Tag Archives: LHC

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.

Nobel Prize for Higgs

On 8 October 2013, the Nobel prize for physics was awarded to Francois Englert and Peter Higgs. In one sense this was a long time coming: the theoretical work that won the prize took place in 1964 (Englert, and his late colleague Robert Brout, working independently of Higgs, published first; a few weeks later Higgs published a paper that explicitly predicted the existence of a scalar boson; another group of physicists – Gerald Guralnik, Carl Hagen and Tom Kibble – published related work later in the same year). In another sense the prize was awarded remarkably quickly: experimental proof of the existence of a fundamental boson was announced on 4 July 2012, and it wasn’t until 14 March 2013 that it was confirmed to be a scalar (spin-0) boson. (If you want to learn more about the Higgs mechanism, you can find a variety of explanations here.)

To my mind, the discovery of the Higgs is one the crowning achievements of human civilisation: it is the culmination of a process that began 2500 years ago with the Greeks. Physicists now have a standard model of fundamental particles: there exists a small number of spin-1/2 point particles (6 quarks; the electron, muon and tau each with their associated neutrino) which interact via the exchange of spin-1 particles that mediate the electroweak and strong (these exchange particles being the photon; W+, W and Z0; 8 gluons). In the ‘pure’ theories underpinning this model the fundamental particles are massless; they acquire mass – and thus in a certain sense their very existence – by interacting with a spin-0 field that pervades the entire universe. This spin-0 field has an associated particle; the Higgs boson. And that’s it. End of story. Except…

We are really just at the beginning of the story. The theories underpinning the standard model are in conflict with the other central pillar of physics: general relativity. The standard model is based on quantum physics; general relativity is a classical theory. Physicists need to develop a quantum theory of gravity. Furthermore, we now know that the standard model applies to only 5% of the universe: 95% of the mass-energy content of the universe resides in the so-called ‘dark’ sector. We desperately need to understand the nature of dark matter and dark energy.

Now that the Large Hadron Collider has discovered the Higgs its next job, when it becomes operational again after its current upgrade, is to shed light on the dark sector.


LHCb and supersymmetry

The Bs meson consists of a strange quark and a bottom antiquark, and once it is produced it quickly decays. Very, very rarely it decays into a muon and an antimuon.

The Standard Model of particle physics predicts the rate of the decay of a Bs meson into muons: for every billion Bs mesons that are produced, about 3 of them will decay into a muon-antimuon pair. (The actual figure is 3.54 plus or minus 0.3.)

LHCb, the Large Hadron Collider beauty experiment, has been studying the decay of Bs mesons. (The beauty quark is another name for the bottom quark. Either way, we’re talking here about b quarks.) The experiment says that, for every billion Bs mesons that are produced, about 3 of them decay into a muon-antimuon pair. (The actual figure is 3.2 plus or minus 1.5.) You can find the paper from this CERN webpage. The team hasn’t claimed this as a discovery: the result is at a 3.5 sigma level, which means that there is about a 1-in-4300 chance that the LHCb would see the same bump in their data just due to random chance. But the result is certainly intriguing.

Bs meson decay to muon antimuon

In May 2012 the LHCb collaboration saw this decay of a Bs meson into a muon-antimuon pair (Credit: CERN, LHCb)

Why should this matter? Isn’t it just another case of the Standard Model being proved right? After all, with the discovery of a Higgs (and we’ll soon know for sure whether it’s the Higgs) the Standard Model is on firmer ground than ever. Well, that’s the whole point! The measurement does agree with the Standard Model. But the decay of the Bs into a muon-antimuon pair is believed to be sensitive to physics beyond the Standard Model. In particular there are several models of supersymmetry which, if they were realised in nature, would have the effect of increasing the rate of Bs decay into muons: in these models LHCb should see more than 3 muon-antimuon pairs per billion Bs decays. If the LHCb result stands, then several models of supersymmetry would appear to be ruled out.

Several recent articles have reported the LHCb finding as a significant blow to the whole idea of supersymmetry. Those articles are, I believe, wrong.

  • First, as the LHCb collects more data it’s possible that deviations from the Standard Model prediction will become evident. Let’s wait and see.
  • Second, there are other models of supersymmetry that aren’t affected by this result. What’s happening here is that the LHC is narrowing the range in which supersymmetry can be found, just as it narrowed the range where a Higgs could be found – and then found it.
  • Third, if the LHCb data confirms the Standard Model then the result poses challenges for all ideas for physics beyond the Standard Model. It’s not just supersymmetry that physicists are investigating here, after all.

The result, if confirmed, does raise one disturbing prospect. Perhaps the LHC may not see physics beyond the Standard Model, even when it starts to run at its highest energies. Supersymmetry could still be a phenomenon that applies at really high energies, but we wouldn’t be able to test it with machines such as the LHC. How frustrating would that be?

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!

Fermi spies a superbubble

One of the mysteries I discuss in New Eyes on the Universe is the origin of ultra-high-energy cosmic rays. These are subatomic monsters, particles that smash into Earth’s atmosphere with macroscopic energy: the famous ‘Oh-my-God’ particle carried 3 x 1020 eV – the kinetic energy of a well-struck tennis ball. What mechanism can accelerate a subatomic particle to that sort of energy? No one knows. However, astronomers are closer to understanding the source of cosmic rays with slightly lower energies (up to about 1015 eV – so still far more energetic than anything the Large Hadron Collider can deliver!)

The Fermi space telescope (previously known as GLAST; as I mention elsewhere, thank heavens that not all astronomy missions are known by acronym) has found evidence for the source of at least some medium-to-high-energy cosmic rays. And thought the details are still to be determined it seems that these cosmic rays are accelerated by shock waves produced when supernovae eject material into space. This model of cosmic ray acceleration, appropriately enough, originated with Enrico Fermi.

Artist's impression of the Fermi space telescope

Artist's impression of the Fermi gamma-ray space telescope.
Credit: NASA

What the Fermi telescope actually found was a source of gamma-rays in the constellation of Cygnus. The source lay along a line between two clusters of stars, the clusters being separated by about 160 light years. One cluster contained over 500 massive stars (the sort of stars that form supernovae), the other cluster contained about 75 massive stars. So how does this relate to cosmic rays?

Well, the clusters contain dense gas clouds – that’s an environment in which massive stars are likely to form – but the stellar wind from a massive star pushes the gas away and creates a ‘bubble’ (When a star explodes as a supernova it also creates a ‘bubble’ around what’s left behind.) These bubbles grow and merge with bubbles around other stars and remnants to form ‘superbubbles’. What Fermi detected (the results are published in Science 334 1103-1107) was high-energy gamma-rays coming from a superbubble in Cygnus. (Since gamma-rays aren’t deflected by magnetic fields, they point straight back to their source; Fermi could thus determine the source of these gamma-rays. Cosmic-rays, being electrically charged, are deflected by the magnetic fields in our Galaxy and around Earth; the arrival direction of a cosmic ray does not necessarily point back to its source.)

The best interpretation of the Fermi data is that cosmic rays were being accelerated by shockwaves in the superbubble; whenever those cosmic rays collided with atoms or molecules inside the superbubble, gamma-rays were produced. The gamma-ray energy distribution was what one would expect from such collisions. Furthermore, the spatial distribution followed the shapes of the gas clouds and cavities. So this is good, strong evidence that some cosmic-rays originate from inside massive-star-forming regions of space.

But what precisely is the acceleration mechanism? An isolated shock wave from a single supernova remnant, or the combined effect of many different shocks? It’s not yet clear. As for the source of the ‘Oh-my-God’ particles – well, God alone knows at present.