Physics at the TeV Scale Discovery Prospects Using the ATLAS Detector at the LHC

Transcription

1 Physics at the TeV Scale Discovery Prospects Using the ATLAS Detector at the LHC Peter Krieger Carleton University Physics Motivations Experimental Theoretical New particles searches Standard Model Higgs Boson Higgs Particles of Supersymmetric Extensions to the SM Supersymmetric Particles Other Scenarios Large Extra Dimensions Summary 1

2 ATLAS and the LHC Large Hadron Collider: proton proton collisions at s = 14TeV L peak = 1 33 cm -2 s Low luminosity running L peak = 1 34 cm -2 s ? High luminosity running 2 minimum bias events per crossing at high luminosity Some physics quantities (e.g. b-tagging efficiency) degraded at high luminosity 2

3 The Standard Model The SM healthy at energies 2 GeV m W [GeV] LEP1, SLD, νn Data LEP2, pp Data 68% CL m H [GeV] Preliminary m t [GeV] Direct measurements of M w and M top agree well with indirect constraints from LEP1 Predict low mass Higgs Higgs boson remains experimentally unobserved χ theory uncertainty α (5) had =.284± ±.26 Direct limit from LEP2 M H > % CL Upper limit from LEP combined electroweak fit 2 M H < % CL Excluded Preliminary m H [GeV] 3

4 Beyond the Standard Model Hierarchy problem (2 fundamental energy scales) M EW / M planck 1 17 Naturalness problem radiative corrections to Higgs mass 2 squared Λ where Λ is the energy scale to which the theory remains valid fine tuning problem with Higgs mass: can be resolved by New physics at the TeV scale Λ 1TeV OR A symmetry protecting the Higgs mass against large radiative corrections (Supersymmetry) If Higgs not discovered with mass < 8 GeV expect the dynamics of WW, ZZ scattering to reveal new structure energies 1 TeV Must see something new at energies 1TeV 4

5 Supersymmetry (SUSY) For each SM fermion (boson) there is a bosonic (fermionic) supersymmetric partner with identical mass and couplings sleptons squarks spin spin ½ spin 1 ~ l q ~ leptons quarks gauginos gluoninos Higgs bosons (5) Higgsinos (5) l q ~, W, Z ~ g~ ~γ gauge bosons Z gluons γ,w, g SM SUSY Charged (neutral) gauginos and Higgsinos mix to form charginos (neutralinos) ~ χ ± ~ i= 1, 2 χ j=1,2,3,4 ordered by mass R-parity quantum number distinguishes SM and SUSY particles Conventional to assume R-parity conservation SUSY particles must be produced in pairs Must be a lightest SUSY particle (LSP) which cannot decay Usually the lightest neutralino ~χ 1 good CDM candidate Experimental signature: large missing transverse energy E/ T 5

6 Supersymmetry Supersymmetry must be a broken symmetry there is no e~ with M ~ e = M e many model parameters (15 extra for MSSM) SUSY solves the naturalness problem if M susy < 1 TeV Allows for gauge coupling unification if M susy < 1 TeV SUSY with M SUSY < 1 TeV is called Weak-Scale SUSY ( ) α µ -1 Inverse coupling constant α -1 ( µ ) 1-1 α ( µ ) 2 α -1 ( µ ) 3 U(1) E.M. Force SU(2) Weak Force SU(3) Strong Force No Supersymmetry Energy Scale, µ [GeV] ( ) α µ -1 Inverse coupling constant U(1) E.M. Force α -1 ( µ ) 1 SU(2) Weak Force α -1 ( µ ) 2 SU(3) Strong Force α -1 ( µ ) ? With Supersymmetry Energy Scale, µ [GeV] SUSY can provide dynamical EW symmetry breaking SUSY may allow unification with gravity (all string theories are inherently supersymmetric) 6

7 MSSM Higgs Sector Two Higgs doublets 5 physical Higgs bosons h, H, A, ± H Assume M SUSY ~ 1 TeV so Higgs SUSY kinematically forbidden All masses and couplings then given as f (tan β, M ) A h Lightest MSSM Higgs M h < M Z at tree level M h < M 14 GeV after loop corrections In limit of large and/or tan β, behaves like A h H SM H A Heavy neutral CP-even, CP-odd Higgs respectively τ + τ µ + µ Decays to, enhanced for high tan β ± H Heavy charged Higgs NB: for moderate (, ) or > 5 GeV A tan β only is observable h M M A 7

8 Higgs Discovery at Tevatron RUN II pp pp pp WH lνbb + ZH l l bb ZH ννbb Required Luminosity (fb -1 ) Combined Results(WH+ZH) Standard cuts (5 σ) NN cuts (5 σ) CDF Int. luminosity target for Run II 15-2 fb -1 / expt. Prior to start of LHC M H (GeV/c 2 ) 5 year running 3-5 σ for M H 13 GeV 8

9 SM Higgs Properties bb _ BR(H) τ + τ cc _ gg WW ZZ tt - γγ Zγ M H [GeV] Large QCD backgrounds: look for final states with high-p T leptons and photons Important channels: low mass intermediate mass high mass { { { H H H H H γγ H bb ZZ WW ZZ (*) WW (*) l l l l l νν l + l ν l ν lν jet jet 9

10 Higgs Production at LHC Production cross-sections at the LHC H Pr W Z ee tt b b ~ g ~ g ( M = 1TeV ) ~ g ( M =.8TeV QCDjets SM p T ocess eν > 2 GeV ) σ 15 nb 1.5nb 8 pb 5 µ b 1pb 1pb 1 nb Events / sec Event / year Direct Higgs production gg fusion or vector boson fusion: Need high p T leptons or photons from Higgs decay Huge QCD background for channels with jets Associated Higgs production g g q - Q t gg fusion Z(W) Z(W) q- Q Vector boson fusion (,) H (,) H High p T leptons from top decays used for triggering Top reconstruction used for QCD background suppression t t - t - t H tth 1

11 Htitle γγ Useful for M H < 14 GeV low luminosity running: use direct production (utilize high p T photons ) large signal, low S/B high luminosity running: add contributions from associated production, WH, ZH, tth (utilize reconstruction of associated particle(s)) small signal, good S/B Backgrounds γγ γj+ jj (irreducible) (reducible) σ γγ σ j ~ 3pb ~ 1 6 σ γγ need σ MH ~ 1% need R j > 1 3 Events / 2 GeV Signal-background, events / 2 GeV M H = 12 GeV 1 fb -1 signal σxbr 5fb m γγ (GeV) m γγ (GeV) Sets severe requirements of the performance of the ATLAS electromagnetic calorimetry 11

12 Hγγ (m H =1 GeV, L=1 34 ) 12

13 title tth, H bb Largest BR for low mass Higgs, but huge QCD background Use associated production with full reconstruction of both top quarks (allows triggering and background suppression) Backgrounds from ttz, Wjjjjjj ttjj etc., (see below) Both top quarks reconstructed Dominant can be measured in tt production H bb t bjj t blν ε 15 ATLAS 1 fb -1 1% Events / 16 GeV 1 ATLAS 3 fb -1 Hig gs mass (GeV) Signal S ttz Wjjjjjj ttjj m bb (GeV) Total backgr ound B S/B S / B.25 S B S H bb /S total S / B 4 Sets stringent requirements on b-tagging performance at high luminosity 13

14 SM Higgs Sensitivity 3 fb -1 3 years of running at low luminosity Signal significance 1 2 H γ γ tth (H bb) H ZZ (*) 4 l H WW (*) lνlν H ZZ llνν H WW lνjj Total significance 1 No single channel discovery for M H =1-13 GeV 1 ATLAS L dt = 3 fb -1 (no K-factors) 5 σ m H (GeV) Full coverage of mass region with significance > 5 Needs combined channels for discovery at low M H Multiple discovery channels for M H > 3 GeV σ 14

15 SM Higgs Sensitivity 1 fb -1 1 year running at high luminosity Signal significance 1 2 H γ γ + WH, tth (H γ γ ) tth (H bb) H ZZ (*) 4 l H WW (*) lνlν H ZZ llνν H WW lνjj Total significance 1 5 σ H γγ N.B now includes associated production channels ATLAS L dt = 1 fb -1 (no K-factors) m H (GeV) Multiple discovery channels for all M H 15

16 Search for MSSM Higgs Bosons h γγ tth, h bb and very important in search for lightest MSSM Higgs (as they are for H SM ) h γγ M SUSY ~ 1 TeV Higgs properties are function of (, tan β ) only M A Contributions to exclusion regions from H/A are small tth, h bb Almost excluded by LEP 16

17 MSSM Higgs Sensitivity 3fb -1 Three years of running at low luminosity H/A important for high tan β Unexcluded region with moderate (, tan β ) M A Combined ATLAS/LEP2 exclusion for most of the Region with moderate (, tan β ) M A (, tan β ) M A remains unexcluded plane N.B. LEP2 exclusion for 2 pb -1 / expt at 2 GeV so should be conservative 17

18 MSSM Higgs Sensitivity 3 fb -1 Three years running at high luminosity Full coverage of the (, tan β ) M A plane Multiple channel coverage for most the plane Most important channels h γγ and tth, h bb 18

19 Supersymmetric Particle Searches Reduce number of SUSY free parameters Assume SUSY broken in some hidden sector at high energy SUSY breaking mediated to visible sector via some interaction Two popular scenarios with different phenomenologies: Gravity-mediated Phenomenology dictated LSP Gravitino G ~ very heavy, phenomenologically unimportant MSUGRA: 5 parameter model, assumes parameter unifications at GUT scale m m 1/ 2 tan β A sign( µ ) Common scalar mass at unification scale Common gaugino mass at unification scale Ratio of Higgs vevs Gauge-mediated Phenomenology dictated NLSP Gravitino is the LSP! M ~ G << 1MeV Next to lightest SUSY particle NLSP can have short or long decay length Minimal model studied by ATLAS has 6 parameters 19

20 Supersymmetric Particle Searches σ(pb) pp collisions at E cm = 14 TeV p M q = 2M g M q = M g sum(qq + qg + gg) p g g g g g p p q g q g q χ + 1 χ 1 - p q q g q q p gluino mass M g [GeV] ~ g ~ g (q) ~ q ~ q (q) χ ~ 2 q q χ ~ 1 l + l - Signature channels l Jets + missing E T 1 l 1 lepton + jets + missing E T 2 l 2 leptons + jets + missing energy SS, OS 3 l 3 leptons + jets + missing energy l + l - jet jet + missing E T final state ~ χ 1 2 (3) l,j 2 or 3 leptons with jet veto + missing E T Search channels for direct gaugino or slepton production 2

21 Supersymmetric Particle Searches SUGRA 5 points in MSUGRA parameter space chosen for study Mass points shown in red m 1/2 (GeV) m 1/2 (GeV) l l SS 3l OS 2l,j 3l,j 1l l tan β = 2, µ < tan β = 2, µ > SS 3l OS 2l,j 3l,j l l 3l SS OS 2l,j 3l,j 1l l tan β = 1, µ < tan β = 1, µ > 3l SS OS 2l,j 3l,j Coverage shown for various lepton and/or jet + missing energy signals m (GeV) m (GeV) 21

22 SUSY Searches (SUGRA) LHC studies choose 5 representative points in parameter space Two shown here in red m 1/2 (GeV) 8 6 l 1l tan β = 1, µ > 3l SS OS 4 2 2l,j 3l,j Experimentally excluded No EW symmetry breaking m (GeV) 1 L = 1fb Mass reach defined by 1 signal events with S / B > 5 For jets + missing energy, mass reach for squarks and gluinos extends to > 2 TeV In multilepton channels reach extends to > 1 TeV Weak scale SUSY easily discovered. Dominant background to a given process is from other SUSY processes 22

23 SUSY Searches (GMSB) Most significant difference phenomenlogically is ~ massless LSP Phenomenology dictated by: NLSP (usually either χ G ~ ~ ~ γ or ± ~ l Gl ± ) NLSP short intermediate long ~χ 1 1 Scale of SUSY breaking (one of model parameters) dictates the gravitino mass and the NLSP lifetime NLSP lifetime photons + E/ T non-pointing photons + E/ As in SUGRA T ~ l leptons + E/ kinked charged tracks + Long lived heavy T E/ T charged particles Unusual signatures in case of intermediate τ NLSP Scenarios have especially low standard model backgrounds Discovery generally straightforward Parameter determination trickier (as for SUGRA) 23

24 Supersymmetric Particle Searches 1-7 LHC Point dσ/dm eff (mb/4 GeV) M SUSY (GeV) M eff (GeV) M eff (GeV) Estimate of M SUSY in jets + missing energy channel M = E + p + p + p + p eff miss T 1 T 2 T 3 T 4 T 24

25 Heavy Higgs H SM coupling to gauge bosons increases with increasing mass Resonance wider interaction stronger eventual violation of unitarity limit No fundamental scalar? Need new physics to EW symmetry breaking, regularization of vector boson couplings and fermion mass generation Study V L V L V L V L scattering (longitudinal gauge bosons are goldstone bosons of symmetry breaking process) Forward-jet tagging important Events/1 GeV m (GeV) Sensitivity to WZ resonance shown for M WZ = 1.2, 1.5 TeV for 3 fb -1 Events/1 GeV m (GeV) Non-resonant V L V L searches more challenging 25

26 Many Other Searches Technicolour Additional gauge bosons Compositeness, leptoquarks, excited quarks Monopoles R-parity violating SUSY (baryon, lepton number violating decays) 26

27 Large Compact Extra Dimensions Recall hierarchy problem M EW / M planck ~ 1 Postulate M planck effective energy scale, not fundamental 17 Assume existence of n compact spatial dimensions of (compactified) radius R 2 () = 2+ n n+ 1 V r m m M pl r 1 1 (4+ n) (r << R) V () r m1m n 2 Mpl(4+ = + 2 n) 1 1 n R r (r >> R) Effective 4-dim planck M planck is then given by Requiring M pl(4+n) ~ M EW R ~ 1 (3/n)-17 cm M = M 2 n+ 2 planck pl(4+ n) R n Various constraints on models with compactification n = 1 R = 1 13 cm (cosmologically excluded) n = 2 R ~.1 1. mm (unexcluded by tests of 1/r 2 nature of gravitation, but excluded by SN1987) Collider limits from missing energy searches (next slide) 27

28 Large Compact Extra Dimensions Model of Arkani-Hamed, Dimopoulos and Dvali: only gravitons propogate freely in the bulk Massless gravitons in 4+n dim massive KK gravitons G M in 4D Missing energy signature Possible signatures at LHC pp pp pp G G G M M M + jet + γ + Z monojet p p q g q q G M Non-kinematic high p T cutoff Other particles localized within 1/M EW in the extra n dimensions In sufficiently hard collisions E esc > M EW particles can acquire momentum in the extra dimensions and disappear from the 4D world upper limit for p T distributions at p T = E esc Such particles may or may not periodically return to and deposit energy in the 4D world 28

29 Constraint Evasion! Randall and Sundrum: can derive the same relationship between a higher dimensional planck mass and the one of our 4-dim world WITHOUT compactification. Evades some astrophysical constraints on compactified models. 29

30 Summary Exciting times ahead! LEP provided Promise of precision tests of the SM Hope for new physics discoveries LHC will provide Promise of discovery Initial parameter determination precision tests will be done at NLC Provided they exist, we will observe SM Higgs or MSSM Higgs (h ) Weak Scale Supersymmetry Sensitivity also to other anticipated new physics not discussed here Possibly (or even probably?) we may discover something entirely unexpected 3