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SUMMARY:Two-brane structure and its manifestation
DTSTART;VALUE=DATE-TIME:20261030T092000Z
DTEND;VALUE=DATE-TIME:20261030T093500Z
DTSTAMP;VALUE=DATE-TIME:20261030T003014Z
UID:indico-contribution-6267@cern.ch
DESCRIPTION:Speakers: Konstantin Prokopev (MEPhI)\nWe study the neutrino s
 ector of an eight-dimensional two-brane model based on $f(R)$ gravity. The
  metric is warped:\n\n$ds^{2} = e^{2\\gamma(u)}\\eta_{\\mu\\nu}dx^{\\mu}dx
 ^{\\nu} - du^{2} - r^{2}(u)\\\,d\\Omega^{2}_{n-1}.$\n\nHere $u$ is the rad
 ial extra coordinate\, and the remaining three coordinates parametrise a c
 ompact internal manifold. The classical field equations admit a countable 
 family of solutions in which $\\gamma(u)$ and $r(u)$ vanish at two endpoin
 ts $u_{1}$ and $u_{2}$. Matter fields accumulate near these endpoints. A p
 air of four-dimensional branes therefore forms spontaneously\, as a soluti
 on of the gravitational dynamics. Nothing is assumed about their existence
 \, position or tension.\n\nWe then derive the consequences of this geometr
 y for the matter sector. Massless bulk fermions obey\n\n$\\left(\\partial_
 {u} + \\frac{\\partial_ur^{}(u)}{2r(u)} + 2\\partial_u\\gamma(u)\\right) Y
 (u) = 0$\n\nalong $u$. The solution is sharply peaked at $u_{1}$ and $u_{2
 }$. On the compact factor\, a normalisable chiral zero mode requires a sui
 table background gauge field. Each bulk fermion then has one chiral zero m
 ode per brane\, and the two sets of modes are effectively decoupled\, beca
 use $|Y(u)|^{2}$ is exponentially small between the branes. The Higgs vacu
 um expectation value differs on the two branes\, so masses and couplings o
 n brane-2 are free up to $m_{D}$. The gauge zero mode behaves differently:
  it is constant along $u$\, so the $Z$ boson is common to both branes.\n\n
 Dimensional reduction of the neutral-current action reflects this structur
 e. Besides the canonical on-brane vertices\, it produces off-diagonal neut
 rino–$Z$ couplings weighted by the overlap of the two profile functions.
  Exponential localisation makes this overlap of order $\\exp\\left[-m(u_{2
 }-u_{1})\\right]$\, that is about $10^{-35}$ for an interbrane distance $u
 _{2}-u_{1} \\approx 80\\\,m_{D}^{-1}$ and $m \\sim 1$. The geometry thus s
 upplies an exponentially small parameter without fine-tuning. A seesaw wit
 h the brane-2 vacuum expectation value places a superheavy sterile neutrin
 o on the hidden brane\, and the off-diagonal vertex lets it decay at a rat
 e suppressed by the squared overlap. The resulting lifetime is cosmologica
 l\, so the particle is a dark-matter candidate.\n\nThe decay $\\nu^{(2)} \
 \to \\nu^{(1)} Z^{\\ast} \\to \\nu^{(1)}\\nu^{(1)}\\bar{\\nu}^{(1)}$ gives
  a quasi-monochromatic flux peaked at $E_{\\nu} \\sim m_{2}$. We estimate 
 the diffuse rate and compare it with IceCube and KM3NeT data\, including K
 M3-230213A at $2.2 \\times 10^{8}\\\,\\mathrm{GeV}$. The competing annihil
 ation channel $\\bar{\\nu}^{(2)}\\nu^{(2)} \\to Z^{\\ast} \\to \\bar{\\nu}
 ^{(1)}\\nu^{(1)}$ falls about 18 orders of magnitude short\, because its p
 roduction rate scales as $m_{2}^{-4}$.\n\nhttps://indico.particle.mephi.ru
 /event/626/contributions/6267/
LOCATION: Conference Hall 1
URL:https://indico.particle.mephi.ru/event/626/contributions/6267/
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