LHC, the mightiest particle accelerator in the world, ready for Run 2

Tunel LHC (fot. CERN) LHC tunnel (source: CERN)

Large Hadron Collider (LHC), the mightiest particle accelerator in the world, is ready for Run 2. Some key accelerator elements have been repaired/modernized, some of accelerator operational parameters have been improved during the two-year long Shutdown 1. Thanks to that researchers (including about 100 scientists from Poland) will be able to explore new frontiers in our understanding of the fundamental structure of matter.

"The single most important outcome of the modernization works is beam energy increased  to 6.5 TeV i.e. collision energy increased to 13 TeV. Such high energy achieved for the first time in history by any machine will enable researchers to look for new, in particular heavy, particles, and to verify theories so far impossible to experimental verification” – explained Dr. Maciej Górski, Head of the NCBJ High Energy Physics Division, member of the CMS collaboration working at LHC. – "Since beams will now collide every 25 nanoseconds, number of collisions will be more than twice as high as during Run 1, and new experimental data will be produced at a correspondingly higher rate than during the 2011-2012 period”.

Scientists hope that LHC Run 2 scheduled till 2017 will allow to acquire some more precise information on:

  • Higgs boson, a manifestation of the Brout-Englert-Higgs mechanisms responsible for acquiring masses by elementary particles. Till 2012 the boson was the last not yet experimentally verified particle predicted by Standard Model, the most widely accepted theory describing fundamental particles and interactions between them. A higher energy of the modernised LHC means a higher probability to produce Higgs bosons in collisions. More bosons mean more opportunities to precisely measure their properties and to explore their more rare decay channels.
  • Dark matter. Gravitational observations indicate that our Universe must be composed mainly of some invisible matter. What can it be? One of the theories says that dark matter is composed of some “super-symmetrical particles”, see below. Experimental data produced by LHC operated at higher energies might shed some light on those mysterious topics.
  • Super-symmetry. A theory proposed to fill some unexplained gaps in the Standard Model. It predicts that every known elementary particle has a more heavy counter-partner. If so, some of those particles should be observed in high-energy collisions produced by the modernized LHC.
  • Additional dimensions. A theory predicting that every standard particle has heavier equivalents existing in additional dimensions (just like every atom normally existing in a low energy ground state may be exited to some higher energy states). Such heavy particles might be observed in high-energy collisions produced by the modernized LHC.
  • Anti-matter. Every elementary particle has anti-particle of identical all properties except sign of its electric charge. For example positrons and electrons are identical by all means except positron has one positive elementary electric charge while electron has one negative elementary electric charge. Matter and anti-matter getting in touch annihilate i.e. sum of their masses is instantly converted into flash of energy. Symmetry between matter and anti-matter on the elementary particle level is nearly perfect; Standard Model predicts very few rare processes that violate that symmetry. However, some observations prove that the Universe is not composed of equal amounts of matter and anti-matter. Precise investigation of decays of charm/beauty particles might help to understand the observed asymmetry and to find some phenomena beyond the Standard Model realm.
  • Quark-gluon plasma. Higher energies made available by the moderni­zed LHC should help to better characterise quark-gluon plasma flood­ing the entire Universe for several millionth parts of a second just after the Big Bang.
  • Exotic particles. Some theories predict the whole world of particles yet undiscovered because they do not interact with electromagnetic fields. However, if such a particle has mass, it has to interact with the Higgs field. That way the Higgs boson becomes a “point of contact” between Standard Model and hypothetical world of exotic particles.

Physicists point out that it may take time to get answers to the above questions. LHC will be re-started in several phases, such as calibration of the equipment, tests of individual subsystems, attaining the rated energy level, gradual increase of the number of packets of accelerated particles, optimization of focussing subsystems etc. It is estimated that targeted beam parameters may be reached only at the turn of 2015/2016. All physicists involved in fundamental research on “new physics”, dark matter, Higgs boson etc. – both theoreticians and experimentalists – are eagerly awaiting for new data. 

“Super-symmetry is a particularly interesting proposition of the ‘new physics’. The theory predicts existence of additional Higgs bosons and dark matter. Within it all these issues are mutually interrelated” – said Professor Leszek Roszkowski from NCBJ, leader of BayesFITS, one of the globally leading teams of theoreticians active in that field. –“Our predictions will soon be verified by international teams of experimentalists analysing new data supplied by the modernized LHC”.

Improvements/new technical solutions introduced/implemented/deployed during the 2 years long Shutdown 1 period in order to achieve goals of LHC modernization include: all superconducting magnets that shape trajectories of the particles have been tested, some of them have been replaced; a new cryogenic plant (that supplies the magnets with liquid helium necessary to keep them in the superconducting state); a more safe vacuum system; new solutions to focus the beams more effectively; new solutions adapted to higher accelerating voltages; more radiation-resistant electronic circuits. LHC is a very complex and advanced facility requiring high accuracy of the conducted works and involvement of a number of experts from various Hi-Tech fields (including some teams from Poland). Successful modernization of such a giant takes time. Value of all done modernization works is estimated to about 150 million Swiss francs.

LHC is planned to run for the next 20 years, with several shutdowns scheduled for necessary upgrades and maintenance works. Future depends on realized scientific discoveries, as well as on organizational/technological/economical capabilities.

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Polish scientists / technicians in LHC modernization works

Formally Poland became a member of CERN (European Organization for Nuclear Research) in 1991. Nowadays nearly 300 experts among 8,000 persons working at all LHC-related projects and experiments came from Poland (more precisely from NCBJ, Warsaw University Physics Faculty, Warsaw University of Technology, AGH University of Science and Technology, Polish Academy of Sciences Institute of Nuclear Physics, and from some other Polish institutions). Some of them held or are still holding high-ranking CERN offices: Professor Agnieszka Zalewska (IFJ PAN) is currently CERN Council President; Professor Ewa Rondio (NCBJ) was a member of CERN Management Board; Professor Ryszard Sosnowski (NCBJ) was CERN Council Deputy President; Professors Helena Białkowska (NCBJ) and Jan Nassalski (NCBJ) were members of CERN Scientific Policy Committees; Professor Jan Nassalski officially represented Polish physicists in CERN Council; Professor Grzegorz Wrochna (NCBJ) coordinated works of an international team of developers one of the subassemblies for the CMS detector; Professor Krzysztof Meissner (Warsaw University) is one of two coordinators of the OSQAR experiment; Professor Michał Turała (AGH) was Head of the Electronics and Computing for Physics CERN Division.

Responsibilities of Polish scientists include/included also design and construction of some key LHC elements and detectors used by LHC collaborations. NCBJ Nuclear Equipment Division (HITEC) participated in modernization of accelerating structures of the first accelerator in the cascade of machines feeding LHC. Special chambers used to accelerate protons and to group them into bunches have also been developed in Świerk. Members of the Warsaw CMS Group (scientists/technicians from Warsaw University, NCBJ, and Warsaw University of Technology) have modernized electronic circuitry of the muon trigger (the circuitry was originally developed by them in 2009). Polish members of the LHCb team developed a large fraction of the so-called straw chambers for the LHCb tracker. Polish members of the ALICE team participated in development of the PHOS (Photon Spectrometer) electromagnetic calorimeter used to reconstruct trajectories of the pi zero mesons and “single” photons. Świerk Computer Centre (CIS) has joined Worldwide LHC Computing Grid, a global network of 160 computer centres involved in analysis of LHC-produced experimental data (the CMS experiment alone is acquiring data at the 100 Mb/s rate). Only two other Polish institutions has joined the Grid: AGH and Warsaw University Interdisciplinary Centre for Mathematical and Computational Modelling. CIS offers about 1,100 processing cores, over 500 TB of storage, some dedicated scientific software, and high-performance network infrastructure (10 Gb/s redundant optical link). 

LHC history and achievements

Scientists started thinking about LHC in the early 1980s, when the previous accelerator, LEP, was not yet running. In December 1994, CERN Council voted to approve construction LHC and in October 1995, the LHC technical design report was published. Contributions from Japan, USA, India and other non-Member States accelerated the process, so that four experiments (ALICE, ATLAS, CMS and LHCb) were officially approved between 1996 and 1998 and construction work started on four designated sites. LHC produced its first beam on September 10, 2008. First beam collisions were observed on November 23, 2009. World record beam energy (1.18 TeV) was exceeded on November 30, 2009, collision energy (2.36 TeV) – on December 16, 2009. Between 2011 and 2012 LHC produced beams of 7 TeV (later 8 TeV) protons. Discovery of Higgs boson was announced on July 4, 2012 . The discovery confirmed the mechanism by way of which elementary particles acquire their masses. 2014 Nobel prize in physics went to François Englert and Peter Higgs, theoreticians who proposed the mechanisms back in 1964.

How does LHC work?

Large Hadron Collider is the most powerful particle accelerator ever built. It’s kind of a microscope that can give insight into the world of elementary particles. It produces two beams of high-energy protons or lead ions moving opposite directions around a large diameter circle (ring) and collides both beams in some designated points of the ring. LHC is fed by cascade of other accelerators, each accelerating the accelerated particles to a higher energy.

All begins from gaseous hydrogen, atoms of which are composed of just one proton and one electron. Every few hours a portion of gas is taken from a small pressurized cylinder and hydrogen atoms are stripped of their electrons (ionized). The remaining protons are injected into a linear accelerator that accelerates them to a velocity of about 30% of light velocity. The next accelerator (PS Booster) increases proton energy almost 30 times. Each of the subsequent accelerators on the protons path (Proton Synchrotron PS, then Proton Super Synchrotron SPS) boost proton energy about 20 times. Finally the protons are injected into the LHC tunnel. Each day LHC consumes merely a few nanograms (10-9 g) of hydrogen. In other words 1 gram of gaseous hydrogen would suffice for about one million years of operation.

LHC forms the injected particles into two beams moving opposite directions. They travel along two parallel circular tubes in a tunnel of 27 km circumference about 100 m below the ground (the ring). Diameter of each of the tubes is a few centimetres. Ultra-high vacuum must be kept at all times inside both tubes or else the particles travelling inside would quickly scatter on gas molecules. Target velocity of protons accelerated in LHC is 0.999999991 of light velocity. At that velocity protons orbit the ring more than 11,000 times every second. Very high magnetic fields are necessary to bend trajectories of high-energy particles along the ring. The field is generated by more than 1,200 mighty dipole electromagnets, coils of which conduct electric currents on the order of several thousand Amperes (comparable to a small lightning). Such currents are impractical unless the magnets are in superconducting state, i.e. their coils exhibit no electrical resistance. To be in superconducting state, magnets must be cooled down to a very low temperature of just 1.9 Kelvin (degrees above absolute zero). It means that interior of LHC is colder than extra-terrestrial open space. Beside dipole magnets that route the particles along the tubes, LHC is also equipped with a number of beam focussing/geometry correcting magnets that prevent beam divergence and focus the beams in designated collision points.

Protons orbit the ring grouped in bunches (packets). There are about 100 billion particles in each bunch. More than 5,600 bunches mutually separated by about 7 metres may orbit simultaneously the ring. Energy of all orbiting bunches corresponds to explosion of 80 kg of TNT, or a train of 800 tonnes mass moving with velocity of 150 km/h. It is a quite challenging task to control so big energies inside so complicated facility.

Particles accelerated to their final energy may orbit the ring for many hours. However, their number is steadily decreasing because periodically beams are intentionally collided within ring sections encircled by detectors, as well as because some particles in the beams inevitable scatter on residue gas molecules within the entire accelerator tubes. After a few hours beams are diverted outside the ring and directed on some graphite blocks where they stop.

Kinetic energy of colliding primary particles (protons or lead nuclei) is converted into new, mostly unstable, particles. They are identified, their parameters (electric charge, velocity, mass, energy) are measured, and their tracks are reconstructed on the basis of various data logged by detectors. Really heavy particles live less than 1 picosecond (10-12 s), therefore they cannot be observed by any detection setup. They can be studied only indirectly by analysis of parameters of products of their decays.

Opposite beams are collided only in sections of the ring encircled by detectors developed by various teams of experimentalists (in physicists’ jargon: experiments or collaborations). The four main LHC detectors include ATLAS, CMS, ALICE and LHCb. 46 m long, 25 m wide, and 25 m tall ATLAS is the largest LHC detector. It weighs 7,000 tonnes. Its 8 superconducting magnetic coils, each 25 m long, form a cylinder around the accelerator tube routed through its centre. CMS is a bit smaller but almost twice as heavy. Total proton-proton collision rate in LHC may reach billions per second, much more that detector data logging capabilities. Besides, majority of the collisions produce only uninteresting events (from the researchers’ point of view). Therefore special electronic circuitry – called trigger – is needed in every experiment to select and log only interesting events. Triggers must be very fast to make decisions on-the-fly.


Tunel LHC (fot. CERN)