Transcript Particle Physics on Noncommutative Spaces

Phenomenology of a
Noncommutative Spacetime
Xavier Calmet
University of Brussels (ULB)
Outline
• Why do we believe in a minimal length
• Motivations and goals
• Local gauge symmetries on noncommutative
spaces
• Bounds on space-time noncommutativity
• Space-Time symmetries of noncommutative spaces
• Gravity on noncommutative spaces
• Conclusions
Why do we believe in a minimal length?
A minimal length from QM and GR
Claim: GR and QM imply
•
that no operational
procedure exists which can
measure a distance less than
the Planck length.
•
Assumptions:
Hoop Conjecture (GR): if an amount of
energy E is confined to a ball of size R,
where R < E, then that region will
eventually evolve into a black hole.
Quantum Mechanics: uncertainty
relation.
Minimal Ball of uncertainty:
Consider a particle of Energy E which is not already a Black hole.
Its size r must satisfy:
where 1/E is the Compton wavelength and E comes from the
Hoop Conjecture. We find:
Could an interferometer do better?
Our concrete model:
We assume that the position operator
has discrete eigenvalues separated by a
distance lP or smaller.
• Let us start from the standard inequality:
• Suppose that the position of a test mass is measured at time
t=0 and again at a later time. The position operator at a later
time t is:
• The commutator between the position operators at t=0 and t is
• so using the standard inequality we have:
• At least one of the uncertainties x(0) or x(t) must be larger
than:
• A measurement of the discreteness of x(0) requires two position
measurements, so it is limited by the greater of
x(0) or x(t):
• This is the bound we obtain from Quantum Mechanics.
• To avoid gravitational collapse, the size R of our measuring device
must also grow such that R > M.
• However, by causality R cannot exceed t.
• GR and causality imply:
• Combined with the QM bound, they require x > 1 in Planck units or
• This derivation was not specific to an interferometer - the result is
device independent: no device subject to quantum mechanics, gravity
and causality can exclude the quantization of position on distances
less than the Planck length.
Motivations
• Space-time noncommutativity is an extension of quantum
mechanics:
Heisenberg algebra:
is extended with new noncommutative (NC) relations:
that lead to new uncertainty relations:
• This is a nice analogy to the Heisenberg uncertainty relations.
• Quantum mechanics and general relativity considered together
imply the existence of a minimal length in Nature: Gauge
theories with a fundamental length are thus very interesting.
• A class of models with a fundamental length are gauge theories
on noncommutative spaces (length ~ ).
• Noncommutative coordinates appear in nature: e.g. electron in
a strong B field (first Landau level can be described in terms of
NC coordinates). Tools which are developed can prove useful
for solid states physics.
• Idea of a noncommutative space-time is not new! It can be
traced back to Snyder, Heisenberg, Pauli etc. At that time the
motivation was that a cutoff could provide a solution to the
infinities appearing in quantum field theory.
• Nowadays, we know that renormalization does the job for
infinites of the Standard Model, but modifying space-time at
short distances will help for quantum gravity.
• Furthermore, the Standard Model needs to be extended if it is
coupled to gravity since it is then inconsistent: noncommutative
gauge theories are a natural candidate to solve this problem.
• Another motivation is string theory where these noncommutative
relations appear. But the situation is in that case very different!
Gauge Symmetries on Noncommutative Spaces
Goals
• How does the Standard Model of particle physics which is a
gauge theory based on the group SU(3)SU(2)U(1), emerge as
a low energy action of a noncommutative gauge theory?
• The main difficulty is to implement symmetries on NC spaces.
• We need to understand how to implement SU(N) gauge
symmetries on NC spaces.
• Are there space-time symmetries (Lorentz invariance) for
noncommutative spaces?
Symmetries and Particle Physics
commutative space-time case
Impose invariance of the action under certain transformations.
Two symmetries are crucial in order to formulate the Standard
Model of particle physics:
- Space-time: Lorentz invariance, and combinations of C, P and
T e.g.:
- Local gauge symmetries
Enveloping algebra approach to NC
QuickTime™ and a
TIFF(Uncompres sed) decompressor
are needed t o see this pict ure.
• Goal: derive low energy effective actions for NC actions
which are too difficult to handle.
• Strategy: map NC actions to an effective action on a
commutative space-time such that higher order operators
describe this special property of space-time.
• There is an alternative to taking fields in the Lie algebra:
consider fields in the enveloping algebra
Definitions and Gauge Transf.
def. 1: consider the algebra
algebra of noncommutative functions
def. 2: generators of the algebra: ``coordinates´´
def. 3:
: elements of the algebra
infinitesimal gauge transformation:
note that the coordinates do not transform under a
gauge transformation:
one has:
that’s not covariant! Introduce a covariant coordinate
such that
i.e.
let’s set
this implies:
that’s the central result: relation between coord. gauge fields
and Yang-Mills fields!
That’s not trivial: problem with direct product!
Star product & Weyl quantization
def: commutative algebra of functions:
aim: construct a vector space isomorphism W. Choose a way to
“decompose” elements of :
(basis):
we need to def. the product (noncommutative multiplication) in
:
Weyl quantization procedure:
Let us use the Campbell-Baker-Hausdorff formula:
We then have:
to leading order:
We now have the first map:
we know how to replace the argument of the functions, i.e. the NC
coordinates by usual coordinates: price to pay is the star product.
This is done using the isomorphism
.
The second map will map the function , this second map
(Seiberg-Witten map) is linked to gauge invariance, more later.
Field Theory
Let us start from the relations:
the Yang-Mills gauge potential
is defined as
has the usual transformation property:
The covariant coordinate leads to the Yang-Mills potential!
Local gauge theories on NC spaces
• Let
be Lie-algebra valued gauge transformations, the
commutator:
is a gauge transformation only for U(N) gauge
transformations in the (anti)fundamental or adjoint
representation.
Problem: Standard Model requires SU(N)!
BUT, it can close for all groups if we take the fields and
gauge transformations to be in the enveloping algebra:
Is there an infinite number of degrees of freedom?
No! They can be reduced using Seiberg-Witten maps!
Consistency condition and Seiberg-Witten map
1.
Replace the noncommutative variable by a
commutative one. Price to pay is the introduction of
the star product:
2.
Let us consider the commutator once again:
Let us now assume that
algebra:
one finds
in 0th order in  and
in the leading order in .
are in the enveloping
Previous partial differential equation is solved by:
Expanding the star product and the fields via the SW
maps in the leading order in theta, one finds:
SM on NC Space-Time
Problems:
a) direct product of groups
b) charge quantization
c) Yukawa couplings
d) “Trace” in the enveloping algebra
Solutions:
a)
One can’t introduce 3 NC gauge potentials:
must remain covariant!
solution: introduce a master field:
SW map for
Note that
b) Charge quantization problem:
solution to charge quantization: introduce n NC photons:
Too many degrees of freedom?
No  Seiberg-Witten map!
there is only one classical photon!
c) Yukawa couplings: left/right makes a difference!
Complication for Yukawa couplings:
is not NC gauge invariant if
on the r.h.s. or l.h.s.
Solution:
Hybrid SW map:
with
transforms only
d) trace for the gauge part of the action:
is a huge matrix. There is not a unique way to fix the trace,
gauge inv. only requires:
Minimal model:
Other choice
How to bound these models?
Tree level
Rigorous but low scale
Quantum level
High energy scale accessible but
It’s maybe not yet clear how to build
a quantum theory: Warning!
Tree level + test of
Lorentz inv.
not a direct test:
Warning!
Quantum level + test of
Lorentz inv.
High energy accessible but
not yet clear how to regularize this
theory and not a direct test: Warning!
• What is θ?
So in
principle
we have
6 scales!

0


  c01
 2
 1
 c02
  2
2

 c03
   2
3
c01
12
c02
 22
0
c13
 24
c13
 2
4
c14
 2
5
0
c24
 2
6
c03 
 23 

c14 
 25 

c24 
 26 


0 

Bounds on NC scale
From colliders:
• Lots of corrections to SM processes, but large background:
search for rare decays.
• Smoking gun for NC: Z-->   or Z--> g g.
• Limit on NC from LEP is around 143 GeV.
Bounds on NC scale
From colliders:
• Lots of corrections to SM processes, but large background:
search for rare decays.
• Smoking gun for NC: Z-->   or Z--> g g.
• Limit on NC from LEP is around 250 GeV
From low energy experiments:
•Bounds on
imply NC ~TeV from atomic clock comparison (Be9).
Note that the bound comes from Lorentz violation, and is thus not a
“direct” test of the noncommutative nature of space-time.
“Quantum Level” Bound
• One loop operator generated in NCQCD:
(Carlson et al. hep-ph/0107291), but there is a problem with that
paper: operator giving the bound
is actually vanishing.
• They considered the one loop correction to the quark mass and
wavefunction renormalization and performed their calculation
using Pauli-Villars regularization:
• They considered 3 operators separately
• Bound from first operator: this is wrong!!! Let us look again
at the matrix element:
• Using the Dirac equation it is obviously vanishing. Quarks
are onshell at this order in perturbation theory.
Quantum Mechanics and EDM
• There are claims in the literature that EDMs can put very tight
bounds on the scale for spacetime noncommutativity.
• A formulation of Quantum Mechanics on a NC spacetime is
needed to address this question.
• Let us start from the QED action on a NC spacetime:
• Two maps lead to the following action:
• And the Dirac eq. easily follows:
• Let us now prepare the non-relativistic expansion:
• And we
• From this it is easy to obtain the low energy Hamiltonian:
• 3 operators are CP violating:
• Let us look at one of them:
• However it is not of the shape:
i.e. there is no spin flip!
• Experiments searching for an EDM are not sensitive to this
operator: there is no bound!
• These experiments measure the energy difference between a
two-levels system. Here the effect cancels out.
Space-time symmetries of NC spaces
Consider NC:
Furthermore, one has the Heisenberg algebra:
Let us now do a variable transformation:
It leads to the following algebra:
Let us consider transformations of the commuting coordinates:
one also has
The invariant length is given by:
It is invariant if
We can now implement this transformation for the NC
coordinates:
The invariant length is given by:
the derivative is given by:
it transforms as
under a noncommutative Lorentz transformation.
The NC Yang-Mills potential transforms as:
and the covariant derivative as:
The field strength transforms as:
and a spinor as:
• This represents an extension of special relativity. The limit
  0 is well defined: one recovers the usual Lorentz
invariance. Note: we do not deform the Poincaré algebra!
• It is easy to verify that the actions discussed previously are
indeed invariant under these transformations.
• This symmetry is important because bounds on space-time
noncommutativity come from bounds on Lorentz violation
(atomic clocks). The bounds will be affected.
• Any operator derived from loop calculations must be
invariant under this symmetry: beware of artifacts of
regularization procedure.
Is microcausality violated?
• Let us look at the light cone of a photon on a NC spacetime:
• which is not
!
• Let us now compute (at equal time) the expectation value of the
commutator between
and
as done
by Greenberg. It is proportional to
He concludes that microcausality is violated. However this precisely
corresponds to our light cone: microcausality is not violated!
Quantization of Noncommutative QED
• Misuse of the term effective theory: mapped theory?
• Seiberg-Witten expansion is an expansion in g .
• If one expands in g and then quantize the theory (expansion in
g can miss important resummation effects.
terms of one
• This is indeed the case because of the vertices phases as we
shall see.
• Let us start from the unexpanded action:
• Fields are representation of the Lorentz group: quantize the
fields which are in the enveloping algebra.
• Add Faddeev-Popov terms
• Feynman rules are then given by:
• It is then straightforward to compute the beta function
• And the renormalized vertex :
• The quantized and renormalized action can then be mapped
on a commutative spacetime.
• The vertex correction is given by
Gravity on Noncommutative Spaces
Gravity on noncommutative spaces
• Hypothesis:  is a constant of nature and it has the same value in every
coordinate frame.
• Well if that is the situation, what are the coordinate transformations
allowed by the NC algebra:
• Let us consider the transformations:
and study the NC algebra:
• It is invariant iff
• The solutions are:
• They form a subgroup of 4-Volume preserving coord. transformations.
• We now want to implement this symmetry for a NC gravity action. We
consider iso(3,1) but restrict ourselves to the coord. transformations that
preserve 
• Consider the enveloping algebra:
• Consistency condition:
• Differential equations:
0th order in  :
1st order in  :
Solution:
Now for the spin connection:
One thus has:
• For the field strength one has:
• classically one has:
• Note that our covariant derivative is torsion free.
• Field strength for the local Lorentz symmetry:
• Noncommutative Riemann tensor:
• we can then define a noncommutative Ricci tensor:
• and a noncommutative Ricci scalar:
• It is easy to see that the leading order correction is vanishing:
•
•
•
•
with
which vanishes!
Second order corrections have to be calculated.
However the result is quite complicated.
It implies a that we need to know solutions to the consistency conditions to
second order in theta.
• OK get ready for the result:
• Action:
• Equations of motion
• Can be massaged into:
• Remarkable: on a canonical NC spacetime: the cosmological constant is
an integration constant uncorrelated to parameters of the action!
Some open questions
• Understand how to formulate gravity on NC spaces (non
constant ).
• CMB signatures of NC physics?
• Do NC black holes have singularities? What about horizon?
Hawking radiation? Information loss?
• Is quantum gravity formulated on NC spaces renormalizable?
• Fine tuning issues and NC physics. Certain short distance
modifications of space-time can modify the high energy
behavior of loops.
• New ideas to break gauge symmetries: after all lots of ideas
come from solid state physics and we have quite a few models
in solid state physics that are described by NC gauge theories.
This will lead to new phenomenology for the LHC.
Conclusions
• Noncommutative gauge theories are examples of non-local
theories with a minimal length.
• SU(N) gauge symmetries, which are crucial for the
Standard Model, can be implemented on noncommutative
spaces.
• Noncommutative Lorentz transformations can be
introduced.
• Constraints are not very severe! Bounds of the order of few
TeV only.
• Lots of open issues e.g. Loops, general relativity: work in
progress!
• Applications to solid state physics, cosmology etc.
• Even if there is a grand desert, Planck scale physics might
be accessible with low energy experiments.
• Exciting field in development.