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.
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.
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
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.
By almost all measures, the Higgs Symposium at the University of Edinburgh, as part of the new Higgs Centre for Theoretical Physics, was a great success. The only negative was that Professor Peter Higgs himself had a bad cold this week, and had to cancel his talk, as well as missing the majority of the talks by others. Obviously all of us in attendance were very disappointed not to hear directly from him, and we wish him a speedy recovery.
Other than this big hole in the schedule, the talks given at the symposium seemed to me to form a coherent summary of where we are right now in our understanding of the Higgs field and particle. They were full of interesting material, and wonderfully complementary to one another. This motivates me to try to provide, for non-experts, some future articles on what the conference attendees had to say. But to write such articles well takes time. So for now, here’s the quick version summarizing the last few talks, along the lines of the summaries I wrote (here and here) of the earlier talks. The slides from all the talks are posted here.
Here we go: Continue reading
Posted in Higgs, LHC Background Info, LHC News, Particle Physics, Quantum Gravity, String Theory
Tagged ExtraDimensions, gravity, Higgs, LHC, LHCb, supersymmetry