A quick note: I’ve had a number of questions from commenters about whether the new Higgs-like particle really has spin 0 (as it must if it is truly a Higgs particle) or whether it might have spin 2. Well, spin 2 (with positive parity) is now strongly disfavored, as a result of new results from the ATLAS and CMS experiments at the Large Hadron Collider. CMS has disfavored it at the 98.5-99.9% confidence level (the number depending on assumptions about whether the particle is produced in collisions of gluons or in collisions of a quark and anti-quark) using their data from the particle’s decays to two lepton/anti-lepton pairs. ATLAS has disfavored it at the 95%-99% confidence level (similarly depending on assumptions) using their data from decays of the new particle to a lepton, anti-lepton, neutrino and anti-neutrino. Meanwhile, there is no reason for a spin-2 particle (especially with negative parity) to have the relative decay probabilities that are observed in the data, so the fact that all these probabilities are similar to those of a simple Higgs particle disfavors spin 2 and favors spin 0. And there’s simply no theory of a spin-2 particle (with either parity) that doesn’t have other observable particles rather nearby in mass. No one of these arguments is definitive, but in combination they are pretty convincing.
Meanwhile all the data is consistent with a spin 0 particle with decay probabilities roughly similar to that of a Standard Model Higgs (the simplest type of Higgs particle.)
So let’s stop spending much bandwidth on spin 2: it is disfavored by both ATLAS and CMS — directly by measurement of the particle’s spin, and indirectly via its relative probabilities to decay to various types of particles — and it is disfavored theoretically. The more important measurement is to check whether this apparently spin-0 particle really has positive parity, or whether it has a mix of positive and negative parity.
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.
The rate for events with two lepton/anti-lepton pairs (data from CMS is the black dots, with uncertainties given by the black bars) as a function of the mass-energy of a particle that might have produced them; the Z particle is the bump around 90 GeV. The bump near 125 GeV due to the Higgs-like particle is now difficult to miss, even if one ignores the blue and red lines which are there to guide the eye.
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.
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.
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
One or two more segments to go.
Day 2 of the Higgs Symposium is flying by, with very interesting presentations… and with little time for me to finish the last details of my own talk for tomorrow. (Tomorrow’s program includes a talk by Professor Peter Higgs himself!) But here’s a quick summary.
In my last post I mentioned a couple of the early talks; here’s a bit more about the later talks from yesterday, and then a bit about the first part of today. Caveat: all descriptions below are brief and necessarily incomplete! Continue reading
I’m enjoying my first visit to Edinburgh, Scotland, United Kingdom, as a visitor at the new Higgs
CenterCentre (it is the UK after all) for Theoretical Physics, recently founded in honor of Professor Peter Higgs and of the discovery of a candidate Higgs particle at the Large Hadron Collider (LHC). Today is the first day of the Higgs Symposium, a three-day workshop organized by the centre (and co-sponsored by the IPPP) that is celebrating the history of and the science behind (and ahead) of the recent discovery. (I’ll be speaking on Friday.)
There was a quick introduction by Richard Ball, the cent
re’s director — who was the first one to show a photo of a blackboard containing the equations of Higgs’ work, along with an great old photo of Peter Higgs from the days when he was writing his famous papers, dug out of the records at the University of Edinburgh. (Experts: For a Higgs doppelganger, see here.) Then a set of hour-long talks began. The first of these was a wonderful historical talk, looking back over 50 years, by Chris Llewellyn Smith. Professor Llewellyn Smith played a significant role in the Higgs discovery, with his contributions ranging from showing in the 1970s why a Higgs particle is necessary if quantum field theory (the types of equations we use today to describe particles) is correct, to pushing for the LHC to be built while head of the CERN Council and then as Director General of CERN in the 1990s. If time permits, I’ll may describe later a few of the fascinating historical twists that he described — though I’m afraid that most of them would be of interest mainly to experts in the field. For the moment, those interested may want to read his article that appeared in the journal Nature in 2007, entitled “How the LHC came to be“, which covers some related issues.
Following this we have so far had talks by Joe Incandela (spokesman of the CMS experiment) and Eilam Gross (co-convener of the Higgs search group at ATLAS) summarizing the experimental situation. As expected, there wasn’t anything new announced here; the talks involved an overview (for the audience of mostly theorists, including quite a few students) of how the measurements are done, and a review of previously announced results. I’ll describe a few interesting details of their talks later today or tomorrow.