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Discovery potential for GMSB models with tau final states

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ATL-PHYS-PUB-2009-089

Abstract

Gauge Mediated Supersymmetry Breaking (GMSB) models provide a possible mechanism to mediate supersymmetry to the visible sector. In these models the lightest supersymmetric particle is usually the gravitino, while the next-to-lightest supersymmetric particle (NLSP) is either a neutralino or a slepton. In the case of a stau NLSP, events with large missing transverse energy, highly energetic jets and up to four tau leptons are expected in pp-collisions at the LHC providing a powerful channel to probe the GMSB theory. In this note we study the expected performance of the ATLAS detector in GMSB scenarios with a stau NLSP for a LHC centre-of-mass energy of sqrt(s) = 10TeV. A cutbased selection is optimised using an example GMSB signal and a scan of the GMSB parameter space is performed to determine the discovery reach as a function of the integrated luminosity. In addition, the invariant mass distribution of two tau leptons is used to study the measurement of masses of supersymmetric particles with larger event samples.

Plots

Figure 1
Nature of the NLSP in an example (Λ-tan β)-plane of the GMSB parameter space (Mm = 250 TeV, N5 = 3, sgn μ = +, Cgrav = 1).
Fig1.png
eps pdf png
Figure 2
SUSY mass spectrum for the benchmark scenario GMSB6. The $\PSgt_1$ (NLSP) has a mass of 102.8 GeV. The quasi-massless gravitino (2.4 eV) is not shown.
Fig2.png
eps pdf png
Figure 3
Comparison of the full (circles) and fast (triangles) simulation:
(a) ETmiss,
(b) number of jets with pT > 20 GeV,
(c) pT of the leading jet,
(d) pT of the second leading jet,
(e) number of τ leptons with pT > 15 GeV,
(f) pT of the leading τ lepton.
Fig3_a.png Fig3_b.png
(a) eps pdf png (b) eps pdf png
Fig3_c.png Fig3_d.png
(c) eps pdf png (d) eps pdf png
Fig3_e.png Fig3_f.png
(e) eps pdf png (f) eps pdf png
Figure 4
Signal preselection:
(a) total event numbers after each step of the preselection;
event distributions after the preselection for the GMSB signal and the various SM backgrounds:
(b) missing transverse energy,
(c) number of τ leptons with pT > 15 GeV (pT > 20 GeV for the leading τ).
Fig4_a.png
(a) eps pdf png
Fig4_b.png Fig4_c.png
(b) eps pdf png (c) eps pdf png
Figure 5
Signal significance (S = NS/sqrt(N)B) as a function of the cut values of ETmiss and Nτ. The maximum significance is obtained for ETmiss > 280 GeV and Nτ ≥ 2.
Fig5.png
eps pdf png
Figure 6
Event distributions after the final selection for the GMSB signal and the various SM backgrounds:
(a) ETmiss after the Nτ ≥ 2 cut,
(b) number of τ leptons with pT > 15 GeV (pT > 20 GeV for the leading τ) after the ETmiss > 280 GeV cut.
Fig6.png Fig6.png
(a) eps pdf png (b) eps pdf png
Figure 7
Total SUSY cross section in pb at sqrt(s) = 10 TeV in the (Λ-tan β)-plane for Mm = 250 TeV, N5 = 3, sign μ = + and Cgrav = 1. It strongly depends on Λ due to the increase of the masses of the SUSY particles with increasing Λ.
Fig7.png
eps pdf png
Figure 8
Expected number of selected signal events for L = 200 pb-1 in the (Λ-tan β)-plane for Mm = 250 TeV, N5 = 3, sgn μ = + and Cgrav = 1. The expected number of corresponding background events is NB = 2.5.
Fig8.png
eps pdf png
Figure 9
Integrated luminosity needed for a signal significance of S = 5 or Zn = 5, respectively, in the (Λ-tan β)-plane for Mm = 250 TeV, N5 = 3, sgn μ = + and Cgrav = 1 using
(a) the simple calculation of the significance following Eq. (2) which neglects the uncertainty on the SM background and
(b) using Eq. (3) which properly includes this uncertainty.
Fig9.png Fig9.png
(a) eps pdf png (b) eps pdf png
Figure 10
Distribution of events in the plane of ETmiss and pT of the leading jet for
(a) the signal and
(b) the SM background for 8 fb-1.
The elliptical cut is indicated by the hashed region. All events outside this region are selected.
Fig10.png Fig10.png
(a) eps pdf png (b) eps pdf png
Figure 11
Invariant mass distribution of two τ leptons originating from different decay processes for the GMSB6 signal:
(a) generator level,
(b) reconstruction level for selected events.
Fig11.png Fig11.png
(a) eps pdf png (b) eps pdf png
Figure 12
Invariant mass distribution of
(a) any two τ leptons for the GMSB6 signal and the SM background after the selection and
(b) the same-sign distribution (SS) subtracted from the opposite-sign distribution (OS) for L = 8 fb-1.
Fig12.png Fig12.png
(a) eps pdf png (b) eps pdf png
Figure 13
Example fits of the invariant mass (OS-SS) distribution using different fit ranges.
Fig13.png
eps pdf png

Tables

Table 1
SM background processes.
Generator Process Cross section [pb]
MC@NLO tt 400
single t 48
di-boson 9.2
ALPGEN Z+jets 4.4 x 103
W+jets 4.8 x 104
Pythia di-jet 1.3 x 1010
γ-jet 2.2 x 105
Table 2
Total numbers of selected events for the signal and various SM background processes at different stages of the event selection for L = 200 pb-1 and sqrt(s) = 10 TeV. The uncertainties given correspond to statistical uncertainties from the limited MC statistics. Some background processes give contributions significantly below one event after the final selection due to the selection cuts on the jet pT and ETmiss and are therefore negligible. The reason for the large rejection of dijets is because only a small fraction of such events have a sufficiently hard scatter to pass all selection requirements
Process Initial Preselection Final selection
GMSB6 240.0 +- 2.5 102.1 +- 1.6 20.4 +- 0.7
tt, single t (8.96 +- 0.01) x 104 1301 +- 11 1.1 +- 0.3
Di-boson (1.84 +- 0.08) x 103 3.7 +- 0.4 -
Z+jets (8.836 +- 0.008) x 105 182.1 +- 4.3 0.2 +- 0.1
W+jets (9.692 +- 0.005) x 106 2058 +- 45 1.1 +- 1.1
Di-jet (2.537 +- 0.004) x 1012 (3.5 +- 1.7) x 103 -
γ-jet (4.40 +- 0.01) x 107 20.1 +- 6.1 -


Major updates:
-- WolfgangEhrenfeld - 26-Nov-2009

Responsible: JanetDietrich WolfgangEhrenfeld
Last reviewed by: Never reviewed

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