Wednesday, June 1, 2022

The Physics Case For A Neutrino Collider

We've had particle colliders that have smashed protons and atoms (both of which are made up of quarks and gluons), such as the Large Hadron Collider (LHC) and Tevatron, and that have smashed electrons and positrons, such as the linear electron-positron collider (LEP). Higher energy hadron colliders and muon colliders are both popular candidates for post-LHC high energy physics experiments. GlueX of Jefferson Labs includes what is basically a photon collider exploring photo-production of fundamental particles and hadrons.

But, the physics case that there will be new physics discoveries or greatly improved measurements of the parameters of the Standard Model and what kind of complex particle interactions are possible, in new higher experiments at energy scales above those already studied is rather weak. 

Notwithstanding the claims of Pollyanna theorists and phenomenologists in high energy physics, the reality is that new physics breakthroughs are not "right around the corner" if we just build an order of magnitude higher energy collider of a type we've already built, and the increased precision we can expect from these higher energy colliders in measuring Standard Model phenomena is likewise real, but quite marginal.

But, high energy physics has not yet built a neutrino collider.

The preprint below, however, makes the case that a neutrino collider could provide lots of interesting and useful results with much lower luminosities than the previous state of the art for hadron and charged lepton colliders, because there is so much more that we don't know about neutrinos at this point.

For example, the LHCb has collected an integrated luminosity of 3/fb at 7-8 TeV and 6/fb at 13 TeV, for a combined 9/fb of integrated luminosity. By comparison, a neutrino collider could start producing interesting and useful results to probe currently unobserved neutrino physics with an integrated luminosity about a million times smaller than the LHCb to date.

The peak energy scale of the proposed neutrino collider in particular collisions would be about 7% of the peak energy scale of LHC and about 50% of the peak energy scale of Tevatron.

This lower luminosity and lower peak collision energy would come with a presumably significantly smaller price tag than proposals for a next generation charged lepton, proton, or hadron collider which could easily cost $10 billion or more to build and hundreds of millions of dollars a year to operate.

The paper and its abstract are as follows:
Addressing the mass origin and properties of neutrinos is of strong interest to particle physics, baryogenesis and cosmology. Popular explanations involves physics beyond the standard model, for example, the dimension-5 Weinberg operator or heavy Majorana neutrinos arising from ``seesaw'' models. 
The current best direct limits on the electron neutrino mass, derived from nuclei beta decay or neutrinoless double beta decay processes, are at the sub-electronvolt level. 
Here we propose a novel neutrino neutrino collider where the neutrino beam is generated from TeV scale muon decays. Such collisions can happen between either neutrinos and anti-neutrinos, or neutrinos and neutrinos. We find that with a tiny integrated luminosity of about 10^−5/fb we can already expect to observe direct neutrino anti-neutrino annihilation, νν¯→Z, which also opens the door to explore neutrino related resonances νν¯→X. The low luminosity requirement can accommodate with relatively large emittance muon beam. 
Such a device would also allow for probing heavy Majorana neutrino and effective Majorana neutrino mass through νν→HH to a competitive level, for both electron and muon types.
Sitian Qian, et al., "The physics case for neutrino neutrino collisions" arXiv:2205.15350 (May 30, 2022).

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