New Interpretation of the ABC Effect in Two-Pion Production in NN collisions

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New Interpretation of the ABC Effect in Two-Pion
Production in NN collisions

• Maria Platonova
• Lomonosov Moscow State University

The 22nd European Conference on Few-Body
Problems in Physics 913 September 2013, Cracow,
Poland
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What is ABC Effect?

• In 1960 A. Abashian, N.E. Booth K.M. Crowe PRL
  5, 258 (1960) 7, 35 (1961) reported an
  observation of a strange enhancement in pd
  fusion to 3He, located near the 2p threshold.

• Later on the similar enhancements were observed
  in the reactions np ? dX and dd ? 4HeX.

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Previous Interpretations of the ABC Effect

1. Resonance formation in pp FSI (K.M. Watson
   A.B.C, 1961)

• Strong pp final state attraction in the
  scalar-isoscalar channel. Possibility of a 0(0)
  resonance (s-meson) formation. Good fit of ABC
  data with pp scattering length as0 23 mp-1.
  However
• Actual pp scattering length as0 0.2 mp-1
• No ABC effect in free pp scattering and pN?ppN
  reaction.
• Prediction of a low-Mpp enhancement due to
  parallel decays of two ?s. Qualitative
  description of some inclusive data.
• Contradicted by recent exclusive experiments!
• We must look for some new
  interpretation.

1. The t-channel ?? model (T. Risser M.D. Shuster,
   1973)

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New Exclusive Experiments of the CELSIUS-WASA
and WASA_at_COSY Collaborations
The first exclusive high-statistics experiments
in full 4p-geometry

• The experimental data clearly show the formation
  of isoscalar dibaryon
• resonance D03 with parameters
• and distinct correlation
• between the np resonance
• and ABC enhancement

ABC peak
P. Adlarson et al.,  PRL 106, 242302, 2011
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The s-channel Resonance Ansatz

• Interpretation of the new experimental results in
  terms of a ?-? deeply bound state (M. Bashkanov
  et al.) a very soft form factor in D03???
  vertex is needed to reproduce the ABC
  enhancement.

• Such a low value of ? means that D03 is a
  deuteron-like object.
• This is incompatible with the observed
  large ?-? binding in the D03 state eB(D03) 90
  MeV.
• The value of ? should be 2 times softer to
  reproduce ABC peak in dd ? 4He(pp)0 with the
  same model.
• Microscopic quark model calculations predict a
  radius for the 0(3) ?-? bound state r(D03)
  0.70.9 fm, i.e., of the order of the nucleon
  size.
• The D03 resonance appears to be the truly
  dibaryon state in which the quark cores of two
  ?s are almost fully overlapped with each other.

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First Prediction of Dibaryon States

• By using SU(6)-symmetry, Dyson and Xuong
  predicted six zero-strangeness low-lying
  dibaryons F.J. Dyson and N.-H. Xuong, PRL 13,
  815 (1964)

• From the simple SU(6) mass formula M
  ABT(T1)J(J1)-2,
• A being the deuteron mass and B 47 MeV,
  the masses of N? and ?? S-wave resonances were
  predicted to be
• M(D12) 2160 MeV
• M(D03) 2350 MeV.
• The deuteron D01 was positioned as NN S-wave
  dibaryon from the same SU(3) multiplet as D03.

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Indications of D12 and other Isovector Dibaryons

• Isovector dibaryons were discovered in late 70ies
  in pp scattering experiments and then confirmed
  in partial wave analyses of pp and pd elastic
  scattering and especially pd ? pp reaction R.A.
  Arndt et al., PRC 48, 1926 (1993).
• All features of the dominant partial wave
  amplitude 1D2P are consistent with production of
  dibaryon resonance D12 with quantum numbers I(JP
  ) 1(2), mass M(D12) 2150 MeV and total width
  G(D12) G(?) 120 MeV.

Contributions of dominant 1D2P, 3F3D and 3P2D
amplitudes to the total X-section of pd ? pp
Argand plot of dominant partial-wave amplitudes
in pd ? pp
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A New Confirmation of Dibaryon Resonances D12
and D03

• From solving exact Faddeev equations for pNN and
  pN? systems the robust dibaryon poles
  corresponding to D12 and D03 were found.
• The pole positions are
• These parameters are in full agreement with Dyson
  and Xuong predictions as well as with
  experimental findings.
• Thus, the D12 pole is located 20 MeV below the
  N? threshold (2170 MeV) and the D03 pole lies
  100 MeV below the ?? threshold (2460 MeV). The
  deuteron, i.e., the NN S-wave dibaryon D01, is
  located near the NN threshold.
• Lets see what happens at short distances, when
  quark d.o.f. come into play.

A. Gal, H. Garcilazo, arXiv1308.2112
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Dibaryon Model of NN Interaction (very briefly
see talk by V.I. Kukulin)

• In dibaryon model, an intermediate dibaryon
  production is assumed to be responsible for the
  basic NN attraction, i.e., for the short-range
  nuclear force.
• The basic meson field surrounding the 6q bag in
  dibaryon model is a scalar (s) field. It arises
  in a 6q transition from a mixed-symmetry 6q
  configuration s4p2 to a fully symmetric s6 (in
  even NN partial waves) s4p2 ? s6 s (Ls 0,2).
  
• The s field stabilizes the quark bag and leads to
  a strong attraction between quarks this results
  in effective attraction between nucleons at
  rNN0.70.8 fm.
• Within the dibaryon model, a very good
  description of NN-scattering phase shifts up to
  EN 1 GeV and of lightest nuclei properties was
  achieved with only a few basic parameters.

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The Deuteron in Dibaryon Model

• The deuteron wave function (d.w.f.) in dibaryon
  model is a two-component Fock column
• The second component of the d.w.f. ?6qs is a
  fully symmetric 6q bag surrounded by s-meson
  cloud, as well as the nucleon is a 3q bag dressed
  with pion cloud. However, closeness to NN
  threshold makes this elementary deuteron to be
  strongly coupled to NN channel.
• As a result, the quark-meson component ?6qs
  gives just a small contribution ( 23) to the
  total d.w.f., however it is still dominant at
  short NN distances, i.e., when two nucleons are
  overlapped with each other.
• Analogously, the D12 and D03 dibaryons at short
  distances may be considered as dressed six-quark
  bags strongly coupled to N? and ?? channels,
  respectively.

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Decay Routes of the D03 Resonance

• So, the dibaryon resonances D12 and D03 may be
  treated as excited states of the deuteron D01, in
  a similar way that baryon resonances ?, N(1440),
  etc., are treated as excited nucleon states.
• Besides the above mentioned decay mode
• we propose two alternative routes for the
  D03 resonance decay which can lead to dpp final
  state
• The mechanisms 1) and 2) resemble the
  respective routes for the Roper resonance decay
  N(1440) ? ? p and N(1440) ? N s.
• NOTE. The mechanisms 1) and 1) lead to very
  similar results in pn?D03?dpp invariant mass and
  angular distributions. However, we consider the
  last mechanism 1) to be the dominant one close
  to D03 peak energy (2.38 GeV), since two ?s in
  D03 are deeply bound and almost fully overlapped
  with each other, so the dibaryon configuration
  should be more probable here.

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Dibaryon Model for the Basic 2p Fusion Reaction
in the ABC Region

• We took the above decay routes 1) and 2) for
  D03 resonance as a basis for our model of p n
  ? d (pp)0 reaction in the ABC region (Tp
  1.01.4 GeV) M.N. Platonova, V.I. Kukulin, PRC
  87, 025202 (2013).
• The D03 (I(JP) 0(3)) dibaryon produced in pn
  collision decays subsequently into the final
  deuteron (i.e., D01 (I(JP) 0(1)) and isoscalar
  pp pair via two interfering processes
• (a) emission of a d-wave s meson, which
  then decays into s-wave pp pair
• (b) sequential emission of two p-wave
  pions via an intermediate isovector
• dibaryon D12 (I(JP) 1(2))
  production.
• 3 model parameters s-meson mass and width
  (presently known with a large uncertainty) and
  the relative weight of the amplitudes (a) and
  (b).

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Results of the Model Calculations.I. Total Cross
Section
Comparison with the WASA_at_COSY Experimental Data
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Results of the Model Calculations.II.
Invariant-mass spectra at E2.38 GeV
Comparison with the WASA_at_COSY Experimental Data

• Each of two mechanisms proposed
• gives a resonance enhancement
• in the respective invariant-mass spectrum
• ABC enhancement is a
• consequence of s-meson production
• The peak in Mdp spectrum reflects
• the isovector dibaryon D12 production.

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Results of the Model Calculations.III. Angular
Distributions at E2.38 GeV
Comparison with the WASA_at_COSY Experimental Data

• The description of deuteron and pion angular
  distributions is not perfect, but still
  reasonable.
• Additional mechanisms, such as two uncorrelated
  pions production, other intermediate resonances
  and non-resonance contributions, should be
  considered for better description.

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Results of the Model Calculations.IV. Energy
Dependence of Mpp Spectrum
Comparison with the WASA_at_COSY Experimental Data
New data from P. Adlarson et al., Phys. Lett.
B721, 229 (2013) renormalized to stot not
fitted

• When approaching the ?? threshold (E2.46 GeV),
  the Mpp spectrum gets closer to pp?dpp0 data
  (scaled to I0 by isospin relations), described
  well by the ?? model.
• The low-mass enhancement almost fully disappears.
  

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Parameters of s-meson

• As extracted from our model fit
• to the ABC peak
• As found from dispersion analysis
• of pp-scattering amplitude
• Is there any contradiction?

M.N. Platonova, V.I. Kukulin, PRC 87,
025202 (2013)
I. Caprini, G. Colangelo, H. Leutwyler, PRL 96,
132001 (2006)
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Chiral Symmetry Restoration

• Numerous theoretical investigations (T. Kunihiro,
  M. Volkov, and others) show that the mass and
  width of the s meson produced in hot and/or dense
  nuclear matter may be signi?cantly shifted
  downwards in comparison with its free-space
  parameters due to the partial Chiral Symmetry
  Restoration (CSR) effect.
• Partial CSR was demonstrated (L. Glozman et al.)
  to take place also in highly excited states of
  isolated hadrons (baryons and mesons). Thus, the
  appearance of approximately degenerate parity
  doublets in the spectra of highly excited baryons
  may be considered as a manifestation of partial
  CSR.

Temperature dependence of Mp, Ms and Gs. D.
Blaschke et al., arXiv0508264
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Chiral Symmetry Restoration

• In fact, the rise of baryon density or nuclear
  matter temperature as well as a high hadron
  excitation energy lead to an increase of quark
  kinetic energy, which results in suppression of
  the chiral condensate in QCD vacuum. This means
  the reduction of the s-meson mass and width for
  the s ? pp decay.
• So, the s meson, being a broad resonance in free
  space, may become a sharp resonance in dense/hot
  nuclear medium or highly excited hadronic states.

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Chiral Symmetry Restoration in Dibaryons

• The D03 resonance
• 1) dense quark matter (r(D03) 0.8 fm ? 6
  times normal nuclear
• density)
• 2) excitation energy of 500 MeV (above the
  deuteron).
• Dibaryon model of NN interaction predicts strong
  CSR effects even in a deuteron, i.e., its
  quark-meson component (due to 2h? excitation of a
  mixed-symmetry 6q configuration s4p2 above a
  fully symmetric s6).
• Thus, the CSR phenomenon is likely to occur
  in the D03 state and also in other dibaryons.
• If so, the s meson produced from the D03 decay
  should have the lower mass and width than those
  for the free s meson.
• One can suggest that the low values for the
  s-meson parameters extracted from the ABC peak
  indicate the partial CSR effect in excited
  dibaryon states.

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Conclusions

• Within the s-dressed dibaryon model, we succeeded
  in description of numerous exclusive data on the
  basic two-pion production reaction p n ? d
  (pp)0.
• The ABC effect, i.e., the low-Mpp enhancement, is
  interpreted as a consequence of the light scalar
  meson production, provided the chiral symmetry is
  partially restored in an excited dibaryon state.
• This means the possibility to study the
  fundamental phenomenon of chiral symmetry
  restoration in few-body sector, through the
  production of light scalar mesons in NN, Nd,
  etc., collisions at intermediate energies E 1
  GeV/u.

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Thank You For Your Attention!