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?
Wednesday, January 30, 2008
Thursday, January 24, 2008
When you're a jet...
you may not be a jet all the way. At least not at the Relativistic Heavy-Ion Collider. "Jets" in this instance refers not to the local NFL team, but instead to streams of hadrons that are formed in the high-energy collisions that take place inside this machine. At high energies the formation of these jets is a prediction of perturbative Quantum Chromodynamics (QCD). Last Thursday's speaker was using the modifications in the patterns of formation of these jets at high temperatures and densities to try an learn about QCD in these extreme conditions. In particular "away-side jet suppression", where jets do not appear as regularly in collisions of gold nuclei as they do in collisions of protons, suggests that the smashing together of the roughly 400 protons and neutrons in two gold nuclei heats up the QCD vacuum a lot more than a proton-proton collision does. Yes, I know this is not a surprise, but the point is that the results are so different that there seems to be a phase transition to a "new state of matter": the so-called Quark-gluon plasma, that some have dubbed "the perfect liquid" because of its low viscosity.
I found it interesting to hear Dr Frantz say that predictions from pQCD do a good job of predicting much of the behaviour of the jets in the proton-proton collisions. But when it comes to the gold-gold collisions the hot "QCD plasma" (or, more conservatively "QCD fluid") that we think gets formed in the collision buffets the jet that has to traverse more of the plasma. Those jets lose a lot of energy to the plasma, and so we tend not to detect them as often. I liked the way that Dr Frantz could test this hypothesis by looking at how photons escape from this plasma. After all, if the jet is interacting, i.e. being accelerated by, the stuff in the plasma, then it should spit off photons too. But once a photon has been spat off, it will tend to get out of the plasma, since it does not carry "QCD charge". So photons, once emitted, can be photons all the way to the detector. And the data Dr Frantz and his colleagues collected bore out this hypothesis.
The future of this research would seem to be at the Large Hadron Collider, which has a heavy-ion program lined up for a period a few years from now when they hope to be done with discovering the Higgs boson, supersymmetry, and extra dimensions. (Yes, I am joking, although I am sure they hope to find all that.) Because the energies at the LHC will be much higher than those at RHIC they can test whether the jets continue to be suppressed in higher-energy collisions. Because QCD is an asymptotically free theory one would expect that the interactions of higher-energy jets with the plasma will be smaller than those of the 4-10 GeV jets they've looked at so far at RHIC. But this is asymptotic freedom, so it may take a while to get there. I guess once the LHC does this experiment we'll have more idea of what energy we have to go to before jets really are jets all the way.
I found it interesting to hear Dr Frantz say that predictions from pQCD do a good job of predicting much of the behaviour of the jets in the proton-proton collisions. But when it comes to the gold-gold collisions the hot "QCD plasma" (or, more conservatively "QCD fluid") that we think gets formed in the collision buffets the jet that has to traverse more of the plasma. Those jets lose a lot of energy to the plasma, and so we tend not to detect them as often. I liked the way that Dr Frantz could test this hypothesis by looking at how photons escape from this plasma. After all, if the jet is interacting, i.e. being accelerated by, the stuff in the plasma, then it should spit off photons too. But once a photon has been spat off, it will tend to get out of the plasma, since it does not carry "QCD charge". So photons, once emitted, can be photons all the way to the detector. And the data Dr Frantz and his colleagues collected bore out this hypothesis.
The future of this research would seem to be at the Large Hadron Collider, which has a heavy-ion program lined up for a period a few years from now when they hope to be done with discovering the Higgs boson, supersymmetry, and extra dimensions. (Yes, I am joking, although I am sure they hope to find all that.) Because the energies at the LHC will be much higher than those at RHIC they can test whether the jets continue to be suppressed in higher-energy collisions. Because QCD is an asymptotically free theory one would expect that the interactions of higher-energy jets with the plasma will be smaller than those of the 4-10 GeV jets they've looked at so far at RHIC. But this is asymptotic freedom, so it may take a while to get there. I guess once the LHC does this experiment we'll have more idea of what energy we have to go to before jets really are jets all the way.
Wednesday, January 23, 2008
Weak is the new strong
In the family of fundamental forces weak interactions are the child we spend time apologizing for because their gifts are not immediately obvious. Electromagnetism gets to bind atoms and be responsible for all of condensed-matter physics. The strong force keeps nuclei together, and governs the interactions that make stars shine. And gravity binds us to the Earth and keeps the celestial spheres moving in their orderly (or not-so-orderly) paths.
But the weak force we usually pass over with a few words about nuclear beta decay, neutrinos, and so forth. We know they are a little peculiar (ooh look, how cute, parity violation) but when pressed, we may have trouble explaining what their purpose in life is. Yesterday's Colloquium provided a nice corrective to this view by explaining how weak interactions are providing a unique window on aspects of proton structure and Standard-Model physics.
These results come out of the parity-violation program at Jefferson Lab which was initiated by a paper written 20 years ago that proposed looking at electrons that scatter from the proton via the weak force (specficially via the exchange of a Z0-boson). The trick was that these electrons would couple to the quarks inside the proton differently than would electrons that scattered from the proton via the exchange of photons. Photon exchange is the usual (i.e. dominant) process by which protons and electrons interact---it mediates the electromagnetic force. But it turns out that one can combine information from photon-exchange and Z0-exchange experiments to infer the distribution of strange quarks inside the proton---something that would be impossible with only electromagnetic-scattering data.
The trick is distinguishing the electrons that scatter via the weak force (Z0-exchange let's say) from those that scatter in the "regular" way (photon exchange). Here the fact that the weak interaction violates parity comes to our aid, since we can eliminate the dominant contribution from the "regular" electrons by constructing the difference of cross sections for electrons with opposite spins. The resulting "asymmetry" has recently been measured at a variety of kinematic points by the G0 collaboration. As a consequence we have important new information on the distribution of strange quarks inside the proton. So we now know for sure (well, at the 68% confidence level) that a proton is not two up quarks and a down quark, like you have been taught. Instead it has a bunch of other stuff in it too, of which the non-zero results measured by G0 are just one manifestation.
But the parity-violating electron scattering people did not stop there. They realized that their new probe gave them access to the "weak charge" of the proton. In just the same way as the charge of the proton determines the scattering of low-energy electrons from it, so too the "weak charge" of the proton determines the low-momentum-transfer result for the parity-violating asymmetry measured by G0 and other groups. (The information on proton structure that these experiments were actually after was obtained at values of the momentum transfer that are not "low"---where details of the proton's weak charge distribution become important.) A new experiment at JLab aims to do the world's best measurement of the proton's weak charge. And since this number---Qweak---is predicted by the Standard Model, they can test if that prediction is correct and perhaps see contributions of beyond-the-standard-model stuff to the proton's weak charge.
Personally I find it amazing that differences in cross sections at the level of 1 ppm are being measured with a few-per-cent-level accuracy. But perhaps more interesting is that the weak force is coming into its own, not just as a peculiarity but as a tool. Since the mass-scale of the weak force is set by the gauge bosons and so is of order 100 GeV, these experiment use 100-GeV-scale physics to learn about proton structure. And ultimately the (we hope) closeness of that 100 GeV scale to the mass of particles that are beyond the standard model is what gives experiments like Qweak the chance to upset the four-force picture of interactions we all grew up hearing about. That would truly be the revenge of the under-appreciated child.
But the weak force we usually pass over with a few words about nuclear beta decay, neutrinos, and so forth. We know they are a little peculiar (ooh look, how cute, parity violation) but when pressed, we may have trouble explaining what their purpose in life is. Yesterday's Colloquium provided a nice corrective to this view by explaining how weak interactions are providing a unique window on aspects of proton structure and Standard-Model physics.
These results come out of the parity-violation program at Jefferson Lab which was initiated by a paper written 20 years ago that proposed looking at electrons that scatter from the proton via the weak force (specficially via the exchange of a Z0-boson). The trick was that these electrons would couple to the quarks inside the proton differently than would electrons that scattered from the proton via the exchange of photons. Photon exchange is the usual (i.e. dominant) process by which protons and electrons interact---it mediates the electromagnetic force. But it turns out that one can combine information from photon-exchange and Z0-exchange experiments to infer the distribution of strange quarks inside the proton---something that would be impossible with only electromagnetic-scattering data.
The trick is distinguishing the electrons that scatter via the weak force (Z0-exchange let's say) from those that scatter in the "regular" way (photon exchange). Here the fact that the weak interaction violates parity comes to our aid, since we can eliminate the dominant contribution from the "regular" electrons by constructing the difference of cross sections for electrons with opposite spins. The resulting "asymmetry" has recently been measured at a variety of kinematic points by the G0 collaboration. As a consequence we have important new information on the distribution of strange quarks inside the proton. So we now know for sure (well, at the 68% confidence level) that a proton is not two up quarks and a down quark, like you have been taught. Instead it has a bunch of other stuff in it too, of which the non-zero results measured by G0 are just one manifestation.
But the parity-violating electron scattering people did not stop there. They realized that their new probe gave them access to the "weak charge" of the proton. In just the same way as the charge of the proton determines the scattering of low-energy electrons from it, so too the "weak charge" of the proton determines the low-momentum-transfer result for the parity-violating asymmetry measured by G0 and other groups. (The information on proton structure that these experiments were actually after was obtained at values of the momentum transfer that are not "low"---where details of the proton's weak charge distribution become important.) A new experiment at JLab aims to do the world's best measurement of the proton's weak charge. And since this number---Qweak---is predicted by the Standard Model, they can test if that prediction is correct and perhaps see contributions of beyond-the-standard-model stuff to the proton's weak charge.
Personally I find it amazing that differences in cross sections at the level of 1 ppm are being measured with a few-per-cent-level accuracy. But perhaps more interesting is that the weak force is coming into its own, not just as a peculiarity but as a tool. Since the mass-scale of the weak force is set by the gauge bosons and so is of order 100 GeV, these experiment use 100-GeV-scale physics to learn about proton structure. And ultimately the (we hope) closeness of that 100 GeV scale to the mass of particles that are beyond the standard model is what gives experiments like Qweak the chance to upset the four-force picture of interactions we all grew up hearing about. That would truly be the revenge of the under-appreciated child.
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