## USLHC | USA

### The Delirium over Beryllium

Flip Tanedo
Thursday, August 25th, 2016

This post is cross-posted from ParticleBites.

Article: Particle Physics Models for the 17 MeV Anomaly in Beryllium Nuclear Decays
Authors: J.L. Feng, B. Fornal, I. Galon, S. Gardner, J. Smolinsky, T. M. P. Tait, F. Tanedo
Reference: arXiv:1608.03591 (Submitted to Phys. Rev. D)
Also featuring the results from:
— Gulyás et al., “A pair spectrometer for measuring multipolarities of energetic nuclear transitions” (description of detector; 1504.00489NIM)
— Krasznahorkay et al., “Observation of Anomalous Internal Pair Creation in 8Be: A Possible Indication of a Light, Neutral Boson”  (experimental result; 1504.01527PRL version; note PRL version differs from arXiv)
— Feng et al., “Protophobic Fifth-Force Interpretation of the Observed Anomaly in 8Be Nuclear Transitions” (phenomenology; 1604.07411; PRL)

Editor’s note: the author is a co-author of the paper being highlighted.

Recently there’s some press (see links below) regarding early hints of a new particle observed in a nuclear physics experiment. In this bite, we’ll summarize the result that has raised the eyebrows of some physicists, and the hackles of others.

## A crash course on nuclear physics

Nuclei are bound states of protons and neutrons. They can have excited states analogous to the excited states of at lowoms, which are bound states of nuclei and electrons. The particular nucleus of interest is beryllium-8, which has four neutrons and four protons, which you may know from the triple alpha process. There are three nuclear states to be aware of: the ground state, the 18.15 MeV excited state, and the 17.64 MeV excited state.

Beryllium-8 excited nuclear states. The 18.15 MeV state (red) exhibits an anomaly. Both the 18.15 MeV and 17.64 states decay to the ground through a magnetic, p-wave transition. Image adapted from Savage et al. (1987).

Most of the time the excited states fall apart into a lithium-7 nucleus and a proton. But sometimes, these excited states decay into the beryllium-8 ground state by emitting a photon (γ-ray). Even more rarely, these states can decay to the ground state by emitting an electron–positron pair from a virtual photon: this is called internal pair creation and it is these events that exhibit an anomaly.

## The beryllium-8 anomaly

Physicists at the Atomki nuclear physics institute in Hungary were studying the nuclear decays of excited beryllium-8 nuclei. The team, led by Attila J. Krasznahorkay, produced beryllium excited states by bombarding a lithium-7 nucleus with protons.

Beryllium-8 excited states are prepare by bombarding lithium-7 with protons.

The proton beam is tuned to very specific energies so that one can ‘tickle’ specific beryllium excited states. When the protons have around 1.03 MeV of kinetic energy, they excite lithium into the 18.15 MeV beryllium state. This has two important features:

1. Picking the proton energy allows one to only produce a specific excited state so one doesn’t have to worry about contamination from decays of other excited states.
2. Because the 18.15 MeV beryllium nucleus is produced at resonance, one has a very high yield of these excited states. This is very good when looking for very rare decay processes like internal pair creation.

What one expects is that most of the electron–positron pairs have small opening angle with a smoothly decreasing number as with larger opening angles.

Expected distribution of opening angles for ordinary internal pair creation events. Each line corresponds to nuclear transition that is electric (E) or magenetic (M) with a given orbital quantum number, l. The beryllium transitionsthat we’re interested in are mostly M1. Adapted from Gulyás et al. (1504.00489).

Instead, the Atomki team found an excess of events with large electron–positron opening angle. In fact, even more intriguing: the excess occurs around a particular opening angle (140 degrees) and forms a bump.

Number of events (dN/dθ) for different electron–positron opening angles and plotted for different excitation energies (Ep). For Ep=1.10 MeV, there is a pronounced bump at 140 degrees which does not appear to be explainable from the ordinary internal pair conversion. This may be suggestive of a new particle. Adapted from Krasznahorkay et al., PRL 116, 042501.

Here’s why a bump is particularly interesting:

1. The distribution of ordinary internal pair creation events is smoothly decreasing and so this is very unlikely to produce a bump.
2. Bumps can be signs of new particles: if there is a new, light particle that can facilitate the decay, one would expect a bump at an opening angle that depends on the new particle mass.

Schematically, the new particle interpretation looks like this:

Schematic of the Atomki experiment and new particle (X) interpretation of the anomalous events. In summary: protons of a specific energy bombard stationary lithium-7 nuclei and excite them to the 18.15 MeV beryllium-8 state. These decay into the beryllium-8 ground state. Some of these decays are mediated by the new X particle, which then decays in to electron–positron pairs of a certain opening angle that are detected in the Atomki pair spectrometer detector. Image from 1608.03591.

As an exercise for those with a background in special relativity, one can use the relation (pe+ + pe)2 = mX2 to prove the result:

This relates the mass of the proposed new particle, X, to the opening angle θ and the energies E of the electron and positron. The opening angle bump would then be interpreted as a new particle with mass of roughly 17 MeV. To match the observed number of anomalous events, the rate at which the excited beryllium decays via the X boson must be 6×10-6 times the rate at which it goes into a γ-ray.

The anomaly has a significance of 6.8σ. This means that it’s highly unlikely to be a statistical fluctuation, as the 750 GeV diphoton bump appears to have been. Indeed, the conservative bet would be some not-understood systematic effect, akin to the 130 GeV Fermi γ-ray line.

## The beryllium that cried wolf?

Some physicists are concerned that beryllium may be the ‘boy that cried wolf,’ and point to papers by the late Fokke de Boer as early as 1996 and all the way to 2001. de Boer made strong claims about evidence for a new 10 MeV particle in the internal pair creation decays of the 17.64 MeV beryllium-8 excited state. These claims didn’t pan out, and in fact the instrumentation paper by the Atomki experiment rules out that original anomaly.

The proposed evidence for “de Boeron” is shown below:

The de Boer claim for a 10 MeV new particle. Left: distribution of opening angles for internal pair creation events in an E1 transition of carbon-12. This transition has similar energy splitting to the beryllium-8 17.64 MeV transition and shows good agreement with the expectations; as shown by the flat “signal – background” on the bottom panel. Right: the same analysis for the M1 internal pair creation events from the 17.64 MeV beryllium-8 states. The “signal – background” now shows a broad excess across all opening angles. Adapted from de Boer et al. PLB 368, 235 (1996).

When the Atomki group studied the same 17.64 MeV transition, they found that a key background component—subdominant E1 decays from nearby excited states—dramatically improved the fit and were not included in the original de Boer analysis. This is the last nail in the coffin for the proposed 10 MeV “de Boeron.”

However, the Atomki group also highlight how their new anomaly in the 18.15 MeV state behaves differently. Unlike the broad excess in the de Boer result, the new excess is concentrated in a bump. There is no known way in which additional internal pair creation backgrounds can contribute to add a bump in the opening angle distribution; as noted above: all of these distributions are smoothly falling.

The Atomki group goes on to suggest that the new particle appears to fit the bill for a dark photon, a reasonably well-motivated copy of the ordinary photon that differs in its overall strength and having a non-zero (17 MeV?) mass.

## Theory part 1: Not a dark photon

With the Atomki result was published and peer reviewed in Physics Review Letters, the game was afoot for theorists to understand how it would fit into a theoretical framework like the dark photon. A group from UC Irvine, University of Kentucky, and UC Riverside found that actually, dark photons have a hard time fitting the anomaly simultaneously with other experimental constraints. In the visual language of this recent ParticleBite, the situation was this:

It turns out that the minimal model of a dark photon cannot simultaneously explain the Atomki beryllium-8 anomaly without running afoul of other experimental constraints. Image adapted from this ParticleBite.

The main reason for this is that a dark photon with mass and interaction strength to fit the beryllium anomaly would necessarily have been seen by the NA48/2 experiment. This experiment looks for dark photons in the decay of neutral pions (π0). These pions typically decay into two photons, but if there’s a 17 MeV dark photon around, some fraction of those decays would go into dark-photon — ordinary-photon pairs. The non-observation of these unique decays rules out the dark photon interpretation.

The theorists then decided to “break” the dark photon theory in order to try to make it fit. They generalized the types of interactions that a new photon-like particle, X, could have, allowing protons, for example, to have completely different charges than electrons rather than having exactly opposite charges. Doing this does gross violence to the theoretical consistency of a theory—but they goal was just to see what a new particle interpretation would have to look like. They found that if a new photon-like particle talked to neutrons but not protons—that is, the new force were protophobic—then a theory might hold together.

Schematic description of how model-builders “hacked” the dark photon theory to fit both the beryllium anomaly while being consistent with other experiments. This hack isn’t pretty—and indeed, comes at the cost of potentially invalidating the mathematical consistency of the theory—but the exercise demonstrates the target for how to a complete theory might have to behave. Image adapted from this ParticleBite.

## Theory appendix: pion-phobia is protophobia

Editor’s note: what follows is for readers with some physics background interested in a technical detail; others may skip this section.

How does a new particle that is allergic to protons avoid the neutral pion decay bounds from NA48/2? Pions decay into pairs of photons through the well-known triangle-diagrams of the axial anomaly. The decay into photon–dark-photon pairs proceed through similar diagrams. The goal is then to make sure that these diagrams cancel.

A cute way to look at this is to assume that at low energies, the relevant particles running in the loop aren’t quarks, but rather nucleons (protons  and neutrons). In fact, since only the proton can talk to the photon, one only needs to consider proton loops. Thus if the new photon-like particle, X, doesn’t talk to protons, then there’s no diagram for the pion to decay into γX. This would be great if the story weren’t completely wrong.

Avoiding NA48/2 bounds requires that the new particle, X, is pion-phobic. It turns out that this is equivalent to X being protophobic. The correct way to see this is on the left, making sure that the contribution of up-quark loops cancels the contribution from down-quark loops. A slick (but naively completely wrong) calculation is on the right, arguing that effectively only protons run in the loop.

The correct way of seeing this is to treat the pion as a quantum superposition of an up–anti-up and down–anti-down bound state, and then make sure that the X charges are such that the contributions of the two states cancel. The resulting charges turn out to be protophobic.

The fact that the “proton-in-the-loop” picture gives the correct charges, however, is no coincidence. Indeed, this was precisely how Jack Steinberger calculated the correct pion decay rate. The key here is whether one treats the quarks/nucleons linearly or non-linearly in chiral perturbation theory. The relation to the Wess-Zumino-Witten term—which is what really encodes the low-energy interaction—is carefully explained in chapter 6a.2 of Georgi’s revised Weak Interactions.

## Theory part 2: Not a spin-0 particle

The above considerations focus on a new particle with the same spin and parity as a photon (spin-1, parity odd). Another result of the UCI study was a systematic exploration of other possibilities. They found that the beryllium anomaly could not be consistent with spin-0 particles. For a parity-odd, spin-0 particle, one cannot simultaneously conserve angular momentum and parity in the decay of the excited beryllium-8 state. (Parity violating effects are negligible at these energies.)

Parity and angular momentum conservation prohibit a “dark Higgs” (parity even scalar) from mediating the anomaly.

For a parity-odd pseudoscalar, the bounds on axion-like particles at 20 MeV suffocate any reasonable coupling. Measured in terms of the pseudoscalar–photon–photon coupling (which has dimensions of inverse GeV), this interaction is ruled out down to the inverse Planck scale.

Bounds on axion-like particles exclude a 20 MeV pseudoscalar with couplings to photons stronger than the inverse Planck scale. Adapted from 1205.2671 and 1512.03069.

• Dark Z bosons, cousins of the dark photon with spin-1 but indeterminate parity. This is very constrained by atomic parity violation.
• Axial vectors, spin-1 bosons with positive parity. These remain a theoretical possibility, though their unknown nuclear matrix elements make it difficult to write a predictive model. (See section II.D of 1608.03591.)

## Theory part 3: Nuclear input

The plot thickens when once also includes results from nuclear theory. Recent results from Saori Pastore, Bob Wiringa, and collaborators point out a very important fact: the 18.15 MeV beryllium-8 state that exhibits the anomaly and the 17.64 MeV state which does not are actually closely related.

Recall (e.g. from the first figure at the top) that both the 18.15 MeV and 17.64 MeV states are both spin-1 and parity-even. They differ in mass and in one other key aspect: the 17.64 MeV state carries isospin charge, while the 18.15 MeV state and ground state do not.

Isospin is the nuclear symmetry that relates protons to neutrons and is tied to electroweak symmetry in the full Standard Model. At nuclear energies, isospin charge is approximately conserved. This brings us to the following puzzle:

If the new particle has mass around 17 MeV, why do we see its effects in the 18.15 MeV state but not the 17.64 MeV state?

Naively, if the new particle emitted, X, carries no isospin charge, then isospin conservation prohibits the decay of the 17.64 MeV state through emission of an X boson. However, the Pastore et al. result tells us that actually, the isospin-neutral and isospin-charged states mix quantum mechanically so that the observed 18.15 and 17.64 MeV states are mixtures of iso-neutral and iso-charged states. In fact, this mixing is actually rather large, with mixing angle of around 10 degrees!

The result of this is that one cannot invoke isospin conservation to explain the non-observation of an anomaly in the 17.64 MeV state. In fact, the only way to avoid this is to assume that the mass of the X particle is on the heavier side of the experimentally allowed range. The rate for emission goes like the 3-momentum cubed (see section II.E of 1608.03591), so a small increase in the mass can suppresses the rate of emission by the lighter state by a lot.

The UCI collaboration of theorists went further and extended the Pastore et al. analysis to include a phenomenological parameterization of explicit isospin violation. Independent of the Atomki anomaly, they found that including isospin violation improved the fit for the 18.15 MeV and 17.64 MeV electromagnetic decay widths within the Pastore et al. formalism. The results of including all of the isospin effects end up changing the particle physics story of the Atomki anomaly significantly:

The rate of X emission (colored contours) as a function of the X particle’s couplings to protons (horizontal axis) versus neutrons (vertical axis). The best fit for a 16.7 MeV new particle is the dashed line in the teal region. The vertical band is the region allowed by the NA48/2 experiment. Solid lines show the dark photon and protophobic limits. Left: the case for perfect (unrealistic) isospin. Right: the case when isospin mixing and explicit violation are included. Observe that incorporating realistic isospin happens to have only a modest effect in the protophobic region. Figure from 1608.03591.

The results of the nuclear analysis are thus that:

1. An interpretation of the Atomki anomaly in terms of a new particle tends to push for a slightly heavier X mass than the reported best fit. (Remark: the Atomki paper does not do a combined fit for the mass and coupling nor does it report the difficult-to-quantify systematic errors  associated with the fit. This information is important for understanding the extent to which the X mass can be pushed to be heavier.)
2. The effects of isospin mixing and violation are important to include; especially as one drifts away from the purely protophobic limit.

## Theory part 4: towards a complete theory

The theoretical structure presented above gives a framework to do phenomenology: fitting the observed anomaly to a particle physics model and then comparing that model to other experiments. This, however, doesn’t guarantee that a nice—or even self-consistent—theory exists that can stretch over the scaffolding.

Indeed, a few challenges appear:

• The isospin mixing discussed above means the X mass must be pushed to the heavier values allowed by the Atomki observation.
• The “protophobic” limit is not obviously anomaly-free: simply asserting that known particles have arbitrary charges does not generically produce a mathematically self-consistent theory.
• Atomic parity violation constraints require that the X couple in the same way to left-handed and right-handed matter. The left-handed coupling implies that X must also talk to neutrinos: these open up new experimental constraints.

The Irvine/Kentucky/Riverside collaboration first note the need for a careful experimental analysis of the actual mass ranges allowed by the Atomki observation, treating the new particle mass and coupling as simultaneously free parameters in the fit.

Next, they observe that protophobic couplings can be relatively natural. Indeed: the Standard Model Z boson is approximately protophobic at low energies—a fact well known to those hunting for dark matter with direct detection experiments. For exotic new physics, one can engineer protophobia through a phenomenon called kinetic mixing where two force particles mix into one another. A tuned admixture of electric charge and baryon number, (Q-B), is protophobic.

Baryon number, however, is an anomalous global symmetry—this means that one has to work hard to make a baryon-boson that mixes with the photon (see 1304.0576 and 1409.8165 for examples). Another alternative is if the photon kinetically mixes with not baryon number, but the anomaly-free combination of “baryon-minus-lepton number,” Q-(B-L). This then forces one to apply additional model-building modules to deal with the neutrino interactions that come along with this scenario.

In the language of the ‘model building blocks’ above, result of this process looks schematically like this:

A complete theory is completely mathematically self-consistent and satisfies existing constraints. The additional bells and whistles required for consistency make additional predictions for experimental searches. Pieces of the theory can sometimes  be used to address other anomalies.

The theory collaboration presented examples of the two cases, and point out how the additional ‘bells and whistles’ required may tie to additional experimental handles to test these hypotheses. These are simple existence proofs for how complete models may be constructed.

## What’s next?

We have delved rather deeply into the theoretical considerations of the Atomki anomaly. The analysis revealed some unexpected features with the types of new particles that could explain the anomaly (dark photon-like, but not exactly a dark photon), the role of nuclear effects (isospin mixing and breaking), and the kinds of features a complete theory needs to have to fit everything (be careful with anomalies and neutrinos). The single most important next step, however, is and has always been experimental verification of the result.

While the Atomki experiment continues to run with an upgraded detector, what’s really exciting is that a swath of experiments that are either ongoing or in construction will be able to probe the exact interactions required by the new particle interpretation of the anomaly. This means that the result can be independently verified or excluded within a few years. A selection of upcoming experiments is highlighted in section IX of 1608.03591:

Other experiments that can probe the new particle interpretation of the Atomki anomaly. The horizontal axis is the new particle mass, the vertical axis is its coupling to electrons (normalized to the electric charge). The dark blue band is the target region for the Atomki anomaly. Figure from 1608.03591; assuming 100% branching ratio to electrons.

We highlight one particularly interesting search: recently a joint team of theorists and experimentalists at MIT proposed a way for the LHCb experiment to search for dark photon-like particles with masses and interaction strengths that were previously unexplored. The proposal makes use of the LHCb’s ability to pinpoint the production position of charged particle pairs and the copious amounts of D mesons produced at Run 3 of the LHC. As seen in the figure above, the LHCb reach with this search thoroughly covers the Atomki anomaly region.

## Implications

So where we stand is this:

• There is an unexpected result in a nuclear experiment that may be interpreted as a sign for new physics.
• The next steps in this story are independent experimental cross-checks; the threshold for a ‘discovery’ is if another experiment can verify these results.
• Meanwhile, a theoretical framework for understanding the results in terms of a new particle has been built and is ready-and-waiting. Some of the results of this analysis are important for faithful interpretation of the experimental results.

What if it’s nothing?

This is the conservative take—and indeed, we may well find that in a few years, the possibility that Atomki was observing a new particle will be completely dead. Or perhaps a source of systematic error will be identified and the bump will go away. That’s part of doing science.

Meanwhile, there are some important take-aways in this scenario. First is the reminder that the search for light, weakly coupled particles is an important frontier in particle physics. Second, for this particular anomaly, there are some neat take aways such as a demonstration of how effective field theory can be applied to nuclear physics (see e.g. chapter 3.1.2 of the new book by Petrov and Blechman) and how tweaking our models of new particles can avoid troublesome experimental bounds. Finally, it’s a nice example of how particle physics and nuclear physics are not-too-distant cousins and how progress can be made in particle–nuclear collaborations—one of the Irvine group authors (Susan Gardner) is a bona fide nuclear theorist who was on sabbatical from the University of Kentucky.

What if it’s real?

This is a big “what if.” On the other hand, a 6.8σ effect is not a statistical fluctuation and there is no known nuclear physics to produce a new-particle-like bump given the analysis presented by the Atomki experimentalists.

The threshold for “real” is independent verification. If other experiments can confirm the anomaly, then this could be a huge step in our quest to go beyond the Standard Model. While this type of particle is unlikely to help with the Hierarchy problem of the Higgs mass, it could be a sign for other kinds of new physics. One example is the grand unification of the electroweak and strong forces; some of the ways in which these forces unify imply the existence of an additional force particle that may be light and may even have the types of couplings suggested by the anomaly.

Could it be related to other anomalies?

The Atomki anomaly isn’t the first particle physics curiosity to show up at the MeV scale. While none of these other anomalies are necessarily related to the type of particle required for the Atomki result (they may not even be compatible!), it is helpful to remember that the MeV scale may still have surprises in store for us.

• The KTeV anomaly: The rate at which neutral pions decay into electron–positron pairs appears to be off from the expectations based on chiral perturbation theory. In 0712.0007, a group of theorists found that this discrepancy could be fit to a new particle with axial couplings. If one fixes the mass of the proposed particle to be 20 MeV, the resulting couplings happen to be in the same ballpark as those required for the Atomki anomaly. The important caveat here is that parameters for an axial vector to fit the Atomki anomaly are unknown, and mixed vector–axial states are severely constrained by atomic parity violation.

The KTeV anomaly interpreted as a new particle, U. From 0712.0007.

• The anomalous magnetic moment of the muon and the cosmic lithium problem: much of the progress in the field of light, weakly coupled forces comes from Maxim Pospelov. The anomalous magnetic moment of the muon, (g-2)μ, has a long-standing discrepancy from the Standard Model (see e.g. this blog post). While this may come from an error in the very, very intricate calculation and the subtle ways in which experimental data feed into it, Pospelov (and also Fayet) noted that the shift may come from a light (in the 10s of MeV range!), weakly coupled new particle like a dark photon. Similarly, Pospelov and collaborators showed that a new light particle in the 1-20 MeV range may help explain another longstanding mystery: the surprising lack of lithium in the universe (APS Physics synopsis).

Could it be related to dark matter?

A lot of recent progress in dark matter has revolved around the possibility that in addition to dark matter, there may be additional light particles that mediate interactions between dark matter and the Standard Model. If these particles are light enough, they can change the way that we expect to find dark matter in sometimes surprising ways. One interesting avenue is called self-interacting dark matter and is based on the observation that these light force carriers can deform the dark matter distribution in galaxies in ways that seem to fit astronomical observations. A 20 MeV dark photon-like particle even fits the profile of what’s required by the self-interacting dark matter paradigm, though it is very difficult to make such a particle consistent with both the Atomki anomaly and the constraints from direct detection.

Should I be excited?

Given all of the caveats listed above, some feel that it is too early to be in “drop everything, this is new physics” mode. Others may take this as a hint that’s worth exploring further—as has been done for many anomalies in the recent past. For researchers, it is prudent to be cautious, and it is paramount to be careful; but so long as one does both, then being excited about a new possibility is part what makes our job fun.

For the general public, the tentative hopes of new physics that pop up—whether it’s the Atomki anomaly, or the 750 GeV diphoton bumpa GeV bump from the galactic center, γ-ray lines at 3.5 keV and 130 GeV, or penguins at LHCb—these are the signs that we’re making use of all of the data available to search for new physics. Sometimes these hopes fizzle away, often they leave behind useful lessons about physics and directions forward. Maybe one of these days an anomaly will stick and show us the way forward.

Here are some of the popular-level press on the Atomki result. See the references at the top of this ParticleBite for references to the primary literature.

### What is “Model Building”?

Flip Tanedo
Thursday, August 18th, 2016

Hi everyone! It’s been a while since I’ve posted on Quantum Diaries. This post is cross-posted from ParticleBites.

One thing that makes physics, and especially particle physics, is unique in the sciences is the split between theory and experiment. The role of experimentalists is clear: they build and conduct experiments, take data and analyze it using mathematical, statistical, and numerical techniques to separate signal from background. In short, they seem to do all of the real science!

So what is it that theorists do, besides sipping espresso and scribbling on chalk boards? In this post we describe one type of theoretical work called model building. This usually falls under the umbrella of phenomenology, which in physics refers to making connections between mathematically defined theories (or models) of nature and actual experimental observations of nature.

One common scenario is that one experiment observes something unusual: an anomaly. Two things immediately happen:

1. Other experiments find ways to cross-check to see if they can confirm the anomaly.
2. Theorists start figure out the broader implications if the anomaly is real.

#1 is the key step in the scientific method, but in this post we’ll illuminate what #2 actually entails. The scenario looks a little like this:

An unusual experimental result (anomaly) is observed. One thing we would like to know is whether it is consistent with other experimental observations, but these other observations may not be simply related to the anomaly.

Theorists, who have spent plenty of time mulling over the open questions in physics, are ready to apply their favorite models of new physics to see if they fit. These are the models that they know lead to elegant mathematical results, like grand unification or a solution to the Hierarchy problem. Sometimes theorists are more utilitarian, and start with “do it all” Swiss army knife theories called effective theories (or simplified models) and see if they can explain the anomaly in the context of existing constraints.

Here’s what usually happens:

Usually the nicest models of new physics don’t fit! In the explicit example, the minimal supersymmetric Standard Model doesn’t include a good candidate to explain the 750 GeV diphoton bump.

Indeed, usually one needs to get creative and modify the nice-and-elegant theory to make sure it can explain the anomaly while avoiding other experimental constraints. This makes the theory a little less elegant, but sometimes nature isn’t elegant.

Candidate theory extended with a module (in this case, an additional particle). This additional model is “bolted on” to the theory to make it fit the experimental observations.

Now we’re feeling pretty good about ourselves. It can take quite a bit of work to hack the well-motivated original theory in a way that both explains the anomaly and avoids all other known experimental observations. A good theory can do a couple of other things:

1. It points the way to future experiments that can test it.
2. It can use the additional structure to explain other anomalies.

The picture for #2 is as follows:

A good hack to a theory can explain multiple anomalies. Sometimes that makes the hack a little more cumbersome. Physicists often develop their own sense of ‘taste’ for when a module is elegant enough.

Even at this stage, there can be a lot of really neat physics to be learned. Model-builders can develop a reputation for particularly clever, minimal, or inspired modules. If a module is really successful, then people will start to think about it as part of a pre-packaged deal:

A really successful hack may eventually be thought of as it’s own variant of the original theory.

Model-smithing is a craft that blends together a lot of the fun of understanding how physics works—which bits of common wisdom can be bent or broken to accommodate an unexpected experimental result? Is it possible to find a simpler theory that can explain more observations? Are the observations pointing to an even deeper guiding principle?

Of course—we should also say that sometimes, while theorists are having fun developing their favorite models, other experimentalists have gone on to refute the original anomaly.

Sometimes anomalies go away and the models built to explain them don’t hold together.

But here’s the mark of a really, really good model: even if the anomaly goes away and the particular model falls out of favor, a good model will have taught other physicists something really neat about what can be done within the a given theoretical framework. Physicists get a feel for the kinds of modules that are out in the market (like an app store) and they develop a library of tricks to attack future anomalies. And if one is really fortunate, these insights can point the way to even bigger connections between physical principles.

I cannot help but end this post without one of my favorite physics jokes, courtesy of T. Tait:

A theorist and an experimentalist are having coffee. The theorist is really excited, she tells the experimentalist, “I’ve got it—it’s a model that’s elegant, explains everything, and it’s completely predictive.”The experimentalist listens to her colleague’s idea and realizes how to test those predictions. She writes several grant applications, hires a team of postdocs and graduate students, trains them,  and builds the new experiment. After years of design, labor, and testing, the machine is ready to take data. They run for several months, and the experimentalist pores over the results.

The experimentalist knocks on the theorist’s door the next day and says, “I’m sorry—the experiment doesn’t find what you were predicting. The theory is dead.”

The theorist frowns a bit: “What a shame. Did you know I spent three whole weeks of my life writing that paper?”

### Live Blogging of the LHCb Bs pi result

Monday, March 21st, 2016

There has been a lot of press about the recent DØ result on the possible $$B_s \pi$$ state. This was also covered on Ricky Nathvani’s blog. At Moriond QCD, Jeroen Van Tilburg showed a few plots from LHCb which showed no signal in the same mass regions as explored by D∅. Tomorrow, there will be a special LHC seminar on the LHCb search for purported tetraquark, where we will get the full story from LHCb. I will be live blogging the seminar here! It kicks off at 11:50 CET, so tune in to this post for live updates.

Mar 22, 2016 -12:23. Final answer. LHCb does not confirm the tetraquark. Waiting for CMS, ATLAS, CDF.

Mar 22, 2016 – 12:24. How did you get the result out so fast? A lot of work by the collaboration to get MC produced and to expedite the process.

Mar 22, 2016 – 12:21. Is the $$p_T$$ cut on the pion too tight? The fact that you haven’t seen anything anywhere else gives you confidence that the cut is safe. Also, cut is not relative to $$B_s$$.

Mar 22, 2016 – 12:18. Question: What are the fractions of multiple candidates which enter? Not larger than 1.2. If you go back to the cuts. What selection killed the combinatoric background the most? Requirement that the $$\pi$$ comes from the PV, and the $$p_T$$ cut on the pion kill the most. How strong the PV cut? $$\chi^2$$ less than 3.5 for the pion at the PV, you force the $$B_s$$ and the pion to come from the PV, and constrain the mass of $$B_s$$ mass.

Mar 22, 2016 – 12:17: Can you go above the threshold? Yes.

Mar 22, 2016 – 12:16. Slide 9: Did you fit with a floating mass? Plan to do this for the paper.

Mar 22, 2016 – 12:15. Wouldn’t $$F_S$$ be underestimated by 8%? Maybe maybe not.

Mar 22, 2016 – 12:13. Question: Will LHCb publish? Most likely yes, but a bit of politics. Shape of the background in the $$B_s\pi$$ is different in LHCb and DØ. At some level, you expect a peak from the turn over. Also CMS is looking.

Mar 22, 2016 – 12:08-12:12. Question: did you try the cone cut to try to generate a peak? Answer: Afraid that the cut can give a biased estimate of the significance. From DØ seminar, seems like this is the case. For DØ to answer. Vincenzo Vagnoni says that DØ estimation of significance is incorrect. We also don’t know if there’s something that’s different between $$pp$$ and $$p \bar{p}$$.

Mar 22, 2016 – 12:08. No evidence of $$X(5568)$$ state, set upper limit. “We look forward to hearing from ATLAS, CMS and CDF about $$X(5568)$$”

Mar 22, 2016 – 12:07. What if the production of the X was the same at LHCb? Should have seen a very large signal. Also, in many other spectroscopy plots, e.g. $$B*$$, look at “wrong sign” plots for B and meson. All results LHCb already searched for would have been sensitive to such a state.

Mar 22, 2016 -12:04. Redo the analysis in bins of rapidity. No significant signal seen in any result. Do for all pt ranges of the Bs.

Mar 22, 2016 – 12:03. Look at $$B^0\pi^+$$ as a sanity check. If X(5568) is similar to B**, then the we expect order 1000 events.

Mar 22, 2016 – 12:02.Upper limits on production given.

Mar 22, 2016 – 12:02. Check for systematics: changing mass and width of DØ range, and effect of efficiency dependence on signal shape are the dominant sources of systematics. All measurements dominated by statistics.

Mar 22, 2016 – 12:00. Result of the fits all consistent with zero. The relative production is also consistent with zero.

Mar 22, 2016 – 11:59. 2 fits with and without signal components, no difference in pulls. Do again with tighter cut on the transverse momentum of the $$B_s$$. Same story, no significant signal seen.

Mar 22, 2016 – 11:58. Fit model: S-wave Breit-Wigner, mass and width fixed to DØ result. Backgrounds: 2 sources. True $$B_s^0$$ with random track, and fake $$B_s$$.

Mar 22, 2016 – 11:56.  No “cone cut” applied because it is highly correlated with reconstructed mass.

Mar 22, 2016 – 11:55. LHCb strategy: Perform 3 independent searches, confirm a qualitative approach, move forward with single approach with Run 1 dataset. Cut based selection to match D∅ strategy. Take home point. Statistics is 20x larger and much cleaner.

Mar 22, 2016 – 11:52. Review of DØ result. What could it be? Molecular model is disfavored. Diquark-Antidiquark models are popular. But could not fit into any model. Could also be feed down of  radiative decays. All predictions have large uncertainties

Mar 22, 2016 –  11:49. LHCb-CONF-2016-004 posted at cds.cern.ch/record/2140095/

Mar 22, 2016 – 11:47. The speaker is transitioning to Marco Pappagallo .

Mar 22, 2016 – 11:44. People have begun entering the auditorium for the talk, at the end of Basem Khanji’s seminar on $$\Delta m_d$$

### LHC 2015: what’s different in four years?

Ken Bloom
Monday, December 21st, 2015

After a long-anticipated data run, LHC proton-proton running concludes in early November.  A mere six weeks later, on a mid-December afternoon, the ATLAS and CMS collaborations present their first results from the full dataset to a packed CERN auditorium, with people all over the world watching the live webcast.  Both collaborations see slight excesses in events with photon pairs; the CMS excess is quite modest, but the ATLAS data show something that could be interpreted as a peak.  If it holds up with additional data, it would herald a major discovery.  While the experimenters caution that the results do not have much statistical significance, news outlets around the world run breathless stories about the possible discovery of a new particle.

December 15, 2015? No — December 13, 2011, four years ago.  That seminar presented what we now know were the first hints of the Higgs boson in the LHC data.  At the time, everyone was hedging their bets, and saying that the effects we were seeing could easily go away with more data.  Yet now we look back and know that it was the beginning of the end for the Higgs search.  And even at the time, everyone was feeling pretty optimistic.  Yes, we had seen effects of that size go away before, but at this time four years ago, a lot of people were guessing that this one wouldn’t (while still giving all of the caveats).

But while both experiments are reporting an effect at 750 GeV — and some people are getting very excited about it — it seems to me that caution is needed here, more so than we did with the emerging evidence for the Higgs boson.  What’s different about what we’re seeing now compared to what we saw in 2011?

I found it instructive to look back at the presentations of four years ago.  Then, ATLAS had an effect in diphotons around an invariant mass of 125 GeV that had a 2.8 standard deviation local significance, which was reduced to 1.5 standard deviations when the “look elsewhere effect” (LEE) was taken into account.  (The LEE exists because if there is a random fluctuation in the data, it might appear anywhere, not just the place you happen to be looking, and the statistical significance needs to be de-weighted for that.)  In CMS, the local significance was 2.1 standard deviations.  Let’s compare that to this year, when both experiments see an effect in diphotons around an invariant mass of 750 GeV.  At ATLAS, it’s a 3.6 standard deviation local effect which reduced to 2.0 standard deviations after the LEE.  For CMS the respective values are 2.6 and 1.2 standard deviations.  So it sounds like the 2015 signals are even stronger than the 2011 ones, although, on their own, still quite weak, when we consider that five standard deviations is the usual standard to claim a discovery because we are sure that a fluctuation of that size would be very unlikely.

But the 2011 signals had some other things going for them.  The first were experimental.  There were simultaneous excesses in other channels that were consistent with what you’d expect from a Higgs decay.  This included in particular the ZZ channel, which had a low expected rate, but also very low backgrounds and excellent mass resolution.  In 2011, both experiments had the beginning of signals in ZZ too (although at a slightly different putative Higgs mass value) and some early hints in other decay channels.  There were multiple results supporting the diphotons, whereas in 2015 there are no apparent excesses in other channels indicating anything at 750 GeV.

And on top of that, there was something else going for the Higgs in December 2011: there was good reason to believe it was on the way.  From a myriad of other experiments we had indirect evidence that a Higgs boson ought to exist, and in a mass range where the LHC effects were showing up.  This indirect evidence came through the interpretation of the “standard model” theory that had done an excellent job of describing all other data in particle physics and thus gave us confidence that it could make predictions about the Higgs too.  And for years, both the Tevatron and the LHC had been slowly but surely directly excluding other possible masses for the Higgs.  If a Higgs were going to show up, it made perfect sense for it to happen right where the early effects were being observed, at just that level of significance with so little data.

Do we have any of that with the 750 GeV effect in 2015?  No.  There are no particular reasons to expect this decay with this rate at this mass (although in the wake of last week’s presentations, there have been many conjectures as to what kind of new physics could make this happen).  Thus, one can’t help but to think that this is some kind of fluctuation.  If you look at enough possible new-physics effects, you have a decent chance of seeing some number of fluctuations at this level, and that seems to be the most reasonable hypothesis right now.

But really there is no need to speculate.  In 2016, the LHC should deliver ten times as much data as it did this year.  That’s even better than what happened in 2012, when the LHC exceeded its 2011 performance by a mere factor of five.  We can anticipate another set of presentations in December 2016, and by then we will know for sure if 2015 gave us a fluctuation or the first hint of a new physical theory that will set the research agenda of particle physics for years to come.  And if it is the latter, I will be the first to admit that I got it wrong.

### What have we learned from the LHC in 2015?

Ken Bloom
Saturday, December 5th, 2015

The Large Hadron Collider is almost done running for 2015.  Proton collisions ended in early November, and now the machine is busy colliding lead nuclei.  As we head towards the end-of-year holidays, and the annual CERN shutdown, everyone wants to know — what have we learned from the LHC this year, our first year of data-taking at 13 TeV, the highest collision energies we have ever achieved, and the highest we might hope to have for years to come?

We will get our first answers to this question at a CERN seminar scheduled for Tuesday, December 15, where ATLAS and CMS will be presenting physics results from this year’s run.  The current situation is reminiscent of December 2011, when the experiments had recorded their first significant datasets from LHC Run 1, and we saw what turned out to be the first hints of the evidence for the Higgs boson that was discovered in 2012.  The experiments showed a few early results from Run 2 during the summer, and some of those have already resulted in journal papers, but this will be our first chance to look at the broad physics program of the experiments.  We shouldn’t have expectations that are too great, as only a small amount of data has been recorded so far, much less than we had in 2012.  But what science might we hope to hear about next week?

Here is one thing to keep in mind — the change in collision energy affects particle production rates, but not the properties of the particles that are produced.  Any measurement of particle production rates is inherently interesting at a new collision energy, as will be a measurement that has never been done before.  Thus any measurement of a production rate that is possible with this amount of data would be a good candidate for presentation.  (The production rates of top quarks at 13 TeV have already been measured by both CMS and ATLAS; maybe there will be additional measurements along these lines.)

We probably won’t hear anything new about the Higgs boson.  While the Higgs production rates are larger than in the previous run, the amount of data recorded is still relatively small compared to the 2010-12 period.  This year, the LHC has delivered about 4 fb-1 of data, which could be compared to the 5 fb-1 that was delivered in 2011.  At that time there wasn’t enough data to say anything definitive about the Higgs boson, so it is hard to imagine that there will be much in the way of Higgs results from the new data (not even the production rate at 13 TeV), and certainly nothing that would tell us anything more about its properties than we already know from the full Run 1 dataset of 30 fb-1.  We’ll all probably have to wait until sometime next year before we will know more about the Higgs boson, and if anything about it will disagree with what we expect from the standard model of particle physics.

If there is anything to hope for next week, it is some evidence for new, heavy particles.  Because the collision energy has been increased from 8 TeV to 13 TeV, the ability to create a heavy particle of a given mass has increased too.  A little fooling around with the “Collider Reach” tool (which I had discussed here) suggests that even as little data as we have in hand now can give us improved chances of observing such particles now compared to the chances in the entire Run 1 dataset as long as the particle masses are above about 3 TeV.  Of course there are many theories that predict the existence of such particles, the most famous of which is supersymmetry.  But so far there has been scant evidence of any new phenomena in previous datasets.  If we were to get even a hint of something at a very high mass, it would definitely focus our scientific efforts for 2016, where we might get about ten times as much data as we did this year.

Will we get that hint, like we did with the Higgs boson four years ago?  Tune in on December 15 to find out!

### Double time

Ken Bloom
Thursday, August 27th, 2015

In particle physics, we’re often looking for very rare phenomena, which are highly unlikely to happen in any given particle interaction. Thus, at the LHC, we want to have the greatest possible proton collision rates; the more collisions, the greater the chance that something unusual will actually happen. What are the tools that we have to increase collision rates?

Remember that the proton beams are “bunched” — there isn’t a continuous current current of protons in a beam, but a series of smaller bunches of protons, each only a few centimeters long, with gaps of many centimeters between each bunch.  The beams are then timed so that bunches from each beam pass through each other (“cross”) inside one of the big detectors.  A given bunch can have 10E11 protons in it, and when two bunches cross, perhaps tens of the protons in each bunch — a tiny fraction! — will interact.  This bunching is actually quite important for the operation of the detectors — we can know when bunches are crossing, and thus when collisions happen, and then we know when the detectors should really be “on” to record the data.

If one were to have a fixed number of protons in the machine (and thus a fixed total amount of beam current), you could imagine two ways to create the same number of collisions: have N bunches per beam, each with M protons, or 2N bunches per beam with M/sqrt(2) protons.  The more bunches in the beam, the more closely spaced they would have to be, but that can be done.  From the perspective of the detectors, the second scenario is much preferred.  That’s because you get fewer proton collisions per bunch crossing, and thus fewer particles streaming through the detectors.  The collisions are much easier to interpret if you have fewer collisions per crossing; among other things, you need less computer processing time to reconstruct each event, and you will have fewer mistakes in the event reconstruction because there aren’t so many particles all on top of each other.

In the previous LHC run (2010-12), the accelerator had “50 ns spacing” between proton bunches, i.e. bunch crossings took place every 50 ns.  But over the past few weeks, the LHC has been working on running with “25 ns spacing,” which would allow the beam to be segmented into twice as many bunches, with fewer protons per bunch.  It’s a new operational mode for the machine, and thus some amount of commissioning and tuning and so forth are required.  A particular concern is “electron cloud” effects due to stray particles in the beampipe striking the walls and ejecting more particles, which is a larger effect with smaller bunch spacing.  But from where I sit as one of the experimenters, it looks like good progress has been made so far, and as we go through the rest of this year and into next year, 25 ns spacing should be the default mode of operation.  Stay tuned for what physics we’re going to be learning from all of this!

### 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.

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

Ken Bloom
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.

### 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.

### Ramping up to Run 2

Ken Bloom
Thursday, March 19th, 2015

When I have taught introductory electricity and magnetism for engineers and physics majors at the University of Nebraska-Lincoln, I have used a textbook by Young and Freedman. (Wow, look at the price of that book! But that’s a topic for another day.) The first page of Chapter 28, “Sources of Magnetic Field,” features this photo:

It shows the cryostat that contains the solenoid magnet for the Compact Muon Solenoid experiment. Yes, “solenoid” is part of the experiment’s name, as it is a key element in the design of the detector. There is no other magnet like it in the world. It can produce a 4 Tesla magnetic field, 100,000 times greater than that of the earth. (We actually run at 3.8 Tesla.) Charged particles that move through a magnetic field take curved paths, and the stronger the field, the stronger the curvature. The more the path curves, the more accurately we can measure it, and thus the more accurately we can measure the momentum of the particle.

The magnet is superconducting; it is kept inside a cryostat that is full of liquid helium. With a diameter of seven meters, it is the largest superconducting magnet ever built. When in its superconducting state, the magnet wire carries more than 18,000 amperes of current, and the energy stored is about 2.3 gigajoules, enough energy to melt 18 tons of gold. Should the temperature inadvertently rise and the magnet become normal conducting, all of that energy needs to go somewhere; there are some impressively large copper conduits that can carry the current to the surface and send it safely to ground. (Thanks to the CMS web pages for some of these fun facts.)

With the start of the LHC run just weeks away, CMS has turned the magnet back on by slowly ramping up the current. Here’s what that looked like today:

You can see that they took a break for lunch! It is only the second time since the shutdown started two years ago that the magnet has been ramped back up, and now we’re pretty much going to keep it on for at least the rest of the year. From the experiment’s perspective, the long shutdown is now over, and the run is beginning. CMS is now prepared to start recording cosmic rays in this configuration, as a way of exercising the detector and using the observed muon to improve our knowledge of the alignment of detector components. This is a very important milestone for the experiment as we prepare for operating the LHC at the highest collision energies ever achieved in the laboratory!