DOI 10.1393/ncc/i2014-11717-3 Colloquia: IFAE 2013
IL NUOVO CIMENTO Vol. 37 C, N. 1 Gennaio-Febbraio 2014
Observation of the new Higgs-like particle in the H
→ ZZ
∗→ 4l
searches with the ATLAS detector
G. Mancini
INFN, Laboratori Nazionali di Frascati - Frascati, Italy and Universit`a di Roma Tor Vergata - Roma, Italy
ricevuto l’1 Ottobre 2013
Summary. — An update on the search results of the newly observed Higgs-like
boson in the H → ZZ∗ → 4l decay channel using 20.7 fb−1 at √s = 8 TeV and
4.6 fb−1at√s = 7 TeV of data recorded with ATLAS detector at the LHC is given.
A clear excess in data has been observed for mH = 124.3 GeV; the event selection, the background estimations with data driven methods and the main results are presented.
PACS 14.80.Bn – Standard-model Higgs bosons.
1. – Introduction
The H → ZZ∗ → 4l decay channel is referred to as the Golden Channel because, despite its low rate (σ× BR (at mH = 125 GeV)∼ 2.5 fb) it has high purity (the ratio Signal/Background being 1.6 at mH = 125 GeV) and the signal is characterized by a
narrow peak in the 4 leptons invariant mass system.
2. – Event selection
The event selection relies on a cut based analysis in order to select Higgs candidates requiring two pair of same flavoured, opposite charged and high pT leptons coming from
the primary vertex and isolated. Final State Radiation (FSR) photons have been also taken into account, the on-shell Z boson mass has been constrained to the PDG value and the number of mis-pairings leptons to Z has been minimized.
3. – Higgs mass and signal strength
The 4 lepton invariant mass for the selected candidates compared to the back-ground expectation is shown in fig. 1 (upper left). This reflects in a deviation from the background-only hypothesis, that is maximum for mH = 124.3 GeV: the local p0-value observed is 2.7× 10−11 or 6.6 σ (fig. 1 (upper right)). Such results confirm
what observed [1] and provides the single channel discovery [2]. The best fit values of the signal strength (μ = σobs/σSM) and mH are shown in fig. 1 together with the
c CERN
300 G. MANCINI [GeV] 4l m 80 100 120 140 160 Events/2.5 GeV 0 5 10 15 20 25 30 -1 Ldt = 4.6 fb ∫ = 7 TeV: s -1 Ldt = 20.7 fb ∫ = 8 TeV: s 4l → (*) ZZ → H Data (*) Background ZZ t Background Z+jets, t =125 GeV) H Signal (m Syst.Unc. Preliminary ATLAS [GeV] H m 110 120 130 140 150 160 170 180 0 Local p -13 10 -11 10 -9 10 -7 10 -5 10 -3 10 -1 10 10 3 10 5 10 Obs 2012 Exp 2012 Obs 2011 Exp 2011 Obs Combination Exp Combination Preliminary ATLAS 4l → (*) ZZ → H -1 Ldt =4.6 fb ∫ =7 TeV: s -1 Ldt =20.7 fb ∫ =8 TeV: s σ 2 σ 3 σ 1 σ 4 σ 5 σ 6 σ 7 [GeV] H m 122 123 124 125 126 127 128 )μ Signal strength ( 0 1 2 3 4 5 4l → (*) ZZ → H Best fit 68% CL 95% CL Preliminary ATLAS -1 Ldt = 4.6 fb ∫ = 7 TeV: s -1 Ldt = 20.7 fb ∫ = 8 TeV: s
without MSS(e) and ) in lighter colours μ MSS( [GeV] 12 m 50 60 70 80 90 100 Events/4 GeV 0 10 20 30 40 50 60 Data fit t Z+jets and t fit t t ZZ Z+jets t t WZ Preliminary ATLAS μ 4 -1 Ldt = 20.7 fb ∫ = 8 TeV: s
Fig. 1. – Upper left: invariant mass of the 4 lepton system. Upper right: local p0 as a function
of the Higgs mass. Lower left: profile likelihood ratio as a function of μ and mH. Lower right: invariant mass of the leading leptons from the CR.
profile likelihood ratio contours corresponding to the 68% CL and the 95% CL: μ (at
mH= 124.3 GeV)= 1.7+0.5−0.4(stat.).
4. – Reducible background (red bkg) from Control Region (CR)
Data driven methods need to be used for the red bkg estimation in the signal region due to the high rejection factor and the theoretical errors on the cross sections; while for the irreducible bkg (ZZ∗), MC samples have been used, since the agreement with data has been verified to be within 10%. CRs (bkg enriched selections obtained inverting isolation and impact parameter requirements) have been defined in order to check the red bkg distribution and to determine its normalization. Figure 1 shows the contribution of the red bkg in the CR, with the Z + b¯b component rescaled with a 1.7 scale factor.
5. – Conclusions
The observation of the Higgs-like particle is fully confirmed in the H → 4l decay channel [2]. The integrated luminosity recorded at√s = 7 TeV and √s = 8 TeV led to
the single channel discovery. REFERENCES
[1] ATLAS collaboration, Phys. Lett. B, 716 (2012) 1.
[2] ATLAS collaboration, Measurements of the properties of the Higgs-like boson in the four
lepton decay channel with the ATLAS detector using 25 fb−1of proton-proton collision data,
