In the steps of the antiproton

On 21 September 1955, Owen Chamberlain, Emilio Segrè, Clyde Wiegand and Tom Ypsilantis found their first evidence of the antiproton, gathered through measurements of its momentum and its velocity. Working at what was known as the “Rad Lab” at Berkeley, they had set up their experiment at a new accelerator, the Bevatron – a proton synchrotron designed to reach an energy of 6.5 GeV, sufficient to produce an antiproton in a fixed-target experiment (CERN Courier November 2005 p27). Soon after, a related experiment led by Gerson Goldhaber and Edoardo Amaldi found the expected annihilation “stars”, recorded in stacks of nuclear emulsions (figure 1). Forty years later, by combing antiprotons and positrons, an experiment at the Low Energy Antiproton Ring (LEAR) at CERN gathered evidence in September 1995 for the production of the first few atoms of antihydrogen.

Over the decades, antiprotons have become a standard tool for studies in particle physics; the word “antimatter” has entered into mainstream language; and antihydrogen is fast becoming a laboratory for investigations in fundamental physics. At CERN, the Antiproton Decelerator (AD) is now an important facility for studies in fundamental physics at low energies, which complement the investigations at the LHC’s high-energy frontier. This article looks back at some of the highlights in the studies of the antiworld at CERN, and takes a glimpse at what lies in store at the AD.

Back at the Bevatron, the discovery of the antineutron through neutral particle annihilation followed in 1956, setting the scene for studies of real antimatter. Initially, everyone expected perfect symmetry between matter and antimatter through the combination of the operations of charge conjugation (C), parity (P) and time reversal (T). However, following the observation of CP violation in 1964, it was not obvious that nuclear forces were CPT invariant and that antinucleons should bind to build antinuclei. These doubts were laid to rest with the discovery of the antideuteron at CERN by a team led by Antonino Zichichi, and at Brookhaven by a team from Columbia University, including Leon Lederman and Sam Ting (CERN Courier May 2009 p15and October 2009 p22). A decade later, evidence emerged for antihelium-3 and antitritium in the WA33 experiment at CERN’s Super Proton Synchrotron, following the sighting of a few candidates at the 70 GeV proton synchroton at the Institute for High Energy Physics near Serpukhov. More recently, the availability of colliding beams of heavy ions has led to the observation of antihelium-4 by the STAR experiment at Brookhaven’s Relativistic Heavy-Ion Collider (CERN Courier June 2011 p8). At CERN, the ALICE experiment at the LHC observes the production of light nuclei and antinuclei with comparable masses and therefore compatible binding energies (figure 2).

Exit baryonium, enter new mesons

Back in 1949, before the discovery of the antiproton, Enrico Fermi and Chen-Ning Yang predicted the existence of bound nucleon–antinucleon states (baryonium), when they noted that certain repulsive forces between two nucleons could become attractive in the nucleon–antinucleon system. Later, quark models based on duality predicted the existence of states made of two quarks and two antiquarks, which should be observed when a proton annihilates with an antiproton. In the 1970s, nuclear-potential models went on to predict a plethora of bound states and resonance excitations around the two-nucleon mass. There were indeed reports of such states, among them narrow states observed in antiproton–proton (pp) annihilation at CERN’s Proton Synchrotron (PS) and in measurements of the pp cross-section as a function of energy (the S meson with a mass of 1940 MeV).

Baryonium was the main motivation for the construction at CERN of LEAR, which ran for more than a decade from 1982 to 1996 (see box). However, none of the baryonium states were confirmed at LEAR. The S meson was not observed with a sensitivity 10 times below the signal reported earlier in the pp total cross-section. Monoenergetic transitions to bound states were also not observed. The death of baryonium was a key topic for the Antiproton 86 Conference in Thessaloniki. What had happened? The high quality of the antiproton beams from LEAR meant that all of the pions had decayed. The high intensity of antiprotons (10⁶/s compared with about 10²/s in extracted beams at the PS) and a high momentum resolution of 10^–3–10^–4 was crucial at low energies for antiprotons stopping with very small range-straggling.

The spectroscopy of mesons produced in pp annihilation at rest in several experiments at LEAR proved to be much more fruitful. This continued a tradition that had begun in the 1960s with antiprotons annihilating in the 81 cm Hydrogen Bubble Chamber at the PS, leading to the discovery of the E meson (E for Europe, now the η(1440)) and the D meson (now the f₁(1285)) in pp → (E, D →  KKπ)ππ. The former led to the long-standing controversy about the existence in this mass region of a glueball candidate – a state made only of gluons – which was observed in radiative J/ψ decay at SLAC’s e⁺e^– collider, SPEAR. With the start up of LEAR, the experiments ASTERIX, OBELIX, Crystal Barrel and JETSET took over the baton of meson spectroscopy in pp annihilation. ASTERIX discovered a tensor meson – the AX, now the f₂(1565) – which was also reported by OBELIX; its structure is still unclear, although it could be the predicted tensor baryonium state.

Crystal Barrel specialized in the detection of multineutral events. The antiprotons were stopped in a liquid-hydrogen target and π⁰ mesons were detected through their γγ decays in a barrel-shaped assembly of 1380 CsI (Tl) crystals. Figure 3 shows the detector together with a Dalitz plot of pp annihilation into π⁰π⁰π⁰, measured by the experiment. The non-uniform distribution of events indicates the presence of intermediate resonances that decay into π⁰π⁰, such as the spin-0 mesons f₀(980) and f₀(1500), and the spin-2 mesons f₂(1270) and f₂(1565). The f₀(1500) is a good candidate for a glueball.

Fundamental symmetries

The CPT theorem postulates that physical laws remain the same when the combined operation of CPT is performed. CPT invariance arises from the assumption in quantum field theories of certain requirements, such as Lorentz invariance and point-like elementary particles. However, CPT violation is possible at very small length scales, and could lead to slight differences between the properties of particles and antiparticles, such as lifetime, inertial mass and magnetic moment.

At LEAR, the TRAP collaboration (PS196) performed a series of pioneering experiments to compare precisely the charge-to-mass ratios of the proton and antiproton, using antiprotons stored in a cold electromagnetic (Penning) trap. The signal from a single stored antiproton could be observed, and antiprotons were stored in the trap for up to two months. By measuring the cyclotron frequency of the orbiting antiprotons with an oscillator and comparing it with the cyclotron frequency of H^– ions in the same trap, the team finally achieved a result at the level of 9 × 10^–11. The experiment used H^– ions instead of protons to avoid biases when reversing the signs of the electric and magnetic fields.

Under the assumption of CPT invariance, the violation of CP symmetry first observed in the neutral kaon system in 1964 implies that T invariance is also violated. However, in 1998 the CPLEAR experiment demonstrated the violation of T in the neutral kaon system without assuming CPT conservation (CERN Courier March 1999 p21). The K⁰ and K⁰ morph into one another as a function of time, and T violation implies that, at a given time t, the probability of finding a K⁰ when initially a K⁰ was produced is not equal to the probability of finding a K⁰ when a K⁰ was produced. CPLEAR established the identity of the initial kaon by measuring the sign of the associated charged kaon in the annihilation pp → K⁺K⁰π^– or K^–K⁰π⁺; that of the kaon at time t was inferred by detecting the decays K⁰ → π⁺e^– ν and K⁰ → π^–e⁺ν. Figure 4 shows that a small asymmetry was indeed observed, consistent with expectations from CP violation, assuming CPT invariance.

The CPT theorem also predicts that matter and antimatter should have identical atomic excitation spectra. Antihydrogen – the simplest form of neutral antimatter consisting of a positron orbiting an antiproton – was observed for the first time in the PS210 experiment at LEAR. The circulating 1.9 GeV/c internal antiproton beam traversed a xenon-cluster jet target, allowing the possibility for an e⁺e^– pair to be produced as an antiproton passed through the Coulomb field of a xenon nucleus. The e⁺ could then be captured by the antiproton to form electrically neutral antihydrogen with a momentum of 1.9 GeV/c, which could be detected further downstream through its annihilation into pions and photons. This production process is rather rare, but nonetheless the PS210 collaboration reported evidence for nine antihydrogen atoms, following about two months of data taking in August–September 1995, and only months before LEAR was shut down. The observation of antihydrogen was confirmed two years later at Fermilab’s Antiproton Accumulator, albeit with a much smaller production cross-section.

At the AD

A new chapter in the story of antihydrogen at CERN opened in 2000 with the start up of the AD, which decelerates antiprotons to 100 MeV/c, before extracting them for experiments on antimatter and atomic physics (CERN Courier November 1999 p17). The PS210 experiment had tried to make antihydrogen in flight, but to study, for example, the spectroscopy of antihydrogen, it is far more convenient to store antihydrogen atoms in electromagnetic traps, just as TRAP had done in its antiproton experiments. This requires antihydrogen to be produced at very low energies, which the AD helps to achieve.

In 2002, the ATHENA and ATRAP experiments at the AD demonstrated the production of large numbers of slow antihydrogen atoms (CERN Courier November 2002 p5and December 2002 p5). ATHENA used absorbing foils to reduce the energy of the antiprotons from the AD to a few kilo-electron-volts. A small fraction of the antiproton beam was then captured in a Penning trap, while positrons from a radioactive sodium source were stored in a second trap. The antiproton and positron clouds were then transferred to a third trap and made to overlap to produce electrically neutral antihydrogen, which migrated to the cryostat walls and annihilated. The antihydrogen detector contained two layers of silicon microstrips to track the charged pions from the antiproton annihilation; an array of 192 CsI crystals detected and measured the energies of the photons from the positron annihilation (figure 5). About a million antihydrogen atoms were produced during the course of the experiment, corresponding to an average rate of 10 antiatoms per second.

Antihydrogen has a magnetic dipole moment (that of the positron), which means that it can be captured in an inhomogeneous magnetic field. The first attempt to do this was carried out at the AD by the ALPHA experiment, which successfully captured 38 antihydrogen atoms in an octupolar magnetic field (CERN Courier March 2011 p13). The initial antihydrogen storage time of 172 ms was increased later to some 15 minutes, thus paving the way to atomic spectroscopy experiments. A sensitive test of CPT is to induce transitions from singlet to triplet spin states (hyperfine splitting, or HfS) in the antihydrogen atom, and to compare the transition energy with that for hydrogen, which is known with very high precision. ALPHA made the first successful attempts to measure the HfS with microwave radiation, managing to flip the positron spin and to eject 23 antihydrogen atoms from the trap (CERN Courier April 2012 p7).

An alternative approach is to perform a Stern–Gerlach-type experiment with an antihydrogen beam. The ASACUSA experiment has used an anti-Helmholtz coil (cusp trap) to exert forces on the antihydrogen atoms and to select those in a given positron spin state. The polarization can then be flipped with microwaves of the appropriate frequency. In a first successful test, 80 antihydrogen atoms were detected downstream from the production region (CERN Courier March 2014 p5).

The ASACUSA collaboration has also tested CPT, using antiprotons stopped in helium. The antiproton was captured by ejecting one of the two orbiting electrons, the ensuing antiprotonic helium atom being left in a high-level, long-lived atomic state that is amenable to laser excitation. By using two counter-propagating laser beams (to reduce the Doppler broadening caused by thermal motion), the group was able to determine the antiproton-to-electron mass ratio with a precision of 1.3 ppb (CERN Courier September 2011 p7). An earlier comparison of the charge-to-mass ratio between the proton and the antiproton had been performed with a precision of 0.09 ppb by the TRAP collaboration at LEAR, as described above. When the results from ASACUSA and TRAP are combined, the masses and charges of the proton and antiproton are determined to be equal at a level below 0.7 ppb.

CPT also requires the magnetic moment of a particle to be equal to (minus) that of its antiparticle. The BASE experiment now under way at the AD will determine the magnetic moment of the antiproton to 1 ppb by measuring the spin-dependent axial oscillation frequency in a Penning trap subjected to a strong magnetic-field gradient. The experimental approach is similar to the one used to measure the magnetic moment of the proton to a precision of 3 ppb (CERN Courier July/August 2014 p8). The collaboration has already compared the charge-to-mass ratios of the antiproton and proton, with a fractional precision of 6.9 × 10^–11 (p7).

The weak equivalence principle (WEP), which states that all objects are accelerated in exactly the same way in gravitational fields, has never been tested with antimatter. Attempts using positrons or antiprotons have so far failed, as a result of stray electric or magnetic fields. In contrast, the electrically neutral antihydrogen atom is an ideal probe to test the WEP. The AEgIS collaboration at the AD plans to measure the sagging of an antihydrogen beam over a distance of typically 1 m with a two-grating deflectometer. The displacement of the moiré pattern induced by gravity will be measured with high resolution (around 1 μm) by using nuclear emulsions (figure 6) – the same detection technique that was used to demonstrate the annihilation of the antiproton at the Bevatron, back in 1956.

The future is ELENA

Future experiments with antimatter at CERN will benefit from the Extra Low ENergy Antiproton (ELENA) project, which will become operational at the end of 2017. The capture efficiency of antiprotons in experiments at the AD is currently very low (less than 0.1%), because most of them are lost when degrading the 5 MeV beam from the AD to the few kilo-electron-volts required by the confinement voltage of electromagnetic traps. To overcome this, ELENA – a 30 m circumference electron-cooled storage ring that will be located in the AD hall – will decelerate antiprotons down to, typically, 100 keV. Fast extraction (as opposed to the slow extraction that was available at LEAR) is foreseen to supply the trap experiments.

One experiment that will profit from this new facility is GBAR, which also aims to measure the gravitational acceleration of antihydrogen. Positrons will be produced by a 4.3 MeV electron linac and used to create positive antihydrogen ions (i.e. an antiproton with two positrons) that can be transferred to an electromagnetic trap and cooled to 10 mK. After transfer to another trap, where one of the positrons is detached, the antihydrogen will be launched vertically with a mean velocity of about 1 m/s (CERN Courier March 2014 p31).

It is worth recalling that the discovery of the antiproton in Berkeley was based on some 60 antiprotons observed during a seven-hour run. The 1.2 GeV/c beam contained 5 × 10⁴ more pions than antiprotons. Today, the AD delivers pure beams of some 3 × 10⁷ antiprotons every 100 s at 100 MeV/c, which makes the CERN laboratory unique in the world for antimatter studies. Over the decades, antiproton beams have led to the discovery of new mesons and enabled precise tests of symmetries between matter and antimatter. Now, the properties of hydrogen and antihydrogen are being compared, and accurate tests will be performed with ELENA. The odds to see any violation of exact symmetry are slim, the CPT theorem being a fundamental law of physics. However, experience shows that – as with the surprising discovery of the non-conservation of parity in 1957 and CP violation in 1964 – experiments will, ultimately, have the last word.