The Dawn of the LHC ERA A Confrontation with Fundamental Questions Michael Dine Quarknet, UCSC, 2008

Aerial view of LHC

Size of LHC In a magnetic field B, a particle of charge q and momentum momentum p travels in a circle of radius R given by At the LHC, the desired beam energy is 7 TeV and the state of the art dipole magnets have a field of 8 Tesla. Plugging in and converting units gives a radius of 3 km and a circumference of 18 km. Addition of quadrupoles, RF cavities, etc., increases the circumference of LHC to 27 km.

Magnet Pictures 2 in 1 superconducting dipole magnet being installed in the CERN tunnel LHC dipoles waiting to be installed.

ATLAS Detector

Tracker Pictures Tracker Inserting silicon detector into tracker Inserting solenoid into calorimeter

Calorimeter Installation

Muon Toroids Muon superconducting toroids.

Endcap muon sector Endcap Muon Sectors

The stored energy in the beams is equivalent roughly to the kinetic energy of an aircraft carrier at 10 knots (stored in magnets about 16 times larger) There will be about a billion collisions per second in each detector. The detectors will record and store “only” approx. 100 collisions per second. The total amount of data to be stored will be 15 petabytes (15 million gigabytes) a year. It would take a stack of CDs 20Km tall per year this much data. SCALE OF THE PROJECT

Today: A Theorist’s View of the LHC Why is this machine, perhaps the largest scientific instrument ever built, interesting? What do we expect to learn? What questions might we hope to answer?

The Standard Model By 1980, the Standard Model of particle physics offered a nearly complete picture of the elementary particles and their interactions. Quarks and leptons, interacting through exchange of gauge particles (photon, W §, Z o, gluons).

quantum field theory, describing interactions between quantum field theory, describing interactions between pointlike spin-1/2 particles (quarks and leptons) pointlike spin-1/2 particles (quarks and leptons) via exchange of spin-1 vector bosons (photon, W and Z, gluon) via exchange of spin-1 vector bosons (photon, W and Z, gluon) fundamental particles (fermions) fundamental particles (fermions) 2 (particle pair) * 2 (particle pair) * 3 (generations)* 3 (generations)* 2 (anti-particles) 2 (anti-particles) The Standard Model (I)

By 1995, the strong and weak interactions were understood at the sort of precision level of QED in the Standard Model was triumphant; no interesting discrepancies. All questions in our list answered (except general relativity)!

The Standard Model Higgs Boson Higgs Search at LEP: mass limits: obs. obs. exp. exp. m h > GeV m h > GeV Last missing particle in SM Last missing particle in SM (EW symmetry breaking – mass) (EW symmetry breaking – mass) Light SM Higgs preferred Light SM Higgs preferred time [year] M H = GeV M H = GeV < 280 GeV (95% CL) < 280 GeV (95% CL)

Puzzles of the Standard Model The Standard Model possesses many parameters. Some are extremely peculiar; e.g. m e /m t = 3 x The electric charges of the quarks and leptons are exact rational multiples of one another (e.g. Q e =Q p ). Why? General relativity cannot be combined sensibly with the Standard Model, without some significant modification. The Standard Model cannot account for most of the energy density of the universe. About 20% dark matter; about 75% dark energy; only 5% baryons. The Standard Model cannot explain why there are baryons at all (baryogenesis).

The Hierarchy Problem (or the failure of dimensional analysis) But, apart from our failure to discover it up to now, the Higgs field presents a deeper puzzle. It may be too heavy to see without an LHC but the real puzzle is that it is so light. Problem is one of dimensional analysis. We know there are large energy scales in nature. Biggest is the “Planck mass”, M p = G N 1/2 = GeV Why isn’t M H = C M p, where C » 1?  H e p i

A possible solution: Supersymmetry... doubled particle spectrum... ☹ Symmetry between Fermions ↔Bosons Fermions ↔ Bosons (matter) (force carrier)

Solves hierarchy problem Now dimensional analysis requires greater care. It turns out that because of the symmetry, M H = C M s New physics at TeV (LHC!) scales Explains dark matter Gives prediction of strong interaction strength

o Interaction Strength in Supersymmetry... BUT some of our puzzles... BUT some of our puzzles solved... solved... Successful unification of Successful unification offorces Lightest susy particle stable, and produced in abundance to be dark matter Readily explains baryon asymmetry 1 TeV without SUSY with SUSY Interaction energy in GeV

q q q q l l l l l l     g Production and decay of superparticles at the LHC. Here, jets, Leptons, missing energy.

I am a fan of the supersymmetry hypothesis; I'm not alone. About 12,000 papers in the SPIRES data base (also a good fraction of your faculty). If true, quite exciting: a new symmetry of physics, closely tied to the very nature of space and time. Dramatic experimental signatures. A whole new phenomenology, new questions. But neither the limited evidence nor these sorts of arguments make it true; there is good experimental as well as theoretical reason for skepticism. This is not the only explanation offered for the hierarchy, and all predict dramatic phenomena in this energy range. Large extra dimensions Warped extra dimensions Technicolor It’s just that way (anthropic?)

Too many parameters Hierarchy Charge quantization Quantum general relativity Dark Matter Dark energy Baryogenesis Hypothetical answers to our fundamental questions: Other proposals have some success with each of the starred items; perhaps fair to see that supersymmetry does best.

STRING THEORY String theory, an extension of the ideas of grand unification, has pretensions to attack the remaining problems on this list: A consistent theory of quantum gravity Incorporates gauge interactions, quarks and leptons, and other features of the Standard Model. Parameters of the model can be calculated, in principle. Low energy supersymmetry emerges naturally – all of this proliferation, which seemed artificial, almost automatic.

Has string theory delivered? String theory is hard. We don’t have a well- understood set of principles. Some problems of quantum gravity are resolved, but many of the challenges remain. String theory seems able to describe a vast number of possible universes, only a small fraction of which are like ours. Until recently, no progress on one of the most difficult challenges to particle physics: the dark energy.

Dark Energy/Cosmological Constant About 3/4 of energy of universe. Satisfies p = -  – an energy density of the vacuum. Dimensional analysis:  » M 4. M p 4 ? M W 4 ? (10 76,10 8 ) Measured: !

Progress and Controversy Many states of string theory now known with properties close to those of the Standard Model. Possibly or more! Among these, a uniform distribution of . So many consistent with observation. Banks, Weinberg: in such a circumstance, only form galaxies in those states with  close to observation. Perhaps universe, in its history, samples all? (This argument actually predicted the observed value of the dark energy).

Can string theorists make other predictions? Supersymmetry at LHC, or not? If yes, spectrum of superpartners? If no, alternatives (“just” a Higgs, large extra dimensions, “warping”?) Cosmology?

We are at the dawn of a very exciting era. We may resolve some of our fundamental questions.

Popular Treatments of String Theory Rhapsodic about string theoryDenounces string theory

Should the public care? Green: too focused on the mathematics of string theory, too little on what we actually see, observe in nature. Given string theory’s limited successes, seems to me this should be ``end of the book” material. Smolin: some valid criticisms, but promoting his own agenda; no more interest in physical phenomena than Green.

Dine rant See handout. Not yet prepared to put on my website.