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Particles over Politics: The More the Merrier

Monday, August 1st, 2016

Earlier last month, Romania became the 22nd Member State of the European Organisation for Nuclear Research, or CERN, home to the world’s most powerful atom-smasher. But the hundred Romanian scientists working on experiments there have already operated under a co-operation agreement with CERN for the last 25 years. So why have Romania decided to commit the money and resources needed to become a full member? Is this just bureaucratic reshuffling or the road to a more fruitful collaboration between scientists?

Image: CERN

On 18th July, Romania became a full member state of CERN. In doing so, it joined twenty one other countries, which over the years have created one of the largest scientific collaborations in the world. Last year, the two largest experimental groups at CERN, ATLAS and CMS, broke the world record for the total number of authors on a research article (detailing the mass of the Higgs Boson).

To meet its requirements for becoming a member, Romania has committed $11mil USD towards the CERN budget this year, three times as much as neighbouring member Bulgaria and more than seven times as much as Serbia, which holds Associate Membership, aiming to follow in Romania’s footsteps. In return, Romania now holds a place on CERN’s council, having a say in all the major research decisions of the ground-breaking organization where the forces of nature are probed, antimatter is created and Higgs Bosons discovered.

Romania’s accession to the CERN convention marks another milestone in the organisation’s history of international participation over the last sixty years. In that time it has built bridges between the members of nations where diplomacy and international relations were less than favourable, uniting researchers from across the globe towards the goal of understanding the universe on its most fundamental level.

CERN was founded in 1954 with the acceptance of its convention by twelve European nations in a joint effort for nuclear research, the year where “nuclear research” included the largest ever thermonuclear detonation by the US in its history and the USSR deliberately testing the effects of nuclear radiation from a bomb on 45,000 of its own soldiers. Despite the Cold War climate and the widespread use of nuclear physics as a means of creating apocalyptic weapons, CERN’s founding convention alongside UNESCO, which member states adhere to today, states:

“The Organization shall provide for collaboration among European States in nuclear research of a pure scientific and fundamental character…The Organization shall have no concern with work for military requirements,”

The provisional Conseil Européen pour la Recherche Nucléaire (European Council for Nuclear Research) was dissolved and its legacy was carried by the labs built and operated under the convention it had laid and the name it bore: CERN. Several years later in 1959, the British director of the Proton Synchrotron division at CERN, John Adams, received a gift of vodka from Soviet scientist Vladimir Nikitin of the Dubna accelerator, just north of Moscow, and at the time the most powerful accelerator in the world. 

The vodka was to be opened in the event the Proton Synchrotron accelerator at CERN was successfully operated at an energy greater than Dubna’s maximum capacity: 10 GeV. It more than doubled the feat, reaching 24 GeV, and with the vodka dutifully polished off, the bottle was stuffed with a photo of the proton beam readout and sent back to Moscow.

John Adams, holding the empty vodka bottle in celebration of the Proton Synchroton’s successful start (Image: CERN-HI-5901881-1 CERN Document Server)

Soviet scientists contributed more than vodka to the international effort in particle physics. Nikitin would later go on to work alongside other soviet and US scientists in a joint effort at Fermilab in 1972. Over the next few decades, ten more member states would join CERN permanently, including Israel, its first non-European member. On top of this, researchers at CERN now join from four associate member nations, four observer states (India, Japan, USA and Russia) and holds a score of cooperation agreements with other non-member states.

While certainly the largest collaboration of this kind, CERN is certainly no longer unique in being a collaborative effort in particle physics. Quantum Diaries is host to the blogs of many experiments all of whom comprise of a highly diverse and internationally sourced research cohort. The synchrotron lab for the Middle East, SESAME, expected to begin operation next year, will involve both the Palestinian and Israeli authorities with hopes it “will foster dialogue and better understanding between scientists of all ages with diverse cultural, political and religious backgrounds,”. It was co-ordinated in part, by CERN.

I have avoided speaking personally so far, but one needs to address the elephant in the room. As a British scientist, I speak from a nation where the dust is only just settling on the decision to cut ties with the European Union, against the wishes of the vast majority of researchers. Although our membership to CERN will remain secure, other projects and our relationship with european collaborators face uncertainty.

While I certainly won’t deign to give my view on the matter of a democratic vote, it is encouraging to take a look back at a fruitful history of unity between nations and celebrate Romania’s new Member State status as a sign that that particle physics community is still, largely an integrated and international one. In the short year that I have been at University College London, I have not yet attended any international conferences, yet have had the pleasure to meet and learn from visiting researchers from all over the globe. As this year’s International Conference on High Energy Physics kicks off this week, (chock-full of 5-σ BSM discovery announcements, no doubt*), there is something comforting in knowing I will be sharing my excitement, frustration and surprise with like-minded graduate students from the world over.

Kind regards to Ashwin Chopra and Daniel Quill of University College London for their corrections and contributions, all mistakes are unreservedly my own.
*this is, obviously, playful satire, except for the case of an announcement in which case it is prophetic foresight.

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The Four-Leafed Clovers of Subatomic Particles: Tetraquarks

Monday, February 29th, 2016

Hadrons, the particles made of quarks, are almost unanimously produced in the two or three quark varieties in particle colliders. However, in the last decade or so, a new frontier has opened up in subatomic physics. Four-quark particles have begun to be observed, the most recent being announced last Thursday by a collaboration at Fermilab. These rare, fleetingly lived particles have the potential to shed some light on the Strong nuclear force and how it shapes our world.

The discovery of a new subatomic particle was announced last Thursday by the DØ (DZero) collaboration at Fermilab in Chicago. DØ researchers analysed data from the Tevatron, a proton-antiproton collider based at Fermilab. The new found particle sports the catchy name “X(5568)” (It’s labelled by the observed mass of 5,568 Megaelectron-volts or MeV. That’s about six times heavier than a proton.) X(5568) is a form of “tetraquark”, a rarer variety of the particles known as hadrons. Tetraquarks consist of two quarks and two antiquarks (rather than the usual three quarks or quark-antiquark pairs that make up hadrons particle physicists are familiar with). While similar tetraquark particles have been observed before, the new addition breaks the mould by consisting of four quarks of totally different flavours: bottom, strange, up and down.

[Regular readers and those familiar with the theory of QCD may wish to skip to the section marked ——]

a) An example of a quark-antiquark pair, known as Mesons. b) An example of a three-quark particle, known as Baryons. c) An example of a tetraquark (four quarks) Source: APS/Alan Stonebraker, via Physics Viewpoint, DOI: 10.1103/Physics.6.69

The particle’s decay is best explained Strong force, aptly named since it’s the strongest known force in the universe[1], which also acts to hold quarks together in more stable configurations such as inside the proton. The Strong force is described by a theory known as Quantum Chromodynamics (QCD for short), a crucial part of the Standard Model of particle physics. The properties of X(5568) will provide precision tests of the Standard Model, as well as improving our understanding of the nature of Confinement. This is a dimly understood process by which quarks are bound up together to form the particles (such as protons) that make up most of the visible matter in the universe.

Quarks are defined by the strong force, being the only particles known to physics that interact via QCD. They were originally conceived of in 1964 by two of the early pioneers of particle physics Murray Gell-Mann and George Zweig, who posited the idea of “quarks” to explain the properties of a plethora of particles that were discovered in the mid-twentieth century. After a series of experiments in the late ‘60s and ‘70s, the evidence in favour of the quark hypothesis grew much stronger[2] and it was accepted that many of the particles that interacted and decayed very quickly (due to the magnitude of the strong force) in detectors were in fact made up of these quarks, which are now known to come in six different varieties known as “flavours”. A more precise model of the strong force, which came to be known as QCD, was also verified in such experiments.

QCD is a very difficult theory to draw predictions from because unlike electromagnetism (the force responsible for holding atoms together and transmitting light between objects), the “force carriers” of QCD known as gluons are self-interacting. Whereas light, or photons, simply pass through one another, gluons pull on one another and quarks in complex ways that give rise to the phenomenon of confinement: quarks are never observed in isolation, only as part of a group of other quarks/antiquarks. These groups of quarks and anti-quarks are what we call Hadrons (hence the name Large Hadron Collider). This self interaction arises from the fact that, unlike light which simply couples to positive or negative charges, QCD has a more complicated structure based on three charges labelled as Red, Green and Blue (which confusingly, have nothing to do with real colours, but are instead based on a mathematical symmetry known as SU(3)).

The hadrons discovered in the twentieth century tended to come in pairs of three quarks or quark-antiquark pairs. Although we now know there is nothing in the theory of QCD that suggests you can’t have particles consisting of four, or even five quarks/antiquarks, such particles were never observed, and in fact even some of the finest minds in theoretical physics (Edward Witten and Sidney Coleman) once thought that QCD would not permit such particles to exist. Like clovers, however, although the fourfold or even fivefold variety would be much rarer to come by it turns out such states did, in fact, exist and could be observed.

——

 

A visualisation of the production and decay of X(5568) to mesons in the Tevatron collider. Source: Fermilab http://news.fnal.gov/

The first hints of the existence of tetraquarks were at the Belle experiment, Japan in 2003, with the observation of a state called X(3872) (again, labelled by its mass of 3872 MeV). One of the most plausible explanations for this anomalous resonance[3] was a tetraquark model, which in 2013, an analysis by the LHCb experiment at CERN found to be a compatible explanation of the same resonance found in their detector. The same year, Belle and the BESIII experiment in China both found a resonance with the same characteristics, labelled Zc(3900), which is now believed to be the first independently, experimentally observed tetraquark. The most recent evidence for the existence of tetraquarks, prior to last Thursday’s announcement, was found by the LHCb experiment in 2014, the Z(4430). This verified an earlier result from Belle in 2007, with an astonishingly high statistical significance of 13.9σ (for comparison, one typically claims a discovery with a significance of 5σ). LHCb would also go on, unexpectedly, to find a pentaquark (four quarks and an antiquark) state in 2015, which could provide a greater understanding of QCD and even a window into the study of neutron stars.

Z(4430) was discovered from the analysis of its decay into mesons (hadrons consisting of quark-antiquark pairs), specifically the ψ’ and π mesons from the decay B0 → K + ψ’  π. In the analysis of the B0 decay, it was found that the Z(4430) was needed as an intermediate particle state to explain the resonant behaviour of the ψ’ and π. The LHCb detector, whose asymmetric design and high resolution makes it particularly well suited for the job, reconstructs these mesons and looks at their kinematic properties to determine the shape and properties of the resonance, which were found to be consistent with a tetraquark model. The recent discovery of X(5568) by the DØ collaboration involved a similar reconstruction from Bs and π mesons, which was used to infer its quark flavour structure (b, s, u, d, though which two are the particles and which two are the antiparticles remains to be determined).

X(5568) is found to have a large width (22 MeV) in the distribution of its decays, implying that it decays very quickly, best explained by QCD. Since quarks cannot change flavours in QCD interactions (while they can do so in weak nuclear interactions), this is what allowed DØ to determine its quark content. The other properties of this anomalous particle, such as its mass and its lack of spin (i.e. S = 0) are measured from the kinematics of the mesons it produces, and can help increase our understanding of how QCD combines the quarks in such an unfamiliar arrangement.

The two models for tetraquarks: Left, a single bound state of four quarks. Right, a pair of mesons bound to one another in orbit, resembing a four quark state. Source: Fermilab http://news.fnal.gov (Particle Physicists have a strange relationship with Comic Sans)

One of the long-standing controversies surrounding tetraquark states is whether the states are truly a joint four particle state or in fact a sort of molecule of two strongly bound mesons, which although they form a bound state of four particles in total, is actually analogous to two separate atoms in a molecule rather than a single, heavy atom. The analysis from DØ, based on X(5568)’s mass seems to imply that it’s the former, a single particle of four quarks tightly bound in an exotic hadron, though the jury is still out on the matter.

DØ’s discovery is based on an analysis of the historic data collected from the Tevatron from the 28 years it was operating, since the collider itself ceased operation 2011. Despite LHCb having found tetraquark candidates in the past and being suited to finding such a particle again, it has not yet independently verified the existence of X(5568). LHCb will now review their own data as well as future data that will recommence being collected later this year, to see if they too observe this unprecedented result and hopefully improve our understanding of its properties and whether they are consistent with the Standard Model. This is definitely a result to look out for later this year and should shed some light on one of the fundamental forces of nature and how it acts to create the particles, such as protons, that make up the world around us.

[1] That is, the dimensionless coupling of the force carrier particle interactions is greater than electromagnetism and the weak nuclear force, both of which in turn are stronger than gravity (consider how a tiny magnet can lift a paper clip against the gravity of the entire Earth). Many theories of Beyond the Standard Model physics predict new forces, and it may turn out that all the forces are unified into a single entity at high energies.

[2] For an excellent summary of the history of quarks and some of the motivations behind the quark model, check out this fantastic documentary featuring none other than the Nobel Prize wining physicists, Richard Feynman and Murray Gell-Mann themselves.

[3] Particles are discovered by the bumps or resonances they leave in the statistical distributions of particle decays/scattering events. See for example, one of the excesses of events that led to the discovery of the Higgs Boson.

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Spun out of proportion: The Proton Spin Crisis

Monday, February 1st, 2016

In the late 1980s, as particle colliders probed deeper into the building blocks of nature, there were hints of a strange and paradoxical behaviour in the heart of atoms. Fundamental particles have a curious quantum mechanical property known as “spin”, which the electron carries in magnitude ½. While the description of electron’s spin is fairly simple, protons are made up of many particles whose “spins” can add together in complicated ways and yet remarkably, its total spin turns out to be the same as the electron: ½. This led to one of the great mysteries of modern physics: how do all the particles inside the proton conspire together to give it a ½ spin? And what might this mean for our understanding of hadrons, the particles that make up most of the visible universe?

[This article is largely intended for a lay-audience and contains an introduction to foundational ideas such as spin. If you’ve had a basic introduction to Quantum Mechanics before, you may wish to skip to section marked —— ]

We’ve known about the proton’s existence for nearly a hundred years, so you’d be forgiven for thinking that we knew all there was to know about it. For many of us, our last exposure to the word “proton” was in high school chemistry, where they were described as a little sphere of positive charge that clumps with neutrons to make atomic nuclei, around which negatively charged electrons orbit to create all the atoms, which make up Life, the Universe and Everything1.

2000px-Proton.svg

The simple, three-quark model of a proton (each coloured circle is a type of “quark”).

Like many ideas in science, this is a simplified model that serves as a good introduction to a topic, but skips over the gory details and the bizarre, underlying reality of nature. In this article, we’ll focus on one particular aspect, the quantum mechanical “spin” of the proton. The quest to measure its origin has sparked discovery, controversy and speculation that has lasted 30 years, the answer to which is currently being sought at a unique particle collider in New York.

The first thing to note is that protons, unlike electrons2, are composite particles, made up from lots of other particles. The usual description is that the proton is made up of three smaller “quarks” which, as far as we know, can’t be broken down any further. This picture works remarkably well at low energies but it turns out at very high energies, like those being reached at the at the LHC, this description turns out to be inadequate. At that point, we have to get into the nitty-gritty and consider things like quark-antiquark pairs that live inside the proton interacting dynamically with other quarks without changing its overall charge. Furthermore, there are particles called gluons that are exchanged between quarks, making them “stick” together in the proton and playing a crucial role in providing an accurate description for particle physics experiments.

So on closer inspection, our little sphere of positive charge turns out to be a buzzing hive of activity, with quarks and gluons all shuffling about, conspiring to create what we call the proton. It is by inferring the nature of these particles within the proton that a successful model of the strong nuclear force, known as Quantum Chromodynamics (QCD), was developed. The gluons were predicted and verfied to be the carriers of this force between quarks. More on them later.

Proton structure

A more detailed model of the proton. The golden chains between the quarks (the coloured spheres) are representations of gluons, transferred between them. Quark anti-quark pairs are also visible with arrows representing spins.

That’s the proton, but what exactly is spin? It’s often compared to angular momentum, like the objects in our everyday experience might have. Everyone who’s ever messed around on an office chair knows that once you get spun around in one, it often takes you a bit of effort to stop because the angular momentum you’ve built up keeps you going. If you did this a lot, you might have noticed that if you started spinning with your legs/arms outstretched and brought them inwards while you were spinning, you’d begin to spin faster! This is because angular momentum (L) is proportional to the radial (r) distribution of matter (i.e. how far out things are from the axis of rotation) multiplied by the speed of rotation3 (v). To put it mathematically L = m × v × r where m is just your constant mass. Since L is constant, as you decrease r (by bringing your arms/legs inwards), v (the speed at which you’re spinning) increases to compensate. All fairly simple stuff.

So clearly, for something to have angular momentum it needs to be distributed radially. Surely r has to be greater than 0 for L to be greater than 0. This is true, but it turns out that’s not all there is to the story. A full description of angular momentum at the quantum (atomic) level is given by something we denote as “J”. I’ll skip the details, but it turns out J = L + S, where L is orbital angular momentum, in a fashion similar to what we’ve discussed, and S? S is a slightly different beast.

Both L and S can only take on discrete values at the microscopic level, that is, they have quantised values. But whereas a point-like particle cannot have L > 0 in its rest frame (since if it isn’t moving around and v = 0, then L = 0), S will have a non-zero value even when the particle isn’t moving. S is what we call Spin. For the electron and quarks, it takes on the value of ½ in natural units.

Spin has a lot of very strange properties. You can think of it like a little arrow pointing in a direction in space but it’s not something we can truly visualise. One is tempted to think of the electron like the Earth, a sphere spinning about some kind of axis, but the electron is not a sphere, it’s a point-like particle with no “structure” in space. While an electron can have many different values of L depending on its energy (and atomic structure depends on these values), it only has one intrinsic magnitude of spin: ½. However, since spin can be thought of as an arrow, we have some flexibility. Loosely speaking, spin can point in many different directions but we’ll consider it as pointing “up” (+½) or “down” (- ½). If we try to measure it along a particular axis, we’re bound to find it in one of these states relative to our direction of measurement.

Spin250

Focus on one of the red faces. When the cube rotates every 360 degrees, the red ribbon appears to go above and below the cube alternatively! Because the cube is coupled to its environment, it takes 720 degrees to return it to it’s original orientation.


One of the peculiar things about spin-½ is that it causes the wave-function of the electron to exhibit some mind bending properties. For example, you’d think rotating any object by 360 degrees would put it back into exactly the same state as it was, but it turns out that doesn’t hold true for electrons. For electrons, rotating them by 360 degrees introduces a negative sign into their wave-function! You have to spin it another 360 degrees to get it back into the same state! There are ways to visualise systems with similar behaviour (see right) but that’s just a sort of “metaphor” for what really happens to the electron. This links into the famous conclusion of Pauli’s that no two identical particles with spin-½ (or any other half-integer spin) can share the same quantum mechanical state.

——

Spin is an important property of matter that only really manifests on the quantum scale, and while we can’t visualise it, it ends up being important for the structure of atoms and how all solid objects obtain the properties they do. The other important property it has is that the spin of a free particle likes to align with magnetic fields4 (and the bigger the spin, the greater the magnetic coupling to the field). By using this property, it was discovered that the proton also had angular momentum J = ½. Since the proton is a stable particle, it was modelled to be in a low energy state with L = 0 and hence J = S = ½ (that is to say, the orbital angular momentum is assumed to be zero and hence we may simply call J, the “spin”). The fact the proton has spin and that spin aligns with magnetic fields, is a crucial element to what makes MRI machines work.

Once we got a firm handle on quarks in the late 1960s, the spin structure of the proton was thought to be fairly simple. The proton has spin-½. Quarks, from scattering experiments and symmetry considerations, were also inferred to have spin-½. Therefore, if the three quarks that make up the proton were in an “up-down-up” configuration, the spin of the proton naturally comes out as ½ – ½ + ½ = ½. Not only does this add up to the measured spin, but it also gives a pleasant symmetry to the quantum description of the proton, consistent with the Pauli exclusion principle (it doesn’t matter which of the three quarks is the “down” quark). But hang on, didn’t I say that the three-quarks story was incomplete? At high energies, there should be a lot more quark-antiquark pairs (sea quarks) involved, messing everything up! Even so, theorists predicted that these quark-antiquark pairs would tend not to be polarised, that is, have a preferred direction, and hence would not contribute to the total spin of the proton.

If you can get the entirety of the proton spinning in a particular direction (i.e. polarising it), it turns out the scattering of an electron against its constituent quarks should be sensitive to their spin! Thus, by scattering electrons at high energy, one could check the predictions of theorists about how the quarks’ spin contributes to the proton.

In a series of perfectly conducted experiments, the theory was found to be absolutely spot on with no discrepancy whatsoever. Several Nobel prizes were handed out and the entire incident was considered resolved, now just a footnote in history. OK, not really.

In truth, the total opposite happened. Although the experiments had a reasonable amount of uncertainty due to the inherent difficulty of polarising protons, a landmark paper by the European Muon Collaboration found results consistent with the quarks contributing absolutely no overall spin to the proton whatsoever! The measurements could be interpreted with the overall spin from the quarks being zero5. This was a complete shock to most physicists who were expecting verification from what was supposed to be a fairly straightforward measurement. Credit where it is due, there were theorists who had predicted that the assumption about orbital angular momentum (L = 0) had been rather ad-hoc and that L > 0 could account for some of the missing spin. Scarcely anyone would have expected, however, that the quarks would carry so little of the spin. Although the nuclear strong force, which governs how quarks and gluons combine to form the proton, has been tested to remarkable accuracy, the nature of its self-interaction makes it incredibly difficult to draw predictions from.

The feynman diagram for Deep Inelastic Scattering (electron line at the top, proton on the bottom). This type of scattering is sensitive to quark spin.

The Feynman diagram for Deep Inelastic Scattering (electron line at the top, proton on the bottom, with a photon exchanged between them). This type of scattering is sensitive to quark spin.

Future experiments (led by father and son rivals, Vernon and Emlyn Hughes6 of CERN and SLAC respectively) managed to bring this to a marginally less shocking proposal. The greater accuracy of the measurements from these collaborations had found that the total spin contributions from the quarks was actually closer to ~30%. An important discovery was that the sea quarks, thought not to be important, were actually found to have measurable polarisation. Although it cleared up some of the discrepancy, it still left 60-70% of spin unaccounted for. Today, following much more experimental activity in Deep Inelastic Scattering and precision low-energy elastic scattering, the situation has not changed in terms of the raw numbers. The best estimates still peg the quarks’ spin as constituting only about 30% of the total.

Remarkably, there are theoretical proposals to resolve the problem that were hinted at long before experiments were even conducted. As mentioned previously, although currently impossible to test experimentally, the quarks may carry orbital angular momentum (L) that could compensate for some of the missing spin. Furthermore, we have failed to mention the contribution of gluons to the proton spin. Gluons are spin-1 particles, and were thought to arrange themselves such that their total contribution to the proton spin was nearly non-existent.

BNL AERIALS

The Brookhaven National Laboratory where RHIC is based (seen as the circle, top right).


The Relativistic Heavy Ion Collider (RHIC) in New York is currently the only spin-polarised proton collider in the world. This gives it a unique sensitivity to the spin structure of the proton. In 2014, an analysis of the data collected at RHIC indicated that the gluons (whose spin contribution can be inferred from polarised proton-proton collisions) could potentially account for up to 30 of the missing 70% of proton spin! About the same as the quarks. This would bring the “missing” amount down to about 40%, which could be accounted for by the unmeasurable orbital angular momentum of both quarks and gluons.

As 2016 kicks into gear, RHIC will be collecting data at a much faster rate than ever after a recent technical upgrade that should double it’s luminosity (loosely speaking, the rate at which proton collisions occur). With the increased statistics, we should be able to get an even greater handle on the exact origin of proton spin. 


The astute reader, provided they have not already wandered off, dizzy from all this talk of spinning protons, may be tempted to ask “Why on earth does it matter where the total spin comes from? Isn’t this just abstract accountancy?” This is a fair question and I think the answer is a good one. Protons, like all other hadrons (similar, composite particles made of quarks and gluons) are not very well understood at all. A peculiar feature of QCD called confinement binds individual quarks together so that they are never observed in isolation, only bound up in particles such as the proton. Understanding the spin structure of the proton can inform our theoretical models for understanding this phenomenon.

This has important implications, one being that 98% of the mass of all visible matter does not come from the Higgs Boson. It comes from the binding energy of protons! And the exact nature of confinement and precise properties of QCD have implications for the cosmology of the early universe. Finally, scattering experiments with protons have already revealed so much to fundamental physics, such as the comprehension of one of the fundamental forces of nature. As one of our most reliable probes of nature, currently in use at the LHC, understanding them better will almost certainly aid our attempts to unearth future discoveries.

Kind regards to Sebastian Bending (UCL) for several suggestions (all mistakes are unreservedly my own).

 

[1] …excluding dark matter and dark energy which constitute the dark ~95% of the universe.

[2] To the best of our knowledge.

[3] Strictly speaking the component of velocity perpendicular to the radial direction.

[4] Sometimes, spins in a medium like water like to align against magnetic fields, causing an opposite magnetic moment (known as diamagnetism). Since frogs are mostly water, this effect can and has been used to levitate frogs.

[5] A lot of the information here has been summarised from this excellent article by Robert Jaffe, whose collaboration with John Ellis on the Ellis-Jaffe rule led to many of the predictions discussed here.

[6] Emlyn was actually the spokesperson for SLAC, though he is listed as one of the primary authors on the SLAC papers regarding the spin structure of the proton.

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Dark Matter: A New Hope

Monday, December 7th, 2015

[Apologies for the title, couldn’t resist the temptation to work in a bit of Star Wars hype]

To call the direct detection of dark matter “difficult” is a monumental understatement. To date, we have had no definite, direct detection on Earth of this elusive particle that we suspect should be all around us. This seems somewhat of a paradox when our best astronomical observations indicate that there’s about five times more dark matter in the universe than the ordinary, visible matter that appears to make up the world we see. So what’s the catch? Why is it so tricky to find?

An enhanced image of the “Bullet Cluster”: two colliding galaxies are observed with ordinary “baryonic” matter (coloured red) interacting as expected and the dark matter from each galaxy, inferred from gravitational lensing (coloured blue), passing straight through one another. Source: NASA Astronomy Picture of the Day 24/08/2006

The difficulty lies in the fact that dark matter does not interact with light (that is, electromagnetically) or noticeably with atoms as we know them (that is, with the strong force, which holds together atomic nuclei). In fact, the only reason we know it exists is because of how it interacts gravitationally. We see galaxies rotate much faster than they would without the presence of some unseen “dark matter”, amongst other things. Unfortunately, none of the particles we know from the Standard Model of particle physics are suitable candidates for explaining dark matter of this sort. There are, however, several attempts in the works to try and detect it via weak nuclear interactions on Earth and pin down its nature, such as the recently approved LUX-ZEPLIN experiment, which should be built and collecting data by 2020.

Direct detection, however, isn’t the only possible way physicists can get a handle on dark matter. In February 2014, an X-Ray signal at 3.5 keV was detected by the XMM-Newton, an X-ray spectroscopy project by the European Space Agency, in orbit around Earth. Ever since, there’s been buzz amongst particle cosmologists that the signal may be from some kind of dark matter annihilation process. One of the strongest candidates to explain the signal has been sterile neutrino, a hypothetical cousin of the Standard Model neutrino. Neutrinos are ghostly particles that also interact incredibly rarely with ordinary matter* but, thanks to the remarkable work of experimentalists, were detected in the late 1950s. Their exact nature was later probed by two famous experiments, SNO and Super-Kamiokande, that demonstrated that neutrinos do in fact have mass, by observing a phenomenon known as Neutrino Oscillations. As reported on this blog in October, the respective heads of each collaboration were awarded the 2015 Nobel Prize in Physics for their efforts in this field.

“Handedness” refers to how a particle spins about the axis it travels along. Standard Model neutrinos (first observed in 1956) are all observed as left handed. Sterile neutrinos, a hypothetical dark matter candidate, would be right-handed, causing them to spin the opposite way along their axes. Image source: ysfine.com

The hope amongst some physicists is that as well as the neutrinos that have been studied in detectors for the last half a century, there exists a sort of heavier “mirror image” to these particles that could act as a suitable dark matter candidate. Neutrinos are only found to “spin” in a certain way relative to the axis of their propagation, while the hypothesised sterile neutrinos would spin the opposite way round (in more technical terms, they have opposite chirality). This difference might seem trivial, but in the mathematical structure underpinning the Standard Model, it would fundamentally change how often these new particles interact with known particles. Although predicted to react incredibly rarely with ordinary matter, there are potentially processes that would allow these sterile neutrinos to emit an X-Ray signal, with half the mass-energy of the original particle. Due to the sheer number of them found in dense places such as the centres of galaxies, where XMM-Newton was collecting data from, in principle such a signal would be measurable from regions with a high density of sterile neutrinos.

This all seems well and good, but how well does the evidence measure up? Since the announcement of the signal, the literature has gone back and forth on the issue, with the viability of sterile neutrinos as a dark matter candidate being brought into question. It is thought that the gravitational presence of dark matter played a crucial role in the formation of galaxies in the early universe, and the best description we have relies on dark matter being “cold”, i.e. with a velocity dispersion such that the particles don’t whizz around at speeds too close to the speed of light, at which point their kinematic properties are difficult to reconcile with cosmological models. However, neutrinos are notorious for having masses so small they have yet to be directly measured and to explain the signal at 3.5 keV, the relevant sterile neutrino would have to have a relatively small mass of ~7 keV/c2, about 15,000 times lighter than the usual prediction for dark matter at ~100 GeV/c2. This means that under the energy predicted by cosmological models for dark matter production, our sterile neutrinos would have a sort of “luke-warm” characteristic, in which they move around at speeds comparable to but not approaching the speed of light.

A further setback has been that the nature of the signal has been called into question, since the resolution of the initial measurements from XMM-Newton (and accompanying X-ray satellite experiments such as Chandra) was not sharp enough to definitively determine the signal’s origin. XMM-Newton built up a profile of X-ray spectra by averaging across measurements from just 73 galaxy clusters, though it will take further measurements to fully rule out the possibility that the signal isn’t from the atomic spectra of potassium and sulpher ions found in hot cosmic plasmas.

But there remains hope.

A recent pre-print to the Monthly Notices of the Royal Astronomical Society (MNRAS) by several leading cosmologists has outlined the compatibility of a 7 keV/c2 sterile neutrino’s involvement with the development of galactic structure. To slow down the sterile neutrinos enough to bring them in line with cosmological observations, “lepton asymmetry” (a breaking of the symmetry between particles and antiparticles) has to be introduced in the model. While this may initially seem like extra theoretical baggage, since lepton asymmetry has yet to be observed, there are theoretical frameworks than can introduce such an asymmetry with the introduction of two much heavier sterile neutrinos at the GeV scale.

A Black Brant XII sounding rocket, similar to the type that could be used to carry microcalorimeters, capable of recording X-ray signals of the type XMM-Newton and Chandra have been observing in galactic nuclei. These rockets are used to conduct scientific experiments in sub-orbital flight, including attempts at dark matter detection. Source: NASA/Wallops

Under such a model, not only could our dark matter candidate be reconciled, but neutrino oscillations could also be explained. Finally, baryogenesis, the description of why there was slightly more matter than antimatter in the early universe, could also find an explanation in such a theory. This would resolve one of the largest puzzles in Physics; the Standard Model predicts nearly equivalent amounts of particles and antiparticles in the early universe which should have annihilated to leave nothing but radiation, rather than the rich and exciting universe we inhabit today. On the experimental side, there are a few proposed experiments to try and measure the X-ray signal more carefully to determine its shape and compare it with the prediction of such models, such as flying rockets around with calorimeters inside to try and pick up the signal by observing a broader section of the sky than XMM or Chandra did.

With the experts’ opinions divided and further research yet to be done, it would be facetious to end this article with any sort of comment on whether the signal can or will gather the support of the community and become verified as a full blown dark matter signal. At time of writing, a paper has been released claiming signal is better explained as an emission from the plasmas found in galactic nuclei. A further preprint to MNRAS, put on arXiv just days ago, claims the sterile neutrino hypothesis is incompatible with the signal but that axions (a dark matter model that supposes a totally different type of particle outside of the Standard Model) remain as a candidate to explain the signal. Perhaps sterile neutrinos, are not the particles we’re looking for.

This kind of endeavour is just one of the hundreds of ways particle physicists and our colleagues in Astrophysics are looking to find evidence of new, fundamental physics. The appeal for me, as someone whose work will probably only have relevance to huge, Earth-bound experiments like the Large Hadron Collider, is the crossover between modelling the birth of colossal objects like galaxies and theories of subatomic particle production, using comparison between the two for consistency. Regardless of whether future rocket-based experiments can gather enough data to fully validate the signal in terms of theories produced by physicists here on Earth, it is a perfect example of breadth of activity physicists are engaged in, attempting to answer the big questions such as the nature of dark matter, through our research.

Kind regards to Piotr Oleśkiewicz (Durham University) for bringing this topic to my attention and for his insights on cosmology, and to Luke Batten (University College London) for a few corrections.

*The oft-quoted fact about neutrinos is that 65 billion solar neutrinos pass through just one square centimetre of area on earth every single second. The vast majority of these neutrinos will whizz straight through you without ever having noticed you were there, but by chance, in your entire lifetime, it’s likely that at least one or two will have the courtesy to notice you and bump off one of your atoms. The other interesting fact is that due to the decay of potassium in your bones, you actually emit about three hundred neutrinos a second.

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