Of Particular Significance

Category: Particle Physics

Theory Killers at the HCP conference

There were many interesting results presented yesterday at the HCP conference in Kyoto, and they were both too numerous and too detailed for me to completely absorb as yet — a follow-up will clearly be needed. But a few are obviously so important that I want to point them out now.

First, both ATLAS and CMS, the two general purpose experiments at the Large Hadron Collider [LHC], produced important new results on “multileptons”. Based on a significant fraction of their 2012 data, they looked for signs of new phenomena that would appear as proton-proton collisions that produce at least three leptons or anti-leptons, or even (in unusual combinations and/or along with other unusual things) two leptons or anti-leptons. (I’ll just summarize this class of studies as “multileptons” for the purpose of this brief post and be more specific at a later date.) ATLAS used about 50% more data than CMS, but CMS had a more intricate analysis of their data, so I believe the results were similar where they can be compared. [By the way, the CMS result was approved to be shown at this conference under extreme conditions; at least two of the major players in the analysis had no power or internet for over a week following Hurricane Sandy!]

The bottom line is simple: neither CMS nor ATLAS sees any significant deviation from what is predicted by the Standard Model. And this now kills off another bunch of variants of many different speculative ideas. The details are extremely complicated to describe, but essentially, what’s dead is any theory variant that leads to many proton-proton collisions containing

two or more top quark/anti-quark pairs
multiple W and Z particles
two or more as-yet unknown moderately heavy particles that often decay to muons, electrons and/or their anti-particles
new moderately heavy particles that decay to many tau leptons

and probably a few others I’m forgetting. While multilepton searches (especially those for 3 or more leptons) are often touted as a great way to look for supersymmetry in particular, that description vastly understates their power — they are a great way to look for many different types of phenomena not predicted in the Standard Model. (This is something that a number of scientists at Rutgers University have been emphasizing in talks and papers.) And both experiments have demonstrated this with various interpretations of their results; CMS has over a dozen of them! (more…)

POSTED BY Matt Strassler

ON November 13, 2012

“Supersymmetry Dealt a Blow”?

One of the challenges of being a science journalist is conveying not only the content of a new scientific result but also the feel of what it means. The prominent article in the BBC about the new measurement by the LHCb experiment at the Large Hadron Collider [LHC] (reported yesterday at the HCP conference in Kyoto — I briefly described this result yesterday) could have been worse. But it has a couple of real problems characterizing the implications of the new measurement, so I’d like to comment on it.

The measurement is of how often B_s mesons (hadrons containing a bottom quark and a strange anti-quark, or vice versa, along with many quark/anti-quark pairs and gluons) decay to a muon and an anti-muon. This process (which I described last year — only about one in 300,000,000 B_s mesons decays this way) has three nice features:

it can be well-predicted in the Standard Model (the equations we use to describe the known particles and forces, including the simplest type of Higgs particle)
it is relatively straightforward to measure, and
it is very sensitive to effects of as-yet unknown particles and forces.

Yesterday the LHCb experiment reported the evidence for this process, at a rate that is consistent (but see below) with the prediction of the Standard Model.

The worst thing about the BBC article is the headline, “Supersymmetry theory dealt a blow” (though that’s presumably the editor’s fault, as much as or more than the author’s) and the ensuing prose, “The finding deals a significant blow to the theory of physics known as supersymmetry.” What’s wrong with it? It’s certainly true that the measurement means that many variants of supersymmetry (of which there are a vast number) are now inconsistent with what we know about nature. But what does it mean to say a theory has suffered a blow? and why supersymmetry?

First of all, whatever this new measurement means, there’s rather little scientific reason to single out supersymmetry. The rough consistency of the measurement with the prediction of the Standard Model is a “blow” (see below) against a wide variety of speculative ideas that introduce new particles and forces. It would be better simply to say that it is a blow for the Standard Model — the model to beat — and not against any speculative idea in particular. Supersymmetry is by no means the only idea that is now more constrained than before. The only reason to single it out is sociological — there are an especially large number of zealots who love supersymmetry and an equal number of zealots who hate it.

Now about the word “blow”. New measurements usually don’t deal blows to ideas, or to a general theory like supersymmetry. That’s just not what they do. They might deal blows to individual physicists who might have a very particular idea of exactly which variant of the general idea might be present in nature; certain individuals are surely more disappointed than they were before yesterday. But typically, great ideas are relatively flexible. (There are exceptions — the discovery of a Higgs particle was a huge blow to the idea behind “technicolor” — but in my career I’ve seen very few.) It is better to think of each new measurement as part of a process of cornering a great idea, not striking and injuring it — the way a person looking for treasure might gradually rule out possibilities for where it might be located.

Then there’s the LHCb scientist who is quoted as saying that “Supersymmetry may not be dead but these latest results have certainly put it into hospital”; well… Aside from the fact that this isn’t accurate scientifically (as John Ellis points out at the end of the article), it’s just not a meaningful or helpful way to think about what’s going on at the LHC. (more…)

POSTED BY Matt Strassler

ON November 13, 2012

First News from Kyoto Conference

The HCP [Hadron Collider Physics] 2012 conference in Kyoto is underway. After opening talks laying out the field’s future, the main topics today have been

Collisions of heavy ions (specifically of lead or gold nuclei) and generic proton-proton collisions
Processes involving “heavy flavor” (meaning in this case the properties of hadrons containing bottom and charm quarks.)

Although there were a number of interesting new results from several experiments, today’s highlight so far has been a presentation on a new measurement by the LHCb experiment, one of the special-purpose experiments at the Large Hadron Collider [LHC], of a rare decay of a B_s meson to a muon and an antimuon. I described this process in some detail, and claims and counterclaims about it, in the first portion of an article last year; the details of the measurements are out of date, but the physics process is, of course, the same.

Today the LHCb experiment, for the first time, announced evidence for the existence of this process, using their data collected in both 2011 and 2012. In the Standard Model (the equations that describe the known particles and forces), it is predicted that about one in about 300,000,000 B_s mesons (hadrons containing a bottom quark and a strange anti-quark, or vice versa) should decay in this fashion. The measurement that LHCb has made is completely consistent with this prediction.

[In detail, the Standard Model predicts (3.54 ± 0.30) × 10^-9 {including mixing effects} and LHCb measures (3.2 +1.5[−1.2])× 10^-9.] The measurement is at the level of 3.5 standard deviations — evidence, but not yet a convincing observation. LHCb excludes a rate of zero at much better than 95% confidence; their 95% confidence lower bound on the process is 1.1 × 10^-9.]

The detailed implications of this result will take a while to work through, but the general implication is easy to state: the Standard Model has survived another test. And the constraints from LHC data on speculative ideas that predict particles and forces beyond those of the Standard Model have become tighter. Many variants of these speculative ideas would have affected this process, and the more precisely the data matches the Standard Model prediction, the more of these variants are excluded by the data.

Note Added: CMS says they a very good chance of being able to confirm LHCb’s result using the full 2012 data; not sure what ATLAS says about their chances.

2nd Note Added: Gino Isidori, in his talk on the theoretical perspective on heavy flavor physics, emphasized that a future high-precision measurement of this process will be very important; it can be predicted with high precision, and interesting variants of speculative ideas often have effects on its rate that are between 10% and 100%.

POSTED BY Matt Strassler

ON November 12, 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

Does the Higgs Field Give the Higgs Particle Its Mass, or Not?

When I wrote my article last week about the relation between the Higgs and gravity, emphasizing that there really was no relation at all, I said that the Higgs field is not the universal giver of mass. I cited four reasons:

The Higgs field does not give an atomic nucleus all of its mass, and since the nucleus is the vast majority of the mass of an atom, that means it does not provide all of the mass of ordinary matter.
Black holes appear at the centers of galaxies, and they appear to be crucial to galaxy formation; but the Higgs field does not provide all of a black hole’s mass. In fact the Higgs field’s contribution to a black hole’s mass can even be zero, because black holes can in principle be formed from massless objects, such as photons.
There is no reason to think that dark matter, which appears to make up the majority of the masses of galaxies and indeed of all matter in the universe, is made from particles that get all of their mass from the Higgs field.
The Higgs field, though it provides the mass for all other known particles with masses, does not provide the Higgs particle with its mass.

Although it doesn’t matter too much to the main point of the Higgs-and-gravity article (since the first three points are not in question), the editor of a leading physics journal, Robert Garisto, took issue with the fourth point, arguing that I was making a statement that really wasn’t right, or at least is too strong. His argument has some merit, though in the end, I stick with my statement. I think it’s worth describing what he had in mind (as best I understand it) and why I feel strongly that one should think about it differently. There are some semantic aspects to the disagreement, but there are also some interesting and important subtle scientific points. I don’t want to suggest that this discussion is really that big a deal — the very fact that we can argue about whether the Higgs field does or doesn’t provide the Higgs particle with its mass distinguishes the Higgs particle from, say, the W particle, whose mass indisputably arises from the Higgs field. But there’s something to learn here about quantum field theory and how the Higgs mechanism works. (more…)

POSTED BY Matt Strassler

ON October 23, 2012

Colloquia Here and There

This Wednesday I was visiting the University of Massachusetts, in the currently colorful town of Amherst, where I gave a colloquium (an hour-long talk aimed at a physics department’s undergraduate majors, graduate students and faculty who are not themselves experts in particle physics) entitled The Quest for the Higgs Boson. It’s similar to the one I gave two weeks ago at the University of Toronto, which is available on-line now. There’s audio and there are slides, but no video, so I’m afraid you’ll have to figure out on your own how the slides and audio fit together; but I think it should be fairly obvious.

If, however, you’re not a physicist or physics student, but you have been following particle physics a little bit, perhaps by reading this blog or Scientific American articles or books for laypeople by, say, Brian Greene or Lisa Randall, then you might instead want to try listening to this lecture I gave recently, which is also in the form of an audio feed plus slides. It makes far fewer assumptions about what audience members are familiar with. And of course there’s always my [in]famous video clips from my March 2011 public lecture on the Large Hadron Collider; a bit out of date since they were made before the new Higgs-like particle was found, but still basically covering what you need to know.

Please note these presentations are under copyright.

POSTED BY Matt Strassler

ON October 19, 2012

New Article on the Non-Zero Value of the Higgs Field

Following on my series of articles on Fields and Particles, I’m building my next series of articles, on How the Higgs Field Works. (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.)

The first article in the new series was an overview of The Basic Idea behind how the Higgs field works. I recently revised it to make it easier to read.

The next article, just completed, is about why and how the Higgs field becomes non-zero — to the extent that we understand it. (The following article will explain how the Higgs particle arises.)

POSTED BY Matt Strassler

ON October 17, 2012

Why the Higgs and Gravity are Unrelated

One of the questions I get most often from my readers is this:

Since gravity pulls on things proportional to their mass, and since the Higgs field is responsible for giving everything its mass, there obviously must be a deep connection between the Higgs and gravity… right?

It’s a very reasonable guess, but — it turns out to be completely wrong. The problem is that this statement combines a 17th century notion of gravity, long ago revised, with an overly simplified version of a late-20th century notion of where masses of various particles comes from. I’ve finally produced the Higgs FAQ version 2.0, intended for non-experts with little background in the subject, and as part of that, I’ve answered this question. But since the question is so common, I thought I’d also put the answer in a post of its own.

As preface, let me bring out my professorial training and correct the question above with a red pen:

Since gravity pulls on things proportional to their ~~mass~~ to a combination of their energy and momentum, and since the Higgs field is responsible of giving ~~everything~~ not everything, just the known elementary particles excepting the Higgs particle itself ~~its~~ mass, there ~~obviously must be a deep connection~~ between the Higgs and gravity… ~~right?~~ wrong.

Now let me explain these corrections one by one. (more…)

POSTED BY Matt Strassler

ON October 15, 2012

Why the Higgs Crew Waits for a Nobel

I was busy with some personal issues over the past few weeks, so I hadn’t even really been following the rampant speculation about the Nobel Prize for this year. Apparently a lot of people (including some of my colleagues) thought that the July discovery, at the Large Hadron Collider [LHC] experiments ATLAS and CMS, of a new particle resembling the long-sought Higgs particle would generate a Nobel Prize in 2012 for Peter Higgs and for the other physicists who predicted the existence of such a particle.

Well, I never thought this notion was very plausible; I was confident it wouldn’t happen before 2013. And that is for several reasons.

The first and most important is that although the evidence that a new particle has been found is very strong, the evidence that it is a Higgs particle of some type (which I’ll describe below) is still only moderate. Personally, I’m convinced that the new particle is a Higgs particle, but that is based partly on the evidence from the data and partly on theoretical prejudice — on my knowledge of theoretical physics and of what the alternatives to the Higgs-particle interpretation of the data are. If I’m wrong, too bad for me, but no harm done. However, the Nobel Prize committee is making a permanent, irrevocable award for the history books, and the bar for evidence from the data alone should be very high. Now here’s the key point: by March 2013 at the latest, the data from all of 2012 will have been analyzed. The amount of data that will be available by then will be about three times as much as was available in July 2012 — enough to change the current moderate evidence to strong evidence, if in fact we’re dealing with a Higgs particle of some type. The Nobel Prize committee is surely well aware of this — that by next year the situation is likely to have qualitatively changed, with the evidence beyond controversy. So it makes sense to wait until 2013, when the case is likely to be closed. (more…)

POSTED BY Matt Strassler

ON October 11, 2012

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