Tag Archives: CERN

A charming new particle

The LHCb experiment at CERN has today announced the discovery of a new baryon: the quark content of the Ξcc++ is ccu — in other words it contains two charm quarks and one up quark.

The commonest baryons contain combinations of u (up) and d (down) quarks. The proton and the neutron, for example, are uud and udd respectively. Baryons containing s (strange) quarks have long been known: the Ω, which was discovered in 1964, has a quark content of sss. But up until now the heavier quarks (charm (c), bottom (b), top (t)) have only ever appeared singly in baryons. The Ξcc++ baryon contains two heavy quarks.

It’s important to note that the LHCb experiment hasn’t discovered a new fundamental particle. The Ξcc++ baryon is a permissible collection of bound quarks. But it is the first time that anyone has seen a baryon containing two heavy quarks. The Ξcc++ should allow physicists to explore the theories behind the Standard Model in ever more detail.

LHC finds the Higgs boson

Today’s announcement at CERN, that the CMS and ATLAS experiments have found a boson consistent with the Standard Model Higgs, is the most exciting find in particle physics since … well, since I can remember. The discovery of charm was before my time, but I was a physics student when news of the W and Z discoveries was made public and I don’t believe those announcements matched today’s press conference for drama and sheer emotion (Peter Higgs had to wipe away a tear).

This is a tremendous day for science. Just think what’s happened here. Over a period of decades, theorists and experimentalists developed a theory of the basic interactions (electromagnetism, weak force, strong force) that govern the behaviour of the fundamental particles (quarks, neutrinos, electron, muon and tau). But in order for the theory to match the observed fact that the fundamental particles have mass, theorists had to add something else into the mix. They used purely mathematical reasoning to deduce something incredible about the Universe: that it’s filled everywhere with a scalar field — the so-called Higgs field. It’s the interaction with this field that gives the fundamental particles mass.

And decades after theorists postulated the existence of this field, CMS and ATLAS have found evidence for the associated boson. They saw hints of the Higgs boson last year. Now it’s definite. It has a mass of around 125 GeV.

This is tremendous news for CMS, ATLAS, CERN and science in general. And it’s the start of a whole new era in physics. Now we know where the Higgs is, the LHC — such a tremendous machine — will be able to investigate its properties in detail. And perhaps for the first time we’ll get a glimpse beyond the Standard Model.

What a great day!

More hints of the Higgs

In December 2012 the ATLAS and CMS teams at the Large Hadron Collider announced that they had seen signals that were consistent with there being a Higgs boson with a mass somwhere in the region of about 124-126 GeV. Statistically, though, they were unable to claim a discovery.

Before Fermilab’s Tevatron collider ceased operations in September 2011 its two experiments – CDF and DZero – generated vast amounts of data that have only now been analysed. On 7 March 2012, scientists announced the results of that analysis at the Rencontres de Moriond conference. The data hint at a Higgs boson with a mass somewhere in the range 115-135 GeV. Again, the statistics fall far short of that required to claim a discovery.

The Tevatron collider at Fermilab, as seen from the air.

The Tevatron collider at Fermilab, as seen from the air. The main ring and main injector are clearly visible. The ponds are there to dissipate waste heat from the machine.
Credit: Fermilab, Reider Hahn

This is tantalising! The ATLAS and CMS teams both make use of high-energy proton-proton collisions produced by the LHC, but they are quite different experiments focusing different things. The CDF and DZero experiments are different again: the Tevatron produced proton-antiproton collisions. So a variety of signals are pointing to a Higgs with a mass somewhere around 125 GeV. But there’s no certainty that it’s there: further data might cause the signal to vanish like the Cheshire Cat.

One thing is certain: by the end of 2012 we will know whether the Higgs exists and, if it does, what its mass is. The LHC is operating so well that there’s now nowhere left for Higgs to hide.

So has the fat lady sang at OPERA?

Well, it had to happen. The OPERA team has identified two potential problems in their measurement of neutrino velocities. (You remember that story, right? The one where it seemed as if the neutrinos were superluminal…)

It turns out that there’s a problem with an atomic clock that they used to get start/stop times for the measurement. (The error here would tend to increase the measured time-of-flight, and thus reduce the measured speed.) There was also a problem with the optical fibre connection between the main clock and the GPS system. (Surprisingly, the error here would tend to increase the measured speed.)

The identification of these two systematic errors means that the OPERA team can no longer claim to have seen superluminal neutrinos. Further experiments later this year, both at OPERA and elsewhere, will surely put the story to bed once and for all.

What has been fascinating here, though, has been the reaction of the scientific community to the claim. I think we all knew that this result was never going to stand. But that doesn’t mean the OPERA team were wrong to publish. Their initial result caught the public imagination, and their identification of systematic errors in the experiment showed the public how science progresses in the real world.

They showed that science is sometimes messy, sometimes confusing. But they also showed that science is transparent, and eventually it gives us knowledge we can rely on. Well done OPERA.

A glimpse of the Higgs?

I’ve just spent the afternoon watching CERN’s live webcast of the latest CMS and ATLAS data (thank you, CERN, for inventing the Web!). After today there’s very little room for the Higgs still to hide.

ATLAS essentially rules out the existence of a Higgs boson, unless the Higgs mass is in the region 115 to 131 GeV. (I guess I should say that, for comparison, the proton mass is about 1 GeV (actually 0.938 GeV)). CMS seems to rule out a Higgs that is more massive than 127 GeV.

What is tantalising is that both experiments saw hints of a Higgs at around about 125 GeV. Unfortunately, the signal was not strong enough to claim a discovery: what they saw might have been a statistical fluke.

The only way to decide the matter is to take more data which is, of course, what the two experiments will do. In a few months time we will know one way or the other. Either the bumps that ATLAS and CMS saw will go away, and we can say that the Higgs doesn’t exist. Or the bumps will get larger and clearer, and we can say that the Higgs exists with a mass of around 125 GeV.

Either way, new physics will be required. Either way, it will be the discovery of the century.

When will the fat lady sing at OPERA?

Some of the world’s finest physicists and cosmologists have in recent weeks been pouring scorn on the now infamous OPERA result. (If you’ve just been released from one of those Mars simulation missions, such as Mars500, then I guess it’s possible that you might have missed what has the potential to be the biggest physics result in a century: the report by the OPERA collaboration that muon neutrinos produced by CERN travelled ever-so-slightly faster than light while on their way to detectors at Gran Sasso.) I’m sure that those scientists, many of whom I admire tremendously, are right: those neutrinos are surely not travelling faster than light. It wasn’t as if the neutrinos acted like resublimated thiotimoline, somehow arriving at the OPERA detectors before they were produced. The OPERA team were making tremendously difficult measurements, and at this point it’s safer to assume that their finding is the result of some unknown source of error in the experiment. But there’s one point on which I think those eminent critics of OPERA have it wrong.

The criticism is that the OPERA team contacted the media and called a press conference before they published their results in a peer-reviewed paper: irresponsible behaviour, clearly, particularly where such a controversial result is involved. Thing is, the OPERA researchers didn’t announce their results at a press conference: they announced them at a CERN seminar. And they didn’t draft a press release: they submitted a technical preprint to arXiv. Surely they did everything that responsible scientists should do?

Once, not many years ago, you could put a preprint on arXiv and you knew you’d be reaching an audience of physicists. We now live in a world of blogs (well, you’re reading this one aren’t you?) and Twitter. Put a preprint on arXiv that says in effect “Einstein was wrong” and you may as well shout it out loud while standing naked at Speaker’s Corner. Perhaps unfortunately for OPERA, in the modern world of social media there’s no way that the original seminar could go unnoticed; the press conferences that followed were inevitable – and then so was the criticism that the collaboration hadn’t followed proper processes.

Neutrino beam going from CERN to Gran Sasso

CERN sends neutrinos directly through the Earth to the Gran Sasso Laboratory, some 730km away Credit: CERN

If there’s a criticism to be made of OPERA it is, I believe, that they hadn’t ruled out all sources of systematic error before giving that initial CERN seminar. Indeed, that’s probably why ten senior members of the collaboration decided not to sign the arXiv submission. One obvious concern with the experiment, which many physicists voiced immediately, is that CERN was sending long neutrino pulses (about 10 microseconds long) to Gran Sasso; the effect they were observing, though, involved a shift that was a tiny fraction of that pulse length (the shift was about 60 nanoseconds). For their analysis to work, the collaboration needed to know the shape of the neutrino pulse quite precisely; but they were only able to infer the neutrino pulse shape. (The neutrinos come from protons smashing into a target; OPERA infer the neutrino pulse shape from the initial proton pulse shape.) Get that inference just a little bit wrong and they would end up seeing things that just aren’t there.

Fortunately, there’s a really simple way to get round this difficulty: repeat the experiment, but send a series of short neutrino pulses separated by large gaps. That way you don’t need to know the neutrino pulse shape: each pulse from CERN is unambiguously linked to the OPERA detector.

The OPERA collaboration has now run precisely this experiment. They asked CERN to generate proton pulses lasting just 3 nanoseconds, and recorded 20 neutrino events. And the result? Well, again the neutrinos reached Gran Sasso about 60 nanoseconds before light itself could have reached there. The anomaly remains.

So when will the fat lady sing at OPERA? When will we know what systematic error is to blame for this bizarre result? (And for what it’s worth I think it will turn out to be a systematic, probably to do the use of GPS in the experiment.) Well, it’s clear that independent checks are required. The first project to be in a position to do those checks is likely to be MINOS at Fermilab. We might get results from MINOS some time in 2012. If MINOS replicates the OPERA result… well, then we’ll be living in interesting times.