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.

/pdf/heavy-quarkonium-progress-puzzles-and-opportunities-2tru0l8jyp.pdf

Heavy quarkonium: progress, puzzles, and opportunities

There are experimental evidences for the existence of narrow states Θ
								+ and Θ
								
 c
 with the same quantum numbers of $uudd\bar s$
 and $uudd\bar c$
 pentaquarks and also NK
								(*) and ND
								(*) molecular states. Their masses deviate from many theoretical estimates of the pure pentaquark and molecular states. In this work we study the possibility that the observed Θ
								+ and Θ
								
 c
 are mixtures of pure pentaquark and molecular states. The mixing parameters are in general related to non-perturbative QCD which are not calculable at present. We determine them by fitting data from known states and then generalize the mechanism to Θ
								b to predict its mass and width. The mixing mechanism can also naturally explain the narrow width for Θ
								+ and Θ
								
 c
 through destructive interferences, even if the pure pentaquark and molecular states have much larger decay widths. We also briefly discuss the properties of the partner eigenstates of Θ
								+ and Θ
								
 c
 and the possibility of experimentally observe them. Moreover, probable consequences of multi-state mixing are also addressed.

/pdf/mixing-of-pentaquark-and-molecular-states-1lufk8042h.pdf

Mixing of pentaquark and molecular states

In this work, we investigate the characteristics of the spin-singlet state $\eta_b$ of the bottomonia family via the radiative decays of $\Upsilon(nS)\to \eta_b+\gamma$. The theoretical estimation of the decay widths is carried out in terms of the light-front quark model (LFQM). Recently CLEO and BaBar collaborations have measured $\mathcal{B}(\Upsilon(3S)\to\gamma\eta_b)$ and the mass of ${\eta_b}$. In terms of the data we fix the concerned input parameters in our calculations of $\Upsilon(nS)\to \eta_b+\gamma$. A special attention is paid on the transition of $\Upsilon(5S)\to \eta_b+\gamma$. The BELLE data showed that the width of $\Upsilon(5S)\to \Upsilon(2S,1S)+\pi\pi$ is two orders larger than that of $\Upsilon(4S)\to \Upsilon(2S,1S)+\pi\pi$, thus some theoretical explanations have been proposed. Among them, it is suggested the inelastic final state interaction (IFSI) $\Upsilon(5S)\to B\bar B\to \Upsilon(1S)+\pi\pi$ may be a natural one. If so, a similar mechanism also applies to $\Upsilon(5S)\to B^{(*)}\bar B^{(*)}\to \eta_b+\gamma$, the precise measurement would serve as a good test whether $\Upsilon(5S)$ possess exotic components. Our calculation in the LFQM indicates that the rate of the direct process $\Upsilon(5S)\to\eta_b+\gamma$ is not anomalous compared to $\Upsilon(mS)\to\eta_b+\gamma (m=1,2,3,4)$, thus if the IFSI does apply, the rate of $\Upsilon(5S)\to\eta_b+\gamma$ should be larger than the others by orders.

Study on the radiative decays of $\Upsilon(nS)\to \eta_b+\gamma$

Assuming a heavy quark-light diquark picture, we systematically study the mass spectra and strong decays of $1P$ and $2S$ charm and charm-strange baryons by the nonrelativistic constituent quark models. Most of the existing charm and charm-strange baryons can be well explained as $1P$ and $2S$ states in the diquark picture. As for the well-determined states, including $\Sigma_c(2455)^{0,+,++}$, $\Sigma_c(2520)^{0,+,++}$, $\Xi^\prime_c(2580)^{0,+}$, $\Xi_c(2645)^{0,+}$, $\Lambda_c(2595)^+$, $\Lambda_c(2625)^+$, $\Xi_c(2790)^{0,+}$, and $\Xi_c(2815)^{0,+}$, the theoretical results are in good agreement with the experimental data. $\Sigma_c(2800)^{0,+,++}$ can be assigned to a $\Sigma_{c2}(3/2^-)$ or $\Sigma_{c2}(5/2^-)$ state. We prefer to interpret the signal $\Sigma_c(2850)^0$ as a $2S(1/2^+)$ state although the possibility can not be thoroughly excluded at present that this is the same state as $\Sigma_c(2800)^0$. $\Lambda_c(2765)^+$ (or $\Sigma_c(2765)^+$) could be explained as the $\Lambda_c^+(2S)$ state and $\Sigma^+_{c1}(1/2^-)$ state, respectively. We propose to measure the branching ratio of $\mathcal{B}(\Sigma_c(2455)\pi)/\mathcal{B}(\Sigma_c(2520)\pi)$ in future, which may disentangle the puzzle of this state. Our results support $\Xi_c(2980)^{0,+}$ as the first radial excited state of $\Xi_c(2470)^{0,+}$ with $J^P=1/2^+$. The assignment of $\Xi_c(2930)^0$ is analogous to $\Sigma_c(2800)^{0,+,++}$. Specifically, $\Xi_c(2930)^0$ could be a $1P(3/2^-)$ or $1P(5/2^-)$ $\Xi_c^\prime$ state. Some typical ratios among partial decay widths are provided for experiments to search for these missing states.

Low-excited charm and charm-strange baryons revisited in the quark-diquark picture

Motivated by the observation of two pentaquark-like resonances and the doubly charmed baryon at LHCb, in this work, we systematically investigate the mass spectra of $QQqq\bar{q}$ ($Q=b,c$; $q=u,d,s$) pentaquark systems in a color-magnetic interaction model. One finds that stable or narrow exotic states are possible and their rearrangement decay patterns are shown. Hopefully, the study is helpful to the experimental search for such pentaquark states.

Doubly heavy pentaquarks

The mass gaps existing in the discovered single heavy flavor baryons are analyzed, which show some universal behaviors. Under the framework of a constituent quark model, we quantitatively explain why such interesting phenomenon happens, when these established excited heavy baryons are regarded as the $$\lambda $$
 -mode excitations. Based on the universal behaviors of the discussed mass gaps, we may have three implications including the prediction of the masses of excited $$\Xi _b^0$$
 baryons which are still missing in the experiment. For completeness, we also discuss the mass gaps of these $$\rho $$
 -mode excited single heavy flavor baryons.

/pdf/universal-behavior-of-mass-gaps-existing-in-the-single-heavy-4g3b2bvtxe.pdf

Xiang Liu

Papers

Mixing of pentaquark and molecular states

Study on the radiative decays of $\Upsilon(nS)\to \eta_b+\gamma$

Low-excited charm and charm-strange baryons revisited in the quark-diquark picture

Doubly heavy pentaquarks

Universal behavior of mass gaps existing in the single heavy baryon family