The discovery of the Higgs Boson on the 4th of July 2012, that explains why atoms and we exist, is an opportunity for science and art to fuse; to speak of a reassessment of our place in the cosmos that has just occurred with the Higgs discovery, to speak of the mysterious and to make it approachable.
By studying the Higgs Boson, we seek to understand why the world is the way it is. It is one of deepest questions humans have ever pursued. Why? Our universe is full of fields invisible but influential: the patterns formed by iron filings near a magnet due to a magnetic field and the orbit of Earth around the Sun due to a gravitational field are two we are familiar with. The Higgs field fills all of space; it’s role understood by analogy to an ocean of water. A barracuda is supremely streamlined, it passes through water almost effortlessly, there is little interaction, it represents a low mass particle: for example an electron. A person wading through deep water does so with great effort, there is lots of interaction. The person represents a high mass particle: for example a top quark. The more interaction a particle has with the Higgs field the more massive it is.
The Higgs boson is the quantum of the Higgs field, the smallest piece, and in the ocean analogy the Higgs boson is a water molecule.
To appreciate the importance of the Higgs it is sufficient to imagine a world without it. If the Higgs field did not exist, particles, including the quarks and the electron and its siblings, could not interact with it and so would have no mass and travel at light speed. Protons and neutrons composed of quarks bound together obtain their mass from the energy associated with binding, so they would still be massive but for subtle reasons protons would now be heavier than neutrons and so disintegrate into them in about 15 picoseconds. There would be no hydrogen atom, and the lightest “nucleus” would be one neutron. A massless electron means that the radius of an atom—half a nanometer in our world—would be infinite. Without compact atoms, chemical bonding would have no meaning. Matter would be insubstantial and life would not exist. This is the reassessment of which we speak.
The discovery of the Higgs Boson represents one of the greatest triumphs of the human intellect, vindicating the construction of one of science’s greatest theories known as “The Standard Model of Particle Physics”.
To tackle this epic goal mankind needed unprecedented scientific collaborations to building the Large Hadron Collider (LHC) at CERN as well as the 4 experiments ALICE, ATLAS, CMS and LHCb. The LHC, the most complicated machine ever built is a 27 km circumference machine that collides counter-rotating beams of protons at almost speed of light. In each of the 4 crossing points where LHC collides the two beams one of the large experiment are installed to capture the products of the particle collisions like big cameras. ATLAS and CMS, the two gigantic multipurpose detectors or cameras, have a size of a 6 story office building but containing in some places of sensors small such as a human hair.
Never before had detectors of such complexity been designed and built. This sophistication was required to match the performance of the LHC, with extreme demands for spatial granularity and resolution, rapidity of response and radiation hardness of sub-detectors. Due to the extremely small production cross section of the Higgs boson, less than a billionth of the proton-proton interaction probability, a billion proton-proton collisions per second are needed to give a chance of observing it and investigating its properties. This data must first be filtered, selected and reduced to a rate of a few hundred potentially ‘interesting’ collisions per second prior to detailed offline analysis
The CMS detector situated 100m below the surface of the French village Cessy in the Pays de Gex has a height of almost 20m, a length of 24m and weights 14.000T.
Its heart, the real backbone around which the entire experiment was designed, is a huge superconducting solenoid magnet, the largest of its kind in the world, which operates at 4 degrees Kelvin producing a uniform 4 Tesla field with 3 Gigajoules of stored energy. It was designed by engineers and scientists from Saclay incorporating numerous technical innovations, and was built with components from all over the world and assembled in Italy.
Outside the magnet cryostat is the iron magnet return yoke. This structure consists of a central barrel part of five huge “wheels” each weighing up to 2000 tons. It was designed in Germany, made from iron cut in Russia, assembled at CERN and held with connector elements from the Czech Republic, everything resting on “feet” built in Pakistan. The two end-caps of CMS, each consisting of three iron disc-like walls, complete the flux return path and hold a variety of sub-detectors. The end-caps themselves were produced in Japan, rest on feet built in China, and are secured by special anti-seismic support-bars built in the USA.
Inside the iron yoke are interleaved four layers of muon detectors, both Drift Tube (DT) and Resistive Plate (RPC) chambers. The DTs were designed and constructed in Germany, Spain and Italy, and the RPCs in Belgium, Bulgaria, Italy, Korea, Pakistan… This central barrel part also houses scintillator detectors built in India, complementing the inner calorimeter. The muon detectors covering the end-cap discs are also organized in four layers; Cathode Strip chambers (CSC) designed and built in the USA, Russia and China, and RPC detectors. The muon system is central to the CMS detector design, giving the experiment its name “Compact Muon Solenoid”. It is the subdetector which the CMS “founding fathers” counted-on for the search for the Higgs boson via the process H => Z + Z bosons decaying each to 2 muons effective over a very broad mass range.
Looking inwards towards the interaction point from the coil cryostat we find three types of detectors; first the hadron then the electromagnetic calorimeters, and at the centre, surrounding the accelerator beam pipe, the central tracker. The hadron calorimeter is a conventional design of alternating layers of absorber (brass) and scintillator plates. It is used to measure particle “jets”, groups of particles, the macroscopic manifestations of quarks and gluons. A curiosity of this detector is that the 1600 tons of absorber brass were recovered by Russian physicists from disused naval artillery cartridges discarded from Russian navy cruisers. The brass after transportation to Bulgaria, was ultimately cut and engineered to a design from Fermilab/USA colleagues in a shipyard in Spain. An excellent example of international cooperation. The barrel, end-cap and very forward hadronic calorimeters were also produced by a collaboration of institutes from USA, Russia, Ukraine, Turkey, Iran, Hungary……
The next inner layer, the electromagnetic calorimeter, one of the most original parts of CMS, comprises some 76000 scintillating crystals made of lead tungstate (PbWO4), each of size 2222 cm3. The crystals are organized in the barrel part as a cylindrical shell with two endcaps, all crystals pointing towards the interaction point at the centre of the detector. They were produced over several years in Russia and China after a five-year-long research and development program, an interesting scientific, political and industrial saga in itself in those post-soviet years. Countries or institutions contributing to various aspects of mechanical design and construction, readout elements of this very sophisticated and high performance detector include CERN, France, Greece, Italy, Japan, Taiwan, UK, USA and Switzerland.
Finally, the innermost detection system is the central tracking detector, the most sophisticated and technologically advanced element. In a cylindrical volume of about 6m length and 2.3m diameter are organized, in cylindrical layers for the central part and in flat discs towards the two ends, 10 million silicon microstrip detectors, typically 6 cm long, 100 to 400 microns wide and 300 microns thick. It has an overall area of 200 square metres, and at the time of its conception and design no Si-detector in the world exceeded 2 square metres! It is complemented in its central part, closest to be beam pipe and interaction point by a pixel detector, 70 million pixels in total of size 100150 microns square, organized in 3 cylindrical barrel layers and by end-cap discs, 3 on each side. The complete tracker system presented extreme requirements of mechanical construction, precision, electronics, radiation hardness, etc., and was a collaborative effort from Austria, Belgium, Germany, Italy, France, Switzerland, USA and CERN. It provides altogether eighty million individual electronic readout channels of high precision track measurements.
To read, select and record the data, CMS has a highly innovative, powerful and flexible data acquisition, triggering, monitoring, control and processing system with both hardware and software components. It is the equivalent of the nervous system of an organism, transforming and making sense from the electrical signals produced by the sub-detectors to produce physical quantities and variables amenable to the physics analysis which finally led to the discovery of the Higgs boson in 2012. The countries or institutions contributing to this system, among others, are CERN, Austria, France, Italy, Germany, Greece, India, Portugal, Spain, Taiwan, UK, USА.
The construction and first year of data taking over 25 years involved 11,000 scientists and engineers from almost 200 institutions, laboratories and universities and 45 countries worldwide. Right now CMS as ATLAS, each of them have about 3,000 active members in their collaboration dedicating their professional work, creativity and passion to run and maintain these experiments. The reader may wonder how this global endeavour with people of different cultures, religions, languages, did not suffer from the Tower of Babel syndrome. How could humans peer into one of the deepest secrets of the universe, the origin of mass, without being confounded by a multitude of languages? The secret is that all were driven by a common purpose with physics and mathematics as universal languages. All used broken-English to communicate, and to give satisfaction to French pride, the metric system of units prevailed, with no inches, feet, yards, pounds, imperial gallons, etc!
With the discovery of the Higgs boson, the LHC story is not over. Years of detailed studies lie ahead to understand its exact nature and properties. Other highly interesting studies and searches with possibly other discoveries are in store; supersymmetry, signs of extra space dimensions, quantum mini black holes, etc. In about 20 years from now, when this research programme will be completed, a new and more powerful machine with more complex detectors may well take over to carry on this incessant quest towards a deeper understanding of Nature allowing future generations to pursue the pleasure of discovery.
The book “CMS - The Art of Science” by Michael Hoch is perceived as a homage to all large scientific collaborations who try to push the limits of science and technology for the benefit of mankind. The document allows general public to discover the beauty and complexity of science architecture at CERN. Furthermore it pays tribute to all involved scientists, engineers, technicians and other passionate professionals who are needed to design, construct and run this experiment
Albert Einstein wrote: “The most beautiful thing we can experience is the mysterious. It is the source of all true art and all science.”
Public who have been fortunate enough to visit a see personally the CMS detector or other experiments at CERN in all its glory, have often commented that it could be displayed as a work of art.
Science requires that we continually reassess our place in the cosmos. Where do we come from? Who are we? Where are we going? It changes our perspective. Great art, literature and music ask these same questions. When we experience a wonderful book, a beautiful film, a painting, sculpture or symphony, it changes our perspective, just as science does.
Given this commonality between art and science it’s especially appropriate and exciting when artists make science their subject. Artists show science from a unique perspective that can be more approachable than science head-on.
There are interdisciplinary programs at CERN who allow access to professional artists, as well as art and educative institutions to collaborate. The goal is to make scientific topics their own and create unique products of diverse fascinating topics related to particle physics. The CERN artist in residence program “ARTS AT CERN” has its main goal to develop expert knowledge in the arts through a connection with fundamental science. The program gives artists the opportunity to encounter the multi-dimensional world of particle physics. http://arts.cern/collide
The art@CMS program https://artcms.web.cern.ch/ establish collaborations between scientists, artists, art institutions and students globally and supports group as well as solo exhibitions. Through these exhibitions and educational workshops the two complementary views merge, stimulating our senses and inspiring our curiosity by highlighting the mystery.
For all who have not visited CERN and or CMS yet, there is a unique chance again between beginning of January until end of March 2017, when the LHC is not in its physics mode but in standby to service and upgrade equipment. The CERN visit service https://visit.cern/ helps you to schedule a visit to this fascinating science laboratory next to Geneva.
Text&photos: Michael Hoch
CMS The Arts of Science / Edition Lammerhuber
Credits for the authors:
Ian Shipsey University of Oxford/ UK
Daniel Denegi SACLAY/ FR
Michael Hoch HEPHY/ AUT