1. LHC Higgs Searches Physics at the LHC, UK HEP Forum, Cosener’s 2011
C. H. Shepherd-Themistocleous
PPD, Rutherford Appleton Laboratory

2. Outline Results from Higgs searches at the LHC
SM Higgs production and Decay
Search for SM Higgs at low, medium and High masses
Searches for non-SM Higgs
EWK results (brief)
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

3. Luminosity ATLAS CMS At EPS reported on ~1 fb-1.
At LP many analyses used close to 1.7 fb-1 and even 2.3 fb-1.

4. Higgs Production
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5. Higgs Decay Analyses can be classed in approximate mass ranges
Low (< 130 GeV) , medium ( GeV) high (> 200 GeV)
Searches are characterized by BR and width.
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6. best overall sensitivity Low mass H → gg , H-> tautau , VH-> bb
High masses
WW and ZZ modes
Intermediate masses
WW key channel with
best overall sensitivity
Low mass
H → gg , H-> tautau , VH-> bb
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

7. Key background cross sections

8. Low mass Higgs searches
Channels for mass range ~ GeV
H →  Best sensitivity at low masses
H →  Useful (potential in SUSY scenarios)
H → bb Large BR but rather challenging

9. H → gg Rare decay through loops Irreducible gg backgrounds
Reducible from jets faking photons. jet-jet supression ~ 108
Require very good mass resolution. ECAL critical.
Photon class (converted or not) plus region of calorimeter
vary resolution. Used to define classes for analyses.
Test of ECAL calibration using Z → ee peak.

10. Resolution comparison
Comparison of resolutions in ATLAS (black) and CMS (blue) using tuned MC.
Best case barrel no conversions
All categories combined

11. H → gg Both experiments divide data into quality classes.
Fit background plus signal shape
No evidence of a signal – set limits
1.08 fb-1
1.6 fb-1

12. H→ gg ATLAS 1.08 fb-1 CMS 1.66 fb-1. No evidence for SM Higgs
Sensitivity currently 3-4 times SM.

13. H → tt
Sensitivity in cleaner VBF (non-VBF also used) modes. Both use ll and l-had final states.
Forward tagging jets suppress SM backgrounds.
Main backgrounds: Z + jets (Z→ tt irreducible), W → ln+jets, diboson, top, QCD.
Background suppression makes use of event topology
CMS use visible mass, ATLAS use collinear approx. and estimate mtt.
Fits peformed to spectra.
ATLAS
ee, em, mm
channels
QCD, W+jets,
Z→ tt from data
Other bg MC.
CMS
e-had example.
QCD, W+jets
from data
Z → tt + rest
from MC

14. H → tt Combination of large number of channels.
Sensitivity some way off SM, but also useful for SUSY.

15. H→ bb Dominant decay mode at low mass BUT QCD background huge.
Interesting because potentially provides information on Higgs to quark coupling.
Searches in VH production and require b-tagging. Backgrounds V+jets, VV, top, QCD.
CMS & ATLAS Z→ ll, W→ ln. CMS also Z → nn
ZH → llbb
CMS sensitivity rather larger than ATLAS’s

16. Intermediate mass Channels for mass range ~ 130-200 GeV
H → WW → llnn Greatest overall sensitivity to SM Higgs

17. H→ WW → llnn Excellent sensitivity where this BR is large 130-200 Gev.
Challenging Higgs mass cannot be reconstructed.
Search both latest results cut based.
Look for 2 opp. sign leptons , missing ET.
Jets 0,1 both; 2 (VBF) CMS.
Scalar Higgs - make use of Df(ll) cut.
CMS Major requirements:
Lepton Pt > 10 GeV, tight ID & Isolation
removes QCD & W+jets contamination
Large MET & Z  μμ, ee veto
removes Drell-Yan contamination
Class # of jets (PT > 30 GeV) & b-jet veto
removes Top contamination
Kinematic discriminants: Mll & ΔΦ (l+l-)
mitigates pp  WW background
MH–dependent cut optimization
ATLAS similar.
Use ETmissrel
After ET and pT
cuts

18. H→ WW → llnn ATLAS Mll after pT cuts on leptons ATLAS MT after all
0 jets
1 jet
ATLAS Mll after pT cuts
on leptons
For 0 jet WW dominates
for 1 jet increased ttbar.
0 jets
1 jet
ATLAS MT after all
other cuts

19. H→ WW → llnn
0 jets
0 jets
1 jet

20. H→ WW → llnn Main backgrounds WW, ttbar, D-Y, W+jets, WZ/ZZ.
Estimated from data for major backgrounds, minor use MC.
ATLAS and CMS similar
WW using high mass ll events where no/little signal.
ttbar invert b tagging cut and extrapolate using tagging efficiencies.
W+jets, QCD using fake rate derived from data.
D-Y using ll events inverting Z veto cut and extrapolating.
Example of ATLAS
Expectations from MC
and observation using
data driven methods

21. H→ WW → llnn SM Higgs 147 < MH < 194 GeV at 95% CL.
Selection change
1.5 fb-1
1.7 fb-1
SM Higgs 147 < MH < 194 GeV at 95% CL.
Expected sensitivity 136 < MH < 200 GeV
SM Higgs 154 < MH < 186 GeV at 95% CL.
Expected sensitivity 135 < MH < 196 GeV

22. High mass Channels for mass range > 200 GeV ZZ → 4l ZZ → 2l2n
ZZ → 2l2q
ZZ → 2l2t
WW→ln2q
WW → 2l2n

23. H → ZZ(*) → 4l
Clean signal 4 isolated leptons from common vertex. One or two Zs on shell.
Full mass reconstruction. (4e, 4m, 2m2e)
Backgrounds: Zbb, tt.
Suppress with impact parameter and isolation requirements
ZZ Continuum:
Shape known at NLO.
Rate obtained from Z yield in data & theoretical prediction for ratio of ZZ to
Z cross sections PT
43 GeV
26 GeV
20 GeV
48 GeV
M4μ = 201 GeV

24. H → ZZ(*) → 4l
CMS
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

25. H → ZZ(*) → 4l Fits to mass spectra performed to extract limits
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

26. H → ZZ→ ll nn Cut and count analyses in ee and 
Production of Z with large missing energy
Cut and count analyses in ee and 
Major backgrounds: Z+jets, ttbar, WZ
Cuts on MET, MT, Δ (MET,jet)
Residual backgrounds:
ZZ (NLO+LO),WZ(NLO) from MC
Z+jets estimated from +jet data.
Non resonant ttbar & WW use e events

27. H → ZZ→ ll nn ATLAS CMS CMS 1.6 fb-1 , ATLAS 1.04 fb-1
Exclusion GeV (CMS) , GeV 95% CL
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

28. H → ZZ(*) → 2l2q Highest rate for Z channel
Events categorized by presence of 0, 1, 2 b-jets
Mass reconstructed from l and jets.
Major background: Z+jets ; ttbar suppressed by MET requirement
CMS Use 5 angles of scalar H  ZZ  2l 2q in a likelihood discriminant
Background shape, normalization  data sideband
CMS
Preliminary
1.04 fb-1
1.6 fb-1
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

29. H → ZZ(*) → 2l2q
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30. H → ZZ(*) → 2l2t Fake rate estimated from Z+jet sample.
New CMS analysis. One Z → ll ( 60 < m < 120 GeV); Z → thth , mth , eth em ( 30 < mvis < 80 GeV)
Backgrounds:
ZZ estimated using observed number of Z and ratio of SM s(ZZ)/s(Z).
Reducible Z+jets, ZW+jets, ttbar ,QCD. non-isolated l or jet can fake th.
Fake rate estimated from Z+jet sample.
event eeet

31. H→ WW → ln qq
x100
x100

32. Non-SM Higgs Tevatron BR limit : 0.15 – 0.2
Searches have started for possible non-SM Higgs.
MSSM SUSY has 5 h,H0,A0,H+/-. CMS F → tt, H+ → tn.
Make use of bbF production (large tanb no VBF)
Significant improvement on
previous results
Tevatron BR limit : 0.15 – 0.2
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

33. Doubly Charged Higgs Doubly charged pair produced H++ → l+l+,t+t+
Arises in models with extra Higgs triplets
F++, F+, F0
Triplet responsible for small neutrino mass
Unknown neutrino mass matrix  unknown branching ratios  broad search
Below M ≈2MW, only leptonic decays
3l f.s.
3 or 4 leptons in final state
used for search

34. All limits
CMS Preliminary

35. CMS combination Expected exclusion mass range: 130 – 440 GeV
Analysed data
between
fb-1
Expected exclusion mass range: 130 – 440 GeV
Observed exclusion mass range: , , GeV
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

36. ATLAS combination Expected exclusion mass range: 130 – 440 GeV
Observed exclusion mass range: , , GeV
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

37. p-values
Probability of an upward background fluctuation as high or higher than observed in data
Interpretation requires “look elsewhere effect” correction
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

38. Summary of Higgs searches
Large number of searches performed by both ATLAS and CMS.
Impressive work on combinations of results
Similar exclusion regions in each of the experiments.
CMS: , , GeV
ATLAS: , , GeV
Excess in low mass range less significant than at time of EPS.
Factor of 2 in integrated luminosity between EPS and LP.
Hope for another similar factor this year.

39. W mass Huge production x-section at 7TeV Pile up a challenge
pdfs affect precision
Kinematics important for precision measurement, pT dist.
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

40. TGCs Will profit from increased stats
Can search for evidence in NP in deviation from expectations
or forbidden vertices. SM vertices WWZ, WWg forbidden ZZZ ZZg
No anomalous coupling observed
WW leptonic
WW leptonic

41. THE END

42. Backup
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43. Search potential wrt Tevatron
Ratio of int. luminosities LHC (40 pb-1) /TEV (5fb-1) ~ 1/100
For ratios of parton luminosities > 100 LHC wins (Fn of initial state)
Pair production of
Intermediate mass
particles from
gg, qg, qq
Production of
massive objects
> ~ 1TeV
Cosener's Forum C.H. Shepherd-Themistocleous - RAL

44. Add table for ATLAS Mode Mass Range Data Used (fb-1) CMS Document
H  γγ
1.7
HIG
H  bb
1.1
HIG (NEW)
H  ττ
HIG
H WW 2l 2ν
1.5
HIG
H  ZZ 4l
HIG
H  ZZ 2l2τ
HIG (NEW)
H  ZZ 2l2j
1.6
HIG
H  ZZ 2l2ν
HIG
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