5
TABLE I. A comparison of theoretical calculations for the ηc(2S) mass, theBF (ψ(3686) γηc(2S)), and thepartial width
B →
Γ(ψ(3686) γηc(2S)).
→
Mass(MeV/c2) B(ψ(3686)→γηc(2S))(×10−4) Γ(ψ(3686)→γηc(2S))(keV)
NRmodel[1] 3630 7.14±0.19 0.21
GImodel[1] 3623 5.80±0.16 0.17
Mesonloopcorrection[2] N/A 2.72±1.00 0.08±0.03
Light-frontquarkmodel[3] 3637 3.9 0.11
Effectivefieldtheory[12] N/A 0.6−36.0 N/A
M1 transition ψ(3686) γη (2S) with η (2S) decaying octagonal flux-return yoke with resistive plate counter
c c
into KK¯π was reporte→ d by BESIII [13]. With a data muon identification modules interleaved with steel. The
sample of 106 106 ψ(3686) events, BESIII measured charged-particle momentum resolution at 1 GeV/c is
theBF (ψ(368× 6) γη (2S))=(6.8 1.1 4.5) 10−4, 0.5%, and the dE/dx resolution is 6% for electrons from
c
B → ± ± ×
where,hereandhereafter,thefirstuncertaintyisstatisti- Bhabha scattering. The EMC measures photon energies
calandthe secondis systematic. Reference [7]extracted with a resolution of 2.5% (5%) at 1 GeV in the barrel
(ψ(3686) γη (2S))=(7+3.4) 10−4 byaglobalfitto (end cap) region. The time resolution in the TOF bar-
B → c −2.5 ×
many experimental measurements and is in good agree- rel region is 68 ps, while that in the end cap region is
ment withthe worldaveragevalue of(7 5) 10−4 [11]. 110 ps. The end cap TOF system was upgraded in 2015
± ×
It is difficult to validate the theoreticalcalculations with using multi-gap resistive plate chamber technology, pro-
the current measurements, since the relative uncertain- viding a time resolution of 60 ps [18].
ties are as large as 70%. Simulated data samples produced with a geant4-
The mass of the η (2S) was predicted to be based [19] Monte Carlo (MC) package, which includes
c
3.630 GeV/c2, 3.623 GeV/c2, and 3.637 GeV/c2 accord- thegeometricdescriptionoftheBESIIIdetector[20]and
ingtotheNRmodel[1],theGImodel[1],andinRef.[3], the detector response, are used to determine the detec-
respectively. Recently, the LHCb collaboration report- tion efficiencies and to estimate backgrounds. The sim-
ed the mass of the η (2S) to be m = (3637.90 ulation models the beam energy spread and initial state
0.54 1.40)
MeV/c2c
and its
widtη hc(2 tS o)
be Γ ± = radiation(ISR) in the e+e− annihilation with the kkmc
±
ηc(2S)
(10.77 1.62 1.08) MeV, via B Kη (2S),η (2S) generator [21]. The inclusive MC sample includes the
c c
K0Kπ±
[14].
T±
he experimental
av→
erages of its
resona→
nt production of the ψ(3686) resonance, the production of
S
parameters are m = (3637.5 1.1) MeV/c2 and J/ψ via ISR, and the continuum processes incorporat-
Γ
=(11.3+3.2η )c( M2S e)
V [11]. ± ed in kkmc. All the known particle decays are mod-
η Tc( h2S e) BESIII e− x2 p.9 eriment has collected (27.08 0.14) eled with evtgen [22] using the BFs either taken from
108 ψ(3686) events [15], which is 25.5 times la± rger tha× n the Particle Data Group [11], when available, or other-
thedatasetusedinthepreviousmeasurement[13]. Using wise estimated with lundcharm [23]. The final state
radiation (FSR) from charged final state particles is in-
this enlarged data sample, we present a comprehensive
corporated using the photos package [24]. We gener-
analysis of the M1 transition ψ(3686) γη (2S) with
c
η (2S)decayingintoKK¯π finalstates. T→ heresonantpa- ate the ψ(3686) γη c(2S) decay using helicity ampli-
c tudes [25] and th→ e η (2S) KK¯π decays with a phase
rametersoftheη (2S)andtheBFofψ(3686) γη (2S) c
c c →
→ space (PHSP) model.
are reported with improved precision.
II. DETECTOR AND MONTE CARLO III. EVENT SELECTION
SIMULATION
In selecting signal events for the process
The BESIII detector [16] recordssymmetric e+e− col- ψ(3686) γKK¯π, we require each candidate event
→
lisions provided by the BEPCII storage ring [17] in the to contain four (two) charged tracks with zero net
center-of-mass energy range from 2.0 to 4.95 GeV, with charge, and at least one (three) photon(s) for the
a peak luminosity of1 1033 cm−2s−1 achievedat √s= γK0K±π∓ (γK+K−π0) mode. Here, KK¯π represents
3.77GeV. BESIIIhasc× ollectedlargedatasamplesinthis onlyS K+K−π0 and K0K±π∓ in Secs. III, IV, and V
S
energyregion[17]. ThecylindricalcoreoftheBESIIIde- for simplicity except explicitly stated. We form K0 (π0)
S
tector covers93%of the full solidangle and consistsof a candidates using pairs of π+π− (γγ).
helium-based multilayer drift chamber (MDC), a plastic Photon candidates are identified using showers in the
scintillator time-of-flight system (TOF), and a CsI(Tl) EMC.The depositedenergy ofthe showermust be more
electromagnetic calorimeter (EMC), which are all en- than25MeVinthebarrelregion( cosθ <0.80)ormore
| |
closed in a superconducting solenoid magnet providing than 50 MeV in the end cap region (0.86 < cosθ <
| |
a 1.0 T magnetic field. The solenoid is supported by an 0.92). To exclude showers that originate from charged