Searches for GMSB and for High ET Di-lepton Pair Events at the LHC

Searches for GMSB and for High ET Di-lepton Pair Events at the LHC

Searches for GMSB and for High ET Di-lepton Pair Events at the LHC

Daniele del Re

Universita’ di Roma “La Sapienza” and INFN Roma

Rencontres de Moriond QCD and High Energy Interactions

8-15 March

03/10/08 Daniele del Re (La Sapienza & INFN) - Moriond QCDDaniele del Re (La Sapienza & INFN) - Moriond QCD 2

Intro and OutlineIntro and Outline• Two striking signatures to discover new physics at the LHC:

1) long lived particles with large masses and low 2) high-mass resonances decaying to two leptons

• Important analyses at beginning of data taking– low luminosity needed for discovery

OUTLINE• Very short intro on GMSB and models with TeV resonances

decaying in two leptons• ATLAS and CMS results and discovery potential for

– GMSB in photons and staus– High-mass resonances in di-electron and di-muon pairs

• Focus on– low statistics scenarios and experimental issues at start-up– techniques to get efficiencies/backgrounds from data


GMSBGMSB• Gauge Mediated Supersymmetry Breaking.

Models for SUSY breaking, alternative to mSUGRA

• SUSY breaking transmitted from Hidden sector to visible sector via

gauge interactions (“messengers”)

• Lightest supersymmetric particle (LSP) is the Gravitino (m≤keV)– light, stable and weakly interacting – possible candidate for Dark Matter

• Depending on parameters N(ext)LSP– neutralino– stau– both cases studied in ATLAS and CMS

• N(ext)LSP lifetime value connected to parameter

Present limits: Tevatron, > 80 TeV, m(neutr.,charg.) > 108/195 GeV

Par. Description

SUSY breaking scale

Mm Messenger mass scale

tan Ratio of Higgs vev

NmNumber of SU(5) messenger

multiplets

sign() from Higgs sector

Cgrav Sets NLSP lifetime


GMSB: final states with ’sGMSB: final states with ’s• If NLSP neutralino 2 in event

• Selection– isolation, cut on pT()– Large MET, Njets > 3

• Main backgrounds– +jets, W+jets– after cuts: S/B > 10

• If lifetime() ≠ 0 non-pointing – experimentally tough measurement

• Lifetime measurement:– transversal (CMS) and longitudinal

(ATLAS) cluster shape to get photon direction

– time measurement in calorimeter (ATLAS) to close kinematics

G~

01

~

01

~

p p

q

q

q

q ~…

…

jet

jet

jet

jet

G~

~


GMSB with ’s: Discovery PotentialGMSB with ’s: Discovery Potential

Low statistics for discovery both in pointing (c=0) and

non-pointing scenarios

for =140 TeVfor =90 TeVL=100fb-1

Lifetime measurement feasible but large statistics needed

Reconstruction of leptons in the event can be used to extract both neutralino and slepton masses

c(cm)


GMSB with stausGMSB with stausPoints used by CMSPoints used by CMS

mass 156 247

Nm 3 3

TeV 50 80

MmTeV) 100 160

Tan 10 10

sign() 1 1

Cgrav 104 104

Arb

itra

ry n

orm

• With a different choice of parameters

(e.g. Nm>3) NLSP is the stau– quasi-stable due to the smallness

of the coupling constant

• One or more staus produced via heavier

SUSY particles (as for final states)

• Large stau mass implies low

• Through tracker and muon detectors– ionization in tracker (dE/dx)– time of flight using muon detector

• Other models predict heavy stable charged particles: Split SUSYR-hadrons, Kaluza-Klein lepton like particles


Trigger and Timing IssuesTrigger and Timing Issues

Due to particle slowness, limited trigger and data acquisition efficiency

Detailed studies on systematic effects ongoing• Studying possibility of taking data of the next bunch crossing and

setting up ad-hoc triggers

Bunch crossing t - 25ns Bunch crossing t- 25ns


and Mass Measurementsand Mass Measurements

Goal: stau mass

1) from time of flight• both ATLAS and CMS use muon drift tubes • timing information and staggering used to extract

delays with respect to muons and calculate

2) from ionization (dE/dx) in tracker (CMS)

for low ’s

TOF and dE/dx tuning on data (CMS):use of Z→, cosmics, high ionizing low pT protons

11

2

pm

500GeV masscharged particles

MIPs

dx

dEK~1

tracking

needed

drift

x

v

c

L

11 averaged over all layers

for 300GeVslepton

expected muon drift

actualstau drift

stau track

x


Discovery PotentialDiscovery Potential

Two options proposed by CMS

TOF + dE/dx combined • sel. criteria: TOF<0.8, DEDX<0.8 and

quality requirements on tracking • almost background free

measurement !• at least 3 events to claim discovery

Standalone tracker (dE/dx only)• as a cross-check

GMSB stau

Stop (split susy)

KK tau

bkg stau

TOF + dE/dx combined

DT DT

trk

trk


High-Mass Resonances: Di-lepton High-Mass Resonances: Di-lepton

• Many extensions of the Standard Model predict resonances decaying to two leptons and with large masses:– super-string inspired, extra dimensions and GUT theories; – left-Right Symmetric Models; – little Higgs Models

• Resonances produced via Drell-Yan process

• Stringent limits from precision EW experiments and direct searches

• Given the striking signature, di-lepton scan over a wide range of masses represents a priority for experiments, regardless of indirect limits and models. Focus on maximizing– mass resolution: significance decreases as the square root of resolution– background rejection

TEVATRON limits ~ 1 TeV


Experimental Issues Experimental Issues

Analysis strategy:• Two leptons (high pT)• QCD and non leptonic backgrounds reduced with isolation (in a

cone around lepton direction):– no additional tracks – no hadronic deposit– no additional electromagnetic deposit

• Invariant mass used for signal extraction (likelihood fit)

Complications:• charge measurement flipped because of large momentum• not perfect momentum resolution at early data taking, affecting

invariant mass measurements

Backgrounds:• Irreducible Drell-Yan (but small at very high masses)• Other contributions are almost negligible


Di-muon AnalysisDi-muon Analysis

Selection criteria:• L1+HLT trigger requirements (single, double muon triggers), ~ 97-98%• Global reconstruction: tracker + muon detector • Tracks with opposite charge

• pT>20GeV

efficiency ~ 80% at 2TeV - misidentification < 1% in barrel

small background contribution (S/B>20)

Efficiencies from data (Z→ tag and probe) – one is reconstructed with tight cuts– cut on Z invariant mass – recoiling probe to measure efficiencies

• Small stat. at high pT. Extrapolate at start-up

New results from CMS

Muon efficiency from data with tag and probe

Z→ method for 100pb-1

Global muon reconstruction


Di-muon: Effect of Misalignment Di-muon: Effect of Misalignment

• Checked impact of misalignment on measurement at beginning of data taking

• Chambers shifted and rotated randomly (±5mm and ±5mrad) to simulate start-up conditions

• Used track based alignment with

muons from W in 100pb-1

Results for 100pb-1:• Mass resolution affected

– ~doubled compared to perfect alignment– small negative shift

• Small impact on background amount

Perfect alignment

With misalignment

in 100pb-1 scenario

Drell Yanbkg

Arb

itra

ry n

orm


Di-muon: Discovery PotentialDi-muon: Discovery Potential

• Use of unbinned maximum likelihood fit with both signal and background normalizations floating

• Discovery luminosity estimated with likelihood ratio

• 1-2 TeV resonances (ZSSM and Z)

discovered with low statistics

ZSSM

Z

Z′→1TeV Z

in 100pb-1

sig+bkg fit

bkg-only fit

Model /M

%

• BR() with interference

1TeV 1.5TeV 2TeV

ZSSM 3.1 620fb 84fb 25fb

Z 0.6 360fb 31fb 13fb


Di-electronDi-electron

4TeV KK Z′

• Di-electron invariant mass resolution improves with e± momentum – dominated by constant term in em

calorimeter for very high E

• ATLAS: di-electron better than di-muon

• CMS: important saturation effects after ~3TeV

• Selection similar to di-muon case

(e± trigger and identification,pT, isolation)

• Example of potential for Randall-Sundrum scenario (Set A,B,C are three different choices of effective coupling constants)


ConclusionsConclusions

• Signatures with long lived particles (both neutral and charged) and di-lepton high-mass resonances, if seen, will represent an undisputable proof of new physics at the LHC

• Discovery possible already at start-up (luminosity<1fb-1)

• Presented last updates on this topic from both ATLAS and CMS experiments

• Studies performed with a realistic simulation of detector – including uncertainties from data-driven calibrations at beginning of

data taking


BACKUP


Total weight 7000 tOverall diameter 25 mBarrel toroid length 26 mEnd-cap end-wall chamber span 46 mMagnetic field 2 Tesla

Total weight 12 500 tOverall diameter 15.00 mOverall length 21.6 mMagnetic field 4 Tesla

Total weight 12 500 tOverall diameter 15.00 mOverall length 21.6 mMagnetic field 4 TeslaDetector subsystems are designed to measure:

energy and momentum of ,e, , jets, missing ET up to a few TeV

Compact Muon SolenoidA Large Toroidal LHC ApparatuS

ATLAS and CMS DetectorsATLAS and CMS Detectors


CMS Detector: TrackerCMS Detector: Tracker

Reconstruct tracks and charged particles momentum

Two different detector types:•Pixel 100x150m2

(r-)~10m, (z)~ 20m•Silicon strips thickness 320-500m pitch 80-500m(r-)~10m, (z)~ 20m

About 220 m2 of Si Sensors 107 Si strips 6.5∙107 pixels

A total of O(108) channels!!!


CMS Detector: EM CalorimetryCMS Detector: EM Calorimetry

Reconstruct photon and electron energy

>75k lead tungstate crystals

crystal lenght~23cm

Front face22x22mm2

PbWO4

30/MeVX0=0.89cm


CMS Detector: Muon DetectorCMS Detector: Muon Detector

Reconstruct muons

Drift Tubes (DT)250 Chambers200K Channels TDC200μm Resolution

Resistive Plate Chambers RPCCourse position, fine timingBarrel 80K channelsEndcap 92K channels

Cathod Strip Chambers (CSC)468 Chambers240K strips150μm Resolution

CSC


GMSB: some parametersGMSB: some parameters

SUSY breaking term

vev superfield

Mass of gaugino and scalars of MSSM(i couplings)


GMSB: S and B MET (CMS)GMSB: S and B MET (CMS)


GMSB: ATLAS angular resolutionGMSB: ATLAS angular resolution


Long Lived: Standalone Long Lived: Standalone


Long Lived: ATLASLong Lived: ATLAS

M(GeV)


Di-lepton in CMSDi-lepton in CMS

KK Z’KK Z’


Z’: discriminate modelsZ’: discriminate models

Distinguish • spin-1 Z(1) from spin-2 G: angular distribution of decay products • spin-1 Z(1) from spin-1 Z’ with SM-like couplings: forward-backward asymmetry

due to contributions of the higher lying states, interference terms and additional √2 factor in its coupling to SM fermions.

The Z(1) can be discriminated for masses up to about 5 TeV with L=300fb-1.

4 TeV Z(1)/(1) or Z’ or RS Graviton? 4 TeV resonances

1000 2000 3000 4000 1000 2000 3000 4000

100 fb-1

1000 2000 3000 4000 1000 2000 3000 4000 M (GeV)


RS: set A,B,C cross-sectionsRS: set A,B,C cross-sections

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Searches for GMSB and for High ET Di-lepton Pair Events at the LHC

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