Fermilab's unique ability to accumulate large numbers of antiprotons makes it possible to directly measure the gravitational force on antimatter for the first time. Such a measurement will be a fundamental test of gravity in a new regime, directly testing both the equivalence principle and the prediction of General Rel ativity that matter and antimatter behave identically in the gravitational field of the earth. VVe propose to decelerate antiprotons in the Main Injector and transfer them into an antihydrogen-production Penning trap. The anti hydrogen will emerge from the trap in a low-velocity beam and pass through an atomic interferometer where the gravitational deflection will be measured. A 1% mea surement should be possible soon after antihydrogen production is established. A possible follow-on phase of the experiment (beyond the scope of this LoI) can use laser-based interferometry techniques to measure much more precisely any difference between the gravitational forces 011 matter and antilllat ter and search . sensitively for a possible "fifth force" significantly weaker than gravity.
The proposed Antihydrogen Gravity experiment at Fermilab (P981) will directly measure the gravitational attraction g between antihydrogen and the Earth, with an accuracy of 1% or better. The following key question has been asked by the PAC:Is a possible 1% difference between g and g already ruled out by other evidence?This memo presents the key points of existing evidence, to answer whether such a difference is ruled out (a) on the basis of direct observational evidence; and/or (b) on the basis of indirect evidence, combined with reasoning based on strongly held theoretical assumptions. The bottom line is that there are no direct observations or measurements of gravitational asymmetry which address the antimatter sector. There is evidence which by indirect reasoning can be taken to rule out such a difference, but the analysis needed to draw that conclusion rests on models and assumptions which are in question for other reasons and are thus worth testing. There is no compelling evidence or theoretical reason to rule out such a difference at the 1% level.
Most recently, a real-time interferometer (RTI) developed by Radiabeam to monitor the bunch length of high-current electron beams in an accelerator was tested. The RTI employs spatial autocorrelation, reflective optics, and a fast response pyro-detector array to obtain a real-time autocorrelation trace of the coherent radiation from an electron beam, thus providing the possibility of online bunch-length diagnostics. A complete RTI system was commissioned at the A0 photoinjector facility to measure sub-mm bunches at 13 MeV. Bunch length variation (FWHM) between 0.8 ps (~0.24 mm) and 1.5 ps (~0.45 mm) has been measured and compared with a Martin-Puplett interferometer and a streak camera. The comparisons demonstrated that the RTI is a viable, complementary bunch length diagnostic for sub-mm high-current electron bunches [21]. An example of measurement appears in Figure 7. References
Abstract.The gravitational acceleration of antimatter,ḡ, has never been directly measured and could bear importantly on our understanding of gravity, the possible existence of a fifth force, and the nature and early history of the universe. Only two avenues for such a measurement appear to be feasible: antihydrogen and muonium. The muonium measurement requires a novel, monoenergetic, low-velocity, horizontal muonium beam directed at an atom interferometer. The precision three-grating interferometer can be produced in silicon nitride or ultrananocrystalline diamond using state-of-the-art nanofabrication. The required precision alignment and calibration at the picometer level also appear to be feasible. With 100 nm grating pitch, a 10% measurement ofḡ can be made using some months of surface-muon beam time, and a 1% or better measurement with a correspondingly larger exposure. This could constitute the first gravitational measurement of leptonic matter, of 2nd-generation matter and, possibly, the first measurement of the gravitational acceleration of antimatter.
We formulate and study a Deck-like model for the reaction
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