arXiv subjects

A. Chen

Publications and source records attributed to A. Chen.

At least 433 records · Page 24Linked to original sources

arXiv2013

Search for CP Violation in the Decay $D^+\rightarrow K^0_S K^+$

We search for CP violation in the decay $D^+\rightarrow K^0_S K^+$ using a data sample with an integrated luminosity of 977 fb$^{-1}$ collected with the Belle detector at the KEKB $e^+e^-$ asymmetric-energy collider. No CP violation has been observed and the CP asymmetry in $D^+\rightarrow K^0_S K^+$ decay is measured to be $(-0.25\pm0.28\pm0.14)%$, which is the most sensitive measurement to date. After subtracting CP violation due to $K^0-\bar{K}^0$ mixing, the CP asymmetry in $D^+\rightarrow\bar{K}^0 K^+$ decay is found to be $(+0.08\pm0.28\pm0.14)%$.

hep-ex↗

arXiv2013

Design of MEBT for the Project X Injector Experiment at Fermilab

The Project X Injector Experiment (PXIE), a test bed for the Project X front end, will be completed at Fermilab at FY12-16. One of the challenging goals of PXIE is demonstration of the capability to form a 1 mA H- beam with an arbitrary selected bunch pattern from the initially 5 mA 162.5 MHz CW train. The bunch selection will be made in the Medium Energy Beam Transport (MEBT) at 2.1 MeV by diverting undesired bunches to an absorber. This paper presents the MEBT scheme and describes development of its elements, including the kickers and absorber.

physics.acc-ph↗

arXiv2013

Measurement of Branching Fraction and First Evidence of CP Violation in B0 --> a1+-(1260) pi-+ Decays

We present a measurement of the branching fraction and time-dependent CP violation parameters in B0 --> a_1^+-(1260) pi^-+ decays. The results are obtained from the final data sample containing 772x10^6 BBbar pairs collected at the Upsilon(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. We also measure an upper limit on the product branching fraction for a possible decay with the same final state B0 --> a_2^+-(1320) pi^-+. In a time-dependent measurement to extract CP asymmetries, we find evidence of mixing-induced CP violation in B0 --> a_1^+-(1260) pi^-+ decays with a 3.1 sigma significance and the rate where the a_1^+-(1260) does not contain the spectator quark is found to dominate the rate where it does at the 4.1 sigma level. However, there is no evidence for either time and flavor integrated direct CP violation or flavor-dependent direct CP violation.

hep-ex↗

arXiv2013

Progress with PXIE MEBT Chopper

A capability to provide a large variety of bunch patterns is crucial for the concept of the Project X serving MW-range beam to several experiments simultaneously. This capability will be realized by the Medium Energy Beam Transport's (MEBT) chopping system that will divert 80% of all bunches of the initially 5mA, 2.1 MeV CW 162.5 MHz beam to an absorber according to a pre-programmed bunch-by-bunch selection. Being considered one of the most challenging components, the chopping system will be tested at the Project X Injector Experiment (PXIE) facility that will be built at Fermilab as a prototype of the Project X front end. The bunch deflection will be made by two identical sets of travelling-wave kickers working in sync. Currently, two versions of the kickers are being investigated: a helical 200 Ohm structure with a switching-type 500 V driver and a planar 50 Ohm structure with a linear 250 V amplifier. This paper will describe the chopping system scheme and functional specifications for the kickers, present results of electromagnetic measurements of the models, discuss possible driver schemes, and show a conceptual mechanical design.

physics.acc-ph↗

arXiv2013

Search for Lepton-Flavor-Violating and Lepton-Number-Violating tau to lhh' Decay Modes

We search for lepton-flavor-violating and lepton-number-violating tau decays into a lepton (l = electron or muon) and two charged mesons (h, h' = pion or Kaon) using 854 fb^{-1} of data collected with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. We obtain 90% confidence level upper limits on the tau to lhh' branching fractions in the range (2.0-8.4)*10^{-8}. These results improve upon our previously published upper limits by factors of about 1.8 on average.

hep-ex↗

arXiv2012

Evidence for the eta_b(2S) and observation of h_b(1P) -> eta_b(1S) gamma and h_b(2P) -> eta_b(1S) gamma

We report the first evidence for the eta_b(2S) using the h_b(2P)->eta_b(2S)gamma transition and the first observation of the h_b(1P)->eta_b(1S)gamma and h_b(2P)->eta_b(1S)gamma transitions. The mass and width of the eta_b(1S) and eta_b(2S) are measured to be m_etab(1S)=(9402.4+-1.5+-1.8)MeV/c^2, m_etab(2S)=(9999.0+-3.5 +2.8-1.9)MeV/c^2 and Gamma_etab(1S)=(10.8 +4.0-3.7 +4.5-2.0)MeV. We also update the h_b(1P) and h_b(2P) mass measurements. We use a 133.4/fb data sample collected at energies near the Upsilon(5S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider.

hep-ex↗

arXiv2012

Measurement of gamma gamma* --> pi0 transition form factor at Belle

We report a measurement of the process gamma gamma* --> pi0 with a 759 fb^{-1} data sample recorded with the Belle detector at the KEKB asymmetric-energy e+e- collider. The pion transition form factor, F(Q^2), is measured for the kinematical region 4 GeV^2 <~ Q^2 <~ 40 GeV^2, where -Q^2 is the invariant mass squared of a virtual photon. The measured values of Q^2|F(Q^2)| agree well with the previous measurements below Q^2 ~= 9 GeV^2 but do not exhibit the rapid growth in the higher Q^2 region seen in another recent measurement, which exceeds the asymptotic QCD expectation by as much as 50%.

hep-ex↗

arXiv2012

Study of the Hadronic Transitions $Υ$(2S)$\rightarrow (η,π^0)Υ$(1S) at Belle

We study the rare hadronic transitions $Υ(2S)\rightarrow Υ(1S)η$ and $Υ(2S)\rightarrow Υ(1S)π^0$ using a sample of 158 $\times 10^6$ $Υ(2S)$ decays collected with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We observe the $η$ meson decay to $γγ$ and $π^+π^-π^0$ final states; the $Υ(1S)$ is reconstructed in the $μ^+μ^-$ and $e^+e^-$ decay modes. We measure the ratios of branching fractions (${\mathcal B}$) $\frac{{\mathcal B}(Υ(2S)\rightarrowΥ(1S)η)}{{\mathcal B}(Υ(2S)\rightarrowΥ(1S)π^+π^-)}$ = (1.99$\pm$0.14 (stat) $\pm$0.11 (syst)) $\times 10^{-3}$ and $\frac{{\mathcal B}(Υ(2S)\rightarrowΥ(1S)π^0)}{{\mathcal B}(Υ(2S)\rightarrowΥ(1S)π^+π^-)} < 2.3 \times 10^{-4}$ at the 90% confidence level (CL). Assuming the value ${\mathcal B}(Υ(2S) \rightarrow Υ(1S)π^-π^+)$ = (17.92$\pm$0.26)%, we obtain $ {\mathcal B}(Υ(2S)\rightarrowΥ(1S)η) = (3.57 \pm 0.25 ({\rm stat})\ \pm 0.21 ({\rm syst}))\times 10^{-4} $ and $ {\mathcal B}(Υ(2S)\rightarrowΥ(1S)π^0) < 4.1\times 10^{-5}\ ({\rm 90%\ CL}). $

hep-ex↗

arXiv2012

Evidence for B- -> Ds+ K- l- nubar and search for B- -> Ds*+ K- l- nubar

We report measurements of the decays B- -> Ds(*)+ K- l- nubar in a data sample containing 657x10^6 BBbar pairs collected with the Belle detector at the KEKB asymmetric-energy e+e- collider. We observe a signal with a significance of 6 sigma for the combined Ds and Ds* modes and find the first evidence of the B- -> Ds+ K- l- nubar decay with a significance of 3.4 sigma. We measure the following branching fractions: BF(B- -> Ds+ K- l nubar) = (0.30 +/- 0.09(stat) +0.11 -0.08(syst)) x 10^-3 and BF(B- -> Ds*+ K- l- nubar) = (0.59 +/- 0.12(stat) +/- 0.15(syst)) x 10^-3 and set an upper limit BF(B- -> Ds*+ K- l- nubar) < 0.56 x 10^-3 at the 90% confidence level. We also present the first measurement of the Ds+K- invariant mass distribution in these decays, which is dominated by a prominent peak around 2.6 GeV/c^2.

hep-ex↗

arXiv2012

Study of Three-Body Y(10860) Decays

We report preliminary results on the analysis of the three-body Y(10860)=> B barB pi, Y(10860) => (B barB* + c.c.) pi and Y(10860)=> B* barB* pi decays including an observation of the Y(10860)=> Zb(10610)+- pi-+ => [B barB*+c.c]+- pi-+ and Y(10860)=> Zb(10650)+- pi-+ => [B* barB*]+- pi-+ decays as intermediate channels. We measure branching fractions of the three-body decays to be Br(Y(10860)=> [B barB*+c.c.]+- pi-+)=(28.3+-2.9+-4.6)x10^{-3} and Br(Y(10860)=> [B* barB*]+- pi-+)=(14.1+-1.9+-2.4)x10^{-3} and set 90% C.L. upper limit Br(Y(10860)=> [B barB]+- pi-+)<4.0x10^{-3}. We also report results on the amplitude analysis of the three-body Y(10860)=>Y(nS)pi+pi-, n=1,2,3 decays and the analysis of the internal structure of the three-body Y(10860)=>hb(mP)pi+pi-, m=1,2 decays. The results are based on a 121.4 1/fb data sample collected with the Belle detector at a center-of-mass energy near the Y(10860).

hep-ex↗

arXiv2012

First observation of CP violation and improved measurement of the branching fraction and polarization of B0 -> D+ D- decays

We report the measurement of the branching fraction, the polarization, and the parameters of the time-dependent CP violation in B0 -> D*+ D*- decays using a data sample of 772 million BB pairs, collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. We obtain a branching fraction of B = (7.82 +- 0.38 +- 0.63) x10^{-4}, a CP-odd fraction of R_\perp = 0.138 +- 0.024 +- 0.006 and, additionally, a fraction of the longitudinal component in the transversity base of R_0 = 0.624 +- 0.029 +- 0.011. The measured values of the parameters of the CP violation are S_{D*+ D*-} = -0.79 +- 0.13 +- 0.03 and A_{D*+ D*-} = 0.15 +- 0.08 +- 0.04.

hep-ex↗

arXiv2012

Evidence for CP Violation in the Decay $D^+\rightarrow K^0_Sπ^+$

We observe evidence for CP violation in the decay $D^+\rightarrow K^0_Sπ^+$ using a data sample with an integrated luminosity of 977 fb$^{-1}$ collected by the Belle detector at the KEKB $e^+e^-$ asymmetric-energy collider. The CP asymmetry in the decay is measured to be $(-0.363\pm0.094\pm0.067)%$, which is 3.2 standard deviations away from zero, and is consistent with the expected CP violation due to the neutral kaon in the final state.

hep-ex↗

arXiv2012

Search for $B^{0}$ decays to invisible final states at Belle

We report a search for $B^{0}$ decays into invisible final states using a data sample of $657 \times 10^{6}$ $B\bar{B}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB $e^{+}e^{-}$ collider. The signal is identified by fully reconstructing a hadronic decay of the accompanying $B$ meson and requiring no other particles in the event. No significant signal is observed, and we obtain an upper limit of $1.3 \times 10^{-4}$ at the 90% confidence level for the branching fraction of invisible $B^{0}$ decay.

hep-ex↗

arXiv2012

First observation of exclusive $Υ(1S)$ and $Υ(2S)$ decays into light hadrons

Using samples of 102 million $Υ(1S)$ and 158 million $Υ(2S)$ events collected with the Belle detector, we study exclusive hadronic decays of these two bottomonium resonances to the three-body final states $ϕK^+ K^-$, $ωπ^+ π^-$ and $K^{\ast 0}(892) K^- π^+ $, and to the two-body Vector-Tensor ($ϕf_2'(1525)$, $ωf_2(1270)$, $ρa_2(1320)$ and $K^{\ast 0}(892) \bar{K}_2^{\ast 0}(1430) $) and Axial-vector-Pseudoscalar ($K_1(1270)^+ K^-$, $K_1(1400)^+ K^- $ and $b_1(1235)^+ π^- $) pairs. Signals are observed for the first time in the $Υ(1S) \to ϕK^+ K^-$, $ωπ^+ π^-$, $K^{\ast 0} K^- π^+$, $K^{\ast0} K_2^{\ast 0}$ and $Υ(2S) \to ϕK^+ K^-$, $K^{\ast 0} K^- π^+$ decay modes. Branching fractions are determined for all the processes, while 90% confidence level upper limits are established on the branching fractions for the modes with a statistical significance less than $3σ$. The ratios of the branching fractions of $Υ(2S)$ and $Υ(1S)$ decays into the same final state are used to test a perturbative QCD prediction for OZI suppressed bottomonium decays.

hep-ex↗

arXiv2012

Evidence for a Zb0(10610) in Dalitz analysis of Y(5S) -> Y(nS) pi0 pi0

We report the first observation of Y(5S) -> Y(1,2S) pi0 pi0 decays. Evidence for the Zb0(10610) with 4.9sigma significance is found in a Dalitz plot analysis of Y(5S) -> Y(2S) pi0 pi0 decays. The results are obtained with a 121.4 fb^-1 data sample collected with the Belle detector at the Y(5S) resonance at the KEKB asymmetric-energy e+e- collider.

hep-ex↗

arXiv2012

Search for B -> phi pi decays

We report on a search for the charmless decays $B^{+} \toϕπ^{+}$ and $B^{0} \toϕπ^{0}$ that are strongly suppressed in the Standard Model. The analysis is based on a data sample of $657 \times 10^6$ $B \bar{B}$ pairs collected at the $Υ(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^+ e^-$ collider. We find no significant signal and set upper limits of $3.3 \times 10^{-7}$ for $B^{+} \to ϕπ^{+}$ and $1.5 \times 10^{-7}$ for $B^0 \to ϕπ^0$ at the 90% confidence level.

hep-ex↗

arXiv2012

First study of η_c, η(1760) and X(1835) production via η'π^+π^- final states in two-photon collisions

The invariant mass spectrum of the η' π^+ π^- final state produced in two-photon collisions is obtained using a 673 fb^{-1} data sample collected in the vicinity of the Υ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. We observe a clear signal of the η_c and measure its mass and width to be M(η_c)=(2982.7 +- 1.8(stat) +- 2.2(syst) +- 0.3(model)) MeV/c^2 and Γ(η_c) = (37.8^{+5.8}_{-5.3}(stat) +- 2.8(syst) +- 1.4(model)) MeV/c^2. The third error is an uncertainty due to possible interference between the η_c and a non-resonant component. We also report the first evidence for η(1760) decay to η' π^+ π^-; we find two solutions for its parameters, depending on the inclusion or not of the X(1835), whose existence is of marginal significance in our data. From a fit to the mass spectrum using coherent X(1835) and η(1760) resonant amplitudes, we set a 90% confidence level upper limit on the product Γ_{γγ} \BR (η' π^+ π^-) for the X(1835).

hep-ex↗

arXiv2012

Measurement of the 18Ne(a,p_0)21Na reaction cross section in the burning energy region for X-ray bursts

The 18Ne(a,p)21Na reaction provides one of the main HCNO-breakout routes into the rp-process in X-ray bursts. The 18Ne(a,p_0)21Na reaction cross section has been determined for the first time in the Gamow energy region for peak temperatures T=2GK by measuring its time-reversal reaction 21Na(p,a)18Ne in inverse kinematics. The astrophysical rate for ground-state to ground-state transitions was found to be a factor of 2 lower than Hauser-Feshbach theoretical predictions. Our reduced rate will affect the physical conditions under which breakout from the HCNO cycles occurs via the 18Ne(a,p)21Na reaction.

nucl-ex↗