Right-handed Neutrino Production Research Articles

Neutrino magnetic moments meet precision Neff measurements

In the early universe, Dirac neutrino magnetic moments due to their chirality-flipping nature could lead to thermal production of right-handed neutrinos, which would make a significant contribution to the effective neutrino number, Neff. We present in this paper a dedicated computation of the neutrino chirality-flipping rate in the thermal plasma. With a careful and consistent treatment of soft scattering and the plasmon effect in finite temperature field theories, we find that neutrino magnetic moments above 2.7 × 10−12μB have been excluded by current CMB and BBN measurements of Neff, assuming flavor-universal and diagonal magnetic moments for all three generation of neutrinos. This limit is stronger than the latest bounds from XENONnT and LUX-ZEPLIN experiments and comparable with those from stellar cooling considerations.

Testing the seesaw mechanisms via displaced right-handed neutrinos from a light scalar at the HL-LHC

We investigate the pair production of right-handed neutrinos from the decay of a light $B-L$ scalar in the $U(1)_{B-L}$ model. The $B-L$ scalar mixes to the SM Higgs, and the physical scalar is required to be lighter than the observed Higgs. The produced right-handed neutrinos are predicted to be long-lived according to the type-I seesaw mechanism, and yield potentially distinct signatures such as displaced vertex and time-delayed leptons at the CMS/ATLAS/LHCb, as well as signatures at the far detectors including the CODEX-b, FACET, FASER, MoEDAL-MAPP and MATHUSLA. We analyze the sensitivity reach at the HL-LHC for the right-handed neutrinos with masses of 2.5 $\sim$ 30 GeV, showing that the active-sterile mixing to muons can be probed to $V_{\mu N} \sim 10^{-5}$ at the CMS/ATLAS/LHCb using the displaced vertex searches, and one magnitude lower at the MATHUSLA/CMS using time-delayed leptons searches, reaching the parameter space interesting for type-I seesaw mechanisms.

Smooth interpolation between thermal Born and LPM rates

In a weakly coupled ultrarelativistic plasma, 1 + n ↔ 2 + n scatterings, with n ≥ 0, need sometimes to be summed to all orders, in order to determine a leading-order interaction rate. To implement this “LPM resummation”, kinematic approximations are invoked. However, in cosmological settings, where the temperature changes by many orders of magnitude and both small and large momenta may play a role, such approximations are not always justified. We suggest a procedure to smoothly interpolate between LPM-resummed 1 + n ↔ 2 + n and Born-level 1 ↔ 2 results, rendering the outcome applicable to a broader range of masses and momenta. The procedure is illustrated for right-handed neutrino production from a Standard Model plasma, and dilepton production from a QCD plasma.

Long-lived TeV-scale right-handed neutrino production at the LHC in gauged U(1)X model

A gauged U(1)X extension of the Standard Model is a simple and consistent framework to naturally incorporate three right-handed neutrinos (RHNs) for generating the observed light neutrino masses and mixing by the type-I seesaw mechanism. We examine the collider testability of the U(1)X model, both in its minimal form with the conventional charges, as well as with an alternative charge assignment, via the resonant production of the U(1)X gauge boson (Z′) and its subsequent decay into a pair of RHNs. We first derive an updated upper limit on the new gauge coupling gX as a function of the Z′-boson mass from the latest LHC dilepton searches. Then we identify the maximum possible cross section for the RHN pair-production under these constraints. Finally, we investigate the possibility of having one of the RHNs long-lived, even for a TeV-scale mass. Employing the general parametrization for the light neutrino mass matrix to reproduce the observed neutrino oscillation data, we perform a parameter scan and find a simple formula for the maximum RHN lifetime as a function of the lightest neutrino mass eigenvalue (mlightest). We find that for mlightest≲10−5 eV, one of the RHNs in the minimal U(1)X scenario can be long-lived with a displaced-vertex signature which can be searched for at the LHC and/or with a dedicated long-lived particle detector, such as MATHUSLA. In other words, once a long-lived RHN is observed, we can set an upper bound on the lightest neutrino mass in this model.

Heavy neutrino production via Z′ at the lifetime frontier

We investigate the pair production of right-handed neutrinos from the decay of an additional neutral ${Z}^{\ensuremath{'}}$ boson in the gauged $B\ensuremath{-}L$ model. Taking into account current constraints on the ${Z}^{\ensuremath{'}}$ mass and the associated gauge coupling ${g}_{1}^{\ensuremath{'}}$, we analyse the sensitivity of proposed experiments at the lifetime frontier, FASER 2, CODEX-b, MATHUSLA as well as a hypothetical version of the MAPP detector to a long lived heavy neutrino $N$ originating in the decays of the ${Z}^{\ensuremath{'}}$. We further complement this study with determining the reach of LHCb and a CMS-type detector for the high-luminosity LHC run. We demonstrate that in a backgroundfree scenario with ${g}_{1}^{\ensuremath{'}}={10}^{\ensuremath{-}3}$ near the current limit, FASER 2 is sensitive to the active-sterile neutrino mixing down to ${V}_{\ensuremath{\mu}N}\ensuremath{\approx}{10}^{\ensuremath{-}4}$, while a reach of ${V}_{\ensuremath{\mu}N}\ensuremath{\approx}{10}^{\ensuremath{-}5}$ can be obtained for CODEX-b and LHCb, in a mass regime of ${m}_{N}\ensuremath{\approx}5--20\text{ }\text{ }\mathrm{GeV}$ and ${m}_{{Z}^{\ensuremath{'}}}\ensuremath{\approx}20--70\text{ }\text{ }\mathrm{GeV}$. Finally, MATHUSLA can probe ${V}_{\ensuremath{\mu}N}\ensuremath{\approx}{10}^{\ensuremath{-}7}$ and cover the mixing regime expected in a canonical seesaw scenario of light neutrino mass generation.

Status of rates and rate equations for thermal leptogenesis

In many realizations of leptogenesis, heavy right-handed neutrinos play the main role in the generation of an imbalance between matter and antimatter in the early Universe. Hence, it is relevant to address quantitatively their dynamics in a hot and dense environment by taking into account the various thermal aspects of the problem at hand. The strong washout regime offers an interesting framework to carry out calculations systematically and reduce theoretical uncertainties. Indeed, any matter–antimatter asymmetry generated when the temperature of the hot plasma [Formula: see text] exceeds the right-handed neutrino mass scale [Formula: see text] is efficiently erased, and one can focus on the temperature window [Formula: see text]. We review recent progress in the thermal field theoretic derivation of the key ingredients for the leptogenesis mechanism: the right-handed neutrino production rate, the CP asymmetry in the heavy-neutrino decays and the washout rates. The derivation of evolution equations for the heavy-neutrino and lepton-asymmetry number densities, their rigorous formulation and applicability are also discussed.

Comment on “Polarized window for left-right symmetry and a right-handed neutrino at the Large Hadron-Electron Collider”

Reference [S. Mondal and S. K. Rai, Phys. Rev. D 93, 011702 (2016).] recently argued that the projected Large Hadron Electron Collider (LHeC) presents a unique opportunity to discover a left-right symmetry since the LHeC has availability for polarized electrons. In particular, the authors apply some basic ${p}_{T}$ cuts on the jets and claim that the on-shell production of right-handed neutrinos at the LHeC, which violates lepton number in two units, has practically no standard model background and, therefore, that the right-handed nature of ${W}_{R}$ interactions that are intrinsic to left-right symmetric models can be confirmed by using colliding beams consisting of an 80% polarized electron and a 7 TeV proton. In this Comment, we show that their findings, as presented, have vastly underestimated the SM background which prevents a Left-Right symmetry signal from being seen at the LHeC.

Relic right-handed Dirac neutrinos and implications for detection of cosmic neutrino background

It remains to be determined experimentally if massive neutrinos are Majorana or Dirac particles. In this connection, it has been recently suggested that the detection of cosmic neutrino background of left-handed neutrinos νL and right-handed antineutrinos ν‾R in future experiments of neutrino capture on beta-decaying nuclei (e.g., νe+H3→He3+e− for the PTOLEMY experiment) is likely to distinguish between Majorana and Dirac neutrinos, since the capture rate is twice larger in the former case. In this paper, we investigate the possible impact of right-handed neutrinos on the capture rate, assuming that massive neutrinos are Dirac particles and both right-handed neutrinos νR and left-handed antineutrinos ν‾L can be efficiently produced in the early Universe. It turns out that the capture rate can be enhanced at most by 28% due to the presence of relic νR and ν‾L with a total number density of 95 cm−3, which should be compared to the number density 336 cm−3 of cosmic neutrino background. The enhancement has actually been limited by the latest cosmological and astrophysical bounds on the effective number of neutrino generations Neff=3.14−0.43+0.44 at the 95% confidence level. For illustration, two possible scenarios have been proposed for thermal production of right-handed neutrinos in the early Universe.

Thermal right-handed neutrino production rate in the relativistic regime

The production rate of right-handed neutrinos from a Standard Model plasma at a temperature above a hundred GeV is evaluated up to NLO in Standard Model couplings. The results apply in the so-called relativistic regime, referring parametrically to a mass M ~ pi T, generalizing thereby previous NLO results which only apply in the non-relativistic regime M >> pi T. The non-relativistic expansion is observed to converge for M > 15 T, but the smallness of any loop corrections allows it to be used in practice already for M > 4 T. In the latter regime any non-covariant dependence of the differential rate on the spatial momentum is shown to be mild. The loop expansion breaks down in the ultrarelativistic regime M << pi T, but after a simple mass resummation it nevertheless extrapolates reasonably well towards a result obtained previously through complete LPM resummation, apparently confirming a strong enhancement of the rate at high temperatures (which facilitates chemical equilibration). When combined with other ingredients the results may help to improve upon the accuracy of leptogenesis computations operating above the electroweak scale.

Thermal 2-loop master spectral function at finite momentum

When considering NLO corrections to thermal particle production in the "relativistic" regime, in which the invariant mass squared of the produced particle is K^2 ~ (pi T)^2, then the production rate can be expressed as a sum of a few universal "master" spectral functions. Taking the most complicated 2-loop master as an example, a general strategy for obtaining a convergent 2-dimensional integral representation is suggested. The analysis applies both to bosonic and fermionic statistics, and shows that for this master the non-relativistic approximation is only accurate for K^2 > (8 pi T)^2, whereas the zero-momentum approximation works surprisingly well. Once the simpler masters have been similarly resolved, NLO results for quantities such as the right-handed neutrino production rate from a Standard Model plasma or the dilepton production rate from a QCD plasma can be assembled for K^2 ~ (pi T)^2.

Right-handed neutrino production at finite temperature: radiative corrections, soft and collinear divergences

The production and decay rate of massive sterile neutrinos at finite temperature receives next-to-leading order corrections from the gauge interactions of lepton and Higgs doublets. Using the Closed-Time-Path approach, we demonstrate that the perturbatively obtained inclusive rate is finite. For this purpose, we show that soft, collinear and Bose divergences cancel when adding the tree-level rates from 1 ↔ 3 and 2 ↔ 2 processes to vertex and wave-function corrections to 1 ↔ 2 processes. These results hold for a general momentum of the sterile neutrino with respect to the plasma frame. Moreover, they do not rely on non-relativistic approximations, such that the full quantum-statistical effects are accounted for to the given order in perturbation theory. While the neutrino production rate is of relevance for Leptogenesis, the proposed methods may as well be suitable for application to a more general class of relativistic transport phenomena.

Supersymmetric seesaw inflation

Supersymmetric Unified theories which incorporate a renormalizable Type I seesaw mechanism for small neutrino masses can also provide slow roll inflection point inflation along a flat direction associated with a gauge invariant combination of the Higgs, slepton and right handed sneutrino superfields. Inflationary parameters are related to the Majorana and Dirac couplings responsible for neutrino masses with the scale of inflation set by a right-handed neutrino mass $M_{\nu^c} \sim 10^6-10^{12}$ GeV. Tuning of the neutrino Dirac and Majorana superpotential couplings and soft Susy breaking parameters is required to enforce flatness of the inflationary potential. In contrast to previous inflection point inflation models the cubic term is dominantly derived from superpotential couplings rather than soft A-terms. Thus since $M_{\nu^c}>>M_{Susy}$ the tuning condition is almost independent of the soft supersymmetry breaking parameters and therefore more stable. The required fine tuning is also less stringent than for Minimal SUSY Standard Model (MSSM) inflation or Dirac neutrino 'A-term" inflation scenarios due to the much larger value of the inflaton mass. Reheating proceeds via `instant preheating' which rapidly dumps all the inflaton energy into a MSSM mode radiation bath giving a high reheat temperature $T_{rh} \approx M_{\nu^c}^{3/4}\, 10^{6}$ GeV $\sim 10^{11}- 10^{15} $ GeV. Thus our scenario requires large gravitino mass $> 50 $ TeV to avoid a gravitino problem. The `instant preheating' and Higgs component of the inflaton also imply a `non-thermal' contribution to Leptogenesis due to facilitated production of right handed neutrinos during inflaton decay. We derive the tuning conditions for the scenario to work in the realistic New Minimal Supersymmetric SO(10) GUT and show that they can be satisfied by realistic fits.

Impact of massive neutrinos on the Higgs self-coupling and electroweak vacuum stability

The presence of right-handed neutrinos in the type I seesaw mechanism may lead to significant corrections to the RG evolution of the Higgs self-coupling. Compared to the Standard Model case, the Higgs mass window can become narrower, and the cutoff scale become lower. Naively, these effects decrease with decreasing right-handed neutrino mass. However, we point out that the unknown Dirac Yukawa matrix may impact the vacuum stability constraints even in the low scale seesaw case not far away from the electroweak scale, hence much below the canonical seesaw scale of 10^15 GeV. This includes situations in which production of right-handed neutrinos at colliders is possible. We illustrate this within a particular parametrization of the Dirac Yukawas and with explicit low scale seesaw models. We also note the effect of massive neutrinos on the top quark Yukawa coupling, whose high energy value can be increased with respect to the Standard Model case.

Thermal right-handed neutrino production rate in the non-relativistic regime

We consider the next-to-leading order thermal production rate of heavy right-handed neutrinos in the non-relativistic regime m_top < pi*T << M, where m_top refers to the electroweak scale. Rephrasing the problem in an OPE language and making use of different techniques than a previous analysis by Salvio et al, we confirm the general structure of their result and many of the coefficients. We also extend the analysis to the next order in the non-relativistic expansion, thereby revealing the leading non-trivial momentum dependence, as well as to NNLO in couplings, revealing the leading sensitivity to thermal resummations. Our results are expressed as a sum of simple "master" structures, which renders them a suitable starting point for determining the next-to-leading order rate also in the relativistic regime pi*T ~ M.

Pair production of right-handed neutrinos in the left-right twin Higgs model at ILC and LHC

In the framework of the left-right twin Higgs model, we study pair production of the right-handed neutrinos at the International Linear Collider (ILC) and the CERN Large Hadron Collider (LHC). Our numerical results show that the production cross section of the process e + e -→ N (tu) is at the level of several tens fb at the ILC. However, the resonance production cross section can be significantly enhanced to the order of pb. The pair production cross section for the right-handed neutrinos is the level of several hundreds of fb at the LHC. In general, pair production of right-handed neutrinos gives rise to 2 l +4 j signal without missing energy. As long as the right-handed neutrinos are not too heavy, we conclude that its pair production might be used to test for the left-right twin Higgs model at the future ILC and LHC experiments.

Right-handed neutrino production in dense and hot plasmas

For Dirac neutrinos with magnetic moment, we compute the production rate for right-handed neutrinos in a hot and dense QED plasma containing an initial population of left-handed neutrinos thermally distributed. The most important mechanisms for ν L depolarization, or production of right-handed neutrinos, are the ν L → ν R chirality flip and the plasmon decay to ν ̄ L+ν R . The rates for these processes are computed in terms of a resummed photon propagator which consistently incorporates the background effects to leading order. Applying the results to the cases of supernovae core collapse and the primordial nucleosynthesis in the early universe, we obtain upper limits on the neutrino magnetic moment.

Dirac neutrino scatterings and nuclear matter effects in supernova core

We consider in detail the production of right-handed neutrinos via helicity-flipping weak-interaction scatterings of neutrinos in the supernova core. We point out the frame dependence of the helicity-flipping scattering cross sections, and present a numerical study of the cross sections in the center-of-mass and laboratory frames. We also discuss briefly the many-body corrections to the scatterings. In particular, we find that for degenerate neutrinos the neutrino-nucleon scattering rate is reduced to be comparable to the neutrino-lepton one.