This article on particle colliders is a guest article by Rishabh Sharma, a third-year integrated Ph.D. student from the Indian Institute of Science Education and Research (IISER) Tirupati, pursuing a Ph.D. in experimental High Energy Physics.
In the year 1928, George Gamow predicted that the energy required to overcome the repulsion of the atomic nuclei and penetrate it will be lower than expected if a phenomenon now called quantum tunneling was to be true. Getting inspired by this idea in 1930, Cockcroft and Walton accelerated protons at the energies of 200,000 electron-Volts and bombarded them onto lithium nuclei hoping that some protons would penetrate the nucleus.
They failed to observe the phenomenon and concluded that protons needed to be accelerated at even greater energies. This began the quest of achieving higher energies which have not come to a halt to this day. In 2015, the Large Hadron Collider (LHC) at the European Council for Nuclear Research (CERN) accelerated protons to record energy of 6.5 million million electron-Volts.
So after almost a century of smashing particles into each other at higher and higher energies, we not only did observe the phenomenon of quantum tunneling but we have come a long way ever since. CERN has recently announced investments in the Future Circular Collider (FCC), an ambitious project that will push the frontiers of particle physics. While people are debating that investing in such ambitious projects is worth the time and money, let us have a look back into the turn of events that resulted in the building of the LHC and why do we want to build an even bigger collider.
The Journey To The LHC
LHC is currently the biggest particle collider with a circumference of 27 km. The story of LHC started in 1961 when CERN decided to build a Large Electron-Positron collider. The history of LHC can be divided into four milestones:
The Proton Synchrotron
A synchrotron is a circular particle collider evolving from a cyclotron, where particles are accelerated by the electric fields and are steered in fixed circular trajectories with the help of synchronized magnetic fields which increases as the energy of the particles is increased. Under the supervision of Sir John Adams, head of the project from 1954 to 1961, the Proton Synchrotron (PS) at CERN became operational in 1959 and accelerated protons at the energy of 24 GeV in a circumference of about half a kilometer. In 1973, the neutrinos produced at PS were used to discover neutral currents – a way in which particles can interact with each other via the weak force.
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The Super Proton Synchrotron
After the start-up of PS, CERN set its eyes to a new Super Proton Synchrotron(SPS) that can be considered as a scaled-up version of PS. However, there was not enough space on the surface to host this 7 km circumference facility and CERN decided to move to a different location. But due to a general lack of consensus about the new location and an exorbitant amount of money that will be spent to build a new lab, the idea of SPS was stuck in limbo.
A way out of this was found by Sir Adams who was invited back in 1970 as SPS project leader in CERN. He suggested building this new facility underground in tunnels at the same location. This let scientists make use of the existing PS and thereby reducing the total cost of the project.
SPS accelerated protons at energies of 400 GeV and in 1983 detected W and Z bosons (force carriers of the weak force), which a year later won Carlo Rubbia and Simon van der Meer the Nobel Prize in physics.
The Large Electron Positron (LEP) Collider
After the discovery of neutral currents by the PS, later solidified by the discovery of W and Z bosons by the SPS, things were looking in good shape for the electroweak theory (describes the weak nuclear possible force). The next step was to measure the masses of W and Z bosons. For this purpose, LEP was built in a circular tunnel of circumference 27 km and was used from 1989 to 2000. It collided electrons and positrons at the energies that reached 209 GeV.
LEP was successful in achieving the purpose it was built for. In the first phase of its application that lasted till ‘95, the collision energy was sufficient to produce Z bosons. However, in phase two of its application, called LEP 2, with certain upgrades it was able to produce pairs of W bosons.
Towards the end of its scheduled run time, data collected by the LEP hinted that the Higgs particle might have been observed. However, the confidence level of this discovery was 91%, lower than the standard set by particle physicists to claim a discovery. Following this cue, there was a suggestion to extend the run time of LEP by a year, which would mean a delayed start of the LHC. Finally around 2001, LEP was dismantled to pave way for the LHC to be built in the same 27 km circumference tunnel.
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The Large Hadron Collider
When LHC first started-up in 2008, it set out to discover the last missing piece of the standard model – a rather dull name to a theory on which the understanding of the microscopic world of particles is based on. The missing piece was the Higgs Boson, which it did discover in 2012. With this discovery the standard model of particle physics was complete. But scientists had other expectations from LHC.
Back in ‘84, in the middle of the construction of LEP, scientists proposed to have a hadron collider which was considered a natural extension of the LEP program. The questions it would pursue were:
- Is the Higgs mechanism responsible for the origin of mass?
- Is there more than one Higgs particle?
- Are their supersymmetry counterparts to the particles described by the standard model? (Supersymmetry is a theory that tries to unify all four forces of nature)
- Is dark matter present in the universe?
- How is electroweak symmetry broken?
- Why is there more matter than antimatter in the universe?
LHC, however, could not answer all the questions it set out to pursue. On one hand, where it did complete the standard model with the discovery of Higgs, discovered exotic particles like tetraquark, pentaquark, found that the amount of Charge-Parity (CP) violation is more for bottom quark than theoretically predicted, tested the neutral current conservation but it also failed to provide evidence for new particles expected to be found in the energy range of 100 GeV to 2 TeV, did not find any supersymmetry candidates, could not provide a conclusive answer to the matter/antimatter asymmetry in the universe.
Even though LHC has so far failed to find new evasive particles, the tremendous amount of data it has generated has helped us to experimentally test the existing theories.
Why Higher Energies?
To study the evolution of particle colliders is to understand the need to move to the higher energies. The reason behind this is two-fold:
1. Higher the energy to which particles are accelerated, smaller the distances we can probe. This result follows from the famous de Broglie relationship which states that higher the momentum (or higher the energy), the smaller will be the wavelength of the particle. Thus, we intend to accelerate these particles to energies which render their wavelengths so small that they become comparable to the size that of the proton leading us to the study of its interior.
2. Higher energies provide us with a window to observe rare exotic particles that decay rather quickly. This follows from the E=mc2, where the initial energies of colliding particles can be converted into mass. Higher the energy, heavier and more exotic are the particles produced.
Going to higher energies has been rewarding in history. When, in fact, Cockcroft and Walton did increase the energy of their generator to accelerate protons at energies higher than 200,000 eV, they successfully were able to split the Lithium nucleus at an energy of 400,000 eV. The decision to build SPS on the existing PS led us to the discovery of W and Z bosons. Increasing the energy of LEP produced pairs of W bosons. Upgrading LEP to LHC led to the discovery of Higgs boson.
So moving from LHC energies to FCC energies looks natural.
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The Road to FCC
The Future Circular Collider (FCC) is a proposed post-LHC collider. It will push the energy limit to 100 TeV. It is hoped that the FCC will provide the answers to the question LHC couldn’t. It plans to look for:
- Dark matter particles
- Is Higgs a fundamental particle or if it is made up of further smaller constituents?
- Answers to the matter-antimatter asymmetry
- FCC will also continue research in heavy-ion collision initiated by the LHC and RHIC in Brookhaven, USA
- FCC will also host electron-proton collisions that will resolve the structure of protons with very high accuracy
FCC will be housed in a tunnel with a circumference ranging from 80 to 100 km. Building such a gigantic machine and running it will require money- a lot of it! The physics community seems to be in a divide when considering the question- is this investment worth it? CERN already has announced investing in the majestic project. One, however, cannot ignore the unmatched precision that we will be able to obtain with such a collider and perhaps hint to any new physics or any new particle can show up in a deviation from the standard model’s predictions at very high precision. If we choose not to build this detector, honestly, we will never know!
The argument against building a future collider is that the money that will be invested in such an endeavor can be spent to study more pressing issues like environmental change and the dynamics of pandemics in the human population.
It remains to be seen whether the FCC will be able to see the light of the day in the form it is recently proposed. This surely will be a huge endeavor in civil engineering and R&D. While the standard model explains most of the observed phenomena but it fails to explain some as the origin of the neutrino mass. It also fails to unify gravity into its framework. The standard model is an extraordinary theory but it is not the entire picture. The experiments to come hold in them an exciting future for particle physics and perhaps provide the answers to the tantalizing questions nature has led in front of us.
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