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Richard Ruiz | Univ. of Pittsburgh | U.S.A.

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Photon Colliders: Making Matter from Light Since 1994

Protons are the brave casualties in the search for new physics, but sometimes everybody lives.

Hi Folks,

The CERN Large Hadron Collider (LHC), history’s largest and most energetic proton collider, is currently being tuned up for another round of new discoveries. A Higgs boson, incredibly rare B meson decays, and evidence for vector boson fusion have already been identified, so there is great anticipation on what we may find during Run II.

Our discoveries, however, come at the cost of protons. During 2012, the ATLAS, CMS, and LHCb experiments collected a combined 48 fb-1 (48 “inverse femtobarns“), and another 12 fb-1 in 2011. To translate, an “inverse femtobarn” is a measure of proton collisions and  is the equivalent of 70 trillion proton-proton collisions. Hence, 60 fb-1 is equivalent to about 4,200 trillion proton-proton collisions, or 8,400 trillion protons. We hope to generate almost twice as much data per year when everything starts back up winter/spring 2015. You know, suddenly, @LHCproton‘s many fears of its day job make sense:

With so many protons spent in the name of science, one can reasonably ask

Is it possible to find new physics at the LHC without destroying a proton?

The answer is


Sometimes, just sometimes, if we are very lucky, two protons can pass each other, interact and make new particles, but remain intact and unbroken. It sounds mind-boggling, but it has been one of the best tests of quantum electrodynamics (QED), the theory of light and matter at small distances and large energies.

From Maxwell to Photon Beams at the LHC

The idea is simple, the consequences are huge, and goes like this: Protons, like electrons, muons and W bosons, are electrically charged, so they can absorb and emit light. Protons, like electrons, do not just radiate light at random. Light is emitted following very specific rules and travel/propagate in very specific directions, dictated by Maxwell’s equations of electrodynamics. However, the rules of quantum mechanics state that at large enough energies and small enough distances, in other words an environment like we have at the LHC, particles of light (called photons) will interact with each other, with a predicted probability. Yes, you read that correctly, quantum mechanics states that light interacts with itself at small enough distances. The more protons we accelerate in the LHC, the more photons are radiated from protons that remain intact, and the more likely two photons will interact with each other, producing matter we can observe with detectors! An example of such a process that has already by observed is the pair production of muons from photons:

Muon pair production from photon scattering via elastic photon emission from protons. Credit: CMS, JHEP 1201 (2012) 052

To understand this more, lets take a look at Maxwell’s Equations, named after Scottish physicist J. C. Maxwell but really represent seminal contributions of several people. Do not worry about the calculus, we will not be working out any equations here, only discussing their physical interpretation. Without further ado, here are the four laws of electrodynamics:


The very first law, Gauss’ law, tells us that since the proton has an electric charge, it also it the source of an electric field (E). The bottom equation, Ampere’s Law, tells us that if we have a moving electric charge (a proton circling the LHC ring for example), then both the moving electric field (E) and the electric current (J) will generate a magnetic field (B). In the LHC, however, we do not just have a moving beam of protons, but an accelerating beam of protons. This means that the magnetic field is changing with time as the proton circles around the collider. The third equation, Faraday’s law, tells us that when a magnetic field (B) changes with time, an electric field (E) is generated. But since we already have an electric field, the two fields add together into something that also changes with time, and we end up back at Ampere’s law (the bottom equation). This is when something special happens. Whenever a charged particle is accelerated, the electric and magnetic fields that are generated feed into each other and create a sort of perpetual feedback. We call it electromagnetic radiation, or light. Accelerating charged particles emit light.


Schematic representation of the strength of the electric (blue) and magnetic (red) fields as light propagates through space. Credit: Wikipedia

Maxwell’s equations  in fact tell us a bit more. They also tell us the direction in which light is emitted. The crosses and dots tell us whether things are perpendicular (at right angles) or parallel to each other. Specifically, they tell us that the generated magnetic field is always at right angles to both the electric field and direction the proton is travelling, and that light travels perpendicular to both the electric and magnetic field.  Since protons are travelling in a circle at the LHC, their tangential velocity, which always points forward, and their radial acceleration, which always points toward the center of the LHC ring, are always at right angles to each other. This crucial bit fixes the direction of the emitted light. As the protons travel in a circle, the generated electric field points in the direction of acceleration (the center); the generated magnetic is perpendicular to both of these, so it points upward if the proton is travelling in a counter-clockwise direction, or downward if the proton is travelling in a clockwise direction. The light must then always travel parallel to the proton! Along side the LHC proton beam is a hyper focused light beam! Technically speaking, this is called synchrotron radiation.


(a) Relative orientation of an electrically charged particle travelling in a circle and its electromagnetic field likes. (b) Synchrotron radiation emitted tangent to a circular path traversed by an electrically charged particle. Credit: Wikipedia

The last but still important step is to remember that all of this is happening at distances the size of a proton and smaller. In other words, at distances where quantum mechanics is important. At these small distances, it is appropriate to talk about individual pieces (quanta) of light, called photons. That beam of synchrotron radiation travelling parallel to the proton beam can appropriately be identified as a beam of photons.  In summary, along side the LHC proton beam is the LHC photon beam! This photon beam is radiated from the protons in the proton beam, but the protons remain intact and do not rupture as long as the momentum transfer to the photon beam is not too large. A very important note I want to make is that the photon beams do not travel in a circle; they travel in straight lines and are constantly leaving the proton beam. Synchrotron radiation continuously drains the LHC beams of energy, which is why the LHC beam must be continuously fed with more energy.

Making Matter from Light Since 1994

Synchrotron radiation has been around for quite sometime. Despite recent claims, the first evidence for direct production of matter from photon beams came in 1994  from the DELPHI experiment at the Large Electron Positron (LEP) Collider, the LHC’s predecessor at CERN. There are earlier reports of photon-photon scattering at colliders but I have been unable to track down the appropriate citations. Since 1994, evidence for photon-photon scattering has been observed by the Fermilab’s CDF experiment at the Tevatron, as reported by the CERN Courier, and there is even evidence for the pair production of muons and W bosons at the CMS Experiment. Excitingly, there has also been so research to a potential Higgs factory using a dedicated photon collider. This image shows a few photon-photon scattering processes that result in final-state bottom quark and anti-bottom quark pairs.

Various photon-photon scattering processes that result in final-state bottom quark and anti-bottom quark pairs. Credit: Phys.Rev. D79 (2009) 033002


We can expect to see much more from the LHC on this matter because photon beams offer a good handle on understanding the stability of proton beam themselves and are a potential avenue for new physics.

Until next time, happy colliding.

– Richard (@BraveLittleMuon)