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Hybrid meson decay widths
A. Giachino (INFN Genova)H. Garcia Tecocoatzi, E. Santopinto and E. Swanson
The 15th International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, MENU-2019Cohon University Center, Carnegie Mellon University, Pittsburgh Pennsylvania
04/06/2019
• 1) Introduction:• Definition of conventional and hybrid mesons;
• 2) Charmonium hybrid multiplets and hybrid mass spectrum from Lattice• 3) Decay amplitudes of the hybrid mesons
• The model of the decay process;• The transition operator;• The gluon dispersion model;• Full amplitude for the decay process• Preliminary results for 2"#(𝐻1) hybrid decay widths in 𝐷(*)𝐷(*)• Hunting for hybrids: MesonEx and Gluex
Conventional mesons
The Constituent Quark Model describes the observed meson spectrum as 𝑞 𝑞′- bound states grouped according to SU(N) flavor multiplets. The heavy/light meson mass spectrum was computed originally by S. Godfrey and N. Isgur in 1985 within the Constituent Quark Model in Phys. Rev D, 32, 1.
Figure taken from PDG.
𝐽/0 = 0"#
𝐽/0 = 1""
SU(4) multiplets showing the 16-‐plets for the pseudoscalar(a) and vector mesons (b) made of the u, d, s, and c quarks.
Constituent Quark Model
If the orbital angular momentum of the 𝑞 𝑞′- state is L, then the parity operator eigenvalues are P=(−1)4#5 and the meson total angular momentum J is given by |L − s| ≤ J ≤ |L + s|,
where s can be 0 (antiparallel quark spins) or 1 (parallel quark spins).A state made of quarks and their own antiquarks 𝑞 𝑞6 is also eigenstate of charge conjugation
operator 𝐶8, whose eigenvalues are given by C = (−1)4#@.
Conventional mesons
Possible 𝑱𝑷𝑪 for 𝑞 𝑞6 mesons
In the quark model the allowed JPC quantum numbers are therefore:
l 𝑳 = 𝟎 S=0, JPC = 0-‐+
S=1, JPC = 1-‐-‐
l 𝑳 = 𝟏 S=0, JPC = 1+-‐
S=1, JPC = 0++, 1++, 2++
l 𝑳 = 𝟐 S=0, JPC = 2-‐+
S=1, JPC = 1-‐ -‐ , 2-‐ -‐ , 3-‐ -‐
Conventional mesons
So one can see that the quantum numbers 𝐽/0 = 0"", 𝑜𝑑𝑑"#, 𝑒𝑣𝑒𝑛#"
are not available to simple quark-‐antiquark systems. Hadrons with these quantum numbers are called “(quantum number) exotic” [1]
On PDG a “(quantum number) exotic” is denoted by a subscript for the total spin, with parity and charge conjugation following that of the corresponding hadron. Thus, for ex., a 𝐽/0 = 1"# isovector hybrid would be denoted by 𝜋5,𝑒. 𝑔. 𝜋5(1400) [2] or 𝜋5(1600) [3] with
[1] Hybrid Mesons C.A. Meyer (Carnegie Mellon U.), E.S. Swanson (Pittsburgh U.). Feb 25, 2015. Prog.Part.Nucl.Phys. 82 (2015) 21-‐58[2] D. R. Thompson et al. (E852 Collaboration), Phys. Rev. Lett. 79, 1630 (1997);[3] E. I. Ivanov et al. (E852 Collaboration), Phys. Rev. Lett. 86, 3977 (2001)
Definition of Hybrid
Such quark-‐antiquark-‐gluon systems admit exotic quantum numbers which are not allowed to simple quark-‐antiquark states because also the gluon quantum numbers are includedLattice simulations of the charmonium spectrum in quenched QCD beyond the Born-‐oppenheimerapproximation also lead to more states then predicted by the quarkonium potential models potentially indicating presence of gluonicexcitations [2]
[1] H. Garcia Tecocoatzi et al., work in preparation.[2] J. J. Dudek, R. G. Edwards, N. Mathur, and D. G.Richards, Phys. Rev. D 77, 034501 (2008).
We define a hybrid state as a bound system made by a heavy quark and a heavy antiquark in a color octet configuration plus some gluonicexcitations [1].
Hybrid multipletsIn the classification of hybrids we follow the notation 𝐻Q originally introduced in [1-‐2] withthe following definitions:• s=0 or 1 is the spin of 𝑞𝑞6;• l=0,1… is the angular momentum between 𝑞 and 𝑞6; ;
• jg =1 is the orbital angular momentum of the quasi-‐gluon
• 𝜉=-‐1 corresponds to TE (unnatural parity) gluon state (at the moment we do not consider gluon in TM mode)
• L=l+Jg;• J= L+s is the total angular momentum of
the hybrid meson.[1] K. J. Juge, J. Kuti, and C. J. Morningstar, Phys. Rev. Lett., 82, 4400 (1999);[2] E. Braaten, C. Langmack, and D. H. Smith, Phys. Rev., D90, 014044 (2014) and Phys. Rev. Lett., 112, 222001 (2014).
Example: the 𝑯𝟏 multipletCoupling the TE gluon with a color octet 𝑞 𝑞′- state in l = 0, one obtains a hybrid state with theintermediate angular momentum L= l + Jg = 1. Adding also the quark spin, s=0 or 1, we obtain four low lying hybrids with 𝐽/0= 1-‐-‐,0-‐+,1-‐+ and 2-‐+ which correspond to the multiplet indicated with H1. The parity and charge conjugation eigenvalues of the hybrid states areare given by:
𝑱𝑷𝑪 multiplets with 𝒍 ≤ 𝟏 for the charmonium hybrid states(table below)
Exotic quantum numbers in red
𝑃 = 𝜉(−1)WX#Y#5,𝐶 = (−1)Y#Z#5
𝑗\ + 𝑙 + 1 = 1 + 0 + 1 = 2 → 𝑃 = −1𝑙 + 𝑠 + 1 = 0 + 0 + 1 = 1 → 𝐶 = −1𝑙 + 𝑠 + 1 = 0 + 1 + 1 = 2 → 𝐶 = +1
Hybrid masses The spectrum of hybrids in the charmonium sector has been calculated in 2012 by the Hadron Spectrum Collaboration [3] using lattice QCD . These masses are used as input in our decay width calculations [4].
[3] L. Liu, G. Moir, M. Peardon, S. M. Ryan, C. E. Thomas, P. Vilaseca, J. J. Dudek, R. G. Edwards, B. Joo, and D. G. Richards (Hadron Spectrum), JHEP, 07, 126 (2012);[4] H. Garcia Tecocoatzi et al., work in preparation.
H1 H3 H2 H4l=0,L=1 l=1,L=0 l=1,L=1 l=1,L=2
Charmonium hybrid spectrum
The model of decay processIn the simplest scheme of a hybrid meson decay process, only the
quark-‐gluon vertex is involved: all the other vertices, in fact, give rise to states not compatible with the initial hybrid meson state, 𝑞 𝑞6 g, and the two mesons of the final states, and so they give zero
contribution to the decay process as one can see in the figure.
Hybrid meson (H1,H2,…)
conventional meson (D,D*,D0,…)
conventional meson(D,D*,D0,…)
the quark-‐gluon vertex is
with
The transition operator• In Coulomb Gauge QCD, the decay operator is given by the integral of the the product of the spatial part of the quark current and the gluon field:
𝐴c is the chromo-‐magnetic field
Ψ is the quark field
𝑖 and 𝑎 are the helicityand color indices, n=1,2 and 3𝑎g# and 𝑎g are the quasi-‐gluon creation and annihilation operators
𝑢 and 𝑣 are Dirac spinorswhich appear in the quark field: 𝑣Z 𝒑 = 𝛾k 𝑢Z(𝑝)
In Dirac-‐Pauli convection:
The transition operatorNow, for the process we are interested in, only the combination b†d†a makes a contribution(we destroy a gluon in the initial state and we create a quark and an antiquark in the final state), so the only terms of the chromo-magnetic field A and the quark fields that survive are, respectively:
and
Replace 𝐴Q ,Ψ and Ψ- into the expression of 𝜃:
One replaced 𝐴Q, Ψ and Ψ-the transition operator 𝜃 is proportional to:
𝜃 ∝
The transition operator• In order to calculate the hybrid decay widths it is useful to expressthe transition operator in the Wigner matrixformalism [7]
where the rotation matrix 𝐷p,pqW is the matrix
representative of the rotation operator R in the irreducible representation of the rotation group labelled by j:
Euler angles
[7] Philip R. Page, Eric S. Swanson and Adam P. Szczepaniak, PHYSICAL REVIEW D, VOLUME 59, 034016
and 𝑑pr,pW (𝜃) is the matrix representative of the rotation about the y
axis:
The Wigner D-‐matrix elements 𝐷p,pqW form a set of orthogonal functions of
the Euler angles 𝛼, 𝛽 and 𝛾 and they satisfy the following normalization condition:
Once rewritten the transition operator in the Wigner matrixformalism the final expression for the decay operator can be written as:
1 is the spin of the quasi-‐gluon;s+s’=𝜆: conservation of the third component of the spin which tells us that the sum of the third component of the quark and the antiquark spin must be equal to the third component of the gluon spin;𝒌 , 𝒑 and 𝒑’ are the momenta of the gluon, the quark and the antiquark respectively;
The transition operator
Gluon Dispersion Model
GeV
What choice for 𝜔(𝑘) ?
We use the expression found by Adam Szczepaniak, Eric S. Swanson et al. in Phys. Rev. Lett. 76, 2011
Learning from superconductivity
A key conceptual element of BCS theory is the pairing of electrons close to the Fermi level into Cooper pairs.This pairing results from a slight attraction between the electrons due to the coupling of the electrons with the lattice vibrations (phonons).
The electron pairs have a slightly lower energy than the free electrons and leave an energy gap around the Fermi energy.
For temperatures such that the thermal energy is less than this band gap, the material exhibits zero resistivity.
≅ 10"z eVLearning from superconductivity
There are no electrons with this energy
Density of states
Understanding the properties of gluons is one of the major challenges of Hadronic physics.
Soft gluons can be studied with the Hamiltonian formulation of QCD in a physical gauge such as the Coulomb gauge [5]: in the Coulomb
gauge all degrees of freedom are physical
[5] Adam P. Szczepaniak, Eric S. Swanson (University of Pittsburgh), Phys. Rev. D 62, 094027
2) QCD Hamiltonian in Coulomb gauge 𝑯;
1) BCS Ansatz for the QCD vacuum |𝛀 >;
Learning from superconductivity
Gluon Dispersion Model
Gluon Dispersion Model [6]
[6] Adam Szczepaniak, Eric S. Swanson, Chueng-‐Ryong Ji, and Stephen R. Cotanch Phys. Rev. Lett. 76, 2011
gap equation: it is the equation that determines the ground state of the systemΩ is the BCS trial vacuum
3) Minimization of the vacuum energy density 𝐄 =< 𝛀 𝑯 𝛀>
The gluon acquire an effective mass
Gluon Dispersion Model [6]
[6] E. S. Swanson and A. P. Szczepaniak, Phys. Rev. D 59, 014035 (1999) doi:10.1103/PhysRevD.59.014035
GeV
GeV
This is the gluon dispersion relation used in our calculations
A fit to the numerical solution of the gap equation brings to the Gluon spectral function
The wave function of the hybrid meson
The hybrid meson wave function depends on the three-‐body Jacobi coordinates 𝝆 and 𝝀 which encode the internal motion of the systemThe hybrid meson wave function in the L-‐S coupling scheme, which is the best scheme to calculate the decay widths
𝝆
𝝀
𝒎
𝑴
𝒎
Full amplitude for the decay process
Wave functions of B, Cand hybridmesons:we use harmonic oscillator wave functions
After performing the coordinate transformation
Full amplitude for the decay process
Hybrid masses from lattice [3] and meson masses from PDG
[3] L. Liu, G. Moir, M. Peardon, S. M. Ryan, C. E. Thomas, P. Vilaseca, J. J. Dudek, R. G. Edwards, B. Joo, and D. G. Richards (Hadron Spectrum), JHEP, 07, 126 (2012);
The spin-‐recoupling coefficients
• 𝑆��� = 𝐽�5 + 𝐽��• 𝐿��� = 𝐽� + 𝑆���• Parity and charge conjugation
must be conserved
Preliminary results: example of 2"#(𝐻1) hybrid decay widths in 𝐷(*)𝐷(*)
… … … … … … … …
Preliminary results: example of 2"#(𝐻1) hybrid decay widths in 𝐷(*)𝐷(*)
10
Γ(MeV
2
xx
16
>th.
>th.
>th.
>th.
x = negligible
Two experiments designed for searching hybrids at Jefferson Lab
Gluex
MesonExprogram with CLAS12
These two experiments are complementary
Photoproduction of high-‐mass mesonic states (consisting of ordinary mesons, hybrids, and mesons with exotic JPC) using Liquid H2 and light nuclear targets;
Experimental technique: coincidence measurement between CLAS12 (final statehadrons) and Forward Tagger facility (low-‐angle scattered electron)
MesonExPhoton tagged by detecting the scattered electron at low angles
Gluex
• The linearly polarized photons travel into the GlueX detector, with some interacting in the liquid hydrogen target → 𝛾 𝑝 collisions
both the energy and intensity of the photon beam are monitored
• Photon polarization provides constraints on production processes and simplifies the PWA
Photon produced from electron Bremsstrahlung in diamond crystal
Thanks for your attention