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The next linear collider

The next linear collider. Stefania Xella Rutherford Appleton Laboratory. Summary. Motivations for a new linear collider of electrons & positrons at high energy (500 GeV or more) Status of different projects

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The next linear collider

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  1. The next linear collider Stefania Xella Rutherford Appleton Laboratory Stefania Xella - RAL

  2. Summary • Motivations for a new linear collider of electrons & positrons at high energy (500 GeV or more) • Status of different projects • Physics potential of the new linear collider: precision measurements of the Higgs boson Stefania Xella - RAL

  3. These are times of great expectation for particle physicists. The LEP/SLD experiments have tested very precisely the Standard Model, and are clearly hinting for a Higgs boson just around the corner. Most likely the physics scenario waiting for us at energies above 200 GeV includes a light Higgs boson and possibly supersymmetric particles WHY? Stefania Xella - RAL

  4. LEP and SLD tell us that 114< Higgs (SM) <210GeV @95% c.l. • Unification for different coupling constants is possible (so far) only introducingSUperSYmmetry • SUSY lightest particle is the most favoured candidated for dark matter • No realistictheory model of electroweak interactions can avoid introducing a Higgs boson Stefania Xella - RAL

  5. Running (now or soon) exp’s Tevatron Run II (p-antip) running. energy : 2TeV, lumi: 2 fb-1 in 2 years • Higgs>115 GeV @95%c.l. with 2 fb-1 Higgs>200 GeV @95%c.l. with 50fb-1 but observed (3 ) with 20fb-1 up to 180 GeV • SUSY: some coverage, rates <= SM (see fig) Stefania Xella - RAL

  6. Running (now or soon) exp’s LHC (p-antip) start 2007. Energy: 14 TeV, lumi:300 fb-1 (6 years) • Higgs detection (5 ) up to 1 TeV with 30 fb-1(3 years, p.e.) • SUSY particles detectable for masses up to 2 TeV (g,q) and to 400 GeV (l) ~ ~ ~ (see fig) Stefania Xella - RAL

  7. So why do we need more? Hadronic machines are very good for discovery because they can go “easily” very high up in energy BUT They are not as good in obtaining clean and precise measurements as e+e- machines are. WHY? Stefania Xella - RAL

  8. pp vs e+e- (I) e+ e+ e- are fundamental particles, so “easy” to go from final state to new particle new particle e- pp are not fundamental particles, so difficult to go from final state to new particle Strong interactions Stefania Xella - RAL

  9. pp vs e+e- (II) in e+e- interactions the energy involved in the interaction between fundamental particles is known => 4-p conservation is important constraint (e.g. final states with ) with pp interactions one cannot use this Stefania Xella - RAL

  10. pp vs e+e- (III) Background in e+e- is EW: (signal)/(backgr) ~ 1 while in pp is mainly QCD -> high, and also high EW (many partons involved) e.g. (ee->ZH) ~ 0.3 (pp->WH,W->l) ~10-4 (ee->ZZ) (pp->Wjj,W->l) Stefania Xella - RAL

  11. pp vs e+e- (IV) Theoretical predictions for signal and background are more precise for e+e- interactions than for pp ones: e+e- ~ 0.1 - 10% pp ~ 10 – 100% (HO QCD, structure functions, ... ) Stefania Xella - RAL

  12. pp vs e+e- (V) • In pp machines the frequency of events where something happens is high -> lots of radiation on detectors -> limits the choice of detectors -> limits the physics potential e.g. CCD pixel detector only at e+e- Stefania Xella - RAL

  13. for precise measurements e+e- is clearly better than pp a high energy(> 500 GeV) e+e- machine is necessary now because: • LHC/TevaTron find Higgs? then they cannot describe accurately its properties (see following) • LHC/TevaTron find nothing? then precision measurements is our only chance to get a hint on what’s beyond e.g. LEP and top mass…and Higgs mass (?) Stefania Xella - RAL

  14. Status of LC R&D projects e+e- machines : lots of advantages BUT one main disadvantage: it is difficult and expensive to go to high energy and high luminosity interesting processes have sometimes low  to cover next energy frontier => proposed machine: e+e- linear collider Stefania Xella - RAL

  15. Energy/luminosity required ~100 more bunches, more particles Luminosity = Frep . Ne2 4 xy Energy 210-300 GeV for Higgs 350 GeV for tt production ?? for susy <TeV new strong interactions Beam vertical dispersion ~1/100 Stefania Xella - RAL

  16. Existing projects (phase 1) R&D work (machine/detector) very active, physics studies well advanced (ECFA/DESY) TESLA @ DESY (T.D.R. 2001) NLC @ SLAC/FNAL (T.D.R. 2003) * start at 500GeV -> 800GeV-1 TeV * possible run at 91.2 GeV (GigaZ) * polarized beams (90% e-, 50% e+) to enhance signal vs background * start envisaged by 2014 (see fig) Stefania Xella - RAL

  17. Stefania Xella - RAL

  18. Existing projects (phase 2) Higher energy/lumi operation than TESLA/NLC requires big step ahead R&D started: CLIC @ CERN -> long way to go to achieve: RF pulse 30 GHz, Gradient 150 MV/m Energy 3-5 TeV, Luminosity 1034 cm-1 s-1 (see fig) Stefania Xella - RAL

  19. LC Physics potential: Higgs The LC can measure precisely SM-like H : • Mass • Coupling to gauge bosons and fermions • Total width • CP : phases, properties • Self coupling (-> Higgs potential) • JPC LHC can discover the Higgs quickly, but only measure 1., and poorly some of 2. Stefania Xella - RAL

  20. LC Physics potential:SUSY extension of SM (I) • MSSM has about 105 free parameters in addition to the SM ones ! => precise measurements are needed • LC one can do energy scan at different prod. thresholds of SUSY particle pairs • Polarized beams greatly improves sensitivity • CP, mixing, q.n. SUSY particles can be studied precisely Stefania Xella - RAL

  21. LC Physics potential:SUSY extension of SM (II) 5. Masses of SUSY particles measured indipendently of model e.g. g -> qq -> qq 20 -> qqll 10 6. sneutrinos can be measured LHC cannot cover points 2. - 6., and cannot avoid using model in interpretation of the results: kinematic is not clean enough ~ ~ ~ ~ Stefania Xella - RAL

  22. Higgs at the LC SM MSSM H h0,H0,A,H± production e Z Same as SM H<-> h0,H0 Z* Low E e H + e  ee->A h0,H0 H High E ee->H+ H- e  Stefania Xella - RAL

  23. Higgs at the LC decay Depends a lot on value of tgb=v2/v1 bb m<140GeV (tt,gg,cc) WW m>140GeV ZZ e.g. Tgb>>1: H<-> h0,H0 tt m>300GeV Stefania Xella - RAL

  24. 500 fb-1 SM Higgs 1. Mass not predicted=> important to measure it well ee -> Z H m<130 GeV qq bb dMH 50MeV ll bb 110 ll qq 70 m>130 GeV qq,ll WW 130 qq,ll “recoil 90 technique” important if H->invisible dominant more precise indipendent of H nature Stefania Xella - RAL (see fig)

  25. SM Higgs 2.Coupling to gauge bosons (W,Z) measured through production Xsection. important also for gHff and Gtot s(ee->HZ) ~ gHZZ #ds/s 2.8 % s(ee->H) ~ gHWW ds/s 4% Br(H->WW*) ~gHWW ds/s 2% # recoil mass technique used -> result independent of model and decay (see fig) Stefania Xella - RAL

  26. SM Higgs • Coupling to fermions (f) gHff ~ mf / v => measurement can tell if H is SM or not Br(H->ff) ~gHff dBr/Br 2.4% (bb) 8%(cc) 5%(gg) 5% (tt) fundamental: optimal flavour tagging -> VXD Uses s(ee->HZ) & s(ee->H) (see fig) Stefania Xella - RAL

  27. SM Higgs 4. Total decay width mH>200 GeV: from H lineshape mH<200 GeV: indirect, from Gtot = Gx Br(H->x) need indep. meas. e.g. x=WW : Gx from s(ee->H) Br => Gtot / Gtot ~ 4% Stefania Xella - RAL

  28. SM Higgs • Higgs potential gHff -> nature of H, but potential needed too V= l (|f|2 –1/2 v2)2 v2=1/(2GF2) measure from physical H potential V= l v2 H2 + l vH3 + l H4 lHH from MH lHHH Difficult For LC e Z H e H Difficult backgrounds, reco, tagging => need optimal vxd DlHHH/lHHH ~ 22% (see fig) Stefania Xella - RAL

  29. (see fig) Stefania Xella - RAL

  30. Conclusions The next linear collider is an essential and unavoidable step in the understanding of physics at energies >200 GeV LET’S BUILD IT ASAP ! Stefania Xella - RAL

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