素粒子実験

伊藤 博士（ポスドク)神戸大

ナビゲータートーク

素粒子物理と標準模型

素粒子： 物質を構成する最小単位

Text

素粒子: 物質を構成する最小単位

標準模型: 素粒子の振舞いを記述する理論体系

クォーク &レプト ン : 物質を構成

ゲージ粒子: 力を媒介

ヒッ グス粒子: 質量起源

• 2 0 1 2年にATLAS& CM S実験で発見

（ ノ ーベル賞へ）

多く の実験結果を精度良く 説明

問題点: ダークマターやニュート リ ノ 質量（ 振動） を説明できないなど ...

⇒ 標準模型を超えた物理があるはず

標準模型の精密検証と 、 新しい物理（ 素粒子） の探索

LH C , Belle, T2 K , Kam Land-Zen , DeeM e, ANKOK, NEW AGE...

など世界各地で様々な実験を行っている

3

u

d s

c t

b W

g

Z

γτ

ντνμ

μe

νe

Hアッ プ

ダウン

チャーム ト ッ プ

スト レンジ ボト ム

電子 ミ ューオン タウオン

電子ニュート リ ノ ミ ューニュート リ ノ タウニュート リ ノ

グルーオン

Wボソン

Zボソン

光子

ヒッ グス

粒子

レプト ン

クォーク ゲージ粒子

標準模型： 素粒子の振舞いを記述する理論

体系

クォーク・レプトン： 物質を構成

ゲージ粒子：力を媒介

ヒッグス粒子：質量起源

多くの実験結果を精度よく説明

問題点：ダークマター、ニュートリノ振動、レプ

トンフレーバー破れなど説明できない

⟹ 標準模型を超える新物理があるはず

標準模型の精密検証、新物理探索

LHC, Belle/Belle II, T2K, Super Kamiokande, XMASS, NEWAGE, MEG, J-PARC

Exp., LEPS, FOREST …

標準模型の精密検証、新物理探索

高エネルギー粒子を衝突させて

新粒子を探索する

(エネルギーフロンティア)

高統計事象の観測によって

新物理を探索する

(ルミノシティフロンティア)

• ダークマター・ダークエネルギー

• ニュートリノ振動

• レプトンフレーバー破れ

• レプトン普遍性破れ

• 希崩壊事象

• ハイパー核

• エキゾティックハドロン

…etc.

標準模型の精密検証、新物理探索

• ダークマター・ダークエネルギー

• ニュートリノ振動

• レプトンフレーバー破れ

• レプトン普遍性破れ

• 希崩壊事象

• ハイパー核

• エキゾティックハドロン

…etc.

Text

素粒子: 物質を構成する最小単位

標準模型: 素粒子の振舞いを記述する理論体系

クォーク &レプト ン : 物質を構成

ゲージ粒子: 力を媒介

ヒッ グス粒子: 質量起源

• 2 0 1 2年にATLAS& CM S実験で発見

（ ノ ーベル賞へ）

多く の実験結果を精度良く 説明

問題点: ダークマターやニュート リ ノ 質量（ 振動） を説明できないなど ...

⇒ 標準模型を超えた物理があるはず

標準模型の精密検証と 、 新しい物理（ 素粒子） の探索

LH C , Belle, T2 K , Kam Land-Zen , DeeM e, ANKOK, NEW AGE...

など世界各地で様々な実験を行っている

3

u

d s

c t

b W

g

Z

γτ

ντνμ

μe

νe

Hアッ プ

ダウン

チャーム ト ッ プ

スト レンジ ボト ム

電子 ミ ューオン タウオン

電子ニュート リ ノ ミ ューニュート リ ノ タウニュート リ ノ

グルーオン

Wボソン

Zボソン

光子

ヒッ グス

粒子

レプト ン

クォーク ゲージ粒子

511 keV/c2 105.6 MeV/c2 1.78 GeV/c2

第一世代 第二世代 第三世代

𝑚𝜈 = ?

ニュートリノ振動

• 太陽ニュートリノ問題の解決 (θ12, Δm122)

• 加速器を用いてニュートリノ生成 (θ23, Δm232)

• ニュートリノがある距離走ると世代が変わる• ニュートリノに質量があることを証明

E

LmP aa

222 27.1

sin2sin1)(

𝐿

p p

120m0m 280m 295 km

off-axism-mon

(p→m )

T2K Exp.: Tokai to Kamioka in Japan

• ニュートリノ質量の精密測定• ニュートリノCP対称性

𝐸𝜈 (GeV)

L=295 km

レプトンフレーバー破れ探索

• レプトン数保存則は破れているのか？• 新物理が存在する

レプトン数𝑁𝑒 = 0𝑁𝜇 = −1

𝑁𝜏 = 0

レプトン数𝑁𝑒 = −1 + 1 = 0𝑁𝜇 = −1

𝑁𝜏 = 0保存

Ne=+1 Nμ=+1 Nτ=+1

e- μ- τ-

ν_e ν_μ ν_τ

𝜇+ → 𝑒+𝛾𝜇+ → 𝑒+𝜈𝑒𝜈𝜇

レプトン数𝑁𝑒 = 0𝑁𝜇 = −1

𝑁𝜏 = 0

レプトン数𝑁𝑒 = −1𝑁𝜇 = 0

𝑁𝜏 = 0非保存

8 JPS 2012 Autumn, 13/Sep/2012

世界最大強度 直流ミューオンビーム @ PSI

特殊勾配磁場による高計数対応 e+ スペクトロメータ

世界最大液体キセノ ン検出器による高精度ガンマ線測定

The MEG Experiment

Crimean Conf, 4/Sep/2011 Yusuke UCHIYAMA, the University of Tokyo 8

~60 collaborators

μ＋ beam

e＋

γ

pE5 beamline @PSI

COBRA SC magnet

Drift chambers

Timing counters

LXe γ-ray detector

8 JPS 2012 Autumn, 13/Sep/2012

世界最大強度 直流ミューオンビーム @ PSI

特殊勾配磁場による高計数対応 e+ スペクトロメータ

世界最大液体キセノ ン検出器による高精度ガンマ線測定

The MEG Experiment

Crimean Conf, 4/Sep/2011 Yusuke UCHIYAMA, the University of Tokyo 8

~60 collaborators

μ＋ beam

e＋

γ

pE5 beamline @PSI

COBRA SC magnet

Drift chambers

Timing counters

LXe γ-ray detector

𝜇+ → 𝑒+𝛾直接探索• MEG実験@ ポールシェラー研究所• BGは𝜇 → 𝑒𝜈𝜈𝛾とアクシデンタル𝛾• 90% C.L. upper limit: 2.4×10−12 in 2012

LFV探索の歴史

標準模型ではレプトンの弱い相互作用の結合定数は種類によらず同じと仮定。

2017/10/03 12R(D)0.2 0.3 0.4 0.5 0.6

R(D

*)

0.2

0.25

0.3

0.35

0.4

0.45

0.5 BaBar, PRL109,101802(2012)

Belle, PRD92,072014(2015)

LHCb, PRL115,111803(2015)

Belle, PRD94,072007(2016)

Belle, PRL118,211801(2017)

LHCb, FPCP2017

Average

SM Predictions

= 1.0 contours2

cD

R(D)=0.300(8) HPQCD (2015)

R(D)=0.299(11) FNAL/MILC (2015)

R(D*)=0.252(3) S. Fajfer et al. (2012)

HFLAV

FPCP 2017

) = 71.6%2cP(

s4

s2

HFLAVFPCP 2017

5

F. Bianchiab, F. De Moriab, A. Filippia , D. Gambaab, and S. Zambitoab

INFN Sezione di Torinoa ; D ipartimento di Fisica Sperimentale, Universita di Torinob , I -10125 Torino, I taly

L. Lanceriab and L. Vitaleab

INFN Sezione di Triestea ; D ipartimento di Fisica, Universita di Triesteb , I -34127 Trieste, I taly

F. Martinez-Vidal, A. Oyanguren, and P. Villanueva-PerezI FIC, Universitat de Valencia-CSIC, E-46071 Valencia, Spain

H. Ahmed, J. Albert, Sw. Banerjee, F. U. Bernlochner, H. H. F. Choi, G. J. K ing, R. Kowalewski,

M . J. Lewczuk, T. Lueck, I . M . Nugent, J. M . Roney, R. J. Sobie, and N. TasneemUniversity of Victoria, Victoria, British Columbia, Canada V8W 3P6

T. J. Gershon, P. F. Harrison, and T. E. LathamDepartment of Physics, University of W arwick, Coventry CV4 7AL, United K ingdom

H. R. Band, S. Dasu, Y . Pan, R. Prepost, and S. L. WuUniversity of W isconsin, Madison, W isconsin 53706, USA

(The BABAR Collaboration)(Dated: March 2, 2013)

Based on the full BABAR data sample, we report improved measurements of the ratios R (D ) =B(B → D τ − ν τ )/B(B → D ℓ− ν ℓ ) and R (D

∗ ) = B(B → D ∗ τ − ν τ )/B(B → D ∗ ℓ− ν ℓ ), where ℓ refers toeither an electron or muon. These ratios are sensitive to new physics contributions in the form of acharged Higgs boson. W e measure R (D ) = 0.440±0.058±0.042 and R (D ∗ ) = 0.332±0.024±0.018,which exceed the Standard Model expectations by 2.0σ and 2.7σ, respectively. Taken together, theresults disagree with these expectations at the 3.4σ level. This excess cannot be explained by acharged Higgs boson in the type I I two-Higgs-doublet model. K inematic distributions presentedhere exclude large portions of the more general type I I I two-Higgs-doublet model, but there aresolutions within this model compatible with the results.

PACS numbers: 13.20.He, 14.40.Nd, 14.80.Da

I . I N T R OD U CT I ON

In the Standard Model (SM), semileptonic decays ofB mesons proceed via first-order electroweak interactionsand are mediated by the W boson [1–3]. Decays involv-ing electrons and muons are expected to be insensitive tonon-SM contributions and therefore have been the basesof the determination of theCabibbo-Kobayashi-Maskawa(CKM) matrix elements |Vcb| and |Vub| [4]. Decays in-volving the higher-mass τ lepton provide additional in-formation on SM processes and are sensitive to addi-tional amplitudes, such as those involving an interme-diate charged Higgs boson [5–9]. Thus, they offer an ex-cellent opportunity to search for this and other non-SM

∗ Now at the University of Tabuk, Tabuk 71491, Saudi A rabia† A lso with Universita di Perugia, D ipartimento di F isica, Perugia,I taly

‡ Now at the University of Huddersfield, Huddersfield HD1 3DH,UK

§Deceased¶ Now at University of South A labama, M obile, A labama 36688,USA

∗∗ A lso with Universita di Sassari , Sassari, I taly†† Now at Universidad T ecnica Federico Santa M aria, Valparaiso,Chile 2390123

contributions.Over the past two decades, the development of heavy-quark effective theory (HQET) and precisemeasurementsof B → D (∗) ℓ− νℓ decays [10] at the B factories [11, 12]have greatly improved our understanding of exclusivesemileptonic decays. The relative rates

R (D ) =B(B → D τ − ντ )

B(B → D ℓ− νℓ ), R (D ∗ ) =

B(B → D ∗τ− ντ )

B(B → D ∗ℓ− νℓ )(1)

are independent of the CKM element |Vcb| and also, toa large extent, of the parameterization of the hadronicmatrix elements. SM expectations [9] for the ratiosR (D ) and R (D ∗ ) have uncertainties of less than 6% and2% , respectively. Calculations [5–9] based on two-Higgs-doublet models predict a substantial impact on the ratioR (D ), and a smaller effect on R (D ∗ ) due to the spin ofthe D ∗ meson.The decay B → D ∗τ− ντ was first observed in 2007by the Belle Collaboration [13]. Since then, both BABARand Belle have published improved measurements, andhave found evidence for B → D τ− ντ decays [14–16]. Upto now, the measured values for R (D ) and R (D ∗) haveconsistently exceeded the SM expectations, though thesignificanceof the excess is low due to the large statisticaluncertainties.

2.J-PARCE36

where l refers to either an e or µ.

3.9σSM

Belle IIが来年度あたり本格始動。今後期待!

2017/10/03 11

2.J-PARCE36

KLOE(2009)(stat) (sys)

NA62(2013)(stat) (sys)

SM

Initialgoal ofE36

where δr is a radiative correction due the internal bremsstrahlung process (IB) which, bydefinition, contributes to RK (unlike the structure dependent process (SD)). A M inimal Super-Symmetric (SUSY) extension of the SM (MSSM) with R-parity conservation has recently beenconsidered as a candidate for new physics to be tested by RK [2–4]. In the case of K l2, inaddition to the W ± exchange, a charged Higgs-mediated SUSY lepton flavor violating (LFV)contribution can be enhanced if accompanied by the emission of a ⌧ neutrino. This e↵ect canbe described as

RL F VK = RSM

K

✓

1 +m4K

M 4H +

·m2⌧m2e

∆ 213 tan6 β

◆

(3)

where M H + is the mass of the charged Higgs and ∆ 13 (. 10− 3) is the term induced by the

exchange of a Bino and a Slepton. Taking ∆ 13 = 5 ⇥ 10− 4, tan β = 40 and M H + = 500 GeV,would yield to a value of RL F V

K = 1.013 ⇥ RSMK . I t is therefore possible to reach a contribution

at the percent level due to possible LFV enhancements arising in SUSY models.

F igure 1. Contributions to RK from (a) SM and (b) LFV SUSY. A charged Higgs-mediatedLFV SUSY contribution can be strongly enhanced by the emission of a ⌧ neutrino.

Recent in-fl ight decay experiments, such as KLOE [5] and NA62 [6], have measured the RK

ratio leading to a current world average of RK = (2.488 ± 0.010) ⇥ 10− 5. The E36 experimentat the Japan Proton Accelerator Complex (J-PARC) by the TREK (Time Reversal Experimentwith Kaons) collaboration aims to provide a competitive measurement of RK with di↵erentsystematics.

2. R K measurem ents using stopped K + at J-PA RCThe E36 experiment, which is part of the TREK program at J-PARC, was set up and fullycommissioned at the K1.1BR kaon beam line between fall 2014 and spring 2015. The datacollection was completed by the end of 2015 using an upgraded version of the KEK-PS E24612-sector superconducting toroidal spectrometer [7] used in a previous T-violation experimentat KEK [8–10]. The incoming K + is tagged by the Fitch Cherenkov counter before stoppingand decaying in the active target, which consists of 256 scintillating fibers (3 ⇥ 3 ⇥ 200 mm3)oriented in the direction of the beam, providing a precise kaon stop location in the transverseplane. W rapped around the fiber target is a Spiral Fiber Tracker (SFT), which consists oftwo pairs of scintillating fiber ribbons of opposite helicity bundled together around the target,providing the longitudinal coordinate of the outgoing decay particles [11]. This target+ SFTsystem is surrounded by 12 time-of-fl ight counters (TOF1) and 12 aerogel [12] counters (AC)aligned with the 12 sectors of the toroidal spectrometer and forms the “Central Detector”.A highy segmented (768 crystals) large acceptance CsI(Tl) photon calorimeter barrel covering

where δr is a radiative correction due the internal bremsstrahlung process (IB) which, bydefinition, contributes to RK (unlike the structure dependent process (SD)). A M inimal Super-Symmetric (SUSY) extension of the SM (MSSM) with R-parity conservation has recently beenconsidered as a candidate for new physics to be tested by RK [2–4]. In the case of K l2, inaddition to the W ± exchange, a charged Higgs-mediated SUSY lepton flavor violating (LFV)contribution can be enhanced if accompanied by the emission of a ⌧ neutrino. This e↵ect canbe described as

RL F VK = RSM

K

✓

1 +m4K

M 4H +

·m2⌧m2e

∆ 213 tan6 β

◆

(3)

where M H + is the mass of the charged Higgs and ∆ 13 (. 10− 3) is the term induced by the

exchange of a Bino and a Slepton. Taking ∆ 13 = 5 ⇥ 10− 4, tan β = 40 and M H + = 500 GeV,would yield to a value of RL F V

K = 1.013 ⇥ RSMK . I t is therefore possible to reach a contribution

at the percent level due to possible LFV enhancements arising in SUSY models.

F igure 1. Contributions to RK from (a) SM and (b) LFV SUSY. A charged Higgs-mediatedLFV SUSY contribution can be strongly enhanced by the emission of a ⌧ neutrino.

Recent in-fl ight decay experiments, such as KLOE [5] and NA62 [6], have measured the RK

ratio leading to a current world average of RK = (2.488 ± 0.010) ⇥ 10− 5. The E36 experimentat the Japan Proton Accelerator Complex (J-PARC) by the TREK (Time Reversal Experimentwith Kaons) collaboration aims to provide a competitive measurement of RK with di↵erentsystematics.

2. R K measurem ents using stopped K + at J-PA RCThe E36 experiment, which is part of the TREK program at J-PARC, was set up and fullycommissioned at the K1.1BR kaon beam line between fall 2014 and spring 2015. The datacollection was completed by the end of 2015 using an upgraded version of the KEK-PS E24612-sector superconducting toroidal spectrometer [7] used in a previous T-violation experimentat KEK [8–10]. The incoming K + is tagged by the Fitch Cherenkov counter before stoppingand decaying in the active target, which consists of 256 scintillating fibers (3 ⇥ 3 ⇥ 200 mm3)oriented in the direction of the beam, providing a precise kaon stop location in the transverseplane. W rapped around the fiber target is a Spiral Fiber Tracker (SFT), which consists oftwo pairs of scintillating fiber ribbons of opposite helicity bundled together around the target,providing the longitudinal coordinate of the outgoing decay particles [11]. This target+ SFTsystem is surrounded by 12 time-of-fl ight counters (TOF1) and 12 aerogel [12] counters (AC)aligned with the 12 sectors of the toroidal spectrometer and forms the “Central Detector”.A highy segmented (768 crystals) large acceptance CsI(Tl) photon calorimeter barrel covering

Phys.Rev.D 74

where δr is a radiative correction due the internal bremsstrahlung process (IB) which, bydefinition, contributes to RK (unlike the structure dependent process (SD)). A Minimal Super-Symmetric (SUSY) extension of the SM (MSSM) with R-parity conservation has recently beenconsidered as a candidate for new physics to be tested by RK [2–4]. In the case of K l2, inaddition to the W ± exchange, a charged Higgs-mediated SUSY lepton flavor violating (LFV)contribution can be enhanced if accompanied by the emission of a ⌧ neutrino. This e↵ect canbe described as

RL F VK = RSM

K

✓

1 +m4K

M 4H +

·m2⌧m2e

∆ 213 tan6 β

◆

(3)

where M H + is the mass of the charged Higgs and ∆ 13 (. 10− 3) is the term induced by the

exchange of a Bino and a Slepton. Taking ∆ 13 = 5 ⇥ 10− 4, tan β = 40 and M H + = 500 GeV,would yield to a value of RL F V

K = 1.013 ⇥ RSMK . I t is therefore possible to reach a contribution

at the percent level due to possible LFV enhancements arising in SUSY models.

F igure 1. Contributions to RK from (a) SM and (b) LFV SUSY. A charged Higgs-mediatedLFV SUSY contribution can be strongly enhanced by the emission of a ⌧ neutrino.

Recent in-flight decay experiments, such as KLOE [5] and NA62 [6], have measured the RK

ratio leading to a current world average of RK = (2.488 ± 0.010) ⇥ 10− 5. The E36 experimentat the Japan Proton Accelerator Complex (J-PARC) by the TREK (Time Reversal Experimentwith Kaons) collaboration aims to provide a competitive measurement of RK with di↵erentsystematics.

2. R K measurem ents using stopped K + at J-PA RCThe E36 experiment, which is part of the TREK program at J-PARC, was set up and fullycommissioned at the K1.1BR kaon beam line between fall 2014 and spring 2015. The datacollection was completed by the end of 2015 using an upgraded version of the KEK-PS E24612-sector superconducting toroidal spectrometer [7] used in a previous T-violation experimentat KEK [8–10]. The incoming K + is tagged by the Fitch Cherenkov counter before stoppingand decaying in the active target, which consists of 256 scintillating fibers (3 ⇥ 3 ⇥ 200 mm3)oriented in the direction of the beam, providing a precise kaon stop location in the transverseplane. W rapped around the fiber target is a Spiral Fiber Tracker (SFT), which consists oftwo pairs of scintillating fiber ribbons of opposite helicity bundled together around the target,providing the longitudinal coordinate of the outgoing decay particles [11]. This target+ SFTsystem is surrounded by 12 time-of-flight counters (TOF1) and 12 aerogel [12] counters (AC)aligned with the 12 sectors of the toroidal spectrometer and forms the “Central Detector”.A highy segmented (768 crystals) large acceptance CsI(Tl) photon calorimeter barrel covering

2017/10/03 11

2.J-PARCE36

KLOE(2009)(stat) (sys)

NA62(2013)(stat) (sys)

SM

Initialgoal ofE36

where δr is a radiative correction due the internal bremsstrahlung process (IB) which, bydefinition, contributes to RK (unlike the structure dependent process (SD)). A M inimal Super-Symmetric (SUSY) extension of the SM (MSSM) with R-parity conservation has recently beenconsidered as a candidate for new physics to be tested by RK [2–4]. In the case of K l2, inaddition to the W ± exchange, a charged Higgs-mediated SUSY lepton flavor violating (LFV)contribution can be enhanced if accompanied by the emission of a ⌧ neutrino. This e↵ect canbe described as

RL F VK = RSM

K

✓

1 +m4K

M 4H +

·m2⌧m2e

∆ 213 tan6 β

◆

(3)

where M H + is the mass of the charged Higgs and ∆ 13 (. 10− 3) is the term induced by the

exchange of a Bino and a Slepton. Taking ∆ 13 = 5 ⇥ 10− 4, tan β = 40 and M H + = 500 GeV,would yield to a value of RL F V

K = 1.013 ⇥ RSMK . I t is therefore possible to reach a contribution

at the percent level due to possible LFV enhancements arising in SUSY models.

F igure 1. Contributions to RK from (a) SM and (b) LFV SUSY. A charged Higgs-mediatedLFV SUSY contribution can be strongly enhanced by the emission of a ⌧ neutrino.

Recent in-fl ight decay experiments, such as KLOE [5] and NA62 [6], have measured the RK

ratio leading to a current world average of RK = (2.488 ± 0.010) ⇥ 10− 5. The E36 experimentat the Japan Proton Accelerator Complex (J-PARC) by the TREK (Time Reversal Experimentwith Kaons) collaboration aims to provide a competitive measurement of RK with di↵erentsystematics.

2. R K measurem ents using stopped K + at J-PA RCThe E36 experiment, which is part of the TREK program at J-PARC, was set up and fullycommissioned at the K1.1BR kaon beam line between fall 2014 and spring 2015. The datacollection was completed by the end of 2015 using an upgraded version of the KEK-PS E24612-sector superconducting toroidal spectrometer [7] used in a previous T-violation experimentat KEK [8–10]. The incoming K + is tagged by the Fitch Cherenkov counter before stoppingand decaying in the active target, which consists of 256 scintillating fibers (3 ⇥ 3 ⇥ 200 mm3)oriented in the direction of the beam, providing a precise kaon stop location in the transverseplane. W rapped around the fiber target is a Spiral Fiber Tracker (SFT), which consists oftwo pairs of scintillating fiber ribbons of opposite helicity bundled together around the target,providing the longitudinal coordinate of the outgoing decay particles [11]. This target+ SFTsystem is surrounded by 12 time-of-fl ight counters (TOF1) and 12 aerogel [12] counters (AC)aligned with the 12 sectors of the toroidal spectrometer and forms the “Central Detector”.A highy segmented (768 crystals) large acceptance CsI(Tl) photon calorimeter barrel covering

where δr is a radiative correction due the internal bremsstrahlung process (IB) which, bydefinition, contributes to RK (unlike the structure dependent process (SD)). A M inimal Super-Symmetric (SUSY) extension of the SM (MSSM) with R-parity conservation has recently beenconsidered as a candidate for new physics to be tested by RK [2–4]. In the case of K l2, inaddition to the W ± exchange, a charged Higgs-mediated SUSY lepton flavor violating (LFV)contribution can be enhanced if accompanied by the emission of a ⌧ neutrino. This e↵ect canbe described as

RL F VK = RSM

K

✓

1 +m4K

M 4H +

·m2⌧m2e

∆ 213 tan6 β

◆

(3)

where M H + is the mass of the charged Higgs and ∆ 13 (. 10− 3) is the term induced by the

exchange of a Bino and a Slepton. Taking ∆ 13 = 5 ⇥ 10− 4, tan β = 40 and M H + = 500 GeV,would yield to a value of RL F V

K = 1.013 ⇥ RSMK . I t is therefore possible to reach a contribution

at the percent level due to possible LFV enhancements arising in SUSY models.

F igure 1. Contributions to RK from (a) SM and (b) LFV SUSY . A charged Higgs-mediatedLFV SUSY contribution can be strongly enhanced by the emission of a ⌧ neutrino.

Recent in-fl ight decay experiments, such as KLOE [5] and NA62 [6], have measured the RK

ratio leading to a current world average of RK = (2.488 ± 0.010) ⇥ 10− 5. The E36 experimentat the Japan Proton Accelerator Complex (J-PARC) by the TREK (Time Reversal Experimentwith Kaons) collaboration aims to provide a competitive measurement of RK with di↵erentsystematics.

2. R K measurem ents using stopped K + at J-PA RCThe E36 experiment, which is part of the TREK program at J-PARC, was set up and fullycommissioned at the K1.1BR kaon beam line between fall 2014 and spring 2015. The datacollection was completed by the end of 2015 using an upgraded version of the KEK-PS E24612-sector superconducting toroidal spectrometer [7] used in a previous T-violation experimentat KEK [8–10]. The incoming K + is tagged by the Fitch Cherenkov counter before stoppingand decaying in the active target, which consists of 256 scintillating fibers (3 ⇥ 3 ⇥ 200 mm3)oriented in the direction of the beam, providing a precise kaon stop location in the transverseplane. W rapped around the fiber target is a Spiral Fiber Tracker (SFT), which consists oftwo pairs of scintillating fiber ribbons of opposite helicity bundled together around the target,providing the longitudinal coordinate of the outgoing decay particles [11]. This target+ SFTsystem is surrounded by 12 time-of-fl ight counters (TOF1) and 12 aerogel [12] counters (AC)aligned with the 12 sectors of the toroidal spectrometer and forms the “Central Detector”.A highy segmented (768 crystals) large acceptance CsI(Tl) photon calorimeter barrel covering

Phys.Rev.D 74

where δr is a radiative correction due the internal bremsstrahlung process (IB) which, bydefinition, contributes to RK (unlike the structure dependent process (SD)). A M inimal Super-Symmetric (SUSY) extension of the SM (MSSM) with R-parity conservation has recently beenconsidered as a candidate for new physics to be tested by RK [2–4]. In the case of K l2, inaddition to the W ± exchange, a charged Higgs-mediated SUSY lepton flavor violating (LFV)contribution can be enhanced if accompanied by the emission of a ⌧ neutrino. This e↵ect canbe described as

RL F VK = RSM

K

✓

1 +m4K

M 4H +

·m2⌧m2e

∆ 213 tan6 β

◆

(3)

where M H + is the mass of the charged Higgs and ∆ 13 (. 10− 3) is the term induced by the

exchange of a Bino and a Slepton. Taking ∆ 13 = 5 ⇥ 10− 4, tan β = 40 and M H + = 500 GeV,would yield to a value of RL F V

K = 1.013 ⇥ RSMK . I t is therefore possible to reach a contribution

at the percent level due to possible LFV enhancements arising in SUSY models.

F igure 1. Contributions to RK from (a) SM and (b) LFV SUSY. A charged Higgs-mediatedLFV SUSY contribution can be strongly enhanced by the emission of a ⌧ neutrino.

Recent in-fl ight decay experiments, such as KLOE [5] and NA62 [6], have measured the RK

ratio leading to a current world average of RK = (2.488 ± 0.010) ⇥ 10− 5. The E36 experimentat the Japan Proton Accelerator Complex (J-PARC) by the TREK (Time Reversal Experimentwith Kaons) collaboration aims to provide a competitive measurement of RK with di↵erentsystematics.

2. R K measurem ents using stopped K + at J-PA RCThe E36 experiment, which is part of the TREK program at J-PARC, was set up and fullycommissioned at the K1.1BR kaon beam line between fall 2014 and spring 2015. The datacollection was completed by the end of 2015 using an upgraded version of the KEK-PS E24612-sector superconducting toroidal spectrometer [7] used in a previous T-violation experimentat KEK [8–10]. The incoming K + is tagged by the Fitch Cherenkov counter before stoppingand decaying in the active target, which consists of 256 scintillating fibers (3 ⇥ 3 ⇥ 200 mm3)oriented in the direction of the beam, providing a precise kaon stop location in the transverseplane. W rapped around the fiber target is a Spiral Fiber Tracker (SFT), which consists oftwo pairs of scintillating fiber ribbons of opposite helicity bundled together around the target,providing the longitudinal coordinate of the outgoing decay particles [11]. This target+ SFTsystem is surrounded by 12 time-of-fl ight counters (TOF1) and 12 aerogel [12] counters (AC)aligned with the 12 sectors of the toroidal spectrometer and forms the “Central Detector”.A highy segmented (768 crystals) large acceptance CsI(Tl) photon calorimeter barrel covering

=(2.477±0.001)×10-5

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