# Variation of stable birational types in positive characteristic

Stefan Schreieder

###### Abstract

Let $k$ be an uncountable algebraically closed field and let $Y$ be a smooth
projective $k$-variety which does not admit a decomposition of the diagonal.
We prove that $Y$ is not stably birational to a very general hypersurface of
any given degree and dimension. We use this to study the variation of the
stable birational types of Fano hypersurfaces over fields of arbitrary
characteristic. This had been initiated by Shinder [Shi19], whose method works
in characteristic zero.

  *     * Keywords. Hypersurfaces, Rationality problems, Variation of stable rationality, Decomposition of the diagonal.

2010 Mathematics Subject Classification. 14J70, 14E08, 14M20, 14D06

[Français]

Titre. Variation du type birationnel stable en caractéristique positive
Résumé. Soient $k$ un corps algébriquement clos non dénombrable et $Y$ une
$k$-variété lisse et projective qui n’admet pas de décomposition de la
diagonale. Nous montrons que $Y$ n’est pas stablement birationnelle à une
hypersuface très générale de degré et de dimension quelconques. Nous mettons
ceci à profit pour étudier la variation du type birationnel stable des
hypersurfaces de Fano sur des corps de caractéristique arbitraire. Ceci a été
initié par Shinder [Shi19], dont la méthode fonctionne en caractéristique
nulle.

###### Contents

  1. 1\. Introduction
  2. 2\. Notations
  3. 3\. Intersection theory on strictly semi-stable schemes
  4. 4\. The diagonal distinguishes stable birational types
  5. 5\. Applications to hypersurfaces
  6. 6\. Questions
  7. References

##  1\. Introduction

Starting with a breakthrough of Voisin [Voi15], an improvement by Colliot-
Thélène and Pirutka [CTP16a], and later a further improvement by the author
[Sch19a], there has recently been major progress in proving that certain
unirational or rationally connected varieties are not stably rational, see
e.g. [AO18, Bea16, BB16, CTP16a, CTP16b, HKT16, HPT18, HPT19, HT16, KO18,
Oka19, Sch18, Sch19a, Sch19b, Tot16, Voi15]. In fact, in all these examples,
it is shown that the variety in question does not admit a decomposition of the
diagonal, which implies stable irrationality, because stably rational
varieties admit such a decomposition. Motivated by a recent result of Shinder
[Shi19] with an appendix by Voisin [Voi19], we prove in this paper that the
non-existence of a decomposition of the diagonal does not only prevent the
variety from being stably rational, but in fact from being stably birational
to a very general hypersurface of any given degree and dimension.

###### Theorem 1.1

Let $X$ and $Y$ be smooth projective varieties over an uncountable
algebraically closed field $k$ of arbitrary characteristic, and let $d,n\geq
1$ be integers. Assume that $Y$ does not admit a decomposition of the diagonal
and that $X\subset\mathbb{P}^{n+1}_{k}$ is a hypersurface of degree $d$ which
is very general with respect to $Y$. Then $X$ and $Y$ are not stably
birational to each other.

Note that $X$ and $Y$ in the above theorem are not necessarily of the same
dimension, and $Y$ is not assumed to be a hypersurface. The condition that the
hypersurface $X\subset\mathbb{P}^{n+1}_{k}$ is very general with respect to
$Y$ means that it lies outside a countable union of proper closed subsets of
the linear series $|\mathcal{O}_{\mathbb{P}^{n+1}_{k}}(d)|$, and these subsets
depend on $Y$. For $d=1$, our theorem specializes to the aforementioned well-
known fact that a smooth projective variety which does not admit a
decomposition of the diagonal is stably irrational. Theorem 1.1 will be
deduced from a more general result, which we prove in Theorem 4.1 below and
which applies to a wide range of varieties other than hypersurfaces as well.
In fact, it applies to any variety which admits a strictly semi-stable
degeneration whose special fibre has universally trivial Chow group of zero-
cycles. In the case of hypersurfaces, we use a degeneration to a general
hyperplane arrangement as in [Shi19]. The above theorem implies in particular
the following.

###### Corollary 1.2

Let $k$ be an uncountable algebraically closed field of arbitrary
characteristic. If there is a hypersurface $Y\subset\mathbb{P}^{n+1}_{k}$ of
degree $d$ which does not admit a decomposition of the diagonal, then two very
general hypersurfaces of degree $d$ in $\mathbb{P}^{n+1}_{k}$ are not stably
birational to each other.

In characteristic zero, a slightly stronger variant of Corollary 1.2 had
previously been proven by Shinder [Shi19, Theorem 1.2], who obtains the same
conclusion under the assumption that $Y$ is smooth and stably irrational.
Improving earlier results of Kollár [Kol95] and Totaro [Tot16], the author
showed in [Sch19b] that over any uncountable field $k$ of characteristic
different from two, a very general hypersurface $X\subset\mathbb{P}_{k}^{n+1}$
of dimension $n\geq 3$ and degree $d\geq\log_{2}(n)+2$ does not admit a
decomposition of the diagonal and so it is not stably rational. By the above
corollary, we thus obtain the following strengthening of that result.

###### Corollary 1.3

Let $k$ be an uncountable field of characteristic different from two. Then two
very general hypersurfaces in $\mathbb{P}^{n+1}_{k}$ of degree
$d\geq\log_{2}(n)+2$ and dimension $n\geq 3$ are not stably birational to each
other.

By [Sch19c, Corollary 1.2], which generalizes the main result of [Sch19b] to
characteristic two, we obtain the following variant in characteristic two.

###### Corollary 1.4

Let $k$ be an uncountable field of characteristic two. Then two very general
hypersurfaces in $\mathbb{P}^{n+1}_{k}$ of degree $d\geq\log_{2}(n)+3$ are not
stably birational to each other.

In [Shi19], Shinder deduced Corollary 1.3 in the case where
$\operatorname{char}(k)=0$ from [Sch19a] with the help of his joint work with
Nicaise [NS19] on the specialization of stable birational types in
characteristic zero. In an appendix to [Shi19], Voisin [Voi19] used
decompositions of the diagonal and unramified cohomology to obtain similar
results in low dimensions. Since [NS19], as well as Kontsevich–Tschinkel’s
generalization in [KT19], rely heavily on the weak factorization theorem, and
hence on resolution of singularities, it is unclear how to generalize
Shinder’s approach to positive characteristic. Similarly, Voisin’s analysis
[Voi19] requires resolution of singularities of the singular hypersurfaces
with unramified cohomology constructed in [Sch19b], which in general is
unknown in positive characteristic. Moreover, Voisin’s approach did not allow
to reprove Shinder’s characteristic zero result in dimensions greater than
nine, because it requires the knowledge of certain unramified cohomology
groups of very general hypersurfaces, which seems out of reach in high
dimensions. Our usage of decompositions of the diagonal is more direct than in
[Voi19] and we do in particular not use unramified cohomology. Instead, our
approach relies on a moving lemma of Gabber, Liu and Lorenzini [GLL13, Theorem
2.3] and intersection theory on strictly semi-stable schemes using Fulton’s
work [Ful98]. While we use a degeneration to a general hyperplane arrangement
as in [Shi19], our approach does not rely on the weak factorization theorem or
Hironaka’s resolution of singularities and does in particular not use the
results in [NS19] and [KT19]. While the above results are formulated over
uncountable fields, Theorem 4.1 below together with [Tot16] and [Sch19b] also
allows to produce explicit examples of Fano hypersurfaces $X$ and $X^{\prime}$
over small fields $k$ (e.g. $k=\mathbb{Q}(t)$ or $k=\mathbb{F}_{p}(t,s)$)
which over the algebraic closure of $k$ are neither stably rational, nor
stably birational to each other. If $k$ is not of characteristic two, the
slopes $\frac{\deg X}{\dim X+1}$ of these examples may be chosen arbitrarily
small, see Theorem 5.2 below. It is worth to compare the results on variation
of stable birational types in [Shi19] and the present paper with the concept
of birational rigidity (see [Kol19] for a recent survey), which allows to
prove that for certain classes of smooth projective Fano varieties of Picard
rank one, any birational equivalence is an isomorphism. In particular, in
these cases the birational types vary as much as possible. By a result of de
Fernex [deF13], this applies for instance to smooth projective hypersurfaces
$X\subset\mathbb{P}^{n+1}_{\mathbb{C}}$ of degree $d=n+1$ and dimension $n\geq
3$. The corresponding result in positive characteristic is open. Birational
rigidity has so far mostly been applied to Fano varieties of index one and two
(see e.g. [Pu16, Pu20] for the index two case) and it is unknown whether the
method applies to hypersurfaces $X\subset\mathbb{P}^{n+1}_{\mathbb{C}}$ of
degree $d\ll n$. Finally, the condition on the Picard rank seems to prevent
applications to questions about stable birational equivalence.

##  2\. Notations

All schemes are separated. An algebraic scheme is a scheme of finite type over
a field. A variety is an integral algebraic scheme. If $k$ is an uncountable
field, a very general point of a $k$-variety $X$ is a closed point outside a
countable union of proper closed subsets. Let $R$ be a ring and let $X$ be an
$R$-scheme and $f$ be a morphism of $R$-schemes. Then for any ring extension
$R\subset A$, we denote the base change of $X$ and $f$ to $A$ by $X_{A}$ and
$f_{A}$, respectively. We denote the Chow group of algebraic cycles of
dimension $r$, resp. codimension $r$, modulo rational equivalence on an
algebraic scheme $X$ by $\operatorname{CH}_{r}(X)$, resp.
$\operatorname{CH}^{r}(X)$. For a standard reference on Chow groups, see
[Ful98]. For a codimension $r$ cycle $\gamma\in Z^{r}(X)$, we denote by
$[\gamma]\in\operatorname{CH}^{r}(X)$ its class in the Chow group and by
$|\gamma|$ its support, which is a reduced closed subscheme of pure
codimension $r$ in $X$. We say that two cycles $\gamma\in Z^{a}(X)$ and
$\gamma^{\prime}\in Z^{b}(X)$ meet properly if $|\gamma|\cap|\gamma^{\prime}|$
has the expected codimension $a+b$ in $X$. We use the same notation for Chow
groups and cycles in the more general setting where $X$ is only a scheme of
finite type over a regular base scheme $S$, replacing the dimension and
codimension by an appropriate relative notion, see [Ful98, Chapter 20]. An
algebraic scheme $X$ of pure dimension $n$ over a field $k$ admits a
decomposition of the diagonal if

$[\Delta_{X}]=[X\times z]+[Z_{X}]\in\operatorname{CH}_{n}(X\times_{k}X),$

where $z$ is a zero-cycle on $X$ and $Z_{X}$ is a cycle on $X\times_{k}X$
which does not dominate the first factor. If $X$ is integral with function
field $K=k(X)$, then this is equivalent to

$[\delta_{X}]=[z_{K}]\in\operatorname{CH}_{0}(X_{K}),$

where $z$ is the zero-cycle on $X$ from above and $\delta_{X}$ denotes the
zero-cycle on $X_{K}$ that is induced by the diagonal $\Delta_{X}$. If $X$ is
a proper variety that is stably rational (_i.e._ $X\times\mathbb{P}^{m}$ is
rational for some $m\geq 0$), then it admits a decomposition of the diagonal,
see [CTP16a, Lemme 1.5] for the case where $X$ is smooth and [Sch19b, Lemma
2.4] in general. We say that a proper algebraic scheme $X$ over a field $k$
has universally trivial Chow group of zero-cycles, if the degree map
$\deg:\operatorname{CH}_{0}(X_{L})\to\mathbb{Z}$ is an isomorphism for any
field extension $L/k$. If $X$ is geometrically integral and smooth over $k$,
then this is equivalent to the fact that $X$ admits a decomposition of the
diagonal, see e.g. [CTP16a, Proposition 1.4]. Let $R$ be a discrete valuation
ring with residue field $k$ and fraction field $K$. For an $R$-scheme
$\mathcal{X}$, we denote by $X_{0}:=\mathcal{X}\times_{R}k$ the special fibre
and by $X_{\eta}:=\mathcal{X}\times_{R}K$ and
$X_{\overline{\eta}}:=\mathcal{X}\times_{R}\overline{K}$ the generic and the
geometric generic fibres of $\mathcal{X}\to\operatorname{Spec}R$,
respectively. If $\mathcal{X}$ is proper and flat over $R$, we say that
$X_{\eta}$ degenerates or specializes to $X_{0}$. If additionally $k$ is
algebraically closed, we also say that for any field extension $L/K$, the base
change $X_{\eta}\times_{K}L$ (e.g. $X_{\overline{\eta}}$) degenerates or
specializes to $X_{0}$, cf. [Sch19a, Section 2.2]. We recall the definition of
strictly semi-stable $R$-schemes, which will be particularly important for us,
see e.g. [Har01, Definition 1.1].

###### Definition 2.1

Let $R$ be a discrete valuation ring and let
$\pi:\mathcal{X}\to\operatorname{Spec}R$ be a proper flat morphism with
$\mathcal{X}$ integral. The $R$-scheme $\mathcal{X}$ (or the morphism $\pi$)
is called strictly semi-stable, if the special fibre $X_{0}$ is a
geometrically reduced simple normal crossing divisor on $\mathcal{X}$, _i.e._
the irreducible components of $X_{0}$ are smooth Cartier divisors on
$\mathcal{X}$ and the scheme-theoretic intersection of $r$ different
components of $X_{0}$ is either empty or smooth and equi-dimensional of
codimension $r$ in $\mathcal{X}$.

The generic fibre $X_{\eta}$ of a strictly semi-stable $R$-scheme is
automatically smooth, because we assumed $\pi$ to be proper in the above
definition. Moreover, the total space $\mathcal{X}$ is regular, because its
generic fibre is smooth and each component of the special fibre is a smooth
Cartier divisor, see [Har01, Remarks 1.1.1 and 1.1.2]. The following lemma is
an immediate consequence of [Har01, Proposition 1.3].

###### Lemma 2.2

Let $R$ be a discrete valuation ring and let $\mathcal{X}$ be a strictly semi-
stable $R$-scheme and $\mathcal{X}^{\prime}$ be a smooth and proper
$R$-scheme. Then, $\mathcal{X}\times_{R}\mathcal{X}^{\prime}$ is also a
strictly semi-stable $R$-scheme. Moreover, if $A$ denotes the local ring of
$\mathcal{X}^{\prime}$ at a generic point of $X^{\prime}_{0}$, then the base
change $\mathcal{X}_{A}:=\mathcal{X}\times_{R}\operatorname{Spec}A$ is a
strictly semi-stable $A$-scheme.

##  3\. Intersection theory on strictly semi-stable schemes

Let $R$ be a discrete valuation ring or a field and let
$\pi:\mathcal{X}\to\operatorname{Spec}R$ be a separated scheme of finite type
over $R$. Let further $\gamma\in Z^{a}(\mathcal{X})$ and $\gamma^{\prime}\in
Z^{b}(\mathcal{X})$ be cycles whose (set-theoretic) intersection lies in the
smooth locus $U\subset\mathcal{X}$ of $\pi$. Then Fulton defines the
intersection $[\gamma]\cdot[\gamma^{\prime}]$ as a class in the Chow group of
$|\gamma|\cap|\gamma^{\prime}|$, see [Ful98, Section 20.2]. In particular, if
$\gamma$ and $\gamma^{\prime}$ meet properly, then the intersection
$\gamma\cdot\gamma^{\prime}$ is defined on the level of cycles and Fulton
proves that the corresponding class in the Chow group of $U$ depends only on
the rational equivalence classes of $\gamma$ and $\gamma^{\prime}$ on $U$. A
cycle $\gamma$ on $\mathcal{X}$ is said to be flat over $R$ if its support (as
a reduced scheme) is flat over $R$. This is equivalent to saying that no
component of $\gamma$ is contained in the special fibre. Recall that in
general there is no intersection product on the Chow group of singular
varieties. For us, it will however be important to compute the intersection of
certain cycles on strictly semi-stable $R$-schemes and in particular on their
special fibres, which are not necessarily smooth and it will not be enough for
us to know the corresponding identities on the smooth locus. In the following
two subsections, we deduce two auxiliary results from Fulton’s theory, which
allow us to overcome this difficulty.

###  3.A. Compatibility of specialization and products in a non-smooth setting

Let $R$ be a discrete valuation ring and let
$\pi:\mathcal{X}\to\operatorname{Spec}R$ be a flat morphism. Taking the
closure of a cycle on $X_{\eta}$ and restricting that cycle to $X_{0}$ yields
a well-defined specialization map

$\operatorname{sp}:\operatorname{CH}^{\ast}(X_{\eta})\to\operatorname{CH}^{\ast}(X_{0}),$

see [Ful98, Section 20.3]. If $\pi$ is smooth, this specialization map
commutes with the product structure on both sides. In the following we discuss
a variant of this compatibility result in a situation where $\pi$ is not
necessarily smooth. Let $\gamma\in Z^{a}(\mathcal{X})$ and $\gamma^{\prime}\in
Z^{b}(\mathcal{X})$ be cycles which meet properly in the smooth locus of $\pi$
and such that $|\gamma|\cap|\gamma^{\prime}|$ is flat over $R$. This implies
that the cycles $\gamma_{0}$ and $\gamma_{0}^{\prime}$ on $X_{0}$ meet
properly in the smooth locus of $X_{0}$. Hence, their intersection

$\gamma_{0}\cdot\gamma_{0}^{\prime}\in Z^{a+b}(X_{0})$

is defined on the level of cycles.

###### Lemma 3.1

In the above notation, we have

$\operatorname{sp}([\gamma_{\eta}]\cdot[\gamma^{\prime}_{\eta}])=[\gamma_{0}\cdot\gamma^{\prime}_{0}]\in\operatorname{CH}^{a+b}(X_{0}).$

  * Proof.

Since $\gamma$ and $\gamma^{\prime}$ meet properly in the smooth locus of
$\pi$, we can define their intersection on the level of cycles

$\gamma\cdot\gamma^{\prime}\in Z^{a+b}(\mathcal{X}).$

By the construction of the specialization homomorphism on Chow groups,

$\displaystyle\operatorname{sp}([\gamma_{\eta}]\cdot[\gamma^{\prime}_{\eta}])=[(\gamma\cdot\gamma^{\prime})_{0}],$
(3.1)

where $(\gamma\cdot\gamma^{\prime})_{0}\in Z^{a+b}(X_{0})$ denotes the
restriction of the cycle $\gamma\cdot\gamma^{\prime}$ to $X_{0}$. We claim
that

$\displaystyle(\gamma\cdot\gamma^{\prime})_{0}=\gamma_{0}\cdot\gamma^{\prime}_{0}\in
Z^{a+b}(X_{0}).$ (3.2)

To see this, note first that $|\gamma_{0}|\cap|\gamma_{0}^{\prime}|$ is
contained in the smooth locus of $X_{0}$. Recall next that for any smooth
$R$-scheme $U$, and cycles $\alpha$ and $\beta$ on $U$, Fulton defines an
intersection product $\alpha\cdot\beta$ as a class in the Chow group of
$|\alpha|\cap|\beta|$ and this construction commutes with specialization, see
[Ful98, Section 20.3]. Applying this to the smooth locus $U\subset\mathcal{X}$
of $\pi$, we find that

$[(\gamma\cdot\gamma^{\prime})_{0}]=[\gamma_{0}]\cdot[\gamma^{\prime}_{0}]\in\operatorname{CH}^{\ast}(|\gamma_{0}|\cap|\gamma_{0}^{\prime}|).$

Since $\gamma_{0}$ and $\gamma_{0}^{\prime}$ meet properly, the above equality
has to hold already on the level of cycles and so (3.2) follows. The lemma
follows now from (3.1) and (3.2). $\Box$

###  3.B. Intersection theory on the special fibre

Let $R$ be a discrete valuation ring and let
$\pi:\mathcal{X}\to\operatorname{Spec}R$ be a strictly semi-stable $R$-scheme.
Let us denote by $X_{0i},X_{0j}\subset X_{0}$ irreducible components of the
special fibre of $\pi$ and assume that $X_{0ij}:=X_{0i}\cap X_{0j}$ is non-
empty. Consider the following diagram, where all morphisms are given by the
natural inclusions:

$\textstyle{X_{0i}\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}$$\scriptstyle{\epsilon_{i}}$ØØ$\scriptstyle{\epsilon_{i}^{0}}$//$\textstyle{X_{0}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}$$\scriptstyle{\iota}$ﬄﬄ$\textstyle{X_{0ij}\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}$
$\scriptstyle{\epsilon^{i}_{ij}}$??
$\scriptstyle{\epsilon^{j}_{ij}}$ØØ$\scriptstyle{\epsilon_{ij}}$//$\textstyle{\mathcal{X}}$$\textstyle{X_{0j}\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}$
$\scriptstyle{\epsilon_{j}}$??
$\scriptstyle{\epsilon_{j}^{0}}$//$\textstyle{X_{0}\ignorespaces\ignorespaces\ignorespaces\ignorespaces}$$\scriptstyle{\iota}$OO

Let $\beta\in Z^{b}(\mathcal{X})$ be a cycle that is flat over $R$. Since
$\mathcal{X}$ is semi-stable over $R$, $X_{0i}$ and $X_{0j}$ are Cartier
divisors on $\mathcal{X}$, and $X_{0ij}$ is a Cartier divisor on $X_{0i}$, as
well as on $X_{0j}$. Hence, $\epsilon_{ij}$, $\epsilon_{i}$ and $\epsilon_{j}$
are regular embeddings and so the pullbacks

$(\epsilon_{ij})^{\ast}[\beta]\in\operatorname{CH}^{b}(X_{0ij}),\ \
(\epsilon_{i})^{\ast}[\beta]\in\operatorname{CH}^{b}(X_{0i})\ \ \text{and}\ \
(\epsilon_{j})^{\ast}[\beta]\in\operatorname{CH}^{b}(X_{0j})$

are well-defined on the level of Chow groups, see [Ful98, Section 20.1]. For
any closed subvariety $Z\subset X_{ij}$, the projection formula thus shows

$(\epsilon_{ij}^{i})_{\ast}([Z]\cdot(\epsilon_{ij})^{\ast}[\beta])=(\epsilon_{ij}^{i})_{\ast}[Z]\cdot(\epsilon_{i})^{\ast}[\beta]\
\ \text{and}\ \
(\epsilon_{ij}^{j})_{\ast}([Z]\cdot(\epsilon_{ij})^{\ast}[\beta])=(\epsilon_{ij}^{j})_{\ast}[Z]\cdot(\epsilon_{j})^{\ast}[\beta].$

Applying the pushforwards $(\epsilon^{0}_{i})_{\ast}$ and
$(\epsilon^{0}_{j})_{\ast}$, respectively, this implies

$\displaystyle(\epsilon^{0}_{i})_{\ast}((\epsilon_{ij}^{i})_{\ast}[Z]\cdot(\epsilon_{i})^{\ast}[\beta])=(\epsilon^{0}_{j})_{\ast}((\epsilon_{ij}^{j})_{\ast}[Z]\cdot(\epsilon_{j})^{\ast}[\beta])\in\operatorname{CH}^{\ast}(X_{0}).$
(3.3)

We use this compatibility, to prove the following.

###### Lemma 3.2

Let $R$ be a discrete valuation ring and let
$\pi:\mathcal{X}\to\operatorname{Spec}R$ be a strictly semi-stable $R$-scheme
with special fibre $X_{0}$. Let $\alpha_{0},\tilde{\alpha}_{0}\in
Z^{a}(X_{0})$ be cycles on $X_{0}$ that are supported on the smooth locus of
$X_{0}$. Let further $\beta\in Z^{b}(\mathcal{X})$ be a cycle on $\mathcal{X}$
that is flat over $R$ and denote its restriction to $X_{0}$ by $\beta_{0}$.
Assume that $\beta_{0}$ meets $\alpha_{0}$ and $\tilde{\alpha}_{0}$ properly.
If $\alpha_{0}$ and $\tilde{\alpha}_{0}$ are rationally equivalent on $X_{0}$,
then so are the cycles

$\alpha_{0}\cdot\beta_{0}\in Z^{a+b}(X_{0})\ \ \text{and}\ \
\tilde{\alpha}_{0}\cdot\beta_{0}\in Z^{a+b}(X_{0}).$

  * Proof.

Let $\tau:\tilde{X}_{0}\to X_{0}$ be the normalization of $X_{0}$. Since $\pi$
is strictly semi-stable, $\tilde{X}_{0}=\sqcup_{i}\tilde{X}_{0i}$ is the
disjoint union of the components $\tilde{X}_{0i}=X_{0i}$ of $X_{0}$. Let
$\tau_{i}:=\tau|_{\tilde{X}_{0i}}$ be the restriction of $\tau$ to the
component $\tilde{X}_{0i}$ and let $\iota:X_{0}\to\mathcal{X}$ be the
inclusion of the special fibre. Then $\iota\circ\tau_{i}$ is a regular
embedding and so we may consider the pullback

$(\iota\circ\tau_{i})^{\ast}[\beta]\in\operatorname{CH}^{b}(\tilde{X}_{0i}).$

Since $\beta$ is flat over $R$, the above pullback is already defined on the
level of cycles. This allows us to define

$\tau^{\ast}\beta_{0}:=\sum_{i}(\iota\circ\tau_{i})^{\ast}\beta\in
Z^{b}(\tilde{X}_{0})=\bigoplus_{i}Z^{b}(\tilde{X}_{0i}).$

Since $\alpha_{0}$ and $\tilde{\alpha}_{0}$ are supported on the smooth locus
of $X_{0}$, the pullbacks
$\tau^{\ast}\alpha_{0},\tau^{\ast}\tilde{\alpha}_{0}\in Z^{a}(\tilde{X}_{0})$
are defined on the level of cycles as well. By construction of these
pullbacks, we have the following identities of cycles on $X_{0}$:

$\displaystyle\tau_{\ast}(\tau^{\ast}\alpha_{0}\cdot\tau^{\ast}\beta_{0})=\alpha_{0}\cdot\beta_{0}\
\ \text{and}\ \
\tau_{\ast}(\tau^{\ast}\tilde{\alpha}_{0}\cdot\tau^{\ast}\beta_{0})=\tilde{\alpha}_{0}\cdot\beta_{0}.$
(3.4)

Assume now that $\alpha_{0}$ and $\tilde{\alpha}_{0}$ are rationally
equivalent on $X_{0}$. To show that the intersections
$\alpha_{0}\cdot\beta_{0}$ and $\tilde{\alpha}_{0}\cdot\beta_{0}$ are
rationally equivalent on $X_{0}$ as well, it suffices by (3.4) to prove the
following: Let $Z\subset\tilde{X}_{0}$ be a closed integral subscheme of
codimension $a$, then the class

$\displaystyle\tau_{\ast}([Z]\cdot[\tau^{\ast}\beta_{0}])\in\operatorname{CH}^{a+b}(X_{0})$
(3.5)

is invariant under the following operations:

    1. 1.

replace $Z$ by a rationally equivalent cycle on $\tilde{X}_{0}$;

    2. 2.

if $Z\subset\tilde{X}_{0i}$ and $\tau(Z)\subset X_{0i}\cap X_{0j}$, then
replace $Z$ by the unique subscheme $Z^{\prime}\subset\tilde{X}_{0j}$ with
$\tau(Z)=Z^{\prime}$.

Since $\tilde{X}_{0}$ is smooth, it is clear that (3.5) is invariant under
(1), see [Ful98, §1.4 and §8]. Moreover, invariance under (2) follows from
(3.3), which concludes the proof of the lemma. $\Box$

##  4\. The diagonal distinguishes stable birational types

It is well-known that the non-existence of a decomposition of the diagonal
prevents a variety from being stably rational, _i.e._ from being stably
birational to a point. The purpose of this section is to prove the following
theorem, which shows that decompositions of the diagonal allow to distinguish
two stable birational types in much greater generality. Partial results in
this direction have previously been obtained by Voisin [Voi19].

###### Theorem 4.1

Let $R$ be a discrete valuation ring with algebraically closed residue field
$k$. Let $\pi:\mathcal{X}\to\operatorname{Spec}R$ and
$\pi^{\prime}:\mathcal{X}^{\prime}\to\operatorname{Spec}R$ be flat projective
morphisms with geometrically connected fibres such that $\pi$ is strictly
semi-stable and $\pi^{\prime}$ is smooth. Assume

  1. 1.

the special fibre $X_{0}$ of $\pi$ has universally trivial Chow group of zero-
cycles;

  2. 2.

the special fibre $X^{\prime}_{0}$ of $\pi^{\prime}$ does not admit a
decomposition of the diagonal.

Then the geometric generic fibres of $\pi$ and $\pi^{\prime}$ are not stably
birational to each other.

If $\pi$ is smooth, then the above theorem is known and follows by similar
arguments as in [Voi19]. However, for most applications (e.g. to
hypersurfaces) it is essential to allow $\pi$ to have singular special fibre.
Theorem 4.1 can be seen as a cycle-theoretic analogue of the main result in
[NS19], where Nicaise and Shinder showed that the geometric generic fibres of
two strictly semi-stable families over a discrete valuation ring of equal
characteristic zero are not stably birational, unless the special fibres have
the same class in the Grothendieck ring of varieties modulo the class of the
affine line. Compared to [NS19, Shi19], the main advantage of the above
theorem is that it works over fields of arbitrary characteristic. Note however
that there is also an advantage in characteristic zero, because the assumption
on the universal $\operatorname{CH}_{0}$-triviality holds in various cases
where Shinder’s assumption does not. For instance, a general arrangement of
$d$ hyperplanes in $\mathbb{P}^{n+1}$ is always universally
$\operatorname{CH}_{0}$-trivial, but its class in the Grothendieck group of
varieties is congruent to $1$ mod $\mathbb{L}$ only if $d\leq n+1$. As an
immediate corollary of Theorem 4.1 and [Sch19a, Lemma 8], we have for
instance.

###### Corollary 4.2

Let $k$ be an uncountable algebraically closed field of arbitrary
characteristic. Let $\pi:\mathcal{X}\to C$ be a flat projective morphism
between smooth $k$-varieties $\mathcal{X}$ and $C$ with $\dim C=1$. Let $0\in
C$ be a closed point and assume that

  1. 1.

the very general fibre of $\pi$ does not admit a decomposition of the
diagonal;

  2. 2.

the special fibre $X_{0}$ is a reduced simple normal crossing divisor on
$\mathcal{X}$ which has universally trivial Chow group of zero-cycles.

Then two very general fibres of $\pi$ are not stably birational to each other.

The key step in the proof of Theorem 4.1 is the following proposition.

###### Proposition 4.3

Let $R$ be a discrete valuation ring with algebraically closed residue field
$k$. Let $\pi:\mathcal{X}\to\operatorname{Spec}R$ and
$\pi^{\prime}:\mathcal{X}^{\prime}\to\operatorname{Spec}R$ be flat projective
schemes with connected fibres such that $\pi$ is strictly semi-stable and
$\pi^{\prime}$ is smooth. Assume

  1. 1.

the generic fibres of $\pi$ and $\pi^{\prime}$ are stably birational to each
other;

  2. 2.

the special fibre $X_{0}$ of $\pi$ has universally trivial Chow group of zero-
cycles.

Then the special fibre $X_{0}^{\prime}$ of $\pi^{\prime}$ admits a
decomposition of the diagonal.

  * Proof of Proposition 4.3.

Up to multiplying $\mathcal{X}$ and $\mathcal{X}^{\prime}$ with some
projective spaces (possibly of different dimensions), we may assume that there
is a birational map $f:X_{\eta}\dashrightarrow X^{\prime}_{\eta}$. In
particular, $\pi$ and $\pi^{\prime}$ are of the same relative dimension, which
we denote by $n$. Let

$\Gamma\subset\mathcal{X}\times_{R}\mathcal{X}^{\prime}$

be the closure of the graph of $f$, which is a cycle of codimension $n$ on
$\mathcal{X}\times_{R}\mathcal{X}^{\prime}$. Note that $\Gamma$ is
automatically flat over $R$. To explain the idea of the proof, assume first
that $\pi$ is smooth and let
$\Gamma^{t}\subset\mathcal{X}^{\prime}\times_{R}\mathcal{X}$ be the transpose
of $\Gamma$, which is nothing but the closure of the graph of $f^{-1}$. Since
$\pi$ is smooth, $\mathcal{X}\times_{R}\mathcal{X}^{\prime}$ is smooth over
$R$ and so we can define the composition of cycles
$\Gamma\circ\Gamma^{t}\in\operatorname{CH}^{n}(\mathcal{X}^{\prime}\times_{R}\mathcal{X}^{\prime})$
as pushforward of

$\displaystyle(\operatorname{pr}_{12}^{\ast}\Gamma^{t})\cdot(\operatorname{pr}_{23}^{\ast}\Gamma)\in\operatorname{CH}^{n}(\mathcal{X}^{\prime}\times_{R}\mathcal{X}\times_{R}\mathcal{X}^{\prime}).$
(4.1)

Since $f\circ f^{-1}=\operatorname{id}$, the restriction of this cycle to the
generic fibre of
$\mathcal{X}^{\prime}\times_{R}\mathcal{X}^{\prime}\to\operatorname{Spec}R$ is
the diagonal. It thus follows from the specialization map on Chow groups
[Ful98] that the special fibre of the above cycle must be rationally
equivalent to the diagonal of $X^{\prime}_{0}$. On the other hand, this cycle
is given by the pushforward of

$\displaystyle(\operatorname{pr}_{12}^{\ast}\Gamma^{t}_{0})\cdot(\operatorname{pr}_{23}^{\ast}\Gamma_{0})\in\operatorname{CH}^{n}(X^{\prime}_{0}\times_{k}X_{0}\times_{k}X^{\prime}_{0})$
(4.2)

to $X^{\prime}_{0}\times_{k}X^{\prime}_{0}$. Since $X_{0}$ has universally
trivial Chow group of zero-cycles, the restriction of $\Gamma^{t}_{0}$ to the
generic fibre $k(X^{\prime}_{0})\times_{k}X_{0}$ of the projection
$X^{\prime}_{0}\times_{k}X_{0}\to X^{\prime}_{0}$ is rationally equivalent to
the base change $k(X^{\prime}_{0})\times z$ of a zero-cycle $z$ on $X_{0}$. As
we already know that the pushforward of (4.2) to
$X^{\prime}_{0}\times_{k}X^{\prime}_{0}$ is rationally equivalent to the
diagonal, this information is enough to conclude that $X^{\prime}_{0}$ admits
a decomposition of the diagonal. In this argument, we used heavily that $\pi$
is smooth, as the intersections in (4.1) and (4.2) are not even defined in the
non-smooth setting. The idea that allows us to bypass this difficulty is to
replace in (4.1) the first factor of
$\mathcal{X}^{\prime}\times_{R}\mathcal{X}\times_{R}\mathcal{X}^{\prime}$ by
the localization of $\mathcal{X}^{\prime}$ at the generic point of
$X^{\prime}_{0}\subset\mathcal{X}^{\prime}$. This way, the intersections in
(4.1) and (4.2) are of relative dimension zero and so we can hope to use a
moving lemma to make sense of these intersections. Moreover, by localizing at
the generic point of $X^{\prime}_{0}\subset\mathcal{X}^{\prime}$ it is still
possible to exploit the information that $k(X_{0}^{\prime})\times X_{0}$ has
trivial Chow group, which will eventually allow us to prove what we want. We
give the details of this argument in what follows. Let
$A:=\mathcal{O}_{\mathcal{X}^{\prime},\eta_{X^{\prime}_{0}}}$ be the local
ring of $\mathcal{X}^{\prime}$ at the generic point of
$X^{\prime}_{0}\subset\mathcal{X}^{\prime}$. This is a discrete valuation
ring, because $X^{\prime}_{0}\subset\mathcal{X}^{\prime}$ is a Cartier
divisor. By Lemma 2.2, the base change
$\mathcal{X}_{A}:=\mathcal{X}\times_{R}\operatorname{Spec}A$ is strictly semi-
stable over $A$. Let

$\tilde{\gamma}\in Z^{n}(\mathcal{X}_{A})$

be the relative zero-cycle that is induced by $\Gamma$. Since $\pi$ is
strictly semi-stable, the smooth locus $U\subset\mathcal{X}$ of $\pi$ meets
each component of any fibre of $\pi$ in a non-empty Zariski open subset. Since
$\mathcal{X}_{A}$ is regular, the moving lemma for relative zero-cycles in
[GLL13, Theorem 2.3] thus shows that

    1. ($\sharp$)

there is a cycle $\gamma\in Z^{n}(\mathcal{X}_{A})$ with
$[\gamma]=[\tilde{\gamma}]\in\operatorname{CH}^{n}(\mathcal{X}_{A})$, which
does not meet the singular locus of
$\pi_{A}:\mathcal{X}_{A}\to\operatorname{Spec}A$ and also not the locus in
$\mathcal{X}_{A}$ over which the projection
$\operatorname{pr}_{1}:\Gamma_{A}\to\mathcal{X}_{A}$ is non-flat, where
$\Gamma_{A}\subset\mathcal{X}_{A}\times_{A}\mathcal{X}^{\prime}_{A}$ denotes
the base change of $\Gamma$ to $A$.

By Lemma 2.2, $\mathcal{X}_{A}\times_{A}\mathcal{X}^{\prime}_{A}$ is a semi-
stable $A$-scheme. Consider the following diagram

$\textstyle{\mathcal{X}_{A}\times_{A}\mathcal{X}^{\prime}_{A}\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces\ignorespaces}$$\scriptstyle{p}$ØØ
$\scriptstyle{q}$¨¨
$\textstyle{\mathcal{X}_{A}}$$\textstyle{\mathcal{X}^{\prime}_{A}}$

where $p$ and $q$ denote the corresponding projections. Since $q$ is flat,
there is a flat pullback map $q^{\ast}$ on the level of cycles. Condition
($\sharp$) implies the following:

    1. ($\sharp^{\prime}$)

$q^{\ast}\gamma$ is a cycle on
$\mathcal{X}_{A}\times_{A}\mathcal{X}^{\prime}_{A}$ which intersects
$\Gamma_{A}$ properly in the smooth locus of
$\mathcal{X}_{A}\times_{A}\mathcal{X}^{\prime}_{A}$ over $A$.

Therefore, the intersection $(q^{\ast}\gamma)\cdot(\Gamma_{A})$ is defined on
the level of cycles, and so we may consider the cycle

$\displaystyle\alpha:=p_{\ast}((q^{\ast}\gamma)\cdot(\Gamma_{A}))\in
Z^{n}(\mathcal{X}^{\prime}_{A}).$ (4.3)

By ($\sharp$), the generic fibre of this cycle is rationally equivalent to

$p_{\ast}((q^{\ast}\tilde{\gamma}_{\eta})\cdot(\Gamma_{\eta})_{F(X^{\prime}_{\eta})})\in
Z^{n}\left((X^{\prime}_{\eta})_{F(X^{\prime}_{\eta})}\right),$

where $F$ denotes the fraction field of $R$. Since $\Gamma$ and
$\tilde{\gamma}$ are induced by the birational map $f:X_{\eta}\dashrightarrow
X^{\prime}_{\eta}$, the above cycle coincides with the zero-cycle
$\delta_{X^{\prime}_{\eta}}\in
Z^{n}\left((X^{\prime}_{\eta})_{F(X^{\prime}_{\eta})}\right)$ that is induced
by the diagonal of $X^{\prime}_{\eta}$. Hence,

$[\alpha_{\eta}]=[\delta_{X^{\prime}_{\eta}}]\in\operatorname{CH}^{n}\left((X^{\prime}_{\eta})_{F(X^{\prime}_{\eta})}\right).$

Applying the specialization map on Chow groups, we thus find

$\displaystyle[\alpha_{0}]=[\delta_{X^{\prime}_{0}}]\in\operatorname{CH}^{n}\left((X^{\prime}_{0})_{K}\right),$
(4.4)

where $K=k(X^{\prime}_{0})$ denotes the residue field of $A$ and
$\delta_{X^{\prime}_{0}}$ denotes the zero-cycle induced by the diagonal of
$X^{\prime}_{0}$. In what follows, we use Lemmas 3.1 and 3.2 to compute
$[\alpha_{0}]$ in another way, which will allow us to conclude that
$X^{\prime}_{0}$ admits a decomposition of the diagonal. By
($\sharp^{\prime}$), Lemma 3.1 applies and we get

$\operatorname{sp}([q^{\ast}\gamma_{\eta}]\cdot[\Gamma_{F(X^{\prime}_{\eta})}])=[(q^{\ast}\gamma_{0})\cdot(\Gamma_{0})_{K}]\in\operatorname{CH}_{0}\left((X_{0})_{K}\times_{K}(X_{0}^{\prime})_{K}\right),$

where $q^{\ast}\gamma_{0}$ is the flat pullback of the zero-cycle
$\gamma_{0}\in Z_{0}((X_{0})_{K})$, given as special fibre of
$\gamma\to\operatorname{Spec}A$ and $(\Gamma_{0})_{K}$ is obtained via base
change to $K$ of the $n$-cycle on $X_{0}\times X^{\prime}_{0}$ given by the
special fibre of $\Gamma\to\operatorname{Spec}R$. Since specialization and
proper pushforward commute (see [Ful98, Proposition 20.3]), this yields

$\displaystyle[\alpha_{0}]=(p_{0})_{\ast}\left([(q_{0}^{\ast}\gamma_{0})\cdot(\Gamma_{0})_{K}]\right)\in\operatorname{CH}_{0}\left((X_{0}^{\prime})_{K}\right).$
(4.5)

Since $X_{0}$ has universally trivial Chow group of zero-cycles by assumption,

$[\gamma_{0}]=[z_{K}]\in\operatorname{CH}^{n}\left((X_{0})_{K}\right),$

where $z\in Z^{n}(X_{0})$ is a zero-cycle with $\deg
z=\deg\gamma_{0}=\deg\gamma_{\eta}=\deg\tilde{\gamma}_{\eta}=1$. By Lemma 3.2,
we conclude from (4.5) that

$[\alpha_{0}]=(p_{0})_{\ast}\left([(z\times
X^{\prime}_{0})_{K}\cdot(\Gamma_{0})_{K}]\right)$

holds in $\operatorname{CH}_{0}((X_{0}^{\prime})_{K})$. This implies in
particular

$[\alpha_{0}]\in\operatorname{im}\left(\operatorname{CH}_{0}(X^{\prime}_{0})\to\operatorname{CH}_{0}\left((X^{\prime}_{0})_{K}\right)\right).$

Comparing this with (4.4), we conclude that $X^{\prime}_{0}$ admits a
decomposition of the diagonal, as claimed. This concludes the proof of the
proposition. $\Box$

Proposition 4.3 deals with stably birational generic fibres. To deal with
stably birational geometric generic fibres, we need the following result of
Hartl [Har01] about the behaviour of strictly semi-stable schemes under base
change.

###### Lemma 4.4

Let $R$ be a discrete valuation ring with algebraically closed residue field
$k$ and let $\pi:\mathcal{X}\to\operatorname{Spec}R$ be strictly semi-stable.
Let $R\subset R^{\prime}$ be a finite extension of discrete valuation rings
and let $\mathcal{X}_{R^{\prime}}$ be the base change of $\mathcal{X}$ to
$R^{\prime}$. Then there is a finite sequence of blow-ups

$\tilde{\mathcal{X}}:=V^{r}\to V^{r-1}\to\dots\to V^{1}\to
V^{0}:=\mathcal{X}_{R^{\prime}},$

where $\tilde{\mathcal{X}}$ is strictly semi-stable over $R^{\prime}$ and
$V^{i}\to V^{i-1}$ is the blow-up of $V^{i-1}$ along an irreducible component
of the special fibre $V^{i}_{0}$ of $V^{i}$ which is not Cartier. Moreover,
pushforward via the natural map induces an isomorphism on Chow groups

$\operatorname{CH}_{0}\left((\tilde{X}_{0})_{L}\right)\stackrel{{\scriptstyle\sim}}{{\lx@xy@svg{\hbox{\raise
0.0pt\hbox{\kern
1.0pt\hbox{\ignorespaces\ignorespaces\ignorespaces\hbox{\vtop{\kern
0.0pt\offinterlineskip\halign{\entry@#!@&&\entry<EMAIL_ADDRESS>0.0pt\hbox{\hbox{\kern 0.0pt\raise 0.0pt\hbox{\hbox{\kern 1.0pt\raise
0.0pt\hbox{$\textstyle{\ignorespaces\ignorespaces\ignorespaces\ignorespaces}$}}}}}}}\ignorespaces\ignorespaces\ignorespaces\ignorespaces{}{\hbox{\kern
1.0pt\vrule width=0.0pt,depth=0.0pt,height=0.0pt}}\ignorespaces{\hbox{\kern
10.99998pt\raise 0.0pt\hbox{\hbox{\kern
0.0pt\raise-2.5pt\hbox{\xy@@atfont\char 47\relax\xy@@btfont\char
47\relax}}}}}{\hbox{\kern 1.0pt\vrule
width=9.99998pt,depth=0.0pt,height=0.0pt}}{\hbox{\kern 21.00002pt\vrule
width=-5.00002pt,depth=0.0pt,height=0.0pt}}{\hbox{\kern 16.0pt\raise
0.0pt\hbox{\hbox{\kern 0.0pt\raise 0.0pt\hbox{\hbox{\kern 1.0pt\raise
0.0pt\hbox{$\textstyle{}$}}}}}}}\ignorespaces}}}}\ignorespaces}}\operatorname{CH}_{0}\left((X_{0})_{L}\right)$

after base change to any extension $L$ of $k$.

  * Proof.

The first assertion is [Har01, Proposition 2.2]. The proof of [Har01,
Proposition 2.2] shows that in each step $V^{i}\to V^{i-1}$, the central fibre
$V^{i}_{0}$ is obtained from $V^{i-1}_{0}$ by glueing in a Zariski locally
trivial $\mathbb{P}^{1}$-bundle. Hence the assertion about the Chow group of
$\tilde{X}_{0}$ follows. $\Box$

We are now able to prove Theorem 4.1.

  * Proof of Theorem 4.1.

Replacing $R$ by its completion, we may assume that $R$ is a complete discrete
valuation ring and we denote the fraction field of $R$ by $K$. For a
contradiction, we assume that $X_{\overline{\eta}}$ is stably birational to
$X^{\prime}_{\overline{\eta}}$. Then there is a finite field extension $L/K$,
such that $X_{\eta}\times_{K}L$ is stably birational to
$X^{\prime}_{\eta}\times_{K}L$. Let $R^{\prime}$ denote the integral closure
of $R$ in $L$. Since $R$ is a complete discrete valuation ring, $R\to
R^{\prime}$ is a finite map of rings and $R^{\prime}$ is a complete discrete
valuation ring as well, see [EGAIV, Théorème 23.1.5 and Corollaire 23.1.6].
Replacing $\pi$ and $\pi^{\prime}$ by the base change to $R^{\prime}$, our
families have all the initial properties, apart from the strict semi-stability
of $\mathcal{X}$, which we will lose in general. However, by Lemma 4.4, there
is projective birational morphism $\tilde{\mathcal{X}}\to\mathcal{X}$ such
that $\tilde{\mathcal{X}}$ is a strictly semi-stable $R$-scheme and such that
the special fibres $\tilde{X}_{0}$ and $X_{0}$ have isomorphic Chow groups of
zero-cycles after any extension of the base field. Hence, $\tilde{X}_{0}$ has
universally trivial Chow group of zero-cycles, because the same holds true for
$X_{0}$ by assumptions. Therefore, Proposition 4.3 shows that $X^{\prime}_{0}$
admits a decomposition of the diagonal, which contradicts our assumptions.
This proves the theorem. $\Box$

##  5\. Applications to hypersurfaces

Theorem 1.1 follows from the following more precise result.

###### Theorem 5.1

Let $k$ be an uncountable algebraically closed field of arbitrary
characteristic and let $Y$ be a smooth projective variety over $k$ which does
not admit a decomposition of the diagonal. For some $d,n\geq 1$, let
$\ell\subset\mathbb{P}(H^{0}(\mathbb{P}^{n+1}_{k},\mathcal{O}(d)))$ be a
general pencil of hypersurfaces of degree $d$ in $\mathbb{P}^{n+1}_{k}$ which
contains a general arrangement of $d$ hyperplanes as one of its members. (The
pencil $\ell$ does not need to be very general, and also not general with
respect to $Y$.) Then $Y$ is not stably birational to a very general member of
the pencil $\ell$.

  * Proof.

Let $\pi:\mathcal{X}\to\ell\simeq\mathbb{P}^{1}_{k}$ be the universal family
of the pencil. By assumptions, there is a point $0\in\ell$ such that
$X_{0}=\\{l_{1}\cdots l_{d}=0\\}$ for general linear polynomials $l_{i}$. By
the genericness assumption on $\ell$, $\mathcal{X}$ is smooth away from the
central fibre $X_{0}$. In [Shi19, Lemma 3.6], Shinder computes a resolution of
singularities $\tau:\widetilde{\mathcal{X}}\to\mathcal{X}$ by repeatedly
blowing-up the proper transforms of $\\{l_{i}=0\\}$ for $i=1,\dots,d$ and
shows that $\tilde{X}_{0}$ is a reduced simple normal crossing divisor on
$\tilde{\mathcal{X}}$, all of whose components are rational. (Shinder’s paper
is written over an uncountable algebraically closed field of characteristic
zero, but that computation holds more generally over an arbitrary field.)
Hence, $\tilde{X}_{0}$ has universally trivial Chow group of zero-cycles.
Consider the families $\tilde{\pi}:\widetilde{\mathcal{X}}\to\ell$ and
$\pi^{\prime}:\mathcal{X}^{\prime}:=Y\times_{k}\ell\to\ell$. Applying Theorem
4.1 to the base change of these families to the local ring of $\ell$ at $0$,
we conclude that the geometric generic fibres
$\widetilde{X}_{\overline{\eta}}$ and $Y\times\overline{k(\ell)}$ are not
stably birational to each other. We thus conclude from [Sch19a, Lemma 8] that
also the very general fibres of $\tilde{\pi}$ and
$\pi^{\prime}:\mathcal{X}^{\prime}\to\ell$ are not stably birational to each
other. That is, the very general fibre of $\tilde{\pi}$ (which coincides with
the very general fibre of $\pi$) is not stably birational to $Y$, as we want.
$\Box$

  * Proof of Corollary 1.2.

Assume that there is a hypersurface $Y\subset\mathbb{P}^{n+1}_{k}$ of degree
$d$ which does not admit a decomposition of the diagonal. By the
specialization map on Chow groups [Ful98, Proposition 11.1], we may assume
that $Y$ is very general and so it is in particular smooth. Hence, Corollary
1.2 follows from Theorem 1.1. $\Box$

  * Proof of Corollary 1.3.

Passing from $k$ to its algebraic closure, we may assume that $k$ is an
uncountable algebraically closed field of characteristic different from two.
By [Sch19b, Theorem 8.1], a very general hypersurface
$X\subset\mathbb{P}_{k}^{n+1}$ of degree $d\geq\log_{2}(n)+2$ and dimension
$n\geq 3$ does not admit a decomposition of the diagonal and so the result
follows from Corollary 1.2. $\Box$

  * Proof of Corollary 1.4.

Passing from $k$ to its algebraic closure, we may assume that $k$ is an
uncountable algebraically closed field of characteristic two. By [Sch19c,
Corollary 1.2], a very general hypersurface $X\subset\mathbb{P}_{k}^{n+1}$ of
degree $d\geq\log_{2}(n)+3$ does not admit a decomposition of the diagonal and
so the result follows from Corollary 1.2. $\Box$

###### Theorem 5.2

Let $k=\mathbb{Q}(t)$ or $k=\mathbb{F}_{p}(s,t)$. There are smooth Fano
hypersurfaces $X$ and $X^{\prime}$ over $k$ of the same dimension and degree,
which over the algebraic closure of $k$ are neither stably rational, nor
stably birational to each other. Moreover, if $k$ is not of characteristic
two, the slopes $\frac{\deg X}{\dim X+1}$ of these examples may be chosen
arbitrarily small.

  * Proof.

We may write $k=F(t)$, where $F=\mathbb{Q}$ or $F=\mathbb{F}_{p}(s)$. Let us
first deal with the case where $k$ has characteristic different from two. Then
[Sch19b, Theorem 8.3] implies that there is a smooth projective Fano
hypersurface $Z$ over $F$ of arbitrarily small slope whose base change to the
algebraic closure $\overline{F}$ of $F$ does not admit a decomposition of the
diagonal. We may write $Z=\\{f=0\\}$ for some irreducible homogeneous
polynomial $f\in F[x_{0},\dots,x_{n+1}]$ of degree $d$. (It is possible to
deduce an explicit description of $f$ from [Sch19b].) Consider the discrete
valuation ring $R:=\overline{F}[t]_{(t)}$ whose residue field is the algebraic
closure of $F$. We then define the flat projective $R$-schemes

$\mathcal{X}:=\\{tf+l_{1}\cdots l_{d}=0\\}\ \ \text{and}\ \
\mathcal{X}^{\prime}:=\\{f+tg=0\\},$

where $g\in F[x_{0},\dots,x_{n+1}]$ is general with $\deg g=d$ and
$l_{1}\cdots l_{d}$ is a product of general linear polynomials $l_{i}\in
F[x_{0},\dots,x_{n+1}]$. Note that the generic fibres $X_{\eta}$ and
$X^{\prime}_{\eta}$ can be defined over $k$. Moreover, the geometric generic
fibres $X_{\overline{\eta}}$ and $X^{\prime}_{\overline{\eta}}$ are smooth and
do not admit decompositions of the diagonal, because they both specialize (via
$t^{-1}\to 0$, resp. $t\to 0$) to $Z_{\overline{F}}$, which is smooth and does
not admit a decomposition of the diagonal. In particular,
$X_{\overline{\eta}}$ and $X^{\prime}_{\overline{\eta}}$ are stably
irrational; it remains to show that they are not stably birational to each
other. As in the proof of Theorem 1.1, Shinder’s computation in [Shi19, Lemma
3.6] shows that there is a projective modification
$\tilde{\mathcal{X}}\to\mathcal{X}$, such that $\widetilde{\mathcal{X}}$ is
strictly semi-stable over $R$ and such that $\tilde{X}_{0}$ has universally
trivial Chow group of zero-cycles. Since $g$ is general,
$\mathcal{X}^{\prime}$ is smooth over $R$. Moreover, $X^{\prime}_{0}\simeq
Z_{\overline{F}}$ does not admit a decomposition of the diagonal. Applying
Theorem 4.1, we thus find that $X_{\overline{\eta}}$ and
$X^{\prime}_{\overline{\eta}}$ are not stably birational to each other. This
concludes the case where $k$ has characteristic different from two. If
$\operatorname{char}k=2$, then we may argue similarly by using Totaro’s result
[Tot16], which produces a smooth projective hypersurface
$Z\subset\mathbb{P}^{n+1}_{\mathbb{F}_{2}(s)}$ of any even degree $d\geq
2\lceil\frac{n+2}{3}\rceil$ which does not admit a decomposition of the
diagonal over the algebraic closure of $\mathbb{F}_{2}(s)$. The rest of the
argument is analogous to the one given above. $\Box$

##  6\. Questions

Let $H_{d,n}$ denote the coarse moduli space of smooth hypersurfaces of degree
$d$ in $\mathbb{P}^{n+1}$. Stable birational equivalence induces an
equivalence relation $\sim$ on the set of $k$-rational points $H_{d,n}(k)$ of
this moduli space and we can consider the quotient $H_{d,n}(k)/\sim$. A
priori, this quotient is only a set, but we can nonetheless study its size in
various ways. For instance, we may define the dimension of $H_{d,n}(k)/\sim$
as

$\min\\{\dim Z\mid Z\subset H_{d,n}(k)\ \text{is closed and}\ Z\to
H_{d,n}(k)/\sim\ \text{is surjective}\\}.$

By Theorem 5.1, we know that for any uncountable algebraically closed field
$k$ such that the very general element of $H_{d,n}(k)$ does not admit a
decomposition of the diagonal, the set $H_{d,n}(k)/\sim$ is uncountable. (By
[Sch19b], this holds for instance if $d\geq\log_{2}(n)+2$, $n\geq 3$ and
$\operatorname{char}(k)\neq 2$.) In particular, $H_{d,n}(k)/\sim$ is positive-
dimensional in these cases and it is natural to ask to improve this result.

###### Question 6.1

What can we say about the dimension of $H_{d,n}(k)/\sim$?

A related question that goes back to Voisin asks about the dimension of the
fibres of the quotient map $H_{d,n}(k)\to H_{d,n}(k)/\sim$.

######  Question 6.2 (Voisin)

What is the maximal dimension of a subvariety $Z\subset H_{d,n}(k)$ such that
all hypersurfaces parametrized by $Z$ are stably birational to each other?

In light of Theorem 5.2, it is also worth to ask the following.

###### Question 6.3

Are all smooth Fano hypersurfaces of given dimension and degree over
$\mathbb{Q}$ stably birational to each other over $\overline{\mathbb{Q}}$?

## Acknowledgements

I am grateful to the excellent referees for corrections and comments which
significantly improved the exposition, and to C. Liedtke for pointing out the
reference [Har01].

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