Post on 16-Dec-2015
“Hidden Valleys” and their
Novel Signals at Colliders
Matthew StrasslerUniversity of Washington
- hep-ph/0604261,0605193 w/ K Zurek- hep-ph/0607160- in preparation
Hidden Valleys – Preview
Theoretical Motivation
Many beyond-the-standard-model theories contain new sectors.
Common in top-down constructions (especially in string theory) Increasingly common in bottom-up constructions (twin Higgs, folded supersymmetry…) Could be home of dark matter Could be related to SUSY breaking, flavor, etc.
New sectors may decouple from our own at low energy SUSY breaking scale? TeV scale?
Learning about these sectors, which may contain many particles, could open up an entirely new view of nature..
Missing these sectors experimentally would be to miss a huge opportunity
Therefore we should ensure that we understand their phenomenological manifestations.
Hidden Valleys – Preview “Hidden Valley” sectors
Coupling not-too-weakly to our sector Containing not-too-heavy particles
may be observable at Tev/LHC
Possible subtle phenomena include High-multiplicity final states (possibly all-hadronic) Highly variable final states Many low-momentum partons
Unusual parton clustering Breakdown of jet/parton matching
Sharp alteration of Higgs decays; new discovery modes
Sharp alteration of SUSY events Usual search strategies may fail, need replacements
Possibly low cross-sections; high efficiency searches needed
Predictions may require understanding non-perturbative dynamics in new sector – theoretical challenge
Hidden Valley Models (w/ K. Zurek)
Basic minimal structure
Standard ModelSU(3)xSU(2)xU(1)
Communicator
Hidden ValleyGv with v-matter
April 06
A Conceptual DiagramEnergy
Inaccessibility
Hidden Valley Models (w/ K. Zurek)
Basic minimal structure
Standard ModelSU(3)xSU(2)xU(1)
Communicator
Hidden ValleyGv with v-matter
Communicators
Standard ModelSU(3)xSU(2)xU(1)
New Z’ fromU(1)’
Hidden ValleyGv with v-matter
Communicators
Standard ModelSU(3)xSU(2)xU(1)
Higgs BosonOr Bosons
Hidden ValleyGv with v-matter
Communicators
Standard ModelSU(3)xSU(2)xU(1)
Lightest StandardModel Superpartner
Hidden ValleyGv with v-matter
Communicators
Standard ModelSU(3)xSU(2)xU(1)
Heavy SterileNeutrinos
Hidden ValleyGv with v-matter
Communicators
Standard ModelSU(3)xSU(2)xU(1)
Loops of ParticlesCharged Under
SM and HV
Hidden ValleyGv with v-matter
Note that the communicator for production need not be the communicator for the decays…
Standard ModelSU(3)xSU(2)xU(1)
Hidden ValleyGv with v-matter
New Z’ fromU(1)’
Higgs Bosons
Communicators
The Hidden Valley (“v”-)Sector
Standard ModelSU(3)xSU(2)xU(1)
Communicator
Hidden ValleyQCD-like Theory
The Hidden Valley (“v”-)Sector
Standard ModelSU(3)xSU(2)xU(1)
Communicator
Hidden ValleyQCD-like Theory
With N ColorsWith n1 Light QuarksAnd n2 Heavy Quarks
The Hidden Valley (“v”-)Sector
Standard ModelSU(3)xSU(2)xU(1)
Communicator
Hidden ValleyGluons only
The Hidden Valley (“v”-)Sector
Standard ModelSU(3)xSU(2)xU(1)
Communicator
Hidden ValleyGluons Plus
Adjoint Matter
The Hidden Valley (“v”-)Sector
Standard ModelSU(3)xSU(2)xU(1)
Communicator
Hidden ValleyKS Throat/RS Model
The Hidden Valley (“v”-)Sector
Standard ModelSU(3)xSU(2)xU(1)
Communicator
Hidden ValleyMultiple Gauge Groups
Many Models, Few Constraints Number of possibilities is huge!
Constraints are limited LEP : production rare or absent Precision tests: new sector is SM-neutral, very small effects Cosmology: few constraints if
Efficient mixing of species One species with lifetime < 1 second to decay to SM
In general, complexities too extreme for purely analytic calculation Event Generation Software Needed!
Reasonable strategy: Identify large class of models with similar experimental signatures Select a typical subset of this class
Compute properties Write event generation software
Explore experimental challenges within this subset Infer lessons valid for entire class, and beyond
This talk Carry out above program for simplest subset of simplest class
General setup Simulation and results
Harder case: no long-lived particles Easier case: long-lived (neutral) particles
Different communicators with simple v-sector Effects on Higgs
[more generally, discovering Higgs via highly-displaced vertices] Effect on SUSY
[more generally, on any model with new global sym] Others…
Other physics in the v-sector Heavy v-quarks One light v-quark Pure YM plus heavy v-quarks SUSY YM And beyond…
Simplest Class of Models
Easy subset of models to understand to find experimentally to simulate to allow exploration of a wide range
of phenomena This subset is part of a wide class of
QCD-like theories
Standard ModelSU(3)xSU(2)xU(1)
New Z’ fromU(1)’
Hidden Valleyv-QCD with
2 (or 3) light v-quarks
Two-flavor (v)QCD
A model with N colors and two light v-quarks serves as a starting point.
The theory is asymptotically free and becomes strong at a scale v
All v-hadrons decay immediately to v-pions and v-nucleons.
All v-hadrons are electric and color neutral, since v-quarks are electric and color-neutral
If v-baryon number is conserved, v-baryons are stable (and invisible)
Two-flavor (v)QCD
All v-hadrons decay immediately to v-pions and the lightest v-baryons
Two of the three v-pions cannot decay via a Z’
But the third one can!
vQ1Q2 stable
vQ2Q1 stable
vQ1Q1 Q2Q2 (Z’)* f f
bb
bb
v Z’Z’
Pseudoscalars: their decays require a helicity flip; branching fractions proportional to fermion masses mf
2
Long lifetimes
The v-hadrons decay to standard model particles through a heavy Z’ boson.
Therefore – no surprise -- these particles may have long lifetimes
Notice the very strong dependence on what are essentially free parameters
LEP constraints are moderate; cosomological constraints weak
Thus displaced bottom-quark pairs and tau pairs are common in such models, but not required.
q q Q Q : v-quark production
Z’Z’
v-quarks
LHC Production Rates for v-Quarks
For a particular model. Others may differ by ~ factor of 10
~ 100 events/year
q q Q Q : v-quark production
Z’Z’
v-quarks
q q Q Q
qqQQ
qq QQ
Z’Z’
v-gluons
q q Q Q
Z’Z’
v, v
;v
q q Q Q
v, v
;v
v-pions
For now, take masses in range 20-350 GeV so that dominant v
decay is to b’s
Z’Z’
q q Q Q
v-pions
Z’Z’
q q Q Q
v-pionsThe v
, vare
invisible and stable
Z’Z’
q q Q Q
v-pions
Z’Z’
q q Q Q
v-pions But the vs
decay in the detector to bb pairs, or rarely tausZ’Z’
How to simulate? Analogy…
Pythia is designed to reproduce data from 70’s/80’s
q q Q Q
q q Q Q
ISR
q q Q Q
ISR
FSR
q q Q Q
ISR
FSRJet
Formation
q q Q Q
ISR
FSR
UnderlyingEvent
JetFormation
Event Display
This is my own event display -- not ideal or bug-free Face on along beampipe – Color indicates angle
(pseudorapidity) Blue – heading forward Red – heading backward Green/Yellow -- central
Notes: No magnetic field; tracks
are straight No tracks below 3 GeV are
shown All photons/neutrals shown
starting at calorimeterCMS
Top quark pair event
Long lifetimes
The v-hadrons decay to standard model particles through a heavy Z’ boson.
Therefore – no surprise -- these particles may have long lifetimes
Notice the very strong dependence on what are essentially free parameters
LEP constraints are moderate; cosomological constraints weak
Thus displaced bottom-quark pairs and tau pairs are common in such models, but not required.
Harder Case – All decays prompt
Events with Multiple jets Some b-tags Possibly taus Some missing energy from invisible v-hadrons
Events fluctuate wildly (despite all being Z’ decays) Events cannot be reconstructed
Kinematic information is scrambled well-beyond repair
Backgrounds? Not computable
What clues may assist with identifying this signal?
LHC : 150 GeV v-pions
LHC : 60 GeV v-pions
LHC : Top quark pairs
Triggering
Should not be a problem in this particular model The Z’ kicks lots of
energy sidewise (big HT)
Many v-hadrons are invisible (big MET)
MET in GeV
1000
Jet HT in GeV
1000 2000
60 GeV v-pions
Jet distributions
Number of jets depends on algorithm, parameters within algorithm
Two IR-safe algorithms in use Cone (multiple variants, some not IR safe) kT (nice at e+e- collider, sensitive to UE)
Studies with cone algorithm reveal some interesting features
Studies with kT not complete
All results shown using Pythia hadron-level output; no detector resolution effects!
Jet-to-Parton (mis)Matching
For any setting of cone algorithm, jets not well correlated with partons
Number of partons above 50 GeV
Number of jets above 50 GeV
Number of partons above 50 GeV
Number of jets above 50 GeV
Top quark pairs 60 GeV v-pions
Midpoint Cone 0.7
Jet-to-Parton (mis)Matching
For any setting of cone algorithm, jets not well correlated with partons
Number of partons above 50 GeV
Number of jets above 50 GeV
Number of partons above 50 GeV
Number of jets above 50 GeV
Top quark pairs 30 GeV v-pions
Midpoint Cone 0.7
Jet-to-Parton (mis)Matching
For any setting of cone algorithm, jets not well correlated with partons
Number of partons above 50 GeV
Number of jets above 50 GeV
Number of partons above 50 GeV
Number of jets above 50 GeV
Top quark pairs 150 GeV v-pions
Midpoint Cone 0.7
Reasons:
Breakdown of jet–parton relation Single boosted v-pion gives one jet –
two partons merge Single slow v-pion often decays to one moderate-pT parton
and one soft parton – one parton is lost
Multiple v-pions have correlated momenta – their partons may overlap
All of these reduce the number of partons per jet
Many final state partons much FSR, esp. heavy v-pions Can bring back a few jets, but relatively small effect
Invariant Mass of Highest-pT Jet
30
Number of jets
Invariant mass of jet
Signal only! No background.
Invariant mass of two hardest jets
Top quark pairs 30 GeV v-pions
150 GeV v-pions60 GeV v-pions
Invariant mass of highest pT jet
Invariant mass of 2nd-highest pT jet
New methods probably needed
This is nice to know, but surely not enough to get good S/B
What else do we need? To use moderate pT “jets”, if possible To use soft hadrons, soft muons, if possible ??!? Technique to classify events as QCD-like or not-QCD-like
What approaches might be available? Jet substructure? Modified use of existing jet algorithms? New algorithms? Move away from jets altogether? Revisit vertexing/b-tagging ?
[a “jet” may contain 2, 3,…, 6 b-quarks?!]
No answers yet…
Easier Case – Long-lived Particles For light v-pions or heavy Z’, get macroscopic v-pion decay lengths
Displaced vertices result, possibly well outside beampipe b pairs or tau pairs in this model Other possible final states in other models
No standard model background! Significant detector-related challenges!!
LHC studies very limited ATLAS undertaking study (Seattle/Rome group) CMS preparing to study LHCb – ideal setting!!! – undertaking first studies
Tevatron searches very limited D0 has search for muon pairs at 5 to 30 cm D0 now undertaking search for displaced jets [more later] CDF -- planning stages? [I’m hoping to learn the status today!]
Tevatron versus LHCCaution: this particular model won’t give highly displaced vertices at Tevatron.
In this model Z’ is communicator for production and decay If heavy, no production rate If lighter, no long-lived v-pions unless v-pions very light Strong LEP constraints on very light v-pions with light Z’ No other v-hadrons decay
However, in other models, no such restriction Example:
FCNC’s can allow for late decay of v, v
Example: if Z’ decays to quarks Q but v-pions are made from quarks Q’ that don’t couple to Z’, then
Q’-Q mixing angles determine v-pion lifetime, or coupling to a heavier Z’ or Higgs boson can determine lifetime
Point being: reasonable to look for such physics at Tevatron, but don’t use this model as benchmark.
Typical Signal: missing energy plus 2, 4, 6 jets with definite jet-pair invariant mass.
LHC : Long-lived v-hadrons
LHC : Long-lived v-hadrons
Summary of this preliminary study Z’ decays to the v-sector give events with
Great variability Many partons Poor jet/parton matching Many b’s, some taus Missing energy
Possibly highly-displaced vertices
Many of these issues apply in other models as well – to be studied
But let’s now consider other “communicators” Higgs LSP
Higgs decays to the v-sector
g
g
v-quarkshh hhvv
mixing
w/ K Zurek, May 06
Higgs mixing in U(1)’ modelSchabinger + Wells 05
Higgs decays to the v-sector
g
g v-pions
hh hhvv
mixing
w/ K Zurek, May 06
bb
bb
bb
bb
See Dermasek and Gunion 04-06 h aa bb bb, bb , , etc. and much follow up work by many authors
Higgs decays to the v-sector
g
g v-pions
hh hhvv
mixing
w/ K Zurek, May 06
bb
bb
bb
bb
Displaced vertex
Displaced vertex
A Higgs Decay
Schematic; not a simulated event!
An Overlooked Discovery ChannelMJS + K. Zurek May 06
This may be how the Higgs is found! Even at small branching fractions, may win at Tevatron -- and LHCb!! Branching fraction for light Higgs may be ~ 1 True for other scalars, esp. those lacking WW decays (e.g. CP-odd Higgs A0),
increasing Tevatron reach toward 200 GeV!
Can happen in many other models with an approx conserved global symmetry MJS & Zurek [weakly-coupled extra real scalar] Fox Cheng Weiner, Fall 05 [weakly-coupled extended-SUSY model]
considered LEP but not Tevatron JHU group, July 06 [R-parity violating model with final-state jet trios]
Also pointed out LHCb connection I can build models with occasional final state lepton resonances
Current status at Tevatron, esp D0 (trigger on muons) – search underway CDF? [I need an update] LHCb (trigger? Perhaps need associated production?) – study in progress CMS? Atlas? Trigger issues under study…
The Challenge:
Higgs decay (CP-odd, 200 GeV 40 GeV)
Andy Haas –
D0 can trigger on soft muons from b decays.
In the inner tracker D0 can see the primary, secondary, and tertiary vertices! This significantly reduces backgrounds and may allow use of events where only one displaced decay to bb is observed.
Higgs decay (CP-odd, 200 GeV 40 GeV)
Second decay occurs too far out for track reconstruction – jet without tracks.
What’s True for Higgs
is True for SUSY
SUSY decays to the v-sector
g
g
q~
q*~
q
q
_
Two neutral particles: Missing Momentum
transverse to beampipe (“MET”)
MJS July 06
SUSY decays to the v-sector
g
g
q~
q*~
q
q
_
But if the Standard Model LSP is heavier than the v-sector LSP (LSvP), then…
Two neutral particles: Missing Momentum
transverse to beampipe (“MET”)
MJS July 06
SUSY decays to the v-sector
g
g
v-(s)quarks
q~
q*~
q
q
Q*Q*~
QQ~_
_
But if the Standard Model LSP is heavier than the v-sector LSP (LSvP), then…!!!
MJS July 06
SUSY decays to the v-sector
g
g
q~
q*~
q
q
_
v-pions
The lightest SUSY v-hadron!
The lightest SUSY v-hadron!
MJS July 06
SUSY decays to the v-sector
The traditional missing energy signal is replaced with multiple soft jets, reduced missing energy, and possibly multiple displaced vertices
MJS July 06
g
g
q~
q*~
q
q
_
v-pions
The lightest SUSY v-hadron!
The lightest SUSY v-hadron!
Comments
Production through LSP decay
V-pion decay through Higgs boson or heavy Z’
The lightest R-parity-odd v-hadron may be stable, and other v-hadrons may be stable, so some MET signal survives
But MET reduced by quite a lot, so SM backgrounds are much larger; need new techniques to find The LSP and/or v-hadrons may give displaced vertices SUSY tag Extra soft jets?!
SUSY tag with Unstable SM LSP
Long history Gauge mediation Hidden sectors R-parity violation RH neutrinos
As in all such models, the SM LSP need not be electrically neutral and/or colorless Implies many possible scenarios Example:
SUSY tag in decays to the v-sectorMJS July 06
g
g
q~
q*~
q
q
_
v-pions
~
~
4 taus in every SUSY event, 2 possibly displaced, plus soft v-hadrons, possibly with displaced decays
SUSY events? How can these SUSY events be identified?
Displaced vertices? Great – but how best to search for them? SUSY tag? Easy if four taus in every event. No displaced vertices? And no SUSY tag?
The v-hadron decay products are much softer than for Z’ case MET may still help; depends on the v-model May need to classify the medium-pT jets as unusual Worry: like SM plus unusual underlying event? This might be very challenging! Needs study.
Cannot currently simulate these models, but in the works
Same issues afflict models with KK-parity, T-parity – indeed, any new global symmetry
Other v-sectors
I will not discuss other possible communicators here Neutrinos Loops
Instead I’d like briefly to consider other v-sectors
This is much harder, since unknown strong dynamics often plays a role
Let’s quickly glance at a few possibilities
Heavier v-quarks?
Heavy v-quarks may be produced in Z’ decays or SUSY events.
Meson spectrum like B meson spectrum Large m-quark approximations apply Most mesons unstable to v-strong decays Last vector meson stable against v-strong decays
Will decay to last pseudoscalar via Z’; No helicity suppression! sometimes muon, electron pairs
Thus Z’ heavy v-quarks generates few v-pions possible vector-to-pseudo decays to jets or leptons
MET plus several rather soft jets, leptons But leptons have a kinematic endpoint
ff
ff
Z’
M*
M
Only one light v-quark?
vQCD with one flavor: very different
Spectrum not precisely known v-omega meson cannot decay to v-hadrons The v-omega can decay to any SM fermions
Including muons, electrons – resonance! Possibly a challenge to detect Should be possible if a sufficiently pure
sample of events can be identified
Cascade decays may be interesting For instance, excited baryon light-lepton production in three-body decays –
kinematic endpoints
Simulation package needed – working with Skands, Mrenna
Better understanding of spectrum, matrix elements needed also, as input to simulation
Analytic and lattice gauge theory needed
w/ K. Zurek, April 06
No light v-quarks?
Low-energy v-hadrons are v-glueball states
Variety of quantum numbers variety of lifetimes, decay chains
Decays depend on communicator(s) Cascade decays?
Additional theoretical study required Simulation package needed
w/ K. Zurek, April 06
MorningstarFigure
Morningstar and Peardon 99
YM glueball spectrum
Conclusions
Models with new sectors: abundant, reasonable, and little studied Many such models produce light neutral bound states,
often several, possibly with heavier charged states
Novel multi-parton final states, with large fluctuations, result Highest pT jets useful Moderate pT jets, soft jets need to be put into play
Other clues might include MET Many b’s, taus Muon/electron resonances or endpoints Highly displaced jet pairs or lepton pairs
Conclusions
Signal identification/Background separation a challenge Easier if displaced vertices are present If not, clues from kinematics, tagging Jet/parton matching breaking down LHCb may have advantages!
May affect Higgs physics, SUSY physics, other models May make detection easier if displaced vertices May impede detection if not
A number of other remarkable phenomenological signals possible
Theoretical work needed for predictions, input to simulations, ideas for signal extraction
Simulation development needed to allow theoretical and experimental studies, searches
Experimental work on several fronts to ensure these different types of signals can all be found.