South-Western Germany. Perfect location to reach Geneva. And Geneva, with CERN, in 2015 is basically physicists’ Mecca.
Although I am not a particle physicist, when I finished my high school in humanities, my love for neutrinos was the reason to start this new adventure in the world of science. Ever since, I have been fascinated by particle physics, making material science (my actual field of study) almost jealous. My secret love for tiny particles is why, about three years ago, I was standing over-excited in front on my laptop waiting for one of the biggest announcement of recent times: the first evidences for the Higgs boson. It was a historical moment, the Woodstock of physics.
The Higgs boson represents the excitation of the Higgs field, thought to be responsible for the mass of fundamental particles. The Standard Model, the big puzzle in which all discovered particles fitted ever since, at first sight did not predict any mass for any of those. The introduction of the Higgs field seems to solve the mystery, associating to a particle a mass proportional to the strength of the interaction with the field. The evidence of a particle of mass ~125 GeV by the two detectors CMS and ATLAS at CERN, confirmed the presence of the field and earned Higgs and Englert a deserved Nobel Prize. Happily ever after. The end.
Apparently, it was just the end of the chapter. That’s why physics is so exciting. Every time something is solved, there is something new to be addressed. In fact, 125 GeV is really a small number for Higgs boson mass, taking into account that it has to interact with all the Standard-Model particles. For me? Well, for me the 4th of July 2012 was the end of the book. And 125GeV were more than enough to be satisfied and go out celebrating with a good beer. Until now….
On March 14th, I finally managed to pay my first visit to Mecca. We did not manage to get a place for the individual visits (if you want to visit CERN, remember to reserve 15 days in advance from their website), but thanks to a friend we managed to get a tour to CMS anyway. CMS, Compact Muon Solenoid, is a detector able to resolve almost everything you could think of, electrons, photons, hadrons, and of course muons. The guide, an enthusiast bright scientist, explained us about its operational principles and its role in the Higgs observation. But at this point, it was still a closed book, like paying a visit to a really expensive, really big, as heavy as the tour Eiffel museum piece. Except one detail, after March 14th no more visits are allowed to the underground tunnels, because a new 3 years experiment is starting.
“So, what’s next?” – said finally aloud one of the visitor, expressing the feeling of all the group.
What’s after the Higgs? What’s the next chapter?
In that precise instant, in the guide’s eyes a sparkle: “Susy!”
Of course, Susy! Susy is brave extension of the Standard Model, that brings the idea of symmetry to next level: SUper SYmmetry (SUSY). It associates to each particle of the Standard Model a Super particle. Where in the Standard Model is a boson, SUSY brings a fermion. Every standard particle we know is a Lois Lane waiting for her Super-Partner. And like Superman is more muscled than Lois, the superparticles seem to be sufficiently more massive to have eluded the detectors since now.
Why do we need them?
Well, first, symmetry is beautiful! Human race is obsessed with it, and it would be cool to discover that the reason we are so much in love with symmetry is because nature itself is perfectly symmetric. But this is philosophy. Scientifically speaking, Susy comes naturally from String Theory maths (or so I have been told), for which the string vibrations that represents fermions and bosons come in couples. Also, the presence of these extra particles would cancel out the expected mass excess for the Higgs, giving as a result the light Higgs boson measured. And finally, the biggest dream for a physicist: if the particles do exist, it can be proven that all the forces that govern our Universe have the same strength at very high energy, leading to a grand unified theory.
During the search for the Higgs, already there was someone starting to look for the emergence of new particles. In fact, it is known that superparticles must be more massive than their ‘standard’ partners, but how much more? This is unknown, and it seems that while I was celebrating the Higgs with beers, some physicists much smarter than me were drinking for the disappointment to not have found some of Susy’s friends already.
This is one of the reason to start new measurements, and collect more data. Of course, some are needed to characterize the newly born Higgs, and see if the particle observed during the first experimental run is really what we suspect. But more data, moreover if at even higher energy, means the probability to spot some superparticles. Actually to spot their absence. No, I am not saying I want the experiment to fail. I am just saying that if they are how we expect them, they will unlikely being detected by any of the equipment in use; at the same time, if they are created in one of the collision, for the conservation of mass/energy, the experimentalists will be able to spot some missing energy between the initial colliding protons and the resulting measured particles. Like spotting that Clark Kent is never there every time Superman is in action. Simple and clever.
What will happen if Susy is “seen”?
A new chapter of physics will start. A completely new chapter, which boundaries will easily fall in sci-fi pages. Susy opens the doors to understand dark matter, and maybe dark energy and new dimensions.
The known matter observed in stars and galaxies seems to account just for the 4% of the Universe matter. Observations of the galaxies dynamics show that much more additional matter is needed to account for the rotation velocity of the galaxy periphery. This matter must have passed undetected, so it cannot interact with the electromagnetic force. No light interaction of any kind, then “dark” matter. Nowadays the lightest form of neutralino, a mixture of three Susy’s fundamental particles wino, bino and higgsino, is the most favourable candidate for the composition of this Dark Matter.
Together with Dark Matter, there is another phenomenon in our Universe that falls in the realm of the Dark side: Dark Energy. Our Universe is expanding faster than expected. Something has to give energy to sustain the acceleration. Just we do not know what. A little bit of fantasy, and a lot of maths, suggest that the answer may be in the presence of extra dimensions. This hypothesis would bring explanations for the weakness of gravitational force compared to the other forces, allowing it to “leak” in other dimensions. The collider at CERN aims to observe the Graviton, the boson mediator for the gravity force field, that looks to be more shy than the Higgs boson was. The graviton, in addition, is expected to rapidly disappears into extra dimensions, so again the absence of some interesting object would be spotted. Extra dimensions. I am not mocking you. This is the science the next run of CERN experiments will investigate.
As a good experimental physicist, I have always been sceptical in regards to the most recent theories. They always seemed to me to be closer to philosophy than physics. It is plenty of them out there, brane cosmology, time travels, black worms, gravity loops, … And I always look at them very carefully. Something fun for a mental exercise, but far from being provable and concrete. Nevertheless, the same evening of my visit, I have spent the night in a pub discussing with the brilliant scientist that gave us the tour at CMS about the realistic scenarios that CERN experiments could open. It was not simply an amusing discussion, but real and tangible new doors in the realm of physics. And this is exciting. It makes you feel like when you were a kid, thinking of everything being possible, dreaming that one day the reality would translate in something similar to the multitude of sci-fi movies you have learnt to love. And this reality is just knocking at our door. Three years is not a long time to wait, moreover for the prize that is waiting for us at the end of this travel.
And what if none of these scenarios happens? Well, if possible, it would be even more exciting. Something completely new will need to be dreamed, to be proposed, to be studied and finally, as it always has to be, to be proved.
In front of the exhibition centre at CERN, there is a monumental homage to the greatest discoveries in physics. And it was almost moving for me. Archimede, Keplero, Galileo, Newton, Maxwell, Plank, Einstein, Schroedinger, Heisenberg. All in there. A short tale of physics made of formulas and passion, from the beginning of human race consciousness to the standard model. Following the connecting wire of curiosity. And there I felt of being part of a never-ending story. Not ever an end. Just chapters to be added, one after the other to the knowledge our mind is able to image and comprehend. And although it may seem scary not to have a closure, for me it almost reassuring knowing that I would never feel that nostalgic bitterness that always accompanies the last page of a great book.