Phenomenology - Daniel Maître Phenomenology Lecture 6: Higgs Daniel Maître IPPP, Durham
Phenomenology - Daniel Maître
Phenomenology
Lecture 6: Higgs
Daniel Maître
IPPP, Durham
Phenomenology - Daniel Maître
The Higgs Mechanism
● Very schematic, you have seen/will see it in SM lectures
● The SM contains spin-1 gauge bosons and spin- 1/2 fermions.
● Massless fields ensure:
– gauge invariance under SU(2)L × U(1)Y
– renormalisability
● We could introduce mass terms “by hand” but this violates gauge invariance
● We add a complex doublet under SU(2)L
Phenomenology - Daniel Maître
Higgs Mechanism● Couple it to the SM
● Add terms allowed by symmetry → potential● We get a potential with infinitely many minima.● If we expend around one of them we get
– Vev which will give the mass tothe fermions and massive gauge bosons
– One radial and 3 circular modes
– Circular modes become thelongitudinal modes of the gaugebosons
Phenomenology - Daniel Maître
Higgs Mechanism
● From the new terms in the Lagrangian we get
● There are fixed relations between the mass and couplings to the Higgs scalar (the one component of it surviving)
Phenomenology - Daniel Maître
What if there is no Higgs boson?
● Consider W+W− → W+W− scattering.● In the high energy limit
● So that we have
Phenomenology - Daniel Maître
Higgs mechanism
● This violate unitarity, so we need to do something● If we add a scalar particle with coupling λ to the W● We get a contribution
● Cancels the bad high energy behaviour if , i.e. the Higgs coupling.
● Repeat the argument for the Z boson and the fermions.
Phenomenology - Daniel Maître
Higgs mechanism
● Even if there was no Higgs boson we are forced to introduce a scalar interaction that couples to all particles proportional to their mass.
● There must be something Higgs-like in the theory!● Now we know there is something that looks very
much like the standard model Higgs, but we want to be sure!
● Width seem to be narrow → we can factorize production and decay
Phenomenology - Daniel Maître
Higgs signal
● The decay modes of the Higgs boson depends on its mass.
● It couples most to heavy particles (top,Z,W)
● This plot is mostly useful if you are looking for another Higgs...
From the LHC Higgs cross section working group
Phenomenology - Daniel Maître
Higgs signal
● But now we know the mass of the Higgs boson
● So we know its (expected) branching ratios– B pair: 56%
– WW : 23%
– Gg: 8.5%
– τ τ : 6.25
– ZZ: 3%
– Photon pair: 0.23%
bb
tautau
mumu
cc
gg
gammagamma
WW
ZZ
Phenomenology - Daniel Maître
Higgs production channels
● Gluon Fusion ● Quark-associated
● Weak Vector Boson Fusion (VBF)
● Higgs Strahlung
19pb
0.13pb
0.69pb0.41pb 1.57pb
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Higgs production channels
● The production channels also depend on the mass
Phenomenology - Daniel Maître
Higgs channels
●
– Very good mass resolution
– Small branching ratio
– Challenges: resolution and pion rejection
●
– Good resolution on mass peak
●
– Larger cross section
– No mass peak
Phenomenology - Daniel Maître
Higgs discovery
● The Higgs boson was discovered in the di-photon and ZZ channels, with some help from the WW channel
Phenomenology - Daniel Maître
Di-photon channel
● Looking for a bump over a smooth background (use side-bands subtraction)
● Challenges are pion conversion of photons
● Diphoton mass resolution: ~ 1GeV
From arXiv:1307.1427
Phenomenology - Daniel Maître
ZZ channel
● Very clean signal,
● Since all final state particles are observed, the mass resolution is very good
● But rate is low for a mass of 125 GeV
From CMS Higgs Physics Results
Phenomenology - Daniel Maître
WW channel
● Provides a direct measurement of the coupling of the Higgs boson with the W
● No mass peak● Consider transverse
mass of leptons● Lepton directions are
correlated, which can be used to reduce the background
From arXiv:1307.1427
Phenomenology - Daniel Maître
ZH Strahlung
● It was long thought that this channel was hopeless because of the high background
● Consider highly boosted configuration● The Higgs decay products (b pair) will be in a
single “fat” jet● Looking for a boosted Z and a fat jet● Problem: the resolution was not good because of
the underlying event contributions● Need to use more information about the jet
structure
Phenomenology - Daniel Maître
ZH Strahlung
● Undo the jet clustering until two jets remain with a small mass, we interpret these two jets as the jets from the b pair decay, and make sure their transverse momenta are not too different
● But the resolution is still poor due to Underlying event contribution
● Solution: filter the jet to keep only the hardest radiation.
● The two hardest jets have to be b-tagged
Phenomenology - Daniel Maître
ZH Strahlung
● reclustering with Cambridge-Aachen algorithm (only angle)
From arXiv:0802.2470
Phenomenology - Daniel Maître
Measuring the properties
● We define the signal strength as the ratio between the observed cross section and the standard model expectation
● One of the challenges is to separate VBF and gluon fusion channels
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Higgs signal strength
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Testing the coupling
● To test the coupling to the Higgs, introduce scaling factors for the coupling of the Higgs to the fermions and vector bosons
From M. Duehrssen's YETI 2013 lecture
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Testing the coupling
● Try to fit the values of the scaling factors
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Testing the coupling
● Try to fit the values of the scaling factors
Phenomenology - Daniel Maître
● Polar angle θ in the Collins-Soper frame (in the rest frame of the di-photon system, z axis is between the the first incoming momentum and – the second incoming momentum )
Measuring spin
Phenomenology - Daniel Maître
Measuring the spin
● Add more channels and obervables– H→WW→lνlν
● Transverse momentum of the lepton pair● Invariant mass of the lepton pair● Azimutal angle difference between the leptons
– H→ZZ→4l● Angles and invariant
– Combine them in a Boosted Decision Tree
● Combine all channels
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Spin measurement
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CP measurement
● Different method at CMS
From arXiv:1212.6639
Phenomenology - Daniel Maître
Future
● Higgs self-coupling, needed to measure the higgs potential– Triple higgs vertex
● Only possible at high luminosity LHC
– Quartic higgs vertex● Probably not possible even at a high luminosity LHC