The Physics of the LHC

1. The Physics of the LHC What do we hope to understand? John Huth Harvard University

2. 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”. John Huth Harvard University

3. 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 All seem to be related John Huth Harvard University

4. The problem with classical electro-magnetism Classical self-energy of the electron: 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? John Huth Harvard University

5. Quantum Field Theory! Electromagnetism+quantum mechanics+special relativity = QED!! (quantum electrodynamics) Implication: A new form of matter emerges called “anti-matter”, which solves the problem of the electron self-energy. How? John Huth Harvard University

6. e Consequence: virtual photon cloud with electron-positron pairs screen the electron’s charge Before QED: After QED: Logarithmic terms can be handled through a process called “renormalization”, but not 1/r John Huth Harvard University

7. 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’s and Z are massive, photon remains massless John Huth Harvard University

8. The W,Z and photon interact with Fermions – leptons and quarks (3 “generations”) Q=0 Leptons Q=-1 Q=2/3 Quarks Q=-1/3 1st 2nd 3rd John Huth Harvard University

9. Fundamental spin-1 objects p p Photon: Massless, Lorentz invariance requires only transverse polarization states W,Z: Massive, add longitudinal polarization state Issue: longitudinal polarization state grows with momentum. What are the implications? John Huth Harvard University

10. 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 John Huth Harvard University

11. 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 John Huth Harvard University

12. 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 John Huth Harvard University

13. 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 John Huth Harvard University

14. Making the Higgs at the LHC Decay modes – WW, ZZ, γγ, pairs of b quarks, perhaps top, if massive enough John Huth Harvard University

15. Hgghigh luminosity (L=10^34) Discovery should be assured by LHC operating parameters John Huth Harvard University

16. 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 John Huth Harvard University

17. u d g g g g g Strong interactions – QCD (Quantum Chromodynamics) Force carrier is the massless gluon – 3 colors, 8 gluons. Dominates action at LHC Quark charge is “anti-screened” u u d d John Huth Harvard University

18. John Huth Harvard University

19. Fine tuning problem with the grand unified scale – supersymmetry predicts new particle species – “sparticles” Before supersymmetry H is supersymmetric cousin of the top quark After supersymmetry H John Huth Harvard University

20. 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 John Huth Harvard University

21. Convergence of force strength Without supersymmetry With supersymmetry John Huth Harvard University

22. Dark Side of the Universe: Dark Matter Gasesous Matter Dark Matter Dark Matter appears to be weakly interacting massive particle Lightest SUSY particle has these properties ! Dark (invisible) matter! John Huth Harvard University 22

23. 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 : Then And Final state is John Huth Harvard University

24. 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)? John Huth Harvard University

25. Looking for Extra Dimensions: Z’ 1 fb-1 John Huth Harvard University 25 T. Virdee, ICHEP08

26. 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! John Huth Harvard University