is 12 times stronger than the previous one. While this is still a billion times larger than what is expected in the Standard Model of particle physics (the equations used for the known elementary particles and forces), there are various types of as-yet unknown particles and forces that could easily produce a much larger electron EDM, through new violations of T symmetry (or, almost equivalently, CP symmetry). These effects could have been large enough to have been discovered by this experiment, so those types of possible phenomena are now more constrained than before. Fortunately, there’s more to look forward to; the method these folks are using can eventually be improved by another factor of 10 or so, meaning that a discovery using this technique is still possible.
This morning the LUX dark matter experiment reported new results, and knocked everyone’s socks off. They have understood their backgrounds from radioactivity much better and more quickly than most of us expected, using new calibration methods and a much better characterization of their backgrounds than has previously been possible. Although they have a detector only a bit larger than XENON100 and have only run the detector underground for three months, compared to the year or so that XENON100 ran previously, their limits on the rate for a dark matter particle to hit a Xenon nucleus beats XENON100’s results by a factor of 2 for a dark matter particle of mass 1000 GeV/c², increasing to about a factor of 3 for a dark matter particle in the 100 GeV/c² mass range, and soaring to a factor of 20 for a dark matter particle in the 10 GeV/c² mass range. Consequently, LUX pretty definitively rules out the possibility, hinted at by several dark matter experiments (as discussed in the second half of the article I wrote about this in April), of a dark matter particle in the 5 – 20 GeV/c² mass range. (See the figure below.) While XENON100 seemed to contradict this possibility already, it didn’t do so by a huge factor, so there were questions raised as to whether their result was convincing. But the sort of ~10 GeV/c² dark matter that people were talking about is ruled out by LUX by such a large factor that finding ways around their result seems nigh impossible. And again, there’s more to look forward to; by 2015 their results should improve by another factor of 5 or so… so they get another shot at a discovery, as will XENON1T, the successor to XENON100.
Congratulations to both groups for their spectacular achievements!
Results from the LUX paper, showing excluded regions as a function of dark-matter particle mass (horizontal axis) and dark-matter/nucleus collision rate (vertical axis), with labels added (hopefully correctly) by me. The shaded blue area is the range that LUX expected to reach, and the blue line their actual limit, which considerably exceeds the XENON100 result (red line) at all masses. The left plot shows the mass range 3 – 4000 GeV/c^2; the right plot is an inset showing details of the 5 – 12 GeV/c^2 mass region, along with the hints of signals from DAMA/LIBRA, COGENT, CMDS and CRESST, all of which now appear entirely implausible.
Particles are just so cool, and so very useful. Scientists can learn about the past — for example, past climate — using “carbon dating”, a combination of biology and nuclear physics.
…the extent to which recent Arctic warming has been anomalous with respect to long-term natural climate variability remains uncertain. Here we use 145 radiocarbon dates on rooted tundra plants revealed by receding cold-based ice caps in the Eastern Canadian Arctic to show that 5000 years of regional summertime cooling has been reversed, with average summer temperatures of the last ~100 years now higher than during any century in more than 44,000 years,…
In a previous post, I told you about how physicists use computers to study how the strong nuclear force combines certain elementary particles — specifically quarks and anti-quarks and gluons — into hadrons, such as protons and neutrons and pions. Computers can also be used to study certain other phenomena that, because they involve the strong nuclear force where it is truly “strong” [in the technical sense described here], can’t be studied using simpler methods of successive approximation. While computers aren’t a panacea, they do allow some important and difficult questions about the strong nuclear force to be answered with precision.
To do these calculations, physicists study an imaginary world, as I described;
all forces except the strong nuclear force are ignored, and
all particles are forgotten except the gluons and the up, down and strange quarks (and their anti-quarks).
On top of this, the up, down and strange quark masses are typically changed. They are taken larger, which makes the calculations easier, and then gradually reduced towards their small values in the real world.
The Notion of “Effective” Quantum Field Theories
There’s one more interesting method for understanding the strong nuclear force that I haven’t mentioned yet, and it too involves changing the quark masses — making them smaller, rather than larger! And weirdly, this doesn’t involve the equations of the quantum field theory for the quarks, antiquarks and gluons at all. It involves a different quantum field theory altogether — one which says nothing about the quarks and gluons, but instead describes the physics of the hadrons themselves. More precisely, its equations are useful for making predictions about the hadrons of lowest mass — called pions, kaons and etas — and it works for processes
with rather low energy — too low to affect the behavior of the quarks and anti-quarks and gluons inside the pions — and
at rather long distance — too long to detect that the pions have a lot of internal structure.
A week at CERN, the laboratory that hosts the Large Hadron Collider [LHC] (where the Higgs particle was discovered), is always extremely packed, and this one was no exception. It’s very typical that on a given day I’ll have four or five intense one-on-one scientific meetings with colleagues, both theorists and experimenters, and will attend a couple of presentations on hot topics — or perhaps ten, if there’s a conference going on (which is almost always.) Work starts at 9 am and typically ends at 7 pm. And of course I have my own work to do — papers to finish, for instance — so after a break for dinner, I keep working til midnight. Squeezing in time for writing blog posts can be tough under these conditions! But at least it is for very good reasons.
Just this morning I’ve just attended two talks related to a future particle physics collider that people are starting to think seriously about… a collider (currently called T-LEP) that would be built in an 80 kilometer-long [50 mile-long] circular tunnel, and in which electrons and positrons [positron = anti-electron] would be smashed together. The physics program of such a machine would be quite broad, including intensive studies of the four heaviest known particles in nature: the Z particle, the W particle, the Higgs particle and the top quark. Any one of them might reveal secrets when investigated in detail. In fact, T-LEP’s extremely precise measurements, made in the 100-500 GeV = 0.1-0.5 TeV energy range, would be used to check the equations that explain how the Higgs field gives elementary particles their masses to one part in a thousand, and to potentially be indirectly sensitive to effects of unknown particles and forces all the way up to 10-30 TeV energy scales.
After that I had a typical meeting with an experimentalist at the CMS experiment, discussing the many ways that one might still make discoveries using the existing 2011-2012 LHC data. The big concern here is that the LHC experimenters are so busy getting ready for the 2015 run of the LHC that they may not fully exploit the data that they already have.
Greetings from Geneva, and CERN, the laboratory that hosts the Large Hadron Collider [LHC], where the Higgs particle was found by the physicists at the ATLAS and CMS experiments. Between jet lag, preparing a talk for Wednesday, and talking to many experimental and theoretical particle physicists from morning til night, it will be a pretty exhausting week.
The initial purpose of this trip is to participate in a conference held by the LHCb experiment, entitled “Implications of LHCb measurements and future prospects.” Its goal is to bring theoretical particle physicists and LHCb experimenters together, to exchange information about what has been and what can be measured at LHCb.
On this website I’ve mostly written about ATLAS and CMS, partly because LHCb’s measurements are often quite subtle to explain, and partly because the Higgs particle search, the highlight of the early stage of the LHC, was really ATLAS’s and CMS’s task. But this week’s activities gives me a nice opportunity to put the focus on this very interesting experiment, which is quite different from ATLAS and CMS both in its design and in its goals, and to explain its important role.
ATLAS and CMS were built as general purpose detectors, whose first goal was to find the Higgs particle and whose second was to find (potentially rare) signs of any other high-energy processes that are not predicted by the Standard Model, the equations we use to describe all the known particles and forces of nature. Crudely speaking, ATLAS and CMS are ideal for looking for new phenomena in the 100 to 5000 GeV energy range (though we won’t reach the upper end of the range until 2015 and beyond.)
LHCb, by contrast, was built to study in great detail the bottom and charm quarks, and the hadrons(particles made from quarks, anti-quarks and gluons) that contain them. These quarks and their antiquarks are produced in enormous abundance at the LHC. They and the hadrons that contain them have masses in the 1.5 to 10 GeV/c² range… not much heavier than protons, and much lower than what ATLAS and CMS are geared to study. And this is why LHCb has been making crucial high-precision tests of the Standard Model using bottom- and charm-containing hadrons. (Crucial, but not, despite repeated claims by the LHCb press office, capable of ruling out supersymmetry, which no single measurement can possibly do.)
Although this is the rough division of labor among these experiments, it’s too simplistic to describe the experiments this way. ATLAS and CMS can do quite a lot of physics at the low mass range, and in some measurements can compete well with LHCb. Less well-known is that LHCb may be able to do a small but critical set of measurements involving higher energies than is their usual target.
LHCb is very different from ATLAS and CMS in many ways, and the most obvious is its shape. ATLAS and CMS look like giant barrels centered on the location of the proton-proton collisions, and are designed to measure as many particles as possible that are produced in the collision of two protons. LHCb’s shape is more like a wedge, with one end surrounding the collision point.
Left: Cut-away drawing of CMS, which is shaped like a barrel with proton-proton collisions occurring at its center. ATLAS’s shape is similar. Right: Cut-away drawing of LHCb, which is shaped something like a wedge, with collisions occurring at one end.
This shape only allows it to measure those particle that go in the “forward” direction — close to the direction of one of the proton beams. (“Backward” would be near the other beam; the distinction between forward and backward is arbitrary, because the two proton beams have the same properties. “Central” would be far from either beam.) Unlike ATLAS and CMS, LHCb is not used to reconstruct the whole collision; many of the particles produced in the collision go into backward or central regions which LHCb can’t observe. This has some disadvantages, and in particular put LHCb out of the running for the Higgs discovery. But a significant fraction of the bottom and charm quarks produced in proton-proton collisions go “forward” or “backward”, so a forward-looking design is fine if it’s bottom and charm quarks you’re interested in. And such a design is a lot cheaper, too. It also means that LHCb is well positioned to make some other measurements where the forward direction is important. I’ll give you one or two examples later in the week.
To make their measurements of bottom and charm quarks, LHCb makes use of the fact that these quarks decay after about a trillionth of a second (a picosecond) [or longer if, as is commonly the case, there is significant time dilation due to Einstein’s relativity effects on very fast particles]. This is long enough for them to travel a measurable distance — typically a millimeter or more. LHCb is designed to make the measurements of charged particles with terrific precision, allowing them to infer a slight difference between the proton-proton collision point, from which most low-energy charged particles will emerge, and the location where some other charged particles may have been produced in the decay of a bottom hadron or some other particle that travels a millimeter or more before decaying. The ability to do precision “tracking” of the charged particles makes LHCb sensitive to the presence of any as-yet unknown particles that might be produced and then decay after traveling a small or moderate distance. More on that later in the week.
A computer reconstruction of the tracks in a proton-proton collision, as measured by LHCb. Most tracks start at the proton-proton collision point at left, but the two tracks drawn in purple emerge from a different point about 15 millimeters away, the apparent location of the decay of a hadron, whose inferred trajectory is the blue line, and whose mass (measured from the purple tracks) indicates that it contained a bottom quark.
One other thing to know about LHCb; in order to make their precise measurements possible, and to deal with the fact that they don’t observe a whole collision, they can’t afford to have too many collisions going on at once. ATLAS and CMS have been coping with ten to twenty simultaneous proton-proton collisions; this is part of what is known as “pile-up”. But near LHCb the LHC beams are adjusted so that the number of collisions at LHCb is often limited to just one or two or three simultaneous collisions. This has the downside that the amount of data LHCb collected in 2011 was about 1/5 of what ATLAS and CMS each collected, while for 2012 the number was more like 1/10. But LHCb can do a number of things to make up for this lower rate; in particular their trigger system is more forgiving than that of ATLAS or CMS, so there are certain things they can measure using data of a sort that ATLAS and CMS have no choice but to throw away.
Just in case you weren’t convinced by yesterday’s post that the shutdown, following on a sequester and a recession, is doing some real damage to this nation’s scientists, science, and future, here is another link for you.
Jonathan Lilly is a oceanographer, a senior research scientist at NorthWest Research Associates in Redmond, Washington, and I can vouch that he is a first-rate scientist and an excellent blogger. He writes in an article entitled
about a wide range of damaging problems affecting this field of study. What’s nice about this post, compared to my own general one from yesterday, is that he has a lot of specific detail.
Here are some other links, demonstrating the breadth and depth of the impact:
