Signatures of Large eXtra Dimensions

• LXD-Group ITP FrankfurtMarcus Bleicher, Sabine Hossenfelder (Tucson), Stefan Hofmann (Stockholm), Lars Gerland (TelAviv), Kerstin Paech, Jörg Ruppert, Christoph Rahmede, Sascha Vogel,

Horst Stöcker, Benjamin Koch

Outline

• Motivation: why eXtra dimensions

• Models with eXtra dimensions ADD & RS

• Observable signatures for eXtra dimensions

• Summary

Very successfull effective quantum-field-theory of electroweak and strong interactions

Problems:

• Gravitation missing

why is gravitation so weak (hierarchy problem)

quantisation of gravity

• Too many parameters to be really fundamental

• Why Higgs so light (finetuning problem)

• ...

The Standard Model (SM)

LXDs

Large eXtra DimensionsModels with extra dimensions:

• Arkani-Hamed, Dimopoulos & Dvali (ADD)

• Randall & Sundrum (RS)

• Universal Extra Dimensions (UXD)

All contain one or more eXtra dimensions that are compactified on a radius that is so small that it could not be observed up to now.

* Large means large compared to Planck length: ...

The ADD - model

• 3+d spacelike dimensions

• d dimensions on d-torus with radii R

• all standard model fields live only in 4-dim. spacetime (brane)

• only gravity propagates in all dimensions (bulk)

This leads to a modification of

Newton´s law:rmm

V

p

112

12

11dd

f rMm

V ddfp RMm 22

Gravitons in accelerator experiments

• excitations of gravitons look on the brane like massive particles:

• From phase space one finds with that the cross section for graviton production for example in behaves like:

• Measurement via energy loss:

0)()(1

2

2

d

i

ii

iA

A

R

n

R

ns max

2

22

~

d

f

p

M

s

s

m

dt

d

)( Gee

Black holes in accelerator experiments

• Modification of Newton´s law at distances smaller than R leads to increase of the Schwarzschild radius

• Particles that get closer than RH form a horizon. up to black holes might be produced at LHC per year.

HR

fmM

M

dMdRfm

m

MR d

d

ffH

pH

42

1

382

101

21)(102

2

910

LXDs in Astrophysics

Strongest constraints on size of eXtra dimensions from supernova and neutron star observations:

• Observed neutrino flux from SN1987a

• Non observed flux of decay products of massive

gravitons from SN and neutron stars

TeVMTeVM dfdf 7,84 32

TeVMTeVM dfdf 30,500 32

Hannestad & Raffelt 2002

Minimal lengthHigher dimensional space-time and the existence ofa minimal length scale are intrinsic features of string theory and quantum gravity

• Minimal length can be modeled by setting

.../)( 33

22 papappkk

• with minimal possible compton wavelength

• this changes basic quantum mechanics

ffc ML /1

2

2

12

]ˆ,ˆ[f

ijji M

ppxikx

Observation of minimal length

• Modification of SM cross sections at high energies

• Modification of the gyromagnetic moment of electrons

and myons. 1110)80116592030(2/)2( ga

GeVM f 577

independent of number of eXtra dimensions

Harbach et al. 2004

SummarySoon we might be able to look behind the SM• Simple models like LXD + minimal length don´t claim to a TOE...• ...but provide useful basis to probe general features of spacetime like - value of new scale - number and size of eXtra dimensions - existence of minimal length• Model yields predictions for astro- & collider-physics• May help to learn about - structure of quantum gravity - mechanism of unification