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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.

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The Higgs Buzz

Posted in The Universe and Stuff with tags , , , , on June 19, 2012 by telescoper

Reaction to rumours about the Higgs, and a not-entirely good-tempered comment thread about the ethics of blogging. All in a day’s work for a particle physicist, I guess! Read the inside story on this post…

..and if you read this article you’ll see where the rumour originated.

Matt Strassler's avatarOf Particular Significance

The rumors about the Higgs particle at the Large Hadron Collider [LHC] have begun again, and since that’s all anyone is going to want to talk about until we actually get the news for real, at the ICHEP conference in Melbourne in a couple of weeks, we may as well get started.

[This is especially true since we learned last year that some well-known non-particle-physicist bloggers have information pipelines directly into the experiments.  It is perhaps inevitable that there are scientists who see it in their best interest to subvert the scientific process.]

The current hot rumor is that the LHC experiments ATLAS and CMS have seen, in the new 2012 data, very roughly what they saw last December in the 2011 data, at least as far as the signal from a Higgs decaying to two photons (particles of light) in the mass range of 125 GeV/c2. 

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Bayes’ Theorem and the Search for Supersymmetry

Posted in The Universe and Stuff with tags , , , , on May 13, 2012 by telescoper

Interesting comments about Bayes’ theorem and the prospects for detecting supersymmetry at the Large Hadron Collider. This piece explains how a non-detection isn’t always “absence of evidence” but can indeed by “evidence of absence”. It’s also worth reading the comments if you’re wondering whether what people say about Lubos Motl is actually true…

Phi G's avatarviXra log

Here’s a puzzle. There are three cups upside down on a table. You friend tells you that a pea is hidden under one of them. Based on past experience you estimate that there is a 90% probability that this is true. You turn over two cups and don’t find the pea. What is the probability now that there is a pea underneath? You may want to think about this before reading on.

Naively you might think that two-thirds of the parameter space has been eliminated, so the probability has gone from 90% to 30%, but this is quite wrong. You can use Bayes Theorem to get the correct answer but let me give you a more intuitive frequentist answer. The situation can be models by imagining that there are thirty initial possibilities with equal probability. Nine of them have a pea under the first cup, nine more under the second and nine more under the third…

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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…

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Is there only one electron in the Universe?

Posted in The Universe and Stuff with tags , , , , , , , , on February 1, 2012 by telescoper

I started teaching Nuclear and Particle Physics to the 3rd year Physics students today. I decided to warm up with a few basics about elementary particles and their properties – all pretty standard stuff and no hairy mathematics. Cue pretty picture:

This doesn’t show the whole picture, of course, because for every particle there is an antiparticle, so there are antiquarks and antileptons. The existence of these was first suggested by Paul Dirac in 1928 based on his investigations into relativistic quantum theory, basically because invariance of special relativity is compatible with the existence of both positive and negative energy states, i.e.

E^2 = p^2c^2 +m^2 c^4

has two sets of solutions, one with E>0 and the other with E<0. Instead of simply assuming the latter set were physically unrealistic, Dirac postulated that they might be real, but completely filled in “empty” space; these filled negative-energy states are usually called the “Dirac Sea”. Injection of an appropriate amount of energy can promote something from a negative state into a positive one, leaving behind a kind of hole (very similar to what  happens in the case of semiconductor). This process creates a pair consisting of a (positive energy) particle and a (negative energy) antiparticle (i.e. a hole in the Dirac Sea). In the case of electrons, the hole is called a positron.

The alternative, and even wackier, explanation of antimatter I usually mention in these lectures derives, I think, from Feynam who noted that in quantum (wave) mechanics the time evolution of particles involves things like

\exp(i\omega t)=\exp(i Et/\hbar),

which have the property that changing E into -E has the same effect as changing t into -t. This is, in essence, the reason why, in Feynman diagrams, antiparticles are usually represented as particles travelling backwards in time…

This is a useful convention from the point-of-view of using such diagrams in calculations, but it allows one also to raise the wacky bar to a higher level still, to a suggestion that, coincidentally, was  doing the rounds very recently – namely whether it is possible that there may really be only one electron in the entire Universe:

….I received a telephone call one day at the graduate college at Princeton from Professor Wheeler, in which he said, “Feynman, I know why all electrons have the same charge and the same mass” “Why?” “Because, they are all the same electron!” And, then he explained on the telephone, “suppose that the world lines which we were ordinarily considering before in time and space—instead of only going up in time were a tremendous knot, and then, when we cut through the knot, by the plane corresponding to a fixed time, we would see many, many world lines and that would represent many electrons, except for one thing. If in one section this is an ordinary electron world line, in the section in which it reversed itself and is coming back from the future we have the wrong sign to the proper time—to the proper four velocities—and that’s equivalent to changing the sign of the charge, and, therefore, that part of a path would act like a positron.”
—Feynman, Richard, Nobel Lecture December 11, 1965

In other words, a single electron can appear in many different places simultaneously if it is allowed to travel backwards and forwards in time…

I think this is a brilliant idea, especially if you like science fiction stories, but there’s a tiny problem with it in terms of science fact. In order for it to work there should be as many positrons in the Universe as there are electrons. Where are they?

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Away to Swansea

Posted in The Universe and Stuff with tags , , , , on January 11, 2012 by telescoper

Just time for a quick post this evening, primarily to make a note of an enjoyable event that took place this afternoon. I long since gave up keeping a proper journal so the old blog will have to play that role.

Today a small group of cosmologists from the School of Physics & Astronomy at Cardiff University made the short trip to Swansea to meet with members of the Physics department there. The idea of the meeting was to explore the possibilites of future research collaboration. For historical reasons there is a pretty strong separation in Wales between research in Particle Physics and Astronomy/Astrophysics; Swansea does the former and Cardiff does the latter. However, cosmology is an area in which there are possible overlaps between some of the – primarily theoretical – research going on at Swansea into, e.g., Quantum Gravity and what we do in Cardiff, e.g. inflationary cosmology.

Anyway we decided to get together for an afternoon of talks by members of both departments to see if anything emerged as potential research topics. In fact, a couple of interesting ideas were discussed and although the main focus of research differs substantially in the two institutions we’re definitely going to get together again to follow up these ideas.

Although I’ve been in Cardiff since 2007, I’d never visited Swansea University before which, considering that it’s only an hour away by train, is admittedly a bit pathetic. In fact I think it’s quite weird the two departments don’t collaborate more in other areas too. I’m certainly very keen to see more joint activities than we have now, so hopefully this is a move in that direction.

Anyway, I’d like to thank Graham Shore at Swansea for hosting us this afternoon and I very much look forward to the planned return leg which will be held in Cardiff in a couple of months.

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In a Physicist’s Mailbag…

Posted in Biographical, The Universe and Stuff with tags , , on January 10, 2012 by telescoper

Among the delights (?) of being a scientist are those priceless pieces of unsolicited mail from members of the public. When I went to collect my mail this morning I found a prime example waiting in my pigeonhole. I knew what it was going to be like before I even opened it because the envelope was addressed (rather inaccurately) using an old-fashioned typewriter. Only a certain kind of person uses a typewriter these days.

I particularly enjoyed the “Emeritus Prof. ” bit. And Cardiff isn’t in “Engand”, by the way. Or even “England”.

Inside were six pieces of paper – all of different sizes – on which fascinating things had been typed and later highlighted with red and black pens in order to enhance both their scientific and artistic impact.

I’m in the middle of a load  of project vivas today so haven’t had time to scan this masterpiece neatly, but it’s such a wonderful piece of correspondence that I couldn’t resist taking a few pictures of various elements for the edification of my vast readership. I think if you click on the images you might be able to read them more clearly but, if you do, I will not accept liability for the consequences.

Unfortunately I’m not sure whether I have them in the right order, as the logic that connects them together escapes me.

I have  a large collection of similar missives but, despite some obvious deficiencie, such as a lack of drawings,  this letter is one of the best and will now take pride of place in the bottom drawer of my filing cabinet. Perhaps one day I’ll write a book about them…

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The Geordie Particle

Posted in The Universe and Stuff with tags , , , , on December 20, 2011 by telescoper

As the media frenzy abates after the latest experimental results from the Large Hadron Collider show tantalising but inconclusive evidence for the existence of the Higgs boson, it’s perhaps now time to focus on the hard facts surrounding this elusive particle. At yesterday’s Christmas lunch I stumbled upon one piece of information of which I was previously unaware and which is clearly of national importance. The eponymous creator of the Higgs particle, Professor Peter Higgs, was in fact born in the fine city of Newcastle upon Tyne, which really is in The North. This fact identifies him as a Geordie, although having just heard him on the radio I think there’s not much sign of it in his accent.

Anyway, in honour of this important discovery I respectfully submit that  The Large Hadron Collider should be given a more appropriate name,  i.e. The Geet Big Hadron Basher. And I’m sure God won’t mind if the Higg’s boson is henceforth known as the Geordie Particle.

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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.

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Neutrinos on Speed

Posted in The Universe and Stuff with tags , , , on September 23, 2011 by telescoper

The internet, twitterdom, blogosphere, and even the mainstream media are all alive today with wild speculations about a curious claim that neutrinos might travel faster than light.

If you’re interested in finding the source of this story, look at the arXiv paper here. I haven’t got time to go through the paper in detail, but I think it must be an instrumental artefact or some other sort of systematic error.

One major reason for doubting the veracity of the claim that neutrinos travel faster than light is provided by astronomical observations. Neutrinos produced by the explosion of Supernova SN1987a were detected when it went boom in 1987, approximately three hours before the visible light from SN 1987A reached the Earth.

The few hours delay between neutrinos and photons is explained by the fact that neutrino emission occurs when the core of the progenitor star collapses, whereas visible light is released only when a shock wave reaches the surface of the imploding object. Three different experiments detected (anti)neutrinos: Kamiokande II found 11 , IMB 8 and Baksan 5, in a burst lasting less than 13 seconds.

If the time delay reported by the OPERA detector over the distance between CERN and Gran Sasso were extrapolated to the distance between Earth and SN1987a then the neutrinos should have arrived not a few hours early, but a few years, and there would not have been coincident arrivals at the different detectors on Earth.

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

UPDATE: Now that I’ve read the paper let me point out that the OPERA result is essentially

δv/c = (2.48 ± 0.28(stat) ± 0.30(syst)) × 10-5,

whereas the constraints from Supernova 1987a work out to be   δv/c < 2 × 10-9 for  neutrino energies of 10 MeV. See the comments below for discussion.

I’ll also mention at this point that the analysis done in the paper is entirely based on frequentist statistics. Somebody needs to do it properly.

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