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 exactlyof 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
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! (more…)
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 centre’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.
A lot of people do put a lot of stock in prophecy, including prophecies of the end of the world that nobody ever made (such as the one not made for today by the Mayans, through their calendar) and others that people made but were wrong (such as those made by Harold Camping last year and by many throughout history who preceded him.) If anyone were any good at prophecy they’d be able to use their special knowledge to become billionaires, so maybe we should be watching Bill Gates and Michael Bloomberg and the Koch brothers and people like that. I haven’t heard any rumors of them building bunkers or spaceships yet. Of course at the end of the year they may get a small tax hike, but that wouldn’t be the end of the world.
The Large Hadron Collider [LHC], meanwhile, has triumphantly reached the end of its first run of proton-proton collisions. Goal #1 of the LHC was to allow physicists at the ATLAS and CMS experiments to discover the Higgs particle, or particles, or whatever took their place in nature; and it would appear that, in a smashing success, they have co-discovered one. But no Higgs particles, or anything like them, will be produced again until 2015. Although the LHC will run for a short while in early 2013, it will do so in a different mode, smashing not protons but the nuclei of lead atoms together, in order to study the properties of extremely hot and dense matter, under conditions the universe hasn’t seen since the earliest stages of the Big Bang that launched the current era of our universe. Then it will be closed down for repairs and upgrades. So until 2015, any additional information we’re going to learn about the Higgs particle, or any other unknown particle that might have been produced at the LHC, is going to be obtained by analyzing the data that has been collected in 2011 and 2012. The total amount of data is huge; what was collected in 2012 was about 4.5 times as much as in 2011, and it was taken at 8 TeV of energy per proton-proton collision rather than 7 TeV as in 2011. I can assure you there will be many new things learned from analyzing that data throughout 2013 and 2014.
Of course a lot of people prophesied confidently that we’d discover supersymmetry, or something else dramatic, very early on at the LHC. Boy, were they wrong! Those of us who were cautioning against such optimistic statements are not sure whether to laugh or cry, because of course it would have been great to have such a discovery early in the LHC program. But there was ample reason to believe (despite what other bloggers sometimes say) that even if supersymmetry exists and is accessible to the LHC experiments, discovering it could take a lot longer than just two years! For instance, see this paper written in 2006 pointing out that the search strategies being planned for seeking supersymmetry might fail in the presence of a few extra lightweight particles not predicted in the minimal variants of supersymmetry. As far as I can tell at present, this very big loophole has only partly been closed by the LHC studies done up to now. The same loophole applies for other speculative ideas, including certain variants of LHC-accessible extra dimensions. I am hopeful that these loopholes can be closed in 2013 and 2014, with additional analysis on the current data, but until they are, you should be very cautious believing those who claim that reasonable variants of LHC-accessible supersymmetry (meaning “natural variants of supersymmetry that resolve the hierarchy problem”) are ruled out by the LHC experiments. It’s just not true. Not yet. The only classes of theories that have been almost thoroughly ruled out by LHC data are those predict on general grounds that there should be no observable Higgs particle at all (e.g. classic technicolor).
One prophecy I made all the way back at the beginning of this blog, July 2011, was that the earliest search strategy for the Higgs, through its decays to a lepton, anti-lepton, neutrino and anti-neutrino, wouldn’t end up being crucial in the discovery; it was just too difficult. (In this experimental context, “lepton” refers only to “electron” or “muon”; taus don’t count, for technical reasons.) In the end, I said, it would be decays of the Higgs to two photons and to two lepton/anti-lepton pairs that would be the critical ones, because they would provide a clean signal that would be uncontroversial. And that prophesy was correct; the photon-based and lepton-based searches were the signals that led to discovery.
Another prophecy I made at the beginning of the year was that Exotic Decays of the Higgs would be a high priority for 2012. You might think this prophesy was wrong, because in fact, so far, there have been very few searches at ATLAS, CMS and LHCb for such decays. But the challenge that required prioritizing these decays wasn’t data analysis; it was the problem of even collecting the data. The problem is that many exotic decays of the Higgs would lead to events that might not be selected by the all-important trigger system that determines which tiny fraction of the LHC’s collisions to store permanently for analysis! At the beginning of 2012 there was a risk that some of these processes would have been dumped by the trigger and irretrievably lost from the 2012 data, making future searches for such decays impossible or greatly degraded. At a hadron collider like the LHC, you have to think ahead! If you don’t consider carefully the analyses you’ll want to do a year or two from now, you may not set the trigger properly today. So although the priority for data analysis in 2012 was to find the Higgs particle and measure its bread-and-butter properties, the fact that the Higgs has come out looking more or less Standard Model-like in 2012 means that focusing on exotic possibilities, including exotic decays, will be one of the obvious places to look for something new, and thus a very high priority for data analysis, in 2013 and 2014. And that’s why, for the trigger — for the collection of the data — exotic decays were a very high priority for 2012. Indeed, one significant use of the new strategy of delayed data streaming at ATLAS and of data parking at CMS (two names for the same thing) was to address this priority. [My participation in this effort, working with experimentalists and with several young theorists, was my most rewarding project of 2012.] As I explained to you, a Higgs particle with a low mass, such as 125 GeV/c2, is very sensitive to the presence of new particles and forces that are otherwise very difficult to detect, and it easily could exhibit one or more types of exotic decays. So there will be a lot of effort put into looking for signs of exotic decays in 2013 and 2014! I’m very excited about all the work that lies ahead of us.
Now, the prophecy I’d like to make, but cannot — because I do not have any special insight into the answer — is on the question of whether the LHC will make great new discoveries in the future, or whether the LHC has already made its last discovery: a Higgs particle of Standard Model type. Even if the latter is the case, we will need years of data from the LHC in order to distinguish these two possibilities; there’s no way for us to guess. It’s clear that Nature’s holding secrets from us. We know the Standard Model (the equations we use to describe all the known particles and forces) is not a complete theory of nature, because it doesn’t explain things like dark matter (hey, were dark matter particles perhaps discovered in 2012?), and it doesn’t tell us why, for example, there are six types of quarks, or why the heaviest quark has a mass that is more than 10,000 times larger than the mass of the lightest quarks, etc. What we don’t know is whether the answers to those secrets are accessible to the LHC; does it have enough energy per collision, and enough collisions, for the job? The only way to find out is to run the LHC, and to dig thoroughly through its data for any sign of anything amiss with the predictions of the Standard Model. This is very hard work, and it will take the rest of the decade (but not until the end of the world.)
In the meantime, please do not fret about the quiet in the tunnel outside Geneva, Switzerland. The LHC will be back, bigger and better (well, at least with more energy per collision) in 2015. And while we wait during the two year shutdown, the experimentalists at ATLAS, CMS, and LHCb will be hard at work, producing many new results from the 2011 and 2012 proton collision data! Even the experiments CDF and DZero from the terminated Tevatron are still writing new papers. In short, fear not: not only isn’t the December solstice of 2012 the end of the world, it doesn’t even signal a temporary stop to the news about the Higgs particle!
Fig. 1: Of the 20 plots shown in yesterday’s post, the five marked in red have no Higgs signal, and contain nothing but statistical fluctuations on a background that, with infinite statistics, would give a flat horizontal histogram.
Well, I hope some of you attempted yesterday’s exercise, which involved looking at a bunch of plots with simulated data, and trying to figure out in each plot
is there a signal of the “Higgs particle” in the plot?
what roughly is the mass of the “Higgs particle” (which I assured you lies, for each plot, in the range 122-127 GeV/c²?)
So now let’s see how well you did, and also what the implications are for the 3 GeV/c² discrepancy between the two measurements by the ATLAS experiment of the Higgs particle’s mass — a discrepancy which led to a big discussion (which ATLAS did not encourage or generate, mind you). Some speculated (and others chose to report this speculation in the news media) that there might be evidence in this data for two Higgs particles. (This is contradicted by CMS’s result from November, but hey, who’s counting?) But the substantive question you might ask is: Is this such a big discrepancy that eitherthere are two Higgs particlesorATLAS has made a big mistake?
With that in the back of our minds, let’s take a look at the exercise I proposed yesterday, and see how things go. Note, however, that this exercise does not have direct or precise implications for ATLAS’s discrepancy. First, these plots are made in crude fashion and without simulating real Higgs particles, though the signal and the background are about the right size to match ATLAS’s current data on a Higgs decaying to two lepton/anti-lepton pairs. Second, ATLAS uses very sophisticated methods to measure the Higgs mass, much more powerful than you could employ by eye, and much more than these graphs could convey. Rather, the point here is to convey limitations of human psychology, and illustrate that humans are not naturally skilled at evaluating the statistical properties of small amounts of data.
Which Plots Have a Higgs Signal and Which Do Not?
15 of the 20 plots shown yesterday(in a larger figure that will be easier for you to read) have a Higgs signal; the others, marked with a red line in Figure 1, do not. That probably wasn’t too hard, except for the uppermost plot in the right column, because the plots without a Higgs have close to 40 events while those with a Higgs have close to 60; and the discrepancy is especially big in the range 120-130 GeV/c2 where I told you that the Higgs is to be found. Some people would have been tripped up by the peak in the bottom left plot, were it not for the fact that I told you the Higgs particle couldn’t be there.
Now on to the next question: What, roughly, is the Higgs mass in each of the 15 plots that have a signal.
Estimate the Higgs’ Mass
This is generally what trips people up the most. The human brain is an absolutely terrible statistician. It takes years of practice to unlearn what your brain wants to tell you — graduate students, and even senior theoretical physicists who haven’t stared at enough data, do an awful job with a problem like this.
For example, take Figure 2, which is one of the plots I showed yesterday. [Recall that the bins are 1 GeV/c² wide, and the bin just to the right of the 2 in “125” is the bin running from 125 to 126 GeV/c².] Your eye wants to tell you there’s a big peak at 126-127 GeV/c² and so that’s where the Higgs mass should be. But you’ve been warned that the Higgs peak is 4 GeV/c² wide at half its maximum height — so it shouldn’t just be a one-bin wide spike! Still, your brain forgets this and your eye misleads you. If you look carefully, there are rather few events to the right of the highest bin, and more to the left. The real Higgs mass is quite a bit lower than 126-127 in this case.
Fig. 2: One of the 15 plots with a Higgs signal.
Now what about Figure 3? This looks like it has a double peak! It has one peak between 123 and 124, and another between 126 and 127! But I assure you this was generated by a signal that has a single peak. Your eye, yet again, is fooled. The real Higgs mass in this case lies between the two peaks.
Fig. 3: Another of the 15 plots with a Higgs signal.
And Figure 4? This case doesn’t look too hard; there are several events in the 125-126 bin and several in the 126-127 bin, just 2 in the 127-128 bin, and none in the 124-125 bin. So clearly the Higgs mass should be between 125 and 127, probably 126. But in fact the actual Higgs mass in this case lies in the empty bin!
Fig. 4: A third plot with a Higgs signal.
The Truth is Revealed
Well, as the experts who are looking closely will already have guessed, every single one of these 15 plots was generated with a Higgs mass of 124.7 GeV/c2. Every one.
If there were 400 times as many events as in each of the plots from yesterday, here’s what you would see; three of the plots in Figure 5 show what the data looks like with a Higgs peak, and the fourth shows what the random background looks like.
Fig. 5: All the plots that contained a Higgs signal had a Higgs at 124.7 GeV/c2, whose shape (with 400 times the amount of data) is shown in three of the above figures, above a flat background. The background alone is shown in the upper right plot.
I assure you there was no funny business in generating the plots from yesterday. No bias was introduced. Try it yourself if you don’t believe me! The computer program was designed to sample this flat background plus smooth peak at random. When you pick only 20 or so events from such a peak, and another 40 or so from the flat background, you’ll get something that looks quite squiggly. The peak in the resulting plot can easily lie one or even two GeV/c2 away from the true mass, as in Figure 2; there can appear to be two peaks, as in Figure 3; the bin with the true mass can actually be empty, as in Figure 4. And the probability of the plot looking weird in some way, given this small amount of data, should not be underestimated — it’s maybe one in four or five.
The Implications
What’s the point? The photon-based and lepton-based measurements of the Higgs mass at ATLAS differ by 3 GeV/c2, which is bigger than we’ve seen here. So what? The photon-based mass measurement is 126.6±0.3±0.7 — the first uncertainty number is due to random statistics, the second is called “systematic” and includes experimental defects and other problems. So it has a systematic uncertainty of nearly a GeV/c2 (which is always ignored by the press, as though uncertainties don’t matter in interpreting what you actually know). Systematic uncertainties are often not random; they may give an overall shift to all the data, moving the peak uniformly up or down. And the lepton-based measurement is 123.5±0.9+0.4-0.2 GeV/c², which has an upward systematic uncertainty of almost half a GeV/c2. So if ATLAS got unlucky and their lepton-based result got pulled down by a couple of GeV/c2 purely due to the statistical effects we’ve seen today, and if they have small systematic problems of nearly a GeV/c2 in one or both of their measurements, they could potentially get a 3 GeV/c2 discrepancy.
Is such a big discrepancy likely? No; it is certainly possible, but it is certainly not very likely.
But here you have to remember how statistics works. If instead of asking “is this particular weird phenomenon likely” you had instead asked “is it likely that somewhere, in all of the measurements that ATLAS is making about the Higgs, one or two of them would have turned out weird”, the answer is “very likely indeed!” I’m writing and you’re reading a post about the mass measurement — and so are other bloggers and Scientific American and all the rest — because currently this is the one that looks odd. It could instead have been that there was a double peak in the photon-based plot, or that the lepton-based plot was showing no peak at all (simply because the signal rate fluctuated down from 20 events to 11, which is just 2 standard deviations.) Or the measurements of the spin and parity of the Higgs could be looking strange. In that case we’d be talking about those things instead. In July people were talking about the hint that the Higgs didn’t seem to be decaying to tau leptons; well that’s over and done.
In other words, there’s a tremendous bias both among scientists and the news media. We always talk about the outliers, the things that look odd to us. We make a big deal about them, and of course, to some extent we should. But we often forget that although these outliers look unlikely,
they aren’t generally as unlikely as they look to our brains, and
the probability of something being an outlier, when many things are being measured, is not small.
I personally think it would be enormously helpful if scientists would remind the news media of this well-known fact, and if the news media would convey it to the public. It would help the public understand how science is really done — and explain why it is so very common that the big story that you read about on the science pages simply disappears, after its 15 minutes of fame, without leaving a trace.
The most interesting outlier is ATLAS’s hint that the Higgs decays more often than expected to two photons; on this everyone (including the Scientific American journalist) agrees. We had that hint last December, in July, and still now. However, although CMS saw something somewhat similar in July, they’ve (somewhat disturbingly) delayed making their most recent data public on this measurement. So right now there’s still no way to know if this is more than a fluke, because the ATLAS result by itself is still not statistically significant.
Ok, everyone; by now you’ve all learned that the ATLAS experiment at the Large Hadron Collider [LHC] announced recently their measurement of the mass of the Higgs particle, in its decay to two photons, is at 126.6±0.3±0.7 GeV/c²; and meanwhile they measure the mass for apparently the same particle, in its decay to two lepton/anti-lepton pairs, to be 123.5±0.9+0.4-0.2 GeV/c², about 3 GeV/c² lower. “So bizarre”, wrote Michael Moyer at Scientific American, bandying about the idea that there are two Higgs-like bosons in this data (though, having pointed out the ambulance to you, and neglecting also to mention CMS’s data from November that directly disfavors this interpretation of the ATLAS data, he tells you later that some physics bloggers say you shouldn’t chase it…)
Well. How bizarre is this 2.7 standard deviation discrepancy really?
At the end of this post are 20 plots showing randomly generated data, in amounts comparable to those used in the current measurement of Higgs decaying to two lepton/anti-lepton pairs (often called “four leptons” for short). [Warning: This certainly hasn’t been done with the level of care needed to match the ATLAS measurement in any precise way; I’ll say a bit more below about the caveats.]
In some of the plots, there’s just a random flat background of about 40 events, similar (though not identical) to what arises in the four-lepton Higgs measurement.
In some of the plots, there’s a Higgs-like peak of about 20 events — a very sharp peak with a perfect detector, but one which is smoothed out a bit by the inevitable imperfections in a real particle detector.
In each plot with a peak, the mass of the Higgs has been chosen to lie somewhere between 122 GeV/c² and 127 GeV/c² [not equally populated].
So:
Can you tell which plots have a Higgs-like signal, and which ones don’t?
In each plot where there is a signal, can you estimate the Higgs mass? Again, in each plot, it lies somewhere between 122 and 127 GeV/c². You’re not going to get it exactly right — that’s impossible — but do your best, and let’s see what happens.
A couple of additional comments to help you:
The resolution on the measurement (i.e., the effect of imperfections in the measurement) is such that with infinite amounts of data, the peak that you’d observe would be a bump whose width, at half the bump’s maximum height, would be about 4 GeV/c².
The bins in each plot are 1 GeV/c² wide; the bin just to the right of the number 2 in “125” runs from 125 to 126, the next from 126 to 127, and so forth.
Lastly, a caveat: in the real ATLAS or CMS measurement, the mass of the Higgs is not measured simply by fitting a Gaussian peak over a flat background. So don’t take this exercise too seriously! It’s just a useful learning experience, and nothing more. What the experiments actually do is far more sophisticated, accounting for the properties of each event separately!!
Here we go: Good Hunting! (If your browser has trouble with the figure, try clicking on it.)
Twenty sets of simulated data, showing number of events versus the “mass” of a putative particle, in GeV/c2. Each contains about 40 events of a non-Higgs-like background that is flat in mass. Some but not all plots contain a signal which is in the form of a peak, comparable to the current size of the expected Higgs particle signal (about 20 events). In each case the mass of the Higgs is chosen between 122 and 127 GeV/c2. Experimental imperfections make the full-width of the signal peak at half its maximum about 4 GeV/c2 wide.
[Note Added: this afternoon the author of the Scientific American article made a few corrections. I leave it to you to judge for yourself whether he addressed the issues raised here.]
There’s been a little silliness floating around (sadly, in Scientific American, whose article contains at least two factual errors) unscientifically speculating that ATLAS’s new results on the Higgs-like particle, from data collected at the Large Hadron Collider [LHC], suggest there are two such particles rather than one. The mass measurement of this particle using the data when it decays to two photons, 126.6±0.3±0.7 GeV/c², is different, by 2.7 standard deviations, from the mass measurement obtained from its decays to two lepton/anti-lepton pairs, 123.5±0.9+0.4-0.2 GeV/c². So… huh… gee… maybe there are two Higgs-like particles, a lighter one which rarely decays to two photons and a heavier one which rarely decays to two lepton/anti-lepton pairs?
[Note Added: I should emphasize, lest anyone blame ATLAS for this implausible line of speculation, that in the ATLAS presentation last week, which was one of several presentations that morning, these two mass measurements were presented simply and responsibly, as results from data. Not a single speculative word was said about there being a hint of two Higgs particles. I don’t know who got the ball rolling on that idea, but it wasn’t ATLAS. And it’s not a plausible idea: see below.]
Take a deep breath. For not only would the two types of particles somehow have to be magically and implausibly arranged to mimic, at first glance and to a rough extent, a single Standard Model Higgs particle (the simplest possible type of Higgs particle), there’s another experiment, which unfortunately the writer of the Scientific American article neglected to consult.
ATLAS’s mass measurement from the events with two lepton/anti-lepton pairs also disagrees with CMS’s mass measurement obtained from the same type of events: 126.2±0.6±0.2 GeV/c². Two similar experimental detectors, same measurement, moderate disagreement. Nature is nature; there’s no way that ATLAS can be making one type of particle all the time, while CMS is making a different one all the time. So there is no evidence here, taking ATLAS and CMS together, favoring the existence of a separate particle with a mass of about 123.5 GeV/c² that decays to two lepton/anti-lepton pairs.
What is behind these discrepancies, then? ATLAS and CMS each have scarcely a dozen of these two lepton/anti-lepton events, and their extraction of the Higgs particle’s mass from each event is somewhat uncertain, which is why many events are required for a good mass measurement. When you still have small amounts of data, funny statistical fluctuations will often occur. We’ve seen this before; back in 1989, when the Stanford Linear Collider (SLC) produced its first few Z particles at the Stanford Linear Accelerator Center, the plot of the Z particle’s mass gave a double resonance peak, instead of the single peak that was expected. A brief moment of speculation occurred, but with more data the anticipated single peak structure emerged. I’ve heard at least one other similar story from an earlier decade. In fact ATLAS and CMS had a 2 GeV mass discrepancy when the first Higgs hints came in; that was just an effect of statistics. Combine a fluctuation of this form with a minor detector calibration problem, and you’ll get discrepancies like this.
Multiple types of Higgs particles are certainly possible; people have considered this scenario for decades, and I’ve written about it here, for instance. Efforts to search for a second type of Higgs particle have been going on since the discovery of the first one. But let’s not manufacture one out of thin air by looking selectively at the data; that’s not how reliable science gets done.
