# Connection (composite bundle)

Composite bundles $Y\to \Sigma \to X$ play a prominent role in gauge theory with symmetry breaking, e.g., gauge gravitation theory, non-autonomous mechanics where $X=\mathbb {R}$ is the time axis, e.g., mechanics with time-dependent parameters, and so on. There are the important relations between connections on fiber bundles $Y\to X$, $Y\to \Sigma$ and $\Sigma \to X$.

## Composite bundle

In differential geometry by a composite bundle is meant the composition
    $\pi :Y\to \Sigma \to X\qquad \qquad (1)$
of fiber bundles
    $\pi _{Y\Sigma }:Y\to \Sigma ,\qquad \pi _{\Sigma X}:\Sigma \to X.$
It is provided with bundle coordinates $(x^{\lambda },\sigma ^{m},y^{i})$, where $(x^{\lambda },\sigma ^{m})$ are bundle coordinates on a fiber bundle $\Sigma \to X$, i.e., transition functions of coordinates $\sigma ^{m}$ are independent of coordinates $y^{i}$.
The following fact provides the above-mentioned physical applications of composite bundles. Given the composite bundle (1), let $h$ be a global section of a fiber bundle $\Sigma \to X$, if any. Then the pullback bundle $Y^{h}=h^{*}Y$ over $X$ is a subbundle of a fiber bundle $Y\to X$.

## Composite principal bundle

For instance, let $P\to X$ be a principal bundle with a structure Lie group $G$ which is reducible to its closed subgroup $H$. There is a composite bundle $P\to P/H\to X$ where $P\to P/H$ is a principal bundle with a structure group $H$ and $P/H\to X$ is a fiber bundle associated with $P\to X$. Given a global section $h$ of $P/H\to X$, the pullback bundle $h^{*}P$ is a reduced principal subbundle of $P$ with a structure group $H$. In gauge theory, sections of $P/H\to X$ are treated as classical Higgs fields.

## Jet manifolds of a composite bundle

Given the composite bundle $Y\to \Sigma \to X$ (1), consider the jet manifolds $J^{1}\Sigma$, $J_{\Sigma }^{1}Y$, and $J^{1}Y$ of the fiber bundles $\Sigma \to X$, $Y\to \Sigma$, and $Y\to X$, respectively. They are provided with the adapted coordinates $(x^{\lambda },\sigma ^{m},\sigma _{\lambda }^{m})$, $(x^{\lambda },\sigma ^{m},y^{i},{\widehat {y}}_{\lambda }^{i},y_{m}^{i}),$, and $(x^{\lambda },\sigma ^{m},y^{i},\sigma _{\lambda }^{m},y_{\lambda }^{i}).$
There is the canonical map
    $J^{1}\Sigma \times _{\Sigma }J_{\Sigma }^{1}Y\to _{Y}J^{1}Y,\qquad y_{\lambda }^{i}=y_{m}^{i}\sigma _{\lambda }^{m}+{\widehat {y}}_{\lambda }^{i}$.

## Composite connection

This canonical map defines the relations between connections on fiber bundles $Y\to X$, $Y\to \Sigma$ and $\Sigma \to X$. These connections are given by the corresponding tangent-valued connection forms
    $\gamma =dx^{\lambda }\otimes (\partial _{\lambda }+\gamma _{\lambda }^{m}\partial _{m}+\gamma _{\lambda }^{i}\partial _{i}),$
    $A_{\Sigma }=dx^{\lambda }\otimes (\partial _{\lambda }+A_{\lambda }^{i}\partial _{i})+d\sigma ^{m}\otimes (\partial _{m}+A_{m}^{i}\partial _{i}),$
    $\Gamma =dx^{\lambda }\otimes (\partial _{\lambda }+\Gamma _{\lambda }^{m}\partial _{m}).$
A connection $A_{\Sigma }$ on a fiber bundle $Y\to \Sigma$ and a connection $\Gamma$ on a fiber bundle $\Sigma \to X$ define a connection
    $\gamma =dx^{\lambda }\otimes (\partial _{\lambda }+\Gamma _{\lambda }^{m}\partial _{m}+(A_{\lambda }^{i}+A_{m}^{i}\Gamma _{\lambda }^{m})\partial _{i})$
on a composite bundle $Y\to X$. It is called the composite connection. This is a unique connection such that the horizontal lift $\gamma \tau$ onto $Y$ of a vector field $\tau$ on $X$ by means of the composite connection $\gamma$ coincides with the composition $A_{\Sigma }(\Gamma \tau )$ of horizontal lifts of $\tau$ onto $\Sigma$ by means of a connection $\Gamma$ and then onto $Y$ by means of a connection $A_{\Sigma }$.

## Vertical covariant differential

Given the composite bundle $Y$ (1), there is the following exact sequence of vector bundles over $Y$:
    $0\to V_{\Sigma }Y\to VY\to Y\times _{\Sigma }V\Sigma \to 0,\qquad \qquad (2)$
where $V_{\Sigma }Y$ and $V_{\Sigma }^{*}Y$ are the vertical tangent bundle and the vertical cotangent bundle of $Y\to \Sigma$. Every connection $A_{\Sigma }$ on a fiber bundle $Y\to \Sigma$ yields the splitting
    $A_{\Sigma }:TY\supset VY\ni {\dot {y}}^{i}\partial _{i}+{\dot {\sigma }}^{m}\partial _{m}\to ({\dot {y}}^{i}-A_{m}^{i}{\dot {\sigma }}^{m})\partial _{i}$
of the exact sequence (2). Using this splitting, one can construct a first order differential operator
    ${\widetilde {D}}:J^{1}Y\to T^{*}X\otimes _{Y}V_{\Sigma }Y,\qquad {\widetilde {D}}=dx^{\lambda }\otimes (y_{\lambda }^{i}-A_{\lambda }^{i}-A_{m}^{i}\sigma _{\lambda }^{m})\partial _{i},$
on a composite bundle $Y\to X$. It is called the vertical covariant differential. It possesses the following important property.
Let $h$ be a section of a fiber bundle $\Sigma \to X$, and let $h^{*}Y\subset Y$ be the pullback bundle over $X$. Every connection $A_{\Sigma }$ induces the pullback connection
    $A_{h}=dx^{\lambda }\otimes [\partial _{\lambda }+((A_{m}^{i}\circ h)\partial _{\lambda }h^{m}+(A\circ h)_{\lambda }^{i})\partial _{i}]$
on $h^{*}Y$. Then the restriction of a vertical covariant differential ${\widetilde {D}}$ to $J^{1}h^{*}Y\subset J^{1}Y$ coincides with the familiar covariant differential $D^{A_{h}}$ on $h^{*}Y$ relative to the pullback connection $A_{h}$.