2016 White Paper for Compute Canada
Computing requirements for TRIUMF Theory
Department
TRIUMF
March 15, 2016
This document describes the computing requirements for the TRIUMF Theory
Department for 2016-2021. It is the first time that such document is
prepared. The TRIUMF theory department has already submitted a contribution
as part of the Subatomic Physics White Paper in 2016. Because our required
needs are considerably different than the rest of the theory community and are
expected to increase dramatically in the next 5 years, we include more
information in this separate document.
Overview
The TRIUMF Theory Department has established world-class expertise in
subatomic theory. We are leaders both in ab initio theories for the nucleus, as
well as in particle phenomenology. High-performance computing is essential
for the scientific viability and overall success of the theory program and to
maintain the current international leadership position of TRIUMF in nuclear
and particle theory. TRIUMF supports our program by providing access a local
theory cluster (with 244 cores and 5GB/core of memory and fast interconnect).
The theory cluster is used to produce, debug and test new computer codes.
For production runs we need external resources, because the theory cluster
cannot satisfy our demands. The applications developed in the TRIUMF Theory
Department are mostly memory intensive and highly parallelized, so that
large memory/node and high-speed interconnect are indispensable. Top
high-performance facilities that meet these requirements are, e.g., Titan at
Oak Ridge National Laboratory in the USA and JURECA in Europe. Theory
department staff members use such external resources obtained through
grant proposals and various established collaborations. It is expected that in
the near future, a large part of such computer resources will be requested to
Compute Canada. This will allow us to maintain leadership in the
collaborations, have better control over use of the resources and push for
Canadian flagship projects.
The TRIUMF theory department is an established hub for nuclear and particle
physics. Every year we host more than 50 scientists who visit us to collaborate
and to participate to our series of successful theory workshops [http://
www.triumf.ca/theory-workshops]. In the last few years, the staff members
together with their postdocs and students have achieved several
breakthroughs, which led to numerous high-impact publications. These
include 2 Nature Physics Papers, 16 Physical Review Letters and 4 Physics
1
Letters B papers, only in the last 3 years. Below we highlight some of our
results.
Highlights - Nuclear Theory
Since its discovery, more than one century ago, the atomic nucleus has been at
the centre of theoretical and experimental studies, playing a fundamental role
in the development of modern physics. The nucleus is a strongly correlated,
quantum many-body system. Its constituents, protons and neutrons, interact
between themselves mainly by strong interactions, giving rise to an ample
variety of phenomena. The main goal of modern nuclear structure and reaction
studies is to explain the unifying mechanisms by which these behaviors
emerge from the underlying strong nuclear interaction between nucleons.
At TRIUMF we have been developing the capability to theoretically describe
light- and medium-mass nuclei as systems of nucleons interacting by forces
derived from QCD. Such first principle approaches are named ab initio
calculations and at TRIUMF we are leaders in this field.
The framework of chiral effective field theory provides us with a systematic
basis for nuclear forces including consistent three-body forces and theoretical
uncertainties. One of the central challenges in nuclear physics is to understand
and predict the properties of stable and exotic nuclei, for which three-nucleon
forces are crucial and present a current frontier. Much of our work has
involved developing three-nucleon forces and powerful many-body methods
to explore strongly interacting systems, from exotic nuclei to neutron stars.
With recent theoretical and computational advances, we are in a key position
to connect the observations made in the laboratory (at TRIUMF and abroad) to
the underlying strong interactions governing the properties of nuclei and
neutron rich matter.
Fig1: Trend of the ab initio calculations for the A-nucleons problem as a function of
time.
2
High performance computing has been key to increase the reach of ab initio
methods to larger mass number A (equal to the sum of the number of protons
and of neutrons). In Fig. 1 we present the trend of ab initio calculations as a
function of time. In the early decades the growth in A was linear, because
while computing power increased exponentially according to Moore’s law, the
used algorithms were also exponentially scaling in A. In recent years, new
generations of algorithms were developed which exhibit polynomial scaling.
These, together with the new developments in the theory of nuclear forces,
have allowed to dramatically progress and reach mass number ~50. We expect
that in the near future we will be able to address even heavier nuclei.
Recently, together with American and European collaborators, we have
performed the first ab initio calculation of the 48Ca nucleus using coupledcluster theory. 48Ca is an asymmetric nucleus in terms of proton and neutron
numbers, as there are 20 protons and 28 neutrons. One of the quantity of
interest is the radial distribution of the protons and the neutrons, which are
quantified by the so called proton and neutron radii, Rp and Rn, respectively.
With more neutrons than protons it is expected that the neutrons will spatially
extend further than the protons and that a neutron skin Rskin will be formed,
with Rskin = Rn - Rp.
3.6
Rp [fm]
3.5
3.4
3.3
3.2
3.1
0.15
0.18
Rskin [fm]
Fig2:
0.21
3.3
3.4
3.5
Rn [fm]
3.6
2.0
2.4
2.8
↵D [fm3]
48Ca
coupled-cluster theory calculations (blue squares) of the point-proton
radius (Rp), the point-neutron radius (Rn), the skin radius (Rskin) and the electric dipole
polarizability. The grey diamonds are density functional theory calculation and the
green line is the experimental value of Rp. Figure adapted from Ref. [1].
Using state-of-the-art algorithms and three-nucleon forces, we computed
those quantities and found that they are very much correlated with each other.
In particular, we also observed that the proton radius Rp correlates very
strongly with the electric dipole polarizability, a quantity which contains
information about the excitation spectrum of the nucleus. This is shown in
Fig. 2, where every blue square corresponds to a different coupled-cluster
calculation. Using the approximately linear correlation and intersecting the
blue band with the green line representing the experimental value for Rp, we
drew narrow constraints for the skin radius and the electric dipole
polarizability [1]. Both quantities are targeted by future experiments. The skin
3
radius of 48Ca is going to be measured at Jefferson Laboratory in the USA
using parity violating electron scattering, while the electric dipole polarizability
is being measured in Japan using inelastic proton scattering. Interestingly, we
find that the skin radius of 48Ca is smaller than previously thought and in
disagreement with other theoretical predictions from density functional
theory, as represented by the grey diamonds in Fig. 2.
TRIUMF contribution was essential in the development of the theory for the
electric dipole polarizability. In fact, by combining integral transform methods
and the many-body coupled-cluster theory, we have recently developed a new
method to tackle electromagnetic break-up reactions in the medium mass
regime. Before 48Ca, our first successful applications of the method have
addressed the photo-absorption of Oxygen isotopes and Calcium isotopes
[2,3].
18
S2n (MeV)
16
14
12
10
AME2003
TITAN
ISOLTRAP
NN+3N
8
6
28
29
30
31
32
53
54
Neutron Number N
Fig.3: Two-neutron separation energy (difference of binding energies) of neutron-rich
Ca isotopes: measurements by TITAN and ISOLTRAP in comparison to the atomic mass
evaluations and state-of-the-art theory calculations (blue line).
Understanding and predicting the formation of shell structure in exotic nuclei
is a central challenge for nuclear theory. Atomic mass measurements
performed at TRIUMF greatly help revealing detailed information of the
effective interaction of nucleons, by providing access to the nuclear binding
energies. The atomic masses are determined with significant precision and
accuracy using the TRIUMF’s ion trap for atomic and nuclear science (TITAN).
Using this system it was possible for the first time to determine the masses of
the very neutron-rich Ca and K isotopes [4] and make interesting comparison
to state-of-the-art theory. Figure 3 shows such a comparison for the Calcium
isotopes. The experimental results found for 52Ca deviated by almost 2 MeV
from the previous measurements but agree well with the predictions from
modern theory [5] where three nucleon forces were included.
More recently, the ISOLTRAP collaboration at ISOLDE/CERN was able to first
confirm the TITAN measurements and further advance the limits of precision
mass measurements out to 54Ca using a new multi-reflection time-of-flight
4
mass spectrometer. The new 53,54Ca masses are in excellent agreement with
modern theoretical predictions and unambiguously establish N = 32 as a shell
closure [6], also shown in Figure 3.
Weakly bound and unbound exotic nuclei produced at TRIUMF can only be
understood using methods that unify the description of both bound and
unbound states. Using the no-core shell model with continuum (NCSMC)
method we can predict the ground- and excited state energies of light nuclei
R
as well as their electromagnetic moments and transitions, including weak
transitions. Further, we can investigate properties of resonances and calculate
GUILLAUME
HUPIN, SOFIA QUAGLIONI, AND PETR NAVRÁTIL
PHYSICAL REVIEW C
cross sections of
nuclear reactions.
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(a)
o
o
Nurmela et al., 4
o
Nurmela et al., 15
o
Kim et al., 20
o
Nagata et al., 20
o
Pusa et al., 20
o
Wang et al., 20
o
Keay et al., 30
dσ/dΩp [b/sr]
ϕp = 4
1.5
o
ϕp = 15
o
ϕp = 20
1.0
1
0.5
4
dσ/dΩp [b/sr]
2.0
4
H(α,p) He
0.6
1
He states
7
6
5
ϕp = 3
4
H(α,p) He
(b)
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ϕp = 30
0
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10
15
Eα [MeV]
20
25
0.2
3
6
Bagl
Bogd
Brow
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Kim
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Eα [MeV]
9
FIG. 4. (Color online) Computed (lines) 1 H(α,p)4 He angular differential cross section at proton recoil angles
1
4He,p)4He angular differential
Fig.4: Computedand
(lines)
cross section
a function
a function
of the incident 4 He energy compared
with data as
(symbols)
from Refs. [9–15,41], and [4
30◦ as H(
of the incident 4He
energy
30◦ , showing, in addition to the most complete results, calculations includ
focus
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proton recoilwith
angledata
ϕp = (symbols).
states.
In the past years we have made a significant progress in the development and
implementation of the NCSMC [7,8] and first demonstrated its power in the
to various data sets over a wide range
of helium incident enof Ref. [23], we also expect a very small
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here underestimate
once again, deteriorates at intermediate energies due to the
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the findings in Ref. [37] for the g.s. of Li, and the substantially
improved convergence of the present results compared to those
beyond the scope of the present work. H
evidence that the present interaction leads t
of the width of the 5 Li g.s. resonance as we
insufficient splitting between this and the
With the ability to further reduce and con
uncertainties spurred by the development o
5
interactions and exascale
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solution of the Schrödinger equation is p
competitive approach to provide guidan
using light-nucleus cross sections.
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very nicely describes
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up to 40%.
Conclusions. We have presented the
6
describe in a unified way the structure of Li and cross sections of
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initio calculation of p-4 He elastic scatte
deuteron-4He scattering
and
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6
asymptotic D- stable
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the calculation
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with the
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nucleus [14].
6
10
∆ [%]
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0
Eα= 9.5 MeV
Highlights - Particle Theory
Modern high-energy particle theory relies heavily on advanced computing
tools. This is particularly true for theoretical work connected to the Large
Hadron Collider (LHC). The LHC is testing our best current description of
elementary particles, the Standard Model (SM), at higher energies than ever
before. It is also searching for particles and forces beyond those of the SM.
Theoretical calculations are essential for directing the experimental searches
towards the most promising areas, and for interpreting the results that are
found.
The TRIUMF theory department investigates the SM and develops and studies
models of new physics beyond the SM that could be discovered at the LHC. To
compare theoretical predictions against current and projected future data, the
LHC signals of particle physics theories are simulated using Monte Carlo
methods. This includes the generation of high-energy collision events,
followed by partonic showering and hadronization, and finally simulation of
the resulting LHC detector response. For each model, large numbers of
collisions events must be simulated, stored, and analyzed to provide
statistically significant kinematic distributions throughout the relevant phase
space. Moreover, it is usually necessary to repeat this procedure for many
different underlying models in order to scan over parameter spaces, and to
generate SM backgrounds with high statistics.
Fig.5: Excluded region in the M2-µ plane of electroweakino parameters derived in Ref.
[1] using current LHC data.
Recent work by the theory department in this area covers a wide range of
topics. The sensitivities of existing LHC searches to the electroweakinos of
the minimal supersymmetric extension of the SM were studied in Ref. [15]. In
supersymmetry, every SM particle is predicted to have a superpartner, and the
electroweakinos are the superpartners of the Higgs and weak vector bosons of
the SM. Existing LHC searches for these and other particles were reinterpreted
6
to derive more general exclusions of the parameter space of the minimal
supersymmetric SM. One such exclusion in the plane of M2 vs. µ, two of the
key parameters that determine the electroweakino masses, is shown in Fig.5. Electroweakinos are also promising candidates for the dark matter of the
Universe, and these exclusions provide significant constraints on this
possibility.
2m, Low Systematics
50
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40
gd
30
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20
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10
20
30
40
50
60
ma @GeVD
70
80
Fig.6: Required LHC luminosity of the 13 TeV LHC for sensitivity to a light
pseudoscalar with mass ma and coupling gd using the dimuon channel discussed in
Ref. [16].
New discoveries at particle colliders may help solve puzzles in astrophysics
and cosmology. The Fermi Space Telescope has observed an excess of cosmic
rays from the region near the galactic centre. Such an excess can arise from
the annihilation of dark matter particles, and fits of dark matter theories to the
signal are suggestive of a new pseudoscalar particle with mass between 20-80
GeV. New techniques to discover such a pseudoscalar at the upcoming 13 TeV
LHC runs were investigated in Ref. [16]. A range of kinematic cuts on final
states with dileptons were studied, and a set of specific combinations were
found that will provide sensitivity to a large fraction of the motivated theory
space with upcoming LHC data. This is illustrated in Fig.6 for a range of
pseudoscalar masses ma and couplings to the SM gd.
Other recent work includes LHC studies of mechanisms for the generation of
neutrino masses that operate near the weak scale [17,18], and theories with
extended Higgs sectors [19]. With the upcoming restart of the LHC at 13 TeV
and the large amount of expected data, it will become increasingly important
to have access to high-power computational resources to make the most of
the new discoveries expected in the years to come.
7
Anticipated Computing Needs
To continue in our leadership roles and to expand our capabilities at TRIUMF
we will need increasing computing resources. It will be a necessity that TRIUMF
theorists can have access to competitive Canadian high performance
computers and secure computing time for themselves, their Canadian highly
qualified personnel, as well as for the international collaborators. Some of us
already are working on Compute Canada but it is expected that in the future
more resources will be asked and other RAC proposals will be submitted.
Below we present a list of anticipated computing needs divided per method or
project.
Coupled-cluster theory for nuclei (S. Bacca)
We have recently developed a new method to tackle electroweak break-up
reactions in nuclei of light and medium-mass number. The idea is based on
the combination of integral transforms and the coupled-cluster theory for
nuclei, which are both well established many-body methods.
An equation of motion needs to be solved in coupled-cluster theory, which
demands considerable computational resources even for sd-shell nuclei and is
expected to substantially increase in the future, when we will address heavier
nuclei.
Codes are written in Fortran90 and utilize MPI and OpenMP parallelization. At
present, production calculations are performed at Oak Ridge National
Laboratory on Titan, which we have access to via our collaborations with USA
scientists. For our last paper, appeared on Nature Physics, we utilized 15
million core hours.
The Titan facility has 18,688 nodes with 16 cores each and 32 GB RAM (with a
Gemini fast interconnect). We typically calculate with about 200 nodes. For
calculating and storing all the matrix elements of the three-body force, we
need instead very large memory/node (ideally 512 GB/node). At the moment
we have access to a few of those nodes in the US, but we expect this not to be
sufficient in the future.
In terms of disk space, typically we need require 10 Tb per person. We have
recently developed a new method and several high impact factor publications
have appeared in the last couple of years from this method. We expect to be
able to reach heavy nuclei such as 90Zn and 132Sn, which will be a
breakthrough. We foresee our group to substantially increase, reaching up to
5 Canadian researchers (including students, postdocs and the PI) to run
coupled-cluster codes. This sets up our computing request for the future to
be access to 1000 nodes (32 cores and 32 GB RAM), 50 nodes (32 cores and
512 GB RAM) and 100 TB of disk space. We expect our requirements to grow
to 5000 core-years by 2020.
8
Hyperspherical Harmonics method for light nuclei (S. Bacca)
The hyperspherical harmonic method is based on a basis function expansion.
We have recently used this method to perform high precision calculations of
nuclear structure corrections in muonic atoms [20-23], which will help
understanding the proton radius puzzle.
After the hyperspherical basis is constructed, a large matrix needs to be
diagonalized using standard linear algebra packages. The computations are
memory intensive, as the size of the matrices is large.
The codes are written in Fortran90 and make use of MPI. The typical needs of
a production run are at least 8 nodes with 12 or 16 cores each, and the
necessary computing time is on the order of days. Typically, 5-6GB/core of
RAM is needed. We run these calculations both on the local TRIUMF cluster and
on computing facilities at the Hebrew University in Israel. We anticipate the
computing needs on this front to increase in the future because we will
address nuclear structure corrections to the hyperfine splitting and we intend
to do several calculations with different nuclear interactions to better assess
the overall precision. We also plan on addressing heavier nuclei, which will
require larger model spaces.
Our algorithms scale well for up to about 8-12 nodes, but for larger numbers
of nodes, the MPI communications slow down the speed-up. That is why we
run on a limited amount of nodes for longer time. A potential problem at
Compute Canada is the wall time limit, which we do not have on the local
TRIUMF cluster and at the Hebrew University’s facility. Hence, a dedicated
queuing system on Compute Canada would be very helpful for this project.
Many-body methods for medium-mass nuclei (J.D. Holt)
This work involves the application of complementary ab initio many-body
techniques, such as many-body perturbation theory, in-medium similarity
renormalization group, and coupled-cluster theory, tailored towards
calculations of properties medium- and heavy-mass exotic nuclei based on
nuclear forces. A significant focus of this work will be to interface with
forefront rare-isotope beam experiments performed at TRIUMF. Very
generally two independent steps are required: the production of two-nucleon
and three-nucleon matrix elements in suitable and suitably large basis spaces
and the actual many-body calculations themselves (all codes are parallelized
using MPI and OpenMP). The latter typically require a high number of nodes
(150-200), with 16+ processors/node, and significant memory available per
node (>64GB). Currently we rely on international computing facilities to
provide such capabilities, primarily the JURECA machine at the Jülich
Supercomputing Center in Germany, where our 2014-2015 project was
awarded one of only two John von Neumann Excellence Projects in 2014 and
granted 4.5M CPU hours (in 2015, we were awarded the equivalent number of
CPU hours on upgraded facilities). The latest configuration features 24 cores/
node with memory options ranging from 128-512GB/node. While this is
anticipated to meet our requirements for the near future, continued access is
9
not guaranteed. As our computing needs would be met ideally through
Canadian computing resources, access to large-scale machine(s) with
specifications similar to the above would adequately serve this research
program for the coming 3-5 years. Beyond this, we anticipate an increasing
need for high-memory nodes, driven by extending the many-body codes to
heavier nuclei, which also requires having nuclear forces available in
increasingly larger basis spaces (three-nucleon forces in particular present
tremendous memory hurdles, and can now only be performed using certain
truncation schemes).
Development and test runs, which comprise the bulk of current PhD student
and postdoc efforts, are essential for the progress of this program but
consume significant resources of a different nature. For these runs, fewer
nodes (10-20) are needed but require immediate starting (for debugging) and
runtimes of up to several days (whereas jobs at Jülich can wait days in a queue
and computing time is restricted to 24 hours/job). Furthermore, production of
starting three-nucleon forces within suitably large basis spaces has long been
unfeasible on supercomputing facilities due to limited walltime. With nextgeneration three-nucleon forces expected to become available within the next
year or two, the ability to have these quickly implemented will ensure this
program remains at the forefront of efforts in the medium- and heavy-mass
region. Finally, the expected increase in needs is driven by extending the
many-body codes to heavier nuclei, while increasing memory requirements
are driven by the need to have nuclear forces in increasingly larger basis
spaces (three-nucleon forces in particular present tremendous memory
hurdles, and can now only be performed using certain truncation schemes). A
typical single run is expected to progress to over 200 nodes and 128+GB/
node within the next few years, access to complementary computing resources
is particularly important for training of undergraduate and graduate students.
We expect our requirements to be 3000 core-years by 2020.
Monte Carlo Simulation of LHC Collisions (D. Morrissey)
Theories of new physics beyond the Standard Model can be tested against data
from the LHC through Monte Carlo simulations of collision events. We use a
suite of collider MC codes to simulate each event: MadGraph is used to model
the initial two-to-two collisions of elementary constituents, Pythia to simulate
the radiation of additional particles and collect them into a set of hadrons, and
DELPHES to model the response of the LHC detectors.
This simulation chain is highly CPU-intensive but only requires modest RAM
(1GB/core). It is also very read-write intensive, and an additional requirement
is fast short-term storage and a much larger long-term storage of the
simulation output. A typical simulation run consists of 100k events with an
output of about 1GB, and multiple runs are needed to fill out distributions of
kinematic observables and to study multiple sets of theory parameters. Longer-term storage requirements are roughly 10 Tb per person. We estimate
that we will need 200 core-years by 2020.
10
No core shell model with continuum (P. Navratil)
This project involves large-scale ab initio nuclear structure and nuclear
reaction calculations, using as input modern two- and three-nucleon forces
derived within chiral effective field theory. Using these forces, the quantum
many-nucleon problem is solved for bound and unbound eigenstates. The
method used is called the no-core shell model with continuum (NCSMC).
For NCSMC calculations, it is important to use a large number of nodes with a
large RAM memory; at least 16 GB per node but optimally substantially more.
The storage requirements are modest (few TB). The codes are written in
Fortran90 or in C and utilize MPI and OpenMP parallelizations. At present,
these calculations are performed at parallel computers at Lawrence Livermore
and Oak Ridge National Laboratories (USA), but the computations are expected
to transition to Canadian facilities in the future. In fact, we already began
calculations on MP2 of Calcul Quebec with a 2016 RAC allocation.
The computing allocation at the Titan machine at ORNL is about 20 million
core hours per year, and the calculations use up to 6000 nodes (96000 cores).
The Titan has 18,688 nodes with 16 CPUs and 32 GB per node. The resources used at LLNL include the Vulcan IBM Blue-Gene Q machine with
24,576 nodes with 16 CPUs (64 threads) and 16 GB per node. In addition a
Xeon based machine with 1232 nodes with 16 CPUs and 32 GB per node is
also available for this project. On Vulcan, we use up to 8000 nodes, On the
Xeon based machines, we typically run on 128 nodes (2048 cores). The total
CPU usage of the NCSMC collaboration exceeded 100 million core hours last
year. In 2016, we were awarded 2500 core-years of CPU time on MP2 machine of
Calcul Quebec. The first exploratory calculations have already been performed
using over 100 nodes.
Despite continuous formal and technical improvements of our codes, our
computing needs will grow in the future, as we plan to tackle more complex
problems, i.e., perform calculations for heavier nuclei (sd-shell and beyond).
Further, we will study the alpha-clustering including the scattering and
reactions of alpha-particles with nuclei. These problems will require a
significant increase of computing power, i.e., by a factor of 10 or more. To
meet our future computing requirements for this project, dedicated machines
for running massively parallel MPI and MPI/OpenMP jobs with several thousand
compute nodes with a fast interconnect and a large memory per node will be
needed. A machine that would allow to run parallel jobs on about 10,000
nodes with 32 cores and 128 GB per node would be ideal. We estimate that we
will need 10,000 core-years in 2020 on Compute Canada.
11
Summary
Most of our computer codes are parallelized either with MPI or OpenMP,
need large memory/core and fast interconnect. Below we present a table
showing the expected evolution of computing needs in the next 5 years
separated into the groups of each scientist. In the near future, we expect more
of us to request accounts on Compute Canada and to send in RAC
applications.
Table 1
Evolution of TRIUMF theory computing needs in time. Units are in core-years.
Group
Total needs for 2016
using external
resources
Total available in 2016 Total needs in 2020 on
in Compute Canada
Compute Canada
Bacca
1,000
default
5,000
Holt
1,000
-
3,000
Morrissey
default
-
200
Navratil
10,000
2500
10,000
Finally, as we anticipate a new hire to replace a retired member, we expect
that the above presented projection of computing needs to 2020 is an
underestimation of TRIUMF Theory future requests to Compute Canada.
Bibliography
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Miorelli, G. Orlandini, C. Drischler, K. Hebeler, J. Simonis, A. Schwenk,
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