mardi 9 septembre 2014

Shut-up and calculate* ... or converse before speculating ?

(A message of) the last of the pioneers of particle colliders

... I may be the last still around of the first generation of pioneers that brought colliding beam machines to reality.  I have been personally involved in building and using such machines since 1957 when I became part of the very small group that started to build the first of the colliders.   While the decisions on what to do next belong to the younger generation, the perspective of one of the old guys might be useful.  I see too little effort going into long range accelerator R&D, and too little interaction of the three communities needed to choose the next step, the theorists, the experimenters, and the accelerator people.  Without some transformational developments to reduce the cost of the machines of the future, there is a danger that we will price ourselves out of the market.
Burton Richter (Stanford University and SLAC National Accelerator Laboratory)
Wed, 3 Sep 2014

The high-energy colliders may not reach to heaven (and high-luminosity ones?)
In early 2015 the LHC will begin operations again at about 13 TeV compared to the 8-TeV operations before its recent shutdown for upgrading. 
The LHC itself is an evolving machine.  Its energy at its restart next year will be 13 TeV, slowly creeping up to its design energy of 14 TeV.  It will shut down in 2018 for some upgrades to detectors, and shut down again in 2022 to increase the luminosity.  It is this high-luminosity version (HL-LHC) that has to be compared to the potential of new facilities.  There has been some talk of doubling the energy of the LHC (HE-LHC) by replacing the 8-tesla magnets of the present machine with 16-tesla magnets, which would be relatively easy compared to the even more talked about bolder step to 100 TeV for the next project.  It is not clear to me why 30-TeV LHC excites so little interest, but that is the case.  
A large fraction of the 100 TeV talk (wishes?) comes from the theoretical community which is disappointed at only finding the Higgs boson at LHC and is looking for something that will be real evidence for what is actually beyond the standard model. Regrettably, there has been little talk so far among the three communities, experimenters, theorists, and accelerator scientists, on what constraints on the next generation are imposed by the requirement that the experiments actually produce analyzable data... 
The most important choice for a new, higher energy collider is its luminosity, which determines its discovery potential.  If a new facility is to have the same potential for discovery of any kind of new particles as had the old one, the new luminosity required is very roughly proportional to the square of the energy because cross sections typically drop as E-2.  A seven-fold increase in energy from that of HL-LHC to a 100-TeV collider therefore requires a fifty-fold increase in luminosity.  If the luminosity is not increased, save money by building a lower-energy machine where the discovery potential matches the luminosity.

String theorists ideas on physics might be popularized only in science fiction magazines ;-)
If you have seen the movie Particle Fever about the discovery of the Higgs boson, you have heard the theorists saying that the only choices today are between Super-symmetry and the Landscape.  Don’t believe them.  Super-symmetry says that every fermion has a boson partner and vice versa.  That potentially introduces a huge number of new arbitrary constants which does not seem like much progress to me.  However, in its simpler variants the number of new constants is small and a problem at high energy is solved.  But, experiments at the LHC already seem to have ruled out the simplest variants.    
The Landscape surrenders to perpetual ignorance.  It says that our universe is only one of a near infinity of disconnected universes, each with its own random collection of force strengths and constants, and we can never observe or communicate with the others.  We can never go further in understanding because there is no natural law that relates the different universes.  The old dream of deriving everything from one constant and one equation is dead.  There are two problems with the landscape idea.  The first is a logic one.  You cannot prove a negative, so you cannot say that there is no more to learn.  The second is practical.  If it is all random there is no point in funding theorists, experimenters, or accelerator builders.  We don’t have to wait until we are priced out of the market, there is no reason to go on 
There is a problem here that is new, caused by the ever-increasing mathematical complexity of today’s theory.  When I received my PhD in the 1950s it was possible for an experimenter to know enough theory to do her/his own calculations and to understand much of what the theorists were doing, thereby being able to choose what was most important to work on.  Today it is nearly impossible for an experimenter to do what many of yesterday’s experimenters could do, build apparatus while doing their own calculations on the significance of what they were working on.  Nonetheless, it is necessary for experimenters and accelerator physicists to have some understanding of where theory is, and where it is going.  Not to do so makes most of us nothing but technicians for the theorists.  Perhaps only the theory phenomenologists should be allowed to publish in general readership journals or to comment in movies. 

*A propos ... 

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