Kiwoon Choi (KAIST) 3 rd GCOE Symposium Feb (Tohoku Univ.)

Transcription

1 Exploring New Physics beyond the Standard Model of Particle Physics Kiwoon Choi (KAIST) 3 rd GCOE Symposium Feb (Tohoku Univ.)

2 We are confronting a critical moment of particle physics with the CERN Large Hadron Collider (LHC) which just began the operation to probe the physics at Tera-electron-Volt (TeV) energy scale.

3 Large Hadron Collider (LHC) is the grandest and most expensive scientific instrument ever built. Lake Leman Geneva Airport proton-proton collider in 27 km long tunnel

4 Why are we so excited with LHC? There are good reasons to speculate that Tera-electron Volt scale (10-19 m) might be the threshold scale of revolutionary new physics. In this talk, I am going to discuss v Why do we expect new physics at the TeV scale? v What would be the implications of those new physics to our understanding of the fundamental physical law?

5 Threshold scales for revolutionary new physics u Atomic scale : m 0:09 ¹m # Atomic spectroscopy: (Balmer, Lyman) = 1 n 2 1 l 2 suggests that interesting new physics might exist at scales around submicro meter. # Rutherford experiment probing the atomic structure (1911) => Discovery of Quantum World * Undeterministic mechanics * Quantized observables

6 u Scale of atomic nucleus: m # Strong nuclear force at scales around m (Yukawa,1935) # Deep inelastic scattering probing the inner structure of the atomic nucleus (1969) => Quantum mechanical strong nuclear force (Quantum Chromo Dynamics) leading to * Confinement, so no macroscopic nuclear force * Spontaneous chiral symmetry breaking, explaining the origin of the mass of atoms

7 u Scale of electroweak unification: m # Fermi theory of the weak nuclear force (1934) # Unification of the electromagnetic and weak forces (1967) # LHC began to probe the inner structure of physics at m (TeV) scale (2010) What will show up at this scale? * The origin of the electroweak symmetry breaking which is the key element of the electroweak unification, and the simplest possibility is the Higgs boson * But there are good reasons to speculate that the Higgs boson is a tip of iceberg, and much more exciting possibility might be waiting for us.

8 Down to the length scale ~ m, all observed phenomena of elementary particles are well-described by the Standard Model (SM) The SM consists of three parts. * Matter: spin = 1/2 quarks and leptons * Force: spin = 1 gauge bosons mediating the electromagnetic, weak and strong forces * Higgs condensation: spin = 0 Higgs boson breaking the electroweak gauge symmetry

9 SM is marvelously successful! * It explains almost all known physical phenomena in our Universe, i.e. all physical phenomena due to the electromagnetic, strong nuclear and weak nuclear forces. * One of its major components, the force part, is based on an elegant symmetry principle. * It is very accurate.

10 v Magnetic Moment of the Muon Electromagnetic, Strong and Weak forces at ` m Theory: Experiment:

11 v e + + e! Z 0! hadrons Weak, Electromagnetic and Strong forces at ` m

12 But SM is still far from being a complete theory. * It can not explain the observed dark matter and the matter-antimatter asymmetry in the Universe * It does not accommodate quantum gravity. * It does not provide a complete framework for the unification of the electromagnetic, weak and strong forces. * Electroweak symmetry breaking in the model is highly unnatural: Hierarchy problem

13 A key component of the SM is the Higgs condensation for electroweak symmetry breaking. Q1: Why is the nice symmetric point at the origin unstable? Why m 2 < 0? Q2: What sets the size of the characteristic scale? What sets the size of m ~ 0.2 TeV?

14 Hierarchy problem In SM, the Higgs boson gets a self energy due to the quantum fluctuations surrounding it, and therefore m 2 = m 2 self + m 2 bare with m 2 self ~ 10-2 Λ 2 ( Λ = highest energy of quantum fluctuation ~ M Planck ~ TeV ) We then need an extremely unnatural fine tuning to have m 2 = m 2 self + m 2 bare ~ (0.2 TeV) 2 => No explanation for m 2 < 0 and a big problem for the magnitude of m.

15 This strongly suggests that there might exist new physics beyond the SM controlling the Higgs boson self-energy at an energy scale around 1 TeV. Proposed ideas: * Supersymmetry (SUSY) * Composite Higgs bounded by a new force (Technicolor) * Extra spatial dimension with Gauge-Higgs unification.

16 u Supersymmetry (SUSY) SUSY is a spacetime symmetry which connects boson and fermion to one another. So the supersymmetric extension of the SM should include the superpartners (= superparticles) of all SM particles. ( Fermion, Boson ) ( quark, squark ) ( lepton, slepton ) ( photino, photon ) ( gluino, gluon ) ( Zino/Wino, Z/W ) ( Higgsino, Higgs )

17 All superparticles must be heavy since none of them is discovered yet: How heavy they are, i.e. where is SUSY? Triple coincidence of the SUSY mass scale A. SUSY at TeV regulates the Higgs mass in a correct way. Without SUSY: m 2 = m 2 self + m 2 bare ~ (0.2 TeV) 2 m 2 self ~ 10-2 M 2 Planck ~ (10 14 TeV) 2 With SUSY: m 2 ~ 10-2 m 2 SUSY log (M Planck / m SUSY ) SUSY at TeV (m SUSY ~ 1 TeV) naturally provides not only a right size of m ~ 0.2 TeV, but also m 2 < 0.

18 B. SUSY at TeV provides a natural candidate for Dark Matter According to the recent observations, the composition of our Universe is given by atom = 4 % = ordinary atoms DM = 23 % = dark matter DE = 73 % = dark energy (probably a vacuum energy) The lightest superparticle (LSP) can be a stable dark matter, and LSP at TeV gives a correct amount of dark matter: DM» 0:1 ³ 1 g 2 m LSP 1 TeV 2» 0:23

19 C. SUSY at TeV leads to the precise unification of the strength of strong, weak and electromagnetic forces at M GUT = 2x10 13 TeV. SM SUSY

20 If superparticles exist at TeV as this triple coincidence suggests, LHC will discover them and eventually will be able to measure their masses. SUSY event = pair-produced superparticles decaying into ordinary particles plus invisible lightest superparticle (LSP) (from B. Webber)

21 If SUSY is indeed at TeV, the two major LHC detectors (ATLAS & CMS) will see the SUSY events. ATLAS (before the full installation)

22 W-boson for weak nuclear force at ATLAS

23 LHC simulation of SUSY event  0 Event characteristics: * Multiple number of energetic jets missing momentum * Large momentum imbalance due to the two invisible LSPs  0

24 In fact, identifying a SUSY event is very nontrivial: ¾ new physics» ¾ TOT SUSY discovery potential depends on the superparticle mass and the LHC energy and luminosity schedules. For glunio mass ~ 1 TeV (3 TeV), 5σ discovery of SUSY will take roughly 1 few years (several 5 years) of running. It will take a quite longer time (at least several 10 years) for mass measurements.

25 Typically superparticle masses are generated at very high energy scale ( ~ TeV ), and logarithmically run down to the TeV scale. sparticle masses Grand unification? M a Extra dimension? String? 1 TeV (LHC) TeV At such high scale ~ TeV, there might be * Grand unification of the electromagnetic, weak and strong forces * Extra spatial dimension * Extended fundamental objects such as string and branes Then the superparticle masses measured at LHC can provide information about grand unification, extra dimension and/or superstring structure.

26 String theory involves 10-D spacetime with 6-D space compactified in very small size ~ m (10 13 TeV). A Popular String Compactification Our world Kachru, Kallosh, Linde, Trivedi (2003) SUSY breaking brane Quark, lepton, gauge boson, superparticles 6-D space with radius m

27 The pattern of resulting superparticle masses depends on Choi, Nilles, Falkowski, Olechowski; Choi, Jeong, Okumura; Endo, Yamaguchi, Yoshioka (a) dynamics to determine the size and shape of 6D space (b) origin of the gauge bosons, quarks and leptons (a) ux (b) Yukawa matter instanton perturbative correction = 0; 1; large n = 1; 1=2; 1=3; ::: gauge = m 2 ~g : m2 ~W : m2 ~B : m2 ~q : m2 ~l (2:5 0:74 ) 2 : (0:83 + 0:08 ) 2 : (0:43 + 0:29 ) 2 : (n + 5:0 3:5 + 0:48 2 ) : (n + 0:5 0:22 0:01 2 )

28 This suggests that, if the superparticle spectra can be measured at the LHC, one might be able to experimentaly test certain predictions of particular string compactification. Predicted superparticle spectra = 0; n = 1=2 = 1; n = 1=2

29 u Summary v We are confronting a critical moment of particle physics. LHC just began to probe the TeV energy scale. v There are good reasons to speculate that revolutionary new physics might exist at the TeV scale. (SUSY or Extra Dim or Technicolor or something else?) v If this speculation is correct, we have an exciting era ahead. There are a bunch of new particles waiting to be discovered at the LHC.

30 v The new particle spectroscopy might provide information about the physics at extremely high energy scales, e.g. grand unification and string compactification. v The whole results will revise our picture of space-time and give a deeper understanding of the origin of our Universe.