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Physics of the Large Hadron Collider Lecture 1: Fundamentals of the LHC Johan Alwall, SLAC Michelson lectures at Case Western Reserve April 13-16, 2009 Outline ● Introduction: What is the LHC? ● Fundamentals of QCD ● Fundamentals of Electroweak Physics ● The Standard Model and the Higgs Johan Alwall - Fundamentals of the LHC 2 What is the LHC Johan Alwall - Fundamentals of the LHC 3 What is the LHC Johan Alwall - Fundamentals of the LHC 4 What is the LHC ● 14 TeV proton-proton collider ● 27 km (17 miles) in circumference ● ● Luminosity: Initial: 1033 /cm2s (10 fb-1/year) Nominal: 1034 /cm2s (100 fb-1/year) 4 detectors: – ATLAS, CMS: general-purpose detectors – ALICE: Specialized for heavy-ion collisions – LHC-B: One-arm large- detector for bottom-quark physics Johan Alwall - Fundamentals of the LHC 5 What is the LHC The CMS (Compact Muon Solenoid) detector cms.cern.ch Johan Alwall - Fundamentals of the LHC 6 What is the LHC The ATLAS detector atlas.ch Johan Alwall - Fundamentals of the LHC 7 What is the LHC Real ATLAS events (from startup run) Images from atlas.ch “Splash” event, when the LHC beam was steered into a magnet upstream Cosmic ray muon event from the detector Johan Alwall - Fundamentals of the LHC 8 What is the LHC Simulated ATLAS events Images from atlas.ch Supersymmetry event with jets and muons Higgs boson event with H→ZZ recoiling agains jet Johan Alwall - Fundamentals of the LHC 9 Fundamentals of QCD ● ● ● The theory of strong interactions; describes interactions between quarks and gluons Represented by a gauge theory with the gauge group SU(3)C (for color) Non-abelian field theory → gluon carries color charge (self-interacting) Johan Alwall - Fundamentals of the LHC 10 Fundamentals of QCD Running of QCD coupling due to quantum loops leads to “asymptotic freedom”: Johan Alwall - Fundamentals of the LHC 11 Fundamentals of QCD Can be understood in terms of screening and anti-screening by vacuum bubbles: Johan Alwall - Fundamentals of the LHC 12 Fundamentals of QCD ● ● At low energy (~1 GeV) QCD is confining – No free color charges – Quarks/gluons always confined in hadrons – Different degrees of freedom at large and small energies (quarks, gluons vs. hadrons, pions) – No analytic proof, but strong evidence from lattice simulations that SU(3) is confining Millennium $1M Prize! Coupling constant s 10 x stronger than electromagnetic coupling EM (at Z mass) Johan Alwall - Fundamentals of the LHC 13 Parton distribution functions ● ● Probability for finding a quark or gluon in a hadron Function of x (momentum fraction of parton) and 2 (momentum transfer in process) Johan Alwall - Fundamentals of the LHC 14 Parton distribution functions ● ● ● ● Probability for finding a quark or gluon in a hadron Function of x (momentum fraction of parton) and 2 (momentum transfer in process) Increases as power-law for small x and logarithmically for large Q2 Behaviour governed by DGLAP equation (Dokshitzer-Gribov-Lipatov-Altarelli-Parisi) Johan Alwall - Fundamentals of the LHC 15 Parton distribution functions ● ● ● ● Probability for finding a quark or gluon in a hadron Function of x (momentum fraction of parton) and 2 (momentum transfer in process) Increases as power-law for small x and logarithmically for large Q2 Behaviour governed by DGLAP equation (Dokshitzer-Gribov-Lipatov-Altarelli-Parisi) Johan Alwall - Fundamentals of the LHC 16 Splitting functions and DGLAP QCD bremsstrahlung – logarithmic divergences as energy fraction z → 0 and virtuality k2 → 0 – Leading contribution from the soft and collinear regions of phase space – Subsequent emissions factorize (no interference in soft/collinear regions) → Allows log resummation to all orders – Integration over multiple emissions gives evolution from interaction scale 2 to hadronic scale ~ 1 GeV Johan Alwall - Fundamentals of the LHC 17 Parton showering ● ● ● Can describe QCD radiation in the soft and collinear limit as subsequent independent emissions, called “parton shower”. Every hard interaction always associated with extra QCD radiation Initial high-energy parton gives showerlike “jet” Johan Alwall - Fundamentals of the LHC 18 Parton showering At end of shower, need hadronization ansatz to pass from partons to color neutral hadrons – Phenomenological models, based on general behaviour of QCD, e.g. the Lund string model or Herwig cluster model Johan Alwall - Fundamentals of the LHC 19 Simulation of LHC collision Hadronization, hadron decay Hard interaction Parton showers Underlying event / multiple interactions Johan Alwall - Fundamentals of the LHC 20 Missing transverse energy ● ● ● ● Fundamental concept for hadron colliders (will be used many times in these lectures) Momentum fraction x1, x2 of incoming partons → event boosted in the lab frame If invisible particle (e.g. neutrino) produced, cannot determine boost along beam direction Only the missing transverse momentum can be measured: where the sum is over all visible particles i Johan Alwall - Fundamentals of the LHC 21 Fundamentals of Elecroweak Physics ● Enrico Fermi explained beta decay of nuclei as 4-fermion interactions n → p e- e with mass scale Mweak~ 90 GeV in denominator (corresponding to intermediate vector boson!) Johan Alwall - Fundamentals of the LHC 22 Fundamentals of Elecroweak Physics ● ● ● ● Enrico Fermi explained beta decay of nuclei as 4-fermion interactions n → p e- e with mass scale Mweak~ 90 GeV in denominator (corresponding to intermediate vector boson!) Looks like SU(2) gauge theory with massive gauge vector bosons Non-renormalizable, gives scattering probability > 1 at high energies (unitarity violation) Solution: Massless gauge bosons getting mass from spontaneous symmetry breaking Johan Alwall - Fundamentals of the LHC 23 The Higgs mechanism ● ● ● Introduce scalar Higgs field which couples to the massless gauge bosons and fermions Let this field get a non-zero vacuum expectation value due to Higgs potential Rewrite Johan Alwall - Fundamentals of the LHC 24 The Higgs mechanism ● ● ● Gives mass to vector bosons and fermions in proportion to their couplings to the Higgs – Gauge fields (gauge couplings): – Matter fields (Yukawa couplings): Three of the four Higgs degrees of freedom become longitudinal components of and mix to give Z and γ Johan Alwall - Fundamentals of the LHC 25 The Standard Model Johan Alwall - Fundamentals of the LHC 26 The Standard Model ● QCD SU(3) x Weak SU(2) x Hypercharge U(1) – ● ● ● Electromagnetic A linear combination of W3 and B Weak SU(2) dynamically broken Three generations of weak doublet fermion fields (left-handed doublets, right-handed singlets) Top quark Yukawa coupling close to 1, all other Yukawas small – no explanation for this hierarchy within the model Johan Alwall - Fundamentals of the LHC 27 The Standard Model Scientific American Johan Alwall - Fundamentals of the LHC 28 The Higgs boson mass Higgs mass (and top and W masses before they were seen) determined by precision measurements – Particle masses affect precision observables through quantum loop contributions – Examples of observables used: ● ● ● ● ● ● Mass and decay widths of the W and Z bosons Branching ratios of Z boson to leptons, hadrons, bottom Weak mixing angle sin2W Forward-backward asymmetries at LEP Top mass EM, weak and QCD couplings EM, Weak and s Johan Alwall - Fundamentals of the LHC 29 Indirect mass determinations SM Higgs mass upper limit: 163 GeV Excluded by LEP Excluded by Tevatron (March 2009) Direct measurements Indirect determination Johan Alwall - Fundamentals of the LHC 30 Higgs boson decays Johan Alwall - Fundamentals of the LHC 31 Higgs hunting at the Tevatron CDF and D0 combined search, March 2009 Johan Alwall - Fundamentals of the LHC 32 Higgs hunting at the Tevatron CDF individual search channels, March 2009 Johan Alwall - Fundamentals of the LHC 33 The Higgs boson mass, cont. Higgs mass only unknown parameter of the Standard Model – 95% CL upper limit from precision measurements at 163 GeV – Must be <800 GeV to conserve unitarity of weak boson scattering (i.e. do its job) – Like all masses in a quantum field theory, mass is given by where mH2 is given by quantum loop corrections Johan Alwall - Fundamentals of the LHC 34 Naturalness and the Higgs ● ● ● ● 't Hooft: A theory is “natural” if the size of corrections not too much larger than bare mass “Fine-tuning”: The precision by which the bare mass term must cancel the corrections Corrections for elementary scalar are quadratic in new-physics cutoff (for fermions and gauge bosons, corrections are logarithmic) Main loop corrections from strongest coupled particles: top, W, Z and Higgs self-couplings Johan Alwall - Fundamentals of the LHC 35 Higgs mass corrections New physics cutoff 1% fine-tuning 10% fine-tuning The “Veltman throat” where loop corrections cancel(excluded by indirect SM bounds at >95% CL) Johan Alwall - Fundamentals of the LHC 36 The hierarchy problem ● ● ● If there is no new physics besides the Standard Model and gravity, the cutoff scale is ~MPl~1018 GeV, giving a finetuning of 10-34 To reduce finetuning to an “acceptable” 10% level, there must be new physics at around 1 TeV, i.e. within reach for the LHC! More about ideas for new physics that solves the hierarchy problem, in the next lecture! Johan Alwall - Fundamentals of the LHC 37 Summary and plan Today I have talked about: – What is the LHC? – Fundamentals of QCD ● ● ● – Fundamentals of Electroweak Physics ● – Parton density functions Parton showering, hadronization Elements of simulations of LHC collisions The Higgs mechanism The Standard Model ● ● Properties of the Higgs boson The Hierarchy problem Johan Alwall - Fundamentals of the LHC 38 Summary and plan Next lecture: New Physics at the LHC – Problems with the Standard Model – Classes of solutions to the hierarchy problem ● ● ● – Supersymmetry Extra dimentions Little Higgs models Other New Physics ideas rd 3 lecture: Simulation at the LHC Johan Alwall - Fundamentals of the LHC 39 Recommended reading ● LHC: – ● QCD: – ● http://public.web.cern.ch/public/en/LHC/LHC-en.html QCD and Collider Physics, Ellis, Sterling, Webber, Cambridge 1996 Electroweak physics, Standard Model: – An introduction to Quantum Field Theory, Peskin, Schröder, Westview 1995 – Gauge Theory of Elementary Particle Physics, Cheng, Li, Oxford 1988 Johan Alwall - Fundamentals of the LHC 40 Recommended reading ● The Higgs boson: – ● Precision measurements of the Standard Model: – ● “Higgs Boson Theory and Phenomenology”, Carena, Haber, Prog.Part.Nucl.Phys.50:63-152,2003 The LEP Electroweak Working Group, http://lepewwg.web.cern.ch/LEPEWWG/ Higgs searches at the Tevatron: – The CDF and D0 Higgs pages: http://www-cdf.fnal.gov/physics/new/hdg/hdg.html http://wwwd0.fnal.gov/Run2Physics/WWW/results/higgs.htm Johan Alwall - Fundamentals of the LHC 41