10 October 2002Stefania Xella - RAL The next linear collider Stefania Xella Rutherford Appleton Laboratory

10 October 2002Stefania Xella - RAL 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

10 October 2002Stefania Xella - RAL 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?

10 October 2002Stefania Xella - RAL LEP and SLD tell us that 114< Higgs (SM) c.l. Unification for different coupling constants is possible (so far) only introducing SUperSYmmetry SUSY lightest particle is the most favoured candidated for dark matter No realistic theory model of electroweak interactions can avoid introducing a Higgs boson

10 October 2002Stefania Xella - RAL Running (now or soon) exp’s Tevatron Run II (p-antip) running. energy : 2TeV, lumi: 2 fb -1 in 2 years Higgs>115 with 2 fb -1 Higgs>200 with 50fb -1 but observed (3  ) with 20fb -1 up to 180 GeV SUSY: some coverage, rates <= SM (see fig)

10 October 2002Stefania Xella - RAL Running (now or soon) exp’s LHC (p-antip) start 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)

10 October 2002Stefania Xella - RAL 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?

10 October 2002Stefania Xella - RAL pp vs e+e- (I) e+ e- are fundamental particles, so e+ e- new particle pp are not fundamental particles, so Strong interactions “easy” to go from final state to new particle difficult to go from final state to new particle

10 October 2002Stefania Xella - RAL 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

10 October 2002Stefania Xella - RAL 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 )

10 October 2002Stefania Xella - RAL pp vs e+e- (IV) Theoretical predictions for signal and background are more precise for e+e- interactions than for pp ones: e+e- ~ % pp ~ 10 – 100% (HO QCD, structure functions,... )

10 October 2002Stefania Xella - RAL 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-

10 October 2002Stefania Xella - RAL 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 (?)

10 October 2002Stefania Xella - RAL 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 to cover next energy frontier interesting processes have sometimes low  => proposed machine: e+e- linear collider

10 October 2002Stefania Xella - RAL Energy/luminosity required Luminosity = F rep. N e 2 4   x  y Energy GeV for Higgs 350 GeV for tt production ?? for susy <TeV new strong interactions Beam vertical dispersion ~1/100 ~100 more bunches, more particles

10 October 2002Stefania Xella - RAL Existing projects (phase 1) R&D work (machine/detector) very active, physics studies well advanced (ECFA/DESY) DESY (T.D.R. 2001) 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)

10 October 2002Stefania Xella - RAL 500 GeVTESLANLC RF cavity Niobium, superconducting Copper, conventional RF pulse 1.3 GHz 11.4 GHz Gradient 23.4 MV/m 50 MV/m Bunches/train Bunch spacing 337 ns 1.4 ns Rep. rate 5 Hz 120 Hz 14 KHz 23 KHz luminosity cm -1 s cm -1 s -1 Charge/bunch length 33 Km 26 Km

10 October 2002Stefania Xella - RAL Existing projects (phase 2) Higher energy/lumi operation than TESLA/NLC requires big step ahead R&D started: CERN -> long way to go to achieve: RF pulse 30 GHz, Gradient 150 MV/m Energy 3-5 TeV, Luminosity cm -1 s -1 (see fig)

10 October 2002Stefania Xella - RAL 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) J PC LHC can discover the Higgs quickly, but only measure 1., and poorly some of 2.

10 October 2002Stefania Xella - RAL LC Physics potential: SUSY extension of SM (I) 1. MSSM has about 105 free parameters in addition to the SM ones ! => precise measurements are needed 2.LC one can do energy scan at different prod. thresholds of SUSY particle pairs 3.Polarized beams greatly improves sensitivity 4.CP, mixing, q.n. SUSY particles can be studied precisely

10 October 2002Stefania Xella - RAL LC Physics potential: SUSY extension of SM (II) 5. Masses of SUSY particles measured indipendently of model e.g. g -> qq -> qq  2 0 -> qqll  sneutrinos can be measured LHC cannot cover points , and cannot avoid using model in interpretation of the results: kinematic is not clean enough ~~~~

10 October 2002Stefania Xella - RAL Higgs at the LC SM MSSM H h 0,H 0,A,H ± production e e e e Z Z* H H Low E High E Same as SM H h 0,H 0 + ee->A h 0,H 0 ee->H + H -

10 October 2002Stefania Xella - RAL Higgs at the LC decay bb m<140GeV ( ,gg,cc) WW m>140GeV ZZ tt m>300GeV Depends a lot on value of tg  =v2/v1 e.g. Tg  H h 0,H 0

10 October 2002Stefania Xella - RAL SM Higgs 1. Mass not predicted=> important to measure it well ee -> Z H m<130 GeV qq bb  M H 50MeV ll bb 110 ll qq 70 m>130 GeV qq,ll WW 130 qq,ll “recoil 90 technique” more precise important if H->invisible dominant indipendent of H nature 500 fb -1 (see fig)

10 October 2002Stefania Xella - RAL SM Higgs 2. Coupling to gauge bosons (W,Z) measured through production Xsection. important also for g Hff and  tot  (ee->HZ) ~ g HZZ #   (ee->H ) ~ g HWW  Br(H->WW*) ~ g HWW  # recoil mass technique used -> result independent of model and decay (see fig)

10 October 2002Stefania Xella - RAL SM Higgs 3.Coupling to fermions (f) g Hff ~ m f / v => measurement can tell if H is SM or not Br(H->ff) ~ g Hff  Br/Br 2.4% (bb) 8%(cc) 5%(gg) 5% (  ) fundamental: optimal flavour tagging -> VXD (see fig) Uses  (ee->HZ) &  (ee->H )

10 October 2002Stefania Xella - RAL SM Higgs 4. Total decay width mH>200 GeV: from H lineshape mH<200 GeV: indirect, from  tot =  x Br(H->x) need indep. meas. e.g. x=WW :  x from  (ee->H ) Br =>   tot /  tot ~ 4%

10 October 2002Stefania Xella - RAL SM Higgs 5.Higgs potential g Hff -> nature of H, but potential needed too V= (|  | 2 –1/2 v 2 ) 2 v 2 =1/(2G F 2 ) measure from physical H potential V=  v 2 H 2 +  v H 3 +  H 4 Difficult For LC  from M H e e Z H H Difficult backgrounds, reco, tagging => need optimal vxd  HHH / HHH ~ 22%  (see fig)

10 October 2002Stefania Xella - RAL M H (GeV)  X/X LHC 2x300 fb -1  X/X LC 500 fb -1 M H  tot g Huu g Hdd g HWW g Huu /g Hdd g Hbb /g HWW g Htt /g HWW g HZZ /g HWW CP  HHH (see fig)

10 October 2002Stefania Xella - RAL 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 !