Category Archives: LHC News

A Few Items of Interest

I was sent or came across a few interesting links that relate to things covered on this blog and/or of general scientific interest.

It was announced yesterday that the European Physical Society 2013 High Energy Physics Prize was awarded to the collaboration of experimental physicists that operate the ATLAS and CMS experiments that discovered a type of Higgs particle, with special mention to Michel Della Negra, Peter Jenni, and Tejinder Virdee, for their pioneering role in the development of ATLAS and CMS.  Jenni and Virdee are both at the LHCP conference in Barcelona, which I’m also attending, and it has been a great pleasure for all of us here to be able to congratulate them in person .

One thing that came up a couple of times regarding weather forecasting (for instance, in forecasting the path of Hurricane Sandy) is that the European weather forecasters are doing a much better job of predicting storms a week in advance than U.S. forecasters are.  And I was surprised to learn that one of the the main reasons is simple: U.S. forecasters have less computing power than their European counterparts, which sounds (and is) ridiculous.  The new director of the U.S. National Weather Service, Louis Uccellini, has been successful in his goal of improving this situation, as reported here[Thanks to two readers for pointing me to this article.]

One of the possible interpretations of the new class of high-energy neutrinos reported by IceCube (see yesterday’s post) is that they come from the slow decay of a small fraction of the universe’s dark matter particles, assuming those particles have a mass of a couple of million GeV/c². [That's much heavier than the types of dark matter particles that most people are currently looking for, in searches that I discussed in a recent article.]  I didn’t immediately mention this possibility (which is rather obvious to an expert) because I wanted a couple of days to think about it before generating a stampede or press articles.  But, not surprisingly, people who were paying more attention to what IceCube has been up to had recently written a paper on this subject[Here's an older, related paper, but at much lower energy; maybe there are other similar papers that I don't know about?]  At the time these authors wrote this paper, only the two highest energy neutrinos — which have energies that, within the uncertainties of the measurements, might be equal (see Figure 2 of yesterday’s post) — were publicly known.  In their paper, they predicted that (just as any expert would guess) in addition to a spike of neutrinos, all at about 1.1 million GeV, one would also find a population of lower-energy neutrinos, similar to those new neutrinos that IceCube has just announced. So yes, among many possibilities, it appears that it is possible that the new neutrinos are from decaying dark matter.  If more data reveals that there really is a spike of neutrinos with energy around 1.1 million GeV, and the currently-observed gap between the million-GeV neutrinos and the lower-energy ones barely fills in at all, then this will be extremely strong evidence in favor of this idea… though it will be another few years before the evidence could become convincing.  Conversely, if IceCube observes any neutrinos near but significantly above 1.1 million GeV, that would show there isn’t really a spike, disfavoring this particular version of the idea.

Regarding yesterday’s post, it was pointed out to me that when I wrote “The only previous example of neutrinos being used in astrophysics occurred with the discovery of neutrinos from the relatively nearby supernova, visible with the naked eye, that occurred in 1987,” I should also have noted that neutrinos were and are used to understand the interior of the sun (and vice versa).  And you could even perhaps say that atmospheric neutrinos have been used to understand cosmic rays (and vice versa.)

In sad news, in the “all-good-things-must-come-to-an-end” category, the Kepler spacecraft, which has brought us an unprecedented slew of discoveries of planets orbiting other stars, may have reached the end of the line (see for example here), at least as far as its main goals.  It’s been known for some time that its ability to orient itself precisely was in increasing peril, and it appears that it has now been lost.  Though this has occurred earlier than hoped, Kepler survived longer than its core mission was scheduled to do, and its pioneering achievements, in convincing scientists that small rocky planets not unlike our own are very common, will remain in the history books forever.  Simultaneous congratulations and condolences to the Kepler team, and good luck in getting as much as possible out of a more limited Kepler.

Opening of LHCP Conference

Greetings from Barcelona, where the LHCP 2013 conference is underway. I wanted to mention a couple of the opening remarks made by CERN’s Sergio Bertolucci and Mirko Pojer, both of whom spoke about the near-term and medium-term future of the Large Hadron Collider [LHC]. Continue reading

Higgs Workshop in Princeton

Today I’m attending the first day of a short workshop of particle theorists and experimentalists at the Princeton Center for Theoretical Science, a sort of “Where are we now and where are we going?” meeting. It’s entitled “Higgs Physics After Discovery”, but discussion will surely range more widely.

What, indeed, are the big questions facing particle physics in the short-term, meaning the next few months? Well, here are a few key ones:

  • A Higgs particle of some type has been discovered by the ATLAS and CMS experiments at the Large Hadron Collider [LHC] (with some contributions from the Tevatron experiments DZero and CDF); is it the simplest possible type of Higgs particle (the “Standard Model Higgs“) or is it more complex? What data analysis can be done on the LHC’s data from 2011-2012 to shed more light on this question?
  • More generally, from the LHC’s huge data set from 2011-2012 — specifically, from the data analysis that has been done so far — what precisely have we learned? (It’s increasingly important to go beyond the rougher estimates that were appropriate last year when the data was still pouring in.) What types of new phenomena have been excluded, and to what extent?
  • What other types of data analysis should be done on the 2011-2012 data, in order to look for other new phenomena that could still be lurking there? (There’s still a lot to be done on this question!) And what types of work should theoretical particle physicists do to help the experimentalists address this issue?
  • Several experiments from the Tevatron and the LHC, notably the LHCb experiment, have learned that newly measured decays of  certain mesons (hadrons with equal numbers of quarks and anti-quarks) that contain heavy quarks are roughly consistent with the Standard Model (the equations we use to describe the known elementary particles and forces, and a simplest type of Higgs field and Higgs particle.) How do these findings constrain the possibility of other new phenomena?
  • Looking ahead to 2015, when the LHC will begin running again at a higher energy per proton-proton collision, what preparations need to be made? Especially, what needs to be done to refine the triggering systems at ATLAS, CMS and LHCb, so that the maximum information can be extracted from the new data, and no important information is unnecessarily discarded?
  • Which, if any, of the multiple (but mostly mutually inconsistent) experimental hints of dark matter should be taken seriously? Which possibilities do the various dark matter experiments, and the LHC’s data, actually exclude or favor?

That might be it for the very near term. There are lots of other questions in the medium- to long-term, among which is the big question of what types of experiments should be done over the next 10 – 20 years. One challenge is that the LHC’s data hasn’t yet given us a clear target other than the Higgs particle itself. An obvious possible experiment to do is to study the Higgs in more detail, using an electron/anti-electron collider — historically this has been a successful strategy that has been used on almost every new apparently-elementary particle. But there are a lot of other possibilities, including raising the LHC’s collisions to even higher energy than we’ll see in 2015, using more powerful magnets currently under development.

If there are other near-term questions I’ve forgotten about, I’m sure I’ll be reminded at the workshop, and I’ll add them in.

Review of the Higgs-to-2-Photon Data

Since it’s been the main news story of the last week, perhaps it would be useful to do a quick summary of what the CMS and ATLAS experiments at the Large Hadron Collider [LHC] have been saying, over the past fifteen months, about their search for the process in which a Higgs particle is produced and decays to two photons.

Before we start, let me remind you that in statements about how uncertain a measurement is (and all measurements have some level of uncertainty — no knowledge is perfect), a “σ”, or “sigma”, is a statistical quantity called a “standard deviation”; a 5σ discrepancy from expectations is impressive, 3σ intriguing; but anything less than 2σ is very typical, and indicative merely of the usual coming and going of statistical flukes and fluctuations of real data around the truth. Note also that the look-elsewhere effect has to be accounted for; but usually a 5σ discrepancy without the look-elsewhere effect is enough to be convincing. And of course a discrepancy may mean either a discovery or a mistake; that’s why it is important that two experiments, not just one, see a similar discrepancy, since it is unlikely that both experiments would make the same mistake.

Ok: here are the results as they came in over time, all the way back to the inconclusive hints of 15 months ago.

December 2011:

  • ATLAS (4.9 inverse fb of data at 7 TeV): excess 2.8σ (where 1.4σ would be expected for a SM Higgs); less than 2σ after accounting for “look-elsewhere effect”.
  • CMS: (4.8 inverse fb of data at 7 TeV): excess just over 2σ (where 1.4σ would be expected for a SM Higgs); much less than 2σ after accounting for “look-elsewhere effect”.

July 2012:

  • ATLAS: (reanalyzing the 7 TeV data and adding 5.9 inverse fb of data at 8 TeV): signal 4.5σ (where 2.4 was expected for a SM Higgs); 3.6σ after “look-elsewhere effect”; best estimate of size of signal divided by that for a SM Higgs: 1.9 ± 0.5 (about 1.8σ above the SM prediction)
  • CMS (reanalyzing the 7 TeV data and adding 5.3 inverse fb of data at 8 TeV): signal 4.1σ (where 2.5 was expected for a SM Higgs); 3.2σ after “look-elsewhere effect”; best estimate of size of signal divided by that for a SM Higgs: 1.6 ± 0.4 (about 1.5σ above the SM prediction)

November/December 2012:

  • ATLAS: (increasing the 8 TeV data to 13.0 inverse fb): signal 6.1σ (3.3 expected for SM Higgs); 5.4σ when look elsewhere is accounted for; best estimate of size of signal divided by that for a SM Higgs: 1.8 ± 0.4 (about 2σ above the SM prediction)
  • CMS: No update

March 2013:

  • ATLAS: (taking the full 7 and 8 TeV data sets): 7.4σ (4.1 expected for a SM Higgs); best estimate of size of signal divided by that for a SM Higgs: 1.65 ± 0.30 (slightly more than 2σ above the SM prediction)
  • CMS: (taking the full 7 and 8 TeV data sets) uses two different methods as a cross-check, one of them complex and (on average) more powerful, the other simpler but (on average) less powerful. For the best estimate of size of signal divided by that for a SM Higgs: one method gives 0.8 ± 0.3 and the other gives 1.1 ± 0.3. Both of these are within 1σ of the SM prediction and within 2σ of the CMS July result.

To understand how consistent the two new CMS results are with each other, you have to consider how the two studies are correlated (since they are selecting events for study from the same pile of data.)  Because the two methods select and discard candidate events in two different ways, they don’t include the exact same data.  CMS’s simulation studies indicate that about 50 percent of the background events and 80 percent of the signal events are common to the two studies. In the end, the conclusion (see the figure below) is that the two results are consistent at 1.5σ (and at 1.8 if one considers only the 8 TeV data) — in other words, reasonably consistent with one another.

You can also ask how consistent are the new results compared to the old ones from July. When you observe that the uncertainty on the July result was very large (1.6 ± 0.4 times the Standard Model prediction, i.e. a 25% uncertainty at 1σ, 50% uncertainty at 2σ) it should not surprise you that CMS claims that their new results are both consistent with the old ones at below the 2σ level.

Slide from Moriond-QCD conference talk presenting CMS's results, and looking at the compatibility of the two results with each other (top two lines in the table) and each of the two results with the previous published results.  Note the conclusion in the last line.

Slide from Moriond-QCD conference talk presenting CMS’s results, and looking at the compatibility of the two results with each other (top two lines in the table) and of each of the two new results with the previous published results. Note the conclusion in the last line.

Meanwhile, all of the ATLAS results are closely compatible with each other. This is more what one would naively expect, but not necessarily what actually happens in real data. Of course ATLAS’s results aren’t giving a consistent mass for the new particle yet, whereas CMS’s are doing so… well, this is what happens with real data, folks.

The real issue is whether ATLAS’s measurements and CMS’s measurements of the two photon rate are compatible with each other. Currently they are separated by at least 2σ and maybe as much as 3σ (a very rough estimate), which is not unheard of but is somewhat unusual. Well, whether the cause is an error or a statistical fluke or both, it unfortunately leaves us in a completely ambiguous situation. On the one hand, CMS’s results agree with the Standard Model prediction to within about 1σ. On the other hand, ATLAS’s results are in tension with the Standard Model prediction by a bit more than 2σ. We have no way to know which result is closer to the truth — especially when we recall that the uncertainty in the Standard Model prediction is itself about 20%. If ATLAS and CMS had both closely agreed with the Standard Model we’d be confident that any deviations from the Standard Model are too small to observe; if they both significantly disagreed in the same way, we’d be excited about the possibility that the Standard Model might be about to break down. But with the current results, we don’t know what to think.

So as far as the Higgs particle’s decays to two photons, we’ve gotten as much (or almost as much) information as we’re going to get for the moment; and we have no choice but to accept that the current situation is ambiguous and to wait for more data in 2015. Of course the Standard Model may break down sooner than 2015, for some other reason that the experimenters have yet to uncover in the 2011-2012 data. But the two-photon measurement won’t be the one to crack the armor of this amazing set of equations.  (For those who got all excited last July; you were warned that the uncertainties were very large and the excess might well be ephemeral.)

From “Higgs-like Particle” to “Standard Model-like Higgs”

As soon as the discovery of that famous new particle was announced at the Large Hadron Collider [LHC] last year, there were already very good reasons to think it was a Higgs particle of some type. I described them to you back then, as part of my “Higgs Discovery” series.  But, as I cautioned, those arguments relied partly on data and partly on theoretical reasoning.

Over the past nine months, with additional data collected through December and analyzed through the present day, it has become more and more convincing that this particle behaves very much like a Higgs particle, along the lines I described following the Edinburgh conference back in January.  One by one the doubters have been giving up, and few remain.  This is a Higgs particle.  That’s my point of view (see last week’s post — you heard it here first), the point of view of most experts I talk to [in a conference I'm currently attending, not one person out of about forty theorists and experimenters has dissented], and now the official point of view of the CERN laboratory which hosts the LHC. Continue reading

CMS sees no excess in Higgs decays to photons

Quick post: the CMS experiment at the Large Hadron Collider [LHC] has updated its measurement of the rate for Higgs particles to be produced and then decay to two photons.  We’ve been waiting for this result with considerable interest.  Recall the history: in July, both CMS and its cousin, the ATLAS experiment, found this process to be in excess of the prediction of the Standard Model [the equations we use to describe the known elementary particles and forces].  Indeed, these excesses were part of why the Higgs particle was discovered a few months earlier than was widely expected.   Although it was exciting that both experiments saw something amiss, the statistical significance of these excesses wasn’t that high, so more data to confirm the excesses was needed before we could take them very seriously. Continue reading