update how neck events are flagged

This commit updates the is_neck_event() function to include the requirement
that 50% of the normal PMTs in the event must have z <= -425.0.

2 files changed, 30 insertions, 6 deletions

diff --git a/src/dc.c b/src/dc.c
index 8a93c12..ff99f3f 100644
--- a/src/dc.c
+++ b/src/dc.c
@@ -454,7 +454,7 @@ int qvnhit(event *ev)
 }
 
 /* Returns 1 if the event is classified as a neck event. The definition of
- * a neck event comes from the SNOMAN companion:
+ * a neck event is a combination of the neck flag in the DAMN word:
  *
  *     This cuts events containing neck tubes. It requires that either both
  *     tubes in the neck fire, or that one of those tubes fires and it has a
@@ -464,6 +464,14 @@ int qvnhit(event *ev)
  *     average ECA time of the PSUP PMTS with z les than 0. After the cable
  *     changes to the neck tubes this time difference changes to 15ns. 
  *
+ * and the SNOOP definition of a neck event which requires that 50% of the
+ * normal PMTs have z <= -425.0.
+ *
+ * I added the requirement that 50% of the normal PMTs be near the bottom since
+ * for the high energy events I'm interested in, it's entirely possible that a
+ * high energy electron or muon travelling upwards could cause multiple neck
+ * PMTs to fire.
+ *
  */
 int is_neck_event(event *ev)
 {
@@ -471,7 +479,7 @@ int is_neck_event(event *ev)
     int n;
     int high_charge;
     double avg_ept_psup;
-    int nhit;
+    int nhit, nhit_bottom;
 
     /* First, we compute the average ECA time for all the PSUP PMTs with z <= 0. */
 
@@ -498,11 +506,16 @@ int is_neck_event(event *ev)
      * PMTs. */
     n = 0;
     high_charge = 0;
+    nhit = 0;
+    nhit_bottom = 0;
     for (i = 0; i < MAX_PMTS; i++) {
-        /* Require good calibrations. */
-        if (!ev->pmt_hits[i].hit || ev->pmt_hits[i].ept <= -100) continue;
+        if (!ev->pmt_hits[i].hit) continue;
+
+        switch (pmts[i].pmt_type) {
+        case PMT_NECK:
+            /* Require good calibrations. */
+            if (ev->pmt_hits[i].ept <= -100) break;
 
-        if (pmts[i].pmt_type == PMT_NECK) {
             n += 1;
 
             /* FIXME: The SNOMAN companion says "Early if defined by the neck
@@ -511,10 +524,15 @@ int is_neck_event(event *ev)
              * place? */
             if ((ev->pmt_hits[i].ehs > 70 || ev->pmt_hits[i].ehs < -110) &&
                 (ev->pmt_hits[i].ept < avg_ept_psup - 70.0)) high_charge = 1;
+            break;
+        case PMT_NORMAL:
+            nhit += 1;
+            if (pmts[i].pos[2] < NECK_BOTTOM_HEIGHT) nhit_bottom += 1;
+            break;
         }
     }
 
-    if (n >= 2 || high_charge) return 1;
+    if ((n >= 2 || high_charge) && nhit_bottom >= nhit*NECK_BOTTOM_FRACTION) return 1;
 
     return 0;
 }
diff --git a/src/dc.h b/src/dc.h
index f613b93..102e47f 100644
--- a/src/dc.h
+++ b/src/dc.h
@@ -33,6 +33,12 @@
 #define DC_OWL_TRIGGER    0x100
 #define DC_FTS            0x200
 
+/* Height of PMTs which are considered to be at the "bottom" of the detector
+ * for the neck cut. Comes from the SNOOP definition of a neck event in SNO. */
+#define NECK_BOTTOM_HEIGHT -425.0
+/* Fraction of normal PMT hits which must be below NECK_BOTTOM_HEIGHT. */
+#define NECK_BOTTOM_FRACTION 0.5
+
 /* Delta-t threshold for PMT pairs in the FTS cut (ns). */
 #define FTS_DT_THRESH 25.0
 /* Distance threshold for PMT pairs in the FTS cut (cm). */