Search for the associated production of the Higgs boson with a top quark pair in multi-lepton final states with the ATLAS detector

ATLAS-CONF-2015-006

17 March 2015

These preliminary results are superseded by the following paper:

HIGG-2013-26
ATLAS recommends to use the results from the paper.

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Main document (CDS record), Physics Briefing - internal pdf from CDS
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Abstract
A search for the associated production of the Higgs boson with a top quark pair is performed in multi-lepton final states using 20.3 fb$^{-1}$ of proton-proton collision data recorded by the ATLAS experiment at $\sqrt{s}=8$ TeV at the Large Hadron Collider. Five final states, targeting the decays $H\to WW^*$, $\tau\tau$, and $ZZ^*$, are examined for the presence of the Standard Model (SM) Higgs boson: two same-charged light leptons ($e$ or $\mu$) without an additional hadronically decaying tau; three light leptons; two same-charged light leptons with an additional hadronically decaying tau; four light leptons; and one light lepton and two hadronically decaying taus. No significant excess of events is observed above the background expectation. The best fit for the $t\bar t H$ production cross section, assuming a Higgs boson mass of 125 GeV, is $2.1 ^{+1.4}_{-1.2}$ times the SM expectation, and the observed (expected) upper limit at the 95% confidence level is 4.7 (2.4) times the SM rate. The $p$-value for compatibility with the background-only hypothesis is $1.8\sigma$; the expectation in the presence of a Standard Model signal is $0.9\sigma$.
Figures
Figure 01a:
The spectrum of the number of jets expected and observed in the ttZ (left) and ttW (right) validation regions (VR). The hatched band represents the total uncertainty on the background prediction in each bin. The "non-prompt" backgrounds are those with a lepton arising from a hadron decay or from a photon conversion in detector material. Rare processes include tZ, ttWW, triboson, tttt, and tH production. The red line corresponds to the ttH signal predicted by the SM.
png (211kB)  eps (27kB)  pdf (9kB) 
Figure 01b:
The spectrum of the number of jets expected and observed in the ttZ (left) and ttW (right) validation regions (VR). The hatched band represents the total uncertainty on the background prediction in each bin. The "non-prompt" backgrounds are those with a lepton arising from a hadron decay or from a photon conversion in detector material. Rare processes include tZ, ttWW, triboson, tttt, and tH production. The red line corresponds to the ttH signal predicted by the SM.
png (190kB)  eps (23kB)  pdf (7kB) 
Figure 02a:
The spectrum of the number of jets expected and observed in each signal region. For display purposes the six 2l0tau categories (ee/eμ/μμ and =4/≥ 5 jets) are combined into one plot, as are the two 4l categories (Z-enriched and Z-depleted). The hatched bands show the total uncertainty on the background prediction in each bin. The non-prompt/charge mis-id background spectra are taken from simulation of ttbar, single top, Z to ll+jets, and other small backgrounds, with normalization as described in the text.
png (128kB)  eps (19kB)  pdf (7kB) 
Figure 02b:
The spectrum of the number of jets expected and observed in each signal region. For display purposes the six 2l0tau categories (ee/eμ/μμ and =4/≥ 5 jets) are combined into one plot, as are the two 4l categories (Z-enriched and Z-depleted). The hatched bands show the total uncertainty on the background prediction in each bin. The non-prompt/charge mis-id background spectra are taken from simulation of ttbar, single top, Z to ll+jets, and other small backgrounds, with normalization as described in the text.
png (128kB)  eps (19kB)  pdf (6kB) 
Figure 02c:
The spectrum of the number of jets expected and observed in each signal region. For display purposes the six 2l0tau categories (ee/eμ/μμ and =4/≥ 5 jets) are combined into one plot, as are the two 4l categories (Z-enriched and Z-depleted). The hatched bands show the total uncertainty on the background prediction in each bin. The non-prompt/charge mis-id background spectra are taken from simulation of ttbar, single top, Z to ll+jets, and other small backgrounds, with normalization as described in the text.
png (109kB)  eps (16kB)  pdf (6kB) 
Figure 02d:
The spectrum of the number of jets expected and observed in each signal region. For display purposes the six 2l0tau categories (ee/eμ/μμ and =4/≥ 5 jets) are combined into one plot, as are the two 4l categories (Z-enriched and Z-depleted). The hatched bands show the total uncertainty on the background prediction in each bin. The non-prompt/charge mis-id background spectra are taken from simulation of ttbar, single top, Z to ll+jets, and other small backgrounds, with normalization as described in the text.
png (79kB)  eps (15kB)  pdf (5kB) 
Figure 02e:
The spectrum of the number of jets expected and observed in each signal region. For display purposes the six 2l0tau categories (ee/eμ/μμ and =4/≥ 5 jets) are combined into one plot, as are the two 4l categories (Z-enriched and Z-depleted). The hatched bands show the total uncertainty on the background prediction in each bin. The non-prompt/charge mis-id background spectra are taken from simulation of ttbar, single top, Z to ll+jets, and other small backgrounds, with normalization as described in the text.
png (174kB)  eps (19kB)  pdf (7kB) 
Figure 03:
Best fit values of the signal strength parameter μ = σttH, obs/σttH, SM. For the 4l Z-depleted category, μ < -0.17 results in a negative expected total yield and so the lower uncertainty is truncated at this point.
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Figure 04:
95% CL limits on μ, determined via the CLs method, for various categories and their combination.
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Figure 05:
Best fit values of the signal strength parameter μ = σtanti-tH, obs/σtanti-tH, SM, for the 2ℓnotau categories, where the 4 jet and ≥ 5 jet categories are combined for each of the ee, eμ, and μμ lepton final states.
png (71kB)  eps (71kB)  pdf (29kB) 
Figure 06:
The invariant mass of the two leptons in the 2ℓnotau signal regions (all six categories are combined).
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Figure 07:
The leading lepton pT in the 2l0tau signal regions (all six categories are combined).
png (232kB)  eps (37kB)  pdf (12kB) 
Figure 08:
The sub-leading lepton pT in the 2l0tau signal regions (all six categories are combined).
png (211kB)  eps (36kB)  pdf (11kB) 
Figure 09:
Number of electrons per event in the 3l signal region.
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Figure 10:
Number of electrons per event in the ttW validation region.
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Figure 11:
The invariant mass of the opposite-sign lepton pair with smaller separation Δ R in the ttZ validation region.
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Figure 12:
Event display for the single candidate event in the 2l1tau category. The red track is the selected muon; the green tower is the selected electron; the yellow cone is the selected tau; the blue cones are the selected b-tagged jets; and the white cones are the selected non-b-tagged jets. The selected μ+ has pT = 42 GeV; the e+ has pT = 16 GeV; the τ- candidate has pT = 52 GeV. The jets have pT 85, 53, 76, and 26 GeV, of which the first two are b-tagged.
png (972kB) 
Figure 13:
Event display for the single candidate event in the 2l1tau category. The red track is the selected muon; the green track is the selected electron; the yellow cone is the selected tau; the blue cones are the selected b-tagged jets; and the white cones are the selected non-b-tagged jets. The selected μ+ has pT = 42 GeV; the e+ has pT = 16 GeV; the τ- candidate has pT = 52 GeV. The jets have pT 85, 53, 76, and 26 GeV, of which the first two are b-tagged.
png (2MB) 
Figure 14:
Event display for a candidate event in the 1l2tau category. The τhadτhad system has mvis = 66 GeV.
png (15MB)  pdf (3MB) 
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2024-05-19 01:21:25