A couple of us are going to try to live-blog the July 4 Higgs update seminars from CERN. This effort will be subject to the whims of internet connectivity, of course, but we’ll do our best. At the moment we have correspondents on at least three different continents (I [Sean] am at CERN, JoAnne is in Melbourne for ICHEP, and I think John is in California…), so hopefully at least one of us will be able to get through. We’ll just be updating this post, so keep refreshing. You are also welcome to try the CERN webcast.
Seminars proper start at 9am Geneva time (3am Eastern time, midnight Pacific time, 5pm Melbourne time). One from ATLAS, by Fabiola Giannoti, and one from CMS, by Joe Incandela. Then a press conference after. Remember what we’re looking for: how significant is the signal, do the two experiments agree with each other, does the rate agree with the Standard Model prediction, are different channels mutually consistent with each other.
If people ask questions in the comments there is some chance that we will try to answer them.
Has there ever been a scientific discovery (if indeed we will be able to call it that) that has been anticipated so far ahead of time? Can’t think of any off the top of my head. Fasten your seatbelts!
11:38 pm Geneva time (Sean): Preliminary thought #1: There is a “nightmare scenario” that particle physicists have worried about for years. Namely: find exactly the Standard Model Higgs and nothing else at the LHC. I personally assign the nightmare scenario very low probability. Not on the basis of any inside info, just on the basis of physics. We know the Standard Model is not right; there is dark matter, there is dark energy, there is baryogenesis, there are the hierarchy and cosmological constant and strong-CP problems. It can’t be the final answer. Seems to me much more likely that there is interesting physics at the weak scale above and beyond the Higgs, than we just get stuck with a vanilla Standard Model. Beyond this physics-informed prediction, there is the wishful hope that the Higgs itself leads directly to new physics. Most obvious example: in many (most?) models of dark matter as weakly-interacting massive particles, the dominant way that dark matter and ordinary matter interact is through exchange of Higgs bosons. If that’s how nature works, the Higgs is literally a portal from our world to another. This isn’t the end of the show, it’s merely an act break (as we say in the movie biz).
11:44 pm Geneva time (Sean): Preliminary thought #2: I am a mere theorist, and let me be as legitimately humble as I can be right here. Beyond the details of whatever may or may not be found, the LHC is a gargantuan effort undertaken by literally thousands of people over the course of years and in many cases decades. This moment, we hope, is something of a payoff for their perseverance. My hat is off to the experimentalists and engineers and technicians who really made it happen.
11:52 pm (Sean): I’m told that there will be a mirror for the webcast at NOVA (PBS).
12:04 am, Geneva time (Sean): Epsilon past midnight, and apparently people are queueing up already. Not me; I’m headed for bed.
5:34 am (Sean): Good morning, world! Anyone got anything going on today?
5:56 am (Sean): Shameless plug alert: physicist and friend-of-the-blog David Kaplan has been producing a feature-film-length documentary about the LHC and the quest for new physics. It’s called Particle Fever, and it’s almost ready to hit the festival circuit. Follow along at the movie’s Facebook page.
6:53 am (Sean): We’re here! Definitely a rock-concert vibe in the air, as folks have been camping out for a while to get into the auditorium. Doors still not open as yet.
7:00 am (Sean): Full disclosure alert: I’m not here in my capacity as a physicist, but my capacity as “media.” (Or just “rabble,” as Ian Sample puts it.) I’m writing a book, of course (did I mention that? Particle at the End of the Universe), but books don’t send you halfway across the world. I’m here with a tiny camera crew as part of filming a special for NOVA on the LHC and the Higgs. Very early in the process, so we don’t have a title or air date as yet — think six months down the road or so. So we didn’t even try to get in the main auditorium — that should be for the experimentalists and the LHC builders.
Looks like they are letting them in!
7:15 am (Sean): Riot narrowly averted as the delicately-organized queue collapsed, and some latecomers tried to cut in front of 100 people who had been camped out. Order temporarily restored!
7:26 am (Sean): From Facebook:
8:10 am (Sean): Had to get press credentials, which involved dashing to the registration building and back, sweet-talking a security guard to let us through a door we weren’t supposed to be going through. Guard: “Why is everyone in a hurry today?”
8:26 am (Sean): Some folks have mentioned, and it’s worth repeating: we call it the “Higgs boson,” but Peter Higgs wasn’t the only one to invent the whole idea back in 1964. This was before electroweak unification, and the issue on people’s minds was whether a broken symmetry necessarily implied massless bosons, as Goldstone’s Theorem would have you think. Massless bosons are phenomenologically bad, because they give rise to long-range forces we don’t see. (QCD is an exception, but that understanding lay in the future.) In 1963 Phil Anderson argued on the basis of an analogy with superconductivity that the massless Goldstone boson could combine with a massless gauge boson to make a massive gauge boson, which is exactly right. But he didn’t have a scalar-field model, and he didn’t speak the relativistic language of particle physicists. So in 1964 three groups came out with relativistic models: one paper by Francois Englert and Robert Brout; two papers by Peter Higgs; and one paper by Gerald Guralnik, Carl Hagen, and Tom Kibble. Those six people shared the Sakurai Prize in 2010 for their work.
Most amusingly of all, because people cared most about getting rid of massless bosons, they didn’t put a lot of emphasis on the extra massive boson we now call “the Higgs.” It was Higgs himself that did draw attention to that in his second paper — and then only because the paper was rejected the first time he submitted it, and he wanted to beef it up a bit before resubmitting to a new journal. That beefing-up was the first explicit discussion of the Higgs boson.
8:51 am (John) Got it up on Evo at home – almost midnight here in California!
8:55 (Sean): Peter Higgs shakes hands with Francois Englert — loud applause.
9:01 am (John)Actually we have both feeds up…cool!
9:04 (Sean): Festivities begin. Master of ceremonies is Rolf Heuer, Director General of CERN.
9:08 am (Sean): Good point that we already know something about what/where the Higgs should be, from indirect measurements.
9:10 am (Sean): They’re looking at five different modes for the Higgs to decay into: bottom/antibottom, tau/antitau, WW, ZZ, two photons. Consistency (and amplitude for each) will be key.
9:16 am (John): And how is the press absorbing all this Sean?
9:17 (Sean): Subtext here: particles in your detector don’t come with little labels telling you what they are, much less how they produced. Remarkable efficiency in identifying particles.
9:23 am (Sean): Press is a little chatty, frankly. 🙂
9:29 am (John): Should have dwelled on the money plot!! That was a nice view of the peak.
9:30 am (Sean): The big bump shown by Joe was in two-photon events (I think … hard to blog and watch). Those are only about 0.2% of Higgs decays, but they stand out above background quite well, unlike events with lots of jets.
9:32 am (Sean): Next-cleanest channel (after two photons) are events with four charged leptons, which come from making two Z’s, each of which decays into electron/positron or muon/antimuon. That’s even more rare, but again extremely clean.
9:34 am (Sean): I think any mention of “sigmas” thus far (four point something) is only for the two-photon channel! Haven’t mentioned combining yet…
9:36 am (Sean): Seeing something in four leptons, maybe 3.2 sigma.
9:37 am (Sean): Combining two photons and ZZ: five sigma! Consistent with 125 GeV Higgs. Applause. (Not in the press room.)
9:39 am (Sean): Now onto two W bosons. The best such events is when each W decays to a charged lepton and a neutrino. But that’s still not at all easy, because the neutrinos themselves are not detected; have to add up the energy and work backwards.
9:41 am (Sean): Slight excess in WW, a bit below expectation (just as ZZ was), but apparently not too much. Small statistics.
9:42 am (Mark): It is ridiculously early here on the East coast of the US, but I’m delighted to have been awake for the important plot, and the audience reaction to the mention of five sigma!
9:43 am (Sean): Now looking at decays to a bottom quark and an anti-bottom. It’s the most common Higgs decay, but very easy to get lost in the background.
Adding channels thus far: 5.1 sigma! (Five sigma, of course, is the informal threshold for “discovery.”)
9:45 am (Sean): And now for decays into a tau lepton and an anti-tau. Another tough one to pick out over the background. Joe is surprised that they did as well as they did.
And … no sign of a Higgs in that channel! Very small significance, but potentially a very intriguing result. Could mean that we have something Higgs-like, but not precisely the Standard Model Higgs.
9:46 am (John): First surprise – where are the tau pair decays?
9:48 am (Sean): Total significance: 4.9 sigma. It went down because of the absence of tau decays. But that could secretly be good news!
Mass = 125.3 plus/minus 0.6 GeV.
9:49 am (Sean): Huge question ongoing: are we seeing a standard Higgs with a couple of statistical fluctuations, or are differences in different channels the sign of something new? Easiest way to make different channels mismatch is to add new particles to your theory that couple to the Higgs, and enter as virtual particles that modify different decay rates. Full employment for both experimentalists and theorists!
9:54 am (Mark): Expect a lot of theory papers in the very near future discussing possible explanations for the missing tau decays.
9:56 am (Sean): ATLAS is going to stick to the two-photon channel and the four-charged-lepton channel, the two most precise ones available. They won’t try to make sense of the messy channels right now. “Not mature enough to be presented today.”
9:59 am (Sean): “Pile-up” refers to the fact that the LHC collides bunches of protons, not just individual particles; at every crossing they get 30 collisions, and need to disentangle them. They weren’t expecting nearly so many collisions.
10:01 am (Sean): “Trigger”: for non-experts, there are far too many collisions and far too much data per collision to possibly save all the data to disk. The experiments throw out something like all but one out of a million events. Not randomly — they try to keep the ones that look interesting upon a very quick glance. That’s the job of the trigger.
10:05 am (Sean): Fabiola is working the crowd, but here in the press room they’ve just handed out the press release. ATLAS has a good result.
10:07 am (Mark): Even to a theorist, it was clear from discussions with our ATLAS group here at Penn what a huge issue pile up was. We’re not used to having a problem with too many collisions!
10:09 am (Sean): And now you can read the press release yourself!
10:11 am (Sean): Great talk, but seriously there shouldn’t be that much information on each slide! Particle experimenters need to do better at this.
10:12 am (Mark): I usually think the same when sitting in experimental particle physics seminars, but I was actually just thinking that these are two of the clearest presentations I’ve seen. Perhaps I’m more focused and excited than usual though.
10:12 am (Sean): Everything up to this point is to convince us that the result they have is a reliable one. They do understand what they’re doing. (Most of us weren’t skeptical.)
10:19 am (Sean): ATLAS result for the two-photon channel: beautifully clear bump in the data at 126 GeV.
10:20 am (Sean): 4.5 sigma in the two-photon channel, once we combine 2011 and 2012.
10:21 am (Sean): Crucial: the bump being seen is larger than expected! By a factor of two, approximately. Huge news. There isn’t a parameter in the Standard Model that you can tweak to explain that. It’s either a cruel fluctuation, or new physics.
10:25 am (Sean): On to four-charged-leptons, coming from Higgs to two Z’s.
10:28 am (Sean): Plot shows a tiny but discernible bump around 125 GeV. I know we’re practically in the post-Higgs era already, but all this consistency is very nice to see. (Consistency in where the peaks are located, I mean … still some issues in reconciling the tau/antitau data from CMS.)
10:33 am (John): Rather weak peak in ZZ for ATLAS! Hmm…still early days.
10:33 am (Sean): ZZ data from ATLAS, by themselves, represent an excess at about 3.4 sigma. Expected in Standard Model: 2.6 sigma. Interesting, or fluctuation? (I’m not including look-elsewhere effect, since I think we know where to look by now.)
10:34 am (Mark): Seminars frequently go over time. But as Gianotti correctly points out, they usually don’t have this kind of final slide to make you stay for!
10:34 am (Sean): Combining both channels from ATLAS: five sigma! Applause.
10:37 am (Sean): Expected achievable significance for SM Higgs: 4.6 sigma. Not sure how to reconcile that with the fact that the two-photon bump was twice the expected size. [Ah: it’s because the error on that height is substantial — maybe we shouldn’t make too much of it.]
10:40 am (Sean): Personal editorializing by me: we’ve found the Higgs, or at least a Higgs. Still can’t be sure that it’s just the vanilla Standard Model Higgs. The discrepancies aren’t quite strong enough to be sure that they really represent beyond-Standard-Model physics… but it’s a strong possibility.
Fortunately, we have a great accelerator working at full speed, and much more data to come! A proud moment for everyone who has worked to get us to this point.
10:37 am (Mark): So we have a five-sigma result from ATLAS as well! This was well-worth getting up for, if only to take part, at great distance, in the joyous applause at this slide.
10:38 am (John): BOTH experiments have a significantly enhanced rate for gamma gamma. My raw impression is that this is not very Standard Model like at all…this is the most important thing I learned tonight, without question.
10:43 am (Sean): Fabiola thanks Nature for putting the Higgs where the LHC could find it.
At the end of her talk, now there’s even applause in the press room!
10:46 am (Mark): Peter Higgs is visibly moved at the final results. I hope people understand, and perhaps this helps make clear, how invested scientists are in this work.
10:47 am (Sean): Not often you get to see history made.
10:49 am (Mark): Yep – it seems we have a Higgs. I’m off to ICHEP in Melbourne in a few hours, and will report in more detail from the presentations there.
10:52 am (Sean): Press release from Edinburgh is passing along this quote from Peter Higgs: “I never expected this to happen in my lifetime and shall be asking my family to put some champagne in the fridge.”
10:55 am (Sean): Here is the plot from CMS for the two-photon channel, from Phil Gibbs.
10:57 am (Sean): Some words from Lyn Evans (who built the LHC), as well as from Francois Englert (who laments the passing of his collaborator Robert Brout) and Peter Higgs. And I think that’s Guralnik and Hagen? (Kibble couldn’t make it.)
10:57 am (Mark): Nice that we’re getting a couple of remarks from the theorists. Lovely tribute from Englert to Brout, and Gerry Guralnik makes a nice point about seeing this kind of joy and excitement about science.
11:01 am (Sean): Now for the press conference. Probably no new revelations, but I’ll keep you posted.
11:10 am (Sean): Scientists enjoying their brief moment of celebrity, mobbed by reporters.
11:22 am (Sean): Joe Incandela says that we’re hoping by the end of the year to say whether the new particle is a scalar or pseudoscalar.
11:23 am (Sean): Peter Higgs gets asked a question, but declines to answer — he thinks this is a day when the experimentalists should be in the spotlight. Have to love that.
11:27 am (Sean): Fabiola and Joe agree that the mass measurements are compatible between the two experiments, given the error bars.
11:28 am (Mark): Interesting, and great, that they may be able to distinguish scalar from pseudoscalar by the end of the year.
11:29 (Sean): Actual news: DG Rolf Heuer reveals that they are now planning to extend this year’s run for another 2-3 months. The plan is to shut down at the end of this year for a two-year upgrade, so this gives a bit more opportunity to collect data.
11:37 am (Sean): Have we found the Higgs boson yet?
11:50 (Sean): A couple of people have mentioned supersymmetry. As Rolf Heuer just said, straightforward SUSY models have a remarkable feature: not a single Higgs boson, but five Higgs bosons. So we may have only have found 20% of the Higgs conglomerate.
Here is the ATLAS two-photon plot.
And here’s the CMS result for four leptons.
And the ATLAS result for four leptons.
12:07 pm (Sean): We made it this far without anyone saying “God particle.” That admirable streak just came to an end.
12:20 pm (Sean): Winding up. What a day. So amazing to see such interest in fundamental physics. Hopefully we have some new puzzles to solve!