## From Quantum Diaries: “The Standard Model: a beautiful but flawed theory”

March 14th, 2014

Pauline Gagnon

This is the first part of a series of three on supersymmetry, the theory many believe could go beyond the Standard Model. First I explain what is the Standard Model and show its limitations. Then I introduce supersymmetry and explain how it would fix the main flaws of the Standard Model. Finally, I will review how experimental physicists are trying to discover “superparticles” at the Large Hadron Collider at CERN.

The Standard Model describes what matter is made of and how it holds together. It rests on two basic ideas: all matter is made of particles, and these particles interact with each other by exchanging other particles associated with the fundamental forces.

The basic grains of matter are fermions and the force carriers are bosons. The names of these two classes refer to their spin – or angular momentum. Fermions have half-integer values of spin whereas bosons have integer values as shown in the diagram below.

The Standard Model of elementary particles, with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

Fermions come in two families. The leptons family has six members, with the electron being the best known of them. The quarks family contains six quarks. The up and down quarks are found inside protons and neutrons. The twelve fermions are the building blocks of matter and each one has a spin value of ½.

These particles interact with each other through fundamental forces. Each force comes with one or more force carriers. The nuclear force comes with the gluon and binds the quarks within the proton and neutrons. The photon is associated with the electromagnetic force. The weak interaction is responsible for radioactivity. It comes with the Z and W bosons. All have a spin of 1.

The main point is: there are grains of matter, the fermions with spin ½, and force carriers, the bosons with integer values of spin.

The Standard Model is both remarkably simple and very powerful. There are complex equations expressing all this in a mathematical way. These equations allow theorists to make very precise predictions. Nearly every quantity that has been measured in particle physics laboratories over the past five decades falls right on the predicted value, within experimental error margins.

So what’s wrong with the Standard Model? Essentially, one could say that the whole model lacks robustness at higher energy. As long as we observe various phenomena at low energy, as we have done so far, things behave properly. But as [particle] accelerators are getting more and more powerful, we are about to reach a level of energy which existed only shortly after the Big Bang where the equations of the Standard Model start getting shaky.

This is a bit like with the laws of physics at low and high speed. A particle moving at near the speed of light cannot be described with the simple laws of mechanics derived by [Isaac] Newton. One needs special relativity to describe its motion.

One major problem of the Standard Model is that it does not include gravity, one of the four fundamental forces. The model also fails to explain why gravity is so much weaker than the electromagnetic or nuclear forces. For example, a simple fridge magnet can counteract the gravitational attraction of a whole planet on a small object.

This huge difference in the strength of fundamental forces is one aspect of the “hierarchy problem”. It also refers to the wide range in mass for the elementary particles. In the table shown above, we see the electron is about 200 times lighter than the muon and 3500 times lighter than the tau. Same thing for the quarks: the top quark is 75 000 times heavier than the up quark. Why is there such a wide spectrum of masses among the building blocks of matter? Imagine having a Lego set containing bricks as disparate in size as that!

The hierarchy problem is also related to the Higgs boson mass. The equations of the Standard Model establish relations between the fundamental particles. For example, in the equations, the Higgs boson has a basic mass to which theorists add a correction for each particle that interact with it. The heavier the particle, the larger the correction. The top quark being the heaviest particle, it adds such a large correction to the theoretical Higgs boson mass that theorists wonder how the measured Higgs boson mass can be as small as it was found.

This seems to indicate that other yet undiscovered particles exist and change the picture. In that case, the corrections to the Higgs mass from the top quark could be cancelled out by some other hypothetical particle and lead to the observed low Higgs boson mass. Supersymmetry just happens to predict the existence of such particles, hence its appeal.

Last but not least, the Standard Model only describes visible matter, that is all matter we see around us on Earth as well as in stars and galaxies. But proofs abound telling us the Universe contains about five times more “dark matter”, a type of matter completely different from the one we know, than ordinary matter. Dark matter does not emit any light but manifests itself through its gravitational effects. Among all the particles contained in the Standard Model, none has the properties of dark matter. Hence it is clear the Standard Model gives an incomplete picture of the content of the Universe but supersymmetry could solve this problem.

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## From CERN at Quantum Diaries: “Grey matter confronted to dark matter”

THIS QUANTUM DIARIES POST IS PRESENTED IN ITS ENTIRETY BECAUSE OF ITS IMPORTANCE.

April 4th, 2013
Pauline Gagnon

“After 18 years spent building the experiment and nearly two years taking data from the International Space Station, the Alpha Magnetic Spectrometer or AMS-02 collaboration showed its first results on Wednesday to a packed audience at CERN. But Prof. Sam Ting, one of the 1976 Nobel laureates and spokesperson of the experiment, only revealed part of the positron energy spectrum measured so far by AMS-02.

Positrons are the antimatter of electrons. Given we live in a world where matter dominates, it is not easy to explain where this excess of positrons comes from. There are currently two popular hypotheses: either the positrons come from pulsars or they originate from the annihilation of dark matter particles into a pair of electron and positron. To tell these two hypotheses apart, one needs to see exactly what happens at the high-energy end of the spectrum. But this is where fewer positrons are found, making it extremely difficult to achieve the needed precision. Yesterday, we learned that AMS-02 might indeed be able to reach the needed accuracy.

The fraction of positrons (measured with respect to the sum of electrons and positrons) captured by AMS-02 as a function of their energy is shown in red. The vertical bars indicate the size of the uncertainty. The most important part of this spectrum is the high-energy part (above 100 GeV or 102) where the results of two previous experiments are also shown: Fermi in green and PAMELA in blue. Note that the AMS-02 precision exceeds the one obtained by the other experiments. The spectrum also extends to higher energy. The big question now is to see if the red curve will drop sharply at higher energy or not. More data is needed before the AMS-02 can get a definitive answer.

Only the first part of the story was revealed yesterday. The data shown clearly demonstrated the power of AMS-02. That was the excellent news delivered at the seminar: AMS-02 will be able to measure the energy spectrum accurately enough to eventually be able to tell where the positrons come from.

But the second part of the story, the punch line everyone was waiting for, will only be delivered at a later time. The data at very high energy will reveal if the observed excess in positrons comes from dark matter annihilation or from “simple” pulsars. How long will it take before the world gets this crucial answer from AMS-02? Prof. Ting would not tell. No matter how long, the whole scientific community will be waiting with great anticipation until the collaboration is confident their measurement is precise enough. And then we will know.

If AMS-02 does manage to show that the positron excess has a dark matter origin, the consequences would be equivalent to discovering a whole new continent. As it stands, we observe that 26.8% of the content of the Universe comes in the form of a completely unknown type of matter called dark matter but have never been able to catch any of it. We only detect its presence through its gravitational effects. If AMS-02 can prove dark matter particles can annihilate and produce pairs of electrons and positrons, it would be a complete revolution.”

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## From US/LHC Blog at Quantum Diaries: “Shutdown? What shutdown?”

Ken Bloom

“The LHC has been shut down for about two months now, but that really hasn’t made anyone less busy. It is true that we don’t have to run the detector now, but the CMS operations crew is now busy taking it apart for various refurbishing and maintenance tasks. There is a detailed schedule for what needs to be done in the next two years, and it has to be observed pretty carefully; there is a lot of coordination required to make sure that the necessary parts of the detector are accessible as needed, and of course to make sure that everyone is working in a safe environment (always our top priority).

A lot of my effort on CMS goes into computing, and over in that sector things in many ways aren’t all that different from how they were during the run. We still have to keep the computing facilities operating all the time. Data analysis continues, and we continue to set records for the level of activity from physicists who are preparing measurements and searches for new phenomena. We are also in the midst of a major reprocessing of all the data that we recorded during 2012, making use of our best knowledge of the detector and how it responds to particle collisions. This started shortly after the LHC run finished, and will probably take another couple of months.

There is also some data that we are processing for the very first time. Knowing that we had a two-year shutdown ahead of us, we recorded extra events last year that we didn’t have the computing capacity to process in real time, but could save for later analysis during the shutdown. This ended up essentially doubling the number of events we recorded during the last few months of 2012, which gives us a lot to do. Fortunately, we caught a break on this — our friends at the San Diego Supercomputer Center offered us some time on their facility. We had to scramble a bit to figure out how to include it into the CMS computing system, but now things are happily churning away with 5000 processors in use.”

See Ken’s complete post here.

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## From ALICE at CERN via Quantum Diaries: “The coolest and hottest fluid”

Pauline Gagnon

“The ALICE experiment is dedicated to the study of the quark-gluon plasma. Each year, the LHC operates for a few weeks with lead ions instead of protons. ALICE collects data both during proton-proton collisions and heavy ions collisions. Even when only protons collide, the projectiles are not solid balls like on a billiard table but composite objects. By comparing what can is obtained from heavy ion collisions with proton collisions, the ALICE physicists must first disentangle what comes from having protons in a bound state inside the nucleus as opposed to “free protons”.

So far, it appears that the quark-gluon plasma only formed during heavy-ion collisions since they provide the necessary energy density over a substantial volume (namely, the size of a nucleus). Some of the effects observed, such as the number of particles coming out of the collisions at different angles or momenta, depend in part on the final state created. When the plasma is formed, it reabsorbs many of the particles created, such that fewer particles emerged from the collision.

By colliding protons and heavy ions, scientists hope to discern what comes from the initial state of the projectile (bound or free protons) and what is caused by the final state (like the suppression of particles emitted when a quark-gluon plasma forms).

A “snapshot” of the debris coming out of a proton-lead ion collision captured by the ALICE detector showing a large number of various particles created from the energy released by the collision.

The ultimate goal is to study the so-called ‘structure function’, which describes how quarks and gluons are distributed inside protons, when they are free or embedded inside the nucleus.

More will be studied during the two-month running period with protons colliding on heavy ions planned for the beginning of 2013.”

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## From US/LHC Blog at Quantum Diaries: “Higgs update, HCP 2012″

Aidan Randle-Conde

“Last week, Seth and I met up to discuss the latest results from the Hadron Collider Physics (HCP) Symposium and what they mean for the Higgs searches. We have moved past discovery and now we are starting to perform precision measurements. Is this the Standard Model Higgs boson, or some other Higgs boson? Should we look forward to a whole new set of discoveries around the corner, or is the Higgs boson the final word for new physics that the LHC has to offer? We’ll find out more in the coming months!

Here are Aiden and Seth in their latest video. The sound is a touch weak, due to the outdoor location; but you can get plenty from what they report.

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## From CERN Blog at Quantum Diaries: “How is new physics discovered?”

IT HAS BEEN A WHILE SINCE I HAVE BEEN ABLE TO PRESENT A POST FROM QUANTUM DIARIES. MY AUDIENCE IS A MORE GENERALIST PUBLIC – INTERESTED, EDUCATED, BUT NOT PROFESSIONAL SCIENTISTS. NOW COMES A POST WHICH I BELIEVE MIGHT BE APPROACHABLE FOR MY READERS.

Pauline Gagnon

2012.09.28
Pauline Gagnon

“Finding an experimental anomaly is a great way to open the door to a new theory. It is such a good trick that many of us physicists are bending over backward trying to uncover the smallest deviation from what the current theory, the Standard Model of particle physics, predicts.

Standard Model

This is the approach the LHCb collaboration at CERN is pursuing when looking at very rare decays. A minute deviation can be more easily spotted for rare processes. One good place to look is in the rate of K meson decays, a particle made of one strange quark s and one anti-down quark d.

Recently, the LHCb collaboration has turned its attention to measuring the decay rate of the short-lived kaons K0S, the only K mesons decaying fast enough to be seen with precision in their detector.”

I hope that is enough to entice you to read further.

LHCb Collaboration

Pauline Gagnon is a very good writer. Read and enjoy the rest of her post here. While you are at it, look around at the various Quantum Diary blogs, Twitter feeds,member organization web sites.

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## From Fermilab at Quantum Diaries: Don Lincoln’s TED Audition

This is a neat 4 minute video of Fermilab’s Don Lincoln’s TED audition. It is worth viewing now. Then, we can hope to see Don at TED.

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## From Flip Tanedo at Quantum Diaries: “Tim Tait: ‘Why look for the Higgs?’”

Flip Tanedo

Flip has taken us to a talk by Tim Tait from UC Irvine at SLAC Lab. This video lecture is all about the Higgs boson and the Higgs field. The video is about one hour. If you are really interested in the Higgs, you will find this talk interesting and informing.

Thank you Flip, for making this video available to the public. Flip’s post at Quantum Diaries is here.

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## From Quantum Diaries – Zeynep Isvan on the Value of Domestic Basic Research

Zeynep Isvan

“We physicists are a very international crowd, and proud of it! The opening slide of conference talks typically has a list of institutions and their home countries, frequently pinned on a Google map. This kind of international collaboration is imperative to advancing science. I can’t think of any scientist who would have it any other way. Is it unreasonable, then, to push for domestic science? Is it insignificant whether an experiment is based in one’s home country (i.e. the primary country in which you have a job as a scientist) or elsewhere in the world, as long as it is somewhere?

I think not.”

Read the whole discussion here. Then, draw your own conclusions. From what I have seen, doing the research for posts to this blog, especially in Physics and Astronomy, doing Basic Research is an impossibility on only a domestic platform.

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## From Burton DeWilde in The US/LHC Blog at Quantum Diaries: “Communicating Science and Its Value, pt. 1″

Burton DeWilde

“In the past I’ve made it known that I’m a politically-engaged person — and not without some commentator controversy. While I generally prefer to keep my science and politics separate, they inevitably intersect in the matter of governmental funding of scientific research and conflicts between groups driving the national dialogue on science policy. Unfortunately, scientists are often left behind in this conversation, resulting in a serious disconnect with the public.

It’s not hard to find embarrassing stories about how Americans are ignorant of basic scientific knowledge: roughly half believe dinosaurs and humans coexisted, 1 in 5 adults believes the Sun revolves around the Earth, and when it comes to acceptance of evolution, we’re out of step with much of the world. On many topical issues — global climate change, nuclear energy, genetically-modified foods, vaccination, cell phones — an abundance of misinformation drowns out the science, or at least muddies the waters. And even worse, many Americans don’t understand how scientists draw their conclusions, i.e. the scientific method, nor do they apply it in their daily lives. A much-quoted survey from 2007 found that 70% of Americans are “scientifically illiterate” (though that distinction, as well as the statistic, is misleading: scientific literacy is not on a binary scale).

I realize that I’m probably preaching to the choir here: You all have made the effort to read a physics blog written by physicists about highly technical topics, which suggests to me that you are either totally awesome science enthusiasts or… scientists. Thanks for reading! :) But from whom does the rest of the country not following Quantum Diaries get its science information? “

This is almost a rhetorical question, our U.S. Press is practically devoid of any kind of prose or video on the subjects which make up Basic Research. Applied Research – where there is a dollar to be made – fares far better.

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