A Testing Time for Antimatter Tator During the Capture Process

The Antiproton Decelerator at CERN used in the preparation of the antiprotonic helium atoms.

n = 31 to n = 40 are probed. The overlap of _ the wave functions of p and He2+ is minimal if the high-n Rydberg states exhibit a large angular momentum—for example, in a state such as n = 38 with l = 36 or 37 (where l de- notes the quantized angular momentum). These circular states have typical lifetimes of microseconds, long enough to perform a laser experiment. Narrow spectral lines can be measured to determine level energies to high accuracy. The Rydberg constant of the heavy- _ Rydberg pHe+ ion pair is so large that the transitions by which the n-quantum number changes by unity lie in the wavelength range of the near-infrared, visible, and ultraviolet, where Hori et al. have narrowband and sta- PHYSICS ble lasers at hand. The second electron of the helium atom remains in its 1s orbital, and it acts as a spec- A testing time for antimatter tator during the capture process. It favorably shields the composite newly built electrically _ Precision measurement of antiprotonic helium neutral epHe system during collisions with

the outside world. This property makes it on November 4, 2016 provides a test of physical laws possible for the antiprotonic helium atom to survive while it undergoes multiple collisions By Wim Ubachs less than a nanosecond. The Heisenberg un- in the surrounding bath of cold helium atoms certainty principle then prohibits a precision at a temperature of 1.5 K above absolute zero. pectroscopy is the most accurate measurement. The capture of antiprotons This buffer-gas cooling technique, originally branch of science. Optical transition in a helium atom, under replacement of an developed for the preparation of cold atoms frequencies in isolated atoms and mol- electron, is the exception (3). The inclusion and molecules (4), is applied to the exotic _ ecules can nowadays be measured to of the antiproton produces a pHe+ ionic core atom bearing an antiparticle in its structure. many-digit accuracies by applying the in a heavy-Rydberg two-particle state. En- After a number of collisions, without annihi- _ tools developed in the atomic phys- ergy conservation dictates that pHe+ emerges lation, the exotic atoms are equilibrated and Sics community: ultrastable lasers, locked via in a quantum state n of typically n = 38; in take up the temperature of the bath. At 1.5 K, frequency-comb lasers to atomic clocks, and the experiment by Hori et al., states with the velocity of the particles is very low, and http://science.sciencemag.org/ the techniques to cool and control the motion hence the usual spectral line broadening due of atoms. Precision measurements on small Proton to the Doppler effect is suppressed, allowing quantum systems can be compared with Neutron for the measurement of narrow spectral lines. theoretical descriptions of these systems at The precision frequency measurements the most fundamental level, allowing physics e– performed by Hori et al. on 13 energy-level theories to be tested and enabling the search transitions in antiprotonic helium, and ob-

for physics beyond the standard model (1). tained with nine-digit precision, can be in- Downloaded from On page 610 of this issue, Hori et al. (2) ap- terpreted in three ways. First, it is a test of ply these tricks of the trade to a small atomic charge-parity-time (CPT) invariance, argu- He2+ quantum system with a built-in antiparticle ably the most fundamental theorem on sym- to perform precise spectroscopic measure- metries in physics. All calculations in particle ment in antiprotonic helium (see the figure). physics would lose validity if CPT symmetry The technique of buffer-gas cooling is dem- p were not valid (3). The exchange of particles onstrated for the first time on a composite by their antiparticles (C), reflection in a mir- matter–antimatter particle. ror (P), and letting time run backwards (T) C IE NCE The laws of physics conspire to create the _ should not cause a change in the measurable S conditions for producing relatively long-lived properties of a physical system. The measure- Structure of the antiprotonic _helium system (epHe). B I CK E L / states in the exotic antiprotonic helium atom The positive He2+ and negative p ions can be considered ments verify that CPT is a valid symmetry, at .

_ ) C (epHe). In all other cases of antiprotons stop- to be bound as a heavy-Rydberg ion (11). For n = 38, the least at the level of accuracy now obtained.

ping in matter, the characteristic annihila- typical state in which the antiproton is captured, this Second, antiprotonic helium is a three- AP HI C G R tion process by which a matter particle and yields a characteristic distance of 0.16 Å. The electron is particle Coulomb system, the stability of an antimatter particle end their lives in a then bound in the field of the two-particle ionic core of which was already investigated by Poincaré ) CE R N ; ( O flash of energy takes place on a time scale of charge 1, similar to the way it is bound in the hydrogen in the framework of classical mechanics. In T

atom at a typical distance of the Bohr radius a0 = 0.5 Å. the fully quantized version of electromagne- S : (P H O Department of Physics, Vrije Universiteit, De Boelelaan 1081, The quantum-level structure of antiprotonic helium can tism, quantum electro-dynamics (QED), the + 1081 HV Amsterdam, Netherlands. Email: [email protected] be calculated to 10-digit precision (6, 7). stability of the helium atom (5) and the H2 C R E D I T

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molecular ion (6) is well established, and the CONSERVATION bound-state level energies can be calculated to 10-digit precision. The same is true for the _ epHe system (6, 7) that is found to withstand Migratory birds under threat a test of QED. Third, the transition frequencies sensitively Habitat degradation and loss, illegal killings, and climate depend on the mass ratios of the constituent particles. Similarly, as in a spectroscopic de- change threaten European migratory bird populations termination of the proton-electron mass ratio from HD+ spectroscopy (8), evaluation of the By Franz Bairlein around 3 billion to 5 billion birds in the data of Hori et al. yields a value for the an- early 19th century. Massive-scale hunting as tiproton–electron mass ratio. We learn that he populations of migratory bird spe- cheap meat resulted in its extinction at the antiprotons weigh the same as protons, up to cies that breed in Europe and overwin- turn of the 20th century (4). the 10th digit. ter in sub-Saharan Africa are declining The frontier of particle physics is com- considerably faster than those of non- HABITAT DEGRADATION AND LOSS monly approached in the realm of high- migratory resident species or of migra- Many European migratory birds that over- energy physics, using particle accelerators tory species that overwinter in Europe winter in sub-Saharan Africa do not travel like the Large Hadron Collider (LHC). The T(1). Likely factors are habitat changes due to across those areas of the eastern Mediterra- detection of the Higgs boson at the LHC changes in land use, illegal killing and tak- nean most affected by illegal killings. Rather, marks the culmination of the standard ing along the northern African coasts, and they use a flyway across the western Mediter- model of physics, but the quest is on to ex- climate-induced changes in timing of migra- ranean where illegal taking is much less in- plore new physics beyond that. Alternative tion and breeding. However, not only Euro- tense (see the figure). Other factors must play approaches exist in the low-energy domain pean trans-Saharan migrants are declining a role in their decline. by performing extreme precision measure- fast. This holds also for North American long- A large number of European migratory ments on small atomic and molecular sys- distance migrants wintering in Central and species overwinter in the dry savannas of

tems for which the quantum-level structure South America. To halt these declines, preser- sub-Saharan Africa. Annual survival of many on November 4, 2016 is calculable (1). This can be done, for ex- vation of remaining habitats and restoration of these species correlates with rainfall in the ample, through the search for an electric of habitats both at breeding and nonbreed- Sahel zone (1). However, despite an increase dipole moment of the electron in molecules ing grounds is essential, as well as stopping in rainfall in the Sahel in recent decades, bird that might reveal signatures of supersym- illegal killing and taking of birds along their populations have continued to decline. Thus, metry (9). Laser precision measurements of migration routes. factors other than rainfall must contribute to optical transitions in molecules constrain the population declines. Land-use and land- the existence of higher dimensions beyond ILLEGAL KILLING AND TAKING cover changes are the most important (5). Be- the 3 + 1 (spatial and time) that we regu- Every year, between 11 million and 36 mil- tween 1975 and 2000, agriculture increased larly observe, or the occurrence of a fifth lion birds are killed or taken illegally in the by 57% in sub-Saharan Africa at the expense force beyond the three forces known in the Mediterranean region (2). The areas of great- of natural vegetation, with nearly 5 million standard model plus gravity (1). Previous est concern are in the eastern and central hectares of forest and nonforest vegetation measurements on antiprotonic helium have Mediterranean, with more than 5 million lost per year (6). Most affected are the Sahel http://science.sciencemag.org/ already set limits on the strength of such birds taken in both Egypt and Italy and an and Guinea Savanna zones where the major- a hypothetical fifth force at sub-ångström estimated 1 million each in Cyprus and Leba- ity of the Eurasian migratory species over- length scale (10); the present data constrain non. Common migratory species such as Eur- winter. An analysis of breeding-bird survey these phenomena even further. asian chaffinch, blackcap, and song thrush trends of 26 long-distance migratory species This exotic atom involving antimatter are most affected, but many less common mi- in the United Kingdom shows that wintering seems a fortunate accident of nature. It ex- gratory species are also taken in substantial habitat is the most important determinant

hibits long-lived (metastable) quantum states numbers, including species of global conser- of population trends, with specialist species Downloaded from that can be probed with lasers, and it survives vation concern such as red kite and Eurasian that occupy either open or woodland habitats the collisional conditions needed to cool its curlew. On average, the annual illegal killings in Africa showing declines (7). kinetic motion, as Hori et al. have demon- and takings of threatened or near-threatened strated. It is likely that these properties may migratory bird species amount to 1.0 to 3.5% CLIMATE CHANGE be further exploited to reveal new physics of their populations (2)—percentages that are Climate change is another major driver for in future experiments on this extraordinary very likely to have considerable impacts on biodiversity changes, including responses of atom-like particle. j the fate of these species. bird populations (8). Many migratory species, Illegal trapping can cause a collapse in including those that overwinter in sub-Saha- REFERENCES population numbers within a short period ran Africa, now arrive earlier at their spring 1. W. Ubachs et al., J. Mol. Spectrosc. 320, 1 (2016). 2. M. Hori et al., Science 354, 610 (2016). of time. For example, the yellow-breasted breeding grounds (9). However, different or- 3. R. S. Hayano et al., Rep. Prog. Phys. 70, 1995 (2007). bunting was abundant in its Eurasian ganisms do not respond to climate change at 4. J. D. Weinstein et al., Nature 395, 148 (1998). breeding range until illegal takings in China the same pace, which has led to an ecologi- 5. K. Pachucki, Phys. Rev. A 74, 062510 (2006). 6. V. I. Korobov et al., Phys. Rev. Lett. 112, 103003 (2014). caused an 84% population decline between cal mismatch between some consumers and 7. V. I. Korobov et al., Hyperfine Int. 233, 75 (2015). 1980 and 2013 (3). Similarly, the passenger their prey (10). 8. J. Biesheuvel et al., Nat. Commun. 7, 10385 (2016). pigeon was once the most abundant mi- Dutch pied flycatchers, which overwin- 9. J. Baron et al., Science 343, 269 (2014). gratory bird in North America, numbering ter in sub-Saharan Africa, do not arrive 10. E. J. Salumbides et al., J. Mol. Spectrosc. 297, 41 (2014). 11. E. Reinhold, W. Ubachs, Mol. Phys. 103, 1329 (2005). earlier at breeding grounds, but the popu- Institute of Avian Research, Wilhelmshaven, Germany. lations of their insect food peak earlier as 10.1126/science.aah6215 Email: [email protected] a result of warmer spring temperatures.

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DA_1104Perspectives.indd 547 11/2/16 10:46 AM A testing time for antimatter Wim Ubachs (November 3, 2016) Science 354 (6312), 546-547. [doi: 10.1126/science.aah6215]

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