How a Trigger Can Potentially Make or Break an LHC Discovery

Triggering is an essential part of the Large Hadron Collider [LHC]; there are so many collisions happening each second at the LHC, compared to the number that the experiments can afford to store for later study, that the data about most of the collisions (99.999%) have to be thrown away immediately, completely and permanently within a … Read more

Final Days of Busy Visit to CERN

I’m a few days behind (thanks to an NSF grant proposal that had to be finished last week) but I wanted to write a bit more about my visit to CERN, which concluded Nov. 21st in a whirlwind of activity. I was working full tilt on timely issues related to Run 2 of the Large Hadron Collider [LHC], currently scheduled to start early next May.   (You may recall the LHC has been shut down for repairs and upgrades since the end of 2012.)

A certain fraction of my time for the last decade has been taken up by concerns about the LHC experiments’ ability to observe new long-lived particles, specifically ones that aren’t affected by the electromagnetic or strong nuclear forces. (Long-lived particles that are affected by those forces are easier to search for, and are much more constrained by the LHC experiments.  More about them some other time.)

This subject is important to me because it is a classic example of how the trigger systems at LHC experiments could fail us — whereby a spectacular signal of a new phenomena could be discarded and lost in the very process of taking and storing the data! If no one thinks carefully about the challenges of finding long-lived particles in advance of running the LHC, we can end up losing a huge opportunity, unnecessarily. Fortunately some of us are thinking about it, but we are small in number. It is an uphill battle for those experimenters within ATLAS and CMS [the two general purpose experiments at the LHC] who are working hard to make sure they have the required triggers available. I can’t tell you how many times people within the experiments — even at the Naturalness conference I wrote about recently — have told me “such efforts are hopeless”… despite the fact that their own experiments have actually shown, already in public and in some cases published measurements (including this, this, this, this, this, and this), that it is not. Conversely, many completely practical searches for long-lived particles have not been carried out, often because there was no trigger strategy able to capture them, or because, despite the events having been recorded, no one at ATLAS or CMS has had time or energy to actually search through their data for this signal.

Now what is meant by “long-lived particles”?

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Our Survey of Exotic Decays of the Higgs is Done

After many months gestation and a difficult labor, a behemoth is born!  Yes, it’s done, finally: our 200 page tome entitled “Exotic Decays of the 125 GeV Higgs Boson“.  Written by thirteen hard-working theoretical particle physicists, this is a paper that examines a wide class of possible decays that our newly found Higgs particle might … Read more

Exotic Higgs Decays: Making the Case

For those of you curious about why posts have been a little sparse this month and have been wondering what I’ve been up to, here’s the latest on what has been an on-going story.

The Large Hadron Collider [LHC] is producing lots of new data, and the search for the Higgs particle continues at the ATLAS and CMS experiments. We’re still within Phase 1 of the search for the Higgs particle (I’ve described the two main phases here and in more detail here) in which the experiments are trying to discover unequivocally, or exclude unequivocally, the simplest possible form of the Higgs particle, which is called the Standard Model Higgs. Phase 1 is well along the way, the experimenters having excluded a Standard Model Higgs particle at any mass except a small range around 125 GeV/c2. Within that range there are hints that a Higgs particle might be showing up (see here, herehere and here). Some would say the evidence is significant and are quite convinced already, while others (quite a few of the experimentalists, and a few cautious theorists such as myself) would say the hints are not yet especially significant and are willing to let more data settle the issue. But everyone agrees there’s a very high chance the issue will be settled in 2012.

At that point, the Higgs search will move toward Phase 2. In fact, in some sense it is already in transition, because either there is a Standard Model Higgs particle with a mass near 125 GeV/c2, or the Higgs particle, if it exists, must be more exotic and complicated — two possibilities on which we can focus our planning.

One of the most important questions that will be asked (and is already being asked) is whether the particle that is showing up in the 2011 data (if it is really there in the first place) is a Standard Model Higgs or a look-alike Higgs particle that is actually more complicated in some way.

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