PHYSICS WITH ULTRA SLOW ANTIPROTON BEAMS
793(2005); http://dx.doi.org/10.1063/1.2121967View Description Hide Description
The antiproton celebrates its 50th birthday this year. Although its existence had been suspected since the discovery of the positron in 1932, there was still doubt in some quarters that such a companion particle to the proton could exist. I will try to trace the scientific history of the antiproton from that time to the publication of the definitive paper by Chamberlain, Segrè, Wiegand and Ypsilantis in November 1955, with a brief look at what happened next. The narrative will be supplemented with thoughts and opinions of some of the main actors, both at the time and in retrospect.
793(2005); http://dx.doi.org/10.1063/1.2121968View Description Hide Description
Low energy antiprotons offer excellent opportunities to study properties of fundamental forces and symmetries in nature. Experiments with them can contribute substantially to deepen our fundamental knowledge in atomic, nuclear and particle physics. Searches for new interactions can be carried out by studying discrete symmetries. Known interactions can be tested precisely and fundamental constants can be extracted from accurate measurements on free antiprotons (p̄’s) and bound two‐ and three‐body systems such as antihydrogen (H̄ = p̄e −), the antprotonic helium ion (He++ p̄)+ and the antiprotonic atomcule (He++ p̄e −) . The trapping of a single p̄ in a Penning trap, the formation and precise studies of antiprotonic helium ions and atoms and recently the production of H̄ have been among the pioneering experiments. They have led already to precise values for p̄ parameters, accurate tests of bound two‐ and three‐body Quantum Electrodynamics (QED), tests of the CPT theorem and a better understanding of atom formation from their constituents. Future experiments promise more precise tests of the standard theory and have a robust potential to discover new physics. Precision experiments with low energy p̄’s share the need for intense particle sources and the need for time to develop novel instrumentation with all other experiments, which aim for high precision in exotic fundamental systems. The experimental programs — carried out in the past mostly at the former LEAR facility and at present at the AD facility at CERN — would benefit from intense future sources of low energy p̄’s. The highest possible p̄ fluxes should be aimed for at new facilities such as the planned FLAIR facility at GSI in order to maximize the potential of delicate precision experiments to influence model building. Examples of key p̄ experiments are discussed here and compared with other experiments in the field. Among the central issues is their potential to obtain important information on basic symmetries such as CPT and to gain insights into antiparticle gravitation as well as the possibilities to learn about nuclear neutron distributions.
793(2005); http://dx.doi.org/10.1063/1.2121969View Description Hide Description
Perhaps the largest gap in our understanding of nature at the smallest scales is a consistent quantum theory underlying the Standard Model and General Relativity. Substantial theoretical research has been performed in this context, but observational efforts are hampered by the expected Planck suppression of deviations from conventional physics. However, a variety of candidate models predict minute violations of both Lorentz and CPT invariance. Such effects open a promising avenue for experimental research in this field because these symmetries are amenable to Planck‐precision tests.
The low‐energy signatures of Lorentz and CPT breaking are described by an effective field theory called the Standard‐Model Extension (SME). In addition to the body of established physics (i.e., the Standard Model and General Relativity), this framework incorporates all Lorentz‐ and CPT‐violating corrections compatible with key principles of physics. To date, the SME has provided the basis for the analysis of numerous tests of Lorentz and CPT symmetry involving protons, neutrons, electrons, muons, and photons. Discovery potential exists in neutrino physics.
A particularly promising class of Planck‐scale tests involve matter‐antimatter comparisons at low temperatures. SME predictions for transition frequencies in such systems include both matter‐antimatter differences and sidereal variations. For example, in hydrogen‐antihydrogen spectroscopy, leading‐order effects in a 1S‐2S transition as well as in a 1S Zeeman transition could exist that can be employed to obtain clean constraints. Similarly, tight bounds can be obtained from Penning‐trap experiments involving antiprotons.
793(2005); http://dx.doi.org/10.1063/1.2121970View Description Hide Description
The structure of matter is related to symmetries on every level of study. CPT symmetry is one of the most important laws of field theory: it states the invariance of physical properties when one simultaneously changes the signs of the charge and of the spatial and time coordinates of particles. Although in general opinion CPT symmetry is not violated in Nature, there are theoretical attempts to develop CPT‐violating models. The Antiproton Decelerator at CERN has been built to test CPT invariance.
Several observations imply that there might be another deep symmetry, supersymmetry (SUSY), between basic fermions and bosons. SUSY assumes that every fermion and boson observed so far has supersymmetric partners of the opposite nature. In addition to some theoretical problems of the Standard Model of elementary particles, supersymmetry may provide solution to the constituents of the mysterious dark matter of the Universe. However, as opposed to CPT, SUSY is necessarily violated at low energies as so far none of the predicted supersymmetric partners of existing particles was observed experimentally. The LHC experiments at CERN aim to search for these particles.
793(2005); http://dx.doi.org/10.1063/1.2121971View Description Hide Description
Ionizing collisions of antiprotons with atoms or molecules at energies between a few keV up to about one MeV provide a unique tool to explore correlated dynamics of electrons at large perturbations on a time scale between several femtoseconds (1 fs = 10−15 s) down to some tens of attoseconds (1 as = 10−18 s). Exploiting and developing many‐particle imaging methods — Reaction‐Microscopes — integrated into a novel ultra‐low energy storage ring (USR) for slow antiprotons will enable to access for the first time fully differential cross sections for single and multiple ionization in such collisions. Moreover, the formation of antiprotonic atoms, molecules or of protonium might be explored in kinematically complete experiments yielding unprecedented information on (n,l)‐distributions of captured antiprotons as well as precise spectroscopic data of the respective energy levels.
In this contribution the present status on single and double ionization by antiproton and ion impact is highlighted pointing to the puzzling discrepancies between experiments and theoretical predictions. The design status of the USR as a central element of the proposed facility for low‐energy antiproton and ion research (FLAIR) at GSI will be shortly presented.
793(2005); http://dx.doi.org/10.1063/1.2121972View Description Hide Description
Differential cross sections for He single ionization in fast p and p̄ impact are presented and compared to theory. To investigate possible correlation effects on the double ionization cross section in p̄ collisions the in momentum space complete differential cross sections for fast p on He transfer ionization processes has been investigated and the influence of correlation effects, i.e. the influence of so called “off‐shell” contributions in the He ground state wave function, has been measured.
793(2005); http://dx.doi.org/10.1063/1.2121973View Description Hide Description
Using results of a previous theoretical treatment of antiproton capture by helium and neon atoms, the energies and angular distributions of the electrons emitted in the capture process are analyzed for various incident antiproton energies. The emitted electron energies are considerably higher for the neon target than for the helium target. The electron energies increase with increasing incident antiproton energies, but this dependence is fairly weak. The angular distributions of the emitted electrons are approximately isotropic. They are similar for helium and neon and depend only weakly on the antiproton energy. The angular distributions of uncaptured antiprotons at similar and somewhat higher collision energies are also presented.
793(2005); http://dx.doi.org/10.1063/1.2121974View Description Hide Description
ATHENA’s first detection of cold antihydrogen atoms relied on their annihilation signatures in a sophisticated particle detector. We will review the features of the ATHENA detector and its applications in trap physics. The detector for a new experiment ALPHA will have considerable challenges due to increased material thickness in the trap apparatus as well as field non‐uniformity. Our studies indicate that annihilation vertex imaging should be still possible despite these challenges. An alternative method for trapped antihydrogen, via electron impact ionization, will be also discussed.
793(2005); http://dx.doi.org/10.1063/1.2121975View Description Hide Description
The ATRAP experiment at the CERN antiproton decelerator AD aims for a test of the CPT invariance by a high precision comparison of the 1s‐2s transition in the hydrogen and the antihydrogen atom.
Antihydrogen production is routinely operated at ATRAP and detailed studies have been performed in order to optimize the production efficiency of useful antihydrogen. The shape parameters of the antiproton and positron clouds, the n‐state distribution of the produced Rydberg antihydrogen atoms and the antihydrogen velocity have been studied. Furthermore an alternative method of laser controlled antihydrogen production was successfully applied.
For high precision measurements of atomic transitions cold antihydrogen in the ground state is required which must be trapped due to the low number of available antihydrogen atoms compared to the cold hydrogen beam used for hydrogen spectroscopy. To ensure a reasonable antihydrogen trapping efficiency a magnetic trap has to be superposed the nested Penning trap. First trapping tests of charged particles within a combined magnetic/Penning trap have started at ATRAP.
793(2005); http://dx.doi.org/10.1063/1.2121976View Description Hide Description
Testing CPT to the highest possible precision using the laser spectroscopy of antiprotonic helium atoms (a neutral three‐body system consisting of an antiproton, a helium nucleus and an electron) is the current goal of ASACUSA collaboration at CERN AD. The present status and future prospects are discussed in the first half of the talk. Our program will be extended in the future to include the microwave spectroscopy of ground‐state hyperfine splitting of antihydrogen. The physics motivations and possible measurement schemes are presented in the second half.
Non‐Neutral Plasma Confinement In A Cusp‐Trap And Possible Application To Anti‐Hydrogen Beam Generation793(2005); http://dx.doi.org/10.1063/1.2121977View Description Hide Description
A new scheme for synthesizing antihydrogen by trapping positrons and antiprotons in a field consisting of a magnetic quadrupole and an electric octupole (cusp ‐trap) is now under investigation. The total electric field of the octupole with the space charge of a nonneutral plasma composed of particles of the same sign of charge, i.e., positrons or mixture of electrons and antiprotons, is expected to form a potential well for particles of the opposite sign of charge. Particles trapped in the well are mixed with the present dense particles, where positrons and antiprotons will combine to produce antihydrogen atoms. A considerable fraction of antihydrogen atoms in low‐field seeking states will be transported outside as a beam.
Experiments on electron confinement in the cusp‐trap were carried out in a strong magnetic quadrupole (3.8T at the maximum on the axis). The confinement time reached 400s for the trapped electron number N 0= 3.6×107. The time decreased with N 0 but it was still about 100s for N 0= 1.6×108.
An electron plasma initially formed around the zero‐field point rapidly expanded and settled down onto a quasi‐stable state. Cross‐sectional density profiles had shapes like a high volcano with a big crater. Analysis of the density profile shows that a potential well for oppositely charged particles (positive ions in this case) is probably formed inside the trapped electrons.
793(2005); http://dx.doi.org/10.1063/1.2121978View Description Hide Description
With a completion of high intensity hadron accelerator in the near future, there will be a realization of high intensity as well as high quality beam of secondary particles like positive muons, anti‐protons, etc. Here, it is suggested that a formation of Muonic Anti‐Hydrogen is not unrealistic at all. Once it is formed, a new type of testing of CPT theorem will be realized by comparing with Muonic Hydrogen; much powerful testing for a short‐range CPT violating interaction in contrast to the case of usual Hydrogen versus Anti‐Hydrogen comparison.
793(2005); http://dx.doi.org/10.1063/1.2121979View Description Hide Description
We propose to use a 13 KeV antiproton beam passing through a dense cloud of positronium (Ps) atoms to produce an H̄+ “beam”. These ions can be slowed down and captured by a trap. The process involves two reactions with large cross sections under the same experimental conditions: the interaction of p̄ with Ps to produce H̄ and the e + capture by H̄ reacting on Ps to produce H̄+. The decelerated H̄+ ions are captured and cooled in a trap. The extra e + is removed with a laser to measure the gravitational acceleration of neutral antimatter in the gravity field of the Earth.
793(2005); http://dx.doi.org/10.1063/1.2121980View Description Hide Description
The present knowledge on strong‐interaction effects in light antiprotonic atoms is reviewed. Data were obtained during the LEAR era, where the high flux made possible the use of high‐resolution devices like semiconductor detectors and a crystal spectrometer. Open questions and possible extensions at the future antiproton facilities are discussed.
793(2005); http://dx.doi.org/10.1063/1.2121981View Description Hide Description
793(2005); http://dx.doi.org/10.1063/1.2121982View Description Hide Description
At the future FAIR project in Darmstadt/Germany low energy antiprotons will be available at FLAIR, the Facility for Low energy Antiproton and Ion Research. Within the FLAIR LoI it is proposed to study the production of strangeness S = −2 baryonic states based on ideas proposed for LEAR.
With stopped antiprotons a very efficient reaction chain for the production of slow Ⅺ hyperons can be initiated. The special feature of this reaction channel is the low momentum of the produced Ⅺ hyperon down to zero MeV/c recoil momentum. These slow Ⅺ particles can go into interacting ⅪN systems which can couple to YY or might also directly connect to the H particle, if it exists.
From the experimental point of view the delayed decays of the strange exit particles allows a highly selective trigger on these reaction channels and the event reconstruction is relatively simple. A non magnetic detection system with track reconstruction ability is sufficient for the complete kinematical reconstruction.
793(2005); http://dx.doi.org/10.1063/1.2121983View Description Hide Description
The nuclear p̄ capture from atomic states is briefly reviewed and several capture modes are compared. All these modes may test neutron density distributions in different regions of nuclei and yield complementary information on the Rms and higher moments of the neutron density profiles. Some advantages and difficulties of experimental methods are indicated and a special attention is paid to the π± emission following p̄ annihilation. It is shown that this useful method may become very powerful if it determines more than the separate π+ and π− multiplicities. Two specific questions are analysed: the ratio of p̄n and p̄p annihilation rates and the reabsorption of π mesons in the residual nuclei.
793(2005); http://dx.doi.org/10.1063/1.2121984View Description Hide Description
Antiprotonic X rays were used to investigate the nuclear matter densities. Neutron densities in 26 isotopes were determined using this method. The information on the nuclear matter density at relatively large radii was then converted to rms radii by the use of a two‐parameter Fermi‐function profile. The obtained systematics of differences of the neutron and proton rms radii is in a fair agreement with theoretical calculations and results of other experimental methods.
An Antiproton Ion Collider (AIC) for Measuring Neutron and Proton Distributions in Stable and Radioactive Nuclei793(2005); http://dx.doi.org/10.1063/1.2121985View Description Hide Description
An antiproton‐ion collider is proposed to independently determine mean square radii for protons and neutrons in stable and short lived nuclei by means of antiproton absorption at medium energies. The experiment makes use of the electron ion collider complex (ELISE) of the GSI FAIR project with appropriate modifications of the electron ring to store, cool and collide antiprotons of 30 MeV energy with 740A MeV energy ions.
The total absorption cross‐section of antiprotons by the stored ions will be measured by detecting their loss by means of the Schottky noise spectroscopy method. Cross sections for the absorption on protons and neutrons, respectively, will be studied by detection of residual nuclei with A‐1 either by the Schottky method or by analysing them in recoil detectors after the first dipole stage of the NESR following the interaction zone. With a measurement of the A‐1 fragment momentum distribution, one can test the momentum wave functions of the annihilated neutron and proton, respectively. Furthermore by changing the incident ion energy the tails of neutron and proton distribution can be measured.
The absorption cross section is at asymptotic energies in leading order proportional to the mean square radius of the nucleus. Predicted cross sections and luminosities show that the method is applicable to nuclei with production rates of about 105 s−1 or lower, depending on the lifetime of the ions in the NESR, and for half‐lives down to 1 second.