Search for the Standard Model Higgs boson in the decay channel H->ZZ(*)->4l with 4.8 fb-1 of pp collision data at sqrt(s) = 7 TeV with ATLAS

TL;DR

This ATLAS study searches for the Standard Model Higgs boson in the H→ZZ(*)→4l decay channel using 4.8 fb^-1 of pp collision data at √s=7 TeV. It employs three four-lepton final states and data-driven background constraints to set 95% CL upper limits on the Higgs production cross section across m_H from 110 to 600 GeV. The results exclude multiple mass ranges (134–156, 182–233, 256–265, 268–415 GeV), with modest local excesses near 125, 244, and 500 GeV that are not significant globally after the look-elsewhere effect. The analysis demonstrates the power of the four-lepton channel and provides stringent constraints on Higgs production in the high-mass region, contributing to the overall Higgs boson search program.

Abstract

This Letter presents a search for the Standard Model Higgs boson in the decay channel H->ZZ(*)->l+l-l'+l'-, where l,l'=e or mu, using proton-proton collisions at sqrt(s) = 7 TeV recorded with the ATLAS detector and corresponding to an integrated luminosity of 4.8 fb^-1. The four-lepton mass distribution is compared with Standard Model background expectations to derive upper limits on the cross section of a Standard Model Higgs boson with a mass between 110 GeV and 600 GeV. The mass ranges 134-156 GeV, 182-233 GeV, 256-265 GeV and 268-415 GeV are excluded at the 95% confidence level. The largest upward deviations from the background-only hypothesis are observed for Higgs boson masses of 125, 244 and 500 GeV with local significances of 2.1, 2.2 and 2.1 standard deviations, respectively. Once the look-elsewhere effect is considered, none of these excesses are significant.

Figures (6)

• Figure 1: Invariant mass distributions for simulated \ref{['fig:sub_higgs4mu']}$H\rightarrow ZZ^{(*)}\rightarrow 4\mu$ and \ref{['fig:sub_higgs4e']}$H\rightarrow ZZ^{(*)}\rightarrow 4e$ events for $m_{H}=130\:\mathrm{\ Ge V} \textrm{Ge V}$. The fitted range for the Gaussian is chosen to be : $-2~\sigma$ to $2~\sigma$ ($-1.5~\sigma$ to $2.5~\sigma$) for the $4\mu$ ($4e$) channel. The reduced mean value of the reconstructed invariant mass in the $4e$ channel arises from energy losses due to bremsstrahlung Aad:2011mk. The fraction of events outside the $\pm 2 \sigma$ region is found to be $15\%$ for $4\mu$ and $18\%$ for $4e$.
• Figure 2: Invariant mass distributions of the lepton pairs in the control sample defined by a $Z$ boson candidate and an additional same-flavour lepton pair. The sample is divided according to the flavour of the additional lepton pair. In \ref{['fig:sub_2crmu']} the $m_{12}$ and in \ref{['fig:sub_crmu']} the $m_{34}$ distributions are presented for $Z(\rightarrow \mu^+\mu^-/e^+e^-)+\mu\mu$ events. In \ref{['fig:sub_2cre']} the $m_{12}$ and in \ref{['fig:sub_cre']} the $m_{34}$ distributions are presented for $Z(\rightarrow \mu^+\mu^-/e^+e^-)+ee$ events. The kinematic selections of the analysis are applied. Isolation requirements are applied to the first lepton pair only.
• Figure 3: Invariant mass distributions \ref{['fig:sub_m12data']}$m_{12}$ and \ref{['fig:sub_m34data']}$m_{34}$ for the selected candidates. The data (dots) are compared to the background expectations from the dominant $ZZ^{(*)}$ process and the sum of $t\bar{t}$, $Zb\bar{b}$ and $Z+\rm{light~jets}$ processes. Error bars represent 68.3% central confidence intervals.
• Figure 4: $m_{4\ell}$ distribution of the selected candidates, compared to the background expectation for \ref{['fig:sub_zoom_range']} the $100-250\:\mathrm{\ Ge V} \textrm{Ge V}$ mass range and \ref{['fig:sub_full_range']} the full mass range of the analysis. Error bars represent 68.3% central confidence intervals. The signal expectation for several $m_{H}$ hypotheses is also shown. The resolution of the reconstructed Higgs mass is dominated by detector resolution at low $m_{H}$ values and by the Higgs boson width at high $m_{H}$.
• Figure 5: The expected (dashed) and observed (full line) 95% CL upper limits on the Standard Model Higgs boson production cross section as a function of $m_H$, divided by the expected SM Higgs boson cross section. The dark (green) and light (yellow) bands indicate the expected limits with $\pm 1\sigma$ and $\pm 2\sigma$ fluctuations, respectively.