Okamura Yusuke Shibata lab.

transcript

“Experimental Observation of Isolated Large Transverse Energy Electrons with Associated

Missing Energy at 　 = 540 GeV”

Okamura YusukeShibata lab.

G. Arnison et al., UA1 CollaborationPhys. Lett. 122B (1983) 103

Contents: 1. Introduction 2. Experimental Method 3. Analysis 4. Summary

Physics ColloquiumJuly 7th, 2008

√s―

1. IntroductionWeak 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 Interactionand 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 .

νe－e‐

Fermi’s Model

Weinberg-Salam’s Model

charged current

neutral current

β-decay

-p + p → W + X

e + ν( －)

We look for the following event ；

u-d-u-

collision

two-body decay

2. Experimental Method

CERN SPS Proton-Antiproton Collider

Accelerator

: proton and antiproton collisions at = 540 GeV

p p－

pE Ep－= 270 GeV= 270 GeV

√ s―

◎ 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

Event Selections

This experiment was carried in a 30-day period .

◎Recorded events

◎Candidate 　 events of W ： 5 events

： 109

conditions:　・ large transverse energy of electron　・ large missing transverse energy (neutrino)　・ no hadron jet

Search for 　 W 　 → e + ν± ± ( －)

はこうやって測定したﾆｭｰﾄﾘﾉ

・ 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

φ angle

Pseudo-rapidity

Φ angle

Pseudo-rapidity η6

Detailed Investigation of the electron-neutrino events

5 candidates events are carefully investigated .

3. Analysis

Following figures are data of one event .

hadronic calorimeter

electromagnetic calorimeterelectron track

charged tracks in the detector

Energy depositions in the calorimeters

・ Pseudo-rapidity η is a function of θ

φ beam axis

particle track

・ φ is angle of spherical coordinatePseudo-rapidity η－

－1.4

－90°

φ angle

－90°

E max 23.7 GeVT

E max 0.5 GeVT

θ = 28° ～ 90 ～ 152( η ＝ -1.4 ～ 0 ～ 1.4 )

beam crossing point

　 This figure shows the correlation between transverse electron energy and the missing transverse energy .

Transverse electron energy

↓20 40 GeV

40 GeV

＃ of events

Momentum balance between electron and neutrino

m is determined as

by correcting for the transverse motion of W .

m = 81 ±5 GeV/cW2

↓beam axis

beam crossing point

ν( －)

Events with large transverse energy

Events with small transverse energy

　 These two energies are proportional.　 This result shows two-body decay of W .

・ 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 1983 .

The physics Nobel prize 1984 was awarded to this discovery .

Energy flow vector

・ Neglecting particle masses・ With an ideal calorimeter response・ With ideal solid-angle coverage

⇒ 　∑ ΔE = 0

Event Selections Expected number of p-p collisions in a 30-day period ：

trigger conditions and other conditions for good data selection ：

the electron trigger

＞ 15 GeV of transverse energy

with a good quality , vertex-associated charged track

9.75 × 10

1.4 × 10

Requirement of Three trigger conditions・ with large transverse energy・ with undetected muon tracks

10 events9

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

◎ 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

⇒ ・ 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 .

Events without jets Events with jets

Electron transverse energy

Missing transverse energyParallel to electron

Missing transverse energynormal to electron

Electrondirection

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

( Fig.2,3 )

Search for electron candidatesWe 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 ,　　　

・ events with a jet have no missing energy・ events with no jets show missing energy

Search for events with energetic neutrinosTaking 2125 events again , we operate conditions.

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

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

Discovery of W±

-p + p → W + X

e + ν( －)

◎ We look for the following event ；p

u-d-u-

collision

two-body decay

νe－

e‐W－

Fig.1 β-decay

Okamura Yusuke Shibata lab.

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