exploring new physics beyond the standard model …...exploring new physics beyond the standard...

Exploring New Physics beyond Exploring New Physics beyond the Standard Model of Particle Physics the Standard Model of Particle Physics

Kiwoon Choi (KAIST)

3rd 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.

Geneva Airport

Lake Leman

Large Hadron Collider (LHC) is the grandest and most expensive scientific instrument ever built.

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# Atomic spectroscopy: (Balmer, Lyman)

suggests that interesting new physics might exist at scalesaround submicro meter.

# Rutherford experiment probing the atomic structure (1911)

=> Discovery of Quantum World

* Undeterministic mechanics * Quantized observables

0:09 ¹m¸ = 1

n2 ¡1l2

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 macroscopicnuclear force

* Spontaneous chiral symmetry breaking, explaining the originof 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, weakand strong forces

* Higgs condensation: spin = 0 Higgsboson breaking the electroweak gauge symmetry

SM is marvelously successful !SM is marvelously successful !

* It explains almost all known physical phenomena in our * It explains almost all known physical phenomena in our

Universe, i.e. all physical phenomena due to the Universe, i.e. all physical phenomena due to the

electromagnetic, strong nuclear and weak nuclear forces. electromagnetic, strong nuclear and weak nuclear forces.

* One of its major components, the force part, is based* One of its major components, the force part, is based

on an elegant symmetry principle.on an elegant symmetry principle.

* It is very accurate. * It is very accurate.

vv Magnetic Moment of the Magnetic Moment of the MuonMuon

Electromagnetic, Strong and Weak forces at

Theory: 1.0011659186+ 0.0000000008Experiment: 1.0011659203+ 0.0000000008

`¸10¡18m

v

Weak, Electromagnetic and Strong forces at `¸10¡18m

e+ + e¡!Z0!hadrons

But SM is still far from being a complete theory.

* It can not explain the observed dark matter and thematter-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 m2 < 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

m2 = m2self + m2

bare with m2self ~ 10-2 Λ2

( Λ = highest energy of quantum fluctuation~ MPlanck ~ 1015 TeV )

We then need an extremely unnatural fine tuning to have

|m2 | = |m2self + m2

bare | ~ (0.2 TeV)2

=> No explanation for m2 < 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 themis 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: |m2| = |m2

self + m2bare| ~ (0.2 TeV)2

|m2self| ~ 10-2 M2

Planck ~ (1014 TeV)2

With SUSY: m2 ~ – 10-2 m2

SUSY log (MPlanck / mSUSY)

SUSY at TeV (mSUSY ~ 1 TeV) naturally provides not only a right size of |m| ~ 0.2 TeV, but also m2 < 0.

B. SUSY at TeV provides a natural candidate for Dark Matter

According to the recent observations, the composition ofour Universe is given by

4 % = ordinary atoms23 % = dark matter73 % = 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:

SUSY and Extra Dimension proposed for natural EWSB provide a stable WIMP with mass = GeV, thus explain the observed DM mass density in a natural manner.

­atom =

­DM =

­DE =

­DM » 0:1³1g2

mLSP

1 TeV

´2» 0:23

C. SUSY at TeV leads to the precise unification of the strength of strong, weak and electromagnetic forcesat MGUT = 2x1013 TeV.

SUSY

SM

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

Event characteristics:

* Multiple number ofenergetic jets

* Large momentum imbalance due to thetwo invisible LSPs

Â0

Â0

missingmomentum

In fact, identifying a SUSY event is very nontrivial:

SUSY discovery potential depends on the superparticlemass 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.

¾new physics » 10¡10¾TOT

Typically superparticle masses are generated at very high energy scale ( ~ 1013 – 1015 TeV ), and logarithmically run down to the TeV scale.

Grand unification ?

At such high scale ~ 1013 – 1015 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.

sparticlemasses

1013 – 1015 TeV1 TeV (LHC)

Ma

Extra dimension ?String ?

String theory involves 10-D spacetime with 6-D space compactified in very small size ~ 10-32 m (1013 TeV).

A Popular String Compactification Kachru, Kallosh, Linde, Trivedi (2003)Our world SUSY

breakingbrane

6-D space withradius 10-32 m

Quark, lepton,gauge boson, superparticles

The pattern of resulting superparticle masses depends onChoi, 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) (b)

m2~g : m

2~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)=

® = 0; 1; large n = 1; 1=2; 1=3; :::

°ux

instanton perturbativecorrection gauge

matter

Yukawa

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 revolutionarynew 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-timeand give a deeper understanding of the origin of ourUniverse.