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Register a new userTriangle singularities in ${J/psirightarrowetapi^0phi}$ and ${pi^0pi^0phi}$
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The BESIII Collaboration recently reported the observation of the $a_0(980)^0-f_0(980)$ mixing in the isospin breaking decay $J/psito etapi^0phi$. In the Dalitz plot for that decay with the $eta$ reconstructed from two photons, there is a band around $1.4$~GeV on the $pi^0 phi$ distribution. In general, this peak can be due to a resonance or a kinematic effect. In this paper, we study the effects of a set of $K^*Kbar K$ triangle diagrams, and show that due to triangle singularities such diagrams can lead to a peak around 1.4~GeV in the $pi^0phi$ invariant mass distribution. The Dalitz plot induced by such a mechanism has a feature consistent with the BESIII observation, namely events along the band accumulate at both ends close to the Dalitz plot boundary. The effect of the same mechanism on the $J/psito pi^0pi^0phi$ and $J/psi to etapi^0 K^+K^-$ decays are also investigated. We suggest to take more data for the $J/psito etapi^0phito etapi^0 K^+ K^-$ and check whether the structure around $1.4$~GeV persists for the $K^+K^-$ invariant mass away from the $phi$ mass region. This is crucial for understanding whether the band is due to triangle singularities or due to a resonance. Were it the latter, the band should remain while it would not if it is due to the former.
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First observation of the decay Bs -> D0bar phi is reported using pp collision data, corresponding to an integrated luminosity of 1.0 fb^-1, collected by the LHCb experiment at a centre-of-mass energy of 7 TeV. The significance of the signal is 6.2 standard deviations. The branching fraction is measured relative to that of the decay Bs -> D0bar K*0bar to be 0.069 +/- 0.013 (stat) +/- 0.007 (syst). The first measurement of the ratio of branching fractions for the decays Bs -> D0bar K*0bar and Bd->D0bar K*0 is found to be 7.8 +/- 0.7 (stat) +/- 0.3 (syst) +/- 0.6 (f_s/f_d), where the last uncertainty is due to the ratio of the Bs and Bd fragmentation fractions.
We have recently observed that hadron triangle singularities, that can mock new exotic hadrons, can be significanttly suppressed in relativistic heavy ion collisions, provided two conditions are met: these are, first, that the fireball lives long enough so that the triangle process has enough time to complete in the Norton-Coleman classical sense, and second, that the mass and/or width of the particles in the triangle diagram are sufficiently modified from their vacuum values. Here we add a very interesting example to the canon, which is $Y(4260) to D_1 D to pi D^* D to pi + J/psi pi$. This reaction has been proposed as a mechanism to explain the appearance of $Z_c(3900)$ in the $J/psi pi$ spectrum. If the two muons and two pions reconstructing the initial-state $Y$ can be isolated from the combinatorial background, then the mechanism can provide a spectroscopy test: presence of $Y(4260)$ but absence of $Z_c(3900)$ would be more indicative of such triangle mechanism, while presence of both would rather point out to $Z_c$ being an exotic hadron.
Using a data sample of $448.1times10^6$ $psi(3686)$ events collected with the BESIII detector operating at the BEPCII, we perform search for the hadronic transition $h_crightarrowpi^+pi^-J/psi$ via $psi(3686)rightarrowpi^0h_c$. No signals of the transition are observed, and the upper limit on the product branching fraction $mathcal{B}(psi(3686)rightarrowpi^0h_c)mathcal{B}(h_crightarrowpi^+pi^-J/psi)$ at the 90% confidence level is determined to be $2.0times10^{-6}$. This is the most stringent upper limit to date.
We analyze possible singularities in the $J/psi Lambda$ invariant mass distribution of the $Xi^-_{b}~to~K^- J/psi Lambda$ process via triangle loop diagrams. Triangle singularities in the physical region are found in 18 different triangle loop diagrams. Among those with $Xi^*$-charmonium-$Lambda$ intermediate states, the one from the $chi_{c1} Xi(2120) Lambda$ loop, which is located around 4628 MeV, is found the most likely to cause observable effects. One needs $S$- and $P$-waves in $chi_{c1} Lambda$ and $J/psi Lambda$ systems, respectively, when the quantum numbers of these systems are $1/2^+$ or $3/2^+$. When the quantum numbers of the $Xi(2120)$ are $J^P=1/2^+$, $1/2^-$ or $3/2^+$, the peak structure should be sharper than the other $J^P$ choices. This suggests that although the whole strength is unknown, we should pay attention to the contributions from the $Xi^*$-charmonium-$Lambda$ triangle diagram if structures are observed in the $J/psi Lambda$ invariant mass spectrum experimentally. In addition, a few triangle diagrams with the $D_{s1}^*(2700)$ as one of the intermediate particles can also produce singularities in the $J/psiLambda$ distribution, but at higher energies above 4.9 GeV.
The spectrum of hadrons is the manifestation of color confinement of quantum chromodynamics. Hadronic resonances correspond to poles of the S-matrix. Since 2003, lots of new hadron resonant structures were discovered in the mass regions from light mesons to hadrons containing a pair of a heavy quark and an antiquark. Many of them are candidates of exotic hadrons, and they are usually observed as peaks in invariant mass distributions. However, the S-matrix also has kinematical singularities due to the on-shellness of intermediate particles for a process, such as two-body thresholds and triangle singularities, and they can produce peaks as well. On the one hand, such singularities may be misidentified as resonances; on the other hand, they can be used as tools for precision measurements. In this paper, we review the threshold cusps and various triangle singularities in hadronic reactions, paying attention to their manifestations in phenomena related to exotic hadron candidates.
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