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Jim Rohlf | USLHC | USA

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LHC: raison d’être

This is my first blog ever. I am not going to make any advanced apologies about my writing. I am not going to attempt to impress you by chirping that I am writing this blog on such-and-such flight, or  explain that I am reporting from a particularly nice part of the world only because I was forced to go there for work. I am going to try to enlighten you, the intelligent reader, about physics and  stimulate interest in the LHC: what we do and how we do it and why. And then I am going to go fishing.

Here is my short take on why we have built the LHC, its true raison d’être. We are searching for the mechanism of electroweak symmetry breaking (ESB). A particle physicist puts this in the context of the so-called gauge bosons that mediate the forces: W, Z, and γ. Why are the W and Z massive (100 times that of the proton) while the photon (γ), the ordinary particle of light, is massless? To make it more intuitive, let’s look at the two fundamental forces involved, the “weak” nuclear force (bad name, but we are stuck with it) and electromagnetism. The weak force allows the sun to shine and you can’t get any more fundamental than that! Those weak interactions, that burn protons by allowing one of them to transform into a neutron which gets fused to another proton to form a deuteron, occur only at extremely short distances, even orders of magnitude smaller than the proton size. On the other hand, electricity has an infinite range, as easily demonstrated by looking at a distant star at night. The electron in the star emits a photon that travels many light years before it is captured by an electron in your eye. Add a sophisticated telescope and one may observe photons that have traveled billions of light years to reach us. So one fundamental force has an extremely TINY range and its close sibling has an INFINITE range!

How does an electron know that it should interact with another electron? Here is the conceptual picture of the interaction. (I was lucky enough to have Richard Feynman explain this to me in his own scruffy manner. Maybe I will write a Feynman blog in the future to relate my own stories.) An electron or any other charge is perpetually surrounded by a cloud of photons that it is continually emitting and absorbing photons. By doing this, the electron is checking if there is another charge around to push or pull. A free electron can’t really emit a photon and conserve energy and momentum. BUT the photon can “borrow” some energy from the electron for a short time as long as the product of borrowed energy times the time interval is smaller than Planck’s constant. This rule is called the uncertainty principle and lies at the heart of quantum mechanics. Although it retains a mysterious je ne sais quoi to this day, it is well tested experimentally. So our electron sends out its messenger photons on a mission to check if there are any other charges to push or pull. Since the photon is traveling on borrowed energy, it can only go large distances because it is massless. It is this masslessness of the photon that gives the electric force an infinite range.

Now how about the weak force? Enter W and Z. In the 1980s I was fortunate to work on the experiment that discovered these massive particles which had been searched for for decades. This discovery experimentally established the quantum nature of the weak force, that quarks and leptons really interact by exchanging W and Z particles. That there are 2 particles has to do with the detailed properties of the weak force: the W changes a quark or lepton into a different type (flavor) while the Z cannot. A quark in the proton sends out a W messenger to see if there is another quark around that wants to play (analogous to the electron sending out its photon messenger). This W can only live for a time allowed by the uncertainty principle. Now comes the big difference between electricity and weak. The W not only has to borrow kinetic energy to move but it also has to borrow some energy for its mass meaning that our W messenger cannot travel very far. The quark in one proton can only interact with the quark in the other proton (via weak force) if the quarks are VERY close together. The large mass of the W gives the weak force its short range. There is our broken symmetry: the W and Z have large mass and the photon is massless. Approximate symmetry can be restored if we can study interactions at an energy scale so large, or equivalently a distance scale so small, that the W and Z mass energies and the short range of the weak force are irrelevant. The forces become unified resulting in one happy electroweak force.

Another way to look at the consequence of the W having mass is that probability for a the weak interaction grows with energy. One can see this on dimensional grounds. This interaction probability cannot grow forever. There is a mathematical bound referred to as the unitarity limit of about 1.7 TeV. This means when Ws and Zs with this energy scale interact, we do not understand much of anything about what will happen. How’s that for exciting (!)? The reader will notice that 1.7 TeV is a VERY large energy for W and Z particles. The LHC will not reach this scale for Ws and Zs for a very, very long time. This is why once upon a time we wanted to build a 40 TeV machine (but don’t get me started on that…). However, all is not so gloomy as the following lesson tells.

There is an elegant historical analogy to ESB. Before the age of modern physics, the classical radius of the electron- the distance beyond which where the electrostatic potential energy exceeds the mass energy of the electron- posed a formidable barrier beyond which classical physics made no sense. This distance is 10^-15 m which corresponds to an electron approaching the GeV/c scale. It turns out, however, that we did not have to get anywhere this limit to discover revolutionary new physics: quantum mechanics was waiting to be discovered at the Bohr radius (10^-10 m) and relativistic quantum field theory at the Compton wavelength (10^-12 m). Okun called the classical electron radius the “paper tiger” and QM and QFT the “real tigers”. Here is a slide that Okun showed on my first trip to Moscow on Oct. 9, 1989:

The LHC was not yet a project and we were designing a detector for the 40 TeV machine. Of the zillion talks I have heard since then on supercollider physics, not one has been as clear and as informative and void of nonsense as the 5 slide talk by Okun. I gave a colloquium at ITEP in on Dec. 3, 2003 at the invitation of Okun and Michael Danilov, the lab director and I showed the 5 slides to the amusement of Okun. So the lesson is: when we collide Ws and Zs at a TeV or so, we WILL learn something exciting BUT if we are lucky we may learn something exciting well before reaching the unitarity limit. Let us hope so or its going to be a very long ride!

For the up and coming experts, a superb technical explanation of the electroweak physics has been given in a series of lectures by Tini Veltman (Nobel Prize, 1999) that have been published in a CERN Yellow Report 1997.

Picture (courtesy Claudia-Elizabeth Wulz) of me with Tini and Carlo Rubbia on the occasion of the later’s 75th birthday.

Having suffered through my explanation of why we have built the LHC, I now owe you something fun. Dark energy- which we shall NOT observe at the LHC- has become increasingly fashionable with the announcement of this year’s Nobel Prize. Dark energy is explained in a brilliant 1 m 39 s video by Sean Carroll:  2011

Time for me to go fishing. More on that later…