Outline
Introduction The LHC HUNTING FOR The ATLAS experiment
SUSY AT ATLAS Supersymmetry Motivations Benchmarks and strategy Early physics reach Antonella De Santo Conclusions Southampton, November 28, 2008
Southampton, 28/11/08 A. De Santo 1
LHC’s Main Goals
Elucidate mechanism for EW symmetry breaking Search or Higgs boson in O(100 GeV)-O(1 TeV) range
If no light Higgs is found, study WW scattering at high mass THE LHC AND ATLAS Look for evidence of new physics at TeV-scale
Deviations from Standard Model predictions in data
Supersymmetry
(and Exotics…)
Southampton, 28/11/08 A. De Santo 3 LHC’s Main Goals The Alps
Lac Léman Elucidate mechanism for EW symmetry breaking Search or HiggsNot boson inToday O(100 GeV)-O(1 TeV)! range If no light Higgs is found, study WW scattering at high mass
The Large Hadron Collider Look for evidence of new physics at TeV-scale @ CERN Deviations from Standard Model predictions in data p-p @ Ecm = 14 TeV Supersymmetry
(and Exotics…) 2πR = 27 km Southampton, 28/11/08 A. De Santo 4 Southampton, 28/11/08 A. De Santo 5
Point 1 @ CERN
Southampton, 28/11/08 A. De Santo 6 Southampton, 28/11/08 A. De Santo 7 ATLAS Modern “Cathedrals”
ATLAS
μ
6.920 m
5.635 m
4.645 m
3.850 m
2.950 m
2.864 m 1.840 m Building 40 1.320 m @ CERN CMS
Y
ϕ X
Towards Center of LHC
CMS-TS-00079 Transverse View Southampton, 28/11/08 A. De Santo 8 Southampton, 28/11/08 A. De Santo 9
The ATLAS Detector Detector Requirements Electromagnetic Muon Calorimeters Detectors Forward Calorimeters Solenoid End Cap Excellent position and momentum resolution in central tracker Toroid b-jets, taus Excellent ECAL performance electrons, photons
v. good granularity (energy and position measurements) p ( 7 TeV )
Good HCAL performance p ( 7 TeV ) jets, Etmiss (neutrinos, SUSY stable LSP, etc) good granularity (energy and position measurements) good η coverage (hermeticity for Etmiss measurements) Excellent muon identification and momentum resolution from “combined” muons in external spectrometer + central tracker Hadronic Barrel Inner Detector Calorimeters Shielding Toroid Southampton, 28/11/08 A. De Santo 10 Southampton, 28/11/08 A. De Santo 11 Collisions at a hadron collider Minimum Bias σinel(pp)~70 mb
x1p Non-single diffractive interactions d d (soft partons) p u p pu p u u x p “Operational” definition of “minimum 2 bias” depends on experimental trigger Etot=? Ptot=? sˆ = x1x2s
No constraints on total initial energy LHC predictions Measured at lower energies (SppS and Tevatron) Broad range of √s Æ good for discovery! CDF Large uncertainties on extrapolation All possible processes are “on” to LHC energies simultaneously PDFs UA5
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The “Minimum Bias” Challenge “Hard Scatters” Only
All reconstructed tracks
Only reconstructed trks w/ PT > 25 GeV
HÆ 4μ + Minimum Bias HÆ 4μ
At design luminosity, on average ~25 low-PT (“minimum bias”) events are Solution: Remove all the uninteresting low-pT tracks from the event superimposed to any rare, high-PT discovery signal – ~1000 low-PT tracks per event !
Southampton, 28/11/08 A. De Santo 14 Southampton, 28/11/08 A. De Santo 15 Design luminosity: σ Design luminosity: σ “Low” = (2x)1033 cm-2 s-1 (O(10 fb-1) / yr ) “Low” = (2x)1033 cm-2 s-1 (O(10 fb-1) / yr ) “High” = 1034 cm-2 s-1 (O(100 fb-1) / yr) tot inel “High” = 1034 cm-2 s-1 (O(100 fb-1) / yr) tot inel pp ~ 110 mb (σ pp ~ 70 mb) pp ~ 110 mb (σ pp ~ 70 mb)
“Minimum Bias” rate: O(109 Hz) !!
#evts Rates(Hz) Process σ (nb) [10 fb−1] [ “high” L ] bb 500 μb 5x1012 5x106 W → eν 15 nb ~108 150 Z → ee 1.5 nb ~107 15 tt 800 pb ~107 10 ~~ gg (1 TeV) ~1 pb ~104 10-2
H (200 GeV) → 4l 2 -4 ~10 fb ~10 10 Must rely on distinctive signatures leptons, photons, b-jets, missing ET, … Southampton, 28/11/08 14 TeV A. De Santo 16 Southampton, 28/11/08 A. De Santo 17
The ATLAS “Brain” The ATLAS “Brain”
At each trigger stage, discard uninteresting events as quickly as possible At each trigger stage, discard uninteresting events as quickly as possible
Data from 40 million events the detector 40 million events per second per second
L1 Trigger Selection 2.5 μs L1 100,000 events 100,000 events Regions per second per second of Interest (RoIs) L2 ~10 ms L2 HLT HLT 1,000 events 1,000 events per second per second EF ~1 s EF 100 events per 100 events per second second Event size Data stored Event size 1-2 MBytes “on tape” 1-2 MBytes Southampton, 28/11/08 A. De Santo 18 Southampton, 28/11/08 A. De Santo 19 The Data Deluge The ATLAS High-Level Trigger
The “Grid” Z(Æee)+jet event “Seeded” and “stepwise” Concorde early rejection of uninteresting events (15 Km) minimum amount of processing maximum flexibility
RoIs “seed” trigger reconstruction chain <10% of the event accessible at L2 1 year of EF seeded by L2 ( “offline” reco) LHC data
(10 PByte) 20 km Reduced CPU/ bandwidth requirements but increased complexity Mt. Blanc (4.8 Km) Avoid biases but retain flexibility to account for the “unexpected”
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The ATLAS High-Level Trigger The ATLAS High-Level Trigger
e.m. clusters “Seeded” and “stepwise” “Seeded” and “stepwise” early rejection of uninteresting events early rejection of uninteresting events minimum amount of processing minimum amount of processing maximum flexibility maximum flexibility
RoIs “seed” trigger reconstruction chain RoIs “seed” trigger reconstruction chain <10% of the event accessible at L2 <10% of the event accessible at L2 EF seeded by L2 ( “offline” reco) EF seeded by L2 ( “offline” reco) RoIs Reduced CPU/ bandwidth requirements Reduced CPU/ bandwidth requirements but increased complexity but increased complexity
Avoid biases but retain flexibility Avoid biases but retain flexibility to account for the “unexpected” to account for the “unexpected”
Southampton, 28/11/08 A. De Santo 22 Southampton, 28/11/08 A. De Santo 23 The ATLAS High-Level Trigger
“Seeded” and “stepwise” early rejection of uninteresting events minimum amount of processing maximum flexibility
RoIs “seed” trigger reconstruction chain <10% of the event accessible at L2 STATUS electron EF seeded by L2 ( “offline” reco) tracks Reduced CPU/ bandwidth requirements but increased complexity
Avoid biases but retain flexibility to account for the “unexpected”
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The world did not come to an end… A Great Start…
Southampton, 28/11/08 SeptemberA. De Santo 10, 2008 26 Southampton, 28/11/08 A. De Santo 27 Unfortunately…
Incident in LHC sector 34 First beam event Geneva, 20 September 2008. During commissioning (without beam) of the final LHC sector (sector 34) at in ATLAS high current for operation at 5 TeV, an incident occurred at mid-day on Friday 19 September resulting in a large helium leak into the tunnel. Preliminary investigations indicate that the most likely cause of the problem was a faulty electrical connection between two magnets, which probably melted at high current leading to mechanical failure. CERN ’s strict safety regulations ensured that at no time was there any risk to people.
A full investigation is underway, but it is already clear that the sector will have to be warmed up for repairs to take place. This implies a minimum of two months down time for LHC operation. For the same fault, not uncommon in a normally conducting machine, the repair time would be a matter of days.
Further details will be made available as soon as they are known.
(R.Aymar, CERN DG)
Southampton, 28/11/08 A. De Santo 28 Southampton, 28/11/08 A. De Santo 29
Unfortunately…
Incident in LHC sector 34 Geneva, 20 September 2008. During commissioning (without beam) of the final LHC sector (sector 34) at high current for operation at 5 TeV, an incident occurred at mid-day on Friday 19 September resulting in a large helium leak into the tunnel. Preliminary investigations indicate that the most likely cause of the problemns was a faulty electrical connection between two magnets,i whichsi oprobably melted at high current leading to mechanical failure.o CERNll’s strict safety regulations ensured that at no time was therers anyt risk c to people. 2009 SUPERSYMMETRY A full investigationF isi underway, but it is alreadyi clearn that the sector will have to be warmed up for repairs to take place.d This implies a minimum of two months down time for LHC operation.cte For the same fault, not uncommon in a normally conductingex machine,pe the repair time would be a matter of days. Further details will be made available as soon as they are known.
(R.Aymar, CERN DG)
Southampton, 28/11/08 A. De Santo 30 Why go Beyond the Standard Model? Hierarchy Problem and Naturalness
Despite its many successes, the Standard Model is widely In SM, loop corrections to Higgs bosonδ mass: α believed to be only an effective theory, valid up to a scale 2 ⎛ ⎞ 2 m H = O⎜ ⎟ Λ Λ << M Planck ⎝ π ⎠ Theory cut-off Natural scale of scalar mass is very large! Gravity not included in SM
Hierarchy/naturalness problem: These corrections, which are large, give rise to fundamental problems when requiring that: MEW << MPlanck Fine-tuning mH << fundamental mass scale (i.e. MPlanck) (hierarchy problem) 2 2 Unification of couplings Corrections δmH to Higgs mass should not be >> mH (naturalness) Need a more fundamental theory of which the SM is only a Need either fine-tuning or protective symmetry! low-energy approximation Southampton, 28/11/08 A. De Santo 32 Southampton, 28/11/08 A. De Santo 33
Unification of Coupling Constants in the SM Supersymmetry
Space-time symmetry that relates fermions (matter) and bosons (interactions) Q boson = fermion and Q boson = fermion
Further doubling of the particle spectrum Every SM field has a “superpartner” with the same mass Spin differs by 1/2 between SUSY and SM partners Or lack of it… Identical gauge numbers Identical couplings Superpartners have not been observed SUSY must be a broken symmetry But SUSY-breaking terms in Lagrangian must not re-introduce quadratic divergences in theory !
Southampton, 28/11/08 A. De Santo 34 Southampton, 28/11/08 A. De Santo 35 Minimal Supersymmetric Standard Model Supersymmetric Solution to Divergences
Supersymmetric Partners Standard Model Particles Bosonicδ and fermionic diagrams now give equal and opposite contributions: and Fields f Interaction Mass S Eigenstates Eigenstates HHπ Symbol Name Symbol Name Symbol Name H quark ~ ~ squark q~ ,q~ squark q = u,d,c, s,t,b qL ,qR 1 2 π ~ ~ ~ ~ 2 2 α l = e, μ, τ lepton slepton l1, l2 slepton ⎛ gf ⎞ ⎛ g ⎞ lR , lL 2 2 2 S 2 2 ⎛ ⎞ 2 2 ~ ~ m =⎜ ⎟ ()Λ +m − ⎜ ⎟ ()Λ +m =O⎜ ⎟ m − m l =ν , ν μ , ν ν ν H 2 f ⎜ 2 ⎟ S S f e τ neutrino sneutrino sneutrino ⎜16 ⎟ 16 4 π ~ ⎝ ⎠ ⎝ ⎠ ⎝ ⎠ g gluon g gluino g~ gluino ± ~ ± If #boson = #fermions and they have equal masses and couplings, the W W-boson W wino ~± χ1,2 + − chargino quadratic divergences cancel charged ~ + ~ − charged Hu , H d Higgs boson H u , H d higgsino 2 2 2 2 2 ~ Higgs mass correction δ m < m if mS − m f <~ TeV B B-field B bino H H 0 W W0-field ~ 0 wino ~0 W χ1,2,3,4 neutralino Gauge boson contribution cancelled by gaugino contribution ~ ~ 0 0 neutral Higgs H 0 , H 0 neutral Hu , H d u d Southampton, 28/11/08 boson A. De Santohiggsino 36 Southampton, 28/11/08 A. De Santo 37
Unification of Coupling Constants in MSSM R-parity
A new symmetry is introduced to cure unwanted effects (e.g. proton decay) from lepton and baryon number violating terms in MSSM:
3 (B−L)+2S Rp = +1 (SM) Rp = ()−1 −1 (SUSY)
Stable LSP (=Lightest SUSY Particle) Rp conservation typically the lightest neutralino (good DM candidate) Pair-production of sparticles in scattering of SM particles (e.g. pp at the LHC)
Now unification of strong, weak and e.m. forces achieved at ~MGUT In the following, will assume R-parity conservation Southampton, 28/11/08 A. De Santo 38 Southampton, 28/11/08 A. De Santo 39 Neutralino as Dark Matter Constituent Benchmarks and Strategy
WMAP 2 Baseline paradigm is Rp-conserving mSUGRA: 0.094 < ΩCDMh < 0.136 (95% CL) 3 (B−L)+2S m0 universal scalar mass Rp =()−1 m universal gaugino mass 1/2 Stable LSP A trilinear soft breaking parameter at GUT scale 0 (good DM candidate) tanβ ratio of Higgs vevs sgn(μ) sign of SUSY Higgs mass term
Other scenarios are also considered (GMSB, AMSB, …)
Full coverage of signatures and topologies in the entire phase space
Neutralino LSP is a good DM candidate stable electrically neutral If it’s there, don’t miss it !! weakly and gravitationally interacting
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mSUGRA Benchmarks SUSY Cross-Sections
Four WMAP-compatible regions, with different mechanisms for neutralino annihilation Cross-section dominated by the and rather different observable phenomenology production of coloured particles
bulk (pb)
neutralino mostly bino, annihilation to tot 3000 ff via sfermion exchange σ
focus point neutralino has strong higgsino (GeV) component, annihilation to WW, ZZ co-annihilation pure bino, small NLSP-LSP mass difference, typically coannihilation with stau
100 Higgs funnel decay to fermion pair through m (GeV) resonant A exchange – high tanβ 250 350 Southampton, 28/11/08 (GeV) A. De Santo 42 Southampton, 28/11/08 A. De Santo 43 Production Mechanisms Possible Final States
Inclusive Squark/Gluino Searches Production Exclusive Searches
Direct Gaugino
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Inclusive Searches Inclusive Searches
p p p p ∼0 ∼0 ~ χ 1 ~ χ 1 ~ q ∼0 ~ ~ q ∼0 ~ g χ 2 l g χ 2 l q q q l l q l l 4 miss ( jet) Meff = ET + ∑ET ()+ pT (lep) ~0 ~0 jet=1 Complex long decay chains to undetected χ1 Complex long decay chains to undetected χ1 Inclusive search: Inclusive search:
high multiplicity of high-pT jets high multiplicity of high-pT jets miss miss large ET (from escaping LSP) large ET (from escaping LSP) ≥ 0 (high-pT) leptons ≥ 0 (high-pT) leptons
Southampton, 28/11/08 A. De Santo 46 Southampton, 28/11/08 A. De Santo 47 A (simulated!) SUSY event in ATLAS Standard Model Backgrounds
0-lepton mode 1-lepton mode
Dominant SM backgrounds: multi-jet QCD Need robust, data-driven background W/Z+jets predictions (as well as predictions ttbar from reliable Monte Carlo simulations) Multi-jet event in dibosons Bulk Region 6 jets Before we can claim a discovery, we must achieve complete confidence in our understanding of the detectors, the trigger, the reconstruction and 2 high-pt muons particle identification algorithms and, above all, of the SM backgrounds Large missing ET Southampton, 28/11/08 A. De Santo 48 Southampton, 28/11/08 A. De Santo 49
Data-Driven Background Estimation – An Example Discovery potential with 1 fb-1 μ μ ν ν Charged lepton replacement in (ZÆll)+jets events
Discovery potential largely dominated by cross-sections and backgrounds
Southampton, 28/11/08 A. De Santo 50 Southampton, 28/11/08 A. De Santo 51 Discovery potential with 1 fb-1 Constraining SUSY – Dilepton Edges
p p ∼0 ~ χ 1 ~ q ∼0 ~ χ g χ 2 l ~ q M2(lχ) M2(χ~0) q Mmax = M(~0) 1− R 1− 1 l l ll 2 2 ~0 2 ~ M ( 2 ) M (lR)
Use flavour-subtracted combination (SFOS-OFOS) to suppress backgrounds
SU3, 1 fb-1
Comparable potential for ATLAS and CMS (largely determined by cross-sections and backgrounds)
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More Edges Fits to Mass Edges
ATLAS, SU3, 1 fb-1 ~ 0 q ~ Already with O(1 fb-1), L χ2 ~ m the fits to these edges l 0 ~ can give reasonably good χ1 q ± estimates of SUSY mass l m differences, and can be l Invariant mass combinations ll + jet mass used to constrain SUSY ll + jet mass lq edge of lepton and jets ~ Under the assumption q ~0 that the considered decay L χ2 ~m chain is the one actually l ~0 realised in data Use them to set extra χ1 constraints on SUSY mass q ± l m l + jet mass (low) differences l l + jet mass (high)
llq edge
Southampton, 28/11/08 A. De Santo 54 Southampton, 28/11/08 A. De Santo 55 104 SU2, “Focus Point” Trileptons in ATLAS Inclusive Trilepton Analysis – I
3 -1 10 Leading jet P Cross T t t 103 mSUGRA Parameters (3-lepATLAS evts) Zb Sections (pb) Dibosons Point Z γ 102 SU4
Events/1 fb SU3 m0 m1/2 A0 tanβ sgn(μ) Tot. 3-lep Crucially simple analysis flow: 2 SU2 ∼∼ 10 SU2 χχ SU2 3550 300 0 10 + 7.2 0.07 10 SU3 100 300 -300 6 + 27.7 0.30 3 isolated leptons (e, μ) (taus are next step!) SU4 200 160 -400 10 + 402.2 2.5 1 3-body decay At least 1 high-pt jet -1 10 0 50 100 150 200 250 300 350 400 450 500 p leading jet [GeV] T Don’t need large missing ET cut B S Signal significance SU4 – good !! 103 ATLAS 2-body decay vs. Leading jet PT SU3 104 104 SU3, “Bulk” Missing E SU4, “Low-mass” SU2 T (after pT(jet) cut) 102
103 103 10
102 102 1 0 50 100 150 200 250 300 350 400 56 p leading jet [GeV] 57 T
Inclusive Trilepton Analysis Exclusive 3-lep Analysis at Focus Point
Potentially an early SUSY electron Dilepton Invariant Mass discovery channel!! 1 fb-1 trk isol. S/√Bvs.
B 10 S IntegratedATLAS Lumi SU4 9 SU3 8 SU2 7 6 muon Missing Transv. Energy 5 trk isol. Lepton track 4 isolation crucial 3 for this analysis 2 1 0 Statistical uncertainties only -2 -1 1010 -2 1010-1 11 -1 ∫ L dt [fb ] 58 59 Exclusive 3-lep Analysis at Focus Point
S/√Bvs. B S 10 IntegratedATLAS Lumi 9 SU2 Exclusive 8 SU2 Inclusive 7 Not an early physics channel 6 5 But potentially the only way to see 4 SUSY if both scalars and gluino 3 are too heavy! 2 1 CONCLUSIONS 0 -1 1010-1 11 1010 Statistical uncertainties only 10 fb-1 ∫ L dt [fb-1]
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Conclusions
With the LHC turn on, new physics at the TeV scale is becoming accessible experimentally for the first time at an accelerator
SUSY provides a well motivated extension to the Standard Model
Very excitingly, we might have the possibility to observe “dark matter in a laboratory” for the first time LET THE SHOW BEGIN !!
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