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

A brief theoretical overview of long-lived states Jessie Shelton ACFI Displaced Workshop November 12, 2015 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 m h • can naturally get m ino << m sfermion 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 operators • lifetime depends on splitting to the fourth power: ◆ 5 ⌘ 4 ✓ TeV ⇣ m ˜ q c τ ≈ 100 µm × 1000TeV m ˜ g c O = g ¯ ˜ q χ q m 2 ˜ q 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 out 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 q c qq 0 q O = g ¯ ˜ m 2 ˜ 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: √ ! 4 ✓ 100 GeV ◆ 5 F c τ ≈ 100 µm × 100TeV m ˜ τ 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) • Hypercharge and Higgs portal couplings with independent strengths ∆ L = V ( S ) +  4 S 2 | H | 2 • Higgs mixing: , + ✏ B µ ν V µ ν h → V D V D h → ss • Kinetic mixing: q ¯ q → V D Thursday, November 12, 15

Multi-state hidden sectors LHC14, 300 / fb, L < 1m 10 - 1 LHC8, DY Electron FT CMS8, h � ZZ D • If Higgs portal coupling CMS7, DY 10 - 2 a � , 5 � Meson Decays EWPT e + e - Colliders a � , ± 2 � favored 10 - 3 dominates, can produce V D a e 10 - 4 through Higgs decays, at 10 - 5 rates not dependent on Electron & Proton � 10 - 6 Beam Dumps potentially small kinetic - 5 10 - 7 - 4 Countours of mixing - 3 Log 10 [ Br ( h � Z D Z D )] - 2 10 - 8 - 1 Supernova 10 - 9 10 - 10 10 - 3 10 - 2 10 2 10 3 0.1 1 10 m Z D [ GeV ] 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) • π ± lightest dark hadrons are v -pions: v , π 0 v • π ± dark approximate flavor symmetry prevents decay: stable v • can decay back to SM through off-shell Z’ : lifetime π 0 v depends on confinement scale, flavor-breaking, can be prompt or displaced 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 operators 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: 1 M 2 | H | 2 F µ ν F µ ν ∆ L = • Then some would-be stable glueballs can decay to others 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 ) 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 φ (for simplicity, say a new BSM state) has an out-of-equilibrium decay that produces a stable DM particle χ • relic abundance depends on m χ Γ φ • If freeze-in occurs during usual radiation-dominated era: get detector-stable φ • 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 φ lifetimes 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 of 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