Monthly Archives: June 2008

Supersymmetry

The success of the unification of the weak- and electromagnetic interactions (see SM post below) soon led to attempts to extend the program to include the strong interaction, i.e. a search for one unified scheme that could describe all three non-gravitational forces (known as Grand Unified Theory) .

However, the GUT program soon ran into serious trouble, with a clutch of ‘no-go’ theorems from mathematicians such as McGlynn, O’Raifeartaigh, Coleman and Mandula showing that such unification could not be achieved using similar gauge methods to that of the electro-weak program. In response, a dramatic new type of symmetry was proposed in the 1970s.

The theory of supersymmetry was a new type of gauge symmetry, and is called ‘super’ in the sense of an ultimate gauge symmetry. Supersymmety (SUSY) supposes a deep connection between two classes of particles that had previously thought to be unrelated – the particles that make up matter (quarks and leptons) and the particles that act as ‘force carriers’ (photons, W and Z bosons ). A very significant difference between the two sets is their spin – quarks and leptons have 1/2 integer spin (called fermions) and obey Fermi-Dirac statistics in consequence. They follow the Pauli Exclusion Principle which states that no two fermions with identical quantum numbers can occupy the same state. ‘Force-carrying’ particles like the photon have integer spin (called bosons ) obey no such rule, and basically behave completely differently.

In essence, supersymmetry posits that every fermion has a corresponding boson sibling and vice versa – in other words, for every quark and lepton there exists a supersymmetric sibling (squarks and sleptons), and every boson also has a supersymmetric partner.

Unfortunately, no-one has ever seen such particles, either in cosmic rays or in particle acceleraor experiments. Hence, if SUSY exists, it must be a broken symmetry, i.e. the supersymmetric partners must have different decay schemes to ‘normal’ particles, and must be much heavier than their ‘normal’ cousins (otherwise we would have seen them). The only way to see if SUSY particles ever existed is to try re-creating them at extremely high energies in particle accelerators (much as we create anti-particles). This is one of the things the new collider at CERN was built to look for.

That said, theoreticians claim that there are indirect hints that SUSY , or something like it, might be right. The first is the convergence of the three non-gravitational forces. While these forces appear completely different at low energy, they have a different energy dependence, and may in fact converge at high enough energies. However, detailed calculations show that they converge to a point only if supersymmetry is allowed for. Unfortunately, this is a purely theoretical conjecture – you can see from the diagram below that the convergence is expected to occur at energies way beyond the reach of current accelerators.

GUT convergence including supersymmetry

The second is a hint from cosmology – we are pretty sure that well over 2/3 of the matter of the universe is ‘dark matter’, i.e. only seen by its gravitational effect (see post below). Such matter must be massive and yet weakly interacting (WIMPS) – an idea that fits supersymmetric particles very nicely. In fact, the favoured candidate for dark matter is the lightest SUSY particle, the neutralino (see post below).

Hence the search for SUSY particles at CERN, and the search for Dark Matter in cosmology are experiments that complement each other. Progress on either front will probaby have implications for the other, a fantastic convergence of particle physics and cosmology.

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The Standard Model

The post below made reference to the theory of supersymmetry and this weblog is long overdue a post on the subject. However, as supersymmetry is proposed as an extension of the Standard Model (SM) of particle physics, we’d better have a few words about the SM first…

As we said before, one of the big discoveries of 20th century physics is that there exist only four independent forces or interactions. These are gravity, electromagnetism (the unification of electricity and magnetism achieved by Maxwell in the 19th century), the strong nuclear force (that holds the protons and neutrons together in the nucleus), and the weak nuclear force (responsible for nuclear decay and radioactivity).

Physicists have long suspected that the four fundamental forces are not truly independent, but deeply connected. The idea is that at the tremendous energies of the Big Bang, a single superforce existed, which gradually split off into the four seperate entities we see today as the universe cooled. This idea received a great boost in the 1970s, when Salaam, Weinberg and Glashow established a strong theoretical connection between the electromagnetic and the weak nuclear interactions, using the methods of gauge symmetry. The theory predicted the existence of new particles (W and Z bosons), which were subsequently discovered in high-energy experiments at CERN in the 1980s…ever since we talk about the electro-weak interaction as a single entity.

Shortly before this, the first comprehensive theory of the strong nuclear force had also emerged – the key idea being Gellman’s prediction that the nuclear particles (protons and neutrons) are in fact made up of quarks, and the strong nuclear force is really an interquark force. This was verified by scattering experiments at Stanford in 1979, and the theory of the strong interaction is now known as quantum chromodynamics

Putting the two theories together gave rise to the Standard Model – a model that has been fantastically accurate at predicting the masses and properties of all particles discovered so far. However, the model contains several shortcomings

– there is no real unification between the electro-weak and strong interactions, they are treated in parallel

gravity doesn’t appear at all

These shortcomings led to new theories that attempted to unify the strong nuclear force with the electro-weak interaction (known as Grand Unified Theories), and even more ambitious attempts to unify all three with gravity (Theories of Everything). To accomplish either of these, some new mathematical approaches would be needed….see next instalment…

Update

I forgot to mention another shortcoming of the Standard Model – namely that one particle, necessary to the model, has never been observed (thanks, tankers!). The Higgs boson plays a central role in the SM as the Higgs field gives the mechanism for other particles to acquire the masses we observe. Unfortunately, no evidence of the Higgs particle has been seen in accelerator experiments so far. Most theoreticians are convinced this is simply because we need higher energies than currently available to create it (i.e. it has a large mass), and expect to see evidence of Higgs bosons in the next round of accelerator experiments due to begin at the new accelerator in CERN next year – the Large Hadron Collider.

The alternative is that we’ll see something quite different, which would be even more interesting!

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Dark Matter at Trinity College

I learned at lunchtime on Monday that Professor Tim Sumner of Imperial College was booked to give a talk in Trinity College that very evening on the search for Dark Matter (DM). Prof Sumner is one of the project directors of the well-known UK Zeplin DM experiment, so I jumped in my car and drove up to Dublin. It’s not every day you get to hear a lecture from a player at that level….

It was certainly worth the drive, it was a cracking lecture. The seminar was organised by Astronomy Ireland, so there were quite a few non-professionals in the audience (I didn’t spot many staff from Trinity Maths or Physics, perhaps the talk wasn’t terribly well advertised). Of course, there’s something slightly ironic about an astronomical society hosting a seminar on Dark Matter as you’re not to likely to see DM through a telescope, but good for AI ! In the event, Tim gave a thorough overview of the whole area before describing current experiments to detect DM.

Recall that Dark Matter is thought to account over 2/3 of the matter of the universe (not to be confused with dark energy). Although we can’t see it, we’re pretty sure it exists because of its gravitational effect on the matter that we can see. I said in a previous post that the phenomenon was first suggested by Fritz Zwicky, but according to Tim, the suggestion first came from a scientist whose name I didn’t catch (Oert?).

The seminar was divided in four parts –

I. Indirect evidence of DM from gravitation effects

II. Indirect evidence of DM from cosmological models

III. DM candidates

IV. Current DM experiments

In part I, Tim gave a comprehensive account of the gravitational evidence, explaining the discrepancy between the expected velocity of stars and galaxies to that measured, working from smaller scales to the largest e.g. local stellar dynamics, galaxy rotation, galaxy cluster dynamics, X-ray halos, gravitational lensing and cluster streaming. I was only aware of a few of these so this was very interesting.

Calculations for galaxy rotation (curve A) and experimental points (curve B)

There was also a brief discussion of the alternative explanation, that our laws of gravity (both Newtonian and Einsteinian) need to be modified (MOND) and why this idea has lost ground recently

Part II concerned the role of DM in analysis of the cosmic microwave backgound (CMB). Tim explained the challenge to relate the temperature perturbations seen in the CMB to galaxy formation, and how all current models rely heavily on the postulate of DM…he also explained how the postulate is necessary to provide enough gravity to explain the geomety of the universe as observed.

Part III concerned the various candidates for DM. Such particles are expected to be weakly interacting (otherwise we would see them) and probably massive – i.e. weakly interacting massive particles or WIMPs. Tim then explained that the most likely candidates are thought to be certain supersymmetric particles. (As we saw before, the theory of supersymmetry (SUSY) arises out of attempts to unify three of the fundamental forces – the theory postulates that every normal particle has a heavier supersymmetric partner). Anyway, it turns out the most likely candidiate for DM is the neutralino, the lightest SUSY particle which cannot decay further.

In part IV, Tim described current experiments. He gave a full description of the recent galactic bullet cluster phenomenon, and was very positive about their results. He also mentioned the DAMA-LIBRA experiment, but was a lot less positive about this. The problem seems to be that their technique is less, not more, sensitive than other experiments, none of which have detected similar results. He confirmed that many in the community are sceptical that the DAMA result is really DM-related at all. Tim then finished with a brief overview of his own group’s attempt to detect WIMPS by their nuclear interactions in underground detectors in a mine over 1km deep, the Zeplin III experiment. There is a very good overview of the Zeplin experiment here .

The photomultiplier tubes of the ZEPLIN III detector

In summary, this was a super overview of the search for Dark Matter. There is always something to learn in such seminars, and things I particularly liked were

1. The lecturer took the time for a thorough overview of the whole area

2. There was time for a description of the experiments of other groups

3. There was great emphasis on the ‘double-whammy”. For many years, many scientists have scoffed at the idea of SUSY particles, as none have so far been seen in our particle detectors. Others have scoffed at the idea of Dark Matter, seeing it as a fudge. If DM turns out to be made up of SUSY particles, that solves both conundrums beautifully – and confirms supersymmetry as the way forward in unified field theory. It would also represent another step in the fantastic convergence of particle physics and cosmology, two of the most fundamental areas of physics.

4. There were plenty of questions afterwards – always interesting. In my case, I asked Tim about mass constraints put on SUSY particles by recent experiments in particle physics (accelerators). In fact, one of his slides showed that the ZEPLIN results so far are in agreement with accelerator experiments, ie. suggest candidate particles lying well within the ‘mass window’ provided by accelerator studies…the key slide was basically an updated version of the slide shown below – the predicted red curve (labelled Zeplin III) is now a reality (note that the vertcial line at 60 GeV is the lower mass limit set by accelerator experiments).

The above is written from my own notes at the talk, I may have missed a few points. Astronomy Ireland will provide a webcast and a DVD of the talk on their website and there is a very good overview of the worldwide search for DM here

Update:

I just read on the Cosmic V ariance blog that the GLAST satellite has just successfully launched (see earlier post on GLAST). Among other things GLAST will look for DM, by looking for gamma-rays produced by DM annihilation…there is a very nice discussion of this on their blog. I meant to ask Prof Sumner about the prospect of success of DM detection by this method but I forgot..

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Summer plans

What are you doing for the summer? Like most academics, I’m asked this question regularly, by people envious of our holidays. I sometimes think they’re more interested in my holidays than I am myself.

But what will I do? I used to head back to my alma mater Trinity College as soon as term ended, doing experimental work in the magnetic resonance lab. These days, I find myself doing more and more writing about science, and less and less labwork. Truth is, I always liked writing papers more than getting the results…

This summer, I intend to make a start on a short book on particle physics, aimed at the layman – The Story of Atoms. I’ve noticed that while there are lots of good introductory books on cosmology, there are fewer such books on particle physics. Also, I’ve always had an interest in the area and l teach an introductory course in high-energy physics. Of course, particle physics probably doesn’t have quite the popular appeal of cosmology – but there’s enough convergence between the two fields to draw in plenty of readers. Plus, it’d be great to get a simple introductory book on particle physics out in time for expected dramatic results at CERN sometime next year. (I’m sure no-one else has thought of this – Ed).

Apparently one needs an outline, chapter headings and at least one full chapter to get a publisher interested. I think I’ll use the summer break to get the structure organised and bang out the first chapter, ready to send off to a few publishers by the time term starts up again.

‘Course I won’t spend the entire summer on it – all work and no play makes Albert a very dull boy. I intend to travel, and hole up somewhere where I can surf in the mornings and work in the afternoons (and socialize in the evenings). Anywhere really, so long as it’s outside Ireland, for God’s sake.

Garret Lisi, the surfer dude with the exceptionally simple theory of everything, has already been in touch with a list of suitable surf spots in California as long as your arm – thanks Garrett!

Mind you, I suspect what Garrett considers ‘suitable’ is probably life-threatening.

Tip – try not to land on the board when you wipeout

So that’s the summer plan.

1. Get started on a pop science book that will eventually make me rich and famous

2. Get back surfing

3. Meet someone nice. You’d be amazed how many academics are single, it’s frightening. All I ask is that a girl can surf and handle complex equations…

Good luck with all that – Ed

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Cold fusion

Incredibly, the cold fusion controversy is with us again. Physics World, normally a reputable source of news in physics, have a posting by Jon Cartwright on their weblog concerning claims that Japanese physicist Yoshiaki Arata of Osaka University may have demonstrated cold fusion.

To understand how startling – and controversial- such a claim is, you only have to call things by their proper names. ‘Cold fusion’ is media-speak for nuclear fusion at low energy, a process most physicists consider pretty much a contradiction in terms (it’s very difficult to achieve nuclear fusion even at extremely high temperatures and energies, with certain well-understood exceptions).

The dream of ‘cold fusion’ first hit the news in 1989, when chemists Fleischmann and Pons claimed to have observed a dramatic, unexplained heating effect in a chemical reaction, and attributed it to nuclear fusion processes ocurring at normal temperatures. The discovery made headlines around the world, because it offered the dream of a clean, cheap energy source on a small scale (nuclear fusion is a very different process from nuclear fission). However, the whole field was controversial from the very start.

Most physicists felt the jump from an unexplained heating effect to the assumption of nuclear fusion was highly speculative. Secondly, the effect was publicized (and funding received) long before the results were published in recognized journals, one of the first times this happened. Worst of all, when physics labs around the world rushed to reproduce the results, no discernible heating effect was found. The end result was a withdrawal of funding and a great career blow to the experimenters…and prompted a serious debate on the importance of peer review before going to the press!

Fusion in a beaker – the Fleischmann apparatus

It’s probably too early to say, but the current Japanese story bears many resemblances to the Fleischmann fiasco – a great deal of talk in the press (now web), a paucity of peer-reviewed results, and a great deal of copy written by non-physicists. In particular, I notice that most descriptions of the experiment focus once more on the benefits of ‘fusion energy’, (cheap, clean energy etc) with only a few lines concerning the skepticism of mainstream scientists.. (see this thread on the Richard Dawkins website for example)

There is also the question of biased opinion. For example, Cartwright’s article states ‘I also received a detailed account from Jed Rothwell, who is editor of the US site LENR (Low Energy Nuclear Reactions) and who has long thought that cold-fusion research shows promise’. Hmm. Not exactly an unbiased opinion, then. Indeed, a glance at the LENR website suggests that the above is not likely to represent the mainstream view…This is exactly the sort of press that caused such a problem the first time around…

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