From 71fe6ea97a4c94611863567bb6b7e517ed486e0f Mon Sep 17 00:00:00 2001
From: Moritz Kiehn <msmk@cern.ch>
Date: Fri, 27 Apr 2018 20:51:31 +0200
Subject: [PATCH] README: mention particle_id=0

---
 README.md | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/README.md b/README.md
index a7cb0cd..5e95db2 100644
--- a/README.md
+++ b/README.md
@@ -170,7 +170,8 @@ particle/track.
 
 *   **hit_id**: numerical identifier of the hit as defined in the hits file.
 *   **particle_id**: numerical identifier of the generating particle as defined
-    in the particles file.
+    in the particles file. A value of 0 means that the hit did not originate
+    from a reconstructible particle, but e.g. from detector noise.
 *   **tx, ty, tz** true intersection point in global coordinates (in
     millimeters) between the particle trajectory and the sensitive surface.
 *   **tpx, tpy, tpz** true particle momentum (in GeV/c) in the global