Lou, Xinchou
Permanent URI for this collectionhttps://hdl.handle.net/10735.1/2305
Dr. Lou’s research interests focus on elementary particle physics experiments at electron-position colliders, high performance cluster computing, and application of data mining techniques to other fields of research. He is currently searching for new elementary particles, measuring charmonium production with the BABAR detector located at the Stanford Linear Accelerator Center.
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Browsing Lou, Xinchou by Subject "Atomic mass"
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Item Measurement of Initial-State-Final-State Radiation Interference in the Processes e⁺e⁻ → μ⁺μ⁻γ and e⁺e⁻ → π⁺π⁻γ(2015-10-28) Lees, J. P.; Poireau, V.; Tisserand, V.; Grauges, E.; Palano, A.; Eigen, G.; Stugu, B.; Izen, Joseph M.; Lou, Xinchou; BaBar Collaboration; 56366505900 (Lou, X); 250674117 (Izen, JM); Lou, XinchouCharge asymmetry in the processes e⁺e⁻ → μ⁺μ⁻γ and e⁺e⁻ → π⁺π⁻γ is measured using 232 fb⁻¹ of data collected with the BABAR detector at e⁺e⁻ center-of-mass energies near 10.58 GeV. An observable is introduced and shown to be very robust against detector asymmetries while keeping a large sensitivity to the physical charge asymmetry that results from the interference between initial-and final-state radiation (FSR). The asymmetry is determined as a function of the invariant mass of the final-state tracks from production threshold to a few GeV/c². It is compared to the expectation from QED for e⁺e⁻ → μ⁺μ⁻γ, and from theoretical models for e⁺e⁻ → π⁺π⁻γ. A clear interference pattern is observed in e⁺e⁻ → π⁺π⁻γ, particularly in the vicinity of the f₂(1270) resonance. The inferred rate of lowest-order FSR production is consistent with the QED expectation for e⁺e⁻ → μ⁺μ⁻γ, and is negligibly small for e⁺e⁻ → π⁺π⁻γ.Item Measurement of the Branching Fraction for ψ(3770) → γχ_{c0}(Elsevier Science BV, 2016-02-10) Ablikim, M.; Achasov, M. N.; Ai, X. C.; Albayrak, O.; Albrecht, M.; Ambrose, D. J.; Amoroso, A.; Lou, Xinchou; BESIII Collaboration; 0000-0003-0867-2189 (Lou, X); Lou, XinchouBy analyzing a data set of 2.92 fb⁻¹ of e⁺e⁻ collision data taken at √s̅ = 3.773 GeV and 106.41 x 10⁻⁶ ψ3686) decays taken at √s̅ = 3.686 GeV with the BESIII detector at the BEPCII collider, we measure the branching fraction and the partial decay width for ψ3770) → γχ(c0) to be B(ψ(3770) → γχ(c0)) = (6.88 ± 0.28 ± 0.67) x 10⁻³ and Γ[ψ(3770) → γχ(c0)] = (187 ± 8 ± 19) keV, respectively. These are the most precise measurements to date.Item Measurement of the e⁺e⁻ → ηJ/ψ Cross Section and Search for e⁺e⁻ → π⁰J/ψ at Center-Of-Mass Energies Between 3.810 and 4.600 GeV(Amer Physical Soc, 2015-06-09) Ablikim, M.; Achasov, M. N.; Ai, X. C.; Albayrak, O.; Albrecht, M.; Ambrose, D. J.; Amoroso, A.; Lou, Xinchou; BESIII Collaboration; 0000-0003-0867-2189 (Lou, X); Lou, XinchouUsing data samples collected with the BESIII detector operating at the BEPCII collider at 17 center-of-mass energies from 3.810 to 4.600 GeV, we perform a study of e⁺e⁻ → ηJ/ψ and π⁰J/ψ The Born cross sections of these two processes are measured at each center-of-mass energy. The measured energy-dependent Born cross section for e⁺e⁻ → ηJ/ψ shows an enhancement around 4.2 GeV. The measurement is compatible with an earlier measurement by Belle.Item Measurement of the Form Factors in the Decay D⁺ → ωe⁺ν_e and Search for the Decay D⁺ → φe⁺ν_e.(Amer Physical Soc, 2015-10-19) Ablikim, M.; Achasov, M. N.; Ai, X. C.; Albayrak, O.; Albrecht, M.; Ambrose, D. J.; Amoroso, A.; Lou, Xinchou; BESIII Collaboration; School of Natural Sciences and Mathematics; Lou, XinchouUsing 2.92 fb⁻¹ of electron-positron annihilation data collected at a center-of-mass energy of √s = 3.773 GeV with the BESIII detector, we present an improved measurement of the branching fraction ℬ(D⁺ → ωe⁺ν_e) = (1.63 ± 0.11 ± 0.08) x 10⁻³. The parameters defining the corresponding hadronic form factor ratios at zero momentum transfer are determined for the first time; we measure them to be r_V = 1.24 ± 0.09 ± 0.06 and r_2 = 1.06 ± 0.15 ± 0.05. The first and second uncertainties are statistical and systematic, respectively. We also search for the decay D⁺ → ωe⁺ν_e. An improved upper limit ℬ(D⁺ → φe⁺ν_e < 1.3 x 10⁻⁵ is set at 90% confidence level.Item Measurement of the Higgs Boson Mass from the H → γγ and H → ZZ* → 4ℓ Channels in pp Collisions at Center-Of-Mass Energies of 7 and 8 TeV with the ATLAS Detector(American Physical Society, 2014-09-09) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael; Lou, Xinchou; Namasivayam, Harisankar; Reeves, Kendall; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouAn improved measurement of the mass of the Higgs boson is derived from a combined fit to the reconstructed invariant mass spectra of the decay channels H → γγ and H → ZZ∗ → 4ℓ. The analysis uses the pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at center-of-mass energies of 7 TeV and 8 TeV, corresponding to an integrated luminosity of 25 fb⁻¹. The measured value of the Higgs boson mass is m_H = 125.36 ± 0.37(stat) ± 0.18(syst) GeV. This result is based on improved energy-scale calibrations for photons, electrons, and muons as well as other analysis improvements, and supersedes the previous result from ATLAS. Upper limits on the total width of the Higgs boson are derived from fits to the invariant mass spectra of the H → γγ and H → ZZ∗ → 4ℓ decay channels.Item Measurement of χ_{cJ} Decaying into η'K⁺K⁻(American Physical Society, 2014-04-11) Ablikim, M.; Achasov, M. N.; Ai, X. C.; Albayrak, O.; Albrecht, M.; Ambrose, D. J.; An, F. F.; Lou, Xinchou; BESIII Collaboration; 0000-0003-0867-2189 (Lou, X.); Lou, XinchouUsing (106.41 ± 0.86) × 106 ψ(3686) events collected with the BESIII detector at BEPCII, we study for the first time the decay χcJ → η'K+K- (J = 1, 2), where η' → γρ0 and η' → ηπ+π-. A partial wave analysis in the covariant tensor amplitude formalism is performed for the decay χc1 → η'K+K-. Intermediate processes χc1 → η' f0 (980), χc1 → η' f0 (1710), χc1 → η' f2' (1525) and χc1 → K∗ 0 (1430)± K∓ (K∗ 0 (1430)± → η' K±) are observed with statistical significances larger than 5σ, and their branching fractions are measured.Item Observation of a Charged Charmoniumlike Structure in e⁺e⁻ → (D*D̅*)^± π^∓ at √s=4.26 GeV(American Physical Society, 2014-04-02) Ablikim, M.; Achasov, M. N.; Albayrak, O.; Ambrose, D. J.; An, F. F.; An, Q.; Bai, J. Z.; Lou, Xinchou; Lou, XinchouWe study the process e⁺e⁻ → (D*D̅*)^± π^∓ at a center-of-mass energy of 4.26 GeV using a 827 pb⁻¹ data sample obtained with the BESIII detector at the Beijing Electron Positron Collider. Based on a partial reconstruction technique, the Born cross section is measured to be (137 ± 9 ± 15) pb. We observe a structure near the (D*D̅*)^± threshold in the π^∓ recoil mass spectrum, which we denote as the Z_c ^±(4025). The measured mass and width of the structure are (4026.3 ± 2.6 ± 3.7) MeV/c² and (24.8 ± 5.6 ± 7.7) MeV, respectively. Its production ratio σ(e⁺ e⁻ → Z_c ^±(4025)π^∓ → (D*D̅*)^± π^∓ / σ(e⁺ e⁻ → ((D*D̅*)^± π^∓ is determined to be 0.65 ± 0.09 ± 0.06. The first uncertainties are statistical and the second are systematic.Item Observation of a Neutral Charmoniumlike State Z_c(4025)⁰ in e⁺e⁻ → (D*D̅*)⁰ π⁰(2015-10) Ablikim, M.; Achasov, M. N.; Ai, X. C.; Albayrak, O.; Albrecht, M.; Ambrose, D. J.; Amoroso, A.; Lou, Xinchou; BESIII Collaboration; 0000-0003-0867-2189 (Lou, X.); Lou, XinchouWe report a study of the process e⁺e⁻ → (D*D̅*)⁰ π⁰ using e⁺e⁻ collision data samples with integrated luminosities of 1092 pb⁻¹ at √s̅ = 4.23 GeV and 826 pb⁻¹ at √s̅ = 4.26 GeV collected with the BESIII detector at the BEPCII storage ring. We observe a new neutral structure near the (D*D̅*)⁰ mass threshold in the π⁰ recoil mass spectrum, which we denote as Z_c(4025)⁰. Assuming a Breit-Wigner line shape, its pole mass and pole width are determined to be (4025.5_{-4.7}^{+2.0) ± 3.1) MeV/c² and (23.0 ± 6.0 ± 1.0) MeV, respectively. The Born cross sections of e⁺e⁻→ Z_c(4025)⁰π⁰ → (D*D̅*)⁰ π⁰ are measured to be (61.6 ± 8.2 ± 9.0) pb at √s̅ = 4.23 GeV and (43.4 ± 8.0 ± 5.4) pb at √s̅ = 4.26 GeV. The first uncertainties are statistical and the second are systematic.Item Observation of a Neutral Structure Near the DD̅* Mass Threshold in e⁺e⁻ → (DD̅*)⁰π⁰ at √s= 4.226 and 4.257 GeV(Amer Physical Soc, 2015-11) Ablikim, M.; Achasov, M. N.; Ai, X. C.; Albayrak, O.; Albrecht, M.; Ambrose, D. J.; Amoroso, A.; Lou, Xinchou; BESII Collaboration; 0000-0003-0867-2189 (Lou, X.); Lou, XinchouA neutral structure in the DD̅* system around the DD̅* mass threshold is observed with a statistical significance greater than 10σ in the processes e⁺e⁻ → D⁺D*π° + c.c. and e⁺e⁻ →D°D̅*⁰ + c.c. At √s = 4.226 and 4.257 GeV in the BESIII experiment. The structure is denoted as Z_c(3885)⁰. Assuming the presence of a resonance, its pole mass and width are determined to be [3885.7_{-5.7}^{+4.3} (stat) ± 8.4(syst)] MeV/c² and [35_{-12}^{+11} (stat) ± 15(syst)] MeV, respectively. The Born cross sections are measured to be σ[e⁺e⁻ → Z_c(3885)°π°, Z_c(3885)° →DD̅*] = [77 ± 13(stat) ± 17(syst)] pb at 4.226 GeV and [47 ± 9(stat) ± 10(syst)] pb at 4.257 GeV. The ratio of decay rates B[Z_c(3885)° → D⁺D*⁻ + c.c.] / 𝓑ℬ[Z_c(3885)° → D°D̅*° + c.c.] is determined to be 0.96 ± 0.18(stat) ± 0.12(syst), consistent with no isospin violation in the process, Z_c(3885)° → DD̅*.Item Observation of e⁺e⁻ → ωχ_{c1, 2} near √s̅ = 4.42 and 4.6 GeV(American Physical Society, 2016-01-14) Ablikim, M.; Achasov, M. N.; Ai, X. C.; Albayrak, O.; Albrecht, M.; Ambrose, D. J.; Amoroso, A.; Lou, Xinchou; BESIII Collaboration; 0000-0003-0867-2189 (Lou, X); Lou, XinchouBased on data samples collected with the BESIII detector operating at the BEPCII storage ring at center-of-mass energies √s̅ > 4.4 GeV, the processes e⁺e⁻ → ωχ(c1,2) are observed for the first time. With an integrated luminosity of 1074 pb⁻¹ near √s̅ = 4.42 GeV, a significant ωχ(c2) signal is found, and the cross section is measured to be (20.9 ± 3.2 ± 2.5) pb. With 567 pb⁻¹ near √s̅ = 4.6 GeV, a clear ωχ(c1) signal is seen, and the cross section is measured to be (9.5 ± 2.1 ± 1.3) pb, while evidence is found for an ωχ(c2) signal. The first errors are statistical, and the second are systematic. Due to low luminosity or low cross section at other energies, no significant signals are observed. In the ωχ(c2) cross section, an enhancement is seen around √s̅ = 4.42 GeV. Fitting the cross section with a coherent sum of the ψ(4415) Breit-Wigner function and a phase-space term, the branching fraction B(ψ(4415) → ωχ(c2)) is obtained to be of the order of 10⁻¹.Item Observation of Z_c (3900)⁰ in e⁺e⁻ → π⁰π⁰J/ψ(2015-09-11) Ablikim, M.; Achasov, M. N.; Ai, X. C.; Albayrak, O.; Albrecht, M.; Ambrose, D. J.; Amoroso, A.; Lou, Xinchou; BESIII Collaboration; 0000-0003-0867-2189 (Lou, X.); Lou, XinchouUsing a data sample collected with the BESIII detector operating at the BEPCII storage ring, we observe a new neutral state Z_c (3900)⁰ with a significance of 10.4σ. The mass and width are measured to be 3894.8 ± 2.3 ± 3.2 MeV/c² and 29.6 ± 8.2 ± 8.2 MeV, respectively, where the first error is statistical and the second systematic. The Born cross section for e⁺e⁻ → π⁰π⁰J/ψ and the fraction of it attributable to π⁰Z_c(3900)⁰ → π⁰π⁰J/ψ in the range E_{c.m.} = 4.19-4.42 GeV are also determined. We interpret this state as the neutral partner of the four-quark candidate Z_c(3900)^±.Item Observation of η′ → ωe⁺e⁻(Amer Physical Soc, 2015-09-15) Ablikim, M.; Achasov, M. N.; Ai, X. C.; Albayrak, O.; Albrecht, M.; Ambrose, D. J.; Amoroso, A.; Lou, Xinchou; BESIII Collaboration; 0000-0003-0867-2189 (Lou, X.); Lou, XinchouBased on a sample of η′ mesons produced in the radiative decay J/ψ → γη′ in 1.31 x 10⁹ J/ψ events collected with the BESIII detector, the decay η′ → ωe⁺e⁻ is observed for the first time, with a statistical significance of 8σ. The branching fraction is measured to be ℬ(η′ → ωe⁺e⁻) = (1.97 ± 0.34(stat) ± 0.17(syst)) x 10⁻⁴, which is in agreement with theoretical predictions. The branching fraction of η′ → ωγ is also measured to be (2.55 ± 0.03(stat) ± 0.16(syst)) x 10⁻², which is the most precise measurement to date, and the relative branching fraction 𝓑(η′ → ωe⁺e⁻) / 𝓑(η′ → ωγ) is determined to be (7.71 ± 1.34(stat) ± 0.54(syst)) x 10⁻³.Item Search for Resonant Diboson Production in the ℓℓqq̅ Final State in pp Collisions at √s̅ = 8 TeV with the ATLAS Detector(Springer New York LLC, 2015-02-10) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, Kendall; Lou, Xinchou; ATLAS Collaboration; 0000-0003-0867-2189 (Lou, X.); 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Meirose, Bernhard; Namasivayam, Harisankar; Reeves, KendallThis paper reports on a search for narrow resonances in diboson production in the ℓℓqq̅ final state using pp collision data corresponding to an integrated luminosity of 20 fb⁻¹ collected at √s̅ = 8 TeV with the ATLAS detector at the Large Hadron Collider. No significant excess of data events over the Standard Model expectation is observed. Upper limits at the 95% confidence level are set on the production cross section times branching ratio for Kaluza-Klein gravitons predicted by the Randall-Sundrum model and for Extended Gauge Model W' bosons. These results lead to the exclusion of mass values below 740 and 1590 GeV for the graviton and W' boson respectively.Item Search for Scalar Diphoton Resonances in the Mass Range 65-600 GeV with the ATLAS Detector in pp Collision Data at √s = 8 TeV(American Physical Society, 2014-10-20) Aad, G.; Abbott, B.; Abdallah, J.; Abdel Khalek, S.; Abdinov, O.; Aben, R.; Abi, B.; Izen, Joseph M.; Leyton, Michael; Xinchou Lou; Namasivayam, Harisankar; Reeves, Kendall; 250674117 (Izen, JM); 250674117 (Izen, JM); Izen, Joseph M.; Leyton, Michael; Namasivayam, Harisankar; Reeves, Kendall; Lou, XinchouA search for scalar particles decaying via narrow resonances into two photons in the mass range 65-600 GeV is performed using 20.3 fb⁻¹ of √s = 8 TeV pp collision data collected with the ATLAS detector at the Large Hadron Collider. The recently discovered Higgs boson is treated as a background. No significant evidence for an additional signal is observed. The results are presented as limits at the 95% confidence level on the production cross section of a scalar boson times branching ratio into two photons, in a fiducial volume where the reconstruction efficiency is approximately independent of the event topology. The upper limits set extend over a considerably wider mass range than previous searches.Item Searches for Isospin-Violating Transitions χ_{c0,2} → π⁰η_c.(Amer Physical Soc, 2015-06-30) Ablikim, M.; Achasov, M. N.; Ai, X. C.; Albayrak, O.; Albrecht, M.; Ambrose, D. J.; Amoroso, A.; Lou, Xinchou; BESIII Collaboration; 0000-0003-0867-2189 (Lou, X); Lou, XinchouWe present the first upper-limit measurement of the branching fractions of the isospin-violating transitions χ{c0,2} → π⁰η{c}. The measurements are performed using 106 x 10⁶ ψ(3686) events accumulated with the BESIII detector at the BEPCII e⁺e⁻ collider at a center-of-mass energy corresponding to the ψ(3686) mass. We obtained upper limits on the branching fractions at a 90% confidence level of B(χ{c0,2} → π⁰η{c}) < 3.2 x 10⁻³.Item Study of the Reaction e⁺e⁻ → ψ(2S)π⁺π⁻ via Initial-State Radiation at BaBar(American Physical Society, 2014-06-25) Lees, J. P.; Poireau, V.; Tisserand, V.; Garra Tico, J.; Grauges, E.; Palano, A.; Eigen, G.; Izen, Joseph M.; Lou, Xinchou; 250674117 (Izen, JM); 250674117 (Izen, JM); Lou, Xinchou; Izen, Joseph M.We study the process e⁺e⁻ → ψ(2S)π⁺ π⁻ with initial-state-radiation events produced at the PEP-II asymmetric-energy collider. The data were recorded with the BABAR detector at center-of-mass energies at and near the Υ(nS) (n=2, 3, 4) resonances and correspond to an integrated luminosity of 520 fb⁻¹. We investigate the ψ(2S)π⁺ π⁻ mass distribution from 3.95 to 5.95 GeV/c², and measure the center-of-mass energy dependence of the associated e⁺e⁻→ ψ(2S)π⁺ π⁻ cross section. The mass distribution exhibits evidence of two resonant structures. A fit to the ψ(2S)π⁺ π⁻ mass distribution corresponding to the decay mode ψ(2S)→J/ψπ⁺ π⁻ yields a mass value of 4340 ±16 (stat) ± 9 (syst) MeV/c² and a width of 94 ± 32 (stat) ±13 (syst) MeV for the first resonance, and for the second a mass value of 4669 ± 21(stat) ± 3 (syst) MeV/c² and a width of 104 ± 48 (stat) ±10 (syst) MeV. In addition, we show the π⁺ π⁻ mass distributions for these resonant regions.