A brief theoretical overview of long-lived states Jessie Shelton - - PowerPoint PPT Presentation

A brief theoretical

• verview of long-lived

states

Jessie Shelton

November 12, 2015

ACFI Displaced Workshop

Thursday, November 12, 15

Making long-lived states at the LHC

• Essential requirement: separate production (appreciable,

for rate), from decays (suppressed, for displacement):

• approximate symmetries, high-dimension operators, tiny

couplings, multiple states

• Option 1: particles carry SM quantum numbers
• E.g.: SUSY
• Option II: particles are SM singlets
• E.g.: Higgs decays, hidden sectors

Thursday, November 12, 15

SUSY (1): Mini-split

• Mini-split SUSY:
• high scale sfermions, weak(ish) scale inos
• Tuned! But:
• can solve flavor problem
• weak-scale inos retains successful gauge unification
• heavy sfermions a minimal explanation for heavy mh
• can naturally get mino << msfermion in models of SUSY-breaking
• independent DM motivation for weak scale inos (strained!)

Thursday, November 12, 15

Displaced decays in Mini-Split SUSY

• Major consequence: potentially collider-accessible

fermions that cannot decay except via higher-order

• perators

O = c m2

˜ q

˜ g¯ qχq

cτ ≈ 100µm × ⇣ m˜

q

1000TeV ⌘4 ✓TeV m˜

g

◆5

• lifetime depends on splitting to the

fourth power:

Thursday, November 12, 15

SUSY (II): RPV

• R-parity violating couplings:
• in superpotential; also in Kahler potential, in soft terms
• can be included or forbidden by hand
• Proton decay constrains combinations of RPV couplings

to be small: does not constrain individual RPV couplings

• possible cosmological motivations for small non-zero RPV

couplings:

• decay would-be LSP before BBN
• prevent baryon-number violating processes from washing
• ut baryon asymmetry

Thursday, November 12, 15

Displaced signatures from RPV SUSY

• Decays become displaced because small couplings are

introduced by hand

• All the familiar prompt RPV decays can be made

displaced O = c m2

˜ q

˜ g¯ qq0q

q

Thursday, November 12, 15

SUSY (III): GMSB

• Gauge-mediated SUSY breaking:
• low SUSY-breaking scale F gives gravitino LSP
• Goldstino coupling means NLSP decays sensitive to

SUSY-breaking scale

• moderate F leads naturally to displaced decays:

cτ ≈ 100µm × √ F 100TeV !4 ✓100 GeV m˜

τ

◆5

Thursday, November 12, 15

Hidden sectors

• Hidden sectors: new SM singlet degrees of freedom,

possibly with their own interactions

• Why weak-scale hidden sectors?
• All the same reasons we might expect any other new physics

at the weak scale:

• directly related to electroweak phase transition
• help to stabilize electroweak scale: neutral naturalness
• responsible for thermal dark matter
• ...because they could be there

Thursday, November 12, 15

Producing hidden sector states

• Chiral structure of SM gauge interactions is

restrictive: relatively few options for producing new SM singlets

• through Higgs
• through Z
• through a BSM particle, eg Z’ -- likely to be heavy
• (will discuss additional BSM possibilities later)

Thursday, November 12, 15

Multi-state hidden sectors

• Presence of multiple states easily leads to displaced

decays

• Simple example: dark Higgs S, dark U(1)

∆L = V (S) +  4 S2|H|2 +✏BµνV µν

• Hypercharge and Higgs portal

couplings with independent strengths

• Higgs mixing: ,
• Kinetic mixing:

h → VDVD

q¯ q → VD h → ss

Thursday, November 12, 15

Multi-state hidden sectors

• If Higgs portal coupling

dominates, can produce VD through Higgs decays, at rates not dependent on potentially small kinetic mixing

10-3 10-2 0.1 1 10 102 103 10-10 10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 mZD [GeV]

• LHC14, 300/fb, L < 1m

EWPT CMS7, DY CMS8, hZZD LHC8, DY Electron & Proton Beam Dumps Supernova a, 5

a,±2 favored

ae Electron FT Meson Decays e+e- Colliders

• 5
• 4
• 3
• 2
• 1

Countours of Log10[Br(hZDZD)]

Thursday, November 12, 15

Hidden valley model I

• The same essential mechanism operates in confining

hidden sectors

• E.g.: two-flavor dark QCD
• Add a Z’ which has couplings to dark “quarks” as well as to

SM (can also give Higgs-dark Higgs mixing)

• lightest dark hadrons are v-pions:
• dark approximate flavor symmetry prevents decay: stable
• can decay back to SM through off-shell Z’: lifetime

depends on confinement scale, flavor-breaking, can be prompt or displaced π±

v , π0 v

π±

v

π0

v

Thursday, November 12, 15

Hidden valley models - generalities

• This model demonstrates several generic features of

confining hidden sectors:

• composite states decay through high mass-dimension
• perators and can thus easily be displaced
• importance of (approximate, discrete, ...) symmetries in

controlling dark state lifetimes: typically these will vary among states in spectrum

• dark showers/cascades can yield high multiplicities if there is

a separation of scales

Thursday, November 12, 15

Hidden valley model II

• A pure glue hidden valley
• lattice: SU(3) with no matter fields has 11 stable glueballs

Thursday, November 12, 15

Hidden valley model II

• At dimension 6, can mix 0++ with SM Higgs:
• Then some would-be stable glueballs can decay to
• thers via (off-shell) Higgs: e.g. 2++ -> 0++ h*
• the lifetime for this decay depends on the mass splitting

through Higgs couplings as well as through HS matrix element: can vary over broad range

• For other glueballs dimension 8 operators offer only

decays (C, P)

∆L = 1 M 2 |H|2FµνF µν

Thursday, November 12, 15

SUSY + hidden sectors

• A conserved quantum number

such as R-parity can force SM- charged BSM states to decay into hidden sectors

• SUSY lepton-jet models
• stealth SUSY
• multiple potential sources of

displacement

Thursday, November 12, 15

And more...

• Dark matter: freeze-in in a non-thermal cosmology
• non-thermal relic abundance: a particle in thermal plasma

(for simplicity, say a new BSM state) has an out-of-equilibrium decay that produces a stable DM particle

• relic abundance depends on
• If freeze-in occurs during usual radiation-dominated era: get

detector-stable

• But if freeze-in occurs during an early matter-dominated era

(e.g.: reheating, moduli decay) then additional entropy injection dilutes DM abundance --> potentially interesting lifetimes φ χ mχΓφ φ φ

Thursday, November 12, 15

A wishlist for LHC searches

• A simplified model program for theories yielding

displaced signatures

• sufficiently general to cover broad range of theories with a

limited number of searches

• sufficiently specific to be powerful
• Broader implementation of displaced decays in Monte

Carlo tools

• both event generation and public detector simulations

Thursday, November 12, 15

A wishlist for LHC searches

• A framework for reporting results of searches that

allows easy reinterpretation

• reliance of displaced searches on detector properties difficult

to simulate with publicly available tools makes reinterpreting very challenging

• Very broad range of possible signatures in multi-state hidden

sectors, combined with freedom in constructing hidden spectrum, makes future re-interpretability important

• Key question for all items: what are the important

variables for signal efficiency?

Thursday, November 12, 15

Characterizing displaced decays

• Focus initially on theories giving two displaced objects X

per event, for simplicity

• Theories characterized by (production mechanism) x

(decay mode) x (lifetime)

• lifetime should be treated as a free parameter
• depending on lifetime, not all possible decay modes are

experimentally distinct signatures: e.g. objects decaying in the HCAL

Thursday, November 12, 15

Characterizing displaced decays

• Production mode
• direct pair-production of X via:
• QCD (e.g. gluino)
• electroweak
• Cases where different choices of electroweak multiplet

would need to be searched for in different ways? E.g. accidental custodial symmetry for winos

• Higgs-portal (off-shell)

Thursday, November 12, 15

Characterizing displaced decays

• Production mode
• resonant pair-production of X through:
• A SM parent: Higgs, Z
• A BSM parent P: scalar, vector
• pair-production of parents P that decay to X + SM
• expect correlated prompt objects, depending on identity
• f P: eg, gluino -> jj neutralino
• As we know from prompt program: can get long

cascades, but do not need to consider every case separately

Thursday, November 12, 15

Characterizing displaced decays

• Decay modes
• Expect cases both with and without detector-stable particles

produced in the displaced decay

• how important is MET for signal reconstruction and

background rejection?

• When is it necessary to define different simplified models

to cover the MET/no-MET cases?

• Classifications for hidden sectors: from portal operators
• Classifications for SM-charged states: related to production

Thursday, November 12, 15

Characterizing displaced decays

• We hope this meeting will serve as a starting point for a

broader program along these lines

• Aim to prepare a white paper:
• surveying status of searches and exclusions
• assess gaps in coverage and strategies to close them
• work toward a broader simplified model program for

displaced searches, with a clear framework for presentation

• f results

Thursday, November 12, 15