− −15 −10 −5 0 5 10 15 20 λ κ 2 − 10 1 − 10 1 HH) [pb] → (pp ggF σ SM HH white-paper Exp. 95% CL limits CMS (arXiv:1811.09689) ATLAS (arXiv:1906.02025) One LHC exp. Run2 Two LHC exp. Run2 Two LHC exp. Run3 Theory prediction
Figure 7.11: Expected limit on the p p → H H cross section as a function of κλfor one of the two LHC experiments and their combination estimated for the Run 2 integrated luminosity (140fb−1) and the Run 2 and 3 (300fb−1).
In the future, more final states will be combined. The orthogonality between the various searches must be carefully ensured, especially when combining similar final states, such as b ¯bW W∗, b ¯bZ Z∗
and b ¯bτ+τ−. They can all result in a signature with two b-jets, two leptons and missing transverse energy.
A potential combination of ATLAS and CMS based on the full Run 2 dataset will be consid- ered, which increases the sensitivity significantly compared to single-experiment results as shown in Fig.7.3. The treatment of systematic uncertainties and correlations between ATLAS and CMS needs to be carefully studied. A useful reference is the ATLAS and CMS Higgs coupling combina- tion [177] and the procedure for this combination outlined in [521] by the LHC Higgs Combination Group. Theoretical uncertainties on the signal process should be correlated, likewise simulation- based background uncertainties may be correlated, while the data-driven background uncertain- ties should not. A harmonised treatment of the signal process in terms of MC generators, theoretical uncertainties, as well as the same mass points, will facilitate such a combination.
By combining the ATLAS and CMS data, the expected upper limit on the non-resonant cross section should reach down to 3.5 times the SM prediction at the end of Run 2 (140fb−1), and to 2.4 times the SM at the end of Run 3 (300fb−1). If instead the impact of the systematic uncertainties will not change, the sensitivity at the end of Run 3 would reach about 5 times the SM expectation, and it would be completely systematic dominated. Concerningκλ, Fig.7.11shows the expected limit on the p p → H H cross section as a function of κλusing Run 2 and Run 3 extrapolations assuming negligible systematic errors. Without important improvements to the analyses but assuming it will be possible to reduce the impact of the systematic errors,κλ is expected to be constrained in the interval −1.2 < κλ< 7.5 at 95% CL at the end of Run 3.
7.6 Constraints onκ
λfrom single Higgs boson measurements
B. Di Micco, S. Manzoni, C. Vernieri
In addition to the direct determination of the Higgs self-coupling through the study of Higgs bo- son pair production, an indirect measurement is also possible exploring the NLO electroweak (EW) corrections to single Higgs measurements, as discussed in detail in Sec.2.3.2. The first experimen-
Measurement Reference L
t ¯tH (H → b ¯b and multileptons final states) [522,523] 36.1 . fb−1
H → γγ (including t ¯tH) [442,464,524] 79.8fb−1
H → Z Z∗(including t ¯tH ) [525,526] 79.8 fb−1
VH H → b ¯b [399,527] 79.8 fb−1
H → W W∗ [528] 36.1 fb−1
H → τ+τ− [529] 36.1 fb−1
Table 7.4: List of the ATLAS measurements of Higgs production and decay modes, combined to derive a constraint on the value of the Higgs boson self-coupling,κλ. The measurements used in this analysis are based on data collected at 13 TeV corresponding to an integrated luminosity of up to 79.8fb−1.
tal constraint onκλ from single Higgs measurements has been determined by the ATLAS experi- ment [134], by fitting data from single Higgs boson analyses taking into account the NLOκλdepen- dence of the cross section and the branching fractions of the ggF, VBF, W H , Z H and t ¯tH production modes and theγγ, W W , Z Z , ττ and b ¯b decay modes, as listed in Table7.4. Differential information has also been exploited through the use of the Simplified Template Cross Section (STXS) categories (described in Sec.6.4).
Each analysis separates the measured events into orthogonal kinematic and topological cate- gories depending on the reconstructed final state. These categories partially account for the kine- matic dependence and they have been optimised to maximise the sensitivity to their associated truth-level region. Although the gluon-gluon fusion production mode is subdivided in bins of jet multiplicity and transverse momentum of the Higgs boson, pTH, differential corrections are not yet available4and therefore the corresponding STXS bins related share the same parameterisation as for the inclusive ggF production. Nevertheless, such contributions have been evaluated in the Heavy Top Quark expansion [176], that is valid for pHT << mt, i.e. pTH < 150 GeV (see Sec. 2.3.2) and result to be small. The t ¯tH production mode is considered inclusively in one single bin, as no differential measurement is available yet. The g g → Z H cross section is not parameterised as a function ofκλ, because the theoretical computation is still missing and it should contribute mostly in high pTHregions where the sensitivity toκλis expected to be small.
The values of the kinematic dependent C1linear coefficients, Eq.(2.34), that parameterise the sensitivity of the measurement toκλhave been derived for each STXS region defined in the mea- surement. The values obtained are reported in Table7.5.
The NLO EW K -factors , that includes one loop EWK correction not involvingκλ, are com- puted inclusively for each production mode and not in STXS bins, as in the regions of phase space where these corrections are most significant (typically for high Higgs boson transverse momen- tum), the sensitivity to the Higgs boson trilinear coupling is minimal [131]. The selection efficien- cies have been evaluated as function ofκλand a negligible dependency has been found, thus they are assumed to be constant. The exception is t ¯tH production mode, which shows a 10% increase forκλ< −10, but in this interval the reduction of the cross section due to the κλ dependence is about 80%, therefore largely dominating over the efficiency variation. This assumption will be re- evaluated in the ggF production mode once a complete computation of the differential NLO EW corrections will become available.
A likelihood fit is performed to constrain the value of the Higgs boson self-couplingκλ, while all 4
7.6. Constraints onκλfrom single Higgs boson measurements 173 STXS region VBF WH ZH C1i× 100 VBF+V(had)H VBF-cuts + pTj 1< 200 GeV, ≤ 2 j 0.63 0.91 1.07 VBF-cuts + pTj 1< 200 GeV, ≥ 3 j 0.61 0.85 1.04 VH-cuts + pTj 1< 200 GeV 0.64 0.89 1.10 no VBF/VH-cuts, pTj 1< 200 GeV 0.65 1.13 1.28 pTj 1> 200 GeV 0.39 0.23 0.28 q q → H`ν pVT < 150 GeV 1.15 150 < pVT < 250 GeV, 0 j 0.18 150 < pVT < 250 GeV, ≥ 1 j 0.33 pVT > 250 GeV 0 q q → H`` p V T < 150 GeV 1.33 150 < pVT < 250 GeV, 0 j 0.20 q q → Hνν 150 < p V T < 250 GeV, ≥ 1 j 0.39 pVT > 250 GeV 0
Table 7.5: C1i coefficients for each region of the STXS scheme for the VBF, WH and ZH production
modes. The definition of the STXS regions can be found in Ref. [134] . In the VBF categories, “VBF- cuts” [19] indicates selections applied to target the VBF di-jet topology, with requirements on the di- jet invariant mass (mj j) and the difference in pseudorapidity between the two jets; the additional ≤ 2 j and ≥ 3 j region separation is performed indirectly by requesting pTH j j ≶25 GeV. “VH-cuts” select the W, Z → j j decays, requiring an mj j value close to the vector boson mass [19]. The C1i
coefficients of the pVT > 250 GeV regions are negligible, O(10−6), and are set to 0.
λ κ 5 − 0 5 10 15 ) Λ -2 ln ( 0 1 2 3 4 5 6 7 8 9 10 Preliminary ATLAS -1 = 13 TeV, 36.1 - 79.8 fb s = 125.09 GeV H m σ 1 σ 2 Stat. only
Stat. + Exp. Sys.
Stat. + Exp. Sys. + Theory Sig.
Total = Stat. + Exp. Sys. + Theory Sig. and Bkg.
other Higgs boson coupling are set to their SM values (κi ,F= κi ,V= 1 in Eq.2.32and2.35). Thus, for a large variety of BSM scenarios, where new physics modifies only the Higgs boson self-coupling, the constraints onκλderived through the combination of single Higgs measurements can be directly compared to the constraints set by double Higgs production measurements. The profile likelihood scan performed as a function ofκλis shown in Fig.7.12. The central value and uncertainty of the modifier of the trilinear Higgs boson self-coupling is determined to be:
κλ= 4.0+4.3−4.1= 4.0+3.7−3.6(stat.)+1.6−1.5(exp.)+1.3−0.9(sig.th.)+0.8−0.9(bkg.th.) (7.3)
where the total uncertainty is decomposed into components for statistical, experimental and the- ory uncertainties on signal and background modelling. The 95% CL allowed interval for κλ is −3.2 < κλ< 11.9 (observed) and −6.2 < κλ< 14.4 (expected). This interval is competitive with the one obtained from the direct H H searches using an integrated luminosity of 36.1 fb−1, which is −5.2 < κλ< 12.1 (observed) and −5.8 < κλ< 12.0 (expected). The dominant contributions to the κλ sensitivity derive from the di-boson decay channelsγγ, ZZ, WW and from the ggF and t ¯tH produc- tion modes. The differential information currently provided by the STXS binning in the VBF, W H and Z H production modes does not improve the sensitivity toκλsignificantly. However, differen- tial information should help most in the t ¯tH production mode. A dedicated optimisation of the kinematic binning, including the most sensitive ggF and t ¯tH production modes, still needs to be fully theoretically and experimentally explored and might improve the sensitivity in the future.
While the sensitivity onκλ derived from single Higgs processes in an exclusive fit is compa- rable to those from H H direct searches, the constraints become significantly weaker when BSM deformations to the single Higgs couplings are taken properly into account. Two additional fit con- figurations with a simultaneous fit to (κλ,κV) or to (κλ,κF) have been considered. These fits target BSM scenarios where new physics could affect only the Yukawa type terms (κV= 1) of the SM or
only the couplings to vector bosons (κF= 1), in addition to the Higgs boson self-coupling (κλ) [272]. This set of results provides a rough indication of the simultaneous sensitivity to both Higgs boson self-coupling and single Higgs boson couplings with the data statistics currently available for the input analyses.
Figure7.13shows negative log-likelihood contours on (κλ,κV) and (κλ,κF). The constraining power of the measurement is reduced by including additional degrees of freedom to the fit. In particular, the sensitivity toκλis degraded by 50% (on the expected lower 95% C.L. exclusion limit) when determining simultaneouslyκVandκλ.
Similarly, the sensitivity toκλfrom double Higgs measurements completely vanishes if the cou- pling to the top quark (κt) is left free to float, due to aκ4t dependence of the total p p → H H cross section (see Sec.7.2and Fig.7.4). Therefore a determination ofκλ which would take into account BSM contributions affectingκt,κF orκV would be possible only through a simultaneous analysis of both single and double Higgs measurements. As the experimental sensitivity increases, the ad- dition of more differential information, in particular for t ¯tH and ggF, would allow the inclusion of more relevant EFT operators in the analysis, asκtand cg g (cf. discussion at the end of Sec.2.3.2).
7.6. Constraints onκλfrom single Higgs boson measurements 175 λ κ 20 − −15 −10 −5 0 5 10 15 20 F κ 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 Preliminary ATLAS -1 = 13 TeV, 36.1 - 79.8 fb s = 1 V κ = 125.09 GeV, H m SM Best Fit 68% CL 95% CL λ κ 20 − −15 −10 −5 0 5 10 15 20 V κ 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 Preliminary ATLAS -1 = 13 TeV, 36.1 - 79.8 fb s = 1 F κ = 125.09 GeV, H m SM Best Fit 68% CL 95% CL
Figure 7.13: Negative log-likelihood contours at 68% and 95% CL as function of (κλ,κF) under the assumption ofκV= 1 (left), and as function of (κλ,κV) under the assumption ofκF= 1) (right). The
Acknowledgements
This research was supported in part by the COST Action CA16201 (“Particleface") of the European Union. Monika Blanke, Simon Kast, Susanne Westhoff and José Zurita are supported in part by the Heidelberg Karlsruhe Strategic Partnership (HEiKA) through the research bridge “Particle Physics, Astroparticle Physics and Cosmology (PAC)”. The research of Monika Blanke and Susanne Westhoff is supported in part by the Deutsche Forschungsgemeinschaft (DFG, German Research Founda- tion) under grant 396021762 – TRR 257 “Particle Physics Phenomenology after the Higgs Discov- ery”. Simon Kast acknowledges the support of the DFG-funded Doctoral School “Karlsruhe School of Elementary and Astroparticle Physics: Science and Technology” (KSETA). Susanne Westhoff is supported in part by the Carl Zeiss foundation through an endowed junior professorship (Junior-
Stiftungsprofessur). The work of Gerhard Buchalla has been supported in part by the Deutsche
Forschungsgemeinschaft (DFG) under grant BU 1391/2-2 (project number 261324988) and the DFG Cluster Exc 2094 "ORIGINS”. Sally Dawson is supported by the U.S. Department of Energy under Grant Contract DE-SC0012704. Christoph Englert is supported by the STFC under grant ST/P000746/1. The research of Arnaud Ferrari is supported by Vetenskapsrådet (DNR 2015-03942). Pier Paolo Gi- ardino is supported by the Spanish Research Agency (Agencia Estatal de Investigacion) through the contract FPA2016-78022-P and IFT Centro de Excelencia Severo Ochoa under grant SEV-2016- 0597. The work of Seraina Glaus is supported by the Swiss National Science Foundation (SNF). Ramona Gröber is supported by the “Berliner Chancengleichheitsprogramm”. Christophe Grojean and Tim Stefaniak are supported by Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany‘s Excellence Strategy – EXC 2121 “Quantum Universe” – 390833306. The research of Stefania Gori is supported by the National Science Foundation under the CAREER grant PHY-1915852. Peisi Huang is supported in part by United States National Science Foun- dation under grant PHY-1820891, and the National Science Foundation supported Nebraska EP- SCoR program under grant number OIA-1557417. Michael Kagan is supported by the US Depart- ment of Energy (DOE) under grant DE-AC02-76SF00515 and by the SLAC Panofsky Fellowship. Jonathan Kozaczuk is supported by NSF grant PHY-1719642. Ian M. Lewis is supported in part by United States Department of Energy grant number DE-SC0017988. Heather E. Logan is sup- ported by the Natural Sciences and Engineering Research Council of Canada and by the H2020- MSCA-RISE-2014 grant no. 645722 (NonMinimalHiggs). Andrew J. Long is supported at the Uni- versity of Michigan by the US Department of Energy under grant DE-SC0007859. The work of Davide Pagani has been supported by the Alexander von Humboldt Foundation, in the frame- work of the Sofja Kovalevskaja Award Project “Event Simulation for the Large Hadron Collider at High Precision”, and by the Deutsche Forschungsgemeinschaft (DFG) through the Collaborative Re- search Centre SFB1258. Michael Peskin is supported by the US Department of Energy (DOE) under grant DE-AC02-76SF00515. Tania Robens is supported by the European Union through the Euro- pean Regional Development Fund - the Competitiveness and Cohesion Operational Programme (KK.01.1.1.06). Nausheen R. Shah is supported by Wayne State University and by the United States Department of Energy under grant number DE-SC0007983. Ambresh Shivaji very much appreci-
ates the support from MOVE-IN Louvain incoming postdoctoral fellowship co-funded by the Marie Curie Actions of the European Commission. For Suyog Shrestha, the work was partially supported by the U.S. Department of Energy through the grant DE-SC0011726. Kuver Sinha is supported in part by United States Department of Energy grant number DE-SC0009956. Matthew Sullivan is supported by the Kansas EPSCoR grant program. Maximilian Swiatlowski is supported by the Uni- versity of Chicago and through the NSF grant PHY-1707981. Rafael Teixeira De Lima is supported by the SLAC Panofsky Fellowship. Caterina Vernieri is supported by the SLAC Panofsky Fellowship.
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