Exploring New Physics beyond the Standard Model of Particle Physics Kiwoon Choi (KAIST) 3 rd GCOE Symposium Feb. 2011 (Tohoku Univ.)
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.
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
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?
Threshold scales for revolutionary new physics u Atomic scale : 10-10 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
u Scale of atomic nucleus: 10-15 m # Strong nuclear force at scales around 10-15 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
u Scale of electroweak unification: 10-19 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 10-19 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.
Down to the length scale ~ 10-18 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
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.
v Magnetic Moment of the Muon Electromagnetic, Strong and Weak forces at ` 10 18 m Theory: 1.0011659186+ 0.0000000008 Experiment: 1.0011659203+ 0.0000000008
v e + + e! Z 0! hadrons Weak, Electromagnetic and Strong forces at ` 10 18 m
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
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?
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 ~ 10 15 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.
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.
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 )
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.
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
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
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)
If SUSY is indeed at TeV, the two major LHC detectors (ATLAS & CMS) will see the SUSY events. ATLAS (before the full installation)
W-boson for weak nuclear force at ATLAS
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
In fact, identifying a SUSY event is very nontrivial: ¾ new physics» 10 10 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.
Typically superparticle masses are generated at very high energy scale ( ~ 10 13 10 15 TeV ), and logarithmically run down to the TeV scale. sparticle masses Grand unification? M a Extra dimension? String? 1 TeV (LHC) 10 13 10 15 TeV At such high scale ~ 10 13 10 15 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.
String theory involves 10-D spacetime with 6-D space compactified in very small size ~ 10-32 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 10-32 m
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 )
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
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.
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.