C. Collard- Search for Extra Dimensions @ LHC

8/3/2019 C. Collard- Search for Extra Dimensions @ LHC

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IIHE Seminar (11/09/2004)C.CollardLLR, Ecole Polytechnique, Paris

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Search forExtra Dimensions

@ LHCC. Collard (LLR, Ecole Polytechnique)


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Figure from Scientific American

The univers where we livecould be a membrane inside ahigher dimensional space-time.

What remains fiction inScience Fiction? Where comesthe idea of extra dimensions andparallel universes from?


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Introduction

Many extensions of the Standard Model are motivatedby the Hierarchy problem :

o Electroweak Scale ~ 100 GeV

MW = 80 GeV, MZ = 91 GeV

o Planck Scale ~ 1019 GeV

Energy where the relativistic and quantum effects are importantfor gravity

o The Higgs mass diverges in the standard model because ofradiative corrections.

MH2 = [MH0]2 + O(2) +

with : the scale of validity for the standard modelFine tuning which can provide cancellations ?


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Introduction

The Standard Model is an effective theory. The newtheory takes over at a scale comparable to the Higgsboson mass, i.e. ~ 1 TeV.

Possible solution?

o Supersymmetry : for each SM particle a susy partner is

introduced. SM and susy particle contributions to Higgs masshave opposite sign.

o Extra Dimensions : strong gravity at TeV scale.

ED already introduced in string theory (theory for describingthe gravitation quantically with 10 or 11 dimensions, in whichextra dimensions are compactified).

o


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How to detect these extra

dimensions?

Astrophysics : Study of the sky activity

Gravitation :Test of the Newtons Law

Particle Physics : Search for theireffects on reactions produced in colliders


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The LHCEcm = 14TeV


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The 2 Experiments of the LHC

ATLAS

CMS

The total mass of CMSisapproximately 12500 tonnes

- double that of ATLAS(even though ATLASis ~8xthe volume of CMS)


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Plan

Large extra dimensionsKaluza-Klein Excitations of gravitons

by direct production or virtual effects

The TeV-1 size extra dimensionsKaluza-Klein Excitations

of gauge bosons

The warped extra dimensionsKaluza-Klein Excitations of gravitons

(The radion)

Sizeoftheextradimensions

1.The cisplanckian physics (s > gravity scale)

- Black holes & string balls )


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Large Extra Dimensions

Model of Arkani-Hamed, Dvali,Dimopoulos: Standard Model

particles are localized on a 3-Dbrane. Gravity propagates inside thebulk (a more dimensional space)

Figure from Scientific American


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Model of Arkani-Hamed, Dvali, Dimopoulos:o World at 4 + n dimensions. Only the gravitons may propagate in extra

dimensions. Gravity appears to be diluted.

The Newtons Law is verified up to distances~0.2 mm. Extra dimensions must be smaller

than 0.2 mm and compactified.The real Planck mass MD= MPL[n+4] :

(MD)(2+n) = (MPL[4])2 Rn

If MD ~ 1 TeV (= no more hierarchy problem):


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Particle in compact extra dimension:o Wavelength set by periodic boundary conditions

o States will be evenly spaced in mass tower of Kaluza-Klein (KK) modes

o Spacing depends on the scale of ED

A tower of massive KK excitations of gravitons:Mn2 = M02 + n2/R2 for the nth state.They couple to Standard Model particles:

L= 1/MPL G(n) T

The large number of states (degenerated in mass)compensates the low coupling (1/M

PL):

~ (s/MD2)n


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Search at LHC: Large Extra

Dimensions Direct Search

o Monojet with a large missingtransverse energy (stable nondetected GKK)

ATLAS

Limit on MD

min:

Validity of the effective theory:s < MD.Above this limit: sensitivity to new physics.

Truncation ofwhens > MD.


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Dimensions

o Isolated Photon with a largemissing transverse energy (nondetected GKK )

Channel which will allow toconfirm the discovery in themonojet channel ATLAS


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Dimensions Indirect Search

o GKK is a propagator. Search fordeviations in or in asymmetriescompared to SM.

o MS is a cut-off (UV divergence).

ATLAS @ 100 fb-1 : MS < 5.1 TeV for the ll channel and MS < 6.6 TeV for


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The TeV-1 size Extra DimensionsVariation of the previous model :In addition to the large

extra dim, smaller ones are introduced (of TeV-1

size) in which gauge bosonscan propagate while fermions are confined on the 4dim branes.

The KK modes of the gauge bosons (n), Z(n), W(n)are massive

Mn2=(n Mc)2 + M02

and their coupling goes like SM (* 2)

Constraints for (1) and Z(1): EW Data: Mc > 4 TeV

At LHC (mostly from ATLAS):

Sensitivity in the peak: Mcmax= 5.8 TeV (100 fb-1)

From interference study: Mcmax=9.5 TeV (e) for 100fb-1 and 13.5 TeV for 300 fb-1 (e+)

(Work of Barbara & Lionel)


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The Warped Extra DimensionsThe Randall-Sundrum Model


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The model of Randall and SundrumThe extra dimension allows to explain

the hierarchy problem (MEW


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o Gravitons at 5 Dim KK excitations of gravitons at 4 Dim

(development in Bessel series and not in Fourier series like for the largeextra dim.)

o 2 free parameters in the model:MG=M1and c=k/MPL

c=1.c=0.5c=0.1c=0.05c=0.01

Mn = k xn e-kr with J1(xn)=0

n= Mn xn2 c2

Si M1=1.5 TeV

G2

G1

G3

The model of Randall and Sundrum


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The model of Randall and Sundrum

Constraints on the two free parameters of the model: MG andc=k/MPL

H.Davoudiasl, J.Hewett,T.Rizzo, hep-ph/0006041

Which part of the plane can be access with CMS?


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The e+e- channel

Signal: pp G e+e- (K Factor =1)

The e+e- decay channel has a low branching ratio (BR=2%) but theclear signal in the electromagnetic calorimeter ECAL allows it to bethe discovery channel for Randall-Sundrum Gravitons.

Background: 2 electrons in the final state

o Drell-Yan: pp/Z e+ e- (K Factor=1.3)

o [ Jet faking an electron: Dijet, -jet, e-jetwhich is negligible in comparison to Drell-Yan ]

q

q_

e+

e-

GKK

g

g e-

GKK

e+


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Electron Reconstruction


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Technical details Generation with PYTHIA (+ inner bremsstrahlung with PHOTOS)

Full Simulation and Reconstruction chainof CMS(CMSIM & ORCA without pile-up):

o Synchrotron radiation is included but found to be negligible incomparison to bremsstrahlung in the tracker

o Work on the electron reconstructiono Possible saturation of the ECAL electronics (ADC overflow) is

studied: Saturation expected at 1.7 TeV in the barrel with measured crystal

light yield (4.5 photo-electrons/MeV)

Study here for saturation at 1.25 TeV (i.e. 6 p.e./MeV) A simple correction is found.


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Saturation of the ECALelectronics

The saturation has a big effect on the mass reconstruction of

heavy resonances. Idea for correction:Correlation betweenRed5=E9-E4and E1

5x5 crystals

Saturation expectedbut not simulated


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Saturation of the ECALelectronics

This correction of the saturation allows to reconstruct heavy mass

resonances.


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Selection Cuts

pp G e+e-

Trigger up to Level 2.5 2 electrons

o Super-Clusters:

pT> 100 GeV,

||


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Search for a resonance

Fit of a Gaussian to the signal distribution

Mass window for NS and NB estimation: 3

For low coupling values: E1 < 1250 GeV (no saturation) For large coupling values: correction of the saturation coming from

the ECAL electronics


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Results for c=0.01


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Results for c=0.01


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Significance

3800 GeV1775 GeV


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Ge+e- : Discovery plane


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Rem : IdentificationThe identification of the particle nature can be done with the study of

the angular distributions and forward-backwardsymmetries in addition tothe search in different decay channels.

100 fb-1, canal ee


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Conclusions on the RS analysis

Full simulation & reconstruction analysis Study of very energetic electrons and search for massive

resonances

Discovery plane for the Randall-Sundrum gravitons G e+ e-:

o With 100 fb-1: the region of interest will be covered by CMS.

o With 1 fb-1: a large part of this region of interest will be

accessible at the first beginning of the LHC running. For the Future: Work on the Identification of the Graviton nature

o Angular Distribution (Graviton is spin 2)

o Other channels:

G is allowed but not Z .

Test the universality of the Graviton couplings.


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Conclusions : The extra dim

A large number of models exist. I have only presented 3

of them here, which propose quite interestingsignatures to be detected at LHC.

If we dont find candidates for new physics, the LHC

will provide strong constraints on the different models. In case of discovery (which may appear quickly), big

efforts should be done to identify the nature of thesignal. Help will come from linear colliders.

Important to be ready for the start of LHC!


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A bad news for the end

Unfortunately, we will not travel in an extra dimension, this is leftto science fiction. In most of the models, only gravitons can

propagate inside the extra dimensions. We will get in the best casean indication of their existence but impossible to visit them duringholidays.

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C. Collard- Search for Extra Dimensions @ LHC