・ Electron was measured with drift chamber and electromagnetic calorimeter. ・ Neutrino was not measured. Momentum of neutrino was determined by momentum imbalance using the electromagnetic calorimeter and hadronic calorimeter. ◎ Expected number of p-p collision in this period ： 9.75 ×10 5 ± ± (－)(－) 行開ける e ± ν (－)(－)

φ angle 270° Pseudo-rapidity Φ angle Pseudo-rapidity η 6 Detailed Investigation of the electron-neutrino events 5 candidates events are carefully investigated. η θ° Analysis Following figures are data of one event. Pseudo-rapidity η is the function of θ, like the following table. hadronic calorimeter electromagnetic calorimeter electron track charged tracks in the detector Energy depositions in the calorimeters ・ Pseudo-rapidity η is a function of θ φ beam axis particle track θ η θ° ・ φ is angle of spherical coordinate η θ° Pseudo-rapidity η － － － 90° φ angle 270° － 90° E max 23.7 GeV T E max 0.5 GeV T θ = 28° ～ 90 ～ 152 ( η ＝ -1.4 ～ 0 ～ 1.4 ) beam crossing point

7 This figure shows the correlation between transverse electron energy and the missing transverse energy. Transverse electron energy ↓ GeV GeV ＃ of events 1 2 Missing transverse energy ＃ of events Momentum balance between electron and neutrino m is determined as by correcting for the transverse motion of W. m = 81 ±5 GeV/c W 2 W ± ± ← ← ↓ beam axis beam crossing point e ± ν (－)(－) e ± ν (－)(－) E T E T E T E T Events with large transverse energy Events with small transverse energy These two energies are proportional. This result shows two-body decay of W.

8 ・ W and Z are intermediate particles of weak interaction. ・ p and p collision at high center-of-mass energy can produce W. ・ Experiment was carried out by UA1 collaboration at CERN-SPS. ・ W decays to electron and neutrino (missing energy) back-to-back. ・ 5 events are consistent with two-body decay of W. ・ m ＝ 81 ±5 GeV/c ・ It agrees with the Weinberg-Salam model 4. Summary Z was also discovered by UA1 collaboration in The physics Nobel prize 1984 was awarded to this discovery. － W 2 ± - ± ± ±

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Energy flow vector 10

11 Energy flow vector ・ Neglecting particle masses ・ With an ideal calorimeter response ・ With ideal solid-angle coverage ⇒ ∑ΔE = 0

12 Event Selections Expected number of p-p collisions in a 30-day period ： trigger conditions and other conditions for good data selection ： 9 the electron trigger ＞ 15 GeV of transverse energy with a good quality, vertex-associated charged track 9.75 × × Requirement of Three trigger conditions ・ with large transverse energy ・ with undetected muon tracks 10 events 9 The fast track must hit a pair of adjacent EM calorimeter modules The Φ information agree with the impact of the track. The energy deposition in the hadronic calorimeters ≦ 600 MeV The energy match the momentum p of other tracks entering the same modules ≧ 2 GeV/c. T with no jets activity 5 events 5 5

◎ e Identification ・ By their charged tracks ・ By the lack of penetration in the hadron calorimeter ◎ ν Identification ・ Only by transverse energy imbalance ( missing transverse energy ) Particle Identification ① ③ ④ ⑤ ② ⇒ ・ Now, we define an energy flow vector ΔE, which is 0 in ideal conditions. ⇒ ・ By using this technique, we detect the missing transverse energy, namely ν. 13 Events without jets Events with jets Electron transverse energy Transverse to electron Parallel to electron Missing transverse energy Parallel to electron Missing transverse energy normal to electron Electron direction

14 Background evaluations Backgrounds to the electron signature for no jets events (1) a high-p charged pion ( hadron ) misidentified as an electron or overlapping with π ⇒ negligible (2) high-p π, η or γ converted to an e e pair with one leg missed ⇒ negligible (3) heavy quark associated production followed by pathological fragmentation and decay configuration ⇒ negligible ① ③ ④ ②

( Fig.2,3 ) 15 Search for electron candidates We require conditions ； ( i ) three conditions on the track for isolated tracks ( 2125 events → 167 events ) ( ii ) two conditions to enhance its electromagnetic nature ( 167 events → 39 events ) + ‐ 3. Analysis ⇒ (1) with no jet activity ( 5 events ) (2) with a jet opposite to the track (11 events ) (3) with two jets or clear e e conversion pairs ( 23 events ) Now, we find that, Fig.2 Fig.3 ・ events with a jet have no missing energy ・ events with no jets show missing energy

16 Search for events with energetic neutrinos Taking 2125 events again, we operate conditions. ① ③ ④ ⑤ ② ⑥ ⑦ ⑧ These events with jet are likely to be hadrons, and without jet electrons. ( Fig.4 ) These jetless events include previous 5 events. ( electron candidates) ⇒ (1) E ≠ 0 ( 10 events ) (2) E = 0 (8 events ) ( i ) two conditions of a high missing transverse energy and the candidate track not part of a jet ( 2125 events → 70 events ) ( ii ) removing undetectable events ( 70 events → 31 events ) ⇒ (1) E ＞ 0.01 E ( 21 events ) (2) E ＜ 0.01 E ( 10 events ) ( iii ) with no high-p track in the small-θ cone ( 31 events → 18 events ) ⇒ (1) without jet ( 7 events ) (2) with jet opposite to the track (11 events ) Events without jets Events with jets Fig.4

17 m (e,ν) = (|p

1. Introduction W ( Intermediate Vector Bosons of weak interaction ) ： cf.) Z also of weak interaction, of electromagnetic interaction, g of strong interaction ・ mediating the β-decay ( Fig.1 ) ・ of very large masses about 80 GeV 18 ± Discovery of W ± - p + p → W + X e + ν (－)(－) ± ± ◎ We look for the following event ； ① ② ③ ④ ⑤ p p - u u d u - d - u - W + ν e e + collision two-body decay np ν e － e ‐ W － Fig.1 β-decay

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