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…

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▫ 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

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• 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 “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


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


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

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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:


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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



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


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


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)


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


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


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)


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


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


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


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


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


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


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


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

Searches for Hidden Sectors Using Lepton Signatures at CMS

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