There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

비만아 부모의 돌봄 경험: q 방법론

A golden age for heavy-quarkonium physics dawned a decade ago, initiated by the confluence of exciting advances in quantum chromodynamics (QCD) and an explosion of related experimental activity. The early years of this period were chronicled in the Quarkonium Working Group (QWG) CERN Yellow Report (YR) in 2004, which presented a comprehensive review of the status of the field at that time and provided specific recommendations for further progress. However, the broad spectrum of subsequent breakthroughs, surprises, and continuing puzzles could only be partially anticipated. Since the release of the YR, the BESII program concluded only to give birth to BESIII; the B-factories and CLEO-c flourished; quarkonium production and polarization measurements at HERA and the Tevatron matured; and heavy-ion collisions at RHIC have opened a window on the deconfinement regime. All these experiments leave legacies of quality, precision, and unsolved mysteries for quarkonium physics, and therefore beg for continuing investigations at BESIII, the LHC, RHIC, FAIR, the Super Flavor and/or Tau-Charm factories, JLab, the ILC, and beyond. The list of newly found conventional states expanded to include h(c)(1P), chi(c2)(2P), B-c(+), and eta(b)(1S). In addition, the unexpected and still-fascinating X(3872) has been joined by more than a dozen other charmonium- and bottomonium-like "XYZ" states that appear to lie outside the quark model. Many of these still need experimental confirmation. The plethora of new states unleashed a flood of theoretical investigations into new forms of matter such as quark-gluon hybrids, mesonic molecules, and tetraquarks. Measurements of the spectroscopy, decays, production, and in-medium behavior of c (c) over bar, b (b) over bar, and b (c) over bar bound states have been shown to validate some theoretical approaches to QCD and highlight lack of quantitative success for others. Lattice QCD has grown from a tool with computational possibilities to an industrial-strength effort now dependent more on insight and innovation than pure computational power. New effective field theories for the description of quarkonium in different regimes have been developed and brought to a high degree of sophistication, thus enabling precise and solid theoretical predictions. Many expected decays and transitions have either been measured with precision or for the first time, but the confusing patterns of decays, both above and below open-flavor thresholds, endure and have deepened. The intriguing details of quarkonium suppression in heavy-ion collisions that have emerged from RHIC have elevated the importance of separating hot- and cold-nuclear-matter effects in quark-gluon plasma studies. This review systematically addresses all these matters and concludes by prioritizing directions for ongoing and future efforts.

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Heavy quarkonium: progress, puzzles, and opportunities

We study the mass splittings of ${Q}_{1}{q}_{2}{\overline{Q}}_{3}{\overline{q}}_{4}$ ($Q=c$, $b$, $q=u$, $d$, $s$) tetraquark states with chromomagnetic interactions between their quark components. Assuming that $X(4140)$ is the lowest ${J}^{PC}={1}^{++}\text{ }\text{ }cs\overline{c}\overline{s}$ tetraquark, we estimate the masses of the other tetraquark states. From the obtained masses and defined measure reflecting effective quark interactions, we find the following assignments for several exotic states: (1) both $X(3860)$ and the newly observed ${Z}_{c}(4100)$ seem to be ${0}^{++}\text{ }\text{ }cn\overline{c}\overline{n}$ tetraquarks; (2) ${Z}_{c}(4200)$ is probably a ${1}^{+\ensuremath{-}}\text{ }\text{ }cn\overline{c}\overline{n}$ tetraquark; (3) ${Z}_{c}(3900)$, $X(3940)$, and $X(4160)$ are unlikely compact tetraquarks; (4) ${Z}_{c}(4020)$ is unlikely a compact tetraquark, but seems the hidden-charm correspondence of ${Z}_{b}(10650)$ with ${J}^{PC}={1}^{+\ensuremath{-}}$; and (5) ${Z}_{c}(4250)$ can be a tetraquark candidate but the quantum numbers cannot be assigned at present. We hope further studies may check the predictions and assignments given here.

Systematic studies of charmonium-, bottomonium-, and B c -like tetraquark states

In this work, we investigate the decay widths and the line shapes of the open-charm radiative and pionic decays of Y(4274) with the $D_{s}\bar{D}_{s0}(2317)$ molecular charmonium assignment. Our calculation indicates that the decay widths of $Y(4274)\to D^{+}_{s}D^{*-}_{s}\gamma$ and $Y(4274)\to D^{+}_{s}D^{-}_{s}\pi^{0}$ can reach up to 0.05 keV and 0.75 keV, respectively. In addition, the result of the line shape of the photon spectrum of $Y(4274)\to D_{s}^{+} {D}_{s}^{*-} \gamma$ shows that there exists a very sharp peak near the large end point of photon energy. The line shape of the pion spectrum of $Y(4274)\to D_{s}^{+} {D}_{s}^{*-} \pi^{0}$ is similar to that of the pion spectrum of $Y(4274)\to D_{s}^{+} {D}_{s}^{*-} \gamma$, where we also find a very sharp peak near the large end point of pion energy. According to our calculation, we suggest further experiments to carry out the search for the open-charm radiative and pionic decays of Y(4274).

The open-charm radiative and pionic decays of molecular charmonium Y(4274)

We study the $P_{cs}(4459)^0$ recently observed by LHCb using the method of QCD sum rules. Our results support its interpretation as the $\bar D^* \Xi_c$ hadronic molecular state of either $J^P=1/2^-$ or $3/2^-$. Within the hadronic molecular picture, the three LHCb experiments observing $P_c$ and $P_{cs}$ states \cite{lhcb,Aaij:2015tga,Aaij:2019vzc} can be well understood as a whole. This strongly supports the existence of hadronic molecules, whose studies can significantly improve our understanding on the construction of the subatomic world. To verify this picture, we propose to further investigate the $P_{cs}(4459)^0$ to examine whether it can be separated into two states, and to search for the $\bar D \Xi_c$ molecular state of $J^P=1/2^-$.

Establishing the first hidden-charm pentaquark with strangeness

We have performed a systematical calculation of the pion ($\ensuremath{\pi}$, $K$, $\ensuremath{\eta}$) and heavy pseudoscalar meson $S$-wave scattering lengths up to $\mathcal{O}({p}^{3})$ in the chiral perturbation theory in the heavy quark symmetry limit. With the three scattering lengths from the lattice simulations as input we estimate the unknown low-energy constants. Then we predict all the other unmeasured scattering lengths. The analytical expressions and predictions may be helpful to future investigations. Especially we note that the $DK$ scattering length is positive. Therefore their interaction is attractive, which helps to lower the mass of the bare charm-strange scalar state in the quark model through the couple-channel effect.

Light pseudoscalar meson and heavy meson scattering lengths

The chiral dynamics of the doubly heavy baryons is solely governed by the light quark. In this paper, we have derived the chiral corrections to the mass of the doubly heavy baryons up to ${\mathrm{N}}^{3}\mathrm{LO}$. The mass splitting of ${\mathrm{\ensuremath{\Xi}}}_{cc}$ and ${\mathrm{\ensuremath{\Omega}}}_{cc}$ at the ${\mathrm{N}}^{2}\mathrm{LO}$ depends on one unknown low energy constant ${c}_{7}$. By fitting the lattice masses of ${\mathrm{\ensuremath{\Xi}}}_{cc}(3520)$, we estimate the mass of ${\mathrm{\ensuremath{\Omega}}}_{cc}$ to be around 3.726 GeV. Moreover, we have also performed a systematical analysis of the chiral corrections to the axial currents and axial charges of the doubly heavy baryons. The chiral structure and analytical expressions will be very useful to the chiral extrapolations of the future lattice QCD simulations of the doubly heavy baryons.

Xiang Liu

Papers

Systematic studies of charmonium-, bottomonium-, and B c -like tetraquark states

The open-charm radiative and pionic decays of molecular charmonium Y(4274)

Establishing the first hidden-charm pentaquark with strangeness

Light pseudoscalar meson and heavy meson scattering lengths

Masses and axial currents of the doubly charmed baryons