Chart to Help You Determine: What Particle Are You?

Sean Carroll has just posted this handy chart to determine what kind of particle you are. (Click on the chart to enlarge)  On a serious note, the chart is helpful in understanding the basic differences between the known fundamental particles.  Enjoy!

New Particle Or Force? And The Beauty Of Independent Experiments.

The latest buzz that we may have evidence of a new particle or force not predicted by any well established physics has reminded me of why I am glad there are several independent experiments looking at the same thing.  As I've discussed earlier, data analysis of complex data sets scares me, and I have learned enough doing my own data analysis that independent checks are crucial.

Tommaso Dorigo, who often has the best write ups of this type of thing, puts it like this:

It so happens that when experimental particle physicists search for something known, they bump into something they do not understand about their data. Most of the times, it is just a bug in their code, so physicists are accustomed to not grow excited in any way, but rather get a big cup of coffee and sit (possibly during nighttime) in front of the computer, painstakingly checking their code.. 

At that point, a more fun phase starts: the effect is studied systematically by using different simulations... 

Sometimes, however, the feature is hard to explain away with bugs or with insufficiently trustable simulations. At that point, a physicist has better start thinking in terms of what new physics model could be producing the effect he or she is seeing in the data. Another obvious thing that the physicist needs to do at this stage is to search for the same signal in other datasets which might likely be sensitive to it.... 

At the end of the lengthy process by means of which the physicists have tried to "kill" the signal they originally saw in the data, they will usually have grown confident enough to publish it.

And as implied above, data analysis is a lengthy problematic exercise that takes time before the person doing the analysis is confident that what he/she is seeing is real.  However, this stuff can be so difficult that even after such time has gone by and confidence is increased at the end of the day the age old maxim still seems to hold true:

Half of all three-sigma detections are false!

That unfortunately all too true rule of thumb is only realized in practice because  even after this painstaking process you still missed some systematic.  That is the worst!!!

However, if other independent groups get exactly the same answer, then you really can start being confident in the result because the odds are that two independent experiments aren't going to suffer from the exact same systematic presenting itself in identically the same way.

So, the Tevetron sees a 3-sigma "evidence" for new physics.  Hurray lets hope it's real!  But I will be even more happy if the two independent groups, CMS and ATLAS at the LHC, see the same thing.  If they do then I will really start believing the result is real.

More Problems With SUSY... and MOND Humor.

Two things.  First, back to problems with supersymmetry.   Tommaso Dorigo has posted this very helpful plot that explains what is meant by "The LHC sees no signs of SUSY".  If you squint closely, on top of all the colors you will see a small red line. That red line represents the prediction for what the LHC should see in the data if only the standard model particles existed at the energies being probed. The dotted black line is the prediction if supersymmetry is real at the energies being probed. The black dots with error bars are what was measured.

As you can see, there is thus far no reason to believe anything but the standard model is happening at these energies from this search optimized for the detection of supersymmetry.  It's still pre-mature as only a small portion of the total data is in. Still, if I was hoping for SUSY I would be a little worried at this point that nothing is leaving a hint anywhere in any bin whatsoever!

Question I have For Particle Experimentalists:  (And here is the reason I said that last sentence.)  My experience with cosmology data is, as more data comes in, the confidence regions change a little but not by several sigma in every bin! Take WMAP for example. Has the confidence intervals for the 7 year data changed in every bin by several sigma from the first year data? No way! Changes are made, but the entire power spectrum has not shifted in every bin by several sigma.

Why would I expect particle data to be any different?  Can anyone help me out here?  I mean, if WMAP came back with every bin being inconsistent with a Lambda-CDM universe would 6 more years of data have changed that!

Now, what it may be is that you only need a detection in one bin, not all bins.  Fine, but again, from my cosmology experience, the error bars will shrink and midpoint change a little over time, but very seldomly have I ever seen the midpoint to change so much that what is initially excluded by a sigma or two is now verified at a 5 sigma level!

So any help here by those who know more is appreciated.

Now to MOND.  By now many of you have read Sean Carroll's post debunking MOND. (A theory that attempts to replace dark matter).  Let's just say MOND doesn't work.  So in commemoration I wanted to post the image above reminding ourselves why some gave MOND a chance at all while at the same time reminding ourselves why nobody pays attention to it any more.

For mor information read Sean's post.

Supersymmetry A Sinking Ship?

I really like supersymmetry, the idea there is some fundamental symmetry between fermions and bosons.  Unfortunately nature doesn't care what I think.  And even more unfortunately, the LHC has been running at very high energies for a while now and nobody is seeing so much as a hint for it.

A few quotes from this Nature article sum up the situation:

The LHC is now rapidly accumulating data at higher energies, ruling out heavier territory for the super particles. This creates a serious problem for SUSY... As the super particles increase in mass, they no longer perfectly cancel out the troubling quantum fluctuations that they were meant to correct. Theorists can still make SUSY work, but only by assuming very specific masses for the super particles — the kind of fine-tuning exercise that the theory was invented to avoid. As the LHC collects more data, SUSY will require increasingly intrusive tweaks to the masses of the particles. 

So far the LHC has doubled the mass limit set by the Tevatron, showing no evidence of squarks at energies up to about 700 gigaelectronvolts. By the end of the year, it will reach 1,000 gigaelectronvolts — potentially ruling out some of the most favoured variations of supersymmetry theory.

So basically, supersymmetry's biggest appeal is that it provides a "natural" solution to many problems in theoretical physics.  (Like why the Higgs mass so small.)  However, enough parameter space is being ruled out by the LHC that is appears that fine-tuning may be required to get SUSY to work correctly.  But the whole point behind SUSY's appeal is that it appeared to be a theory where fine tuning was not needed.

So by saying: "well supersymmetry may still exist if we do a bunch of fine tuning" seems to destroy the whole point for why we thought SUSY was a good idea in the first place.

Next:

Privately, a lot of people think that the situation is not good for SUSY... This is a big political issue in our field... For some great physicists, it is the difference between getting a Nobel prize and admitting they spent their lives on the wrong track... [Some have] been working on it for almost 30 years now, and I can imagine that some people might get a little bit nervous.

Now, before I get too hard on the theory, we are only in the first few years of the LHC operating at high energies.  Still, after 30 years you would hope that if SUSY was as "natural" as people have assumed, you would hope by now you would have a hint.  I mean, it's one thing to say we don't have enough to claim discovery but we don't even have enough evidence to suggest a hint!

So, is SUSY a sinking ship?  I think abandoning SUSY is still pre-mature.  However, if the LHC cannot see so much as even a hint in the next few years, I will think things will start looking really bad indeed.

Thoughts?

How Particle Accelerators Like The LHC Work.

The above video shows the best "layman" explanation for how particle accelerators, like the Large Hadron Collider (LHC), work that I have ever seen. I highly encourage you to watch it as it is very clear and straight forward.

Just remember, and this may be the most confusing part of the whole video, words like GeV and TeV refer to how much energy the particles have. Lets just say for particles to have that much energy is extraordinarily!

The Recession Has Finally Caught Up To UCI's Physics Department.

                                Image via Wikipedia

I have never had to pay any extra student fees at UC Irvine because the physics department has always covered them out of their own pocket.  I guess the recession has finally caught up to the department as, starting next year, the physics department will no longer be covering our fees out of their pocket.

However, only TAs and graders are affected as research assistants still have this covered somehow.  So personally I am still safe! :)

Here is the email:

Dear Graduate Students,

Every quarter, graduate students are assessed $256.50 in local fees (see below). Students who are hired as GSR's (Graduate Student Researcher) have this fee covered through GSR remission paid from the research grant, which covers 100% of fees and tuition. Unfortunately this does not apply to TA's and readers/graders. TA's and readers/graders receive only a partial fee remission, which does not include the local fees. In the past, the department has covered the cost of these local fees from our graduate student fellowship budget. A deficit in the graduate student fellowship fund has arisen because of the current university budget reductions, and as a consequence the department can no longer pay for the cost of the local fees. This is a regrettable but necessary decision.

Effective Winter 2011, graduate students hired as a TA or reader/grader will be responsible for covering their own local fees each quarter. Fees for Winter 2011 are due December 15. Information, as well as a detailed explanation of the fees and tuition, is provided below.

CMB Power Spectrum and Harmonic Space.

I would like to quickly return to my post on cosmologists loving Fourier/Harmonic space and share another example:  the power spectrum of the CMB.  The famous power spectrum of the CMB, from which we infer things like: dark matter density, baryon density, expansion rate of the universe, etc... is plotted in harmonic space.

Look at the plot above .  On the "top" x-axis you see in fact that the power spectrum is plotted in L space.  On the "bottom" x-axis you see what length scales in the sky those L modes correspond to.  As you can see, the small L modes represent physics that affected very large scales and the large L modes represent the very small scales.

For example, take the first peak.  Physically the first peak is at scales where the universe was just coming into casual contact for the first time.  What do I mean.  I mean: go outside at night with your protractor.  Find two points in the night sky that are separated by one degree.  The power spectrum is telling us that, at the time of emission of the CMB, those two points were coming into casual contact for the first time.  Until that time, those two points were existing as if the other point didn't exist.

Points in the sky separated by more than one degree were not in casual contact at the time the CMB was created and yet if you look at the power spectrum for these larger scales you will notice there is still some  structure.  This is what is predicted by inflation.  In fact, for the lowest L modes, or angular scales close to 90 degrees, inflation predicts that the power spectrum should be nearly flat with a slight downward tilt which is exactly what is seen.

Lastly, for the high L modes, or for points in the sky less than one degree apart, we see oscillations happening.  This makes sense since these points were in casual contact for some time and so the photon-baryon plasma between the two points had plenty of time to interact and cause oscillations that manifest themselves n the oscillatory behavior of the power spectrum for high L modes.

So again, the large and small scale physics at play in the universe is easily visualized in harmonic/Fourier space and cosmologists love this.
(Plot Credit: WMAP Team)

Could The Planck Satellite Discover A New Species Of Neutrino?

It has been known for some time that the WMAP data is more consistant with the existence of four neutrino species than three. Nevertheless, most cosmologists shrug this off as three is by no means ruled out. However, Hamann et al. 2010 demonstrate that such a dismissal may be a mistake.

It turns out, when WMAP 7 year data is combined with Sloan data, the three neutrino species model is ruled out by nearly two sigma. The best fit number of neutrino species becomes 4.78 +/- 1.79 at 95% confidence. Furthermore, big bang nucleosynthesis (BBN) data involving Heluim abundances seems to confirm that such an excess better fits the data.

With this in mind, Hamann et al. 2010 decides to test just how many extra neutrinos are needed to fit the combined data of "the WMAP 7-year data release, small-scale CMB observations from ACBAR, BICEP and QuAD, the 7th data release of the Sloan Digital Sky Survey, and measurement of the Hubble parameter from Hubble Space Telescope observations".  Their findings are plotted above.  They confirm that when all data is added together, the existence of one or two extra neutrinos provides a much better fit than only the standar three.

If this is real it would be major news! On one hand such a "4th" or even "5th" neutrino would have to exist at low energies as it has clearly affected both BBN and CMB physics.  However, extra neutrinos at such low energies have alluded modern particle accelerators.  Therefore, such much neutrinos must be "sterile" in that they do not couple to the rest of the standard model the way normal neutrinos do.  Furthermore, they must not have a lepton partner the same way other neutrinos do. (Example: like the electron neutrino does with the electron.)

Interestingly, if there are an extra one or two of such neutrinos in nature, the Planck satellite has a good chance of making a 5-sigma discovery!  (See plot below).  If this happens, the discovery of such interesting low energy neutrinos could well go down as one of Planck's greatest contributions to science.

Jan Hamann, Steen Hannestad, Georg G. Raffelt, Irene Tamborra, & Yvonne Y. Y. Wong (2010). Cosmology seeking friendship with sterile neutrinos Eprint arXiv: 1006.5276v1

A Great History Of The Evidence For Dark Matter.

In the paper Dark Matter: A Primer Garrett and Dudagives give a nice historical background to the accumulating evidence for dark matter.  Lets go through the history they lay out.

1.  J. H. Oort:  Astronomers have come to tust what is known as the mass to light ratio, M/L, that does a good job telling you what the mass of luminous matter should be based off of the luminosity of that matter.  This relation is normalized such that for the sun, M/L = 1.  In the 1930, Oort found that the stars moving on the galactic plane were moving faster than the galaxy's escape velocity!  He knew what the mass of the visible matter should be from M/L and discovered stars in the galatic plane are moving too fast to be bound by that much mass.   He postulated more mass must be present in the galaxy than can be attributed to the visible matter.

2.    F. Zwicky:  Zwicky studied the Coma Cluster and found that the stars had much more kinetic energy than they should from the viral theorem, KE = - 1/2 PE, assuming that the cluster had te amount of mass predicted by M/L.  He then worked out how much mass this cluster must be to have that high of a kinetic energy and found the mass should be about 10 times more mass than the visible matter. (Again, from M/L measurements.)

3.  Vera Rubin:  Vera Rubin studied the rotation curves for 60 galaxies.  These curves should obey the well known relation v(r) = sqrt(G m(r)/r) where v is the velocity, r is the radius, m is the mass and G is the gravitational constant.  Instead, she found that the velocity was not consitant with the amount of mass seen in visable matter.  Instead, new unseen matter is needed to explain the rotation curves.  (See plot above.)

4.  D. Walsh et al:  in 1979, D. Walsh et al. were among the first to detect gravitational lensing.  They watched how the light was bent by certain distant galaxies.  The problem was that the galaxies had to have more mass to bend the light as profoundly as it did than come from the M/L relation.  Dark matter could explain this discrepancy.

5.  Microlensing:  Several MACHOS studies went into effect and all came to the same conclusion: the missing matter could not be attributed to brown dwarfs, neutron stars, black holes, planets or other "dark" objects made of matter that we are familiar with.  This extra mass had to be coming from some exotic type matter thus far unknown.

6.  BBN:  Big Bang Nucleosynthesis is one of the great achievements of modern cosmology.  It turns out, the Deuterium to Hydrogen ratio (D/H) is heavily influenced by the overall density of baryons in the universe.  Using D/H, one finds that the amount of baryons in the universe is much smaller than the total baryonic matter.  The rest must be coming from some extra dark matter.

7.  The CMB: The power spectrum taken from the Cosmic Microwave background is highly sensitive to the amount of baryonic matter in the universe.  See the plot above.  As the amount of baryons, \Omega_b, changes, so does the power spectrum... by a lot!  The red error bars show the measured value.  As can be seen, baryonic matter only makes up 4.6% of the universe.  From the same power spectrum on finds that the total matter in the universe is more like 25% of the universe indicating that the vast majority of the matter is matter we don't understand.

8:  N-Body Simulations and SDSS:  Numerical simulations of large structure formation have been performed.  Only those that include dark matter give results that match what we observe from large structure surveys such as the Sloan Digital Sky Survey.

9.  The Bullet Cluster:  "Smoking gun" evidence for dark matter, as some would say, came from a recent experiment involving the Bullet Cluster.  The Bullet Cluster recently collided with a larger galaxy.    In such a collision, dark matter should just pass through without interacting and the visible matter heated up giving a tremendous amount of X-Ray emissions.  It was clear that the matter causing the majority of the lensing was not centered in the same spots as the luminous matter.  This showed convincingly that the amount of baryonic matter in galaxies is not as large as the amount of dark matter.  Furthermore, in 2007 another team confirmed a ring-like structure of dark matter was found after the collision of two massive galaxies.

10.  Penny et al:  In 2009, Penny et al. found that a significant amount of dark matter would be needed to hold certain galaxies together that were experiencing a significant amount of tidal forces.  These galaxies were surprisingly stable given how little luminous mass they had.

As you can tell.  This is a great article and I recommend everyone read it.

All images taken from the article cited.

Katherine Garrett, & Gintaras Duda (2010). Dark Matter: A Primer Eprint arXiv: 1006.2483v1