# KOPIO EXPERIMENT

## Introduction

Design Overview

Expected Signal

Expected Background

Enhancing Signal

Summary

![](media/image1.png){width="2.9833333333333334in"
height="1.5291666666666666in"}

### **CKM MATRIX**

##### Electroweak Interactions & CP Violation

> CP violation in the Standard Model arises from the
>
> complex phase in the CKM matrix

Unitarity of the CKM matrix, e.g. V~ud~V~ub~^\*^ + V~cd~V~cb~^\*^ +

> V~td~V~tb~^\*^ = 0, allows one to draw the unitary triangle (shown
> below in the Wolfenstein representation)

![](media/image2.png)

This triangle can be determined by measuring two rare K

> decays or through measurements in the B system

Comparison of CP violation determined by K decays to

> that measured in the B system provides a crucial test of the SM

#### **RARE K DECAY**

**Predictions for the K~L~ → π^0^ ν ν decay:**

The K~L~ → π^0^ ν ν decay is dominated by CP violating, one

loop diagrams involving s → d ν ν

![](media/image3.png)

SM gives a precise (\~ 2%) prediction for the branching

ratio of the K~L~ → π^0^ ν ν decay mode:

B(K^+^ → π^0^ e^+^ ν ) 3 α^2^ τ(K~L~)

B(K~L~ → π^0^ ν ν ) = 9.44 • ------------------------ ---------------
--------- \[X(x~t~) ImV~ts~^\*^ V~td~\]^2^

│V~us~│^2^ 2π^2^ sin^4^θ~w~ τ(K^+^)

Using current values for CKM phenomenology and m~t~

yields a branching ratio of (3.1 ± 1.3) • 10^--11^

###   {#section .unnumbered}

### **BASIC DESIGN CRITERIA** 

**Beam design criteria:**

Optimizing the number of usable decays per micro bunch

gives 14 million K~L~ decays / sec

![](media/image4.png)

**Detector design criteria:**

##  Measurement of the photon energy Eγ

Reconstruction of π^0^ vertex

Measurement of E~KL~

Hermetic vetoing

### **DESIGN OVERVIEW**

![](media/image5.wmf)

###   {#section-1 .unnumbered}

### **CANONICAL SIGNAL** 

**A canonical signal event in the detector:**

Canonical event: 2--γ's from a signal π^0^ decay with

> showers entering the Pre-Radiator (PR) \< ±1.5m from the beam
> geometric center

![](media/image6.png)

Reconstructed π^0^ vertex must be 50cm downstream of the

> decay tank entrance and 50cm upstream of the PR, due to expected
> neutron background

Of the signal π^0^'s within the decay tank about \~ 24% have

both photons heading into the PR

###   {#section-2 .unnumbered}

### **K~L~^0^ DECAY BACKGROUND** 

**K~L~ background decay modes:**

Several decay modes of the K~L~ tend to dominate the

trigger rate:

K~L~^0^ → π^0^ π^0^ π^0^ \~ 21% decays

K~L~^0^ → π^0^ π^+^ π ^--^ \~ 10% decays

K~L~^0^ → π^0^ π^0^ \~ 0.09% decays

![](media/image7.png)

With competing K~L~ decay branching ratios of O(10^10^)

> greater than what is predicted for the signal, much of the detector
> design focuses on background rejection

Fortunately, these modes involve additional gammas or

charged particles, that can also be detected

######### Other decay modes are of concern but not a triggering 

issue

### **HERMETIC PHOTON VETO** 

![](media/image8.png)

##### What must be detected?

Since the K~L~ can decay into several modes with multiple

> π^0^'s and/or charged particles, one must effectively detect 2 gammas
> and no other particles in the detector

Photon and charged vetos surround the decay region

###   {#section-3 .unnumbered}

### **DOWNSTREAM REGION** 

![](media/image9.png)

##### Particles entering the downstream volume:

##  A magnet just downstream of the calorimeter sweeps

## charged particles from the beam for vetoing

The Downstream Photon Vetos (DPV) adjacent to the

beam are arranged to detect photons that escape from

the decay region

######### At the end of the beam, is a "neutron blind" photon veto,

the "catcher"

###   {#section-4 .unnumbered}

### **RECONSTRUCTED SIGNAL** 

**MC predicted reconstructed signal events:**

Using the TOF for the K~L~, the momentum of the π^0^ in the

K~L~ rest frame can be reconstructed

![](media/image10.png)

Plot shows MC signal distribution in Eπ^0^ versus

\|Eγ1 -- Eγ2\| space

###### The events in green populate a low background region

###   {#section-5 .unnumbered}

### **RECONSTRUCTED BACKGROUND** 

**MC predicted reconstructed background events:**

![](media/image11.png)

##  Kinematic constraints on the background give rise to the

## sparsely populated band between Eπ^0^ 170 and 190 MeV

###   {#section-6 .unnumbered}

### **SIGNAL vs BACKGOUND** 

**MC predicted signal vs background events:**

########  Exploiting the background kinematics, the signal events

######## will be extracted from the region shown below in green

![](media/image12.png)

From this region we expect some 50 ± 10 signal events

with 25 ± 5 background events

###   {#section-7 .unnumbered}

### **LEVEL 0 TRIGGERING** 

**Triggering and Background Rejection:**

At the lowest level, KOPIO aims to suppress the

background trigger rates to around 25kHz

![](media/image13.png)

To keep the timing gates for each Photon Barrel Veto

> (PBV) section as narrow as possible, it is necessary to use the
> natural divisions of both the PR and Calo for the L0 trigger

Using the PBV, PR, Calo and DPV elements together

allows KOPIO to effectively count gammas at L0

###   {#section-8 .unnumbered}

### **ENHANCING THE SIGNAL**

**Methods to enhance the signal rate include:**

#######  Improving the AGS

Investigating alternative aspect ratios

Modification of L0 trigger to use the photon veto

information for signal identification

![](media/image14.png)

Using veto information to improve signal identification:

Predicted Signal: 2 PR tracks \~ 40-50 events

1 PR + 1PRV \~ 60-75 events

1 PR + 1PBV \~ 100-125 events

###   {#section-9 .unnumbered}

### **SUMMARY** 

**KOPIO results in terms of the Standard Model:**

The measured B(K~L~ → π^0^ ν ν ) can unambiguously be

interpreted in terms of SM

Any observed deviation from the predicted branching

ratio hints at new physics

KOPIO and the E787/E949 results allow an independent

determination of the Unitarity Triangle that can be

compared with that obtained from B measurements