We show that the recently observed suppression of the yield ratio of deuteron to proton and of helium-3 to proton in p+p collisions compared to those in p+Pb or Pb+Pb collisions by the ALICE ...Collaboration at the Large Hadron Collider (LHC) can be explained if light nuclei are produced from the coalescence of nucleons at the kinetic freeze-out of these collisions. This suppression is attributed to the non-negligible sizes of deuteron and helium-3 compared to the size of the nucleon emission source in collisions of small systems, which reduces the overlap of their internal wave functions with those of nucleons. The same model is also used to study the production of triton and hypertriton in heavy-ion collisions at the LHC. Compared to helium-3 in events of low charged particle multiplicity, the triton is less suppressed due to its smaller size and the hypertriton is even more suppressed as a result of its much larger size.
Based on the coalescence model for light nuclei production, we show that the yield ratio Op-d-t=NH3Np/Nd2 of p, d, and 3H in heavy-ion collisions is sensitive to the neutron relative density ...fluctuation Δn=〈(δn)2〉/〈n〉2 at kinetic freeze-out. From recent experimental data in central Pb+Pb collisions at sNN=6.3 GeV, 7.6 GeV, 8.8 GeV, 12.3 GeV and 17.3 GeV measured by the NA49 Collaboration at the CERN Super Proton Synchrotron (SPS), we find a possible non-monotonic behavior of Δn as a function of the collision energy with a peak at sNN=8.8 GeV, indicating that the density fluctuations become the largest in collisions at this energy. With the known chemical freeze-out conditions determined from the statistical model fit to experimental data, we obtain a chemical freeze-out temperature of ∼144 MeV and baryon chemical potential of ∼385 MeV at this collision energy, which are close to the critical endpoint in the QCD phase diagram predicted by various theoretical studies. Our results thus suggest the potential usefulness of the yield ratio of light nuclei in relativistic heavy-ion collisions as a direct probe of the large density fluctuations associated with the QCD critical phenomena.
The reduction of carbon dioxide (CO2) has been considered as an approach to mitigate global warming and to provide renewable carbon‐based fuels. Rational design of efficient, selective, and ...inexpensive catalysts with low overpotentials is urgently desired. In this study, four cobalt(II) tripodal complexes are tested as catalysts for CO2 reduction to CO in a MeCN/H2O (4:1 v/v) solution. The replacement of pyridyl groups in the ligands with less basic quinolinyl groups greatly reduces the required overpotential for CO2‐to‐CO conversion down to 200–380 mV. Benefitting from the low overpotentials, a photocatalyst system for CO2‐to‐CO conversion is successfully constructed, with an maximum turnover number (TON) of 10 650±750, a turnover frequency (TOF) of 1150±80 h−1, and almost 100 % selectivity to CO. These outstanding catalytic performances are further elucidated by DFT calculations.
Electric/light orchestration: Four cobalt complexes with tripodal ligands are utilized as high‐performance molecular electro‐ and photocatalysts for the reduction of CO2 to CO in a water‐containing system. By the introduction of less basic aromatic nitrogen donors in the tripodal ligands, the overpotentials can be reduced down to record low values of 200–380 mV, leading to high efficiency and selectivity for photocatalytic reduction of CO2 to CO.
It is generally believed that the quark-hadron transition at small values of baryon chemical potentials μB is a crossover but changes to a first-order phase transition with an associated critical ...endpoint (CEP) as μB increases. Such a μB-dependent quark-hadron transition is expected to result in a double-peak structure in the collision energy dependence of the baryon density fluctuation in heavy-ion collisions with one at lower energy due to the spinodal instability during the first-order phase transition and another at higher energy due to the critical fluctuations in the vicinity of the CEP. By analyzing the data on the p, d and 3H yields in central heavy-ion collisions within the coalescence model for light nuclei production, we find that the relative neutron density fluctuation Δρn=〈(δρn)2〉/〈ρn〉2 at kinetic freeze-out indeed displays a clear peak at sNN=8.8GeV and a possible strong re-enhancement at sNN=4.86GeV. Our findings thus provide a strong support for the existence of a first-order phase transition at large μB and its critical endpoint at a smaller μB in the temperature versus baryon chemical potential plane of the QCD phase diagram.
Using the nucleon coalescence model based on kinetic freeze-out nucleons from the hybrid model of MUSIC hydrodynamics and UrQMD hadronic transport model, we study the production of antideuteron and ...its event-by-event fluctuation in Pb+Pb collisions at sNN=5.02 TeV. We find a clear suppression of the antideuteron to antiproton yield ratio in peripheral collisions, which is in accordance with the measurements from the ALICE Collaboration. Also found is a Poissonian event-by-event fluctuation of the antideuteron multiplicity in all collision centralities, which is different from the prediction of a simple coalescence model calculation which assumes that all antiproton and antineutron pairs have the same probability to form an antideuteron. However, due to the lack of baryon number conservation at the particlization when the viscous fluid in the hydrodynamic stage of MUSIC is converted to a hadronic matter, our model fails to describe the net-proton number fluctuation and the negative correlation between the antiproton and antideuteron multiplicities at large pseudorapidity acceptance observed in ALICE experiments. Both can nevertheless be well reproduced after selecting a suitable subset of events to fit the measured net-proton number fluctuation.
Using the nucleon coalescence model, which can naturally take into account the correlations in the nucleon density distribution, we study the effects of QCD critical point on light nuclei production ...in relativistic heavy-ion collisions. We find that the yield ratio NtNp/Nd2 of proton (p), deuteron (d) and triton (t) increases monotonically with the nucleon density correlation length, which is expected to increase significantly near the critical point in the QCD phase diagram. Our study thus demonstrates that the yield ratio NtNp/Nd2 can be used as a sensitive probe of the QCD critical phenomenon. We further discuss the relation between the QCD phase transitions in heavy-ion collisions and the possible non-monotonic behavior of NtNp/Nd2 in its collision energy dependence.
Using the nucleon coalescence model based on kinetic freeze-out nucleons from the 3D MUSIC+UrQMD and the 2D VISHNU hybrid model with a crossover equation of state, we study the multiplicity ...dependence of deuteron (d) and triton (t) production from central to peripheral Au+Au collisions at sNN= 7.7, 14.5, 19.6, 27, 39, 62.4 and 200 GeV and Pb+Pb at sNN=2.76 TeV, respectively. It is found that the ratio NtNp/Nd2 of the proton yield Np, deuteron yield Nd and triton yield Nt exhibits a scaling behavior in its multiplicity dependence, i.e., decreasing monotonically with increasing charged-particle multiplicity. A similar multiplicity scaling of this ratio is also found in the nucleon coalescence calculation based on kinetic freeze-out nucleons from a multiphase transport (AMPT) model. The scaling behavior of NtNp/Nd2 can be naturally explained by the interplay between the sizes of light nuclei and the nucleon emission source. We further argue that the multiplicity scaling of NtNp/Nd2 can be used to validate the production mechanism of light nuclei, and the collision energy dependence of this yield ratio can further serve as a baseline in the search for the QCD critical point in relativistic heavy-ion collisions.
There is a demand to develop molecular catalysts promoting the hydrogen evolution reaction (HER) with a high catalytic rate and a high tolerance to various inhibitors, such as CO and O2. Herein we ...report a cobalt catalyst with a penta‐dentate macrocyclic ligand (1‐Co), which exhibits a fast catalytic rate (TOF=2210 s−1) in aqueous pH 7.0 phosphate buffer solution, in which proton transfer from a dihydrogen phosphate anion (H2PO4−) plays a key role in catalytic enhancement. The electrocatalyst exhibits a high tolerance to inhibitors, displaying over 90 % retention of its activity under either CO or air atmosphere. Its high tolerance to CO is concluded to arise from the kinetically labile character of undesirable CO‐bound species due to the geometrical frustration posed by the ligand, which prevents an ideal trigonal bipyramid being established.
Highly tolerant: A cobalt(II) complex of a pentadentate ligand can efficiently electrocatalyze hydrogen evolution in an aqueous phosphate buffer at pH 7.0, affording a fast catalytic rate of 2210 s−1. This complex has a high tolerance to both CO and air, displaying over 90 % retention of its catalytic activity under CO‐ and air‐saturated conditions.
Light nuclei production is sensitive to the baryon density fluctuations and can be used to probe the QCD phase transition in relativistic heavy-ion collisions. In this work, we studied the production ...of proton, deuteron, triton in central Au+Au collisions at sNN = 5, 7.7, 11.5, 14.5, 19.6, 27, 39, 54.4, 62.4 and 200 GeV from a transport model (JAM). Based on the coalescence production of light nuclei, we calculated the energy dependence of rapidity density dN/dy and particle ratios (d/p, t/p, and t/d). More importantly, the yield ratio Nt×Np/Nd2, which is sensitive to the neutron density fluctuations, shows a flat energy dependence and cannot describe the non-monotonic trend observed by the STAR experiment. Based on the nucleon coalescence, this work can provide constraint and reference to search for the QCD critical point and/or first order phase transition with light nuclei production in future heavy-ion collision experiments.
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