Tag Archives: quarks

A Few More Facts About Protons

[Reminder: I’ll be interviewed today at 5 p.m. Eastern time, at http://www.blogtalkradio.com/virtuallyspeaking/2012/02/15/matt-strassler-tom-levenson-virtually-speaking-science , 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 otherswon 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.  Should be fun!]

Since a number of readers were surprised to learn, from yesterday’s article about the benefits of increasing the energy of the protons at the Large Hadron Collider [LHC], that protons are very complicated and have a lot more in them than just two up quarks and a down quark, I thought I’d put up a plot or two that gives some indication of how particles are distributed inside a proton. Caution: the answers you get, and the physical intuition you obtain, depends in some subtle ways on exactly what you ask, so you should pay some attention to precisely which question I’m answering below. The details matter.

Two plots, differing only in the range for the vertical axis, showing the relative likelihood of striking a gluon or an up or down quark or antiquark carrying a fraction x of the proton's energy. At low x gluons dominate (and quarks and antiquarks become equally likely, and numerous, though far less so than gluons), while quarks dominate (but are very rare) at moderate x. Plotted using a Mathematica package (Trout and Olness, 2000) based on CTEQ5L results; somewhat out of date, but accurate enough for today's purposes.

The two plots in the Figure show exactly the same thing, just with a different vertical scale, so that certain things that are hard to see on one plot are clearer on the other. And what they show is this: if a proton is flying toward you in a Large Hadron Collider [LHC] proton beam, and you strike something inside that proton, how likely are you to have hit an up quark, or down quark, or gluon, or up antiquark, or down antiquark, that carries a fraction x of the proton’s energy? From these plots we can learn: Continue reading

The Benefits of 8 TeV Collisions Over 7 TeV.

Yesterday, a commenter asked me a very good question that I realized I hadn’t yet addressed on this site.  Answering it gives us a chance to look at real data from the Large Hadron Collider [LHC], and to see what differences will arise the machine’s energy is increased from 7 TeV to 8.

The protons that are smashed together at the LHC are made from many quarks, gluons and antiquarks. The proton-proton collisions take place at a definite energy: 7 TeV = 7000 GeV in 2011, 8 TeV = 8000 GeV  in 2012.  But what we’re mainly interested in — what can really create new physical phenomena for us to observe — are the collisions of a quark in one proton with an antiquark in the other proton, or the collision of two gluons, etc. These “mini-collisions” carry only a fraction — typically a very small fraction — of the total proton-proton collision energy. How high a fraction can they carry?  and what are the motivations for increasing the energy from 7 TeV per collision to 8 TeV?  Click here for the answer.