Minutes etc. Antihydrogen Meeting 4/23/99 Present: Christian, Hahn, Mandelkern, Melissinos, Munger 1. How to do the experiment. a. Only the laser method makes sense. The laser beam size must be small; nominal value for w_0 (1/e radius) of 0.5 mm. A power of 100 kW seems feasible. b. The antihydrogen beam size sigma_b must be comparable to the laser beam size. Since the stored antiproton beam is not small, the target must be small, placed at a point of relatively large beta (horizontal or vertical) abd not too far from the laser cavity. Various technologies are possible. 2. The Booster Test a. We don't think the foil excitation method makes sense now. The yield is low and method inferior to that of the laser which we now think is possible. b. At the Booster we would be fighting background problems that are irrelevant to an eventual run in the Antiproton Source. AH has made a background study described in his memo of 3/2/99. c. We don't think the Booster test is worth pursuing. A study of spider web fiber integrity in the Antiproton Source will allow us to maintain our momentum. We will write a letter to Beams Division spelling out our progress and asking for permission to do the latter. 3. Calculations of excitation rates. a. ACM has refined the calculation and provides curves giving the following result. For a sigma_b=0 antihydrogen beam, laser power of 100 kW and sigma_p/p=0, we have an excitation probability P of 58% independent of w_0. For sigma_b=w_0 P=28%. For finite sigma_p/p, P decreases as w_0 increases (detuning). For sigma_p/p=2 10^(-4) and w_0=0.5mm P decreases by a factor 0.56 for a final P of 15%. The best sigma_p/p that has been done at the Antiproton Source is just greater than 10^(-4). 2 10^(-4) is plausible. 4. Production targets. a. The antihydrogen is formed in the target and the beam diverges as it passes out of the machine. AH has identified a location for the target just upstream of B503 where the horizontal beta function is 30.5 m. Is this a zero dispersion point? We assume a 10 m distance from the target to the laser cavity. b. The target size and antiproton beam divergence determines the size of the antihydrogen beam at the laser cavity. The rms horizontal angular width of the antiproton beam, sigma_theta_h, determined by the emittance and the horizontal beta function, is approximately 1 10^(-4) r. AH argues in his memo of 3/2/99 that for a wire (or thin gas jet) target, because of phase space depletion at large x and x', the angular divergence for the antihydrogens may be substantially smaller. Mike Church agrees. If sigma_theta is 0.5 10^(-4) we will achieve a antihydrogen beam of 0.5 mm rms at the laser cavity. c. A gas jet target has the advantages that it is so sparce that there is essentially no loss of antihydrogen by ionization. It is also self-healing. The near absence of ionization losses allows us to take advantage of the Z^2 increase of the production cross section (more rapid than the decrease of radiation length with Z) by using a high Z gas. Can a 1 mm diameter cluster jet of suitable thickness be made by scraping while at the same time maintaining a low ambient gas pressure? A brief conversation with Gabriele Garzoglio provides the following: Once formed, clusters propagate ballistically without significant evaporation over distances of a meter or greater. The maximum E835 hydrogen jet of thickness ~ 2 10^14 cm^(-2) and full width 0.7 cm is produced using a final scraper aperture of 1 mm followed by a baffle, using a nossle pressure of 100 psi at 30 degrees K. Clusterization can be obtained at pressures of a minimum of 3-5 psi. The ambient pressure in the beam pipe (at full density) is about 10^(-7). In principle a smaller jet can be formed by further aperturing using scrapers. Significant problems for gas jets include the space requirements of the huge pumps required, cost and vacuum degradation. Assume that the Antiproton Source and hydrogen gas jet can be run at a luminosity of 10^32. For a production cross section of 1.7 pb that yields 14.6 antihydrogens in a "nominal" day. We use a 1 mm Xe jet placed off- center with the same effective thickness in radiation lengths, thus multiplying the nuclear thickness by 8.48/61.28*5*2/131=0.0106 where 5 is the ratio of target to beam diameters and 2 accounts for the off-center beam. The event rate per nominal day is then 14.6*0.0106*54^2/(5*2)=45. The nuclear density in the 1 mm Xe jet is then 0.0742 that in the 7 mm hydrogen jet. Since the latter is 3.2 10^(14), the required Xe nuclear density is ~2.4 10^(12) which is in the range of the CERN Xe gas jet. d. We are considering fibers from spider webs. These have diameters of as small as 0.5 micron, are stronger than carbon fibers and are used is fusion experiments to support D-T pellets. We may plate with a thin layer of a metal in order to make the fiber heat conductive and supply some reflectivity so the experiment can be aligned. We will propose to place such fibers in the Antiproton Source to determine whether they can survive bombardment. We may also be able to learn something about phase depletion etc. from such a test. ACM plans on providing a fiber(s) on a mount for installation in the Antiproton Source as early as mid-May. Even a 0.5 micron carbon filament is much thicker than a gas jet and causes significant antihydrogen losses due to ionization. However a wire system taked up very little space and is low cost. e. There are further-out options. Magneto-optical traps will confine alkali metal atoms such as Cs. Can enough atoms be confined. Can one make a charged or neutral atomic beam with emough density and suitable dimensions? Can one levitate (magnetically? electrostatically? by laser?) tiny spheres in the beam? Can one inject droplets into the beam? We'll pursue these thoughts. 5. Plan Get approval for and plan the spiderweb test. Continue to think about gas jets. Mauro Marinelli will be here for 10 days starting April 30. I suggest that Dave talk to him generally about targetry. He's broadly knowledgable. I'll try to talk to him. Work on the laser stuff. It's likely that the crunch of work at FNAL will prevent doing an experiment until Collider Run II is well into its running.