Peter Watkins (Professor in the particle physics group at Birmingham University) presented a fascinating talk entitled “Searching for Particles at the Large Hadron Collider(LHC) at CERN” at a recent Café Scientifique event.
A very engaging speaker, Peter did a great job of explaining the subject.
He began his talk by explaining what matter is made of, pointing out that everything you see around you is made of atoms, that atoms are composed of a nucleus surrounded by electrons and that the nucleus is composed of neutrons and protons.
So far, so GCSE - but then Prof Watkins dug down deeper and explained that neutrons and protons are made of “Up” and “Down” quarks, which are the basic building blocks of matter and can’t be broken down any further.
As well as the quarks there are also ghostly particles called neutrinos which, because they have no charge, barely interact with matter at all. Millions of them are constantly passing straight through the earth without being hindered in the slightest.
So, we have four basic elementary particles : the Electron, the Neutrino, the “Up” quark and the “down quark.
However, in the heat of the very early universe, heavier versions of these particles were able to exist and these conditions can be recreated in facilities like the LHC.
The mass of particles is usually expressed as the amount of energy required to create them, measured in millions of electron volts(MeV). To give a feel for how much energy is in a MeV, a few calculations show that it is the same energy as grain of sand has after falling 2mm.
As well as these “building block” particles, there are also “force carrier” particles. Perhaps unsurprisingly, these carry the forces that hold the natural world together. The Strong and Weak Nuclear forces work within the nucleus of atoms, whilst Gravity and the Electromagnetic forces can work at very long ranges.
Peter commented on the strange fact that, at an atomic level, the electromagnetic force is 10e40 (i.e. 1 followed by 40 zeros) times stronger than the gravity force. That is a very big difference and there is speculation that this might be due to the gravity force leaking out to other dimensions.
As Neil Pye might say - “That’s pretty heavy, man!”
This seemingly bonkers notion has some support because Super String theory, a concept that aims to connect all the forces into one mathematical framework, requires 10 dimensions to work.
So, these elementary particles. How to describe them without it being confusing?
But lets have a go by summarising them in three tables. . .
|Leptons : Cheeky particles that whizz about all by themselves|
|Muon Neutrino||below 0.2||0||1962|
|Tau Neutrino||below 15.5||0||2000|
|Note that only the Electron and the Electron Neutrino exist under normal circumstances. Also, the mass of the Electron Neutrino is not zero, but below a few millionths of a MeV|
|Quarks : Needy particles that join together to form larger things like Protons or Neutrons.|
|Note that only the Down and Up exist under normal cirrcumstances|
|Murray Gell-Mann, together with George Zweig proposed the existence of Quarks. |
For a laugh, Gell-Mann would sometimes pretend to be Sid James at parties
|Force Carriers : Spooky particles that, weirdly, carry the forces we see around us|
|Z and W bosons||~80000||Weak Nuclear||1983|
|Note that light rays are Photons and that the discovery date of 1905 refers to Einstein's description of light as being packaged in "quanta" (now called photons)|
|Sir Peter Higgs, who predicted the existence of the Higgs Boson, |
pictured here demonstrating his knowledge of all the STEPS dance moves.
Finding out whether there is a Higgs Boson - and what its mass is - is one of the aims of the LHC. Thus far, they know that, if it exists, its mass is somewhere in the range 110-150MeV as previous experiments have ruled out the possibility of masses outside this range.
Starting at the beginning, it is perhaps worth mentioning that the “Hadron” in “Large Hadron Collider” is a name given to entities, like protons and neutrons, that are made up from quarks. Given that neutrons are neutral and therefore a tad tricky to accelerate using electromagnetic fields, it probably didn’t take the LHC team too long to realise that they were better off focussing on using protons as the particle that they would accelerate around the famous 8km diameter underground ring.
Now, whilst the LHC gets a lot of press coverage, to a layperson, it’s portrayal in the media seem to raise more questions than answers.
One question that had been troubling NSB was that surely, if you have two beams of particles going round a tube, they can’t get more than half way round without hitting each other!
Prof Watkins explained that the two beams are in separate, parallel tubes that intersect at four points around the CERN ring. At each of these locations is a different detector ATLAS, CMS, ALICE, LHCb
He also described how the protons were in bunches a few centimetres long and a about 100th of a millimetre in diameter. Each “bunch” contains about a million million protons and there are a number of these bunches circulating at any one time. In fact, bunches collide at the centre of the ATLAS every 25 nanoseconds. Of course, only a few of the protons from each bunch actually collide and the detectors record about 100 collisions per second.
Clearly, the LHC is a pretty technical piece of kit - but just how technical only became apparent when Prof Wilkins recalled the calibration process for the LHC’s predecessor the LEP (which used the same circular tunnel as the LHC). As the calibration went on, the operators noticed a recurring daily variability. After some headscratching, they eventually worked out that this was due to the tidal movements of the earth, which cause the diameter of the globe to oscillate by some 15cm every day.
Having accounted for this in their calculations, they carried on with their calibration process and found that there was some other effect that was subtly affecting the performance of the beams. This time it took a lot of headscratching, but after much drawing of diagrams and checking of possibilities, it was found that the problems were being caused by current leaking from the TGV trains on the surface and affecting the magnets around the ring deep underground.
Which leaves one with the curious vision of CERN staff arranging their experiments so that they do not co-incide with the 11.27 from Paris!
The ATLAS Detector
Prof Watkins spent some time explaining the workings of the ATLAS detector with the first point being that IT IS VERY BIG. You can get an idea of just how big when you see how tiny the people are in the drawing below.
A larger version of this image can be found here.
Happily for you, gentle reader, NSB does not need to try and relate the detail of Prof Watkins very interesting explanation of the ATLAS detector, and can instead point you towards the rather groovy explantatory slideshows at CERN (see here and here).
The CERN site is a real goldmine of information, and gives a feel for the technical challenge faced by ATLAS as it describes how ATLAS has to filter out the interesting collision events from the routine ones
“When protons collide, some events are "interesting" and may tell us about exciting new particles or forces, whereas many others are "ordinary" collisions (often called "background"). The ratio of their relative rates is about 1 interesting event for 10 million background events. One of our key needs is to separate the interesting events from the ordinary ones."
"Furthermore the information must be sufficiently detailed and precise to allow eventual recognition of certain "events" that may only occur at the rate of one in one million-million collisions, a very small fraction of the recorded events, which are a very small fraction of all events. The term "event" here denotes a single proton-proton collision that may lead to an interesting configuration of outgoing particles."
The site also gives a feel for the scale of the computing challenge faced by ATLAS when it points out that:
“If all the data from ATLAS would be recorded, this would fill 100,000 CDs per second. This would create a stack of CDs 450 feet high every second, which would reach to the moon and back twice each year. . . . ATLAS actually only records a fraction of the data (those that may show signs of new physics) and that rate is equivalent to 27 CDs per minute.”