With the end of the High-Luminosity LHC (HL-LHC) run, the

physics potential of pp colliders at an energy of about 14 TeV will be completely exploited. Nevertheless important fundamental questions about the Standard Model, in case of missing evidences of new physics, still would need to be addressed. One possible option to continue the investiagtion of fundamental physics is the building up of higher-energy e+e− and pp machines. The physics potential and motivations

of such machines will be summarised in this contribution

Di Micco, B.

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DOI 10.1393/ncc/i2018-18011-2Colloquia: IFAE 2017IL NUOVO CIMENTO 41 C (2018) 11Future circular collider studiesB. Di MiccoUniversità degli Studi di Roma Tre e Sezione INFN - Roma, Italyreceived 21 April 2018Summary. — With the end of the High-Luminosity LHC (HL-LHC) run, thephysics potential of pp colliders at an energy of about 14 TeV will be completely ex-ploited. Nevertheless important fundamental questions about the Standard Model,in case of missing evidences of new physics, still would need to be addressed. Onepossible option to continue the investiagtion of fundamental physics is the buildingup of higher-energy e+e− and pp machines. The physics potential and motivationsof such machines will be summarised in this contribution.1. – The status of the Standard Model at the end of HL-LHCIn the present work we assume that no new physics effect will be seen in the currentLHC Run-2 run and the following HL-LHC program. Such assumption is not based on ascientific prejudice about the LHC outcome, but it is just a working assumption, beingevident that in case new physics will show up at HL-LHC, it is very difficult to figureout now what would be the best choice to study such new phenomena.HL-LHC will be able to constrain Higgs boson couplings to bosons, b quark and τleptons at few % level, observe the h → μ+μ− decay channel, probably have a firstevidence of di-Higgs production.Concerning supersimmetry, it should be possible to exclude s-top and gluino massesup to 3 TeV.2. – The next physics target after the Higgs boson discoveryAfter the Higgs boson has been discovered, all particle needed for the Standard Modeldescription of the law of physics have been found. Neverthless, the validation of the Stan-dard Model description itself is still failry incomplete. While Higgs couplings to photon,Z and W and the Higgs boson spin and parity have been completely estabilished, thecouplings to leptons and quarks are still largely experimentally unknown. In particu-lar top, b and τ lepton couplings should have been completely certified at the end ofRun-2 and the coupling to muon should have been observed at HL-LHC. Nevethless, theCreative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0) 12 B. DI MICCOcoupling of the Higgs boson to electrons, c and light quarks will be completely untested.Furthermore, the Higgs symmtry breaking mechanism relies on the presence of an energypotential for the Higgs scalar field that has a well-defined shape. The SM strictly pre-dicts the shape of such potential that depends only from the Higgs mass and the Higgsvacuum expectation value. The last quantity is determined from the Fermi couplingconstant extracted from the muon decay Gμ. Neverthless, nowadays, and most likelyeven at the end of HL-LHC, this last fundamental piece of the fundamental theory ofnature will remain untested.3. – Relevance of the Higgs potential for cosmologyIt has been known for several years [1] that due to the running of the Higgs self-coupling constant with the energy scale at which it is computed, the Higgs potential getsa shape that deviates from the simple λΦ4 form and acquires a non-trivial Φ-dependence.Assuming no new physics at least up to the Planck scale, the present values of the topand Higgs masses imply the presence of a second, and deeper, minimum in the Higgspotential. Therefore, there is a non-null probability that the Higgs boson field has atransition to the stable minimum changing the law of physics as we know them in thepresent universe. The lifetime of such transition is extremely long and many orders ofmagnitude longer than the age of the Universe, neverthless it is interesting to know ifthe present Universe really has been shaped in such instability condition. Furthermore,it is known that to explain all comsologial parameters today, an inflationary epoch needsto be postulated during which the Universe undertook an exponential expantion andbecame extremely cold and uniform. Inflation models postulate the existence of a scalarboson with a properly shaped potential. Being the Higgs boson a scalar with its relativepotential term, it is a natural inflaton candidate [2,3]. The goodness of the Higgs bosonas an inflaton candidate strongly depends on the potential shape, and at the momentthere is an open discussion about the goodness of the Higgs potential to explain all theaspects of the inflationary theories. It is therefore important to have a direct probe ofthe potential, in order to determine its shape and test it against inflationary theories.4. – How to determine the Higgs potentialThe Higgs potential can be probed at pp and e+e− colliders by looking at the di-Higgsproduction processes. Di-Higgs production can be mediated by a top-box diagram, withthe emission of a Higgs boson at each of the two box diagram vertices, and by a toptriangle diagram with the Higgs boson propagating from the initial to the di-Higgs finalstate, with a coupling constant proportional to the λ parameter of the λΦ4 SM potentialterm. In pp interactions the hh production strongly increases with the center-of-massenergy, going from 40 fb at 13 TeV to 1750 fb at 100 TeV. This huge cross-section increasemakes a 100 TeV pp collider an ideal candidate for studies of di-Higgs boson production.5. – The 100 TeV pp collider projectIn the recent years, a strong effort has been started to study the feasibility andthe physics potential of a 100 TeV proton-proton collider. It is quite clear that in therelatively far future, 16 T Nb3Sn could become available for industrial productions andstudies are on-going to obtain 20 T high-temperature superconducting coil magnets.FUTURE CIRCULAR COLLIDER STUDIES 3Extrapolating the LHC energies with these configurations, a maximum energy of 100 TeVor 130 TeV is reachable depending on the used magnet technology.5.1. Physics deliverables. – The physics reaches of a 100 TeV collider have been deeplyexploited in [4]. Using the γγbb final state, a fractional error of 13% can be reached onthe hh production cross-section that translates to a 2.5% error on the self-coupling λusing a total integrated luminosity of 30 ab−1. Such study has been performed withouttaking into account pile-up effects and a realistic detector simulation, therefore, it willneed to be redone in the near future. Neverthless it clearly sets the measurement ofthe Higgs self-coupling as a clear reachable target of a 100 TeV pp collider. In order totry improving this result, with the target to reduce the needed integrated luminosityand, therefore, the running time of the machine, more channels are under study, like theWWbb and the ZZbb decay channels. A 100 TeV collider is also a unique opportunityto extend the explorable region for new particles up to 10 TeV, for example the s-quarksand gluinos supersymmetric particles.6. – The e+e− 100 km optionThe timeline needed to develop and industrialise magnets able to provide a magneticfield up to 16–20 T is still uncertain, therefore it is of interest to explore the physics reachof other options. An e+e− 100 km collider would allow to start the building up of thetunnel and the facilities for a following pp collider when the technology and the higherbudget needed for the pp collider would become available. A circumference larger thanthe previous LEP circumference allows to reduce the impact of photon radiation on thebeam energy loss. Studies for the e+e− option are performed assuming a constant powerconsumption of 100 MW. With such power a luminosty of 1× 1034 cm−2s−1 is reachableat the tt̄ production threshold, 2 × 1035 at the Z mass and 1034 at the Zh threshold.A collider of this type could set precision measurements on the Higgs couplings at sub-percent level, a measurement of the W mass at 0.5 MeV level and of the top massat 10 MeV level, to compare with the present, systematic dominated measurements of15 MeV for the W mass and 500 Mev for the top mass.7. – Current on-going studies and conclusionsThe possibility to build up an e+e− collider followed by a pp collider is under con-sideration by both European and Chinese physics communities. The two projects arenowadays becoming more and more similar, with a 100 km circumference length in bothcases and a design of the e+e− collider based on a double-ring for the accumulation ofthe beams plus a booster for the acceleration to high energies. For the pp case, while theEuropean community is concentrating on the Nb3Sn-based magnets, the Chinese com-munity is working more on the high-temperature superconductors. The e+e− and the ppoptions look nowadays the only two possibilities to boost forward the knowledge in parti-cle physics at the energy frontier, the first one setting the best measurements of the Higgscouplings, the second allowing to probe new physics up to the 10 TeV scale and beingable to probe the Higgs boson potential through the Higgs self-coupling measuerementat percent level.4 B. DI MICCOREFERENCES[1] Degrassi G. et al., JHEP, 08 (2012) 098.[2] Isidori G. et al., Phys. Rev. D, 77 (2008) 025034.[3] Masina G. et al., Phys. Rev. D, 89 (2014) 123505.[4] Mangano M. et al., arXiv:1606.09408, Physics at the FCC-hh, Chapt. 2: Higgs and EWsymmetry breaking studies, Report Number CERN-TH-2016-113 (2016).

Future circular collider studies

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Future circular collider studies

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