What's LHC up to?

future tense The biggest achievement of the Large Hadron Collider is the discovery of the Higgs boson. But what does it have in store for us next, que

What's LHC up to?

A  few years ago, CERN, the European Organisation for Nuclear Research restarted the world’s biggest particle accelerator, the Large Hadron Collider (LHC). It had been shut for two years for various refits and overhauls after the announcement of its successful discovery of the long elusive Higgs boson in 2012. The particle smashing collisions are however expected to start in June.

The LHC accelerates proton beams travelling in opposite directions to energies of seven trillion (tera) electron volts (7 TeV). At such energies, protons would be 7,000 times more energetic than their rest mass energy and their velocity would differ from that of light by just about 3m/s. These energetic proton beams would undergo a billion collisions a year making 15,000 thousand laps every second around the LHC’s 27 km ring.

The counter-rotating rings meeting head on at four points in the ring where the giant detectors (ATLAS, CMS, ALICE and LHC-b) have been located thoroughly, analyse and pore over the debris of the particle wreckage caused by the collisions. These titanic collisions would cause the particles to be smashed into smithereens, with their constituents forming entirely new types of particles. They could generate the largest concentration of energy ever generated in an experiment across a region that’s hardly a micro nanometre.

To smash a composite particle like the proton into its tightly bound constituents (quarks, gluons etc), one must collide them at high energies. Energetic collisions of nucleii (accelerated by strong electromagnetic fields) became part of routine particle physics experiments leading to nuclear transmutations, generation of antiparticles etc. To produce an antiproton, the energetic proton smashing into a proton at rest must have seven times the proton rest mass energy. Only when six gigavolt accelerators like the Bevatron were developed, did this become possible.

High energy “probe particles” showed that the proton is made up, in turn, of much smaller constituents. The constituents would form a plethora of new particles (mostly short-lived or transient). Each time a bigger accelerator was developed, more particles were discovered. Ever since Maxwell unified electric and magnetic forces into one unified theory and Einstein (and others) tried to do the same for gravity and electromagnetism, physicists have been trying to unify the fundamental forces which underlie all interactions between particles. The electromagnetic interactions of condensed matter are mediated by the massless photon, making them long range forces.

However the weak interactions mediated by W and Z bosons describe nuclear beta decay and associated interactions of neutrinos, but are effective only over a very short range because of the large masses of the W and Z. How can one unify interactions with such diverse properties? A great triumph of particle physics was in explaining why W and Z are heavy and the photon is massless.

Above some energy all would be massless, but at some well defined energy, the symmetry which makes particles massless is broken; giving W and Z masses while the photon remains massless. This needed the presence of a new particle, the so called Higgs boson which ‘gives’ mass to the W and Z.

The correct prediction of the W and Z masses was verified in 1983 at CERN when these particles were discovered. The detailed properties of more particles and their decays were studied particularly in the Large Electron-Positron (LEP) collider at CERN and at SLAC. LEP reached energies well over 200 Giga electron volts. The discovery of the predicted tau-lepton and the top quark (at Fermilab) marked the triumph of the so-called standard model of particle Physics. Only the Higgs was waiting to be discovered. This was not found possible at the Tevatron (the previous largest accelerator) but was finally discovered soon after the LHC started functioning.

What next questions
What else waits to be discovered at the LHC? What are scientists hoping to discover? An important goal is to search for the so called super-symmetric particles. Super symmetry unites both bosons and fermions which were earlier considered completely separate entities. Bosons named after the Indian physicist S N Bose have integer intrinsic spin.

Fermions have spins which are half-integer, electrons and protons have spin half, others have spin 3/2 etc. Therefore, super symmetry (a very elegant symmetry) implies that each of the known particles has a corresponding bosonic or fermionic partner. For example, photino is the fermionic counterpart of a photon; selectron is the bosonic counterpart of the electron, etc.

If super symmetry were an exact symmetry, all these particles would have the same mass as their corresponding super partners. However, none of the super symmetric particles have yet been seen. This implies that super symmetry is broken at some large energy making the super partners much heavier and difficult to detect.

For various reasons, the energy at which this symmetry is broken is expected to be around TeV energies. So the LHC should be able to produce such particles copiously. Indeed the heavier super symmetric particles are expected to decay into the lightest super symmetric particle (LSP) which however interacts so weakly that it will fly straight out of the detectors carrying energy and momentum. They will look or this missing energy as a possible sign of the LSP. Moreover the LSP (and some others) are favourite candidates for the dark matter (DM) which dominates the halos of galaxies and outweighs ordinary matter (like us) by a factor of five (in the universe).

The idea is that the LHC energies mimics the conditions in the early universe, one picosecond after the big bang when energies were several TeV and so these particles would have been produced copiously and now must be hanging around as DM. Missing energy could also imply that a particle has disappeared into an extra dimension. So this is another goal -  look for extra spatial dimensions (of the order of an attometre or nano-nano metres). Some of the theories expect gravity to become as strong as other interactions at this scale when extra space dimensions manifest themselves.

Our macroscopic world as is well known, has three space dimensions and several remarkable physical laws follow as a direct consequence like the inverse square laws for gravity (and electrostatics) and diminishing of luminosity from distant objects as inverse square of the distance. In higher space dimensions gravity forces fall of much faster. This strengthening of gravity could lead to formation of mini black holes of TeV energies (they would decay by Hawking radiation in a pico-pico second).

At 27 km long, LHC is the largest machine in the world producing the highest energy particles. However at less than two degrees Kelvin, LHC is also the coldest ring in the universe (the microwave background is about three degree Kelvin). This low temperature was required to maintain the most powerful superconducting magnets.

The ATLAS has 10,000 ton magnet and its five tesla field (one lakh times the Earth’s field) generates an outward force of 100 atmospheres. Helium becomes a superfluid at two degree Kelvin with its strange properties. Tiny gaps between thousands of segments or broken electronic connections could give illusory results. Such low temperatures are required so that the magnets can carry much higher currents. They learnt the lesson from the abandoned SSC (Superconducting Super Collider) as the higher four degree Kelvin made its magnets not powerful enough.

Temperature variations
However the much lower LHC ring temperature implies that for repairs it would take several weeks to warm to room temperature and later its 50,000 tons of magnets would need to be cooled again to two degree requiring six more weeks. About 10,000 tons of liquid nitrogen and 150 tons of super fluid helium are used. It would need a book to describe all the engineering marvels going into the design.

What if the LHC, by some chance is unable to make any pathbreaking or exciting discovery? This might call for a complete rethink on the subject. The highest energies conceived of in particle physics is the so called Planck energy but this would require an accelerator several light years long. One hopes the LHC makes some crucial breakthrough which would clearly indicate how future ideas, whether in superstrings, dark matter or extra dimensions evolve.

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