MacDirectory Magazine

INTERVIEW fibres are used to transport light produced in plastic scintillating tiles when ionizing particles traverse them. In this way we exploited all our knowledge about fibres for light transmission and about photomultipliers as light detectors. MA > It looks like your wave-length shifter fibres knowledge was a key factor for you to get into CERN. BDG > In such big projects, both knowledge and skills are needed and exploited. When I started to work in ATLAS my first original contributions were for the measurement of the response time of the fibres and for the measurement of the absolute quantum efficiency of the photomultipliers. I have been extremely lucky in my career because after a (very fruitful) parenthesis of about three years spent at SLAC in the BaBar experiment, I had the possibility to move to CERN as a Fellow and work again in ATLAS. Since then I have participated into several projects within ATLAS. That gave me a lot of knowledge and skill development. I have also developed my skills in mechanical engineering, financial management and crisis solving to help the experiment in few challenging last-minute fixes. Maybe it is worth mentioning that I helped few colleagues with their Macs as well! MA > The collider finally crashed particles into each other recently but how was the decision made to smash “protons” instead of other subatomic particles? How can we be certain that the Big Bang did not include other elements? BDG > When the decision about which particles to smash is made, it takes into account many factors. In the case of the LHC one of the needs is to have a very high rate of events because the physics we are looking for is quite rare and not just only accessible at high energy. Smashing electrons, for example, is very nice because the events are, in our jargon, “very clean”. However electrons suffer important energy losses when constrained to fly in a circular accelerator…. We are certain that the Big Bang produced many other particles and in reality the protons are not elementary particles because they are made out of quarks. However, it is not possible to isolate the quarks, and by colliding the protons we essentially make the quarks collide, as they are the intimate components of the protons. In short, protons are the best particles we know to obtain dense population of particles in each beam. And with a reasonable effort we can quickly and finally obtain high rates of collisions. MA > The bigger the accelerator, the bigger the crash. So why was the Hadron collider built exactly 17 miles, not bigger? BDG > The LHC is re-using the same tunnel used by LEP, the former electron positron accelerator at CERN. To minimize the cost of the project the LHC has been conceived since the beginning as a machine housed in that tunnel and all the efforts have been made to have the best possible machine with that length. Building a new tunnel is a major civil engineering exercise and, to have a sizeable benefit, it should be really much larger. MA > Since you mentioned cost, how are you able to obtain cost-effective energy to operate LHC? I was told that the Collider requires 3.5 trillion electron volts per proton! BDG > The protons are accelerated with strong electromagnetic fields to accelerate the protons and bring them up to the desired energy. Therefore the energy is coming from the electrical network. MA > Once you get the protons to the designed energy, what happens next? A big loud explosion inside the Collider? BDG > Sound waves are propagating through the air, but in the beam pipes we have very high level of vacuum. If you direct a beam to a metallic block, then you would get shock waves in the metal and these would propagate through the metal, eventually to the outside and that would create a bang. You need air to make these sounds to propagate, as you have in our world. MA > One of the top goals of the collider is to identify and learn more about dark matter. How do you create a device to measure this strange element that has never been measured before? BDG > We cannot build what we don’t know yet. The approach to building detectors is to build a device that we understand fully and that we are able to calibrate to obtain a very high level of comprehension of their work. 122 MacDirectory

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