ECFA meeting on e+e- to ZH angular measurements

ECFA meeting on e+e- to ZH angular measurements
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Chris Hays
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CP violation in the Higgs section (Henning Bahl)


- CPV in HVV at loop level vs tree level in Hff via mixing

- EFTs limited to empirical CP constraints (hard to connect to matter-antimatter asymmetry)

   o Study of strong 1st order phase transitions using EFTs is limited

- Complex 2HDM is a benchmark model

   o CP-odd couplings induced via mixing with CP-odd A boson

   o Does the mixing allow CP-odd HVV couplings at tree level? No, cannot have direct AVV coupling

- EDMs provide constraints but direct searches needed to break degeneracies

   o Q: Are there tools to calculate EDMs?  Should we use EFTs?

      A: EFTs useful, need to run up to scale for neutron EDM, smaller effect for electron EDM

- Q: What about spontaneous CP violation?  A: usually happens at high temperature

- Q: What about breaking vacuum at zero temperature?  A: can occur in the dark sector


CP and entanglement in H->VV decays (J. A. Aguilar Saavedra)


- Spin density operator has 8 polarisations each for V1 and V2, 64 spin correlations

  o coefficients can be measured from charged lepton distributions

  o density operator translates into a 4D distribution dependent on spherical harmonics

    x L=1 terms have suppressed effect for ZZ

- Within narrow bins of mV* the VV pair is produced in an almost pure state

  o the problem is reduced to a binned measurement of general helicity amplitudes a00, a11, a-1-1

- When testing entanglement you could assume CP conservation (CP violation must be small in HVV)

  o then a11 = a-1-1 and there is a normalization constraint to reduce the number of parameters to one

- Entanglement measurements involving orbital angular measurements are rare and have not been done in HEP

- Spins highly entangled, orbital angular momentum less so

- H->ZZ does not look competitive with LHC due to low event yields

  o H->ZZ->lltautau may be competitive using post-decay entanglement

- CP violation can be measured using certain polarizations, can be done in semileptonic decays

  o Likely best done in H->WW

- Q: How well do you need to reconstruct the neutrinos in H-.WW->lvlv? A: Need to study

- Q: How do you explain entanglement?  A: Inseparable states and density matrices


Polarized beams for CP tests (Cheng Li)


- Need both beams to be transversely polarized to directly probe CP violation using beam polarization

  o can also probe CP-violation via triple product of final-state particles

- Define asymmetries for polarized and unpolarized beams

- Longitudinally polarized beams give slightly better constraints than transversely polarized beams

  o due to higher cross section and smaller statistical uncertainty

- Limit on cHZZ~ is O(10^-2), limit on fCP is O(10^-5)

- Q: What is the ILC polarization scenario? A: Conservatively 80% for electrons and 30% for positrons

(the latter reducing to 20% at 1 TeV).  An upgrade could give 90% for electrons and 60% for positrons.


Beam polarization at CEPC (Zhe Duan)


- Time to polarize beam longer than beam lifetime

 o motivates a polarized injector for CEPC

 o also trying to use beam self-polarization to generate polarized beams

- Depolarization in the booster can occur through spin precession and crossing through spin resonancnes

 o however CEPC has weak spin resonance strength

- Potential to achieve 50%-70% polarization for electron bunches at Z pole

 o studying positron polarization and longitudinal polarization at WW threshold

- Simulation suggests 10-30% polarization possible for ZH production

 o need to reduce the spin resonance strength by a factor of 10

- Need to study effect of beam-beam interaction

- Q: does the polarization affect beam spread? We do not see any effect

- Q: what about for an energy of 62.5 GeV?  A: should be similar to 45.6 GeV

  o would see an effect if used wigglers


Probing CP properties at the CEPC in ee->ZH->llH using optimal variables (Qiyu Sha)


- Use EFT framework and assume CP-even dim-6 Wilson coefficients are zero

- General differential cross section includes three angular coefficients that are CP-odd

- Define two observables that are the ratios of the AZ odd-to-even and ZZ-odd-to-even angular coefficients

- Perform log likelihood fit to the distributions of the observables

- constrain Wilson coefficients to |cZa| < 0.3 and |cZZ| < 0.06, as well as fCP <~ 9.2 x 10^-6

- Q: are the likelihood scans 1D? A: yes

- Q: Do you consider systematics such as cross-section uncertainty? A: no

- Q: did you compare the sensitivity of the likelihood fit to a simple asymmetry? A: no

- Comment: it would be intersting to perform a 2d fit of c~ vs c to see the impact of the CP-even operator


Updates to FCC-ee ZH CP studies using kinematic observables (Valdis Slokenbergs)


- added a correction for the interference normalization that reduces the resolution

- added Z->ee channel

- sensitivity of combined Z->ll channels is fCP ~ 4 x 10^-5 at 68% CL

  • Q: what about direct fits to Wilson coefficients?  A: first priority is to add Z->qqbar


Higgs self-coupling sensitivity at the ILC (Bryan Bliewert)


- previous study at 500 GeV gave a relative uncertainty of 26.6% on the Higgs self-coupling

- a range of ZHH analysis bottlenecks can be addressed by state-of-the-art tools

- Expect ~20% improvement from updating sensitivity projections using these tools

- Q: what about H->tau-tau? does it help reject background?  Comment: it could help by selecting tau-tau

and avoiding jet clustering

There are minutes attached to this event. Show them.
    • 2:00 PM 2:05 PM
      Introduction 5m
      Speaker: Chris Hays (University of Oxford (GB))
    • 2:10 PM 2:30 PM
      CP violation in the Higgs sector 20m
      Speaker: Henning Bahl
    • 2:35 PM 2:55 PM
      CP and entanglement in H to VV decays 20m
      Speaker: Juan Antonio Aguilar Saavedra (Consejo Superior de Investigaciones Científicas (ES))
    • 3:00 PM 3:20 PM
      Polarized beams for CP tests 20m
      Speaker: Cheng Li
    • 3:25 PM 3:45 PM
      Beam polarization at CEPC 20m
      Speaker: Duan,Zhe duanz
    • 3:50 PM 4:10 PM
      CP at CEPC 20m
      Speaker: Qiyu Sha (Chinese Academy of Sciences (CN))
    • 4:15 PM 4:35 PM
      FCC-ee ZH CP studies 20m
      Speaker: Valdis Slokenbergs (Johns Hopkins University (US))
    • 4:40 PM 5:00 PM
      Higgs self-coupling sensitivity at the ILC 20m
      Speaker: Bryan Bliewert (Deutsches Elektronen-Synchrotron (DE))