Charm physics at BESIII Weidenkaff, P
Journal of physics. Conference series,
01/2019, Volume:
1137, Issue:
1
Journal Article
Peer reviewed
Open access
The study of mesons and baryons which contain at least one charm quark is referred to as open charm physics. It offers the possibility to study up-type quark transitions. Since the quark can not be ...treated in any mass limit, theoretical predictions are difficult and experimental input is crucial. BESIII collected large data samples of e+e− collisions at several charm thresholds. The at-threshold decay topology offers special opportunities to study open charm decays. We present a selection of recent BESIII results: The measurement of the branching fraction D s + → p n ¯ , the form factor of neutral D decays to K−µ+νµ as well as the search for rare decays D → h(h′)e+e−. Recent results on Λ c + decays are presented in 1.
Charm physics at BESIII Weidenkaff, P
Journal of physics. Conference series,
11/2016, Volume:
770, Issue:
1
Journal Article
Peer reviewed
Open access
The study of mesons and baryons which contain at least one charm quark is referred to as open charm physics. It offers the possibility to study up-type quark transitions. Since the c quark can not be ...treated in any mass limit, theoretical predictions are difficult and experimental input is crucial. BESIII collected large data samples of e+e- collisions at several charm thresholds. The at-threshold decay topology offers special opportunities to study open charm decays. We present a selection of recent BESIII results. The D+s decay constant is measured using the leptonic decays to μ+ν and τ+ ν. Using the semi-leptonic decays of D0 and D± to Ke+νe and πe+νe, a measurement of the form factors f+K (q2) and f+π (q2) is performed and furthermore, we show preliminary results of a model independent measurement of the strong phase difference between D0 and D0 in the channel D0 → Ks0π+π- which is an experimental input to the measurement of the CKM angle γ/φ3.
We report precision measurements of hypernuclei $^3_ΛH$ and $^4_ΛH$ lifetimes obtained from Au + Au collisions at $\sqrt{sNN}$ = 3.0 GeV and 7.2 GeV collected by the STAR experiment at the ...Relativistic Heavy Ion Collider, and the first measurement of $^3_ΛH$ and $^4_ΛH$ midrapidity yields in Au + Au collisions at $\sqrt{sNN}$ = 3.0 GeV. $^3_ΛH$ and $^4_ΛH$, being the two simplest bound states composed of hyperons and nucleons, are cornerstones in the field of hypernuclear physics. Their lifetimes are measured to be 221 ± 15 (stat) ± 19 (syst) ps for $^3_ΛH$ and 218 ± 6 (stat) ± 13 (syst) ps for $^4_ΛH$. The pT-integrated yields of $^3_ΛH$ and $^4_ΛH$ are presented in different centrality and rapidity intervals. It is observed that the shape of the rapidity distribution of $4_ΛH$ is different for 0%–10% and 10%–50% centrality collisions. Thermal model calculations, using the canonical ensemble for strangeness, describes the $^3_ΛH$ yield well, while underestimating the $^4_ΛH$ yield. Transport models, combining baryonic mean-field and coalescence (jam) or utilizing dynamical cluster formation via baryonic interactions (phqmd) for light nuclei and hypernuclei production, approximately describe the measured $^3_ΛH$ and $^4_ΛH$ yields. Our measurements provide means to precisely assess our understanding of the fundamental baryonic interactions with strange quarks, which can impact our understanding of more complicated systems involving hyperons, such as the interior of neutron stars or exotic hypernuclei.
We report the beam energy and collision centrality dependence of fifth and sixth order cumulants (C_{5}, C_{6}) and factorial cumulants (κ_{5}, κ_{6}) of net-proton and proton number distributions, ...from center-of-mass energy (sqrts_{NN}) 3 GeV to 200 GeV Au+Au collisions at RHIC. Cumulant ratios of net-proton (taken as proxy for net-baryon) distributions generally follow the hierarchy expected from QCD thermodynamics, except for the case of collisions at 3 GeV. The measured values of C_{6}/C_{2} for 0%-40% centrality collisions show progressively negative trend with decreasing energy, while it is positive for the lowest energy studied. These observed negative signs are consistent with QCD calculations (for baryon chemical potential, μ_{B}≤110 MeV) which contains the crossover transition range. In addition, for energies above 7.7 GeV, the measured proton κ_{n}, within uncertainties, does not support the two-component (Poisson+binomial) shape of proton number distributions that would be expected from a first-order phase transition. Taken in combination, the hyperorder proton number fluctuations suggest that the structure of QCD matter at high baryon density, μ_{B}∼750 MeV at sqrts_{NN}=3 GeV is starkly different from those at vanishing μ_{B}∼24 MeV at sqrts_{NN}=200 GeV and higher collision energies.
The chiral magnetic effect (CME) refers to charge separation along a strong magnetic field due to imbalanced chirality of quarks in local parity and charge-parity violating domains in quantum ...chromodynamics. The experimental measurement of the charge separation is made difficult by the presence of a major background from elliptic azimuthal anisotropy. This background and the CME signal have different sensitivities to the spectator and participant planes, and could thus be determined by measurements with respect to these planes. We report such measurements in Au+Au collisions at a nucleon-nucleon center-of-mass energy of 200 GeV at the Relativistic Heavy-Ion Collider. It is found that the charge separation, with the flow background removed, is consistent with zero in peripheral (large impact parameter) collisions. Some indication of finite CME signals is seen with a significance of 1–3 standard deviations in mid-central (intermediate impact parameter) collisions. Furthermore, significant residual background effects may, however, still be present.
The STAR Collaboration reports measurements of back-to-back azimuthal correlations of di-π0s produced at forward pseudorapidities (2.6<η<4.0) in p+p, p+Al, and p+Au collisions at a center-of-mass ...energy of 200 GeV. We observe a clear suppression of the correlated yields of back-to-back π0 pairs in p+Al and p+Au collisions compared to the p+p data. The observed suppression of back-to-back pairs as a function of transverse momentum suggests nonlinear gluon dynamics arising at high parton densities. Furthermore, the larger suppression found in p+Au relative to p+Al collisions exhibits a dependence of the saturation scale $Q^{2}_{s}$ on the mass number A . A linear scaling of the suppression with A1/3 is observed with a slope of -0.09±0.01.
The polarization of $Λ$ and $\bar{Λ}$ hyperons along the beam direction has been measured relative to the second and third harmonic event planes in isobar Ru + Ru and Zr + Zr collisions at ...$\sqrt{^SNN}$ = 200 GeV. This is the first experimental evidence of the hyperon polarization by the triangular flow originating from the initial density fluctuations. The amplitudes of the sine modulation for the second and third harmonic results are comparable in magnitude, increase from central to peripheral collisions, and show a mild pT dependence. The azimuthal angle dependence of the polarization follows the vorticity pattern expected due to elliptic and triangular anisotropic flow, and qualitatively disagree with most hydrodynamic model calculations based on thermal vorticity and shear induced contributions. The model results based on one of existing implementations of the shear contribution lead to a correct azimuthal angle dependence, but predict centrality and pT dependence that still disagree with experimental measurements. Thus, our results provide stringent constraints on the thermal vorticity and shear-induced contributions to hyperon polarization. Finally, comparison to previous measurements at RHIC and the LHC for the second-order harmonic results shows little dependence on the collision system size and collision energy.
Elliptic flow measurements from two-, four-, and six-particle correlations are used to investigate flow fluctuations in collisions of U+U at sqrts_{NN}=193 GeV, Cu+Au at sqrts_{NN}=200 GeV and ...Au+Au spanning the range sqrts_{NN}=11.5-200 GeV. The measurements show a strong dependence of the flow fluctuations on collision centrality, a modest dependence on system size, and very little if any, dependence on particle species and beam energy. The results, when compared to similar LHC measurements, viscous hydrodynamic calculations, and trento model eccentricities, indicate that initial-state-driven fluctuations predominate the flow fluctuations generated in the collisions studied.
The STAR collaboration reports a measurement of the transverse single-spin asymmetries, AN , for neutral pions produced in polarized proton collisions with protons ( pp ), with aluminum nuclei ( pAl ...) and with gold nuclei ( pAu ) at a nucleon-nucleon center-of-mass energy of 200 GeV. Neutral pions are observed in the forward direction relative to the transversely polarized proton beam, in the pseudorapidity region 2.7<η<3.8 . Results are presented for π0 s observed in the STAR forward meson spectrometer electromagnetic calorimeter in narrow Feynman x ( xF ) and transverse momentum ( pT ) bins, spanning the range 0.17<xF<0.81 and 1.7<pT<6.0 GeV/c . For fixed xF<0.47 , the asymmetries are found to rise with increasing transverse momentum. For larger xF , the asymmetry flattens or falls as pT increases. Parametrizing the ratio r ( A ) ≡AN ( pA ) / AN ( pp ) =AP over the kinematic range, the ratio r ( A ) is found to depend only weakly on A , with ( P ) =-0.027±0.005 . No significant difference in P is observed between the low- pT region, pT<2.5 GeV/c , where gluon saturation effects may play a role, and the high- pT region, pT>2.5 GeV/c . It is further observed that the value of AN is significantly larger for events with a large- pT isolated π0 than for events with a nonisolated π0 accompanied by additional jetlike fragments. The nuclear dependence r ( A ) is similar for isolated and nonisolated π0 events.
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