## Posts Tagged ‘LHC’

### Finding a five-leafed clover

Wednesday, July 15th, 2015

Photo Credit: Cathy Händel, Published on http://www.suttonelms.org.uk/olla12.html

Sometimes when you’re looking for something else, you happen across an even more exciting result. That’s what’s happened at LHCb, illustrated in the paper “Observation of $$J/\psi p$$ resonances consistent with pentaquark states in $$\Lambda_b^0\to J/\psi K^-p$$ decays”, released on the arXiv on the 14th of July.

I say this is lucky because the analysts found these states while they were busy looking at another channel; they were measuring the branching fraction of $$B^0\to J/\psi K^+ K^-$$. As one of the analysts, Sheldon Stone, recalled to me, during the review of the $$B^0$$ analysis, one reviewer asked if there could be a background from the decay $$\Lambda_b^0\to J/\psi K^- p$$, where the proton was misidentified as a kaon. As this was a viable option, they looked at the PDG to see if the mode had been measured, and found that it had not. Without a certain knowledge of how large this contribution would be, the analysts looked. To their surprise, they found a rather large rate of the decay, allowing for a measurement of the lifetime of the $$\Lambda_b^0$$. At the same time, they noticed a peak in the $$J/\psi p$$ spectrum. After completing the above mentioned analysis of the $$B^0$$, they returned to the channel.

It’s nice to put yourself in the analysts shoes and see the result for yourself. Let’s start by looking at the decay $$\Lambda_b^0\to J/\psi p K^-$$. As this is a three body decay, we can look at the Dalitz Plots.

Dalitz plots from the decay $$\Lambda_b^0\to J/\psi K^- p$$. Compiled from http://arxiv.org/abs/1507.03414

The above Dalitz plots show all combinations of possible axes to test. In the one on the left, around $$m^2=2.3$$ GeV$$^2$$, running vertically, we see the $$\Lambda(1520)$$ resonance, which decays into a proton and a kaon. Running horizontally is a band which does not seem to correspond to a known resonance, but which would decay into a $$J/\psi$$ and a proton. If this is a strong decay, then the only option is to have a hadron whose minimum quark content is $$uud\bar{c}c$$. The same band is seen on the middle plot as a vertical band, and on the far right as the sloping diagonal band. To know for sure, one must perform a complete amplitude analysis of the system.

You might be saying to yourself “Who ordered that?” and think that something with five quarks hadn’t been postulated. This is not the case. Hadrons with quark content beyond the minimum were already thought about by Gell-Mann and Zweig in 1964 and quantitatively modeled by Jaffe in 1977  to 4 quarks and 5 quarks by Strottman in 1979. I urge you to go look at the articles if you haven’t before.

It appears as though a resonance has been found, and in order to be sure, a full amplitude analysis of the decay was performed. The distribution is first modeled without any such state, shown in the figures below.

Projections of the fits of the$$\Lambda_b^0\to J/\psi K^- p$$ spectrum without any additional components. Black is the data, and red is the fit. From http://arxiv.org/abs/1507.03414

Try as you might, the models are unable to explain the invariant mass distribution of the $$J/\psi p$$. Without going into too much jargon, they wrote down from a theoretical standpoint what type of effect a five quark particle would have on the Dalitz plot, then put this into their model. As it turns out, they were unable to successfully model the distribution without the addition of two such pentaquark states. By adding these states, the fits look much better, as shown below.

Mass projection onto the $$J/\psi p$$ axis of the total fit to the Dalitz plot. Again, Black is data, red is the fit. The inset image is for the kinematic range $$m(K p)>2 GeV$$.
From http://arxiv.org/abs/1507.03414

The states  are called the $$P_c$$ states. Now, as this is a full amplitude analysis, the fit also covers all angular information. This allows for determination of the total angular momentum and parity of the states. These are defined by the quantity $$J^P$$, with $$J$$ being the total angular momentum and $$P$$ being the parity. All values for both resonances are tried from 1/2 to 7/2, and the best fit values are found to be with one resonance having $$J=3/2$$ and the other with $$J=5/2$$, with each having the opposite parity as the other. No concrete distinction can be made between which state has which value.

Finally, the significance of the signal is described by under the assumption $$J^P=3/2^-,5/2^+$$ for the lower and higher mass states; the significances are 9 and 12 standard deviations, respectively.

The masses and widths turn out to be

$$m(P_c^+(4380))=4380\pm 8\pm 29 MeV$$

$$m(P_c^+(4450))=4449.8\pm 1.7\pm 2.5 MeV$$

With corresponding widths

Width$$(P_c^+(4380))=205\pm 18\pm 86 MeV$$

Width$$(P_c^+(4450))=39\pm 5\pm 19 MeV$$

Finally, we’ll look at the Argand Diagrams for the two resonances.

Argand diagrams for the two $$P_c$$ states.
From http://arxiv.org/abs/1507.03414

Now you may be saying “hold your horses, that Argand diagram on the right doesn’t look so great”, and you’re right. I’m not going to defend the plot, but only point out that the phase motion is in the correct direction, indicated by the arrows.

As pointed out on the LHCb public page, one of the next steps will be to try to understand whether the states shown are tightly bound 5 quark objects or rather loosely bound meson baryon molecule. Even before that, though, we’ll see if any of the other experiments have something to say about these states.

### LARP completes first successful test of High-Luminosity LHC coil

Monday, June 22nd, 2015

Steve Gould of the Fermilab Technical Division prepares a cold test of a short quadrupole coil. The coil is of the type that would go into the High-Luminosity LHC. Photo: Reidar Hahn

Last month, a group collaborating across four national laboratories completed the first successful tests of a superconducting coil in preparation for the future high-luminosity upgrade of the Large Hadron Collider, or HL-LHC. These tests indicate that the magnet design may be adequate for its intended use.

Physicists, engineers and technicians of the U.S. LHC Accelerator Research Program (LARP) are working to produce the powerful magnets that will become part of the HL-LHC, scheduled to start up around 2025. The plan for this upgrade is to increase the particle collision rate, or luminosity, by approximately a factor of 10, so expanding the collider’s physics reach by creating 10 times more data.

“The upgrade will help us get closer to new physics. If we see something with the current run, we’ll need more data to get a clear picture. If we don’t find anything, more data may help us to see something new,” said Technical Division’s Giorgio Ambrosio, leader of the LARP magnet effort.

LARP is developing more advanced quadrupole magnets, which are used to focus particle beams. These magnets will have larger beam apertures and the ability to produce higher magnetic fields than those at the current LHC.

The Department of Energy established LARP in 2003 to contribute to LHC commissioning and prepare for upgrades. LARP includes Brookhaven National Laboratory, Fermilab, Lawrence Berkeley National Laboratory and SLAC. Its members began developing the technology for advanced large-aperture quadrupole magnets around 2004.

The superconducting magnets currently in use at the LHC are made from niobium titanium, which has proven to be a very effective material to date. However, they will not be able to support the higher magnetic fields and larger apertures the collider needs to achieve higher luminosities. To push these limits, LARP scientists and engineers turned to a different material, niobium tin.

Niobium tin was discovered before niobium titanium. However, it has not yet been used in accelerators because, unlike niobium titanium, niobium tin is very brittle, making it susceptible to mechanical damage. To be used in high-energy accelerators, these magnets need to withstand large amounts of force, making them difficult to engineer.

LARP worked on this challenge for almost 10 years and went through a number of model magnets before it successfully started the fabrication of coils for 150-millimeter-aperture quadrupoles. Four coils are required for each quadrupole.

LARP and CERN collaborated closely on the design of the coils. After the first coil was built in the United States earlier this year, the LARP team successfully tested it in a magnetic mirror structure. The mirror structure makes possible tests of individual coils under magnetic field conditions similar to those of a quadrupole magnet. At 1.9 Kelvin, the coil exceeded 19 kiloamps, 15 percent above the operating current.

The team also demonstrated that the coil was protected from the stresses and heat generated during a quench, the rapid transition from superconducting to normal state.

“The fact that the very first test of the magnet was successful was based on the experience of many years,” said TD’s Guram Chlachidze, test coordinator for the magnets. “This knowledge and experience is well recognized by the magnet world.”

Over the next few months, LARP members plan to test the completed quadrupole magnet.

“This was a success for both the people building the magnets and the people testing the magnets,” said Fermilab scientist Giorgio Apollinari, head of LARP. “We still have a mountain to climb, but now we know we have all the right equipment at our disposal and that the first step was in the right direction.”

Diana Kwon

### Starting up LHC Run 2, step by step

Thursday, June 11th, 2015

I know what you are thinking. The LHC is back in action, at the highest energies ever! Where are the results? Where are all the blog posts?

Back in action, yes, but restarting the LHC is a very measured process. For one thing, when running at the highest beam energies ever achieved, we have to be very careful about how we operate the machine, lest we inadvertently damage it with beams that are mis-steered for whatever reason. The intensity of the beams — how many particles are circulating — is being incrementally increased with successive fills of the machine. Remember that the beam is bunched — the proton beams aren’t continuous streams of protons, but collections that are just a few centimeters long, spaced out by at least 750 centimeters. The LHC started last week with only three proton bunches in each beam, only two of which were actually colliding at an interaction point. Since then, the LHC team has gone to 13 bunches per beam, and then 39 bunches per beam. Full-on operations will be more like 1380 bunches per beam. So at the moment, the beams are of very low intensity, meaning that there are not that many collisions happening, and not that much physics to do.

What’s more, the experiments have much to do also to prepare for the higher collision rates. In particular, there is the matter of “timing in” all the detectors. Information coming from each individual component of a large experiment such as CMS takes some time to reach the data acquisition system, and it’s important to understand how long that time is, and to get all of the components synchronized. If you don’t have this right, then you might not be getting the optimal information out of each component, or worse still, you could end up mixing up information from different bunch crossings, which would be disastrous. This, along with other calibration work, is an important focus during this period of low-intensity beams.

But even if all these things were working right out of the box, we’d still have a long way to go until we had some scientific results. As noted already, the beam intensities have been low, so there aren’t that many collisions to examine. There is much work to do yet in understanding the basics in a revised detector operating at a higher beam energy, such as how to identify electrons and muons once again. And even once that’s done, it will take a while to make measurements and fully vet them before they could be made public in any way.

So, be patient, everyone! The accelerator scientists and the experimenters are hard at work to bring you a great LHC run! Next week, the LHC takes a break for maintenance work, and that will be followed by a “scrubbing run”, the goal of which is to improve the vacuum in the LHC beam pipe. That will allow higher-intensity beams, and position us to take data that will get the science moving once again.

### 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.

### New magnet at Fermilab achieves high-field milestone

Friday, April 3rd, 2015

This magnet recently achieved an important milestone, reaching its design field of 11.5 Tesla. It is the first successful niobium-3-tin, twin-aperture accelerator magnet in the world. Photo: Sean Johnson

Last month, a new superconducting magnet developed and fabricated at Fermilab reached its design field of 11.5 Tesla at a temperature nearly as cold as outer space. It is the first successful twin-aperture accelerator magnet made of niobium-3-tin in the world.

The advancements in niobium-3-tin, or Nb3Sn, magnet technology and the ongoing U.S. collaboration with CERN on the development of these and other Nb3Sn magnets are enabling the use of this innovative technology for future upgrades of the Large Hadron Collider (LHC). They may also provide the cornerstone for future circular machines of interest to the worldwide high-energy physics community. Because of the exceptional challenges — Nb3Sn is brittle and requires high-temperature processing — this important milestone was achieved at Fermilab after decades of worldwide R&D efforts both in the Nb3Sn conductor itself and in associated magnet technologies.

Superconducting magnets are at the heart of most particle accelerators for fundamental science as well as other scientific and technological applications. Superconductivity is also being explored for use in biosensors and quantum computing.

Thanks to Nb3Sn’s stronger superconducting properties, it enables magnets of larger field than any in current particle accelerators. As a comparison, the niobium-titanium dipole magnets built in the early 1980s for the Tevatron particle collider produced about 4 Tesla to bend the proton and antiproton beams around the ring. The most powerful niobium-titanium magnets used in the LHC operate at roughly 8 Tesla. The new niobium-3-tin magnet creates a significantly stronger field.

Because the Tevatron accelerated positively charged protons and negatively charged antiprotons, its magnets had only one aperture. By contrast, the LHC uses two proton beams. This requires two-aperture magnets with fields in opposite directions. And because the LHC collides beams at higher energies, it requires larger magnetic fields.

In the process of upgrading the LHC and in conceiving future particle accelerators and detectors, the high-energy physics community is investing as never before in high-field magnet technologies. This creative process involves the United States, Europe, Japan and other Asian countries. The latest strategic plan for U.S. high-energy physics, the 2014 report by the Particle Physics Project Prioritization Panel, endorses continued U.S. leadership in superconducting magnet technology for future particle physics programs. The U.S. LHC Accelerator Research Program (LARP), which comprises four DOE national laboratories — Berkeley Lab, Brookhaven Lab, Fermilab and SLAC — plays a key role in this strategy.

The 15-year investment in Nb3Sn technology places the Fermilab team led by scientist Alexander Zlobin at the forefront of this effort. The Fermilab High-Field Magnet Group, in collaboration with U.S. LARP and CERN, built the first reproducible series in the world of single-aperture 10- to 12-Tesla accelerator-quality dipoles and quadrupoles made of Nb3Sn, establishing a strong foundation for the LHC luminosity upgrade at CERN.

The laboratory has consistently carried out in parallel an assertive superconductor R&D program as key to the magnet success. Coordination with industry and universities has been critical to improve the performance of the next generation of high-field accelerator magnets.

The next step is to develop 15-Tesla Nb3Sn accelerator magnets for a future very high-energy proton-proton collider. The use of high-temperature superconductors is also becoming a realistic prospect for generating even larger magnetic fields. An ultimate goal is to develop magnet technologies based on combining high- and low-temperature superconductors for accelerator magnets above 20 Tesla.

The robust and versatile infrastructure that was developed at Fermilab, together with the expertise acquired by the magnet scientists and engineers in design and analysis tools for superconducting materials and magnets, makes Fermilab an ideal setting to look to the future of high-field magnet research.

Emanuela Barzi

### CERN Had Dark Energy All Along; Uses It To Fuel Researchers

Tuesday, March 31st, 2015

I don’t usually get to spill the beans on a big discovery like this, but this time, I DO!

# CERN Had Dark Energy All Along!!

That’s right. That mysterious energy making up ~68% of the universe was being used all along at CERN! Being based at CERN now, I’ve had a first hand glimpse into the dark underside of Dark Energy. It all starts at the Crafted Refilling of Empty Mugs Area (CREMA), pictured below.

One CREMA station at CERN

Researchers and personnel seem to stumble up to these stations at almost all hours of the day, looking very dreary and dazed. They place a single cup below the spouts, and out comes a dark and eerie looking substance, which is then consumed. Some add a bit of milk for flavor, but all seem perkier and refreshed after consumption. Then they disappear from whence they came. These CREMA stations seem to be everywhere, from control rooms to offices, and are often found with groups of people huddled around them. In fact, they seem to exert a force on all who use them, keeping them in stable orbits about the stations.

Q. How much of this dark stuff do you consume on a daily basis?

A. At least one cup in the morning to fuel up, I don’t think I could manage to get to lunchtime without that one. Then multiple other cups distributed over the day, depending on the workload. It always feels like they help my thinking.

Q. Do you know where it comes from?

A. We have a machine in our office which takes capsules. I’m not 100% sure where those capsules are coming from, but they seem to restock automatically, so no one ever asked.

Q. Have you been hiding this from the world on purpose?

A. Well our stock is important to our group, if we would just share it with everyone around we could run out. And no one of us can make it through the day without. We tried alternatives, but none are so effective.

Q. Do you remember the first time you tried it?

A. Yes, they hooked me on it in university. From then on nothing worked without!

Q. Where does CERN get so much of it?

A. I never thought about this question. I think I’m just happy that there is enough for everyone here, and physicist need quite a lot of it to work.

In order to gauge just how much of this Dark Energy is being consumed, I studied the flux of people from the cafeteria as a function of time with cups of Dark Energy. I’ve compiled the results into the Dark Energy Consumption As Flux (DECAF) plot below.

Dark Energy Consumption as Flux plot. Taken March 31, 2015. Time is given in 24h time. Errors are statistical.

As the DECAF plot shows, there is a large spike in consumption, particularly after lunch. There is a clear peak at times after 12:20 and before 13:10. Whether there is an even larger peak hiding above 13:10 is not known, as the study stopped due to my advisor asking “shouldn’t you be doing actual work?”

There is an irreducible background of Light Energy in the cups used for Dark Energy, particularly of the herbal variety. Fortunately, there is often a dangly tag hanging off of the cup  to indicate to others that they are not using the precious Dark Energy supply, and provide a clear signal for this study to eliminate the background.

While illuminating, this study still does not uncover the exact nature of Dark Energy, though it is clear that it is fueling research here and beyond.

### Excited Quarks and Early Discoveries at LHC Run II

Wednesday, February 4th, 2015

The LHC turns back on this year for Run II. What might we see day 1?

The highest-p_T jet event collected by the end of September 2012 (Event 37979867, Run 208781): the two central high-p_T jets have an invariant mass of 4.47 TeV; the highest-p_T jet has a p_T of 2.34 TeV, and the subleading jet has a p_T of 2.10 TeV. The missing E_T and Sum E_T for this event are respectively 115 GeV and 4.97 TeV. Only tracks with p_T> 0.7 GeV are displayed. The event was collected on August 17th, 2012. Image and caption credit: ATLAS

In seven weeks CERN’s Large Hadron Collider (LHC), the largest and most energetic particle accelerator in history, is scheduled to turn back on. The LHC has been shutdown since December 2012 in order for experimentalists to repair and upgrade the different detector experiments as well as the collider itself. When recommissioning starts, the proton beams will be over 60% more energetic than before and probe a regime of physics we have yet to explore directly. With this in mind, today’s post is about a type of new physics that, if it exists, we can potentially see in the first days of LHC Run II: excited quarks.

# Excited Quarks and Composite Quarks

Excited quarks are interesting little beasts and are analogous to excited atoms in atomic physics. When light (a photon) is shined onto an atom, electrons orbiting the nucleus will become energized and are pushed into higher, metastable orbits. This is called an excited atom.

After some estimable and often measurable period of time, an electron will radiate light (photon) and drop down to its original orbit. When this happens, we say that an excited atom has relaxed to its ground state.

In analogy, if quarks were bound states of something smaller, i.e., if they were composite particles, then we can pump energy into a quark, excite it, and then watch the excited quark relax back into its ground state.

Feynman diagram representing heavy excited quark (q*) production from quark (q)-gluon (g) scattering in proton collisions.

Observing an excited quark would tell us that the quark model may not be the whole story after all. Presently, the quark model is the best description of protons and neutrons, and it certainly works very, very well, but this does not have to be the case. Nature may have something special in store for us. However, this is not why I think excited quarks are so odd and interesting. What is not obvious is that excited quarks, if they exist, could show up immediately after turning the LHC back on.

# Early Dijet Discoveries at LHC Run II

Excited quarks participate in the strong nuclear force (QCD) just like ordinary quarks, which means they can absorb and radiate gluons with equal strength. This is key because protons at the LHC are just brimming with highly energetic quarks and gluons. Of particles in a proton carrying a small-to-medium fraction of the proton’s total energy, gluons are the most commonly found particle in a proton (red g curve below). Of those particles carrying a large fraction of the proton’s energy, the up and down quarks are the most common particles (blue u and green d curves below). Excited quarks, if they exist, are readily produced because their ingredients are the most commonly found particles in the proton.

Distributions of partons in a proton. The x-axis represents the fraction of the proton’s energy a parton has (x=1 means that the parton has 100% of the proton’s energy). The y-axis represents the likelihood of observing a parton. The left (right) plot corresponds to low (high) energy collisions. Credit: MSTW

When an excited quark decays, it will split back into quark and gluon pair. These two particles will be very energetic (each will have energy equal to half the mass of the excited quark due to energy conservation), will be back-to-back (by linear momentum conservation), and will each form jets (hadronization in QCD). Such collisions are called “dijet” events (pronounced: die-jet) and look like this

Display for the event with the highest dijet mass (5.15 TeV) observed in CMS data. Image and caption credit: CMS

Although gluons and quarks in the Standard Model can mimic the signal, one can add up the energies of the two jets (which would equal the excited quark’s mass due to energy conservation) and expect to see a bump in the data centered about the mass of the excited quark. Unfortunately, the data (below) do not show such a bump, indicating that excited quarks with masses below a couple TeV do not exist.

Inclusive dijet mass spectrum from wide jets (points) compared to a fit (solid curve) and to predictions including detector simulation of multijet events and signal resonances. The predicted multijet shape (QCD MC) has been scaled to the data. The vertical error bars are statistical only and the horizontal error bars are the bin widths. For comparison, the signal distributions for a W resonance of mass 1.9 CMS.TeV and an excited quark of mass 3.6 CMS.TeV are shown. The bin-by-bin fit residuals scaled to the statistical uncertainty of the data are shown at the bottom and compared with the expected signal contributions. Image and caption credit: CMS

However, this does not mean that excited quarks do not or cannot exist at higher masses. If they do, and if their masses are within the energy reach of the LHC, then excited quarks are very much something we might see in just a few months from now.

Happy Colliding,

Richard Ruiz (@BraveLittleMuon)

Appreciation to Ms. Frost and her awesome physics classes at Whitney M. Young High School in Chicago, Illinois for motivating this post. Good luck on your AP exams!

### Lepton Number Violation, Doubly Charged Higgs Bosons, and Vector Boson Fusion at the LHC

Wednesday, January 21st, 2015

Doubly charged Higgs bosons and lepton number violation are wickedly cool.

Hi Folks,

The Standard Model (SM) of particle physics is presently the best description of matter and its interactions at small distances and high energies. It is constructed based on observed conservation laws of nature. However, not all conservation laws found in the SM are intentional, for example lepton number conservation. New physics models, such as those that introduce singly and doubly charged Higgs bosons, are flexible enough to reproduce previously observed data but can either conserve or violate these accidental conservation laws. Therefore, some of the best ways of testing if these types of laws are much more fundamental may be with the help of new physics.

## Observed Conservation Laws of Nature and the Standard Model

Conservation laws, like the conservation of energy or the conservation of linear momentum, have the most remarkable impact on life and the universe. Conservation of energy, for example, tells us that cars need fuel to operate and perpetual motion machines can never exist. A football sailing across a pitch does not suddenly jerk to the left at 90º because conversation of linear momentum, unless acted upon by a player (a force). This is Newton’s First Law of Motion. In particle physics, conservation laws are not taken lightly; they dictate how particles are allowed to behave and forbid some processes from occurring. To see this in action, lets consider a top quark (t) decaying into a W boson and a bottom quark (b).

asdasd

A top quark cannot radiate a W+ boson and remain a top quark because of conservation of electric charge. Top quarks have an electric charge of +2/3 e, whereas W+ bosons have an electric charge of +1e, and we know quite well that

(+2/3)e ≠ (+1)e + (+2/3)e.

For reference a proton has an electric charge of +1e and an electron has an electric charge of -1e. However, a top quark can radiate a W+ boson and become a bottom quark, which has electric charge of -1/3e. Since

(+2/3)e = (+1)e + (-1/3)e,

we see that electric charge is conserved.

Conservation of energy, angular momentum, electric charged, etc., are so well-established that the SM is constructed to automatically obey these laws. If we pick any mathematical term in the SM that describes how two or more particles interact (for example how the top quark, bottom quark, and W boson interact with each other) and then add up the electric charge of all the participating particles, we will find that the total electric charge is zero:

The top quark-bottom quark-W boson interaction terms in the Standard Model. Bars above quarks indicate that the quark is an antiparticle and has opposite charges.

## Accidental Conservation Laws

However, not all conservation laws that appear in the SM are intentional. Conservation of lepton number is an example of this. A lepton is any SM fermion that does not interact with the strong nuclear force. There are six leptons in total: the electron, muon, tau, electron-neutrino, muon-neutrino, and tau-neutrino. We assign lepton number

L=1 to all leptons (electron, muon, tau, and all three neutrinos),

L=-1 to all antileptons (positron, antimuon, antitau, and all three antineutrinos),

L=0 to all other particles.

With these quantum number assignments, we see that lepton number is a conserved in the SM. To clarify this important point: we get lepton number conservation for free due to our very rigid requirements when constructing the SM, namely the correct conservation laws (e.g., electric and color charge) and particle content. Since lepton number conservation was not intentional, we say that lepton number is accidentally conserved. Just as we counted the electric charge for the top-bottom-W interaction, we can count the net lepton number for the electron-neutrino-W interaction in the SM and see that lepton number really is zero:

The W boson-neutrino-electron interaction terms in the Standard Model. Bars above leptons indicate that the lepton is an antiparticle and has opposite charges.

However, lepton number conservation is not required to explain data. At no point in constructing the SM did we require that it be conserved. Because of this, many physicists question whether lepton number is actually conserved. It may be, but we do not know. This is indeed one topic that is actively researched. An interesting example of a scenario in which lepton number conservation could be tested is the class of theories with singly and doubly charged Higgs boson. That is right, there are theories containing additional Higgs bosons that an electric charged equal or double the electric charge of the proton.

Models with scalar SU(2) triplets contain additional neutral Higgs bosons as well as singly and doubly charged Higgs bosons.

Doubly charged Higgs bosons have an electric charge that is twice as large as a proton (2e), which leads to rather peculiar properties. As discussed above, every interaction between two or more particles must respect the SM conservation laws, such as conservation of electric charge. Because of this, a doubly charged Higgs (+2e) cannot decay into a top quark (+2/3 e) and an antibottom quark (+1/3 e),

(+2)e ≠ (+2/3)e + (+1/3)e.

However, a doubly charged Higgs (+2e) can decay into two W bosons (+1e) or two antileptons (+1e) with the same electric charge,

(+2)e = (+1)e + (+1)e.

but that is it. A doubly charged Higgs boson cannot decay into any other pair of SM particles because it would violate electric charge conservation. For these two types of interactions, we can also check whether or not lepton number is conserved:

For the decay into same-sign W boson pairs, the total lepton number is 0L + 0L + 0L = 0L. In this case, lepton number is conserved!

For the decay into same-sign leptons pairs, the total lepton number is 0L + (-1)L + (-1)L = -2L. In this case, lepton number is violated!

Doubly charged Higgs boson interactions for same-sign W boson pairs and same-sign electron pairs. Bars indicate antiparticles. C’s indicate charge flipping.

Therefore if we observe a doubly charged Higgs decaying into a pair of same-sign leptons, then we have evidence that lepton number is violated. If we only observe doubly charged Higgs decaying into same-sign W bosons, then one may speculate that lepton number is conserved in the SM.

## Doubly Charged Higgs Factories

Doubly charged Higgs bosons do not interact with quarks (otherwise it would violate electric charge conservation), so we have to rely on vector boson fusion (VBF) to produce them. VBF is when two bosons from on-coming quarks are radiated and then scatter off each other, as seen in the diagram below.

Diagram depicting the process known as WW Scattering, where two quarks from two protons each radiate a W boson that then elastically interact with one another.

If two down quarks, one from each oncoming proton, radiate a W- boson (-1e) and become up quarks, the two W- bosons can fuse into a negatively, doubly charged Higgs (-2e). If lepton number is violated, the Higgs boson can decay into a pair of same-sign electrons (2x -1e). Counting lepton number at the beginning of the process (L = 0 – 0 = 0) and at the end (L = 0 – 2 = -2!), we see that it changes by two units!

Same-sign W- pairs fusing into a doubly charged Higgs boson that decays into same-sign electrons.

If lepton number is not violated, we will never see this decay and only see decays to two very, very energetic W- boson (-1e). Searching for vector boson fusion as well as lepton number violation are important components of the overarching Large Hadron Collider (LHC) research program at CERN. Unfortunately, there is no evidence for the existence of doubly charged scalars. On the other hand, we do have evidence for vector boson scattering (VBS) of the same-sign W bosons! Additional plots can be found on ATLAS’ website.  Reaching this tremendous milestone is a triumph for the LHC experiments. Vector boson fusion is a very, very, very, very, very rare process in the Standard Model and difficult to separate from other SM processes. Finding evidence for it is a first step in using the VBF process as a probe of new physics.

Same-sign W boson scattering candidate event at the LHC ATLAS experiment. Slide credit: Junjie Zhu (Michigan)

We have observed that some quantities, like momentum and electric charge, are conserved in nature. Conservation laws are few and far between, but are powerful. The modern framework of particle physics has these laws built into them, but has also been found to accidentally conserve other quantities, like lepton number. However, as lepton number is not required to reproduce data, it may be the case that these accidental laws are not, in fact, conserved. Theories that introduce charged Higgs bosons can reproduce data but also predict new interactions, such as doubly charged Higgs bosons decaying to same-sign W boson pairs and, if lepton number is violated, to same-sign charged lepton pairs. These new, exotic particles can be produced through vector boson fusion of two same-sign W boson pairs. VBF is a rare process in the SM and can greatly increase if new particles exist. At last, there is evidence for vector boson scattering of same-sign W bosons, and may be the next step to discovering new particles and new laws of nature!

Happy Colliding

– Richard (@BraveLittleMuon)

### The Ties That Bind

Sunday, January 18th, 2015

Beneath the ATLAS detector – note the well-placed cable ties. IMAGE: Claudia Marcelloni, ATLAS Experiment © 2014 CERN.

A few weeks ago, I found myself in one of the most beautiful places on earth: wedged between a metallic cable tray and a row of dusty cooling pipes at the bottom of Sector 13 of the ATLAS Detector at CERN. My wrists were scratched from hard plastic cable ties, I had an industrial vacuum strapped to my back, and my only light came from a battery powered LED fastened to the front of my helmet. It was beautiful.

The ATLAS Detector is one of the largest, most complex scientific instruments ever constructed. It is 46 meters long, 26 meters high, and sits 80 metres underground, completely surrounding one of four points on the Large Hadron Collider (LHC), where proton beams are brought together to collide at high energies.  It is designed to capture remnants of the collisions, which appear in the form of particle tracks and energy deposits in its active components. Information from these remnants allows us to reconstruct properties of the collisions and, in doing so, to improve our understanding of the basic building blocks and forces of nature.

On that particular day, a few dozen of my colleagues and I were weaving our way through the detector, removing dirt and stray objects that had accumulated during the previous two years. The LHC had been shut down during that time, in order to upgrade the accelerator and prepare its detectors for proton collisions at higher energy. ATLAS is constructed around a set of very large, powerful magnets, designed to curve charged particles coming from the collisions, allowing us to precisely measure their momenta. Any metallic objects left in the detector risk turning into fast-moving projectiles when the magnets are powered up, so it was important for us to do a good job.

ATLAS is divided into 16 phi sectors with #13 at the bottom. IMAGE: Steven Goldfarb, ATLAS Experiment © 2014 CERN

The significance of the task, however, did not prevent my eyes from taking in the wonder of the beauty around me. ATLAS is shaped somewhat like a large barrel. For reference in construction, software, and physics analysis, we divide the angle around the beam axis, phi, into 16 sectors. Sector 13 is the lucky sector at the very bottom of the detector, which is where I found myself that morning. And I was right at ground zero, directly under the point of collision.

To get to that spot, I had to pass through a myriad of detector hardware, electronics, cables, and cooling pipes. One of the most striking aspects of the scenery is the ironic juxtaposition of construction-grade machinery, including built-in ladders and scaffolding, with delicate, highly sensitive detector components, some of which make positional measurements to micron (thousandth of a millimetre) precision. All of this is held in place by kilometres of cable trays, fixings, and what appear to be millions of plastic (sometimes sharp) cable ties.

Scaffolding and ladder mounted inside the precision muon spectrometer. IMAGE: Steven Goldfarb, ATLAS Experiment © 2014 CERN.

The real beauty lies not in the parts themselves, but rather in the magnificent stories of international cooperation and collaboration that they tell. The cable tie that scratched my wrist secures a cable that was installed by an Iranian student from a Canadian university. Its purpose is to carry data from electronics designed in Germany, attached to a detector built in the USA and installed by a Russian technician.  On the other end, a Japanese readout system brings the data to a trigger designed in Australia, following the plans of a Moroccan scientist. The filtered data is processed by software written in Sweden following the plans of a French physicist at a Dutch laboratory, and then distributed by grid middleware designed by a Brazilian student at CERN. This allows the data to be analyzed by a Chinese physicist in Argentina working in a group chaired by an Israeli researcher and overseen by a British coordinator.  And what about the cable tie?  No idea, but that doesn’t take away from its beauty.

There are 178 institutions from 38 different countries participating in the ATLAS Experiment, which is only the beginning.  When one considers the international make-up of each of the institutions, it would be safe to claim that well over 100 countries from all corners of the globe are represented in the collaboration.  While this rich diversity is a wonderful story, the real beauty lies in the commonality.

All of the scientists, with their diverse social, cultural and linguistic backgrounds, share a common goal: a commitment to the success of the experiment. The plastic cable tie might scratch, but it is tight and well placed; its cable is held correctly and the data are delivered, as expected. This enormous, complex enterprise works because the researchers who built it are driven by the essential nature of the mission: to improve our understanding of the world we live in. We share a common dedication to the future, we know it depends on research like this, and we are thrilled to be a part of it.

ATLAS Collaboration members in discussion. What discoveries are in store this year? IMAGE: Claudia Marcelloni, ATLAS Experiment © 2008 CERN.

This spring, the LHC will restart at an energy level higher than any accelerator has ever achieved before. This will allow the researchers from ATLAS, as well as the thousands of other physicists from partner experiments sharing the accelerator, to explore the fundamental components of our universe in more detail than ever before. These scientists share a common dream of discovery that will manifest itself in the excitement of the coming months. Whether or not that discovery comes this year or some time in the future, Sector 13 of the ATLAS detector reflects all the beauty of that dream.

### 2015: The LHC returns

Saturday, January 10th, 2015

I’m really not one for New Year’s resolutions, but one that I ought to make is to do more writing for the US LHC blog.  Fortunately, this is the right year to be making that resolution, as we will have quite a lot to say in 2015 — the year that the Large Hadron Collider returns!  After two years of maintenance and improvements, everyone is raring to go for the restart of the machine this coming March.  There is still a lot to do to get ready.  But once we get going, we will be entering a dramatic period for particle physics — one that could make the discovery of the Higgs seem humdrum.

The most important physics consideration for the new run is the increase of the proton collision energy from 8 TeV to 13 TeV.  Remember that the original design energy of the LHC is 14 TeV — 8 TeV was just an opening step.  As we near the 14 TeV point, we will be able to do the physics that the LHC was meant to do all along.  And it is important to remember that we have no feasible plan to build an accelerator that can reach a higher energy on any near time horizon.  While we will continue to learn more as we record more and more data, through pursuits like precision measurements of the properties of the Higgs boson, it is increases in energy that open the door to the discovery of heavy particles, and there is no major energy increase coming any time soon.  If there is any major breakthrough to be made in the next decade, it will probably come within the first few years of it, as we get our first look at 13 TeV proton collisions.

How much is our reach for new physics extended with the increase in energy?  One interesting way to look at it is through a tool called Collider Reach that was devised by theorists Gavin Salam and Andreas Weiler.  (My apologies to them if I make any errors in my description of their work.)  This tool makes a rough estimate of the mass scale of new physics that we could have access to at a new LHC energy given previous studies at an old LHC energy, based on our understanding of how the momentum distributions of the quarks and gluons inside the proton evolve to the new beam energy.  There are many assumptions made for this estimate — in particular, that the old data analysis will work just as well under new conditions.  This might not be the case, as the LHC will be running not just at a higher energy, but also a higher collision rate (luminosity), which will make the collisions more complicated and harder to interpret.  But the tool at least gives us an estimate of the improved reach for new physics.

During the 2008 LHC run at 8 TeV, each experiment collected about 20 fb-1 of proton collision data.  In the upcoming “Run 2″ of the LHC at 13 TeV, which starts this year and is expected to run through the middle of 2018, we expect to record about 100 fb-1 of data, a factor of five increase.  (This is still a fairly rough estimate of the future total dataset size.)  Imagine that in 2008, you were looking for a particular model of physics that predicted a new particle, and you found that if that particle actually existed, it would have to have a mass of at least 3 TeV — a mass 24 times that of the Higgs boson.  How far in mass reach could your same analysis go with the Run 2 data?  The Collider Reach tool tells you:

Using the horizontal axis to find the 3 TeV point, we then look at the height of the green curve to tell us what to expect in Run 2.  That’s a bit more than 5 TeV — a 70% increase in the mass scale that your data analysis would have sensitivity to.

But you are impatient — how well could we do in 2015, the first year of the run?  We hope to get about 10 fb-1 this year. Here’s the revised plot:

The reach of the analysis is about 4 TeV. That is, with only 10% of the data, you get 50% of the increase in sensitivity that you would hope to achieve in the entire run.  So this first year counts!  One year from now, we will know a lot about what physics we have an opportunity to look at in the next few years — and if nature is kind to us, it will be something new and unexpected.

So what might this new physics be?  What are the challenges that we face in getting there?  How are physicists preparing to meet them?  You’ll be hearing a lot more about this in the year to come — and if I can keep to my New Year’s resolution, some of it you’ll hear from me.