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Ken Bloom | USLHC | USA

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Double time

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!


Starting up LHC Run 2, step by step

Thursday, June 11th, 2015

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

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

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

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

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


Ramping up to Run 2

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!

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

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!

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!


2015: The LHC returns

Saturday, January 10th, 2015

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

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

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

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


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

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


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

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


Where the wind goes sweeping ’round the ring?

Thursday, October 23rd, 2014

I travel a lot for my work in particle physics, but it’s usually the same places over and over again — Fermilab, CERN, sometimes Washington to meet with our gracious supporters from the funding agencies.  It’s much more interesting to go someplace new, and especially somewhere that has some science going on that isn’t particle physics.  I always find myself trying to make connections between other people’s work and mine.

This week I went to a meeting of the Council of the Open Science Grid that was hosted by the Oklahoma University Supercomputing Center for Education and Research in Norman, OK.  It was already interesting that I got to visit Oklahoma, where I had never been before.  (I think I’m up to 37 states now.)  But we held our meeting in the building that hosts the National Weather Center, which gave me an opportunity to take a tour of the center and learn a bit more about how research in meteorology and weather forecasting is done.

OU is the home of the largest meteorology department in the country, and the center hosts a forecast office of the National Weather Service (which produces forecasts for central and western Oklahoma and northern Texas, at the granularity of one hour and one kilometer) and the National Severe Storms Laboratory (which generates storm watches and warnings for the entire country — I saw the actual desk where the decisions get made!).  So how is the science of the weather like and not like the science that we do at the LHC?

(In what follows, I offer my sincere apologies to meteorologists in case I misinterpreted what I learned on my tour!)

Both are fields that can generate significant amounts of data that need to be interpreted to obtain a scientific result.  As has been discussed many times on the blog, each LHC experiment records petabytes of data each year.  Meteorology research is performed by much smaller teams of observers, which makes it hard to estimate their total data volume, but the graduate student who led our tour told us that he is studying a mere three weather events, but he has more than a terabyte of data to contend with — small compared to what a student on the LHC might have to handle, but still significant.

But where the two fields differ is what limits the rate at which the data can be understood.  At the LHC, it’s all about the processing power needed to reconstruct the raw data by performing the algorithms that turn the voltages read out from millions of amplifiers into the energies and momenta of individual elementary particles.  We know what the algorithms for this are, we know how to code them; we just have to run them a lot.  In meteorology, the challenge is getting to the point where you can even make the data interpretable in a scientific sense.  Things like radar readings still need to be massaged by humans to become sensible.  It is a very labor-intensive process, akin to the work done by the “scanner girls” of the particle physics days of yore, who carefully studied film emulsions by eye to identify particle tracks.  I do wonder what the prospects are in meteorology for automating this process so that it can be handed off to humans instead.  (Clearly this has to apply more towards forefront research in the field about how tornadoes form and the like, rather than to the daily weather predictions that just tell you the likelihood of tornado-forming conditions.)

Weather forecasting data is generally public information, accessible by anyone.  The National Weather Service publishes it in a form that has already had some processing done on it so that it can be straightforwardly ingested by others.  Indeed, there is a significant private weather-forecasting industry that makes use of this, and sells products with value added to the NWS data.  (For instance, you could buy a forecast much more granular than that provided by the NWS, e.g. for the weather at your house in ten-minute intervals.)  Many of these companies rent space in buildings within a block of the National Weather Center.  The field of particle physics is still struggling with how to make our data publicly available (which puts us well behind many astronomy projects which make all of their data public within a few years of the original observations).  There are concerns about how to provide the data in a form that will allow people who are not experts to learn something from the data without making mistakes.  But there has been quite a lot of progress in this in recent years, especially as it has been recognized that each particle physics experiment creates a unique dataset that will probably never be replicated in the future.  We can expect an increasing number of public data releases in the next few years.  (On that note, let me point out the NSF-funded Data and Software Preservation for Open Science (DASPOS) project that I am associated with on its very outer edges, which is working on some aspects of the problem.)  However, I’d be surprised if anyone starts up a company that will sell new interpretations of LHC data!

Finally, here’s one thing that the weather and the LHC has in common — they’re both always on!  Or, at least we try to run the LHC for every minute possible when the accelerator is operational.  (Remember, we are currently down for upgrades and will start up again this coming spring.)  The LHC experiments have physicists on on duty 24 hours a day, monitoring data quality and ready to make repairs to the detectors should they be needed.  Weather forecasters are also on shift at the forecasting center and the severe-storm center around the clock.  They are busy looking at data being gathered by their own instruments, but also from other sources.  For instance, when there are reports of tornadoes near Oklahoma City, the local TV news stations often send helicopters out to go take a look.  The forecasters watch the TV news to get additional perspectives on the storm.

Now, if only the weather forecasters on shift could make repairs to the weather just like our shifters can fix the detector!


Teaming up on top and Higgs

Monday, October 6th, 2014

While the LHC experiments are surely turning their attention towards the 2015 run of the collider, at an energy nearly double that of the previous run, we’re also busy trying to finalize and publish measurements using the data that we already have in the can.  Some measurements just take longer than others, and some it took us a while to get to.  And while I don’t like tooting my own horn too much here at the US LHC blog, I wanted to discuss a new result from CMS that I have been working on with a student, Dan Knowlton, here at the University of Nebraska-Lincoln, along with collaborators from a number of other institutions.  It’s been in the works for so long that I’m thrilled to get it out to the public!

(This is one of many CMS results that were shown for the first time last week at the TOP 2014 conference.  If you look through the conference presentations, you’ll find that the top quark, which has been around for about twenty years now, has continued to be a very interesting topic of study, with implications for searches for new physics and even for the fate of the universe.  One result that’s particularly interesting is a new average of CMS top-quark mass measurements, which is now the most accurate measurement of that quantity in the world.)

The LHC experiments have studied the Higgs boson through many different Higgs decay modes, and many different production mechanisms also.  Here is a plot of the expected cross sections for different Higgs production mechanisms as a function of Higgs mass; of course we know now that the Higgs has a mass of 125 GeV:

The most common production mechanism has a Higgs being produced with nothing else, but it can also be produced in association with other particles.  In our new result, we search for a Higgs production mechanism that is so much more rare that it doesn’t even appear on the above plot!  The mechanism is the production of a Higgs boson in association with a single top quark, and in the standard model, the cross section is expected to be 0.018 pb, about an order of magnitude below the cross section for Higgs production in association with a top-antitop pair.  Why even bother to look for such a thing, given how rare it is?

The answer lies in the reason for why this process is so rare.  There are actually two ways for this particular final state to be produced. Here are the Feynman diagrams for them:


In one case, the Higgs is radiated off the virtual W, while in the other it comes off the real final-state top quark.  Now, this is quantum mechanics: if you have two different ways to connect an initial and final state, you have to add the two amplitudes together before you square them to get a probability for the process.  It just so happens that these two amplitudes largely destructively interfere, and thus the production cross section is quite small.  There isn’t anything deep at work (e.g. no symmetries that suppress this process), it’s just how it comes out.

At least, that’s how it comes out in the standard model.  We assume certain values for the coupling factors of the Higgs to the top and W particles that appear in the diagrams above.  Other measurements of Higgs properties certainly suggest that the coupling factors do have the expected values, but there is room within the constraints for deviations.  It’s even possible that one of the two coupling values has the exact opposite sign from what we expect.  In that case, the destructive interference between the two amplitudes would become constructive, and the cross section would be almost a factor of 13 larger than expected!

The new result from CMS is a search for this anomalous production of the Higgs in association with a single top quark.  CMS already has a result for a search in which the Higgs decays to pair of photons; this new result describes a search in which the Higgs decays to bottom quarks.  That is a much more common Higgs decay mode, so there ought to be more events to see, but at the same time the backgrounds are much higher.  The production of a top-antitop pair along with an extra jet of hadrons that is mis-identified as arising from a bottom quark looks very much like the targeted Higgs production mechanism.  The top-antitop cross section is about 1000 times bigger than that of the anomalous production mechanism that we are looking for, and thus even a tiny bottom mis-identification rate leads to a huge number of background events.  A lot of the work in the data analysis goes into figuring out how to distinguish the (putative) signal events from the dominant background, and then verifying that the estimations of the background rates are correct.

The analysis is so challenging that we predicted that even by throwing everything we had at it, the best we could expect to do was to exclude the anomalous Higgs production process at a level of about five times the predicted rate for it.  When we looked at the data, we found that we could exclude it at about seven times the anomalous rate, roughly in line with what we expected.  In short, we do not see an anomalous rate for anomalous Higgs production!  But we are able to set a fairly tight limit, at around 1.8 pb.

What do I like about this measurement?  First, it’s a very different way to try to measure the properties of the Higgs boson.  The measurements we have are very impressive given the amount of data that we have so far, but they are not very constraining, and there is enough wiggle room for some strange stuff to be going on.  This is one of the few ways to probe the Higgs couplings through the interference of two processes, rather than just through the rate for one dominant process.  All of these Higgs properties measurements are going to be much more accurate in next year’s data run, when we expect to integrate more data and all of the production rates will be larger due to the increase in beam energy.  (For this anomalous production process, the cross section will increase by about a factor of four.)  In this particular case, we should be able to exclude anomalous Higgs couplings through this measurement…or, if nature surprises us, we will actually observe them!  There is a lot of fun ahead for Higgs physics (and top physics) at the LHC.

I’ve also really enjoyed working with my CMS colleagues on this project.  Any measurement coming out of the experiment is truly the work of thousands of people who have built and operated the detector, gotten the data recorded and processed, developed and refined the reconstruction algorithms, and defined the baselines for how we identify all kinds of particles that are produced in the proton collisions.  But the final stages of any measurement are carried out by smaller groups of people, and in this case we worked with colleagues from the Catholic University of Louvain in Belgium, the Karlsruhe Institute of Technology in Germany, the University of Malaya in Malaysia, and the University of Kansas (in Kansas).  We relied on the efforts of a strong group of graduate students with the assistance of harried senior physicists like myself, and the whole team did a great job of supporting each other and stepping up to solve problems as they arose.  These team efforts are one of the things that I’m proud of in particle physics, and that make our scientists so successful in the wider world.


ICHEP at a distance

Friday, July 11th, 2014

I didn’t go to ICHEP this year.  In principle I could have, especially given that I have been resident at CERN for the past year, but we’re coming down to the end of our stay here and I didn’t want to squeeze in one more work trip during a week that turned out to be a pretty good opportunity for one last family vacation in Europe.  So this time I just kept track of it from my office, where I plowed through the huge volume of slides shown in the plenary sessions earlier this week.  It was a rather different experience for me from ICHEP 2012, which I attended in person in Melbourne and where we had the first look at the Higgs boson.  (I’d have to say it was also probably the pinnacle of my career as a blogger!)

Seth’s expectations turned out to be correct — there were no earth-shattering announcements at this year’s ICHEP, but still a lot to chew on.  The Standard Model of particle physics stands stronger than ever.  As Pauline wrote earlier today, the particle thought to be the Higgs boson two years ago still seems to be the Higgs boson, to the best of our abilities to characterize it.  The LHC experiments are starting to move beyond measurements of the “expected” properties — the dominant production and decay modes — into searches for unexpected, low-rate behavior.  While there are anomalous results here and there, there’s nothing that looks like more than a fluctuation.  Beyond the Higgs, all sectors of particle physics look much as predicted, and some fluctuations, such as the infamous forward-backward asymmetry of top-antitop production at the Tevatron, appear to have subsided.  Perhaps the only ambiguous result out there is that of the BICEP2 experiment which might have observed gravitational waves, or maybe not.  We’re all hoping that further data from that experiment and others will resolve the question by the end of the year.  (See the nice talk on the subject of particle physics and cosmology by Alan Guth, one of the parents of that field.)

This success of the Standard Model is both good and bad news.  It’s good that we do have a model that has stood up so well to every experimental test that we have thrown at it, in some cases to startling precision.  You want models to have predictive power.  But at the same time, we know that the model is almost surely incomplete.  Even if it can continue to work at higher energy scales than we have yet explored, at the very least we seem to be missing some particles (those that make up the dark matter we know exists from astrophysical measurements) and it also fails to explain some basic observations (the clear dominance of matter over antimatter in the universe).  We have high hopes for the next run of the LHC, which will start in Spring 2015, in which we will have higher beam energies and collision rates, and a greater chance of observing new particles (should they exist).

It was also nice to see the conference focus on the longer-term future of the field.  Since the last ICHEP, every region of the world has completed long-range strategic planning exercises, driven by recent discoveries (including that of the Higgs boson, but also of various neutrino properties) and anchored by realistic funding scenarios for the field.  There were several presentations about these plans during the conference, and a panel discussion featuring leaders of the field from around the world.  It appears that we are having a nice sorting out of which region wants to host which future facility, and when, in such a way that we can carry on our international efforts in a straightforward way.  Time will tell if we can bring all of these plans to fruition.

I’ll admit that I felt a little left out by not attending ICHEP this year.  But here’s the good news: ICHEP 2016 is in Chicago, one of the few places in the world that I can reach on a single plane flight from Lincoln.  I have marked my calendar!