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 check Fundamental Physics Prize today. (more…)
Category: Particle Physics
POSTED BY Matt Strassler
ON March 20, 2013
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.
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.)
POSTED BY Matt Strassler
ON March 18, 2013
A busy week and a computer crash has delayed my report on a number of new results on the Higgs particle from the current Moriond conference on particle physics, but the quiet not only on my blog but on some others should be a clue: the new results shown do not significantly change what we have previously known, and to the extent they do, they do not point to anything unexpected.
As a summary before I mention a few details, let me say that all in all, I think it is pretty safe now to award the Nobel prize to the theoretical physicists behind this story; last year was too early, but this year is not. Confidence is steadily growing that this “Higgs-like” particle really is a type of Higgs (Brout-Englert) (Guralnik-Hagen-Kibble) boson [what’s a boson?], and most alternatives are now significantly disfavored. Whether it is the one and only type of Higgs particle in nature, and whether it is exactly of Standard Model type (the simplest possible type of Higgs particle), we cannot yet be sure, but its properties are more or less in line with what Higgs and friends proposed, enough to give them credit for having correctly imagined (to greater and lesser degrees) how nature might provide mass to force-carrier particles like the W and Z particles, and how we might test this notion experimentally. We should also remember some theorists who came before them and some who came after, but that’s a story for another day.
POSTED BY Matt Strassler
ON March 8, 2013
As many of you will have already noticed, today’s Science Times section of the New York Times newspaper is devoted to articles by Dennis Overbye on the search for the Higgs particle. At first read, the articles seem pretty good; several key players are interviewed (though inevitably, given page constraints, a number of important players in the experiments are not mentioned) and the science seems mostly accurate, with a few small errors, omissions, or misleading ways of saying things in the glossary and elsewhere. I’m busy preparing a new public talk for tomorrow, so I’ll have to reserve any detailed comments for later in the week.
But one thing you will notice, if you read the long article which describes the ins and outs of the search process, is that several of the responsible scientists quoted indicate, directly or indirectly, that the December 2011 data did not convince them that a Higgs particle had yet been found. That was the position I took on this blog, and I reported to you that most responsible scientists I had spoken to (which didn’t happen to include any of the ones quoted in the Science Times today) viewed the December data as inconclusive — meaning that it was still quite possible that the apparent signal of a Higgs particle might evaporate. Almost every other major particle physics blogger disagreed with me, both on my opinion and on my characterization of others’ opinions. But I stand by my statements: that though the data reported in July 2012 was essentially definitive, the data in December 2011 was, not only from my perspective but from that of many serious scientists, suggestive yet inconclusive. And you can now read that in the New York Times.
POSTED BY Matt Strassler
ON March 5, 2013
The Structure of Matter series continues: last week’s article on the basics of atomic nuclei is now supplemented with an article discussing the “residual” strong nuclear force which binds protons and neutrons inside of nuclei. It further explains why nuclei are so small compared to atoms. Or rather, it explains it in part, because I have to also explain why protons and neutrons themselves are so small — which I will do soon enough.
As always, readers are encouraged to comment on things that don’t seem clear or correct. And any nuclear physics experts who want to weigh in on my presentation — suggesting how it might be improved or extended, or identifying misconceptions on my part — are encouraged to speak up (publicly or privately as you prefer).
Meanwhile, we’re entering the March conference season, when many new results from the Large Hadron Collider [LHC] (based on analysis of last year’s data) and from other important experiments will start appearing. Since the LHC’s proton-proton collisions went til December in 2012 (in 2011 they stopped in October) the time for LHC data analysis has been rather short. I therefore think it likely that any really surprising results from the LHC will be delayed for extra scrutiny — and may not appear until late spring or summer, when there are other conferences. But I could be wrong! And one thing we’re all waiting for is the measurement by CMS (one of the two general purpose LHC experiments) of the rate for the recently discovered Higgs particle to decay to two photons. However, we won’t see that result until CMS is absolutely confident in it.
POSTED BY Matt Strassler
ON March 4, 2013
Posts have been notably absent, due mainly to travel with very limited internet; apologies for the related lack of replies to comments, which I hope to correct later this week.
Meanwhile I’ve been working on a couple of articles related to the nuclei of atoms, part of my Structure of Matter series, which serves to introduce non-experts to the basics of particle physics. The first of these articles is done. In it I describe why it was so easy (relatively speaking) to figure out that nuclei are made from certain numbers of protons and neutrons, and how it was understood that nuclei are very small compared to atoms. Comments welcome as always!
A related article, which should appear later this week, will clarify why nuclei are so tiny relative to atoms, and describe the force of nature that keeps them intact.
POSTED BY Matt Strassler
ON February 25, 2013
A quick reminder that tonight at 6 Pacific/9 Eastern, Sean Carroll and I will be interviewed by Alan Boyle on the online radio show “Virtually Speaking Science”. Topics will cover the LHC and other hot issues in physics, astrophysics, gravity and cosmology, as well as the scientific process. See Monday’s post for the link to the show and other details.
Continuing my more careful summary of the Higgs Symposium (held January 9-11 at the University of Edinburgh, as part of the new Higgs Center for Theoretical Physics), and improving on my quick blog posts that I put up during and just after the symposium (#1, #2 and #3), I’ve finished another article about our current knowledge and ignorance concerning the recently discovered Higgs-like particle. The new article
covers a topic that I spoke about extensively at the Symposium. The other completed articles in this series are
- 0. Introduction: Taking Stock of the “Higgs”
- 1. Is the Newly Discovered Particle a Higgs Particle of Some Type?
- 2. If the New Particle is a Higgs Particle, Might it be Composite?
- 3. Might the New Particle be a Higgs of the Simplest Possible Type (the Standard Model Higgs)?
- 4. Might there be more than one type of Higgs particle? Has our recent discovery been just the first of several that are still to come?
- 5. Supersymmetry: What Does The New Particle Teach Us About This Possibility?
One or two more segments to go.
POSTED BY Matt Strassler
ON February 6, 2013
On Wednesday February 6th, at 9 pm Eastern/6 pm Pacific time, Sean Carroll and I will be interviewed by Alan Boyle on “Virtually Speaking Science”. The link where you can listen in (in real time or at your leisure) is http://www.blogtalkradio.com/virtually-speaking-science/2013/02/07/sean-carroll-matt-strassler-alan-boyle
What is “Virtually Speaking Science“? It is an online radio program that presents, according to its website:
- Informal conversations hosted by science writers Alan Boyle, Tom Levenson and Jennifer Ouellette, who explore the explore the often-volatile landscape of science, politics and policy, the history and economics of science, science deniers and its relationship to democracy, and the role of women in the sciences.
Sean Carroll is a Caltech physicist, astrophysicist, writer and speaker, one of the founders of the blog Cosmic Variance, who recently completed an excellent popular book (which I highly recommend) on the Higgs particle, entitled “The Particle at the End of the Universe“. Our interviewer Alan Boyle is a noted science writer, author of the book “The Case for Pluto“, winner of many awards, and currently NBC News Digital’s science editor [at the blog “Cosmic Log“].
I was interviewed on Virtually Speaking Science once before, by Tom Levenson, about the Large Hadron Collider (here’s the link). Also, my public talk “The Quest for the Higgs Particle” is posted in their website (here’s the link to the audio and to the slides).
POSTED BY Matt Strassler
ON February 4, 2013
There’s been a lot of reporting recently on a puzzle in particle physics that I haven’t previously written about. There have been two attempts, a preliminary one in 2010 and a more detailed one reported just this month, to measure the size of a proton by studying the properties of an exotic atom, called “muonic hydrogen”. Similar to hydrogen, which consists of a proton orbited by an electron (Figure 1), this atom consists of a proton and a short-lived heavy cousin of the electron, called the muon (Figure 2). A muon, as far as we have ever been able to tell, is just like an electron in all respects except that it is heavier; more precisely, the electromagnetic force and the strong and weak nuclear force treat electrons and muons in exactly the same way. Only the first two of these forces should play a role in atoms (and neither gravity nor any force due to the Higgs field should matter either). So because we have confirmed our understanding of ordinary hydrogen with very high precision, we believe we also understand muonic hydrogen very well also. But something’s amiss. (more…)
POSTED BY Matt Strassler
ON January 31, 2013
Ten days ago I started providing you with a more careful summary of the Higgs Symposium (held January 9-11 at the University of Edinburgh, as part of the new Higgs Center for Theoretical Physics), to make up for the quick blog posts that I put up during and just after the symposium (#1, #2 and #3). The aim was to provide a more coherent overview, appropriate for non-expert readers, of our current knowledge and ignorance concerning the recently discovered Higgs-like particle. Initially I put up about half of the summary, organized as follows
- 0. Introduction: Taking Stock of the “Higgs”
- 1. Is the Newly Discovered Particle a Higgs Particle of Some Type?
- 2. If the New Particle is a Higgs Particle, Might it be Composite?
- 3. Might the New Particle be a Higgs of the Simplest Possible Type (the Standard Model Higgs)?
- 4. Might there be more than one type of Higgs particle? Has our recent discovery been just the first of several that are still to come?
As of today, I have added the following segment, mainly based on Professor Nima Arkani-Hamed’s talk about the implications for supersymmetry of the (presumed)-Higgs particle’s mass and properties .
At least two more segments will follow soon.
POSTED BY Matt Strassler
ON January 28, 2013
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