arXiv subjects

C. Hanhart

Publications and source records attributed to C. Hanhart.

At least 37 records · Page 2Linked to original sources

arXiv2019

Remarks on the Heavy-Quark Flavour Symmetry for doubly heavy hadronic molecules

The possibility for a common effective field theory for hadronic molecules with different heavy-quark flavours is examined critically. It is argued that such a theory does not allow one to draw definite conclusions for doubly heavy molecules. In particular, it does not allow one to relate binding energies for the molecules in the c-quark and b-quark sectors with controlled uncertainties. Therefore, while this kind of reasoning does not preclude from employing heavy-quark spin symmetry for charmonium- and bottomonium-like states separately within a well established effective field theory framework, relations between different heavy-quark sectors can only be obtained using phenomenological approaches with uncontrolled uncertainties.

hep-ph↗

arXiv2019

Feasibility Study for an EDM Storage Ring

This project exploits charged particles confined as a storage ring beam (proton, deuteron, possibly $^3$He) to search for an intrinsic electric dipole moment (EDM, $\vec d$) aligned along the particle spin axis. Statistical sensitivities can approach $10^{-29}$~e$\cdot$cm. The challenge will be to reduce systematic errors to similar levels. The ring will be adjusted to preserve the spin polarization, initially parallel to the particle velocity, for times in excess of 15 minutes. Large radial electric fields, acting through the EDM, will rotate the polarization ($\vec d \times\vec E$). The slow rise in the vertical polarization component, detected through scattering from a target, signals the EDM. The project strategy is outlined. It foresees a step-wise plan, starting with ongoing COSY activities that demonstrate technical feasibility. Achievements to date include reduced polarization measurement errors, long horizontal-plane polarization lifetimes, and control of the polarization direction through feedback from the scattering measurements. The project continues with a proof-of-capability measurement (precursor experiment; first direct deuteron EDM measurement), an intermediate prototype ring (proof-of-principle; demonstrator for key technologies), and finally the high precision electric-field storage ring.

physics.acc-ph↗

arXiv2018

The line shapes of the $Z_b(10610)$ and $Z_b(10650)$ in the elastic and inelastic channels revisited

The most recent experimental data for all measured production and decay channels of the bottomonium-like states $Z_b(10610)$ and $Z_b(10650)$ are analysed simultaneously using solutions of the Lippmann-Schwinger equations which respect constraints from unitarity and analyticity. The interaction potential in the open-bottom channels $B^{(*)}\bar{B}^{*}+\mbox{c.c.}$ contains short-range interactions as well as one-pion exchange. It is found that the long-range interaction does not affect the line shapes as long as only $S$ waves are considered. Meanwhile, the line shapes can be visibly modified once $D$ waves, mediated by the strong tensor forces from the pion exchange potentials, are included. However, in the fit they get balanced largely by a momentum dependent contact term that appears to be needed also to render the results for the line shapes independent of the cut-off. The resulting line shapes are found to be insensitive to various higher-order interactions included to verify stability of the results. Both $Z_b$ states are found to be described by the poles located on the unphysical Riemann sheets in the vicinity of the corresponding thresholds. In particular, the $Z_b(10610)$ state is associated with a virtual state residing just below the $B\bar{B}^{*}/\bar B{B}^{*}$ threshold while the $Z_b(10650)$ state most likely is a shallow state located just above the $B^*\bar{B}^{*}$ threshold.

hep-ph↗

arXiv2018

The branching ratio $ω\to π^+π^-$ revisited

We analyze the most recent data for the pion vector form factor in the timelike region, employing a model-independent approach based on dispersion theory. We confirm earlier observations about the inconsistency of different modern high-precision data sets. Excluding the BaBar data, we find an updated value for the isospin-violating branching ratio $\mathcal{B}(ω\to π^+π^-) = (1.46\pm 0.08) \times 10^{-2}$. As a side result, we also extract an improved value for the pion vector or charge radius, $\sqrt{\langle r_V^2\rangle} = 0.6603(5)(4)\text{fm}$, where the first uncertainty is statistical as derived from the fit, while the second estimates the possible size of nonuniversal radiative corrections. In addition, we demonstrate that modern high-quality data for the decay $η' \to π^+π^-γ$ will allow for an even improved determination of the transition strength $ω\toπ^+π^-$.

hep-ph↗

arXiv2018

Importance of d-wave contributions in the charge symmetry breaking reaction $dd \to {}^4\text{He}π^0$

This letter reports a first quantitative analysis of the contribution of higher partial waves in the charge symmetry breaking reaction $dd \to {}^4\text{He}π^0$ using the WASA-at-COSY detector setup at an excess energy of $Q = 60$ MeV. The determined differential cross section can be parametrized as $\text{d}σ/\text{d}Ω= a + b\cos^{2}θ^*$, where $θ^*$ is the production angle of the pion in the center-of-mass coordinate system, and the results for the parameters are $a = \left(1.55 \pm 0.46 (\text{stat}) ^{+0.32}_{-0.8} (\text{syst}) \right)$ pb/sr and $b = \left(13.1 \pm 2.1 (\text{stat}) ^{+1.0}_{-2.7} (\text{syst})\right)$ pb/sr. The data are compatible with vanishing p-waves and a sizable d-wave contribution. This finding should strongly constrain the contribution of the $Δ$ isobar to the $dd \to {}^4\text{He}π^0$ reaction and is, therefore, crucial for a quantitative understanding of quark mass effects in nuclear production reactions.

nucl-ex↗

arXiv2018

Search for $C$ violation in the decay $η\rightarrowπ^0+e^++e^-$ with WASA-at-COSY

We report on the investigation of the rare decay $η\rightarrowπ^0+\text{e}^++\text{e}^-$ which is of interest to study both $C$ violation in the electromagnetic interaction and to search for contributions from physics beyond the Standard Model, since the allowed decay via a two-photon intermediate state is strongly suppressed. The experiment has been performed using the WASA-at-COSY installation, located at the COSY accelerator of the Forschungszentrum Jülich, Germany. In total $3\times10^7$ events of the reaction $\text{p}+\text{d}\rightarrow{^3\text{He}}+η$ have been recorded at an excess energy of $Q = 59.8\,\textrm{MeV}$. Based on this data set the $C$ parity violating decay $η\rightarrowπ^0+γ^*\rightarrowπ^0+\textrm{e}^++\textrm{e}^-$ via a single-photon intermediate state has been searched for, resulting in new upper limits of $Γ\left(η\rightarrowπ^0+\textrm{e}^++\textrm{e}^-\right)/ Γ\left(η\rightarrowπ^++π^-+π^0\right) < 3.28\times10^{-5}$ and $Γ\left(η\rightarrowπ^0+\textrm{e}^++\textrm{e}^-\right)/ Γ\left(η\rightarrow\textrm{all}\right) < 7.5\times10^{-6}\ (\textrm{CL} = 90\,\%)$, respectively.

hep-ex↗

arXiv2018

Total and differential cross sections of $η$-production in proton-deuteron fusion for excess energies between $Q_η=13\;\text{MeV}$ and $Q_η=81\;\text{MeV}$

New data on both total and differential cross sections of the production of $η$ mesons in proton-deuteron fusion to ${}^3\text{He}\,η$ in the excess energy region $13.6\;\text{MeV}\leq Q_η\leq 80.9\;\text{MeV}$ are presented. These data have been obtained with the WASA-at-COSY detector setup located at the Forschungszentrum Jülich, using a proton beam at 15 different beam momenta between $p_p = 1.60\;\text{GeV}/c$ and $p_p = 1.74\;\text{GeV}/c$. While significant structure of the total cross section is observed in the energy region $20\;\text{MeV}\lesssim Q_η\lesssim 60\;\text{MeV}$, a previously reported sharp variation around $Q_η\approx 50\;\text{MeV}$ cannot be confirmed. Angular distributions show the typical forward-peaking that was reported elsewhere. For the first time, it is possible to study the development of these angular distributions with rising excess energy over a large interval.

nucl-ex↗

arXiv2017

Theory Concepts for Heavy Exotic Mesons

Some of the currently most popular conjectures for the structure of the recently discovered heavy mesons that do not find a place in the quark model quarkonium spectrum are sketched. Furthermore, some observables are identified that should allow one to identify the most prominent components of individual states.

hep-ph↗

arXiv2017

Theory of hadronic molecules applied to the XYZ states

In recent years data have been accumulated at various experiments about states in the heavy quarkonium mass range that seem to be inconsistent with the most simple variants of the quark model. In this contribution it is demonstrated that most of those data are consistent with a dominant molecular nature of those states. It is also discussed which kind of observables are sensitive to the molecular component and which are not.

hep-ph↗

arXiv2017

Spin partners of the $Z_b(10610)$ and $Z_b(10650)$ revisited

We study the implications of the heavy-quark spin symmetry for the possible spin partners of the exotic states $Z_b(10610)$ and $Z_b(10650)$ in the spectrum of bottomonium. We formulate and solve numerically the coupled-channel equations for the $Z_b$ states that allow for a dynamical generation of these states as hadronic molecules. The force includes short-range contact terms and the one-pion exchange potential, both treated fully nonperturbatively. The strength of the potential at leading order is fixed completely by the pole positions of the $Z_b$ states such that the mass and the most prominent contributions to the width of the isovector heavy-quark spin partner states $W_{bJ}$ with the quantum numbers $J^{++}$ ($J=0,1,2$) come out as predictions. Since the accuracy of the present experimental data does not allow one to fix the pole positions of the $Z_b$'s reliably enough, we also study the pole trajectories of their spin partner states as functions of the $Z_b$ binding energies. It is shown that, once the heavy-quark spin symmetry is broken by means of the physical $B$ and $B^*$ masses, especially the pion tensor force has a significant impact on the location of the partner states clearly demonstrating the need of a coupled-channel treatment of pion dynamics to understand the spin multiplet pattern of hadronic molecules.

hep-ph↗

arXiv2017

Can X(3915) be the tensor partner of the X(3872)?

It has been proposed recently (Phys. Rev. Lett. 115 (2015), 022001) that the charmoniumlike state named X(3915) and suggested to be a $0^{++}$ scalar, is just the helicity-0 realisation of the $2^{++}$ tensor state $χ_{c2}(3930)$. This scenario would call for a helicity-0 dominance, which were at odds with the properties of a conventional tensor charmonium, but might be compatible with some exotic structure of the $χ_{c2}(3930)$. In this paper, we investigate, if such a scenario is compatible with the assumption that the $χ_{c2}(3930)$ is a $D^*\bar D^*$ molecular state - a spin partner of the $X(3872)$ treated as a shallow bound state. We demonstrate that for a tensor molecule the helicity-0 component vanishes for vanishing binding energy and accordingly for a shallow bound state a helicity-2 dominance would be natural. However, for the $χ_{c2}(3930)$, residing about 100 MeV below the $D^*\bar D^*$ threshold, there is no a priori reason for a helicity-2 dominance and thus the proposal formulated in the above mentioned reference might indeed point at a molecular structure of the tensor state. Nevertheless, we find that the experimental data currently available favour a dominant contribution of the helicity-2 amplitude also in this scenario, if spin symmetry arguments are employed to relate properties of the molecular state to those of the X(3872). We also discuss what research is necessary to further constrain the analysis.

hep-ph↗

arXiv2017

How to employ $\bar{B}^0_d \to J/ψ(πη,\bar{K}K)$ decays to extract information on $πη$ scattering

We demonstrate that dispersion theory allows one to deduce crucial information on $πη$ scattering from the final-state interactions of the light mesons visible in the spectral distributions of the decays $\bar{B}^0_d \to J/ψ(π^0η,K^+K^-,K^0\bar{K}^0)$. Thus high-quality measurements of these differential observables are highly desired. The corresponding rates are predicted to be of the same order of magnitude as those for $\bar{B}^0_d \to J/ψπ^+π^-$ measured recently at LHCb, letting the corresponding measurement appear feasible.

hep-ph↗

arXiv2017

First Model-Independent Measurement of the Spin Triplet $pΛ$ Scattering Length from Final State Interaction in the $\vec{p}p \rightarrow pK^{+}Λ$ Reaction

The $\vec{p}p \rightarrow pK^{+}Λ$ reaction has been measured with the COSY-TOF detector at a beam momentum of $2.7\,\mathrm{GeV}/c$. The polarized proton beam enables the measurement of the beam analyzing power by the asymmetry of the produced kaon ($A_N^{K}$). This observable allows the $pΛ$ spin triplet scattering length to be extracted for the first time model independently from the final-state interaction in the reaction. The obtained value is $a_{t} = (-2.55 ^{+0.72}_{-1.39} {}_{\textrm{stat.}} \pm 0.6_{\textrm{syst.}} \pm 0.3_{\textrm{theo.}})\mathrm{fm}$. This value is compatible with theoretical predictions and results from model-dependent analyses.

nucl-ex↗

arXiv2017

New method for a continuous determination of the spin tune in storage rings and implications for precision experiments

A new method to determine the spin tune is described and tested. In an ideal planar magnetic ring, the spin tune - defined as the number of spin precessions per turn - is given by $ν_s = γG$ (gamma is the Lorentz factor, $G$ the magnetic anomaly). For 970 MeV/c deuterons coherently precessing with a frequency of ~120 kHz in the Cooler Synchrotron COSY, the spin tune is deduced from the up-down asymmetry of deuteron carbon scattering. In a time interval of 2.6 s, the spin tune was determined with a precision of the order $10^{-8}$, and to $1 \cdot 10^{-10}$ for a continuous 100 s accelerator cycle. This renders the presented method a new precision tool for accelerator physics: controlling the spin motion of particles to high precision is mandatory, in particular, for the measurement of electric dipole moments of charged particles in a storage ring.

physics.acc-ph↗

arXiv2016

Remarks on meson loop effects on quark models

We investigate the effect of meson loops on the spectrum of quark states. We demonstrate that in general quark states do not tend to get very broad if their coupling to the continuum increases, but instead they decouple from the latter in the large coupling limit. We ascribe this effect to the essentially nonperturbative unitarization procedure involved. In the meantime, some quark resonances behave very differently and demonstrate collectivity in the sense that their pole trajectories span a wide, as compared to the level spacing, region therefore acquiring contributions from multiple bare poles rather than from the closest neighbors. While the actual calculations are done within particular, very simplified models, it is argued that the findings might well be general.

hep-ph↗

arXiv2016

Heavy-quark spin symmetry partners of the $X(3872)$ revisited

We revisit the consequences of the heavy-quark spin symmetry for the possible spin partners of the $X(3872)$. We confirm that, if the $X(3872)$ were a $D\bar{D}^*$ molecular state with the quantum numbers $J^{PC}=1^{++}$, then in the strict heavy-quark limit there should exist three more hadronic molecules degenerate with the $X(3872)$, with the quantum numbers $0^{++}$, $1^{+-}$, and $2^{++}$ in line with previous results reported in the literature. We demonstrate that this result is robust with respect to the inclusion of the one-pion exchange interaction between the $D$ mesons. However, this is true only if all relevant partial waves as well as particle channels which are coupled via the pion-exchange potential are taken into account. Otherwise, the heavy-quark symmetry is destroyed even in the heavy-quark limit. Finally, we solve the coupled-channel problem in the $2^{++}$ channel with nonperturbative pions beyond the heavy-quark limit and, contrary to the findings of previous calculations with perturbative pions, find for the spin-2 partner of the $X(3872)$ a significant shift of the mass as well as a width of the order of 50 MeV.

hep-ph↗

arXiv2016

A new Parameterization for the Pion Vector Form Factor

A new approach to the parameterization of pion form factors is presented and for illustration applied to the pion vector form factor. It has the correct analytic structure, is at low energies consistent with recent high accuracy analyses of $ππ$ scattering phase shifts and, at high energies, maps smoothly onto the well--known, successful isobar model.

hep-ph↗

arXiv2016

Interplay of quark and meson degrees of freedom in near-threshold states: A practical parametrisation for line shapes

We propose a practical parametrisation for the line shapes of near-threshold states compatible with all requirements of unitarity and analyticity. The coupled-channel system underlying the proposed parametrisation includes bare poles and an arbitrary number of elastic and inelastic channels treated fully nonperturbatively. The resulting formulae are general enough to be used for a simultaneous analysis of the data in all available production and decay channels of the (system of) state(s) under consideration for a quite wide class of reactions. As an example, we fit the experimental data currently available for several decay channels for the charged $Z_b^{(\prime)}$ states in the spectrum of bottomonia and find a good overall description of the data. We find the present data to be consistent with the $Z_b(10610)$ as a virtual state and with the $Z_b(10650)$ as a resonance, both residing very close to the $B\bar{B}^*$ and $B^*\bar{B}^*$ threshold, respectively.

hep-ph↗