New Physics at the TeV Scale ?New Physics at the TeV Scale ? (What do we expect at LHC(What do we expect at LHC ?) ?)

Kiwoon Choi (KAIST)

(CTP Colloquium @ SNU)

LHC is a proton + proton collider with c.m energy 14 TeV and

the luminosity 1034 cm-2 s-1 in the 26.6 km tunnel at CERN, Geneva.

LHC (Large Hadron Collider) is coming.

Swiss Alps

Airport

Geneva

Lac Leman

CERN

LHC will probe for the first time the TeV energy scale

(~ 10-19 m) which is expected to be the threshold scale

of revolutionary new physics.

# Why do we expect new physics at TeV ?

# What do we expect (hope?) to see at LHC ?

# What would be the implications of LHC discoveries

for more fundamental physics ?

Threshold scales of new physics

(c = h/2π = 1)

# Scale of atomic spectroscopy: α2em me ~ 10 eV

⇒ Atomic structure and Quantum Mechanics manifesting

themselves at scales ≥ αem me ~ 103 eV

(uncertainty principle, wavefunction description of

physical state, force as an exchange of particle, …)

# Scale of nuclear force: mπ ~ 100 MeV

⇒ Quarks, gluons and Quantum Chromodynamics (QCD)

manifesting at scales ≥ ΛQCD ~ 1 GeV

(asymtotic freedom, confinement, chiral symmetry

breaking, …)

# Scale of weak force due to electroweak symmetry

breaking (EWSB): MW,Z ~ 100 GeV

We don’t know yet what is the underlying physics of

EWSB, just expect it can be as rich as the new physics

encountered at lower threshold scales.

There are many reasons to believe that there exists the

so-called Higgs boson providing the simplest description

of EWSB.

However this minimal Higgs description of EWSB suffers

from a fine tuning problem, implying that more nontrivial

structures should exist , and this is why we expect there

exist rich new physics at the TeV scale.

Unnatural Higgs boson mass Self energy due to the cloud of virtual top-antitop pairs surrounding Higgs

δm2H = - 3y2

tΛ2/2π2 (yt ~ 1)

(Λ = maximal energy of the top-antitop fluctuations)

m2H = (m2

H)bare + δm2H ~ - M2

W,Z

We always like to have a description valid over a large scale,

e.g. valid up to Λ ≫ MW , however it requires an unnatural

fine tuning at the level of M2W/Λ2 ≪ 1 .

top

Higgs

y t

y t

Similar problem has been encountered in # Electron mass in non-relativistic QED (or classical electrodynamics) # Cosmological constant of our Universe

Electron mass:

Electromagnetic self energy due to Coulomb-type virtual photons surrounding an electron of radius re

δme = e2/(4π2re)

me = (m e)bare + δme = (m e)bare + e2/(4π2re) Unnatural fine tuning is required also if this theory of electron and photon does work up to Λ = 1/re ≫ me .

Hypothetical physicist knowing only the low energy world

at scales ≤ me can imagine three different possibilities.

A) Composite electron

Electron is a fat guy with radius

re ~ 1/me ~ 4x10-13 m δm⇒ e ~ e2/(4π2re ) < me

Without directly probing the physics at the composite length

scale, one might be able to exclude this possibility by

precise low energy multipole expansion data.

Anomalous magnetic moment of the electron δμ :

Composite structure at re δμ ~ e⇒ re ~ e/me

B) More Universes and Anthropic Selection

re 1/m≪ e and δme ~ e2/(4π2re ) m≫ e

However there exist huge number of different Universes

having all possible values of (me)bare and thus of me,

and we are living in a special Universe realizing a

fine cancellation between (me)bare and δme , yielding

me = (m e)bare + δme = 0.51 MeV,

since any sizable deviation of me from this value does

not allow the DNA helix to replicate itself, thus not allow

us to exist now. (T. Regge, 1971)

C) More Symmetry There appears a new symmetry at scales ~ me , eliminating

the linearly divergent part ( ~ 1/re) of δme , thereby reducing

the self energy as δme ≤ me even for Λ = 1/re m≫ e .

Indeed relativistic QED has a new symmetry

γ5 |e, E, up> = |e, -E, down> = |-e, E, up>

(electron) (positron)

δme =

= [ e2Λ/(4π2) - e2Λ/(4π2 ) ] + 3e2meln(Λ/me)/(16π2)

( γ5 cancellation ) (V. F. Weisskopf, 1934)

≤ me as long as Λ ≤ mee500 ~ 10210 GeV

Cosmological Constant (Vacuum Energy Density)

Vacuum energy density of generic quantum field:

δΩvac = ±Σ ( zero point energy ) ~ Λ4

(Λ = maximum energy of quantum fluctuations)

Ωvac = (Ωvac)bare +δΩvac = (3x10-3 eV)4

The success of quantum field theory up to scales ~ MW

suggests Λ ≥ MW ~ 1011 eV , then an extreme fine tuning

(better than 10-50) is required to get the observed vacuum

energy density of our Universe.

Again we can imagine three possibilities.

A) Composite graviton at sub-millimeter

Graviton is a composite object at scale ~ 10-3 eV, so is

blind to quantum fluctuations at higher energy scales.

(R. Sundrum, 2003)

This might be tested by looking for a modification of gravity

at sub-millimeter scale.

(Principle of general covariance appears as an emergent

feature of Nature at sub-millimeter.)

B) More Universe and Anthropic Selection

Ωvac = (Ωvac)bare + Λ4 = (3x10-3 eV)4

There exist huge number of different Universes having all

possible values of (Ωvac)bare and thus of Ωvac , and we are

living in a special Universe realizing the extreme fine tuning

(better than 10-50) for Ωvac = (3x10-3 eV)4 , since only

a Universe with

- (2x10-3 eV)4 ≤ Ωvac ≤ (10-2 eV)4

can form a galaxy and a life in it. (S. Weinberg,1987)

C) More Symmetry

At present, we don’t know any symmetry reducing the

vacuum energy density Ωvac down to the observed

value ~ (3x10-3 eV)4 . (cf. Kaplan and Sundrum (2005)?)

#################################################

Now, we know definitely that more symmetry(γ5 symmetry)

is the correct answer to the electron mass puzzle, while

at the moment more Universe is considered as the

most plausible answer to the cosmological constant puzzle

because we don’t know any theoretical scheme to realize

the other possibilities (fat graviton or symmetry) .

Remarks on multiverse and anthropic principle The multiverse and anthropic principle assume that there

exist huge number of different universes including any

kind of Universe under the consideration, and our universe

has certain particular feature since only a universe with

such feature can accommodate life.

On the other hand, we are looking for a fundamental

principle selecting our universe as a unique possibility.

So, talking about the anthropic principle, some of your

friends might say “you are abandoning to be a physicist ”. S. Weinberg...A physicist talking about the anthropic principle runs the same risk

as a cleric talking about pornography : no matter how much you say you areagainst it, some people will think you are a little too much interested.

Until very recently, we didn’t take the multiverse and

anthropic principle as a serious explanation for any fine

tuning problem in physics.

But the recent discovery of dark energy which is most

likely to be a small but nonzero cosmological constant

dramatically changed our attitude (but not the attitude of

religious people who believes in the God’s design for

unique Universe).

The realization that string theory admits huge number of

different vacua giving different universes (string landscape)

also boosted this change of our attitude.

How much are some prominent physicists now confident about the multiverse explanation of the

cosmological constant ?

Martin Rees : I bet my

dog’s life!

Andrei Linde : I bet my own life

!

Steven Weinberg : I bet the lives of both A.Linde & M.Ree

s’s dog!

Christoph Schoenborn (Cardinal Archbishop of Vienna)

“Now, at the beginning of the 21st century, faced with scientific claims like neo-Darwinism and the multiverse hypothesis in cosmology invented to avoid the overwhel ming evidence for purpose and design found in modern science, the Catholic Church will again defend human nature by proclaiming that the immanent design evident in nature is real. Scientific theories that try to explain away the appearance of design as the result of`chance and necessity are not scientific at all, but, as John Paul put it, an abdication of human intelligence.”

What will be the correct answer to the Higgs

mass puzzle:

mH2 = (mH

2)bare + Λ2 ~ - (300 GeV)2 ?

A) Composite Higgs at Λ ~ 1 TeV ?

Highly disfavored by the measured multipole properties

of the W and Z bosons

B) Multiverse and Anthropic selection with Λ 1 TeV ?≫ Might be the answer as a Universe with

mH2 > 0 or mH

2 < - (1 TeV)2

cannot form complex elements for life. (Agrawal et al, 1997)

(No exciting new physics at TeV other than the boring Higgs,

so a disaster for us !)

C) More Symmetry ?

Yes, we have a beautiful symmetry (SUPERSYMMETRY)

naturally reducing the Higgs self energy as δmH2 ~ MW

2.

SUSY | fermion > = | boson > ( SUSY |quark> = |squark> )

SUSY | boson > = | fermion > ( SUSY |photon> = |photino> )

δmH2 = -3yt

2 [ Λ2 – Λ2 + mst2 ln (Λ/mst) ] / (2π2)

(SUSY cancellation)

SUSY at TeV ( mst = stop mass ~ 1 TeV) gives the desired

mH ~ MW ~ 100 GeV without fine tuning.

top

Higgs

y t

y t

stop

Higgs

+

SUSY is the unique possible extension of the Lorentz symmetry, so no doubt about its existence. The real question is at which scale it does appear, and it is very likely that TeV is the right scale for SUSY to appear as it gives a natural EWSB. In TeV scale SUSY scenario, superpartners of known particles have a mass ~ 1 TeV:

Fermion SUSY Boson quark ⇔ squark lepton ⇔ slepton photino ⇔ photon gluino ⇔ gluon

In addition to providing natural EWSB , TeV scale SUSY has more attractive features !

# Lightest superparticle (typically photino) is stable and a good Dark Matter candidate. Direct Evidence of Dark Matter (Galaxy cluster : 1E 0657-56)

# Unification of the strong, weak and electromagnetic gauge couplings

If SUSY exists at TeV, LHC will discover it! p p ⇒ gluino or squark pairs

⇒ many jets + leptons + missing ET

Glunio Mass ≤ 1 TeV : few months of running Gluino Mass ≤ 3 TeV :

several years of running (5σ discovery)

LHC can measure sparticle masses accurately.(with an accuracy of order few to 10%)

# Cascade decays of sparticle

Hinchliffe et al

Dilepton invariant mass distribution

~ 77 GeV

# Transverse mass of sparticle pair (MT2) Lester and Summers

Gluino MT2 for the process:gluino + gluino⇒ q q χ + q q χ (q=quark, χ=photino) Cho, KC, Kim, Park (2007)

(minimization over all possible splitting of the observed missing ET) = Max of mT2(gluino) over all events = a function of the trial photino mass

: transverse mass and momentum of qq system

: trial mass and transverse momentum of χ

The gluino MT2max(mχ) has a CUSP when

trial photino mass = true photino mass, with which one can determine precisely both the gluino mass and the photino mass. Cho, KC, Kim, Park (2007)

LHC measurement of gluino, photino, squark, and slepton masses: Gluino MT2 can make the accuracy much bette

r!

Sparticle masses are generated at very high scale and logarithmically run down to low energy scale. So, accurately measured sparticle spectra at TeV provide an window to more fundamental physics such as grand unification or superstring structure which might exist at extremely high energy scale.

Mass

1016 103

(GeV)

~g~q

~W~~B

Ma

m2

GUT?

String?

KKLT-type string compactification at scale 10-33 m Kachru, Kallosh, Linde, Trivedi (2003)

Our world

Anti-brane

6D CY spacewith radius 10-33m

Quark, lepton,gauge boson, superpartners

* The structure of string flux vacuum at 10-33m can beread from the pattern of superparticle masses at 10-19m.

Choi, Falkowski, Nilles, Olechowski (2005)

Our world

SUSY breaking brane

CY volume modulus

graviton

Messengers of supersymmetry breaking

Mechanism to mediate SUSY breakingMechanism to mediate SUSY breakingin KKLT-type string compactificationsin KKLT-type string compactifications

Superparticle masses in such mediation scheme show a highly distinctive feature !

Choi, Jeong, Okumur

a (2005)

Mirage MediationMirage Mediation질량

10-33m10-19m

거리

~g~q

~W~~B

Ma

m2

질량

10-33m10-19m

거리

~g

~q~W~

~B

Ma

m2

10-25m

Conventional scenarios

Mirage mediation

Transmission of SUSY breaking to our world shouldbe an important ingredient of the fundamental theory such as superstring or supergravity theory, which can be tested by the sparticle spectra measured by LHC.

Four mediation schemes predicting distinctive patterns of sparticle spectra at TeV: # Gravity mediation ⇒ M1 : M2: M3 = 1 : 2 : 6 # Gauge mediation ⇒ M1 : M2: M3 = 1 : 2 : 6 (but with light gravitino)

# Anomaly mediation ⇒ M1 : M2: M3 = 3.3 : 1 : 9 # Mirage mediation ⇒ M1 : M2: M3 = 1 : 1.3 : 2.5

Sparticle spectra from these mediation schemes

LHC will be able to test the predictions of these mediation schemes, and

exclude some or all of them.

Conclusion and Summary

# We are confronting a critical moment of particle physics.

# From July, 2008, LHC will be probing the TeV scale where exciting new physics is expected to be waiting for us.

# TeV SCALE SUPERSYMMETRY appears to be the most plausible candidate for the new physics at TeV.

Conclusion and Summary

# LHC will be able to determine sparticle spectra, thereby providing an window to more fundamental physics such as grand unification, supergravity and superstring structure which might exist at extremely high energy scales.