Helicity versus canonical
Contents
Helicity versus canonical#
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import logging
import graphviz
import matplotlib as mpl
import numpy as np
import qrules
import sympy as sp
from IPython.display import HTML, Math
from matplotlib import cm
from rich.table import Table
import ampform
LOGGER = logging.getLogger()
LOGGER.setLevel(logging.ERROR)
In this notebook, we have a look at the decay
in order to see the difference between a HelicityModel
formulated in the canonical basis and one formulated in the helicity basis. To simplify things, we only look at spin projection \(+1\) for \(D_1(2420)^0\), because the intensities for each of the spin projections of \(D_1(2420)^0\) are incoherent, no matter which spin formalism we choose.
First, we use qrules.generate_transitions()
to generate a ReactionInfo
instance for both formalisms:
def generate_transitions(formalism: str):
reaction = qrules.generate_transitions(
initial_state=("D(1)(2420)0", [+1]),
final_state=["K+", "K-", "K~0"],
allowed_intermediate_particles=["a(1)(1260)+"],
formalism=formalism,
)
builder = ampform.get_builder(reaction)
return builder.formulate()
cano_model = generate_transitions("canonical-helicity")
heli_model = generate_transitions("helicity")
From components
and parameter_defaults
, we can see that the canonical formalism has a larger number of amplitudes.
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table = Table(show_edge=False)
table.add_column("Formalism")
table.add_column("Coefficients", justify="right")
table.add_column("Amplitudes", justify="right")
table.add_row(
"Canonical",
str(len(cano_model.parameter_defaults)),
str(len(cano_model.components) - 1),
)
table.add_row(
"Helicity",
str(len(heli_model.parameter_defaults)),
str(len(heli_model.components) - 1),
)
table
Formalism β Coefficients β Amplitudes βββββββββββββββββββββββββββββββββββββββ Canonical β 3 β 8 Helicity β 3 β 3
The reason for this is that canonical basis distinguishes amplitudes over their \(LS\)-combinations. This becomes clear if we define \(a\) to be the amplitude without coefficient (\(A = C a\)), and consider what the full, coherent intensity looks like.
If we write the full intensity as \(I = \left|\sum_i A_i\right|^2\), then we have, in the case of the canonical basis:
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def extract_amplitude_substitutions(model, colorize=False):
amplitude_to_symbol = {}
amplitude_names = sorted(c for c in model.components if c.startswith("A"))
n_colors = len(amplitude_names)
color_map = cm.brg(np.linspace(0, 1, num=n_colors + 1)[:-1])
color_iter = (mpl.colors.to_hex(color) for color in color_map)
for name in amplitude_names:
expr = model.components[name]
for par in model.parameter_defaults:
if par in expr.args:
expr /= par
name = "a" + name[1:]
if colorize:
color = next(color_iter)
name = Rf"\color{{{color}}}{{{name}}}"
symbol = sp.Symbol(name)
amplitude_to_symbol[expr] = symbol
return amplitude_to_symbol
cano_amplitude_to_symbol = extract_amplitude_substitutions(cano_model)
heli_amplitude_to_symbol = extract_amplitude_substitutions(heli_model)
def render_amplitude_summation(model, colorize=False):
amplitude_to_symbol = extract_amplitude_substitutions(model, colorize)
collected_expr = sp.collect(
model.expression.subs(amplitude_to_symbol).args[0].args[0],
tuple(model.parameter_defaults),
)
terms = collected_expr.args
latex = ""
latex += R"\begin{align}"
latex += Rf"\sum_i A_i & = {sp.latex(terms[0])}\\"
for term in terms[1:]:
latex += Rf"& + {sp.latex(term)} \\"
latex += R"\end{align}"
return Math(latex)
render_amplitude_summation(cano_model, colorize=True)
In the helicity basis, the \(LS\)-combinations have been summed over already and we can only see an amplitude for each helicity:
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render_amplitude_summation(heli_model)
Amplitudes in the canonical basis are formulated with regard to their \(LS\)-couplings. As such, they contain additional Clebsch-Gordan coefficients that serve as expansion coefficients.
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def extract_amplitudes(model):
return {
expr: sp.Symbol(name)
for name, expr in model.components.items()
if name.startswith("A")
}
cano_amplitudes = extract_amplitudes(cano_model)
heli_amplitudes = extract_amplitudes(heli_model)
expression, symbol = next(iter(cano_amplitude_to_symbol.items()))
display(symbol, Math(Rf"\quad = {sp.latex(expression)}"))
In the helicity basis, these Clebsch-Gordan coefficients and Wigner-\(D\) functions have been summed up, leaving only a Wigner-\(D\) for each node in the decay chain (two in this case):
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See formulate_wigner_d()
and formulate_clebsch_gordan_coefficients()
for how these Wigner-\(D\) functions and Clebsch-Gordan coefficients are computed for each node on a StateTransition
.
We can see this also from the original ReactionInfo
objects. Letβs select only the transitions
where the \(a_1(1260)^+\) resonance has spin projection \(-1\) (taken to be helicity \(-1\) in the helicity formalism). We then see just one StateTransition
in the helicity basis and three transitions in the canonical basis:
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def render_selection(model):
transitions = model.reaction_info.transitions
selection = filter(lambda s: s.states[3].spin_projection == -1, transitions)
dot = qrules.io.asdot(selection, render_node=True, render_final_state_id=False)
return graphviz.Source(dot)
display(
HTML("<b>Helicity</b> basis:"),
render_selection(heli_model),
HTML("<b>Canonical</b> basis:"),
render_selection(cano_model),
)
Coefficient names#
In the previous section, we saw that the HelicityAmplitudeBuilder
by default generates coefficient names that only contain helicities of the decay products, while coefficients generated by the CanonicalAmplitudeBuilder
contain only \(LS\)-combinations. Itβs possible to tweak this behavior with the naming
attribute. Here are two extreme examples, where we generate coefficient names that contain \(LS\)-combinations, the helicities of each parent state, and the helicity of each decay product, as well as a HelicityModel
of which the coefficient names only contain information about the resonances:
reaction = qrules.generate_transitions(
initial_state=("D(1)(2420)0", [+1]),
final_state=["K+", "K-", "K~0"],
allowed_intermediate_particles=["a(1)(1260)+"],
formalism="canonical-helicity",
)
builder = ampform.get_builder(reaction)
builder.naming.insert_parent_helicities = True
builder.naming.insert_child_helicities = True
builder.naming.insert_ls_combinations = True
model = builder.formulate()
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amplitudes = [c for c in model.components if c.startswith("A")]
assert len(model.parameter_defaults) == len(amplitudes)
sp.Matrix(model.parameter_defaults)
builder.naming.insert_parent_helicities = False
builder.naming.insert_child_helicities = False
builder.naming.insert_ls_combinations = False
model = builder.formulate()
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assert len(model.parameter_defaults) == 1
display(*model.parameter_defaults)