1. 880.P20 Winter 2006 Richard Kass 1 Detector Systems momentumenergy A typical detector beam looks something like: BaBar, CDF, STAR, ATLAS, GLAST…… particle ID Let’s look at the ATLAS Detector as an example of a “real” system

2. 880.P20 Winter 2006 Richard Kass 2 A Toroidal LHC ApparatuS ATLAS is one of the 4 major experiments planned for the LHC Goal is to find Higgs + physics beyond the standard model (SUSY??) Turns on in 2007 The ATLAS detector stresses: good charged particle tracking pixel+SCT+TRT good momentum resolution 2 magnets good calorimetry (EM+Had) electron ID TRT+ECAL muon ID ATLAS has limited (none?)  /K/p ID ATLAS superimposed on the 5 story building 40 at CERN

3. 880.P20 Winter 2006 Richard Kass 3 ATLAS is Under Construction Now

4. 880.P20 Winter 2006 Richard Kass 4 A Systems View of ATLAS

5. 880.P20 Winter 2006 Richard Kass 5 ATLAS Data Flow At the LHC at full luminosity the interesting event rates will be: 10 7 B’s/sec (but most go down the beampipe) several thousand W’s/sec several hundered Z’s/sec ~10 tops/sec ~1 Higgs/sec (m H =150 GeV) many SUSY particles/sec write out ~ 100Hz of data

6. 880.P20 Winter 2006 Richard Kass 6 How Can ATLAS Discover the Higgs? Standard Model makes predictions for the Higgs production cross section. Many process contribute depends on Higgs mass. In 1 st 3years of data taking ATLAS is expected to accumulate 100 fb -1 of data #evts=L  L=luminosity=100x10 -15 barns  = cross section~30 pb@100GeV #evts~3x10 7 Higgs What the Higgs decays into depends on the value of it mass. Best guess is m H ~120GeV If m H ~120 GeV then the discovery modes may be H  ZZ, H  H  Z 

7. 880.P20 Winter 2006 Richard Kass 7 For these decay modes it is possible to measure the energy and/or momentum of all of its decay products: H  measure  ’s in EM calorimeter H  ZZ with Z  e + e - or Z  +  - measure e’s and  ’s tracking+calorimeter(+muon) How Can ATLAS Discover the Higgs? What is so good about the decay modes H  ZZ, H  Z  and H  If we measure the energy/momentum of the decay products then we can measure (reconstruct) the mass of the Higgs. The H  ZZ is more complicated, we must reconstruct the 2 Z’s. Need Z decay modes that do not involve neutrinos or jets. Use Z decays to electrons or muons: Other Higgs decay modes involve “jets” and or decays with neutrinos: H  W + W - or  +  - : Always a neutrino in the decay products as a W or  decay includes ’s and ’s do not interact in ATLAS (not enough material) H  b b or c c or gg: We can not measure the quarks (b, c) or gluons (g) directly. The quarks and gluons turn into many other particles collectively called a “jet”. The jet may contain neutrinos and/or some particles from the jet may not be measured (efficiency and acceptance)

8. 880.P20 Winter 2006 Richard Kass 8 How Can ATLAS Discover the Higgs? Simulation of H  ZZ Simulation of H 

9. 880.P20 Winter 2006 Richard Kass 9 How Can ATLAS Discover the Higgs? Simulation of H  b b a b-quark “jet”

10. 880.P20 Winter 2006 Richard Kass 10 How Can ATLAS Discover the Higgs? modepixelSCTTRTECALHCALmuonreconstruct m H H  veto yesno yes H  ZZ Z  e + e -     yes yes/eID yes no yes HbbHbb yes + vertex yes eID? yes if e’s yes if  ’s maybe H  W + W - W  e  W  W  quark yes yes/eID yes no yes no yes no     yes + vertex yes eID? yes if e’s yes if hadrons yes if  ’s no

11. 880.P20 Winter 2006 Richard Kass 11 How Can ATLAS Discover the Higgs? From ATLAS detector simulations: The statistical significance of Higgs observation as function of Higgs mass and decay mode MC simulation of H  with 3 years of data collection There can be > 1 Higgs….

12. 880.P20 Winter 2006 Richard Kass 12 How Well Can ATLAS Determine Higgs Properties? m H measurement Higgs Branching Fraction It is not enough to just show that the Higgs particle(s) exist! Must measure their properties too.

13. 880.P20 Winter 2006 Richard Kass 13 Supersymmetry predicts lots of new particles: bosonsfermions squarksquarkssleptons HiggsHiggsinos gluongluinos W, Zwinos photonphotino SUSY particles have not yet been observed. Either too massive to produce at today’s accelerators or they don’t exist About 100 free parameters in Minimal SUSY (MSSM) Because of “R-Parity” SUSY particles to be produced in pairs. Lightest SUSY particle stable. How Can ATLAS Find Supersymmetry? Supersymmetry: A grand unified theory with symmetry between fermions and bosons With SUSY the force couplings unify at very high energy Also: string theories no proton decay R=(-1) 3(B-L)+2S with B=baryon #, L=lepton #, S=spin, R=1 for SM, -1 for SUSY particles

14. 880.P20 Winter 2006 Richard Kass 14 How Can ATLAS Find Supersymmetry? Several SUSY particles might be stable and not interact with matter Lightest SUSY particle might be responsible for dark matter… Consider the following decay chain (one of many…) We can measure the electrons (or muon) energy and momentum Also look for the presence of two b-quark jets Key is the invariant mass of the lepton pair: there is a sharp cutoff in m(e + e - ) that depends on the masses of the neutralinos: Evaluate in the frame where  2 is at rest m 2 will be max when E  1 is min.  p  1 =0 so E  1 =m  1 assume