Detector Effects and Backgrounds

1. Detector Effects and Backgrounds In the following, I examine the following possible sources of spurious signals which could simulate the parity signal: Recentering as the sole “corrector” of detector inefficiencies. The effect of resonances in producing a spurious signal. The effects of jets in producing a spurious parity signal.

2. Detector Effects and Backgrounds(Cont’d) • To anticipate the result, we find that these effects are not likely to have produced a spurious signal. • We will also present results from a variety of simulation programs (HIJING,URQMD,AMPT,MEVSYM,PYTHIA) while none of these fully describe the data on HI heavy ion collisions, we look to them for possible clues to how a spurious signal might be produced. • We do not find that they reproduce the observations.

3. Recentering • Although recentering has been used in the flow analysis and has theoretically been shown to be adequate for the correction of the correlation function. We recognize that the observed correlation function is small (~10-5) and we wanted a more complete check of its function.

4. Simulation re:acceptance effects • Use basic simulation with pT, v2 vs nch roughly matched to data. • Add efficiency for +,- vs. φ,pT parameterized from data (also make deliberately much worse) • Look at effect of acceptance on correlator before and after centering of Q-vector. Pt>0.15 Pt>1

5. Sample Results-with 50% sector 2 efficiency x10-3 After centering Before centering

6. Resonances • Simple kinematic studies show it is very hard to get ++,-- signal without turning v2 negative. • To check contribution from known resonances, we’ve run MEVSIM with roughly experimental particle distributions (including resonances) and v2, and then again with 10x resonances.

7. The Effects of Jets • Basic approach is to use the extensive STAR data to estimate the effects. • STAR has observed jets with leading particles with Pt as low as 3 GeV/c, and measured their rate (yield). • In addition STAR has also measured their production relative to the reaction plane. • Finally, STAR has measured jets in p-p collisions at high Pt. This data can be used to set an upper limit on the effects of jets.

8. in-plane fS=0 out-of-plane fS=90o Jet contribution estimate Yield #low pT associated particles(~20) Average (reaction plane independent) correlation v2 PRL 91,172302 PRL 95,152301 A. Feng, QM ‘08 p+p jet patch trigger data, looping only over ‘jet’ particles

9. The Effects of Jets • We can calculate an upper limit on the effect of jets by taking the two body correlation measured with jets from p-p, at large Pt, (~10 GeV/c, from jet patch triggers) as follows: • Taking .1 as the p-p correlation, 300 particles as the average number of tracks for the midcentral events which have the signal, and v2=.3 we get, for the jets with >4 GeV/c leading particles

10. Summary • It seems very unlikely that detector effects or resonance or jet backgrounds can produce a spurious effect that mimics the parity signal. • “Standard” simulation programs also do not predict the signal, although it would be good to have improved statistics (which we will).

11. Au-Au (and p+p) event generator studies Data (no error bars) for +- ++ and -- +- ++ -- Result: no generator we’ve looked at matches cos(phi1+phi2) or cos(phi1-phi2) correlations (note: same and opposite sign correlations typically very similar in these models). AMPT shows some resemblance and probably bears further study.

12. Summary(Cont’d) • There are other phenomena which we should understand better. • One is the effect of V1 fluctuations (probably much too small). • Another is the question of “minijets”. If there are many small (few hundred MeV) type of jets, perhaps that could produce a fake signal. Each one would have rather small angular correlation between its particles but there may be enough of them to make a fake signal. • As there is no extant model for these we cannot at this time estimate their effect if indeed they do exist. Interestingly, there is a possibility that they are a manifestation of the parity effect!

13. Summary(Cont’d) • The future: • More analysis of the data (run 7 with many more events), More facets of the data to be studied to better identify the Chiral Magnetic effect, etc. • What would be most helpful would be additional theoretical analysis of just how the effect would manifest itself in realistic HI collisions. • One of the reasons for wanting to publish the results to date is to inspire such further theoretical work.