Tag Archives: Physics

New LHCb Results Available

Over the past couple days, the LHCb collaboration has released a number of papers. LHCb is probably the least well known of the four large experiments at the LHC. ATLAS and CMS are general purpose detectors meant to search for a wide variety of physical processes. ALICE focuses on collisions of heavy ions. Unlike these detectors, which try to cover as much of the total 4π solid angle as possible and typically look for events with a large transverse (perpendicular to the beam) momentum, LHCb is a forward spectrometer focusing on events near the beam direction. One of the principal goals of LHCb is to study the properties of B mesons, which are mesons containing a b-quark.

The LHCb papers include one measuring CP violation in a particular decay of the B0s meson, one measuring the lifetime of the B0s using a particular decay channel, and a third measuring the production of two charmonium (charm-anticharm mesons) states in proton proton collisions.

The most interesting paper to non-specialists is the fourth paper, which presents evidence of direct CP violation in the decay of the B+ to a K+ and a proton-antiproton pair. Direct CP violation is basically just looking for differences between particles and their antiparticles. In this case, it was found that in some regions of the kinematic phase space of the final state, decays of the B+ and B mesons seem to occur at different rates. This can occur due to interference between different diagrams leading to the same final state. The result is not significant enough to claim a discovery of direct CP violation in B decays. If the result holds up, it would represent the first measurement of direct CP violation in B decays using a decay channel involving baryons.


Physicists Aren’t Experts on Everything

Here is a great example of how not to support a cause. This guy managed to become a physics professor at Princeton but shows an appalling grasp of logic if he thinks trying to reduce CO2 emissions is equivalent to being a Nazi.

Also, it’s not at all clear if he even has any relevant expertise to talk about global warming. His current research doesn’t seem to have anything to do with atmospheric science. Just because he’s a physicist doesn’t mean he’s qualified to weigh in on any science question.

New OPERA Tau Neutrino Event

OPERA has a new paper on the arXiv. They report finding a fourth candidate ντ event.

The tau neutrino (ντ) is probably the rarest particle in the Standard Model. Well, that technically isn’t true. The tau neutrino should be extremely common but is the hardest to identify, so very few candidate events have ever been found. In years of running, OPERA only has four candidate events.

There are several things that make the ντ hard to find:

  1. Neutrinos don’t interact very much, so huge numbers of them are required to have measurable numbers of events. The mean free path for neutrino interactions is often quoted as being more than a light year in lead.
  2. Neutrinos can interact via neutral current (the neutrino stays a neutrino) and charged current (the neutrino turns into a charge lepton). Neutral current events cannot be used to distinguish the neutrino type on an event-by-event basis.
  3. The τ (tau) is much heavier than the other charged leptons. It has over 10 times the mass of the muon and over 1000 times the mass of an electron. Thus, much more energy is needed for charged current interactions to occur. A high energy proton synchrotron is needed to create neutrinos with energies high enough to produce taus.
  4. Accelerator-based neutrino experiments primarily produce muon neutrinos from decays of mesons produced in hadronic showers. A tau neutrino experiment must then look for ντ appearance due to the muon neutrinos (and any electron neutrinos) to oscillate into tau neutrinos. This requires the detector to be at an appropriate distance from the target to maximize the number of tau neutrinos passing through the detector.
  5. Even if taus are produced in charged current events, they quickly decay to other particles. The neutrino detector must be able to identify tau candidates, which will typically require a high enough energy and fine enough spatial resolution to distinguish the primary interaction vertex from the secondary vertex created when the tau decays.

This result further strengthens OPERA’s case for definitively confirming the discovery of νμ to ντ oscillations.

Recent MALBEK Result on the ArXiv

About a week ago, the Majorana collaboration released a new MALBEK conference proceeding presenting a preliminary WIMP dark matter limit curve. MALBEK uses p-type point contact germanium detectors to search for new physics. In particular, the MALBEK detectoris basically identical to the germanium detectors used by the CoGeNT collaboration. A few years ago, CoGeNT famously released a result that appeared to display hints of the annually modulating event rate expected from a galactic WIMP dark matter halo. With the same type of detector in a different location, MALBEK could potentially hope to confirm or reject the existence of the proposed CoGeNT signal.

There’s nothing groundbreaking here, but MALBEK has obtained results that reject the CoGeNT result with certain analysis choices but not with others. They use a wavelet-based pulse shape discrimination variable to remove surface events but don’t have a very good way yet to quantify the efficiency for bulk nuclear recoils or the contamination from background.

Juan Collar (from CoGeNT) has already responded. He criticizes the rather opaque nature of the MALBEK pulse shape analysis. MALBEK resorted to these methods because it was found that a simple variable such as the pulse rise time does not separate surface and bulk events at energies below 2 keV. CoGeNT is able to separate these events due to a much lower amount of electronics noise.

At any rate,  the best results in the low mass region from CDMS and LUX already seem to rule out the CoGeNT dark matter hypothesis by a fairly wide margin. CDMS even uses germanium so one can’t argue that maybe the Ge cross section is enhanced compared to the standard assumptions for the Xe cross section.

New Video on T2K Long Baseline Neutrino Oscillation Experiment

In case you were wondering what I work on, KEK (the Japanese high energy physics lab) released a video on the T2K experiment last week. I think it gives a pretty good general explanation for laypeople of the experiment and the physics it’s studying.

Sadly, I don’t think I appear anywhere in the video (maybe in one of the group pictures), but I know some of the people who do.

The short explanation is that there are 3 flavors (types) of neutrinos (electron, muon, and tau neutrinos), which have very small but non-zero mass. As they travel, the neutrinos can change from one flavor to another. We create a beam composed mostly of muon neutrinos (or antineutrinos) at the J-PARC facility in Tokai, Ibaraki and measure neutrino interactions a few hundred meters from the target and again a few hundred kilometers away at the Super-Kamiokande detector in the Kamioka mine near Toyama. This lets us study how the neutrinos change flavor. This is generally known as neutrino oscillations and is one of several active neutrino research topics in high energy and nuclear physics.

DoE and NSF Announce Support for Next-Gen Dark Matter Searches

Yesterday, the Department of Energy and National Science Foundation announced the major dark matter searches that they will be supporting for the coming years. The experiments to be supported are LZ (LUX-ZEPLIN), SuperCDMS (Cryogenic Dark Matter Search), and ADMX (Axion Dark Matter Search). Additional funding will be available for smaller R&D efforts.

My take on this is that the funding agencies have gone with the most conservative approach, supporting proven technologies and experiments based in North America.

LZ is a planned multiton dual-phase xenon TPC. It’s basically a scaled-up version of detectors like LUX and XENON-100, which have been getting some of the best WIMP search results in recent years. Currently, LUX is the most sensitive direct detection experiment for spin-independent interactions across a wide range of WIMP masses. These experiments use liquid xenon as their target material and pull electrons left from ionization into a gaseous region, where a high electic field causes electroluminescence that can be measured by photodetectors. The scintillation left from a nuclear recoil can also be measured by those detectors, giving them two energy channels to use for position and energy reconstruction and particle ID. LZ will be a continuation of the noble gas TPC program, and is expected to be constructed at the Homestake Mine in South Dakota.

SuperCDMS is a scaled-up version of CDMS, an experiment using cryogenic germanium detectors that are sensitive to both ionization and thermal excitation.Germanium detectors have less mass than the TPCs but potentially have a much lower energy threshold allowing for better sensitivity to low mass WIMPs. SuperCDMS is planned to be constructed at SNOLAB in Ontario, which has a long history of operating large experiments.

I’m not entirely sure what these choices mean for other experiments such as XENON-1ton, COUPP, DarkSide, DEAP/CLEAN. These experiments will not be getting US funding, but many of them have significant support from other countries. They’ll have to get enough non-US support to keep going. If US collaborators are unable to secure funding, it’s quite likely that some of these experiments will end up being forced to merge with others or will just shut down completely due to a lack of personnel.

DoE and NSF will continue to support ADMX, which searches for axion dark matter and is basically the only player in the axion DM field right now. This is not surprising, as it’s an easy way for the US to host a world-leading experiment. Finally, while there will continue to be funding for R&D efforts the specific groups and projects have not been announced, so it will be interesting to see what technologies are developed over the next few years.