Of Particular Significance

Category: Higgs

Two Higgs Bosons? No Evidence for That

[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.

POSTED BY Matt Strassler

ON December 17, 2012

Another Higgs Update from CERN

At the CERN laboratory today, there’s an ongoing report to the CERN council that oversees the lab, and this includes talks from the Large Hadron Collider [LHC] accelerator operators, and from the experimentalists who built and operate the detectors (ATLAS, CMS, LHCb, ALICE, TOTEM) that are designed to detect and interpret the debris from the LHC’s proton-proton collisions. Among the results being presented today are some measurements of the properties of the Higgs-like particle whose discovery was announced in July, including ones that were notably missing from the HCP conference presentations in Kyoto last month.

Here are some highlights, to be fleshed out in more detail later, if warranted.

LHC accelerator operations report:

An excellent year. In the best week the LHC produced 1.35 inverse femtobarn (fb) of data for both ATLAS and CMS; as of December 5, LHC produced 23.2 inverse fb for the year per experiment (note each experiment will have somewhat less recorded, due to normal losses), slightly above the target for the year.

Biggest problems: beam instability (much bigger problem in 2012 than 2011); stray high-energy particles affecting electronics in the tunnel; dust falling out of the beampipe into the beam, potentially a significant problem for 2015.

After the 2013-2014 shutdown, what will be the likely running conditions in 2015? The current intention is to go to 25 nanoseconds between collisions (the design) rather than the 50 nanoseconds used in 2012, and to start at 13 TeV per collision.

ATLAS: New two-photon and four-lepton measurements; new spin and parity measurements.

Two photons: new categories of events added, with either 1 lepton or of two jets at low invariant mass, characteristic of production of a Higgs with a Z or W.

This channel now shows 6.1 standard deviation significance (3.3 expected) by itself: discovery of a Higgs-like particle in a single decay mode. Mass 126.6±0.3±0.7 GeV/c²

Signal remains high: 1.8 ± 0.3 + 0.29 – 0.21 times the Standard Model expectation. (But note the expectation depends somewhat on the assumed mass of the Higgs; ~~not sure yet which mass was taken here.~~ 126.6 GeV was assumed.)

Four leptons: 4.1 standard deviations, signal strength 1.3 ± 0.4 times Standard Model expectation. Mass 123.5±0.9+0.4-0.2 GeV/c² — notably lower than two photons.

The two mass measurements are 3 GeV apart (surprising but not impossible given the amount of data; or perhaps there is a technical problem somewhere, though I’m sure they looked very, very hard for one). They are compatible only at 2.7 standard deviations. Combined mass: 125.2 +- 0.3 +- 0.6 GeV/c²

Spin (from photons): spin 2 disfavored at the 91% level; compatible with spin 0.

Spin (from leptons): spin 2 disfavored only at the 85% level; compatible with spin 0.

Parity (from leptons): Exclusion of odd-parity spin-0 particle at 99%.

CMS:

First limit on Higgs decay to Z + photon: although still ~20 times the Standard Model expectation, this limit is good enough to rule out various non-standard interpretations of the Higgs-like particle.

No update of Higgs decay to two photons. This is too bad. It’s somewhat exciting that ATLAS’s result on Higgs decay to two photons remains somewhat high compared to expectations. But the excess is still not yet 3 standard deviations. Deviations of this size do come and go. And we don’t have confirmation from CMS. So the situation remains tantalizing but unfortunately not yet very convincing. We may not learn anything more from CMS or ATLAS til March, when they have analyzed the full 2012 data set.

I did not catch anything new from LHCb or ALICE; generally I haven’t had time to cover ALICE’s research program here. TOTEM, a special purpose detector for measuring things I haven’t discussed on this website, is still getting rolling.

POSTED BY Matt Strassler

ON December 13, 2012

My Version of the TIME Higgs Paragraph

A few commenters have complained that I’m too hard on science journalists, who have a tough job; it’s hard to explain difficult concepts in a few words. To paraphrase them: “if it’s so easy, you do it! Rather than merely complain about the erroneous TIME magazine paragraph on the Higgs boson, write your own explanation of the Higgs particle for readers of TIME magazine.”

Well, first of all, I have never once suggested science journalism is easy; far from it! A big part of the challenge is to find ways to explain complex ideas that are simple, compelling and accurate (and not two out of three.)

Second, I have written an article suitable for non-expert readers; it’s just over a page long, and is called Why the Higgs Particle Matters. It’s gotten about 30,000 hits; some people seem to really like it, so try it out on your friends.

And third, for those who point out that the above-mentioned article is much longer than a paragraph, and that I shouldn’t be so critical of the TIME journalist who had to fit so much into a such a small space, here is my version of the TIME paragraph: six sentences rather than five, but scarcely longer. I have borrowed the style and the feel of the TIME journalist’s writing, and I have removed some inaccurate content and replaced it with different accurate content.

Take a moment to thank the Higgs field for all the work it does, because without it, you’d explode. This field pervades the universe and supplies electrons (and many other particles) with their mass, thus preventing ordinary matter from disintegrating into a ghastly vapor. It was in the 1960s that British physicist Peter Higgs (and a few others) first posited the existence of this field. But it was not until last summer that two huge teams of researchers at Europe’s Large Hadron Collider at last sealed the deal by discovering a new particle — the Higgs boson — which confirms the Higgs field exists. You see, the particle is a consequence of the field wiggling a bit; and just as sound, a ripple in the air, can’t be heard unless there’s air in the room, there wouldn’t be Higgs particles to discover unless Higgs and friends were right all along about their famous field. Now the Higgs — as most particles do — decays in an instant to other particles, so it wouldn’t be able to attend the award ceremony; however, the scientists would surely be happy to appear in its stead.

Although not everything I’ve written here is 100% accurate — that would indeed be impossible in a paragraph for a wide readership — I believe none of it is fundamentally wrong (but my colleagues should feel free to complain!) Yes, science journalism is difficult; but is it really inevitable that profound errors concerning the science must appear in articles for the public?

POSTED BY Matt Strassler

ON December 5, 2012

TIME for a Little Soul-Searching

Yes, it was funny, as I hope you enjoyed in my post from Saturday; but really, when we step back and look at it, something is dreadfully wrong and quite sad. Somehow TIME magazine, fairly reputable on the whole, in the process of reporting the nomination of a particle (the Higgs Boson; here’s my FAQ about it and here’s my layperson’s explanation of why it is important) as a Person (?) of the Year, explained the nature of this particle with a disastrous paragraph of five astoundingly erroneous sentences. Treating this as a “teaching moment” (yes, always the professor — can’t help myself) I want to go through those sentences carefully and fix them, not to string up or further embarrass the journalist but to be useful to my readers. So that’s coming in a moment.

But first, a lament.

Who’s at fault here, and how did this happen? There’s plenty of blame to go around; some lies with the journalist, who would have been wise to run his prose past a science journalist buddy; some lies with the editors, who didn’t do basic fact checking, even of the non-science issues; some lies with a public that (broadly) doesn’t generally care enough about science for editors to make it a priority to have accurate reporting on the subject. But there’s a history here. How did it happen that we ended up a technological society, relying heavily on the discoveries of modern physics and other sciences over the last century, and yet we have a public that is at once confused by, suspicious of, bored by, and unfamiliar with science? I think a lot of the blame also lies with scientists, who collectively over generations have failed to communicate both what we do and why it’s important — and why it’s important for journalists not to misrepresent it. (more…)

POSTED BY Matt Strassler

ON December 3, 2012

Particles are People Too

Much has already been written about the hilarious proposal that the recently-apparently-discovered particle called the Higgs Boson (you know, [what’s a boson?] the one sometimes called “God Particle” by people who don’t know much about particles) be selected as Time magazine’s “Person of the Year” for 2012. Well, I hope it wins, so that, as justice requires, it can finally be allowed to donate unlimited amounts of its money to U.S. election campaigns, just like U.S corporations, which, as you know, are people too. And I think this would also mean that Higgs bosons could finally get married, though I’m not entirely sure about the legality of whether they could marry each other… (recall the Higgs particle is its own anti-particle.)

But the hilarity became that much greater when Time magazine listed its various nominees and (as many have pointed out) described the Higgs particle in five glowing sentences, each of which contains at least one spectacular scientific or historical error. (Indeed, when I saw this, I assumed I was reading the Onion — maybe that’s where Time, following the lead of China’s famous newspaper Particles’ Daily, got this description? — though the Onion has been strangely quiet on the subject of the Higgs up to now.) The Higgs boson is candidate #18 out of 40, and I encourage you, in order to fully appreciate the company that it now keeps in the upper echelons of our society, to look at #17 and #19. Nevertheless, it’s worth noting that Time avoids the term “God Particle”, which would be praiseworthy under better circumstances.

So — a quiz for the reader. Can you identify five God-Particle-worthy errors in this piece from Time? Extra credit if you can find six or more. It’s an open-book test; you can use this website for insights.

The Higgs Boson

By Jeffrey Kluger

Monday, Nov. 26, 2012

Take a moment to thank this little particle for all the work it does, because without it, you’d be just inchoate energy without so much as a bit of mass. What’s more, the same would be true for the entire universe. It was in the 1960s that Scottish physicist Peter Higgs first posited the existence of a particle that causes energy to make the jump to matter. But it was not until last summer that a team of researchers at Europe’s Large Hadron Collider — Rolf Heuer, Joseph Incandela and Fabiola Gianotti — at last sealed the deal and in so doing finally fully confirmed Einstein’s general theory of relativity. The Higgs — as particles do — immediately decayed to more-fundamental particles, but the scientists would surely be happy to collect any honors or awards in its stead.

POSTED BY Matt Strassler

ON December 1, 2012

Why The Electron Can’t Have a Mass Without the Higgs Field

After a hiatus for a hurricane and a trip to a conference in Asia, I am adding one more article to my series on How the Higgs Field Works, following my series of articles on Fields and Particles. (These sets of articles require a little math and physics background, the sort you’d get in your first few months of a beginning university or pre-university physics class. I’m still thinking about how to structure a similar set of articles that require no math or physics; that’s much harder, of course!)

The first article in the series explained the basic Idea behind how the Higgs field works. Then came an article about why and how the Higgs field becomes non-zero, and a third concerning how the Higgs particle arises as the quantum of waves that oscillate around the non-zero value of the Higgs field. The new article tries to clarify why there’s no alternative to introducing a Higgs field, explaining that it’s otherwise impossible to reconcile two apparently contradictory features of our world: a mass for the electron (and many other types of known particles) and the properties of the weak nuclear force.

This article contains the most elaborate equations and concepts that I’ve had to introduce to my readers, so it won’t be suitable for everyone (though it still only requires some first-year physics/math.) But on the other hand, it seems necessary for me to write it, since it’s the only place that I’ve explained not only why the Higgs field can give mass to the known particles, but why it (or something very much like it) must do so.

(Note that in these articles I’m mainly concentrating on the simplest type of Higgs, the Standard Model Higgs field and particle. However, most of the basic concepts in these articles apply even for more complicated cases.)

POSTED BY Matt Strassler

ON November 27, 2012

Summary of New Higgs Results

As I warned you to expect, there wasn’t anything particularly dramatic in the new results, reported at the HCP conference in Kyoto, on the Higgs-like particle. There just wasn’t enough new data relative to what was available in July for that to happen — a bit less than double for most measurements. That said, there were some interesting developments. Here’s are the summary plots from ATLAS (left) and CMS (right); things are a bit more consistent with the Standard Model Higgs hypothesis (especially for decays to tau’s, b’s and W’s) than they were before, but the precision of the measurements is still pretty low.

HCP_ATLAS_CMS_Higgs — The measurements looking for signals of a Higgs particle at ATLAS (left) and CMS (right). For each process, the dot is the measurement of the rate relative to the Standard Model expectation, and the horizontal bar gives the one-standard-deviation uncertainty. Note all measurements are within two standard deviations of expectations. Some measurements (photons for CMS and ATLAS, ZZ* –> 4 leptons for ATLAS) were not updated for this conference. For details, see the previous post.

Also notable: that the Tevatron experiments’ measurement of Higgs decaying to bottom quarks is now smaller than before and more in line with Standard Model expectations; that the ratio of W to Z coupling strengths is consistent with a simple generic Higgs [i.e custodial symmetry works with uncertainties of 30%]; and at 2.5 standard deviations this is not a parity-odd spin-zero particle, while it remains consistent with being the parity-even spin-zero particle that we’d expect a simple Higgs to be.

For many more details, see my previous post from Kyoto.

POSTED BY Matt Strassler

ON November 14, 2012

Higgs Results at Kyoto

At the HCP conference in Kyoto, we’re expecting to see some updated (and perhaps new) measurements concerning the recently discovered Higgs-like particle. Presentations from the ATLAS and CMS experiments at the Large Hadron Collider [LHC] are coming shortly.

First things first — based on the questions I’m getting from reporters and other non-experts, I need to emphasize right away that it is very unlikely that anything dramatic is going to happen here. ATLAS and CMS only just co-discovered this particle in July; the amount of data that is available now is only about twice as much as was available in July, and that is nowhere near enough for the big questions about the Higgs-like particle to be resolved. We will not know after today whether it is a Higgs at all, whether it is a Standard Model Higgs or not, or whether any particular speculative idea beyond the Standard Model is now excluded. What we are likely to see is some incremental changes in what we know.

Knowledge about nature does not come easy. We discovered the top quark in 1995, and we are still learning about its properties today — there were over a dozen talks about the top quark earlier in this conference. And we will still be learning important things about the Higgs during the coming few decades. We’ve no choice but to be patient.

But here are the new results (below, SM is short-hand for “Standard Model”); see here for a discussion of the main July results: (more…)

POSTED BY Matt Strassler

ON November 14, 2012

Meanwhile, On the Other Side of the Planet

This week I’ve escaped the power outages, the gasoline shortages, the cold and the snow of my home town, and have been visiting warm and humid Hong Kong. Today I gave a colloquium (same one, more or less, as I gave last month at the University of Toronto, on-line here) at the physics department of the Chinese University of Hong Kong (CUHK). CUHK is one of three universities in Hong Kong that are banding together to try to establish a particle physics presence at the Large Hadron Collider (LHC) and beyond.

I’m in Asia because of next week’s Hadron Collider Physics Symposium (HCP 2012), a conference that will be held in Kyoto, Japan. Among other things, many new measurements using 2012 data from the LHC will be presented at this conference, along with updates of the measurements of the properties of the new Higgs-like particle whose discovery was announced in July. Stay tuned for posts on the new LHC results next week.

POSTED BY Matt Strassler

ON November 9, 2012

Garisto’s Reply to Previous Post

Robert Garisto sent me a reply to my previous post; here it is. [A “vev” is shorthand for a non-zero value in the vacuum of space, what I call a “non-zero average value”.]

—

Matt – Thanks for your extensive reply to my comment! Of course I agree that a scalar field without a vev can have a hard mass term. And I do agree that how the Higgs boson gets mass is at least somewhat different than how the W does.

Let’s agree to define a Higgs as a scalar field with a vev. Then I think you agree that the mass of the excitation about the vev, the Higgs boson, is not a hard mass term, one obtains it by finding the minimum of the potential as you did above. Now if there are other scalars with vevs, the mass of the Higgs boson we are concentrating on can depend on those too. But isn’t it correct to say that the mass of such a Higgs boson goes to zero in the limit that all of those vevs go to zero? If so, I would say that the Higgs boson mass is provided by the Higgs fields (all scalars with nonzero vevs).

Anyway, the main reason I made the comment is that for the purposes of explaining to the public electroweak symmetry breaking, I think it makes sense to say that the Higgs boson mass comes from the Higgs field, because it is, in the SM, proportional to the vev. It’s also kind of neat, I think.

—

We disagree, that’s all there is to it. What Garisto says about the Standard Model is a simple consequence of dimensional analysis, not a fundamental relation that applies widely. And no, it is not correct to say that the mass of a Higgs boson always goes to zero in the limit that all vevs go to zero; there can be first order phase transitions in which, as the parameters change, the Higgs field’s vev jumps from non-zero to zero abruptly, and the mass of the Higgs particle is never zero. So I think to tell the public that the Higgs particle gets its mass from the Higgs field is to confuse them into thinking that the Higgs particle gets its mass the same way the other known particles do — which is false.

But in any case, we agree it’s not that big a deal. The thing which is important for the public to understand is that the Higgs field does not give mass to all massive objects — such as atomic nuclei and black holes. And the thing which it is important for particle physics students to understand is that the Higgs mass is not generically proportional to the vev of the Higgs field.

POSTED BY Matt Strassler

ON October 24, 2012

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The Higgs Boson

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