[Note from Shaun: Here is Higgs hunter Mikko Voutilainen's account of the recent search for the Higgs. You can find the teaser to this post here. And my own, partially cynical, but ultimately upbeat, account of Higgs-things, here.]
Here it is, finally
[I assume the readers of this blog are somewhat familiar with the Higgs boson; if not, there's a nice summary on the CMS pages here]
So, this is the follow-up to the teaser I wrote a week ago. Now that everybody knows we found a Higgs boson at \(125.3\pm 0.6\) GeV, I'm free to talk about our finding, what it means and how we got there. Note the intentional use of 'a' Higgs there: although we, beyond reasonable doubt (less than one in a million chance of an error, to be precise), found a new particle, it's not 100% sure yet if it's *the* Higgs boson predicted by the standard model, or one of its many twins predicted by the hundreds of theories out there. There's even a tiny chance of it being an altogether different particle yet.
We actually already know a fair deal about this new particle besides the rather impressively precise estimate of its mass: it seems to be produced at a rate that matches the standard model prediction within about 20% uncertainty, it decays into bosons (W, Z and photon) and fermions (b-quarks and tau-leptons) roughly in the ratios predicted by the standard model, and in particular it decays into W and Z bosons in the ratio predicted by the standard model. The last point is rather important, because the Higgs mechanism, and the Higgs boson along with it, was invented to give mass to the W and Z bosons, and leave the photon massless. This also fixes the ratio of the decay rates to W and Z. If the new particle didn't decay into Z's and W's in just the right ratio, it couldn't be the Higgs boson we predicted.
We've also had a stab at determining the more abstract properties of the particle such as a quantum number called parity, but the statistics are low and the results still inconclusive. Predictions say we should be able to tell by the end of the current run, when we've collected 2--3 times the amount of data we have now. At this point we should also have more precise determination of the particle's decay rates in all the different channels, in order to gain more confidence in calling the particle a Higgs boson or something else.
So, is this the end, or the beginning of something new? I'm really hoping for the latter. If the new particle turns out to be 'just' the standard model Higgs boson and there's nothing new to be found, that would be fairly boring. If instead it's a Higgs twin, we may have just opened a window into a new landscape of particles.
At the moment it's too early to tell for sure, but there are a few interesting features to the way the new particle decays. It seems to decay into photons more often than expected, and to tau-leptons less often than expected. Taking all the decays to fermions together, they only seem to add up to about half of the rate predicted by the standard model, albeit with an error of about 50% as well. That coincidence is causing a bit of excitement nevertheless.
It might not be too bad for the standard model, though, it could just indicate that it's 'non-minimal'. While the Higgs coupling to W and Z is pretty tightly constrained, all the other particle masses are more of an ad-hoc addition to the theory, and there's some freedom to adjust how these particles couple to the Higgs boson without breaking everything else. Another good example of something that would require a 'non-minimal' standard model are the neutrino masses, which in the simplest expectation are exactly zero. We now know they are not zero, although we've still to nail down exactly how much they weigh (it's very very little in any case).
What for me was most interesting in this was to see first-hand how things have evolved towards a big discovery. Things started rolling about six months ago, when the first results from LHC Higgs boson searches were presented last December. Back then both ATLAS and CMS saw a hint of a Higgs at 125 GeV, with about 2-2.5 sigma statistical confidence. If you were a Higgs-believer, you could have given the signal more than 95% chance of being true.
After December it was decided that we wouldn't look at the 2012 data in the signal region before we had enough to confirm or refute the hint seen in 2011. This process is called blinding, and its important for making sure the analyzers are not unconciously affected by their prior expectations. Blinding is also one of the reasons we've tried to keep a lid on the results until today's seminar so that the experiments would not affect each other's findings between opening their signal box opening and presenting the final results. I think we were fairly successful in the end, although rumors started circulating on the blogs within days, and by yesterday almost every major newspaper (including Nature) had run a story on Higgs.
Between opening the signal box and seeing the first evidence of a new particle there was a whole lot of work going on for 2--3 weeks to prepare for ICHEP. The analyses added around 50% more data, the particle properties were studied in more detail, the CMS management had regular meetings with both ATLAS and CERN directors, people were working day and night to scrutinize the results, prepare documentation, etc. The final days were spent polishing plots, rehearsing presentations and fine-tuning press releases. Although I didn't happen to be at CERN during that period (I did attend the signal box opening in the beginning, though), I could at least participate through the almost daily video meetings and by keeping my own small piece of CMS running (I'm responsible for a team calibrating jets).
Just two days prior to the seminar there was also a presentation of the Tevatron Higgs results at Fermilab. The Tevatron people had done a superb job in squeezing every last bit of sensitivity out of their data and fell just a hair's width short of claiming evidence for the Higgs (they got 2.94 sigma by the most optimistic count, and needed 3.0). The Tevatron experiments collected data for ten years before shutting down last summer, and have the same amount of data (10 fb-1) available for analysis as the LHC experiments now. The lower collision energy of the Tevatron, 2 TeV versus 8 TeV at LHC, means roughly ten times less Higgs bosons are produced, but they still have better sensitivity in one single channel, the Higgs decaying into two b-quarks. I was watching that live on video, too, cheering for my old colleagues (I did my PhD on D0, one of the two experiments at the Tevatron).
And then, finally, today we had a chance to see how our colleagues and rivals at ATLAS were doing with their Higgs search. According to blog rumors, newspaper leaks and sensitivity estimate just a tad behind CMS, but never far. As it turned out, both CMS and ATLAS came up with the same significance in the end, within 0.1 sigma precision. Both experiments have now just made it to the 5-sigma milestone, and it's pretty clear that the signal has been effectively confirmed by at least three experiments (counting D0 and CDF together as a single Tevatron experiment).
P.S. I wrote a lengthy story about the box opening the same evening when I was at CERN, and stored it on a time capsule on my e-mail account. I'm not sure if it's interesting anymore, but at least I shouldn't be breaking any confidentiality rules by releasing it. [Shaun speaking: I now have this item in my possession, so if anyone wants to see it please let me know and I will upload it in a few days.]
Wednesday, July 4, 2012
Tuesday, July 3, 2012
On its own, a Higgs discovery would be grim (II)
[...continued from yesterday]
This is like, instead of mapping the entire globe, the ocean explorers found that they had simply reached the edge of all navigable land, and, as far as any vessel could see, beyond that, there was just apparently endless ocean. The explorers would know, from measuring the curvature of the Earth, that Earth was a globe and had a finite extent, but the radius of the Earth would be so enormous that they would never be able to come close to traversing it by boat. It would also be as if, on the last few islands this civilisation discovered, there were all sorts of indications that there must be new land out there somewhere. Only there was no way of knowing where, or how far away, it was. The Higgs, for these explorers, would be one, last, island, discovered far into the wilderness of this ocean, farther from the mainland than anything else except the top quark (another island, alone in the wilderness). To reach either island would require the finest ship imaginable and would require a journey of decades.
Such a civilisation would be left to wonder, 'what is it that is out there in that wilderness?' But, they would be unable to answer their question until the invention of the aeroplane hundreds of years later. The next land might be just over the horizon, or it could be on the other side of the globe. This world, is where particle physics will find itself if the LHC finds the Higgs and nothing else.
The LHC's great, great grandparent in this journey of exploration was Ernest Rutherford who fired alpha particles at gold and discovered the atomic nucleus. Where Rutherford was the first of this kind, the LHC (or ILC) might be the last. For just over 100 years, collision experiments have been one of the driving forces of fundamental physics. The photos interspersed throughout this post show a collection of some of the more famous colliders during this period. But, just as the days of the ocean explorer had to eventually come to an end and the romantic tales of discovery that came with them ceased to be written, so might we have to fare colliders well and accept that the Higgs is the last of its kind.
If such an event occurs, a thought should be spared for all the map-makers of this oceanic world (the theoretical physicsists of the last thirty years), who, for decades, have built ever more complicated maps showing that Higgs island would not be alone. They had fascinating and compelling arguments for why Higgs island should be surrounded by exotic new islands, completely different to anything we've encountered before, many maps even showed new continents. The map-makers will have built entire careers making those maps, but if the islands and continents turn out not to be next to Higgs island, they're simply not there; however much we thought they should be. Of course, these continents may very well still exist, somewhere out of HMS Large Hadron Collider's range, but the map makers themselves would never get the chance to know.
This is like, instead of mapping the entire globe, the ocean explorers found that they had simply reached the edge of all navigable land, and, as far as any vessel could see, beyond that, there was just apparently endless ocean. The explorers would know, from measuring the curvature of the Earth, that Earth was a globe and had a finite extent, but the radius of the Earth would be so enormous that they would never be able to come close to traversing it by boat. It would also be as if, on the last few islands this civilisation discovered, there were all sorts of indications that there must be new land out there somewhere. Only there was no way of knowing where, or how far away, it was. The Higgs, for these explorers, would be one, last, island, discovered far into the wilderness of this ocean, farther from the mainland than anything else except the top quark (another island, alone in the wilderness). To reach either island would require the finest ship imaginable and would require a journey of decades.
 |
| The Large Electron Positron Colllider (most accurate measurer of the precision electroweak parameters) |
Such a civilisation would be left to wonder, 'what is it that is out there in that wilderness?' But, they would be unable to answer their question until the invention of the aeroplane hundreds of years later. The next land might be just over the horizon, or it could be on the other side of the globe. This world, is where particle physics will find itself if the LHC finds the Higgs and nothing else.
The LHC's great, great grandparent in this journey of exploration was Ernest Rutherford who fired alpha particles at gold and discovered the atomic nucleus. Where Rutherford was the first of this kind, the LHC (or ILC) might be the last. For just over 100 years, collision experiments have been one of the driving forces of fundamental physics. The photos interspersed throughout this post show a collection of some of the more famous colliders during this period. But, just as the days of the ocean explorer had to eventually come to an end and the romantic tales of discovery that came with them ceased to be written, so might we have to fare colliders well and accept that the Higgs is the last of its kind.
If such an event occurs, a thought should be spared for all the map-makers of this oceanic world (the theoretical physicsists of the last thirty years), who, for decades, have built ever more complicated maps showing that Higgs island would not be alone. They had fascinating and compelling arguments for why Higgs island should be surrounded by exotic new islands, completely different to anything we've encountered before, many maps even showed new continents. The map-makers will have built entire careers making those maps, but if the islands and continents turn out not to be next to Higgs island, they're simply not there; however much we thought they should be. Of course, these continents may very well still exist, somewhere out of HMS Large Hadron Collider's range, but the map makers themselves would never get the chance to know.
Monday, July 2, 2012
On its own, a Higgs discovery would be grim
 |
| Rutherford, with the first ever particle collider |
Why the Higgs is cool
If rumours are to be believed, then, in two days time, CERN will announce the discovery of a new particle and it will be called Higgs. To the degree that the discovery of any new particle is a pretty big deal, this will be a pretty big deal.
To put things into perspective, not only will this be the discovery of an entirely new particle, if the standard model of particle physics is correct, this will also be the discovery of an entirely new fundamental particle. That is, it won't be made up of any constituent pieces. Also, the field that it will be excited from will not have been directly detected ever before. And that's not even it. Other aspects of the Higgs are also completely new. For example, the way it behaves when you rotate it will be unique amongst all the fundamental particles we've discovered so far, which is quite curious because its rotational properties will be the simplest (i.e. it has no spin at all).
So, irrespective of everything I'm about to write I want to first stress the following: the discovery of a Higgs-like particle is pretty damn cool and a great achievement of exploration for humanity.
Beyond the hype
However, the Higgs is no God particle and it is not the origin of all the mass in the universe (or even a significant proportion of it). No great mysteries of the universe are about to be solved on Wednesday. The Higg's significance in our understanding of the universe is similar to the understanding gained when the last piece of a jigsaw is finally placed in a puzzle. Placing that last piece produces an enormous amount of cathartic pleasure (more so than any other individual piece). But, the image in the puzzle has become clear long before that final piece is placed. The role the Higgs plays in the standard model of particle physics is to break a certain symmetry in nature, the electroweak symmetry. All the other pieces of this broken symmetry have been found, some quite a long time ago.
Monday, June 25, 2012
The Higgs: To be, or not to be?
[Note from Shaun: The following is a guest post from Higgs Hunter, Mikko Voutilainen. Mikko is a colleague of mine here in Helsinki. He is a postdoc working on the CMS experiment at the LHC in CERN. Below, he rhetorically asks the Higgs boson whether it exists or not. The irony of this is that Mikko asks this question, non-rhetorically, for a living and it is quite possible that he has already received an answer. He cannot (unfortunately) tell us this answer, yet. You should consider the following a teaser for what will follow next Wednesday when CERN unveils its latest results to the world. On that date, Mikko has promised to give us another guest post where he will reveal everything he knows about, The Higgs... (I've even heard rumours that this follow-up post has already been written)]
To be, or not to be?
That's a question for the Higgs boson to answer, and we might know soon enough. CERN just (well, three days ago really, but everybody here was out in the countryside celebrating Midsummer) published a press release about having a seminar on the new results on Wednesday 4th of July.
Coincidence that it's also Independence Day for the folks in the US? Probably yes, although my collaboration, the Compact Muon Solenoid (CMS) experiment at CERN, does have a strong representation from the States, including our spokesperson Joe Incandela.
The real reason, though, is that the 4th of July is also the eve of a major particle physics conference, ICHEP, starting in Melbourne. The ATLAS and CMS experiments will deliver the preliminary results of their 2012 data analysis there, and the seminar will be a kickoff for these presentations (you can see the live broadcast at webcast.cern.ch).
The experiments at the Large Hadron Collider stopped collecting data only on the 18th of June, and everybody is now busily analysing this dataset. We actually collected quite a nice bunch of data, just over 6/fb, which is a bit better than last year. The collision energy was also raised from 7 TeV to 8 TeV, which should increase the production rate of possible Higgs bosons by 20--30%.
People are really eager to see the new results, and for a reason. The data collected in 2011 showed some hints of a Higgs boson in the 124-126 GeV range. The amount of data collected this year is nearly equal to that collected last year so the results are directly comparable. We should be able to see whether the earlier trends are still there, or whether they've gone away. Either way, it should be pretty exciting.
The predictions made earlier indicate that a combination of the 2011 and 2012 datasets should get pretty close to five sigma, the traditional standard for a discovery in the field. Or, we should be able to rule the existence of the Higgs boson out at a 95% confidence level from the whole remaining mass window.
What happens in a week depends both on the hard work of the physicists, who are improving the sensitivity of their analysis, and, due to statistical fluctuations, pure luck. If we're unlucky, the existence of the Higgs boson may still remain a mystery, but if we're lucky, we might end the quest earlier than expected.
So, what if we find the Higgs or not? Is it the answer to Life, the Universe, and Everything? Or a piece in the puzzle of the origin of mass for the elementary particles? The latter, more likely.
If we find that the Higgs boson lacks existence, much of the theoretical work done in particle physics for the past few decades will end up in the dustbin. It's not all that bad, really, because it will allow the theorists to start from a clean slate, and that's often been a very fruitful thing. The experimentalists will continue to hunt for other particles that could replace the Higgs boson.
If the Higgs boson is found, it's properties will have to be scrutinized carefully. There are many theories out there besides the Standard Model of particle physics that predict the Higgs boson (or bosons) so determining it's precise identity might take a while. Many of the alternative theories also predict other particles, leaving plenty of work to be done for the experimentalists.
[Note: Mikko writes for a Finnish language blog, Higgs Hunters. This post is an English translation of his latest post at Higgs Hunters.]
To be, or not to be?
That's a question for the Higgs boson to answer, and we might know soon enough. CERN just (well, three days ago really, but everybody here was out in the countryside celebrating Midsummer) published a press release about having a seminar on the new results on Wednesday 4th of July.
Coincidence that it's also Independence Day for the folks in the US? Probably yes, although my collaboration, the Compact Muon Solenoid (CMS) experiment at CERN, does have a strong representation from the States, including our spokesperson Joe Incandela.
The real reason, though, is that the 4th of July is also the eve of a major particle physics conference, ICHEP, starting in Melbourne. The ATLAS and CMS experiments will deliver the preliminary results of their 2012 data analysis there, and the seminar will be a kickoff for these presentations (you can see the live broadcast at webcast.cern.ch).
The experiments at the Large Hadron Collider stopped collecting data only on the 18th of June, and everybody is now busily analysing this dataset. We actually collected quite a nice bunch of data, just over 6/fb, which is a bit better than last year. The collision energy was also raised from 7 TeV to 8 TeV, which should increase the production rate of possible Higgs bosons by 20--30%.
 |
| The amount of data collected in 2010, 2011 and 2012. One fb-1 amounts to almost 100 trillion proton-proton collisions. |
People are really eager to see the new results, and for a reason. The data collected in 2011 showed some hints of a Higgs boson in the 124-126 GeV range. The amount of data collected this year is nearly equal to that collected last year so the results are directly comparable. We should be able to see whether the earlier trends are still there, or whether they've gone away. Either way, it should be pretty exciting.
The predictions made earlier indicate that a combination of the 2011 and 2012 datasets should get pretty close to five sigma, the traditional standard for a discovery in the field. Or, we should be able to rule the existence of the Higgs boson out at a 95% confidence level from the whole remaining mass window.
 |
| Predictions for the significance of a Higgs signal as a function of the boson mass. The combination of 2011 (5 fb-1, 7 TeV) and 2012 (5 fb-1, 8 TeV) data will correspond to roughly the average of the two red lines. |
What happens in a week depends both on the hard work of the physicists, who are improving the sensitivity of their analysis, and, due to statistical fluctuations, pure luck. If we're unlucky, the existence of the Higgs boson may still remain a mystery, but if we're lucky, we might end the quest earlier than expected.
So, what if we find the Higgs or not? Is it the answer to Life, the Universe, and Everything? Or a piece in the puzzle of the origin of mass for the elementary particles? The latter, more likely.
If we find that the Higgs boson lacks existence, much of the theoretical work done in particle physics for the past few decades will end up in the dustbin. It's not all that bad, really, because it will allow the theorists to start from a clean slate, and that's often been a very fruitful thing. The experimentalists will continue to hunt for other particles that could replace the Higgs boson.
If the Higgs boson is found, it's properties will have to be scrutinized carefully. There are many theories out there besides the Standard Model of particle physics that predict the Higgs boson (or bosons) so determining it's precise identity might take a while. Many of the alternative theories also predict other particles, leaving plenty of work to be done for the experimentalists.
[Note: Mikko writes for a Finnish language blog, Higgs Hunters. This post is an English translation of his latest post at Higgs Hunters.]
Monday, June 18, 2012
The human machine: pistons and ratchets
In
my last post I talked about how we are all powered by tiny spinning
motors that work tirelessly to convert the chemical energy of our food into
electrical potential energy, and then back into chemical energy in the form of
the cellular energy currency, ATP . This week, I thought it would be
interesting to look at how that ATP gets used up in a process that will be very
familiar to you but that you probably know little about: muscle contraction.
As
I mentioned last time, ATP is used by just about every active process that
takes place in your cells. Most of these processes siphon only tiny amounts of
ATP from the ever-replenished pool that is available in your cells – as far as
they’re concerned there is an infinite amount of ATP available because they
could never use is all up on their own. For this reason, a lot of our cellular
machinery is, frankly, wasteful. The mechanisms that regulate DNA repair, movement of organelles within cells, and many other processes
consume ATP with gay abandon because their impact is so miniscule on the total
energy consumed by your body as a whole. There has been no need to evolve more
cost-efficient mechanisms, such as those employed by our single-celled relatives
for whom every ATP counts!
For
some components of the human machine, however, their impact on total ATP
consumption is far from insignificant. One often overlooked example is neural transmission, which is why the brain consumes around 20%
of your total calories on an average day, and why you may find yourself famished after a
tough exam even though you’ve just been sat down for three hours! Nonetheless,
the clear winner in the ATP-expenditure competition is muscle contraction. Your
muscles are responsible for using nearly 60% of your total calories on an average
day – potentially far more if you’re very active (or very muscular!). Motile
animals have to consume far more energy to survive than non-motile animals or
plants, and this is primarily down to their muscles. Fortunately, this is very
much worth it because being able to move gives you far broader options in terms
of finding food in the first place. Moreover, because of its huge significance
in terms of total energy consumption, the molecular basis of muscle contraction
has evolved to be a highly efficient affair. In fact, in terms of work achieved
per ATP used, muscle contraction is one of the most efficient processes in your
body – it’s simply the fact that it’s used on a huge scale that makes it such
an ATP-hungry mechanism.
Monday, June 11, 2012
3 Quarks Daily's science writing prize. (Vote for us)
I nominated my favourite of James' posts (The War of the Immune Worlds) for the science writing prize at 3 Quarks Daily. If you want to support the blog, it would be great if you would go and vote for it!
To vote, click this link, scroll down to "The Trenches of Discovery" and click away!
If you're super cool, you'll also try to convince your friends and family to vote for James' post. This first round will basically be a popularity contest. There are 100 odd entries and nobody is going to read them all. The top 20 in this round go through to the next round where the 3 Quarks Daily editors actually read and vote on every entry. So, by helping James' post get to this next round you help us to compete against the bigger, more established blogs who've been around much longer than us.
In any case, it is actually a really cool post, so, while you'll clearly be doing us a favour by sharing this post with others, you'll also be doing a favour to anyone who ends up reading it too. In that vein, if you haven't read the post yet yourself, go check it out, it's really cool.
Voting closes June 15 so vote now!
To vote, click this link, scroll down to "The Trenches of Discovery" and click away!
If you're super cool, you'll also try to convince your friends and family to vote for James' post. This first round will basically be a popularity contest. There are 100 odd entries and nobody is going to read them all. The top 20 in this round go through to the next round where the 3 Quarks Daily editors actually read and vote on every entry. So, by helping James' post get to this next round you help us to compete against the bigger, more established blogs who've been around much longer than us.
In any case, it is actually a really cool post, so, while you'll clearly be doing us a favour by sharing this post with others, you'll also be doing a favour to anyone who ends up reading it too. In that vein, if you haven't read the post yet yourself, go check it out, it's really cool.
Voting closes June 15 so vote now!
Monday, June 4, 2012
The 2012 Transit of Venus
 |
| David Peck Todd, photograph of the 1882 Transit of Venus |
- 1631 (not witnessed) & 1639
- 1761 & 1769
- 1874 & 1882
- 2004 & 2012
Basically when Venus crosses the Sun we know that [Venus], the Sun and the Earth are all in a straight line. Very slight differences in the viewing angle from two observers on the Earth can then be used along with our basic knowledge of trigometry to measure the distance to the Sun. For over 100 years, the distance to the Sun measured this way was the most accurate measurement we had.
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