Searches for Hidden Sectors Using Lepton Signatures at CMS
description
Transcript of Searches for Hidden Sectors Using Lepton Signatures at CMS
Searches for Hidden Sectors Using Lepton Signatures at CMS
Alexei Safonov Texas A&M Universityfor the CMS Collaboration
LHC Workshop, UChicago, November 2012
A. Safonov, LHC Workshop, UChicago, November 2012 2
Why Searching for Hidden Sectors?•Dark matter is one big reason
▫If satellite experiments excesses have something to do with the dark matter, these could signify presence of dark sectors
•Higgs “problems”:▫While SUSY can be the answer to the hierarchy
problem, MSSM isn’t that great of a candidate Fine tuning, the m-problem etc. …and in all likelihood it’s wrong anyway
▫NMSSM can help some of these problems Yields a more complex higgs sector with new
fields weakly coupling to SM particles •Finally, they just might be there…
3
▫ Unknown pulsar? Cosmic rays interacting with giant molecular clouds?
▫ Or heavy dark matter annihilation in the galactic halo with a large x-section: Light dark photon : an attractive long-
distance force between slow WIMPs Sommerfeld enhancement
can weakly couple to SM via kinetic mixing with photon
As no antiproton excess observed, M( ≲ O(1 GeV)
TeV Scale Dark Matter
arXiv:1109.0521v1
X
X
m-, e-
m+, e+
m-, e-
m+, e+
• PAMELA and Fermi observe rising positron fraction towards higher energy:
A. Safonov, LHC Workshop, UChicago, November 2012
4
• Modified superpotential:▫ MSSM: ▫ NMSSM:
• NMSSM less fine tuning and solves m-problem:▫ m is generated by singlet field VEV and naturally
has EW scale• More complex Higgs sector:
▫3 CP-even higgses h1,2,3, 2 CP-odd higgses a1,2
▫ a1 is hidden as it is mostly singlet and weakly couples to SM particles except through h1
• Experimentally relevant decays: (Branchings depend on mixing) (standard higgs hierarchy)
Couplings are weak but it has to decay somewhere
NMMSM Phenomenology
A. Safonov, LHC Workshop, UChicago, November 2012
A. Safonov, LHC Workshop, UChicago, November 2012 5
A “Long Living” Example•A separate hidden strongly interacting
sector coupling to SM only through a heavy Z’▫Visible higgs(es) can naturally mix with the
hidden higgs
One can have models with higgs-like decays too
•Striking signatures, relatively easy to look at
Strassler, Zurek, PLB 661 (2008)
If Z’ is heavy, “hidden pions” can easily have decay lengths O(0-100 cm)
▫Z-like decay hierarchy for new hidden bosons
A. Safonov, LHC Workshop, UChicago, November 2012 6
Hidden Sectors Search Strategies• Aim to produce something that links visible and
hidden sectors and look for evidence of new hidden states:▫In the dark SUSY the “stable” visible LSP has no
choice but to decay to hidden states even if small couplings If we can make the LSP either through squark/gluino
production or Higgs, we can see its decay products▫In the NMSSM new higgs states can have very weak
coupling to SM, but appreciable coupling to the SM-like higgs due to mixing – look for exotic higgs decays Similar story for the “long living” example model
• Brute force: make hidden sector particles▫Because of typically small couplings, need high
luminosity and clean final states
A. Safonov, LHC Workshop, UChicago, November 2012 7
Dark Photons in SUSY Cascades•SUSY with squarks/gluinos
accessible by LHC:
•Dark photons decay as SM g
Branching fraction of arxiv:1002.2952
▫MSSM LSP is a neutralino decaying to dark neutralino and light gdark/hdark
▫MSSM LSP is a squark decaying to q and light dark fermion and gdark/hdark
8
Selections• Data:
▫ 35 pb-1 of 2010 LHC data▫ Inclusive muon trigger pT>15 GeV
• Offline:▫ Require at least 1 muon with pT>15 GeV, |h|<0.9▫ Identify all other muons with pT>5 GeV, |h|<2.4▫ Reconstruct muon jets and categorize▫ No isolations, cluster using pairwise mass of muons
• Assume new bosons produced on-shell:
A. Safonov, LHC Workshop, UChicago, November 2012
9
name description Lead m-Jet pT BackgroundsR1
2 Single dimuon+X >80 GeV/c 2m’s from a b-jet, Drell YanR1
4 Single quadmuon+X
no explicit cut 2m’s from a b-jet + 2 fakes
R22 2 Two dimuons+X no explicit cut bb-bar+X, 2m’s from each
bRN
5 + All other categories
no explicit cut Rare, from bb-bar+X/fakes
• No events with consistent masses of dimuons in higher order categories
Topologies: Data and Backgrounds
A. Safonov, LHC Workshop, UChicago, November 2012
A. Safonov, LHC Workshop, UChicago, November 2012 10
Model-Independent Interpretation• Use three simplest topologies to set
“conservative” model independent limits:▫Dimuon+X▫Two-dimuon+X▫Quadmuon+X
• Limits of applicability:▫Mean pT(m-jet)≤250GeV
• Easy to apply to other models:▫Follow analysis steps to calculate branching and
acceptance for a specific final state assuming an ideal detector
▫Compare with the limit plot Complex topologies can be reduced to one of these
three
A. Safonov, LHC Workshop, UChicago, November 2012 11
Models with TeV Scale Dark Matter
•MSSM LSP is a squark decaying to a quark, light dark fermion and either gdark (left) or hdark (right)
Model from JHEP 04 (2009) 014.
More details in CMS-EXO-11-013 and JHEP07 (2011) 098
A. Safonov, LHC Workshop, UChicago, November 2012 12
Search for Displaced Lepton Pairs•Generic search for H0XX, using leptonic
decays Xll, with X having substantial lifetime▫2011 data: L=4 fb-1 for l=e, L=5 fb-1 for l=m
•Selections:▫Displaced e/m candidate defined as a track
within |h|<2 with pT>41/33 GeV and d0/sd>3/2▫Require at least one X-candidate per event:
A common vertex with c2/ndf>4, displaced more than 8(5)svtx-fit from the beamline for e(m) channel
M(ee/mm)>15 GeV, DR(mm)>0.2, pT(e) from ECAL Isolation: SpT <4 GeV counting tracks w/ pT>1 GeV
in DR(trk,e/m)<0.4 around each lepton (but not counting the other lepton in the X-candidate)
A. Safonov, LHC Workshop, UChicago, November 2012 13
Search for Displaced Lepton Pairs• Avoid standard lepton ID:
▫Inefficient for displaced tracks• Efficiency driven by tracking
reconstruction efficiency▫Cross-checked with cosmic
muon data
• Backgrounds dominated by Drell-Yan events▫ Shape from simulation cross
checked with data• Normalization from the fit of the
vertex Lxy/s distribution▫ B= and for e/m channel
A. Safonov, LHC Workshop, UChicago, November 2012 14
Search for Displaced Lepton Pairs•Signal region (significant Lxy): no excess
▫N0 events with Xmm candidates▫4 events Xee candidates )
•Limits as a function of the new boson mass
A. Safonov, LHC Workshop, UChicago, November 2012 15
Search for Displaced Lepton Pairs•Limits vs lifetime for M(H)=200 GeV and
1 TeV▫Reflect track reconstruction efficiency
dependence on decay path length▫More details in CMS-EXO-11-101 (public
note)
A. Safonov, LHC Workshop, UChicago, November 2012 16
Light Dark Sectors and Higgs•NMSSM:
•Dark SUSY with light dark photons:
▫Similar signature, but softer dimuons and missing energy
▫Either h1 or h2 (or both) can decay to a1a1, BR depends on the singlet component
▫Production cross-section for h and BR highly model dependent
A. Safonov, LHC Workshop, UChicago, November 2012 17
Light Dark Sector Higgs Limits• Spin-off of the 35 pb-1 muon jet analysis
▫~5fb-1 of 2011 data• Focus on the topology with two muon pairs of
consistent mass▫Di-muon trigger with pT>17 and pT>8 GeV
Following Phys. Rev. D 81 (2010) 075021.
▫Same selections, but apply loose track-based isolation High signal efficiency,
strong suppression of bb backgrounds
Insensitive to pile-up▫No pairs with
consistent mass found Expect ~ 1 event
A. Safonov, LHC Workshop, UChicago, November 2012 18
Exotic Higgs Limits• Express in terms of limits on production
▫NMSSM: pp h1 or h2 a1a1 4m▫Dark SUSY: pp h c1c1 gd gd c1c1 4m + MET
• Plots use SM higgs production cross-section ▫Most of time not true, but convenient benchmarking
A. Safonov, LHC Workshop, UChicago, November 2012 19
NMSSM Parameter Space• To gauge what it does
to the NMSSM parameter space, scan NMSSM parameter space▫Focus on ma<2mt
▫Use actual NMSSM cross-sections Not SM
▫See EXO-12-012 for details
• Make deep inroads into the allowed space
A. Safonov, LHC Workshop, UChicago, November 2012 20
Search for Direct NMSSM a1 Production• Production via gluon fusion
▫Large cross-section if mixing with MSSM A is large Suppressed as a1 has to be highly
singlet to abide experimental constraints (cos2qA <1)
▫Usual enhancement with tanb• Search for resonances in dimuon
spectrum▫5.5<m<8.8 and 11.5<m<14 GeV
Avoid region dominated by large Upsilon backgrounds
▫Early 2011 data (1.3 fb-1) Had a dedicated very low pT di-
muon trigger (prescale = 2)
cos2qA =1
A. Safonov, LHC Workshop, UChicago, November 2012 21
Search for Direct NMSSM a1 Production• Analysis selections:
▫ Muon pT>5.5 GeV and |h|<2.4▫ Isolation (per muon):
• In the two signal regions, fit for the sum of Crystal Ball (signal) and a 1st order polynomial (background) ▫ Plus the radiative tail of Upsilon for the low mass region
A. Safonov, LHC Workshop, UChicago, November 2012 22
Search for Direct NMSSM a1 Production•No significant excesses in data•Limits vs m(a) on the production rate
▫Further interpretation in terms of cosqA and tanb
• LHC limits start superseding those from BaBar
A. Safonov, LHC Workshop, UChicago, November 2012 23
Summary•Several CMS analyses aiming at searches for
hidden sectors▫Different scenarios for production mechanisms
and the lifetime of the new hidden bosons▫Electron channels starting gaining ground▫When possible, results presented in a quasi model
independent fashion to allow future interpretations
•No discoveries, but the new ground in sensitivity▫Important complementarity to the SM Higgs
searches as the searches for exotic higgs decays can rule out many non-SM scenarios
•The data keeps coming in, so stay tuned