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DOI 10.1393/ncc/i2015-11868-7

Colloquia_{: LaThuile14}

Search for supersymmetry at ATLAS

E. Mustoon behalf of the ATLAS Collaboration

Technion, Israel Institute of Technology - Technion City, Haifa 32000, Israel

ricevuto il 31 Luglio 2014

Summary. — These proceedings summarize the searches for Supersymmetry (SUSY) with the ATLAS detector at the Large Hadron Collider (LHC), focusing on the results obtained with the full proton-proton collisions data set recorded at a center-of-mass energy of√s = 8 TeV, corresponding to an integrated luminosity

of approximately 20 fb−1. Various searches were carried out in order to explore a large variety of production models and the results matched the Standard Model background expectation, therefore exclusion limits were derived on the production of new physics.

PACS 11.30.Pb – Supersymmetry. PACS 12.60.Jv – Supersymmetric models.

PACS 14.80.Ly – Supersymmetric partners of known particles. PACS 14.80.Nb – Neutralinos and charginos.

1. – Introduction

Supersymmetry (SUSY) [1-8] is one of the most promising theory to describe Physics beyond the Standard Model (SM). It predicts the existence of new particles, “super-partners” of SM particles, that have the same quantum numbers and masses but obey diﬀerent spin-statistics. In particular, this new symmetry associates a complex scalar to each SM fermion, and a fermion to each gauge boson. Since SUSY particles have not been observed yet, this symmetry is assumed to be broken.

Some SUSY scenarios foresee the conservation of R-parity, a quantum number deﬁned

as (−1)3(B−L)+2S_{, where B is the baryon number, L is the lepton number and S is the}

spin. This conservation implies that the Lightest Supersymmetric Particle (LSP) is stable and that SUSY particles are produced in pairs at colliders; in some R-parity conserving

(RPC) models the LSP is identiﬁed with the lightest neutralino ( ˜χ01), which is a massive,

neutral, weakly interactive particle suitable to be a good Dark-Matter candidate. In R-parity–violating (RPV) models the LSP is no longer stable and can decay to SM particles.

c

CERN on behalf of the ATLAS Collaboration

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1.1. SUSY searches at LHC . – At the LHC [9] SUSY can be tested up to the TeV scale. The large variety of SUSY breaking models led to a general search approach in simpliﬁed scenarios involving a reduced number of parameters. Early searches targeted

the inclusive production of strong particles like gluinos (˜g) and ﬁrst/second generation

squarks (˜q), since the production cross-sections for these particles dominates the total

SUSY production cross-section at the LHC [10]. Final states for such processes contain

energetic jets, missing transverse energy (Emiss

T ) and sometimes leptons.

The sensitivity to intermediate production cross-sections increased as the integrated

luminosity increased, so the search for third ˜q generation has become more relevant;

third ˜q generation production involves in the ﬁnal state b-jets, E_Tmissand leptons, while

top events represent the main SM background. In some models, also supported by the

“natural” stabilization of the Higgs boson mass argument, light stops (˜t) and sbottoms

(˜b) are expected. Lower cross-sections correspond to the ElectroWeak (EW) production

of SUSY particles; the latter can be the dominant production mechanism if colored sparticles are too heavy to be produced. These processes are very interesting because are

often characterized by multi-lepton and Emiss

T signatures and very low SM background.

1.2. ATLAS search strategy. – The ATLAS [11] search strategy is designed to cover a

broad spectra of the different SUSY breaking models in both RPC and RPV scenarios. For every search, Signal Regions (SRs) are defined using Monte Carlo (MC) simulation and are individually optimized to achieve the best sensitivity. Different Control Regions (CRs) featuring similar signatures as the SRs but with negligible signal contamination are used to evaluate the dominant background processes and to extract normalization factors for MC. Rare background processes are often evaluated using MC simulation, and possible discrepancies are addressed by using correction factors obtained from data.

As general rule, to avoid bias the data remain blinded until the analysis procedure is fully validated. Once the analysis is considered well established, the SRs are unblinded and if no discrepancies between the observed data and the SM expectations are found, limits are set on the SUSY models using the CLs [12] prescription.

2. – Inclusive searches for 1st- and 2nd-generation squarks and gluinos The search for strong production of 1st and 2nd generation of squarks and gluinos posed strong constraints on several SUSY models.

Figure 1 left summarizes the results of the searches performed in both fully hadronic and semi-leptonic ﬁnal states, interpreted in the MSUGRA/CMSSM model [15]: squarks and gluinos are excluded for masses up to 1.35 TeV and the highest sensitivity is provided

by ﬁnal states with high jet multiplicity and large Emiss

T .

One interesting example of these searches is reported in [16], where the ﬁnal states contain at least one isolated lepton (electron or muon), jets (with or without b-jet

re-quirements) and large E_Tmiss: in particular, the SR containing only one “soft” lepton

(i.e. having transverse momentum (pT) < 25 GeV) provided high sensitivity for the

com-pressed spectra where the mass diﬀerence between ˜g and ˜χ0

1(LSP) is relatively low.

The results in the Emiss

T + jets and no leptons ﬁnal states have been also interpreted

in a simpliﬁed model framework [14], where only the strong production of gluinos and

1st and 2nd generation of squarks (of common mass) with direct decay to the LSP ( ˜χ01)

was allowed. In this case ten SRs with diﬀerent requirements on the jets multiplicity

or Emiss

T have been deﬁned; among them, the most sensitive ones have been used to set

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[GeV] 0 m 0 1000 2000 3000 4000 5000 6000 [GeV] 1/2 m 300 400 500 600 700 800 900 1000 (2400 GeV)q~ (1600 GeV) q ~ (1000 GeV) g ~ (1400 GeV) g ~ h (122 GeV) h (124 GeV) h (126 GeV) Expected Observed Expected Observed Expected Observed Expected Observed Expected Observed Expected Observed > 0 μ , 0 = -2m 0 ) = 30, A β

MSUGRA/CMSSM: tan( Status: SUSY 2013

ATLAS Preliminary = 8 TeV s , -1 L dt = 20.1 - 20.7 fb ∫ τ∼ LSP not included. theory SUSY σ 95% CL limits. 0-lepton, 2-6 jets 0-lepton, 7-10 jets 0-1 lepton, 3 b-jets 1-lepton + jets + MET 1-2 taus + jets + MET 3 b-jets ≥ 2-SS-leptons, 0 - ATLAS-CONF-2013-047 arXiv: 1308.1841 ATLAS-CONF-2013-061 ATLAS-CONF-2013-062 ATLAS-CONF-2013-026 ATLAS-CONF-2013-007

gluino mass [GeV]

800 1000 1200 1400 1600 1800 2000 2200 2400

squark mass [GeV]

800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800Squark-gluino-neutralino model =8 TeV s , -1 L dt = 20.3 fb ∫ ATLASPreliminary 0-lepton combined ) theory SUSY σ 1 ± ) = 0 GeV Observed limit (

1 0 χ∼ m( ) exp σ 1 ± ) = 0 GeV Expected limit (

1 0

χ∼ m(

) = 395 GeV Observed limit

1 0

χ∼ m(

) = 395 GeV Expected limit

1 0

χ∼ m(

) = 695 GeV Observed limit

1 0

χ∼ m(

) = 695 GeV Expected limit

1 0 χ∼ m( ) = 0 GeV Observed 1 0 χ∼ ) m( -1 7TeV (4.7fb

Fig. 1. – Left: Exclusion limits at 95% CL for 8 TeV analysis in the (m0, m1/2)-plane for the

MSUGRA/CMSSM model where tan β = 30, A0 =−2 m0, μ > 0. Part of the model plane

accommodates a lightest neutral scalar Higgs boson mass of 125 GeV. The dashed and solid lines show the expected and observed limits, respectively, including all uncertainties except the theoretical signal cross-section uncertainties [13]. Right: Exclusion limits for a simpliﬁed model framework with only strong production of gluinos and 1st/2nd-generation squarks (of common mass), with direct decays to jets and ˜χ01(LSP) for diﬀerent hypothesis on mχ˜0

1. The dashed lines

show the expected limits at 95% CL, with the light (yellow) band indicating the 1σ experimental and background-theory uncertainties on the m_χ_˜0

1 = 0 limit. Observed limits are indicated by

solid curves, while the dotted lines represent the mχ˜0

1 = 0 observed limits obtained by varying

the signal cross-section by the theoretical scale and PDF uncertainties [14].

3. – Searches for 3rd-generation squarks

The exclusions up to the TeV scale of heavy squarks and gluinos has drawn the attention to lighter sparticles production, and the search for bottom and top squarks is currently one of the most active areas in the search for SUSY. RPC models foresee the pair production of third generation squarks at LHC; this production can be either gluino mediated or direct. The outcome of these searches has been interpreted in the simpliﬁed models picture, where only the particles of interest can be produced and only the most relevant decay modes are considered with 100% branching ratio (BR), while the other particles are set to very high masses.

3.1. Gluino-mediated production. – Directly produced gluino pairs are assumed to

decay to on-shell or oﬀ-shell stops or sbottoms (Gtt model: ˜g→ ˜tt, Gbb model: ˜g → ˜bb),

which then decay to t or b and ˜χ0

1(LSP). Final states contain large ETmiss, multiple jets,

b-jets and leptons. Figure 2, left, shows one of discriminating variables used for the search [17] for gluino mediated stop production in ﬁnal states with 7 jets, among which at least three are b-jets, and with either one or no leptons. This variable, called eﬀective

mass, is deﬁned as the sum of the E_Tmiss and of the pT of the leading jets in the events,

and would give indication of the SUSY mass scale if any excess would be observed. This analysis posed the strongest constrains on the Gtt model, as can be seen also from fig. 2, right, where the results of the searches for gluino mediated stop production in final states containing different flavor/multiplicity of jets and and lepton are summarized.

3.2. Direct stop production. – Figure 3 shows the results obtained by diﬀerent analyses

looking for top squark direct production as a function of the stop and lightest neutralino (LSP) masses for simpliﬁed models. The left part of the plot summarizes the searches

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Events / 100 GeV 1 10 2 10 3 10 Data 2012 SM total Reducible bkg (MM) Irreducible bkg (MC) 10 × ) (1300, 100) GeV 0 1 χ∼ g, ~ Gtt ( = 8 TeV s , -1 Ldt = 20.1 fb ∫

0-lepton baseline selection 3 b-jets > 30 GeV ≥ 7jets > 30 GeV, ATLASPreliminary [GeV] incl eff m 400 600 800 1000 1200 1400 1600 1800 2000 data / exp 0.5 1 1.5

Fig. 2. – Left: Inclusive eﬀective mass distribution for the gluino-mediated stop production in the fully hadronic decay channel with 7 jets, among which at least 3 are b-jets. Data (black points) are compared to the background expectation (ﬁlled area) and to one signal point from the Gtt model (dotted line). The ratio between the observed event yield and background prediction is also shown [17]. Right: Exclusion limits at 95% CL for 8 TeV analyses in the (mg˜, mχ˜0

1)-plane

for the Gtt simpliﬁed model where a pair of gluinos decays promptly via oﬀ-shell stop to four top quarks and two lightest neutralinos (LSP). The dashed and solid lines show the expected and observed limits, respectively, including all uncertainties except the theoretical signal cross-section uncertainties [13]. [GeV] 1 t ~ m 100 200 300 400 500 600 700 1 0 χ ∼ +m t < m 1 t ~ m 1 0 χ ∼ + m W + m b < m 1 t ~ m 1 0 χ ∼ + m c < m 1 t ~ m 200 300 400 500 600 ) 1 0 χ∼ m × = 2 1 ± χ∼ ( m 1 ± χ ∼ +m b < m 1 t ~ m < 106 GeV 1 ± χ∼ m ( = 150 GeV) 1 ± χ∼ > m 1 0 χ∼ m +5 GeV) 1 0 χ ∼ = m 1 ± χ ∼ ( m 1 ± χ ∼ +m b < m 1 t ~ m < 103.5 GeV 1 ± χ∼ m [GeV]0_χ∼1 m 0 100 200 300 400 500 600 Observed limits Expected limits All limits at 95% CL [1203.4171] -1 CDF 2.6 fb ATLAS Preliminary production 1 t ~ 1 t ~ Status: SUSY 2013 =8 TeV s -1 = 20 - 21 fb int L _{= 4.7 fb}-1s=7 TeV int L 0L ATLAS-CONF-2013-024 1L ATLAS-CONF-2013-037 2L ATLAS-CONF-2013-065 2L ATLAS-CONF-2013-048 0L mono-jet/c-tag, CONF-2013-068 0L 1308.2631 -1L CONF-2013-037, 0L 1308.2631 2L ATLAS-CONF-2013-048 1L CONF-2013-037, 2L CONF-2013-048 0L [1208.1447] 1L [1208.2590] 2L [1209.4186] -2L [1208.4305], 1--2L [1209.2102] -1-2L [1209.2102] 1 0 χ∼ (*) W → 1 ± χ∼ , 1 ± χ∼ b → 1 t ~ 1 0 χ∼ t → 1 t ~ / 1 0 χ∼ W b → 1 t ~ / 1 0 χ∼ c → 1 t ~ 0L, 1L, 2L, 2L, 0L, 0L, 1-2L, 1L, 2L, 1-2L, 1 0 χ∼ t → 1 t ~ 1 0 χ∼ t → 1 t ~ 1 0 χ∼ t → 1 t ~ 1 0 χ∼ W b → 1 t ~ 1 0 χ∼ c → 1 t ~ mono-jet/c-tag, + 5 GeV 1 0 χ∼ = m ± 1 χ m = 106 GeV ± 1 χ , m 1 ± χ∼ b → 1 t ~ = 150 GeV ± 1 χ , m 1 ± χ∼ b → 1 t ~ - 10 GeV 1 t ~ = m ± 1 χ , m 1 ± χ∼ b → 1 t ~ 1 0 χ∼ m × = 2 ± 1 χ , m 1 ± χ∼ b → 1 t ~

Fig. 3. – Exclusion limits at 95% CL for diﬀerent 8 TeV and 7 TeV searches for stop pair produc-tion in the (m˜t1, mχ˜01)-plane. The dashed and solid lines show the expected and observed limits,

respectively, including all uncertainties except the theoretical signal cross-section uncertainty (PDF and scale) [13].

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[GeV] 1 b ~ m 100 200 300 400 500 600 700 800 [GeV] 0_χ∼1 m 0 100 200 300 400 500 600 forbidden 0 1 χ ∼ b → 1 b ~ -1 CDF 2.65 fb -1 D0 5.2 fb =7 TeV s , -1 ATLAS 2.05 fb 0 1 χ∼ b → 1 b ~ Sbottom pair production,

=8 TeV s , -1 Ldt = 20.1 fb

∫

ATLAS theory) SUSY σ 1 ± Observed limit ( ) exp σ 1 ± Expected limit ( All limits at 95% CL [GeV] 1 b~ m 300 350 400 450 500 550 600 650 [GeV]±_χ∼1 m 100 200 300 400 500 600 forbidden 1 ± χ ∼ t → b ~ ) = 60 GeV 1 0 χ∼ , m( 1 ± χ∼ t → 1 b~ production, b ~ -b ~

2 same-charge leptons + jets =8 TeV s , -1 L dt = 20.7 fb

∫

298 194 149 127 108 95 82 86 76 73 77 66 66 67 55 62 56 56 55 50 50 90 72 46

Numbers give 95% CL excluded model cross sections [fb]

) theory SUSY σ 1 ± Observed limit ( ) exp σ 1 ± Expected limit ( =8TeV s , -1 4 jets, 13.0fb ≥ 3-leptons, ATLAS Preliminary All limits at 95% CL [GeV] 1 b ~ m 200 300 400 500 600 700 800 900 1000 [GeV] 0 2 χ∼ m 200 300 400 500 600 700 800 900 1000 1100 0 2 χ∼ b+ → 1 b ~ production, 1 b ~ -1 b ~ ) = 60 GeV 0 1 χ∼ m( 0 1 χ∼ h + → 0 2 χ∼ forbidden 0 2 χ ∼ b+ → 1 b ~ =8 TeV s , -1 = 20.1 fb int L

0 and 1 lepton + 3 b-jets channels

All limits at 95% CL

ATLAS Expected limit ±1 σexp

Theory SUSY σ 1 ± Observed limit Preliminary

Fig. 4. – Exclusion limits at 95% CL for direct sbottom production search in the b ˜χ01 (top

left) [18], in the t ˜χ±₁ (top right) [17] and in the b ˜χ0

2 (bottom) [19] decay modes. The bands

around the expected limits show the±1σ uncertainties. The dotted lines around the observed limits represent the results obtained when moving the nominal signal cross-section up or down by±1σ theoretical uncertainty.

for ˜t decaying to a bottom and a lightest chargino ( ˜χ±₁), where the ˜χ±₁ further decays

into the LSP and a W boson, while the right part the searches for the decays in the t ˜χ0

1,

in the c ˜χ0

1 and in the W b ˜χ01 channels. For massless ˜χ01, stop masses are excluded up to

700 GeV, while for massive neutralinos of 250–300 GeV these limits fall to 450–500 GeV.

3.3. Direct sbottom production. – The dominant decay modes of the sbottom involve

either the neutralinos and a b quark (˜b→ b˜χ0₂, ˜b→ b˜χ0₁) or the lightest chargino and a

top quark (˜b→ t˜χ±₁). The sbottom pair decays into two lightest neutralinos and b¯b are

targeted by analyses requiring lepton veto, two b-jets and a signiﬁcant amount of Emiss

T .

If the second lightest neutralino is produced, the decays involve a Z or the Higgs boson and additional leptons and b-jets are foreseen in the ﬁnal states. Finally, if charginos

are produced, they decay into W and ˜χ0

1: this signal scenario is explored by analyses

requiring two same-sign leptons or three leptons in the ﬁnal state. Some of the results of these searches [17-19] are shown in ﬁg. 4.

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) [GeV] 2 0 χ∼ (=m 1 ± χ∼ m 100 200 300 400 500 600 [GeV] 0_χ∼1 m 0 50 100 150 200 250 300 350 400 450 500 Expected limits Observed limits ATLAS-CONF-2013-035 , μ , 3e/ ν∼ / L l ~ , via 0 2 χ∼ ± 1 χ∼ → pp ATLAS-CONF-2013-049 , μ , 2e/ ν∼ / L l ~ , via -1 χ∼ + 1 χ∼ → pp ATLAS-CONF-2013-028 , τ , 2 τ ν∼ / L τ∼ , via 0 2 χ∼ ± 1 χ∼ → pp ATLAS-CONF-2013-028 , τ , 2 τ ν∼ / L τ∼ , via -1 χ∼ + 1 χ∼ → pp ATLAS-CONF-2013-035 , μ , via WZ, 3e/ 0 2 χ∼ ± 1 χ∼ → pp ATLAS-CONF-2013-093 bb, μ , via Wh, e/ 0 2 χ∼ ± 1 χ∼ → pp

=8 TeV Status: SUSY 2013 s , -1 = 20.3-20.7 fb int Preliminary L ATLAS ) 2 0 χ∼ + m 1 0 χ∼ = 0.5(m ν∼ / L τ∼ / L l ~ m 1 0 χ ∼ < m 2 0 χ ∼ m Z + m 1 0 χ ∼ = m 2 0 χ ∼ m h + m 1 0 χ ∼ = m 2 0 χ ∼ m 1 0 χ ∼ = 2m 2 0 χ ∼ m ) [GeV] 1 0 χ∼ m ( 100 200 300 400 500 600 700 800 ) [GeV]₂ 0 χ∼ , 1 1±_χ∼ m ( 300 400 500 600 700 800 900 1000 17 15 4.2 2.9 11 9.2 3.8 3.2 2.8 8.6 10 3.6 2.9 2.3 6.6 7.9 3.5 2.5 2.1 5.5 6.4 3 3 2.4 1.9 4.9 5.8 2.8 2.6 2.3 1.9 ) Theory SUSY σ 1 ± Observed ( ) Exp. σ 1 ± Expected ( ATLAS Preliminary = 8 TeV s , -1 L dt = 20.3 fb ∫ < 0.1 mm τ = 1.5, c β GGM: bino-like neutralino, tan

) Forbidden 2 0 χ ∼ , 1 1 ± χ ∼ ) > m ( 1 0 χ ∼ m(

Numbers give 95% CL upper limits on the production cross-section [fb]

Fig. 5. – Left: Summary of ATLAS searches for electroweak production of charginos and neu-tralinos for diﬀerent 8 TeV analyses. Exclusion limits at 95% CL are shown in the (m_χ_˜±

1 , mχ˜0

1

)-plane. The dashed and solid lines show the expected and observed limits, respectively, including all uncertainties except the theoretical signal cross-section uncertainties [13]. Right: Expected and observed 95% CL lower limits on the wino mass as a function of the neutralino mass in the GGM model with a bino-like lightest neutralino (NLSP). The lower bound of the excluded cross-section (in fb−1) is displayed at each grid point. The green band shows the expected-limit (±1σ), while the three contours correspond to three observed-limits: one for the nominal

wino production cross-section, and one each for a cross-section augmented or diminished by one

standard deviation of the cross-section uncertainty [21].

4. – EW production

The most recent and powerful ATLAS result on the direct production of ˜χ±₁ and ˜χ0

2

is reported in [20]. The search is performed in ﬁnal states containing three leptons and

Emiss

T . Four SRs have been used to target ˜χ±1 and ˜χ02 decays to ˜χ01 (LSP) via either

all three generations of sleptons, staus only, gauge bosons, or Higgs bosons in simpliﬁed

SUSY scenarios where ˜χ0

1 is 100% bino-like while ˜χ±1 and ˜χ02 are 100% wino-like and

mass degenerate. Data were found in agreement with the SM expectations and limits

have been set in the (m_χ_˜±

1, mχ˜01/ ˜χ±1) mass plane for each SR. In the limit of massless ˜χ

0 1,

˜

χ±₁/ ˜χ0

2are excluded up to 700 GeV, 380 GeV, 345 GeV, or 148 GeV respectively. Previous

searches for charginos and neutralinos are summarized in ﬁg. 5.

Limits on the ˜χ0

1 mass in the GGM [22] scenario are set in [21]. In this case ˜χ01 is

the next to lightest supersymmetric particle (NLSP) and is mostly bino-like, while the LSP is the gravitino, therefore the search is performed in ﬁnal states involving two real

photons and Emiss

T .

5. – Searches for long-lived particles

Long-lived particles having a proper decay length cτ > 1 mm are predicted in both

RPC and RPV SUSY models. For instance, the LSP ( ˜χ0

1) in RPV SUSY models is

allowed to decay; for lifetimes in the picosecond-to-nanosecond range, the expected ﬁnal states involve a muon and many charged tracks originating from a single displaced vertex (DV). The results of the ATLAS search [23] for such signals were found to be consistent with the background expectations, thus limits were set, for instance, as a function of the neutralino lifetime for a range of neutralino masses and velocities.

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[GeV] 1 ± χ∼ m 100 150 200 250 300 350 400 450 500 550 600 [ns]±_χ∼1 τ -1 10 1 10 ATLAS -1 L dt = 20.3 fb ∫ = 8 TeV, s ) theory σ 1 ± Observed 95% CL limit ( ) exp σ 1 ± Expected 95% CL limit ( , EW prod.) -1 = 7 TeV, 4.7 fb s ATLAS (

ALEPH (Phys. Lett. B533 223 (2002))

± 1 χ∼ ‘Stable’ > 0 μ = 5, β tan

Fig. 6. – Left: Constraints on long-lived charginos in the (τ_χ_˜±

1

-m_χ_˜±

1

)-plane. The black dashed line shows the expected limits at 95% CL, with the surrounding shaded bands indicating the 1σ exclusions due to experimental uncertainties. Observed limits are indicated by the solid bold contour representing the nominal limit and the surrounding shaded bands are obtained by varying the cross-section by the theoretical scale and PDF uncertainties [25]. Right: Bayesian lower limits on ˜g mass versus its lifetime in the ATLAS stopped gluino search [26].

In AMSB models [24] as well the LSP is the lightest neutralino, and in this case it

is nearly mass degenerate with lightest chargino. The small mass splitting (∼ 160 MeV)

between these two particles results in a considerable chargino lifetime, and therefore its

decays, mainly into ˜χ±₁ π∓ pairs, are expected to be observed as “disappearing” tracks

outside the tracker volume, as the π∓ is not reconstructed. The constraint posed on such

events by ATLAS [25] are shown in ﬁg. 6, right.

Color-singlet states of R-mesons (˜gq ¯q), R-baryons (˜gqqq) and R-gluinoballs (˜gg),

gen-erally called R-hadrons, may occur in some RPC scenarios like split SUSY [27], where gluinos are much lighter than squarks and their decays are suppressed. In RPV SUSY

long-lived squarks may also form an R-hadron (e.g. ˜tq) having similar phenomenology

as in the RPC R-hadrons but smaller production cross-sections. The ATLAS search for long-lived R-hadrons which may loose energy and stop in the detector material can be found in [26]. A strong suppression of the main sources of background (cosmic rays, beam halo) is obtained by selecting empty bunch crossings trigger events with some calorimeter activity, energetic jets in the central pseudorapidity region and no tracks in the Muon Spectrometer. Also in this case no excess is found and limits are set on the mass of the

long-lived hadronizing particles: as an example, ﬁg. 6 shows the limits obtained for ˜g

masses as a function of ˜g lifetime.

Finally, a search for long-lived charged sleptons using 16 fb−1of integrated luminosity

collected during 2012 is reported in [28]; in this case the sleptons are expected decay out-side the detector volume and appear as if they are heavy muons, charged and penetrating.

Mass limits are set on long-lived ˜τ (NLSP) production in the GMSB scenario [29]; model

independent limits are also set on the direct production of long-lived sleptons and on the direct production of gauginos decaying to long-lived staus.

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6. – Summary

The search for SUSY is one of the most important part of the LHC physics programme. Several searches have been carried out in ATLAS to cover a wide multiplicity of SUSY scenarios, and in all of them the results were found in agreement with the background expectations. Data so far collected allowed to exclude strong production of 1st- and 2nd-generation squarks and gluinos up to the TeV scale. Naturalness arguments favor the existence of light top and bottom squarks; exclusions for the masses of these particles

are set up to∼ 700 GeV. EW SUSY production of chargino and neutralinos have been

investigated in multi-lepton ﬁnal states, and the most sensitive analysis posed limits on

the mass of these particles up to ∼ 700 GeV. Several searches for long-lived particles

have been performed; all the results, consistent with the background expectations, have been interpreted both in RPC and RPV scenarios.

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