The Higgs boson mass and σ(ZH) from the recoil mass with leptonic Z decays

Abstract

A precision on mH as small as 5 MeV can be achieved from a fit to the distribution of the mass recoiling to a leptonically-decaying Z boson (Z → e+e− or μ+μ−) in the e+e− → ZH process at √s = 240 GeV. The requirements on the detector design (electron energy and muon momentum resolution, in particular) to achieve this statistical precision will be checked with this channel in the FCC-ee context. The feasibility of a calibration of the method – to reduce systematic effects due to, e.g., momentum scale determination and stability – will be ascertained with the e+e− → ZZ → l+l−X process. The need for calibration data at the Z pole will also be estimated (frequency, number of events).

See also the determination of the centre-of-mass energy √s with a precision of O(1 MeV) which is a key ingredient for the determination of the recoil mass.

• The corresponding Snomass LOI

Contributors

• Clement Helsens (CERN)
• Ang Li, Gregorio Bernardi (APC)
• Sylvie Braibant, Valentina Diolaiti, Paolo Giacomelli (Bologna), Giacomo Ortona (Torino)
• Markus Klute, Jan Eysermans, Tianyu Justin Yang (MIT)

Bibliography

• Precision Higgs physics at the CEPC Fenfen An et al 2019 Chinese Phys. C 43 043002
• Measurement of the Higgs boson mass and ee → ZH cross section using Z→μμ and Z→ee at the ILC J. Yan et al, Phys Rev D94 (2016) 113002
• EPJ+ essay, Measuring the mass and production cross section with ultimate precision at FCC-ee, P. Azzurri et al, June 2021

List of Monte-Carlo samples (spring2021)

See the compilation from Jan at this googledoc

Recommended nominal samples in the spring2021 campaign :

• ZH signal: Use the Whizard samples (proper description of the BES), wzp6_ee_XXH_ecm240 with XX = mumu, tautau, ee, nunu, or qq.
• WW background:
  • mumu analysis: better use p8_ee_WW_mumu_ecm240, which includes W -> mu and W -> tau -> mu
  • otherwise, the lower statistics inclusive sample p8_ee_WW_ecm240
• ZZ background : p8_ee_ZZ_ecm240
• Fermion pair production:
  • Dileptons: wzp6_ee_mumu_ecm240, wzp6_ee_tautau_ecm240 and wzp6_ee_ee_Mee_30_150_ecm240. The mumu and tautau samples correspond to ee -> Z/gamma* -> mumu or tautau. The third sample includes the s- and the t-channel (important !), and was generated within 30 < Mee < 150 GeV and 15 deg < theta < 165 deg. The statistics corresponds to O(2x) that expected in 5 ab-1.
  • Hadrons: p8_ee_Zqq_ecm240, corresponds to ee -> Z / gamma* -> qq , all quark flavors.
• Other small backgrounds generated in the mumu channel :
  • “single Z”, i.e. ee -> e(e)Z with Z -> mumu: use both wzp6_egamma_eZ_Zmumu_ecm240 and wzp6_gammae_eZ_Zmumu_ecm240
  • gamma gamma background: wzp6_gaga_mumu_60_ecm240 and wzp6_gaga_tautau_60_ecm240, generated within M(mumu) or M(tautau) > 60 GeV

Results obtained for the FCC week, July 2021

See Jan’s talk

Early version of the analysis (December 2020)

• Clement Helsens: Developer, Sample production, Preliminary FCCAnalyses design, Preliminary recoil fit analysis

To run the analysis, should use the corresponding FCCAnalyses configurations located in FCCAnalyses-config/mumu and FCCAnalyses-config/ee. This preliminary analysis i using the input files produced in the edm4hep data format, and produced with this sample production. FlatNtuples and histograms will be produced and used to fit the recoil mass. FCCAnalyses output files can also be directly accessed on eos /eos/experiment/fcc/ee/analyses/case-studies/higgs/mH-recoil/FlatNtuples/.

arguments and examples to run the fitting macro could be seen by running

python utils/massFit.py
usage:   python massFit.py BASEDIR HISTONAME SELECTION BINLOW=120 BINHIGH=140
example: python massFit.py /eos/experiment/fcc/ee/analyses/case-studies/higgs/mH-recoil/FlatNtuples/ZH_Zee/ leptonic_recoil_m_zoom3 sel1
example: python massFit.py /eos/experiment/fcc/ee/analyses/case-studies/higgs/mH-recoil/FlatNtuples/ZH_Zmumu/ leptonic_recoil_m_zoom4 sel0 122 128

As an example the fit result of running the following command

python utils/massFit.py /eos/experiment/fcc/ee/analyses/case-studies/higgs/mH-recoil/FlatNtuples/ZH_Zee/ leptonic_recoil_m_zoom4 sel1 123 127:

is shown below (to be re-done with including beam energy spread)

  EXT PARAMETER                                INTERNAL      INTERNAL  
  NO.   NAME      VALUE            ERROR       STEP SIZE       VALUE   
   1  alpha       -8.67272e-01   4.57686e-02   3.90224e-02   7.11703e-01
   2  cbmean       1.25094e+02   6.79656e-03   3.51532e-03   1.87562e-02
   3  cbsigma      2.81768e-01   6.89490e-03   5.74469e-02  -6.08122e-02
   4  lam         -3.43241e-06   1.01039e-02   3.40174e-01   1.56709e+00
   5  n            1.27209e+00   1.72834e-01   2.30740e-02  -1.34474e+00
   6  nbkg         7.59189e+03   2.15350e+02   1.34305e-03  -1.51568e+00
   7  nsig         9.05996e+03   2.19379e+02   1.28389e-03  -1.51059e+00