Of Particular Significance

Why a Lightweight Higgs is a Sensitive Creature — Part 2

POSTED BY Matt Strassler

POSTED BY Matt Strassler

ON 02/13/2012

[Note added:  It is official — as expected, at this year’s Chamonix workshop, where the Large Hadron Collider’s [LHC’s] future is planned out each year, it was decided that the LHC’s energy will be increased by 14% next year (from 3.5 TeV energy per proton and 7 TeV energy per collision in 2010-2011 to 4 TeV per proton and 8 per collision.) Also the time between collisions will remain at 50 nanoseconds.  I’ll have some things to say about the pros and cons of this decision, in particular the challenges for the experiments, over the next few days.]

On Monday last week, I gave you half the explanation as to why a lightweight Higgs particle is a sensitive creature, one that is easily altered by new phenomena — by particles and/or forces that we might not yet know about.  It all had to do with an analogy between a violin string (or a guitar string or a xylophone key) and the properties of the Higgs particle.   Today, on the same webpage as the first half, I have provided the second half of the story. (If you have already read the first half, just look for the boldface words “The Diverse Modes of a Higgs’ Demise”, which separate last week’s prose from the new stuff.)  I’ve also added, for particle physicists and for those laypersons who want to go a little deeper, a short quantitative discussion of my main points.

Also: I will have the honor to be interviewed on Wednesday at 5 p.m. Eastern time, at


which you can listen to either live or later.  My interviewer, Tom Levenson, is an eminent science journalist who has written fascinating and surprising books on Einstein and on Newton, among others, won awards for his work on television (e.g. NOVA), has a great blog (and also posts here), and is a professor of science writing at MIT.  In short, he’s a bright and interesting dude whom you should consider following on Twitter, or in whatever way floats your boat in the ocean of social media.  For this reason I suspect that the conversation is going to be a lot deeper and more interesting than the average interview, with the interviewer making at least as many interesting comments about the topic as the interviewee.

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3 Responses

  1. What is amazing about the Higgs business in general is that either knows how to hide – or – perhaps we are looking in a wrong place. If one kind of Higgs were at or around 125 GeV – well the top quark has shown to be heavier and we discovered it. . That would only tell us that Higgs must be one precarious character we are dealing with, since we should have enough energy at our disposal to notice it. After suffusing the vacuum with weak charge and breaking the symmetry in the process, it is as if it dressed up into a costume and we are watching Verdi’s Un ballo in maschera ..

  2. Great job Matt! I really appreciate you taking the time you do. Increasing the LHC energy got me wondering…Because LHC uses Protons and the Protons consist of 2up & 1down quark (plus gluons, etc) not all 8 TeV goes into the products of the collision right? But exactly what percentage does? In other words, what is the upper energy range the LHC can actually explore operating at 8 TeV? Is there a fixed number that 8 TeV should be divided by to get upper energy limit of the products?

    1. Well, it is really important here that a proton is not just made from two up quarks and one down quark — that rather it is made from a huge number of gluons, a huge number of quarks and antiquarks (one antiquark for each quark), and finally, two additional up quarks and one additional down quark. For those who haven’t done so, read my explanation of this here:


      which gives you a more accurate picture of a proton than do many websites.]

      Most proton-proton collisions at the LHC involve the mini-collisions of gluons with each other, or of gluons with quarks or antiquarks. Quark-quark collisions are not the dominant ones observed… which is a good thing, since the dominant production of the Higgs particle is in gluon-gluon mini-collisions.

      The probability that the mini-collision of a quark or gluon or antiquark from one proton with another quark or gluon or antiquark from the other proton will have an energy E falls rapidly as you consider larger and larger values of E. But it does so smoothly; there is no sharp cut-off, and no a sharp upper limit (other than 8 TeV).

      The rare high-energy collisions with the very highest E are most likely to be quark-quark collisions. But most quark-quark collisions are boring (though there is the interesting process quark + quark –> quark quark Higgs )

      What we typically do is this: we ask, “if we have this amount of data, and we look for that particle, how massive a version of that particle would be we able to discover with this amount of data?” And the answer is different for each type of particle, making the story complicated. Many types of particles can be discovered in 2012 up to 2 or 3 TeV. Others can only be discovered up to a half a TeV.

      I’ve decided this deserves an article; stay tuned for it.

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