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Hunting for SUSY at ATLAS Antonella De Santo (RHUL) Royal Holloway HEP Seminar 18 th June 2007.

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Presentation on theme: "Hunting for SUSY at ATLAS Antonella De Santo (RHUL) Royal Holloway HEP Seminar 18 th June 2007."— Presentation transcript:

1 Hunting for SUSY at ATLAS Antonella De Santo (RHUL) Royal Holloway HEP Seminar 18 th June 2007

2 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 2 Outline  The case for Supersymmetry –theoretical motivations –R-parity and Dark Matter connection –mSUGRA  The LHC and ATLAS  SUSY searches at ATLAS –inclusive searches –exclusive searches  Conclusions

3 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 3 Why go Beyond the Standard Model?  Despite its many successes, Standard Model widely accepted to be only an effective theory, valid up to a scale  << M Planck –Gravity not included in SM –Hierarchy/naturalness problem: M EW << M Planck Fine-tuning –Unification of couplings  Need a more fundamental theory of which SM is only a low-energy approximation

4 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 4 Hierarchy Problem and Naturalness  These corrections, which are large, introduce two sets of problems when requiring that: –m H << fundamental mass scale (i.e. M Planck ) (hierarchy problem) –Corrections  m H 2 to Higgs mass should not be >> m H 2 (naturalness) Theory cut-off Natural scale of scalar mass is very large!  In SM, loop corrections to Higgs boson mass: Need either fine-tuning or protective symmetry!

5 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 5 Unification of Coupling Constants in SM Or lack of it… Slope of 1/  lines depends on matter and couplings of entire theory

6 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 6 Supersymmetry (SUSY)  Space-time symmetry that relates fermions (matter) and bosons (interactions)  Further doubling of the particle spectrum –Every SM field has a “superpartner” with same mass –Spin differs by 1/2 between SUSY and SM partners –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 !

7 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 7 Minimal Supersymmetric Standard Model (MSSM) – I Standard Model Particles and Fields Supersymmetric Partners Interaction Eigenstates Mass Eigenstates SymbolNameSymbolNameSymbolName quarksquark leptonslepton neutrinosneutrino gluongluino W-bosonwino chargino charged Higgs boson charged higgsino B-fieldbino neutralino W 0 -fieldwino neutral Higgs boson neutral higgsino

8 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 8 Standard Model Particles SpinSuperpartnersSpin quarks1/2squarks0 leptons1/2sleptons0 gauge bosons 1gauginos1/2 Higgs bosons 0higgsinos1/2 Minimal Supersymmetric Standard Model (MSSM) – II  Gauginos and higgsinos mix 2 charginos and 4 neutralinos  Two Higgs doublets 5 physical Higgs bosons (h,H; A; H±)

9 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 9  If #boson = #fermions and they have equal masses and couplings, the quadratic divergencies cancel –Higgs mass correction if Supersymmetric Solution to Divergencies  Now two diagrams, one bosonic and one fermionic, give equal contributions but with opposite signs: HH f H S  Gauge boson contribution cancelled by gaugino contribution

10 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 10 Unification of Coupling Constants in MSSM Now unification of strong, weak and e.m. forces at ~M GUT

11 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 11 R-parity  This can be prevented by introducing a new symmetry in the theory, called R-parity:  MSSM contains L- ad B-violating terms, which could in principle allow proton decay via sparticle diagrams:  R=+1 (SM particles), R=-1 (SUSY particles)  Two important consequences: –LSP (=Lightest SUSY Particle) is stable – typically neutralino –sparticles can only be produced in pairs (in scattering of SM particles)

12 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 12 In the following, assume R-parity conservation R-parity Stable LSP (neutralino)

13 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 13 WMAP Neutralino as Dark Matter Constituent <  CDM h 2 < (95% CL)  Neutralino LSP is a good DM candidate –stable –electrically neutral –weakly and gravitationally interacting

14 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 14  SUSY-breaking lagrangian terms do not re-introduce quadratic divergencies (“soft” breaking)  Empirically we know that SUSY must be a broken symmetry (no sparticles with same mass as particles)  Spontaneous breaking not possible in MSSM –otherwise sparticles with mass less than their SM partners would exist (Ferrara-Girardello-Palumbo)  SUSY breaking must be confined to a hidden sector, which communicates indirectly with the visible one via flavour-blind interactions (i.e. gravity) SUSY Soft Breaking

15 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 15 mSUGRA (or CMSSM)  Soft SUSY breaking mediated by gravitational interaction at GUT scale  Only five parameters: –m 0 — universal scalar mass –m 1/2 — universal gaugino mass –A 0 — trilinear soft breaking parameter at GUT scale –tan  — ratio of Higgs vevs –sgn(  ) — sign of SUSY Higgs mass term (|  |determined by EW symmetry breaking)  Highly predictive –masses determined mainly by m 0 and m 1/2

16 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 16  Universal boundary conditions at some high scale (GUT)  Evolution down to EW scale through Renormalisation Group Equations (RGE)  Radiative EW symmetry breaking – impose correct value of M Z at EW scale RGE Evolution Universal scalar mass Universal gaugino mass gluino, squarks charginos, neutralinos, sleptons

17 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 17 mSUGRA Parameter Space  Four regions compatible with WMAP value for  h 2, different mechanisms for neutralino annihilation: (GeV) –bulk neutralino mostly bino, annihilation to ff via sfermion exchange –focus point neutralino has strong higgsino component, annihilation to WW, ZZ –co-annihilation pure bino, small NLSP-LSP mass difference, typically coannihilation with stau –Higgs funnel decay to fermion pair through resonant A exchange – high tan 

18 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 18 The LHC u u d p u u d p p p E tot =? P tot =? “Broad band” parton beam – broad range of event √s (and all processes on simultaneously!)

19 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 19 The ATLAS Detector Muon Detectors Electromagnetic Calorimeters Hadronic Calorimeters Inner Detector Barrel Toroid End Cap Toroid

20 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 20 LHC Challenges Design luminosity: 14 TeV Minimum Bias (MB) =inelastic pp interactions Process  nb  #evts [10 fb  ] Rates (Hz) [ “high” L ] 500  b5x x nb~ nb~ pb~ ~1 pb~ ~10 fb~ “Needle in haystack” MB rate : O(10 9 Hz) !!

21 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 21 Minimum Bias and Pile-up  At nominal high (low) luminosity, on average ~23 (2.3) minimum bias events superimposed on any rare discovery signal –25 ns bunch crossing –only 2835/3564 “full” bunches –Minimum Bias rate ~7 x 10 8 (7 x 10 7 ) Hz  And ~1000 (100) low-pt tracks per event !  Moreover, due to finite detector response time, out-of-time pile-up from different bunch crossings –need “time stamp” to distinguish events  Trigger system ought to be very fast and extremely selective: –40 MHz input  O(100 Hz) “on tape” (the right 100 events/s !!) –selection at the <10 -4 /10 9 = level, with virtually zero dead-time !

22 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 22 ATLAS Triggers – A Schematic Overview EF L2 L1 Three levels (L2 + EF = HLT ) HLT (HLT = High-Level Trigger) L1: Hardware based (calo+  ’s) HLT: Software based 2.5  s ~10 ms ~1 s O(100 Hz) output rate (on tape) O(1 kHz) L2 rate (ATLAS only) Regions of Interest (RoIs) O(100 kHz) L1 rate 40 MHz input rate (bunch crossing) Event size 1-2 MBytes

23 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 23 ATLAS HLT  “Seeded” and “stepwise” –early rejection of uninteresting events, with minimum amount of processing –maximum flexibility  RoIs “seed” trigger reconstruction chain –L2 gets access only to a fraction (<10%) of the event (with full granularity) –EF seeded by L2 ( “offline” reco)  Minimizes CPU and bandwidth –but adds complexity  Care must be paid to –avoid biases –account for the “unexpected” RoIs electron tracks e.m. clusters

24 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 24  tot (pb) m (GeV) SUSY Cross-Sections Cross-section dominated by production of coloured particles

25 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 25 Direct Gaugino Production Squark/Gluino Production Production Mechanisms

26 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 26 Possible Final States

27 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 27 Inclusive Searches  In most scenarios, squark and gluino production dominates  Squarks and gluinos typically heaviest particles (heavier than sleptons, gauginos, etc)  Complex long decay chains to undetected neutralino (stable in RPC models) – Inclusive search: –high multiplicity of high-p T jets –large E T miss (from escaping LSP) –≥ 0 (high-pT) leptons l q q l g ~ q ~ l ~  ~  ~ p p

28 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 28 The Needle in the Haystack…

29 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 29 … a SUSY event in ATLAS…  6 jets  2 high-pt muons  Large missing E T Multi-jet event in Bulk Region

30 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 30 SUSY Mass Scale M eff (GeV) d  /dM eff (mb/400 GeV)  SUSY mass scale defined as cross- section weighted mean of masses of initial SUSY particles produced in pp collision  Effective SUSY mass scale M eff SUSY takes into account mass of undetected LSP (neutralino)  M eff a good estimator for M eff SUSY

31 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 31 ATLAS mSUGRA Reach m 0 (GeV) m 1/2 (GeV) 10 fb  reach — tan  =10,  >0, A=0 m 1/2 (GeV) m 0 (GeV) Etmiss signature — tan  =10,  >0, A=0 g(2500) q(2500) g(1000) q(1000) g(1000) q(1000) g(2500) q(2500) For ideal case of perfectly understood backgrounds and detector effects !!

32 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 32 If Only Life Were That Easy…  Significant discrepancy observed between PS (e.g. Pythia) and ME (e.g. ALPGEN) calculations of multiparton emission amplitudes  Reliance on MC simulated events must be minimized and backgrounds estimated using data-driven techniques  Also need good understanding of detector response  Background rate larger in ME generator, which also predicts a more similar shape to that of signal

33 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 33 Example of Data-Driven Bkgd Estimation   Z+jets (Z  ) background from Drell-Yan (Z  ee,  )   Red: Z  Blue: Estimated

34 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 34  can undergo chain two-body decay to : Dilepton Edge  Sharp Same-Flavour Opposite-Sign (SFOS) dilepton invariant mass edge –sensitive to sparticle mass differences

35 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 35 More Decay Chains (and edges)  Decay chains originating from squarks: lq edge llq edge  Combine constraints from different decay chains to extract information of individual sparticle masses  Consider invariant mass combination of lepton and jets llq edge lq edge

36 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 36 Exclusive Channels – An Example  Trilepton signal (from direct chargino-neutralino production and decay) –two different points in the mSUGRA parameter space (GeV) “SU2” — Focus Point –heavy scalars No decays through intermediate sleptons “SU3” — Bulk –“typical” spectrum Decays through intermediate sleptons are allowed

37 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 37 SU2 and SU3 Points m 0 = 3550 GeV m 1/2 = 300 GeV A = 0 tan  = 10  > 0 m 0 = 100 GeV m 1/2 = 300 GeV A = -300 GeV tan  = 6  > 0 SU2SU3  = 4.9 pb  = 18.6 pb

38 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 38 Direct Gaugino Production – SU2 2 SFOS leptons another lepton missing E T SU2 – No intermediate sleptons m(    103, 160, 180, 296  GeV m(  ± ) =149, 288 GeV  Relatively low-p T leptons due to small mass differences  Preliminary results indicate discovery possible with few 100s fb -1 SM bkgd from ZW, ZZ, ttbar, Z/W+jets

39 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 39  Decays through intermediate sleptons are now allowed Direct Gaugino Production – SU3 2 SFOS leptons another lepton missing E T m(    116, 224, 460, 478  GeV m(  ± ) =224, 476 GeV m(l R,L ) =152, 232 GeV  Preliminary results show that the statistics needed for discovery in this channel in the SU3 scenario would be prohibitive (1000s fb -1 !)

40 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 40 Trileptons +jets (+ Etmiss) signature Jets 1,2 leptons + Etmiss (x2) >=3 leptons (+Etmiss)  Relevant for “low-mass” SUSY (SU3, SU4), not in Focus Point region  An interesting channel for mass edges, but largest background is from SUSY itself  explore discovery potential of inclusive 3-lep search !

41 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 41  Signal = any decay originating from SUSY particles that gives 3 3 leptons (+ jets + Etmiss) in the final state  Simple analysis flow: –3 isolated leptons (including e,  from taus) – not necessarily SFOS –at least 1 high-pt jet (>200 GeV) –high Etmiss requirement can help, but not crucial  Preliminary results for 5s discovery reach: –SU2 (“focus point”) : O(10-15 fb -1 ) –SU3 (“bulk”) : O(1-2 fb -1 ) –SU4 (“low-mass”) : O( pb -1 ) (SU4: similar to SU3, but lower mass (more statistics), within Tevatron reach) Trileptons +jets (SU3) – Inclusive

42 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 42 Trilepton Inclusive – SU3 OSSF Dilepton Edge (GeV) theoretical endpoint at ~ 100 GeV ATLAS preliminary SU3 Signal only SUSY07

43 RHUL HEP Seminar 18 June, 2007Antonella De Santo (RHUL) 43 Conclusions  SUSY provides a well motivated extension to the Standard Model –hierarchy/naturalness –unification of couplings –dark matter candidate  LHC (and ATLAS) *THE* place to search for SUSY –extremely rich phenomenology  Inclusive and exclusive searches –early discovery and detailed measurements programme  These are exciting times for particle physics –and what a chance for the younger generation !


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