From 90e6269f71e301517344fe89756d6c76236cb1d8 Mon Sep 17 00:00:00 2001
From: Sajal Kumar Jana <kakalijana1254@gmail.com>
Date: Fri, 2 Oct 2026 02:23:18 +0000
Subject: [PATCH] fix(event-display): keep jivexml backward track theta and
 charge

JiveXML gives a track's polar angle as cotTheta, and the loader turns it
into theta with atan(1 / cotTheta). For a backward track (cotTheta < 0,
eta < 0) that is negative, and the shift into (0, PI) only happened after
dparams had already been built from it. So backward tracks were stored
with a negative theta in dparams[3] and, since |p| = pT / sin(theta)
came out negative, with the sign of q/p in dparams[4] flipped.

That gave every backward track the wrong charge wherever it is read from
dparams (the q shown for a track, "Color by Charge", the kinematics
panel), a wrong theta/angle, and made the Runge-Kutta extrapolation of
tracks without polylines bend the wrong way.

Shift theta into (0, PI) before dparams is built, and add tests for
forward and backward tracks.
---
 .../src/loaders/jivexml-loader.ts             |  7 ++-
 .../src/tests/loaders/jivexml-loader.test.ts  | 48 +++++++++++++++++++
 2 files changed, 51 insertions(+), 4 deletions(-)
 create mode 100644 packages/phoenix-event-display/src/tests/loaders/jivexml-loader.test.ts

diff --git a/packages/phoenix-event-display/src/loaders/jivexml-loader.ts b/packages/phoenix-event-display/src/loaders/jivexml-loader.ts
index 1b4f58d68..0c69da1e6 100644
--- a/packages/phoenix-event-display/src/loaders/jivexml-loader.ts
+++ b/packages/phoenix-event-display/src/loaders/jivexml-loader.ts
@@ -299,6 +299,9 @@ export class JiveXMLLoader extends PhoenixLoader {
           track.author = trackAuthor[i];
 
         let theta = Math.atan(1 / cotTheta[i]);
+        if (theta < 0) {
+          theta += Math.PI;
+        }
 
         track.pT = Math.abs(pT[i]) * 1000; // JiveXML uses GeV
         const momentum = track.pT / Math.sin(theta);
@@ -318,10 +321,6 @@ export class JiveXMLLoader extends PhoenixLoader {
         //   storeTrack = true;
         // }
 
-        if (theta < 0) {
-          theta += Math.PI;
-          // TODO - check if we need to flip phi here?
-        }
         // FIXME - should probably handle this better ... what if phi = 4PI for example?
         if (track.phi > Math.PI) {
           track.phi -= 2.0 * Math.PI;
diff --git a/packages/phoenix-event-display/src/tests/loaders/jivexml-loader.test.ts b/packages/phoenix-event-display/src/tests/loaders/jivexml-loader.test.ts
new file mode 100644
index 000000000..3b453b7c5
--- /dev/null
+++ b/packages/phoenix-event-display/src/tests/loaders/jivexml-loader.test.ts
@@ -0,0 +1,48 @@
+/**
+ * @jest-environment jsdom
+ */
+import { JiveXMLLoader } from '../../loaders/jivexml-loader';
+
+describe('JiveXMLLoader', () => {
+  describe('getTracks', () => {
+    const getTracks = () => {
+      const xml = `<Event eventNumber="1" runNumber="1" lumiBlock="1" dateTime="2026-01-01">
+        <Track count="2" storeGateKey="SomeTracks">
+          <chi2>1.0 1.0</chi2>
+          <numDoF>3 3</numDoF>
+          <pt>10.0 10.0</pt>
+          <d0>0.1 0.1</d0>
+          <z0>1.0 1.0</z0>
+          <phi0>0.5 0.5</phi0>
+          <cotTheta>1.0 -1.0</cotTheta>
+        </Track>
+      </Event>`;
+
+      const loader = new JiveXMLLoader();
+      loader.process(xml);
+      return loader.getEventData().Tracks.SomeTracks;
+    };
+
+    it('should give forward and backward tracks a theta in (0, PI)', () => {
+      const [forward, backward] = getTracks();
+
+      expect(forward.dparams[3]).toBeCloseTo(Math.PI / 4);
+      expect(backward.dparams[3]).toBeCloseTo((3 * Math.PI) / 4);
+    });
+
+    it('should keep the charge of backward tracks', () => {
+      const [forward, backward] = getTracks();
+      const momentum = 10000 / Math.sin(Math.PI / 4);
+
+      expect(forward.dparams[4]).toBeCloseTo(1 / momentum, 10);
+      expect(backward.dparams[4]).toBeCloseTo(1 / momentum, 10);
+    });
+
+    it('should give backward tracks a negative eta', () => {
+      const [forward, backward] = getTracks();
+
+      expect(forward.eta).toBeCloseTo(0.8814, 4);
+      expect(backward.eta).toBeCloseTo(-0.8814, 4);
+    });
+  });
+});