Observation of two charged bottomoniumlike resonances in Υ(5S) decays

Evidence of h_{b}(2P)→ϒ(1S)η Decay and Search for h_{b}(1P,2P)→ϒ(1S)π^{0} with the Belle Detector

We report the first evidence for the h_{b}(2P)→ϒ(1S)η transition with a significance of 3.5 standard deviations. The decay branching fraction is measured to be B[h_{b}(2P)→ϒ(1S)η]=(7.1_{-3.2}^{+3.7}±0.8)×10^{-3}, which is noticeably smaller than expected. We also set upper limits on π^{0} transitions of B[h_{b}(2P)→ϒ(1S)π^{0}]<1.8×10^{-3}, and B[h_{b}(1P)→ϒ(1S)π^{0}]<1.8×10^{-3}, at the 90% confidence level. These results are obtained with a 131.4  fb^{-1} data sample collected near the ϒ(5S) resonance with the Belle detector at the KEKB asymmetric-energy e^{+}e^{-} collider.

Observation of a Resonant Structure near the D_{s}^{+}D_{s}^{-} Threshold in the B^{+}→D_{s}^{+}D_{s}^{-}K^{+} Decay

An amplitude analysis of the B^{+}→D_{s}^{+}D_{s}^{-}K^{+} decay is carried out to study for the first time its intermediate resonant contributions, using proton-proton collision data collected with the LHCb detector at center-of-mass energies of 7, 8, and 13 TeV. A near-threshold peaking structure, referred to as X(3960), is observed in the D_{s}^{+}D_{s}^{-} invariant-mass spectrum with significance greater than 12 standard deviations. The mass, width, and the quantum numbers of the structure are measured to be 3956±5±10  MeV, 43±13±8  MeV, and J^{PC}=0^{++}, respectively, where the first uncertainties are statistical and the second systematic. The properties of the new structure are consistent with recent theoretical predictions for a state composed of cc[over ¯]ss[over ¯] quarks. Evidence for an additional structure is found around 4140 MeV in the D_{s}^{+}D_{s}^{-} invariant mass, which might be caused either by a new resonance with the 0^{++} assignment or by a J/ψϕ↔D_{s}^{+}D_{s}^{-} coupled-channel effect.

Compositeness and several applications to exotic hadronic states with heavy quarks

Several methods for studying the nature of a resonance are applied to resonances recently discovered in the bottonomium and charmonium sectors. We employ the effective-range expansion, the saturation of the width and compositeness of a resonance, as well as direct fits to data. The latter stem from generic S -matrix parameterization that account for relevant dynamical features associated to channels that couple strongly in an energy region around the resonance masses, in which their thresholds also lie. We report on results obtained with these methods for the resonances Zb(10610), Zb(10650), Zcs(3985), Zc(3900), X(4020), X(6900), X(6825), and Pcs(4459).

Charmonium and charmoniumlike states at the BESIII experiment

Charmonium is a bound state of a charmed quark and a charmed antiquark, and a charmoniumlike state is a resonant structure that contains a charmed quark and antiquark pair but has properties that are incompatible with a conventional charmonium state. While operating at center-of-mass energies from 2 to 5 GeV, the BESIII experiment can access a wide mass range of charmonium and charmoniumlike states, and has contributed significantly in this field. We review BESIII results involving conventional charmonium states, including the first observation of the M1 transition ψ(2S) → γη c (2S) and the discovery of the ψ2(3823) state; and report on studies of charmoniumlike states, including the discoveries of the Z c (3900) and Z c (4020) tetraquark candidates, the resolution of the fine structure of the Y(4260) state, the discovery of the new production process e + e - → γX(3872) and the uncovering of strong evidence for the commonality among the X(3872), Y(4260) and Z c (3900) states. The prospects for further research at BESIII and proposed future facilities are also presented.

Predicting the D‾s(*)Ds(*) bound states as the partners of X(3872)

In this work, we investigate the SU(3) flavor symmetry, heavy quark spin symmetry and their breaking effects in the di-meson systems. We prove the existence of the [Formula: see text] , [Formula: see text] , and [Formula: see text] bound states as the consequence of two prerequisites in the SU(3) flavor symmetry and heavy quark spin symmetry. The first prerequisite, the X(3872) as the weakly D‾^∗D/D‾D^∗ bound state is supported by its mass and decay branching ratios. The second prerequisite, the existence of the [Formula: see text] bound state is supported by the lattice QCD calculation and the observation of χ_c0(3930) by the LHCb Collaboration. We hope the future experimental analyses can search for these bound states in the B→D_(s)^(*)D‾_(s)^(*)h processes (h denotes the light hadrons). The [Formula: see text] bound state is also expected to be reconstructed in the J/ψϕ final state in the B→J/ψϕK decay.

Explaining the Many Threshold Structures in the Heavy-Quark Hadron Spectrum

Tremendous progress has been made experimentally in the hadron spectrum containing heavy quarks in the last two decades. It is surprising that many resonant structures are around thresholds of a pair of heavy hadrons. There should be a threshold cusp at any S-wave threshold. By constructing a nonrelativistic effective field theory with open channels, we discuss the generalities of threshold behavior, and offer an explanation of the abundance of near-threshold peaks in the heavy quarkonium regime. We show that the threshold cusp can show up as a peak only for channels with attractive interaction, and the width of the cusp is inversely proportional to the reduced mass relevant for the threshold. We argue that there should be threshold structures at any threshold of a pair of heavy-quark and heavy-antiquark hadrons, which have attractive interaction at threshold, in the invariant mass distribution of a heavy quarkonium and light hadrons that couple to that open-flavor hadron pair. The structure becomes more pronounced if there is a near-threshold pole. Predictions of the possible pairs are also given for the ground state heavy hadrons. Precisely measuring the threshold structures will play an important role in revealing the heavy-hadron interactions, and thus understanding the puzzling hidden-charm and hidden-bottom structures.

Dynamical Approach to Decays of XYZ States

We review the existing results on the exotic XYZ states and their decays obtained within the confined covariant quark model. This dynamical approach is based on a non-local Lagrangian of hadrons with quarks, has built-in quark confinement, and is suited well for the description of different multiquark states, including the four quark ones. We focus our analysis on the various decay modes of five exotic states, X ( 3872 ) , Z c ( 3900 ) , Y ( 4260 ) , Z b ( 10610 ) , and Z b ′ ( 10650 ) , aiming to clarify their internal quark structures. By considering mostly branching fractions and decay widths using the molecular-type or the tetraquark-type interpolating currents, conclusions about the nature of these particles are drawn: the molecular structure is favored for Z c ( 3900 ) , Z b ( 10610 ) , and Z b ′ ( 10650 ) and the tetraquark for X ( 3872 ) and Y ( 4260 ) .

Deuteronlike Heavy Dibaryons from Lattice Quantum Chromodynamics

We report the first lattice quantum chromodynamics (QCD) study of deuteronlike (np-like) dibaryons with heavy quark flavors. These include particles with the following dibaryon structures and valence quark contents: Σ_{c}Ξ_{cc}(uucucc), Ω_{c}Ω_{cc}(sscscc), Σ_{b}Ξ_{bb}(uububb), Ω_{b}Ω_{bb}(ssbsbb), and Ω_{ccb}Ω_{cbb}(ccbcbb), and with spin (J) parity (P), J^{P}≡1^{+}. Using a state-of-the art lattice QCD calculation, after controlling relevant systematic errors, we unambiguously find that the ground state masses of dibaryons Ω_{c}Ω_{cc}(sscscc), Ω_{b}Ω_{bb}(ssbsbb), and Ω_{ccb}Ω_{cbb}(ccbcbb) are below their respective two-baryon thresholds, suggesting the presence of bound states that are stable under strong and electromagnetic interactions. We also predict their masses precisely. For dibaryons Σ_{c}Ξ_{cc}(uucucc), and Σ_{b}Ξ_{bb}(uububb), we could not reach to a definitive conclusion about the presence of any bound state due to large systematics associated with these states. We also find that the binding of these dibaryons becomes stronger as they become heavier in mass. This study also opens up the possibility of the existence of many other exotic nuclei, which can be formed through the fusion of heavy baryons, similar to the formation of nuclei of elements in the periodic table.

Emergence of a Complete Heavy-Quark Spin Symmetry Multiplet: Seven Molecular Pentaquarks in Light of the Latest LHCb Analysis

A recent analysis by the LHCb Collaboration suggests the existence of three narrow pentaquarklike states-the P_{c}(4312), P_{c}(4440), and P_{c}(4457)-instead of just one in the previous analysis [the P_{c}(4450)]. The closeness of the P_{c}(4312) to the D[over ¯]Σ_{c} threshold and the P_{c}(4440)  and  P_{c}(4457) to the D[over ¯]^{*}Σ_{c} threshold suggests a molecular interpretation of these resonances. We show that these three pentaquarklike resonances can be naturally accommodated in a contact-range effective field theory description that incorporates heavy-quark spin symmetry. This description leads to the prediction of all the seven possible S-wave heavy antimeson-baryon molecules [that is, there should be four additional molecular pentaquarks in addition to the P_{c}(4312), P_{c}(4440), and P_{c}(4457)], providing the first example of a heavy-quark spin symmetry molecular multiplet that is complete. If this is confirmed, it will not only give us an impressive example of the application of heavy-quark symmetries and effective field theories in hadron physics, it will also uncover a clear and powerful ordering principle for the molecular spectrum, reminiscent of the SU(3)-flavor multiplets to which the light hadron spectrum conforms.

X, Y, Z Search at Belle II

Search for exotics has increased importance since the observation of the X(3872), 13 years ago, announced by the Belle Collaboration. The observation of pentaquark states by LHCb, and the Z-charged states observed at Belle and BES III have raised even more the attention to the field. Presently several states are observed that do not fit potential models, and looking for them in different production mechanisms and search for their decay modes it is important, as well as to do precise measurement of their mass, width, lineshape. We shortly report in this note about the plan in searching for exotics at Belle II at KEK (Tsukuba, Japan), that just ended the Phase-II running period, and show the first re-discovery results using 5 pb−1 integrated luminosity.

XYZ states as hadronic molecules

In the past decade, a lot of new hadrons containing heavy quarks were discovered which do not fit into the scheme provided by the traditional quark models. Such states are known as the XYZ states and they are conventionally referred to as exotic ones. At present, there is no consensus on their nature, and different models and approaches have been suggested to explain their unusual properties. The talk is devoted to a brief overview of the molecule model for such exotic states.

Heavy-quark spin-symmetry partners of Zb(10610) and Zb(10650) molecules

Heavy-quark spin-symmetry (HQSS) partners of the isovector bottomonium like states Zb(10610) and Zb(10650) are predicted within the molecular picture. Treating both Zb’s as shallow bound states, we solve the system of coupled-channel integral equations for the contact plus one-pion exchange (OPE) potentials to predict the location of the partner states with the quantum numbers J++ (J = 0,1,2). In particular, we predict the existence of a narrow tensor 2++ state residing a few MeV below the B∗ B¯∗ threshold. It is emphasised that the tensor part of the OPE potential in combination with HQSS breaking due to the nonvanishing B∗ -B mass splitting has a significant impact on the location of this partner state.

X, Y, Z search at Belle II

The Belle II experiment has successfully concluded Phase-2 of data taking in July 2018, and just started Phase-3, with the complete detector setup. Great perspectives and unique physics cases are enhanced in the Belle II physics program. In the sector of charmonium and spectroscopy, Belle II will investigate several physics processes: ISR physics for the vector states, bottomonium search and the study of the X(3872) line shape are some examples where the contribution of the Belle II experiment will help in better understanding QCD, for which several open issues still remain unsolved. First simulations and data analyses will be shown, with the first available dataset (2018), and the plan for spectroscopy search at Belle II will be discussed.

Exotic molecular states in the decays of vector bottomonia

The most recent experimental data for the decays of the vector bottomonium γ(10860) proceeding through the formation of the states Zb(10610) and Zb(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^*} $ contains short-range interactions as well as the one-pion exchange; both types of the interaction are taken into account fully nonperturbatively. This way, all parameters of the interaction are fixed directly from the data and the pole positions for the Zb’s are determined as a prediction. In particular, both Zb states are found to be described by resonance poles located on the unphysical Riemann sheets in the vicinity of the corresponding thresholds. The heavy quark spin symmetry (HQSS) is employed to predict, in a parameter-free way, the pole positions and the line shapes in the elastic and inelastic channels for the Zbs’ spin partner states WbJ with the quantum numbers J++ (J = 0, 1, 2). Such spin partners can be produced in radiative decays of the vector bottomonium Υ(10860) and are expected to be detected in the Belle-II experiment.

Properties of Zb(10610) and Zb(10650) from an analysis of experimental line shapes

The experimental line shapes available in the B$ \bar {B} $*, B*$ \bar {B} $*, hb(1P)π and hb(2P)π channels are analysed using a theoretical EFT-based framework manifestly consistent with unitarity and analyticity. The line shapes are calculated using a system of coupled channel integral equations with the potential consisting of the one-pion and one-eta meson exchange interactions from the lightest Goldstone boson octet as well as of several contact terms at leading and subleading orders which are adjusted to minimise the overall chi squared. The pole positions of the Zb(10610) and Zb(10650) are extracted for the best fits corresponding to χ2/d.o.f. of the order of one.

XYZ at Belle

Recent results of exotic states with heavy quarks, denoted as XYZ states, are presented. The results include searches for the Y(4260) in B meson decays, a spin partner of Y(4630), and the strange Pentaquark $ P_s^ + $. An addition, the measurements of the absolute branching fraction of X(3872) and search for double Zc production is dicussed.

Precise Predictions of Charmed-Bottom Hadrons from Lattice QCD

We report the ground state masses of hadrons containing at least one charm and one bottom quark using lattice quantum chromodynamics. These include mesons with spin (J) and parity (P), (J^{P}): 0^{-}, 1^{-}, 1^{+}, and 0^{+} and the spin 1/2 and 3/2 baryons. Among these hadrons, only the ground state of 0^{-} is known experimentally, and therefore our predictions provide important information for the experimental discovery of all other hadrons with these quark contents.

Heavy light tetraquarks from Lattice QCD

We present preliminary results from a lattice calculation of tetraquark states in the charm and bottom sector of the type udbb, usbb, udcc and scbb. These calculations are performed on Nf = 2 + 1 + 1 MILC ensembles with lattice spacing of a = 0:12 fm and a = 0:06 fm. A relativistic action with overlap fermions is employed for the light and charm quarks while a non-relativistic action with non-perturbatively improved coefficients is used in the bottom sector. Preliminary results provide a clear indication of presence of energy levels below the relevant thresholds of different tetraquark states in the double bottom sector while a scattering state is observed in the charm sector.

bb̅ud̅ four-quark systems in the Born-Oppenheimer approximation: prospects and challenges

We summarize previous work on b̅b̅ud four-quark systems in the Born-Oppenheimer approximation and discuss first steps towards an extension to the theoretically more challenging bb̅ud̅ system. Strategies to identify a possibly existing bb̅ud̅ bound state are discussed and first numerical results are presented.

Dynamical picture for the exotic XYZ states

We present a dynamical approach for description of the multi-quark states that is based on an effective interaction Lagrangian describing the coupling of hadrons to their constituent quarks. First, we explore the consequences of treating the X(3872) meson as a tetraquark bound state. We calculate the decay widths of the observed channels and conclude that for reasonable values of the size parameter of the X(3872) one finds consistency with the available experimental data. Then we have critically checked the tetraquark picture for the Zc(3900) state by analyzing its strong decays. We found that Zc(3900) has a much more stronger coupling to DD* than to J/Ψπ that is in discord with experiment. As an alternative we have employed a molecular-type four-quark current to describe the decays of the Zc(3900) state as the charged particle in the isotriplet. We found that a molecular-type current gives the values of the above decays in accordance with the experimental observation. By using moleculartype four-quark currents for the recently observed resonances Zb(10610) and Zb(10650), we have calculated their two-body decay rates into a bottomonium state plus a light meson as well as into B-meson pairs.

Exotic and Conventional Bottomonium Physics Prospects at Belle II

The Belle II experiment, being constructed at the KEK laboratory in Japan, represents a substantial upgrade of both the Belle detector and the KEKB accelerator. It is expected that Belle II will collect 50 times more data than existing B-Factory samples beginning in 2019. Belle II is uniquely capable of studying the so-called "XYZ" particles: heavy exotic hadrons consisting of more than three quarks. First discovered by Belle, these now number in the dozens, and represent the emergence of a new category within Quantum Chromodynamics. In this article we summarize the capabilities of Belle II to explore both exotic and conventional bottomonium physics, with a particular focus on the physics reach of the first data, where opportunities exist to make an immediate impact in this area.

Energy Scan of the e^{+}e^{-}→h_{b}(nP)π^{+}π^{-} (n=1, 2) Cross Sections and Evidence for ϒ(11020) Decays into Charged Bottomoniumlike States

Using data collected with the Belle detector at the KEKB asymmetric-energy e^{+}e^{-} collider, we measure the energy dependence of the e^{+}e^{-}→h_{b}(nP)π^{+}π^{-} (n=1, 2) cross sections from thresholds up to 11.02 GeV. We find clear ϒ(10860) and ϒ(11020) peaks with little or no continuum contribution. We study the resonant substructure of the ϒ(11020)→h_{b}(nP)π^{+}π^{-} transitions and find evidence that they proceed entirely via the intermediate isovector states Z_{b}(10610) and Z_{b}(10650). The relative fraction of these states is loosely constrained by the current data: The hypothesis that only Z_{b}(10610) is produced is excluded at the level of 3.3 standard deviations, while the hypothesis that only Z_{b}(10650) is produced is not excluded at a significant level.

Evidence for a B_{s}^{0}π^{±} State

We report evidence for a narrow structure, X(5568), in the decay sequence X(5568)→B_{s}^{0}π^{±}, B_{s}^{0}→J/ψϕ, J/ψ→μ^{+}μ^{-}, ϕ→K^{+}K^{-}. This is evidence for the first instance of a hadronic state with valence quarks of four different flavors. The mass and natural width of this state are measured to be m=5567.8±2.9(stat)_{-1.9}^{+0.9}(syst)  MeV/c^{2} and Γ=21.9±6.4(stat)_{-2.5}^{+5.0}(syst)  MeV/c^{2}. If the decay is X(5568)→B_{s}^{*}π^{±}→B_{s}^{0}γπ^{±} with an unseen γ, m(X(5568)) will be shifted up by m(B_{s}^{*})-m(B_{s}^{0})∼49  MeV/c^{2}. This measurement is based on 10.4  fb^{-1} of pp[over ¯] collision data at sqrt[s]=1.96  TeV collected by the D0 experiment at the Fermilab Tevatron collider.

Searching for the rules that govern hadron construction

Just as quantum electrodynamics describes how electrons are bound in atoms by the electromagnetic force, mediated by the exchange of photons, quantum chromodynamics (QCD) describes how quarks are bound inside hadrons by the strong force, mediated by the exchange of gluons. QCD seems to allow hadrons constructed from increasingly many quarks to exist, just as atoms with increasing numbers of electrons exist, yet such complex constructions seemed, until recently, not to be present in nature. Here we describe advances in the spectroscopy of mesons that are refining our understanding of the rules for predicting hadron structure from QCD.

Observation of Z_{b}(10610) and Z_{b}(10650) Decaying to B Mesons

We report the analysis of the three-body e^{+}e^{-}→BB[over ¯]π^{±}, BB[over ¯]^{*}π^{±}, and B^{*}B[over ¯]^{*}π^{±} processes, including the first observations of the Z_{b}^{±}(10610)→[BB[over ¯]^{*}+c.c.]^{±} and Z_{b}^{±}(10650)→[B^{*}B[over ¯]^{*}]^{±} transitions that are found to dominate the corresponding final states. We measure Born cross sections for the three-body production of σ(e^{+}e^{-}→[BB[over ¯]^{*}+c.c.]^{±}π^{∓})=[17.4±1.6(stat)±1.9(syst)]  pb and σ(e^{+}e^{-}→[B^{*}B[over ¯]^{*}]^{±}π^{∓})=[8.75±1.15(stat)±1.04(syst)]  pb and set a 90% C.L. upper limit of σ(e^{+}e^{-}→[BB[over ¯]]^{±}π^{∓})<2.9  pb. The results are based on a 121.4  fb^{-1} data sample collected with the Belle detector at a center-of-mass energy near the ϒ(10860) peak.

Practical Parametrization for Line Shapes of Near-Threshold States

Numerous quarkonium(like) states lying near S-wave thresholds are observed experimentally. We propose a self-consistent approach to these near-threshold states compatible with unitarity and analyticity. The underlying coupled-channel system includes a bare pole and an arbitrary number of elastic and inelastic channels treated fully nonperturbatively. The resulting analytical parametrization is ideally suited for a combined analysis of the data available in various channels that is exemplified by an excellent overall description of the data for the charged Z(b)(10610) and Z(b)(10650) states.

First Observation of the Hadronic Transition ϒ(4S)→ηh(b)(1P) and New Measurement of the h(b)(1P) and η(b)(1S) Parameters

Using a sample of 771.6×10(6) ϒϒ(4S) decays collected by the Belle experiment at the KEKB e(+)e(-) collider, we observe, for the first time, the transition ϒ(4S)→ηh(b)(1P) with the branching fraction B[ϒ(4S)→ηh(b)(1P)]=(2.18±0.11±0.18)×10(-3) and we measure the h(b)(1P) mass M(h(b)(1P))=(9899.3±0.4±1.0)  MeV/c(2), corresponding to the hyperfine (HF) splitting ΔM(HF)(1P)=(0.6±0.4±1.0)  MeV/c(2). Using the transition h(b)(1P)→γη(b)(1S), we measure the η(b)(1S) mass M(η(b)(1S))=(9400.7±1.7±1.6)  MeV/c(2), corresponding to ΔM(HF)(1S)=(59.6±1.7±1.6)  MeV/c(2), the η(b)(1S) width Γ(η(b)(1S))=(8(-5)(+6)±5)  MeV/c(2) and the branching fraction B[h(b)(1P)→γη(b)(1S)]=(56±8±4)%.

New Exotic Meson and Baryon Resonances from Doubly Heavy Hadronic Molecules

We predict several new exotic doubly heavy hadronic resonances, inferring from the observed exotic bottomoniumlike and charmoniumlike narrow states X(3872), Z_{b}(10610), Z_{b}(10650), Z_{c}(3900), and Z_{c}(4020/4025). We interpret the binding mechanism as mostly molecularlike isospin-exchange attraction between two heavy-light mesons in a relative S-wave state. We then generalize it to other systems containing two heavy hadrons which can couple through isospin exchange. The new predicted states include resonances in meson-meson, meson-baryon, baryon-baryon, and baryon-antibaryon channels. These include those giving rise to final states involving a heavy quark Q=c,b and antiquark Q[over ¯]^{'}=c[over ¯],b[over ¯], namely, DD[over ¯]^{*}, D^{*}D[over ¯]^{*}, D^{*}B^{*}, B[over ¯]B^{*}, B[over ¯]^{*}B^{*}, Σ_{c}D[over ¯]^{*}, Σ_{c}B^{*}, Σ_{b}D[over ¯]^{*}, Σ_{b}B^{*}, Σ_{c}Σ[over ¯]_{c}, Σ_{c}Λ[over ¯]_{c}, Σ_{c}Λ[over ¯]_{b}, Σ_{b}Σ[over ¯]_{b}, Σ_{b}Λ[over ¯]_{b}, and Σ_{b}Λ[over ¯]_{c}, as well as corresponding S-wave states giving rise to QQ^{'} or Q[over ¯]Q[over ¯]^{'}.

Observation of Z_{c}(3900)^{0} in e^{+}e^{-}→π^{0}π^{0}J/ψ

Using a data sample collected with the BESIII detector operating at the BEPCII storage ring, we observe a new neutral state Z_{c}(3900)^{0} with a significance of 10.4σ. The mass and width are measured to be 3894.8±2.3±3.2  MeV/c^{2} and 29.6±8.2±8.2  MeV, respectively, where the first error is statistical and the second systematic. The Born cross section for e^{+}e^{-}→π^{0}π^{0}J/ψ and the fraction of it attributable to π^{0}Z_{c}(3900)^{0}→π^{0}π^{0}J/ψ in the range E_{c.m.}=4.19-4.42  GeV are also determined. We interpret this state as the neutral partner of the four-quark candidate Z_{c}(3900)^{±}.

Observation of a New Narrow Axial-Vector Meson a1(1420)

The COMPASS Collaboration at CERN has measured diffractive dissociation of 190  GeV/c pions into the π(-)π(-)π(+) final state using a stationary hydrogen target. A partial-wave analysis (PWA) was performed in bins of 3π mass and four-momentum transfer using the isobar model and the so far largest PWA model consisting of 88 waves. A narrow peak is observed in the f0(980)π channel with spin, parity and C-parity quantum numbers J(PC)=1(++). We present a resonance-model study of a subset of the spin-density matrix selecting 3π states with J(PC)=2(++) and 4(++) decaying into ρ(770)π and with J(PC)=1(++) decaying into f0(980)π. We identify a new a1 meson with mass (1414(-13)(+15))  MeV/c2 and width (153(-23)(+8))  MeV/c2. Within the final states investigated in our analysis, we observe the new a1(1420) decaying only into f0(980)π, suggesting its exotic nature.