Minutes (Hours) from May 6 Meeting

Discussion today focused on the summaries of the new intiatives workshop that we just survived (May 2-4), in particular what the detector issues are. One new detector technology that was discussed at the workshop came from Ken Heller and consists of a fine-grained water-liquid scintillator detector. The absorber is cheap, and adds stability to the liquid scintillator, which is itself known to be significantly cheaper than solid scintillator. Ken Heller's talk in the working group session can be found in LiquidScintillator_Detector.pdf. Mayda reported that Michal was looking into what happens if you take the steel--solid scintillator--air design they have now and change it such that the air gap is filled with water, and the scintillator is now liquid scintillator and not solid. They will have new performance results for that detector soon, although it was noted that if there's 0.45mm of steel combined with 3cm of water and still only one readout plane, then the total mass of the detector per readout plane has gone up by almost a factor of 1.7 and the segmentation has been increased a little above 1/3 a radiation length. Michal is also looking into the performance of Ken's design, which would only contain water and scintillator, and different transverse segmentation than the initial design. Both sets of results should be very interesting...

The summary talks from the detector working group at the New Initiatives session can be found in newinit.ps (Debbie's talk) and Detectorsummary.pdf (Ken's talk).

The discussion at the workshop after these two talks (and at today's meeting) centered around the following two approaches: On the subject of how well delta m2 has to be known before you would pick a site, Mayda sent email to the group last month with this plot fom.eps showing the figure of merit (signal/sqrt(signal+background))for the fine-grained steel-scintillator detector as a function of delta m^2 (delta m2 ranging between 2e-3 and 4e-3eV2). Things don't change very quickly within that range, for several different angles and baseline distances. A similar plot can be found basevdm2.eps , which shows the limit versus baseline length for dm2 = 2,3,4eV^2. However, there are several problems with these kinds of plots, as pointed out by Kevin but iterated by others (I added my own capitalization...): Many believe that there is a strong case for going ahead quickly now and proposing something that has a reach of 10x the CHOOZ limit, and trying to get funding such that the detector can be built ASAP and start running as early in the MINOS run as possible. While we all agree that the first measurement of a non-zero theta_13 is exciting and interesting, we also all agree that it's only the first step. The sooner we go ahead with an experiment and a detector technology choice, the sooner we get to that first step, but it might be argued (Debbie's editorializing here) that the sooner we make the detector decision on the first step the less likely it is that the second step, namely the upgraded detector, looks identical to the first one. On the other hand, maybe the NC cross sections and rejection factors we are using now turn out to be correct, and the next detector looks almost identical to the first one. Also, in the model that Stan has suggested where we just start building and just keep adding more detector as the funding allows, this would make changing detector technologies in the middle really painful.

Finally, Maury lead a discussion on comparing detector costs--both for the "first" and "second" generation detector, where the assumption is that the second detector is a factor of 5 or 25 in mass past the first detector. Executive summary: 25 times anything is a big number The idea is: if the proton driver upgrade is something like 300M$ and gives you a factor of 5 in intensity, how much do you spend for upgrading your program in two scenarios: making a detector a factor of 5 more massive plus a proton driver upgrade, or making a detector a factor of 25 more massive with no proton driver upgrade? Although the answer of which option you would prefer is not surprising to anyone, the magnitude of the factor by which a proton driver upgrade is cheaper was surprising to most of us in the room. Steve Geer commented that in some of these detectorx25 options, you're starting to talk about an experiment that is as expensive as a neutrino factory. (At which point many of us in the room said "yes, that's why we've wanted a neutrino factory all along!). The other important point everyone has to remember of course is that with a proton driver you get lots of other physics too, whereas with a more massive detector near the surface you're still not going to be able to do proton decay, solar neutrinos, etc.

In comparing different detector technologies, the cost of the detector for a given sensitivity in Ue3^2 (i.e. you need less mass for an ICARUS-type detector, more mass for a water cerenkov detector) ranges from 30M all the way to 100M. (Aside: when Jeff Nelson used the spread sheet for what MINOS would cost for a 20kton detector, but with 0.45mm steel segmentation he got 313M, which includes labor, materials, etc. There are cheaper readout options being explored by Bruce King, and also Michal is now adding water and liquid scintillator, as mentioned above). But anyway, if the detector is 30M to 100M, then a 5 times more massive detector is 150M to 500M So a 5x more massive detector plus a PD upgrade is (very very roughly) 450M to 800M. Just an aside for the water cerenkov fans (who probably know this already): 5 to the 2/3 power is only 3, and 25 to the 2/3 power is only 9. Compare this with a x25 more massive detector, which would be 750M to 2.5B!!! Of course with most of these detector technologies, with a factor of 25 more exposure you do have to start worrying about the systematic errors, so a factor of 25 in exposure might not get you a factor of 5 in overall reach or precision! As noted above, this last region is where the neutrino factory with its "measly" 50kton minos-type detector starts looking comparable in cost.
Deborah Harris
Last modified: Thu May 9 12:41:28 CDT 2002