CMS and “Hidden Valleys”Phenomenology, Potential Pitfalls, and Event Generation Matthew Strassler, University of Washington
Hidden Valley Papers and Website • hep-ph/0604261 : Echoes of a hidden valley at hadron colliders.(with Kathryn Zurek) • hep-ph/0605193 : Discovering the Higgs through highly-displaced vertices.(with Kathryn Zurek) • Other relevant papers with similar phenomenology • Example mentioned in hep-ph/0511250, Naturalness and Higgs decays in the MSSM with a singlet. Chang, Fox and Weiner • hep-ph/0607204 : Reduced fine-tuning in supersymmetry with R-parity violation. Carpenter, Kaplan and Rhee • hep-ph/0607160 : Possible effects of a hidden valley on SUSY phenomenology. • Hidden Valley Website:http://www.phys.washington.edu/~strasslr/hv/hv.htm • Papers • Talks • Benchmark Models (preliminary) • Event Generator (preliminary)
What is a hidden valley, and should CMS experimentalists care? • It is almost 2007; time is running out! • There is no time to waste on theorists’ models unless they • Are consistent with existing experiments • Are well-motivated theoretically • Give signatures not currently covered by existing CMS studies • Affect CMS experiment in ways that must be dealt withNOW! • Impact HLT architecture • Impact basic reconstruction algorithms • Impact quality control filtering • I will first argue that hidden valley models satisfy the first three criteria • I will then suggest that some h.v. models (and models with related phenomenology) may satisfy the last criterion • Very preliminary ATLAS discussions suggest there are gaps in their trigger strategy that it may in some cases be possible to close
Hidden Valley Models (w/ K. Zurek) April 06 • Basic minimal structure Communicator Hidden Valley Gv with v-matter Standard Model SU(3)xSU(2)xU(1)
A Conceptual Diagram Energy Inaccessibility
Hidden Valley Models (w/ K. Zurek) • Basic minimal structure Z’, Higgs, LSP, sterile neutrinos, loops of charged particles,… Communicator Hidden Valley Gv with v-matter Standard Model SU(3)xSU(2)xU(1) Limited only by your imagination (?)…
What kind of things might happen? • Imagine you can only measure leptons, photons, and have never seen a hadron • Now turn on LEP: you discover all of QCD in one experiment… • Pions – light, some long-lived • Kaons – light, some long-lived, sometimes decay to pions • Short-lived resonances – rho, omega, phi • D and B mesons – heavy, long-lived • Quarkonium states • Nucleons – (effectively) stable • Jets of hadrons
What kind of things might happen? • This could happen to us when we turn on the LHC… • A hidden valley involves a new (mostly or all neutral) “valley sector” or “v-sector” • Many new “v-particles” (2? 5? 30?) • With range of masses (1 GeV? 10 GeV? 100 GeV? 1 TeV?) • And range of lifetimes (fs? ps? ns? ms?) • Strong dynamics and/or cascade decays often occur in valley • May lead to high- or very-high-multiplicity events, very active • Example: 14 b quarks and 2 taus, plus MET • Example: 6 light quarks, 2 b quarks, 2 muons • Variety of lifetimes for the many new particles • Implies reasonable probability of some events with long-livedparticle decays • Long-lived particles may be light, not produced at threshold, so not necessarily slow • Example: event with four jets, one jet-pair appearing at 30 cm from primary vertex • Example: event with 4 jet-pairs and a mu-pair appearing at different places in detector
What kind of things might happen? • This could happen to us when we turn on the LHC… • A hidden valley involves a new (mostly or all neutral) “valley sector” or “v-sector” • Many new “v-particles” (2? 5? 30?) • With range of masses (1 GeV? 10 GeV? 100 GeV? 1 TeV?) • And range of lifetimes (fs? ps? ns? ms?) • Strong dynamics and/or cascade decays often occur in valley • May lead to high- or very-high-multiplicity events, very active • Example: 14 b quarks and 2 taus, plus MET • Example: 6 light quarks, 2 b quarks, 2 muons • Question: how will jets reconstruction, tau/mu/e isolation work at trigger level? • Variety of lifetimes for the many new particles • Implies reasonable probability of some events with long-livedparticle decays • Long-lived particles may be light, not produced at threshold, so not necessarily slow • Example: event with four jets, one jet-pair appearing at 30 cm from primary vertex • Example: event with 4 jet-pairs and a mu-pair appearing at different places in detector • Question: “quality” of displaced objects may appear to be poor; will HLT discard the event?
Theoretically Motivated Consistent with Experiment Might not always be able to afford low-efficiency trigger strategies that rely on existing triggers Extends beyond existing studies Might cause problems for existing trigger strategies
Too many possibilities to handle? The number of possible v-sectors is huge, their phenomenology enormously variable • Can the trigger be made efficient for the majority of those which could in principle be observed at the LHC? • Clearly the only way to find out is to explore the space of possibilities intelligently. • This can be done with • a series of benchmark hidden-valley-type models chosen to systematically challenge the trigger system • event generation programs that can simulate these models • Benchmark models appropriate for stress-testing the CMS trigger system will need to be chosen • need collaboration of CMS trigger experts w/ theorists • Good News: • For some models, event generation is possible NOW! • For many other models, event generation will be possible very soon
Some examples: • Let’s consider a handful of the many possibilities • Higgs decays to two [or more] long-lived particles • Aside on classes of possible decays of new particles • Z’ decays to the v-sector: • Final state with many particles, possibly long-lived • LSP decays to the v-sector • Degradation of MET signal • Wide array of complex final states • Quick look at the signature and possible trigger issues • Apology in advance! • I am just learning now the CMS experiment’s proposed trigger strategies and hardware constraints • All of the remarks below are preliminary and some have not been vetted by CMS experts; many of them will surely turn out to be misguided • Hopefully some of them will be useful, or at least thought-provoking • Thanks to Maria Spiropulu, Greg Landsberg, Chris Tully for discussions
Higgs decays to the v-sector w/ K Zurek, May 06 b g h hv b b g v-particles b mixing • See Dermasek and Gunion 04-06 h aa bb bb, bb tt, tttt, etc. and much follow up work by many authors
Higgs decays to the v-sector Displaced vertex w/ K Zurek, May 06 b g h hv b b g v-particles b mixing Displaced vertex
A Higgs Decay to four b’s Schematic; not a simulated event!
Higgs decays to displaced vertices • Actually something like this can happen in many models • At least one already appeared in the past, though implications for discovery at Tevatron/LHC seem to have been missed • hep-ph/0511250 : Chang, Fox and Weiner • Zurek and I wrote down another class, in addition to hidden valley models • New examples recently involving R-parity-violating SUSY • hep-ph/0607204 : Carpenter, Kaplan and Rhee • This can be a discovery channel! • For light higgs Br could be 1, 10, 100 % • No Backgrounds! Easier than tau tau, gamma gamma?! • For Higgs ~ 160-180 GeV • Br could be only a few times smaller than Br(hWWdilepton) • It has no SM background, unlike h WW! • Could it even beat ZZ? Probably not; needs high trigger efficiency… • For elusive A0 (CP-odd Higgs) discovery channel even if Br is small; Br could be 1, 10, 100 % • Let’s consider a couple of classes of events
A Higgs Decay to four b’s Schematic; not a simulated event! 4 b’s few percent pass Level 1 dimuon HLT Fail mu isolation? Fail mu tracking? Fail b tagging? Fail quality control filters?
A Higgs Decay to two b’s, two taus Schematic; not a simulated event! 2 b’s 2 taus ~10% pass Level 1 dimuon Tau “volunteer”? HLT Fail mu tracking? Fail tau tracking? Fail b tagging? Fail quality control filters? t t m
A Higgs Decay to two jets, two mus (Occurs in another class of models from previous case) Schematic; not a simulated event! 2 jets, 2 ms pass Level 1 dimuon HLT Fail mu tracking? Fail b tagging? Fail quality control filters? m m
Efficiently trigger on these events? • Displaced dimuon: • Outside-in tracking from muon system might be used to detect that two muon tracks may cross somewhere far from primary vertex • Possible intersection detected? Then search for vertex in tracker. • Can something like this work for displaced taus also? • Assume one tau decays to muon… • Can one use pointing pi-zeros to estimate direction of other tau? • Displaced jet pairs are trickier: • If jet pair in HCAL then ECAL/HCAL energy ratio anomalously small • If jet pair in outer HCAL then muon hits, strange HCAL deposition • Cosmic/Punchthrough bkgds – L1 issues? HLT: may need some cuts • If jet pair is in inner tracker – write b-trigger algorithm so as not to reject! • Need to cut down on pion-nucleus collisions in solid material. • If jet pair in outer tracker…??? – • if b-trigger is activated, find no stiff pixel tracks…? Ideas developed in discussions with Andy Haas, D0; Stefano Giagu, ATLAS; Chris Tully, CMS
Higgs decay (CP-odd, 200 GeV 40 GeV) Andy Haas – DZero can trigger on soft muons from b decays. In the inner tracker DZero 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. Note pixel detector alone cannot find tracks in upper right.
Can quick/dirty tracking help? • Possible algorithm for events with displaced jets • Take leading jet and apply b-trigger • Highly displaced vertex found? Keep event. • B-vertex found? Follow b-vertex trigger path. • No tracks found? Apply b-trigger to next jet. • Highly displaced vertex found? Keep event. • B-vertex found? Follow b-vertex trigger path. • No tracks found? Check tracker is working. • If so, either keep event • or do special purpose tracking to look for highly displaced vertex in outer tracker? • look for odd pattern of hits in outer tracker? • use shape of jet in HCAL/ECAL as clue, …?
Can the whole trigger break down? • Overall event issues: • Is Primary Vertex difficult to identify? • If so, might this ruin all tracking? • Would quality of the objects or of the entire event be too low to pass HLT? • Can one trigger on events passing multiple L1 triggers paths but failing in unusual (and perhaps correlated) ways at HLT? • Last Resort Trigger: • before final rejection, check HLT decisions for multiple unusual failure modes or multiple odd-looking objects • Useful way to track common detector/trigger failure modes • Can be adjusted over time so that only uncommon failures trigger • Useful long-term strategy for allowing sensitivity to surprising physics • Thanks Greg Landsberg for discussions
Let’s think more generally • Suppose h X X: • What can X decay to? • Two body X f f [like K m n ] • Flavor-democratic • X q q, X b b, X n n, X e e, X m m, X t t • Heavy-flavor-weighted • X b b, X t t • Gluonic • X g g • Three body X f f Y [like K e n p ] • Flavor-democratic • Heavy-flavor-weighted • Gluonic • Multi-body: X Y Y Y, then Y f f [like K ppp decay] • May be worth going through possibilities systematically to ensure the trigger can handle them all with reasonable efficiency
Bigger Challenges • What if h XX (gg)(gg) • Now the L1 trigger path is not clear; too few electrons, muons, taus • Maybe only detect with recoiling jet, vector boson fusion, associated W or Z? • To lower threshold, might need algorithm (such as suggested earlier) to look for jets with displaced vertices or no tracks • What if h X X Y Y Y Y (bb)(bb)(bb)(bb) • Prompt? Chang, Fox, Weiner 05 • B’s will not make jets; soft dimuons may not even pass L1 • If decays are prompt, then • even offline, danger of losing higgs in underlying event • Almost certainly have to trigger on vector boson fusion jets, or on lepton from Wh, Zh. • If X or Y decays are displaced, then • possibility of using tracking offline; • any chance of triggering on the vertices?
Event Generation • A wide variety of scenarios in which to test out the limitations of current CMS trigger strategies. • Studies of detector response to these scenarios can be started now. • Generally trivial to modify Pythia to allow all these h, A0 decays. • Can vary masses, lifetimes, branching ratios, decay modes at will • Care needed for spin, color flow • This approach is being used for studies of Higgs displaced vertices at D0, CDF, LHCb, ATLAS • A small caution: GEANT has limitations • For example, presumably does not treat B mesons in material correctly • Might be worthwhile to develop extensive list of trigger-oriented benchmarks, associated Pythia cards
High-Multiplicity Events • Let’s consider a simple model: • The v-sector consists of a QCD-like theory • The communicator is a Z’ • An example is in the new MC package. New Z’ from U(1)’ Hidden Valley v-QCD-like theory with v-quarks and v-gluons Standard Model SU(3)xSU(2)xU(1)
q q Q Q : v-quark production v-quarks Q q Z’ q Q
q q Q Q v-gluons Q q Z’ q Q
q q Q Q q Q Z’ q Q
q q Q Q v-hadrons q Q Z’ q Q
q q Q Q v-hadrons q Q Z’ q Q
q q Q Q Some v-hadrons are stable and therefore invisible v-hadrons But some v-hadrons decay in the detector to visible particles, such as bb pairs, tau pairs, etc. q Q Z’ q Q
Production Rates for v-Hadrons Easily differs from model to model by factors of 10 Cannot afford low-efficiency trigger
Triggering: Summed-ET or HT, plus MET • Should not be a problem in this kind of model • The Z’ kicks lots of energy sidewise (big HT) • Some v-hadrons are usually invisible or metastable (big MET) 3 TeV Z’ 60 GeV v-pions MET in GeV 1000 1000 2000 Jet HT in GeV
But it could be a bit more subtle… • Even without displaced vertices, some subtleties/issues in high-multiplicity environment • Despite large number of partons, number of jets is often small • Parton merging • Parton soft hadrons • Jet merging algorithm • Jets will have strange substructure • Response of L1 and HLT jet-related triggers (including “HT” trigger) may be surprising; worth a look • Busy events with substantial activity • loss of efficiency for tau-, mu-, e-isolation requirement • Sometimes a jet contains 5 b-partons, 30 tracks • does tracking and/or vertexing lose its efficiency? • Many Soft Particles • Can one trigger on (or integrate into a trigger) a strange distribution of soft particles – a weird-looking underlying event ? • Useful for measuring tails on underlying event distribution? • Distinguish weird UE from new physics? [theory/MC/data challenge]
What if low HT/MET? • In some extreme cases both HT and observed MET (<HT) are small • If most v-hadrons invisible or have lifetimes > nsec, then fraction decaying in detector may be small • If Z’ is very weakly coupled and very light, then v-hadrons softer (but lower multiplicity expected) • Example: Suppose only 2 v-hadrons decay in detector, but displaced • Not so different from Higgs decay; may be acoplanar and/or more energetic • Solving trigger issues for Higgs decays probably covers this case…? • Example: 4 v-hadron decays, 1 in muon chamber, 2 in HCAL, 1 in outer tracker • Moderate HT, MET; • No tracks reconstructed; • Poor quality on two jets; • One strange muon chamber event • Will this event fail quality control? • Example: 3 v-hadrons decay promptly to b-quark pairs • Moderate HT, MET; 3-6 jets • Depend on (nonisolated) muons • Taus could help too • Really need b-tagging to beat backgrounds
Simulation package • These ideas can be checked using a simulation package that I have written • simple modification of Pythia, easy to understand • It simulates Z’ decays to a two-quark-flavor v-QCD model • produces standard Les Houches Accord output • or particle-level output • It can simulate a variant of the model • It will soon be able to do many more v-sector models. • working w/ S. Mrenna, P. Skands
SUSY decays to the v-sector July 06 ~ Q* q c v-(s)hadrons ~ q g Q v-(s)quarks _ Q ~ q* g 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!!!
The decaying Standard Model LSP • Note SM LSP no longer has to be a neutralino. • There are various possibilities • All decays are prompt • SM LSP decays with a displaced vertex to promptly decaying v-particles • SM LSP decays promptly to long-lived v-particles • Both SM LSP and v-particles have displaced decays • Cases similar to second possibility have been considered widely in theoretical literature, much less by expts at Tevatron and LHC • Neutralino photon + invisible widely studied • Long-lived stau LSP (stable or decaying in detector) studied • Long-lived gluino (exiting detector) studied • Neutralino Z + invisible or h + invisible (displaced jets, muons) not studied (?) • Long-lived gluon or squark (decaying in detector to displaced jets) not studied (?) • Etc. • But the other possibilities are new and might need to be looked at.
Triggering for decaying SM LSP? • Main SUSY cascade decays are unaltered until the last step • Traditional high-pT triggers aimed at SUSY will probably still work • Degradation of MET signal will hurt trigger efficiency, but probably not disastrous • Displaced vertices could be found offline, if they are present • If no displaced vertices, the problem is not triggering but background subtraction offline – • Theoretical study needed. • If SM LSP is a stau, then each event will still have two taus – a SUSY-tagging signal • Easy if the taus are promptly produced, • Only slightly more difficult if they are produced at a displaced vertex • What if no displaced vertices, very little MET? • then the large number of soft partons will have to be used somehow to separate from the SM • but this will probably be an offline issue, not a trigger issue • High-mass SUSY; low rates, few jets? • Ability to detect LSP pair-production, electroweak pair-production could be important • Displaced vertices could be essential; trigger strategies probably similar to Higgs case • Extra leptons and MET obviously would help trigger • Without displaced vertices – very challenging to remove SM background. • Theoretical study needed.
Simulation package • These ideas can be checked using a simulation package that I have written • simple modification of Pythia, easy to understand • It simulates Z’ decays to a two-quark-flavor v-QCD model • produces standard Les Houches Accord output • or particle-level output • It can simulate a variant of the model • It will soon be able to do many more v-sector models. • working w/ S. Mrenna, P. Skands • It will soon be able to do SUSY with LSP decays • to the two-flavor v-QCD model, • and eventually to other v-sector models.
Summary and Outlook • Considered candidate scenarios with hidden-valley or hidden-valley-like physics • Focused on trigger, especially displaced vertices, where tracking and other algorithms could fail • Discussed three examples • Higgs decays to displaced vertices [moderate rate, low pT] • Z’ decays to high multiplicity events, possibly displaced vertices [low rate, high pT] • LSP decays to moderate multiplicity, possibly displaced vertices [high rate, moderate pT] • Many possible Higgs decays can be quite challenging for the trigger; • Perhaps useful to explore systematically • The Z’ and LSP decays are probably relatively easy to trigger on – • High multiplicity environment could cause surprising behavior in L1 and HLT • If low HT/MET and displaced vertices, problems may be similar to those of Higgs decays • If low HT/MET and decays are prompt, an interesting challenge is to somehow use the moderate-to-soft partons – needs much more study – may not be possible • Of these scenarios, • The Higgs can be simulated in depth now • The Z’ can be simulated in some examples now, more soon • The LSP decay scenario will be simulable soon • Other scenarios will become available later