Particle types in HIJING and in the EPD

We checked with Roland, what types of primary particles cause hits in the EPD (i.e. for each hit, the primary particle was checked, to which the EPD hit can be traced back, potentially through several daughters - this means that some primary tracks are multiple counted, if they caused multiple EPD hits through their daughters). Here is a summary plot, also showing the fractions for all primaries (single counted, and not just those that leave an EPD hit):

FIG 1: ALL HIJING PRIMARIES, BEFORE DECAYS	FIG 2: ALL HIJING PARTICLES, AFTER DECAYS OF PRIMARIES
FIG 3: EPD HITS BY CAUSING HIJING PARTICLE, BEFORE DECAYS	FIG 4: EPD HITS BY CAUSING HIJING PARTICLE, AFTER DECAYS

In these plots, "primary causes" mean that we track each EPD hit back to a primary, where primary can be understood (a la HIJING) to be before or after decays.

To give the numbers as well, the major contributors of EPD hits are (again, this is in fraction of EPD hits, so particles are "multiple counted" if they cause multiple hits through their shower or decay history)Where the "before decays" list goes on with Sigmas, Xis, Ds, leptons..., but the "after decays" list is complete, there are no other "primaries" (after decays).

BEFORE DECAY			AFTER DECAY
pi0	23.35%		gamma	25.61%
pi-	22.04%		pi-	25.33%
pi+	22.03%		pi+	24.23%
P	7.88%		P	8.05%
K0S	4.74%		N	5.24%
N	4.42%		K-	2.93%
K+	2.89%		Pbar	2.76%
K-	2.79%		K+	2.75%
Pbar	2.05%		Nbar	1.23%
gamma	1.69%		K0L	0.94%
K0L	1.08%		e-	0.48%
Nbar	0.91%		e+	0.44%
OTHER	4.14%		OTHER	0.00%

Particle types ring-by-ring

Now how does this look ring by ring? Here are the plots with the "before decays" version (left plot is for totals, right is for percentage):

FIG 5: RING-BY-RING HIT CAUSING PRIMARIES, TOTALS	FIG 6: RING-BY-RING HIT CAUSING PRIMARIES, PERCENTAGES

Apparently there is some ring dependence, for the middle rings (around eta 3-4) the fraction of charged pions is much larger, and for the outer rings, pi0 fraction is larger. Note furthermore that the "OTHER" category includes Lambdas, Sigmas, Xis, Ds, etc.

Here are the same plots summarized for particle classes:

FIG 7: RING-BY-RING HIT CAUSING PARTICLE TYPES, TOTALS	FIG 8: RING-BY-RING HIT CAUSING PARTICLE TYPES, PERCENTAGES

Again, one can see that in the outer rings (1&16) less than 50% is charged hadron, while in the inner rings (around ring 8), more than 70% of the hits come from charged hadrons. Similarly, neutral hadron fraction is barely more than 20% in the central rings (around ring 8), while it is more than 50% in the outer rings (1&16). In the meanwhile, the lepton and photon fraction is very small, maximally a few %. Note again that this is the primary particle's type that in the end leads to the given EPD hit.

GEANT particles hitting the EPD

Just for completeness, let me show here the composition of particles, hijing+secondary (mostly secondary, i.e. GEANT), that cause EPD hits. It shows that more than 80% of hits are directly caused by e+- and pi+-, 10% by p and pbar, and 2% by gammas.

FIG 9: EPD HITS BY DIRECTLY HITTING PARTICLE TYPE

Particles born in the EPD and also leaving a hit in the EPD

Another thing we checked was the type distributon of those particles (secondaries) that create a hit in the EPD but are also born in the EPD (where "inside the EPD" means for simplicity that
zvertex is +-2 cm within the 374 cm distance, east or west, and 4.5 cm < r < 90 cm). These represent roughly a 17% of all hits, i.e. out of 240k hits, roughly 41k hits are caused by particles that are born inside the EPD.

FIG 10: EPD HITS CREATED BY PARTICLES BORN INSIDE THE EPD

One further thing we checked was how far these particles fly after they are born inside the EPD. Note that most particles being born in the EPD do not have an end vertex (not sure what this means), so the below histograms only show those 2.5k particels that do have one (out of the 41k). This is shown here, along with a 2D plot where flight distance vs ending vertex z-coordinate are shown. Recall that these are tracks for which the start vertex is inside the EPD.

FIG 11: FLIGHT DISTANCE OF PARTICLES BORN INSIDE THE EPD	FIG 12: FLIGHT DISTANCE VS Z-VERTEX END

While there is a large peak at zero (i.e. some many of these particles are also dying inside the EPD), there are many entries at larger flight distances. This really means that a significant fraction of these particles travel far away, so are not dying inside the EPD. The peak at ~3.5 meters is due to something at roughly +7 meters, as there is a local maximum on the 2D histogram at roughly (350 cm, 720 cm). So these fly away from the plus side EPD, and then die at z=7.2 meters.

We furthermore investigated what kind of interactions create these tracks being born inside the EPD and giving a hit at the same time. We found that out of those 41k hits by particles born inside the EPD, 30k have a direct parent that is also hitting the EPD. The types of interactions that do point #2 above (particle creates EPD hit, daughter created inside EPD and also creates hit) are these (in descending order of frequency):