ATLAS: New signals from a “Hidden Valley” Matt Strassler, U Washington Theoretical Motivation Many top-down models, such as string theory or extended grand.
Download ReportTranscript ATLAS: New signals from a “Hidden Valley” Matt Strassler, U Washington Theoretical Motivation Many top-down models, such as string theory or extended grand.
ATLAS: New signals from a “Hidden Valley” Matt Strassler, U Washington Theoretical Motivation Many top-down models, such as string theory or extended grand unified models, typically predict many sectors beyond the standard model. Such sectors are also appearing regularly in solutions to the hierarchy problem (twin Higgs, folded supersymmetry…) New sectors could be involved in SUSY-breaking, flavor, dark matter, … Often these sectors continue to interact with our own down to low scales Constraints on such sectors from LEP, cosmology, Tevatron are rather limited Learning about these sectors, which may contain many particles, could open up an entirely new view of nature.. But as we will see… Cross-sections may be low, Signals are very unusual; novel phenomenology, special challenges Push the limit of (but do not exceed) the experiments’ capabilities Experimental Motivation “Hidden valley”: signal that challenges the usual assumptions about how the LHC detectors are supposed to function – Typical signatures are do not get high-pT jets and isolated leptons Complex non-QCD-like multi-jet events Extreme event-to-event fluctuations Probably some missing energy (possibly a lot) Probably some heavy flavor (possibly a lot) Perhaps displaced jets Perhaps nonisolated moderate pT leptons Could drastically affect “standard signals” such as standard Higgs supersymmetry little Higgs Hidden Valley Models (w/ K. Zurek) April 06 Basic minimal structure Communicator Standard Model SU(3)xSU(2)xU(1) Hidden Valley Gv with v-matter Energy A Conceptual Diagram Inaccessibility Hidden Valley Models (w/ K. Zurek) Basic minimal structure Communicator Standard Model SU(3)xSU(2)xU(1) Hidden Valley Gv with v-matter Communicators New Z’ from U(1)’ Standard Model SU(3)xSU(2)xU(1) Hidden Valley Gv with v-matter Communicators Higgs Boson Or Bosons Standard Model SU(3)xSU(2)xU(1) Hidden Valley Gv with v-matter Communicators Lightest Standard Model Superpartner Standard Model SU(3)xSU(2)xU(1) Hidden Valley Gv with v-matter Communicators Heavy Sterile Neutrinos Standard Model SU(3)xSU(2)xU(1) Hidden Valley Gv with v-matter Communicators Loops of Particles Charged Under SM and HV Standard Model SU(3)xSU(2)xU(1) Hidden Valley Gv with v-matter Communicators Note that the communicator for production need not be the communicator for the decays… New Z’ from U(1)’ Hidden Valley Gv with v-matter Standard Model SU(3)xSU(2)xU(1) Higgs Bosons The Hidden Valley (“v”-)Sector Communicator Standard Model SU(3)xSU(2)xU(1) Hidden Valley QCD-like Theory The Hidden Valley (“v”-)Sector Communicator Standard Model SU(3)xSU(2)xU(1) Hidden Valley Gluons only The Hidden Valley (“v”-)Sector Communicator Standard Model SU(3)xSU(2)xU(1) Hidden Valley QCD-like Theory With N Colors With n1 Light Quarks And n2 Heavy Quarks The Hidden Valley (“v”-)Sector Communicator Standard Model SU(3)xSU(2)xU(1) Hidden Valley Gluons Plus Adjoint Matter The Hidden Valley (“v”-)Sector Communicator Standard Model SU(3)xSU(2)xU(1) Hidden Valley Multiple Gauge Groups Simplest Class of Models Clearly the number of possibilities is huge! Cannot address them one by one. Key is to identify typical signatures of large classes of models. Easiest model to understand … and simulate… is: New Z’ from U(1)’ Standard Model SU(3)xSU(2)xU(1) Hidden Valley v-QCD with two light v-quarks Simplest Class of Models This model is typical of a large class: QCD-like theory with a few light quarks and no heavy quarks Other models can be quite different in their details; we’ll discuss a couple of them later. For now, let’s explore this one in detail, since it’s the one in the current MC package. New Z’ from U(1)’ Standard Model SU(3)xSU(2)xU(1) Hidden Valley v-QCD with two light v-quarks The Simple Model in the Program Structure of the model Spectrum of the “v-hadrons” Decays of the v-hadrons Production of the v-hadrons Events Along the way: Simulation techniques 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 Lv 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! pv+ ~ Q1Q2 ~ stable pv- ~ Q2Q1 ~ stable pv0 ~ Q1Q1 - Q2Q2 (Z’)* f f pv0 Z’ b b Pseudoscalars: their decays require a helicity flip; branching fractions proportional to fermion masses mf2 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 v-quarks q q Q Z’ Q qqQQ v-gluons q q Q Z’ Q qqQQ q q Z’ Q Q qqQQ v-pions pv+ , pv- ;pvo q Z’ q Q Q For now, take masses in range 20-350 GeV so that dominant pvo decay is to b’s pv+ , pv- ;pvo qqQQ v-pions q q Z’ Q Q qqQQ v-pions The pv+ , pv- are invisible and stable q q Z’ Q Q qqQQ v-pions q q Z’ Q Q qqQQ v-pions q q Z’ Q Q But the pvos decay in the detector to bb pairs Production Rates for v-Hadrons Other interesting processes This is going to be [almost] the process addressed by the current simulation package But first let’s step back… To keep perspective on what we will be able to achieve with my current software, and what we cannot do directly but should have in the back of our minds, let’s consider other possible phenomena that would arise in other models, or even in this one… What if Q_2 decays to Q_1 Z’* q Z’ q Q2 FCNC; model dependent Q1 K+ K- Kaons or other soft hadrons or leptons too soft to observe; essentially a decay to bottom quarks plus very soft stuff… Z’ pv+ pv0 b Z’ b qqQQ v-pions q q Z’ Q Q Now all or most v-pions decay in the detector Higgs decays to the v-sector w/ K Zurek, May 06 Q g h hv g v-quarks Q mixing Higgs vpion vpion two displaced jet-pairs A Discovery Channel at Tevatron! At LHC, trigger?! Possibly in associated production or VBF? Needs study… July 06 SUSY decays to the v-sector ~ q g Q* c ~ q Q v-(s)hadrons v-(s)quarks _ g Q ~ q* c _ q ~ Q If the standard model LSP is heavier than the v-sector LSP,then the former will decay to the latter (a v-squark or v-gluino in simplest models) The traditional missing energy signal is replaced with multiple soft jets, reduced missing energy, and possibly multiple displaced vertices Many possibilities!!! Other v-sector models QCD with one flavor has a very different spectrum The spectrum is not precisely known but the omega meson is stable against decay to hadrons The v-omega can decay to any standard model fermion pair including muons and electrons However its production will be accompanied by the production and decay of other v-hadrons, making it a challenge to detect the v-omega resonance in electron/muon pairs Still this should be possible if the a sufficiently pure sample of events can be identified Cascade decays of stable scalar and spin-two particles may be interesting and allow additional light-fermion production in three-body decays Simulation package needed – w/ Skands April 06 qqQQ Back to our original model and our original process… v-pions q q Z’ Q Q But the pvos decay in the detector to bb pairs Returning to v-quark production Our two-v-quark model is a very simple model to understand But it is not simple to simulate; since it is different from QCD, it requires a new simulation package For our current purposes, it is useful to consider a model which is almost exactly like QCD… …“exactly” as far as the quarks and gluons are concerned, but with electroweak physics turned off, and with all mass scales scaled up by a constant factor… … so that we can use existing Pythia software, suitably adjusted. A third v-quark Let’s add a third v-quark analogous to the s quark, but without allowing it to couple to the Z’ boson. (Charm and bottom quark physics will be a small effect; rarely produced in Pythia showering, hadronization) Then The Z’ does not decay directly to the third v-quark. Any v-hadrons containing the third v-quark cannot decay… except through annihilation to v-pions. v-pion production is almost the same as in QCD v-Kaons are stable and invisible But the v-eta is different in this model than the eta is in Pythia. In QCD the eta decays to two photons But there are no v-photons, so the v-eta decays to v-pions – which Pythia does not currently simulate Similarly there are few differences among other v-hadrons A third v-quark Some of these differences between Pythia’s simulation of QCD and a perfect simulation of the v-sector could be adjusted for by changing hadron decay settings in Pythia However these differences are rather small effects and I do not believe they will cause serious errors Since the v-sector is not going to be exactly like QCD anyway, I see attempts to refine the simulation to this level as overkill At worst there will be a few percent overestimate of the missing energy signal and a few percent underestimate of the vpion production in this particular model Variations from model to model will be much larger than this! A third v-quark So the claim is that in this model with Two light quarks coupling to the Z’, and a third quark not coupling to the Z’ Masses arranged to that all meson masses and decay constants, relative to Lv, are the same as in QCD, The use of a Pythia QCD simulation is a very good model of the showering and hadronization that would occur in this vQCD sector, except that v-pi-zeros decay not to v-photons [which don’t exist] but to standard model fermion pairs, through the Z’. v-eta’s decay incorrectly – indeed all radiative decays are incorrect [except v-pi-zero decays which are corrected for.] Some v-Kaon decays are not consistent within the model Simulating this process To simulate the full process, we would need three steps 1. 2. 3. The first is no problem. Z’ production is as always, though it depends on charges of SM particles under Z’ Decay of Z’ to v-quarks is like decay to quarks, but depends on charges of vquarks under Z’ The last is no problem. Simulate Z’ production and decay to v-quarks Simulate v-showering and v-hadronization and the formation of a final state of vpions and v-baryons Simulate decays of the v-pi-zeros and the subsequent formation of standard model b jets, tau final states; add in ISR and the UE. Since v-pions are spin-zero, decays are isotropic and are very similar to Higgs boson decays The current program uses Higgs bosons as stand-ins for v-pions [but this may change if ATLAS software requires it.] Pythia adds ISR, UE when event is generated The second step is tricky, and compromises are necessary at present. The procedure in step 2 qq Z’ QQ leaves us with a v-quark pair of invariant mass MQQ ~ mZ’ We scale down the mass MQQ by a factor L / Lv : m = MQQ L / Lv For example: if mZ’ ~ 3 TeV, Lv ~ 90 GeV, mpv ~ 45 GeV, then m ~ 10 GeV We simulate (using Pythia) the showering and hadronization of an ordinary quark-antiquark pair of invt mass m. Caution! If m lies too close to a bottomonium, charmonium, or light-quark resonance, answers will be badly distorted. No current check to prevent problems! Then scale all the particle energies by Lv /L so that the invt mass of the hadronization products is again MQQ Look in Pythia event record and grab all pions, throw away all other particles The resulting event record (in new LHA format) can be uploaded into Pythia Store pi-zeros in event record as h0 bosons [these always decay] Store pi-plus/minus in event record as H0 bosons [these are usually stable, but not in all variants of the model…] (with a simple Pythia card setting that turns off unwanted h 0 and H0 decays and sets the lifetimes of these particles equal to v-pion lifetimes.) This allows v-pion-decays/QCD-showers/QCD-hadronization to be simulated. The interesting phenomena Case 1: particle lifetimes are short Multiple moderate-to-low pT overlapping partons make an unusual looking event Mapping of jets to partons very poor Light v-pions often make a single jet Boosted b-jets overlap Not boosted only one jet is moderate pT, other soft Many b quarks, but at moderate-to-low pT and overlapping, pose a tagging challenge Case 2: particle lifetimes are long – displaced jets, displaced tau pairs and in many models, other displaced possibilities Mixture of these is possible of course Displaced jets Questions I can’t answer but would like to: Decays in beampipe – Tevatron expts would record these as b-tagged jets. Can one do better? What distinguishes them? How much background is there? Decays in inner tracker – Any hints at trigger level? Muons that miss the beampipe? Is there a better strategy? Any hints at reconstruction level? Vertices with hints of wide-angle tracks? Decays in outer tracker, calorimeter – In a scatter plot of the number of reconstructed tracks versus the hadron/em ratio in the calorimeter, late decays will be out on a tail (no tracks, normal had/em ratio). Can this be used? Study needed… Extra hits in outer tracker near jet with no tracks? [what can TRT do at ATLAS?] Since many events have multiple decays, it is important to combine these strategies be combined in a single analysis! Trigger on ISR – how efficient? Can this be used to grab a few events even when majority cannot be triggered on?