seminar, qmultracey berry (rhul) 1 1 searches for in the dilepton (and diphoton) final states with...

Seminar, QMUL Tracey Berry (RHUL) 11

Searches for

in the dilepton (and diphoton) final

states with the ATLAS detector

Dr Tracey BerryRoyal Holloway

University of London


Overview

• Introduction:

• Beyond the Standard Model

• Fundamental Symmetries: Heavy Gauge Bosons Z’

(ee, mm)

• Extra Dimensions (ee, + mm gg)

• Contact Interactions (ee, mm)

• ATLAS & LHC

• Searches for new physics

• Summary and Outlook


The Standard Model

Motivation for searching for something beyond the SM….

Gravity is not included!

The SM : particles + forces


Forces in Nature

MEW (103 GeV) << MPlanck (1019

GeV)?

Gravity is very weak! → Hierarchy Problem

Table from Cigdem Issever, Oxford


Extra Dimensions: Motivations

if compact space (Rd) is large

MPl2 ~ RdMPl(4+d)

(2+d)

Effective MPl ~ 1TeV

if warp factor kRc ~11-12

Planck TeV brane

Arkani-Hamed, Dimopoulos, Dvali, Phys Lett B429 (98)

Randall, Sundrum, Phys Rev Lett 83 (99)

MEW (1 TeV) << MPlanck (1019 GeV)?

Many (d) large compactified EDsIn which G can propagate

1 highly curved EDGravity localised in the ED

In the late 90’s Large Extra Dimensions (LED) were proposed as a solution to the hierarchy problem

Some of these models can be/have been experimentally tested at high energy colliders

Since then, new Extra Dimensional models have been developed and been used to solved other problems:

Dark Matter, Dark Energy, SUSY Breaking, etc

= Mple-kRc

~ TeV


The “extra” dimensions could be hidden to us: E.g. To a tightrope walker, the tightrope is one-dimensional:

he/she can only move forward or backward

If ED exist, why haven’t we observed them?

• But an ant can go around the tightrope as well …

• The “extra” dimensions may be too small to be detectable at energies less than ~ 1019 GeV (E.g. they are small that only extremely energetic particles could fit into them (so we need high energies to probe them))

– But to an ant, the rope has an extra dimension: the ant can travel around the rope as well


Or only some kinds of matter are able to move in the extra dimensions, and we are confined to our world.

• The “extra” dimensions may be too small to be detectable at energies less than ~ 1019 GeV

like something that was forced to reside on the surface of a tabletop, being unaware of any such thing as up or down.

G

Our Brane/ World

Extra Dimensions


KK towers/particles

When particles go into the extra dimensions….

Mn = √(M02+n2/R2))

Like QM particles in a box

Spacing & (summation of ) KK towers determines the search signature: • narrow resonance (RS) or• broad increase in cross-section

(ADD)


Questions remaining with the Standard Model

Fundamental symmetries:

Are there more symmetries beyond SU(3)C SU(2)L U(1)Y?

GUTs with larger symmetry group? Left-right symmetry?

How can we address the hierarchy problem? / Include Gravity into the SM?

Are there warped extra dimensions? (RS model)

Are there large extra dimensions? (ADD model)

Quark and lepton generations:

Why are there 3 generations? Fermions composite?

Is there a lepto(n)-quark symmetry?

More than 3 generations of quarks & leptons?

Reasons to search for new physics…


Introduction Standard Model describes data

very well, but is only a low energy effective theory

Physics Processes at LHC: Total cross-section SM Processes: W->ev New Physics: hard

lepton/photon (Z’->ee, etc)

New Physics is needed at the weak scale ~ 1 TeV


Questions remaining with the Standard Model

Fundamental symmetries: New Gauge Bosons?

Extra Dimensions?

Contact Interactions ?

®Search for evidence of beyond the SM physics

in dilepton + diphoton data...…

& Beyond!schooljotter.com

empireonline.com


Signature for BSM in ATLAS

Resonance

Broad Increase in Cross-sectionG*: ADD

CI

G*: RS


Introduction Standard Model describes data

very well, but is only a low energy effective theory

New Physics is needed at the weak scale ~ 1 TeV

Physics Processes at LHC: Total cross-section SM Processes: W->ev New Physics: hard

lepton/photon (Z’->ee, etc) Need large luminosity and

efficient background rejection to see interesting signal events: & effective lepton/photon identification


LHC data

2010 data: collected in one day in 2011 running (Dan’s chapter)

2010 data: PLB700: 163-180, 2011 (~40 pb-1)2011 data 200 pb-1 update: ATL-CONF-2011-0832011 data: ~1 fb-1 :


ATLAS Detector


A Toroidal LHC AppartuS (ATLAS) DETECTOR

Precision Muon Spectrometer,

s/pT 10% at 1 TeV/c

Fast response for trigger

Good p resolution

(e.g., Z’ )

EM Calorimeters, /E 10%/E(GeV) 0.7%

excellent electron/photon identification

Good E resolution (e.g., Ggg)

Hadron Calorimeters,

/E 50% / E(GeV) 3%

Good jet and ET miss performance

Inner Detector:

Si Pixel and strips (SCT) &

Transition radiation tracker (TRT)

s/pT 5 10-4 pT 0.001

Good impact parameter res.

(d0)=15m@20GeV

Magnets: solenoid (Inner Detector) 2T, air-core toroids (Muon Spectrometer) ~0.5T

Full coverage for ||<2.5


ATLAS Detector


Dilepton BSM Searches

Resonance Searches Z’ RS model Extra Dimensions G

Non-Resonance Searches Contact Interactions ADD model Extra Dimensions (to come...)


Signal and Backgrounds

Select events with two leptons of same flavor (ee, )

Search for excess above Standard Model expectations in high invariant mass region

● Main backgrounds:

— SM Z (irreducible, primary background)

— QCD (electron channel only)

— Top quark pair production

— SM W+jets (electron channel only)

— Dibosons (WW, WZ, ZZ)

● Negligible backgrounds for muon channel:

— QCD, SM W+jets

— Cosmic rays


Selection

Electron channel

● Trigger requiring single Medium electron with ET > 20 GeV● 2 electrons with:— pT > 25 GeV— || < 2.47, exclude crack region 1.37 < || < 1.52— Medium Electron ID— Hit in first pixel layer (“Blayer”)

Muon channel

● Trigger requiring single Muon with pT > 22 GeV● Primary vertex with |z| < 200 mm● 2 muons with:— pT > 25 GeV— || < 2.4— Hits in all 3 muon stations— Hit in nonbending plane— Veto on overlapping hits in barrel and endcaps— |d0| < 0.2 mm, |z0| < 1 mm— Isolation: within a cone of R < 0.3— Opposite charge

ΣpTtrk<0.05 pT


Muons

Muon tracks are reconstructed independently in both the inner detector and muon spectrometer, and their momenta are determined from a combined fit to these two measurements.

To optimize the momentum resolution, each muon candidate is required to pass quality cuts in the inner detector and to have at least three hits in each of the inner, middle, and outer layers of the muon system.

Muons with hits in both the barrel and the endcap regions are discarded because of residual misalignment between these two parts of the muon spectrometer.

The effects of misalignments and intrinsic position resolution are included in the simulation.

The pT resolution at 1 TeV ranges from 15% to 44% (for |η| > 2).


Electron QCD Background

Estimated in data using two techniques, extrapolated to high mass search region Inverted identification method reverses Medium

ID cuts, uses template fit in invariant mass Isolation fit method requires standard Medium

ID, reverses other ID cuts, uses template fit in calorimeter isolation


Z' Electron Kinematics

Good agreement with background expectations


Calorimeter Isolation


Z' Muon Kinematics


The small excess in the tails of the distributions may be explained by the imperfect modelling of highly energetic jets in PYTHIA, since the agreement is much better with ALPGEN which generates more high energy jets which can boost the dilepton system in the Z+jets events. Of the ten highest p_T muons, only four belong to a dimuon pair with mass greater than 300 GeV.


PseudorapidityThe dips in the pseudorapidity distribution are caused by the requirement that hits are measured in all three layers of the muon chambers. Most chambers in the middle station near |eta|=1.2 have not been installed yet, which causes the two gaps. The central dip is due to the passage of inner detectors' (tracker and calorimeter) services.

|eta|=1.45 transition region between the barrel and endcap calorimeters


Rapidity


Resonant Searches

Z’ RS model Extra Dimensions G


2010 data: PLB700: 163-180, 20112011 data 200 pb-1 update: ATL-CONF-2011-083


Z’ Theoretical Motivations

New heavy gauge bosons are predicted in several extensions of the Standard Model

Benchmark model for these searches is the Sequential Standard Model (SSM) Z’ has the same couplings as SM Z Z’ width assumed comparable to detector

resolution

Also consider string theory –inspired E6 models


Invariant Mass Distributions


Data & SM Background


Highest mass ee eventET 257 GeV ,(eta, phi) (-0.76, 1.14). ET 207 GeV (eta, phi) of (2.05, -2.05). Mee= 993 GeV.

Only tracks with p_T > 1 GeV are shown.


Highest Mass mm eventPT of 510 GeV , (\eta, phi) = (0.37, 3.01). PT of 437 GeV , (eta, phi) =(0.72, -0.12). Mmm=959 GeV.

.

Only tracks with P_T > 0.5 GeV are shown


Signal?

The significance of a signal is summarized by a p-value,the probability of observing an excess at least as signal likeas the one observed in data, in the absence of signal.

The outcome of the search is ranked using a likelihood ratio,which is scanned as a function of Z′ cross section andmZ′ over the full considered mass range.

The data are consistent with the SM hypothesis, with p-values of 54% for e+e− and 24% for the μ+μ− channels.


Limits Setting and Errors

Normalize MC to data in Z peak region

(70 < mℓℓ < 110 GeV) Luminosity and other mass independent systematics

cancel Uncertainties treated as correlated across all bins


Z' Individual Channel Limits

an upper limit on the signal cross section is determined at the 95% confidencelevel (C.L.) using a Bayesian approach [41] with a flatprior on the signal cross section.

Limits set using template shape fit — Bayesian method

Observed (Expected) 95 % C.L. mass lower limit in TeV on Z’SSM resonance


Z’ Combined Limits

Mass Limits Z' (GeV/c2)

Observed (Expected) 95 % C.L. mass lower limit in TeV on Z’SSM resonance

arXiv:1108.1582v1 (hep-ex)


Comparison of Z’SSM Limits

The Tevatron experiments exclude a Z′ SSM with a mass lower than

1.071 TeV .

Recent measurements from the LHC experiments, based on 40 pb−1 of data recorded in 2010, exclude a Z′ SSM with a mass lower than

1.042 TeV (ATLAS) and 1.140 TeV (CMS)

Indirect constraints from LEP extend these limits to

1.787 TeV.


Other Z’ models

In the Z′ are the Sequential Standard Model (SSM) the Z’ has the same couplings to fermions as the Z boson

Also when the E6 grand unified symmetry group is broken into SU(5) and two additional U(1) groups leads to new neutral gauge fields ψ and χ.

The particles associated with the additional fields can mix in a linear combination to form the Z′ candidate:

Z′(θE6 ) = Z′ψ cos θE6 + Z′χ sin θE6 ,

where θE6 is the mixing angle between the two gauge bosons.

The pattern of spontaneous symmetry breaking and the value of θE6 determine the Z′ couplings to fermions;

six well motivated choices of θE6 lead to the specific Z′states named:


Combined Limits

Mass Limits Z' (GeV/c2)


Z’ limits vs Other Experiments


G*


Z’ compared to G* Muon/Electron Kinematics



G* limits


G* limits

Observed 95 % C.L. mass lower limit in TeV on RS Gravitons


G* limits

k/MPl 0.010 0.020 0.025 0.030 0.050 0.070 0.100e+e- 385 - 595 740 701 745 993

m+m- 333 - 493 780 746 824 1000gg 230 - 500 - 694 782 850

e+e- + m+m- 425 - - 850 729 - 1080

e+e- + gg 560 750 580 - 935 820 1050

ATLAS CMS CDF D0

Mass Limits RS Graviton (GeV/c2)


Combination of ee +mm with updated gg result

Coming Soon!Under Approval


Tetiana Berger-Hryn’ova , on behalf of the ATLAS collaborationEPS, Grenoble, France, 21 July 2011Update Coming Soon!


Non-Resonant Searches

Contact Interactions ADD model Extra Dimensions


Contact Interactions

Four-fermion contact interactions (CI) at low energy limit describe phenomena as: Large Extra Dimension ADD model Quark-lepton compositeness

Benchmark composite model is left-left isoscalar model


Contact Interactions (mm)

Taking the integral

No excess, limits at 95 % CL:L- > 4.9 TeV for constructive interferenceL+> 4.5 TeV for destructive interference arxiv:1104.4398, PRD


Contact Interaction SearchCombination of ee + mm

with ~ 1fb-1

Coming Soon!Under Approval

for mid Nov HCP conference


Conclusion

LHC is working very well ATLAS detector is efficiently collecting data We are searching for New Physics

many limits are reaching ~1TeV range distributions so far consistent with SM

expectations previous limits substantially improved

We already have more than 5fb-1 of data, more exciting results from ATLAS to come… using full 2011 dataset


Thank youfor inviting me!


Sensitivity: 10 Signal Events

D. Olivito’s PLHC 2011 talk, June 10 2011


Sensitivity: Limit Setting

seminar, qmultracey berry (rhul) 1 1 searches for in the dilepton (and diphoton) final states with...

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