Outline – Standard Model – LHC and the Experiments – pp – Heavy ions – Outlook
A brief history of collider physics Energy 50 years of colliders with steadily increasing energy. Discoveries come with increasing energy
2009
Energy sets the distance scale for which interactions are studied λ= h/p LHC now occupies the high energy frontier.
λ
What we know: Standard Model
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SM: The known and expected subatomic particles Gauge theory of three forces: em, weak (electroweak) and strong. Standard Model Electromagnetic Weak Strong Log(Momentum transfer, Q(GeV) )
Still to be discovered
Energy scales of interest Electroweak 100 GeV - ~TeV Higgs mass Mass generation Dark matter (WIMP)
Grand unified scale Postulated grand unified theories inc em, weak, strong
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LHC Standard Model
Electromagnetic
Weak
Strong
Energy scale (log(GeV))
Planck scale GR collides with quantum physics. Gravity becomes important.
Large Hadron Collider 27km ring at CERN, Geneva Protons accelerated to 3.5 TeV 7 TeV c.m energy collisions 2011 - 50ns bc, 1033 cm-2s-1 To be upgraded to 25ns, 14 TeV, 1034 cm-2s-1
2 large general purpose experiments: ATLAS and CMS 2 large experiments with some general capability but more specific program: Alice (heavy ions) and LHCB (Bphysics) + 2 small experiments
ATLAS Detector Multipurpose detector with inner tracking, calorimetry and muon detection Collaboration consists of several thousand physicists and engineers
Cross section
Luminosity – what we have and what we need
LHC running extremely well. So far accumulated ~1fb-1 at 7 TeV c.m. Expect ~ 100 fb-1 per year at 14 TeV over coming years. Large lumi required for detailed study of high mass/energy processes
Publication output of ATLAS
Data analysis Representative of the LHC multipurpose experiments Does not include papers on detector operation, preliminary results etc.
Tests of the SM • SM is a predictive theory • Important for early data to measure well understood processes and check for discrepancies: new physics ☺, new detector • Strong
• electromagnetic and weak forces
Testing the SM – strong force
Measuring and testing the strong sector of the SM up to ~TeV scale
Cross sections for SM processes compared with theory
Higgs • Within SM the Higgs mechanism accounts for the masses of the fundamental particle • Prediction of a Higgs scalar boson with mass ~100-200 GeV
Precision electroweak fits and previous searches tightly bound possible Higgs mass Not much room left..
Higgs hunting
Many different possible decay channels. Large analysis effort required to ”cover all bases”.
How close are we to discovery or rejection ? First sets of LHC limits produced. Techniques established. Aim for more concrete conclusions for data taken during 2012.
Assessing the Standard Model and looking beyond Criteria
Predictivity and testability
Rating
– Collider measurements – electron anomalous dipole moment g-2/2 = 1159652180.7+- 0.3 x 10^-12 (exp) g-2/2 = 1159652154+-28 x 10^-12 (th)
Completeness
X – no quantum theory of gravity ? – unifies weak and em forces, what about the strong force ? --Dark matter ?
Compactness
Based on 19 free parameters – not bad for describing EM,weak and strong forces below ~ 1TeV. FK7003
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Dark matter 23% of universe's energy budget. Astrophysical observations - galaxy cluster rotations - velocity dispersion of galaxies - gravitational lensing - structure formation Cold Dark Matter (non-relativistic) Weakly interacting massive particle (WIMP) Most models of WIMPs with masses from 10 GeV -> ~10 TeV
Hierarchy problem – why is gravity so weak ? Gravity can't be ignored for energy scales > Planck scale Λ pl ∼ 1019 GeV Renormalisation: particle mass calculations contains contributions from decoupled regions: SM ( energy scales < Λ pl ) and "new physics" region (energy scales > Λ pl ) m physical = SM + new physics theory at high momenta=δ m + m(Λ pl ) Fermion, eg electron me ∼ δ me ∼ m(Λ pl ) Higgs mH2 ∼ 1002 GeV 2 = Ο(1019 ) 2 − Ο(1019 ) 2 Extraordinary fine tuning required !!
Supersymmetry
Every Standard Model has a supersymmetry partner (~TeV mass) Symmetry between bosons and fermion Symmetry is broken otherwise SM and SUSY particles (sparticles) would have the same mass.
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FK7003
Why look for SUSY ? Many reasons for looking for SUSY, amongs them... (1) It predicts a dark matter candidate: i.e. a WIMP with mass ∼ TeV.
Standard Model Electromagnetic
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Neutralino: χɶ 0 a mixed state of SUSY partners of the Higgs, Z and γ . (2) Unification of the couplings is more exact if SUSY sparticles exist.
(3) Introduces cancellations which suppress fine tuning/hierarchy problem. 20
FK7003
SUSY searches (1)
Look for events in which two neutralinos escape undetected and leave ”missing” transverse momentum. So far no SUSY for masses up to ~500-1000 GeV (model-dependent)
SUSY searches (2)
Look for events in which the lightest SUSY particles are electrically charged. Model dependent limits on masses up to ~600 GeV.
Extra spatial dimensions Original ideas on extra dimensions from T. Kaluza and O. Klein (1921). Several different models incorporating extra dimensions on the market today. Large Extra Dimensions. Hierarchy problem → gravity is weak since it propagates in extra dimensions (bulk) and we see a diluted form of it in our 3+1 dimension world (brane). 1 Gravitational potential V ( r ) ∼ n +1 (15.08) where r < R r n = number of extra dimensions. R = distance scale for interactions at which the effects of extra dimensions are observed. n ≥ 2 ⇒ R
