DOC: explain rest frame of decaying particle

.cspell.json

@@ -139,6 +139,7 @@
     "nbformat",
     "nbins",
     "nbody",
+    "nbsp",
     "nbsphinx",
     "ncalls",
     "ncols",

docs/amplitude-analysis.ipynb

@@ -473,7 +473,40 @@
     "raw_mimetype": "text/restructuredtext"
    },
    "source": [
-    "The resulting phase space sample is a {obj}`dict` of final state IDs to an {obj}`~numpy.array` of four-momenta. In the last step, we converted this sample in such a way that it is rendered as an understandable {class}`pandas.DataFrame`."
+    "The resulting phase space sample is a {obj}`dict` of final state IDs to an {obj}`~numpy.array` of four-momenta. In the last step, we converted this sample in such a way that it is rendered as an understandable {class}`pandas.DataFrame`.\n",
+    "\n",
+    ":::{admonition} Four-momentum arrays\n",
+    "{doc}`Kinematic expressions <ampform:usage/kinematics>` from AmpForm that involve four-momenta should be formatted as $\\left(E, p_x, p_y, p_z\\right)$. In addition, the shape of input arrays should be `(n, 4)` with `n` the number of events.\n",
+    ":::\n",
+    "\n",
+    ":::{warning}\n",
+    "When using the helicity formalism, the sum of the four-momenta should be in the rest frame, that is $\\sum_i p_i = \\left(m_A, 0, 0, 0\\right)$ with $m_A$ the mass of the decaying particle&nbsp;$A$. This is because the helicity formalisms boosts through the decay chain starting from particle&nbsp;$A$. **Take care to boost your experimental data into the rest frame**, optionally following the {doc}`kinematics classes provided by AmpForm <ampform:usage/kinematics>`.\n",
+    ":::"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "tags": []
+   },
+   "outputs": [],
+   "source": [
+    "import numpy as np\n",
+    "\n",
+    "p = np.array(list(phsp_momenta.values()))\n",
+    "p.shape"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {
+    "tags": []
+   },
+   "outputs": [],
+   "source": [
+    "p.sum(axis=0).round(decimals=14)"
    ]
   },
   {
@@ -637,6 +670,10 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
+    "::::{note}\n",
+    "Check the remark about four-momentum format and rest frame of the decaying particle [here](compwa-step-2.1).\n",
+    "::::\n",
+    "\n",
     "The {obj}`.DataSample` is a mapping of kinematic variables names to a 1-dimensional array of values. The numbers you see here are final state IDs as defined in the {class}`~ampform.helicity.HelicityModel` member of the {class}`~ampform.helicity.HelicityModel`:"
    ]
   },