[Halld-physics] Eta Proposal

[Matthew Shepherd]

Hi Liping,

On May 1, 2012, at 10:51 AM, Gan, Liping wrote:

> As pointed out in Dave's note, he wanted to study the upper limit only. Therefore, he did not apply any rare decay specific trigger and cuts at all. In the eta rare experiment, we take beam above 9 GeV. There should be a hardware cut in the trigger which requires the total energy deposit in the FCAL greater than 8 GeV, which will remove most of background in Dave's figure 2.  In the offline analysis, the coincident window will not be 100 ns, it should be at level of 4 ns! We used about 4 ns coincident window in PrimEx-I analysis even without individual TDC. With Flash ADC in Hall D, we can only do better. 

You have shown that the upper limit (with some crude logarithmic extrapolation) suggestions thousands to tens of thousands of events in your signal region.  That doesn't say anything about the feasibility of the measurement.  I agree that, if the upper limit showed that there was definitely no problem, then anything better than the upper limit would be fine -- I don't think you are in that scenario.  The upper limit argument only works if it produces a limit that is safely below your proposed signal.  Rather than argue about cuts that were not made or windows that are not applied that would reduce background, why not just do the simulation for the realistic running configuration?

> As for eta -> pipi, I don't not aware any  particular models for non-SM CP violation prediction, neither  one would see an effect in eta -> 2 pi0 but not in eta -> pi+ pi-.

I can't decipher the grammar here -- can you clarify?

> New experiment results may encourage new theoretical calculations.   I do not agree with you that BESIII result can make final statement on those channels. I have discussed with the group who are working on this subject last year . They agreed that their background will be orders of magnitude worse than Jlab.  With same number of eta decays, our actual detection efficiency will be much higher than any facilities, including BESIII, WASA, CB-MIMI, and  future KLOE-2.

You may be right -- let's avoid speculation about what will happen at future facilities.  

Independent of that, the limit for pi+ pi- channel is already 30 times lower than then pi^0 pi^0 channel.  Since you are exploring just the pi^0 pi^0 channel you are only sensitive to effects that may appear in pi^0 pi^0 but not pi^+ pi^- (since a partial width to pi^+ pi^- at your level of sensitivity has already been excluded).  My question was:  what type of models would predict such a thing?  If the answer is "none," then it seems like the goal for your experiment needs to be push the limit well (an order of magnitude) below the current pi+ pi- limit -- your current proposal will produce a limit that is about 1.5 times lower than the pi+ pi- limit (assuming your backgrounds are properly estimated).  

Maybe there are completely different models that predict the pi^0 pi^0 channel would be more sensitive.  If so, it would be good to highlight this.  You are proposing a very expensive detector and this is your flagship measurement, it would be good to know what models for beyond SM physics are being tested.

> I agree with you that  to distinguish eta->pi0+2gamma and eta-2pi0 is not a easy job. However, we are not the first one to do this business either.

Do you have a reference for the proposed analysis method?  The pi^0 pi^0 upper limit is actually higher than the measured rate for eta -> pi^0 2 gamma.  This implies that no one has been able to limit the pi^0 pi^0 rate to anything less than than the pi^0 -> 2 gamma rate.  Your goal is 8 x 10^-6 which is over an order of magnitude below the pi^0 gamma gamma rate.  It may be possible, but I can't follow all the numbers in the text to try to figure out how you plan to get there and how will handle the cross feed from the larger pi^0 2 gamma.  A few plots would be very helpful.

Thanks,

Matt