Boosted Signatures from BSM at Boosted Signatures from BSM at the LHC the LHC (with an emphasis on Extra Dimensional (with an emphasis on Extra Dimensional

ModelsModels… and a bias towards ATLAS)… and a bias towards ATLAS)

MMüüge Karagge Karagöözz(U. Oxford)(U. Oxford)

Liverpool HEP SeminarLiverpool HEP Seminar

October 28,October 28, 20102010

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Outline

• Introduction to LHC and ATLAS• Introduction to extra dimensions• Why boosted objects? techniques with

examples• Boosted signatures in ED: status and

prospects• Where we are with current data?• Conclusion

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LHC: the Current Frontier

• Proton-proton collider

• 27 km circumference

• 4 interaction regions with experiments– CMS, ATLAS, – Alice, LHCb

• November 2009 saw the first LHC collisions!

Design Initial

Energy (c.m.) 14 TeV 900 GeV (2.36 TeV) [ 7 TeV ]

Luminosity (cm-2s-

1)

1034 ~7 x 1026 [ ~2 x 1032]

Bunches/Beam 2808 4 (2) [348] (colliding in ATLAS)

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44 m

22 m

ATLAS Detector Specifics

• Inner Tracking (||<2.5, 2T solenoid) :• Silicon pixels and strips• Transition Radiation Detector (e/ sep’n)

• Calorimetry (||<5) :• EM : Pb-LAr, Accordion shape• HAD: Fe/scint (central), Cu/W-LAr (fwd)

• Muon Spectrometer (||<2.7, 4T toroid) : • air-core toroids w/ muon chambers

A toroidal LHC apparatus

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The aim: Extending the PP Map

Rediscovery background

Higgs SUSY

Heavy exotics

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0 10+1 10-2

10-6 10-38

“Apparent” gravity is weak, governed by Planck scale (MPl=1019 GeV)How to unify forces & solve the hierarchy problem (MPl >>MEW)?

A motivation: the Unbearable Lightness of Being

TeV-1 ED

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Extra Dimensions: Not a Flatland

• In 1920’s Kaluza&Klein: attempt to unify EM with gravity in 5D

• In 1990’s, models to solve the hierarchy problem: actual gravity is stronger & its scale can be as low as ~ TeV

• Many ED models: • flat (ADD, TeV-1, UED)• warped (RS)

• various particles escaping into “bulk” while SM is confined to our 3-brane

1/r2-law valid for R=44 μm @ 95% CL

Size of the ED from gravitational potential

MPl ~ 1019 GeV, MPl(4+n)~MEW

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(4 )

11/

( )N DPL n

G MM

2 2 n nPl DM M R

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RS Warped ED: Baseline Picture

222 dydxdxeds vuuv

ky

Randall-Sundrum (Type I) PRL83/3370/99PRL83/3370/99

• Brane metric scales as function of bulk position

• Only KK graviton in bulk• Coupling constant: c= k/M’Pl, k:

curvature scale• Well separated narrow-width

graviton mass spectrum with masses

mn=kxnekrcπ (J1(xn)=0)

Bulk (y)

TeV

Plan

ck

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0RS KK Graviton Reach in

Dielectrons

ATLAS: 900 GeV G* can be discovered with 1.0 fb-1, for k/MPl = 0.01(Tevatron reach is around 300 GeV with 1 fb-1)

• Dileptons may be first discovery channels• Cross-section varies from ~200-20 fb for 0.5-1.4 TeV Graviton (@14 TeV).• Spin-2 nature of G*: powerful discriminant @ high luminosity

hep-ph/0006114hep-ph/0006114• Most stringent limits from Tevatron (k/MPl > 0.1):

– CDF : mG > 921 GeV (, 2.3 fb-1) PRL102/091805/09 PRL102/091805/09 – D0 : mG > 900 GeV (diEM,1 fb-1) PRL100/091802/08PRL100/091802/08

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Realistic RS Models: Bulk RS• Current favourite model building in RS • EWSB with bulk matter fields & KK modes for SM particles• Solves more than gravity hierarchy problem

– Gauge hierarchy problem, Fermion mass hierarchy, Gauge Higgs Unification, ….

• New physics couples with stronger coupling to heavier SM particles (Top, H, VL)

• Arrange zero modes (couplings) such that– Light fermions close to the UV

brane to protect precision EW corrections

– Top (tR) near IR (TeV) brane (where Higgs resides) in order to produce its observed heavy mass

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What is Boost and Why?• LHC is a heavy and boosted object factory!• Sources and reasons:

– Something heavy (e.g. Z’) decays to something lighter (t,W/Z,H,. . .), which is then naturally boosted

– A new light particle (H,0, . . .) emerges more clearly above backgrounds when produced boosted

• Signatures: – Merged/collimated decay products, large displaced

vertices, ...

• Concerns: – Standard algorithms may fail

• Considerations:– Signatures with boost– Backgrounds to boost– Methods for boost

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0Boosted Objects in the ATLAS

Detector• ATLAS is well-prepared for boosted objects

– High granularity calorimeter and precision tracking to exploit new techniques for efficient reconstruction

• Some examples:– Vector Boson Scattering (VBS): 2 light quarks– SUSY neutralino: 3 light quarks Not shown todayNot shown today– Higgs: 2 b-quarks– Heavy Resonances into tops: semi-leptonic 3-body

decays of top pairs

• I will use these examples to illustrate boosted objects techniques

• There is a lot of ongoing experimental work for leptonic jets (“lepton-jets”) within boosted framework. I am not covering those here…

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Boosted Jet Techniques

•Reminder: ATLAS jet finding default is anti-kT (R=0.4 or 0.6)– Infra-red safe and considered robust against noise, etc..

•For a parent with m and pT, merging starts showing at R> 2m/pT

– Use jet mass for parent and jet substructure to resolve merging

•Recombination algorithms favoured for jet substructure

•kT algorithm:

– recombination intrinsically ordered in pT scale:

dij = min(pTi2,pTj

2) * Rij2/R2

– For subjet analysis, undo last merging:

– Define y-value yn = dij / m where dij is the kT splitting level from the last (n-th last) merging

– Get the y-scale at which the jet would split into 2 subjets.

•Cambridge/Aachen algorithm (C/A):– Similar to kT but ordering is in angles, not pT.

– Clustering stops when all jets separated by a prescribed -φdistance R

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0Hadronic Vector Bosons in Vector Boson Scattering - I ((CERN-OPEN-2008-CERN-OPEN-2008-

020)020)• Higgsless models with vector

boson resonances decaying into 2 vector bosons.

• Scattering at high mass means VBs at high momenta.

• Dibosons in semi-leptonic decay mode:

– V ll: Standard reconstruction– V qq: Quarks are boosted =>

can be merged

• Final analysis approach:– Use topology of the event: select

2 forward ‘tag’ jets and veto central jets and tops.

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0Hadronic Vector Bosons in Vector Boson Scattering - II ((CERN-OPEN-2008-CERN-OPEN-2008-

020)020)• Hadronic VBs reconstructed

from 1 or 2 kT jets (R=0.6)

• In each event: Take highest pT jet. Jet mass close to W/Z ?

• Yes: The jet is the VB candidate. Apply cut on jet substructure.

– Consider the y-scale from kT merging at last step.

• Require pT > 300 GeV

– Define Y= ET,jet√y. Y ≈ O (mV ) if jet comes from a boosted VB

• Require 30 < Y < 100 GeV

• No: Loop over all jet pairs. Find one with highest combined pT. This is the VB candidate. Apply further cuts.

Discovery @ 14 TeV: 60 fb-1 required for a 800GeV WZ resonance

decayingsemi-leptonically with = 0.65 fb.

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Higgs searches in bb channel - I(ATL-PHYS-PUB-2009-088)(ATL-PHYS-PUB-2009-088)

• Associated production of Higgs with a VB, for a low mass Higgs (a difficult channel at LHC)

• pp HV, with H bb and VB leptons– Use leptons from VB as event tag and combine VBs at

end– b-quarks from Higgs will be boosted to merge into one

single jet

• Start with Cambridge-Aachen jets (R=1.2)– Split the jet until a large drop in mass is found– Filter the jet by rerunning C/A with smaller R– Apply b-tagging

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Higgs searches in bb channel - II(ATL-PHYS-PUB-2009-088)(ATL-PHYS-PUB-2009-088)

• Select jets with pT>200 GeV, | |<2.5• For each jet j:

• Undo last clustering step => 2 subjets: j1, j2, mj1>mj2

– Significant mass drop? (mj1 < 1/3mj)

– & No asymmetric split? (y > ycut (=0.1 )?)

– => j is composite (bb)

– Else, start over, with j1

• If j is composite– Filter the jet by rerunning C/A, with Rfilt <

Rbb (Rfilt = min(0.3, Rbb/2)), Rbb : distance between b quarks

• Take the hardest 3 subjets– j is a Higgs candidate if 2 hardest subjets

are b-tagged

• Require H candidate pT > 200 GeV

• Combine V channels for sensitivity

S/√B = 3.7 for combination for ℒ = 30 fb-1, √s = 14 TeV: Comparable to all ATLAS low mass Higgs channels (LO only, no pile-up)

WH→lvbb

S/√B = 3.0 at 30fb-1

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Boosted Top Quarks• Top quark plays a special role in many EWSB BSM, due to mtop~ MEWSB

• LHC energy and luminosity: physics involving energetic tops in the final state possible

• At the LHC, top pT may be so high that it gets reconstructed as one fat jet (~75% within dR<0.4 for 1TeV top)

• Various “top-tagging” techniques developed, eg, JHEP0807/092/08JHEP0807/092/08.

B. Esposito (INFN)

Standard semileptonic ttbar selection

T. Isobe

Merged top jet in a single cone

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Boosted Top Quarks• Top quark plays a special role in many EWSB BSM, due to mtop~ MEWSB

• LHC energy and luminosity: physics involving energetic tops in the final state possible

• At the LHC, top pT may be so high that it gets reconstructed as one fat jet (~75% within dR<0.4 for 1TeV top)

• Various “top-tagging” techniques developed, eg, JHEP0807/092/08JHEP0807/092/08.

B. Esposito (INFN)

Standard semileptonic ttbar selection

T. Isobe

Merged top jet in a single cone

ATL-PHYS-PUB-2009-081ATL-PHYS-PUB-2009-081

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0Resonances in semileptonic ditops

- I(ATL-PHYS-PUB-2010-008)(ATL-PHYS-PUB-2010-008)

• Top pair resonances in semileptonic channel for fully-resolved, partially-merged and fully merged (mono-jet) events

• Concentrate on mono-jets, expected at Mttbar > 1.5 TeV

• Resonance signals (1 ≤ M ≤ 2 TeV):– Z’: narrow, spin 1, colour singlet– RS graviton: narrow, spin 2, colour singlet– RS gluon: wide, spin 1, colour octet

• Use “tog-tagging” for background discrimination– Hadronic top: a 3- prong fat jet with

substructure– Leptonic top: a merged lepton + a b-

quark

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0Resonances in semileptonic ditops

- II(ATL-PHYS-PUB-2010-008)(ATL-PHYS-PUB-2010-008)

Hadronic top mono-jet reconstruction

• Anti-kT jet algorithm (a large jet size of R =1.0)

• Selection on a fat-jet exploring variables for 3 subjet structure– Techniques avoid combinatorics, increase sensitivity and

provide good performance down to ~ 1 TeV reach

• Tagging variables used:– Qjet : mass of the hadronic top

jet

– zcut : energy sharing between subjects

– QW : invariant mass of the subjet pair with lowest mass, after splitting into 3 subjets

Bas

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0Resonances in semileptonic ditops

- III(ATL-PHYS-PUB-2010-008)(ATL-PHYS-PUB-2010-008)

Leptonic top reconstruction• Search for a lepton (e or ) and a jet (b-quark)• Leptons define the trigger path• Selection exploring variables for leptonic top

structure

• Tagging variables used:– Qvis : mass of the leptonic top

jet– DR (l,j)

– xl: invariant mass carried by leptonic activity

– zl = El / Ej

– iso – relative energy in a 0.2 cone around the lepton

Bas

elin

e

Bas

elin

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0Resonances in semileptonic ditops

- IV(ATL-PHYS-PUB-2010-008)(ATL-PHYS-PUB-2010-008)

Overall efficiency and rejection rates

Signal reconstruction efficiency &(QCD dijet) background rejection (R = 1−e) for hadronic top decay

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0Heavy and Boosted Objects in

ED• ED particles will decay into SM paticles which can

be boosted• They can occur in RS, UED or TeV-1 models:

– Exclusive resonance searches into ttbar/VV: • KK gluons• KK gravitons• string resonances

– Higher multiplicities: • KK heavy fermions & leptons• spin 3/2 string states

– etc...

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0New fermions in Bulk RS Models:

An example with KK Heavy Fermion

• RS + bulk gauge symmetry SU(2)LxSU(2)RxU(1)X (hep-ph/0612048hep-ph/0612048)– custodial and a L-R symmetry

to protect EW precision observables

– Light degenerate KK quarks (“custodians”) (with no zero modes but with chiral mixing) including a q5/3

• Investigate feasibility for KKHF and related signatures through energetic multi-W events (hep-hep-ph/0701158ph/0701158)– Uncommon in SM processes– Initially try to stay as inclusive

as possible– Good source of Boosted Ws

4W + 2b-jets

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KKHF Boosted Signature

LL HH HH

LLLL HH HH

LL

Signature: 2L + 2H + 2b-jets

ttbar ttbar

• Hadronic W counting after reducing dominant ttbar background by dilepton requirement

• Remaining hadronic Ws mostly from non-SM sources• Hadronic Ws can be reconstructed as dijets or single jets• Fat-jet W-tagging especially important at high masses

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2L

KKHF Fat-Jet W Counting -14 TeV

• KKHF W bosons have higher pT than those from ttbar

• W/t identification using single-jet mass (e.g., Skiba&T-Smith, hep-ph/0701247hep-ph/0701247)

• Greater reach than the standard W/top reconstruction in dilepton selected events

• Analysis to be repeated with 7 TeV ATLAS dataMbR = 500 GeV

MbR = 1 TeV

2L

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New Custodial Leptons in Bulk RS(arXiv:1007.4206)(arXiv:1007.4206)

• KK Tau lepton pair production• Large coupling to SM tau• EWK coupling means reach requires > 10 fb-1

• Very collimated (boosted) final states

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0New bosons in Realistic RS

Models• Heavy, broad new vector bosons with

reduced couplings to light SM particles and enhanced BR to tops and longitudinal gauge bosons– KK gluon (M < 4 TeV)

• advantage due to strong coupling

– KK Z/A (M < 2 TeV)– KK W (M < 3 TeV)– Radion

• (See Les Houches 2009 BSM report for a review arXiv:1005.1229arXiv:1005.1229 )

• All are good source of boosted tops/Vs!

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• Scalar field needed to generate potential to stabilize ED radius.

• Radion search as an ED resonance:– KK gauge bosons have stringent EWK limits and production

rates in diboson channels ~ O(10fb)– Radions have less stringent limits, can be light

• Particle similar to Higgs (sometimes called a Higgs’)

RS Bulk Radions(arXiv:0705.3844)(arXiv:0705.3844)

100 pb-1, 14 TeV

Significance ratio of radion/Higgs search for 100 pb-1 @ 14 TeV

RS1• High mass radions

prefer to decay into VV/hh and tt : • r->WW~50%,

hh~20%, ZZ~20%, tt~10%

• Good source of boosted vector bosons and tops

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RS Bulk KK Gluon

• Large coupling to top and reduced coupling to light quarks source of boosted tops

• Large LHC yield: σ=O(10pb), for MKKg = 1 TeV

• Broad resonances (width ~20% of mass)

B. Lillie et. al

JHEP0709/074/07

Ruled out by using Tevatron data

up to 800 GeV hep-ph/0703060hep-ph/0703060

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0ATLAS Limits on KK Gluons from

Tops(ATL-PHYS-PUB-2010-008)(ATL-PHYS-PUB-2010-008)

• Exclusion possible with 200 pb-1 @ 10 TeV for KK gluon at ~ 1 TeV mass. – compatible with 1 fb-1 @ 7 TeV

• New cross section limits for Z'-like resonances can be set to ~4 (~2) pb for M =1 (2) TeV. (Slightly better limits for spin-2 KK Gravitons)

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Boosted Jets and Tops at CDF

• Boosted top search in high pT jets in 5.95 fb-1

• With other quark and gluon contributions suppressed, ttbar production is > 50% of the signal for pT > 400 GeV objects

• 103 candidate events with 2 massive jets or a massive jet + high MET, over a background of 76±10(stat)+26-20(syst) events.

95% CL upper limit of 54 fb on SM ttbar event production with Ntop > 0 with pT > 400 GeV

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• High pT jets are observed on a daily basis

• Start to make jet substructure/fat-jet analysis for boosted objects!

ATLAS Jet Measurements (arXiv:1009.5908)(arXiv:1009.5908)

Inclusive jet differential cross section.Luminosity uncertainty of 11% is not shown.

Dijet inv. Mass used to derive BSM limits95% CL exclusion for 0.50< mq*< 1.53 TeV

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First W/Zs from ATLAS(arXiv:1010.2130v1)(arXiv:1010.2130v1)

• Validating ATLAS with SM Vector Bosons• 2250 W and 179 Z candidate events• No signs of boosted Ws yet

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• ATLAS searched for events consistent with top quark pair production in 280 nb−1

• 9 top candidates in Nlep>1 channels, compatible with NLO

In “Search” for top quarks(ATLAS-CONF-2010-063)(ATLAS-CONF-2010-063)

• Lepton + jets event selection:– Primary vertex with 5 tracks– Exactly 1 lepton with pT>20

GeV– At least 4 jets with pT > 20

GeV– One jet bttaged– Missing ET>20 GeV

• Top resonance searches will follow

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Conclusion & Outlook

• LHC energies open up new channels and signatures for BSM.

• ATLAS is ready for the energetic era of pp collisions in various hadronic and leptonic signatures– Exploit techniques to efficiently reconstruct heavily-

boosted particles – Looking at 7 TeV data and preparing for > 13 TeV

• ATLAS has a very rich discovery potential for ED signatures and TeV-Scale gravitational effects. Work ongoing on many fronts.

• For further details on boosted objects, please see Boost2010 (Oxford, 22-25 June, 2010) agenda, especially ATLAS talk by E. Bergeaas Kuutmann.

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What we need is…

Lotsa collisions@ high energies

Thank you!