What do we hope to understand? The Physics of the LHC What do we hope to understand? 3 Dec. 2008 John Huth Harvard University

Right now, the theorists are in the driver’s seat, but 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 All seem to be related 3 Dec. 2008 John Huth Harvard University

classical electro-magnetism 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? 3 Dec. 2008 John Huth Harvard University

QED!! (quantum electrodynamics) 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? 3 Dec. 2008 John Huth Harvard University

Consequence: virtual photon cloud with electron-positron pairs screen the electron’s charge Before QED: e After QED: Logarithmic terms can be handled through a process called “renormalization”, but not 1/r 3 Dec. 2008 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’s and Z are massive, photon remains massless 3 Dec. 2008 John Huth Harvard University

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 3 Dec. 2008 John Huth Harvard University

Fundamental spin-1 objects 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? 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gg 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

u u d d Strong interactions – QCD (Quantum Chromodynamics) u d g Force carrier is the massless gluon – 3 colors, 8 gluons. Dominates action at LHC Quark charge is “anti-screened” u d g u u d d 3 Dec. 2008 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 H is supersymmetric cousin of the top quark After supersymmetry H 3 Dec. 2008 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 Without supersymmetry With supersymmetry 3 Dec. 2008 John Huth Harvard University

Dark Side of the Universe: Dark Matter Dark (invisible) matter! Dark Matter Gasesous Matter Dark Matter appears to be weakly interacting massive particle Lightest SUSY particle has these properties ! 3 Dec. 2008 John Huth Harvard University 22 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 : Then And Final state is 3 Dec. 2008 John Huth Harvard University

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 University T. Virdee, ICHEP08 25 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