Transcript Slide 1
The Physics of the LHC What do we hope to understand? 3 Dec. 2008 John Huth Harvard University Martinus Veltman – 1980 Right now, the theorists are in the driver’s seat, but in thirty years, to make any progress at all in particle physics, we absolutely need input from experiments. Context – this was when a high energy hadron collider was envisaged as a “world machine” to explore the energy scale of 100 GeV to 1 TeV, the “symmetry breaking sector”. 3 Dec. 2008 John Huth Harvard University How did we get here? Progress toward a unified theory of nature. Fundamental particles Fundamental interactions Space, time Quantum mechanics The structure of the Universe 3 Dec. 2008 John Huth Harvard University All seem to be related The problem with classical electro-magnetism Classical self-energy of the electron: ECoulomb e e2 re Given the current limits on the “size” of the electron, some new physics has to intervene to keep its mass small (relative to known scales), yet give it a finite mass. What new physics? 3 Dec. 2008 John Huth Harvard University Quantum Field Theory! Electromagnetism+quantum mechanics+special relativity = QED!! (quantum electrodynamics) e e Implication: A new form of matter emerges called “anti-matter”, which solves the problem of the electron self-energy. How? 3 Dec. 2008 John Huth Harvard University Consequence: virtual photon cloud with electron-positron pairs screen the electron’s charge Before QED: e2 re ECoulomb e After QED: 1 E e me log me re 2 3 Dec. 2008 Logarithmic terms can be handled through a process called “renormalization”, but not 1/r John Huth Harvard University This might be the end of the story, But… Gravity: a relativistic quantum treatment is difficult Relevant scale: Planck mass 1019 times the proton mass Weak interactions: Experiment: from β decay, charged current interaction part of an isotriplet state, where the photon is included. W o W Z 3 Dec. 2008 W’s and Z are massive, photon remains massless John Huth Harvard University The W,Z and photon interact with Fermions – leptons and quarks (3 “generations”) Q=0 Q=-1 Q=2/3 Q=-1/3 3 Dec. 2008 e e u d c s t b 1st 2nd 3rd John Huth Harvard University Leptons Quarks Fundamental spin-1 objects p Photon: Massless, Lorentz invariance requires only transverse polarization states p W,Z: Massive, add longitudinal polarization state Issue: longitudinal polarization state grows with momentum. What are the implications? 3 Dec. 2008 John Huth Harvard University ISSUE: processes like WW scattering exceed unitarity above energy of 1 TeV Cannot have a consistent theory with massive spin-1 particles. The solution? An initially massless theory, where mass arises as a result of interactions 3 Dec. 2008 John Huth Harvard University One version: the Higgs boson The Higgs boson is a spin 0 object that interacts with the spin 1 force carriers and gives them mass – longitudinal polarization states. Quarks and leptons, too. Shape of interaction potential 3 Dec. 2008 John Huth Harvard University Peculiarities of the Higgs model Coupling strength is proportional to mass. Mass is inertial mass (what about gravity?) The potential is a minimum with a non-zero field (so-called “vacuum expectation value” – VEV), denoted by Λ Λ has been invoked to explain the “flatness” of the universe – inflation. But, at a much different scale – 1015 GeV, not 103 GeV Likewise another value of Λ has been used to explain dark energy – milli eV 3 Dec. 2008 John Huth Harvard University Data prefer light Higgs Combination of precision data – masses of W, Z, top quark and other fits – Conclude that: Mh< 207 GeV Direct search limit from e+e-Zh 3 Dec. 2008 John Huth Harvard University Making the Higgs at the LHC Decay modes – WW, ZZ, γγ, pairs of b quarks, perhaps top, if massive enough 3 Dec. 2008 John Huth Harvard University H high luminosity (L=10^34) Discovery should be assured by LHC operating parameters 3 Dec. 2008 John Huth Harvard University Possible problems with the Higgs • Unappealing – “The toilet of the standard model” • Alternatives abound – Mass generated dynamically – Technicolor, gravity • Naturalness – If unification includes the strong force, problems arise – similar to the self-energy of the electron 3 Dec. 2008 John Huth Harvard University Strong interactions – QCD (Quantum Chromodynamics) Force carrier is the massless gluon – 3 colors, 8 gluons. Dominates action at LHC u d Quark charge is “anti-screened” g g g g 3 Dec. 2008 u g d u u d d John Huth Harvard University 3 Dec. 2008 John Huth Harvard University Fine tuning problem with the grand unified scale – supersymmetry predicts new particle species – “sparticles” _ Before supersymmetry t ht2 EH rH H ~t t is supersymmetric cousin of the top quark After supersymmetry 1 EH h mt log mt rH 2 t H 3 Dec. 2008 ~t John Huth Harvard University Consequences of SUSY • Preservation of “low” masses of particles compared to the grand unified scale • Unification of forces actually line up • Doubling of number of particle species – Mirrored by spin – ½ change • Lighest supersymmetric partner consistent with dark matter 3 Dec. 2008 John Huth Harvard University Convergence of force strength Evolution of Coupling Constants in the SM 70 60 50 40 1/ a Without supersymmetry 30 20 a3 a2 a1 10 0 0 10 5 10 10 10 Mass(GeV) 15 10 Evolution of Coupling Constants in SUSY 70 a3 a2 a1 60 50 With supersymmetry 1/ a 40 30 20 10 0 0 10 3 Dec. 2008 5 10 10 10 Mass(GeV) 15 10 20 10 John Huth Harvard University Dark Side of the Universe: Dark Matter Dark (invisible) matter! Gasesous Matter Dark Matter 3 Dec. 2008 Dark Matter appears to be weakly interacting massive particle Lightest John HuthSUSY particle has these properties ! Harvard University 22 Example of a SUSY event at the LHC Use SUSY cascades to the stable LSP to sort out the new spectroscopy. Decay chain used is : 1o 20 Then And b 20 b g b b Final state is bb 3 Dec. 2008 John Huth Harvard University 10 Burning questions: • Is there a Higgs? What is its mass • Is there another symmetry breaking mechanism? • Is nature supersymmetric? – If so, in what way? • Tie ins to cosmology • Is gravity involved (hidden spatial dimensions)? 3 Dec. 2008 John Huth Harvard University Looking for Extra Dimensions: Z’ 1 fb-1 3 Dec. 2008 John Huth Harvard T. Virdee,University ICHEP08 25 Summary • The energy scale probed at the LHC offers the answers to a large number of questions that have perplexed physicists for over forty years. • Only experiment can clear up these issues! 3 Dec. 2008 John Huth Harvard University