Re: Overview Talk

[Matthew Shepherd <mashephe@indiana.edu>]

Hall D PID Mail List:


Hi Curtis,

I'm reading through your talk.  There are still several typographical  
errors and such that I'll send in a separate email.  However, I have  
one or two "big" concerns that I would like to pick out for discussion.

On slides 14 and 15 --

slide 14:

the a4 is 3% of the a2 as observed in E852
the a2 has a 0.5 mub cross section in photoproduction
therefore GlueX should detect 3% * 0.5 mub or 15 nb things with E852  
stats

I think this is faulty logic.

First of all there is no guarantee that the background in photo  
production and pion production are the same.

More importantly, if a2 is suppressed in photoproduction this  
artificially pushes your claimed sensitivity down.  I think to make  
any claim you would have say that a4 production in 3 pi in E852 is X %  
of the total hadronic cross section.  Then one might roughly estimate  
X * 120 mu-b for sensitivity in GlueX.

The E852 example also is in a relatively background free channel.  A  
high multiplicity final state that is fighting off background from  
other broken events will certainly be more of a challenge.  Recall in  
the all of the recent calorimetry work we were comfortable making a  
loose statement about being able to isolate from background a state  
produced at the level of a hundred nb and decaying into the "key"  
channels.  This included an analysis of leakage from other inclusively  
produced but mis-reconstructed events.

slide 15:

"GlueX will find exotic hybrids if they exist at a few % of normal  
mesons."

This statement is not justified anywhere that I can see.  It may be  
true under certain qualifying assumptions (hybrids are narrow, they  
have some considerable 3 pi partial width, etc.).  Since this is the  
key point for GlueX physics I would avoid making such a statement that  
cannot be defended.  In my opinion I would categorize this as a sound  
bite that is very effective in selling the experiment, but we now we  
are at the final design and need to be serious with ourselves and also  
with reviewers.  This is the opening talk -- if one of them would  
latch onto that statement and really press you to defend it, I'm  
afraid it could start things off on the wrong foot.  I also think the  
panel will be on the lookout for what appears to be essential concrete  
statements like this and attempting to verify their validity.  We  
don't want to give them any that can't be backed up.

Don't get me wrong; I'm not throwing a wet blanket on GlueX physics.   
I think you should emphasize the potential for discovery -- that's  
what gives me goosebumps -- but I would back off from making strong  
quantitative statements about sensitivity that we cannot clearly  
defend.  (If you can rehash your argument for 15 nb using E852  
comparisons that would be great, but as is, I don't quite see the  
logic.)

I'll make other notes on the slides and send them to you in the next  
hour.

-Matt



On Mar 24, 2008, at 9:42 PM, Curtis A. Meyer wrote:

> I have reposted the slides to the overview talk to the GlueX
> portal. It is now consistent with Elton's talk on PID, but I
> will need to make another pass after Eugene posts his
> talk.
>
> I tried to get all the sensible changes that were recomended
> today. The reordering of things required a bit more rearranging than
> I had expected, but I think it is roughly right now. I will be going
> through it again carefully in the morning.
>
>  Cheers - Curtis
> -- 
> Professor Curtis A. Meyer        Department of Physics
> Phone:  (412) 268-2745          Carnegie Mellon University
> Fax:    (412) 681-0648            Pittsburgh PA 15213-3890
> cmeyer@ernest.phys.cmu.edu  http://www.curtismeyer.com/