HidSec benchmark points

From Wiki Les Houches 09

Jump to: navigation, search

Hidden Sector Benchmarks

[References need to be added]

The group investigating hidden sector benchmarks focused on hidden valley models in general (with considerable discussion of dark-matter motivated models with light particles [$<$ few GeV] that have a large branching fraction to muons and/or electrons, often leading to lepton-jets) and on quirks in certain regimes (including dark-matter-motivated examples.)

There were two types of benchmarks that were discussed actively.

1) Benchmarks to challenge the trigger and reconstruction systems at the LHC experiments.

The key challenges of which we are aware involve one or more of the following:

a) Long-lived particles decaying in flight (which pose many different
types of challenges)
b) Highly boosted light particles
c) Unusual clustering of particles
d) Ultra-soft signatures (involving many standard model particles
below a few GeV).
e) Tracking challenges (which we did not discuss actively).

M. Strassler has been coordinating with F. Moortgat and R. Brunliere, among others, to generate various data sets and ensure they will run through ATLAS and CMS software. These data sets involve problematic regimes of many of the models listed below, as well as others. (The software used is not fully validated as far as total cross-sections and branching fractions as a function of model parameters, and is not appropriate for analysis-level studies, but should be sufficient for stressing the systems.) A web page with the data sets will be established, along with an explanation of the models and their phenomenology. The specific models will be adjusted, replaced or expanded as issues emerge, or do not emerge, with triggering, reconstruction, and data storage.

2) Benchmarks to serve as springboards for experimental analysis during the early running (a few hundred inverse pb of data.)

The signatures from hidden sectors for which models seem especially appropriate for early running are

a) >2 lepton or photon signatures.
b) dilepton or diphoton resonances
c) dilepton or diphoton edges/endpoints
d) multiple taus? to show up in dilepton or lepton + tau?
e) multiple b's?
...

We aimed to produce models that have a significant possibility of producing such signatures.

I) Higgs decays to hidden sectors typically involve 4 or more SM partons in the final state. Simple examples include H decays to two pseudoscalars (giving heavy flavors or lepton-jets), to two vectors (giving pairs of lepton pairs), or two fermions (giving four-leptons+MET final states). Decays to multiple lepton-pairs or to photon pairs are possible. The decay products may be long-lived, etc.

Questions:

a) which final states could potentially be observed in early running? An example would be a 4-photon or 4-lepton final state for a light Higgs boson.

b) for these cases, is a benchmark model needed? or is the process so simply simulated, with so few parameters, that one can express limits as functions of obvious parameters (such as the Higgs mass, the mass of the new particles to which it decays, the branching fraction times cross-section, and possibly the lifetime of the particles if appropriate.)

P. Fox will try to determine the answer to these questions and make specific recommendations as to how to simulate models of this type. The leptophobic Higgs model of Dobrescu, Landsberg and Matchev may deserve special discussion.

II) Supersymmetric models with a hidden sector to which the LSP can decay. Important features of this scenario include the possible reduction of MET (which would make traditional SUSY searches less effective), high rates for production of unusual objects such as long-lived or clustered particles, and/or production of unusual objects in association with hard jets, leptons, or large MET that may make background-reduction easier

It is recommended that benchmark models of this type are recommended involve attaching a new hidden sector to an existing SUSY benchmark model.

The hidden sectors for which benchmarks are needed include

a) A broken abelian hidden sector, which easily produces an isolated

dilepton resonance (though the leptons may not be isolated from each other.) D. Morrissey will coordinate the process of bringing together experts on this model, which include I. Yavin, S. Thomas, D. Shih, L.-T. Wang, and others, to ensure that a version of this model is encoded correctly in existing software. Discussion as to how to select one or two benchmark points (for example, one with the dark photon mass at 0.5 GeV, one at 15 GeV) will have to ensue. Long-lifetime benchmark cases may also be necessary.

b) A broken non-abelian hidden sector, which may naturally produce

clusters of new particles, and thus dilepton resonances which are not isolated from each other and/or from other particles. This requires a parton shower in the hidden sector with massive gauge bosons. (A hidden parton shower has been implemented in HERWIG and SHERPA, by P. Richardson and S. Schumann, during this meeting.) This model has been studied by P. Meade, M. Pappucci and T. Volansky, who have been in contact with S. Schumann; it is natural to continue this discussion to bring the simulation software to an acceptable form and to allow selection of a benchmark model. The criteria may be similar to the previous case.

c) Confining nonabelian hidden sectors: we focus on theories with

matter in the fundamental representation of the gauge group. For SU(N) there are three critical cases

 i) Nf > 1; this is the QCD-like case.  All hidden mesons decay

rapidly to hidden pions, some of which may decay to SM particles. Pythia 6-based software to generate hidden sector hadronization exists, but is not fully validated; at this workshop, F. Krauss and S. Schumann discussed implementing hidden sector hadronization in SHERPA, while P. Richardson is implementing this in HERWIG.

 ii) Nf = 1; the same software will work for this case, but the hadron

spectrum is very different, because there are no light pseudoscalars if the number of colors is small. This makes many hadrons stable against hadron-to-hadron decays. J. Wacker is working to ensure this case can be simulated There are three key steps. The hidden hadron spectrum must be estimated; no lattice data is available. The most likely hadron-to-hadron decays much be encoded in a decay table. Finally, the most likely hadron-to-SM decays will have to be computed. The last step requires additional theoretical work.

 iii) Nf = 0; this case has very different hadronization, many stable

glueballs, and a variety of final states. The decays of the hidden glueballs to the SM has been estimated by M. Strassler and two students in a particular theory, and other theories are under consideration. However, the decays of the LSP to this type of sector have not been carefully studied. Since the glueballs must be fairly heavy (>50 GeV?) for their lifetimes to be short enough for LHC observation, and the LSP is probably below a few hundred GeV, a dynamically-detailed simulation of glueball formation may not be necessary; it may be enough to make a model for LSP decays to a given set of hidden glueballs (plus the hidden LSP), using phase-space decays.

b') Abelian sector with clustered dark photons: Another model that

produces non-isolated lepton pairs involves the decay of light hidden psuedoscalars into lighter dark photon pairs. Whether an analysis benchmark is needed for this model is not clear, but it is useful for current applications, including trigger/reco challenges and near-term studies of clustered objects, such as lepton-jets. It might also be qualitatively different from case b), but this requires study. M. Strassler currently can simulate this model; other more reliable simulation tools will soon be available, see below.


Note that in all these cases we have assumed here that supersymmetry is significantly broken in the hidden sector. If it is very weakly broken, then some modification of the simulation approach might be needed.


III) Quirk models: Many models have heavy particles which is charged under both the SM and under the hidden sector. Such particles can be produced in pairs in the usual way. If the hidden sector has no light matter in the fundamental representation, then the heavy particles may be strongly confined by flux tubes. Such particles are often called "quirks". When produced, these particles are permanently joined together by a flux tube with a tension of order the confinement scale. They typically will gradually lose energy, and when they reach or approach their ground state, they will annihilate into SM or hidden-sector particles

Quirks may exhibit a number of unusual phenomena, depending on the hidden confinement scale:

a) Very low scales: unique tracking signatures (not considered here);
b) Low scales: a displaced vertex at the point of annihilation;
c) Moderate scales: if electrically charged, a large amount of
ultra-soft photon emission; if colored, a large number of ultra-soft
pions;
d) Large scales: production of hidden glueballs that decay within the detector,
possibly in large numbers and possibly with displaced vertices.

G. Kribs and C. Henderson will consider case c), and understand whether this case can now be done fully in Pythia 6 or 8, as well as whether there are interesting trigger opportunities involving events with exceptional distributions or amounts of electromagnetic energy deposition.

For case (d), M. Strassler has proposed a method that could be implemented in any generator by treating the tower of quirkonium states as a large number of new particles with various cascade decays. We did not consider cases (a) or (b).


IV) Z' models: We did not discuss these sufficiently yet, but clearly there is a need for a reasonable model of a relatively light Z', accessible at 8 to 10 TeV energies with 200 inverse pb, that has a substantial branching fraction into a hidden sector. Such decays may produce high-energy, high-multiplicity events, possibly with large MET, possibly with dilepton pairs, many b's, or many jets and photons. Any of the hidden sectors discussed above would potentially be suitable for a benchmark model.


Experimental commitments:

CMS: C. Shepherd-Themistocleous will focus on lepton-jet-type studies; various types of software are available. I. Tomalin and Y. Gershtein are pursuing complementary approaches to searches for long-lived particles. F. Moortgat is involved in the trigger/reconstruction challenge.

ATLAS: R. Brunliere is involved with the trigger/reconstruction challenge.

We do not have anyone directly involved with searches for hidden sectors using more standard objects (e.g., searches for resonances in isolated lepton pairs, searches for three-photon events, etc.)


Things that were not discussed that deserve further thought: Benchmarks for invisible hidden sectors, if discovery is ever possible in early data. Macroscopic quirks, monopoles and R-hadrons require overly specialized analysis lying outside our conversation.

Personal tools