ThreeBodyDecay.jl

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ThreeBodyDecay.jl

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Julia Implementation of the Dalitz plot decomposition

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# ThreeBodyDecay

> [!NOTE]
> soon DEPRICATED
>
> use [`ThreeBodyDecays.jl`](https://github.com/mmikhasenko/ThreeBodyDecays.jl)
> 
> that is going to be a registered package shortly

![Build Status](https://github.com/mmikhasenko/ThreeBodyDecay.jl/actions/workflows/ci.yaml/badge.svg)
[![Codecov](https://codecov.io/gh/mmikhasenko/ThreeBodyDecay.jl/branch/master/graph/badge.svg)](https://codecov.io/gh/mmikhasenko/ThreeBodyDecay.jl)
[![arXiv article](https://img.shields.io/badge/article-PRD%20101%2C%20034033-yellowgreen)](https://arxiv.org/abs/1910.04566)

A framework for the amplitude analysis of multibody decay chains.
The main focus of the project is the three-body decay and reactions required the Dalitz Plot analysis.
The latter includes reactions with more then three particles, which however can be factorized to a product
of sequential decays with $≤3$ products due to the lifetime of the long-lived particles.
All particles can have arbitrary spin.

The framework is based on the publication, "Dalitz-plot decomposition for three-body decays" by JPAC Collaboration (M Mikhasenko at al.) [(arxiv)](http://inspirehep.net/record/1758460).
The code inherits notations of the paper:

- Particles are numbered 1,2,3, and 0 for the decay products and the mother particle, respectively.
- `s` is a total invariant mass of three particles,
- `σ` is a two-particle invariant mass squared, `σₖ = (pᵢ+pⱼ)²`,
- `θᵢⱼ` is a scattering angle, an angle between `vec pᵢ` and `- vec pₖ`.
- `hat θₖ₍ⱼ₎` is a isobar angle (the Wigner angle of the 0-particle), an angle of `vec pⱼ+pⱼ` with respect the the chain `j`.
- `ζᵏᵢ₍₀₎` is the Wigner angle for the final-state particle (see the paper for the definition).

## API for describing the decay

A type `ThreeBodySystem` is used to keep information of the three-body system (masses, spins, and parities).
The `DecayChain` structure contain information on the spin of the intermediate resonance, its lineshape
and $LS$ Clebsch-Gordan coefficient in the production and decay vertices.

Here is an example for amplitude description for Λb ⟶ Jψ p K,
the reaction where the pentaquarks candidates were observed for the first time.

```julia
using ThreeBodyDecay # import the module
#
# decay Λb ⟶ Jψ p K
ms = (Jψ = 3.09, p=0.938, K = 0.49367, Lb = 5.62) # masses of the particles
# create two-body system
tbs = ThreeBodySystem(ms.Jψ, ms.p, ms.K; m0=ms.Lb,   # masses m1,m2,m3,m0
            two_js=ThreeBodySpins(2, 1, 0; two_h0=1])) # twice spin
Concerving = ThreeBodyParities('-',  '+',  '-'; P0='+')
Violating  = ThreeBodyParities('-',  '+',  '-'; P0='-')
```

`ThreeBodySystem` creates an immutable structure that describes the setup.
Two work with particles with non-integer spin, the doubled quantum numbers are stored.

The following code creates six possible decay channels.
The lineshape of the isobar is specified by the second argument,
it is a simple Breit-Wigner function in the example below.

```julia
# chains-1, i.e. (2+3): Λs with the lowest ls, LS
Λ1520  = DecayChainLS(1, σ->BW(σ, 1.5195, 0.0156); two_s = 3/2|>x2, parity = '+', Ps=Concerving, tbs=tbs)
Λ1690  = DecayChainLS(1, σ->BW(σ, 1.685,  0.050 ); two_s = 1/2|>x2, parity = '+', Ps=Concerving, tbs=tbs)
Λ1810  = DecayChainLS(1, σ->BW(σ, 1.80,   0.090 ); two_s = 5/2|>x2, parity = '+', Ps=Concerving, tbs=tbs)
Λs = (Λ1520,Λ1690,Λ1810)
#
# chains-3, i.e. (1+2): Pentaquarks with the lowest ls, LS
Pc4312 = DecayChainLS(3, σ->BW(σ, 4.312, 0.015); two_s = 1/2|>x2, parity = '+', Ps=Concerving, tbs=tbs)
Pc4440 = DecayChainLS(3, σ->BW(σ, 4.440, 0.010); two_s = 1/2|>x2, parity = '+', Ps=Concerving, tbs=tbs)
Pc4457 = DecayChainLS(3, σ->BW(σ, 4.457, 0.020); two_s = 3/2|>x2, parity = '+', Ps=Concerving, tbs=tbs)
Pcs = (Pc4312,Pc4440,Pc4457)
#
A(σs,two_λs,cs) = sum(c*amplitude(dc,σs,two_λs) for (c, dc) in zip(cs, (Λs...,Pcs...)))
```

Amplitudes for the decay chains are added coherently with complex constants.

- the invariant variables, `σs = [σ₁,σ₂,σ₃]`,
- helicities `two_λs = [λ₁,λ₂,λ₃,λ₀]`
- and complex couplings `cs = [c₁,c₂,...]`

The intensity (and probability) is a squared amplitude summed over the summed over helicities for the case the decay particle is unpolarized.

```julia
I(σs,cs) = sum(abs2(A(σs,two_λs,cs)) for two_λs in itr(tbs.two_js))
#
I(σs) = I(σs,[1, 1.1, 0.4im, 2.2, 2.1im, -0.3im]) # set the couplings
#
σs = randomPoint(tbs.ms) # just a random point of the Dalitz Plot
@show I(σs) # gives a real number - probability
```

# Plotting API

Visualization discuss below exploits the `Plots.jl` module with `matplotlib` backend.

```julia
using Plots
pyplot()
```

A natural way to visualize the three-body decay with two degrees of freedom
is a correlation plot of the subchannel invariant masses squared.
Kinematic limits can visualized using the `border` function.
Plot in the σ₁σ₃ variables is obtained by

```julia
plot(
  plot(border31(tbs), xlab="σ₁ (GeV²)", ylab="σ₃ (GeV²)"),
  plot(border12(tbs), xlab="σ₂ (GeV²)", ylab="σ₁ (GeV²)"))
```

![border31](https://raw.githubusercontent.com/mmikhasenko/ThreeBodyDecay.jl/main/example/plot/border31_12.png)

A phase-space sample is generated using the `flatDalitzPlotSample` function.
By weighting the sample one gets the intensity distribution.
The intensity function can also be plotted on the Dalitz plan using a grid.

```julia
using TypedTables # converint inteface of selecting columns
#
σsv = Table(flatDalitzPlotSample(tbs.ms; Nev = 10_000))
weights = I.(σsv) # dot is a broadcast
#
plot(layout=(900,350), layout=grid(1,2), xlab="σ₁ (GeV²)", ylab="σ₃ (GeV²)")
histogram2d!(sp=1, σsv.σ1, σsv.σ3, weights=weights, lab="weighted phase space"))
plot!(sp=2, I, tbs.ms; iσx=1, iσy=3, lab="on a grid")
```

![Scatter and Histogram](https://raw.githubusercontent.com/mmikhasenko/ThreeBodyDecay.jl/main/example/plot/dalitz31.png)

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