2006 EEMC Neutral Pions: Transverse Single-spin Asymmetry (388 Runs)

Steve has produced trees for a large fraction of the ppTransverse run. In this update, I show asymmetry figures for 388 runs. Please, refer to my previous update for a bit of background discussion. Again, note, the asymmetries are not corrected for polarization. Also, errors are purely statistical and potentially azimuthally dependent backgrounds are not corrected.

Figure 1

In Fig. 1 we see various plots for di-photons. There is obviously quite a bit of background under the pion peak which will need to be removed. The xF vs. pT kinematic distribution shows almost no events above xF = 0.5.

Figure 2

pT xF

The top row shows the extracted asymmetry while the bottom row shows the constant of the fit. Asymmetries for xF > 0 are in blue circles, while asymmetries for xF < 0 are in red triangles. In the pT plots I have shifted the blue points by 0.25 to attempt to make the figures easier to read. Again, I emphasize the asymmetries are not corrected for polarization. There do not appear to be large effects in either blue beam, yellow beam, or in the p0's. However, it does seem like the xF asymmetries are systematically above the horizontal. It will be important to examine, again, the individual fits to make sure they are sensible.

Figure 3

Fitted Asymmetries as a Function of pT (xF > 0)

One other interesting check is to look at the pT dependence, if any, of the asymmetry. One would expect an effect driven by pQCD to fall off as ∼1/pT. In Fig. 3, on the left, I plot a p0/pT fit to the xF > 0 asymmetries; and on the right I fit with a function of the form p0+p1/pT. There is a 2σ dependence with a not-so-crazy χ2. Again, I would caution against any conclusions until we check not only individual fits but also systematics and background corrections.

Figure 4: Fit Results for pT Bins

0 < pT < 4 GeV/c
4 < pT < 5 GeV/c
5 < pT < 6 GeV/c
6 < pT < 7 GeV/c
7 < pT < 8 GeV/c
8 < pT < 9 GeV/c
9 < pT < 10 GeV/c
10 < pT < 12 GeV/c
12 < pT < 16 GeV/c

In Fig. 4 I show the fits for the individual pT bins as well as the instrumental and luminosity asymmetries. Instrumental asymmetries seem to be below 30% and typically below 10%. Luminosity asymmetries also appear to be fairly unremarkable.

Figure 5: Fit Results for xF Bins

0 < xF < 0.1
0.1 < xF < 0.2
0.2 < xF < 0.3
0.3 < xF < 0.5

One thing to note about the xF asymmetries is that the χ2 values appear to be way too good. It seems as though I could fit each of them with a constant and do just as well. Most points are statistically consistent with zero.