S.-K. Choi et al., Belle Collaboration Observation of a Narrow Charmoniumlike State in Exclusive B± → K± π+ π− J/ψ Decays S.-K. Choi et al., Belle Collaboration Phys. Rev. Lett. 91, 262001 (2003) Contents 1. Introduction 2. KEKB accelerator 3. Belle detector 4. Analysis 5. Result 6. Summary Shibata lab. 12B16340 Yuuki Fujii

1. Introduction The purpose of Belle experiment is to observe CP violation using the difference between B meson decay and anti-B meson decay. Another purpose is to perform hadron spectroscopy. Belle collaboration produced many charged and neutral B mesons. B mesons have many decay modes. This paper reports B± → K± π+ π− J/ψ decays. J/ψ π+ B+ π- K+ e- e+ J/ψ π+ B- π- K-

2. KEKB accelerator KEKB accelerator is the asymmetric e+e− collider at KEK in Tsukuba. e− energy : 8 GeV e+ energy : 3.5 GeV center-of-mass energy : = 10.6 GeV KEKB ring Belle e- e+ ⇒ LINAC 𝑠 1km KEKB collider produces a lot of B meson and anti-B meson pairs. Therefore, this collider is called B Factory. KEKB collider achieved the luminosity of 2.11 × 10 34 cm −2 s −1 in June 2009.

3. Belle detector Belle detector is composed as follows from inside to outside: Silicon Vertex Detector (SVD) Central Drift Chamber (CDC) Aerogel Cherenkov Counters (ACC) Time-of-flight counters (TOF) Electromagnetic calorimeter (ECL) KL meson and muon detector (KLM) 7 m 7.7 m Nucl. Instrum. Methods A 479, 117 (2002) These detectors determine momentum, energy and trajectory of particles. ⇒

4. Analysis Belle collaboration determined invariant mass of π+ π− J/ψ in B± → K± + π+ π− J/ψ decay process. The invariant mass is calculated from energy and momentum of decay particles: 𝑀 = 𝑖 𝐸 𝑖 2 − 𝑖 𝑝 𝑖 2 J/ψ meson decays to e+e- or μ+μ-. (J/ψ : charmonium state. It is bound state of charm quark and anti-charm quark. ) J/ψ 𝑐 𝑐 Invariant mass of lepton pair was evaluated first. Lepton pair was identified as J/ψ by . 𝑀 ℓ + ℓ − 𝑀 ℓ + ℓ − Invariant mass was evaluated next. 𝑀 π + π − ℓ + ℓ − 𝑀 ℓ + ℓ − : invariant mass of J/ψ (ℓ+ℓ- : e+e- or μ+μ-) 𝑀 π + π − ℓ + ℓ − : invariant mass of π + π − J/ψ The mass difference is plotted in next page. 𝑀 π + π − ℓ + ℓ − − 𝑀 ℓ + ℓ −

Monte Carlo simulation 5. Result B± → K± + π+ π- J/ψ There are two peaks at 0.589 GeV and 0.775 GeV in real data. Monte Carlo simulation is smooth except for the 0.589 GeV peak. 0.589 GeV peak corresponds to ψ′ mass. 　　　　　 (ψ′ : an excited state of J/ψ) M π + π − ℓ + ℓ − − M ℓ + ℓ − GeV M π + π − ℓ + ℓ − − M ℓ + ℓ − GeV experiment data Monte Carlo simulation 0.775 GeV peak is small, but this signal has statistical significance of 10.3σ. The signal at 0.775 GeV corresponds to 𝑀 = 𝑀 π + π − ℓ + ℓ − − 𝑀 ℓ + ℓ − + 𝑀 J/ψ = 3872 MeV. ( 𝑀 J/ψ : PDG value of J/ψ mass) Belle collaboration named this state as X(3872).

Belle collaboration first thought X(3872) was S=1, L=2, J=2 charmonium state ( ), ( 3D2 state ). Mass value of this state has been theoretically calculated. c c 𝑀 3 𝐷 2 = 3810 MeV decay width of this state has been theoretically calculated. ( 　: photon , 　　: one of the charmonium states ) 3 𝐷 2 →γ χ 𝑐1 γ χ 𝑐1 Γ ( 3 𝐷 2 →γ χ 𝑐1 ) > 5 × Γ ( 3 𝐷 2 → π + π − J/ψ) Experimental values are not consistent with the calculated values. 𝑀 X(3872) = 3872 MeV Γ (X(3872)→γ χ 𝑐1 ) 0.89 × Γ (X(3872)→ π + π − J/ψ) < X(3872) is not a normal charmonium state.

Another interpretation X(3872) X(3872) mass value is close to the . (3871.1 ± 1.0 MeV) X(3872) is suggestive of a loosely bound multiquark “molecular state”, as proposed by a theory. (M. Bander, G.L. Shaw, P. Thomas et al.) 𝑀 𝐷 0 + 𝑀 𝐷 ∗0 𝐷 0 𝐷 ∗0 c u c u 𝐷 0 𝐷 ∗0 u u c c K+ J/ψ B+ W+ s c c X(3872) b c u u π+ c d X(3872) u u d π− u u B+ → K+ + X(3872) X(3872) → π+ + π- + J/ψ X(3872) is not fully understood yet. Further study is needed.

6. Summary The purpose of Belle experiment is to observe CP violation using the difference between B meson decay and anti-B meson decay. Another purpose is to perform hadron spectroscopy. KEKB accelerator is the e+e- collider. Belle detector determines momentum, energy and trajectory. Belle collaboration observed a strong signal for a state. It decays to π+ π- J/ψ. M = 3872 MeV Belle collaboration named this state as X(3872). X(3872) mass value is close to the . X(3872) is suggestive of a loosely bound multiquark “molecular state”, as proposed by a theory. 𝑠 = 10.6 GeV 𝑀 𝐷 0 + 𝑀 𝐷 ∗0 𝐷 0 𝐷 ∗0

補足

CP violation in B mesons B中間子でのCP対称性の破れは、寿命の違いという形で現れ る。そのため、B中間子と反B中間子の崩壊までの時間を正確に 測定する必要があり、その測定のためにBelle実験は計画され た。 Belle実験は、2001年にB中間子における「CP対称性の破れ」が 存在することを発見した。

Spectroscopy スペクトロスコピーとは、特に励起状態の崩壊生成物を判 定する実験を意味する。それにより励起状態の性質や構成 要素の間の相互作用について知ることができる。 系が空間的に小さいほど系の励起エネルギーが大きく、励 起状態を生成するためには高エネルギーの粒子が必要と なってくる。

KEKB accelerator B+ + B- e+ + e- → γ* → Υ(4S) → エネルギーが非対称・・・e- – 8 GeV , e+ – 3.5 GeV ⇒　生成物がe+の方向に運動量を持つ ⇒　相対論的効果により生成物の寿命が延びる 𝑠 = 𝐸 𝑒 − + 𝐸 𝑒 + 2 − 𝑝 𝑒 − + 𝑝 𝑒 + 2 = 𝐸 𝑒 − 2 +2 𝐸 𝑒 − 𝐸 𝑒 + + 𝐸 𝑒 + 2 − 𝑝 𝑒 − 2 −2 𝑝 𝑒 − 𝑝 𝑒 + − 𝑝 𝑒 + 2 ≃ 2 𝐸 𝑒 − 𝐸 𝑒 + +2 𝑝 𝑒 − 𝑝 𝑒 + 2 ≃ 4 𝐸 𝑒 − 𝐸 𝑒 + ∴ 𝑠 ≃ 4×8×3.5 ≃ 10.6 GeV B中間子生成までの過程 主な崩壊モード B+ + B- e+ + e- → γ* → Υ(4S) → u b b u B 0 + B 0 b b 質量：10.5794±0.0012 GeV d b b d

Luminosity of KEKB accelerator 1nb = 10 −33 cm 2

Belle detector Belle検出器は、B中間子崩壊に おけるCP対称性の破れを観測 するために設計された。 エネルギーが非対称なため、 衝突点が測定器の中心から少し ずれている。 ・荷電粒子の通過した位置を測定： Silicon Vertex Detector, Central Drift Chamber ・粒子のエネルギー測定：Electromagnetic calorimeter • 荷電粒子の識別： Central Drift Chamber, Aerogel Cherenkov Counters, KL meson and muon detector • 荷電粒子の飛行時間の測定： Time-of-flight counters

charmonium S: スピン (中間子なのでS=0 or 1) L: 軌道角運動量 (L=0,1,2,…はそれぞれL=S,P,D,…で表される) J: 全角運動量 (|L-S| < J < L+S に従う) 2S+1LJのように表記される場合もある。 例えばJ/ψは 3S1という状態であり、量子数はそれぞれS=1, L=0, J=1である。 S=0のチャーモニウムの状態はηcと呼ばれるが、e+e-衝突で直接 作られることはない。

X(3872) Quantity ψ' X(3872) Signal events 489 ± 23 35.7 ± 6.8 M peak 3685.5 ± 0.2 MeV 3871.5 ± 0.6 MeV σ 3.3 ± 0.2 MeV 2.5 ± 0.5 MeV 𝑀 X(3872) = 𝑀 X measured − 𝑀 ψ′ measured + 𝑀 ψ′ PDG = 3872.0 ± 0.6(stat) ± 0.5(syst) MeV 𝑀 ψ′ PDG − 𝑀 J/ψ PDG = 589.07 ± 0.13 MeV 𝑀 J/ψ PDG = 3096.916 ± 0.011 MeV

Meson 粒子記号 構成クォーク 不変質量[MeV] B+ 5,279.15±0.31 K+ 493.677±0.016 π+ 139.57018±0.00035 J/ψ 3,096.916±0.011 D0 1,864.84 ± 0.17 D*0 2,006.97 ± 0.19 u b u s u d c c c u c u