searches for in the dilepton (and diphoton ) final states with the atlas detector
DESCRIPTION
New Physics. Searches for in the dilepton (and diphoton ) final states with the ATLAS detector . Dr Tracey Berry Royal Holloway University of London. And Prospects for the LHC. Extra Dimensions. Overview. Introduction: Beyond the Standard Model - PowerPoint PPT PresentationTRANSCRIPT
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Searches for
in the dilepton (and diphoton) final
states with the ATLAS detector
Dr Tracey BerryRoyal Holloway
University of London
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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
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The Standard Model
Motivation for searching for something beyond the SM….
Gravity is not included!
The SM : particles + forces
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Forces in Nature
MEW (103 GeV) << MPlanck (1019
GeV)?
Gravity is very weak! → Hierarchy Problem
Table from Cigdem Issever, Oxford
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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
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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
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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
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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)
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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…
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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
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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
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Signature for BSM in ATLAS
Resonance
Broad Increase in Cross-sectionG*: ADD
CI
G*: RS
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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
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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 :
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ATLAS Detector
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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’ mm)
EM Calorimeters, s/E 10%/E(GeV) 0.7%
excellent electron/photon identification
Good E resolution (e.g., Ggg)
Hadron Calorimeters,
s/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.
s(d0)=15mm@20GeV
Magnets: solenoid (Inner Detector) 2T, air-core toroids (Muon Spectrometer) ~0.5T
Full coverage for ||<2.5
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ATLAS Detector
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Dilepton BSM Searches
Resonance Searches Z’ RS model Extra Dimensions G
Non-Resonance Searches Contact Interactions ADD model Extra Dimensions (to come...)
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Signal and BackgroundsSelect events with two leptons of same flavor (ee, mm)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
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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
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MuonsMuon 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).
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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
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Z' Electron Kinematics
Good agreement with background expectations
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Calorimeter Isolation
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Z' Muon Kinematics
Good agreement with background expectations
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.
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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
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Rapidity
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Resonant Searches
Z’ RS model Extra Dimensions G
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2010 data: PLB700: 163-180, 20112011 data 200 pb-1 update: ATL-CONF-2011-083
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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
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Invariant Mass Distributions
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Data & SM Background
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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.
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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
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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.
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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
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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
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Z’ Combined LimitsMass Limits Z' (GeV/c2)
Observed (Expected) 95 % C.L. mass lower limit in TeV on Z’SSM resonance
arXiv:1108.1582v1 (hep-ex)
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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.
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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:
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Combined Limits
Mass Limits Z' (GeV/c2)
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Z’ limits vs Other Experiments
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G*
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Z’ compared to G* Muon/Electron Kinematics
Good agreement with background expectations
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G* limits
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G* limits
Observed 95 % C.L. mass lower limit in TeV on RS Gravitons
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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 - 1080e+e- +
gg 560 750 580 - 935 820 1050
ATLAS CMS CDF D0
Mass Limits RS Graviton (GeV/c2)
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Combination of ee +mm with updated gg result
Coming Soon!Under Approval
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Tetiana Berger-Hryn’ova , on behalf of the ATLAS collaborationEPS, Grenoble, France, 21 July 2011Update Coming Soon!
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Non-Resonant Searches
Contact Interactions ADD model Extra Dimensions
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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
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Contact Interactions (mm)Taking the integral
No excess, limits at 95 % CL:- > 4.9 TeV for constructive interference+> 4.5 TeV for destructive interference arxiv:1104.4398, PRD
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Contact Interaction SearchCombination of ee + mm
with ~ 1fb-1
Coming Soon!Under Approval
for mid Nov HCP conference
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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
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Thank youfor inviting me!
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Sensitivity: 10 Signal Events
D. Olivito’s PLHC 2011 talk, June 10 2011
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Sensitivity: Limit Setting