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Scientific Report 2007-2009
Theoretical physics
T1. The New Hadrons
Ordinary matter is made of bound states of three
quarks (baryons) or of two quarks (mesons). Although
bound states of a larger number of constituents are possible
in the theory describing strong interactions (QCD),
there is hardly any evidence of such states.
Since decades the light scalar mesons are candidate
four-quark states, but their experimental evidence has
been questioned since recently. The situation was basically
stalled until the B-Factories, followed by Tevatron
experiments, started observing, in 2003, states containing
at least an heavy quark anti-quark pair, that did
not have the characteristics of mesons. Systems that
include heavy quark-antiquark pairs (quarkonia) are an
ideal laboratory for probing both the high energy regimes
of QCD, where an expansion in terms of the coupling
constant is possible, and the low energy regimes, where
nonperturbative effects dominate. The detailed level of
understanding of the quarkonia mass spectra is therefore
such that a particle mimicking quarkonium properties,
but not fitting any quarkonium level, is most likely to be
considered to be of a different nature.
The activity of this group, composed of both theorists
and experimentalists proceeded in parallel in a
deeper theoretical understanding and in a reanalysis of
experimental data to have a uniform and global picture
of the observations. In particular, since the observed
states have signatures which indicate the presence of
four quarks, the group concentrated in understanding
whether there is evidence of tetra-quarks: bound states
of a diquark and anti-diquark, a diquark being an agglomerate
of two quarks.
On the theoretical side the work has been concentrated
on two fields: the prediction of the spectra of tetraquark
states with a non-relativistic, quark constituent model
for the calculation of the masses [1,2], and the understanding
of the interaction between diquarks, with particular
attention to the interpretation of the light scalar
mesons [3]. There is also an ongoing discussion on
whether the molecular option, where the four quarks
mostly bind into quark-antiquark pairs, is to be preferred
to the tetraquark one in particular cases [4]. Recent
work of the group has gone in the direction of studying
the production mechanism for the molecular option and
showing its incompatibility with the data.
On the experimental side, a systematic study of the
evidences has been carried out, with particular attention
to states which are claimed as different in different publications
because they are close but not identical in mass.
In the case of states subject to strong interactions and
therefore short-living, there is a significant uncertainty
on their mass and width depending on the assumptions
on the distribution of the invariant mass of the decay
products. Recently we realized that experimental observations
in different final states were attributed to different
exotic states by different research groups although
σ(ψ(2S) π π)(pb)
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Figure 1: Invariant mass distributions and corresponding
likelihood fits as evidence of baryonium, a bound
state of two baryons and that therefore can decay both
in ψ(2S)π + π − (top) and in Λ c ¯Λc (bottom) with strong
preference for the latter, baryonic, final state
they were indeed the same state if studied in a consistent
frame. The most striking case is shown in Fig. 1: our reanalysis
of the Belle data showed that the two states observed
in the ψ(2S)π + π − and Λ c ¯Λc final states were actually
the same and that the ratio of the branching fraction
is BF (Y → Λ c ¯Λc )/BF (Y → ψ(2S)ππ) = 117±44).
This has very important implications because the smoking
gun of tetraquarks is a strong preference for baryonic
decay when kinematically allowed.
The group is now in the process of writing a review
where all the measurements are treated uniformly, work
that will allow indentifying analyses that need to be
performed on existing data and areas where the next
generation of experiments at higher luminosity is needed.
References
1. N.V. Drenska et al., Phys. Rev D79, 077502 (2009).
2. L. Maiani et al., Phys. Rev. Lett 99, 182003 (2007).
3. G. ’t Hooft et al., Phys. Lett. B662, 424 (2008).
4. C. Bignamini et al., Phys. Rev. Lett 103, 162001 (2009).
Authors:
N.V. Drenska, R. Faccini, L. Maiani, A.D. Polosa 1 , V.
Riquer 1 , C. Sabelli, R. Jora 1 , T. Burns 1
Sapienza Università di Roma 24 Dipartimento di Fisica