DOING HIGH ENERGY PHYSICS EXPERIMENTS ---TIMELINE OF A TYPICAL
EXPERIMENT, BECOMING A PHYSICIST, DOING PHYSICS ANALYSIS
(Draft) version 0.93, John Yoh, 12/29/04
In this note, we provide some additional information on High Energy
Physics experiments in general
Timeline for a typical High Energy Physics Experiment
Becoming a Physicist
TIMELINE FOR A TYPICAL LARGE HIGH ENERGY PHYSICS EXPERIMENT
Trying to make a major discovery in Particle (aka High Energy) Physics is
a long drawn-out multi-year process. We need to
- Have an idea for an experiment, work out the experimental apparatus,
do lots of preliminary studies (using computing--Monte Carlo or
what-if's studies) in order to optimize the detector design
and determine how well we could measure our objectives,
find collaborators (often from multiple universities, labs),
work out possible funding and other support (such as engineers, etc.),
write a proposal
- Have the proposal approved (oftentimes a long drawn-out process, quite
often with "shot-outs" with competing experiment proposals in front
of a "wise-man" committee (call "Program advisory committee) which will
effectively decide on the winning proposal),
- Once our experiment is approved (sometimes a many-stage process), we need
to get the funding --from government agencies, universitites, etc.,
hire the necessary people (Post doctoral fellows, graduate students,
engineers, technicians, etc--
a continueing process--even after the experiment is in the middle of
data-taking), come out with a full design report
detailing everything. Many more computing studies are made to further
optimize the design, often requiring choices of technologies (again,
sometimes shoot-outs among competing proponents), electronics
development (we often have to design our own electronics, since
commercial electronics are not advance enough--many of our physicists
love to work with engineers to design specialize electronics), mechanics
(for support of our often-heavy detector), auxilliary items such as
gas system, specialized detector developments, etc..
- Start construction of all the components of the detector, test them (and
often make further modifications to improve them),
install the detector, make a engineering run. Physicists works with
engineers and technicians to cover all aspects--involving mechianical,
electrical, chemical, etc. projects. We really have to be "renaissance"
man to work on these project, like Archimedes, or Leanardo di Vinci !!!
- In the meantime, the machine (accelerator or collider) needed to be
constructed or improved to provide the necessary interactions or
collisions, often requiring changes in the beam line suitable for
that particular experiment. The accelerator physicists often need
to do work that are as complicated as the experiment.
- Once the various components have been tested and found working (lots
of repairs, perhaps even on-the-fly modifications to take care of
unexpected problems--e.g., electronics noise such as ground-loops)
Data-taking begins. Physicists and others are needed to run shifts,
make sure data-taking is smooth (fixing problems on the fly),
Collect the data.
- First stage analysis include reconstruction of the events, determining
the various tracks, energy deposit, etc. divide the reconstructed
data into different data samples according to selection criteria.
- One critical aspect, requring much time and effort, is that of
"calibration". Just as race car drivers need their race cars "tuned"
to obtain the best performance, we need to calibrate our equipment
and detectors to optimize the performance and understand the quanalties
of our electronic signals--we need to make sure that such and such
an electrical signal (digitized into computing bits) correspond
to a particular track signal, or an energy deposited in our calorimeter.
- Now, the final stage anaylsis, with lots of Computer (Monte carlo)
studies, determing the efficiency and biases of the various electronics
cuts (trigger), selection criterias, detector efficiencies, etc.
- We finally arrive a physics result for a particular mode, we then need
to compare it with some physics model (old known physics as well as
possibly new physics), determine the uncertianties (both statistical
due to the finite number of events, and systematic, due to lack
of understanding of efficiencies, biases, etc.)--these require even
more computing studies.
- The Physics result must then be interpreted, whether its a discovery of
a new physics (such as Top), or the measurement of a known process.
Hopefully, even a old physics results will validate (or invalidate)
some physics model, and thus improve our knowledge of Nature.
- A paper is written (often a multi-step iteration process, in order to
satisfy all the co-authors), and submitted to a journal (short papers
to Physical review Letters, or Physics Letters; longer papers submitted
to Physical Review, etc.). The Journal send the paper to referees,
who will then either accept the paper (Often requring some small
modification to satisfy some reservations), or reject the paper.
Paper are rejected usually because the referee feels that the paper
is not of sufficient importance--whether the result is not important
enough, or is no better than previous publications, or is actually
- Once the paper is published, all the rest of Physics world can read it
and perhaps even comment on it. One of the best measure of the
importance of the paper, or the physics results it contained, is
the "citation" index--how many other subsequent physics pubilcations
reference this paper.
- Oftentimes, even when the results is published, this is not the end of
--The results could point to the need for a further experiment, whether
to measure similar processes with a higher precision, or to study
other aspects of the physics process. This would require a
new proposal, and the cycle continues.
--The results could be controversial, so other experiments would be runned
to verify the result, usually with a different group of physicists.
Unfortunately, many so-called "discoveries" which are not totally
convincing are often proven to be wrong (whether a statistical
fluctuation, such as turning up 7 heads in a row on flipping a coin--
or perhaps even some wrong analysis, such as not realizing a background
that would mimic the signal one thought one has found)
--Sometimes, theorists would take the result and suggest some new
consequences, which would lead to new experiments on different
- The time scale of a typical experiment could last from 1-2 years, to
5 or even 10+ years (as is the case for CDF and D0), during which
many physicists would leave the collaboration and go to the next
stage of their careers, and many others would join--new graduate
students, new postdocs, and even senior physicists. These longer
"experiments" often involve long time-consuming upgrades where one
actually rip out much of the detector of the previous stage and
replace them with upgraded detectors of improved capabilities--
this also applies to the machine that the detector is using.
- Depending on the nature and complexity of the experiment, the number of
physicists involved could vary from a few, to hundreds. There is
usually a good mix--perhaps 1/3 each of graduate students, postdocs,
and senior physicists.
- What many of us like about the field is that there are lots of room for
innovation. Even the most junior of physicists can make important
contributions, and oftentimes have ideas that turn out to be very
important and innovative. For example, one of the most important
developments in CDF, that of B physics using the J/Psi dimuon tag,
was pushed by a young graduate student--now dozens of physicists
in both CDF and D0 are following up on this important area.
- While many physicists are wary about "large experiments" consisting of
hundreds of physicists (such as CDF and D0), in actuality, much
of the work that a typical physicist do in these experiment involves
working in a small "working group" of 1 to 5 physicists, whether
doing the analysis of a particular physics process or objective,
making sure that a piece of the detector is working properly, or
other important aspects. Thus, such large experiment is NOT like
some impersonal regimented army of hundreds of people with one
leader who tells everyone what to do--but more like a cooperative
of small "families" inside a village--helping other groups when it
is useful (such as providing data for a particular study that you
have done which would be useful for other "working groups"--so that
they need not repeat what you have already done).
BECOMING A HIGH ENERGY PHYSICIST
How do we become High Energy (aka Particle) experimental Physicists ??
Here's some comments about how most of us become physicists
- FORMULATIVE STAGE --
First of all, most of us have some aptitude for Physics--and Science
in general. We all got good to excellent grades in High School and
College. Physics, Math, etc. courses.
- Many of us are "bitten" by the science "bug"--whether it's Space
exploration (for many of us, it's the Moon shot), TV programs or Movies
on Science (or even pseudo science, such as Star Trek or Star wars),
Scientific American or other Science Magazines, or even the love of
- Many of us decide on Science, and even perhaps Physics in particular,
as a career--often fairly early, in the first years of college or
before. We may have had some especially inspring teachers, which
interest us in Physics--or even have summer jobs at some Physics
(in CDF and D0, there are typically dozens of undergraduate students
that spent a summer at Fermilab working on the experiment--you can
contact the professors in your college that works on CDF and/or D0
to find out more)
- After the hard work in college that leads to a Bachelor in Physics, or
related field, we decided to go to graduate school to try to get a
Doctorate (Ph. D) in Physics
- GRADUATE STUDENT STAGE--
Most Graduate students in Physics would have teaching assistantships
that help pay the way for the first 2 years or so, when one have to
take course work. After finishing your course-work and passing the
qualifying exam, you can choose an advisor to work on your thesis
topic--along with a getting a research assistantship.
- Choosing among the various Physics fields for a thesis topic is quite
difficult. There are many sub-fields to choose from. Aside from
High Emergy (aka Particle) Physics, there are Atomic, Astronomy,
Astrophysics, Biological, Chemical/Molecular, Computational,.
Environment, Fluids, Geophysics, Material Science, Nuclear, Optics,
Plasma/Fusion, Quantum, Space, Solid state, etc. Even in a particular
field, one have to decide whether to do theory or experiment.
Some of us know
fairly certainly that we want to do High Energy Physics; many others
talked with the various professors in the department before deciding
on a thesis advisor, thesis topic, etc.
- Now the hard work begins. Once a graduate student is committed to a
particular experiment, he (or she) practically live on the experiment.
- During the construction phase of an experiment, the student is stationed
where the construction is--whether at the home institution, or at a
remote construction site--often supervising or working on construciton
of equipment or electronics, testing and assembling.
- During data-taking, the student is often stationed at the detector
to work on data-taking, fixing equipment, providing other support;
he can also be working (part time, of full-time if his service
committment is fulfilled) on his thesis topic, which would usually
be the study of a particular physics objectives involving one (or
perhaps a few) processes.
- The Thesis work stage, usually involving 2+ years, usually involve
a learning stage--when the student understand the work of previous
analysis, perhaps even doing some minor projects to "learn the ropes".
Most other more senior students and postdocs are often very helpful
in this stage and worked as mentors.
- After a lot of hard work, and discussion (perhaps presentation) with
other physicists in the group. Many large experiments have special
working groups (specializing in a particular area of research)
who meet often --once every week or 2 weeks--and listen
to presentation from anyone who have preliminary results--with
discussion and suggestions coming from people after the presentation.
- With further hard work, the results become more solid, with all the
various studies of efficiencies, biases, etc. worked out thoroughly.
A final presentation is made before the working group; in the case
of CDF, a formal request is made for a "blessing"--that is, asking
for all present in the working group to okay the result so that it
may be presented in outside meetings as a result of CDF (certain
senior elected/selected members of the collaborations--"wise man"
also need to okay these results.
- With the result ready for presentation, the next stage is the writing
of a paper to be submitted to a physics journal. In CDF, there
is a committee of 3 appointed to "godfather" the paper, to make sure
that the paper is of good quality. Also, a collaboration approval
must be obtained for each paper--before it can be submitted.
- In the meantime, the student is writing his thesis on this result.
Much more details are needed, much more than the paper.
- The final stage of a graduate student's effort is the thesis defense,
in which he stands in front of a selected group of professors in
his school and actively defend his thesis to any questions from this
group of professors. Hopefully, the professor approve his effort,
and he will thus obtain a Ph. D degree.
- POSTDOC STAGE (Post doctoral Research Assocate)--Once the Graduate student
finishes his thesis, and pass his thesis defense, he will receive his
Ph. D. (Doctor of Philosophy) degree. He must now decide on the next
stage of his career. Many HEP Physicists decide to pursue other fields,
such as programing, Medical physicist (often dealing with radialogy),
or even Wall Street (working on programing needed for financial
instruments and analysis)--hundreds of different careers--the training
one receives as a HEP physicist is extremely useful for many careers,
often due to what we learn in computing needed for the thesis research
--as well as the project-oriented process which enable us to understand
the entire process of a physics analysis project--often applicable
to business projects, etc..
Many of these careers are very lucrative, and could lead to management
or even senior level capacities--many High-Tech company CEO's are
Physics Ph. D.'s.
- A large fraction of HEP Physics Ph. D. will decide to stay in the field--
they like to search for knowledge, the give-and-take between junior
and senior physicists, the atmosphere, and the research--and, of course,
they hope to become one of the giants of the field--HEP Physicists
captures a large fraction of the Physicst Nobel prizes. Despite the
fact that they will typically get less money than if they leave the
field and work in industry or financial fields--along with longer
hours of work they will typically put in as HEP physicists--they
decide to continue in the field.
- After getting a doctorate, the next step for almost all of us is a
"Postdoc"--or a Post-Doctoral Research Fellow, either with a university,
or with a National Lab, such as Fermilab. Some of us can get a
postdoc at locations outside of the USA--such as CERN in Geneva
Switzerland, Japan, etc. I personally gotten a NATO postdoc and
spend 3 years at CERN, Geneva Switzerland !!!
- A postdoc will ber attached to an experiment, and will often do similar
things as a graduate student, but with more responsibility and
room for initiative.
- Many of the postdocs works long hours--trying to come up with physics
results that will be outstanding, and ease their way into the next
stage of their careers.
- The analysis are similar to that describe above (in the Graduate student
phase). In general, since the Postdoctoral appointment is good only
for 3 years or so, the senior physicists typically allow the postdoc
to have a large fraction of his time working on physics analysis,
and only a minority of time working on service jobs.
- After one stint as a postdoc (or perhaps 2), he is ready to try for the
- SENIOR PHYSICIST STAGE--After several years being a Postdoc/Research
Associate, young physicists who wish to continue in Physics research
look for the next step--possibilities are
--Tenure-tracked positions at a University, which, hopefully, will
lead to tenure as a professor
--senior positions at a research Laboratory
--Other senior positions
--Competition for getting these positions are very tough--one have to
has a very good reputation and record to obtain such positions--
unfortunately, many people would not be able to get these jobs.
- For those who do get these senior positions, they often become among
the leaders of their reserarch group--and needs to direct the work
of more junior members (students and postdocs). There are also other
duties, such as teaching, writing proposals to funding agencies
(as well as make presentation and defending your proposals), other
university chores. In the case of CDF, one problem is that most
universities are far from Fermilab--so, the senior physicists who
need to teach must commute to Fermilab, on a weekly, bi-weekly,
or other basis.
- (to be continued)
ANALYZING PHYSICS RESULTS