arXiv subjects

Christoph Hanhart

Publications and source records attributed to Christoph Hanhart.

At least 73 records · Page 4Linked to original sources

arXiv2017

Effects of $Z_b$ states and bottom meson loops on $Υ(4S) \to Υ(1S,2S) π^+π^-$ transitions

We study the dipion transitions $Υ(4S) \rightarrow Υ(nS) π^+π^-$ $(n=1,2)$. In particular, we consider the effects of the two intermediate bottomoniumlike exotic states $Z_b(10610)$ and $Z_b(10650)$ as well as bottom meson loops. The strong pion-pion final-state interactions, especially including channel coupling to $K\bar{K}$ in the $S$-wave, are taken into account model-independently by using dispersion theory. Based on a nonrelativistic effective field theory we find that the contribution from the bottom meson loops is comparable to those from the chiral contact terms and the $Z_b$-exchange terms. For the $Υ(4S) \rightarrow Υ(2S) π^+π^-$ decay, the result shows that including the effects of the $Z_b$-exchange and the bottom meson loops can naturally reproduce the two-hump behavior of the $ππ$ mass spectra. Future angular distribution data are decisive for the identification of different production mechanisms. For the $Υ(4S) \rightarrow Υ(1S) π^+π^-$ decay, we show that there is a narrow dip around 1 GeV in the $ππ$ invariant mass distribution, caused by the final-state interactions. The distribution is clearly different from that in similar transitions from lower $Υ$ states, and needs to be verified by future data with high statistics. Also we predict the decay width and the dikaon mass distribution of the $Υ(4S) \rightarrow Υ(1S) K^+ K^-$ process.

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arXiv2015

The nature of near-threshold XYZ states

We demonstrate that the recently observed $X$, $Y$, $Z$ states cannot be purely from kinematic effect. Especially the narrow near-threshold structures in elastic channels call for nearby poles of the $S$-matrix which are qualified as states. We propose a way to distinguish cusp effects from genuine states and demonstrate that (not all of) the recently observed $X$, $Y$, $Z$ states cannot be purely from kinematic effects. Especially, we show that the narrow near-threshold structures in elastic channels call for nearby poles of the $S$-matrix, since the normal kinematic cusp effect cannot produce that narrow structures in the elastic channels in contrast to genuine $S$-matrix poles. In addition, it is also discussed how spectra can be used to distinguish different scenarios proposed for the structure of those poles, such as hadro-quarkonia, tetraquarks and hadronic molecules. The basic tool employed is heavy quark spin symmetry.

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arXiv2015

Origin of the structures observed in $e^+e^-$ annihilation into multipion states around the $\bar pp$ threshold

We analyze the origin of the structures observed in the reactions $e^+e^-\to 3(π^+π^-)$, $2(π^+π^-π^0)$, $ωπ^+π^-π^0$, and $e^+e^-\to 2(π^+π^-)π^0$ around the antiproton-proton ($\bar pp$) threshold. We calculate the contribution of the two-step process $e^+e^-\to \bar NN \to$ multipions to the total reaction amplitude. The amplitude for $e^+e^-\to \bar NN$ is constrained from near-threshold data on the $e^+e^-\to \bar pp$ cross section and the one for $\bar NN \to$ multipions can be likewise fixed from available experimental information, for all those $5 π$ and $6π$ states. The resulting amplitude for $e^+e^-\to$ multipions turns out to be large enough to play a role for the considered $e^+e^-$ annihilation channels and, in three of the four reactions, even allows us to reproduce the data quantitatively near the $\bar NN$ threshold. The structures seen in the experiments emerges then as a threshold effect due to the opening of the $\bar NN$ channel.

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arXiv2015

Employing spin symmetry to disentangle different models for the XYZ states

In order to test different models proposed for some states discovered recently in the charmonium mass range that do not fit into the pattern predicted by the conventional quark model, we derive predictions for the spectrum within the hadro-charmonium picture, the tetraquark picture as well as the hadronic molecular approach. We exploit heavy quark spin symmetry for the hadro-charmonium and hadronic molecule scenarios. The patterns that emerge from the different models turn out to be quite distinct. For example, only within the hadro-charmonium picture a pseudoscalar state emerges that is lighter than the Y(4260). Possible discovery channels of these additional states are discussed.

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arXiv2015

Could the near-threshold $XYZ$ states be simply kinematic effects?

We demonstrate that the spectacular structures discovered recently in various experiments and named as $X$, $Y$ and $Z$ states cannot be purely kinematic effects. Their existence necessarily calls for nearby poles in the $S$--matrix and they therefore qualify as states. We propose a way to distinguishing kinematic cusp effects from genuine $S$--matrix poles: the kinematic threshold cusp cannot produce a narrow peak in the invariant mass distribution in the elastic channel in contrast with a genuine $S$--matrix pole.

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arXiv2014

$Y(4260)$ as the first $S$-wave open charm vector molecular state?

Since its first observation in 2005, the vector charmonium $Y(4260)$ has turned out to be one of the prime candidates for an exotic state in the charmonium spectrum. It was recently proposed that the $Y(4260)$ should have a prominent $D_1\bar{D}+c.c.$ molecular component that is strongly correlated with the production of the charged $Z_c(3900)$. In this paper we demonstrate that the nontrivial cross section line shapes of $e^+e^-\to J/ψππ$ and $h_cππ$ can be naturally explained by the molecular scenario. As a consequence we find a significantly smaller mass for the $Y(4260)$ than previous studied. In the $e^+e^-\to \bar D D^*π+c.c.$ process, with the inclusion of an additional $S$-wave $\bar D^*π$ contribution constrained from data on the $D\bar D^*$ invariant mass distribution, we obtain a good agreement with the data of the angular distributions. We also predict an unusual line shape of $Y(4260)$ in this channel that may serve as a smoking gun for a predominantly molecular nature of $Y(4260)$. Improved measurements of these observables are therefore crucial for a better understanding of the structure of this famous resonance.

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arXiv2014

Strong and radiative decays of the $D_{s0}^*(2317)$ and $D_{s1}(2460)$

Since their discovery in 2003, the open charm states $D_{s0}^*(2317)$ and $D_{s1}(2460)$ provide a challenge to the conventional quark model. In recent years, theoretical evidence has been accumulated for both states in favor of a predominantly $DK$ and $D^*K$ molecular nature, respectively. However, a direct experimental proof of this hypothesis still needs to be found. Since radiative decays are generally believed to be sensitive to the inner structure of the decaying particles, we study in this work the radiative and strong decays of both the $D_{s0}^*(2317)$ and $D_{s1}(2460)$, as well as of their counterparts in the bottom sector. While the strong decays are indeed strongly enhanced for molecular states, the radiative decays are of similar order of magnitude in different pictures. Thus, the experimental observable that allows one to conclusively quantify the molecular components of the $D_{s0}^*(2317)$ and $D_{s1}(2460)$ is the hadronic width, and not the radiative one, in contradistinction to common belief. We also find that radiative decays of the sibling states in the bottom sector are significantly more frequent than the hadronic ones. Based on this, we identify their most promising discovery channels.

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arXiv2014

B_l4 decays and the extraction of |V_ub|

The Cabibbo-Kobayashi-Maskawa matrix element |V_ub| is not well determined yet. It can be extracted from both inclusive or exclusive decays, like B --> pi (rho) l nu-bar_l. However, the exclusive determination from B --> rho l nu-bar_l, in particular, suffers from a large model dependence. In this paper, we propose to extract |V_ub| from the four-body semileptonic decay B --> pi pi l nu-bar_l, where the form factors for the pion-pion system are treated in dispersion theory. This is a model-independent approach that takes into account the pi-pi rescattering effects, as well as the effect of the rho meson. We demonstrate that both finite-width effects of the rho meson as well as scalar pi-pi contributions can be considered completely in this way.

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arXiv2013

Y(4260): hadronic molecule versus hadro-charmonium interpretation

In this paper we confront both the hadronic molecule and the hadro-charmonium interpretations of the $\y$ with the experimental data currently available. We conclude that the data support the $\y$ being dominantly a $D_1\bar D+c.c.$ hadronic molecule while they challenge the hadro-charmonium interpretation. However, additional data is necessary to allow for stronger conclusions.

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arXiv2013

Probing the nature of $Y(4260)$ and $Z_c(3900)$ in the isospin violating process $Y(4260) \to J/ψηπ^0$

The isospin violation process $Y(4260) \to J/ψηπ^{0}$ is studied assuming that $Y(4260)$ is a $D_{1} \bar{D}+c.c.$ hadronic molecule. In association with the production of the $Z_c(3900)$, which is treated as a $D \bar{D}^{*}+c.c.$ hadronic molecule, this process can help us distinguish their molecular natures from other scenarios, since the incomplete cancellation between the charged and neutral--meson loops, which are prominent in the molecular picture only, produces a peak in the $e^+e^-\to Y(4260)\to J/ψηπ^0$ cross section at the $D_{1} \bar{D}+c.c.$ threshold and a very prominent peak in the $J/ψη$ invariant mass spectrum in between the $D \bar{D}^{*}+c.c.$ thresholds; the latter being much narrower than the corresponding one in the isospin conserving channel, i.e. $J/ψπ^+ π^{-}$. The partial width of $Y(4260)\to J/ψηπ^0$ is about $4 \times 10^{-4}$ of that of $Y(4260)\to J/ψπ^+π^-$. The cross section of $e^+e^-\to Y(4260)\to J/ψηπ^0$ at the $D_{1} \bar{D}+c.c.$ threshold is about $0.05 \ \mathrm{pb}$ which is much larger than that produced by the nearby resonances. These features are the direct consequences of the assumed nature of these two states which might be accessible at the high-statistics experiments such as BESIII and LHCb.

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arXiv2013

Meson loop singularity, exotic states' productions and decays near threshold

We investigate in detail the "triangle singularity" regions of meson loops where the corresponding intermediate mesons are nearly-on-shell and have relatively small momenta, and the two heavy mesons strongly interact with each other in an $S$ wave. This $S$-wave interaction will make such kind of meson loops always enhanced and can explain experimental observations of threshold enhancements. We show that the production of the $Y(4260)$ and the recently observed $Z_c(3900)$ can be related to each other by this mechanism which will allow for a possible understanding of the nature of these threshold enhancements.

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arXiv2013

Supernova Constraints on MeV Dark Sectors from e+ e- Annihilations

Theories with dark forces and dark sectors are of interest for dark matter models. In this paper we find the region in parameter space that is constrained by supernova cooling constraints when the models include dark sector particles with masses around 100 MeV or less. We include only interactions with electrons and positrons. The constraint is important for small mixing parameters.

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arXiv2013

Effective Field Theory calculations of $NN\to NNπ$

In this review we present the recent advances for calculations of the reactions $NN\to NNπ$ using chiral effective field theory. Discussed are the next-to-next-to leading order loop contributions with nucleon and Delta-isobar for near threshold s-wave pion production. Results of recent experimental pion-production data for energies close to the threshold are analyzed. Several particular applications are discussed: (i) it is shown how the measured charge symmetry violating pion-production reaction can be used to extract the strong-interaction contribution to the proton-neutron mass difference; (ii) the role of $NN\to NNπ$ for the extraction of the pion-nucleon scattering lengths from pionic atoms data is illuminated.

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arXiv2013

Decoding the riddle of Y(4260) and $Z_c(3900)$

The observation of $Z_c(3900)$ by the BESIII collaboration in the invariant mass spectrum of $J/ψπ^\pm$ in $e^+e^-\to J/ψπ^+π^-$ at the center of mass 4.260 GeV suggests the existence of a charged $\bar D D^*+D\bar D^*$ molecular state with $I(J^P)=1(1^+)$, which could be an isovector brother of the famous X(3872) and an analogue of $Z_b(10610)$ claimed by the Belle Collaboration. We demonstrate that this observation provides strong evidence that the mysterious Y(4260) is a $\bar{D}D_1(2420) +D\bar{D}_1(2420)$ molecular state. Especially, we show that the decay of this molecule naturally populates low momentum $\bar DD^*$ pairs and leads unavoidably to a cusp at the $\bar DD^*$ threshold. We discuss the signatures that distinguish such a $\bar DD^*$ cusp from the presence of a true resonance.

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arXiv2013

Systematic study of the singularity mechanism in heavy quarkonium decays

We investigate in detail the role of heavy meson loops in the transition from $J^{PC}=1^{--}$ sources to candidates for QCD "exotics", such as $Z_c(3900)$, $Z_b(10610)$ and $Z_b'(10650)$. We demonstrate that, if a vector state strongly couples to a heavy meson pair in an $S$--wave and this system decays to another heavy meson pair (e.g. via pion emission), again in an $S$-wave, the pertinent diagrams get enhanced significantly, if the intermediate states are (near) on--shell and have small relative momenta. In a limited kinematic range this mechanism generates "singularity regions" that lead to the creation of a large number of low energy heavy meson pairs, providing an ideal environment for the formation of hadron-hadron bound states or resonances. For instance, we predict that the signals for $Z_b$ and $Z_b'$ should be a lot stronger in $Υ(6S)$ decays due to this mechanism, if these states are indeed hadron-hadron resonances. The findings of this work should be valuable for deepening our understanding of the nature of the mentioned states.

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arXiv2013

Production of the X(3872) in charmonia radiative decays

We discuss the possibilities of producing the X(3872), which is assumed to be a D \bar D^* bound state, in radiative decays of charmonia. We argue that the ideal energy regions to observe the X(3872) associated with a photon in e^+e^- annihilations are around the Y(4260) mass and around 4.45 GeV, due to the presence of the S-wave D \bar D_1(2420) and D^* \bar D_1(2420) threshold, respectively. Especially, if the Y(4260) is dominantly a D \bar D_1 molecule and the X(3872) a D \bar D^* molecule, the radiative transition strength will be quite large.

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arXiv2013

Confirming the molecular nature of the $Z_b(10610)$ and the $Z_b(10650)$

The decays of the $Z_b(10610)$ and the $Z_b(10650)$ to $Υ(nS)π$, $h_b(mP)π$ and $χ_{bJ}(mP)γ$ ($n=1,2,3$, $m=1,2$ and $J=0,1,2$) are investigated within a nonrelativistic effective field theory. It is argued that, while the decays to $Υ(nS) π$ suffer from potentially large higher order corrections, the $P$-wave transitions of the $Z_b$ states offer the best possibility to confirm the nature of the $Z_b$ states as molecular states and to further study their properties. We give nontrivial and parameter-free predictions for the ratios of various partial widths for final states with $h_b(mP)π$ and $χ_{bJ}(mP)γ$. In addition, the branching fractions for the neutral $Z_b$-states to $χ_{bJ}γ$ are predicted to be of order $10^{-4}$--$10^{-3}$. This provides a fine test of the molecular nature in future high-luminosity experiments.

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arXiv2012

Isospin breaking in pion-deuteron scattering and the pion-nucleon scattering lengths

In recent years, high-accuracy data for pionic hydrogen and deuterium have become the primary source of information on the pion-nucleon scattering lengths. Matching the experimental precision requires, in particular, the study of isospin-breaking corrections both in pion-nucleon and pion-deuteron scattering. We review the mechanisms that lead to the cancellation of potentially enhanced virtual-photon corrections in the pion-deuteron system, and discuss the subtleties regarding the definition of the pion-nucleon scattering lengths in the presence of electromagnetic interactions by comparing to nucleon-nucleon scattering. Based on the pi^{+/-} p channels we find for the virtual-photon-subtracted scattering lengths in the isospin basis a^{1/2}=(170.5 +/- 2.0) x 10^{-3} mpi^{-1} and a^{3/2}=(-86.5 +/- 1.8) x 10^{-3} mpi^{-1}.

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