E. von Törne, Overview of the ATLAS Higgs Measurements

1Eckhard von Toerne, Bonn ATLAS Higgs Measurements
Talk at BW 2013 – Beyond the Standard Models
25 – 29 April, 2013, Vrnjačka Banja, Serbia
Overview of the
ATLAS Higgs
Measurements
Eckhard von Toerne
University of Bonn
on behalf of the
ATLAS Collaboration

• “Observation of a new particle”
in July 2012
• Overview of properties: mass,
spin/parity, (couplings)
• A Note:
While existence of new boson at mass of
~125 GeV is undisputed, its nature is
under active study.
Boson exhibits Higgs-like properties.
For brevity the term “Higgs-like boson” is
shortened to Higgs boson or Higgs
(working hypothesis).
PLB 716(2012) 1-29,
http://arxiv.org/abs/1207.7214
Introduction
See also: ATLAS Coll., Science 338, 1576 (2012)

• x
Production of SM Higgs @ LHC
Cross sections at 8 TeV [pb]
ggH 19.4
VB Fusion 1.6
WH+ZH 1.1
ttH 0.1
mH = 125.5 GeV

Decay of SM Higgs
56.9 % bb
22.3 % WW
6.2 %
2.8 % ZZ
0.23 % γγ
@ mH=125.5 GeV

• x
LHC runs in 2011 + 2012
Z→μμ event with
25 reconstructed vertices

ATLAS detector

• Two isolated high-pt
photons (ET>40, 30 GeV)
• Mass peak with
resolution 1.77 GeV
above large backgr.
(S/B=3%).
• Reducible backgr. j, jj
(~25%)
• Irreducible bgd from
QED reaction qq
(~75%)
• Example event with
m =126.6 GeV
H

• Two isolated high-pt
photons (ET>40, 30 GeV)
• Mass peak with
resolution 1.77 GeV
above large backgr.
(S/B=3%).
• Reducible backgr. j, jj
(~25%)
• Irreducible bgd from
QED reaction qq
(~75%)
• Example event with
m =126.6 GeV
H
Zoom into
Calorimeter cluster
• Layered
Calorimeter 
• Photon direction
• Selection of vertex

local significance:
- observed: 7.4σ
-expected
-(SM Higgs): 4.1σ
Best fit mass: 126.8 ± 0.2 (stat) ± 0.7 (syst) GeV
Signal Strength µ = obs/ SM = 1.65 ± 0.24(stat) ± 0.22(syst)
H
Prob. of a background
fluctuation to be more signal-
like than observed signal.

H
• 2.0 excess for VBF
production over bgd.
• 14 categories to optimize
sensitivity (different S/B ratios)
• and to categorize ggF, VBF, VH

HZZ(*)
ATLAS-CONF-2013-013
mH = 124.3 ± 0.6 (stat) ± 0.4 (syst) GeV
μ (124.3 GeV) = 1.7 ± 0.4
Need to reconstruct low pt leptons

HZZ(*)
ATLAS-CONF-2013-013
mH = 124.3 ± 0.6 (stat) ± 0.4 (syst) GeV
μ (124.3 GeV) = 1.7 ± 0.4

• x
H  ZZ(*)  4 µ
m12=86.3 GeV, m34=31.6 GeV, m4μ = 125.1 GeV, ATLAS-CONF-2012-169

Higgs mass combination
Combined ATLAS result
Difference reduced by
700 MeV compared to
December result
Considering all
systematics and their
correlations, compatibility
of both results is at 1.5%
(2.4σ level)
With alternative
treatment of systematics,
compatibility rises to 8%
Main systematics:
energy scale
mH = 125.5 ± 0.2 (stat) ± 0.6 (syst) GeV

• limited mass sensitivity due to neutrino contribution
• Transverse mass
• S/B ~ 15%
HWW

• Observed significance (125 GeV) 3.8σ (3.7σ expected)
• Signal strength at 125 GeV μ = 1.01 ± 0.21(stat) ± 0.23(syst)
[ 0.21(stat) ± 0.19(theo) ± 0.12(exp) ± 0.04(lumi) ]
HWW

• x
Hbb
• Limits derived from Mbb distribution (~16% mass resolution)
• 95%CL limit at 125 GeV: 1.8 x SM (expected limit 1.9)
• μ(125) = -0.4 ± 0.7(stat) ± 0.8(syst),
• Analysis still in flux and not all of 2012 data used
• Main benchmark analysis yields 4.0σ observation of boson
pairs WZ(bb)/ZZ(bb)
Combination of search channel WH, Wl / ZH, Zll / ZH, Z

• x
Hbb candidate

• x
H candidate

• Global compatibility
between presented
channels and the SM
expectation is 8%
• Dependence of signal
strength on mass is
weak
ATLAS combined signal strength
ATLAS-CONF-2013-034

ATLAS combined Higgs signal significance
Moriond 2013 status, ATLAS-CONF-2013-034
• Combination of all
channels
• Local p0 = 10-23
•  ~10 excess
around 125 GeV

Spin and Parity
• SM predicts JP=0+
• Spin studies in three different modes
• H : sensitivity decay angle in Higgs rest frame *
• HWW decy angles no directly reconstructable due to
neutrinos, however other kinematic vars. quite sensitive
due to V-A nature of W decays.
• HZZ fully reconstructed, decay plane is sensitive
• Spin 1 disfavored by Landau-Yang
theorem and H decay
• Spin 2 case may be quite evolved
• Here: tensor with minimal
couplings to SM particles (2+
m model)
• Dependency on 2+
m production mode.
 free parameter fqq, fqq+fgg=1

• Spin information from cosθ* shape, not sensitive to parity
• Minimize correlation between cosθ* and mass for bgd by
applying absolute pt cuts
• control the background cosθ* shape via mass sidebands
• Largest sensitivity if 2+
m is created by 100% gg fusion
Spin and Parity: H

Spin and Parity: HZZ(*)
Combine all kinematic variables in a Boosted Decision
Tree to optimize sensitivity
sensitive to parity

Spin and Parity: HWW
ATLAS-CONF-2013-031
0+ Hyp. 2+ Hyp.
BDT to combine all sensitive variables, most sensitive: fqq=100%

• the 3 channels combined exclude 2+
m scenario at 99.9% CL
• ZZ analysis excludes the 0-, 1+ and (1-) hypotheses at the
>95% (94%) CL
Spin and Parity: combined
ATLAS-CONF-2013-040

• Overwhelming evidence for a new boson at
• M = 125.5 ± 0.2 (stat) ± 0.6 (syst) GeV
• Measurements of signal strength and spin/parity in
agreement with SM hypothesis.
• Not covered in this talk: Meas. of couplings also in good
agreement.
• Improved measurements of pre-shutdown data and after
shutdown will further investigate this particle.
• Search for rare decays
• Search for additional states
• Moving towards precision Higgs Physics
• …if something walks like a duck and quacks like a duck,
it is probably a duck. This particle is looking more and
more like a Higgs boson.
Conclusion and Outlook

• x
https://twiki.cern.ch/twiki/bin/view/AtlasPublic/HiggsPublicResults

Additional Slides

• x
Higgs Mass

• ATLAS-CONF-2012-101
ATLAS ET-mis Performance

Higgs couplings Overview
Good agreement with SM prediction
If µVBF tested alone  3.1 evidence for VBF production

Fermion vs. vector couplings
2-parameter model, group fermion and vector couplings together
κV = κZ = κW ; κF = κt = κb = κ = κg = κ
We assume only SM-loop contributions to kg and k
One overall un-observable sign, chose kV>0
Sign sensitivity mostly from interference in Hgg (t vs W in loop)
Only slight preference for kF>0  no conclusions yet
2D compatibility with SM 8%

• Custodial symmetry protects ratio of W, Z couplings (λWZ).
W vs Z couplings
λWZ = 0.80 ± 0.15 in agreement with SM prediction

HZZ(*) spin variables,
example: decay plane phi

HWW spin variables, example: deltaPhi

E. von Törne, Overview of the ATLAS Higgs Measurements

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