arXiv:1204.6736v1 [hep-ex] 30 Apr 2012
EUROPEAN ORGANISATION FOR NUCLEAR RESEARCH (CERN)
CERN-PH-EP-2012-097
Submitted to: Physics Letters B
Search for Scalar Top Quark Pair Production in Natural Gauge Mediated Supersymmetry Models with the ATLAS Detector in pp
Collisions at √
s = 7 TeV
The ATLAS Collaboration
Abstract
The results of a search for pair production of the lighter scalar partners of top quarks (t˜1) in 2.05fb−1ofppcollisions at√
s = 7TeV using the ATLAS experiment at the LHC are reported. Scalar top quarks are searched for in events with two same flavour opposite-sign leptons (e, µ) with invariant mass consistent with the Z boson mass, large missing transverse momentum and jets in the final state. At least one of the jets is identified as originating from a b-quark. No excess over Standard Model expectations is found. The results are interpreted in the framework of R-parity conserving, gauge mediated Supersymmetry breaking ‘natural’ scenarios, where the neutralino (χ˜01) is the next-to- lightest supersymmetric particle. Scalar top quark masses up to 310 GeV are excluded for 115 GeV<
mχ˜01 <230 GeV at 95% confidence level, reaching an exclusion ofm˜t1 <330 GeV formχ˜01 = 190GeV.
Scalar top quark masses below240GeV are excluded for all values ofmχ˜01 > mZ.
Search for Scalar Top Quark Pair Production in Natural Gauge Mediated Supersymmetry Models with the ATLAS Detector in pp Collisions at √
s = 7 TeV
The ATLAS Collaboration
Abstract
The results of a search for pair production of the lighter scalar partners of top quarks (˜t1) in 2.05 fb−1of pp collisions at
√s = 7 TeV using the ATLAS experiment at the LHC are reported. Scalar top quarks are searched for in events with two same flavour opposite-sign leptons (e, µ) with invariant mass consistent with the Z boson mass, large missing transverse momentum and jets in the final state. At least one of the jets is identified as originating from a b-quark. No excess over Standard Model expectations is found. The results are interpreted in the framework of R-parity conserving, gauge mediated Supersymmetry breaking ‘natural’ scenarios, where the neutralino ( ˜χ01) is the next-to-lightest supersymmetric particle. Scalar top quark masses up to 310 GeV are excluded for 115 GeV < mχ˜01 < 230 GeV at 95% confidence level, reaching an exclusion of m˜t1 < 330 GeV for mχ˜01 = 190 GeV. Scalar top quark masses below 240 GeV are excluded for all values of mχ˜01 > mZ.
1. Introduction
Supersymmetry (SUSY) [1–9] provides an extension to the Standard Model (SM) which can resolve the hierar- chy problem. For each known boson or fermion, SUSY introduces a particle (sparticle) with identical quantum numbers except for a difference of half a unit of spin.
The non-observation of the sparticles implies that SUSY is broken and the superpartners are generally heavier than the SM partners. In the framework of a generic R-parity conserving minimal supersymmetric extension of the SM (MSSM) [10–14], SUSY particles are produced in pairs and the lightest supersymmetric particle (LSP) is stable.
The scalar partners of right-handed and left-handed quarks, ˜qR and ˜qL, can mix to form two mass eigenstates.
In the case of the scalar top quark (˜t, stop), large mixing effects due to the Yukawa coupling, yt, and the trilinear coupling, At, can lead to one stop mass eigenstate, ˜t1, that is significantly lighter than other squarks. Consequently, the ˜t1 could be produced with large cross sections at the LHC via direct pair production.
Light stop masses are favoured by arguments of ‘natu- ralness’ of electroweak symmetry breaking [15], because of the possibly large coupling between the ˜t and the Higgs boson, h. In particular, radiative corrections to the Higgs boson mass mainly arise from the stop-top loop diagrams including top Yukawa and three-point stop-stop-Higgs in- teractions.
In gauge mediated SUSY breaking (GMSB) models [16–
21], gauge interactions (messengers) are responsible for the appearance of soft supersymmetry breaking terms. If the characteristic scale of the masses of the messenger fields is about 10 TeV, an upper bound on m˜t1of about 400 GeV is found when imposing the absence of significant (∼ 10%)
fine tuning [15].
In GMSB, the gravitino ˜G is the LSP (in general mG˜ ≪ 1 keV). The experimental signatures are largely determined by the nature of the next-to-lightest SUSY particle (NLSP). For several GMSB models the NLSP is the lightest neutralino, ˜χ01, promptly decaying to its lighter SM partner through gravitino emission. Neu- tralinos are mixtures of gaugino ( ˜B, ˜W0) and higgsino ( ˜Hu0, ˜Hd0) gauge-eigenstates, and therefore the lightest neutralino decays to either a γ, Z or Higgs boson. If the ˜χ01 is higgsino-like, it decays either via ˜χ01 → h ˜G or ˜χ01 → Z ˜G. Light higgsinos lead to a large higgsino component in ˜χ01 and a small mass difference between ˜χ01 and ˜χ±1. In particular, if the higgsino mass (|µ|) is much smaller than the gaugino masses (pure higgsino case), ˜χ01 and ˜χ±1 are almost degenerate such that the (f f′) system resulting from the chargino decay ˜χ±1 → ˜χ01f f′is very soft.
In this Letter, a search for direct stop pair production is presented, assuming a GMSB model where the ˜χ01 is purely higgsino-like and is lighter than the ˜t1 [22]. The model parameters are
mq˜3 = mu˜3 = −At/2; tanβ = 10, (1) where mq˜3 and mu˜3 are the soft SUSY breaking masses for the left- and right-handed third-generation squarks, re- spectively, and tanβ is the ratio of the vacuum expectation values of up-type and down-type Higgs field. In these sce- narios, masses of first and second generation squarks and gluinos (superpartners of the gluons) are above 2 TeV, the
˜t mass eigenstates are such that m˜t2 ≫ m˜t1 and only ˜t1 pair production is considered in what follows. Stops de- cay either via ˜t1 → b ˜χ+1 or, if kinematically allowed, via
˜t1 → t ˜χ01(2). For the scenarios considered, the subsequent
decay ˜χ01→ Z ˜G has a branching ratio (BR) between 1 and 0.65 for mχ˜01 between 100 GeVand 350 GeV [23]. Thus, the expected signal is characterised by the presence of two jets originating from the hadronisation of the b-quarks (b- jets), decay products of Z (or h) bosons and large missing transverse momentum — its magnitude is here referred to as ETmiss— resulting from the undetected gravitinos.
This search uses data recorded between March and Au- gust 2011 by the ATLAS detector at the LHC. After the application of beam, detector, and data quality require- ments, the dataset corresponds to a total integrated lumi- nosity of 2.05 ± 0.08 fb−1 [24, 25]. To enhance the sen- sitivity to the aforementioned SUSY scenarios, events are required to contain energetic jets, of which one must be identified as a b-jet, large ETmiss and two opposite-sign, same flavour leptons (ℓ = e, µ) with invariant mass consis- tent with the Z boson mass, mZ. This is the first search for scalar top quarks decaying via Z bosons in GMSB models.
2. The ATLAS Detector
The ATLAS detector [26] consists of inner tracking de- vices surrounded by a superconducting solenoid, electro- magnetic and hadronic calorimeters, and a muon spec- trometer with a toroidal magnetic field.
The inner detector system, in combination with the 2 T field from the solenoid, provides precision tracking of charged particles for |η| < 2.5 1. It consists of a sil- icon pixel detector, a silicon microstrip detector and a straw tube tracker that also provides transition radiation measurements for electron identification. The calorimeter system covers the pseudorapidity range |η| < 4.9. It is composed of sampling calorimeters with either liquid ar- gon (LAr) or scintillating tiles as the active media. The muon spectrometer surrounds the calorimeters. It consists of a set of high-precision tracking chambers placed within a magnetic field generated by three large superconducting eight-coil toroids. The spectrometer, which has separate trigger chambers for |η| < 2.4, provides muon identifica- tion and measurement for |η| < 2.7.
3. Simulated Event Samples
Simulated event samples are used to aid in the descrip- tion of the background, as well as to determine the de- tector acceptance, the reconstruction efficiencies and the expected event yields for the SUSY signal.
1ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis coinciding with the axis of the beam pipe. The x- axis points from the IP to the centre of the LHC ring, and the y- axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the beam pipe.
The pseudorapidity is defined in terms of the polar angle θ as η =
−ln tan(θ/2). The distance ∆R in the η − φ space is defined as
∆R =p
(∆η)2+ (∆φ)2.
The signal samples are simulated with the HERWIG++ [27]
v2.4.2 Monte Carlo (MC) program at fixed ˜t1 and ˜χ01 masses, obtaining the desired values by varying the mq˜3
and |µ| parameters. The particle mass spectra and decay modes are determined using ISASUSY from the ISAJET [28]
v7.80 program. The SUSY sample yields are normalised to the results of next-to-leading-order (NLO) calculations as obtained using PROSPINO [29] v2.1 including higher-order supersymmetric QCD corrections and the resummation of soft-gluon emission at next-to-leading-logarithmic (NLL) accuracy [30]. An envelope of cross-section predictions is defined using the 68% C.L. ranges of the CTEQ6.6M [31]
(including αsuncertainty) and MSTW2008 [32] parton dis- tribution function (PDF) sets, together with variations of the factorisation and renormalisation scales, set to the stop mass. The nominal cross section is taken to be the midpoint of the envelope and the uncertainty assigned is half the full width of the envelope, following closely the PDF4LHCrecommendations [33]. NLO+NLL cross sections vary between 80 pb and 0.1 pb for stop masses between 140 GeVand 450 GeV.
For the backgrounds the following SM processes are considered. Top quark pair and single top quark pro- duction are simulated with MC@NLO [34], setting the top quark mass to 172.5 GeV, and using the NLO PDF set CTEQ6.6 [35]. Additional samples generated with POWHEG[36] and ACERMC [37] are used to estimate the event generator systematic uncertainties. Samples of W +jets, Z/γ∗+jets with light- and heavy-flavour jets, and t¯t with additional b-jets, t¯tb¯b, are generated with ALPGEN [38]
and the PDF set CTEQ6L1 [39]. The fragmentation and hadronisation for the ALPGEN and MC@NLO samples are per- formed with HERWIG [40], using JIMMY [41] for the under- lying event. Samples of Zt¯t and W t¯t are generated with MADGRAPH[42] interfaced to PYTHIA [43]. Diboson (W W , W Z, ZZ) samples are generated with HERWIG. For the comparison to data, all SM background cross sections are normalised to the results of higher-order calculations using the same values as Ref. [44].
The MC samples are produced using PYTHIA and HERWIG/JIMMY parameters tuned as described in Ref. [45]
and are processed through a detector simulation [46] based on GEANT4 [47]. Effects of multiple proton–proton inter- actions [45] are included in the simulation and MC events are re-weighted to reproduce the mean number of collisions per bunch crossing estimated from data.
4. Object Reconstruction
Jet candidates are reconstructed using the anti-kt jet clustering algorithm [48, 49] with a radius parameter of 0.4. The inputs to the algorithm are three-dimensional calorimeter energy clusters [50] seeded by cells with energy calibrated at the electromagnetic energy scale significantly above the measured noise. The jet energy is corrected for inhomogeneities and for the non-compensating nature of the calorimeter using pT- and η-dependent correction
factors derived using simulated multi-jet events (following Ref. [51] and references therein). Only jet candidates with pT> 20 GeV and |η| < 2.8 are retained.
A b-tagging algorithm [52] is used to identify jets con- taining a b-hadron decay. The algorithm is based on a mul- tivariate technique based on properties of the secondary vertex, of tracks within the jet and of the jet itself. The nominal b-tagging efficiency, computed on t¯t MC events, is on average 60%, with a misidentification (mis-tag) rate for light-quark/gluon jets of less than 1%. These b-jets are identified within the nominal acceptance of the inner detector (|η| < 2.5) and are required to have pT> 50 GeV.
Electron candidates are required to have pT > 20 GeV and |η| < 2.47, and are selected to satisfy the ‘tight’ shower shape and track selection criteria of Ref. [53]. The candi- date electron must be isolated, such that the pT sum of tracks (ΣpT, not including the electron track), within a cone in the (η, φ) plane of radius ∆R = 0.2 around the candidate must be less than 10% of the electron pT.
Muons are reconstructed using an algorithm [54] which combines the inner detector and the muon spectrometer information (combined muons). A muon is selected for the analysis only if it has pT > 10 GeV and |η| < 2.4, and the sum of the transverse momenta of tracks within a cone of ∆R = 0.2 around it is less than 1.8 GeV. To reject cosmic rays, muons are required to have longitudinal and transverse impact parameters within 1 mm and 0.2 mm of the primary vertex, respectively.
Following the object reconstruction described above, overlaps between jet candidates and leptons are resolved.
Any jet within a distance ∆R = 0.2 of a candidate electron is discarded. Any remaining lepton within ∆R = 0.4 of a jet is discarded.
The ETmiss is calculated from the vectorial sum of the transverse momenta of jets (with pT > 20 GeV and
|η| < 4.5), electrons and muons — including non-isolated muons [55]. The four-vectors of calorimeter clusters not belonging to other reconstructed objects are also included.
During 40% of the data-taking period, a localised elec- tronics failure in the LAr barrel calorimeter created a dead region in the second and third calorimeter layers (∆η × ∆φ ≃ 1.4 × 0.2) in which, on average, 30% of the incident energy is not measured. If a jet with pT> 50 GeV or an electron candidate falls in this region, the event is rejected. The loss in signal acceptance is less than 10% for the models considered.
5. Event Selection
The data are selected with a three-level trigger system based on the presence of leptons. Two trigger paths are considered: a single electron trigger, reaching a plateau efficiency for electrons with pT≥ 25 GeV, and a combined muon+jet trigger, reaching a plateau efficiency for muons with pT≥ 20 GeV and jets with pT≥ 60 GeV.
Events must pass basic quality criteria against detector noise and non-collision backgrounds [51] are required to
have a reconstructed primary vertex associated with five or more tracks; when more than one such vertex is found, the vertex with the largest summed p2T of the associated tracks is chosen
The selections applied in this analysis are listed below.
• To ensure full efficiency of the trigger, events are selected if they contain at least one electron with pT> 25 GeV or one muon with pT> 20 GeV.
• Exactly two same flavour opposite-sign leptons (ee, µµ) are required, such that their invariant mass mℓℓ is within the Z mass range (86 GeV < mℓℓ< 96 GeV). Events with additional electron or muon can- didates are vetoed.
• Events must include at least one jet with pT> 60 GeV and one additional jet with pT> 50 GeV.
• At least one jet with pT > 50 GeV and |η| < 2.5 is required to be b-tagged.
Two signal regions, referred to as SR1 and SR2, are de- fined using two different ETmiss threshold requirements in order to maximise the sensitivity across the ˜t1- ˜χ01 mass plane. For SR1, ETmiss> 50 GeV is required and it is cho- sen for models with ∆m = m˜t1−mχ˜01 larger than 100 GeV or mt˜1 < 300 GeV, where moderate missing transverse momentum is expected. SR2 is optimised for small ∆m scenarios and events are required to have ETmiss> 80 GeV.
The signal efficiencies, which include the Z → ee, µµ BR, acceptance and detector effects, vary across the ˜t1- ˜χ01 mass plane. For SR1 (SR2) the efficiencies are found to lie between 0.03% and 2.1% (0.01% and 1.7%) as the stop mass increases from 140 GeV to 400 GeV, and between 0.6% and 2.0% (0.5% and 1.7%) for ∆m between 300 GeV and 100 GeV at a stop mass of 400 GeV.
6. Background Estimation
The main SM processes contributing to the background are, in order of importance, top quark pair and single top quark production, followed by the associated production of Z bosons and heavy-flavour jets — referred to as Z+hf.
The top background is evaluated using control regions (CRs) that are the same as the SRs with the exception of the mℓℓrequirement (modified to 15 GeV < mℓℓ< 81 GeV or mℓℓ> 101 GeV). Depending on the corresponding sig- nal region, CRs are labelled as CR1 and CR2. In both cases, negligible yields from the targeted SUSY signals are expected. The background estimation in each SR is ob- tained by multiplying the number of events observed in the corresponding CR — corrected using simulations for non-top backgrounds — by a transfer factor, defined as the ratio of the MC-predicted yield in the signal region to that in the control region:
NSRtop =NSRtop,MC NCRtop,MC
NCRobs− NCRnon-top,MC
(2)
[GeV]
mll
50 100 150 200 250
Events / 20 GeV
1 10 102
103
104
> 50 GeV
miss
ET
= 7 TeV s
-1, L dt = 2.05 fb
∫
Data 2011 SM Total top
+hf Z fake-lepton Others
ATLAS
Figure 1: The distribution of mℓℓ in CR1 for the sum of ee and µµ channels. The dashed band shows the experimental systematic uncertainties including effects due to JES, b-tagging and lepton ID efficiency. The last mℓℓbin includes the number of overflow events for both data and SM expectation.
where NCRobsdenotes the observed yield in the CR. For each CR, the contribution from other SM processes accounts for less than 10% of the total. The estimate based on this approach benefits from a cancellation of systematic uncertainties that are correlated between SRs and CRs.
The distribution of mℓℓfor CR1 is shown in Figure1. The experimental uncertainties, described in Section7, are dis- played. They include effects due to jet energy scale and resolution [51] (JES), b-tagging [52] and lepton identifica- tion (ID) efficiencies [53,54,56]. The number of expected events for 2.05 fb−1 of integrated luminosity as predicted by the MC simulation is in good agreement with data for both CRs without introducing data/MC scaling factors.
The topology of Z+hf production events is similar to that of the signal, especially in low ˜t1- ˜χ01 mass scenarios.
Therefore the background from the Z+hf process is esti- mated from MC simulation and validated in a control re- gion where events passing all SR selection criteria except for a reversed EmissT cut (ETmiss < 50 GeV) are consid- ered. Possible signal contamination in the control region varies across the ˜t1- ˜χ01 mass range. As an example, for mχ˜01 ≃ 100 GeV, the contamination is 5% (80%) of the total predicted SM background for mt˜1 ≃ 350 (150) GeV.
In Figure 2 the ETmiss distribution is shown in the range 0–50 GeV for ee+µµ final states. The number of events observed in data is in good agreement with the SM ex- pected yields within experimental uncertainties.
Backgrounds from W+jets and multi-jet production, re- ferred to as “fake-lepton” contributions, are subdominant.
In this case, events passing the selection contain at least one misidentified or non-isolated lepton (collectively called
“fakes”). The fake-lepton background estimate is obtained using the data-driven approach described in Ref. [57]. The probability of misidentifying a jet as a signal lepton is es- timated in control regions dominated by multi-jet events
[GeV]
miss
ET
0 5 10 15 20 25 30 35 40 45 50
Events / 10 GeV
0 50 100 150 200 250 300 350 400
< 50 GeV
miss
ET
= 7 TeV s
-1, L dt = 2.05 fb
∫
Data 2011 SM Total top
+hf Z fake-lepton Others
ATLAS
Figure 2: The distribution of ETmissfor the Z+hf validation region for the sum of ee and µµ channels. By construction, only events with ETmissbelow 50 GeV are displayed. The dashed band represents the experimental uncertainties including effects due to JES, b-tagging and lepton ID efficiency.
where exactly one pre-selected lepton, at least one b-tagged jet and low ETmissare required.
Finally, background contributions from diboson, Zt¯t, W t¯t and t¯tb¯b events — referred to as ‘Others’ — are es- timated from MC simulation. They account for less than 3% of the total SM background in either SR.
7. Systematic Uncertainties
Various systematic uncertainties affecting the back- ground rates and signal yields have been considered. The values quoted in the following refer to ee and µµ channels summed.
Systematic uncertainties on the top background expec- tations vary between 11% and 13% depending on the SR and are dominated by the residual uncertainties on the shape of the kinematic distributions of top quark events.
The uncertainties are evaluated using additional MC sam- ples. ACERMC [37] is used to evaluate the impact of initial and final state radiation parameters (varied as in Ref. [58]), PYTHIAfor the choice of fragmentation model, POWHEG [36]
for the choice of generator. Experimental uncertainties on the b-tagging efficiency, JES and lepton ID efficiency account for about 4% in either SR.
The dominant uncertainties on the Z+hf background es- timates from simulation arise from the uncertainty on the production cross section used to normalise the MC yields.
A ±55% uncertainty on the total production cross section is evaluated from the direct Z+hf inclusive measurement described in Ref. [59] and takes into account differences be- tween data, MCFM [60] and ALPGEN predictions. The extrap- olation to each following jet multiplicity in Z + hf +N jets events increases this uncertainty by an additional 24% [61].
Other uncertainties due to JES, b-tagging efficiency and lepton ID efficiency are found to be about 25% and 35%
for SR1 and SR2, respectively.
[GeV]
miss
ET
0 50 100 150 200 250 300 350
Events / 30 GeV
10-1
1 10 102
103
104
> 50 GeV
miss
ET
= 7 TeV s
-1, L dt = 2.05 fb
∫
Data 2011 SM Total top
+hf Z fake-lepton Others
100GeV
0
χ∼1
250GeV,
~t
220GeV
0
χ∼1
250GeV,
~t
ATLAS
Figure 3: The ee+µµ ETmissdistribution for SR1 compared to the SM predictions, shown by the light (red) solid line, and SM+signal predictions, shown by the dark (black) solid and dashed lines. The dashed band represents the total systematic uncertainty. The last EmissT bin includes the number of overflow events for both data and SM expectation.
The estimated fake-lepton background is affected by sys- tematic uncertainties related to the determination of the lepton misidentification rate and to the subtraction of non- multi-jet contributions to the event yield in the multi-jet enhanced regions. The estimated uncertainty is 50% and 60% in SR1 and SR2, respectively. Finally, a conservative 100% uncertainty is taken into account on the contribu- tions from ‘Others’.
For the SUSY signal processes, uncertainties on the renormalisation and factorisation scales, on the PDF and on αs affect the cross section predictions. PDF and αs
uncertainties are between 10% and 15% depending on m˜t1
for the mass range considered. The variation of renormal- isation and factorisation scales by a factor of two changes the nominal signal cross section by 9–13% depending on the stop mass. The impact of detector-related uncertain- ties, such as JES, b-tagging and lepton ID efficiency, on the signal event yields varies between 10% and 25% and is dominated by the uncertainties on the JES.
8. Results and Interpretation
The numbers of observed and expected SM background events in the two SRs are summarised in Table 1, for ee and µµ channels summed. The ee and µµ contributions are also shown separately for illustration. In all SRs, the SM expectation and observation agree within uncertainties.
In Figure3the distributions of ETmissin SR1 (full spec- trum) and SR2 (ETmiss >80 GeV), summing the ee and µµ channels, are shown. For illustrative purposes, the dis- tributions expected for two signal (˜t1, ˜χ01) scenarios with masses of (250,100) GeV and (250,220) GeV, respectively, are added to the SM predictions.
The results are translated into 95% confidence level (C.L.) upper limits on contributions from new physics
SR1 SR2
ee channel
Data 39 20
SM 36.2±8.5 14.1±3.0
top 23.8±4.8 11.9±2.8
Z+hf 9.4±7.0 0.9±0.8
fake lepton 2.4±0.9 1.1±0.6 Others 0.5 ± 0.5 0.2 ± 0.2
µµ channel
Data 47 23
SM 55 ± 12 26.6±5.1
top 40.4 ± 6.2 22.9 ± 4.3
Z+hf 14.2±9.9 3.3±2.6
fake lepton 0.00±0.08 0.00±0.07 Others 0.7 ± 0.7 0.3 ± 0.3
ee+µµ
Data 86 43
SM 92±19 40.7 ± 6.0
top 64.3±7.7 34.8 ± 5.0
Z+hf 24±16 4.2±3.2
fake lepton 2.4 ± 0.9 1.1 ± 0.6 Others 1.2 ± 1.2 0.6 ± 0.6 95% C.L. upper limits: observed (expected)
events 37.2 (40.6) 19.8 (17.8) visible σ [fb] 18.2 (19.8) 9.7 (8.7)
Table 1: Expected and measured number of events in SR1 and SR2 for ee and µµ channels (separately and summed) for an integrated luminosity of 2.05 fb−1. Rows labelled as ‘Others’ correspond to the subdominant SM backgrounds estimated from MC simulation. The total systematic uncertainties are also displayed. At the bottom, model-independent observed and expected limits at 95% C.L on the number of events and visible cross sections are shown summing the ee and µµ channels.
using the CLs prescription [62]. The SR with the bet- ter expected sensitivity at each point in parameter space is adopted as the nominal result. Systematic uncertain- ties are treated as nuisance parameters and their corre- lations are taken into account. Figure 4 shows the ob- served and expected exclusion limits at 95% C.L. in the
˜t1- ˜χ01 mass plane, assuming direct stop pair production in the framework of GMSB models with light higgsinos.
The ±1σ contours around the median expected limit are also shown. Stop masses up to 310 GeVare excluded for 115 GeV< mχ˜01< 230 GeV. The exclusion extends to stop masses of 330 GeVfor a neutralino mass of about 190 GeV.
Stop masses below 240 GeV are excluded for mχ˜01 > mZ. The two SRs are used to set limits on the number of events and the visible cross section, σvis, of new physics models, without corrections for the effects of experimental reso- lution, acceptance and efficiency. The observed and ex- pected excluded values at 95% C.L. are reported in Ta- ble1.
[GeV]
t1
m~
100 150 200 250 300 350 400
[GeV]0 1χ∼m
50 100 150 200 250 300
350stop pair in GMSB Natural model
= 7 TeV s
-1, L dt = 2.05 fb
∫
ATLAS
Reference points
+ χ1
m∼ b+ < m
t1
m~
mZ
<
0
χ∼1
m
observed limit (95% C.L.) CLs
expected limit (95% C.L.) CLs
σ
±1 limit Expected CLs
Figure 4: Expected and observed exclusion limits and ±1σ varia- tion on the expected limit in the ˜t1- ˜χ01 mass plane. The reference points indicated on the plane correspond to the (˜t1, ˜χ01) scenarios of (250,100) GeVand (250,220) GeV, respectively.
9. Conclusions
In summary, results of a search for direct scalar top quark pair production in pp collisions at √
s = 7 TeV, based on 2.05 fb−1 of ATLAS data are reported. Scalar top quarks are searched for in events with two same flavour opposite-sign leptons (e, µ) with invariant mass consistent with the Z boson mass, large missing transverse momen- tum and jets in the final state, where at least one of the jets is identified as originating from a b-quark. The results are in agreement with the SM prediction and are interpreted in the framework of R-parity conserving ‘natural’ gauge me- diated SUSY scenarios. Stop masses up to 310 GeVare ex- cluded for 115 GeV < mχ˜01 < 230 GeVat 95% C.L., reach- ing an exclusion of mt˜1 < 330 GeVfor mχ˜01 = 190 GeV.
Stop masses below 240 GeV are excluded for mχ˜01> mZ.
10. Acknowledgements
We thank CERN for the very successful operation of the LHC, as well as the support staff from our institutions without whom ATLAS could not be operated efficiently.
We acknowledge the support of ANPCyT, Argentina;
YerPhI, Armenia; ARC, Australia; BMWF, Austria;
ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CON- ICYT, Chile; CAS, MOST and NSFC, China; COL- CIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF, DNSRC and Lundbeck Foundation, Denmark; ARTEMIS, European Union;
IN2P3-CNRS, CEA-DSM/IRFU, France; GNAS, Georgia;
BMBF, DFG, HGF, MPG and AvH Foundation, Ger- many; GSRT, Greece; ISF, MINERVA, GIF, DIP and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands;
RCN, Norway; MNiSW, Poland; GRICES and FCT, Por- tugal; MERYS (MECTS), Romania; MES of Russia and
ROSATOM, Russian Federation; JINR; MSTD, Serbia;
MSSR, Slovakia; ARRS and MVZT, Slovenia; DST/NRF, South Africa; MICINN, Spain; SRC and Wallenberg Foundation, Sweden; SER, SNSF and Cantons of Bern and Geneva, Switzerland; NSC, Taiwan; TAEK, Turkey;
STFC, the Royal Society and Leverhulme Trust, United Kingdom; DOE and NSF, United States of America.
The crucial computing support from all WLCG part- ners is acknowledged gratefully, in particular from CERN and the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Sweden), CC-IN2P3 (France), KIT/GridKA (Germany), INFN-CNAF (Italy), NL-T1 (Netherlands), PIC (Spain), ASGC (Taiwan), RAL (UK) and BNL (USA) and in the Tier-2 facilities worldwide.
References
[1] H. Miyazawa,Prog. Theor. Phys. 36 (6), 1266 (1966).
[2] P. Ramond,Phys. Rev. D 3, 2415 (1971).
[3] Y. A. Golfand and E. P. Likhtman, JETP Lett. 13, 323 (1971), [Pisma Zh.Eksp.Teor.Fiz.13:452-455,1971].
[4] A. Neveu and J. H. Schwarz,Nucl. Phys. B 31, 86 (1971).
[5] A. Neveu and J. H. Schwarz,Phys. Rev. D 4, 1109 (1971).
[6] J. Gervais and B. Sakita,Nucl. Phys. B 34, 632 (1971).
[7] D. V. Volkov and V. P. Akulov,Phys. Lett. B 46, 109 (1973).
[8] J. Wess and B. Zumino,Phys. Lett. B 49, 52 (1974).
[9] J. Wess and B. Zumino,Nucl. Phys. B 70, 39 (1974).
[10] P. Fayet,Phys. Lett. B 64, 159 (1976).
[11] P. Fayet,Phys. Lett. B 69, 489 (1977).
[12] G. R. Farrar and P. Fayet,Phys. Lett. B 76, 575 (1978).
[13] P. Fayet,Phys. Lett. B 84, 416 (1979).
[14] S. Dimopoulos and H. Georgi,Nucl. Phys. B 193, 150 (1981).
[15] R. Kitano and Y. Nomura,Phys. Rev. D 73, 095004 (2006).
[16] M. Dine and W. Fischler,Phys. Lett. B 110, 227 (1982).
[17] L. Alvarez-Gaume et al.,Nucl. Phys. B 207, 96 (1982).
[18] C. R. Nappi and B. A. Ovrut,Phys. Lett. B 113, 175 (1982).
[19] M. Dine and A. E. Nelson,Phys. Rev. D 48, 1277 (1993).
[20] M. Dine et al.,Phys. Rev. D 51, 1362 (1995).
[21] M. Dine et al.,Phys. Rev. D 53, 2658 (1996).
[22] M. Asano et al.,JHEP 12, 019 (2010).
[23] S. Dimopoulos et al.,Nucl. Phys. B 488, 39 (1997).
[24] ATLAS Collaboration,Eur. Phys. J. C 71, 1630 (2011).
[25] ATLAS Collaboration, ATLAS-CONF-2011-116 (2011), https://cdsweb.cern.ch/record/1376384.
[26] ATLAS Collaboration,JINST 3, S08003 (2008).
[27] M. Bahr et al.,Eur. Phys. J. C 58, 639 (2008).
[28] F. E. Paige et al.,arXiv:hep-ph/0312045 (2003).
[29] W. Beenakker et al.,Nucl. Phys. B 515, 3 (1998).
[30] W. Beenakker et al.,JHEP 08, 098 (2010).
[31] D. Stump et al.,JHEP 10, 046 (2003).
[32] A. Martin et al.,Eur. Phys. J. C 63, 189 (2009).
[33] M. Botje et al.,arXiv:1101.0538 [hep-ex] (2011).
[34] S. Frixione and B. R. Webber,arXiv:hep-ph/0601192 (2006).
[35] P. Nadolsky et al.,Phys. Rev. D 78, 013004 (2008).
[36] S. Frixione et al.,JHEP 11, 070 (2007).
[37] B. Kersevan and E. Richter-Was,arXiv:hep-ph/0405247 (2004).
[38] M. Mangano et al.,JHEP 07, 001 (2003).
[39] J. Pumplin et al.,JHEP 07, 012 (2002).
[40] G. Corcella et al.,JHEP 01, 010 (2001).
[41] J. M. Butterworth et al.,Z. Phys. C 72, 637 (1996).
[42] J. Alwall et al.,JHEP 06, 128 (2011).
[43] T. Sj¨ostrand, S. Mrenna, and P. Skands,JHEP 05, 026 (2006).
[44] ATLAS Collaboration, Submitted to Phys. Rev. D, arXiv:1203.6193 [hep-ex] (2012).
[45] ATLAS Collaboration, ATL-PHYS-PUB-2011-008 (2011), https://cdsweb.cern.ch/record/1345343.
[46] ATLAS Collaboration,Eur. Phys. J. C 70, 823 (2010).
[47] GEANT4 Collaboration,Nucl. Inst. Meth. A 506, 250 (2003).
[48] M. Cacciari and G. Salam,Phys. Lett. B 641, 57 (2006).
[49] M. Cacciari et al.,JHEP 04, 063 (2008).
[50] W. Lampl et al., ATL-LARG-PUB-2008-002 (2008), https://cdsweb.cern.ch/record/1099735.
[51] ATLAS Collaboration, Submitted to Eur. Phys. J. C, arXiv:1112.6426 [hep-ex] (2011).
[52] ATLAS Collaboration,ATLAS-CONF-2011-102 (2011).
[53] ATLAS Collaboration,Eur. Phys. J. C 72, 1909 (2011).
[54] ATLAS Collaboration, ATLAS-CONF-2011-063 (2011), https://cdsweb.cern.ch/record/1345743.
[55] ATLAS Collaboration,Eur. Phys. J. C 72, 1844 (2012).
[56] ATLAS Collaboration, ATLAS-CONF-2011-021 (2011), https://cdsweb.cern.ch/record/1336750.
[57] ATLAS Collaboration,Eur. Phys. J. C 71, 1577 (2011).
[58] ATLAS Collaboration,Phys. Lett. B 707, 459 (2012).
[59] ATLAS Collaboration,Phys. Lett. B 706, 295 (2012).
[60] J. M. Campbell et al.,Phys. Rev. D 69, 074021 (2004).
[61] J. Alwall et al.,Eur. Phys. J. C 53, 474 (2008).
[62] A. L. Read,J. Phys. G 28, 2693 (2002).
The ATLAS Collaboration
G. Aad48, B. Abbott111, J. Abdallah11, S. Abdel Khalek115, A.A. Abdelalim49, O. Abdinov10, B. Abi112, M. Abolins88, O.S. AbouZeid158, H. Abramowicz153, H. Abreu136, E. Acerbi89a,89b, B.S. Acharya164a,164b, L. Adamczyk37,
D.L. Adams24, T.N. Addy56, J. Adelman176, S. Adomeit98, P. Adragna75, T. Adye129, S. Aefsky22,
J.A. Aguilar-Saavedra124b,a, M. Aharrouche81, S.P. Ahlen21, F. Ahles48, A. Ahmad148, M. Ahsan40, G. Aielli133a,133b, T. Akdogan18a, T.P.A. ˚Akesson79, G. Akimoto155, A.V. Akimov94, A. Akiyama66, M.S. Alam1, M.A. Alam76, J. Albert169, S. Albrand55, M. Aleksa29, I.N. Aleksandrov64, F. Alessandria89a, C. Alexa25a, G. Alexander153, G. Alexandre49, T. Alexopoulos9, M. Alhroob164a,164c, M. Aliev15, G. Alimonti89a, J. Alison120, B.M.M. Allbrooke17, P.P. Allport73, S.E. Allwood-Spiers53, J. Almond82, A. Aloisio102a,102b, R. Alon172, A. Alonso79, B. Alvarez Gonzalez88, M.G. Alviggi102a,102b, K. Amako65, C. Amelung22, V.V. Ammosov128, A. Amorim124a,b, G. Amor´os167, N. Amram153, C. Anastopoulos29, L.S. Ancu16, N. Andari115, T. Andeen34, C.F. Anders20, G. Anders58a, K.J. Anderson30,
A. Andreazza89a,89b, V. Andrei58a, X.S. Anduaga70, A. Angerami34, F. Anghinolfi29, A. Anisenkov107, N. Anjos124a, A. Annovi47, A. Antonaki8, M. Antonelli47, A. Antonov96, J. Antos144b, F. Anulli132a, S. Aoun83, L. Aperio Bella4, R. Apolle118,c, G. Arabidze88, I. Aracena143, Y. Arai65, A.T.H. Arce44, S. Arfaoui148, J-F. Arguin14, E. Arik18a,∗, M. Arik18a, A.J. Armbruster87, O. Arnaez81, V. Arnal80, C. Arnault115, A. Artamonov95, G. Artoni132a,132b, D. Arutinov20, S. Asai155, R. Asfandiyarov173, S. Ask27, B. ˚Asman146a,146b, L. Asquith5, K. Assamagan24,
A. Astbury169, B. Aubert4, E. Auge115, K. Augsten127, M. Aurousseau145a, G. Avolio163, R. Avramidou9, D. Axen168, G. Azuelos93,d, Y. Azuma155, M.A. Baak29, G. Baccaglioni89a, C. Bacci134a,134b, A.M. Bach14, H. Bachacou136, K. Bachas29, M. Backes49, M. Backhaus20, E. Badescu25a, P. Bagnaia132a,132b, S. Bahinipati2, Y. Bai32a, D.C. Bailey158, T. Bain158, J.T. Baines129, O.K. Baker176, M.D. Baker24, S. Baker77, E. Banas38, P. Banerjee93, Sw. Banerjee173, D. Banfi29, A. Bangert150, V. Bansal169, H.S. Bansil17, L. Barak172, S.P. Baranov94,
A. Barbaro Galtieri14, T. Barber48, E.L. Barberio86, D. Barberis50a,50b, M. Barbero20, D.Y. Bardin64, T. Barillari99, M. Barisonzi175, T. Barklow143, N. Barlow27, B.M. Barnett129, R.M. Barnett14, A. Baroncelli134a, G. Barone49, A.J. Barr118, F. Barreiro80, J. Barreiro Guimar˜aes da Costa57, P. Barrillon115, R. Bartoldus143, A.E. Barton71,
V. Bartsch149, R.L. Bates53, L. Batkova144a, J.R. Batley27, A. Battaglia16, M. Battistin29, F. Bauer136, H.S. Bawa143,e, S. Beale98, T. Beau78, P.H. Beauchemin161, R. Beccherle50a, P. Bechtle20, H.P. Beck16, S. Becker98, M. Beckingham138, K.H. Becks175, A.J. Beddall18c, A. Beddall18c, S. Bedikian176, V.A. Bednyakov64, C.P. Bee83, M. Begel24,
S. Behar Harpaz152, P.K. Behera62, M. Beimforde99, C. Belanger-Champagne85, P.J. Bell49, W.H. Bell49, G. Bella153, L. Bellagamba19a, F. Bellina29, M. Bellomo29, A. Belloni57, O. Beloborodova107,f, K. Belotskiy96, O. Beltramello29, O. Benary153, D. Benchekroun135a, K. Bendtz146a,146b, N. Benekos165, Y. Benhammou153, E. Benhar Noccioli49, J.A. Benitez Garcia159b, D.P. Benjamin44, M. Benoit115, J.R. Bensinger22, K. Benslama130, S. Bentvelsen105, D. Berge29, E. Bergeaas Kuutmann41, N. Berger4, F. Berghaus169, E. Berglund105, J. Beringer14, P. Bernat77, R. Bernhard48, C. Bernius24, T. Berry76, C. Bertella83, A. Bertin19a,19b, F. Bertolucci122a,122b, M.I. Besana89a,89b, N. Besson136, S. Bethke99, W. Bhimji45, R.M. Bianchi29, M. Bianco72a,72b, O. Biebel98, S.P. Bieniek77, K. Bierwagen54, J. Biesiada14, M. Biglietti134a, H. Bilokon47, M. Bindi19a,19b, S. Binet115, A. Bingul18c, C. Bini132a,132b, C. Biscarat178, U. Bitenc48, K.M. Black21, R.E. Blair5, J.-B. Blanchard136, G. Blanchot29, T. Blazek144a, C. Blocker22, J. Blocki38, A. Blondel49, W. Blum81, U. Blumenschein54, G.J. Bobbink105, V.B. Bobrovnikov107, S.S. Bocchetta79, A. Bocci44, C.R. Boddy118, M. Boehler41, J. Boek175, N. Boelaert35, J.A. Bogaerts29, A. Bogdanchikov107, A. Bogouch90,∗, C. Bohm146a, J. Bohm125, V. Boisvert76, T. Bold37, V. Boldea25a, N.M. Bolnet136, M. Bomben78, M. Bona75, M. Bondioli163, M. Boonekamp136, C.N. Booth139, S. Bordoni78, C. Borer16, A. Borisov128, G. Borissov71, I. Borjanovic12a, M. Borri82, S. Borroni87, V. Bortolotto134a,134b, K. Bos105, D. Boscherini19a, M. Bosman11, H. Boterenbrood105, D. Botterill129, J. Bouchami93, J. Boudreau123, E.V. Bouhova-Thacker71, D. Boumediene33, C. Bourdarios115, N. Bousson83, A. Boveia30, J. Boyd29, I.R. Boyko64, N.I. Bozhko128, I. Bozovic-Jelisavcic12b, J. Bracinik17, P. Branchini134a, A. Brandt7, G. Brandt118, O. Brandt54, U. Bratzler156, B. Brau84, J.E. Brau114, H.M. Braun175, B. Brelier158, J. Bremer29, K. Brendlinger120, R. Brenner166, S. Bressler172, D. Britton53,
F.M. Brochu27, I. Brock20, R. Brock88, E. Brodet153, F. Broggi89a, C. Bromberg88, J. Bronner99, G. Brooijmans34, W.K. Brooks31b, G. Brown82, H. Brown7, P.A. Bruckman de Renstrom38, D. Bruncko144b, R. Bruneliere48,
S. Brunet60, A. Bruni19a, G. Bruni19a, M. Bruschi19a, T. Buanes13, Q. Buat55, F. Bucci49, J. Buchanan118, P. Buchholz141, R.M. Buckingham118, A.G. Buckley45, S.I. Buda25a, I.A. Budagov64, B. Budick108, V. B¨uscher81, L. Bugge117, O. Bulekov96, A.C. Bundock73, M. Bunse42, T. Buran117, H. Burckhart29, S. Burdin73, T. Burgess13, S. Burke129, E. Busato33, P. Bussey53, C.P. Buszello166, B. Butler143, J.M. Butler21, C.M. Buttar53,
J.M. Butterworth77, W. Buttinger27, S. Cabrera Urb´an167, D. Caforio19a,19b, O. Cakir3a, P. Calafiura14, G. Calderini78, P. Calfayan98, R. Calkins106, L.P. Caloba23a, R. Caloi132a,132b, D. Calvet33, S. Calvet33, R. Camacho Toro33,
P. Camarri133a,133b, D. Cameron117, L.M. Caminada14, S. Campana29, M. Campanelli77, V. Canale102a,102b, F. Canelli30,g, A. Canepa159a, J. Cantero80, L. Capasso102a,102b, M.D.M. Capeans Garrido29, I. Caprini25a, M. Caprini25a, D. Capriotti99, M. Capua36a,36b, R. Caputo81, R. Cardarelli133a, T. Carli29, G. Carlino102a,
