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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

Adam Davis
Wednesday, July 15th, 2015
Photo Credit: Cathy Händel, Published on http://www.suttonelms.org.uk/olla12.html

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

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. From http://arxiv.org/abs/1507.03414

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...  From http://arxiv.org/abs/1507.03414

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

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

Adam Davis
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.

In order to find out a little bit more about this mysterious substance and its dispersion, I asked a graduating student, who wished to remain unnamed, a little bit about their experiences:

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!


Video contest: Rock the LHC!

Ken Bloom
Tuesday, March 17th, 2015

Do you think the Large Hadron Collider rocks? I sure do, and as the collider rocks back to life in the coming weeks (more on that soon), you can celebrate by entering the Rock the LHC video contest. It’s simple: you make a short video about why you are excited about research at the LHC, and submit it to a panel of physicists and communication experts. The producer of the best video will win an all-expenses paid trip for two to Fermilab in Batavia, IL, the premier particle physics laboratory in the United States, for a VIP tour. What a great way to celebrate the restart of the world’s most powerful particle collider!

This contest is funded by the University of Notre Dame, and please note the rules — you must be over 18 and a legal U.S. resident currently residing in the 50 states or the District of Columbia to enter. The deadline for entries is May 31.

As we get ready for the collider run of a lifetime, let’s see how creative you can be about the exciting science that is ahead of us!



Behind the scenes of our “Big Bang Theory” post

Ken Bloom
Tuesday, February 10th, 2015
Well, that was fun!

At 8 PM ET on February 5, 2015, Quantum Diaries ran a post that was tied to “The Troll Manifestation”, an episode of “The Big Bang Theory” (TBBT) that was being aired at exactly that time.  This was generated in partnership with the show’s writers, staff and advisers. What happens when you couple a niche-interest website to one of the most popular TV shows in the United States? The QD bloggers and support staff had a great time getting ready for this synergistic event and tracking what happened next.  Here’s the story behind the story.

I’ve mentioned previously, in my largely unheralded essay about the coffee culture at CERN, that I have known David Saltzberg, UCLA faculty member and science adviser to TBBT, for a very long time, since we were both students in the CDF group at The University of Chicago.  On January 14, David contacted me (and fellow QD blogger Michael DuVernois) to say that Quantum Diaries was going to be mentioned in an episode of the show that was going to be taped in the coming week.  David wanted to know if I could sign a release form allowing them to use the name of the blog.

I couldn’t — the blog is not mine, but is operated by the InterAction Collaboration, which is an effort of the communications organizations of the world’s particle physics laboratories.  (They signed the release form.)  But I did come up with an idea.  David had said that the show would refer to a Quantum Diaries blog post about a paper that Leonard and Sheldon had written.  Why not actually write such a post and put it up on the site?  A real blog post about a fake paper by fake scientists.  David was intrigued; he discussed it with the TBBT producers, and they liked the idea too.  The show was to air on February 5.  Game on!

David shared the shooting script with me, and explained that this was one of the rare TBBT episodes in which he didn’t just add in some science, but also had an impact on the plot.  He had described his own experience of talking about something with a theorist colleague, and getting the response, “That’s an interesting idea — we should write a paper about it together!”  I myself wouldn’t know where to get started in that situation.  This gave me the idea for how to write about the episode.  The script had enough information about Leonard and Sheldon’s paper for me to say something intelligible about it.  The fun for me in writing the post was in figuring out how to point to the show without giving it all away too quickly.  I ran my text by David, who passed it on to the show’s producers, and everyone enjoyed it.  We knew there was some possibility that the show’s social media team would promote the QD site through their channels; their Facebook page has 33 million likes and their Twitter account has 3.1 million followers.

Meanwhile, the Quantum Diaries team sprung into action.  Kelen Tuttle, the QD webmaster, told the other bloggers for the site about our opportunity to gain national recognition for the blog, and encouraged everyone to generate some exciting new content.  Regular QD readers might have noticed all the bloggers becoming very voluble in the past week!  Kevin Munday and his team at Xeno Media prepared the site for the possible onslaught of visitors  — remember, twenty million people watch TBBT each week! — by migrating the site to the CloudFlare content delivery network, with 30 data centers worldwide, and protecting the site against possible security issues.

We all crossed our fingers for Thursday night.  I spent Thursday at Fermilab, and was flying back to Lincoln in the afternoon, scheduled to land at 6:43 PM, a few minutes before the 7 PM air time.  When I got home, I started keeping an eye on the computer.  The blog post was up, but was TBBT going to say anything about it?  Alas, their Twitter feed was quiet during the show.  (No, I didn’t watch — I have to admit that we watch so little television that we couldn’t figure out which channel it might be on!)

All of us involved were a bit disappointed that evening.  But David took up the case again with the CBS interactive team the next day, and was told that they’d put out a tweet as long as we changed our blog post to link to the archive of the show.  We did that, and then at 12:45 Central Time, we got the shout-out that we were hoping for:

So what happens when a TV audience of around 20 million people hear a website (which may or may not be real) mentioned in a show?  Or when 3.1 million people who are fans of a TV show receive a tweet pointing to a blog post?  The Quantum Diaries traffic metrics tell the tale. Here is a plot of the number of visitors to the site during the past four weeks, including the February 5 air date and the February 6 tweet date:
When the blog was mentioned on the air, there was a definite spike in activity, and an even bigger spike on the day after, when the tweet went out. Traffic on the site was up by a factor of four thanks to TBBT!

However, the plot doesn’t show the absolute scale. On February 6, the site had about 4600 visitors, compared to a typical level of 800-1000 visitors. This means that only 0.1% of people who saw the TBBT tweet actually went and clicked on the link that took them to QD. This is nowhere near the level of activity we saw when the Higgs boson was discovered. TBBT may be a great TV show, but it’s no fundamental scientific discovery.

However, the story did have some pretty strong legs in Nebraska.  My employer, the University of Nebraska-Lincoln, graciously wrote a story about my involvement in the show and promoted it pretty heavily through social media.  This led to a couple of appearances on some news programs that enjoy making local links to national stories (if you could call this a national story).  I found it a bit surreal and was reminded that I need to get a haircut and clean my desk.

Thank you to David Salzberg for making this possible, and to the TBBT producers and writers who were supportive, and of course to all of my colleagues at Quantum Diaries who did a lot of writing and technical preparation last week.  (A special shout-out to Kelen Tuttle, who left QD for a new position at Invitae this week; at least we sent her off with a Big Bang!) And if you have discovered this blog because of “The Troll Manifestation”, I hope you stay for a while!  These are great times for particle physics — the Large Hadron Collider starts up again this year, we’re planning an exciting international program of neutrino physics that will be hosted in the United States, and we’re scanning the skies for the secrets of cosmology.  We particle physicists are excited about what we do and want to share some of our passion with you.  And besides, now we know that Stephen Hawking reads Quantum Diaries — shouldn’t you read it too?


Theory and experiment come together — bazinga!

Ken Bloom
Thursday, February 5th, 2015

Regular readers of Quantum Diaries will know that in the world of particle physics, there is a clear divide between the theorists and the experimentalists. While we are all interested in the same big questions — what is the fundamental nature of our world, what is everything made of and how does it interact, how did the universe come to be and how might it end — we have very different approaches and tools. The theorists develop new models of elementary particle interactions, and apply formidable mathematical machinery to develop predictions that experimenters can test. The experimenters develop novel instruments, deploy them on grand scales, and organize large teams of researchers to collect data from particle accelerators and the skies, and then turn those data into measurements that test the theorists’ models. Our work is intertwined, but ultimately lives in different spheres. I admire what theorists do, but I also know that I am much happier being an experimentalist!

But sometimes scientists from the two sides of particle physics come together, and the results can be intriguing. For instance, I recently came across a new paper by two up-and-coming physicists at Caltech. One, S. Cooper, has been a noted prodigy in theoretical pursuits such as string theory. The other, L. Hofstadter, is an experimental particle physicist who has been developing a detector that uses superfluid liquid helium as an active element. Superfluids have many remarkable properties, such as friction-free flow, that can make them very challenging to work with in particle detectors.

Hofstadter’s experience in working with a superfluid in the lab gave him new ideas about how it could be used as a physical model for space-time. There have already been a number of papers that posit a theory of the vacuum as having properties similar to that of a superfluid. But the new paper by Cooper and Hofstadter take this theory in a different direction, positing that the universe actually lives on the surface of such a superfluid, and that the negative energy density that we observe in the universe could be explained by the surface tension. The authors have difficulty generating any other testable hypotheses from this new theory, but it is inspiring to see how scientists from the two sides of physics can come together to generate promising new ideas.

If you want to learn more about this paper, watch “The Big Bang Theory” tonight, February 5, 2015, on CBS. And Leonard and Sheldon, if you are reading this post — don’t look at the comments. It will only be trouble.

In case you missed the episode, you can watch it here.

Like what you see here? Read more Quantum Diaries on our homepage, subscribe to our RSS feed, follow us on Twitter, or befriend us on Facebook!


A particle detector in your pocket

Kyle Cranmer
Wednesday, February 4th, 2015

Do you love science and technology and sometimes wish you could contribute to a major discovery? I’ve got good news: “there’s an app for that.” With the Crayfis app, you can join a world-wide network of smartphones designed to detect ultra-high energy cosmic rays.

Cosmic rays were discovered by Victor Hess in 1912, for which he received the Nobel Prize in Physics in 1936. They are constantly raining down on us from space; typically atomic nuclei that hit the upper atmosphere leading to a huge shower of particles, some of which make it to the Earth’s surface.

Just last year a team of scientists published a result based on data from the Fermi gamma-ray space telescope indicating that lower-energy cosmic rays are associated to supernovae. However, the origin of the most energetic ones remains a mystery.

The highest energy cosmic rays are amazing, they have about as much kinetic energy as a 60 mph (~100 km/h) baseball packed into a single atomic nucleus! This is much higher energy than what is probed by the LHC, but these kinds of ultra-high energy cosmic rays are very rare. To get a feel for the numbers, the Pierre Auger Observatory, which is about the size of Rhode Island or Luxembourg, observes one of these ultra-high energy cosmic rays roughly every four weeks. What could possibly be responsible for accelerating particles to such high energies?

Untangling the mystery of these ultra-high energy cosmic rays will require observing many more, which means either a very long-running experiment or a very large area. Current arrays with large, highly-efficient devices like Auger cannot grow dramatically larger without becoming much more expensive. This motivates some out of the box thinking.

Smartphones are perfect candidates for a global cosmic ray detector. Phones these days are high-tech gadgets. The camera sensor is a lot like the pixel detectors of ATLAS and CMS, so they are capable of detecting particles from cosmic ray showers (check out the video for a quick demo). In addition, most phones have GPS to tell them where they are, wifi connections to the internet, and significant processing power. Perhaps most importantly, there are billions of smartphones already in use.

Late last year a small team led by Daniel Whiteson and Michael Mulhearn put out a paper making the case for such a world-wide network of smartphones. The paper is backed up by lab tests of the smart phone cameras and simulations of ultra-high energy cosmic ray showers. The results indicate that if we can have roughly a thousand sq. km clusters each with a thousand phones that the exposure time would be roughly equivalent to the Pierre Auger observatory. The paper quickly garnered attention as indicated by the “altmetric” summary below.

After the initial press release, more than 50,000 people signed up to the Crayfis project! That’s a great start. The Crayfis app for iOS and android are currently in beta testing and should be ready soon. I’ve joined this small project by helping develop the iOS app and the website, which are both a lot of fun. All you have to do is plug your phone in and set it camera down, probably at night when you are sleeping. If your phone thinks it has been hit by a cosmic ray, it will upload the data to the Crayfis servers. Later, we will look for groups of phones that were hit simultaneously, which indicates that its not just noise but a real cosmic ray shower.

The image below shows a screenshot of the live monitor of the Crayfis network so far — check it out, it’s fun to play with. As you can see Crayfis is already a world-wide network and may soon have the claim for the world’s largest detector.

Crayfis: A global network of smartphones

Crayfis: A global network of smartphones


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Kyle Cranmer is a Professor of physics and data science at New York University. His blog is at theoryandpractice.org.


Looking Forward to 2015: Analysis Techniques

Adam Davis
Tuesday, January 27th, 2015

With 2015 a few weeks old, it seems like a fine time to review what happened in 2014 and to look forward to the new year and the restart of data taking. Along with many interesting physics results, just to name a few, LHCb saw its 200th publication, a test of lepton universality. With protons about to enter the LHC, and the ALICE and LHCb detectors recording muon data from transfer line tests between the SPS and LHC (see also here), the start of data-taking is almost upon us. For some implications, see Ken Bloom’s post here. Will we find supersymmetry? Split Higgs? Nothing at all? I’m not going to speculate on that, but I would like to review two techniques which played a key role in two results from LHCb and a few analysis techniques which enabled them.

The first result I want to discuss is the \(Z(4430)^{-}\). The first evidence for this state came from the Belle Collaboration in 2007, with subsequent studies in 2009 and in 2013. BaBar also searched for the state, and while they did not see it, they did not rule it out.

The LHCb collaboration searched for this state, using the specific decay mode \(B^0\to \psi’ K^{+} \pi^{-} \), with \(\psi’\) decaying to two muons. For more reading, see the nice writeup from earlier in 2014. As in the Belle analyses, which looked using muons or electrons in the final \(\psi’\) state, the trick here is to look for bumps in the \(\psi’ \pi^{-}\) mass distribution. If a peak appears which is not described  by the conventional 2 and 3 quark states, mesons and baryons, we know and love, it must be from a state involving a \(c \overline{c}d\overline{u}\) quark combination. The search is performed in two ways: a model-dependent search, which looks at the \(K\pi\) and \(\psi’\pi\) invariant mass and decay angle distributions, and a “model independent” search which looks for structure induced in the \(K\pi\) system induced by a resonance in the \(\psi’\pi\) system and does not invoke any exotic resonances.

At the end of the day, it is found in both cases that the data are not described without including a resonance for the \(Z(4430)^-\).

Now, it appears that we have a resonance on our hands, but how can we be sure? In the context of the aforementioned model dependent analysis, the amplitude for the \(Z(4430)^{-}\) is modeled as a Breit-Wigner amplitude, which is a complex number. If this amplitude is plotted in the imaginary plane as a function of the invariant mass of the resonance, a circular shape is traced out. This is characteristic of a resonance. Therefore, by fitting the real and imaginary parts of the amplitude in six bins of \(\psi’\pi\) invariant mass, the shape can be directly compared to that of an exected resonance. That’s exactly what’s done in the plot below:

The argand plane for the Z(4430)- search. Units are arbitrary.

The argand plane for the Z(4430)- search. Units are arbitrary.

What is seen is that the data (black points) roughly follow the outlined circular shape given by the Breit-Wigner resonance (red). The outliers are pulled due to detector effects. The shape quite clearly follows the circular characteristic of a resonance. This diagram is called an Argand Diagram.


Another analysis technique to identify resonances was used to find the two newest particles by LHCb:

Depiction of the two Xi_b resonances found by the LHCb Collaboration. Credit to Italic Pig (http://italicpig.com/blog/)

Depiction of the two Xi_b resonances found by the LHCb Collaboration. Credit to Italic Pig

Or perhaps seen as


Xi_b resonances, depicted by Lison Bernet.

Xi_b resonances, depicted by Lison Bernet.

Any way that you draw them, the two new particles, the \(\Xi_b’^-\) and \(\Xi_b^{*-}\) were seen by the LHCb collaboration a few months ago. Notably, the paper was released almost 40 years to the day that the discovery of the \(J/\psi\) was announced, sparking the November Revolution, and the understanding that mesons and baryons are composed of quarks. The \(\Xi_b’^-\) and \(\Xi_b^{*-}\) baryons are yet another example of the quark model at work. The two particles are shown in \(\delta m \equiv m_{candidate}(\Xi_b^0\pi_s^-)-m_{candidate}(\Xi_b^0)-m(\pi)\) space below.

Xi_b'^- and Xi_b^{*-} mass peaks shown in delta(m_candidate) space.

\(\Xi_b’^-\) and \(\Xi_b^{*-}\) mass peaks shown in \(\delta(m_{candidate})\) space.

Here, the search is performed by reconstructing \(\Xi_b^0 \pi^-_s\) decays, where the \(\Xi_b^0\) decays to \(\Xi_c^+\pi^-\), and \(\Xi_c^+\to p K^- \pi^+\). The terminology \(\pi_s\) is only used to distinguish between that pion and the other pions. The peaks are clearly visible. Now, we know that there are two resonances, but how do we determine whether or not the particles are the \(\Xi_b’^-\) and \(\Xi_b^{*-}\)? The answer is to fit what is called the helicity distributions of the two particles.

To understand the concept, let’s consider a toy example. First, let’s say that particle A decays to B and C, as \(A\to B C\). Now, let’s let particle C also decay, to particles D and F, as \(C\to D F\). In the frame where A decays at rest, the decay looks something like the following picture.

Simple Model of A->BC, C->DF

Simple Model of \(A\to BC\), \(C\to DF\)

There should be no preferential direction for B and C to decay if A is at rest, and they will decay back to back from conservation of momentum. Likewise, the same would be true if we jump to the frame where C is at rest; D and F would have no preferential decay direction. Therefore, we can play a trick. Let’s take the picture above, and exactly at the point where C decays, jump to its rest frame. We can then measure the directions of the outgoing particles. We can then define a helicity angle \(\theta_h\) as the angle between the C flight in A’s rest frame and D’s flight in C’s rest frame. I’ve shown this in the picture below.

Helicity Angle Definition for a simple model

Helicity Angle Definition for a simple model

If there is no preferential direction of the decay, we would expect a flat distribution of \(\theta_h\). The important caveat here is that I’m not including anything about angular momentum, spin or otherwise, in this argument. We’ll come back to that later. Now, we can identify A as the \(\Xi_b’\) or \(\Xi_b^*\) candidate, C as the \(\Xi_b^0\) and D as the \(\Xi_C\) candidates used in the analysis. The actual data are shown below.

Helicity angle distributions for the Xi_b' and Xi_b* candidates (upper and lower, respectively).

Helicity angle distributions for the \(\Xi_b’ \)and \(\Xi_b*\) candidates (upper and lower, respectively).

While it appears that the lower mass may have variations, it is statistically consistent with being a flat line. Now the extra power of such an analysis is that if we now consider angular momentum of the particles themselves, there are implied selection rules which will alter the distributions above, and which allow for exclusion or validation of particle spin hypotheses simply by the distribution shape. This is the rationale for having the extra fit in the plot above. As it turns out, both distributions being flat allows for the identification of  the \(\Xi ‘_b^-\) and the \(\Xi_b^{*-}\), but do not allow for conclusive ruling out of other spins.

With the restart of data taking at the LHC almost upon us (go look on Twitter for #restartLHC), if you see a claim for a new resonance, keep an eye out for Argand Diagrams or Helicity Distributions.