Enhance the dec file "Particle" statement to allow for optional widths (

#414)

* Make the width optional in a 'Particle' statement, as in EvtGen

* Test the new functionality

* Enhance code to deal with optional width parameter

* Fix for MyPy

src/decaylanguage/data/decfile.lark

@@ -26,7 +26,7 @@ cdecay : "CDecay" LABEL
 
 define : "Define" LABEL SIGNED_NUMBER
 
-particle_def: "Particle" LABEL SIGNED_NUMBER SIGNED_NUMBER // Set the mass and width of a particle (in GeV)
+particle_def: "Particle" LABEL SIGNED_NUMBER SIGNED_NUMBER? // Set the mass and width (optional) of a particle (in GeV)
 
 alias : "Alias" LABEL LABEL
 

src/decaylanguage/dec/dec.py

@@ -455,14 +455,17 @@ def dict_charge_conjugates(self) -> dict[str, str]:
     def get_particle_property_definitions(self) -> dict[str, dict[str, float]]:
         """
         Return a dictionary of all particle property definitions
-        in the input parsed file, of the form "Particle <PARTICLE> <MASS> <WIDTH>",
+        in the input parsed file, of the form "Particle <PARTICLE> <MASS> <WIDTH>"
+        or "Particle <PARTICLE> <MASS>",
         as {'PARTICLE1': {'mass': MASS1, 'width': WIDTH1},
             'PARTICLE2': {'mass': MASS2, 'width': WIDTH2}, ...}.
 
         Note
         ----
-        Particles are often defined via aliases and post-processing may be needed
-        to match the mass and width to the actual particle.
+        1) Particles are often defined via aliases and post-processing may be needed
+           to match the mass and width to the actual particle.
+        2) The mass (width) parameter is compulsory (optional).
+           When not specified, the width is taken from the particle or alias.
         """
         self._check_parsing()
         return get_particle_property_definitions(self._parsed_dec_file)
@@ -1287,7 +1290,7 @@ def find_charge_conjugate_match(
     from "ChargeConj" statements in a decay file.
 
     The name `ChargeConj(pname)` is returned if all matching "routes" fail,
-    see `charge_conjugate_name(...) function.`
+    see `charge_conjugate_name(...)` function.
     """
     # Check the list of particle-antiparticle matches provided ;-)
     if dict_cc_names:
@@ -1623,20 +1626,45 @@ def get_charge_conjugate_defs(parsed_file: Tree) -> dict[str, str]:
 def get_particle_property_definitions(parsed_file: Tree) -> dict[str, dict[str, float]]:
     """
     Return a dictionary of all particle property definitions
-    in the input parsed file, of the form "Particle <PARTICLE> <MASS> <WIDTH>",
+    in the input parsed file, of the form "Particle <PARTICLE> <MASS> <WIDTH>"
+    or "Particle <PARTICLE> <MASS>",
     as {'PARTICLE1': {'mass': MASS1, 'width': WIDTH1},
         'PARTICLE2': {'mass': MASS2, 'width': WIDTH2}, ...}.
 
+    Note
+    ----
+    1) Particles are often defined via aliases and post-processing may be needed
+       to match the mass and width to the actual particle.
+    2) The mass (width) parameter is compulsory (optional).
+       When not specified, the width is taken from the particle or alias.
+
     Parameters
     ----------
     parsed_file: Lark Tree instance
         Input parsed file.
     """
+    # Get particle aliases since "Particle" statements often use name aliases
+    aliases = get_aliases(parsed_file)
+
+    def get_set_width_or_default(children: list[Tree]) -> float:
+        if len(children) > 2:
+            return float(children[2].value)
+
+        # Get the particle name or the alias
+        token_name: str = children[0].value
+        try:
+            pname: str = aliases.get(token_name, token_name) if aliases else token_name
+            return Particle.from_evtgen_name(pname).width  # type: ignore[return-value]
+        except Exception as err:
+            raise RuntimeError(
+                f"Particle name/alias {token_name!r} not found! Check your dec file(s)!"
+            ) from err
+
     try:
         return {
             tree.children[0].value: {
                 "mass": float(tree.children[1].value),
-                "width": float(tree.children[2].value),
+                "width": get_set_width_or_default(tree.children),
             }
             for tree in parsed_file.find_data("particle_def")
         }

tests/data/defs-aliases-chargeconj.dec

@@ -280,6 +280,9 @@ LSNONRELBW rho0
 BlattWeisskopf rho0 3.0
 IncludeBirthFactor rho0 no
 IncludeDecayFactor rho0 yes
+# Redefined particles only need to specify the mass, and the width is optional
+Alias MyRho0 rho0
+Particle MyRho0 0.8
 
 # Pythia 8 parameter modifications
 # (Very important that there are no blank spaces in the parameter string!)

tests/dec/test_dec.py

@@ -157,7 +157,7 @@ def test_aliases_parsing():
     p = DecFileParser(DIR / "../data/defs-aliases-chargeconj.dec")
     p.parse()
 
-    assert len(p.dict_aliases()) == 135
+    assert len(p.dict_aliases()) == 136
 
 
 def test_model_aliases_parsing():
@@ -197,6 +197,7 @@ def test_particle_property_definitions():
         "MyK*0": {"mass": 0.892, "width": 0.051},
         "MyPhi": {"mass": 1.02, "width": 0.004},
         "rho0": {"mass": 0.8, "width": 0.2},
+        "MyRho0": {"mass": 0.8, "width": 149.1},
     }