October 3, 2005 @ ICFP 2005

Takao INAGAKI 　（ KEK)

for the E391a collaboration

Status of E391a experiment for the rare decay

Theoretically it is very pure and clean, but it is a very rare decay

Recently confirmed that the theoretical uncertainty is also very small in most of calculations of new physics. hep-ph/050517

Possible Scenario

Experimentally very challenging

• Small branching fraction

Any tiny effects are possible sources of background

• Weak kinematical constraint (a

ll neutrals, three-body decay with invisible neutrinos, etc) Not easy to separate the signal from background

This is the reason why the present limit stays at 5.9X10-7 (KTeV, 2000) and why E391a is the first dedicated experiment in spite of a theoretically dream channel.

Strategy: step-by-step approach

E391a at KEK 12 GeV PS O(10-10)

An experiment at JPARC 50 GeV PS O(10-14)

Collaboration and status of E391a

● 　 The third data taking (Run-III) is scheduled in Nov/2005

• E391a collaboration • 11 institutes, ~50 members from 5 countries

Univ. of Chicago, JINR, KEK. Kyoto Univ., NDA, National Taiwan Univ.,

Osaka Univ., Pusan National Univ., RCNP, Saga Univ., Yamagata Univ.

• Status of experiment • First data taking was carried out from Feb/2004 to July/2004

• Second data taking (Run-II) was carried out from Feb/2005 to April/2005

0.25 GeV/c

0.13 GeV/c

Three important tools

Additional discussion about detection inefficiency, which causes photon veto miss

・ The inefficiency strongly depends on 　 gamma energy. It is large at low energy

　　　　　　　　　　　　↓

Hi ｇｈ PT selection

・ The inefficiency strongly depends on the detection threshold

　　　　　　　　　　　　　↓

Low detection threshold

Concern ： how we can lower it under high rate environment. Understand the detector response at low energy deposit.

Final plot

Using 10% sample of Run-1

What we have learned

BackgroundBy understanding the sources of side-band events

Core neutron

Halo neutron

Kpi2: negligibly small

Back splash

It is clearly observed for the main barrel having both-end readout.

Mean time

Tim

e d

iffere

nce

(p

ositio

n)

Introduction of J-PARCour next playground

Configuration of the accelerators

NeutronMuon

Layout

K0 line is included in the construction plan

Construction Schedule

Linac

3 GeV

50 GeV

Materials + Life

Nuclear-Particle

Neutrino (plan)

Others

Salt Farms

Bldg. construction

Equip. constructionBeamtest

Bldg. construction

Equip. constructionBeamtest

Bldg. construction

Equip. constructionBeamtest

Bldg. construction

Equip. constructionBeamtest

Bldg. construction

Equip. constructionBeamtest

Bldg. construction

Equip. constructionBeamtest

Construction

Archelogical studies

Construction Schedule (as of Oct., 2003)

FY2001 FY2002 FY2003 FY2004 FY2005 FY2006 FY2007 FY2008

BeamConstruction Start

study report

Summary

1. is one of the pure and clean channels in the field of quark flavor physics. It provides a critical and unambiguous check of the Standard Model.

2. E391a, which is the first dedicated experiment, has started at KEK 12-GeV PS successfully.

3. Using 10 % sample of Run I, we obtained a new limit of 2.9×10-7 (90%cl).

4. We are learning many things, background sources, the rate of extremely low energy deposit etc. They must be quite important for a future extension.

5. J-PARC, which is expected to be our next play-ground, is now being constructed steadily. We are preparing a proposal.