Problems
431
At about the same time, the CDF and D0 collaborations at the Tevatron
reported the combined results of their searches for a SM Higgs boson produced
in association with a W or a Z boson, and subsequently decaying to a b pair.
b ¯
The data corresponded to an integrated luminosity of 9.7 fb
−1 . An excess of
events was observed in the mass range 120–135 GeV, at a significance of 3σ,
which was interpreted as evidence for a new particle, consistent with the SM
Higgs boson (Aaltonen et al. 2012). This provided the first strong indication
for the decay of the new particle to a fermion–antifermion pair at a rate
consistent with the SM prediction.
Is the particle discovered by the ATLAS and CMS collaborations the Higgs
boson of the Standard Model? The decay to two photons implies that its
spin cannot be unity (Landau 1948, Yang 1950), but spin-0 has not yet been
established. Even so, this already implies that the particle is different from all
the other SM particles. The decay modes γγ, ZZ
∗ , WW
∗ have been observed
by ATLAS and CMS, and b ¯
b by CDF/D0. The τ
+ τ
− mode has not yet been
seen. A measure of the compatibility of the observed boson with the SM
Higgs boson is provided by the best-fit value of the common signal strength
parameter μ defined by
μ = σ · BR/(σ · BR) SM
(22.213)
where σ is the boson production cross section and BR is the branching ratio
of the boson to the observed final state. ‘SM’ denotes the SM prediction, so
that the value μ = 1 is the SM hypothesis. ATLAS reported a best-fit μ-value
of μ = 1.4 ± 0.3 for m H = 126 GeV; the μ-values for the individual channels
were all within one standard deviation (s.d.) of unity. CMS reported a best-fit
value of μ = 0.87 ± 0.23 at m H = 125.5 GeV, and again the individual values
in the observed channels were within 1 s.d. of unity. The conclusion is that
these results are consistent, within uncertainties, with the predictions for the
SM Higgs boson.
We end this book with a discovery which opens a new era in particle
physics, in which the electroweak symmetry-breaking (Higgs) sector will be
rigorously tested. The aim will be to measure the couplings of the new boson
to the other SM particles with increasing accuracy, so as to reveal possible
deviations from the SM values. The level of precision required to provide
clear pointers to physics beyond the SM may be very high (see for example
Gupta et al. 2012). The LHC will continue running until early 2013, when
it will be shut down for machine improvements needed to allow operation at
√
s = 14 TeV and higher luminosity; beyond that, the High Luminosity LHC
is planned to begin data-taking in 2022. However, just as the discovery of the
W and Z bosons at the CERN p¯ p collider was followed by precision studies
+
at the e e
− colliders LEP and SLC, a lepton collider is likely to be needed on
the next stage of this fundamental exploration.
431
At about the same time, the CDF and D0 collaborations at the Tevatron
reported the combined results of their searches for a SM Higgs boson produced
in association with a W or a Z boson, and subsequently decaying to a b pair.
b ¯
The data corresponded to an integrated luminosity of 9.7 fb
−1 . An excess of
events was observed in the mass range 120–135 GeV, at a significance of 3σ,
which was interpreted as evidence for a new particle, consistent with the SM
Higgs boson (Aaltonen et al. 2012). This provided the first strong indication
for the decay of the new particle to a fermion–antifermion pair at a rate
consistent with the SM prediction.
Is the particle discovered by the ATLAS and CMS collaborations the Higgs
boson of the Standard Model? The decay to two photons implies that its
spin cannot be unity (Landau 1948, Yang 1950), but spin-0 has not yet been
established. Even so, this already implies that the particle is different from all
the other SM particles. The decay modes γγ, ZZ
∗ , WW
∗ have been observed
by ATLAS and CMS, and b ¯
b by CDF/D0. The τ
+ τ
− mode has not yet been
seen. A measure of the compatibility of the observed boson with the SM
Higgs boson is provided by the best-fit value of the common signal strength
parameter μ defined by
μ = σ · BR/(σ · BR) SM
(22.213)
where σ is the boson production cross section and BR is the branching ratio
of the boson to the observed final state. ‘SM’ denotes the SM prediction, so
that the value μ = 1 is the SM hypothesis. ATLAS reported a best-fit μ-value
of μ = 1.4 ± 0.3 for m H = 126 GeV; the μ-values for the individual channels
were all within one standard deviation (s.d.) of unity. CMS reported a best-fit
value of μ = 0.87 ± 0.23 at m H = 125.5 GeV, and again the individual values
in the observed channels were within 1 s.d. of unity. The conclusion is that
these results are consistent, within uncertainties, with the predictions for the
SM Higgs boson.
We end this book with a discovery which opens a new era in particle
physics, in which the electroweak symmetry-breaking (Higgs) sector will be
rigorously tested. The aim will be to measure the couplings of the new boson
to the other SM particles with increasing accuracy, so as to reveal possible
deviations from the SM values. The level of precision required to provide
clear pointers to physics beyond the SM may be very high (see for example
Gupta et al. 2012). The LHC will continue running until early 2013, when
it will be shut down for machine improvements needed to allow operation at
√
s = 14 TeV and higher luminosity; beyond that, the High Luminosity LHC
is planned to begin data-taking in 2022. However, just as the discovery of the
W and Z bosons at the CERN p¯ p collider was followed by precision studies
+
at the e e
− colliders LEP and SLC, a lepton collider is likely to be needed on
the next stage of this fundamental exploration.