1. ATLAS Standard Model Higgs Results
Andrew Mehta
For the ATLAS Collaboration
LHCP, Barcelona, 13 May 2013
13/05/2013
Andrew Mehta, ATLAS SM Higgs LHCP2013

2. Andrew Mehta, ATLAS SM Higgs LHCP2013
Introduction
10 Months since observation of
a new particle
Since then:
More data taken
More channels analysed
Final state observables used to measure properties
Focus now on understanding particle’s properties
In this talk:
Mass and signal strength
Couplings
Spin/Parity
Rare Decays
Note: This talk is overview only. Many more details in parallel session talks and:
13/05/2013
Andrew Mehta, ATLAS SM Higgs LHCP2013

3. Mass and Signal Strength
13/05/2013
Andrew Mehta, ATLAS SM Higgs LHCP2013

4. Evidence for a new Particle
γγ, ZZ and WW channels updated with full data
Clear evidence for a new particle seen in these channels
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Andrew Mehta, ATLAS SM Higgs LHCP2013

5. Combined significance
Combined significance almost at 10 σ.
No doubt about discovery of a new particle
But is it the SM Higgs Boson?
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Andrew Mehta, ATLAS SM Higgs LHCP2013

6. Andrew Mehta, ATLAS SM Higgs LHCP2013
Mass Measurement
Accurate mass from ZZ and γγ channels.
mH=125.5± 0.2 (stat) ± 0.6 (sys) GeV
mγγ-mZZ=2.3 ±0.7 (stat) ±0.6 (sys) GeV
Mass difference compatibility 1.5%
(2.4σ).
With alternative conservative systematic treatment compatibility rises to 8%
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Andrew Mehta, ATLAS SM Higgs LHCP2013

7. Andrew Mehta, ATLAS SM Higgs LHCP2013
Signal Strengths
Signal strength μ = σ/σSM measured assuming mH =125.5 GeV
No large deviation from SM prediction
Compatibility of combined μ value and SM μ=1 is 9%
Global compatibility between 5 measured μ values and μ=1 is 8%
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Andrew Mehta, ATLAS SM Higgs LHCP2013

8. Andrew Mehta, ATLAS SM Higgs LHCP2013
Couplings
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Andrew Mehta, ATLAS SM Higgs LHCP2013

9. Andrew Mehta, ATLAS SM Higgs LHCP2013
Production Modes
gluon-gluon fusion (ggF) dominant
Several measurements of vector-boson fusion (VBF)
Data not yet very sensitive to VH and ttH
Combine VBF and VH (both scale with V-H coupling)
Combine ggF and ttH (both scale with t-H coupling)
μVBF+VH/μggF+ttH =
3.1 σ significance of non-vanishing VBF
13/05/2013
Andrew Mehta, ATLAS SM Higgs LHCP2013

10. Fermion and Vector Couplings
Vector coupling (κV) measured indirectly and directly in many channels
Fermion coupling (κF) measured:
- directly in H→bb and H→ττ – not well measured yet
- indirectly via loop gg→H
Combined –no assumption on Higgs Width
Measure ratio of fermion to vector couplings:
λFV=
Fermion and vector couplings non-zero and consistent with SM.
Still have a sign ambiguity
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Andrew Mehta, ATLAS SM Higgs LHCP2013

11. Andrew Mehta, ATLAS SM Higgs LHCP2013
Spin
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Andrew Mehta, ATLAS SM Higgs LHCP2013

12. Andrew Mehta, ATLAS SM Higgs LHCP2013
Measurement of Spin
SM prediction for Higgs is JP =0+
J=1 unlikely after H→γγ observation but possible in some models
Decay angles e.g. cosθ* (polar angle of daughters in H rest frame) reconstructed in H→γγ and H→ZZ
H→WW also sensitive but decay angles not directly reconstructed – use other final state variables
Results analysed in a graviton inspired model
which has an unknown production mechanism – fraction of qq to gg
Spin 2+ hypothesis tested in all channels
H→ZZ used to test 0- and 1+ scenarios
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Andrew Mehta, ATLAS SM Higgs LHCP2013

13. Andrew Mehta, ATLAS SM Higgs LHCP2013
Spin from H→γγ
Reconstruct decay angle from photons
Sensitive to spin but not parity
Fit background and signal in bins of cosθ*
Spin 0 favoured.
14/05/2013
Andrew Mehta, ATLAS SM Higgs LHCP2013

14. Andrew Mehta, ATLAS SM Higgs LHCP2013
Spin from H→WW
Combine several variables in a multivariate discriminant
(boosted decision tree)
Variables used: mll, PTll, Δφll, mT 0+ 2+
Spin 0 favoured.
13/05/2013
Andrew Mehta, ATLAS SM Higgs LHCP2013

15. Andrew Mehta, ATLAS SM Higgs LHCP2013
Spin from H→ZZ
Combine several variables using a boosted decision tree
Variables used: Φ, θ1, θ2, m12, m34, θ*, Φ1
Sensitive to parity and spin. Spin 0+ favoured.
13/05/2013
Andrew Mehta, ATLAS SM Higgs LHCP2013

16. Andrew Mehta, ATLAS SM Higgs LHCP2013
Spin Summary
Test statistic is ratio of likelihoods of 2 spin hypotheses
Compatibility with spin 0
Compatibility with spin 2
3 channels combined exclude 2+ at 99.9% CL independent of production mode
H→ZZ excludes 0-, 1+ and (1-) and >95% (94%) CL.
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Andrew Mehta, ATLAS SM Higgs LHCP2013

17. Andrew Mehta, ATLAS SM Higgs LHCP2013
Rare Decays
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Andrew Mehta, ATLAS SM Higgs LHCP2013

18. Andrew Mehta, ATLAS SM Higgs LHCP2013
Search for H→Zγ
Reconstruct Z→ll and photon
Take mass difference between H and Z
Observed limit at MH=125 GeV is 18.2 × SM cross section (95% CL)
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Andrew Mehta, ATLAS SM Higgs LHCP2013

19. Andrew Mehta, ATLAS SM Higgs LHCP2013
Search for H→μμ
Look for a peak in μμ mass spectrum
Observed limit at MH=125 GeV is 9.8 × SM cross section (95% CL)
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Andrew Mehta, ATLAS SM Higgs LHCP2013

20. Search for H→invisible
100 % H→invisible BF
Assuming SM ZH production rate (MH=125 GeV)
H→invisible BF <65% observed (84% expected) at 95% CL.
13/05/2013
Andrew Mehta, ATLAS SM Higgs LHCP2013

21. Andrew Mehta HSG5 Meeting
Summary
2012 data confirm beyond doubt the existence of a new particle.
Signal strengths compatible with SM
Mass is measured to mH=125.5± 0.2(stat) ± 0.6 (sys) GeV
>3σ observation of VBF production mode
Non-zero fermion and vector couplings
Couplings consistent with SM
Non-SM spin/parity disfavoured
Particle looks very much like the SM Higgs
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Andrew Mehta HSG5 Meeting

22. Andrew Mehta HSG5 Meeting
References
Higgs couplings: Higgs mass: Higgs spin: γγ: γγ spin : ZZ: WW: WW spin: ττ: bb: μμ: Zγ: ZH→invisible:
13/15/2013
Andrew Mehta HSG5 Meeting

23. Andrew Mehta HSG5 Meeting
Backup
17/04/2019
Andrew Mehta HSG5 Meeting

24. Andrew Mehta, ATLAS SM Higgs LHCP2013
Couplings Summary
14/05/2013
Andrew Mehta, ATLAS SM Higgs LHCP2013

25. Andrew Mehta, ATLAS SM Higgs LHCP2013
Couplings Summary
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Andrew Mehta, ATLAS SM Higgs LHCP2013

26. Andrew Mehta, ATLAS SM Higgs LHCP2013
Mass Measurement
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Andrew Mehta, ATLAS SM Higgs LHCP2013