Investigation of meson properties with the Belle detector

We report on recent results from the Belle detector related to studies of meson properties. They include an observation of new mesons of light quarks in γγ collisions, a discovery of a newD state in theψ family and observations of new states in the bottomonium family.


Introduction
Although B factories were designed to study CP violation in the B B system at Υ(4S ) energy, since already ARGUS and CLEO times it has been known that much richer physics in this and other energy domains was accessible in specific initial states and /or with special methods of analysis, e.g., in γγ → light quark mesons, decays of τ leptons and charm particles, production and decay of narrow Υ etc.
Huge statistics collected by BaBar (∼550 fb −1 ) and Belle (∼1030 fb −1 ) strengthened this understanding and resulted in principally new studies, e.g., γγ → cc, initial-state radiation to q q and cc.The combination of various methods and ideas led to spectacular observations in charmonium and bottomonium systems with many new states found as well as to detailed studies of various mesons of light quarks.
Progress of experiment stimulated theory resulting in many models: tetraquark, hybrid, molecules, hadrocharmonium or, alternatively, effects of close thresholds, coupled channels and rescattering.
Here we report on some selected results in the field of meson studies recently obtained at Belle.
2D angular analysis for various J P (0 + , 0 − , 2 + , 2 − ) reveals a mixture of spin-0 and spin-2 components for all modes.Table 1 lists the positions (masses) of the new states as well as the corresponding cross sections at the peak.
In the charmonium energy range Γ γγ B(R → VV) are measured with improved precision for the η c , χ c0 , χ c2 → φφ, η c → ωω and upper limits for other decays to ωω, ωφ are the first measurements.
4-quark, t-channel factorization, one-pion exchange models fail to explain the position and height of the peaks.

h b (1P) and h b (2P) states
Figure 3 shows the distribution of the missing mass to π + π − .A variety of states with different quantum numbers J P can be seen.
In addition to Υ(1S , 2S , 3S ), we observe 3S → 1S and 2S → 1S transitions, find weak evidence for the Υ(1D) state with 2.4σ significance and discover h b (1P) and h b (2P), for the latter -the first observation.In Table 2 we list the properties of the h b (nP) states.
It is also interesting to determine the value of the hyperfine splitting ∆M HF =< M(n 3 P J ) > −M(n 1 P 1 ), where < M(n 3 P J ) > is the spin-weighted average mass of the P-wave triplet states.Here a triplet n 3 P J − χ bJ (nP) and a singlet Thus, the hyperfine splitting is consistent with zero that can be compared to 0.00 ± 0.15 MeV for the h c (1P).Another interesting observation following from a rather large cross section is that a spin flip of the b quark is not suppressed as could be expected.

Observation of Charged
There are two Z b states at 10610 MeV and 10650 MeV, which both decay into Υ(nS Two Z b states are charged and obviously exotic.
For each Z b state masses, widths, relative amplitudes measured in different decay modes are consistent.Relative phases are swapped for the Υ and h b final states as expected in the molecular model.Averaging the results from different decay modes one obtains the values of mass and width shown in Table 4. Results from five decay modes are also shown in Fig. 4.

η b (1S ) and η b (2S ) states
First claim for the observation of the η b (1S ) came in 2002 from ALEPH that reported an excess of six-and eight-prong events in 200 GeV e + e − collisions [4].A position of the excess corresponded to a mass of 9300 ± 20 ± 20 MeV.However, as follows from the current measurements, most probably this was a fluctuation since its mass is significantly different from the recent values of BaBar [5,6] and CLEO [7] that studied the Υ(2S , 3S ) → η b (1S )γ decays.

01001-p.2
It is tempting to search for h b (nP) → η b (mS )γ with 50k of h b (1P) and 84k of h b (2P) at Belle for which theory predicts sizable branchings [11].Belle did that first with 121.4 fb −1 and observed the η b (1S ) [12], then the analysis of the full data sample of 133.4 fb −1 gave first evidence for the η b (2S ) [13].
In the ideal case all events group in the center, in reality there is finite resolution as well as background/fake π and γ.The horizontal band for ∆M miss (π + π − γ) corresponds to the η b , true γ and background π + π − .The vertical band for M miss (π + π − ) corresponds to h b , true π + π − and background γ.In Tables 5 and 6 we summarize our results on the parameters of the η b (1, 2S ) and h b (1, 2P) states as well as on the branching fractions of the h b radiative transitions to η b obtained from the full data sample at the Υ(10860).

Conclusions
-Huge data samples collected at B factories together with various methods of analysis give access to rare processes in e + e − annihilation, γγ, B and Υ(5S ) decays.-Many new mesons of light and heavy quarks were discovered, some expected and many with surprising or even exotic properties.-Impressive progress is observed in the charmonium family studies, where about 20 new meson states were observed, but only a few fully understood and identified.-In many cases detailed analysis of charmonium-like states is limited by statistics, a breakthrough is expected at SuperB-factories, PANDA and LHC.-Various new states were discovered recently in the bottomonium family: -Theoretical interpretation is very far from final and new interesting experimental observations are coming.
invariant mass of the χ c1 γ system.In addition to the obvious signal from the decay chain B + → ψ(2S )K + , ψ(2S ) → χ c1 γ, a new state can be seen at 3823.5 ± 2.8 MeV with 4.2σ significance.There is no signal at 3872 MeV.

Table 1 .
Positions of the new states and peak cross sections of their production peak , nb 0.27 ± 0.05 0.30 ± 0.04 5.30 ± 0.42

Table 2 .
Properties of the h b states

Table 3 .
Hyperfine splitting and cross sections of h b (nP) production

Table 4 .
Mass and width of the Z b states

Table 5 .
Results on η b and h b states from the full data sample