Category Archives: Particle physics

Peter Watkins and Z bosons at Trinity College Dublin

last night, I attended a terrific lecture on recent developments at CERN’s Large Hadron Collider, hosted by Astronomy Ireland at Trinity College Dublin. The lecture was presented by Professor Peter Watkins, a former leader of the particle physics group at University of Birmingham and a member of the ATLAS collaboration at the LHC. Professor Watkins was a member of one of the experimental teams that discovered the Z boson at the LEP at CERN in 1983. He is also very well-known for his work in bringing particle physics to the public and is the author of ‘The story of the W and Z, one of my favourite books on particle physics.

I try to go to as many of these public lectures as I can, in order to see how others present physics to the public. In this case, the lecture was superb, very easy to understand yet at quite a high level. It was loosely divided into five sections;

– an introduction to the building blocks of matter

–  a description of what the LHC is looking for

– a description of experimental setup of the LHC and the ATLAS detector

– a description of the methods of searching for particles

– a discussion of recent discoveries at the LHC

The first section gave a brief introduction to the standard model of particle physics. However, rather than present the audience with a list of quarks and leptons, Peter described our view of ordinary matter in terms of up and down quarks, electrons and neutrinos. Only after this did he mention the higher generations, an approach that worked really well. On the next slide, he gave a description of the fundamental forces, explaining along the way how electricity and magnetism were unified into the unified framework of electromagnetism many years ago, and how the latter interaction was more recently unified with the weak force to form the electroweak interaction. There followed a very nice discussion of the force-carrying particles, and the subsequent search for the W and Z bosons. This section finished with an overview of the role of the Higgs field in determining the mass of the particles – about as succinct an introduction to particle physics as I’ve seen!

The second section of the talk described what the LHC will search for; from the Higgs boson to supersymmetric particles, from investigations of the slight asymmetry in matter and antimatter decay to candidates for dark matter. Professor Watkins was also careful to explain that the LHC may yield great surprises, from missing energy that might constitute evidence of hidden dimensions to possible hints of new forces.

An experimental overview of the LHC and the ATLAS detector was presented in the third part of the talk. The technical challenges of LHC operation were clearly laid out, from the need for ultra-low temperatures to the problem of establishing an ultra-high vacuum on this scale, from issues with beam focusing to problems with superconducting magnets. This section included a great overview of the ATLAS detector, with each component described carefully.

The ATLAS detector, not the LHC as many newspapers seem to think

The fourth section of the talk was most unusual, where Peter gave a clear description of how the existence of elusive particles is inferred from those beautiful patterns on computer monitors.  Starting with E2 = p2c2 + m2c4, he gave a few examples where measurements of momentum and energy in the detector lead to an estimate of the mass of the parent particle. This section included a great description of the search for the Higgs via the ZZ and photon-photon decay channels.

In the last part of the talk, the speaker gave a clear description of recent work at the LHC. Touching briefly on the initial accident of 2008, he explained how ATLAS and CMS have gradually been closing the window on mass ranges for the Higgs (including earlier data from LEP). He had a nice surprise for many in the audience when he mentioned that ATLAS has already discovered its first new particle – a new state of the chi-b particle . The lecture finished with a discussion of the famous ‘bump’ in the ATLAS data at 126 GeV announced two weeks ago, and the possible significance of the discovery.

Hints of a higgs in the ATLAS measurements ? (Dec 2011)

I found this a superb lecture overall. The speaker outlined difficult concepts extremely clearly and gave a great description of how concepts emerge, rather than presenting ‘facts’ as fixed dogma. The audience certainly thought so too and there were dozens of questions afterwards. As always with Astronomy Ireland lectures, the discussion continued in the pub across the road. At one point, Peter explained that part of the current excitement is due to where the bump is; if the 126 GeV result stands, this relatively low mass for the Higgs may be compatible with extensions to the standard model such as supersymmetry….a good time to be a particle physicist!

Update

Rumours are circulating that the CMS bump has not disappeared on further analysis, but is converging on the ATLAS result, exciting times

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Neutrinos and string theory at Trinity College Dublin

I gave a few more talks on the Gran Sasso neutrino experiments last week, in Waterford on Wednesday and in Trinity College Dublin on Saturday. I really enjoy giving these talks; it’s not often one gets an excuse to present the theory of relativity to the hapless public. Journalists talk about the ‘hook’ – well this is a hook from heaven. I even got a 20-minute interview on Ireland’s premier radio show Today with Pat Kenny . You can find the podcast and the slides I used for the lecture here.

There was a real buzz in the air at Saturday’s lecture, thanks to the latest results from OPERA. As you probably know, the team announced on Friday that the superluminal result has passed its first major test: a repeat experiment using a much shorter proton pulse. This time they used pulses only 3 nanoseconds long, separated by by gaps ten time larger. This is vastly shorter than before (10 microseconds) and obviates the statistical approach used for matching transmitted and received pulses used in the original experiment. Like most physicists, I am still pretty certain the result will eventually turn out to be an anomaly, but I certainly hope it survives for another few months! See here for the new OPERA paper.

The lecture was hosted by Astronomy Ireland,a very interested audience that always turn out in droves. The theatre was jammers, quite a few audience members had to stand throughout. As always, I particularly enjoyed the questions and answers afterwards. It’s also fun to be come home; as a postgraduate student, I spent many long years in the magnetic resonance lab next door!

David Moore of Astronomy Ireland presents me with something (?)

Afterwards, some of us all legged it over to another Trinity lecture theatre, to hear the annual statutory lecture of the School of Theoretical Physics of the Dublin Institute for Advanced Studies. This year, the speaker was well-known string theorist Cumrun Vafa from Harvard. Titled ‘Geometric Physics’, the lecture was an excellent introduction to string theory today.

String theorist Cumrun Vafa from Harvard

And after all that, there was a reception to celebrate the fact that Werner Nahm, the director of the DIAS School, was recently made a fellow of the Royal Society. What a weekend

Update

On Sunday, Werner gave a fascinating talk on ancient astronomy at the Dublin Institute of Advanced Studies. After the seminar, many of us remained in the fading light in that famous seminar room, discussing the neutrino result and other experiments at CERN. As so often, I was struck by the depth and detail of knowledge the theorists had of particle experiments. I also enjoyed the way the discussion wandered into German for a while, then seamlessly back to English – only at DIAS!

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Faster than the speed of light

So. A respected experimental group, doing respected work, the OPERA neutrino experiment at Grand Sasso in Italy, have reported a startling result; they have measured a velocity for neutrinos that is in excess of the speed of light (a fractional increase of about of 1 in 100,000). The result is getting a huge amount of publicity because it appears to be in conflict with Einstein’s theory of relativity. ‘Einstein wrong‘ always makes headlines. I’m certainly getting a lot of calls and emails on the subject, not least because I had an article on relativity in Thursday’s Irish Times (see here).

In the OPERA experiment, a beam of neutrinos travels underground from CERN travel to Gran Sasso in Italy

The OPERA paper has been posted on the ArXiv here. Most physicists (including the participants) are calling the result an ‘anomaly’ and expect to find a hidden error, for two reasons

1. Thousands and thousands of experiments on elementary particles suggest that the speed of light represents a natural speed limit for material bodies, no matter how much energy you whack them with

2. There are deep mathematical reasons for believing that the speed of light in vacuum represents an absolute limit, from arguments of symmetry to the principle of least action. Basically, all sorts of mathematics suggests that the speed of photons- massless particles –  is the highest speed achievable. In addition, the principle underlies a great deal of observed physics, far beyond the remit of relativity.

So what is going on?

Science is a skeptical activity and scientists are slow to throw out a successful theory at the first sign of trouble -especially a theory as successful and as central as special relativity. Most scientists adopt a ‘wait and see’ approach when an experiment like this is reported.

For example, we know a great deal more about relativity than we do about neutrinos. It is only a few years since it was discovered that neutrinos have mass, and the phenomenon of neutrino oscillation – the transformation of one type of neutrino to another – is still not well understood. So it is possible that this experiment is an artefact of some unknown neutrino process.

A more prosaic possibility is that there is a systematic error in the extremely precise time/distance measurements necessary for the experiment. For example, the time of flight of the neutrinos is measured using a sophisticated version of GPS – perhaps there is a hitherto undetected error lurking in this method that is affecting the measurement. A few years ago, it was discovered that the moon has an effect on the curvature of the LHC tunnel, as does the TGV arriving at Geneva – these effects only show up because of the unprecedented precision involved in the experiments.

Finally, it is always possible that this result may turn out to be a real effect. In this case, we could be looking at some exciting new physics; not a violation of relativity, but the first evidence of hidden dimensions. String theorists have long mooted the possibility that the three familiar three dimensions of space may be accompanied by other dimensions, tiny ones that are curled up so that they are undetectable at normal energies. In principle, a particle that takes a shortcut through such a dimension could arrive early! This may sound like a rather fantastic explanation, but it is possible that an experiment at the unprecedented energy and precision of OPERA could see this effect for the first time. Certainly, it would not contradict any previous theory or experiment.

So an exciting wait, but my money is on a systematic error in the measurement of distance or time

Technical note

I keep hearing in the media that ‘relativity forbids travelling at speeds faster than the speed of light in vacuum.’ Actually, it doesn’t, as Einstein was fond of pointing out. Special relativity suggests that it is impossible for  body to be accelerated from subluminal to superluminal speed. Thus particles that travel faster than light are possible in principle so long as they always travel at that speed (known as tachyons). However, such behaviour has implications for time (it would run backwards) and for causality, and is therefore thought unlikely. Also, no such particles have been observed  in five decades of experimentation in particle physics .

Last weekend, I was quoted (well misquoted) in The Irish Times, making the last point above; you can read it here, it’s quite a good article.

Update

If it is a systematic error, what could it be?  Looking at the paper, my own guess is that it is significant that the group do not measure the time-of-flight of individual neutrinos, but massive bunches of the particles. Essentially they measure the beginning and end of a bunch, and apply statistics to get the mean time. A messy enough procedure, considering the accuracy required..

Update II

I have a letter on the experiment in The Irish Times today, you can read it here

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MIT, the LHC and a royal wedding

It’s always a pleasure to pop over to MIT for the afternoon, and on Friday I attended a seminar titled ‘MIT and the World’s Largest Science Experiment: Hunting the Higgs Boson”, given by Markus Klute, Assitant Professor of Physics at MIT and member of the CMS (compact muon solenoid) collaboration at CERN. It seems MIT has quite a big involvement with the LHC, with a large group working at the CMS detector and a smaller group at ATLAS; about 40 researchers overall.

MIT and the World’s Largest Experiment: Hunting the Higgs Boson

The talk was aimed at a wide audience, and much of it was a fairly standard introduction to particle physics and the experiments at the Large Hadron Collider. I always try and attend such talks whenever I can, partly to pick up any new information but also to see how the real players present the story.

Starting with a review of the nucleus and its particles, Markus gave a succinct overview of the Standard Model. I liked the way he linked the theory to today’s news; in describing the way particles are believed acquire mass (the Higgs mechanism), he invited the audience to imagine Charles and Kate entering the auditorium, how people would interact (i.e. cluster around them) to different degrees, thus acquiring different masses. This is a nice twist to a common analogy and it never hurts to connect with current events. (Being Irish, I’d be a neutrino with almost zero interaction, though I wish the couple well).

Professor Klute then gave a nice overview of the four main detectors at the LHC, and then some details about his group’s contribution to the CMS experiment, particularly in the area of building the tracking system. I won’t repeat the details but you can find a good review of US involvement at the CMS detector here. I particularly enjoyed the emphasis on the ‘rediscovery’ of the particles of the Standard Model at CMS, beautifully summarized in the plot below. I think particle physicists should emphasize this chart more, it gives great confidence in the methods of particle physics (and shows how sociologists such as Shapin and Schaffer underestimate the reproducibility of big science experiments, see ‘Leviathan and the Air Pump’).

Summary chart of particles rediscovered at CMS

I liked the speaker’s simple description of particle detectors: a camera with a hundred million pixels and a shutter speed of 400 million times per second – not to mention the filtering. He also placed great emphasis on the computing challenges thrown up by the data, giving a nice overview of the Worldwide LHC Computing Grid. I also liked the way he described particle physics experiments  in terms of four components: accelerators, detectors,  computing and people!

Finally, there was a nice overview of the challenge of the hunt for the Higgs, explaining that

– it is rarely produced

– decays almost immediately

– its mass is not known, hence neither are the main production or decay channels

Professor Klute then gave a very quick review of the main production and decay channels for the Higgs and explained how CMS will look for them.

Higgs production via gluon fusion; a dominant process for a range of mass

The decay channel depends on the Higgs mass

Another nice plot was a summary slide showing the masses already ruled out by previous accelerator experiments.

The window is closing

Finally, the speaker gave a quick synopsis of the possibility of observing physics beyond the Standard Model, concentrating on the possibility of the detection of supersymmetric  particles, in particular the possibility of supersymmetric Higgses. After questions and answers, there was a poster session and reception, with some very impressive posters by MIT postgrads at CMS.

All in all, a very enjoyable LHC talk with a useful description of US participation in the project. If you want to more about this, the US LHC blog is well worth following.

Update

Markus points out I said Charles and Kate instead of William! This is such a whopping error (of planetary magnitude) I think I’ll leave it. Re Higgs production and decay channels, there is an really nice overview here by a group at Imperial College also involved with the CMS experiment. This week there was a big meeting at Notre Dame concerning US participants in the CMS experiment, you can link to it here

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Heuer at Harvard

Rolf Heuer, the Director General of CERN, gave a talk on the Large Hadron Collider at the weekly physics colloqium at Harvard this week. The talk, “The Large Hadron Collider: Entering a New Era of Fundamental Science“, was aimed primarily at undergraduates and postgraduates in the physics department and it certainly lived up to expectation.

Professor Rolf-Dieter Heuer

The talk was roughly structured in 4 parts:

– a brief introduction to quarks and the standard model of particle physics

– a brief description of the LHC experiment; the four main detectors and their main purposes

– a brief review of results so far: luminosity successes, top quark production, quark gluon plasma using heavy-ion collision etc

– a  brief overview of possible new territory; from the Higgs boson to physics beyond the Standard Model (supersymmetric particles  etc)

What struck me most was the speaker’s emphasis on the link between the world of the sub-atomic and the universe at large. From the very first slide, Prof Heuer explained the symbiosis between particle physics, astrophysics and cosmology, pointing out that the Standard Model of particle physics addresses just 5% of the the universe (since it is now thought that dark matter and dark energy make up the rest). This theme came up many times and is indicative of how much has changed in the world of particle physics in the last few decades. Another major theme was how the individual detectors overlap and complement one another, giving a result that is greater than the sum of its parts.

The big news is that due to its excellent performance to date, the LHC is to run through until the end of 2012 (and not to go into a long technical upgrade at the end of 2011 as perviously planned). This means there will be enough data collected (albeit at 7 TeV) that some exciting new physics may have been discovered by 2013.  This is very good news for particle physics, given that the Tevatron is due to cease operations in September.

Schematic of the four main detectors of the LHC

Overview of the 27-km LHC ring

Heuer also introduced a historical perspective, pointing out that this year is the centenary of the Rutherford’s discovery of the atomic nucleus, the first public announcement of this I’ve heard so far. There was also a reference to the fact that 2011 is also the centenary of the discovery of superconductivity, without which the LHC would not be possible (superconducting materials are used in the giant magnets that guide the proton beams in the LHC).  On the experiments, I particularly liked that he payed special attention to the LHCb experiment; although it is on a much smaller scale than ATLAS and ALICE, this is my favourite of the four detectors, because of the connection with antimatter (LHCb will seek to answer why matter predominates in our universe by probing the asymmetry between matter and antimatter decay in beauty quarks, see website here or a nice blogpost on it here). It is also the only major CERN experiment that has an Irish connection, see here.

Event display of a pp collision in the LHCb detector producing two b-hadrons.

Some quotes I really liked were

We know everything about the Higgs boson, except whether it exists”

Within the next 2 years, we will have found a Higgs boson between 114 and 200 GeV, or ruled it out”

“Ruling things out is important, but we hope to have some discoveries too” (on supersymmetry)

At question time afterward, questions were deftly handled, with clear and succinct answers. I asked a question on dark energy; what did Prof Heuer have in mind when he said he hoped that LHC experiments could shed light on dark energy? His answer was that if the Higgs boson is found, it will be the first scalar boson observed (the W and Z bosons are vector bosons). Hence the idea of  a scalar field, required by cosmologists for dark energy, while not the same field, becomes more tangible. On string theory, he declined to dismiss the theory as philosophy (as suggested by someone in the audience), pointing out that the detection of supersymmetric particles at the LHC could offer some support for ST.

All in all, it was a great talk, pitched at exactly the right level for students and really conveying the excitement of discovery. I was slightly surprised the speaker didn’t call more attention to the international aspect of CERN, that an inter-european project involving a motley collection of sparring nations has emerged as the world’s premier centre for experimental particle physics (perhaps he was being polite). Just as the detectors complement one another and offer a result that is greater than the sum of its parts, so can genuine co-operation of individual nations working in harmony (and all the more reason for Ireland to join, as I have said many times in public and in the press).

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Stuck at the airport: that in-between time again

I’m still hoping to get back to Ireland for Christmas Day, but it’s now midday on Christmas Eve and I haven’t made it any further than Boston airport. The problem is, as everybody knows, many European airports like Dublin and Heathrow are experiencing large snowfalls and feezing conditions this week, and simply can’t keep the runways clear; planes can’t get in or out which causing travel chaos.

‘What? Snow in winter?’, you cry sarcastically. Yes, but heavy snow in December is relatively uncommon in countries like Ireland and the UK. Worse, it comes at a time of huge passenger volumes. Many airports simply can’t cope with the double whammy, with knockon effects for all air travel. Delays at Heathrow, in particular, have caused chaos at airports around Europe and elsewhere.

Dublin airport in the snow

As for me, I’m happy enough. I like these in-between times, where one is neither working nor on holiday. A good time to think. Also, Logan airport is very nice, sensibly divided into small terminals (unlike Dublin airport). Of course, travel delays are relatively easy if you don’t have tired kids, financial worries, or have to sleep on the airport floor. Here in Boston, Aer Lingus passengers are being put up in the nearby Hilton while we wait for the next available flight; pretty decent, considering the airline can hardly be held responsible for the weather.

I’m using the time to read through Piers Bizony’s ATOM, the only interesting book I could find in the tiny airport bookstore. The book is based on the excellent BBC TV series of the same name and it’s a very entertaining read, if a little surprising. From the title, I had expected a brief history of particle physics for the layman; from the discovery of the nucleus to the quark etc. Instead, the book concentrates mainly on the story of the evolution of quantum physics. Which is no harm. But it does remind me that there are remarkably few books out there that tell the story of the discovery of the atom and subatomic particles as a simple phenomenological tale . Hmm…

Something to think about. Right now, it’d be nice to get home sometime soon.

Update

We’re finally about to get on a plane to Dublin (arriving 3 am Christmas Day!) and I still haven’t finished Bizony’s book.  I’ve enjoyed my airport sojourn but others are less sanguine. Some passengers are utterly fed up and are less than polite to the airline staff.  It’s amazing how some need someone to blame in situations like this, even when it’s perfectly obvious that it’s weather, not the airline, that is the problem. Human nature I guess…

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Antimatter trapped at CERN

The Daily Telegraph has a story today with the headline

Antimatter captured by CERN scientists in dramatic physics breakthrough

accompanied by the picture below and the usual razzmatrazz of antimatter-powered spaceships, antimatter bombs, Angels and Demons etc.

I first came across this strange story on Facebook early this morning and the Daily Telegraph headline iis equally puzzling. As every schoolgirl knows, antimatter is an exotic form of matter made up of particles of opposite electric charge to that of everyday matter (see post on this here). What is puzzling about the story is that physicists have been producing antiparticles in high-energy accelerator experiments since the 1950s and have been able to manufacture whole atoms of antimatter for over a decade now. (Atoms of anti-hydrogen are manufactured in accelerators by allowing anti-protons to capture anti-electons, see here).

About a third of the way down the article in the Telegraph, one discovers the real nature of the breakthrough –  the ALPHA experiment at CERN have reported that they have managed to produce atoms of anti-hydrogen that are relatively longlived (see paper in Nature here). Up to now anti-atoms were extremely shortlived because antimatter is instantly annihilated when it encounters matter (e.g. the container walls). What the Alpha group has done is to trap anti-hydrogen atoms in complex magnetic fields for up to a tenth of a second. Hence the word ‘capture‘ in media headlines, I guess.  It is certainly an important breakthrough as it should enable a detailed study of subtle differences between atoms of hydrogen and anti-hydrogen. (For example, in what way does the spectrum of anti-hydrogen differ from that of ordinary hydrogen?)

The Alpha experiment – don’t try this at home

This is an important area of study because any differences in the spectrum of anti-hydrogen vs ordinary hydrogen could shed light on one of the greatest mysteries of particle physics and cosmology; why is our universe made of matter? What subtle imbalance occurred in the early universe that led to the survival of ordinary matter over antimatter? From the point of view of particle physics, it wll be very interesting to see if CPT symmetry is conserved in the case of anti-hydrogen: if not this has implications for the standard model of particle physics.

Almost everybody in the particle physics universe is blogging on this breakthrough today so I won’t comment further – there is an excellent summary of the experiment on the Symmetry Breaking blog

Update

Kate McAlpine (author of the great LHC rap) has an excellent article on the above in this week’s edition of New Scientist . It’s well worth a look, especially her explanation of how neutral anti-atoms can be trapped in a magnetic field.

Update II

When the film Angels and Demons came out, Dan Brown was widely criticized for suggesting that enough antimatter could be trapped long enough to form a stable bomb (see post and review of A&D here). Looks like Brown wasn’t quite so far off the mark after all – at least about entrapment, if not about a feasible amount of antimatter for a bomb. My guess is that he had some serious discussions with the CERN group, just as he claimed at the time..

Not quite so daft

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