Category Archives: Other Collider News

What if the Large Hadron Collider Finds Nothing Else?

In my last post, I expressed the view that a particle accelerator with proton-proton collisions of (roughly) 100 TeV of energy, significantly more powerful than the currently operational Large Hadron Collider [LHC] that helped scientists discover the Higgs particle, is an obvious and important next steps in our process of learning about the elementary workings of nature. And I described how we don’t yet know whether it will be an exploratory machine or a machine with a clear scientific target; it will depend on what the LHC does or does not discover over the coming few years.

What will it mean, for the 100 TeV collider project and more generally, if the LHC, having made possible the discovery of the Higgs particle, provides us with no more clues?  Specifically, over the next few years, hundreds of tests of the Standard Model (the equations that govern the known particles and forces) will be carried out in measurements made by the ATLAS, CMS and LHCb experiments at the LHC. Suppose that, as it has so far, the Standard Model passes every test that the experiments carry out? In particular, suppose the Higgs particle discovered in 2012 appears, after a few more years of intensive study, to be, as far the LHC can reveal, a Standard Model Higgs — the simplest possible type of Higgs particle?

Before we go any further, let’s keep in mind that we already know that the Standard Model isn’t all there is to nature. The Standard Model does not provide a consistent theory of gravity, nor does it explain neutrino masses, dark matter or “dark energy” (also known as the cosmological constant). Moreover, many of its features are just things we have to accept without explanation, such as the strengths of the forces, the existence of “three generations” (i.e., that there are two heavier cousins of the electron, two for the up quark and two for the down quark), the values of the masses of the various particles, etc. However, even though the Standard Model has its limitations, it is possible that everything that can actually be measured at the LHC — which cannot measure neutrino masses or directly observe dark matter or dark energy — will be well-described by the Standard Model. What if this is the case?

Michelson and Morley, and What They Discovered

In science, giving strong evidence that something isn’t there can be as important as discovering something that is there — and it’s often harder to do, because you have to thoroughly exclude all possibilities. [It’s very hard to show that your lost keys are nowhere in the house — you have to convince yourself that you looked everywhere.] A famous example is the case of Albert Michelson, in his two experiments (one in 1881, a second with Edward Morley in 1887) trying to detect the “ether wind”.

Light had been shown to be a wave in the 1800s; and like all waves known at the time, it was assumed to be a wave in something material, just as sound waves are waves in air, and ocean waves are waves in water. This material was termed the “luminiferous ether”. As we can detect our motion through air or through water in various ways, it seemed that it should be possible to detect our motion through the ether, specifically by looking for the possibility that light traveling in different directions travels at slightly different speeds.  This is what Michelson and Morley were trying to do: detect the movement of the Earth through the luminiferous ether.

Both of Michelson’s measurements failed to detect any ether wind, and did so expertly and convincingly. And for the convincing method that he invented — an experimental device called an interferometer, which had many other uses too — Michelson won the Nobel Prize in 1907. Meanwhile the failure to detect the ether drove both FitzGerald and Lorentz to consider radical new ideas about how matter might be deformed as it moves through the ether. Although these ideas weren’t right, they were important steps that Einstein was able to re-purpose, even more radically, in his 1905 equations of special relativity.

In Michelson’s case, the failure to discover the ether was itself a discovery, recognized only in retrospect: a discovery that the ether did not exist. (Or, if you’d like to say that it does exist, which some people do, then what was discovered is that the ether is utterly unlike any normal material substance in which waves are observed; no matter how fast or in what direction you are moving relative to me, both of us are at rest relative to the ether.) So one must not be too quick to assume that a lack of discovery is actually a step backwards; it may actually be a huge step forward.

Epicycles or a Revolution?

There were various attempts to make sense of Michelson and Morley’s experiment.   Some interpretations involved  tweaks of the notion of the ether.  Tweaks of this type, in which some original idea (here, the ether) is retained, but adjusted somehow to explain the data, are often referred to as “epicycles” by scientists.   (This is analogous to the way an epicycle was used by Ptolemy to explain the complex motions of the planets in the sky, in order to retain an earth-centered universe; the sun-centered solar system requires no such epicycles.) A tweak of this sort could have been the right direction to explain Michelson and Morley’s data, but as it turned out, it was not. Instead, the non-detection of the ether wind required something more dramatic — for it turned out that waves of light, though at first glance very similar to other types of waves, were in fact extraordinarily different. There simply was no ether wind for Michelson and Morley to detect.

If the LHC discovers nothing beyond the Standard Model, we will face what I see as a similar mystery.  As I explained here, the Standard Model, with no other particles added to it, is a consistent but extraordinarily “unnatural” (i.e. extremely non-generic) example of a quantum field theory.  This is a big deal. Just as nineteenth-century physicists deeply understood both the theory of waves and many specific examples of waves in nature  and had excellent reasons to expect a detectable ether, twenty-first century physicists understand quantum field theory and naturalness both from the theoretical point of view and from many examples in nature, and have very good reasons to expect particle physics to be described by a natural theory.  (Our examples come both from condensed matter physics [e.g. metals, magnets, fluids, etc.] and from particle physics [e.g. the physics of hadrons].) Extremely unnatural systems — that is, physical systems described by quantum field theories that are highly non-generic — simply have not previously turned up in nature… which is just as we would expect from our theoretical understanding.

[Experts: As I emphasized in my Santa Barbara talk last week, appealing to anthropic arguments about the hierarchy between gravity and the other forces does not allow you to escape from the naturalness problem.]

So what might it mean if an unnatural quantum field theory describes all of the measurements at the LHC? It may mean that our understanding of particle physics requires an epicyclic change — a tweak.  The implications of a tweak would potentially be minor. A tweak might only require us to keep doing what we’re doing, exploring in the same direction but a little further, working a little harder — i.e. to keep colliding protons together, but go up in collision energy a bit more, from the LHC to the 100 TeV collider. For instance, perhaps the Standard Model is supplemented by additional particles that, rather than having masses that put them within reach of the LHC, as would inevitably be the case in a natural extension of the Standard Model (here’s an example), are just a little bit heavier than expected. In this case the world would be somewhat unnatural, but not too much, perhaps through some relatively minor accident of nature; and a 100 TeV collider would have enough energy per collision to discover and reveal the nature of these particles.

Or perhaps a tweak is entirely the wrong idea, and instead our understanding is fundamentally amiss. Perhaps another Einstein will be needed to radically reshape the way we think about what we know.  A dramatic rethink is both more exciting and more disturbing. It was an intellectual challenge for 19th century physicists to imagine, from the result of the Michelson-Morley experiment, that key clues to its explanation would be found in seeking violations of Newton’s equations for how energy and momentum depend on velocity. (The first experiments on this issue were carried out in 1901, but definitive experiments took another 15 years.) It was an even greater challenge to envision that the already-known unexplained shift in the orbit of Mercury would also be related to the Michelson-Morley (non)-discovery, as Einstein, in trying to adjust Newton’s gravity to make it consistent with the theory of special relativity, showed in 1913.

My point is that the experiments that were needed to properly interpret Michelson-Morley’s result

  • did not involve trying to detect motion through the ether,
  • did not involve building even more powerful and accurate interferometers,
  • and were not immediately obvious to the practitioners in 1888.

This should give us pause. We might, if we continue as we are, be heading in the wrong direction.

Difficult as it is to do, we have to take seriously the possibility that if (and remember this is still a very big “if”) the LHC finds only what is predicted by the Standard Model, the reason may involve a significant reorganization of our knowledge, perhaps even as great as relativity’s re-making of our concepts of space and time. Were that the case, it is possible that higher-energy colliders would tell us nothing, and give us no clues at all. An exploratory 100 TeV collider is not guaranteed to reveal secrets of nature, any more than a better version of Michelson-Morley’s interferometer would have been guaranteed to do so. It may be that a completely different direction of exploration, including directions that currently would seem silly or pointless, will be necessary.

This is not to say that a 100 TeV collider isn’t needed!  It might be that all we need is a tweak of our current understanding, and then such a machine is exactly what we need, and will be the only way to resolve the current mysteries.  Or it might be that the 100 TeV machine is just what we need to learn something revolutionary.  But we also need to be looking for other lines of investigation, perhaps ones that today would sound unrelated to particle physics, or even unrelated to any known fundamental question about nature.

Let me provide one example from recent history — one which did not lead to a discovery, but still illustrates that this is not all about 19th century history.

An Example

One of the great contributions to science of Nima Arkani-Hamed, Savas Dimopoulos and Gia Dvali was to observe (in a 1998 paper I’ll refer to as ADD, after the authors’ initials) that no one had ever excluded the possibility that we, and all the particles from which we’re made, can move around freely in three spatial dimensions, but are stuck (as it were) as though to the corner edge of a thin rod — a rod as much as one millimeter wide, into which only gravitational fields (but not, for example, electric fields or magnetic fields) may penetrate.  Moreover, they emphasized that the presence of these extra dimensions might explain why gravity is so much weaker than the other known forces.

Fig. 1: ADD's paper pointed out that no experiment as of 1998 could yet rule out the possibility that our familiar three dimensional world is a corner of a five-dimensional world, where the two extra dimensions are finite but perhaps as large as a millimeter.

Fig. 1: ADD’s paper pointed out that no experiment as of 1998 could yet rule out the possibility that our familiar three-dimensional world is a corner of a five-dimensional world, where the two extra dimensions are finite but perhaps as large as a millimeter.

Given the incredible number of experiments over the past two centuries that have probed distances vastly smaller than a millimeter, the claim that there could exist millimeter-sized unknown dimensions was amazing, and came as a tremendous shock — certainly to me. At first, I simply didn’t believe that the ADD paper could be right.  But it was.

One of the most important immediate effects of the ADD paper was to generate a strong motivation for a new class of experiments that could be done, rather inexpensively, on the top of a table. If the world were as they imagined it might be, then Newton’s (and Einstein’s) law for gravity, which states that the force between two stationary objects depends on the distance r between them as 1/r², would increase faster than this at distances shorter than the width of the rod in Figure 1.  This is illustrated in Figure 2.

Fig. 2: If the world were as sketched in Figure 1, then Newton/Einstein's law of gravity would be violated at distances shorter than the width of the rod in Figure 1.  The blue line shows Newton/Einstein's prediction; the red line shows what a universe like that in Figure 1 would predict instead.  Experiments done in the last few years agree with the blue curve down to a small fraction of a millimeter.

Fig. 2: If the world were as sketched in Figure 1, then Newton/Einstein’s law of gravity would be violated at distances shorter than the width of the rod in Figure 1. The blue line shows Newton/Einstein’s prediction; the red line shows what a universe like that in Figure 1 would predict instead. Experiments done in the last few years agree with the blue curve down to a small fraction of a millimeter.

These experiments are not easy — gravity is very, very weak compared to electrical forces, and lots of electrical effects can show up at very short distances and have to be cleverly avoided. But some of the best experimentalists in the world figured out how to do it (see here and here). After the experiments were done, Newton/Einstein’s law was verified down to a few hundredths of a millimeter.  If we live on the corner of a rod, as in Figure 1, it’s much, much smaller than a millimeter in width.

But it could have been true. And if it had, it might not have been discovered by a huge particle accelerator. It might have been discovered in these small inexpensive experiments that could have been performed years earlier. The experiments weren’t carried out earlier mainly because no one had pointed out quite how important they could be.

Ok Fine; What Other Experiments Should We Do?

So what are the non-obvious experiments we should be doing now or in the near future?  Well, if I had a really good suggestion for a new class of experiments, I would tell you — or rather, I would write about it in a scientific paper. (Actually, I do know of an important class of measurements, and I have written a scientific paper about them; but these are measurements to be done at the LHC, and don’t involve a entirely new experiment.)  Although I’m thinking about these things, I do not yet have any good ideas.  Until I do, or someone else does, this is all just talk — and talk does not impress physicists.

Indeed, you might object that my remarks in this post have been almost without content, and possibly without merit.  I agree with that objection.

Still, I have some reasons for making these points. In part, I want to highlight, for a wide audience, the possible historic importance of what might now be happening in particle physics. And I especially want to draw the attention of young people. There have been experts in my field who have written that non-discoveries at the LHC constitute a “nightmare scenario” for particle physics… that there might be nothing for particle physicists to do for a long time. But I want to point out that on the contrary, not only may it not be a nightmare, it might actually represent an extraordinary opportunity. Not discovering the ether opened people’s minds, and eventually opened the door for Einstein to walk through. And if the LHC shows us that particle physics is not described by a natural quantum field theory, it may, similarly, open the door for a young person to show us that our understanding of quantum field theory and naturalness, while as intelligent and sensible and precise as the 19th century understanding of waves, does not apply unaltered to particle physics, and must be significantly revised.

Of course the LHC is still a young machine, and it may still permit additional major discoveries, rendering everything I’ve said here moot. But young people entering the field, or soon to enter it, should not assume that the experts necessarily understand where the field’s future lies. Like FitzGerald and Lorentz, even the most brilliant and creative among us might be suffering from our own hard-won and well-established assumptions, and we might soon need the vision of a brilliant young genius — perhaps a theorist with a clever set of equations, or perhaps an experimentalist with a clever new question and a clever measurement to answer it — to set us straight, and put us onto the right path.

A 100 TeV Proton-Proton Collider?

During the gap between the first run of the Large Hadron Collider [LHC], which ended in 2012 and included the discovery of the Higgs particle (and the exclusion of quite a few other things), and its second run, which starts a year from now, there’s been a lot of talk about the future direction for particle physics. By far the most prominent option, both in China and in Europe, involves the long-term possibility of a (roughly) 100 TeV proton-proton collider — that is, a particle accelerator like the LHC, but with 5 to 15 times more energy per collision.

Do we need such a machine? Continue reading

It’s (not) The End of the World

The December solstice has come and gone at 11:11 a.m. London time (6:11 a.m New York time). That’s the moment when the north pole of the Earth points most away from the sun, and the south pole points most toward it. Because it’s followed by a weekend and then Christmas Eve, it marks the end of the 2012 blogging season, barring a major event between now and year’s end. But although 11:11 London time is the only moment of astronomical significance during this day (clearly the universe does not care where humans set our international date line and exactly how we set our time zones, so destruction was never going to be at local midnight — something the media doesn’t seem to get) it obviously wasn’t the end of the world.

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).

While we’re on the subject, I’ve been looking back at how I did on prophecy this year. It’s been a remarkably good year, probably my best ever — though admittedly I only made very easy (though not necessarily common) predictions. First, the really easy one:  I assured you, as did most of my colleagues, that 2012 would be the Year of the Higgs — at least, the Year of the Simplest Possible Higgs particle, called the “Standard Model Higgs”. It would be the year when Phase 1 of the Higgs Search would end — when we’d either find a Higgs particle of Standard Model type (or something looking vaguely like it), or, if not, we’d know we’d have to move to a more aggressive search in Phase 2, in which we’d look for more complicated versions of the Higgs particle that would have been much harder to find. We started the year with ambiguous hints of the Higgs particle, too flimsy to be sure of, but certainly tantalizing, at around a mass of 125 GeV/c2. In July the hints turned into a discovery — somewhat faster than expected for a Standard Model Higgs particle, because the rate for this particle to appear in collisions that produce two photons was higher than anticipated. The excess in the photon signal means either the probability for the Higgs particle to decay to photons is larger than predicted for a Higgs of Standard Model type, or both CMS and ATLAS experienced a fortunate statistical fluctuation that made the discovery easier. We still don’t know which it was; though we’ll know more by March, this ambiguity may remain with us until 2015.

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.

Now we’ve reached December, and the data seems to imply that except possibly for this overabundance of photons, which still tantalizes us, the various measurements of how the Higgs-like particle is produced and decays are starting to agree, to a precision which is still only moderate, with the predictions of the Standard Model for a Higgs of this mass. Fewer and fewer experts are still suggesting that this is not a Higgs particle. But it will be some years yet — 2018 or later — before measurements are precise enough to start convincing people that this Higgs particle is really of Standard Model type. Many variants of the Standard Model, with new particles and forces, predict that the difference of the real Higgs from a Standard Model Higgs may be subtle, with deviations at the ten percent level or even less. Meanwhile, other Higgs-like particles, with different masses and different properties, might be hiding in the data, and it may take quite a while to track them down. Many years of data collecting and data analysis lie ahead, in Phase 2 of the Higgs search.

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!

—-

One last personal note (just for those with some interest in my future.)

End of the OPERA Story

In case you haven’t yet heard (check my previous post from this morning), neutrinos traveling 730 kilometers from the CERN laboratory to the Gran Sasso laboratory do arrive at the time Einstein’s special relativity predicts they would.

Of course (as the press mostly seems to forget) we knew that.  We knew it because

So the news from the Neutrino 2012 conference in Kyoto, on new data from May 2012 taken by OPERA and three nearby experiments, is no surprise to anyone who was paying attention back in March and early April; it’s exactly what we were expecting.

One thing that almost no one is reporting, as far as I can tell, is that CERN’s research director Sergio Bertolucci did not give the first talk on neutrino speeds in Kyoto.  That talk was given by Marcos Dracos, of OPERA.  Dracos presented both OPERA’s corrected 2011 results (with corrections based on the detailed investigation shown in March of the problems reported back in February) and also the new 2012 results, which were taken with a kind of short-pulse beams similar to that used in OPERA-2.  (A short pulse beam allows for a neutrino speed measurement to be made rather easily and quickly, at the expense of OPERA’s neutrino oscillation studies, which were the main purpose of building the OPERA experiment.)

Following Dracos’ talk, Bertolucci spoke next, and reported the results of the neighboring Borexino, LVD and ICARUS experiments on the May 2012 data, which along with OPERA are all bathed in the same CERN-to-Gran Sasso neutrino beam, and collected their data simultaneously.  All of the results are preliminary so the numbers below will change in detail.  But they are not going to change very much.  Here they are: neutrinos arrive at a time that differs from expectation by:

  • Borexino: δt = 2.7 ± 1.2 (stat) ± 3 (sys) ns
  • ICARUS: δt = 5.1 ± 1.1 (stat) ± 5.5 (sys) ns
  • LVD: δt = 2.9 ± 0.6 (stat) ± 3 (sys) ns
  • OPERA: δt = 1.6 ± 1.1 (stat) [+ 6.1, -3.7] (sys) ns

(Here “ns” means nanoseconds, and “stat” and “sys” mean statistical and systematic uncertainty.)  The original OPERA result was an early arrival of about 60 nanoseconds, about six standard deviations away from expectations.  You see that all the experiments are consistent with zero early/late arrival to about 1 standard deviation — almost too consistent, in fact, for four experiments.

So there is no longer any hint of any evidence whatsoever of a problem with the predictions of special relativity, and in particular with the existence of a universal speed limit.

A summing up is called for, but I want to write that carefully.  So unless something else comes up, that’s all for today.

Guess What?! Neutrinos Travel Just Below the Speed of Light

Five out of five experiments agree: neutrinos do not travel faster than the speed limit.

Or more precisely: to within the uncertainties of current measurements, neutrino speed, for neutrinos with energies far larger than their masses, is experimentally indistinguishable from the speed of light in vacuum.  This is just as expected in standard Einsteinian special relativity, which would predict they move just below light speed, by an amount too small to measure with current experiments.

http://press.web.cern.ch/press/PressReleases/Releases2011/PR19.11E.html

Based on data taken in May 2012 using a beam of neutrinos sent from the CERN laboratory to the Gran Sasso lab, the four experiments ICARUS, LVD, Borexino and even OPERA (the source of  all the excitement) find results consistent with the speed of light, with uncertainties (at one-standard-deviation) about 10 times smaller than OPERA’s original measured deviation of neutrino speed from the speed of light.  The new results are consistent with ICARUS’s result from 2011 data.  Moreover, OPERA’s mistaken result from September and November 2011 — a claimed six standard deviations away from the expected speed — has now been corrected, following their detective work presented in March.  Even MINOS, a U.S. experiment, has revised their older result, which was previously slightly discrepant from the speed of light by a small amount (two standard deviations), and they find now that their data too are quite consistent with neutrinos traveling with light speed, though with much less precision in the measurement.

And so with a final quintet, sung in unison, this melodramatic comic OPERA buffa comes to a close.  As with all classic operatic comedies, there’s been crisis, chaos, and a good bit of hilarity, all the while with wise voices speaking reason to no avail, but in the end the overzealous are chastened, the righteous are made whole, everyone (even the miscreant) is happy, and all is well with the world.

Curtain!! Applause!!  Science Triumphant!!

Favorable review to follow when time permits.

A Violation of Lepton Universality?

A brief mention today of a new measurement from the BABAR experimental collaboration, which tests lepton universality (to be explained below) and finds it wanting.

The Basic Story (Slightly Oversimplified)

Within the Standard Model of particle physics (the equations that describe and predict the behavior of all of the known particles and forces), the effects of the weak nuclear force on the three leptons — the electron, the muon and the tau — are all expected to be identical. This called “lepton universality”. Continue reading

OPERA’s Timing Issue Confirmed? Yes!

[QUICK UPDATE April 2: I’ve now finished an article giving more details of how OPERA, with LVD’s help, solved the mystery.]

[UPDATE March 31 2 a.m.: following study of the slides from a mini-workshop recording the results of investigations by OPERA and LVD, I now have the information to remove all the guesswork from my original post; you’ll see outdated information crossed out and newer and more precise information written in orange.  I’ve also added figures from the talks.]

March 30 5:30 p.m. Two main scientists at OPERA, one leading the OPERA team as a whole and the other leading the neutrino speed measurement, resigned their leadership positions today.  The suggestion from the press is that this is due to personal and scientific conflicts within the OPERA experiment, rather than due directly to the errors made in the neutrino speed experiment; but of course the way the measurement was publicized by OPERA caused serious internal conflicts at the time and are surely part of the issue.    [Oh, and meanwhile, back over at the CERN lab, some good news: collisions at the Large Hadron Collider with 8 TeV of energy per collision were achieved this afternoon.]

The mystery surrounding OPERA, the Gran Sasso experiment which (apparently through a technical problem) measured that neutrinos sent from the CERN lab to the Gran Sasso lab in Italy arrived earlier than expected by 60 nanoseconds, seems to be on the verge of being is resolved.  Statements made by an OPERA scientist in the Italian language press, pointed out to me by commenters (Titus and A.K.), seem to imply that OPERA has more or less confirmed that the problematic fiber optic cable (along with the clock problem, to a lesser extent) was responsible for a 60 nanosecond (billionth-of-a-second) shift in the timing, creating the false result.  We do not yet have official information from OPERA about this, but talks given at a mini-workshop a couple of days ago make clear that this is the case.

The way this was done if I/we understand the Italian correctly is something like is the following  with all details still very uncertain. Continue reading

Taking Stock: Where is the Higgs Search Now?

Today, we got new information at the Moriond conference on the search for the Higgs particle (in particular, Phase 1 of the search, which involves the search for the simplest possible Higgs particle, called the “Standard Model Higgs”) from the Tevatron and the Large Hadron Collider [LHC], the Tevatron’s successor.  With those results in hand, and having had a little time to mull them over, let me give you a short summary.  If you want more details, read today’s earlier post and yesterday’s preparatory post.

Before I do that, let me make a remark.  There is a big difference between healthy skepticism and political denialism.  I get the impression that some people who are writing or reading other blogs misinterpret my caution with regard to experimental results as being somehow a political and unreasonable bias against the Higgs particle being present, either at a mass of 125 GeV/c2 or at all.  That’s ridiculous.  All that is going on is that I simply am not convinced yet by the data.  I’m a careful scientist… period.  And you’ll see that I’m consistent; later in this post I will advise you not to over-react negatively to what ATLAS didn’t see.

What happened today at the Moriond conference?

What did we learn?

The Tevatron experiments see a combined 2.2 standard deviation [2.2 “sigma”] excess in their search, consistent with a Standard Model Higgs particle with a mass anywhere in the range of 115 to 135 GeV/c2.  This is not inconsistent with the Higgs hints that we saw in December from the LHC experiments.  Here I am being perhaps overly careful in not saying, more positively, “it is consistent with the Higgs hints…” only because this measurement is intrinsically too crude to allow us to narrow in on 124-126 GeV, where ATLAS and CMS see their hints.  In short, the Tevatron measurement could, in the end, turn out to indicate a Higgs at a different mass than the one indicated by the current ATLAS and CMS hints.  Anyway, it’s a minor and mostly a semantic point.

The results from ATLAS were a bit of a shock.  In all three processes on which ATLAS reported, CMS has presented results already, and in each case CMS saw a small excess (1 standard deviation [1″sigma”], which is  small indeed.)  But ATLAS reported today that it sees essentially no excess in any of the three, and even a deficit in one of them for low mass.  This has a big effect.

  • First, it allows ATLAS to exclude a Standard Model Higgs all the way up to 122 GeV/c2 (except for a little window 1 GeV/c2 wide centered at 118) and down to 129 GeV/c2.  The only large window left for the Standard Model Higgs particle is 122-129, more or less centered around the hint at 126 GeV/c2 that they saw in December.
  • But second, the significance of the December hint, when combined with the new data that shows no excesses in these three new processes, drops by about a full standard deviation.  That’s a pretty big drop.

What does it all mean?

I think it basically means, roughly, status quo.  We got some positive information and some negative information today, and none of it is that easy to interpret.  So I think we are roughly where we were before, except that we probably no longer have to worry about any Standard Model Higgs below 122 GeV/c2.  Before today we had a decent hint of a Standard Model-like Higgs particle with a mass around 125 GeV/c2; we still have it.  Let me explain what I mean.

There are easy (relatively!) searches for the Higgs, and there are hard ones.  The easy searches are the ones where the backgrounds are relatively simple and the signal is a narrow peak on a plot.  There are two:

  1. Higgs decaying to  photons
  2. Higgs decaying to two lepton/anti-lepton pairs (often called “four leptons” for short)

Results on these were presented by both ATLAS and CMS back in December.  The hard searches are the ones where the backgrounds are rather complicated and the signal is quite broad, so that a mistake in estimating a background can either create a fake signal or hide a real one.    There are three (mainly) for a lightweight Higgs:

  1. Higgs decaying to a lepton, an anti-lepton, a neutrino and an anti-neutrino
  2. Higgs decaying to a tau lepton/anti-lepton pair
  3. Higgs decaying to a bottom quark/anti-quark pair

These are the three that ATLAS reported on today (where they saw no sign of a Higgs signal), and that CMS presented back in December (and saw a small excess in all three.)  [ATLAS presented a result on the first one in December, but only using part of their data; it showed a small excess at the time, but not now.]  The third process is the main one in which CDF and DZero reported an excess today, though the first one also plays a role in interpreting that excess.

In other words, everything we learned today had to do with the difficult searches — the ones that are hard to perform, hard to interpret, and hard to check.  And everything we learned was 1 or 2 sigma information; not very compelling even statistically.

For this reason,

  • I would not conclude that the new Tevatron results make the 125 GeV Higgs case much stronger
  • I would not conclude that the new ATLAS results make the 125 GeV Higgs case much weaker

For the same reason, when I explained why I was skeptical of the evidence back in December, I told you that in my view the CMS excesses in the difficult searches did not make the case for a 125 GeV Higgs much more compelling.  Since the easy searches at CMS do not show as large excesses as ATLAS’s do, I wasn’t really comfortable with the whole case from CMS.   Their case improved in January, when they added a bit more information from their easy search for two photons.

If, like me, you discount the difficult Higgs searches somewhat relative to the easy Higgs ones, then almost nothing has changed, as far as the current Higgs hints, after today’s up and down information.  The excess in the two easy searches at ATLAS is still there, and there are excesses at CMS at least in the two-photon search.  Even from the beginning, I gave you good reasons to think the ATLAS’s easy-search excesses were a bit larger than they should be, probably due to an upward statistical fluctuation in the background.    Conversely I think now that one should not overstate how bad today’s ATLAS news is for the Higgs hints.  It’s still quite reasonable to think there may be a Standard Model Higgs there at 125 GeV/c2.  There’s some evidence in its favor, and it’s certainly not ruled out at this point. (Whereas now, almost all other masses are.)

So as usual I advise patience and calm and no hyperventilating; the 2012 data will settle the issue.  Either there is a Standard Model Higgs with a mass within a few percent of 125 GeV/c2 , or we’ll soon be fanning out in Phase 2 of the Higgs search, looking for all the other types of Higgs particles that might be out there.