WARPLAB

What am I looking at?

What you are looking at

Each point on the histogram counts real collisions that CMS recorded at CERN in 2011 and 2012 and that left four electrons or muons. Their combined mass tells you what made them: a single Z near 91 GeV, pairs of Z bosons above 180 GeV, and the Higgs boson near 125 GeV. Move the cuts and watch which bump survives, then ask the fit below how sure you can be that the Higgs bump is real.

Controls

On this device:

Mouse

  • Drag the three cut sliders: Z₁ mass above (40 to 90 GeV), Z₂ mass above (12 to 60 GeV) and every lepton's pT above (5 to 30 GeV).
  • Click 4μ, 4e or 2e2μ to drop that channel or bring it back.
  • Click a Try button to set all the cuts at once; The analysis cuts puts them back.
  • Replay adds the 278 collisions again over 6 seconds, in the order CMS recorded them. The still version shows all of them at once.
  • Under Is the bump real?, click 1. Background, 2. Local significance or 3. Look elsewhere to read each step.
  • Click the badge beside Events kept for the data's source, licence and date.

Keyboard

  • Tab to a cut slider; the arrow keys move it 1 GeV, Home and End jump to its ends.
  • Tab to a channel, a Try button or an explainer step; Enter or Space presses it.
  • Replay and the source badge take Enter or Space too.
  • With a screen reader, the histogram reads out how many collisions pass and where the peak sits.
  • H or ? opens this panel. Esc closes it.
Other devices

Touch

  • Slide a cut with your finger: Z₁ mass, Z₂ mass or every lepton's pT.
  • Tap 4μ, 4e or 2e2μ to drop that channel or bring it back.
  • Tap a Try button to set all the cuts at once; The analysis cuts puts them back.
  • Tap Replay to watch the histogram fill again, collision by collision.
  • Tap 1, 2 or 3 under Is the bump real? to read the background fit, the local significance and the look-elsewhere step.
  • Tap the badge beside Events kept for the data's source.

Glossary

Collision (event)
Two protons hitting head on inside the LHC, and everything the CMS detector saw come out. The module calls each recorded collision an event.
Why it matters Only a tiny share of collisions makes a Higgs boson, so you need many of them before a bump shows.
CMS Open Data, records 5200 and 5500
Record 5200 is the 278 four-lepton collisions from 2011 and 2012 that CMS released for teaching, free to use (CC0). Record 5500 is the analysis example whose cuts selected them.
Why it matters Every point on the histogram is a real collision you can download yourself.
Lepton
A light particle that the detector measures very cleanly. Here it means an electron (e) or a muon (μ), a heavier cousin of the electron.
Why it matters Four clean leptons are one of the clearest traces a Higgs boson can leave.
Channels: 4μ, 4e, 2e2μ
Which leptons a collision left: four muons, four electrons, or two of each. With the analysis cuts there are 113 in 4μ, 48 in 4e and 117 in 2e2μ.
Why it matters Electrons and muons are measured differently, so each channel is a separate check.
Four-lepton mass m₄ℓ
Add up the four leptons' energies and momenta and you get one number, their invariant mass. It is the mass of whatever made them, whichever way they flew.
Why it matters Collisions that made the same particle pile up at the same mass, so a particle shows as a bump.
GeV
Giga-electronvolt, the unit for energy and mass in particle physics. A proton weighs about 0.94 GeV; the Higgs about 125.
Z boson
A heavy particle that carries the weak force, with a mass of 91.2 GeV. It lives too briefly to see, and often decays to two leptons of the same kind and opposite charge.
Why it matters A Higgs boson decays to two Z bosons here, and each Z to a lepton pair.
Z₁
Of the two lepton pairs in a collision, the one whose mass is nearest the Z's, 91.2 GeV. The Z₁ mass above slider throws out collisions whose Z₁ is lighter than you set.
Why it matters Raise it above 75 GeV and the bump at 91 GeV goes, while most Higgs collisions stay.
Z₂
The other lepton pair. In a Higgs decay it is usually far lighter than a real Z, because a 125 GeV Higgs leaves it at most about 34 GeV.
Why it matters That is why asking Z₂ for 40 GeV removes the Higgs bump.
pT (transverse momentum)
How much momentum a lepton carries across the beam, in GeV. The analysis keeps muons above 5 GeV and electrons above 7; the slider sets one floor for every lepton.
Why it matters Leptons with little pT are more often fakes or come from other processes.
Cuts
Rules a collision must pass to count, such as Z₁ heavier than 40 GeV. The analysis cuts of record 5500 already kept these 278; your sliders can only make them tighter, never looser.
Why it matters Good cuts remove collisions that only look like a Higgs and keep most real ones.
Try buttons
Four ready-made cut settings. Remove the Z peak sets Z₁ above 75 GeV, Ask Z₂ for 40 GeV sets Z₂ above 40, Hard leptons only sets pT above 20 GeV, and The analysis cuts goes back to the start.
Bin (Events / 3 GeV)
The mass axis from 70 to 181 GeV is cut into slices 3 GeV wide, as in the CMS 2012 figure. Each point is the number of kept collisions whose mass falls in that slice. The faint outline is all 278 with the analysis cuts.
Error bars (68%)
The range of true average counts that could have given the count seen, 68 times in 100. They are exact Poisson intervals: 10 events give 6.89 to 14.27.
Why it matters With so few events per bin, a single bin can jump by several just by chance.
Collisions shown and events kept
Collisions shown counts the collisions on the plot so far, up to 278. Events kept is how many of them pass your cuts.
121–130 GeV, below 110, above 181
Counts of kept events in three mass ranges. 121–130 GeV is the window round the Higgs, where the analysis cuts keep 12; below 110 GeV holds the single-Z bump; above 181 GeV, off the plot, sit pairs of real Z bosons.
Peak, 110–150 GeV
Where the bump sits: the highest bin between 110 and 150 GeV and its two neighbours, averaged by their counts. With the analysis cuts it lands at about 125 GeV; none means no event is left in that range.
Higgs boson, 125.3 GeV
The particle that gives the W and Z bosons their mass, found by CMS and ATLAS in 2012. The dashed H line marks the mass CMS published then, 125.3 GeV.
Why it matters If your peak lands on this line, the data you are cutting are finding the same particle.
Background fit
The dashed curve: a smooth falling shape, e^(a + bm), fitted to the kept masses from 100 to 170 GeV in 1 GeV bins. It stands for everything that is not a Higgs. With fewer than 5 events in that range there is no fit.
Why it matters A bump only means something against what you would see without it.
Bump (background + bump fit)
The solid curve adds to the background a bump 2 GeV wide, about the detector's resolution, whose size and mass the fit chooses. Bump mass is where it lands; Bump, fitted is how many events it holds.
Background, 121–130 GeV
How many events the background curve alone expects in the Higgs window. With the analysis cuts it is about 5.6, where 12 were seen.
Why it matters The gap between expected and seen is what the significance measures.
Local significance Z
How much better the bump explains the data than the background alone, in standard deviations (σ), from the ratio of the two fits' likelihoods (Wilks' formula). With the analysis cuts it is 2.8σ at 125 GeV, between record 5500's about 2σ and CMS 2012's 3.2σ in this channel.
Why it matters It says how often the background alone would fake a bump this big at this one mass.
σ (standard deviations)
A scale for how unlikely a result is by chance. 1σ happens about 1 time in 6, 3σ about 1 in 740, and 5σ about 1 in 3.5 million.
Why it matters Physicists claim a discovery only at 5σ.
Global significance and the look-elsewhere effect
The fit tried every mass from 110 to 150 GeV, and a fluke could have turned up at any of them. The global figure pays for that search by counting how often the scan climbs past 1σ (Gross and Vitells): 2.8σ local becomes 1.5σ global with the analysis cuts.
Why it matters The more places you look, the easier it is to find a bump by chance.
Cut settings tried
Each new mix of the three sliders and the channels you try counts once. The explainer gives the best local significance among them on all 278 collisions, against the analysis cuts'.
Why it matters Tuning cuts until the bump grows is looking elsewhere too, which is why an analysis fixes its cuts before it looks.

All help and questions

Find the Higgs

278 real CMS collisions with four leptons. Move the cuts and watch a bump form near 125 GeV: the Higgs.

05101520258090100110120130140150160170180Four-lepton mass m₄ℓ (GeV)Events / 3 GeVZH
  • Collisions your cuts keep, with 68% error bars
  • All 278 with the analysis cuts
  • Background fit
  • Background + bump fit
  • CMS 2012 Higgs mass: 125.3 GeV

Collisions shown0 of 278

Cuts

Z₁ is the lepton pair nearest the Z mass, 91.2 GeV.

Z₂ is the other pair.

pT is momentum across the beam. The analysis keeps muons above 5 GeV and electrons above 7.

Events kept
0 of 0
121–130 GeV
0
Below 110 GeV
0
Peak, 110–150 GeV
none
Above 181 GeV
0

Is the bump real?

Local significance
too few
Bump mass
too few
Bump, fitted
too few
Background, 121–130 GeV
too few
Global, 110–150 GeV
too few

Fewer than 5 events are left between 100 and 170 GeV: too few to fit.

The record has no simulated background, so the dashed curve is a smooth one, e^(a + bm), fitted to the kept masses from 100 to 170 GeV. The fit is binned: 70 bins of 1 GeV, each a Poisson count. It runs twice: with the curve alone, and with a 2 GeV wide bump on top whose size and mass are free (the solid curve). Move a cut and both fits run again.

What you are looking at

Each point counts real collisions CMS recorded in 2011 and 2012 that left four electrons or muons. Add up the four leptons' energies and momenta and you get their invariant mass: the mass of whatever made them. A Higgs boson decays to two Z bosons and each Z to a lepton pair, so Higgs collisions pile up at one mass.

Reading the bumps

The bump at 91 GeV is a single Z whose leptons gave off a second pair: raise Z₁ above 75 GeV and it goes, while most Higgs events stay. Above 180 GeV sit pairs of real Z bosons. A 125 GeV Higgs leaves at most about 34 GeV for the second Z, so asking Z₂ for 40 GeV removes it. The CMS selection already kept these 278 events; your cuts only tighten it.

Data and cuts

Events: CMS Collaboration, CERN Open Data Portal record 5200, DOI 10.7483/OPENDATA.CMS.YGG7.333M, released under CC0. Cuts: record 5500, the Higgs-to-four-lepton analysis example. CMS selected this subset for education: it is not suitable for a full physics analysis, and it has no simulated background, so the background here is fitted to the data. Look-elsewhere: E. Gross and O. Vitells, Eur. Phys. J. C 70 (2010) 525.

Record 5200Record 5500arXiv:1207.7235