ATLAS and the Higgs Boson

Weighed to 0.09%

In 2023 the ATLAS Collaboration, using data from two runs, put the mass of the Higgs boson at 125.11 ± 0.11 GeV, a precision of 0.09%. The GeV is the usual unit of mass in particle physics; a proton is about 0.94 GeV.

Nobody has ever seen the particle itself. The Standard Model predicts it lives about 1.6 × 10⁻²² s, less time than light needs to cross 50 femtometres. A detector records what it decays into, two photons, say, or four leptons, and works its mass back out from their energies and directions.

125.11 ± 0.11 GeV (ATLAS, 2023) · lifetime about 1.6 × 10⁻²² s (Standard Model)

A 27-Kilometre Ring

The Large Hadron Collider (LHC) lies underground near Geneva, 26,659 metres round. Two beams of protons travel in opposite directions in two pipes side by side, going round 11,245 times a second.

They are bent by 1,232 superconducting dipole magnets, each 15 m long, designed for 8.33 tesla. The magnets sit in superfluid helium at 1.9 K, colder than the 2.7 K of space. Since 5 July 2022 each beam has carried 6.8 TeV, so the protons collide at 13.6 TeV. Superconducting coils are also on the plate “Tokamak”.

Circumference 26,659 m · 1,232 dipoles · 1.9 K · 6.8 TeV per beam

Energy Into Mass

A proton is made of quarks and gluons. When two protons collide, two of these actually meet, carrying part of the protons’ energy. That energy can become particles that were not there before; by E = mc², the more energy, the heavier the particles it can make.

Most heavy particles decay at once. What reaches the detector is electrons, photons, muons, sprays of hadrons, and neutrinos. Add up the energies and momenta of the decay products and work out their “invariant mass”: however fast the parent was flying, it comes out as the parent’s mass.

m²c⁴ = (ΣE)² − |Σp c|²: the invariant mass of the products is the mass of the parent

Forty Million a Second

The protons fly in bunches 25 ns apart, about 7.5 m. At the centre of ATLAS the bunches cross 40 million times a second; in the third run, from 2022, each crossing brings on average up to about 60 proton collisions.

All of it cannot be kept. The first-level trigger is hardware: it decides within 2.5 µs and keeps at most 100,000 crossings a second. The second level is a computer farm; in the third run it writes about 3,000 a second on average, in 2012 about 400. The rest are lost. In CERN’s words, a Higgs boson is made about once in a billion collisions.

40 million a second → 100,000 a second (within 2.5 µs) → about 3,000 a second written (Run 3)

Cut Open: 7,000 Tonnes

ATLAS is a cylinder lying on its side, 46 m long and 25 m across by ATLAS’s public figures, and it weighs about 7,000 tonnes. It stands in a cavern about 100 m underground. This model is about 1 : 40; the figure on the floor is 1.75 m tall.

From the centre outwards, layer within layer: round the beam pipe, the inner detector inside a superconducting solenoid; then the liquid-argon electromagnetic calorimeter and the hadronic calorimeter of steel and scintillating tiles; outermost, eight toroid coils with the muon chambers hung between them. About 3,000 km of cable carry about 100 million readout channels out.

46 × 25 m · 7,000 t · model about 1 : 40 · feet, struts and chamber layout schematic

Tracks in Two Tesla

The inner detector fills a cylinder 1.15 m in radius and 7 m long. Its innermost layer is silicon pixels, only about 3.3 cm from the beam, each 50 × 250 µm; then silicon strips; then about 300,000 straw tubes 4 mm across. A charged particle leaves one point in each layer.

The solenoid makes a 2-tesla field, which bends a charged particle’s path into an arc. The radius is the transverse momentum divided by 0.3 times the field: a 1 GeV particle curves with a radius of about 1.7 m; one of 50 GeV is nearly straight. How much it bends gives its momentum.

Radius R = p_T / (0.3 B): at B = 2 T, 1 GeV gives about 1.7 m

Stopping Particles

An electron or a photon that hits lead turns into a growing spray of weaker electrons and photons, an electromagnetic shower. ATLAS’s electromagnetic calorimeter is lead folded like an accordion, in liquid argon at about −184 °C, at least 22 radiation lengths deep; the shower stops in it. The charge collected is proportional to the particle’s energy.

Hadrons such as protons, neutrons and pions get further. The tile calorimeter outside uses steel plates as the absorber with hundreds of thousands of plastic scintillating tiles between them, 64 wedges to a barrel. Hadrons shower in it, the tiles glow, and about 9,500 photomultipliers turn the light into signals. At about 2,900 tonnes it is the heaviest part of ATLAS.

Electromagnetic: lead + liquid argon, accordion · hadronic: steel + tiles, 64 wedges

Magnets Full of Air

Hardly anything stops a muon; it passes through the calorimeters. The muon spectrometer outside measures it. The eight barrel toroid coils are each 25.3 m long and together 20.1 m across; at 20.5 kA they store 1.08 GJ. At each end sits an end-cap toroid.

Between the coils there is air, not iron, so a passing muon is scattered very little. The field runs round the beam, so it bends muons along the beam’s direction. Three layers of chambers, at radii of about 5, 7.5 and 10 m, measure the bend with drift tubes 3 cm across; the wheels at the ends are about 25 m in diameter.

Barrel toroid: 8 coils · 25.3 m · 20.1 m across · about 0.5 T where the muons fly

Where Each One Stops

Each kind of particle leaves its own trace. An electron leaves a track and stops in the electromagnetic calorimeter; a photon, with no charge, leaves no track and stops there too; quarks and gluons turn into a spray of hadrons called a jet, a few tracks with most of the energy in the tile calorimeter; a muon leaves a track, passes all the calorimeters and leaves a few more points in the muon chambers.

A neutrino leaves nothing. Across the beam the total momentum before the collision is zero, so the momenta seen after it should also add up to zero. What is missing, the missing transverse momentum, was carried off by what cannot be seen.

Electron · photon · muon · jet · neutrino, and the layer each one stops in

4 July 2012

On this day ATLAS and CMS each reported at a seminar at CERN a new particle near 125 GeV. ATLAS, with 4.8 fb⁻¹ of 7 TeV data from 2011 and 5.8 fb⁻¹ at 8 TeV from 2012, found a mass of 126.0 GeV and a local significance of 5.9 σ; CMS found 125.3 GeV and 5.0 σ.

The signal came mainly from two decays. Two photons: between 100 and 160 GeV ATLAS selected 59,039 photon pairs, of which about 190 were expected from a Higgs boson, and they piled up into a small bump near 126.5 GeV. Four leptons: 13 events between 120 and 130 GeV, where background alone predicted about 4.9. A local 5.9 σ means that background alone would throw up so large a bump with a probability of about 1.7 × 10⁻⁹.

ATLAS 5.9 σ (γγ 4.5 σ · 4ℓ 3.6 σ · WW 2.8 σ) · CMS 5.0 σ

The 2013 Nobel Prize

The 2013 Nobel Prize in Physics went to François Englert and Peter W. Higgs “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider”. In 1964 Englert, with the late Robert Brout, and Higgs proposed the theory independently.

The prize was for the mechanism; the experiments confirmed the particle. The Higgs field gives mass to the W and Z bosons and to fundamental particles such as electrons and quarks, but it is not where all mass comes from. The three quarks in a proton add up to only about 1% of its mass; almost all the rest is the energy of the strong interaction binding quarks and gluons together. Nor did the theory predict how heavy the Higgs boson itself is.

1964 theory · 2008 LHC start · 2010 7 TeV collisions · 2012 discovery · 2013 prize

Taken Apart

The beam pipe, the inner detector, the solenoid, the liquid-argon calorimeter, the tile calorimeter, the eight barrel coils, the muon chambers, the end-cap calorimeters, the end-cap toroids, the muon wheels and the shielding, one by one.

Point at a part and it lifts out alone, the rest fading; at rest, every part is named.