Archive for lhc

The Low-down on the LHC Boson

Posted in Open Access, The Universe and Stuff with tags , , , , , , on August 2, 2012 by telescoper

Although it’s a little late I thought I’d just put up a brief post to draw your attention to the news that a couple of technical papers have appeared on the arXiv giving updated details of the recent discovery at the Large Hadron of a new scalar particle that could be the Higgs boson. I don’t think it’s yet absolutely proven that this is what the new particle is, which is why I’ve called it the “LHC boson” in the title.

The ATLAS paper reports the detection of a Higgs-like particle with a 5.9 sigma confidence level, up from the 5.0 sigma reported on July 4. Here’s the abstract:

A search for the Standard Model Higgs boson in proton-proton collisions with the ATLAS detector at the LHC is presented. The datasets used correspond to integrated luminosities of approximately 4.8 fb^-1 collected at sqrt(s) = 7 TeV in 2011 and 5.8 fb^-1 at sqrt(s) = 8 TeV in 2012. Individual searches in the channels H->ZZ^(*)->llll, H->gamma gamma and H->WW->e nu mu nu in the 8 TeV data are combined with previously published results of searches for H->ZZ^(*), WW^(*), bbbar and tau^+tau^- in the 7 TeV data and results from improved analyses of the H->ZZ^(*)->llll and H->gamma gamma channels in the 7 TeV data. Clear evidence for the production of a neutral boson with a measured mass of 126.0 +/- 0.4(stat) +/- 0.4(sys) GeV is presented. This observation, which has a significance of 5.9 standard deviations, corresponding to a background fluctuation probability of 1.7×10^-9, is compatible with the production and decay of the Standard Model Higgs boson.

The paper from CMS reinforces the discovery of a Higgs-like particle with a mass of 125 GeV at a 5-sigma level of confidence:

Results are presented from searches for the standard model Higgs boson in proton-proton collisions at sqrt(s)=7 and 8 TeV in the CMS experiment at the LHC, using data samples corresponding to integrated luminosities of up to 5.1 inverse femtobarns at 7 TeV and 5.3 inverse femtobarns at 8 TeV. The search is performed in five decay modes: gamma gamma, ZZ, WW, tau tau, and b b-bar. An excess of events is observed above the expected background, a local significance of 5.0 standard deviations, at a mass near 125 GeV, signalling the production of a new particle. The expected significance for a standard model Higgs boson of that mass is 5.8 standard deviations. The excess is most significant in the two decay modes with the best mass resolution, gamma gamma and ZZ; a fit to these signals gives a mass of 125.3 +/- 0.4 (stat.) +/- 0.5 (syst.) GeV. The decay to two photons indicates that the new particle is a boson with spin different from one.

I’ll refrain from commenting on the use of frequentist language in both these papers, but instead just comment that these extremely important papers are available for free on the arXiv. Open access, we call it.

PS. There’s an interesting blog post related to these papers, about citations in particle physics here.

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.

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.

More Boring Than Advertised? (via Occasional Musings of a Particle Physicist)

Posted in The Universe and Stuff with tags , , , , , on July 29, 2011 by telescoper

My (probably ill-informed) earlier post about particle physics seems to have generated quite a lot of traffic, so I thought I’d reblog this short article (by a real particle physicist) for the benefit of those people who want to find out about the latest results from someone who actually knows what they’re talking about.

You would be forgiven for seeing the headlines from EPS-HEP 2011 and thinking the LHC is less interesting than maybe you were led to believe. A year or so ago you might have expected hints of supersymmetry, black holes, extra dimensions or even something more exotic to have been found in the ever increasing LHC datasets. But the current story is that the Standard Model is still describing all data analysed so far pretty damn well. There may or ma … Read More

via Occasional Musings of a Particle Physicist

LHC Hasn’t Destroyed Earth Yet (via Today’s New Reason to Believe)

Posted in The Universe and Stuff with tags , , , , , , on June 19, 2011 by telescoper

I thought I’d reblog this as it relates to the pronouncements about the LHC by Otto Rössler I mentioned yesterday.

LHC Hasn’t Destroyed Earth Yet As I predicted nearly two-and-a-half years ago, it looks like the Earth will survive the most powerful accelerator ever built. A recent article validates my prediction. On September 8, 2008, I recorded a Science News Flash podcast addressing concerns that the Large Hadron Collider (LHC) would produce a shower of black holes that would, in turn, consume Earth from the inside out. I predicted that Earth would not only survive the experiments perfor … Read More

via Today's New Reason to Believe

You might also want to read this older blog post about the kerfuffle when the LHC was switched on. I quote:

Rössler turns out to be quite a strange fellow. He is an MD who stayed in academia, moved into biochemistry, and then made a name in the relatively new field of chaos theory. He seems to think of himself as a visionary, having founded a new field of physics called “endophysics,” which is supposed to take into account the observer’s inner state. Or something like that. Have you heard of it? Neither had I.

Recently, at the age of sixty-eight, Rössler, despite having no particle physics or blackhole physics credentials, announced that he had found important new results, alarmingly relevant to the destructive potential of microscopic black holes in LHC proton-proton collisions. Rössler variously estimates the likelihood of such blackhole production by LHC as being from 10% to 50% though he appears to have pulled these numbers out of a hat.

And there’s also this most excellent video that John Butterworth told me about because he’s in it…