Since they interact with matter only via the weak force, neutrinos provide a probe of the internal structure of nucleons and nuclei that is complementary to the electromagnetic probes used at, say, JLab. The Minerva experiment will attempt to exploit this complementarity so that we learn more about both neutrino interactions with nuclei and the internal structure of neutrons and protons.
Minerva is in fact a detector that will see events generated "parasitically" from the beam being used for the MINOS experiment. In MINOS (which we had a colloquium on last year) where neutrinos are shot from Fermilab to Minnesota in efforts to measure theta_{13}---a key undetermined parameter in our description of neutrino mixing. You need a lot of neutrinos to do that measurement, and a small fraction of these neutrinos can scatter off the iron, carbon, etc. in the Minerva detector. The fraction that do this is small enough that it will not affect MINOS' ability to do its work, hence the term "parasitic". So Minerva hopes to provide us---essentially for free, OK, for about $10M---with (a) more information on neutrino-nucleus cross sections and (b) data on neutrinos scattering from nucleons. (a) actually means that Minerva's relationship to MINOS is symbiotic, not parasitic, since uncertainties in these very neutrino-nucleus cross sections limit MINOS' ability to model their detectors. And lack of understanding of how the neutrinos interact with MINOS' detectors is in turn is a significant systematic error in the limit that MINOS places on theta_{13}.
As for (b), at high energies and momentum transfers we think that the neutrinos scatter almost all the time from individual neutrons and protons inside the nuclei. (Why?) Consequently we can look at events where a neutrino converts a neutron in the nucleus into a muon and a proton. Dr Schulte explained how Minerva could use such events to infer the behavior of the "nucleon axial form factor" with momentum transfer (Q^2). This quantity tells us something about how "axial charge" is distributed inside neutrons and protons. Since we now know (thanks to JLab) that the electric and magnetic form factors of the proton fall off differently at Q^2s above 1 GeV^2 it would be interesting to know which one the axial form factor is like---or if it is different from both G_E and G_M.
Minerva will not deliver data for a few years, which is partly because it is still under development, and partly because neutrino experiments take a while. They don't call them weak interactions for nothing. Even when you have several tons of material it takes a long time to collect enough events to measure these cross sections. Neutrino physicists must be good at delayed gratification.
Which leads me to ask: would you be prepared to put up with waiting this long for your data? If you thought it was important? Can you think of other reasons why we might want to know about neutrino interactions with nucleons and nuclei at high energies?
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2 comments:
I have a question related to her talk that I think someone might be able to give me an insight into. So, Prakash asked her for some physical motivation for the ~5% difference in form factors between light nuclei and heavy nuclei and she didn't really have an answer. I was wondering if anyone knew the reason for this? Furthermore, is the reason for adding a Helium-3 chamber in front of the detector an effort to better refine this measurement?
Also, with regard to the axial form factor. It seems to me as if the ability to determine the axial form factor is significantly related to our understanding of G_E and G_M. But, as she her self demonstrated, data on these quantities is all over the place (she showed an image very similar to this one I found at JLab - first image on the page). What I'm wondering is this: are the people at the Minerva collaboration hoping that new, better data on G_E and G_M will come out by the time they start analyzing data (perhaps from the experimental proposal I linked to), or is this error just not that large of a contribution?
The Helium-3 chamber I do not understand. Maybe someone else can comment?
The physical motivation for the modification of nucleon properties "in the nuclear medium" has to do with the picture of the nucleus where individual protons and neutrons inside it move in the 'mean-field' generated by all the other nucleons. (Think Hartree-Fock for atoms.) The idea in the model Elaine showed results from in her talk is that it's not just the nucleons that respond to that field, but also the quarks inside them. It's quite natural to think that placing a bound state of quarks inside a strong potential will distort the structure of the bound state. It's that distortion by the "mean-field potential" inside the nucleus that might produce modification of the nucleon's magnetic moment, radius, and axial properties too.
I think the answer to your G_E and G_M question comes in two parts: (1) From what I hear pretty much everyone now places more faith in the lower data on the ratio G_E/G_M. That's because we now understand that two-photon-exchange corrections affect the extraction of G_E that was employed to obtain the data sets that showed G_E approximately equal to G_M above Q^2=1 GeV^2. (Calculations seem to indicate that two-photon exchange corrections do not significantly affect the G_E/G_M data obtained at JLab by polarization transfer.) So I think that as far as the proton goes, there is not as much uncertainty as that figure might lead you to think there is.
(2) But neutron form factors are still quite uncertain at these Q^2s, so your question remains pertinent. (Not impertinent....) That was indeed the question I wanted, but did not get, an answer to. If I heard Elaine right she said that G_E and G_M are thought to fall off faster than G_A and so somewhere above Q^2=1 GeV^2 the cross section for the "quasi-elastic scattering" will have only a small piece that depends on our knowledge of G_E/G_M.
If you want a better answer have a look at the original Minerva proposal. (I love the archive....) Try p. 55 of the postscript file.
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