In November of 2010, the ALPHA collaboration at CERN’s Antiproton Decelerator (AD) grabbed the world’s headlines by trapping a handful of atoms of antihydrogen (CERN Courier January/February 2011 p7). The result demonstrated that it was, indeed, possible to produce trappable antihydrogen atoms. Now, the ALPHA team has shown that it can hold on to the trapped antiatoms for up to 1000 seconds and has succeeded in measuring the energy distribution of the trapped antihydrogen (ALPHA collaboration 2011).
Antihydrogen has been produced at CERN since 2002 by allowing antiprotons from the AD to mix with positrons in a Penning trap comprised of a strong solenoid magnet and a set of hollow, cylindrical electrodes for manipulating the particles. However, being neutral, the antiatoms are not confined by the fields of the Penning trap and annihilate in the apparatus. It has taken eight years to learn how to trap the antihydrogen, mainly because of the weakness of the magnetic dipole interaction that holds the antiatoms. The antihydrogen must be produced with a kinetic energy, in temperature units, of less than 0.5 K, otherwise it will escape ALPHA’s “magnetic bottle”. By contrast, the plasma of antiprotons used to synthesize the antihydrogen begins its time in ALPHA with an energy of up to 4 keV (about 50 million K).
The ALPHA antiatom trap consists of a transverse octupole magnet and two short solenoid or “mirror” coils – all fabricated at the Brookhaven National Laboratory (figure 1). This configuration produces a magnetic minimum at the centre of the device (CERN Courier March 2011 p13). Antihydrogen forms at the magnetic minimum and cannot escape if its energy is below 0.5 K. To see if there is any antihydrogen in the trap, the team rapidly shuts down the magnets (9 ms time constant). Any escaping antiatoms are revealed by their annihilation, which is registered in a three-layer, silicon vertex detector. In 2010, antiatoms were trapped for 172 ms, the minimum time necessary to make certain that no bare antiprotons remained in the trap, and the experiment detected 38 events consistent with the release of trapped antihydrogen.
The ALPHA team has subsequently worked to improve the trapping techniques, succeeding in particular in increasing by a factor of five the number of antiatoms trapped in each attempt; the total number trapped has now risen to 309. The improvements include the addition of evaporative antiproton cooling and optimization of the autoresonant mixing that helps to produce the coldest-possible antiatoms. The team then made measurements in which they increased the time in the trap from 0.4 to 2000 s, yielding 112 detected annihilations in 201 attempts (figure 2). The probability that the detected events are background from cosmic rays is less than 10–15 (8 σ) at 100s, and 4 × 10–3 (2.6 σ) at 2000s. Calculations indicate that most of these trapped antiatoms reach the ground state – which is crucial for future studies with laser and microwave spectroscopy.
The distributions in space and time of the annihilations of the escaping antiatoms are already providing information about their energy distribution in the trap. This can be compared with a theoretical model of how the team thinks the antihydrogen is being produced in the first place.
The long storage time implies that the team can begin almost immediately to look for resonant interactions with antihydrogen – even if only one or two atoms occupy the trap at any given time. For example, resonant microwaves will flip the spin of the positron in the trap, causing a trapped atom to become untrapped, and annihilate. The ALPHA collaboration hopes to begin studies with microwaves in 2011, aiming for the first resonant interaction of an antiatom with electromagnetic radiation. In the longer term, the ALPHA2 device will allow laser interaction with the trapped antiatoms in 2012 – the first step in what the team hopes will be a steady stream of laser experiments with ever-increasing precision.