Proceedings of the Fifth Annual LHCP
ATL-PHYS-PROC-2017-106
November 7, 2018
Searches for rare and exotic Higgs decays with ATLAS
Marija Marjanovi´
c
On behalf of the ATLAS Collaboration,
Laboratoire de Physique de Clermont-Ferrand (LPC)
Universit´
e Clermont Auvergne, CNRS/IN2P3
Clermont-Ferrand, France
ABSTRACT
Searches for rare and exotic Higgs decays using proton-proton collision data with
the center-of-mass energy of 8 TeV and 13 TeV collected with the ATLAS
detector are presented. Various final states are considered. No significant
deviations from the Standard Model expectations are found. The results are
interpreted in different Beyond Standard Model theories.
PRESENTED AT
The Fifth Annual Conference
on Large Hadron Collider Physics
Shanghai Jiao Tong University, Shanghai, China
May 15-20, 2017
1
Introduction
Standard Model (SM) of particle physics predicts many possible decay modes for the Higgs boson. Up to now Higgs boson was observed in many of them [1, 2]. Some predicted Higgs boson decay modes have very low branching ratios (BR) [3], and are not observed until now. It is not expected that with the current datasets collected with the ATLAS detector [4] there is sensitivity to all of the predicted Higgs boson decay modes. However, it is still interesting to probe them in order to verify if there is an excess in any of them as it would be clear indication of new physics. Existing measurements constrain the non-SM or ”exotic” branching ratio of the Higgs boson decays to less than approximately 30% at 95% confidence level (CL). Some Beyond Standard Model (BSM) theories predict new decay modes of the Higgs boson in addition to those predicted by the SM. Exotic Higgs boson decays include new light resonances, as well as flavour violating decays and invisible decays.
2
Search for Higgs boson decays to φγ
A search [5] for the decays of the Higgs boson to a φ meson and a photon is performed with a pp collision data
sample corresponding to an integrated luminosity of 2.7 fb−1 collected at √s=13 TeV. The expected SM
branching fraction is B(H → φγ) = (2.3 ± 0.1) × 10−6[6], and there is no direct experimental evidence about
this decay mode currently. The decay φ → K+K− is reconstructed from pairs of oppositely charged inner
detector tracks. The background shape is generated from the templated final state particles kinematics and correlations. This background model is validated with data in samples with relaxed kinematic and isolation
criteria. Figure 1 on the left shows the distribution of mK+K−γ in data compared to the prediction of the
background model for a validation control sample
The mK+K−γ distributions of the selected φγ candidates, along with the results of the
maximum-likelihood fit with background-only model is shown in Figure 1 on the right. No significant excess of events is observed above the background, and 95% confidence level upper limit on the branching fraction of the
Higgs boson decay to φγ of 1.4 × 10−3 is obtained (600 times the expected SM branching fraction).
[GeV] γ -K + K m 0 50 100 150 200 250 300 Events / 5 GeV 0 20 40 60 80 100 120 140 160 180 200 -1 = 13 TeV, 2.7 fb s Data Background Model Model Shape Uncertainty
ATLAS Events/ 5 GeV 20 40 60 80 100 120 ATLAS -1 =13 TeV, 2.7 fb s Data Background Fit -3 )=10 γ φ → B(H -6 )=10 γ φ → B(Z [GeV] γ -K + K m 0 50 100 150 200 250 300 Data/Fit 0 0.51 1.52
Figure 1: The distribution of mK+K−γ in data compared to the prediction of the background model for a
validation control sample (on the left). The mK+K−γ distributions of the selected φγ candidates, along with
the results of the maximum- likelihood fit with background-only model (on the right) [5].
3
A search for the decays of the Higgs boson to J/Ψγ and Υ(nS)γ
Rare decays of the Higgs boson to a quarkonium state and a photon may offer unique sensitivity to both the magnitude and sign of the Yukawa couplings of the Higgs boson to quarks. The expected SM branching
fractions for these decays have been calculated to be B(H → J/Ψγ) = (2.8 ± 0.2) × 10−6 and B(H →
A search [8] for the decays of the Higgs boson to J/Ψγ and Υ(nS)γ is performed with pp collision data
samples corresponding to integrated luminosity of up to 20.3 fb−1 collected at √s=8 TeV. The decays
J/Ψ → µ+µ− and Υ(nS) → µ+µ− are used to reconstruct the quarkonium states. The main background
from inclusive multijet processes is modeled with a non-parametric data-driven approach using templates
to describe the kinematic distributions as described in Section 2. Figure 2 on the left presents the mµµγ
and pµµγT distributions of the selected J/ψγ candidates, along with the results of the unbinned maximum
likelihood fit to the signal and background model.
No significant excess of events is observed above expected backgrounds and 95% C.L. upper limits are placed on the branching fractions as shown in Figure 2 on the right. In the J/Ψγ final state the limits are
1.5 × 10−3 for the Higgs boson decays, while in the Υ(1S, 2S, 3S)γ final states the limits are (1.3, 1.9, 1.3) ×
10−3 respectively. [GeV] γ µ µ m 40 80 120 160 200 Events / 4 GeV 0 2 4 6 8 10 12 14 16 18 20 22 24 ATLAS =8 TeV s -1 Ldt = 19.2 fb ∫ Data S+B Fit Background ] -3 H [B=10 ] -6 Z [B=10 [GeV] γ µ µ T p 0 50 100 150 200 Events / 4 GeV 5 10 15 20 25 ATLAS =8 TeV s -1 Ldt = 19.2 fb ∫ Data S+B Fit Background ] -3 H [B=10 ] -6 Z [B=10 H→ J/ψγ H→ J/ψγ 19.2 fb−1 H→ Υ(1 S)γ H→ Υ(1S)γ 20.3 fb−1 H→ Υ(2 S)γ H→ Υ(2S)γ 20.3 fb−1 H→ Υ(3 S)γ H→ Υ(3S)γ 20.3 fb−1 H→ Υ(n S)γ H→ Υ(nS)γ 20.3 fb−1 Z→ J/ψγ Z→ J/ψγ 19.2 fb−1 Z→ Υ(1 S)γ Z→ Υ(1S)γ 20.3 fb−1 Z→ Υ(2 S)γ Z→ Υ(2S)γ 20.3 fb−1 Z→ Υ(3 S)γ Z→ Υ(3S)γ 20.3 fb−1 Z→ Υ(n S)γ Z→ Υ(nS)γ 20.3 fb−1 95 % C L up pe rl im it on B ra nc hi ng Fr ac tio n 10−6 10−5 10−3 10−2 H/Z→ Qγ Observed Expected (±1, 2σ) 95% CLs upper limit on Branching Fraction
.
ATLAS √s= 8 TeV
Figure 2: The mµµγ and pµµγT distributions of the selected J/ψγ candidates, along with the results of the
unbinned maximum likelihood fit to the signal and background model are shown on the left and in the middle. Summary of the expected and observed branching fraction limits in the various channels studied is shown on the right [8].
4
Search for the Zγ decay mode of the Higgs boson
This analysis [9] searches for the Zγ decay of the Higgs boson exploiting Z boson decays to pairs of electrons
or muons. It uses 36.1 fb−1of pp collisions at√s=13 TeV. The branching ratio for the Higgs boson decay to
Zγ is predicted by the SM to be B(H → Zγ) = (1.54 ± 0.09) × 10−3for a Higgs boson mass of 125.09 GeV.
Events are split into 6 exclusive event categories which are optimised to improve the sensitivity of the search and show 20% improvement in sensitivity with respect to the Run1 categories. Figure 3 shows the
invariant mass distributions mZγ for the ee and µµ channels which are displayed with the background-only
fit performed in the range of 115 < mZγ < 150 GeV. The variable pT t is the orthogonal component of the
transverse momentum of the Z system when projected onto the axis given by the difference of the 3-momenta of the Z boson and the photon candidate. No evidence of a localised excess is visible near the anticipated Higgs mass.
The observed p-value is 0.16 under the background-only hypothesis, in which the dominant contribution
comes from the µµ low-pT tcategory. The observed (expected - assuming SM pp → H → Zγ production and
decay) upper limit on the production cross section times the branching ratio for pp → H → Zγ is 6.6 (5.2) times the SM prediction at the 95% confidence level for a Higgs boson mass of 125.09 GeV.
5
Search for the Higgs boson produced in association with a W
boson and decaying to four b-quarks via two spin-zero particles
A dedicated search [10] for exotic decays of the Higgs boson to a pair of new spin-zero particles, H → aa,
[GeV] γ ll m Events / GeV 0 100 200 300 400 500 600 700 800 Data Background fit 20 × Signal = 125 GeV H m ATLAS -1 =13 TeV, 36.1 fb s Tt ee low p [GeV] γ Z m 115 120 125 130 135 140 145 150 Data - Fit −50 0 50 mllγ [GeV] Events / GeV 0 200 400 600 800 1000 Data Background fit 20 × Signal = 125 GeV H m ATLAS -1 =13 TeV, 36.1 fb s Tt low p µ µ [GeV] γ Z m 115 120 125 130 135 140 145 150 Data - Fit −50 0 50
Figure 3: The invariant Zγ mass (mZγ) distributions of events satisfying the H → Zγ selection in data for
the ee and µµ channels for the low pT t category [9].
recorded in 2015, corresponding to an integrated luminosity of 3.2 fb−1. The decay channel a → bb is the
preferred one when m(a) > 2m(b). The search is performed in events where the Higgs boson is produced in association with a W boson, giving rise to a signature of a lepton (electron or muon), missing transverse
momentum (Emiss
T ), and multiple jets from b-quark decays. The W H process is chosen because the charged
lepton in the final state allows to efficiently trigger and identify these events against the background process of strong production of four b-jets. The analysis uses several kinematic variables combined in a multivariate discriminant in signal regions.
The best fit of the background predictions to data in the binned maximum-likelihood fit is shown in Figure 4. No significant excess of events above the SM prediction is observed, and a 95% confidence-level upper limit is derived for the product of the production cross section for pp → W H times the branching ratio for the decay H → aa → 4b. Assuming the SM pp → W H cross section, it is not possible to set limits on the branching fraction with the amount of data used.
BDT output (4j, 4b) 0.6 − −0.4 −0.2 0 0.2 0.4 0.6 0.8 Data / Pred. 0.25 0.55 0.85 1.15 1.45 1.75 Events / 0.1 3 − 10 2 − 10 1 − 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 ATLAS -1 = 13 TeV, 3.2 fb s 4 jets, 4 b-tags = 60 GeV a 4b, m → 2a → H Data 2015 WH + light t t c + c t t b + b t t t Non-t [GeV] a m 20 30 40 50 60 BR [pb] × (WH) σ 95% C.L. upper limits on 0 10 20 Observed 95% CLs σ 1 ± Expected 95% CLs σ 2 ± Expected 95% CLs (WH) SM σ ATLAS s = 13 TeV, 3.2 fb-1
Figure 4: Comparison between the data and prediction for the distribution of the BDT discriminant used in the signal regions after the fit is performed on data under the background-only hypothesis (left). Upper
limit at 95% CL on σ(W H) × BR, where BR = B(H → aa) × B(a → bb)2, as a function of m
a [10].
6
Search for lepton-flavour-violating decays of the Higgs boson
Direct searches [11] for lepton flavour violation (LFV) in decay of the Higgs boson to H → eτ and H → µτ are performed based on the data sample of pp collisions corresponding to an integrated luminosity of 20.3
fb−1 at a centre-of-mass energy of√s = 8 TeV. The first study is a search for H → eτ decays in the final
state with one electron and one hadronically decaying τ -lepton, τhad. The second analysis is a simultaneous
search for the LFV H → eτ and H → µτ decays in the final state with a leptonically decaying τ -lepton, τlep.
A combination of results of the earlier ATLAS search for the LFV H → µτhad decays and the two searches
described here is also presented. The LFV signal is searched by fitting mMMC (for H → eτhad) and mcoll
(for H → eτlep and H → µτhad). Missing mass calculator (MMC) is a version of the collinear approximation
where relative orientations of the neutrino and other τ -lepton decay products are chosen to be consistent with the mass and kinematics of a τ -lepton decay.
No significant excess is observed, and upper limits on the LFV branching ratios are set at the 95 % confidence level: B(H → eτ ) < 1.04% , B(H → µτ ) < 1.43% as shown in Figure 5.
), % τ e → H 95% CL upper limit on Br( 0 2 4 6 8 10 12 14 ATLAS -1 L dt = 20.3 fb ∫ = 8 TeV s , Comb τ e , Comb lep τ e withJets , SR lep τ e noJets , SR lep τ e , Comb had τ e , SR2 had τ e , SR1 had τ e σ 1 ± Expected σ 2 ± Expected Observed Excluded ), % τ µ → H 95% CL upper limit on Br( 0 2 4 6 8 10 12 14 ATLAS -1 L dt = 20.3 fb ∫ = 8 TeV s , Comb τ µ , Comb lep τ µ withJets , SR lep τ µ noJets , SR lep τ µ , Comb had τ µ , SR2 had τ µ , SR1 had τ µ σ 1 ± Expected σ 2 ± Expected Observed Excluded
Figure 5: Upper limits on LFV decays of the Higgs boson in the H → eτ hypothesis (left) and H → µτ hypothesis (right). The limits are computed under the assumption that either B(H → µτ ) = 0 or B(H →
eτ ) = 0 [11]. The µτhadchannel is from [12].
7
Search for new phenomena in the Z(→ ll) + E
Tmissfinal state
A study [13] of the ll + Emiss
T (l = e, µ) final state is performed using 13.3 fb−1 of 13 TeV pp collision data in
2015 and the first half of 2016. The analysis searches for an invisibly decaying Higgs boson in the channel
ZH, Z → ll, H(→ invisible). The SM Higgs boson with a measured mass of mH = 125.09 ± 0.21(stat)
± 0.11(syst) GeV is predicted to have a small BF to invisible particles, ∼ 0.1% in the H → ZZ → νννν channel [3], which is far below the experimental sensitivity of the current analyses.
New physics is searched for as an excess over the SM predictions in the ZZ transverse mass mZZT (Eq. 1)
(mZZT )2= q m2 Z+ pll T 2 + q m2 Z+ Emiss T 22 − ~p ll T + ~E miss T 2 (1)
distribution and in the Emiss
T distribution. Figure 6 left presents the mZZT distributions in one signal region
for the combined ee + µµ channels. Results are found to be compatible with SM expectations. Figure 6 right presents a distribution of the confidence levels corresponding to each value of upper limits on σ(Z(→
ll)H(→ invisible)) divided by the SM prediction of the ZH production cross-section (with mH=125 GeV)
scanned from 0 to 1.4. The shown confidence levels can be interpreted as that on the upper limits of B(H → invisible), for the region with the x-axis value less than one. The expected and observed upper limit on B(H → invisible) at 95% CL is 65% and 98%, respectively.
[GeV] ZZ T m Events/50 GeV 3 − 10 2 − 10 1 − 10 1 10 2 10 3 10 4 10 5 10 [GeV] ZZ T m 0 200 400 600 800 1000 1200 1400 Data/Pred. 0 0.5 1 1.5 2 Data ZZ WZ Other Bkgs =300 GeV) H ggH(m =600 GeV) H ggH(m =1000 GeV) H ggH(m non-resonant-ll )+jets µ µ Z(ee)/Z( Fake Lepton Stat.+Sys. ATLAS Preliminary -1 =13 TeV, 13.3 fb s HMSR ee+µµ SM ZH σ inv.) / → BF(H × ZH σ 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1-CL 2 − 10 1 − 10 1 Observed Expected Median σ 1 ± Expected σ 2 ± Expected 95% CL 68% CL ATLAS Preliminary -1 = 13TeV, 13.3 fb s Figure 6: mZZ
T distributions in one signal region for the combined ee + µµ channels (left). The stacked
histograms represent the background predictions, while the blue, pink and cyan curves give the predicted signal distributions for a heavy Higgs boson with different masses. Confidence levels corresponding to upper
limits as a function of σZH× B(H → inv.) / σZHSM [13].
8
Conclusions
Searches for rare and exotic Higgs decays with the ATLAS detector with 8 TeV and 13 TeV data are presented. No significant deviations from the Standard Model are found. Limits on different models of Beyond Standard Model physics are extended or set for the first time. There are still more possibilities to explore to cover the full spectrum.
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