From: "Thomas Phillips" To: "Young-Kee Kim" Subject: Input for steering committee: antimatter gravity experiment Date: Thursday, June 07, 2007 12:04 AM Dear Young-Kee, This note is in response to the call for input you sent to the DPF email list on May 14th. For some time, I have been interested in making a direct measurement of the gravitational force on antimatter by measuring the gravitational phase shift of a slow beam of antihydrogen in an atomic interferometer. Basically, the interference pattern shifts by the same amount that the antihydrogen atoms fall as they traverse the interferometer, so it is possible to efficiently measure very small (sub-micron) deflections. The technology for making atomic-beam interferometers is well developed, and recently a beam of antihydrogen with the ideal velocity distribution has been produced at CERN's AD (PRL 97, 143401 figure 1b), but the AD produces too few antiprotons to make the antihydrogen interferometer practical. On the other hand, Fermilab could easily produce enough antiprotons to make the measurement if you decide to have a low-energy antiproton program. There is a wide range of possibilities for slowing antiprotons at Fermilab. It has already been demonstrated that the main injector is capable of decelerating antiprotons to the point where they could be loaded into a trap with an expected efficiency of 0.1% using a degrader. While many infrastructure options have already been investigated, the installation of a previously designed beamline up an existing carrier pipe at MI-9 along with the construction of a modest service building at the pipe's end has already been documented to be a very modestly priced option. A significantly higher trapping efficiency could be achieved by building a small decelerating ring that could reduce the antiproton energy to the range where an RFQ could be used to load the trap. This ring could be part of a broader physics program using low and medium energy antiprotons. Of course, coupled with Fermilab's high pbar production capability, the ability to trap antiprotons would make Fermilab the world's premier facility for the community of physicists who work with low-energy antiprotons, and it would make new experiments possible that cannot be done with the limited number of antiprotons available at the AD. The physics case for making the gravity measurement can be divided into the initial measurement that would determine the gravitational force on antimatter to the percent level, and a high-precision measurement of the difference between the gravitational force on matter and antimatter. The first measurement can be done with existing technology and it would tell us whether our current understanding of gravity is fundamentally incorrect with respect to antimatter. While any anomalous result would clearly cause many theoretical problems, finding a repulsive force would explain two of the biggest mysteries of cosmology. First, it would explain the apparent asymmetry between matter and antimatter in the universe, since the universe could have equal amounts of matter and antimatter that have segregated into separate non-overlapping regions. Second, the net gravitational force from these separate regions would be repulsive, so it would explain the observed accelerating expansion of the universe. In my opinion, the possibility to explain these mysteries justifies making the measurement despite the strong theoretical bias we have that we already know how antimatter will fall, especially considering the fact that the fundamental incompatibility between general relativity and quantum mechanics already tells us that something we think we "know" is incorrect. A sufficiently high precision comparison between the gravitational force on matter and antimatter can test for new forces that are weaker than gravity which couple differently to matter and antimatter. Equivalence-principle experiments already put (model- dependent) limits on such forces, so reaching the level of precision necessary to explore beyond these limits would require some development effort. On the other hand, this would be a direct measurement while the the equivalence-principle experiments are indirect (in effect using the minute amount of virtual antimatter present in matter), so the antimatter experiment needs much less measurement precision than the equivalence-principle experiments to explore the same physics. The precision experiment would require significantly more antiprotons than the initial measurement not only for reducing statistical uncertainties, but also in order to understand and control the systematic uncertainties. The apparatus that would be required for this experiment consists of a Penning trap for capturing the antiprotons, a positron source (produced either with a radioactive source or an accelerator, perhaps an ILC prototype?), and an atomic interferometer. It may be possible to use some existing equipment for part of this apparatus; I have access to a trap which should work for the antiprotons and I have a partially-constructed interferometer intended for a demonstration experiment using hydrogen that could potentially be used for the initial measurement with antihydrogen. This interferometer uses microfabricated gratings, while I expect the high-precision measurement will require lasers to produce standing-wave optical gratings. I would like to point out that the antimatter gravity experiment would have great public-relations value in addition to its scientific interest. While the public has trouble understanding the significance of Bs oscillations, for example, they do understand gravity and they are fascinated with antimatter. Furthermore, they are not burdened with a prejudice that they already know what the experiment will find. Particle physics has not always been good about raising public awareness and interest in our work, and yet we continue to ask for (and receive) significant amounts of public financing to continue our research. I think we would be wise to take this opportunity to perform an experiment that the public can understand and perhaps get excited about, especially considering that it can be done at a modest cost. Sincerely, Thomas Phillips Duke University