There are a number of physics processes that can contribute to the signal in the W kinematical region either because of their inherent similarlity to W production or because of the lack of the hermiticity of the detector. They are listed here and their relative contributions to the W signal region will be filled in as the studies are completed. (Needed as well : Their possibility of contaminating the asymmetry will also be discussed.) The hope is to have a table of processes and their statistical contamination in the signal region. In some places I have put in ways of measuring this or using a data-driven method to estimate it.
1) Z production - There are a number of ways that Z production could potentially contribute to the background in the W signal region: dijet production, heavy flavor production, and mainly electron-positron production where either the electron or positron is not reconstructed because of detector innefficiencies or lack of hermiticity. The relative contributions from each of these processes should be talleyed. The electron/positron contamination can partly be measured by looking at the Z spectrum that we measure and then extrapolating for the missing Endcap,which only gives you a small part of phase space, but we can use that as a constraint on further PYTHIA studies that should be done.
2) Heavy flavor production - It is possible in the initial 2->2 scattering to create a pair of heavy quarks. There are a number of ways this process can mimic a W signal: one of the quarks fragments into a highly collimated jet where the other quark's decay products fall out of the acceptance or semi-leptonically into particles that don't interact strongly with the detector elements, and where one of the quarks semi-leptonically decays into an electron or positron and the other quark undergoes a similar lifetime to the previous example. There is also the possibility of intrinsic charm/bottom in the proton, but its effect will not be discussed here. This is likely to be a small source of background, but nontheless a number should be calculated. This can be estimated using PYTHIA as well as the published cross sections in the literature.
3) Heavy lepton production - In the case of W/Z production there is a non-negligible probabliity that it will decay through the heavy leptonic channels (mu,tau). These heavy leptons can then semi-leptonically decay to an electron or positron that could then subsequently mimic a W decay. For this to pass the algorithm the decay would have to be early in the lifetime of the heavy lepton. All the neutrinos won't be detected so this is a potential source of background. This is also likely to be a small source.
4) 2->2 dijets - Dijets will be the dominant source of background as the 2->2 process will be the majority of all dijets, this will be the dominant background. These will be broken up into three kinematical/geometrical categories.
- mid-mid - (midrapidity jet + midrapidity jet) - In this background you have both jets of the dijet pair fragmenting towards midrapidity. One of the jets must be highly collimated to mimic the electron signal (or just have fragmented almost fully to an electron) and the opposite jet must have done one of the following : fragmented to a diffuse jet or to a high-z hadron, fragmented into a region of dead BEMC (can we constrain using BSMD?), or be on the edge of the detector where half of the jet is being radiated outside the acceptance (should look at the ratio of events being rejected for opposite ET in the center of the BEMC versus those that are being rejected for ET near the edge of the barrel to see if there is an effect and how much it is... possibly implement a rapidity dependent cut?). There should be a study in this about the other jet leaking outside the crack between the barrel and the endcap.
- mid-endcap - (midrapidity jet + endcap rapidity jet) - This background is similar to the mid-mid background in that it suffers from all the same effects and many of the same studies (ESMD check on bad t0 sectors and dead regions/eta depedence of veto cuts) should be done. We are going to use Justin's/Scott's idea to statisically subtract the background due to those dijets where the opposite jet was in the missing endcap.
- mid-beampipe - (midrapidity jet + beampipe rapidity jet) - We have no real working detectors in this region (the ZDC should give nothing? no chance of the FMS?) so to estimate the background in this region we should simulate these events in PYTHIA (as well as the mid-mid and mid-endcap) and then taking ratios of the signals in MC use the size of the signal in the missing Endcap as an estimation of the background (I wouldn't do an absolute subtraction with the MC yet because it hasn't been shown to reproduce our background shape/size at all and I would prefer something more data motivation/constrained).
5) 2->3 dijets - These should be a smaller fraction of the dijet signal than the 2->2 dijets (how much?), though because two of the dijets will inherently have less energy and be less back-to-back than than in the case of the 2->2 dijets they have the chance of being a non-negligible background. The various types of 2->3 dijet events to worry about are outlined below.*
- mid-mid-mid (midrapidity jet + midrapidity jet (or out of acc)+ midrapidity jet (out of acc)) - In this background you again have one highly collimated jet in the BEMC mimicing an electron/positron and then two opposite jets, both falling into the BEMC accetpance, which are at a "large" angle to backwards direction from the highly collimated jet. Because of this they can either fall outside the acceptance of the opposite veto in phi (We are designing cuts to treat this now...). They can also fall into the class where one of the three jets falls outside the detector acceptance (into the crack between the BEMC/EEMC) or on a dead patch of the BEMC (again check with the BSMD?) or on the edges of the detector
- You can also have any other combination where you have one of the jets falls into the endcap acceptance or out of the acceptance. There are a large number of combinations mid-(mid/endcap/out)-(mid/endcap/out) and the only real way to cut these out now is to consider more robust veto cuts (???)