LETTER New physics and signal-background interference in associated pp → HZ production To cite this article: Christoph Englert et al 2016 EPL 114 31001   View the article online for updates and enhancements. Related content The HiggsTools handbook: a beginners guide to decoding the Higgs sector M Boggia, J M Cruz-Martinez, H Frellesvig et al. - Precision measurements of Higgs couplings: implications for new physics scales C Englert, A Freitas, M M Mühlleitner et al. - Topical Review W Bernreuther - Recent citations Dimension-six electroweak top-loop effects in Higgs production and decay Eleni Vryonidou and Cen Zhang - This content was downloaded from IP address 186.217.236.55 on 22/04/2019 at 21:39 https://doi.org/10.1209/0295-5075/114/31001 http://iopscience.iop.org/article/10.1088/1361-6471/aab812 http://iopscience.iop.org/article/10.1088/1361-6471/aab812 http://iopscience.iop.org/article/10.1088/0954-3899/41/11/113001 http://iopscience.iop.org/article/10.1088/0954-3899/41/11/113001 http://iopscience.iop.org/article/10.1088/0954-3899/41/11/113001 http://iopscience.iop.org/article/10.1088/0954-3899/35/8/083001 http://dx.doi.org/10.1007/JHEP08(2018)036 http://dx.doi.org/10.1007/JHEP08(2018)036 May 2016 EPL, 114 (2016) 31001 www.epljournal.org doi: 10.1209/0295-5075/114/31001 New physics and signal-background interference in associated pp → HZ production Christoph Englert 1 , Rogerio Rosenfeld 2 , Michael Spannowsky 3 and Alberto Tonero 2 1 SUPA, School of Physics and Astronomy, University of Glasgow - Glasgow G12 8QQ, UK 2 ICTP-SAIFR & IFT UNESP - Rua Dr. Bento Teobaldo Ferraz 271, 01140-070, São Paulo, Brazil 3 Institute for Particle Physics Phenomenology, Department of Physics, Durham University Durham DH1 3LE, UK received 25 March 2016; accepted in final form 16 May 2016 published online 31 May 2016 PACS 13.85.-t – Hadron-induced high- and super-high-energy interactions (energy > 10GeV) PACS 12.60.-i – Models beyond the standard model Abstract – We re-investigate electroweak signal-background interference in associated Higgs pro- duction via gluon fusion in the presence of new physics in the top Higgs sector. Considering the full final state pp → bb̄�+�− (� = e, μ), we discuss how new physics in the top Higgs sector that enhances the ZZ component can leave footprints in the HZ limit setting. In passing we investi- gate the phenomenology of a class of new physics interactions that can be genuinely studied in this process. Copyright c© EPLA, 2016 Introduction. – After the Higgs discovery in 2012 and initial property measurements [1,2] in the so- called κ framework, the phenomenology community has now moved towards understanding constraints in the dimension-six effective field theory (EFT) extension of the Standard Model (SM), which provides a theoretically clean and well-defined approach to constrain the presence of new physics interactions with minimal assumptions [3–7]. The field of Standard Model EFT has seen a rapid development recently. Not only have the run-I mea- surements by ATLAS and CMS been interpreted in terms of the dimension-six EFT extension [8–26], but the EFT framework has also been extended to next-to- leading order [27–35]. Measurement strategies that take into account these corrections via renormalization group improved calculations have been presented in [36,37]. Due to the large number of effective operators that are relevant to Higgs physics, it becomes essential to col- lect information from all possible processes related to the Higgs boson, especially at the LHC run II and the future high-luminosity phase. Since a single effective op- erator can contribute to different processes, there are cor- relations among them that can be used to find bounds on the Wilson coefficients of different operators. Mea- surements of the associated Higgs production [20,36,38], Higgs+jet production [39–45], top-quark–associated and multi-Higgs [46–50] production and the recently devel- oped Higgs off-shell measurements in gg → ZZ [51–53] will be pivotal to obtain a fine-grained picture of poten- tial compatibility of the Higgs discovery with the SM ex- pectation. In particular, the latter production mechanism has been motivated as an excellent candidate to constrain new physics effects by exploiting large momentum trans- fers to break degeneracies of new physics interactions in the on-shell Higgs phenomenology [54–57]. Similarly, high momentum transfers in the associated Higgs production pp → HZ are sensitive probes of new interactions [20,58–60]. The reason is the existence of a destructive interference between the triangle and box contributions in the SM that can be lifted by new or anomalous couplings. Furthermore, the high momentum transfer provides another avenue to discriminate the Higgs signal from the background relying on jet substructure methods [61–65]. While jet substructure analyses provide an extremely versatile and adaptable tool in new physics and Higgs searches, the mass resolution of Higgs decays H → bb̄ in such a search is a limiting factor. This becomes a chal- lenge especially if cross-sections or beyond the SM-induced deviations thereof become small for large backgrounds. It is known that the gluon fusion-induced associated Higgs production [66–68], while only contributing ∼ 10% of the inclusive HZ production cross-section [69–81], becomes relevant at large momentum transfers due to the top quark threshold [58,59]. A similar argument ap- plies to the non-decoupling of gg → H → ZZ at 31001-p1 Christoph Englert et al. high momentum transfers [51,52,82]. Therefore, the same type of physics can enhance both pp → HZ and pp → ZZ. We are therefore tempted to ask the following question: when studying the full final state pp → bb̄�+�− as signal for pp → H(→ bb̄) Z(→ �+�) (see footnote 1) for kinematics that allow the discovery of the Higgs boson in the associated produc- tion, how important is the irreducible pp→ Z(→ bb̄)Z(→ �+�) background, keeping in mind an imperfect H → bb̄ resolution? To answer this question we organise this letter as fol- lows. First we introduce a minimal set of operators which impact the two contributions pp→ HZ and pp→ ZZ in a different way, but necessarily related through gauge invari- ance. We then investigate the phenomenology of high-pT final states at the parton level. Subsequently, we show how our findings translate to the fully hadronized final state before we conclude. New physics effects in gluon-initiated HZ pro- duction. – Gluon-initiated associated production has been shown to contribute significantly to pp→ HZ in the boosted regime at the LHC and important consequences for new physics searches can be obtained by looking at this process [58,59,79]. New physics can potentially mod- ify the associated Higgs production both in the quark- and gluon-initiated channels. The quark-initiated channel may be altered at leading order through modified Higgs couplings [37] or at next to leading order through the influ- ence of new particles or effective operators in loops [59,83]. Similarly, the gluon-initiated channel may receive cor- rections through modified Higgs and top couplings to SM states. In principle, all dimension-six operators that are rele- vant for the Higgs sector should be considered since at the very least they can change the Higgs width, which af- fects the full partonic final state. However, several of these operators are already constrained from other observables, such as the Z-pole properties measured at LEP1. In order to keep our discussion transparent, we will focus on only two operators that are weakly constrained and are rele- vant for Higgs production (we adopt the parameterisation of [7,84,85]): OHt = ic̄Ht υ2 (t̄RγμtR)(Φ†←→D μΦ), (1) Ot = − c̄t υ2 ytΦ†Φ Φ† · Q̄L tR + h.c. (2) with Hermitian covariant derivative Φ†←→D μΦ = Φ†(DμΦ)− (DμΦ)†Φ, and Φ being the weak doublet that contains the physical Higgs Φ ⊃ H . The operator in eq. (1) modifies the coupling of the right-handed top quark to the Z boson t̄RtRZ by a factor 1The Higgs decay to leptons, i.e. pp → HH → bb̄�+�−, is numerically negligible. (a) g g H Z t t t (b) g g H ZZ q q q (c) g g H Z q q q q (d) g g Z ZH q q q (e) g g Z Z q q q q Fig. 1: Representative Feynman diagrams contributing to pp → (H,Z)Z → bb̄�+�−; we suppress the Higgs and Z boson decays. proportional to the c̄Ht coefficient, 2 3 g s2 W cW → 2 3 g s2 W cW + g c̄Ht 2cW . (3) It affects the Ztt coupling but not Htt and introduces a new ttHZ coupling. As required by gauge invariance, the derivative coupling of the top quark to the neutral Goldstone boson gets also shifted by the same quantity. Couplings to left-handed quark doublets are constrained by data on Z → bb̄ and will not change the qualitative outcome of our discussion2. Operators of this form but involving light fermions are constrained by precision elec- troweak measurements |cHu| � 2% and assuming a trivial flavor structure of the UV dynamics will directly constrain the interaction of eq. (1), which is otherwise unconstrained at the tree level by electroweak precision data and has no impact on Higgs decays (see, e.g., [7] for a comprehensive discussion). Higher-order corrections, however, re-induce a dependence, see [87]. We will ignore this potential con- straint for the time being, but will come back to it later. The operator in eq. (2) modifies the top Yukawa cou- pling by a factor proportional to the Wilson coefficient c̄t, yt → yt(1 + c̄t), while leaving the top mass as in the SM with a simple redefinition of the top quark field. The non-derivative couplings of the top quark to the neutral Goldstone boson are unchanged. We show in fig. 1 the relevant Feynman diagrams for pp → HZ and pp → ZZ ignoring the diagrams involv- ing the unphysical Goldstone bosons. Note, in particular, the new effective vertex t̄tHZ introduced by the operator in eq. (1), not present in the SM, which gives rise to the Feynman diagram contribution to the gluon-initiated am- plitude shown in fig. 1(a), and which may affect the cancel- lation between triangle and box diagrams for pp→ HZ in the SM, leading to an enhanced cross-section. This cancel- lation is also impacted by the change in the top Yukawa coupling introduced by the operator in eq. (2). In fact, the effect of a flipped top Yukawa coupling (i.e., with a coupling of opposite sign with respect to the SM, corre- sponding to c̄t = −2) on pp→ HZ was studied in [60]. 2Interactions of this type can typically arise in composite Higgs scenarios [86], which will also leave footprints in qq̄ → HZ as a function of the fine-tuning parameter v2/f2, where f is the pion decay constant analogue. 31001-p2 New physics and signal-background interference in associated pp→ HZ production gg → (ZZ + HZ) gg → ZZ gg → HZ 2mt m(bb̄�+�−) [TeV] dσ /d m [a b/ 20 G eV ] 0.70.60.50.40.30.2 10 1 0.1 Fig. 2: (Colour online) Invariant mass distribution of the bb̄�+�− (in this plot � = μ) system for the final state gg → bb̄�+�− in the SM and the phase space region pT (�+�−) � 100 GeV relevant for a boosted H → bb̄ analysis. Together these operators provide a parameterisation that allow us to “template” the gg → ZZ and gg → HZ components of the full partonic final state pp → bb̄l+l− in a gauge-invariant fashion, and, therefore, gives us a well-defined approach to study the signal-background in- terference in this final state. Note that since these opera- tors only modify the ttH and ttZ couplings, they do not affect the tree-level qq̄ → HZ process. Only the operator in eq. (2) changes the Higgs branching ratios (by a few percent in the relevant BR(H → bb̄) in the cases explored here) and it has been taken into account. The new interactions arising from eq. (1) and eq. (2) were implemented using FeynRules [88]. We calculate the one-loop gluon-initiated gg → (HZ + ZZ) → bb̄�+�− production amplitudes using the FeynArts, FormCalc and LoopTools [89,90] framework which we interface with Vbfnlo [91] to perform the phase space integra- tion and generate events in the Les Houches standard and keep the full quark mass dependences throughout. We pass these events to Herwig++ [92] for showering and hadronization. The qq̄-initiated process is simulated with MadGraph5 [93] using an identical input parame- ter setting and passed through Herwig++ to obtain the full hadronic final state. The respective samples are nor- malised to the NLO QCD predictions of the SM [68,69]. We use a K-factor of 1.2 and 1.8 for qq and gg-initiated processes, respectively. We focus on collisions at 13 TeV centre-of-mass energy. Parton level analysis. Before we analyse the full hadron level, it is worthwhile to re-investigate the order of magnitude of the expected interference effects between the gg → HZ and gg → ZZ parts in the full pp→ HZ + ZZ final state (see also [79] for an earlier discussion). To this end, we show in fig. 2 the parton level comparison of the invariant mass distribution between HZ and ZZ produc- tion for the gluon-initiated bb̄�+�− (in this case � = μ) production. Note the rise of the cross-section near the 2mt threshold. For these selection requirements we find a SM cross-section of 0.9 fb (including the flat K-factor). A choice of c̄Ht = 1, c̄t = 0 increases this cross-section c̄Ht = 1 c̄Ht = 0 (SM) pT (bb̄) [GeV] dσ /d p T [a b/ 10 G eV ] 450400350300250200150 10 1 0.1 Fig. 3: (Colour online) Transverse Higgs pT distribution and its sensitivity to the operator c̄Ht. It can be seen that the boosted regime pT (�+�−) � pT (bb̄) � 150 GeV is highly sensitive to the operator in eq. (1) which also modifies the continuum ZZ production. by 70%. A quantitatively identical enhancement can be achieved for c̄Ht = 0, c̄t � 0.33. Signal-background interference between the two contri- butions is in general a small effect and the relative size of HZ dominates over ZZ as a consequence of the relative branching ratio suppression of H → bb̄ (60%) and Z → b̄b (15%). This is left unchanged for changes in c̄t [79], how- ever, there will be modifications from eq. (2). In order to obtain a first estimate of the sensitivity to the effective operators, we consider first the process pp→ (HZ +ZZ)→ bb̄�+�− again at parton level. Based on the event simulation described above, we select events with pT (�+�−) > 150 GeV, 110 GeV < m(bb̄) < 140 GeV. (4) As an example, we show in fig. 3 the effect of c̄Ht = 1. One can see that this operator can dramatically impact the boosted Higgs regime due to the lifting of the SM cancellation and also the derivative nature of the induced coupling [85]. In order to derive exclusion regions in the (c̄t, c̄Ht)-plane we perform a log-likelihood hypothesis test based on a shape comparison of the pT (bb̄) distribution using the CLs method [94–96]. In fig. 4 we show the expected exclusion for a luminos- ity of 100 fb−1 based on our parton level results. While the resonant and continuum ZZ contributions are largely suppressed, the gauge-invariant extension of the top loop- induced gg → ZZ diagram3 introduces the tt̄HZ interac- tion. The result of fig. 4 indicates that the modification according to the operator in eq. (1), even for small choices in agreement with precision analyses [7], can in princi- ple impact the limit setting procedure in the associated Higgs production through sculpting the pT (bb̄) distribu- tion, especially when marginalising over eq. (2) in a global fit where degenerate operator directions will influence the expected exclusion. 3One can understand the modification of the Ztt̄ interaction as replacing H → 〈H〉. 31001-p3 Christoph Englert et al. 0.3 0.2 0.1 0.0 0.1 0.2 0.3 0.6 0.4 0.2 0.0 0.2 0.4 0.6 ct cHt s � 13 TeV Fig. 4: (Colour online) Projected sensitivity of the boosted parton level analysis of pp → bb̄�+�− in the conventions of eqs. (1) and (2); the shaded region is excluded at 95% confi- dence level for the ideal parton level setting described in the text, for L = 100 fb−1. One might worry about the validity of an effective field theory in our analysis. This issue has been a subject of recent discussion, see, e.g., [37,97]. The coefficients of the dimension-6 operators can be related to the scale M where new physics appears by c̄ ≈ g2v2/M2, where g is a coupling constant of the heavy states with SM parti- cles. Further suppression factors arise in the case in which an operator is generated at loop level. We can therefore put an upper bound in the new mass scale from requir- ing that the underlying theory is strongly coupled, i.e., g = 4π: M < 4πv/ √ c ≈ 3 TeV for c = O(1). Since our analysis relies on pT < 1 TeV we do not violate this upper bound. Showering and hadronization. The results of the par- ton analysis detailed in the previous section are known to change substantially when we turn to the full hadron level final state and perform a realistic reconstruction [58]. Based on the event generation strategy outlined above, we apply typical HZ final state selection cuts by i) requiring exactly 2 oppositely charged same-flavor leptons satisfying |η�| < 2.5, pT (�) > 30 GeV, ii) requiring that these leptons are compatible with the Z boson mass: 80 GeV < m(�+�−) < 100 GeV, iii) and requiring boosted topologies pT (�+�−) > 200 GeV. iv) We then perform a typical BDRS analysis [61]: All the remaining hadronic activity is clustered using FastJet [98] into a Cambridge-Aachen fat jet with R = 1.2. The boosted Higgs candidate jet has to satisfy pT,j > 200 GeV and at least one such object is required in |η| < 2.5. The fat jet is filtered, mass- dropped and double b-tagged with a b-tag efficiency of 60% (2% fake rate), yielding a total efficiency of 36%. 0.3 0.2 0.1 0.0 0.1 0.2 0.3 0.6 0.4 0.2 0.0 0.2 0.4 0.6 ct cHt s � 13 TeV Fig. 5: (Colour online) Projected exclusion at 95% CLs (blue shaded region) of the boosted hadron level analysis of pp → bb̄�+�− at 3 ab−1 integrated luminosity. v) Higgs candidates are required to be compatible with 110 GeV < m(bb̄) < 140 GeV evaluated on the b-tagged subjets. While the high-pT selection is enough to remove the biggest background tt̄ almost entirely, jet substructure ap- proaches remove the QCD-induced bb̄ production modes from the selection to a large extent, leaving the Z+jet pro- duction as a dominant background (or calibration tool). The Higgs mass resolution quoted in v) is a key factor in the boosted analysis to allow signal vs. background ex- traction in the first place (and veto SM qq̄-induced ZZ production). However, as mentioned before, the gluon- induced ZZ contribution could in principle be enhanced through the operator discussed previously, thus adding more significantly to the region v) than expected in the SM and at parton level due to shower and hadronization effects. After these analysis steps one typically obtains a cross- section of ∼ 0.2 fb for the SM which includes both qq̄- and gg-initiated processes. And again we find the impact of HZ far more dominant than ZZ. As expected, the low- ered statistical yield when taking into account the full re- construction efficiencies requires a larger luminosity to set limits. Setting limits, we obtain a result comparable to the parton analysis of the previous section for 3 ab−1, see fig. 5. This means that when including the constraints from com- plementary Higgs measurements at this luminosity, which are expected to limit |c̄t| � 10−2 [26], the presence of c̄Ht for trivial flavor structures, i.e. at the level of c̄Ht = c̄Hu is difficult to constrain and can practically be neglected when working with this assumption. However, the associated Higgs production provides a test of non-trivial beyond the SM flavour structures, which can be combined with di- rect tt̄Z searches (see, e.g., [85,87,99–103]). Comparing to the projections of [99], −0.13 < c̄Ht < 0.64, we see that the associated Higgs production can be expected to provide an additional discriminating power to complemen- tary tt̄Z searches. It should be noted that our results do not reflect systematic uncertainties from both theoretical 31001-p4 New physics and signal-background interference in associated pp→ HZ production and experimental sources and are therefore very likely to worsen, in particular in a global fit when more operators are included. In particular, the theoretical uncertainties due to missing higher orders in gg → HZ are currently large for boosted kinematics ∼ O(30%) [68]. Potential im- provements in particular related to experimental system- atics are hard to foresee at this stage in the LHC program, but our results suggest that the boosted Higgs analysis should continue to receive attention. Summary and conclusions. – In this letter we have re-investigated electroweak signal-background interference in the gluon-initiated associated Higgs production in the light of the expected efficiencies and selection requirements of the fully hadronized final state. While the HZ + ZZ signal-background interference is suppressed, new physics effects that impact pp → ZZ can also leave footprints in the boosted analyses pp → HZ through new interactions related by gauge invariance. However, a robust limit set- ting in this channel will require a large luminosity. Even at these large luminosities the constraints on c̄Ht will not be competitive with electroweak precision constraints under the assumption of a trivial flavor structure (as commonly done in Higgs fits at this stage in the LHC phenomenol- ogy program). Relaxing this assumption, the associated Higgs production via gluon fusion can act as a test of this hypothesis, especially when other measurements point to- wards the SM. ∗ ∗ ∗ CE thanks the organisers of the 2014 ICTP-SAIFR GOAL Workshop, where this work was initiated and Marco Farina for helpful discussions. MS is supported by the European Commission through ITN PITN-GA- 2012-316704 (“HiggsTools”). 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