1. Tatia Engelmore, Columbia University
Electron - Muon Correlations in p+p Collisions at s=√200 GeV
Tatia Engelmore, Columbia University
Experimental Method
Motivation
Charm production is an important tool for studying the early stages of a heavy ion collision. Despite numerous PHENIX and STAR measurements of leptons produced by charm decays, much remains unknown about the methods of production and suppression by the medium. A new method for studying charm yields is to use correlated electron-muon pairs, which are the semi-leptonic decay products of pairs of D mesons. These have the advantage of being immune to many types of background affecting dielectron and dimuon measurements, such as resonance decays and Drell-Yan production.
Electrons tracks are found in the drift chamber, and are identified using the Ring Imaging Cerenkov Detector and the Electromagnetic Calorimeter. An important method of identification is to look at the ratio of energy over momentum of the particle, cutting on anything that is not close to 1. Electrons are detected in the region between <  < 0.35, with 180 degree acceptance in .
Also because the measurement requires a combination of the forward and central detectors in PHENIX, it will allow us to probe charm production in an intermediate rapidity region that has not yet been studied.
The PHENIX muon detectors are located in the large rapidity region, between |1.2| <  < |2.4|. The muon arms find and fit tracks, and identify muons as those particles that are not absorbed by the steel absorbers.
There is one additional source of correlated e- pairs, and those originate from the semi-leptonic decay of B mesons. These will appear similar to the charm-produced pairs, but will have a higher invariant mass. It may be possible to separate these two sources and use them to study the charm/bottom ratio at RHIC.
If an electron and a muon are the result of the semi-leptonic decay of a pair of D mesons, they will have opposite signs. Many of the backgrounds though, including direct and thermal lepton production produce equal amounts of same sign and opposite sign pairs. Therefore to look at only pairs from D decays we subtract the like sign distribution from the opposite sign distribution. Further backgrounds from Dalitz 0 decay and charged  and K decay to muons will be the main sources of background.
Simulations
To determine what phase space to look in, simulations of p+p events were performed using Pythia. Signal events in which charm production was forced were compared to hard scattering events, in which charm production occurs at a rate normally expected in p-p collisions. The minimum pT was set to 4 GeV/c, and D mesons were forced to decay semi-leptonically.
Diagram of difference acceptance regions for electrons and muons.
It is apparent that e-  pairs from D decay (left) are produced back to back, with a strong peak at =. This is because cc pairs are produced back to back in the initial hard scattering, and the decay products are produced somewhat collinearly with the initial trajectory.
Future Work
In hard scattering, though (right), electrons and muons are produced with a more randomized distribution of , with a small peak at  due to jet contributions (a peak at  = 0 is ruled out due to the differing acceptance of the e's and  's).
Analysis of the p+p dataset from RHIC Run-6 has already begun. This includes 2.3 billion events, and will be used as a baseline for future measurements. After that is completed both d+Au and Au+Au datasets will be analyzed. The charm yield, and possibly the bottom yield will be calculated.
The relative azimuthal angle will be the most useful variable in this analysis. Invariant mass distributions are also being studied, but since there is no sharp peak in signal it is a difficult variable to discriminate with. It may be useful though in studying the relative contributions of D vs. B.