4.4 Particle reconstruction
5.1.1 Physics Benchmarks
The requirements on the Phase-II upgrades to the ATLAS trigger system are driven by the physics goals of theHL-LHC. These include measurements of the new scalar particle observed in 2012 to assess its compatibility with theSMHiggs boson. In addition to precisely measuring its properties and couplings to the otherSM particles, theHL-LHCpresents the first opportunity to access the Higgs self-interaction through di-Higgs production. The large data set will also allow for improved measurements of the top-quark mass, scattering of vector bosons produced in association with high-mass jet pairs (V V jj), and electroweak precision observables in singleW andZ events. The
6References to individualTechnical Design Reports (TDRs)are provided in the relevant sections below.
5. Hardware Tracking for the Trigger 54
Figure 5.1: An overview of theLHCschedule, from the initial 7 TeV run to the ultimately-planned 14 TeV runs for theHL-LHC. At the time of writing, Run 2 has concluded, entering the period of Phase-I upgrades to the ATLAS detector. Run 3 will recommence at a planned energy of 14 TeV in 2021, after which data-taking will stop once more to allow for major Phase-II upgrades in 2024. This marks the beginning of the HL-LHC project, aiming to collect 3 ab−1 of collision data over 10-12 years. Figure is reproduced from Ref. [10].
discovery potential for signatures of New Physics will also be significantly extended, particularly for scenarios with electroweak production cross-sections, compressed mass spectra, and unconventional signatures. For many of these targeted examples, the quality of future searches measurements relies on maximizing the trigger acceptance to each signature. As representative examples, we consider the case ofSM(non-resonant) di-Higgs production and compressedSUSYin further detail.
The SM predicts a value of the Higgs boson self-coupling
λhhh= 1 2 mh v 2 ≈0.13, (5.1)
where mh is the observed value of the Higgs mass and v the Higgs VEV (for a more detailed
discussion see Section2.2). The parameter can be extracted by measuring the production of pairs of Higgs bosons, which dominantly occurs through two diagrams and destructively interfere. These are shown in Figure 5.2. The first involves an s-channel h∗ decaying to two (dominantly on-shell) Higgs bosons and the second a box loop of top quarks. Only the first is proportional to λhhh.
The most promising channels to measure the rate of such events are in decayshh→b¯bb¯b,b¯bτ+τ−, and b¯bγγ. The sensitivity of the b¯bbb¯and b¯bτ+τ− channels will completely rely on the ability to efficiently trigger on relatively low-energybjets and hadronic tau candidates. Figure5.3shows the sensitivity of thehh→b¯bb¯bandb¯bτ+τ− analyses to thep
Trequirement on the least-energetic jet or hadronic tau object. This dependence is used to set the requirements on the multi-jet and hadronic tau triggers to be used to collectHL-LHCdata.
5. Hardware Tracking for the Trigger 55
Table 11.7:
Summary of the final states investigated in the search for Higgs boson
pair production by ATLAS and CMS. (**) denotes results obtained with the 2015
dataset corresponding to an integrated luminosity of approximately 3 fb−1, (*)
denotes results obtained with a partial 2016 dataset corresponding to an integrated
luminosity of approximately 12 fb−1
and the other results reported correspond to
the full 2016 dataset. Results are 95% CL upper limits on the observed (expected)
SM signal strengths.
Channel
ATLAS
CMS
bbγγ
117 (161)** [187]
19 (17) [188]
bbbb
29 (38)* [189]
342 (308)** [190]
bbτ
+τ
−—
30 (25) [191]
bbW
+W
−—
79 (89) [192]
W
+W
−γγ
747 (386)* [193]
—
whose amplitude is not negligible compared to the former. These diagrams interfere
negatively making the overall production rate smaller than what would be expected in
the absence of a trilinear coupling.
Figure 11.5:
Feynman diagrams contributing to Higgs boson pair production
through (a) a top- and b-quark loop and (b) through the self couplings of the Higgs
boson.
III.4.1. Searches for Higgs boson pair production
The searches for Higgs boson pair production both resonant and non-resonant are very
interesting probes for a variety of theories beyond the SM, and can be done in a large
number of Higgs boson decay channels. At Run 1 the ATLAS and CMS collaborations
have searched for both resonant and non resonant Higgs boson pair production in the
following channels: (i)
HH
→
bbγγ
[181]; (ii)
HH
→
bbτ+τ−
[182]; (iii)
HH
→bbbb
[183];
and (iv)
HH
→
W W∗γγ
[182]. (iv) in final states containing multiple leptons (electrons
or muons) covering the
W W∗W W∗,
W W∗ZZ∗,
ZZ∗ZZ∗,
ZZ∗τ+τ−,
W W∗τ+τ−,
ZZ∗bb,
τ+τ−τ+τ−
channels [184]; (v)
γγτ+τ−
channels [184].
At Run 2 most of these channels have been updated both by the ATLAS and CMS
collaborations and the results are summarized in Table 11.7.
June 5, 2018 19:47
Figure 5.2: Feynman diagrams are shown depicting the dominant contributions to the gg → hh
process. The box diagram with intermediate top quarks is shown at left, while the diagram with
s-channel Higgs is shown at right. Only the second depends on the Higgs self-coupling. Figure reproduced from Ref. [1].
[GeV] T Minimum offline jet p
30 40 50 60 70 80 90 100 110 SM HH σ / HH σ 95% CL exclusion limit on 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 ATLAS Preliminary -1 = 14 TeV, 3000 fb s
Projection from Run 2 data b b b b → HH No systematic uncertainties
Figure 5.3: At left, the sensitivity of thehh →b¯bb¯b analysis to thepT requirement on the fourth most energetic jet is shown. This is quantified by the corresponding excluded value of the di- Higgs production cross section with respect to the SM prediction σhh/σhhSM as determined by a
projected analysis of the full HL-LHC data set [115]. No systematic uncertainties are included and no combination is made with other decay channels. At right, the relative acceptance of the
hh → b¯bτ+τ− process as a function of the leading and sub-leading hadronic tau p
T thresholds is shown. Acceptances are highlighted which correspond to the capabilities of the current trigger system and proposed upgrades [10].
So-called “natural” scenarios of SUSY predict relatively light supersymmetric partners of the gluon, top quark, and electroweak bosons [116]. In general, the limits on gluino and stop sparticle masses will not significantly improve with additional data. This is because they may be pair- produced via the strong interaction and thus have large enough cross sections that large numbers of events should have already been seen, even for TeV-scale sparticle masses. However, limits on the allowed masses of the super-partners of the electroweak bosons are much weaker in scenarios where they are the only light sparticles, as they must be produced froms-channelW andZ bosons. In the naturalness-motivated scenario with a Higgsino LSP, a triplet of states is predicted with
5. Hardware Tracking for the Trigger 56 (a) ) [GeV] 0 2 χ∼ m( 100 150 200 250 300 350 400 450 ) [GeV] 0 1 χ∼ , 0 2 χ∼ m( ∆ 1 10 ) σ 1 ± Expected limit ( discovery σ 5
Run 2 Limit (arXiv:1712.0811) LEP excluded > 0 µ = 5, β production, tan ± 1 χ∼ 0 2 χ∼ , ± 1 χ∼ ± 1 χ∼ , 0 1 χ∼ 0 2 χ∼ Pure Higgsino
Soft Lepton analysis All limits at 95% CL
-1 =14 TeV, 3000 fb s
ATLAS Simulation Preliminary
(b)
Figure 5.4: At left, an illustration is shown of the ˜χ02χ˜±1 production process that leads to signatures with three leptons and missing transverse momentum. At right, the projected sensitivity of analyses of the fullHL-LHCdata set are shown as a function of the mass splitting of the electroweak super- partners [117].
of electroweak bosons, picture in Figure5.4a, leads to a challenging signature with moderateEmiss T and multiple low-pT leptons. The projected sensitivity to such scenarios was studied in Ref. [117] and is summarized in Figure5.4b. Depending on the details of the model, triggers requiringEmiss
T , leptons, or a combination of both can provide the best signal acceptance. These are shown for benchmarkSUSYmodel withmχ˜0
2−mχ˜01 = 20 and 40 GeV in Figure5.5.