Of Particular Significance

Tag: cms

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

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

Remember That “Blow” to Supersymmetry (And Other Theories)?

According to the BBC, it was a heavy blow. According to a member of the LHCb experiment quoted in the article, it put the theory “in the hospital.” The reality? Nobody even suffered a scratch.

On Monday I wrote about a new measurement by the LHCb experiment at the Large Hadron Collider [LHC] of a rare process, reported at the HCP 2012 conference in Kyoto (a link to the talks can be found at this link), in which a B_s meson decays to a muon and an anti-muon (click here for more details of the physics process.) It’s a very important measurement, definitely! But whilelistening to theorist Gino Isidori’s talk in which he briefly discussed this measurement, I was a little puzzled about an inconsistency between what LHCb had done and said in the past, and what they had done and were saying now, in particular as was reported/implied by the BBC.

When I chatted with him later, Isidori reminded me exactly what LHCb reported in March, and how it compares to what they report now.

March: LHCb reported that at most 4.5 per billion B_s mesons decay this way (at 95% confidence)
November: LHCb reports about 3.5 per billion B_s mesons decay this way, and at 95% confidence the rate is at least 1.1 per billion and at most 6.4 per billion.

Notice that the new measurement raises the upper limit on how often this process occurs. This upward shift is not an indication of a problem; it’s probably just an ordinary statistical effect that arises from having small amounts of data. But it makes the constraints from this measurement — on the many variants of supersymmetry, and on other theories — a little weaker, if anything. Most of the supersymmetric (and other) models that would be constrained by this measurement lead to a higher rate than in the Standard Model (where it is predicted to be about 3.2 ± 0.3 per billion). So a higher upper limit means fewer of these variants are excluded by the data. All in all, the constraints on supersymmetric (and other) models are little changed, and perhaps somewhat weaker than they were in March. Isidori and his colleagues have worked this out, and I’ll try to get details from them next week.

No need to take my word for it, or even Isidori’s. Professor Michelangelo Mangano of CERN, apparently having spoken independently to other experts, made exactly the same point on page 41 of his summary talk concluding the conference. (By the way, it was a great talk, and I recommend that experts read it.)

Well! So much for the big BBC headline! [Most likely the public will never learn about this; a news report describing more accurately what this really means for supersymmetry etc. probably will not appear at all on the BBC, and even if it does, it certainly won’t get a big attention-grabbing headline. It’s sad that this inherent bias in media reporting ensures the public gets an unhealthy dose of incorrect scientific information and rarely gets the antidote.]

None of this at all diminishes LHCb’s accomplishment! They’ve made a great measurement, for which they deserve big congratulations. A round of applause, please!! But let’s not overstate its immediate impact. As the measurement becomes more precise, its impact will gradually become greater, and even more so when it is combined with similar measurements from ATLAS and CMS.

But meanwhile, there were powerful and truly new constraints on supersymmetry (and other theories) reported at this conference, and they came from ATLAS and CMS, in their searches for effects from superpartner (and other) particles. I told you about a small number of these searches a couple of days ago (by the way, I learned meanwhile that CMS has a search that is similar to the one I mentioned from ATLAS involving bottom quarks) and maybe I’ll point out a few others next week, if I have the energy.

POSTED BY Matt Strassler

ON November 16, 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

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

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

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

From String Theory to the Large Hadron Collider

The huge Milner prizes for nine well-known scientists, and the controversy they generated, have motivated me to relate a story. It happened during the theorist/experimentalist workshop that was held in early August (see also here) at the Perimeter Institute in Waterloo, Canada. And it illuminates something that many scientists, science commentators and science journalists, as well as science fans in the public, seem to be unaware of, but ought to know.

Before I start, I want to make one thing clear. I am by no means a flag waver for the string theory community; the theory’s been spectacularly over-hyped, and the community’s political control of high-energy physics in many U.S. physics departments has negatively impacted many scientific careers, including my own. On the other hand, I am also not going to tell you that string theory, as a theory, is somehow evil incarnate; I have done a certain amount of string theory research, and not only have I learned a great deal from it that I could not have learned any other way, doing the research had a positive effect on my career. So I feel it is unfortunate that string theory has been a political football, with two violent teams trying to kick the ball toward their opponents’ goal posts. From my perspective, the game is irrational and preposterous, reasonable people were long ago refusing to play it, and it is high time the ball were grabbed by the referee and placed quietly in the middle of the field where it belongs.

My story takes place on the evening of Friday, August 3rd, following the second day of the workshop, which brought together theorists and CMS experimentalists for discussions concerning research strategies at the Large Hadron Collider [LHC]. (more…)

POSTED BY Matt Strassler

ON August 15, 2012

The Trigger and the Parking Lot

I told you a few days ago about the workshop I attended recently at the Perimeter Institute (PI), which brought a number of particle theorists together with members of the CMS experiment, one of the two general purpose experiments operating at the Large Hadron Collider [LHC]. I described the four main areas of discussion, and mentioned a fifth issue that underlies them all: triggering. Today I’m going to explain to you one of two big advances in triggering at CMS that have recently been made public, the one called “Data Parking”. And I’ll also describe my small role in Data Parking over the last ten months, which will explain, in retrospect, some of the other articles that have appeared on this site during that period.

[This article is almost entirely about CMS, but CMS’s competitor, ATLAS, has also made public that they have something essentially identical (I think!) to Data Parking, which they call “Delayed Data Streaming”.]

Click here to read the rest of this article.

POSTED BY Matt Strassler

ON August 13, 2012

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