“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

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

Physics ColloquiumJuly 7th, 2008

√s―

2

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 .

n p

νe－e‐

n p

νe－e‐

Fermi’s Model

±

W‐

Weinberg-Salam’s Model

p

νe

Z

p

νe

±

charged current

neutral current

β-decay

β-decay

3

-p + p → W + X

e + ν( －)

±

±

We look for the following event ；

pp-u

ud

u-d-u-

W+

νe

e+

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―

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◎ 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

270°

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

θ

・ φ is angle of spherical coordinatePseudo-rapidity η－

1.4

+1.4

－1.4

+1.4

－90°

φ angle

270°

－90°

E max 23.7 GeVT

E max 0.5 GeVT

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

beam crossing point

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

Transverse electron energy

↓20 40 GeV

20

40 GeV

0

0

24

＃ of events

12

Mis

sing

tra

nsve

rse

ene

rgy

＃ o

f eve

nts

Momentum balance between electron and neutrino

m is determined as

by correcting for the transverse motion of W .

m = 81 ±5 GeV/cW2

W

±

±

←

←

↓beam axis

beam crossing point

e±

ν( －)

e±

ν( －)

ET

ET

ET

ET

Events with large transverse energy

Events with small transverse energy

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

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・ 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 .

W2

±

-±

±

±

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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

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

trigger conditions and other conditions for good data selection ：

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the electron trigger

＞ 15 GeV of transverse energy

with a good quality , vertex-associated charged track

9.75 × 10

1.4 × 10

28000

2125

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

1106

276

167

7239

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

⇒ ・ 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 .

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Events without jets Events with jets

Electron transverse energy

Tran

sver

se to

ele

ctro

nP

aral

lel t

o el

ectro

n

Mis

sing

tra

nsve

rse

ene

rgy

Missing transverse energyParallel to electron

Missing transverse energynormal to electron

Electrondirection

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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 )

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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 ,　　　

Fig.2

Fig.3

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

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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

Fig.4

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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

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±

Discovery of W±

-p + p → W + X

e + ν( －)

±

±

◎ We look for the following event ；p

p-uud

u-d-u-

W+

νe

e+

collision

two-body decay

n p

νe－

e‐W－

Fig.1 β-decay

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