What would be the next realistic 'major' target in particle physics, given confirmation that the Higgs boson has been discovered?

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• 
  Marta Felcini · CERN

  Next steps: confirm, or disprove, that this is indeed the scalar particle as expected in the standard model, with the expected properties, and keep searching for new phenomena beyond the standard model paradigm. Among these, one should keep searching for additional Higgs-like particles, as predicted in many models beyond the standard model, including supersymmetry. Discovery of supersymmetric particles or other phenomena, like eg the existence of extra dimensions, would be a major step in understanding what lies beyond the standard model Also, understanding gravity at the quantum level is one of the main goals of physics in the 21st century. How to do this experimentally, in particular at high energy colliders, is still unclear because we lack models of quantum gravity so we cannot make predictions of what could be observed. However, the LHC and other colliders could also discover something totally unexpected and unexplainable by all existing models, that may put us on the right track to fundamentally further our understanding, in particular quantum gravity. It is evident that the discovey of a Higgs-like particle is a first step. Now we are moving into an exploratory phase of more unchartered territories. As for the LHC, it is planned to collect something like four times the present amount of data by the end of 2012. Then it will resume operation in 2015 at a much larger collision energy (14 TeV as compared to 7 last year and 8 this year). Higher energy, never attained before, will open up the possibility to discover high mass particles or new interactions that have not been probed yet. An even higher luminosity LHC operation phase with upgraded detectors is also planned. The complete LHC operation programme is expected to span 20 years. As for planning of new colliders, this is an important question, that is being addressed in the particle physics community. In my modest opinion, we should have some indication of physics beyond the standard model before deciding on what kind of experimets we need beyond the LHC. These new signs may come soon, eg from a non standard properties of the newly discovered bosons or observation of new phenomena. But clearly new discoveries at the LHC cannot be predicted with certitude. However the great performance of the LHC and the experiments, ATLAS and CMS, is certainly holding great promises.

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  Next steps: confirm, or disprove, that this is indeed the scalar particle as expected in the standard model, with the expected properties, and keep searching for new phenomena beyond the standard model paradigm. Among these, one should keep searching for additional Higgs-like particles, as predicted in many models beyond the standard model, including supersymmetry. Discovery of supersymmetric particles or other phenomena, like eg the existence of extra dimensions, would be a major step in understanding what lies beyond the standard model
  Also, understanding gravity at the quantum level is one of the main goals of physics in the 21st century. How to do this experimentally, in particular at high energy colliders, is still unclear because we lack models of quantum gravity so we cannot make predictions of what could be observed. However, the LHC and other colliders could also discover something totally unexpected and unexplainable by all existing models, that may put us on the right track to fundamentally further our understanding, in particular quantum gravity.
  It is evident that the discovey of a Higgs-like particle is a first step. Now we are moving into an exploratory phase of more unchartered territories.
  As for the LHC, it is planned to collect something like four times the present amount of data by the end of 2012. Then it will resume operation in 2015 at a much larger collision energy (14 TeV as compared to 7 last year and 8 this year). Higher energy, never attained before, will open up the possibility to discover high mass particles or new interactions that have not been probed yet. An even higher luminosity LHC operation phase with upgraded detectors is also planned. The complete LHC operation programme is expected to span 20 years.
  As for planning of new colliders, this is an important question, that is being addressed in the particle physics community. In my modest opinion, we should have some indication of physics beyond the standard model before deciding on what kind of experimets we need beyond the LHC. These new signs may come soon, eg from a non standard properties of the newly discovered bosons or observation of new phenomena. But clearly new discoveries at the LHC cannot be predicted with certitude. However the great performance of the LHC and the experiments, ATLAS and CMS, is certainly holding great promises.

  Jul 4, 2012
• 
  F. Vissani · INFN - Istituto Nazionale di Fisica Nucleare

  HI, I would like to offer my point of view on one question raised by our host What are the predictions from physics beyond the standard model that could be realistically tested in the near future? Let me begin saying clearly that: The theory that did predictions till now is just the standard model. Tini Veltman has discussed since the beginning that, in the standard model, the range for Higgs mass was fixed and not arbitrary. The electroweak precision tests indicated the correct range. Now, many other things are discussed in the literature. But I am not sure we should speak of "physics beyond the standard model" without using more precise terms. We should either speak of some drawback of the standard model or of different models that make predictions. Trying to be conservative, I begin with the first point. The two most serious open questions from the point of view of the standard model, in my view, are 1-the nature of the masses of the neutrinos 2-the nature of dark matter (if it exists). Moreover, other pending issues are 3-absence of strong CP violation 4-nature of the cosmic matter asymmetry. Other issues, as the existence of sterile neutrinos, the naturalness of the standard model, the inclusion of "quantum gravity", and even the existence of supersymmetry/extra dimensions at the TeV scale, etc. seem to me still vague, premature or dictated by our wish, more than by true needs. I hope to be exceedingly conservative here and in the end just wrong, but from this point of view, I would tend to agree with Alain on the list of priorities, possibly widening a bit the context. Finally I do not think it is useful to isolate forcefully particle physics from the rest of physics. E.g, the study of the matter distribution in the galaxy clusters, the luminosity of type IA supernovae at cosmic distances, the study of atmospheric neutrinos could have been dubbed as irrelevant for particle physics--while they have been entrance doors to the physics beyond the standard model.

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  HI, I would like to offer my point of view on one question raised by our host
  
  What are the predictions from physics beyond the standard model that could be realistically tested in the near future?
  
  Let me begin saying clearly that: The theory that did predictions till now is just the standard model. Tini Veltman has discussed since the beginning that, in the standard model, the range for Higgs mass was fixed and not arbitrary. The electroweak precision tests indicated the correct range.
  
  Now, many other things are discussed in the literature. But I am not sure we should speak of "physics beyond the standard model" without using more precise terms. We should either speak of some drawback of the standard model or of different models that make predictions. Trying to be conservative, I begin with the first point. The two most serious open questions from the point of view of the standard model, in my view, are 1-the nature of the masses of the neutrinos 2-the nature of dark matter (if it exists). Moreover, other pending issues are 3-absence of strong CP violation 4-nature of the cosmic matter asymmetry. Other issues, as the existence of sterile neutrinos, the naturalness of the standard model, the inclusion of "quantum gravity", and even the existence of supersymmetry/extra dimensions at the TeV scale, etc. seem to me still vague, premature or dictated by our wish, more than by true needs. I hope to be exceedingly conservative here and in the end just wrong, but from this point of view, I would tend to agree with Alain on the list of priorities, possibly widening a bit the context.
  
  Finally I do not think it is useful to isolate forcefully particle physics from the rest of physics. E.g, the study of the matter distribution in the galaxy clusters, the luminosity of type IA supernovae at cosmic distances, the study of atmospheric neutrinos could have been dubbed as irrelevant for particle physics--while they have been entrance doors to the physics beyond the standard model.

  Jul 4, 2012