√ s = 8 TeV with the ATLAS detector at the LHC

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Nuclear Physics B Proceedings Supplement 00 (2014) 1–3

Nuclear Physics B Proceedings Supplement

Search for pair-produced third-generation squarks in compressed supersymmetric scenarios using monojet-like final states in pp collisions at

√ s = 8 TeV with the ATLAS detector at the LHC

Roger Caminal Armadans^a

aUniversitat Aut`onoma de Barcelona - Institut de F´ısica d’Altes Energies (IFAE), Campus UAB, Edifici Cn, E-08193 Bellaterra, Barcelona, Spain

Abstract

Searches for third-generation squarks are reported, using 20.3 fb⁻¹ of proton-proton collisions at √

s = 8 TeV recorded by the ATLAS experiment at the LHC. The analysis has been optimized for the stop decaying into a charm quark plus a neutralino SUSY signal and it is carried out in several signal regions differing by the requirements on the pTof the leading jet and the missing transverse energy. No excess above the Standard Model background expectation is observed. Limits are set on the visible cross-section of new physics within the requirements of the search. The results of this analysis are interpreted in terms of compressed scenarios for top squark pair production in the decay channels ˜t1 → c+χ˜⁰₁ and ˜t1 → b+ f f⁰+χ˜⁰₁, and in terms of bottom squark pair production in the decay channel b˜1 →b+χ˜⁰₁. Similar exclusions are found for the nearly mass-degenerated third-generation squarks and the lightest neutralino. In particular, masses of the ˜t1and ˜b1up to 240 GeV are excluded at 95% CL, thus significantly extending the previous collider results.

Keywords: SUSY, Stop, Sbottom, Compressed

Supersymmetry (SUSY) is a theoretical favored model for physics beyond the Standard Model (BSM) which naturally solves the hierarchy problem and pro- vides a possible candidate for dark matter in the Uni- verse. In scenarios for which∆m=mt˜1−m_χ_˜0

1<m_b+m_W, the “four-body” decay mode ˜t1→b+f f⁰+χ˜⁰₁competes with the stop decay to a charm quark and the lightest supersymmetric particle (LSP), ˜t1 → c+χ˜⁰₁. In case of sbottom pair production, each sbottom is assumed to decay exclusively in the ˜b1 → b+χ˜⁰₁ mode. The corresponding final states to these processes are character- ized by the presence of two jets from the hadronization of the charm or bottom quarks, missing transverse mo- mentum (denoting its magnitude byE^miss_T ) from the two undetected LSPs and two fermions (the latest only in the case of the four-body decay). If the mass difference between the third-generation squark and the LSP,∆m, is small, the transverse momenta of the Standard Model

(SM) decay products is too low to be reconstructed.

Therefore, a monojet analysis strategy is followed, mak- ing use of the presence of initial-state radiation jets to identify signal events with an energetic jet.

The data sample used in this contribution [1] was col- lected with the ATLAS detector [2] in the LHC, and cor- responds to a total integrated luminosity of 20.3 fb⁻¹. The events are selected using an inclusiveE^miss_T trigger.

They are required to haveE^miss_T >150 GeV and at least one jet with p_T >150 GeV. Further conditions on the vertex and the jet quality are imposed to ensure that the events originate from a proton-proton collision [2].

In order to reject events coming from the production of W or Z bosons in association with jets, the events are required to have no electrons or muons in the final state. No more than three jets with p_T > 30 GeV and |η| < 2.8 are allowed in the event. In order to suppress multijet events, the direction of the E^miss_T of the event is forced to be well separated from each jet,

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Preselection Primary vertex

E^miss_T >150 GeV Jet quality requirements

At least one jet withpT>150 GeV and|η|<2.8 Lepton vetoes

At most three jets withpT>30 GeV and|η|<2.8

∆φ(jet,pmiss

T )>0.4

Monojet-like selection

Signal region M1 M2 M3

Minimum leading jetpT[GeV] 280 340 450 MinimumE_T^miss[GeV] 220 340 450

Table 1: Event selection criteria applied for the regions M1, M2 and M3.

∆φ(E_T^miss,pT) > 0.4. Finally, three signal regions are defined, named M1 to M3, by imposing different conditions on thepTof the leading jet and theE_T^miss. In particular, for the M1 selection, the events are required to have E^miss_T >220 GeV and leading jetpT >280 GeV, while for the M2 (M3) selection the thresholds are increased toE^miss_T >340 GeV (E^miss_T >450 GeV) and leading jet p_T > 340 GeV (p_T > 450 GeV). Table 1 summarizes the event selection criteria for each signal region.

The Standard Model background contributions are dominated by the production of Z and W bosons in association with jets. These processes account for a 95%, 96% and 95% of the total background for M1, M2 and M3 signal regions respectively, with the largest contribution being the irreducibleZ(→ νν) + jets process.

TheW/Z+jets backgrounds are normalized with ded- icated control regions in which the presence of leptons is required. Other processes are considered, for example the diboson whose contribution to the total background is 2%, 3% and 3% for M1-M3, respectively, and is determined from MC. The top quark production processes are small (about 2% in all the regions) and is en- tirely determined from MC. The multi jet background is also considered and it is estimated in a data-driven way and it constitutes less than 1% of the total background.

Finally, the non-collision background is also estimated with data-driven techniques and is found to be negligi- ble in all the selections.

Different sources of systematic uncertainties are considered in the analysis: the absolute jetp_Tand theE_T^miss energy scale and resolution, the pileup corrections, the lepton identification efficiencies, the modeling of parton showers and hadronization in the simulation, and the

statistical uncertainties on the control samples used to constrain the boson+jets contributions. Model uncertainties related to potential differences betweenW+jets andZ+jets processes are also considered. This leads to a total systematic uncertainty of about 3% for M1, M2 selections and about 5% for M3 selection. As an example, Figure 1 shows the distributions of theE^miss_T distribution for the M1 signal regions. Good agreement is observed between the data and the Standard Model prediction.

400 600 800 1000 1200 1400

[Events/GeV]miss TdN/dE

10-2

10-1

1 10 102

103

104 Signal Region M1

Data 2012 Standard Model

) + jets ν ν Z(→

) + jets ν

→ l W(

ll) + jets

→ Z(

dibosons (+X) + single top t

t multijets

) = (200, 195) GeV χ0

, ∼

~t m(

) = (200, 125) GeV χ0

, ∼

~t m(

ATLAS

∫Ldt = 20.3 fb^-1^,^s^{= 8 TeV}

[GeV]

miss

ET

400 600 800 1000 1200 1400

Data / SM

0.5 1 1.5

Figure 1: MeasuredE^miss_T and leading jetpTdistributions for the M1 selection compared to the SM predictions. For illustration purposes, the distribution of two different SUSY scenarios are included. The error bands in the ratios include both the statistical and systematic uncertainties on the background predictions. [1].

The results are translated into 95% CL limits on different SUSY final states. Experimental uncertainties on the signal vary between 3% and 7%, depending on the masses of the SUSY particles. An extra 2.8% systematic uncertainty in the luminosity is also included. Un- certainties effecting the acceptance times efficiency vary between 8% and 12% depending on the stop/sbottom and neutralino masses. Finally, the theoretical uncertainty on the cross section varies between 14% and 16%.

For the exclusions, the signal region with the best 95%

CL expected limit is employed. The M1 signal region dominates the exclusion plane and M2 and M3 serve to enhance the sensitivity at very large stop and neutralino masses.

For the stop pair production assuming the decay ˜t1→ c+χ˜⁰₁ with a branching ratio of 100%, masses for the stop up to 260 GeV are excluded for almost degenerated stop and LSP masses. Figure 2 shows the exclusion plane at 95% CL as a function of stop and neutralino

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masses.

[GeV]

t1

m~

100 150 200 250 300 350

[GeV]0 1χ∼m

50 100 150 200 250 300 350

=8 TeV s

-1, L dt = 20.3 fb

∫

monojet-like selection: M1, M2, M3

All limits at 95% CL

ATLAS

) = 1

1

χ∼0

→ c t1

production, BR(~ t1

~ t1

~

theory) σSUSY

±1 Observed limit (

exp) σ

±1 Expected limit (

°) θ = 0 LEP (

-1) CDF (2.6 fb

+ mc 0 χ1

< m∼ t1

m~

+ mW + mb 0 χ1

> m∼ t1

m~

Figure 2: Exclusion plane at 95% CL as a function of stop and neutralino masses. The observed (red line) and expected (blue line) upper limits from this analysis are compared to previous results from Teva- tron experiments, and from LEP experiments at CERN with squark mixing angleθ=0◦. The dotted lines around the observed limit indicate the range of observed limits corresponding to±1σvariations on the NLO SUSY cross section predictions. The shaded area around the expected limit indicates the expected±1σranges of limits in the absence of a signal. A band form_t_˜₁−m_χ_˜0

1<2 GeV indicates the region in the phase space for which the stop can become long-lived [1].

In the scenario with the stop decay ˜t1→b+f f⁰+χ˜⁰₁ with BR(100%), masses for the stop up to 255 GeV are excluded at 95% CL for almost degenerated stop and neutralino, as shown in Figure 3.

Finally, for the sbottom pair production assuming b˜1 → b+χ˜⁰₁ with BR=100%, masses for the sbottom up to 255 GeV can be excluded at 95% CL for almost degenerated sbottom and neutralino masses. Figure 4 shows the exclusion plane for this model.

References

[1] G. Aadet al.[ATLAS Collaboration], “Search for pair-produced third-generation squarks decaying via charm quarks or in compressed supersymmetric scenarios in pp collisions at √

s = 8 TeV with the ATLAS detector,” Phys. Rev. D 90 (2014) 052008 [arXiv:1407.0608 [hep-ex]].

[2] G. Aadet al.[ATLAS Collaboration], “The ATLAS Experiment at the CERN Large Hadron Collider,” JINST3, S08003 (2008).

[GeV]

t1

m~

100 150 200 250 300 350

[GeV]0 1χ∼m

0 50 100 150 200 250 300 350

=8 TeV s

-1, L dt = 20.3 fb

∫

ATLAS

theory) σSUSY

exp) σ

) = 1

1

χ∼0

b f f’

1→

~t production, BR(

t1

~ t1

~

+ mb 0 χ1

< m∼ t1

m~

+ mW + mb 0 χ1

> m∼ t1

m~

Figure 3: Exclusion plane at 95% CL as a function of stop and neutralino masses for the decay channel ˜t₁→b+f f⁰+χ˜⁰₁(BR=100%).

The dotted lines around the observed limit indicate the range of observed limits corresponding to ±1σvariations on the NLO SUSY cross-section predictions. The shaded area around the expected limit indicates the expected±1σranges of limits in the absence of a signal.

A band form_t_˜₁−m_χ_˜0

1<2 GeV indicates the region in the phase space for which the stop can become long-lived [1].

[GeV]

b1

m~

100 150 200 250 300 350 400

[GeV]0 1χ∼m

0 50 100 150 200 250 300 350 400

=8 TeV s

-1, L dt = 20.3 fb

∫

ATLAS

) = 1

1

χ∼0

→ b b1

production, BR(~ b1

~ b1

~

theory) σSUSY

exp) σ

miss ATLAS 0 leptons + 2 b-jets + ET CDF 2.65 fb-1

D0 5.2 fb-1

+ mb 0 χ1

< m∼ b1

m~

Figure 4: Exclusion plane at 95% CL as a function of sbottom and neutralino masses for the decay channel ˜b1 →b+χ˜⁰₁(BR=100%).

The observed (red line) and expected (blue line) upper limits from this analysis are compared to previous results from CDF, D0, and ATLAS. For the latter, the area below the dashed-dotted line is excluded. The dotted lines around the observed limit indicate the range of observed limits corresponding to±1σvariations on the NLO SUSY cross-section predictions. The shaded area around the expected limit indicates the expected±1σranges of limits in the absence of a signal.

A band form_b_˜

1−m_χ_˜0

1<2 GeV indicates the region in the phase space for which the sbottom can become long-lived. [1].