LHC results on Higgs
boson physics
G.Unal (CERN)
Review of ATLAS and CMS results

TGSW, Tsukuba, September 30th 2015

1

The Higgs boson in the Standard Model•

The Standard Model of particle physics•

Matter is made of fermions (quarks and leptons)•

Electromagnetic, Weak and Strong interactions are described by gauge theories => Predicts masslessintermediate vector bosons (like the photon)•

W,Z bosons are very massive (~80-90 GeV). How to preserve the "good" features of gauge symmetry ?

•Spontaneous Symmetry Breaking => Brout-Englert-Higgs mechanism for gauge theories.

•

Applying this mechanism to the Standard Model predicts one massive scalar boson: The Higgs boson•

Understanding electroweak symmetry breaking and studying the Higgs boson properties (if it exists) wasa key goal of the CERN Large Hadron Collider (LHC)•

In the Standard Model, fermion masses are also generated by this mechanism though Yukawa
interactions between the Higgs fields and the fermions2Some properties of the Higgs boson•

Coupling proportional to particle mass
-7•

Short lifetime (10 fs for a mass of ~125 GeV)•

Scalar particle•

Its mass is not protected by any fundamental symmetry•

If new physics at scale Λ ~Planck mass, difficult to keep m(H) ~100 GeV

Radiative corrections to Higgs boson mass:+loops with W,Z,ha

Except if new physics at mass ~ 1 TeV• For instance supersymmetry• Contributions from new particles cancel Standard Model contributions•

3How to produce Higgs boson:
The LHC accelerator at CERN
pp (or PbPb) collider

8.3 T dipoles to bend
proton trajectories
NbTi @ T=1.9K

Integrated luminosity: 5 fb-1 at √s=7 TeV in 201120 fb-1 at 8 TeV in 2012
Higgs boson results are obtained thanks to the very good
LHC performance4Ordrede grandeur
des principaux
processus
Main
processes
in high
energy pp
collisions
109/sσ(inélastique)σ(inelastic)σ (pb)

b quarkquarkproduction
production

102/s

QCD jetjet,EtEt>100
QCD> 100 GeV
GeV
W
top

0.1/s

Z

QCD
processes
Pure QCD-> Quarks
quarks et andgluons dans étatgluons
in
final
final
state

-> e et µenergy
de hauteeénergie
High
and μ

Higgs
masse
= 120@
GeV
Higgs
boson
120 GeV mass

Note: Tevatron run 1
σ(top) ~ 5pb
σ(tot) ~ 60 mb

34cm
-2-2s
-1
Taux dévénements
s-1
Event
rate @ 10à3410cm=> Online selection critical to record ~ 1 kHz event rate to disk for offline analysis
5σ(pp → H+X) [pb]

102
pp →

s= 8 TeV
H (N

NLO

+NN

LL
QC
D

10
pp →

1

pp

pp

→

pp
ZH

→

NL
O

→

ttH

NL

O

QC

(N

QC

D)

10

OQ

CD +

(N

D

LO

-1

(NNL

W

H

(N

qqH

+N

LO

NLO
E

EW

LHC HIGGS XS WG 2012

Higgs boson production at the LHC
)

W)

QC

D
+
LO NLO
EW
EW
)
)

+N

10-2
80 100gluon fusion

200

300

vector boson fusion

6

400

1000
MH [GeV]WH, ZH

ttH

1

WW
bb

10-1

gg

ττ

ZZ

LHC HIGGS XS WG 2013

Higgs BR + Total Uncert [%]Higgs boson decay

cc

10-2
γγ

Zγ

120

140

10-3
µµ

10-480100

160

Main decay modes: To heaviest particles
kinematically allowed
Important exception: H→γγ (via loop with W or
top quarks)
7

180
200
MH [GeV]Strategy to search for the Higgs boson and to study oit
Combine production mode * decay mode to get a final state
which can be separated from backgroundgg fusion

VBF

H➔γγ
H➔ZZ*➔4l
H➔WW*➔2l2ν
H➔ττ
H➔bb
H➔μμ
H➔Zγ
(l = e or μ)
8

VH

ttH

Which detectors to observe the Higgs boson ?

The CMS detector

9The ATLAS detector during installation in 2005

10

Standard Model processes are measured
and well understood
CMS Preliminary

Production Cross Section, σ [pb]July 2015

7 TeV CMS measurement (L ≤ 5.0 fb-1)
8 TeV CMS measurement (L ≤ 19.6 fb-1)
7 TeV Theory prediction
8 TeV Theory prediction

105
104

CMS 95%CL limit

103

Higgs

102
10
1
10−1
10−2
10−3

W

Z

Wγ

Zγ

WW

WZ

ZZ

All results at: http://cern.ch/go/pNj7

γγ→
EW
EW
qqW
qqZ
WW
W →lν , Z→ll, l=e,µ

EW
ssWW

11

WV γ

tt

t t-chtWt s-ch

ttγ

ttW

ttZ

ggH

VBF
qqH

VH

Th. ∆ σH in exp. ∆ σttHOne example analysis: Higgs boson decay to two photons

γ candidate
First step: Identify two high energy photons
Need to reject photons coming from decay of
hadrons (like π0), otherwise overwhelmed bybackground processes
Exploits fine granularity of the electromagnetic
calorimeter to achieve this goal
Example in ATLAS: very fine segmentation of
first calorimeter layer

12

π0 candidate

Example pp collisions with
two high energy photon
candidates

Second step: Measure invariant
mass of the two photons

M2 = 2 E1 E2 (1-cosθ)M(γγ) (2011 data)
H→γγ gives a peak at M(H)=> a very good resolution isneeded to achieve a goodsensitivity
Résolution on M ~1%
13

Discovery of a new boson in July 2012

14

CERN seminar July 4th, 20122013 Nobel price for physics toEnglert et Higgs

This is the beginning of the studies of this new boson
15

Characterizing the boson••

Mass measurement•

Not predicted in the Standard Model (but can be constrained through precision
measurements)•

Once the mass is determined, all properties of the Higgs boson are predicted in theStandard Model

Lifetime measurement (of decay width)••

Difficult to measure experimentally in the Standard Model (but new idea emerged
recently)

Measurement of spin and parity+••

Should be 0 for the boson linked to the BEH mechanism=> The alternative hypothesis
have been strongly rejected by the analysis of the LHC data

Measurement of the coupling between the boson and other particles•

Sensitive to deviations from the Standard Model•

Higgs decay width in the Standard Model is not that large => could be quite sensitive to
new physics contributions
16

Mass measurement
Use the two channels with the best mass reconstruction
resolution
H→γγ
H→ZZ*→4l (l=e or μ)

The analysis exploit the data as much as possible using categories withdifferent S/B, different mass resolutions, etc..
17Results for the mass measurement

Accuracy < 0.2%
18

One implication of the mass measurement
In the Standard Model MW=f(MZ,Mtop,MH) via radiative corrections to the W
and Z bosons propagators=> MW et Mtop can be predicted from precision measurements done at LEP
and elsewhere, especially if the Higgs mass is known

Good consistency=> a spectacular success for the Standard Model
19

Study of the Higgs boson couplings•

Measure as much (production channel)*(decay mode) as possible•

Some ratio measurements also allow one to reduce systematic
uncertainties•

Assume only one boson with M ~125 GeV and spin 0•Same Lagrangian structure as in the Standard Model•

Define multiplicative coupling modifiers κ••

κ=1 for the Standard Model•

Investigate different assumptions for the κ's: one per particle, or a smaller
set of κ, and also different assumptions on particles running in loops•

Not yet sensitive to Higgs coupling to itself (HH pair production)

Can also allow "exotic" Higgs boson decays•

Complementary of direct searches H→(invisible particles) (Dark Matterparticles for instance), where no excess is observed (yet)20

Look for signal in each five main decay channels for ~almost all production
modes
Refined analysis (multivariate techniques) to maximize sensitivity

H→WW→llννH→ττ

Clearly observed

ATLAS+CMS Combination> 5 σ

Signal significance ~2-3 σ pour
VH (H→bb)
21

Separation by production mode:
Candidate H(→ττ) produced by vector boson fusion

Difficulty: Correctly accounts the crosscontamination between different production
modes
22

Summary of the signal yield in eachcategory compared to Standard Model
predictions

Summary of cross-section
measurement (assumingStandard Model decay
yields)=σ(measured)/σ(predicted)- Good consistency- ttH production not yet established, but some hint of an excess- To look closely with the run 2 data
- Uncertainty are mainly limited by data statistical uncertainties
23

Fit with two coupling modifiersκF for all fermionsκV for all bosons(No new decay mode nor new particles in loop)

Example:yield (gg→H→γγ) ~ κF2 * (1.6 κV2+0.07 κF2-0.66 κF κV) / (0.75* κF2+0.25* κV2)

Γ(H)
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Fit with one coupling modifier per Standard Model particle(no new decay mode, no new particle in loops)

Typical accuracies on coupling modifiers are O(15-40)%
for more "flexible" models
25Other Higgs bosons ?
A richer scalar sector is possible
Main example: Supersymmetric theories
predict 2 electroweak scalar doublets=> 5 spin 0 particles after electroweak
symmetry breaking:h,H,A,H±h = lightest scalar, could have properties
very close to h(MS)H,A,H± typically heavier
Look for instance for H,A→ττ

26

Complementarity between direct searches and precision
measurements of h(125) properties in the context of some minimal
supersymmetric model27

Prospects•

Analysis of run 1 (2010-2012) LHC data is ~ final••

Futur at the LHC•

With runs 2 and 3 (until ~ 2022) => O(10) more data at higher collision energy•

With the HL-LHC program => O(100) more data••

A preliminary combination of ATLAS and CMS results was released few weeks ago•

Significant improvement in accuracy of signal yield measurements (between ~ 1% and10%)•

Precise measurement of ttH process•

Also can observe rare decays (like H→μμ) and starts to explore HH production

Also search for heavier particles in scalar sector and other sectors (-> See H.Okawa's
talk for di-boson resonance searches at high mass)-

The 125 GeV Higgs boson can be also accessed at future e+e colliders•

Would like O(1%) accuracy on coupling measurements to probe many realistic beyond
the Standard Model theories
28

Conclusions• The discovery of "a" Higgs boson has been the main result of the first LHC run• But should not shadow the many other interesting and important physics results• This discovery confirms the Standard Model mechanism for generating masses to elementary

particles• Several alternative theories are excluded• But this scalar particle opens many unanswered questions• It is the first elementary particle with spin 0• Mass stability ?
• Portal toward physics beyond the Standard Model ?
• Links with other scalar fields and cosmology ?
• Precision measurement of the properties open a new field of exploration complementary

to direct searches at higher masses• With run 1 LHC data, Higgs boson coupling have been probed with~ 15-40 % accuracy• A much better accuracy will be achieved in the future, this is only the beginning of Higgs

boson measurements

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Few references
•

•

•

•

Résultats ATLAS et CMS•

http://twiki.cern.ch/twiki/bin/view/AtlasPublic/HiggsPublicResults•

http://cms-results.web.cern.ch/cms-results/public-results/publications/HIG/index.html

Mécanisme BEH•

F. Englert and R. Brout, Broken symmetry and the mass of gauge vector mesons, Phys. Rev. Lett. 13 (1964) 321.

•

P. W. Higgs, Broken symmetries, massless particles and gauge fields, Phys. Lett. 12 (1964) 132.

•

P. W. Higgs, Broken symmetries and the masses of gauge bosons, Phys. Rev. Lett. 13 (1964) 508.

•

G. S. Guralnik, C. R. Hagen, and T. W. B. Kibble, Global conservation laws and massless particles,Phys. Rev. Lett. 13 (1964) 585.

Modèle Standard•

S. L. Glashow, Partial-symmetries of weak interactions, Nucl. Phys. 22 no. 4, (1961) 579.

•

S. Weinberg, A Model of Leptons, Phys. Rev. Lett. 19 (1967) 1264.

•

A. Salam, “Weak and electromagnetic interactions”, in Elementary particle theory: relativistic groups and analyticity, N. Svartholm, ed., p. 367. Almqvist &Wiksell, 1968. Proceedings of the eighth Nobel symposium.

•

G. ’t Hooft and M. Veltman, Regularization and Renormalization of Gauge Fields, Nucl. Phys. B44 (1972) 189.

Test de précision du Modèle Standard
•

•

•

•

M. Baak, J. Cuth, J. Haller, A. Hoecker, R. Kogler, K. Mönig, M. Schott, J. Stelzer Eur. Phys. J. C 74, 3046 (2014)

Metastabilité•

Degrassi, Di Vita, Elias-Miro', Espinosa, Giudice, G.I., Strumia arXiv 1205.6497•

V. Branchina, E. Messina, M. Sher, Phys.Rev.D91 (2015) 1, 013003

Production off-shell•

N. Kauer and G. Passarino, Inadequacy of zero-width approximation for a light Higgs boson signal, JHEP 08 (2012) 116, arXiv:1206.4803.

•

F. Caola and K. Melnikov, Constraining the Higgs boson width with ZZ production at the LHC, Phys. Rev. D 88 (2013) 054024, arXiv:1307.4935•

J. M. Campbell, R. K. Ellis, and C. Williams, Bounding the Higgs width at the LHC using full analytic results for gg → e−e+μ−μ+, JHEP 04 (2014) 060, arXiv:
1311.3589•

J. M. Campbell, R. K. Ellis, and C. Williams, Bounding the Higgs width at the LHC: complementary results from H → WW, Phys. Rev. D 89 (2014) 053011,
arXiv:1312.1628.

Calcul section efficaces et BR du Higgs•

S. Dittmaier et al., Handbook of LHC Higgs Cross Sections: 1. Inclusive Observables (2011), arXiv:1101.0593 [hep-ph].

•S. Dittmaier et al., Handbook of LHC Higgs Cross Sections: 2. Differential Distributions (2012), arXiv:1201.3084 [hep-ph].

•S. Heinemeyer et al., Handbook of LHC Higgs Cross Sections: 3. Higgs Properties (2013), arXiv:1307.1347 [hep-ph].

•+ references dans ces articles•

C. Anastasiou, C. Duhr, F. Dulat, F. Herzog and B.Mistlberger, arXiv:1503.06056 (cacul N3LO)

30

Lifetime / Decay width
Γ(MS) ~4 MeV (lifetime ~ 1.6 10-7 fs, cτ
~50 fm)Direct measurement of width throughwidth of MH(measured)=> Limit O(GeV) (experimental
resolution on M)
Can exclude very long lifetime bychecking that Higgs boson decays at
the position where it is produced=> lifetime cτ < 57 μm
i.e Γ > 3.6 10-9 MeV

31"Off-shell" events
A new idea to constrain the width (with some caveat)

Interférence with "backgroun

q ~MH σ~1/ΓH *(couplings)q>>MH σ~(couplings)=> Γ/Γ(MS) < ~ (5-7)
32Spin /CP
H→ZZ* is the ideal decay channel to measure these properties

5 angular variables + 2 kinematic ones=> combine all the informations for best sensitivity
33

Results for spin/CP
The O+ hypothesis is always favoredcompared to alternative hypothesis

34

Z→ee

Calibration of e and muon energy
measurement:
Use Z→ee ou μμ
M(Z) known to ~2.10-5 thanks to LEP(former e+e- collider at CERN)
Candle to adjust energy scale
Then check stability and extrapolatione→γ

Z→ee et J/ψ→ee

Z→llγ

35

Another implication of the mass measurement
V(φ) ~λ(φ) φ4
In the SM, the Higgs potential has radiative
corrections depending on Mtop=> The potential can be unstable at highvalues of the field=> Vacuum instability or "metastability) (Ifthe tunneling time is > the age of theuniverse))
The MH value is close to this limit. Is it anaccident ? λ(MPlanck) ~0 ?

36