CMS and “Hidden Valleys” Phenomenology, Potential Pitfalls, and Event Generation Matthew Strassler, University of Washington.
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CMS and “Hidden Valleys” Phenomenology, Potential Pitfalls, and Event Generation Matthew Strassler, University of Washington 1 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) 2 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 with NOW! 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 3 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 4 Energy A Conceptual Diagram 5 Inaccessibility Hidden Valley Models (w/ K. Zurek) Basic minimal structure Z’, Higgs, LSP, sterile neutrinos, loops of charged particles,… Communicator Standard Model SU(3)xSU(2)xU(1) Hidden Valley Gv with v-matter Limited only by your imagination (?)… 6 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 7 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” Strong dynamics and/or cascade decays often occur in valley 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?) 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-lived particle 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 8 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” Strong dynamics and/or cascade decays often occur in valley 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?) 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-lived particle 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? 9 MOTIVATION SUSY Extra Dims Little Higgs Hidden Valley Solution to Hierarchy Problem Yes Warped, yes No, merely delays Not usually, but can coexist with any known solution also twin Higgs, folded SUSY Common within String Theory? Yes Yes(?) No? Yes Contraints from Precision EW? Weak Strong Strong Very Weak Constraints from FCNCs? Strong Strong Strong Very Weak Yes Some models Yes Yes Masses on new exotic particles? > 50-100 GeV 100s GeV 100s GeV > few GeV Cross-Sections Typically large (new colored particles) UED: large, Warped: small Moderate Typically small (no new colored particles); depends on production mechanism Phenomenology Diverse: high pT jets, MET, leptons, stable or unstable LSP Diverse; UED similar to SUSY; MET; warped gives electroweak resonances Diverse, similar to SUSY, or electroweak resonances; tops Extremely Diverse, can be very different from SUSY; typically no production resonances; and can change the 10 signatures of SUSY etc. Can Provide Dark Matter Candidate? MOTIVATION SUSY Extra Dims Little Higgs Hidden Valley Solution to Hierarchy Problem Yes Warped, yes No, merely delays Not usually, but can coexist with any known solution Theoretically Motivated No? Common within String Theory? Yes Yes(?) Contraints from Precision EW? Weak Strong Constraints from FCNCs? Strong Strong Yes Some models Can Provide Dark Matter Candidate? Masses on new exotic particles? Cross-Sections Phenomenology > 50-100 GeV Strong Consistent with Experiment Strong Yes 100s GeV 100s GeV Might not always be able to afford low-efficiency trigger strategies Typically large UED: Moderate that relylarge, on existing triggers (new colored Warped: small particles) Extends beyond Diverse: Diverse; existingDiverse, studies similar to high pT jets, UED similar to SUSY, or MET, leptons, SUSY; MET; electroweak stable or Might cause for warped gives problems resonances; unstable LSP existing trigger strategies electroweak tops resonances also twin Higgs, folded SUSY Yes Very Weak Very Weak Yes > few GeV Typically small (no new colored particles); depends on production mechanism Extremely Diverse, can be very different from SUSY; typically no production resonances; and can change the 11 signatures of SUSY etc. 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 Benchmark models appropriate for stress-testing the CMS trigger system will need to be chosen a series of benchmark hidden-valley-type models chosen to systematically challenge the trigger system event generation programs that can simulate these models 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 12 Some examples: Let’s consider a handful of the many possibilities Higgs decays to two [or more] long-lived particles Z’ decays to the v-sector: Aside on classes of possible decays of new particles 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 13 Higgs decays to the v-sector w/ K Zurek, May 06 b g h hv b b g v-particles b mixing Dermasek and Gunion 04-06 h aa bb bb, bb tt, tt tt, etc. and much follow up work by many authors See 14 Higgs decays to the v-sector Displaced vertex g w/ K Zurek, May 06 b h hv b b g v-particles mixing b Displaced vertex 15 A Higgs Decay to four b’s Schematic; not a simulated event! 16 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 % For Higgs ~ 160-180 GeV No Backgrounds! Easier than tau tau, gamma gamma?! 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 17 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? 18 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 tm 19 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 20 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? Ideas developed in discussions with Andy Haas, D0; Stefano Giagu, ATLAS; Chris Tully, CMS 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 If jet pair is in inner tracker – write b-trigger algorithm so as not to reject! Cosmic/Punchthrough bkgds – L1 issues? HLT: may need some cuts 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…? 21 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. 22 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, …? 23 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 24 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 Heavy-flavor-weighted Xgg Three body X f f Y X b b, X t t Gluonic X q q, X b b, X n n, X e e, X m m, X t t [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 25 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) B’s will not make jets; soft dimuons may not even pass L1 If decays are prompt, then Prompt? Chang, Fox, Weiner 05 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? 26 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 27 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)’ Standard Model SU(3)xSU(2)xU(1) Hidden Valley v-QCD-like theory with v-quarks and v-gluons 28 q q Q Q : v-quark production v-quarks q q Q Z’ Q 29 qqQQ v-gluons q q Q Z’ Q 30 qqQQ q q Z’ Q Q 31 qqQQ q Z’ q v-hadrons Q Q 32 qqQQ v-hadrons q q Z’ Q Q 33 qqQQ Some v-hadrons are stable and therefore invisible v-hadrons q q Z’ Q Q But some vhadrons decay in the detector to visible particles, such as bb pairs, tau pairs, etc. 34 Production Rates for v-Hadrons Easily differs from model to model by factors of 10 Cannot afford low-efficiency trigger 35 Triggering: Summed-ET or HT, plus MET 3 TeV Z’ 60 GeV v-pions MET in GeV Should not be a 1000 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) 1000 2000 Jet HT in GeV 36 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 Jets will have strange substructure loss of efficiency for tau-, mu-, e-isolation requirement Sometimes a jet contains 5 b-partons, 30 tracks Response of L1 and HLT jet-related triggers (including “HT” trigger) may be surprising; worth a look Busy events with substantial activity Parton merging Parton soft hadrons Jet merging algorithm 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] 37 What if low HT/MET? In some extreme cases both HT and observed MET (<HT) are small Example: Suppose only 2 v-hadrons decay in detector, but displaced 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) 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 38 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 39 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!!! 40 The decaying Standard Model LSP Note SM LSP no longer has to be a neutralino. There are various possibilities Cases similar to second possibility have been considered widely in theoretical literature, much less by expts at Tevatron and LHC 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 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. 41 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 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? 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. 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. 42 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. 43 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 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 – 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] 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 moderateto-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 44