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v3.3.3-pre
Short summary of decisions taken so far on monojet models - A. Boveia, S. Mrenna, S. Malik, S. Lowette, C. Doglioni
DM type:
Mediator types, s-channel
Mediator types, t-channel
Couplings to SM and DM
Width
Other outstanding questions:
Differences/overlap with SUSY models: the coupling between squark and WIMP needs to be a free parameter --> need dedicated samples incorporating diagrams important for large, but could use existing samples for small couplings.
Vector, s-channel, spin-2 mediator: there is the option include a spin-2 mediator in the list of models (eg RS graviton), leading to potentially new analysis techniques given the distinctive angular distributions [hep-ph:0211205] Follow-up point: Start on implementation may already exist, to be followed up with interested theorists if sufficient time and interest.
How to proceed:
Discussion on monojet-like models and widths - S. Belyaev, V. Khoze, A. Boveia, P. Pani
Consistency with gauge invariance
Q Should we consider a model only if consistent with gauge invariance, meaning that there is a mechanism for the mediators to acquire mass? Should we be concerned on what that mechanism is?
A: General consensus: no, as long as we are sure that the simplified model provides a consistent parameterizes the full theory and we have an idea of the parameter space we are choosing when setting a given mass. We should be also aware of other possible states due to specific mass generation mechanisms (e.g. EW model with Higgs mixing). This can be followed up on the mailing list.
Parameter scan
Q: The usual dimensionality of a scan is 4 or 5: Two coupling constants, mass of mediator, mass of DM, mediator width. What choices do we make to reduce it?
A: we could use e.g. gDM=gSM for non-s-channel models (it might be important to avoid constraints from direct mediator searches)
Width and kinematics
Q: How to scan the width?
A: Both absolute width and width/mass are important parameters. We can first compute the minimal width based on the couplings, and then scale that by a multiplicative factor. We shouldn't get to the point where the width becomes greater than the mass of the mediator.
The width is important in the case of scalar and pseudoscalar mediators: they look like a Higgs, and if it is increased by 2 or 3x (eg to allow the possibility of other decays from the dark sector) the model is still consistent with perturbation theory, and the kinematic changes quite dramatically. We could have a minimal width, an intermediate width and MMed ~ width. Possible concrete proposal: minimal width, width/M = 0.3 or 0.5, MMed~width?. In the choice of grid points for the widths, we should also consider that the sensitivity depends on the width. Follow-up point: different groups have different results on kinematics when changing the width.
Scan strategy
Once we have a set of points and a decision on the grid, we should generate the models, shower them and look at the kinematic distributions. This will allow us to potentially reduce even more the set of signal samples experiments have to produce in full simulation. Follow-up point: there are a few volunteers (ATLAS Monojet analysers, SB and collaborators already doing this for a 2-higgs doublet model)
Repositories
1. SVN repository at CERN - stable, CERN-managed location, preferred by CERN collaborations.
2. HEPMDB (High Energy Physics Model Database) is also available at Southampton, and interfaces model files to a cluster.
Monojet-like models, based on (updated) http://arxiv.org/abs/1411.0535 - V. Khoze
Update to paper: third jet veto fixed, no visible difference in plots.
Study details:
S/P mediators: dominant production by gg through top loop, explicitly calculated and implemented/mostly publicly available in MCFM. The authors will make sure that everything is available to the experiments on a short timescale. Follow-up point: the P might not be there, to be checked with MCFM authors.
Mediator masses: left as free parameters.
DM/SM couplings: generally following literature. Follow-up point: is the scalar the same as Matt Buckley's model, beyond the DM Yukawas?
S/P mediators: Higgs-like for both SM and DM couplings (due to MCFM implementation), the DM coupling is proportional to the Yukawa coupling by a factor (which could be 1, and be Higgs-like - not desirable, as pointed out in http://arxiv.org/abs/1409.2893). This can however be reparameterized to obtain a total coupling value that makes these plots comparable across collaborations.
The model has 4-5 parameters, and the phase space can be reduced if the signal is proportional to g_DM * g_SM. With this assumption, 8 and 14 TeV LHC results are simulated, and example limits are obtained.
Results:
V and A mediators show a difference depending on the minimal width (values of 2x/5x/10x are used) that is not just a uniform x-sec rescaling. The difference is noticeable when superimposing non-collider bounds. The lower the minimal width the greater the signal: this is the maximal reach of the collider, but it's important to investigate the larger-width region too. Caveats apply to collider/non-collider comparisons: at colliders, we don't necessarily need to be seeing cosmologically stable DM, it might be that we are seeing decays of the mediator into heavier states of the dark sector instead. So it's important to study models with larger widths.
A mu factor is used to indicate how much the cross section should be multiplied by for an observation at the LHC (if mu>1 means the signal needs to be enhanced).
For V and A mu < 1 (observation is possible). For V the LHC is not competitive with Direct Detection (DD), but for A the LHC has a greater reach.
For S and P mediators, mu > 1, so with present sensitivity one would need to increase the signal or the couplings. If mu=100, for S the LHC complements and goes beyond the DD limits, while for P mediators colliders fare better than ID. How to achieve a factor of mu = 100? We could rescale the choice of the couplings by DM by a factor of 10 and reduce the DM mass by a factor of 10, which still means not leaving the perturbative regime for low-mass DM (the coupling parameterization for this paper is g_DM*m_DM/v). We should also be careful not to go beyond width ~ mass, but this is not a problem for narrow S and P. However we could also avoid such a large signal enhancement by increasing our search sensitivity - which may happen in future searches.
Q&A:
Q (S. Belyaev): Is there a coupling of Z boson to DM?
A (V. Khoze): no, we could however extend S and P with a Higgs that couples to DM in addition to the mediators. This is a more complex model, with 5 parameters, and it can be investigated as a next step.
Q (S. Belyaev): Can we make more realistic scenarios? Having a Higgs there is a step forward.
A (A. Boveia): The aim of this Forum is to provide simple building blocks rather than more realistic but more complicated scenarios. We can write up about how to go beyond the simple choices - it would be great to have volunteers for this. To be followed up: both S. Belyaev and V. Khoze have the 2-scalar simplified model [CD: the pdf has already been sent, and the comment was that this is one of Linda's models]
Q (Zhiqing Zhang): Comparing 8 and 14 TeV limits (Slide 7), the limit at high mass is better at high energy, but it gets worse at low energy. Do we expect this?
A (V. Khoze): we should follow this up offline, with P. Harris as well.
Q (Steve Mrenna): What propagator was used? Do you have a running, energy-dependent propagator? An s-hat dependent width?
A: (V. Khoze): this is just a standard proagator with the mass and the width. \( p^2 - m^2 + i M \Gamma \), where \( \Gamma \) is the minimal width calculated as a function of the mass of the mediator, not a function of the transverse momentum. It is not a running parameter.
Q (Steve Mrenna): would it be possible to check whether reweighting the minimal width sample to a larger width works?
A: This can be checked, but we'd have to reweight event by event as it's kinematic-dependent Follow-up point: ask whether this is possible, include Phil Harris and S. Belyaev in the thread.
ATLAS Monojet contribution: discussion on ongoing work - D. Salek
Tools:
Powheg, discussions with Felix Kahlhoefer and Emanuele Re.
Goal:
1) validate and propose choice for mediator width.
2) find whether VV/VA/AV/AA couplings are distinguishable for vector mediator
Plan:
Choice of minimal widths:
Scan mDM/mMed to cover full phase space (paying attention to resonant/non-resonant region: mMed vs 2*mdM), allow comparison to contact interaction.
Present ratio plots for quantitative comparison.
Q&A/open questions:
S and P would need the full loop calculation, and it would be good to compare with the EFT vertex.
It is important to keep the changes in the overall rate (coming from changing the couplings) separate from the changes in the kinematics (from changing the width). However, couplings and (minimal/maximal) width are connected, so the scans in couplings and width need to be linked, to avoid both duplications and unphysical regions.
Monophoton models - B. Gomber, M-H. Genest, L. Carpenter, U. Haisch
Models considered:
parameters: k1, k2 (control coupling to EW bosons), mDM and weak mixing angle
Plan: agree on grid parameters, scan models for various values, checking the kinematics.
First results shown: comparisons agree on cross-sections with inclusive samples, apply analysis selection at a later stage.
Q&A/Open questions:
Where is a monophoton analysis most sensitive for Run-2? Would the analysis strategy differ in different regions of parameter space where one or the other model could be more important?
We should keep the monophoton modes synchronized with the mono-W/Z/H.
Meeting summary - A. Boveia, S. Mrenna, S. Malik, S. Lowette, C. Doglioni
The short term goalis to converge on the list of simplified models. For this reason, we invite the collaborators to contact us organizers in order to commit to SVN the model implementation, as that's the starting point for all other detailed studies.
The choices and discussions done so far will be documented in the write-up. The organizers will prepare a skeleton and some initial text to be circulated to the mailing list.
The next meeting will be dedicated to converging on a list of EW models.