XXXV Reunión Bienal de la Real Sociedad Española de Física
Título Simposio 1
Search for Supersymmetry in events with a Z boson, jets and large missing transverse momentum in √s = 8 TeV pp collisions with the ATLAS detector
V.A. Mitsou1, E. Romero Adam1,*, E. Torró Pastor1
1Instituto de Física Corpuscular (IFIC), 46980 Paterna (Valencia), España
Introduction
Supersymmetry (SUSY) [1] is one of the most popular extensions of the Standard Model (SM) that can provide solutions to several of its shortcomings. It introduces supersymmetric particles (sparticles) that differ from their SM counterpart by half a unit of the spin. If strongly interacting sparticles have masses around the TeV scale, they should be produced at the LHC.
This contribution summarises a search for SUSY in final states containing a leptonically- decaying Z boson, jets and large missing transverse momentum (ETmiss) [2]. The proton-proton collision data used in this search were collected at a centre-of-mass energy √s = 8 TeV by the ATLAS detector [3] at the Large Hadron Collider (LHC) and correspond to an integrated luminosity of 20.3 fb-1.
An excess of events above the expected SM background is observed, with a significance of three standard deviations. The results are interpreted in the context of a generalised gauge-mediated (GGM) supersymmetry-breaking model where the gravitino is the lightest supersymmetric particle (LSP) and the next-to-lightest SUSY particle (NLSP) is a higgsino-like neutralino.
Event selection
Events are required to contain at least two same-flavoured leptons (electrons or muons) with opposite electric charge. If more than two leptons are present, the two with the largest values of pT
are selected. The leading (highest pT) lepton, must have a pT > 25 GeV, whereas the subleading lepton pT can be as low as 10 GeV. Their invariant mass must fall within the Z boson mass window, here considered as 81 < mll < 101 GeV. In addition, all events are required to contain at least two jets of pT > 35 GeV and |η| < 2.5 (signal jets) and to have ETmiss > 225 GeV and HT > 600 GeV, where HT is the scalar sum of the pT of all signal jets and the two leading leptons. Furthermore, the azimuthal angle between each of the two leading jets and the ETmiss is required to be ∆φ > 0.4.
Background estimation
A great effort has been made to accurately estimate the number of SM events that survive the previous selection. The dominant background processes and those that are expected to be most difficult to model using MC simulation are estimated using data-driven techniques. A brief description of the considered backgrounds and their estimation methods is given below.
The dominant backgrounds come from so-called “flavour-symmetric” processes. Here the branching fractions to ee, µµ and eµ have a 1:1:2 ratio such that the same-flavour contributions can be estimated with data using information from the different-flavour contribution. This group of backgrounds is dominated by ttbar and also includes WW, Single Top (Wt) and Z →ττ, and makes up ~ 60% of the predicted SM background.
Diboson backgrounds with real Z boson production contribute up to 25% of the total background. These are estimated using MC simulation, as are “Rare Top” backgrounds, which include ttbar + W, ttbar + Z, and t + Z processes. The Rare Top contribution is less than 5%.
Processes with “fake leptons”, i.e. jets mis-reconstructed as leptons, contribute up to 10% and are estimated using the Matrix Method [4], which is a data-driven method widely used in most of ATLAS analyses.
XXXV Reunión Bienal de la Real Sociedad Española de Física
Título Simposio 2
Finally, there is the special case of the Z/γ* + jets background. Since only Z → ee and Z → µµ (and not Z →ττ) decays are selected, no real ETmiss can be produced in these events. Consequently, high ETmiss in events from this background (usually called instrumental or fake ETmiss) is due to jet mismeasurements. Given that the Z/γ* + jets background could mimic a possible signal, particular care has been taken to suppress it as much as possible (the Δφ cut described in the previous section reduces this background to negligible levels) and to estimate its remaining, although negligible, contribution as precisely as possible. To perform this estimation, given the difficulties of modelling instrumental ETmiss in MC simulation, the data-driven “jet smearing” method [5] is applied. This method provides an estimate for the contribution from events containing both fake ETmiss, from object mismeasurements, and real ETmiss, from neutrinos in heavy flavour quark decays.
Results and interpretation
The resulting background estimates along with the observed event yields are displayed in Table 1, showing that the data exceeds the background expectations with a significance of 3.0, 1.7 and 3.0 standard deviations (σ) in the electron, muon and combined channels respectively. This significance is calculated, using pseudo-experiments, as the local probability for the background estimate to produce a fluctuation greater than or equal to the excess observed in the data.
The results are interpreted in a GGM model where the gravitino is the LSP and a higgsino-like neutralino is the NLSP.
The higgsino mass (µ) and the gluino mass are free parameters. The U(1) and SU(2) gaugino mass parameters, M1 and M2, are fixed to be 1 TeV, and the masses of all other sparticles are set at ~ 1.5 TeV.
In addition, µ is set to be positive and the ratio of the vacuum expectation value for
the two Higgs doublets (tanß) is set to 1.5. Figure 1 shows the expected and observed exclusion contours for this GGM model. The ±1σexp and ±2σexp experimental uncertainty bands indicate the impact on the expected limit of all uncertainties considered on the background processes. The
±1σSUSYtheory uncertainty lines around the observed limit illustrate the change in the observed limit as the nominal signal cross section is scaled up and down by the theoretical cross-section uncertainty. Given the observed excess of events with respect to the SM prediction, the observed limits are weaker than expected. Updating these interesting results, once the LHC Run2 data become available, is among the priorities of the ATLAS SUSY Working Group.
References
[1] S.P. Martin, arXiv:hep-ph/9709356 (1997).
[2] ATLAS Collaboration, arXiv:1503.03290 [hep-ex], CERN-PH-EP-2015-038.
[3] ATLAS Collaboration, JINST 3 (2008) S08003, doi:10.1088/1748-0221/3/08/S08003.
[4] ATLAS Collaboration, Eur. Phys. J. C 71, (2011) 1577, doi:10.1140/epjc/s10052-011-1577-6.
[5] ATLAS Collaboration, Phys. Rev. D 87 (2013) 012008, doi:10.1103/PhysRevD.87.012008.
Table 1: Number of observed data events, total number of expected background events and Gaussian significance for the electron, muon and combined channels.
Uncertainties are statistical and systematic combined.
Channel ee µµ ee+µµ
Observed 16 13 29
Expected background 4.2 ± 1.6 6.4 ± 2.2 10.6 ± 3.2 Gaussian significance 3.0σ 1.7σ 3.0σ
Figure 1: The 95% CL exclusion limit from the combined same-flavour channels in the higgsino mass (µ) versus gluino mass plane in the GGM model with tanß=1.5.
[GeV]
µ
200 400 600 800 1000
) [GeV]g~m(
600 700 800 900 1000 1100 1200 1300 1400
0) χ1
∼ ) < m(
~g m(
200 300 400 500 600 700 0) [GeV]
χ1 m(∼ )=1.5 TeV q~ = 1 TeV, m(
= M2
= 1.5, M1
; tanβ
0
χ1
GGM: higgsino-like ∼
=8 TeV, 20.3 fb-1
s µ SR-Z ee+µ
ATLAS theory)
σSUSY
1
± Observed limit (
exp) 1 σ Expected limit (±
exp) σ 2
± Expected limit (
