Jobs – Page 166 of 530

Illuminating antimatter

The physics programme at CERN’s Antiproton Decelerator (AD) is concerned with fundamental studies of the properties and behaviour of antimatter. Diverse experiments endeavour to study the basic characteristics of the antiproton (BASE, ATRAP), the spectra of antiprotonic helium (ASACUSA) and antihydrogen (ALPHA, ASACUSA, ATRAP), and gravitational effects on antimatter (GBAR, AEGIS, ALPHA-g). These innovative experiments at the AD – itself a unique facility in the world – can test fundamental symmetries such as charge–parity–time (CPT) and search for indications of physics beyond the Standard Model involving systems that have never before been studied.

Lurking in the background to all this is the baryon asymmetry problem: the mystery of what happened to all the antimatter that should have been created after the Big Bang. This mystery forces us to question whether antimatter and terrestrial matter really obey the same laws of physics. There is no guarantee that AD experiments will find any new physics, but if you can get your hands on some antimatter, it seems prudent to take a good, hard look at it.

We live in interesting times for antimatter. In addition to experiments at the AD, physicists study potential matter–antimatter asymmetries at the energy frontier at the LHCb experiment, and search for evidence of primordial antimatter streaming through space using the AMS-02 spectrometer onboard the International Space Station. Antihelium-4 nuclei were observed for the first time at Brookhaven’s Relativistic Heavy Ion Collider (RHIC) in 2011, while the LHC’s ALICE collaboration observed and studied anti-deuterons and antihelium-3 nuclei in 2015. By contrast, the experiments at the AD are low-energy affairs: we are essentially dealing with antimatter at rest.

One of the unique advantages of AD physics, therefore, is that we can address antimatter using precision techniques from modern atomic and ion-trap physics. Following three decades of development in advanced experimental techniques by the low-energy antimatter community, the ALPHA collaboration has recently achieved the major goal of examining the spectrum of antihydrogen atoms for the first time. These results herald the start of a new field of inquiry that should enable some of the most precise comparisons between matter and antimatter ever attempted.

Fig. 1. A schematic cutaway of the ALPHA-2 apparatus, showing the Penning-trap electrodes (yellow), the atom-trap octupole and mirror coils (copper), and the three-layer silicon vertex detector. The mirrors for the Fabry–Pérot cavity are not to scale, and the internal diameter of the Penning-trap electrodes is about 44.5 mm. The lower part of the image illustrates the shape of the magnetic trapping potential in which antihydrogen can be confined.

Unprecedented precision

If you want to measure something precisely, you should probably ask an atomic physicist. For example, the measured frequency of the electronic transition between the ground state and the first excited state in hydrogen (the so-called 1S–2S transition) is 2 466 061 413 187 035 (10) Hz, corresponding to an uncertainty of 4.2 × 10^–15, and the measurement is referenced directly to a cesium time standard. Sounds impressive, but, to quote a recent article in Nature Photonics, “Atomic clocks based on optical transitions approach uncertainties of 10⁻¹⁸, where full frequency descriptions are far beyond the reach of the SI second”. In other words, the current time standard just isn’t good enough anymore, at least not for matter. For comparison, the current best value for the mass of the Higgs boson is 125.09 ± 0.24 GeV/c², representing an uncertainty of about 2 × 10^–3.

To be fair, scientists had already been observing hydrogen’s spectrum for about 200 years by the time the Higgs was discovered. Fraunhofer is credited with mapping out absorption lines, some of which are due to hydrogen, in sunlight in 1814. From there we can trace a direct path through Kirchhoff and Bunsen (1859/1860), who associated Fraunhofer lines with emission lines from distinct elements, to Rydberg, Balmer, Lyman and ultimately to Niels Bohr, who revolutionised atomic physics with his quantum theory in 1913. It is no exaggeration to say that physicists learned modern atomic physics by studying hydrogen, and we are therefore morally obligated to subject antihydrogen to all of the analytical tools at our disposal.

Anti-atomic spectra are not the only hot topic in precision physics at the AD. In 2015 the BASE collaboration determined that the charge-to-mass ratios for the proton and antiproton agree to 69 parts per trillion. The following year, the ASACUSA experiment – which has been making precision measurements on antiprotonic helium for more than a decade – reported that the antiproton-to-electron mass ratio agrees with its proton counterpart to a level of 8 × 10^–10 (CERN Courier December 2016 p19). One of the long term and most compelling goals of the AD programme has always been to compare the properties of hydrogen and antihydrogen to precisions like these.

A word of caution is in order here. In searching for deviations from existing theories, it is tempting to use dimensionless uncertainties such as Δm/m, Δf/f or Δq/q (corresponding to mass, frequency or charge) to compare the merits of different types of measurements. Yet, it is of course not obvious that a hitherto unknown mechanism that breaks CPT or Lorentz invariance, or reveals some other new physics, should create an observable effect that is proportional to the mass, frequency or charge of the state being studied. An alternative approach is to consider the absolute energy scale to which a measurement is sensitive. There is good historical precedent for this in the quantum mechanics of atoms. Roughly speaking, atomic structure, fine structure, hyperfine structure and the Lamb shift reflect different energy scales describing the physical effects that became apparent as experimental techniques became more precise in the 20th century.

At the time of the construction of the AD in the late 1990s, the gold standard for tests of CPT violation was the neutral kaon system. The oft-quoted limit for the fractional difference between the masses of the neutral kaon and anti-kaon was of the order 10^–18. Although there are many other tests of CPT using particle/antiparticle properties, this one in particular stands out for its precision. In the most recent review of the Particle Data Group, the kaon limit is presented as an absolute mass difference of less than 4 × 10^–19 GeV. Although purists of metrology will argue that nothing has actually been measured with a precision of 10^–18 here, the AD physics programme needed a potential goal that could compete, at least in principle, with this level of precision.

The holy grail

Thus the hydrogenic 1S–2S transition became a kind of “holy grail” for antihydrogen physics. The idea was that if the transition in antihydrogen could be measured to the same precision (10^–15) as in hydrogen, any difference between the two transition frequencies could be determined with a precision approaching that of the kaon system. On an absolute scale, the 1S–2S transition energy is about 10.2 eV, so a precision of 10^–15 in this value corresponds to an energy sensitivity of 10^–14 eV (10^–23 GeV). Other features in hydrogen such as the ground-state hyperfine splitting or the Lamb shift have even smaller energies, on the order of µeV. They are also of fundamental interest in antihydrogen and test different types of physical phenomena than the 1S–2S transition. The BASE antiproton experiment probes CPT invariance in the baryon sector at the atto-electron volt scale – 10^–27 GeV – and recently measured the magnetic moment of the antiproton to a precision of 1.5 parts-per-billion. Amazingly, the result was better than the most precise measurement of the proton at the time.

It is sobering to reflect on the state of antihydrogen physics when the AD started operations in 2000. The experiments at CERN’s Low Energy Antiproton Ring (LEAR) in 1996 and at the Accumulator at Fermilab in 1998 had detected nine and 66 relativistic atoms of antihydrogen, respectively, which were produced by interactions between a stored antiproton beam and a gas-jet target. These experiments proved the existence of antihydrogen, but they held no potential for precision measurements.

The pioneering TRAP experiment had already developed the techniques needed for stopping and trapping antiprotons from LEAR, and demonstrated the first capture of antiprotons way back in 1986. The PS200 collaboration succeeded in trapping up to a million antiprotons from LEAR, and TRAP compared the charge-to-mass ratio of protons and antiprotons to a relative precision of about 10^–9. However, no serious attempt had yet been made to synthesise “cold” antihydrogen by the time LEAR stopped operating in 1996.

In 2002 the ATHENA experiment won the race to produce low-energy antihydrogen and the global number of antihydrogen atoms jumped dramatically to 50,000, observed over a few weeks of data taking. This accomplishment had a dramatic effect on world awareness of the AD via the rapidly growing Internet, and it even featured on the front page of the New York Times. Today in ALPHA, which succeeded ATHENA in 2005, we can routinely produce about 50,000 antihydrogen atoms every four minutes.

The antihydrogen atoms produced by ATHENA, and subsequently by ATRAP and ASACUSA, were not confined; they would quickly encounter normal matter in the walls of the production apparatus and annihilate. It would take until 2010 for ALPHA to show that it was possible to trap antihydrogen atoms. Although antihydrogen atoms are electrically neutral, they can be confined through the interaction of their magnetic moments with an inhomogeneous magnetic field. Using superconducting magnets, we can trap antihydrogen atoms that are created with a kinetic energy of less than 43 μeV, or about 0.5 K in temperature units.

In ALPHA’s milestone 2010 experiment, we could trap on average one atom of antihydrogen every eight times we tried, with a single attempt requiring about 20 minutes. Today, in the second-generation ALPHA-2 apparatus, we trap up to 30 atoms in a procedure that takes four minutes. We have also learned how to “stack” antihydrogen atoms. In December 2017 we accumulated more than 1000 anti-atoms at once – limited only by the time available to mess about like this without measuring anything useful! It is no exaggeration to say that no one would have found this number credible in 2000 when the AD began running.

Since the first demonstration of trapped antihydrogen, we have induced quantum transitions in anti-atoms using microwaves, probed the neutrality of antihydrogen, and carried out a proof-of-principle experiment on how to study gravitation by releasing trapped antihydrogen atoms. These experiments were all performed with a trapping rate of about one atom per attempt. In 2016 we made several changes to our antihydrogen synthesis procedure that led to an increase in trapping rate of more than a factor of 10, and we also learned how to accumulate multiple shots of anti-atoms. At the same time, the laser system and internal optics necessary for exciting the 1S–2S transition were fully commissioned in the ALPHA-2 apparatus, and we were finally able to systematically search for this most sought-after spectral line in antimatter.

Antihydrogen’s colours

The ALPHA-2 apparatus for producing and trapping antihydrogen is shown in figure 1. It involves various Penning traps that utilise solenoidal magnetic fields and axial electrostatic wells to confine the charged antiprotons and positrons from which antihydrogen is synthesised. Omitting 30 years of detail, we produce cold antihydrogen by gently merging trapped clouds of antiprotons and positrons that have carefully controlled size, density and temperature. The upshot is that we can combine about 100,000 antiprotons with about two million positrons to produce 50,000 antihydrogen atoms. We trap only a small fraction of these in the superconducting atom trap, which comprises an octupole for transverse confinement and two “mirror coils” for longitudinal confinement.

Anti-atoms that are trapped can be stored for at least 1000 s, but we have yet to carefully characterise the upper limit of the storage lifetime, which depends on the quality of the vacuum. The internal components of ALPHA are cooled to 4 K by liquid helium, and antihydrogen annihilations are detected using a three-layer silicon vertex detector (SVD) surrounding the production region. The SVD senses the charged pions that result from the antiproton annihilation, and event topology is used to differentiate the latter from cosmic rays, which constitute the dominant background (figure 2).

Fig. 2. The dominant background in ALPHA when searching for antihydrogen annihilations comes from cosmic rays. The left panel shows a typical topology of pion tracks resulting from an antiproton annihilation, while the right panel illustrates a typical cosmic-ray event.

A tough catch

Trapping antihydrogen is extremely challenging because the trapped, charged particles that are needed to synthesise it start out with energies measured in eV (in the case of positrons) or keV (antiprotons), whereas the atom can only be confined if it has sub-meV energy. The antihydrogen is trapped due to the interaction of its magnetic moment, which is dominated by the positron spin, with an inhomogeneous magnetic field. Even with very careful preparation of the trapped positron and antiproton clouds in a cryogenic trap, only a small fraction of the produced antiatoms are “cold” enough to be trapped. The good news is that once you have trapped them, the antiatoms stick around for long enough to perform experiments.

Compared to atomic physics with normal matter, one has to somehow make up for the dramatic reduction – at least 20 orders of magnitude – in particle number at the source. The key to this is twofold: the long interaction times available with trapped particles, and the single-atom detection sensitivity afforded by antimatter annihilation. The annihilation of an antihydrogen atom is a microscopically violent event, releasing almost 2 GeV of mass-energy that can be easily detected. This is perhaps the only good thing about working with antihydrogen: if you lose it, even just one atom of it, you know it. Conversely, the loss of a single atom of hydrogen in an equivalent experiment would go unnoticed and un-mourned if there are, say, 10¹² remaining (a typical number for trapped hydrogen). Thus, the two experiments recently reported by ALPHA are conceptually simple: trap some antihydrogen atoms; illuminate them with electromagnetic radiation that causes the anti-atoms to be lost from the trap when the radiation is on-resonance; sit back and watch what falls out.

Let’s consider first the “holy grail” (1S–2S) transition, which is excited by two, counter-propagating ultraviolet photons with a wavelength of 243 nm. The power from our Toptica 243 nm laser is enhanced in a Fabry–Pérot cavity formed by two mirrors inside the cryogenic, ultra-high vacuum system. (This cavity owes its existence to the paucity of atoms available; without the optical power buildup achieved, the experiment would not be currently possible.) The 1S–2S transition has a very narrow linewidth – this is what makes it interesting – so the laser frequency needs to be just right to excite it. The other side of the same coin is that the 2S state lives for a relatively long time, about one eighth of a second, so there can be time for an excited antihydrogen atom to absorb a third photon, which will ionise it. Stripped of its positron, the antiproton is no longer confined in the magnetic trap and is free to escape to the wall and annihilate. There is also a chance that an un-ionised 2S state atom will suffer a positron spin-flip in the decay to the ground state, in which case the atom is also lost.

In the actual experiment, we illuminate trapped antihydrogen atoms with a laser for about 10 minutes, then turn off the trap (in a period of 1.5 s) and use the SVD to count any remaining atoms as they escape. Also, using the SVD we can observe any antihydrogen atoms that are lost during the laser illumination. In this way, we obtain a self-consistent picture of the fate of the atoms that were initially trapped. The evidence for the laser interaction comes from comparing what happens when the laser has the “right” frequency, compared to what happens when we intentionally de-tune the laser to a frequency where no interaction is expected (for hydrogen). As a control, and to monitor the varying trapping rate, we perform the same sequence with no laser present. The whole thing can be summarised in a simple table (figure 3), which shows the results of 11 trials of each type.

Fig. 3. The number of raw events that are used by ALPHA to measure the antihydrogen 1S–2S transition must be scaled by an overall detection efficiency to represent the actual numbers of atoms. In this table, the detection efficiencies are different for the first column (0.376) and the third column (0.688) because the algorithms used to distinguish annihilations from cosmic rays are tuned differently in order to reflect the very different observation times (600 s versus 1.5 s for each trial).

A quick glance reveals that the off-resonance and no-laser numbers are consistent with each other and with “nothing going on”. In contrast, the on-resonance numbers show excess events due to atoms knocked out when the laser is on, and a dearth of events left over after the exposure. If we consider the overall inventory of antihydrogen atoms and compare the on- and off-resonance data only, we see that about 138 atoms (79–27)/0.376 have been knocked out, and 134 atoms (159–67)/0.688 are missing from the left-over sample, so our interpretation is self-consistent within the uncertainties.

This initial “go/no-go” experiment demonstrates that the transition is where we expect it to be for hydrogen and localises it to a frequency of about 400 kHz (the laser detuning for the off-resonance trials) out of 2.5 × 10¹⁵ Hz. That’s a relative precision of about 2 × 10^–10, or 2 × 10^–18 GeV in absolute energy units, just for showing up, and this was achieved by employing a total of just 650 or so trapped atoms. The next step is obviously to measure more frequencies around the resonance to study the shape of the spectral line, which will allow more precise determination of the resonance frequency. Note that CPT invariance requires that the shape must be identical to that expected for hydrogen in the same environment. Determination of this lineshape was the main priority for ALPHA’s 2017 experimental campaign, so stay tuned.

To hyperfine splitting and beyond

A similar strategy can be used to study other transitions in antihydrogen, in particular its hyperfine splitting. With ALPHA we can drive transitions between different spin states of antihydrogen in the magnetic trap. In a magnetic field, the 1S ground state splits into four states that correspond, at high fields, to the possible alignments of the positron and antiproton spins with the field (figure 4). The upper two states can be trapped in ALPHA’s magnetic trap and, using microwaves at a frequency of about 30 GHz, it is possible to resonantly drive transitions from these two states to the lower energy states, which are not trappable and are thus expelled from the trap.

Fig. 4. A Breit–Rabi diagram showing the hyperfine energy levels of hydrogen in a magnetic field. The arrows in the state labels refer to the electron (left) and proton spin orientations, while the yellow shaded region illustrates the range of magnetic field strength in the ALPHA trap.

We concentrate on the two transitions |d〉 →|a〉 and |c〉 →|b〉, which in the ALPHA trapping field (minimum 1 T) correspond to positron spin flips. We had previously demonstrated that these transitions are observable, but in 2016 we took the next step and actually characterised the spectral shapes of the two discrete transitions in our trap. We are now able to accumulate antihydrogen atoms, scan the microwave frequency over the range corresponding to the two transitions, and watch what happens using the SVD. The result, which may be considered to be the first true antihydrogen spectrum, is shown in figure 5.

Fig. 5. The number of detected annihilation events plotted versus microwave frequency. The two spectral lines represent the c to b (left) and the d to a transitions (see main text and figure 4).

The difference between the onset frequencies of the two spectral lines gives us the famous ground-state hyperfine splitting (in hydrogen, the ground-state hyperfine transition is the well known “21 cm line”, so beloved of radioastronomers and those searching for signs of extraterrestrial life). From figure 5 we extract a value for this splitting of 1420.4 ± 0.5 MHz, for a relative precision of 3.5 × 10^–4; the energy sensitivity is 2 × 10^–18 GeV. In normal hydrogen this number has been measured to be 1420.405751768 (2) MHz – that’s 1.2 × 10^–12 relative precision or a shockingly small 10^–26 GeV. ALPHA is busily improving the precision of the antihydrogen hyperfine measurement, and the ASACUSA collaboration at the AD hopes to measure the same quantity to the ppm level using a challenging antihydrogen-beam technique; an analogous experiment on hydrogen was recently reported (CERN Courier December 2017 p23).

The antihydrogen atom still holds many structural secrets to be explored. Near-term perspectives in ALPHA include the Lyman-alpha (1S–2P) transition, with its notoriously difficult-to-produce 121.5 nm wavelength in the vacuum ultraviolet. We are currently attempting to address this with a pulsed laser, with the ultimate goal to laser-cool antihydrogen for studies in gravitation and for improved resolution in spectroscopy. To give a flavour of the pace of activities, a recent daily run meeting saw ALPHA collaborators actually debate which of the three antihydrogen transitions we should study that day, which was somewhat surreal. In the longer term, even the ground-state Lamb shift should be accessible using ALPHA’s trapped antiatoms.

It is clearly “game on” for precision comparisons of matter and antimatter at the AD. It is fair to say that the facility has already exceeded its expectations, and the physics programme is in full bloom. We have some way to go before we reach hydrogen-like precision in ALPHA, but the road ahead is clear. With the commissioning of the very challenging gravity experiments GBAR, AEGIS and ALPHA-g over the next few years, and the advent of the new low-energy ELENA ring at the AD (CERN Courier December 2016 p16), low-energy antimatter physics at CERN promises a steady stream of groundbreaking results, and perhaps a few surprises.

Putting the Pauli exclusion principle on trial

Portrait of a young Pauli at Svein Rosseland’s institute in Oslo in the early 1920s, when he was thinking deeply on the applications of quantum mechanics to atomic physics. Image credit: PAULI-ARCHIVE-PHO-011-1

If we tightly grasp a stone in our hands, we neither expect it to vanish nor leak through our flesh and bones. Our experience is that stone and, more generally, solid matter is stable and impenetrable. Last year marked the 50th anniversary of the demonstration by Freeman Dyson and Andrew Lenard that the stability of matter derives from the Pauli exclusion principle. This principle, for which Wolfgang Pauli received the 1945 Nobel Prize in Physics, is based on ideas so prevalent in fundamental physics that their underpinnings are rarely questioned. Here, we celebrate and reflect on the Pauli principle, and survey the latest experimental efforts to test it.

The exclusion principle (EP), which states that no two fermions can occupy the same quantum state, has been with us for almost a century. In his Nobel lecture, Pauli provided a deep and broad-ranging account of its discovery and its connections to unsolved problems of the newly born quantum theory. In the early 1920s, before Schrödinger’s equation and Heisenberg’s matrix algebra had come along, a young Pauli performed an extraordinary feat when he postulated both the EP and what he called “classically non-describable two-valuedness” – an early hint of the existence of electron spin – to explain the structure of atomic spectra.

Part of a letter written by Pauli to Fierz in October 1949, where Pauli discusses Leibnitz’s principle of the “identity of the indiscernibles” in connection with the exclusion principle. Image credit: Pauli Archive, holding: fierz_0092-064

At that time the EP met with some resistance and Pauli himself was dubious about the concepts that he had somewhat recklessly introduced. The situation changed significantly after the introduction in 1925 of the electron-spin concept and its identification with Pauli’s two-valuedness, which derived from the empirical ideas of Lande, an initial suggestion by Kronig, and an independent paper by Goudsmit and Uhlenbeck. By introducing the picture of the electron as a small classical sphere with a spin that could point in just two directions, both Kronig, and Goudsmit and Uhlenbeck, were able to compute the fine-structure splitting of atomic hydrogen, although they still missed a critical factor of two. These first steps were followed by the relativistic calculations of Thomas, by the spin calculus of Pauli, and finally, in 1928, by the elegant wave equation of Dirac, which put an end to all resistance against the concept of spin.

A conceptual scheme of the Ramberg and Snow experiment. “Anomalous” electrons in the copper bar can be captured by copper atoms, followed by the emission of X-rays. The extent of EP violation can be estimated from the number of detected X-rays. The same conceptual scheme is used by the VIP experiment at LNGS.

However, a theoretical explanation of the EP had to wait for some time. Just before the Second World War, Pauli and Markus Fierz made significant progress toward this goal, followed by the publication in 1940 by Pauli of his seminal paper “The connection between spin and statistics”. This paper showed that (assuming a relativistically invariant form of causality) the spin of a particle determines the commutation relations, i.e. whether fields commute or anticommute, and therefore the statistics that particles obey. The EP for spin-1/2 fermions follows as a corollary of the spin-statistics connection, and the division of particles into fermions and bosons based on their spins is one of the cornerstones of modern physics.

Beguilingly simple

The EP is beguilingly simple to state, and many physicists have tried to skip relativity and find direct proofs that use ordinary quantum mechanics alone – albeit assuming spin, which is a genuinely relativistic concept. Pauli himself was puzzled by the principle, and in his Nobel lecture he noted: “Already in my original paper I stressed the circumstance that I was unable to give a logical reason for the exclusion principle or to deduce it from more general assumptions. I had always the feeling and I still have it today, that this is a deficiency. …The impression that the shadow of some incompleteness fell here on the bright light of success of the new quantum mechanics seems to me unavoidable.” Even Feynman – who usually outshone others with his uncanny intuition – felt frustrated by his inability to come up with a simple, straightforward justification of the EP: “It appears to be one of the few places in physics where there is a rule which can be stated very simply, but for which no one has found a simple and easy explanation… This probably means that we do not have a complete understanding of the fundamental principle involved. For the moment, you will just have to take it as one of the rules of the world.”

Of special interest

After further theoretical studies, which included new proofs of the spin-statistics connection and the introduction of so-called para-statistics by Green, a possible small violation of the EP was first considered by Reines and Sobel in 1974 when they reanalysed an experiment by Goldhaber and Scharff in 1948. The possibility of small violations was refuted theoretically by Amado and Primakoff in 1980, but the topic was revived in 1987. That year, Russian theorist Lev Okun presented a model of violations of the EP in which he considered modified fermionic states which, in addition to the usual vacuum and one-particle state, also include a two-particle state. Okun wrote that “The special place enjoyed by the Pauli principle in modern theoretical physics does not mean that this principle does not require further and exhaustive experimental tests. On the contrary, it is specifically the fundamental nature of the Pauli principle that would make such tests, over the entire periodic table, of special interest.”

The VIP2 experimental setup installed at the underground LNGS laboratory in Gran Sasso, Italy.

Okun’s model, however, ran into difficulties when attempting to construct a reasonable Hamiltonian, first because the Hamiltonian included nonlocal terms and, second, because Okun did not succeed in constructing a relativistic generalisation of the model. Despite this, his paper strongly encouraged experimental tests in atoms. In the same year (1987), Ignatiev and Kuzmin presented an extension of Okun’s model in a strictly non-relativisitic context that was characterised by a “beta parameter” |β| << 1. Not to be confused with the relativistic factor v/c, β is a parameter describing the action of the creation operator on the one-particle state. Using a toy model to illustrate transitions that violate the EP, Ignatiev and Kuzmin deduced that the transition probability for an anomalous two-electron symmetric state is proportional to β²/2, which is still widely used to represent the probability of EP violation.

This non-relativistic approach was criticized by A B Govorkov, who argued that the naive model of Ignatiev and Kuzmin could not be extended to become a fully-fledged quantum field theory. Since causality is an important ingredient in Pauli’s proof of the spin-statistics connection, however, Govorkov’s objections could be bypassed: later in 1987, Oscar Greenberg and Rabindra Mohapatra at the University of Maryland introduced a quantum field theory with continuously deformed commutation relations that led to a violation of causality. The deformation parameter was denoted by the letter q, and the theory was supposed to describe new hypothetical particles called “quons”. However, Govorkov was able to show that even this sleight of hand could not trick quantum field theory into small violations of the EP, demonstrating that the mere existence of antiparticles – again a true relativistic hallmark of quantum field theory – was enough to rule out small violations. The take-home message was that the violation of locality is not enough to break the EP, even “just a little”.

The connection between the intrinsic spin of particles and the statistics that they obey are at the heart of quantum field theory and therefore should be tested. A violation of the EP would be revolutionary. It could be related either to the violation of CPT, or violation of locality or Lorentz invariance, for example. However, we have seen how robust the EP is and how difficult it is to frame a violation within current quantum field theory. Experiments face no lesser difficulties, as noted as early as 1980 by Amado and Primakoff, and there are very few experimental options with which to truly test this tenet of modern physics.

One of the difficulties faced by experiments is that the identicalness of elementary particles implies that Hamiltonians must be invariant with respect to particle exchange, and, as a consequence, they cannot change the symmetry of any given state of multiple identical particles. Even in the case of a mixed symmetry of a many-particle system, there is no physical way to induce a transition to a state of different symmetry. This is the essence of the Messiah–Greenberg superselection rule, which can only be broken if a physical system is open.

Breaking the rules

The first dedicated experiment in line with this breaking of the Messiah–Greenberg superselection rule was performed in 1990 by Ramberg and Snow, who searched for Pauli-forbidden X-ray transitions in copper after introducing electrons into the system. The idea is that a power supply injecting an electric current into a copper conductor acts as a source of electrons, which are new to the atoms in the conductor. If these electrons have the “wrong” symmetry they can be radiatively captured into the already occupied 1S level of the copper atoms and emit electromagnetic radiation. The resulting X-rays are influenced by the unusual electron configuration and are slightly shifted towards lower energies with respect to the characteristic X-rays of copper.

Ramberg and Snow did not detect any violation but were able to put an upper bound on the violation probability of Β²/2 < 1.7 × 10^–26. Following their concept, a much improved version of the experiment, called VIP (violation of the Pauli principle), was set up in the LNGS underground laboratory in Gran Sasso, Italy, in 2006. VIP improved significantly on the Ramberg and Snow experiment by using charge-coupled devices (CCDs) as high-resolution X-ray detectors with a large area and high intrinsic efficiency. In the original VIP setup, CCDs were positioned around a pure-copper cylinder; X-rays emitted from the cylinder were measured without and with current up to 40 A. The cosmic background in the LNGS laboratory is strongly suppressed – by a factor of 10⁶ thanks to the overlying rock – and the apparatus was also surrounded by massive lead shielding.

Setting limits

After four years of data taking, VIP set a new limit on the EP violation for electrons at β²/2 < 4.7 × 10^–29. To further enhance the sensitivity, the experiment was upgraded to VIP2, where silicon drift detectors (SDDs) replace CCDs as X-ray detectors. The VIP2 construction started in 2011 and in 2016 the setup was installed in the underground LNGS laboratory, where, after debugging and testing, data-taking started. The SDDs provide a wider solid angle for X-ray detection and this improvement, together with higher current and active shielding with plastic scintillators to limit background, leads to a much better sensitivity. The timing capability of SDDs also helps to suppress background events.

The experimental programme testing for a possible violation of the EP for electrons made great progress in 2017 and had already improved the upper limit set by VIP in the first two months of running time. With a planned duration of three years and alternating measurement with and without current, a two-orders-of-magnitude improvement is expected with respect to the previous VIP upper bound. In the absence of a signal, this will set the limit on violations of the EP at β²/2 < 10^–31.

Experiments like VIP and VIP2 test the spin-statistics connection for one particular kind of fermions: electrons. The case of EP violations for neutrinos was also theoretically discussed by Dolgov and Smirnov. As for bosons, constraints on possible statistics violations come from high-energy-physics searches for decays of vector (i.e. spin-one) particles into two photons. Such decays are forbidden by the Landau–Yang theorem, whose proof incorporates the assumption that the two photons must be produced in a permutation-symmetric state. A complementary approach is to apply spectroscopic tests, as carried out at LENS in Florence during the 1990s, which probe the permutation properties of ¹⁶O nuclei in polyatomic molecules by searching for transitions between states that are antisymmetric under the exchange of two nuclei. If the nuclei are bosons, as in this case, such transitions, if found, violate the spin-statistics relation. High-sensitivity tests for photons were also performed with spectroscopic methods. As an example, using Bose–Einstein-statistics-forbidden two-photon excitation in barium, the probability for two photons to be in a “wrong” permutation-symmetry state was shown by English and co-workers at Berkeley in 2010 to be less than 4 × 10^–11 – an improvement of more than three orders of magnitude compared to earlier results.

To conclude, we note that the EP has many associated philosophical issues, as Pauli himself was well aware of, and these are being studied within a dedicated project involving VIP collaborators, and supported by the John Templeton Foundation. One such issue is the notion of “identicalness”, which does not seem to have an analogue outside quantum mechanics because there are no two fundamentally identical classical objects.

This ultimate equality of quantum particles leads to all-important consequences governing the structure and dynamics of atoms and molecules, neutron stars, black-body radiation and determining our life in all its intricacy. For instance, molecular oxygen in air is extremely reactive, so why do our lungs not just burn? The reason lies in the pairing of electron spins: ordinary oxygen molecules are paramagnetic with unpaired electrons that have parallel spins, and in respiration this means that electrons have to be transferred one after the other. This sequential character to electron transfers is due to the EP, and moderates the rate of oxygen attachment to haemoglobin. Think of that the next time you breathe!

Shaping science in South-East Europe

Delegates at a scientific forum at the ICTP in Trieste in January explored a new world-class research infrastructure for South-East Europe. Image credit: Government of Montenegro

In the autumn of 2016, at a meeting in Dubrovnik, Croatia, trustees of the World Academy of Art and Science discussed a proposal to create a large international research institute for South-East Europe. The facility would promote the development of science and technology and help mitigate tensions between countries in the region, following the CERN model of “science for peace”. A platform for internationally competitive research in South-East Europe would stimulate the education of young scientists, transfer and reverse the brain drain, and foster greater cooperation and mobility in the region.

The South-East Europe initiative received first official support by the government of Montenegro, independent of where the final location would be, thanks to the engagement of Montenegro science minister Sanja Damjanovic, who is also a physicist with a long tradition working at CERN.

On 25 October last year at a meeting at CERN, ministers of science or their representatives from countries in the region signed a Declaration of Intent (DOI) to establish a South-East Europe International Institute for Sustainable Technologies (SEEIIST) with the above objectives. The initial signatories were Albania, Bosnia and Herzegovina, Bulgaria, Kosovo^*, The Former Yugoslav Republic of Macedonia, Montenegro, Serbia and Slovenia. Croatia agreed in principle, while Greece participated as an observer. CERN’s role was to provide a neutral and inspirational venue for the meeting.

The signature of the DOI was followed by a scientific forum on 25–26 January at the International Centre for Theoretical Physics (ICTP) in Trieste, Italy, held under the auspices of UNESCO, the International Atomic Energy Agency (IAEA) and the European Physical Society. The forum attracted more than 100 participants ranging from scientists and engineers at universities to representatives of industry, government agencies and international organisations including ESFRI and the European Commission. Its aim was to present two scientific options for SEEIIST: a fourth-generation synchrotron light source that would offer users intense beams from infrared to X-ray wavelengths; and a state-of-the-art patient treatment facility for cancer using protons and heavy ions, also with a strong biomedical research programme. The concepts behind each proposal were worked out by two groups of international experts.

With SEEIIST’s overarching goal to be a world-class research infrastructure, the training of scientists, engineers and technicians is essential. Whichever project is selected, it will require several years of effort, during which people will be trained for the operation of the machines and user communities will also be formed. Capacity-building and technology-transfer activities will further trigger developments for the whole region, such as the development of powerful digital networks and big-data handling.

Reports and discussions from the ICTP forum have provided an important basis for the next steps. Representatives of IAEA declared an interest in helping with the training programme, while European Union (EU) representatives are also looking favourably at the project – potentially providing resources to support the preparation of a detailed conceptual design and eventual concrete proposal.

The initiative is gathering momentum. On 30 January the first meeting of the SEEIIST steering committee, chaired initially by the Montenegro science minister, took place in Sofia, Bulgaria. Sofia was chosen at the invitation of Bulgaria since it currently holds the EU presidency, and the meeting was introduced by Bulgarian president Rumen Radew, who expressed strong interest in SEEIIST and promised to support the initiative. Officials have underlined that a decision between the two scientific options should be taken as soon as possible – a task that we are now working towards.

SEEIIST wouldn’t be the first organisation to be inspired by the CERN model. The European Southern Observatory, European Molecular Biology Laboratory and the recently operational SESAME facility in Jordan – a third-generation light source governed by a council made up of representatives from eight members in the Middle East and surrounding region – each demonstrate the power of fundamental science to advance knowledge and bring people and countries together.

• This designation is without prejudice to positions on status and is in line with UNSC 1244/1999 and the ICJ opinion on the Kosovo Declaration of Independence.

Raoul Gatto 1930–2017

The passing of Raoul Raffaele Gatto in Meyrin, Geneva, on 30 September is a big loss for science and for a whole generation of particle theorists. After graduating at the Scuola Normale in Pisa, and a short stay at La Sapienza (Rome), Gatto held prominent positions at Berkeley and Frascati before occupying, successively, the chair of theoretical physics in Cagliari, Florence, Padua, Rome and, eventually, at the University of Geneva.

A member of the Accademia dei Lincei, the Accademia delle Scienze of Turin and the American Physical Society, he received numerous recognitions such as the Enrico Fermi medal and the prize of the President of the Italian Republic. For several decades he was editor of Physics Letters B and deputy director of the Rivista del Nuovo Cimento.

Raoul Gatto transmitted Fermi’s legacy to the next generation. Image credit: Gatto family

Gatto’s contributions to theoretical physics are too many to be listed here. We may just recall his joint work with Cabibbo on the muon neutrino and on weak hyperon decays (which formed the basis of Cabibbo’s discovery of the angle that carries his name), the Ademollo-Gatto theorem on the absence of first-order breaking of flavour symmetry in weak hadronic decays, his pioneering work on scale and conformal invariance in quantum field theory, and a series of papers on composite Higgs models.

While in terms of scientific achievements Gatto clearly belonged to the class of the theorists of his generation, he was head and shoulders above the crowd as a teacher. It is not easy to pin down the secret of his success in attracting young researchers to theoretical physics and in helping them grow and develop their own individual qualities. Both Luciano Maiani and myself, for instance, were dragged from experimental high-energy physics to theory by his charming, attractive personality. Luciano had already graduated as an experimentalist before joining Gatto’s group in 1964. I had to go through a long period of study and work before being accepted, but it was worthwhile.

When Gatto came to Florence, a group of very promising young researchers followed him one after another: Altarelli, Buccella, Celeghini, Gallavotti, Maiani and Preparata. Gatto created a stimulating, healthy, competitive atmosphere by distributing among us original, challenging research projects. We had to work things out without much help from him, except for letting us know, occasionally and very gently, that there was something that had to be changed in our approach. The whole group (soon dubbed the “gattini”) grew in strength and reputation, and soon we became capable of doing independent research. More senior theorists who were already in Florence (among them Ademollo, Chiuderi and Longhi) were also integrated in the new structure, together with students like myself, Casalbuoni and Dominici. This success story repeated itself when Gatto moved to Padue (with Sartori, Tonin and Feruglio) and then again in Rome (with Ferrara and Parisi).

It is often said that Enrico Fermi created the Italian school of particle physics after World War Two. I believe that, for theoretical physics, Raoul Gatto was the heir of Fermi, who best transmitted his legacy to the next generation.

Exact Solutions in Three-Dimensional Gravity

By Alberto A García-Díaz
Cambridge University Press

As stated by the author himself, this book is the result of many years of work and has the purpose of providing a comprehensive, but concise, account of exact solutions in three-dimensional (or 2+1) Einstein gravity. It presents the theoretical frameworks and the general physical and geometrical characteristics of each class of solutions, and includes information about the researchers who discovered or studied them.

These solutions are identified and ordered on the basis of their geometrical invariant properties, their symmetries and their algebraic classifications, or according to their physical nature. They are also examined from different perspectives.

Emphasis is given to solutions to the Einstein equation in the presence of matter and fields, such as: point particle solutions, perfect fluids, dilatons, inflatons and cosmological space-times.

The second part of the book discusses solutions to vacuum topologically massive gravity with a cosmological constant.

Overall, this text serves as a thorough catalogue of exact solutions in (2+1) Einstein gravity and is a very valuable resource for graduate students, as well as researchers in gravitational physics.

Mosquitoes

by Lucy Kirkwood
National Theatre, London 18 July–28 September 2017

Lucy Kirkwood’s play Mosquitoes is an ambitious piece of theatre. It combines the telling of an eclectic family drama with the asking of a variety of questions ranging from personal relationships to the remit of science. Mosquitoes tells the story of CERN scientist Alice (Olivia Williams), and the fractious relationship she has with her sister Jenny (Olivia Colman). After working for 11 years at CERN on the French–Swiss border, Alice is visited by Jenny just as work on discovering the Higgs boson is nearing its peak. Conflict between Jenny and Alice’s challenged son, Luke (Joseph Quinn), drives much of the plot. Domestic scenes between these three characters are interspersed with glimpses of Luke’s absent father, who momentarily turns the theatre into a planetarium while waxing lyrical over the science which the play is set against.

The spectacle of these brief moments is a highlight of the play; contrasting wonderfully with the often mundane lives of the characters. Kirkwood also makes a poignant contrast between the characters’ personal and professional lives. Alice, despite exuding a certain confidence in her professional life as a scientist, often struggles to relate personally to those around her. Chief amongst those is her son Luke who, despite showing the occasional interest in his mother’s work, is ultimately critical of it for a number or reasons. He questions the environmental impact of what she is doing, believing that the LHC poses existential risks. He also frequently bemoans his mother’s commitment to her work, which he believes comes at the expense of himself. Through the play, it becomes apparent that Luke and his mother previously lived in the UK, and that he was made to follow her to Switzerland, but he would like to go back to England.

These personal relationships are played out in front of the sisters’ ailing mother Karen (Amanda Boxer). A former physicist herself now suffering from dementia, Karen frequently laments missing out on her chances at winning a Nobel Prize. Karen’s character, who provides the audience with a glimpse of her daughter Alice’s future, adds a sense of futility to Alice’s work.

Overall, Mosquitoes – the title coming from a line of dialogue in which protons smashing in the Large Hadron Collider are compared to mosquitoes hitting each other head on – is a stunning piece of work. Not just for the way it weaves together story lines to explore a range of complex questions, but also for the immensely high quality of acting talent which it boasts. This is bettered only by the faultless light, sound, and set design, which complement each other perfectly during the play’s most dramatic moments.

Fermilab at 50

By Swapan Chattopadhyay and Joseph David Lykken (eds.)
World Scientific

On the occasion of the 50th anniversary of its foundation, the management of Fermilab asked leading scientists and supporters, whose careers and life paths crossed at the US laboratory, to share their memories and thoughts about its past, present and future. The short essays received have been collected in this commemorative book.

Among the many prestigious contributors are Nobel laureates T D Lee, Burton Richter and Jack Steinberger; in addition to present and former Fermilab directors (Nigel Lockyer, Piermaria Oddone and John Peoples); present and former CERN Directors-General (Fabiola Gianotti and Rolf Heuer), as well as many other important physicists, scientific leaders and even politicians and businessmen.

Through the recollections of the authors, key events in Fermilab’s history are brought to life. The milestone of 50 years of research are also retraced in a rich photo gallery.

While celebrating its glorious past, Fermilab is also looking towards its future, as highlighted in the book. Many experiments are ongoing, or planned at the laboratory and its scientific programme includes research on neutrinos; accelerator science; quantum computing; dark matter and the cosmic background radiation, as well as a continuous participation in the LHC physics, especially in the CMS experiment.

A light read, this book will appeal to all the scientists who at some point in their career stepped on the floor of Fermilab. It will also appeal to those readers who are interested in discovering more about the history of the laboratory through the records of the people who participated in it, whether it was directly or indirectly.

Loop Quantum Gravity: The First 30 Years

By Abhay Ashtekar and Jorge Pullin (eds.)
World Scientific

This book, which is part of the “100 Years of General Relativity” series of monographs, aims to provide an overview of the foundations and recent developments of loop quantum gravity (LQG).

This is a theory that merges quantum mechanics and general relativity in an effort to unify gravity with the other three fundamental forces. In the approach of LQG, space–time is not a continuum, but it is quantised, and is considered as a dynamic entity. Different from string theory, loop quantum gravity is a “background-independent” theory, which aims to explain space and time instead of being plugged into an already existing space–time structure.

The book comprises eight chapters, distributed in three parts. The first is a general introduction that sets the scene and anticipates what will be discussed in detail in the following sections. The second part, comprising five chapters, introduces the conceptual, mathematical and physical foundation of LQG. In part three, the application of this theory to cosmology and black holes is discussed, also introducing predictions that might be testable in the foreseeable future.

Written by young theoretical physicists who are expert in the field, this volume is meant both to provide an introduction to the field and to offer a review of the latest developments, not discussed in many other existing books, for senior researchers. It will also appeal to scientists who do not work directly on LQG but are interested in issues at the interface of general relativity and quantum physics.

I am the Smartest Man I Know: A Nobel Laureate’s Difficult Journey

By Ivar Giaever
World Scientific

At the end of his last semester studying mechanical engineering at the Norwegian Institute of Technology, Ivar Giaever gained a grade of 3.5 for a thesis on the efficiency of refrigeration machines – just a little better than the 4.0 needed to pass. The thesis had been hastily written as the machines worked badly, and he and his friend had had little time to collect their data. But they both scraped through and, as Giaever writes, “maybe sometimes life is a little bit fair after all?”.

It’s a reference to the opening words of his light-hearted autobiography: “Life is not fair, and I, for one, am happy about that.” The title sounds provocative, but

the book is a reflection on how life’s little twists and turns can have extremely important consequences.

Giaever calls this “luck” and admits that he has had more than his share of it – from relatively humble beginnings in Norway to a Nobel prize and beyond.

In many respects Giaever had been a “bad” student. Good at cards, billiards, chess – and drinking – he had little interest in mechanical engineering. He finished with a grade of 4.0 in both physics and mathematics; but had at least married Inger, his long-time sweetheart.

His first job was at the patent office in Oslo, but apartments were hard to find, so the couple decided to emigrate to Canada. A few twists led Giaever to General Electric (GE), where he had the chance to study again through the company’s “A, B and C” courses.

This second chance to learn proved pivotal. Seeing how the studies related to GE’s production of generators, motors and such like, made learning exciting, and Giaever graduated as the best student on the A course. But GE in Canada offered only the A course and, eager to learn more, he moved to GE’s Research Laboratory in Schenectady in the US.

There he completed the B and C courses, and also began studying for a master’s degree in physics at the Rensselaer Polytechnic Institute (RPI). He was to remain with GE for the next 30 years, after being offered a permanent job, even though he did not yet have a PhD.

As a fully-fledged member of the research lab, Giaever needed a project. John Fisher proposed that he look into quantum mechanical tunnelling between thin films, which Giaever went on to do with great success in 1959.

Then, during his studies at RPI, he learned about the new Bardeen–Cooper–Schrieffer (BCS) theory of superconductivity, which predicted the appearance of a forbidden energy gap near the Fermi level when a metal becomes superconducting. Giaever realised that he could measure this gap using his tunnelling apparatus, and so provide crucial verification of the BCS theory. He also realised that tunnelling between two superconductors with different energy gaps would produce a negative resistance, and could allow for active devices such as amplifiers. He worried that if he talked about his work, others would realise this before he had done the relevant experiment.

To his surprise nobody did, hence his comment to his family: “I am the smartest man I know!”. His children thought he was being big-headed, but in 1973 the whole family went with him to Stockholm when he was rewarded with a share of the Nobel Prize in Physics in 1973 for his work on tunnelling in superconductors.

Giaever, of course, covers much more of his life story in this book. There is little technical detail, but a plethora of anecdotes that provide fascinating insight into a person who has made the most of his life.

Two impressions stand out: he is lucky to have found in Inger a partner with whom he has been able to share his long life; and he is lucky to have had a second chance to study and discover that he is smarter than many people thought.

Particle physics meets quantum optics

Photo of Sergio Bertolucci, John Womersley and Victor Matveev — The roundtable discussion, with panel members including former CERN director of research, Sergio Bertolucci (left), and two directors-general of major laboratories: John Womersley of the ESS (middle) and Victor Matveev of JINR (right).

The sixth International Conference on New Frontiers in Physics (ICNFP) took place on 17–29 August in Kolymbari, Crete, Greece, bringing together about 360 participants. Results from LHC Run 2 were shown, in addition to some of the latest advances in quantum optics.

A mini-workshop dedicated to “highly-ionising avatars of new physics” brought together an ever-growing community of theorists, astroparticle physicists and collider experimentalists. There were also presentations of advances in the theory of highly ionising particles as well as light monopoles, with masses accessible to LHC and future colliders, and discussions included experimental searches both extraterrestrial and terrestrial, including results on magnetic monopoles from MoEDAL-LHC experiment that have set the strongest limits so far on high-charge monopoles at colliders.

In the “quantum” workshops, this year dedicated to the 85th birthday of theorist Yakir Aharonov, leading experts addressed fundamental concepts and topics in quantum mechanics, such as continuous variables and relativistic quantum information measurement theory, collapse, time’s arrow, entanglement and nonlocality.

In the exotic hadron workshop the nature of the exotic meson X(3872) was discussed in considerable detail, especially with regard to its content: is it a mixture of a hadronic molecule and excited charmonium, or a diquark–antidiquark state? Detailed studies of the decay modes and p_T dependence of the production cross section in proton–proton collisions emerged as two most promising avenues for clarifying this issue. Following the recent LHCb discovery of doubly-charmed Χ_cc baryon, new results were reported including the prediction of a stable bbbud tetraquark and a quark-level analogue of nuclear fusion.

Presentations on the future low-energy heavy-ion accelerator centres, FAIR in Darmstadt and NICA at JINR in Dubna, showed that the projects are progressing on schedule for operation in the mid-2020s. Delegates were also treated to the role of non-commutative geometry as a way to unify gauge theories and gravity, self-interactions among right-handed neutrinos with masses in the warm-dark-matter regime, and the subtle physics behind sunsets and the aurora.

The conference ended with two-day workshops on supergravity and strings, and a workshop on the future of fundamental physics. Major future projects were presented, together with visionary talks about the future of accelerators and the challenges ahead in the interaction of fundamental physics and society. The conference also hosted a well-attended special session on physics education and outreach. The next ICNFP conference will take place on 4–12 July 2018 in Kolymbari, Crete.

indico.cern.ch/event/559774

Topics

Reset your password

Registration complete