Download PhD dissertation - Pierre

Phenomenology of bulk scalar
production at the LHC
A PhD oral examination
By
Pierre-Hugues Beauchemin
A Small Title Analysis

Jargon:
Phenomenology:
Study of the essential structure
of nature that relates empirical
observations of phenomena to
each other in a way which is
consistent with a fundamental
theory, but without being


directly derived from it.

The structures (ingredients) of
our theoretical framework is
motivated by the more
fundamental String Theory
Conceptual framework
designed to relate Dark Energy
measurement to possible High
Energy physics observations
From this, physical predictions
can be made for a specific
experimental context
My work stand on the bridge between theory and
observation
What kind of predictions are under
concern here?

Jargon
Scalar production:
Controlled production or
“creation” of a specific
type of particle that
haven’t been observed
yet but is predicted by our
theoretical framework.


This scalar is gravitational
interacting spinless particle
The physical predictions
consist in deriving the
Feynman rules of their
coupling to ordinary field and
to compute their cross section,
ie their rate of production for a
specific experiment
Part of my work consisted in using the theoretical
framework to compute these quantities for scalar
direct production processes

Jargon
At the LHC
The LHC is the
experimental setup
which will provides the
empirical observations
that could reveal the
phenomena under
study.

The LHC is a ProtonProton collider at 14 TeV
so it can probe high mass
and high PT states.

To “see” the collision
products, 4 detectors will
be build on the 27 km LHC
ring.

The LHC and Atlas
detector design are
optimized for new physics
search
The second part of my work consist in determine if the
physics predictions that I computed from our theoretical
framework can experimentally be tested with the ATLAS
detector at the LHC
However…

There are already existing studies of the “phenomenology of
scalar production at the LHC”.
e.g.: Higgs search
So what is new, different and interesting about what I
studied???
ANSWER:

BULK

Extra dimensions
Completely new theoretical framework,
phenomena, set of predictions and
experimental signatures
Does it make sense to consider
that the world is 4+n dimensional?
OF COURSE!
There are examples of everyday life of objects
with more dimensions than we see.

What do we need to assert that the
fundamental structure of the Universe have
such a extra dimensions?

Some theoretically motivated concept or
mechanism that account for the fact that so far
precise particle physics experiment haven’t seen
such extra dimensions.

A way to justify that we can probe them.
The solution come from String Theory!
String Theory predicts the existence of branes
Our world as a 3-brane on which every SM fields
are confined.
Gravity is not confined to such brane.
Moreover…

If we add supersymmetry to our framework, we will have the
ingredients to solve probably the most annoying problems of
fundamental physics:
The cosmological constant problem

Recall the problem:
The vacuum energy predicted by quantum field theory is much
bigger (10E60 times) than our Dark Energy measurement in the
c.m.b. anisotropy (WMAP)
This new, well-motivated, rich and explanatory theoretical
framework will be called the SUPERSYMMETRIC LARGE
EXTRA DIMENSIONS (SLED) scenario
What is SLED involves?

In order to be able to solve the cosmological
constant problem, SLED requires:
2 extra dimensions of O(10mm)
 SM particles stuck to a 3-brane
 N=2 SUperGRAvity in the bulk
 SUSY strongly broken on the brane
 Bulk SUSY breaking scale of O(10-3eV)
 Exactly
We can use these structures to write down a low-energy
4D effective quantum field theory that generically couple
the KK-states of a massless bulk scalar to the brane SM
fields and draw physical predictions from it!
MY WORK THUS CONSIST IN:
MAKING THESE PREDICTIONS
and
USE THEM TO SHOW HOW
SUCH THEORY IS TESTABLE
AT THE LHC
Plan of the analysis

Write the effective 4D low-energy Lagrangian
describing the coupling of a SLED bulk scalar to SM
field

Concentrate on the lowest mass dimension
interaction term to:



Quarks and gluons
Higgs bosons
Evaluate the possibility to observe such scalar with
the ATLAS detector by studying:


Jet+ETmiss:
H+ETmiss:
Beauchemin et al. (J. Phys. G: Nucl. Part. Phys. 31)
Beauchemin et al. (J. Phys. G: Nucl. Part. Phys. 30)
LAGRANGIAN
This formal object summarize all the structures
contained in SLED and describe the dominant
coupling of a bulk scalar f with the SM fields.
Coupling to quarks and gluons:
Coupling to Higgs bosons:
Cross sections
From this we can predict the distributions for all the
experimental observables: PT, h, ETmiss, etc.
Extra dimensions phase space factor:
Where we followed the conventions of [hep-ph/9912459]:
After having compute these differential cross section, I wrote
Monte Carlo programs using PYTHIA to perform the integrations
numerically and to generate the events used to simulate ATLAS
detector’s outcomes.
Jet+ETmiss Experimental Analysis

The main standard background (ie. known events that leave
the same jet+ETmiss signature in the detector) are




pp→jet+Z(→nn) (277.6 fb)
pp→jet+W(→ene) (364.2fb)
pp→jet+W(→mnm) (363.7 fb)
pp→jet+W(→tnt) (363.3 fb)
Proc.:
Jet+…
Z→nn
W→en
W→mn
W→tn
Bulk
scalar
Total
27760
36420
36370
36330
30960
No
lepton
27100
5224
957
24600
30090
|fj1-fj2|
24940
1430
866
9459
27720
≤2.83
PTjet ≥ 500 GeV
Results
Discovery criteria:

99.99994% certainty that
it is not a statistical
fluctuation
With the 36700 background events (PT≥500 GeV), it is required that:
S ≥ 970 events  σ(pp→jet+φ) ≥ 10.9 fb
c  5.1  10 3 TeV 2
g  7.1  10
2
TeV
-1
where
n2
2
c  cMD
n
2
and g  gM D

To be valid and testable at the LHC, any model of bulk
scalar must satisfy the following inequalities that show
the ATLAS sensitivity to this new physics:
1  g  g (M )
(n)
obs
min
D
n
2
and 1  c  c (M )
(n)
obs
min
D
n2
2
gg+ETmiss Experimental Analysis

The main standard background are:






qq→gg
gg→gg
QCD jet-jet
QCD g-jet
qq→hZ→ggnn
qq→tth (h→gg)
(56.2 pb)
(49.0 pb)
( 7.0 pb)
(15.0 pb)
(1.22E-3 pb)
(1.28E-3 pb)
After applying standard cuts for h→gg search:
But looking to these graphs, we can see that we can get a
Better sensitivity of we also apply a cut on ETmiss
Conclusion

SLED scenario offers a fundamentally new
understanding of high energy physics and thus deserves
to be carefully studied

SLED has a rich phenomenology. In particular, it predicts
coupling of bulk scalars to SM particles.

I computed physical predictions for fairly generic bulkscalar and bulk scalar-Higgs interactions

I then showed that this theory can be tested at the LHC

Using cut on ETmiss, I evaluate ATLAS sensitivity to:
 qqf:
0.26 ≲g≲1
 ggf:
0.06≲c≲1
 hhf:
0.09≲a≲1
Back-up slides
What do gain from these considerations?

This brane technology allows for extra
dimensions as big as O(10mm) across.


Experimental test of the Newton law (classical) at distance
scales smaller than the radius R of the extra dimensions.
Explain why gravity is so weak.
The D-dimensional Planck scale will be:
M
2
Pl
R
n
M
n 2
D
R  10mm  R-1  1meV  MD  TeV
We can study quantum gravity in colliders!
Feynman rules
qqf:
 i(g  ig 5g 5 )
ggf:
 4i[c(p.q)g mn  cpn q m  c~ mn p q  ] ab
gggf:
 4 g 3 f abc [cg m n (p   q  )  cgn (kn  pn )  cgn (q m  k m )  c~ m n(p  q  k  )]