Physics at 13 TeV – Cranking Up the LHC

A composite picture showing inside the underground tunnel at CERN, and a small portion of the giant particle accelerator in artificial colours, with the words 13 TeV superimposed on it. Image: NaturPhilosophieA Vernesque Feat of Human Engineering

Deep down, in huge subterranean caverns… Underneath the Franco-Swiss border… 300 feet underground… lies a beast of unprecedented power… and mystery.  The Large Hadron Collider (LHC) that man summons to  explore the uncharted corners of the sub-atomic realm…  After two years of a deep slumber, the mighty beast has awoken…

The Large Hadron Collider is the World’s largest and most powerful particle collider – the largest and most complex experimental facility ever built by the European Organization for Nuclear Research (CERN) between 1998 and 2008, in collaboration with over 10,000 scientists and engineers from over 100 countries, as well as hundreds of universities and laboratories around the World.  It is the largest single machine ever constructed.

A sort of 21st century Vernesque feat of human engineering…

 

Large Hadron Collider (LHC)

A composite aerial photograph showing the site of CERN across the Franco-Swiss border, and an outline of the super collider superimposed on it, with the location of the four detectors: ALICE, ATLAS, LHCb and CMS.
Aerial photograph of CERN’s LHC Site  Source: CERN

In a tunnel 27 kilometres (17 miles) in circumference, at depths of 175 metres (574 ft), the LHC lies beneath the Franco-Swiss border, near Geneva in Switzerland.  The collider is contained in a 3.8-metre (12 ft) wide concrete-lined circular tunnel, constructed between 1983 and 1988, formerly used to house the Large Electron-Positron Collider.  CERN’s LHC crosses the border between Switzerland and France at four points, with most of it in France.

Surface buildings hold ancillary equipment: compressors, ventilation equipment, control electronics and refrigeration plants.

A diagram detailing the site at CERN, and all the numbered locations and sections of the particle collider, with the four main experiments: ALICE, ATLAS, CMS and LHCb.
A diagram of the underground installation. Source: CERN

The collider tunnel contains two adjacent parallel beamlines (or beam pipes) that intersect at four different points.  Each line contains a beam, which travels in opposite directions to its counterpart around the ring.

Some 1,232 dipole magnets keep the beams on their circular path.  An additional 392 quadrupole magnets are used to keep the beams focused, to maximise the chances for interaction between particles at the four intersection points, where the beams cross.

 

Crossing Beams

Seven detectors were constructed at the Large Hadron Collider, located underground in large caverns excavated at the LHC’s intersection points.  ATLAS and the Compact Muon Solenoid (CMS) experiments are large, general purpose particle detectors.  ALICE and LHCb have more specific roles.  The last three are TOTEM, MoEDAL and LHCf, much smaller and designed for very specialised research.

Before injection into the main accelerator ring, the particles are prepared successively by a series of systems that increase their energy:

  • the LINear particle ACcelerator (LINAC 2) generates 50 MeV protons

  • the Proton Synchrotron Booster (PSB) accelerates protons to 1.4 GeV

  • the Proton Synchrotron (PS) accelerates the protons to 26 GeV

  • the Super Proton Synchrotron (SPS) further increases the energy to 450 GeV

A cross-section diagram of the LHC's relative beam sizes in collisions.
Relative Beam Sizes in Collisions (ATLAS)  Source: CERN

At that point, protons are injected into the main ring.  The proton bunches are accumulated over a period of 20 minutes and accelerated to their peak energy.  Finally, proton bunches keep circulating for 5 to 24 hours, while particle collisions occur at the four intersection points.

Over 1,600 superconducting magnets are installed, most of which weigh over 27 tonnes.  Around 96 tonnes of superfluid helium-4 is needed to keep the copper-clad niobium-titanium magnets at their operating temperature of 1.9 K (−271.25 °C), making the LHC the largest cryogenic facility in the World at liquid helium temperature.

 

Operational Activity

The Large Hadron Collider went live on 10th September 2008 for the first time, but the initial testing was delayed following a magnet quench incident – the sudden loss of a superconducting magnet’s superconducting ability due to warming or electric field effects – that caused extensive damage.

Its very first research run took place from 30 March 2010 to 13 February 2013.

The LHC collided two opposing particle beams of protons at an initial energy of 3.5 teraelectronvolts3.5 TeV per beam (7 TeV total), almost 4 times over the previous world record for a collider, rising up to 4 TeV per beam (8 TeV total) from 2012.

An infographic explaining CERN's LHC activity until 2013: 7 x 10^12 electronvolts beam energy; 10^34 squared centimetres per second; 2635 bunches/beam; 10^11 protons/bunch; 7 teraelectronvolts proton-proton colliding beams; Bunch crossing 4 x 10^7 Hertz; Proton/Parton collisions 10^9 Hertz; New particle production (Higgs, SUSY...) 10^-5 Hertz; Selection of 1 event in 10,000,000,000,000
Large Hadron Collider (LHC) Activity until 2013 Source: CERN

Although the LHC program is mainly based on proton-proton collisions, shorter running periods include heavy-ion collisions.  While lighter ions are considered as well, the baseline scheme deals with lead ions (ALICE – A Large Ion Collider Experiment).  The aim of the heavy-ion program is to investigate quark-gluon plasma, which existed in the early universe.

On 13th February 2013 the LHC’s initial run officially ended, and it was shut down for planned upgrades.  This first run included the discoveries of the long sought-after Higgs boson, several hadrons, the first creation of a quark-gluon plasma, and the first observations of the rare Bs-meson decays (B^0 _s \longrightarrow \mu^+ \mu^-), which already challenged the validity of existing models of supersymmetry.

 

Colliding Protons at 13 TeV

‘Test’ collisions restarted in the upgraded collider on 5 April 2015, reaching 6.5 TeV per beam on 20 May 2015.  That is 13 TeV in total for the two beams – the current world record for particle collisions.  Its second run began on schedule, on 3rd June 2015.

 

Proton-Proton Inelastic Collisions

A diagram explaining how bosons, and other sub-atomic particles, are created as a result of the collision of two proton beams - one polarized, the other unpolarized.
Production of Bosons during Proton-Proton Collisions Source: Nanotech Now

Analysing high-speed relativistic collisions requires using the relativistic equations for momentum and energy, rather than their Newtonian counterparts.

According to Einstein’s theory, the momentum p of a particle with mass m and velocity v is given by

\boldsymbol p = \frac{m \boldsymbol v}{\sqrt{1 - \frac{v^2}{c^2}}}

and the total energy Etot  

E_{tot} = \frac{m c^2}{\sqrt{1 - \frac{v^2}{c^2}}}

with c as the constant speed of light.

 

In an elastic collision, all of the quantities defined will be conserved.  That is, momentum, rest energy, kinetic energy and total energy.  But many high-energy collisions are actually inelastic collisions.  In a high-energy inelastic collision, the only quantities that are conserved for certain are the momentum and the total energy.  Generally, neither kinetic energy, nor rest energy are necessarily conserved.

A diagram illustrating the interactions of constituents of the colliding protons, the so-called partons (quarks, gluons).
A Proton-Proton Collision Diagram Source: ATLAS Physics Master Classes

In an inelastic collision, it is possible for particles to be created or destroyed, thereby increasing or decreasing the mass energy.

The conservation of total energy means that any change in mass energy must be accompanied by a compensating change in kinetic energy.

Particles may be created at the expense of kinetic energy.

 

The boost to collisions at 13 TeV has come after two years of repairs and upgrades, including the re-soldering of thousands of connections between the LHC’s superconducting magnets after flaws were discovered.

The changes and modifications had one aim – to run the particle collider at higher energies.  The massive superconducting magnets require considerable magnet training to handle the high currents involved without losing their superconducting ability.  The process involves repeatedly running the magnets with lower currents to provoke any quenches or minute movements that may result.

In both experimental runs, the LHC has been initially run at energies below its planned operating energy, and ramped up:

  • initial energy of 2 x 3.5 TeV energy on its first run, final at 2 x 4TeV;
  • initial energy of 2 x 6.5 TeV on its second run, with final intended energy 2 x 7 TeV.

Now, the proton beams used by the LHC carry as much energy as a speeding train!  Each beam contains billions of subatomic particles.  But, only a fraction of those will collide at the crossing points.

A humoristic cartoon showing two alien scientists busy experimenting in a lab. The caption reads: 13.8 billion years ago, a few seconds before the creation of our Universe... One of the scientists says to the other: "All set. Let's fire up this Large Hadron Particle Collider and see what happens!"

The energy of two protons colliding in the LHC has been compared to that of a dozen mosquitoes in flight, but with that energy packed into a minuscule space, billions of times smaller than a single mosquito!

The collider is thus capable of recreating energy densities close to those that existed just after the Big Bang – allowing physicists to probe the very fabric of the Cosmos…

 

The Near-Future…

When running the LHC at its full design energy of 7 TeV per beam, once or twice a day, as the protons are accelerated from 450 GeV to 7 TeV, the field of the superconducting dipole magnets will be increased from 0.54 to 8.3 teslas (T).

A humoristic cartoon about the Large Hadron particle Collider. Two scientists are talking to each other in one of CERN's underground tunnels. The first one, an engineer, says: "Smashing particles for two weeks now, Sir, and the planet hasn't exploded yet." The other one, a physicist, replies: "I trust the poets, son. The World will end not with a Bang but a whimper...". The second slide shows our Solar system, where the Earth suddenly goes "POOF!" and disappears.
Alarmists…

Each proton will then reach an energy of 7 TeV, giving a total collision energy of 14 TeV.  At this energy, the protons move at about 0.999999991 c, that is only about 3 metres per second slower than the speed of light (c).  It will take less than 90 microseconds for a proton to travel once around the main ring of the Large Hadron Collider, giving a proton speed of about 11,000 revolutions per second.

Rather than continuous beams, the protons will be bunched together, into up to 2,808 bunches, with 115 billion protons in each bunch.  Interactions between the two beams will thus take place at discrete intervals, mainly 25 nanoseconds apart, providing a bunch collision rate of around 40 MHz.

 

 

“Okay, but…  What’s the Point?!”

One of the first supersymmetric particles to be detected might be one called the gluino – a novel addition to the cosmic zoo of building-block particles.

The gluino is the super-partner of the gluon, which is thought to “glue” quarks together inside protons and neutrons.  This might be the particle responsible for dark matter, which makes up some 27% of the Universe.

A humoristic cartoon about the potential backlash of running the LHC. The caption reads: "Inevitable Protests by Fundamentalists". Three people are taking part in a demonstration and holding up placards with the following slogans. The first one reads "I'm not made of string". The second reads: "Jesus is the higher dimension". Finally, the third placard reads: "Thou shalt not comingle the boson and the fermion for it is an abomination in My Eyes - Deuteronomy 59:12".
Fundamentalists…

Since dark matter is expected to be “invisible” at sub-atomic levels, as well as on astronomical scales.  Therefore, physicists will have to look for indirect evidence of its production.

A key sign that dark matter may have been generated is an apparent imbalance in momentum before and after a particle collision known as “missing transverse energy“.

If such a signature is detected at the LHC, which cannot be explained by the Standard Model of Particle Physics, it would be an indication that normal matter turned into dark matter.  If that is indeed the case, the LHC would be acting as a dark matter factory.

A rather cool idea?

 

Why All That Matters…

When the LHC collaboration discovered the famous Higgs boson, and confirmed its position in the Standard Model of physics, it was an extraordinary achievement in its own right – it proved the existence of an invisible process that performs the fundamentally important role of giving all other particles their mass or substance.

“Big deal!” you may say…  “Does it change anything tangible to our everyday lives?”

Well, no…  But that is not how Science works.  This is merely one more step on a fantastic journey towards understanding how our Universe works.  And there is much more to come.

A screenshot image from the BBC News Channel, with the habitual typographical error. The onscreen caption reads: "Hardon Collider Restart".
BBC…

Future collisions of protons may reveal something about the majority of existing matter has yet to be uncovered – the stuff known as dark matter.  Perhaps they will uncover conclusive evidence for the weird notion that there are extra dimensions, or legions of previously unseen particles that form pairs with the ones we have come to know about.

Any of this would open our eyes to a new way of perceiving the fabric of everything we see and touch.

How is it made?

What holds it together?

Unresolved Problems of High-Energy Particle Physics

 

  • Higgs mechanism

Are the branching ratios of the Higgs boson consistent with the Standard Model?  Is there only one type of Higgs boson?

  • Hierarchy problem 

Why is gravity such a weak force?  Is the solution supersymmetry, extra dimensions, or anthropic fine-tuning?

  • Magnetic monopoles 

Did particles that carry “magnetic charge” exist in some past, higher-energy epoch?

  • Proton decay and spin crisis

Is the proton fundamentally stable?  Or does it decay with a finite lifetime, as predicted by some extensions to the Standard Model?  How do the quarks and gluons carry the spin of protons?

  • Supersymmetry

Is spacetime supersymmetry realized at TeV scale?  Does the lightest supersymmetric particle (LSP) comprise dark matter?

  • Generations of matter

Why are there three generations of quarks and leptons?

  • Neutrino mass

What is the mass of neutrinos?  Is mass hierarchy normal or inverted?

  • Colour confinement

Why has there never been measured a free quark or gluon, but only objects that are built out of them, like mesons and baryons?

  • Strong CP problem and axions

Why is the strong nuclear interaction invariant to parity and charge conjugation?  Is the Peccei-Quinn theory the solution to the problem?

  • Anomalous magnetic dipole moment

Why is the experimentally measured value of the muon’s anomalous magnetic dipole moment significantly different from the theoretically predicted value of that physical constant?

  • Proton size puzzle

What is the electric charge radius of the proton?  How does it differ from gluonic charge?

 

Initially, the focus was on investigating the possible existence of the Higgs boson, a key part of the Standard Model of Particle Physics predicted by theory, but previously neither observed nor proven to exist, due to its elusive nature and exceedingly brief lifespan.

The LHC also allowed for the search for supersymmetric particles and other hypothetical particles as possible unknown areas of physics.  Extensions to the Standard Model predict additional particles, such as the heavy W’ and Z’ gauge bosons, also believed to be within reach of the LHC to discover.

Astounding though these discoveries may be, they would not by themselves alter anything tangible about how we get up the next day, to face our work and daily lives.  But that’s how Science goes.

A humoristical cartoon by Chappatte. The scene takes place inside the tunnels of CERN, where two French-speaking physicists are giving a press conference. One of them declares: "On a découvert de quoi est fait le RIEN! (We discovered what nothing is made of!)". The other one concurs: "... C'est quand même QUELQUE CHOSE! (It's actually quite something!)".
Skeptics… Everytime fundamental research hits the media headlines you can be sure someone, somewhere, will question its worth.

Each new insight can potentially unlock another door in our knowledge and leave it up to future generations of researchers to choose whether to venture through it or not, and sometimes only decades later develop practical applications.  We live in an age of electronics that did not come about as a single discovery overnight.  It can be traced back to the fundamental research undertaken by such giants as James Clerk Maxwell in the 19th century.

In CERN’s case, you could argue that you would have never read this webpage if it had not been for the advances of a few scientists keen on sharing their data, which ultimately brought us the World Wide Web…