Transcript liverpool seminar

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
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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!
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)
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
4
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
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
5
The aim: Extending the PP Map
Rediscovery
background
Higgs
SUSY
Heavy exotics
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
6
A motivation:
the Unbearable Lightness of Being
TeV-1 ED
+1
10
-2
10
10-38
10-6
“Apparent” gravity is weak, governed by Planck scale (MPl=1019 GeV)
How to unify forces & solve the hierarchy problem (MPl >>MEW)?
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
7
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
GN 
1
(M PL (4 n ) )
2
 1/ M D2
M Pl2  M D2n Rn
MPl ~ 1019 GeV, MPl(4+n)~MEW
Size of the ED from
gravitational potential
1/r2-law valid for
R=44 μm @ 95% CL
M.Karagöz – Liverpool Sem.
28/10/2010
• 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)
TeV
Boosted Signatures from BSM
Randall-Sundrum (Type I) PRL83/3370/99
Planck
8
RS Warped ED: Baseline Picture
Bulk (y)
ds2  e2kyuv dxu dxv  dy2
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
9
RS KK Graviton Reach in Dielectrons
•
•
•
•
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/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
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)
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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
• ATLAS is well-prepared for boosted objects
– High granularity calorimeter and precision tracking to exploit
new techniques for efficient reconstruction
• Some examples:
–
–
–
–
Boosted Signatures from BSM
28/10/2010
M.Karagöz – Liverpool Sem.
Boosted Objects in the ATLAS Detector
12
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…
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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-φ distance R
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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.
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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.
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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
–
–
–
–
WH→lvbb
S/√B = 3.0 at 30fb-1
Significant mass drop? (mj1 < 1/3 mj)
& 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 btagged
• 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)
•
•
•
•
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.
Boosted Signatures from BSM
28/10/2010
M.Karagöz – Liverpool Sem.
Boosted Top Quarks
18
B. Esposito (INFN)
Standard semileptonic ttbar selection
T. Isobe
Merged top jet in a single cone
•
•
•
•
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.
Boosted Signatures from BSM
28/10/2010
M.Karagöz – Liverpool Sem.
Boosted Top Quarks
19
ATL-PHYS-PUB-2009-081
B. Esposito (INFN)
Standard semileptonic ttbar selection
T. Isobe
Merged top jet in a single cone
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
20
Resonances in semileptonic ditops - I
(ATL-PHYS-PUB-2010-008)
• Top pair resonances in semileptonic
channel for fully-resolved, partiallymerged 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
(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
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
21
Resonances in semileptonic ditops - II
(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
– 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 e
• Tagging variables used:
Baseline m
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
22
Resonances in semileptonic ditops - III
(ATL-PHYS-PUB-2010-008)
Overall efficiency and rejection rates
Boosted Signatures from BSM
28/10/2010
M.Karagöz – Liverpool Sem.
Resonances in semileptonic ditops - IV
23
Signal reconstruction efficiency &
(QCD dijet) background rejection
(R = 1−e) for hadronic top decay
M.Karagöz – Liverpool Sem.
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:
Boosted Signatures from BSM
28/10/2010
– Exclusive resonance searches into ttbar/VV:
24
• KK gluons
• KK gravitons
• string resonances
– Higher multiplicities:
• KK heavy fermions & leptons
• spin 3/2 string states
– etc...
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
25
New fermions in Bulk RS Models:
An example with KK Heavy Fermion
• RS + bulk gauge symmetry
SU(2)LxSU(2)RxU(1)X (hepph/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-ph/0701158)
– Uncommon in SM processes
– Initially try to stay as inclusive as
possible
– Good source of Boosted Ws
4W + 2b-jets
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
26
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
ttbar
L
L
ttbar
H
H
L
L
Signature: 2L + 2H + 2b-jets
H
H
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
27
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&TSmith, hep-ph/0701247)
• Greater reach than the standard W/top reconstruction
in dilepton selected events
• Analysis to be repeated with 7 TeV ATLAS data
2L
MbR = 500 GeV
2L
MbR = 1 TeV
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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
M.Karagöz – Liverpool Sem.
28/10/2010
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)
Boosted Signatures from BSM
• advantage due to strong coupling
29
– 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!
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
30
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
RS1
Significance ratio of
radion/Higgs search for
100 pb-1 @ 14 TeV
100 pb-1, 14 TeV
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
31
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/0703060
(ATL-PHYS-PUB-2010-008)
Boosted Signatures from BSM
28/10/2010
M.Karagöz – Liverpool Sem.
ATLAS Limits on KK Gluons from Tops
32
• 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)
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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 fb on SM ttbar event production with
Ntop > 0 with pT > 400 GeV
(arXiv:1009.5908)
• High pT jets are observed on a daily basis
• Start to make jet substructure/fat-jet analysis for
boosted objects!
Boosted Signatures from BSM
28/10/2010
M.Karagöz – Liverpool Sem.
ATLAS Jet Measurements
34
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
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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
M.Karagöz – Liverpool Sem.
28/10/2010
Boosted Signatures from BSM
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
Boosted Signatures from BSM
28/10/2010
M.Karagöz – Liverpool Sem.
What we need is…
Lotsa collisions
@ high energies
Thank you!