
CERN starts first lead collisions in upgraded Large Hadron Collider - Tomte
http://arstechnica.com/science/2015/11/cern-starts-first-lead-collisions-in-upgraded-large-hadron-collider/
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raverbashing
What would be the advantages of only colliding electrons rather than bigger
nucleus?

Seems that since the collisions are more energetic the chance of new
discoveries would be increased, no?

Edit: one of the comments on the article mention " A much more useful measure
of the energy is the center of momentum energy per nucleon (either a proton or
neutron). This is the energy that would be available between a single
scattering of two nucleons. We only accelerate charged particles, so when we
collide Pb 208 which contains 82 protons at the equivalent of proton-proton
collisions at 13 TeV, have 13 TeV * 82 / 208 = 5.125 TeV/nucleon" so it seems
the energy per particle is smaller

~~~
untilted
An electron is a truly elementary particle. This means that in an e-e
collision, you know precisely the center-of-mass energy of the resulting
interaction. A proton, on the other hand, is _not_ a fundamental particle:
it's composed of quarks and gluons, so even if we know that the proton's
energy is 7 TeV, we don't know which fraction is carried by each of the
proton's constiuents. And since interactions happen between the constituents,
the center-of-mass energy is not fixed, and generally not known with high
precision.

It's therefore much better to probe for unknown phenomena using a hadron
collider (e.g., a proton-proton or ion-ion) since with a single beam energy
you can potentially produce interactions at a wide range of center-of-mass
energy. However, if you know the center-of-mass energy of the processe(s) you
need to study, a lepton (e.g. electron-electron) collider is more suitable
since you can precisely tune the center-of-mass energy of the interactions.

It's also possible to collide electrons with protons. This was done for
example at the HERA collider at DESY in Hamburg in the 90's/00's to study the
inner structure of the proton.

~~~
yakshemash
Not native speaker, so apologies for the bad english.

Just wanted to to add to untitled's comment: electron-on-proton collisions are
still being done at Jefferson Lab in Newport News, VA. It just doesn't get as
much press as CERN with its impressive PR machine (which I have nothing
against). The motivation is the same as at HERA: understand the structure of
protons, with the added possibility of polarizing the electron beam or the
proton target, which allows to access components of the nucleon structure that
are otherwise not accessible. As a bonus, they also have photon beams that
collide with proton target exciting different interaction channels.

To come back to gp's question, one can in principle collide anything with
anything, but the ideal tool really depends on what one wants to study.
Attaining ever higher energy is not necessarily the ultimate goal. Even though
it is useful to explore new boundaries, there are also a lot of unanswered
questions that can be addressed with much lower energy collisions. For example
the Jefferson Lab beam maxes out at 12 GeV, orders of magnitude lower than
that available at the LHC, but it's still an extremely useful machine.

Also the comment quoted by gp from the article is wrong. Even though only the
protons respond to the electromagnetic fields of the accelerator, the whole
nucleus is accelerated. Therefore if the pairs of protons attain a center-of-
mass energy of 13TeV, the same holds true for the neutrons, because (1)
neutrons have very similar mass as protons (2) neutrons are dragged along for
the ride, as they are bound to the protons by a force that is much much
stronger than that of the electromagnetic forces involved in acceleration. (It
is not true that only protons are accelerated). So the available energy per
pair of colliding nucleon (whether it is proton or neutron) is still 13TeV,
and the total energy potentially available in the center-of-mass system is 13
TeV * 208 = 2.704 PeV. However, not all of it necessarily released, since the
number of nucleon-nucleon collisions that can take place is dependent on the
area overlap between the two.

