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And the 2013 Nobel Prize in Physics goes to… October 8, 2013

Posted by apetrov in Particle Physics, Physics, Science, Uncategorized.
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Today the 2013 Nobel Prize in Physics was awarded to François Englert (Université Libre de Bruxelles, Belgium) and Peter W. Higgs (University of Edinburgh, UK). The official citation is “for the theoretical discovery of a mechanism that contributes to our understanding of the origin of mass of subatomic particles, and which recently was confirmed through the discovery of the predicted fundamental particle, by the ATLAS and CMS experiments at CERN’s Large Hadron Collider.” What did they do almost 50 years ago that warranted their Nobel Prize today? Let’s see (for the simple analogy see my previous post from yesterday).

The overriding principle of building a theory of elementary particle interactions is symmetry. A theory must be invariant under a set of space-time symmetries (such as rotations, boosts), as well as under a set of “internal” symmetries, the ones that are specified by the model builder. This set of symmetries restrict how particles interact and also puts constraints on the properties of those particles. In particular, the symmetries of the Standard Model of particle physics require that W and Z bosons (particles that mediate weak interactions) must be massless. Since we know they must be massive, a new mechanism that generates those masses (i.e. breaks the symmetry) must be put in place. Note that a theory with massive W’s and Z that are “put in theory by hand” is not consistent (renormalizable).

The appropriate mechanism was known in the beginning of the 1960′s. It goes under the name of spontaneous symmetry breaking. In one variant it involves a spin-zero field whose self-interactions are governed by a “Mexican hat”-shaped potential

MexicanHatIt is postulated that the theory ends up in vacuum state that “breaks” the original symmetries of the model (like the valley in the picture above). One problem with this idea was that a theorem by G. Goldstone required a presence of a massless spin-zero particle, which was not experimentally observed. It was Robert Brout, François Englert, Peter Higgs, and somewhat later (but independently), by Gerry Guralnik, C. R. Hagen, Tom Kibble who showed a loophole in a version of Goldstone theorem when it is applied to relativistic gauge theories. In the proposed mechanism massless spin-zero particle does not show up, but gets “eaten” by the massless vector bosons giving them a mass. Precisely as needed for the electroweak bosons W and Z to get their masses!  A massive particle, the Higgs boson, is a consequence of this (BEH or Englert-Brout-Higgs-Guralnik-Hagen-Kibble) mechanism and represents excitation of the Higgs field about its new vacuum state.

It took about 50 years to experimentally confirm the idea by finding the Higgs boson! Tracking the historic timeline, the first paper by Englert and Brout, was sent to Physical Review Letter on 26 June 1964 and published in the issue dated 31 August 1964. Higgs’ paper, received by Physical Review Letters on 31 August 1964 (on the same day Englert and Brout’s paper was published)  and published in the issue dated 19 October 1964. What is interesting is that the original version of the paper by Higgs, submitted to the journal Physics Letters, was rejected (on the grounds that it did not warrant rapid publication). Higgs revised the paper and resubmitted it to Physical Review Letters, where it was published after another revision in which he actually pointed out the possibility of the spin-zero particle — the one that now carries his name. CERN’s announcement of Higgs boson discovery came 4 July 2012.

Is this the last Nobel Prize for particle physics? I think not. There are still many unanswered questions — and the answers would warrant Nobel Prizes. Theory of strong interactions (which ARE responsible for masses of all luminous matter in the Universe) is not yet solved analytically, the nature of dark matter is not known, the picture of how the Universe came to have baryon asymmetry is not cleared. Is there new physics beyond what we already know? And if yes, what is it? These are very interesting questions that need answers.

Higgs mechanism for electrical engineers October 7, 2013

Posted by apetrov in Particle Physics, Physics, Science, Uncategorized.
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Since the Higgs boson’s discovery a little over a year ago at CERN I have been getting a lot of questions from my friends to explain to them “what this Higgs thing does.” So I often tried to use the crowd analogy that is ascribed to Prof. David Miller, to describe the Higgs (or Englert-Brout-Higgs-Guralnik-Hagen-Kibble) mechanism. Interestingly enough, it did not work well for most of my old school friends, majority of whom happen to pursue careers in engineering. So I thought that perhaps another analogy would be more appropriate. Here it is, please let me know what you think!

Imagine Higgs field as represented by some quantity of slightly magnetized iron filings, i.e. small pieces of iron that look like powder, spread over a table or other surface to represent Higgs field that permeates the Universe. Iron filings are common not only as dirt in metal shops, they are often used in school experiments and other science demonstrations to visualize the magnetic field.  It is important for them to be slightly magnetized, as this represents self-interaction of the Higgs field. Here they are pictured in a somewhat cartoonish way:

ImageHow can Higgs field generate mass? Moreover, how can one field generate different masses for different types of particles? Let us first make an analogue of fermion mass generation. If we take a small magnet and put it in the filings, the magnet would pick up a bunch of filings, right? How much would it pick up? It depends on the “strength” of that magnet. It could be a little:

Image

…or it could be a lot, depending on what kind of magnet we use — or how strong it is:

Image

If we neglect the masses of our magnets, as we assumed they are small, the mass of the picked up mess with the magnets inside is totally determined by the mass of the picked filings, which in turn is determined by the interaction strength between the magnets and the filings. This is precisely how fermion mass generation works in the Standard Model!

In the Standard Model the massless fermions are coupled to the Higgs field via so-called Yukawa interactions, whose strength is parametrized by a number, the Yukawa coupling constant. For different fermion types (or flavors) the couplings would be numerically different, ranging from one to one part in a million. As a result of interaction with the Higgs field (NOT the boson!) in the form of its vacuum expectation value, all fermions acquire masses (ok, maybe not all — neutrinos could be different). And those masses would depend on the strength of the interaction of fermions with Higgs field, just like in our example with magnets and iron filings!

Now imagine that we simply kicked the table! No magnets. The filings would clamp together to form lumps of filings. Each lump would have a mass, which would only depend on how strong the filings attract to each other (remember that they are slightly magnetized?). If we don’t know how strong they are magnetized, we cannot tell how massive each lamp will be, so we would have to measure their masses.

ImageThis gives a good analogy of the fact that Higgs boson is an excitation of the Higgs field (the fact that was pointed out by Higgs), and why we cannot predict its mass from the first principles, but need a direct observation at the LHC!

Notice that this picture (so far) does not provide direct analogy to how gauge bosons (W’s and Z bosons) receive their masses. W’s and Z are also initially massless because of the gauge (internal) symmetries required by the construction of the Standard Model. We did know their mass from earlier CERN and SLAC experiments — and even prior to those, we knew that W’s were massive from the fact that weak interactions are of the finite range.

To extend our analogy, let’s clean up the mess — literally! Let’s throw a bucket of water over the table covered with those iron filings and see what happens. Streams of water would pick up iron filings and flow from the table. Assuming that that water’s mass is negligible, the total mass of those water streams (aka dirty water) would be completely determined by the mass of picked iron filings, just like masses of W’s and Z are determined by the Higgs field.

This explanation seemed to work better for my engineering friends! What do you think?

LHCb reports observation of CP-violation in charm. Welcome New Physics? Or not? November 14, 2011

Posted by apetrov in Particle Physics, Physics, Science.
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One of CERN collaborations, the LHCb, has reported observation of direct CP-violation in the decays of charmed mesons at the Hadronic Collider Physics Symposium 2011 (HCP 2011) in Paris today. This is a fantastic news! While I am not at HCP 2011, kind folks at LHCb let me know about this fantastic measurement — since charm physics is my specialty.

So, what are we talking about here?

First things first. CP (or Charge Parity) is a set of (discrete) transformations performed on a theory’s Lagrangian — a function that describes what particles we have in a theory and how they interact. If your Lagrangian is symmetric  under this transformation, then particles and antiparticles — matter and antimatter — have the same properties. If not — interactions of matter particles are different from interactions of antimatter particles.  This possible difference is a crucial property of a theory because, according to three Sakharov criteria, the Universe could evolve in what we see around us only if matter and antimatter have different interaction properties. Otherwise, at best, we’d have big chanks of antimatter floating around — or at worst would not not exist at all.

This is why many huge experiments built to study CP violation. Big national labs’ flagship experiments were designed to search and study CP-violation (BaBar at SLAC, Belle at KEK, LHCb at CERN), with hopes to see glimpses of New Physics that could explain matter-antimatter asymmetry in the Universe. This new result from LHCb can in principle provide one.

So, what did LHCb see? The reported analysis looks at the difference of a difference — i.e. a difference of CP-violating asymmetries in kaons and pions. The CP-violating asymmetry is defined as the difference between decay widths (roughly speaking, decay probabilities) of a neutral D-meson to decay into a final state, say positive K-meson and a negative K-meson and the same quantity for the D-anti-particle to decay to the same final state. This quantity is also defined for the final state of two pions — and it is CP-violating!

The structure of this CP-violating asymmetry, aCP, is not that simple. Because D0 is a neutral particle it can, in principle, mix with its antiparticle (see here) — and this antiparticle can also decay into the same final state! This process can be also CP-violating (this type of CP-violation is called indirect CP-violation). So the result would depend on both types of CP-violation!

Moreover, experimentally, the asymmetries like this are not easy to measure — there are experimental systematics associated with D-production asymmetries, difference of interactions of positive and negative kaons with matter, etc. For this reason, experimentalists at LHCb decided to report the difference of CP-violating asymmetries, in which many of those effects, like productions asymmetries, would cancel. So, here is the result:

ΔaCP = -0.82 ± 0.21 (stat) ±0.11 (syst) %

In other words, this quantity is 3.5 sigmas away from being zero. The first question that one should ask is whether this quantity is consistent with previous measurements. The biggest question, however, is whether this quantity is consistent with Standard Model expectations.

There is a bunch of previous measurements available for aCP (KK) and aCP (ππ) separately. The thing is that

aCP (KK) = – aCP (ππ)

or approximately so. So by subtracting those quantities we not only subtract the experimental uncertainties, but also enhance the signal! However, looking at the table on page 6 of the talk, one can immediately realize that this measurement is at least consistent with the previous ones.

Is it a sign of something beyond the Standard Model? This one is hard to answer. I usually put an upper bound on the SM value (that is, absolute value) of asymmetries like aCP (KK) at 0.1% — which would make ΔaCP to be about 0.2%. Is it consistent with LHCb findings? Maybe. The size of this asymmetry is notoriously difficult to estimate due to hadronic effects. Maybe it is a sign of New Physics — this could be an exciting conclusion, as we have never seen CP-violation in up-quark sector.

It is interesting that the first “big” result from LHC comes in the realm of charm physics, not Higgs searches. Moreover, all “big” results in the last decade were from the experiments searching for New Physics indirectly, in the “intensity frontier” (this is lingo of US Department of Energy) — with most of them coming from charm physics. Maybe at the very least LHC-b should be renamed as LHC-c?

We have a job… or two! October 21, 2011

Posted by apetrov in Particle Physics, Physics, Science.
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Depending on how the budget for the new year looks like, we (the high energy particle theory group at WSU) will have two new postdoc positions. Please apply, if you are interested! Here is the ad.

—————————————————————————-

The high energy theory group at Wayne State University ( http://www.physics.wayne.edu/heptheory ) anticipates making TWO postdoctoral research appointments to start September 1, 2012, subject to budgetary approval. The initial appointments will be for one year, and may be extended for one or more years depending on the performance and availability of funding.

The group consists of faculty Gil Paz and Alexy A. Petrov, as well as a postdoc and several students. Research interests of the group include particle phenomenology, physics beyond the Standard Model, effective field theori es, heavy quark physics, CP violation, Dark Matter phenomenology and particle astrophysics. The group has close ties to the nuclear theory group of Sean Gavin and Abhijit Majumder. The WSU Department of Physics and Astronomy offers a unique opportunity of close interaction with experimental high energy particle and nuclear physics groups.

Applications including CV, a list of publications, a brief statement of research interests and three letters of recommendation should be submitted to Academic Jobs Online at http://academicjobsonline.org/ajo/jobs/1128

or by mail to

Prof. Gil Paz
Department of Physics and Astronomy
Wayne State University
Detroit, Michigan, 48201

or

Prof. Alexey A. Petrov
Department of Physics and Astronomy
Wayne State University
Detroit, Michigan, 48201

or electronically to gilpaz@wayne.edu or apetrov@wayne.edu. The deadline for application is January 15, 2012. Later applications will be considered until the positions are filled. Informal inquiries are welcomed.

Wayne State University is an affirmative action/equal opportunity employer. Women and members of minority groups are encouraged to apply.

Why do physicists go to Aspen? September 1, 2011

Posted by apetrov in Near Physics, Particle Physics, Physics, Science.
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While the most obvious answer to this question is “to ski”, it is, nonetheless, not the correct one. Yes, skiing is great here in the winter (and hiking is great in the summer), but most of the time physicists come here to work. The reason is Aspen Center for Physics. I write “here” because I’m currently participating in one of the programs organized by the Center (the program is called “Flavor Origins” — it brought together theorists working on the problems of neutrinos, heavy and light quarks, CP-violation, etc.). The Center, which exists here since 1961, organizes workshops and conferences. But the main reason that theorists (and occasional experimentalists) come here is to talk to other theorists. In short, it is as if you are visiting a huge theory group — you can work individually or with your colleagues, but you can always knock on an office door and bounce your ideas off someone else visiting the Center, etc. It is great to have such a concentration of theorists of different trades. And it leads to breakthroughs and simply good papers. As it is said on the Center’s website:

“Many seminal papers have been written in Aspen, which has grown to be the largest center for theoretical physics in the world during its summer sessions. Among many other subjects, the theories of superstrings, chaos, evolution of stars and galaxies, and high temperature superconductivity have all made large strides in recent Aspen seasons.”

There is almost always someone with an expertise in a subject that you have a question about. And that makes this Center great. And, of course, hiking and skiing is also good. The only “downside” (note the quotes) is that you can meet a real bear (even at the Center) or other wildlife. Today a snake came to check out a lecture on conformal field theories…

P.S. Also check out my blog on Quantum Diaries

Congratulations Dr. Yeghiyan! July 26, 2011

Posted by apetrov in Near Physics, Particle Physics, Physics, Science, Uncategorized.
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Today my third graduate student at WSU, Gagik Yeghiyan, defended his Ph.D. thesis. Congratulations Dr. Yeghiyan! Good luck to you in your new life as an Assistant Professor at Grand Valley State University!

Site selected for the Italian Super-B factory, the Cabibbo Lab June 13, 2011

Posted by apetrov in Particle Physics, Physics, Science.
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As I blogged some time ago, Italian government decided to fund a new accelerator for precision studies of New Physics in decays of heavy-flavored mesons, the so-called SuperB factory, a high-intensity B-factory, which is designed to look for glimpses of New Physics in rare decays of B- and D-mesons (for professional description of the physics case, see here; for Conceptual design Report (CDR) see here).

Last week a decision was made for a location of the site of the new machine. It will be built on campus of the University of Rome ‘Tor Vergata’. Here is the picture of the proposed site (shamelessly taken from the talk of Roberto Petronzio, President of the Italian National Institute for Nuclear Physics at XVII SuperB Workshop and Kick Off Meeting – La Biodola (Isola d’Elba) Italy):

 

The (“green”) site is located reasonably close (4.5 km) to another well-known Italian National Lab in Frascati, Laboratori Nazionali di Frascati (LNF). The new lab will be a CERN-like consortium. The name for the lab was proposed: Cabibbo Lab, after the great Italian physicist Nicola Cabibbo whose name is associated with some of the most important objects in flavor physics.
The new lab will bring lots of talent from all over the world and, besides experiments in high energy physics, will be used as a light source for other physics experiments. It is great that even at the time when finances are tight, European governments realize that fundamental physics is important for the future of their countries. These are exciting times for the European physics!

Science and politics: to the attention of Michigan Congressional delegation February 13, 2011

Posted by apetrov in Near Physics, Particle Physics, Physics, Science.
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I usually don’t comment about politics in this blog. But today I’ll make an exception. Maybe someone from Michigan Congressional delegation will read it. I’ll be happy to answer any questions regarding this situation.

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Each developed country in the world has a stake in an interdependent triad that builds up its wealth and independence: fundamental research, applied research and industry. It is only the combination of excellence in those three fields that has kept the United States at the forefront of technological revolutions of the past 50 years. Elimination of one of those components will spell trouble for the remaining two: for example, defunding fundamental and applied science in the Russian Federation in the early 1990’s led to a quick demise of that country’s high tech industry.

The new Continuing Resolution (CR) bill announced on 02/08/2011 by House Appropriations Chairman Hal Rogers [1] imposes deep cuts on Department of Energy’s Office of Science (DOE OS), National Science Foundation (NSF), NASA and National Institutes of Health (NIH) that simply threaten US fundamental research. The cut to DOE OS’ budget of $5.12B is $1.1B. It is proposed to happen half way through the current budget year. To keep things in perspective, the amount needed to implement this cut would be equivalent to closing down all US National Laboratories for a continuous period of time this year.

Among other things, DOE’s Office of Science supports fundamental and applied research done by the University groups all over the country. In the state of Michigan that includes University of Michigan, Michigan State University, Michigan Tech and Wayne State University. This funding is neither redundant nor wasteful: each grant issued by DOE’s Office of Science, NSF or NIH is reviewed by several independent experts and expert panels. It is this funding that helps us train the next generation of scientists and engineers that will keep America prosperous in coming years. It is this funding that the new CR proposal would severely cut.

To compare, Chinese government’s spending on science and technology was slated to rise 8% to $24 billion in 2010, of which $4 billion is basic R&D [2]. By contrast, the cuts included in the proposed Continuing Resolution bill reduce funding to basic and applied research made by DOE’s Office of Science by 18%. Liberal and conservatives commentators alike voiced concerns about how the US is losing its edge in math and sciences. This budget cut signals that there is no reason for young Americans to pursue careers in science.

The fundamental research done by particle scientists might not have immediate applications to industry. But not all basic research projects are “long shots.” The first Internet browser developed by high energy physicists at CERN (the site of currently running Large Hadron Collider) for the needs of the experiment designed to understand the basic building blocks of Nature in 1991 made possible creation of the World Wide Web and revolutionized the US and world’s commerce.

Balancing our country’s budget is an important and noble goal, but it should not be done at the expense of the future.

 

References:

[1] House appropriation committee website http://republicans.appropriations.house.gov/index.cfm?FuseAction=PressReleases.Detail&PressRelease_id=259

[2] Physics Today http://blogs.physicstoday.org/politics/2010/03/china-increases-science-fundin.html

A picture on a wall? February 12, 2011

Posted by apetrov in Funny, Particle Physics, Physics, Science.
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I was moving old pictures from my camera to my computer today and found this image. Here is a funny picture of a reflection on my neighbor’s wall. What does it look like?

There was a picture

To a particle physicist, this is just a pair of Feynman graphs for 2 -> 2 scattering amplitudes… with the left one in an external field :-). Enjoy.

Bye-bye, Tevatron! January 10, 2011

Posted by apetrov in Particle Physics, Physics, Science.
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Well, it is now official: DOE has decided not to pursue the extension of the Tevatron running until the year 2013. The operations of the Tevatron, the largest US hadron accelerator,  will end at the end of this year, 2011. The details of the DOE decision can be found here.

To remind you, the original idea to extend the Tevatron running until 2013 came out because of the LHC shutdown schedule (and physics, of course), Tevatron might have been competitive with the LHC in the search for light (~ 120 GeV) Higgs. Now we have to rely solely on the LHC.

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