Archive for CERN

Short but sweet – Higgs (1964)

Posted in The Universe and Stuff with tags , , , , on August 31, 2012 by telescoper

In the light of all this Malarkey about the (claimed) discovery of the Higgs Boson at the Large Hadron Collider, I thought you might be interested to see the original paper by Higgs (1964) in its entirety. As you can see, it’s surprisingly small. The paper, I mean, not the boson…

p.s. The paper is freely available to download from the American Physical Society website; no breach of copyright is intended.

p.p.s. The manuscript was received by Physical Review Letters on 31st August 1964, i.e. 48 years ago today.

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The Higgs? A Definite Maybe..

Posted in The Universe and Stuff with tags , , , , , , on July 4, 2012 by telescoper

This is really something for expert particle physicists to blog about, but I couldn’t resist saying something about this morning’s dramatic physics news.

Well, after yesterday’s preview here is the actual press release from CERN:

Geneva, 4 July 2012. At a seminar held at CERN1 today as a curtain raiser to the year’s major particle physics conference, ICHEP2012 in Melbourne, the ATLAS and CMS experiments presented their latest preliminary results in the search for the long sought Higgs particle. Both experiments observe a new particle in the mass region around 125-126 GeV.

“We observe in our data clear signs of a new particle, at the level of 5 sigma, in the mass region around 126 GeV. The outstanding performance of the LHC and ATLAS and the huge efforts of many people have brought us to this exciting stage,” said ATLAS experiment spokesperson Fabiola Gianotti, “but a little more time is needed to prepare these results for publication.”

“The results are preliminary but the 5 sigma signal at around 125 GeV we’re seeing is dramatic. This is indeed a new particle. We know it must be a boson and it’s the heaviest boson ever found,” said CMS experiment spokesperson Joe Incandela. “The implications are very significant and it is precisely for this reason that we must be extremely diligent in all of our studies and cross-checks.”

“It’s hard not to get excited by these results,” said CERN Research Director Sergio Bertolucci. “ We stated last year that in 2012 we would either find a new Higgs-like particle or exclude the existence of the Standard Model Higgs. With all the necessary caution, it looks to me that we are at a branching point: the observation of this new particle indicates the path for the future towards a more detailed understanding of what we’re seeing in the data.”

The results presented today are labelled preliminary. They are based on data collected in 2011 and 2012, with the 2012 data still under analysis.  Publication of the analyses shown today is expected around the end of July. A more complete picture of today’s observations will emerge later this year after the LHC provides the experiments with more data.

The next step will be to determine the precise nature of the particle and its significance for our understanding of the universe. Are its properties as expected for the long-sought Higgs boson, the final missing ingredient in the Standard Model of particle physics? Or is it something more exotic? The Standard Model describes the fundamental particles from which we, and every visible thing in the universe, are made, and the forces acting between them. All the matter that we can see, however, appears to be no more than about 4% of the total. A more exotic version of the Higgs particle could be a bridge to understanding the 96% of the universe that remains obscure.

“We have reached a milestone in our understanding of nature,” said CERN Director General Rolf Heuer. “The discovery of a particle consistent with the Higgs boson opens the way to more detailed studies, requiring larger statistics, which will pin down the new particle’s properties, and is likely to shed light on other mysteries of our universe.”

Positive identification of the new particle’s characteristics will take considerable time and data. But whatever form the Higgs particle takes, our knowledge of the fundamental structure of matter is about to take a major step forward.

There’s a hive of internet activity related to this announcement, and I can’t possibly link to all the excellent expert commentary going on, but for details you can do no better that Sean Carroll’s live blog from Geneva or the Guardian’s live blog.

In a nutshell, there’s definitely something in both CMS and Atlas data which, if it really is a new particle,  is definitely a boson and which weighs in around 125 GeV. The two-photon decays are consistent with what a standard model Higgs boson would be expected to produce, for example. The consistency between the two experiments is very compelling.

The overall level of significance is around 5σ. I’ll refrain from making churlish comments about the frequentist language and just say that the LHC certainly seems to have detected something that could definitely be the Higgs. This is genuinely exciting because it has come more quickly than most people expected. That’s a tribute to the LHC teams, I’d say.

However, it isn’t yet proven that the Higgs what this particle is. If it’s a new particle that’s not the Higgs that could be even more interesting. To establish the identity of the particle that has been discovered will require a lot more work,  looking at much more detailed aspects of its behaviour as revealed by collision data. But it’s certainly possible that it is the Higgs, and I venture to suggest that’s what most particle physicists think it is.

So a discovery. A palpable discovery. Now comes the exploration…

Higgs Preview

Posted in Science Politics, The Universe and Stuff with tags , , , , , , on July 3, 2012 by telescoper

I’m a bit slow to post anything about the ongoing bout of Higgs-steria that’s been engulfing the interwebs in recent days. Even Andy Lawrence got there ahead of me.  What’s caused all the commotion is an announcement about an announcement from CERN at a special seminar tomorrow (Wednesday 4th July) at 9am CEST, which is 8am British “Summer” Time.  Here’s a bit of the press release:

CERN will hold a scientific seminar at 9:00 CEST on 4 July to deliver the latest update in the search for the Higgs boson. At this seminar, coming on the eve of this year’s major particle physics conference, ICHEP, in Melbourne, the ATLAS and CMS experiments will deliver the preliminary results of their 2012 data analysis.

“Data taking for ICHEP concluded on Monday 18 June after a very successful first period of LHC running in 2012,” said CERN’s Director for Accelerators and Technology, Steve Myers. “I’m very much looking forward to seeing what the data reveals.”

The 2012 LHC run schedule was designed to deliver the maximum possible quantity of data to the experiments before the ICHEP conference, and with more data delivered between April and June 2012 than in the whole 2011 run, the strategy has been a success. Furthermore, the experiments have been refining their analysis techniques to improve their efficiency in picking out Higgs-like events from the millions of collisions occurring every second. This means that their sensitivity to new phenomena has significantly increased for both years’ data sets.  The crunching of all this data has been done by the Worldwide LHC Computing Grid, which has exceeded its design specifications to handle the unprecedented volume of data and computing.

“We now have more than double the data we had last year,” said CERN Director for Research and Computing, Sergio Bertolucci, “that should be enough to see whether the trends we were seeing in the 2011 data are still there, or whether they’ve gone away. It’s a very exciting time.”

I won’t try to repeat what’s been said better and more authoritatively elsewhere; a nice collection of video material at the STFC website and a piece by Sean Carroll (also here) are worth mentioning if you’re not up on why the Higgs Boson is so important.

I wrote  a rather facetious post about the last episode of Higgs-mania way back in December because I found the actual announcement to be a bit of a damp squib and the associated hype rather irritating. This time there are even more rumours flying around – not to everyone’s approval – but it’s obviously best to wait and see what is actually announced rather than comment on them.

The main question in my mind is whether it’s sufficiently interesting to get up in time to watch the seminar 8am tomorrow morning…

Brian Cox is 44.

A New Baryon on the Block

Posted in The Universe and Stuff with tags , , , , , on April 29, 2012 by telescoper

I just chanced upon the news that a new particle has been discovered at the Large Hadron Collider. This is probably old hat for people who work at CERN, but for those of us following along in their wake it definitely belongs to the category of things marked Quite Interesting.

The new particle is a baryon, which means that it consists of three quarks. These quarks are held together by the colour force (which I refuse to spell the American way); baryonic states exist by virtue of the colours of constituent quarks being a red-green-blue mixture that is colourless.

Quarks are fermions with spin 1/2. The new particle has spin 3/2 which contrasts with the most familiar baryons, the proton and the neutron, which also consist of three quarks but which have spin 1/2. The difference can be understood from basic quantum mechanics: spins have to be added like vectors, so the three individual quark spins can be added to produce total spin 3/2 or 1/2.

The most familiar spin 3/2 baryons are made from the lightest quarks (the up, down and strange) as shown in the diagram below:

The top row contains no strange quarks, only up and down. In fact the Δ0 and Δ+ contain exactly the same quark compositions as the proton and the neutron (udd and uud respectively), but differ in spin. The next row down contains one strange quark (e.g. uds) , the one below two (e.g uss), and the particle at the bottom is a very famous one called the Ω which is entirely strange (sss). For reasons I’ve never really understood, a strange quark carries a strangeness quantum number S=-1 (why not +1?) and the electrical charge is labelled by q in the diagram.

There are six quark flavours altogether so one can construct further baryonic states by substituting various combinations of heavier quarks (c,b and t) in the basic configurations shown above. There are also excited states with greater orbital energy; all the particles shown above have quarks in the lowest state of orbital angular momentum (L=O). There is then a potential plethora of baryonic particles,  but because all are unstable you need higher and higher energies to bring them into existence. Bring on the LHC.

The new particle is called the Ξb*, and it consists of a combination of up, strange and bottom quarks that required collision energies of 7 TeV to make it. The nomenclature reflects the fact that this chap looks a bit like the particles in the third row of the figure, but with one strange quark replaced by a much more massive bottom quark; this one has zero electrical charge because the charges on the u, s and b are +2/3, -1/3 and -1/3 respectively.

Anyway, here’s the graph that represents the detection of the new baryon on the block:

Only 21 events, mind you, but still pretty convincing. For technical details, see the arXiv preprint here.

Whether you really think of this as a new particle depends on how fundamental you think a particle should be. All six quark species have been experimentally detected and in a sense those are the real particles. Things like the Ξb* are merely combinations of these states. You probably wouldn’t say that an excited state of the hydrogen atom (say with the electron in the 2s energy level) is actually a different particle from the ground state so why do different permutations of the same quarks warrant distinct names?

The answer to this I guess is the fact that the mass of an excited hydrogen atom differs from the ground state by only a tiny amount; electronic energy levels correspond to electron-volt scales compared to the 1000 MeV or so that is the rest-mass energy of the nucleus. It’s all very different when you’re talking about energy levels of quarks in baryonic particles. In such situations the binding energies of the quarks are comparable to, or even larger than, their rest masses because the colour force is very strong and the quarks are whirling around inside baryons  with correspondingly enormous energies. When two creatures have enormously different masses, it’s difficult to force yourself to think of them as different manifestations of the same beast!

Anyway, the naming of this particle isn’t really the important thing. A rose by any other name would smell as sweet. What matters is that existence of this new quark state provides another example of a test of our understanding of quark-quark interactions based on the theory of quantum chromodynamics. You might say that it passed with flying colours…

Neutrino Timing Glitch?

Posted in The Universe and Stuff with tags , , , , on February 23, 2012 by telescoper

You may recall the kerfuffle last September when physicists connected with the OPERA experiment at the Gran Sasso National Laboratory in Italy produced a paper suggesting that neutrinos might travel at speeds greater than that of light. I posted on that story myself and even composed a poem specially for the occasion at no extra charge:

Do neutrinos go faster than light?
Some physicists think that they might.
In the cold light of day,
I am sorry to say,
The story is probably shite

Well news began to break last night that OPERA scientists had identified an error. The first story I read was a bit shaky on the question of attribution, so I decided to sleep on it and see whether anything emerged that seemed sounder before posting on here. Later on last night an item in Nature News appeared which looks a bit better grounded:

But according to a statement OPERA began circulating today, two possible problems have now been found with its set-up. As many physicists had speculated might be the case, both are related to the experiment’s pioneering use of Global Positioning System (GPS) signals to synchronize atomic clocks at each end of its neutrino beam. First, the passage of time on the clocks between the arrival of the synchronizing signal has to be interpolated and OPERA now says this may not have been done correctly. Second, there was a possible faulty connection between the GPS signal and the OPERA master clock.

We should wait for a more definitive announcement from OPERA about these possible errors, but if it does turn out that technical glitches are responsible for the neutrino speed result then it won’t be entirely unexpected. A faulty cable connection does sound a bit lame, however. I hope they weren’t relying on a USB connection….

Anyway, as I mentioned in a comment elsewhere the arXiv paper from OPERA has now received about 230 citations, although it has not appeared in a refereed journal.  If it turns out to have been a completely wrong result, what does that tell you about the use of citations to measure “quality”?

UPDATE: There is now an official press release from CERN, confirming the unofficial reports mentioned above:

The OPERA collaboration has informed its funding agencies and host laboratories that it has identified two possible effects that could have an influence on its neutrino timing measurement. These both require further tests with a short pulsed beam. If confirmed, one would increase the size of the measured effect, the other would diminish it. The first possible effect concerns an oscillator used to provide the time stamps for GPS synchronizations. It could have led to an overestimate of the neutrino’s time of flight. The second concerns the optical fibre connector that brings the external GPS signal to the OPERA master clock, which may not have been functioning correctly when the measurements were taken. If this is the case, it could have led to an underestimate of the time of flight of the neutrinos. The potential extent of these two effects is being studied by the OPERA collaboration. New measurements with short pulsed beams are scheduled for May.

Higgs-mania Day

Posted in Science Politics, The Universe and Stuff with tags , , , , , on December 13, 2011 by telescoper

I woke up this morning to the BBC Radio News at 7am announcing that scientists at CERN were going to report “hints” of the discovery of the Higgs Boson at the Large Hadron Collider;  you can find a longer discussion by the BBC here. This was later accompanied by articles tackling the important questions of the day such as whether the discovery of the Higgs would justify the enormous expense of Brian Cox the LHC.

Prize for the most  inaccurate science report goes to  the Daily Fail:

‘God’ particle found:

Atom smasher reveals Higgs boson, the key to the universe

Evidence soon emerged however that this particular squib might be of the damp variety. Consistent with previous blogospheric pronouncements, a paper on the arXiv this morning suggested no convincing detection of the Higgs had actually been made by the ATLAS experiment.

I then had to make an important choice between watching the live webcast of the CERN seminar at which detailed information on the Higgs searches was to be presented or to accept a free lunch with the examiners of a PhD candidate. I chose the latter.

Catching up on events after lunch confirmed the underwhelming nature of the Higgs “detection”, but with some intriguing evidence an excess signal at around 126 GeV at the 2.3 sigma level, in the frequentist parlance favoured by particle physicists and others who don’t know how to do statistics properly. In the words of the late John Bahcall:  “half of all three-sigma detections are false“. Of course if they used proper Bayesian language, scientists wouldn’t make so many nonsensical statements. Personally, I just don’t do sigmas.

My attention then switched to the CMS experiment. As a point of information you should be aware that CMS stands for Compact Muon Solenoid, where “compact” is a word used by particle physicists to mean “fucking enormous”. CMS makes  pictures like this:

Anyway, it seems from the CMS part of the presentation that they find a bit of a peak at a similar mass ~ 125 GeV but spread out over a larger range, this time at a level of – sigh – 2.6 sigma.

All in all, it’s a definite maybe. Putting the results together in the way only a frequentist can the result is a 2.4 sigma detection. In other words,  nothing any serious scientist would call convincing.

It’s interesting how certain these particle physicists are that the Higgs actually exists. It might, of course, and I think these results may be pointing the way to more convincing evidence based on more data. However,  I still think we should bear in mind the words of Alfred North Whitehead:

There is no more common error than to assume that, because prolonged and accurate mathematical calculations have been made, the application of the result to some fact of nature is absolutely certain.

If there is a Higgs boson with a mass of 125 GeV then that would of course be an exciting discovery, but if there isn’t one at all wouldn’t that be even more exciting?

Final word from the Director of CERN:

We have not found it yet, we have not excluded it yet, stay tuned for next year.

Thunder and hail descended on Cardiff just as the webcast finished, which is clearly not a coincidence although I couldn’t say how many sigmas were involved.

And a final, final word from the Chief Executive of the Science & Technology Facilities Council, John Womersley:

There is still some way to go before the existence of the Higgs boson can be confirmed or not, but excitement is mounting. UK physicists and engineers have played a significant role in securing today’s results, and will continue to be at the forefront of exploring the new frontiers of knowledge opened by the results coming from the LHC. This is an incredibly exciting time to be involved in physics!

Brian Cox is 43.

Never mind the Higgs, where’s the Supersymmetry?

Posted in The Universe and Stuff with tags , , , , , on July 25, 2011 by telescoper

There’s been a big conference on High Energy Physics going on in Grenoble since last Thursday, which I’ve been following a little bit via Tweets from various participants and links to blog articles contained therein. The media seem to be almost exclusively focussed on the Higgs boson but, as is made clear in a Guardian blog article by John Butterworth, the situation is that the latest data from the Large Hadron Collider do not provide clear evidence for it yet. Strangely, though, the Guardian ran another piece at the weekend claiming that “CERN scientists suspect a glimpse of the Higgs”, which appears to have been based on a blog article which offers various possible interpretations of a set of measurements which lie at the margin of statistical significance. It must be very frustrating not having a clear detection, but this  strikes me as clutching at straws. Far better to wait for more data before speculating in public. Nobody really expected to see the Higgs so soon, so it’s surely better to wait for more data rather than  over-interpreting what’s there. Let’s put it down to overenthusiasm.

However the real point of the latest news is not in my view the lack of, or marginal nature of, evidence for the Higgs Boson. It’s the extremely strong limits that have been placed on supersymmetry. This is of particular (geddit?) interest to me as a cosmologist because supersymmetric theories provide us with plausible candidates for the non-baryonic dark matter we think pervades the Universe.  The possibilities include fermionic counterparts of the bosons that mediate the standard model interactions. The photon, for example, which is a boson, mediates the electromagnetic interaction between charged particles; in SUSY theories it would have a fermionic partner called a photino. There would also be the Higgsino (assuming there is a Higgs!), gluino, gravitino and so on. Supersymmetry is a beautiful idea and many theorists love it to bits, but there isn’t a shred of evidence that has anything to do with the way nature is.

The search for supersymmetry is thus more directly relevant to my work than the Higgs, in fact, but the Large Hadron Collider was largely “sold” to politicians and the public in terms of the quest for the Higgs.  That’s the MacGuffin, as Alfred Hitchcock would have said. Actually the LHC will do many other things, but I guess it’s easier to make the case for funding to government if you have one Big Idea rather than lots of smaller ones.

Anyway, a piece from New Scientist today hits the nail on the head. While the Higgs search may or may not be producing tantalising clues, the searches for supersymmetry has drawn a complete blank. Zilch. Nada. Not the merest smidgeon of a scintilla. The class of supersymmetric theories is broad and no doubt many possibilities remain viable; the current measurements only rule out the “minimal” variety. But I think this is a timely reminder not to take nature for granted. Perhaps an  ugly fact is about to slay a beautiful hypothesis…

UPDATE: Bookmaker Paddy Power has shortened the odds on a Higgs discovery this year from 12-1 against to 3-1 on.