1. Constraints on Higgs width using H*(126) → ZZ events
Roberto Covarelli ( University of Rochester )
Seminar at CEA Saclay, 12 May 2014
R. Covarelli

2. Outline The LHC and the CMS detector After Higgs discovery
Theory review of off-shell Higgs production
Experimental techniques and combination with on-shell analysis
Monte Carlo simulations
Analysis of H* → ZZ → 2l2l’
Analysis of H* → ZZ → 2l2n
Limits on Higgs width and combination
Future plans
R. Covarelli

3. LHC and CMS: operationsaa
LHC Access Point 1:
ATLAS
Geneva
CERN
Meyrin
LHC Access Point 5:
CMS
R. Covarelli
11/15/2018

4. The CMS detector
R. Covarelli
11/15/2018

5. Looks more and more like the SM Higgs boson
After Higgs discovery
Great progress since new boson discovery in CMS
Observation in boson channels
Evidence in fermion channels
Mass determination
CMS H → ZZ → 4l measurement: ± 0.4(stat.) ± 0.2(syst.) GeV
Spin/parity studies
Angular analyses favor the JPC = 0++ hypothesis
arXiv:
arXiv:
Looks more and more like the SM Higgs boson

6. Property measurements - width
Direct decay width measurements at the peak limited by experimental resolution:
f(m) ~ BW(m, G) R(m, s)
If G << s, not possible to disentangle natural width
SM Higgs width at mH = GeV is GH = 4.15 MeV
Experimental resolution is s ~ 1-3 GeV for H → ZZ → 4l
arXiv:
ΓH < % CL

7. A different idea…
Assume a dummy (relativistic BW) resonance “R” with m = 100 and variable width
On-shell:
Off-shell:
Ratio of the two gives G!
Experimentally, this never worked before because of tiny off-shell yields and backgrounds
“on-shell”
region
“off-shell”
region
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8. H  WW and ZZ
D. De Florian
@ Higgs Couplings 2013
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9. The idea in detail Off-shell H* → VV (V = W, Z)
Peculiar cancellation between BW trend and decay amplitude creates an enhancement of H(126) cross-section at high mVV
About 7.6% of total cross-section in the ZZ final state, but can be enhanced by experimental cuts WW ZZ
gluon-gluon fusion production
H(126) peak
Threshold effects
at 2mZ and 2mt
Recover CPS
(~BW) trend
N. Kauer and G. Passarino, JHEP 08 (2012) 116
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10. Constraint on width
F. Caola, K. Melnikov (Phys. Rev. D88 (2013) )
J. Campbell et al. (arXiv: )
Once the “signal strength” m is fixed from an independent source a determination of r is obtained
N.B. r-scaling while keeping m fixed is equivalent to coupling scaling
Caution: the interference with continuum gg → ZZ is not negligible at high mZZ
Can be used to set a constraint on the total Higgs width:
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11. Monte Carlo simulation
gluon-gluon fusion
Using latest versions of gg2VV and MCFM (LO in QCD)
Including signal H(125.6), background and interference
“Running” QCD scales (= mZZ/2) + scale and PDF variations for systematics
Signal mZZ-dependent k-factors (NNLO/LO) applied G. Passarino (arXiv: )
Using results from M. Bonvini et al (Phys. Rev. D88 (2013) ), use kcontinuum = ksignal, assigning an additional 10% uncertainty on this assumption
other production modes
VBF production is 7% of the total at H(126) peak
Slightly enhanced at high mass by trend of sVBF(mZZ) ~ 10%
Using PHANTOM to model it, with same settings
VH and ttH do not contribute to tail effect
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12. Analysis procedure Fit r, using one or more variables:
P are MC- or data-derived templates for variables in each analysis
For a self-contained ZZ analysis use m from CMS on-peak 4-lepton analysis CMS collab. , arXiv: :
SM width/couplings evaluated at mH = GeV
Use observed signal strength (“m observed”, )
N.B. An additional assumption we must make is that mggF = mVBF = m (necessary because couplings are in principle different in the two processes, but mVBF not enough constrained by present ZZ data)
Expected results are provided also for m = (“m expected”, expected uncertainty from low-mass analysis)
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13. The 4l and 2l2n final states
Generator-level distributions
with approximated CMS
experimental cuts
4l final state (l = e, m)
At high mass, basically only background is qq → ZZ (known at NLO, QCD uncertainties at the level of %)
Fully reconstructed state  can use matrix element probabilities of lepton 4-vectors to distinguish between gg and qq production
2l2n final state (l = e, m)
Much larger BR (x6) but smaller acceptance (tight pT selection)
Rely on transverse mass distributions
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N. Kauer and G. Passarino, JHEP 08 (2012) 116
11/15/2018

14. H → ZZ → 2l2l’

15. Analysis overview
Same event reconstruction and selection as those used in the previous measurement of Higgs boson properties (arXiv: )
Event selections:
Two pairs of leptons (electrons or muons), isolated and prompt, of opposite sign and same flavor
Z1: closest to the Z boson mass Z2: the remaining with highest scalar sum of pT
At least one lepton has pT > 20 GeV, and another has pT > 10 GeV
40 < mZ1 < 120 GeV; 12 < mZ2 < 120 GeV
Off-shell analysis region: 220 < m4l < 1600 GeV
Background:
Irreducible background is qq→ZZ, modeled from MC
Reducible background is Z+X (Z and WZ, at least one lepton is non-prompt): much smaller, evaluated using a “fake rate” method, with control regions in data

16. MELA discriminant
No changes in selection w.r.t. CMS collab. , arXiv:
Lepton pT cuts, Z invariant masses, impact parameter significance, loose isolation
In the matrix element likelihood approach (MELA), design a specific discriminant for gg → ZZ production:
Built with 7 variables completely describing kinematics (mZ1, mZ2, five angles)
Pgg,(qq) are joint probabilities for gg → ZZ, signal + background + interference (qq → ZZ) from MCFM matrix elements
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17. Input variables to Dgg

18. m4l and Dgg distributions / yields
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19. m4l and Dgg distributions / yields
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20. H → ZZ → 2l2n
Missing ET (ETmiss)

21. Analysis overview Analysis variable is transverse mass:
6 times higher branching fraction compared to 4l final state
Branching ratio matters in high mass region where cross section is low
No access to Higgs on-shell production  m taken from 4l analysis
Z+jets background is several orders of magnitude higher (fake ETmiss due to hadronic energy mis-measurement)
Other backgrounds
Irreducible: non-resonant ZZ, WZ
Non-resonant (not involving a Z boson): top production, WW
Analysis variable is transverse mass:

22. Event selection Z + large ETmiss signature
First select a Z → ll: a pair of isolated electrons or muons, pT > 20 GeV, |m(ll) – mZ| < 15 GeV
In order to reject WZ: veto 3rd lepton (pT > 10 GeV)
In order to reject top processes: veto b-tagged jet or soft muon close to jets (pT > 3 GeV)
In order to reject Z+jets: ETmiss > 80 GeV; azimuthal angle between ETmiss and the closest jet: Δϕ > 0.5
To improve sensitivity, selected events are categorized according to number and topology of jets (pT > 30 GeV)
VBF / 0 jet / ≥1 jet (but non-VBF)
VBF is defined as m(jj) > 500 GeV and |Δη(jj)| > 4

23. Background estimations
qq → ZZ, WZ estimated from MC
Non-resonant background (tt, tW, WW)
Estimated from data using lepton flavor symmetry: compute the ee/eμ and μμ/eμ ratios in sidebands, and apply the ratios to eμ events in signal region
Z+jets background
Modeled by photon+jets events in data: reweight photon pT spectrum to match that of dilepton in data, and model ETmiss with photon sample

24. mT / ET,miss distributions
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25. mT / ET,miss distributions
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26. Signal enriched region: ETmiss > 100 GeV and mT > 350 GeV
Event yields
Signal enriched region: ETmiss > 100 GeV and mT > 350 GeV

27. Systematic uncertainties (I)
Theoretical uncertainties
gg → ZZ processes: QCD renormalization and factorization scales varied by a factor of two both up and down, and applied corresponding NNLO K factors; PDF variations by using CT10, MSTW2008 and NNPDF2.1
Additional 10% on continuum gg → ZZ background, accounting for limited knowledge on its NNLO K factor
QCD scales and PDF uncertainties on qq → ZZ and WZ backgrounds
In the 4l analysis, uncertainty of VBF shapes to account for approximate simulation
In the 2l2n analysis, theoretical uncertainties on jet-binning

28. Systematic uncertainties (II)
Experimental uncertainties
Lepton trigger, identification, isolation
In the 2l2n analysis, uncertainties on lepton momentum scale and jet energy scale are propagated to ETmiss; b-tagging efficiency
Background estimations from data
Integrated luminosity
Limited statistics in MC or data control samples
For systematics affect both normalization and shape, variations of shape are taken into account
Many are correlated with m measurement  reduced effect on G determination

29. Results
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30. 4l: results using only m4l or Dgg
r < 26.3 (17.0 expected)
r < 7.1 (12.7 expected)
With m4l observed limit is 1s worse than expected
Slight excess of events at high mass
Kinematic discriminant strongly suppresses the signal hypothesis for this excess
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31. 4l: 2-dim. result Observed (expected) 95% CL limit: r < 6.6 (11.5)
Best fit value:
r =
Equivalent to
Γ < 27.4 MeV
Γ = MeV

32. 1-dim. fit using mT or ETmiss
Results in 2l2ν analysis
1-dim. fit using mT or ETmiss
Observed (expected) 95% CL limit:
r < 6.4 (10.7)
Best fit value:
r =
Equivalent to
Γ < 26.6 MeV
Γ = MeV
ee-only : r < 6.9 (14.3 expected)
μμ-only : r < 14.0 (13.7 expected)
Counting analysis in “signal-enriched region”:
r < 12.4 (16.4 expected)

33. Combined results Observed (expected) 95% CL limit: r < 4.2 (8.5)
p-value = 0.02
Best fit value:
r =
Equivalent to
Γ < 17.4 (35.3) MeV
Γ = MeV

34. Conclusions
First experimental constraint on Higgs total width using H*(126) → ZZ events has been presented
Mild model-dependence
Just based on Higgs propagator structure
Assumptions on gg → ZZ continuum production beyond LO
Assumption of SM production of qq → ZZ and, in general, no other BSM sources enhancing high-mass ZZ yields
Combining 4l and 2l2n final states
Using variables related to ZZ inv. mass and kinematic discriminants
Small deficits in signal regions observed in both channels
Combination results:
G/GSM < 4.2 (8.5 95% CL
 G < 17 MeV (35 MeV 95% CL
Direct measurements at the peak set a limit of G < 3.4 GeV
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35. Future plans Addition of 7 TeV CMS data taken in 2011
Implementation of NLO electro-weak corrections for qq → ZZ and WZ backgrounds
recently available from several authors: Baglio et al. (arXiv: ), Bierweiler et al. (arXiv: ), Gieseke (arXiv: )
Remove jet binning from 2l2n analysis
Recent calculations show that jet bin migration uncertainties may be underestimated
Make a joint fit with low-mass 4l analysis
Exact correlation of systematic uncertainties
R. Covarelli

36. Back up

37. 4l mass

38. 4l control regions

39. Input to Dgg in signal-enriched region

40. 4l: limits per final state

41. Yields vs width (loose Missing ET cut)

42. Event yields

43. Systematics

44. Effect of G / coupling scalings
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45. PHANTOM settings LO generation Central scale mZZ/√2
NNLO/LO k-factor is 6% and independent on mZZ (from CERN Yellow Report 3)
Do not apply explicitly, normalize cross-section at the peak relatively to ggF
Central scale mZZ/√2
Same scale and PDF variations as ggF  effect much smaller (1-2%)
Signal, background, interference not available separately. Generate total amplitudes with r = 1, 10, 25 (and equal coupling scalings) and extract the 3 components from:
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46. Full formula of MELA Dgg
Depends on parameter a (relative weight of signal in the likelihood ratio). Since the expected exclusion is r ~ 10, use a = 10
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47. 2l2n: breakdown by channel
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