Ok, testing you to see if you’re on your toes: when was the first Higgs particle produced by humans? We can’t know the exact date, but make your best estimate. 🙂
[Hint: how many Higgs particles will be produced at the Large Hadron Collider in 2012?]
Tag: LHC
The time’s come for me to return home to the United States. It is almost impossible to convey the intensity of the last few weeks. I’m excited, exhilarated and exhausted.
Even in more normal times, a visit to the CERN laboratory that built and operates the Large Hadron Collider [LHC] always wipes me out. When I’m there, several times a day I run into physicists I know who work on the LHC experiments, and so I’m constantly getting into impromptu conversations, both about the most recent scientific results and about planning how to investigate the data in future. And then there are lots of theoretical physicists of all stripes to talk to and learn from, including not only CERN faculty but also many, many visitors from all around the world.
This visit, of course, was unique. Not only did was there the historic presentation July 4th that convinced the particle physics community that a new particle, most likely a type of Higgs particle, has been found, it was followed (as was anticipated) by days and days of incessant discussions about interpreting the data that demonstrated the particle’s presence, and about future strategies to learn more about it. Then there was a five-day workshop at CERN (over a weekend!) concerning not only the Higgs search but also all of the other new results (of which there are many, though not as newsworthy) that the LHC experiments have produced. [For that workshop, I was asked to put together a presentation on a subject on which I don’t feel entirely an expert, which was an interesting but rather stressful experience.] And finally there was a three-day Higgs Hunting Workshop on the outskirts of Paris, entirely focused on the Higgs, where we saw some new Higgs data presented by the ATLAS experiment, a full review of all the previously presented Higgs data from all the relevant experiments, lots of theoretical talks about how to calculate the properties of the simplest type of Higgs particle with very high precision, discussions of the implications of the current data for whether the new particle might be a Higgs of a more complicated type, and also presentations reviewing the history of the Higgs search and looking forward into the near-term and long-term future. All Higgs All The Time! [Yet again, I had to prepare a presentation that I found very stressful, and struggled with until almost the last moment. Between my departure from Geneva Tuesday afternoon until the Higgs Hunting Workshop ended late Friday afternoon, I don’t think I took a break except for meals.]
So I think you can understand why I’ve been a little slow to post and why I’m a little slow to answer your questions and comments just now. I expect to be ready to start answering questions and putting more pedagogical stuff up on the site over the coming week. In fact, for those of you who’ve had a bit of a freshman physics course, I’ve figured out how to explain how the Higgs field does its thing, namely, how it gives mass to other particles. Maybe as I put that explanation together I’ll figure out how to explain it even for those of you who haven’t had freshman physics, though that will be a lot trickier.
But right now I’m still winding down, and trying to clear my brain. Much work lies ahead of us in particle physics — for the LHC is still in its early stages, and we need many years of study of the new particle before it will teach us what we need to know about the Higgs field(s).
POSTED BY Matt Strassler
ON July 23, 2012
[2:40 Paris time: UPDATED ]
The ATLAS experiment (in a talk by Olivier Arnaez at the Higgs Hunting workshop in Orsay, France that I’m attending) has made public its search for Higgs particles decaying through real and virtual W particles to a lepton (electron or muon), anti-lepton, neutrino and anti-neutrino. [CMS made its result for this search public on July 4th at the big presentations at which discovery of the Higgs-like particle was announced.] The importance of this search is now no longer to tell whether the Higgs-like particle exists — we are confident that it does — but to try to measure how often these particles are produced and then decay in this particular way.
This is a very difficult measurement, with a small signal and a complex and large background, and it only allows a measurement of a rate of the process; the mass of the particle producing an observed excess cannot be determined to better than 30-50%, because the neutrino and anti-neutrino are not observed, and the lepton and anti-lepton are not enough to figure out the mass of the parent particle. Meanwhile, small amounts of data can play tricks. After last summer’s data, ATLAS saw a sign of something. By March, with the full 2011 data, they said they didn’t. Now, including 2012 data, they see it — in fact, they see slightly more than expected for the simplest possible type of Higgs particle (“the Standard Model Higgs) of mass 125 GeV/c2.
More details to follow shortly, but the news is:
- ATLAS observes in 2012 data a 3.1 sigma deviation from the no-Higgs hypothesis
- Combined with 2011 data (which showed almost no excess) there is now a 2.8 sigma deviation from the no-Higgs hypothesis
- the excess looks like a Higgs particle; it is quite consistent in its transverse-mass distribution with the hypothesis of a Higgs-like particle at a mass of 125 GeV/c2
- the fit to the signal is 1.4+-0.5 times larger than the Standard Model prediction for a Higgs particle of mass of 125 GeV/c2 (though one should not forget that this measurement isn’t good for telling us the mass; just by itself, it would also be consistent with a lighter Higgs particle produced with an expected rate or a heavier Higgs particle with a much lower than expected rate.)
- this in turn is in good agreement with what ATLAS observes in the two-photon and four-lepton searches, both of which are a bit higher than the prediction for the simplest Higgs with mass of 125 GeV/c2
- this also is consistent specifically with the prediction, which holds for most expected types of Higgs particles, that the ratio of the strengths of the interaction of the new particle with W and Z particles is equal to the ratio of the masses of the W and Z.
These results are also roughly consistent, within the uncertainties, with those from CMS, which sees a slightly smaller production rate than expected, something like 0.6 +- 0.4 (don’t quote me, I’m reading it off a plot) times what is predicted for a simplest Higgs of mass 125 GeV/c2.
NOTE ADDED: Did I mention this is a difficult measurement? The excess in the 2012 data looks a lot like a Higgs particle signal, but a mis-estimated background would look pretty similar (though admittedly it would have to be a big mis-estimate — but then again, the signal in 2012 is itself more than twice as big as expected), and so we’re reliant on the estimates of the systematic uncertainties on the backgrounds given by the experimenters. There are differences between the 2011 and 2012 analysis techniques (improvements, surely) that I don’t understand yet. So… an impressive result, but my own view of it is still a little murky. I’m glad this excess wasn’t essential for the claim of Higgs particle discovery.
POSTED BY Matt Strassler
ON July 18, 2012
I got a question after yesterday’s post that motivates me to make a comment.
My post yesterday said that “the LHC is gradually moving from broad searches to precision tests.”
The question I received was “Does this mean that the LHC experiments are giving up on looking for supersymmetry [for now]?”
The answer is: “Of course not.” There’s a complete logical disconnect between those two statements. The first would imply the second ONLY if it were true that the way to find supersymmetry (or anything else new) was in broad searches rather than in precision measurements. But that premise is false.
New particles and forces (such as, but not limited to, those predicted by supersymmetry) are easy to find in broad searches if they generate collisions that look very distinctive and are much more common than similar collisions predicted by known phenomena.
New particles and forces (such as, but not limited to, those predicted by supersymmetry) are impossible to find in broad searches if they generate collisions that are either not so distinctive or are not very common compared to similar events predicted by known phenomena. For these you need to measure and predict known phenomena much more precisely.
Some variants of supersymmetry (including many of the more popular ones) generate large distinctive signals. Some don’t. Broad searches only rule out the first class (and I should mention that not all the broad searches have even been done yet.)
The same goes for many other theories with as-yet unknown particles and forces. There’s nothing special about supersymmetry in this regard.
So no, the new phase of the LHC research program is not about giving up on looking for this or that. It’s about working even harder than before, in order to find what might be hiding a bit below the surface. In fact, that was the major topic of this weekend’s workshop (including my own talk).
POSTED BY Matt Strassler
ON July 17, 2012
Today I’m attending the start of a several day workshop at the CERN laboratory (host of the Large Hadron Collider [LHC]). This is bringing LHC experimentalists and theoretical particle physicists together to hear about and discuss not only results from the (successful) search for the Higgs particle but also from many other searches (so far unsuccessful, but still important and instructive for our understanding of nature) for other new particles and/or forces, as well as relatively high-precision tests of the Standard Model itself. This should help those of us who were distracted for the past week by the discovery of the Higgs-like particle to catch up with everything else that the experiments reported at the ICHEP conference. Will update today or over the next few days if anything striking is presented.
POSTED BY Matt Strassler
ON July 13, 2012
I’ve finished my third and fourth articles on the Higgs discovery (the first two being on the data itself and why it is convincing, and on the great skill and the bits of luck that led to a discovery occurring so soon.) The new articles are about why I personally think the new particle is almost certainly a Higgs particle of some type, and about the outstanding question of whether this particle is the simplest possible type of Higgs (the Standard Model Higgs particle) or not.
I believe that nothing I’ve written in these two new articles is original; it has all (or almost all) been stated in papers or preprints by other people. That said, what’s in these articles represents my own personal views (since at this early date I have no way to know what fraction of my colleagues agree with me), and those views are not yet set in stone… so experts with well-reasoned arguments are encouraged to try to change my mind.
POSTED BY Matt Strassler
ON July 12, 2012
Super-busy at the CERN laboratory outside Geneva; lots of new data becoming public as it appears at the ICHEP conference, lots of theory papers flying around (not all of them accurate and uncontroversial) trying to interpret the Higgs measurements, and lots of work to do preparing the experiments for their further measurements of the Higgs and for their searches for other particles and forces. Updates to this website are getting slotted in when possible. In addition to my recent summary post from Thursday, I put up an article Friday about the data itself and why it is convincing [updated today with some new plots], and a new article today about how a lot of skill and a little luck led to the beautiful results we saw on Wednesday. Enjoy! [If you have friends asking you why the Higgs particle matters, you can have them read my article about it.]
POSTED BY Matt Strassler
ON July 9, 2012
[July 6 UPDATE: very busy with work here, but I have finished a short article briefly describing the data, and why it is convincing that a new particle has been found.]
July 5: A wild day at the CERN laboratory yesterday, continuing into today and surely the rest of the week. [If you’re a lay-person wondering what all the fuss is about, you might want to read my article “Why The Higgs Particle Matters“, or watch my year-old video clips that explain how one searches for the Higgs particle, or read the Higgs FAQ.] [Very well-written and almost entirely accurate articles at both the New York Times and the BBC.]
The contrast from last December is profound. After last December’s presentation on the search for the Higgs particle at the Large Hadron Collider [LHC], whose results I called “inconclusive” and other bloggers called “firm”, most ATLAS and CMS experimentalists I talked to were expressing, in public and in private, something between extreme caution and cautious optimism. I can’t count the number of times that senior experimentalists told me a story about a 3 sigma result that they’d seen disappear because of a subtle mistake. Though many theorists were convinced, many other theorists (despite what you hear from some other bloggers) shook their heads and expressed deep caution. Discussion at the CERN cafeteria and in the hallways focused on which parts of the data were most trustworthy and whether the discrepancies between what ATLAS and CMS had observed made the result unstable to small changes. All in all, despite what all the other bloggers said, I personally did not talk to a single experimentalist who felt the result was secure — which is not to say (this is an important distinction, please read carefully and please do not misquote) that anyone (including me) thought the hints were clearly a mirage. The problem, as I explained at the time, was that you could run an argument that made you confident in the hints, and you could also run an argument that made you lack confidence. Well, evidence isn’t firm until the argument in favor is a lot stronger than the argument against, and that wasn’t true in December. The evidence did improve somewhat by March, though the improvements were kind of up and down.
By enormous contrast, after yesterday’s presentation I did not speak to a single person who expressed serious doubts that ATLAS and CMS had made a discovery. (The most negative comment was from an experimentalist who wished ATLAS and CMS hadn’t been forced, by the early-July scheduling of the ICHEP conference, to do their data analysis so soon; he would have preferred to have twice as much data and a result that left him with absolute confidence. Well, if that’s the worst thing anyone can say…) What’s changed from December is that every argument you can make comes out in favor of the result being real; I’ve been unable to think of any argument against it, nor have I spoken to anyone who has proposed one. All the conversation now is on what the discovery means and on what to do in coming months.
How did a result that was inconclusive in December become so convincing in July, with just a doubling of the amount of data? The answer, as far as I can tell, is that a number of things that could have gone wrong all went right. I’ll go through this in more detail later, but suffice it to say for now that the experts who operate the accelerator and the two detectors all proved, yet again, that they are extremely smart and capable, and on top of that, they had some good luck that brought them to results almost as strong as anyone could hope for. Only time will tell whether the two experiments both got a bit lucky with a statistical fluke or whether the Higgs is actually a bit easier to find than theorists expected — but on top of this, the LHC produced somewhat more collisions than estimated in advance, the experimenters dealt admirably with the extreme collision rates, many new techniques for improving the measurements were developed, and these numerous small bits of good news combined for a nearly best-case scenario. Yesterday’s results were close to the optimal that could have been possible, given the amount of data; to have both experiments see clear signs of a new particle with a mass of about 125 GeV/c2 in both of their “easy” searches (for Higgs particles decaying to two photons, and for Higgs particles decaying to two lepton/anti-lepton pairs) was a lot to ask for, but it’s what we got!
My goal for today and tomorrow, if I can squeeze it in — obviously my highest priority right now is conversations with my professional colleagues about interpreting the result and discussing what needs to be on the agenda for the rest of 2012 and 2013 — is to explain to you:
- What does the current data actually show?
- What in the data makes the evidence in favor of a new particle so convincing?
- What elements of first-rate experimental technique and plain-old luck combined to make the result so strong?
- Why I am personally so convinced (more than the experimentalists seem to be right now) that the new particle is a Higgs particle, as opposed to something else?
- To what extent is this particle consistent with the simplest Higgs (a “Standard Model Higgs”) as opposed to a more complicated type?
Maybe getting through this list will take me into next week. At some point I’ll write more about the implications of this being a simplest Higgs or not, but probably not before the middle of next week. And hey, it’s also time to revise the Higgs FAQ!!!!
So stay tuned to this channel! I’ll alert you when I finish writing the answer to each of these questions.
POSTED BY Matt Strassler
ON July 5, 2012
[This paragraph was my introductory paragraph from this morning, before the presentation. Details of the presentation follow.] IndependHiggs Day has dawned, warm and hazy. Yesterday, at the CERN laboratory that hosts the Large Hadron Collider [LHC], all the discussion at lunch, and over afternoon coffee and evening beer, centered on the Higgs particle (here’s my Higgs FAQ, my video clips for non-experts about how the Higgs search is done, and a recent article for non-experts explaining its larger importance;more detailed background on the Higgs is linked from here), and on today’s presentation to update the search for it. The mood is ebullient and excited; everyone is anticipating a very big step forward, perhaps a definitive one. Gone is the uncertain mood of December, when CERN preceded its presentations with a warning that the results shown then would not be conclusive — a warning that is notably absent this time. It’s a party atmosphere, and a media circus; many luminaries are here, invited by the director to witness the event. The press will fill an entire auditorium all their own; physicists will be spread out over perhaps a dozen auditoriums. It’s been suggested that CERN should have rented a soccer stadium in Geneva; the small size of the main auditorium at the lab reflects how much smaller particle physics experiments were just 30 years ago.
What I will be watching for in the presentations today is the following (to be updated when I know the answers): Here’s what happened in the presentation today:
[8:50 Professor Higgs has entered the room.]
[9:00 CERN Director General Rolf Heuer very brief opening remarks]
First, CMS (presented by spokesperson Joseph Incandela, from the University of California Santa Barbara): (5.2 inverse fb of data, with 5.6 for muon analyses, improvements on many fronts)
- Is there a new particle apparent, as a bump in a plot, in the search for a Higgs of mass near 125 GeV/c2 decaying to two photons? Yes!! 4.1 sigma, a bit above standard model expectation though not too much
- Is there supporting evidence of a bump, at around the same mass as for the photon search, in the search for a Higgs decaying to two lepton/anti-lepton pairs? Yes! 5 or 6 events above background, 3 sigma, a bit below expectation but quite consistent with standard model
- TOTAL 5 SIGMA!!! LOUD APPLAUSE…
- Is there supporting evidence for an excess of some sort in the searches for a Higgs decaying (a) to a tau lepton/anti-lepton pair, (b) to a bottom quark/anti-quark pair, and (c) to a lepton, anti-lepton, neutrino and anti-neutrino? (a) not at all, but very low statistics (b) not incompatible, but still low statistics (c) preliminary technique, yes
- TOTAL 4.9 sigma, EXPECTED 5.9 sigma ;
- MASS 125.3 +- 0.6, preliminary
- Is the 8 TeV collision-energy data from 2012 roughly consistent with what was seen in the 7 TeV data from 2011? Yes
- Does the evidence in favor of a new particle support the idea that this is a Higgs particle? Absolutely!!!! As I have explained elsewhere, an observation of a signal in both photons and leptons, with even roughly the ratio predicted by the Standard Model, is a very strong argument in favor of the new particle being a Higgs particle.
- Does evidence in favor of this particle suggest that it is a Higgs, but not a Higgs of the simplest type (i.e., that it is not a Standard Model Higgs)? No signs of (statistically significant) non-Standard Model behavior
Next, ATLAS (presented by spokesperson Fabiola Gianotti, from CERN): (up to 5.9 inverse fb, and various improvements, some quite important)
- Is there a new particle apparent, as a bump in a plot, in the search for a Higgs of mass near 125 GeV/c2 decaying to two photons? Yes!! 4.5 sigma , 3.6 sigma with look-elsewhere effect over 110-150 GeV: (cross-section almost twice as big as expected! but only 2 sigma excess); mass about 126.5 GeV
- Is there supporting evidence of a bump, at around the same mass as for the photon search, in the search for a Higgs decaying to two lepton/anti-lepton pairs? Yes!! 6 or 7 events above background, 3.4 sigma (2.5 sigma after look-elsewhere over 110-140), cross-section again a bit large, 1.3 +- 0.6 of Standard Model expectation
- Is there supporting evidence for an excess of some sort in the searches for a Higgs decaying (a) to a tau lepton/anti-lepton pair, (b) to a bottom quark/anti-quark pair, and (c) to a lepton, anti-lepton, neutrino and anti-neutrino? Nothing from 8 TeV collisions shown today
- TOTAL 5.0 SIGMA!!!!!! LOUD APPLAUSE…
- MASS 126.5 GeV (from photons only; leptons are lower; no error bar given, so measurement of mass is not complete) with 1.2 of standard model production rate
- Is the 8 TeV collision-energy data from 2012 roughly consistent with what was seen in the 7 TeV data from 2011? Yes
- Does the evidence in favor of a new particle support the idea that this is a Higgs particle? Absolutely!!
- Does evidence in favor of this particle suggest that it is a Higgs, but not a Higgs of the simplest type (i.e., that it is not a Standard Model Higgs)? No significant deviations from Standard Model though two-photon rate is currently 2 sigma high. Two sigma is not enough to be excited about. This is especially since the Standard Model prediction for the overall cross-section is a very tricky calculation; its current precision is estimated at 15%, but perhaps one might worry about that estimate.
Then:
- Is what CMS observes largely consistent with what ATLAS observes? Absolutely!!
- Are there any notable discrepancies between the two experiments that might undermine confidence in their results? None! The discrepancy of about 1.2 GeV in the masses quoted is not official, since ATLAS does not yet have a final mass measurement with a proper uncertainty estimate. Even if I take the ATLAS estimate as if it were a mass measurement, if I put a similar uncertainty on it as CMS has quoted, the discrepancy between the two experiments is below 2 sigma, not enough to cause concern at this time, especially given the strength of the signal in each experiment separately.
WOW!!!! CONGRATULATIONS LHC, ATLAS and CMS !!!
[Heuer celebrates a global success… A standing ovation for all…]
[No one has any questions; the data is so clean and so convincing!]
[Some nice words from the inventors of the idea…]
POSTED BY Matt Strassler
ON July 4, 2012
Excitement is reaching fever pitch as the presentation at CERN on the Higgs search approaches! What’s important is that they measure By now the experiments have actually finished their analyses, and the press is reporting (harmless) rumors that both of the big Large Hadron Collider experiments, ATLAS and CMS, are seeing something. Presumably, assuming the rumors aren’t wrong, what they are observing, using this year’s data, is signs of a Higgs-like particle with a mass of 125 GeV/c2 that are roughly similar to what they reported in December, January and March using last year’s data. The details will be fascinating to see!
UPDATE: The CDF and DZero experiments at the now-closed Tevatron just updated their Higgs searches, and they find some amount of evidence in favor of a Higgs particle being produced through its interaction with W and Z particles and decaying to bottom quark/anti-quark pairs. Resonaances has a summary. This process is interesting because neither this production mechanism nor this decay mode are yet accessible to ATLAS and CMS; they don’t have enough data collected yet. One should be a bit cautious about such difficult analyses, but taken at face value these measurements are complementary to what ATLAS and CMS can tell us on Wednesday.
What’s all this fuss about? For the non-technical reader (and/or for your non-technical friends) I’ve written an article trying to put this historic moment in perspective, using language that is as non-technical and unfrightening as I can manage. If you agree that this is a good article for non-experts, please forward the link to others!
For the reader who wants to follow more details: Let me remind you of the sharpest questions that we’ll be asking on Wednesday, many of which are the same or almost the same as those we asked last December. (You can review my various background articles on the Higgs if you want to understand why these are the right questions):
For both ATLAS and CMS, we will ask: (more…)
POSTED BY Matt Strassler
ON July 2, 2012
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