M ü ge Karag ö z (U. Oxford) Liverpool HEP Seminar October 28, 2010

1. Boosted Signatures from BSM at the LHC (with an emphasis on Extra Dimensional Models… and a bias towards ATLAS) Müge Karagöz (U. Oxford) Liverpool HEP Seminar October 28,2010

2. 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

3. 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!

4. 22 m 44 m ATLAS Detector Specifics A toroidal LHC apparatus • 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

5. Rediscovery background SUSY Higgs Heavy exotics The aim: Extending the PP Map

6. TeV-1 ED 10+1 10-2 10-38 10-6 A motivation: the Unbearable Lightness of Being “Apparent” gravity is weak, governed by Planck scale (MPl=1019 GeV) How to unify forces & solve the hierarchy problem (MPl >>MEW)?

7. MPl ~ 1019 GeV, MPl(4+n)~MEW Size of the ED from gravitational potential 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

8. TeV Planck Bulk (y) RS Warped ED: Baseline Picture • Randall-Sundrum (Type I)PRL83/3370/99 • 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)

9. Dileptons may be first discovery channels Cross-section varies from ~200-20 fb for 0.5-1.4 TeVGraviton (@14 TeV). Spin-2 nature of G*: powerful discriminant @ high luminosity hep-ph/0006114 Most stringent limits from Tevatron (k/MPl > 0.1): CDF : mG >921 GeV (mm, 2.3 fb-1) PRL102/091805/09 D0 : mG > 900 GeV(diEM,1 fb-1) PRL100/091802/08 RS 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)

10. 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

11. 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

12. Boosted 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 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…

13. 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,pTj2) * DRij2/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 η-φdistance R

14. Hadronic Vector Bosons in Vector Boson Scattering - I (CERN-OPEN-2008-020) • 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.

15. Hadronic Vector Bosons in Vector Boson Scattering - II (CERN-OPEN-2008-020) • 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 800 GeV WZ resonance decaying semi-leptonically with  = 0.65 fb.

16. Higgs searches in bb channel - I(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

17. Higgs searches in bb channel - II(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 WH→lvbb S/√B = 3.0 at 30fb-1 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)

18. B. Esposito (INFN) T. Isobe Merged top jet in a single cone Standard semileptonic ttbar selection 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, JHEP0807/092/08.

19. 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, JHEP0807/092/08. ATL-PHYS-PUB-2009-081 B. Esposito (INFN) T. Isobe Merged top jet in a single cone Standard semileptonic ttbar selection

20. Resonances in semileptonic ditops - I(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

21. Resonances in semileptonic ditops - II(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 Baseline cut

22. Resonances in semileptonic ditops - III(ATL-PHYS-PUB-2010-008) Leptonic top reconstruction • Search for a lepton (e or μ) 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 Baseline  Baseline e

23. Resonances in semileptonicditops - IV(ATL-PHYS-PUB-2010-008) Overall efficiency and rejection rates Signal reconstruction efficiency & (QCD dijet) background rejection (R = 1−e) for hadronic top decay

24. Heavy 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...

25. 4W + 2b-jets New fermions in Bulk RS Models: An example with KK Heavy Fermion • RS + bulk gauge symmetry SU(2)LxSU(2)RxU(1)X (hep-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-Wevents (hep-ph/0701158) • Uncommon in SM processes • Initially try to stay as inclusive as possible • Good source of Boosted Ws

26. ttbar ttbar H H L L L L H H Signature: 2L + 2H + 2b-jets KKHF Boosted Signature • 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

27. 2L 2L MbR = 1 TeV 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/0701247) • Greater reach than the standard W/top reconstruction in dilepton selected events • Analysis to be repeated with 7 TeV ATLAS data MbR = 500 GeV

28. New Custodial Leptons in Bulk RS(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

29. New 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:1005.1229 ) All are good source of boosted tops/Vs!

30. RS1 RS Bulk Radions(arXiv:0705.3844) • 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’) • 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 Significance ratio of radion/Higgs search for 100 pb-1 @ 14 TeV 100 pb-1, 14 TeV

31. B. Lillie et. al JHEP0709/074/07 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) Ruled out by using Tevatron data up to 800 GeVhep-ph/0703060

32. ATLAS Limits on KK Gluons from Tops(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)

33. 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 fbon SM ttbar event production with Ntop > 0 with pT > 400 GeV

34. ATLAS Jet Measurements (arXiv:1009.5908) • High pT jets are observed on a daily basis • Start to make jet substructure/fat-jet analysis for boosted objects! Inclusive jet differential cross section. Luminosity uncertainty of 11% is not shown. Dijet inv. Mass used to derive BSM limits 95% CL exclusion for 0.50< mq*< 1.53 TeV

35. First W/Zs from ATLAS(arXiv:1010.2130v1) • Validating ATLAS with SM Vector Bosons • 2250 W and 179 Z candidate events • No signs of boosted Ws yet

36. In “Search” for top quarks(ATLAS-CONF-2010-063) • ATLAS searched for events consistent with top quark pair production in 280 nb−1 • 9 top candidates in Nlep>1 channels, compatible with NLO • 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

37. 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.

38. What we need is… Lotsa collisions @ high energies Thank you!