The Higgs boson was discovered primarily in two channels: H -> gamma gamma and H -> ZZ* -> 4 leptons. Neither of these is the dominant Higgs decay mode (which is H -> bb at 58%). Why were these rare channels the discovery modes?
ABecause H -> bb has too low a cross section
BBecause H -> gamma gamma (branching ratio ~0.2%) has excellent mass resolution from the two photon energies, and H -> ZZ* -> 4l (branching ratio ~0.01%) has very low background and excellent mass resolution from the four lepton momenta — despite their low rates, the signal-to-background ratio is far superior to H -> bb, which is overwhelmed by the enormous QCD bb-bar production background at the LHC
CBecause photons and leptons are easier to detect than b quarks
DBecause only these channels were predicted by the Standard Model
At the LHC, the QCD production of b-quark pairs has a cross section about 10^7 times larger than Higgs production, making H -> bb nearly invisible in the inclusive channel. The diphoton channel benefits from the excellent electromagnetic calorimeter resolution of ATLAS and CMS (mass resolution ~1-2 GeV), producing a narrow peak above a smooth continuum background. The four-lepton channel has the best signal-to-background ratio of any Higgs channel (S/B ~ 2:1 near the peak) because requiring four isolated leptons with the right invariant mass is extremely selective.
Question 2 True / False
The Standard Model predicts that the Higgs coupling to a particle is proportional to its mass (for fermions: g_Hff = m_f/v; for gauge bosons: g_HVV proportional to M_V^2/v). This has been tested by measuring Higgs production and decay rates. The agreement with the mass-proportional prediction is at the 10-20% level for the measured couplings.
TTrue
FFalse
Answer: True
The LHC has measured Higgs couplings to W, Z, top, bottom, tau, and muon. The coupling-mass relationship follows the predicted pattern: the largest couplings are to the top quark (y_t ~ 1) and W/Z bosons, with progressively smaller couplings to bottom, tau, charm, and muon. Signal strength measurements (mu = observed rate / SM prediction) are consistent with 1 for all measured channels. The precision is currently 10-20% for most couplings and will improve to a few percent at the HL-LHC, which is where deviations from the SM prediction would indicate new physics in the Higgs sector.
Question 3 Short Answer
The Higgs boson has spin 0 and CP-even (scalar) quantum numbers. How were these quantum numbers determined experimentally?
Think about your answer, then reveal below.
Model answer: The spin and CP properties were determined from angular distributions of the Higgs decay products. In H -> ZZ* -> 4l, the angles between the two Z decay planes and the Z production angles are sensitive to the Higgs spin and parity. A spin-0 CP-even particle (J^P = 0+) produces a specific pattern of angular correlations that differs from spin-0 CP-odd (0-), spin-1, or spin-2 hypotheses. The observed distributions at both ATLAS and CMS strongly favor 0+ over all alternatives, with the 0- hypothesis excluded at more than 99.9% confidence level. The H -> gamma gamma channel also constrains the spin: the Landau-Yang theorem forbids a massive spin-1 particle from decaying to two photons, so the observation of H -> gamma gamma rules out spin 1.
Determining the Higgs quantum numbers was essential for confirming that the discovered particle is indeed the Standard Model Higgs boson and not an impostor with different spin or parity. Ongoing measurements search for small CP-odd admixtures, which would indicate CP violation in the Higgs sector -- a beyond-SM effect.
Question 4 Multiple Choice
The dominant Higgs production mechanism at the LHC is gluon-gluon fusion (gg -> H), even though the Higgs does not couple directly to gluons. How does this process occur?
AThrough tree-level quark exchange in the s-channel
BThrough a top quark loop — two gluons couple to a virtual top quark loop, which couples to the Higgs through the large top Yukawa coupling; the cross section is proportional to alpha_s^2 * y_t^2 and the top loop gives an effective ggH coupling
CThrough W boson fusion
DThrough direct Higgs-gluon coupling from higher-dimensional operators
The ggH process proceeds through a fermion triangle loop, dominated by the top quark (because y_t ~ 1). Despite being loop-induced, it is the dominant production mechanism (~87% of Higgs production at 13 TeV) because the gluon PDFs are very large at the LHC. The next-largest production modes are vector boson fusion (VBF, ~7%), associated production with W/Z (VH, ~4%), and associated production with top quarks (ttH, ~1%). Each production mode provides complementary information about the Higgs couplings.