## Posts Tagged ‘new physics’

### In search of the secret passage

Wednesday, June 3rd, 2015

Today begins the second operation period of the Large Hadron Collider (LHC) at CERN. By declaring “stable beams”, the LHC operators signal to physicists it is now safe to turn all their detectors on. After more than two years of intensive repair and consolidation work, the LHC now operates at higher energy. What do we hope to achieve?

The discovery of the Higgs boson in July 2012 completed the Standard Model of particle physics. This theoretical model describes all matter seen around us, both on Earth and in all stars and galaxies. But this is precisely the problem: this model only applies to what is visible in the Universe, namely 5% of its content in matter and energy. The rest consists of dark matter (27%) and dark energy (68%), two absolutely unknown substances. Hence the need for a more encompassing theory. But what is it and how can it be reached?

By operating the LHC at 13 TeV, we now have much more energy available to produce new particles than during the 2010-2012 period, when the proton collisions occurred at 8 TeV. Given that energy and mass are two forms of the same essence, the energy released during these collisions materialises, producing new particles. Having more energy means one can now produce heavier particles. It is as if one’s budget just went from 8000 euro to 13000 euro. We can “afford” bigger particles if they exist in Nature.

The Standard Model tells us that all matter is built from twelve basic particles, just like a construction set consisting of twelve basic building blocks and some “connectors” linking them together. These connectors are other particles associated with the fundamental forces. Since none of these particles has the properties of dark matter, there must still be undiscovered particles.

Which theory will allow us to go beyond the Standard Model? Will it be Supersymmetry, one of the numerous theoretical hypotheses currently under study. This theory would unify the particles of matter with the particles associated with the fundamental forces. But Supersymmetry implies the existence of numerous new particles, none of which has been found yet.

Will the LHC operating at 13 TeV allow us to produce some of these supersymmetric particles? Or will the entrance of the secret passage towards this “new physics” be revealed by meticulously studying a plethora of quantities, such as the properties of the Higgs boson. Will we discover that it establishes a link between ordinary matter (everything described by the Standard Model) and dark matter?

These are some of the many questions the LHC could clarify in the coming years. An experimental discovery would reveal the new physics. We might very well be on the verge of a huge scientific revolution.

### Top quark still raising questions

Wednesday, October 15th, 2014

Why are scientists still interested in the heaviest fundamental particle nearly 20 years after its discovery? Photo: Reidar Hahn, Fermilab

“What happens to a quark deferred?” the poet Langston Hughes may have asked, had he been a physicist. If scientists lost interest in a particle after its discovery, much of what it could show us about the universe would remain hidden. A niche of scientists, therefore, stay dedicated to intimately understanding its properties.

Case in point: Top 2014, an annual workshop on top quark physics, recently convened in Cannes, France, to address the latest questions and scientific results surrounding the heavyweight particle discovered in 1995 (early top quark event pictured above).

Top and Higgs: a dynamic duo?
A major question addressed at the workshop, held from September 29 to October 3, was whether top quarks have a special connection with Higgs bosons. The two particles, weighing in at about 173 and 125 billion electronvolts, respectively, dwarf other fundamental particles (the bottom quark, for example, has a mass of about 4 billion electronvolts and a whole proton sits at just below 1 billion electronvolts).

Prevailing theory dictates that particles gain mass through interactions with the Higgs field, so why do top quarks interact so much more with the Higgs than do any other known particles?

Direct measurements of top-Higgs interactions depend on recording collisions that produce the two side-by-side. This hasn’t happened yet at high enough rates to be seen; these events theoretically require higher energies than the Tevatron or even the LHC’s initial run could supply. But scientists are hopeful for results from the next run at the LHC.

“We are already seeing a few tantalizing hints,” says Martijn Mulders, staff scientist at CERN. “After a year of data-taking at the higher energy, we expect to see a clear signal.” No one knows for sure until it happens, though, so Mulders and the rest of the top quark community are waiting anxiously.

A sensitive probe to new physics

Top and antitop quark production at colliders, measured very precisely, started to reveal some deviations from expected values. But in the last year, theorists have responded by calculating an unprecedented layer of mathematical corrections, which refined the expectation and promise to realigned the slightly rogue numbers.

Precision is an important, ongoing effort. If researchers aren’t able to reconcile such deviations, the logical conclusion is that the difference represents something they don’t know about — new particles, new interactions, new physics beyond the Standard Model.

The challenge of extremely precise measurements can also drive the formation of new research alliances. Earlier this year, the first Fermilab-CERN joint announcement of collaborative results set a world standard for the mass of the top quark.

Such accuracy hones methods applied to other questions in physics, too, the same way that research on W bosons, discovered in 1983, led to the methods Mulders began using to measure the top quark mass in 2005. In fact, top quark production is now so well controlled that it has become a tool itself to study detectors.

Forward-backward synergy

With the upcoming restart in 2015, the LHC will produce millions of top quarks, giving researchers troves of data to further physics. But scientists will still need to factor in the background noise and data-skewing inherent in the instruments themselves, called systematic uncertainty.

“The CDF and DZero experiments at the Tevatron are mature,” says Andreas Jung, senior postdoc at Fermilab. “It’s shut down, so the understanding of the detectors is very good, and thus the control of systematic uncertainties is also very good.”

Jung has been combing through the old data with his colleagues and publishing new results, even though the Tevatron hasn’t collided particles since 2011. The two labs combined their respective strengths to produce their joint results, but scientists still have much to learn about the top quark, and a new arsenal of tools to accomplish it.

“DZero published a paper in Nature in 2004 about the measurement of the top quark mass that was based on 22 events,” Mulders says. “And now we are working with millions of events. It’s incredible to see how things have evolved over the years.”

Troy Rummler

### Two anomalies worth noticing

Monday, July 14th, 2014

The 37th International Conference on High Energy Physics just finished in Valencia, Spain. This year, no big surprises were announced: no new boson, no signs from new particles or clear phenomena revealing the nature of dark matter or new theories such as Supersymmetry. But as always, a few small anomalies were reported.

Looking for deviations from the theoretical predictions is precisely how experimentalists are trying to find a way to reveal “new physics”. It would help discover a more encompassing theory since everybody realises the current theoretical model, the Standard Model, has its limits and must be superseded by something else. However, all physicists know that small deviations often come and go. All measurements made in physics follow statistical laws. Therefore deviations from the expected value by one standard deviation occur in three measurements out of ten. Larger deviations are less common but still possible. A two standard deviation happens 5% of the time. Then there are systematic uncertainties that relate to the experimental equipment. These are not purely statistical, but can be improved with a better understanding of our detectors. The total experimental uncertainty quoted with each result corresponds to one standard variation. Here are two small anomalies reported at this conference that attracted attention this year.

The ATLAS Collaboration showed its preliminary result on the production of a pair of W bosons. Measuring this rate provides excellent checks of the Standard Model since theorists can predict how often pairs of W bosons are produced when protons collide in the Large Hadron Collider (LHC). The production rate depends on the energy released during these collisions. So far, two measurements can be made since the LHC operated at two different energies, namely 7 TeV and 8 TeV.

CMS and ATLAS had already released their results on their 7 TeV data. The measured rates exceeded slightly the theoretical prediction but were both well within their experimental error with a deviation of 1.0 and 1.4 standard deviation, respectively. CMS had also published results based on about 20% of all data collected at 8 TeV. It exceeded slightly the theoretical prediction by 1.7 standard deviation. The latest ATLAS result adds one more element to the picture. It is based on the full 8 TeV data sample. Now ATLAS reports a slightly stronger deviation for this rate at 8 TeV with 2.1 standard deviations from the theoretical prediction.

The four experimental measurements for the WW production rate (black dots) with the experimental uncertainty (horizontal bar) as well as the current theoretical prediction (blue triangle) with its own uncertainty (blue strip). One can see that all measurements are higher than the current prediction, indicating that the theoretical calculation fails to include everything.

The four individual measurements are each reasonably consistent with expectation, but the fact that all four measurements lie above the predictions becomes intriguing. Most likely, this means that theorists have not yet taken into account all the small corrections required by the Standard Model to precisely determine this rate. This would be like having forgotten a few small expenses in one’s budget, leading to an unexplained deficit at the end of the month. Moreover, there could be common factors in the experimental uncertainties, which would lower the overall significance of this anomaly. But if the theoretical predictions remain what they are even when adding all possible little corrections, it could indicate the existence of new phenomena, which would be exciting. It would then be something to watch for when the LHC resumes operation in 2015 at 13 TeV.

The CMS Collaboration presented another intriguing result. They found some events consistent with coming from a decay of a Higgs boson into a tau and a muon. Such decays are prohibited in the Standard Model since they violate lepton flavour conservation. There are three “flavours” or types of charged leptons (a category of fundamental particles): the electron, the muon and the tau. Each one comes with its own type of neutrinos. According to all observations made so far, leptons are always produced either with their own neutrino or with their antiparticle. Hence, the decay of a Higgs boson in leptons should always produce a charged lepton and its antiparticle, but never two charged leptons of different flavour. Violating a conservation laws in particle physics is simply not allowed.

This needs to be scrutinised with more data, which will be possible when the LHC resumes next year. Lepton flavour violation is allowed outside the Standard Model in various models such as models with more than one Higgs doublet or composite Higgs models or Randall-Sundrum models of extra dimensions for example. So if both ATLAS and CMS confirm this trend as a real effect, it would be a small revolution.

The results obtained by the CMS Collaboration showing that six different channels all give a non-zero value for the decay rate of Higgs boson into pairs of tau and muon.

Pauline Gagnon

### The International Linear Collider: More than Just a Higgs Factory

Friday, August 23rd, 2013

The ILC site has been chosen. What does this mean for Japan?

The two ILC candidate sites: Sefuri in the South and Kitakami in the North. Credit: linearcollider.org

Hi Folks,

It is official [Japanese1,Japanese2]: the Linear Collider Collaboration and the Japanese physics community have selected the Kitakami mountain range in northern Japan as the site for the proposed International Linear Collider. Kitakami is a located in the Iwate Prefecture and is just north of the Miyagi prefecture, the epicenter of the 2011 Tohoku Earthquake. Having visited the site in June, I cannot aptly express how gorgeous the area is, but more importantly, how well-prepared Iwate City is for this responsibility.

Science is cumulative: new discoveries are used to make more discoveries about how nature works, and physics is no different. The discovery of the Higgs boson at the Large Hadron Collider was a momentous event. With its discovery, physicists proved how some particles have mass and why others have no mass at all. The Higgs boson plays a special role in this process, and after finally finding it, we are determined to learn more about the Higgs. The International Linear Collider (ILC) is a proposed Higgs boson factory that would allow us to intimately understand the Higgs. Spanning 19 miles (31 km) [310 football pitches/soccer fields], if constructed, the ILC will smash together electrons and their antimatter partners, positrons, to produce a Higgs boson (along with a Z boson). In such a clean environment (compared to proton colliders), ultra-precise measurements of the Higgs boson’s properties can be made, and thereby elucidate the nature of this shiny new particle.

The general overview schematic of the International Linear Collider. Credit: linearcollider.org

However, the ILC is more than just a experiment. Designing, constructing, and operating the machine for 20 years will be a huge undertaking with lasting effects. For staters, the collider’s Technical Design Report (TDR), which contains every imaginable detail minus the actual blueprints, estimates the cost of the new accelerator to be 7.8 billion USD (2012 dollars). This is not a bad thing. Supposing 50% of the support came from Asia, 25% from the Americas, and 25% from Europe, that would be nearly 2 billion USD invested in new radio frequency technology in England, Germany, and Italy. In the US, it would be nearly 2 billion USD invested in coastal and Midwestern laboratories developing new cryogenic and superconducting technology. In Asia, this would be nearly 4 billion USD invested in these technologies as well as pure labor and construction. Just as the LHC was a boon on the European economy, a Japanese-based ILC will be a boon for an economy temporarily devastated  by an historic earthquake and tsunami. These are just hypothetical numbers; the real economic impact will be  larger.

I had the opportunity to visit Kitakami this past June as a part of a Higgs workshop hosted by Tohoku University. Many things are worth noting. The first is just how gorgeous the site is. Despite its lush appearance, the site offers several geological advantages, including stability against earthquakes of any size. Despite its proximity to the 2011 earthquake and the subsequent tsunami, this area was naturally protected by the mountains. Below is a photo of the Kitakami mountains that I took while visiting the site. Interestingly, I took the photo from the UNESCO World Heritage site Hiraizumi. The ILC is designed to sit between the two mountains in the picture.

The Kitamaki Mountain Range as seen from the UNESCO World Heritage Site in Hiraizumi, Japan. Credit: Mine

What I want to point out in the picture below is the futuristic-looking set of tracks running across the photo. That is the rail line for the JR East bullet train, aka the Tohoku Shinkansen. In other words, the ILC site neighbours a very major transportation line connecting the Japanese capital Tokyo to the northern coast. It takes the train just over 2 hours to traverse the 250 miles (406.3 km) from Tokyo station to the Ichinoseki station in Iwate. The nearest major city is Sendai, capital of Miyagi, home to the renown Tohoku University, and is only a 10 minute shinkansen ride from Ichinoseki station.

The Kitamaki Mountain Range as seen from the UNESCO World Heritage Site in Hiraizumi, Japan. Credit: Mine

What surprised me is how excited the local community is about the collider. After exiting the Ichinoseki station I discovered this subtle sign of support:

There is much community support for the ILC: The Ichinoseki Shinkansen Station in Iwate Prefecture, Japan. Credit: Mine

The residents of Iwate and Miyagi, independent of any official lobbying organization, have formed their own “ILC Support Committee.” They even have their own facebook page. Over the past year, the residents have invited local university physicists to give public lectures on what the ILC is; they have requested that more English, Chinese, Korean, and Tagalog language classes be offered at local community centers; that more Japanese language classes for foreigners are offered in these same facilities; and have even discussed with city officials how to prepare Iwate for the prospect of a rapid increase in population over the next 20 years.

Despite all this, the real surprises were the pamphlets. Iwate has seriously thought this through.

Pamphlets showcasing the Kitakami Mountain Range in Iwate, Japan. Credit: Mine

The level of detail in the pamphlets is impressive. My favourite pamphlet has the phrase, “Ray of Hope: Tohoku Is Ready to Welcome the ILC” on the front cover. Inside is a list of ways to reach the ILC site and the time it takes. For example: it takes 12 hours 50 minutes to reach Tokyo from Rome and 9 hours 40 minutes from Sydney. The brochure elaborates that the Kitakami mountains maintain roughly the same temperature as Switzerland (except in August-September) but collects much more precipitation through the year. Considering that CERN is located in Geneva, Switzerland, and that many LHC experimentalists will likely become ILC experimentalists, the comparison is very helpful. The at-a-glance annual festival schedule is just icing on the cake.

“Ray of Hope” pamphlet describing how to each different ILC campuses by train.  Credit: Mine

Now that the ILC site has been selected, surveys of the land can be conducted so that blue prints and a finalized cost estimate can be established. From my discussions with people involved in the site selection process, the decision was very difficult. I have not visited the Fukuoka site, though I am told it is a comparably impressive location. It will be a while still before any decision to break ground is made. And until that happens, there is plenty of work to do.

Happy Colliding

– Richard (@bravelittlemuon)

### The Take Away Message of Snowmass on the Mississippi

Friday, August 9th, 2013

Snowmass Came and Passed. What have we learned from it?

Skyline of Minneapolis, home of the University of Minnesota and host city of the Community Summer Study 2013: Snowmass on the Mississippi.

Hi All,

Science is big. It is the systematic study of nature, so it has to be big. In another way, science is about asking questions, questions that expands our knowledge of nature just a bit more. Innocuous questions like, “Why do apples fall to the ground?”, “How do magnets work?”, or “How does an electron get its mass?” have lead to understanding much more about the universe than expected. Our jobs as scientists come down to three duties: inventing questions, proposing answers (called hypotheses), and testing these proposals.

As particle physicists, we ask “What is the universe made of?” and “What holds the universe together?”  Finding out that planets and stars only make up 5% of the universe really makes one pause and wonder, well, what about everything else?

From neutrino masses, to the Higgs boson, to the cosmic microwave background, we have learned  much about the origin of mass in the Universe as well as the origin of the Universe itself in the past 10 years. Building on recent discoveries, particle physicists from around the world have been working together for over a year to push our questions further. Progress in science is incremental, and after 10 days at the Community Summer Study 2013: Snowmass on the Mississippi Conference, hosted by the University of Minnesota, we have a collection of questions that will drive and define particle physic for the next 20 years. Each question is an incremental step, but each answer will allow us to expand our knowledge of nature.

I had a chance to speak with SLAC‘s Michael Peskin, a convener for the Snowmass Energy Frontier study group and author of the definitive textbook on Quantum Field Theory, on how he sees the high energy physics community proceeding after Snowmass. “The community did a lot of listening at Snowmass. High energy physics is pursuing a very broad array of questions.  I think that we now appreciate better how important all of these questions are, and that there are real strategies for answering them.”  An important theme of Snowmass, Peskin said, was “the need for long-term, global planning”.  He pointed to the continuing success of the Large Hadron Collider, which is the result of the efforts of thousands of scientists around the world.  This success would not have happened without such a large-scale, global  effort.  “This is how high energy physics will have to be, in all of its subfields, to answer our big questions.”

Summary presentations of all the work done for Snowmass are linked below in pdf form and are divided into two categories: how to approach questions (Frontiers) and what will enable us to answer these questions. These two categories represent the mission of the US Department of Energy’s Office of Science. A summary of the summaries is at the bottom.

What is the absolute neutrino mass scale? What is the neutrino mass ordering? Is CP violated in the neutrino sector? What new knowledge will neutrinos from astrophysical sources bring?

What is dark matter? What is dark energy? Why more matter than anti-matter? What is the physics of the Universe at the highest energies?

Where are the new particles that modify the Higgs, t, W couplings? What particles comprise the dark matter? Why is the Higgs boson so light?

The growth in data drives need for continued R&D investment in data management, data access methods, networking. Challenging resource needs require efficient and flexible use of all resources HEP needs both Distributed High-Throughput computing (experiment program) and High-Performance computing (mostly theory/simulation/modeling)

Encourage and enable physicists to be involved in and support local, national and world-wide efforts that offer long–term professional development and training opportunities for educators (including pre-service educators), using best practice and approaches supported by physics education research. and Create learning opportunities for students of all ages, including classroom, out-of-school and online activities that allow students to explore particle physics

Our vision is for the US to have an instrumentation program for particle physics that enables the US to maintain a scientific leadership position in a broad, global, experimental program; and develops new detection capabilities that provides for cutting edge contributions to a world program

Is dark energy a cosmological constant? Is it a vacuum energy? From where do ultra high energy cosmic rays originate? From where do ultra high energy neutrinos originate?

How would one build a 100 TeV scale hadron collider? How would one build a lepton collider at >1 TeV? Can multi-MW targets survive? If so, for how long?

To provide a conduit for untenured (young) particle physicists to participate in the Community Summer Study. To facilitate and encourage young people to get involved.
Become a long term asset to the field and a place where young peoples voices can be heard

Several great posts from QD (Family, Young, Frontierland), Symmetry Magazine (Push, Q&A, IceSlam, Decade), and even real-time updates from QD’s Ken Bloom (@kenbloomunl) and myself (@bravelittlemuon) via #Snowmass are available. All presentations can be found at the Snowmass Indico page.

Until next time, happy colliding.

– Richard (@bravelittlemuon)

Community Summer Study: Snowmass 2013 Poster

### The Definite Article Problem

Tuesday, June 4th, 2013

A Little Bit of the Higgs Boson for Everyone

Hi All,

This post is long overdue but nonetheless I am thrilled to finally write it. We have discovered the a some  ??? Higgs boson, and it is precisely my trouble writing this very sentence that inspires a new post. CERN‘s press office has keenly presented a new question in particle physics known as the Definite Article Problem:

Have we discovered “a” Higgs boson or “the” Higgs boson?

We can express the Article problem in another way:

Are there more Higgs bosons?

Before I touch upon that problem, I want to explain about why the Higgs boson is important. In particular, I want to talk about the Sun! Yes, the Sun.

## The Higgs Boson and Electroweak Symmetry Breaking is Important because the Sun Shines.

Okay, there is no way to avoid this: I really like the sun.

Slide Credit: Mine. Image Credit: GOES Collaboration

It shines. It keeps the planet warm. There is liquid water on Earth, and some very tasty plants too.

Slide Credit: Mine. Image Credit: NobelPrize.org

At the heart of the Sun is a ranging nuclear furnace and involves two types of processes: (1) those that involve the Strong nuclear force and (2) those that involve the Weak nuclear force (look for the neutrinos!). The two types of processes work together in a solar relay race to complete a circuit, only to do it over and over again for billions of years. And just like a real relay race, the speed at which the circuit is finished is set by the slowest member. In this case, the Weak force is the limiting factor and considerably slows down the rate at which the sun could theoretically operate. If we make the Weak force stronger, then the Sun would shine more brightly. Conversely, if we make the Weak force even weaker, the Sun would be dimmer.

Slide Credit: Mine. Image Credit: NobelPrize.org

From studying the decays of radioactive substances, we have learned that the rate of Weak nuclear processes is set by a physical constant called Fermi’s Constant. Fermi’s Constant is represented by symbol GF. From study the Higgs boson and the Higgs Mechanism, we have learned that Fermi’s Constant is literally just another constant, v, in disguise. This second physical constant (v) is called the Higgs “vacuum expectation value” , or “vev” for short, and is the amount of energy the Higgs field has at all times relative to the vacuum.

The point I want to make is this: If we increase the Higgs vev, Fermi’s Constant gets smaller, which reduces the rate of Weak nuclear interactions. In other words, a larger Higgs vev would make the sun shine less brightly. Going the other way, a smaller Higgs vev would make the sun shine more brightly. (This is really cool!)

Slide Credit: Mine. Image Credit: Jacky-Boi

The Higgs vev is responsible for some other things, too. It is a source of energy from which all elementary particles can draw. Through the Higgs Mechanism, the Higgs field provides mass to all elementary particles and massive bosons. One would think that for such an important particle we would have a firm theoretical understanding it, but we do not.

Credit: Mine

We have a very poor theoretical understanding of the Higgs boson. Among other things, according to our current understanding of the Higgs boson, the particle should be much heavier than what we have measured.

Credit: Mine

## The Definite Article Problem

There are lots of possible solutions to the problems and theoretical inconsistencies we have discovered relating to the Standard Model Higgs boson. Many of these ideas hypothesize the existence of other Higgs bosons or particles that would interact like the Higgs boson. There are also scenarios where Higgses have identity crises: the Higgs boson we have observed could be a quantum mechanical combination (superposition) of several Higgs bosons.

I do not know if there are additional Higgses. Truthfully, there are many attractive proposals that require upping the number Higgs bosons. What I do know is that our Higgs boson is interesting and merits much further studying.

Credit: Mine

Happy Colliding

– richard (@bravelittlemuon)

PS In case anyone is wondering, yes, I did take screen shots from previous talks and turn them into a DQ post.

### Huge impact from a tiny decay

Wednesday, November 14th, 2012

The Hadron Collider Physics Symposium opened on November 12 in Kyoto on a grand note. For the first time, the LHCb collaboration operating at the Large Hadron Collider (LHC) at CERN showed evidence for an extremely rare type of events, namely the decay of a Bs meson into a pair of muons (a particle very similar to the electron but 200 times heavier). A meson is a composite class of particles formed from a quark and an antiquark. The Bs meson is made of a bottom quark b and a strange quark s. This particle is very unstable and decays in about a picosecond (a millionth of a millionth of a second) into lighter particles.

Decays into two muons are predicted by the theory, the Standard Model of particle physics, that states it should occur only about 3 times in a billionth of decays. In scientific notation, we write (3.54±0.30)x10-9 where the value of 0.30 represents the error margin on this theoretical calculation. Now, the LHCb collaboration proudly announced that they observed it at a rate of (3.2+1.5-1.2)x10-9 , a value very close to the theoretically predicted value, at least within the experimental error.

Here is the plot shown by the LHCb collaboration for the number of events found in data as a function of the combined mass of the two muons. The solid blue line represents the sum of all types of events from known phenomena containing two muons. The dashed curve in red shows the number of events coming from a Bs meson. With the current error margin on the measurement (shown by the

vertical and horizontal bars on the data points), the data seem to agree with all expected contributions from known sources, leaving little room for new phenomena.

This represents a great achievement, not only because this is the rarest process ever observed, but because it puts stringent limits on new theories. Here is why.

Theorists are convinced that a theory far more encompassing than the Standard Model exists even though we have not detected its presence yet. As if the Standard Model is to particle physics what the four basic operations (addition, multiplication, division and subtraction) are to mathematics. They are sufficient to tackle daily problems but one needs algebra, geometry and calculus to solve more complex problems. And in particle physics, we do have problems we cannot solve with the Standard Model, such as explaining the nature of dark matter and dark energy.

A good place to catch the first signs of “new physics” is where the Standard Model predicts very faint signals such as in Bs mesons decaying into two muons. These decays occur extremely rarely because the Standard Model only has limited ways to produce them. But if an additional mechanism comes into play due to some new theory, we would observe these decays at a rate different from what is expected within the Standard Model.

This is a bit like using the surface of a lake to detect the presence of an invisible creature, hoping its breath would create a ripple on the water surface. It would only work if the lake were extremely calm or disturbed only by an occasional tiny fish.  Here the Standard Model acts like all known little animals creating ripples on the water surface.  The hope was to detect other ripples in the absence of known causes (fish, frogs or mosquitoes). The LHCb result reveals no extra ripples yet. So either the new creature does not breathe as expected or we need to find another method to see it. It will be easier to know once the error margin is reduced with more data.

This new result pushes the reach for new physics even further. Nevertheless, it will help theorists eliminate faulty models like on the plot below and eventually zoom on the right solution. Meanwhile, experimentalists will have to devise yet more stringent tests to be able to discover the way to this new physics.

This plot shows how this measurement (horizontal axis) shown earlier this year reduced the space where new physics could be seen. With this new result, the constraints will even be stronger.

(For more details, see LHCb website)

Pauline Gagnon

### How is new physics discovered?

Friday, September 28th, 2012

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.

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.

There are in fact two sorts of K mesons: short-lived ones, K0S (called K-short) and long-lived ones, K0L (“K-long”). In the early 1970’s, scientists discovered that the K0L were decaying into a pair of muons 10 000 times less often than the theory predicted. At the time, the theory knew of only three quarks: u, d and s. This hinted three theorists, Sheldon Glashow, John Iliopoulos and Luciano Maiani to propose a mechanism that required the existence of a new, unknown quark, the charm quark c, to explain how this rate could be so suppressed. This explanation is now called the GIM mechanism, an acronym based on their last names.

This major breakthrough on a theoretical level was soon confirmed by the discovery of the charm quark in 1974.

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.

To make this measurement, they had to select billions of muon pairs and see if any was coming from the decay of a K0S. One can reconstruct the mass of a decaying particle by adding together the mass and momentun of all its fragments. If these muons were coming from the decays of K0S, the reconstructed mass would be the K0S mass. An accumulation of events would appear near this value in the distribution of all the recombined masses.

But as can be seen in the figure below, no such accumulation appears in the region around 500 MeV, the K0S mass value. This allowed the LHCb collaboration to estimate how often a K0S can decay into two muons, a quantity called the branching ratio. They placed a limit at less than 9 times in a billion, or in scientific notation, BR(K0S →μμ ) < 9 x 10-9 with 90% confidence level using all of the 2011 data. Since no peak appears anywhere on this curve, it means the muon pairs were produced in a variety of decays where other particles were also produced.

They have a long way to go since it is still about 2000 times larger than what the Standard Model predicts, namely a branching ratio of 5×10-12. Nevertheless, LHCb is getting closer to the theoretical prediction and eventually, given enough data, they might be able to test it.

Not easy to get to the next layer of the theory when the current one makes predictions requiring thousands of billions of events to be tested.

Pauline Gagnon

### Why The Higgs Boson Should Not Exist and Why This Is a Good Thing

Thursday, September 13th, 2012

Theoretically, the Higgs boson solves a lot of problems. Theoretically, this Higgs boson is a problem.

Greetings from the good ol’ U.S. of A.

Now that Fall is here, classes are going, holidays are wrapping up, and research programs are in full steam. Unfortunately, all is not well in the Wonderful World of Physics. To refresh, back on 4th of July, the LHC experiments announced the outstanding and historical discovery of a new particle with properties consistent with the Standard Model Higgs boson. No doubt, this is a fantastic feat by the experiments, a triumph and culmination of a decades-long endeavor. However, there is deep concern about the existence of a 125 GeV Higgs boson. Being roughly 130 times the proton’s mass, this Higgs boson is too light. A full and formal calculation of the Higgs boson’s mass, according to the theory that predicts it, places the Higgs mass pretty close to infinity. Obviously, the Higgs boson’s mass is less than infinite. So let’s talk mass and why this is still a very good thing for particle physics.

For an introduction to the Higgs boson, click here, here, or here (This last one is pretty good).

## The Standard Higgs

The Standard Model of Particle Physics (SM) is the theory that describes, well, everything with the exception of gravity (Yes, this is admittedly a pretty big exception).  It may sound pompous and arrogant, but the SM really does a good job at explaining how things work: things like the lights in your kitchen, or smoke detectors, or the sun.

Though if this “theory of almost-everything” can do all this, then when written out explicitly, it must be pretty big, right? Yes. The answer is yes. Undeniably, yes. When written out fully and explicitly, the “Lagrangian of the Standard Model” looks like this (click to enlarge):

Figure 1: The Standard Model Lagrangian in the Feynman Gauge. Credit: T.D. Gutierrez

This rather infamous and impressive piece of work is by Prof. Thomas Gutierrez of Cal Poly SLO. Today, however, we only care about two terms (look for the red circles):

Figure 2: The Standard Model Lagrangian in the Feynman Gauge with the Higgs boson tree-level mass and 4-Higgs vertex interactions terms circles. Original Credit: T.D. Gutierrez

The first term is pretty straightforward. It expresses the fact that the Higgs boson has a mass, and this can represented by the Feynman diagram in Fig 3. (below). As simple and uneventful as this line may appear, its existence has a profound impact on the properties of the Higgs boson. For example, because of its mass, the Higgs boson can never travel at the speed of light; this is the complete opposite for the massless photon, which can only travel at the speed of light. The existence of the diagram if Fig. 3 also tells us exactly how a Higgs boson (denoted by h) travels from one place in the Universe, let’s call is x, to another place in the Universe, let’s call it y. Armed with this information, and a few other details, we can calculate the probability that a Higgs boson will travel from point x to point y, or if it will decay at some point in between.

Figure 3: The tree-level Feynman diagram the represents a SM Higgs boson (h) propagating from a point x in the Universe to a point y somewhere else in the Universe. Credit: Mine

The second term is an interesting little fella. It expresses the way the Higgs boson can interact with other Higgs bosons, or even itself. The Feynman diagram associated with this second term is in Fig. 4. It implies that there is a probability a Higgs boson (at position w) and a second Higgs boson (at position x) can collide into each other at some point in the Universe, annihilate, and then produce two Higgs bosons (at point z and y). To recap: two Higgses go in, two Higgses go out.

Figure 4: The tree-level Feynman diagram the represents two SM Higgs bosons (h) at points w and x in the Universe annihilating and producing two new SM Higgs bosons at points z and y somewhere else in the Universe. Credit: Mine

This next step may seem a little out-of-the-blue and unmotivated, but let’s suppose that one of the incoming Higgs bosons was also one of the outgoing Higgs bosons. This is equivalent to supposing that w was equal to z. The Feynman diagram would look like Fig. 5 (below).

Figure 5: By making an incoming Higgs boson (h) the same as an outgoing Higgs boson in the 4-Higgs interaction term, we can transform the tree-level 4-Higgs interaction term into the 1-loop level correction to the Fig. 1, the diagram the represents the propagation of a Higgs boson in the Universe. Credit: Mine

In words, this “new” diagram states that as a Higgs boson (h) at position x travels to position y, it will emit and absorb a second Higgs boson somewhere in between x and y. Yes, the Higgs boson can and will emit and absorb a second Higgs boson.

If you look carefully, this new diagram has the same starting point and ending point at our first diagram in Fig. 3, the one that described the a Higgs boson traveling from position x to position y. According to the well-tested rules of quantum mechanics, if two diagrams have the same starting and ending conditions, then both diagrams contribute to all the same processes and both must be included in any calculation that has the same stating and ending points. In terms of Feynman diagrams, if we want to talk about a Higgs boson traveling from point x to point y, then we need to look no further than Fig. 6.

Figure 6: The tree-level (L) and 1-loop level (R) contributions to a Higgs boson (h) traveling from point x to point y. Credit: Mine

## What Does This All Mean?

Now that I am done building things up, let me quickly get to the point. The second diagram can be considered a “correction” to the first diagram. The first diagram is present because the Higgs boson is allowed to have mass (mH). In a very real sense, the second diagram is a correction to the Higgs boson’s mass. In a single equation, the two diagrams in Fig. 6 imply

Equation 1: The theoretical prediction for the SM Higgs boson's observed mass, which includes the "tree-level" contribution ("free parameter"), and 1-loop level contribution ("cutoff"). Credit: Mine

In Eq. (1), term on the far left is the Higgs boson’s mass that has been experimentally measured, i.e., 125 GeV. Hence the label, “what we measure.” The term just right of that (the “free parameter”) is the mass of the Higgs boson associated with the first term in the SM Lagrangian (Fig. 2 and 3). When physicists talk about the Standard Model not predicting the mass of the Higgs boson, it is this term (the free parameter) that we talk about. The SM makes no mention as to what it should be. We have to get down, dirty, and actually conduct an experiment get the thing. The term on the far right can be ignored. The term “Λ” (the “cutoff scale“), on the other hand, terrifies and mystifies particle physicists.

Λ is called the “cutoff scale” of the SM. Physically, it represents the energy at which the SM stops working. I mean it: we stop calculating things when we get to energies equal to Λ. Experimentally, Λ is at least a few hundred times the mass of the proton. If Λ is very LARGE, like several times larger than the LHC’s energy range, then the observed Higgs mass gets an equally LARGE bump. For example, if the SM were 100% correct for all energies, then Λ would be infinity. If this were true, then

(the Higgs boson’s mass) = (something not infinity) + (something infinity) ,

which comes out inevitably to be infinity. In other words, if the Standard Model of Physics were 100% correct, then the Higgs boson’s mass is predicted to be infinity. The Higgs boson is not infinity, obviously, and therefore the Standard Model is not 100%. Therefore, the existence of the Higgs boson is proof that there must be new physics somewhere. “Where and at what energy?,” is a whole different question and rightfully deserves its own post.

Happy Colliding

– Richard (@bravelittlemuon)

### How to discover new physics

Saturday, June 2nd, 2012

The biggest news at CIPANP 2012 for particle physicists seems to be coming from the “low” energy frontier, at energies in the ballpark of 10GeV and lower. This may come as a surprise to some people, after all we’ve had experiments working at these energies for a few decades now, and there’s a tendency to think that higher energies mean more potential for discovery. The lower energy experiments have a great advantage over the giants at LHC and Tevatron, and this is richer collection of analyses.

There’s a big difference between discovering a new phenomenon and discovering new physics, which is something that most people (including physicists!) don’t appreciate enough. Whenever a claim of new physics is made we need to look at the wider implications of the idea. For example, let’s say that we see the decay of a $$\tau$$ lepton to an proton and a $$\pi^0$$ meson. The Feynman diagram would look something like this:

tau lepton decay to a proton and a neutral pion, mediated by a leptoquark

The “X” particle is a leptoquark, and it turns leptons into quarks and vice versa. Now for this decay to happen at an observable rate we need something like this leptoquark to exist. There is no Standard Model process for $$\tau\to p\pi^0$$ since it violates baryon number (a process which is only allowed under very special conditions). So suppose someone claims to see this decay, does this mean that they’ve discovered new physics? The answer is a resounding “No”, because if they make a claim of new physics they need to look elsewhere for similar effects. For example, if the leptoquark existed the proton could decay with this process:

proton decay to an electron and neutral pion, mediated by a leptoquark

We have very stringent tests on the lifetime of the proton, and the lower limits are currently about 20 orders of magnitude longer than the age the universe. Just take a second to appreciate the size of that limit on the lifetime. The proton lasts for at least 20 orders of magnitude longer than the age of the universe itself. So if someone is going to claim that they have proven the leptoquark exists we need to check that what they have seen is consistent with the proton lifetime measurements. A claim of new physics is stronger than a claim of a new phenomena, because it must be consistent with all the current data, not just the part we’re working.

How does all this relate to CIPANP 2012 and the low energy experiments? Well it turns out that there are a handful of large disagreements in this regime that all tend to involve the same particles. The $$B$$ meson can decay to several lighter particles and the BaBar experiment has seen the decays to the $$\tau$$ lepton are higher than they should be. The disagreement is more than $$3\sigma$$ disagreement with the Standard Model predictions for $$B\to D^{(*)}\tau\nu$$, which is interesting because it involves the heaviest quarks in bound states, and the heaviest lepton. It suggests that if there is a new particle or process, that it favors coupling to heavy particles.

Standard model decays of the B mesons to τν, Dτν, and D*τν final states

In another area of $$B$$ physics we find that the branching fraction $$\mathcal{B}(B\to\tau\nu)$$ is about twice as large as we expect from the Standard Model. You can see the disagreement in the following plot, which compares two measurements ($$\mathcal{B}(B\to\tau\nu)$$ and $$\sin 2\beta$$) to what we expect given everything else. The distance between the data point and the most favored region (center of the colored region) is very large, about $$3\sigma$$ in total!

The disagreement between B→τν, sin2β and the rest of the unitary triangle measurements (CKMFitter)

Theorists love to combine these measurements using colorful diagrams, and the best known example is the unitary triangle. If the CKM mechanism describes all the quark mixing processes then all of the measurements should agree, and they should converge on a single apex of the triangle (at the angle labeled $$\alpha$$). Each colored band corresponds to a different kind of process, and if you look closely you can see some small disagreements between the various measurements:

The unitary triangle after Moriond 2012 (CKMFitter)

The blue $$\sin 2\beta$$ measurement is pulling the apex down slightly, and green $$|V_{ub}|$$ measurement is pulling it in the other direction. This tension shows some interesting properties when we try to investigate it further. If we remove the $$\sin 2\beta$$ measurement and then work out what we expect based on the other measurements, we find that the new “derived” value of $$\sin 2\beta$$ is far off what is actually measured. The channel used for analysis of $$\sin 2\beta$$ is often called the golden channel, and it has been the main focus of both BaBar and Belle experiments since their creation. The results for $$\sin2\beta$$ are some of the best in the world and they have been checked and rechecked, so maybe the problem is not associated with $$\sin 2\beta$$.

Moving our attention to $$|V_{ub}|$$ the theorists at CKMFitter decided to split up the contributions based on the semileptonic inclusive and exclusive decays, and from $$\mathcal{B}(B\to\tau\nu)$$. When this happens we find that the biggest disagreement comes from $$\mathcal{B}(B\to\tau\nu)$$ compared to the rest. The uncertainties get smaller when $$\mathcal{B}(B\to\tau\nu)$$ is combined with the $$B$$ mixing parameter, $$\Delta m_d$$, which is well understood in terms of top quark interactions, but these results still disagree with everything else!:

Disagreement between B→τν, Δmd and the rest of the unitary triangle measurments (CKMFitter)

What this is seeming to tell us is that there could be a new process that affects $$B$$ meson interactions, enhancing decays with $$\tau$$ leptons in the final state. If this is the case then we need to look at other processes that could be affected by these kinds of processes. The most obvious signal to look for at the LHC is something like production of $$b$$ quarks and $$\tau$$ leptons. Third generation leptoquarks would be a good candidate, as long as they cannot mediate proton decay in any way. Searching for a new particle of a new effect is the job of the experimentalist, but creating a model that accommodates the discoveries we make is the job of a theorist.

That, in a nutshell is the difference between discovering a new phenomenon and discovering new physics. Anyone can find a bump in a spectrum, or even discover a new particle, but forming a consistent model of new physics takes a long time and a lot of input from all different kinds of experiments. The latest news from BaBar, Belle, CLEO and LHCb are giving us hints that there is something new lurking in the data. I can’t wait to see wait to see what our theorist colleagues do with these measurements. If they can create a model which explains anomalously high branching fractions $$\mathcal{B}(B\to\tau\nu)$$, $$\mathcal{B}(B\to D\tau\nu)$$, and $$\mathcal{B}(B\to D^*\tau\nu)$$, which tells us where else to look then we’re in for an exciting year at LHC. We could see something more exciting than the Higgs in our data!

(CKMFitter images kindly provided by the CKMfitter Group (J. Charles et al.), Eur. Phys. J. C41, 1-131 (2005) [hep-ph/0406184], updated results and plots available at: http://ckmfitter.in2p3.fr)