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Slide 2 : Light is everywhere around us and is the source of life in our planet. Also, we can see and admire the world around us because of light. So, you seeing an object is light reflecting from that object into your eyes. Slide 3 : Have you ever wonder if light can see light ? in other words can light interact with light ? Slide 4 : the short answer to that is YES . And now you are wondering how ?. Slide5. Light does interact with light in many ways, but I will only talk about this unique and elegant one. Which is the light-by-light scattering. This process was predicted almost 90 years ago by the theorist and is part of a theory that is used in daily basis by particle physicist. So its really important to be able to measure it experimentally. However, observing this process experimentally has remained a challenge all these years because of its small probability and to walk you through it. We have 2 light particles which are the photon1 and 2 will scatter each other via electrically charged particles such as electrons which is the red box and produce the photon3 and 4. And inside the same box we can also have particles that have not been discovered yet such as some dark matter candidate and that’s another reason why this process is so important to us. Now the simplest experiment u can think off is laser beams but unfortunately none of the laser beams are powerful enough to probe this process. Slide 6 : what I will show you today is an unconventional way we used to observe the process Slide 7 : Well it all start at the large hadron collider. And you can think of it as when kids break their toys to figure out what they are made off that’s them being physicist. So, what we basically do is break particles to see what they are made off and how these components interact with each other. Slide 8 : Here is a more realistic picture of the machine its 27 km cercle with 4 experiments around it and mine is ATLAS . Slide 9 : Inside this tunnel we accelerate and collide typically 2 things proton which comes from the hydrogen atoms and these kind of collision is what was used for the famous Higgs particle discovery. We also collide heavy ions typically lead nucleus after removing the electrons. Slide 10 : That leave us with a lead nucleus made of protons and neutrons traveling at almost the speed of light. Slide 11 : The protons since they carry an electric charge they create a huge electromagnetic field which is the key to the observation of this process Slide 12 : The field contains the photons also knows as light particles . Slide 13 : although when you build a collider you want your particles to smash head on but occasionally that’s not the case and we have what we call ultra-peripheral collisions where the 2 lead ions will only pass by each other and only interact via the surrounding electromagnetic field and that’s when the light by light scattering occur and we can see the 2 resulting photons inside our detector. Slide 14 : In summary we have analyzed billion of events and found 59 event like one I just showed and this mark the first ever light by light scattering observation. Slide 15 : Thank you