Tag Archives: cms

At the Naturalness 2014 Conference

Greetings from the last day of the conference “Naturalness 2014“, where theorists and experimentalists involved with the Large Hadron Collider [LHC] are discussing one of the most widely-discussed questions in high-energy physics: are the laws of nature in our universe “natural” (= “generic”), and if not, why not? It’s so widely discussed that one of my concerns coming in to the conference was whether anyone would have anything new to say that hadn’t already been said many times.

What makes the Standard Model’s equations (which are the equations governing the known particles, including the simplest possible Higgs particle) so “unnatural” (i.e. “non-generic”) is that when one combines the Standard Model with, say, Einstein’s gravity equations. or indeed with any other equations involving additional particles and fields, one finds that the parameters in the equations (such as the strength of the electromagnetic force or the interaction of the electron with the Higgs field) must be chosen so that certain effects almost perfectly cancel, to one part in a gazillion* (something like 10³²). If this cancellation fails, the universe described by these equations looks nothing like the one we know. I’ve discussed this non-genericity in some detail here.

*A gazillion, as defined on this website, is a number so big that it even makes particle physicists and cosmologists flinch. [From Old English, gajillion.]

Most theorists who have tried to address the naturalness problem have tried adding new principles, and consequently new particles, to the Standard Model’s equations, so that this extreme cancellation is no longer necessary, or so that the cancellation is automatic, or something to this effect. Their suggestions have included supersymmetry, warped extra dimensions, little Higgs, etc…. but importantly, these examples are only natural if the lightest of the new particles that they predict have masses that are around or below 1 TeV/c², and must therefore be directly observable at the LHC (with a few very interesting exceptions, which I’ll talk about some other time). The details are far too complex to go into here, but the constraints from what was not discovered at LHC in 2011-2012 implies that most of these examples don’t work perfectly. Some partial non-automatic cancellation, not at one part in a gazillion but at one part in 100, seems to be necessary for almost all of the suggestions made up to now.

So what are we to think of this? Continue reading

Day 2 At CERN

Day 2 of my visit to CERN (host laboratory of the Large Hadron Collider [LHC]) was a pretty typical CERN day for me. Here’s a rough sketch of how it panned out:

  • 1000: after a few chores, arrived at CERN by tram. Worked on my ongoing research project #1. Answered an email about my ongoing research project #2.
  • 1100: attended a one hour talk, much of it historical, by Chris Quigg, one of the famous experts on “quarkonium” (atom-like objects made from a quark or anti-quark, generally referring specifically to charm and bottom quarks). Charmonium (charm quark/antiquark atoms) was discovered 40 years ago this week, in two very different experiments.
  • 1200: Started work on the talk that I am giving on the afternoon of Day 3 to some experimentalists who work at ATLAS. [ATLAS and CMS are the two general-purpose experimental detectors at the LHC; they were used to discover the Higgs particle.] It involves some new insights concerning the search for long-lived particles (hypothesized types of new particles that would typically decay only after having traveled a distance of at least a millimeter, and possibly a meter or more, before they decay to other particles.)
  • 1230: Working lunch with an experimentalist from ATLAS and another theorist, mainly discussing triggering, and other related issues, concerning long-lived particles. Learned a lot about the new opportunities that ATLAS will have starting in 2015.
  • 1400: In an extended discussion with two other theorists, got a partial answer to a subtle question that arose in my research project #2.
  • 1415: Sent an email to my collaborators on research project #2.
  • 1430: Back to work on my talk for Day 3. Reading some relevant papers, drawing some illustrations, etc.
  • 1600: Two-hour conversation over coffee with an experimentalist from CMS, yet again about triggering, regarding long-lived particles, exotic decays of the Higgs particle, and both at once. Learned a lot of important things about CMS’s plans for the near-term and medium-term future, as well as some of the subtle issues with collecting and analyzing data that are likely to arise in 2015, when the LHC begins running again.

[Why triggering, triggering, triggering? Because if you don’t collect the data in the first place, you can’t analyze it later!  We have to be working on triggering in 2014-2015 before the LHC takes data again in 2015-2018]

  • 1800: An hour to work on the talk again.
  • 1915: Skype conversation with two of my collaborators in research project #1, about a difficult challenge which had been troubling me for over a week. Subtle theoretical issues and heavy duty discussion, but worth it in the end; most of the issues look like they may be resolvable.
  • 2100: Noticed the time and that I hadn’t eaten dinner yet. Went to the CERN cafeteria and ate dinner while answering emails.
  • 2130: More work on the talk for Day 3.
  • 2230: Left CERN. Wrote blog post on the tram to the hotel.
  • 2300: Went back to work in my hotel room.

Day 1 was similarly busy and informative, but had the added feature that I hadn’t slept since the previous day. (I never seem to sleep on overnight flights.) Day 3 is likely to be as busy as Day 2. I’ll be leaving Geneva before dawn on Day 4, heading to a conference.

It’s a hectic schedule, but I’m learning many things!  And if I can help make these huge and crucial experiments more powerful, and give my colleagues a greater chance of a discovery and a reduced chance of missing one, it will all be worth it.

Off to CERN

After a couple of months of hard work on grant writing, career plans and scientific research, I’ve made it back to my blogging keyboard.  I’m on my way to Switzerland for a couple of weeks in Europe, spending much of the time at the CERN laboratory. CERN, of course, is the host of the Large Hadron Collider [LHC], where the Higgs particle was discovered in 2012. I’ll be consulting with my experimentalist and theorist colleagues there… I have many questions for them. And I hope they’ll have many questions for me too, both ones I can answer and others that will force me to go off and think for a while.

You may recall that the LHC was turned off (as planned) in early 2013 for repairs and an upgrade. Run 2 of the LHC will start next year, with protons colliding at an energy of around 13 TeV per collision. This is larger than in Run 1, which saw 7 TeV per collision in 2011 and 8 TeV in 2012.  This increases the probability that a proton-proton collision will make a Higgs particle, which has a mass of 125 GeV/c², by about a factor of 2 ½.  (Don’t try to figure that out in your head; the calculation requires detailed knowledge of what’s inside a proton.) The number of proton-proton collisions per second will also be larger in Run 2 than in Run 1, though not immediately. In fact I would not be surprised if 2015 is mostly spent addressing unexpected challenges. But Run 1 was a classic: a small pilot run in 2010 led to rapid advances in 2011 and performance beyond expectations in 2012. It’s quite common for these machines to underperform at first, because of unforeseen issues, and outperform in the long run, as those issues are solved and human ingenuity has time to play a role. All of which is merely to say that I would view any really useful results in 2015 as a bonus; my focus is on 2016-2018.

Isn’t it a bit early to be thinking about 2016? No, now is the time to be thinking about 2016 triggering challenges for certain types of difficult-to-observe phenomena. These include exotic, unexpected decays of the Higgs particle, or other hard-to-observe types of Higgs particles that might exist and be lurking in the LHC’s data, or rare decays of the W and Z particle, and more generally, anything that involves a particle whose (rest) mass is in the 100 GeV/c² range, and whose mass-energy is therefore less than a percent of the overall proton-proton collision energy. The higher the collision energy grows, the harder it becomes to study relatively low-energy processes, even though we make more of them. To be able to examine them thoroughly and potentially discover something out of place — something that could reveal a secret worth even more than the Higgs particle itself — we have to become more and more clever, open-minded and vigilant.

Some Higgs News from the LHCP Conference

Some news on the Higgs particle from the ATLAS and CMS experiments, the two general purpose experiments at the Large Hadron Collider. I just mention a few highlights. Continue reading

Higgs Experts: A Small But Important Correction to a Previous Post

I have to admit that this post is really only important for experimentalists interested in searching for non-Standard Model decays of the Higgs particle.  I try to keep these technical posts very rare, but this time I do need to slightly amend a technical point that I made in an article a few weeks ago. Continue reading

A 100 TeV Proton-Proton Collider?

During the gap between the first run of the Large Hadron Collider [LHC], which ended in 2012 and included the discovery of the Higgs particle (and the exclusion of quite a few other things), and its second run, which starts a year from now, there’s been a lot of talk about the future direction for particle physics. By far the most prominent option, both in China and in Europe, involves the long-term possibility of a (roughly) 100 TeV proton-proton collider — that is, a particle accelerator like the LHC, but with 5 to 15 times more energy per collision.

Do we need such a machine? Continue reading

Could the Higgs Decay to New Z-like Particles?

Today I’m continuing with my series, begun last Tuesday (click here for more details on the project), on the possibility that the Higgs particle discovered 18 months ago might decay in unexpected ways.

I’ve finished an article describing how we can, with current and with future Large Hadron Collider [LHC] data, look for a Higgs particle decaying to two new spin one particles, somewhat similar to the Z particle, but with smaller mass and much weaker interactions with ordinary matter.  [For decays to spin zero particles, click here.] Just using existing published plots on LHC events with two lepton/anti-lepton pairs, my colleagues and I, in our recent paper, were able to put strong limits on this scenario: for certain masses, decays to the new particles can occur in at most one in a few thousand Higgs particles.  The ATLAS and CMS experiments could certainly do better, perhaps even to the point of making a discovery with existing data, if this process is occurring in nature.

The Higgs could decay to two new spin-one particles, here labelled ZD, which in turn could each produce a lepton/anti-lepton pair.  The resulting signature would be spectacular, but neither ATLAS nor CMS has done a optimizal search for this signal covering the full allowed ZD mass range.

The Higgs could decay to two new spin-one particles, here labelled ZD, which in turn could each produce a lepton/anti-lepton pair (e = electron, μ = muon). The resulting signature would be spectacular, but neither ATLAS nor CMS has yet published an optimal search for this signal across the full allowed ZD mass range.

You might wonder how particle physicists could have missed a particle with a mass lower than that of the Z particle; wouldn’t we already have observed it? A clue as to how this can occur: it took much longer to discover the muon neutrino than the muon, even though the neutrino has a much lower mass. Similarly, it took much longer to discover the Higgs particle than the top quark, even though the Higgs has a lower mass. Why did this happen?

It happened because muon neutrinos interact much more weakly with ordinary matter than do muons, and are therefore much harder to produce, measure and study than are muons. Something similar is true of the Higgs particle compared to the top quark; although the top quark is nearly 50% heavier than the Higgs, the Large Hadron Collider [LHC] produces 20 times as many top quarks and anti-quarks as Higgs particles, and the signature of a top quark is usually more distinctive. So new low-mass particles to which the Higgs particle can perhaps decay could easily have been missed, if they interact much more weakly with ordinary matter than do the Z particle, top quark, bottom quark, muon, etc.

The muon neutrino was discovered not because these neutrinos were directly produced in collisions of ordinary matter but rather because muons were first produced, and these then decayed to muon neutrinos (plus an electron and an electron anti-neutrino).  Similarly, new particles may be discovered not because we produce them directly in ordinary matter collisions, but because, as in the above figure, we first produce a Higgs particle in proton-proton collisions at the LHC, and the Higgs may then in turn decay to them.

I should emphasize that direct searches for these types of new particles are taking place, using both old and new data from a variety of particle physics machines (here’s one example.) But it is often the case that these direct searches are not powerful enough to find the new particles, at least not soon, and therefore they may first show up in unexpected exotic decays of the Higgs… especially since the LHC has already produced a million Higgs particles, most of them at the ATLAS and CMS experiments, with a smaller fraction at LHCb.

I hope that some ATLAS and CMS experimenters are looking for this signal… and that we’ll hear results at the upcoming Moriond conference.

More Examples of Possible Unexpected Higgs Decays

As I explained on Tuesday, I’m currently writing articles for this website that summarize the results of a study, on which I’m one of thirteen co-authors, of various types of decays that the newly-discovered Higgs particle might exhibit, with a focus on measurements that could be done now with 2011-2012 Large Hadron Collider [LHC] data, or very soon with 2015-2018 data.  See Tuesday’s post for an explanation of what this is all about.

On Tuesday I told you I’d created a page summarizing what we know about possible Higgs decays to two new spin-zero particles, which in turn decay to quark pairs or lepton pairs according to our general expectation that heavier particles are preferred in spin-zero-particle decays. A number of theories (including models with more Higgs particles, certain non-minimal supersymmetric models, some Little Higgs models, and various dark matter models) predict this possibility.

Today I’ve added to that page (starting below figure 4) to include possible Higgs decays to two new spin-zero particles which in turn decay to gluon or photon pairs, according to our general expectation that, if the new spin-zero particles don’t interact very strongly with quarks or leptons, then they will typically decay to the force particles, with a rate roughly related to the strengths of the corresponding forces.  While fewer known theories directly predict this possibility compared to the one in the previous paragraph, the ease of looking for Higgs particles decaying to four photons motivates an attempt to do so in current data.

I have a few other classes of Higgs particle exotic decays to cover, so more articles on this subject will follow shortly!