The big news overnight for science was the best measurement yet of the Cosmic Microwave Background [CMB], by the Planck Satellite. The CMB consists of microwave photons (particles of light with microwave wavelengths) that are the tell-tale leftover glow from the universe’s hot period, the Big Bang. These photons are almost entirely uniform across the sky, and consistent with a glowing object of temperature 2.7 degrees Kelvin (or Centigrade) [poorly written] above absolute zero, the temperature where everything moves as slowly as allowed by quantum mechanics. (Note added: A change of 1 degree Kelvin is the same as a change of 1 degree Centigrade, but absolute zero is 0° Kelvin and -273.15° Centigrade. Centigrade and Celsius are the same.) But they aren’t quite uniform! And those slight non-uniformities, which speak volumes about the universe, have now been read with the greatest precision ever achieved.
The Cosmic Microwave Background – as seen by Planck and its predecessor, WMAP. Credit: ESA and the Planck Collaboration; NASA / WMAP Science Team
Today my chores prevent my writing a proper post, and it doesn’t help that Planck released over a dozen papers overnight… it will take a while to sift through this. But the bullet points that everyone is talking about are (more…)
As I think most of us in the field expected, professor Alexander Polyakov was selected from among the nominees as the winner of a cool $3 million checkFundamental Physics Prize today. (more…)
One of the big challenges facing journalists writing about science is to summarize a scientific subject accurately, clearly and succinctly. Sometimes one of the three requirements is sacrificed, and sadly, it is often the first one.
So here is my latest (but surely not last) attempt at an accurate, succinct, and maybe even clear summary of why the Higgs business matters so much.
`True’ Statements about the Higgs
True means “as true as anything compressed into four sentences can possibly be” — i.e., very close to true. For those who want to know where I’m cutting important corners, a list of caveats will follow at the end of the article.
Our very existence depends upon the Higgs field, which pervades the universe and gives elementary particles, including electrons, their masses. Without mass, electrons could not form atoms, the building blocks of our bodies and of all ordinary matter.
Last July’s discovery of the Higgs particle is exciting because it confirms that the Higgs field really exists. Scientists hope to learn much more about this still-mysterious field through further study of the Higgs particle.
Is that so bad? These lines are almost 100% accurate… I’m sure an experienced journalist can cut and adjust and amend them to make them sound better and more exciting, but are they really too long and unclear to be useable?
Professor Michio Kaku, of City College (part of the City University of New York), is well-known for his work on string theory in the 1960s and 1970s, and best known today for his outreach efforts through his books and his appearances on radio and television. His most recent appearance was a couple of days ago, in an interview on CBS television, which made its way into this CBS news article about the importance of the Higgs particle.
Unfortunately, what that CBS news article says about “why the Higgs particle matters” is completely wrong. Why? Because it’s based on what Professor Kaku said about the Higgs particle, and what he said is wrong. Worse, he presumably knew that it was wrong. (If he didn’t, that’s also pretty bad.) It seems that Professor Kaku feels it necessary, in order to engage the imagination of the public, to make spectacular distortions of the physics behind the Higgs field and the Higgs particle, even to the point of suggesting the Higgs particle triggered the Big Bang.
In doing this, Professor Kaku sows confusion among journalists and the public, and undermines the efforts of serious particle physicists to explain and convey, both vividly and accurately, the science and the excitement of our time. And on what grounds does he justify this? Doesn’t the taxpaying public deserve the truth? Isn’t the truth already exciting enough? And what will the public think of science if, in this information era, the promulgation of falsehoods and near-falsehoods on national media is unanswered by complaints from other scientists?
I’m so frustrated with Professor Kaku’s unfortunate remarks that rather than write more today, I’ll simply direct you to Sean Carroll’s blog — Sean’s response was much more measured and polite than mine would be if I spoke my mind. For now I’ll just conclude by suggesting that Professor Kaku has some serious explaining to do — to his scientific colleagues, to the science journalist that he misled, and to the public.
(Perhaps you will ask me the same question: “Why DOES the Higgs particle matter?” Here’s my own article from July giving the answer; it’s short and condensed, but it’s not false, as my colleagues will attest! For a longer explanation with more details and fewer shortcuts, you can try Sean Carroll’s book or Lisa Randall’s book, or you can poke around on my website for various related articles; there’s the Higgs FAQ, the story of the Higgs discovery, an article on why the Higgs is not related to gravity, or if you’re really ambitious you can try this set of articles [which requires you first read this set] which is suitable for people who once took a little first-year college physics.)
Since it’s been the main news story of the last week, perhaps it would be useful to do a quick summary of what the CMS and ATLAS experiments at the Large Hadron Collider [LHC] have been saying, over the past fifteen months, about their search for the process in which a Higgs particle is produced and decays to two photons.
Before we start, let me remind you that in statements about how uncertain a measurement is (and all measurements have some level of uncertainty — no knowledge is perfect), a “σ”, or “sigma”, is a statistical quantity called a “standard deviation”; a 5σ discrepancy from expectations is impressive, 3σ intriguing; but anything less than 2σ is very typical, and indicative merely of the usual coming and going of statistical flukes and fluctuations of real data around the truth. Note also that the look-elsewhere effect has to be accounted for; but usually a 5σ discrepancy without the look-elsewhere effect is enough to be convincing. And of course a discrepancy may mean either a discovery or a mistake; that’s why it is important that two experiments, not just one, see a similar discrepancy, since it is unlikely that both experiments would make the same mistake.
Ok: here are the results as they came in over time, all the way back to the inconclusive hints of 15 months ago.
December 2011:
ATLAS (4.9 inverse fb of data at 7 TeV): excess 2.8σ (where 1.4σ would be expected for a SM Higgs); less than 2σ after accounting for “look-elsewhere effect”.
CMS: (4.8 inverse fb of data at 7 TeV): excess just over 2σ (where 1.4σ would be expected for a SM Higgs); much less than 2σ after accounting for “look-elsewhere effect”.
July 2012:
ATLAS: (reanalyzing the 7 TeV data and adding 5.9 inverse fb of data at 8 TeV): signal 4.5σ (where 2.4 was expected for a SM Higgs); 3.6σ after “look-elsewhere effect”; best estimate of size of signal divided by that for a SM Higgs: 1.9 ± 0.5 (about 1.8σ above the SM prediction)
CMS (reanalyzing the 7 TeV data and adding 5.3 inverse fb of data at 8 TeV): signal 4.1σ (where 2.5 was expected for a SM Higgs); 3.2σ after “look-elsewhere effect”; best estimate of size of signal divided by that for a SM Higgs: 1.6 ± 0.4 (about 1.5σ above the SM prediction)
November/December 2012:
ATLAS: (increasing the 8 TeV data to 13.0 inverse fb): signal 6.1σ (3.3 expected for SM Higgs); 5.4σ when look elsewhere is accounted for; best estimate of size of signal divided by that for a SM Higgs: 1.8 ± 0.4 (about 2σ above the SM prediction)
CMS: No update
March 2013:
ATLAS: (taking the full 7 and 8 TeV data sets): 7.4σ (4.1 expected for a SM Higgs); best estimate of size of signal divided by that for a SM Higgs: 1.65 ± 0.30 (slightly more than 2σ above the SM prediction)
CMS: (taking the full 7 and 8 TeV data sets) uses two different methods as a cross-check, one of them complex and (on average) more powerful, the other simpler but (on average) less powerful. For the best estimate of size of signal divided by that for a SM Higgs: one method gives 0.8 ± 0.3 and the other gives 1.1 ± 0.3. Both of these are within 1σ of the SM prediction and within 2σ of the CMS July result.
To understand how consistent the two new CMS results are with each other, you have to consider how the two studies are correlated (since they are selecting events for study from the same pile of data.) Because the two methods select and discard candidate events in two different ways, they don’t include the exact same data. CMS’s simulation studies indicate that about 50 percent of the background events and 80 percent of the signal events are common to the two studies. In the end, the conclusion (see the figure below) is that the two results are consistent at 1.5σ (and at 1.8 if one considers only the 8 TeV data) — in other words, reasonably consistent with one another.
You can also ask how consistent are the new results compared to the old ones from July. When you observe that the uncertainty on the July result was very large (1.6 ± 0.4 times the Standard Model prediction, i.e. a 25% uncertainty at 1σ, 50% uncertainty at 2σ) it should not surprise you that CMS claims that their new results are both consistent with the old ones at below the 2σ level.
Slide from Moriond-QCD conference talk presenting CMS’s results, and looking at the compatibility of the two results with each other (top two lines in the table) and of each of the two new results with the previous published results. Note the conclusion in the last line.
Meanwhile, all of the ATLAS results are closely compatible with each other. This is more what one would naively expect, but not necessarily what actually happens in real data. Of course ATLAS’s results aren’t giving a consistent mass for the new particle yet, whereas CMS’s are doing so… well, this is what happens with real data, folks.
The real issue is whether ATLAS’s measurements and CMS’s measurements of the two photon rate are compatible with each other. Currently they are separated by at least 2σ and maybe as much as 3σ (a very rough estimate), which is not unheard of but is somewhat unusual. Well, whether the cause is an error or a statistical fluke or both, it unfortunately leaves us in a completely ambiguous situation. On the one hand, CMS’s results agree with the Standard Model prediction to within about 1σ. On the other hand, ATLAS’s results are in tension with the Standard Model prediction by a bit more than 2σ. We have no way to know which result is closer to the truth — especially when we recall that the uncertainty in the Standard Model prediction is itself about 20%. If ATLAS and CMS had both closely agreed with the Standard Model we’d be confident that any deviations from the Standard Model are too small to observe; if they both significantly disagreed in the same way, we’d be excited about the possibility that the Standard Model might be about to break down. But with the current results, we don’t know what to think.
So as far as the Higgs particle’s decays to two photons, we’ve gotten as much (or almost as much) information as we’re going to get for the moment; and we have no choice but to accept that the current situation is ambiguous and to wait for more data in 2015. Of course the Standard Model may break down sooner than 2015, for some other reason that the experimenters have yet to uncover in the 2011-2012 data. But the two-photon measurement won’t be the one to crack the armor of this amazing set of equations. (For those who got all excited last July; you were warned that the uncertainties were very large and the excess might well be ephemeral.)
As soon as the discovery of that famous new particle was announced at the Large Hadron Collider [LHC] last year, there were already very good reasons to think it was a Higgs particle of some type. I described them to you back then, as part of my “Higgs Discovery” series. But, as I cautioned, those arguments relied partly on data and partly on theoretical reasoning.
Over the past nine months, with additional data collected through December and analyzed through the present day, it has become more and more convincing that this particle behaves very much like a Higgs particle, along the lines I described following the Edinburgh conference back in January. One by one the doubters have been giving up, and few remain. This is a Higgs particle. That’s my point of view (see last week’s post — you heard it here first), the point of view of most experts I talk to [in a conference I’m currently attending, not one person out of about forty theorists and experimenters has dissented], and now the official point of view of the CERN laboratory which hosts the LHC. (more…)
