Collider Signals of Large Extra Dimension

Nobuchika Okada (KEK)

The 7th Acfa Workshop

November 9-12, Taipei, Taiwan

1. Introduction

Large extra-dimension (ADD) scenario

(Arkani-Hamed-Dimopoulos-Dvali, ’98)

gravitonSM fields

compact extra-dimensions

“3-brane”

We are confined on “3-brane” graviton lives in the bulk

Basic picture:

( with common radii r )

dimensional model

Alternative solution to the gauge hierarchy problem without SUSY, TC, etc.

Low scale gravity model

Gauss law

We can take with

We can expect collider signal!

2. Theoretical Framework

: graviton in dimensions

Kaluza-Klein (KK) mode decomposition in 4 dimensions

Ex) if 6 dim.

+ periodic boundary conditions

4D description

Infinite tower of KK modes

KK graviton:

KK gravi-scalar:

KK vectors

KK scalars do not couple to SM fields

: negligible at high energies

Reduced Planck mass

Characteristic features: infinite tower of KK gravitons

universal couplings

Feynman rules

ex) QED + KK graviton

Phenomenology of extra-dimension scenario

= Phenomenology of graviton Kaluza-Klein modes

Detection of Extra-dimension @ future colliders

detection of KK graviton

direct KK graviton emission processes

indirect KK graviton mediated processes

First detection of spin 2 particle !

3. Collider signal of large extra dimension

KK graviton emission process

Emitted KK graviton non-interacting & stable particle missing energy event

3-1. direct detection

Ex) Each process is suppressed by

is very small can be sizable

Example:

SM background:

Giudice, Rattazzi & Wells,

NPB 544 (1999) 3

KK graviton emission process @ linear colliders

Example:

SM background:

Giudice, Rattazzi & Wells,

NPB 544 (1999) 3

KK graviton emission process @ LHC

a: integrating the cross section over

b: all

3-2. indirect detection

KK graviton mediated process

Need regularization

Naïve: Cut Off by

I: total cross section

　　　 new physics evidence

II: angular dependence of cross section

　 effects due to spin 2 particle exchange

Important points:

deviation from the SM collider energy

LC < LHC

precise measurements of angular dependence

LC > LHC

process

KK graviton exchange is dominant

SM background free very interesting

if this cross section is large enough

N. Delerue, K. Fujii & N. Okada

hep-ex/0403029, to appear in PRD

Example:

Comparable to

Angular dependence of cross section

Qualitative understanding of angular dependence

Initial state helicity:

or

Final state helicity: 0

Orbital angular momentum is needed

to make up spin 2 of intermediate KK gravitons

Angular distribution reflects the spin 2 nature of KK gravitons

Monte Carlo Simulation

process

two pair of b-jets

SM background: processes producing two pair of jets

Invariant mass distributions

Invariant mass distributions

Event selection

Criteria: More than 25 tracks with an energy > 100 MeV

A visible energy larger than 600GeV

Transverse momentum larger than 50 GeV

More than 4 jets

Invariant mass less than 16 GeV away from Higgs mass

One of the jets of each Higgs candidate to be b-taged

Number of remaining evens per

700 Higgs Pair events @ 1TeV LC

integrated luminosity 500

Essentially No SM backgrounds!

15/17

Reconstruction of Angular Distribution (after selection)

integrated luminosity 500

4. Summary

Large extra dimension Scenario is an interesting candidate

for new physics beyond the standard model

The existence of extra dimensions may be revealed through

processes involving the KK gravitons,

KK graviton emission processes

KK graviton mediated processes

LC has an advantage to measure the angular dependence

of the cross section of the KK graviton mediated process,

which reflects the spin 2 nature of the intermediate KK gravitons