Heavy Ion Collisions at 574 TeV

The Large Hadron Collider has succeeded in colliding lead nuclei at a centre of mass energy of 574 TeV. That is by far the highest energy collisions ever seen in any particle accelerator exceeding previous records of the RHIC collider by a factor of 14.

The process of commissioning heavy Ion collisions at the CERN accelerator started less than three days ago so the speed with which they have started collisions is impressive. In fact the physical process of colliding heavy ions is almost identical to colliding protons. The lead nucleus has a charge of exactly 82 times the charge on a proton, but the curvature of a particle in a magnetic field depends only on the ratio of charge to energy. So if the lead ions are accelerated to exactly 82 times the energy of the protons they will follow the same path and the setup used to collide protons works just as well for ions.

The luminosities used last night were just a fraction of the records set with proton collisions a few days ago, but the lead ions have a larger cross-section so some interesting data may already have been collected in last night’s run which lasted three hours. Two bunches per beams were used to provide collisions in CMS, ATLAS and ALICE, but it is the ALICE detector that is best suited to studying the showers of thousands of particles that these collisions produce.

Here is a simulated picture of a collision in ALICE. Hopefully some real pictures will be available soon. Update: they are, see below.

Update: Collisions have been underway again this afternoon. Here is a picture of a collision from CMS. This was taken from the Live CMS fireworks display.

And another 3D picture from CMS

Here is a superb animation of a 3D recording of one of the first Heavy Ion events seen today in ALICE

More pictures from ALICE are here

15 Responses to Heavy Ion Collisions at 574 TeV

  1. Ulla says:

    There is a denser ‘ring’ some way from the center. What is that?

  2. Ulla says:

    “It (heavy ion physic) will be like for low luminosity proton-proton [running], in that the event rate will be relatively low,” explains Peter, “but imagine 100 or so collisions piled up on top of each other, for every single event: it looks quite complicated!”

    The number of particles produced in the collision of lead ions is still a hotly debated detail. “It turns out if you ask our community of theorists how many particles should come out, the answers vary by a factor of two to three,” says Peter. “For the proton-proton program, people were arguing over 20 per cent differences. For heavy ions though – all bets are off.”

    The problem is complicated by having to consider the fact that nucleons undergo more than one collision amongst the crowd of nucleons as they pass through the oncoming nucleus. And when they do, do they have the same chance of producing jets or mini-jets each time? “That’s the kind of hypothesis that will be tested,” Peter assures. “Within a few hours, we’ll have enough data to actually know.”

    How this ‘multiplicity’ depends on energy will likely be the first big result coming out of the programme. Following that, probing the bulk features – viscosity, temperature, density – of the now-infamous QGP medium is on the agenda. Studies from RHIC show that it reaches thermal equilibrium quickly. “So it very quickly turns from individual particles into a collective state of matter,” explains Peter.

    In this state, in which the constituent particles are strongly coupled, the system behaves like a tiny drop of liquid, in the sense that it expands geometrically in a way related to its original shape. “How the system responds to changing the shape is actually described by hydrodynamics, which seems surprising because hydrodynamics is not a theory of particles per se,” Peter considers. “So we care about things like the systematic changes induced as we change the geometry of the collision … and we’re very interested in characterising this ultra-dense matter with a variety of tools.”

    Measurements of photons, muons, and electrons – particles able to pass right through the QGP matter – will help to build a picture of what is going on inside it as it evolves.

  3. Bornerdogge says:

    There’s one small mistake: for now ions do not collide in LHCb…

    I don’t know if they plan to turn it on again in the next days.

    • Philip Gibbs says:

      You are right, I will correct it. I think LHCb is not a suitable detector for this kind of collision and with it off it is much easier to arrange the filling schemes. 4 bunches in each beam spaced evenly round the collider will give four collisions per turn in CMS, ALICE and ATLAS.

  4. Bill K says:

    “The Large Hadron Collider has succeeded in colliding lead nuclei at a centre of mass energy of 574 TeV.”

    Right, in other words half the 1100 TeV previously stated.

    “That is by far the highest energy collisions ever seen in any particle accelerator exceeding previous records of the RHIC collider by a factor of 6.”

    Even better than that. This is 1.38 TeV/nucleon/beam. The RHIC has reached 250 GeV/nucleon/beam, but only with polarized protons. When colliding Au ions, they get 100 GeV/nucleon/beam.

    “The process of commissioning heavy Ion collisions at the CERN accelerator started less than three days ago and the speed with which they have started collisions was not expected.”

    Yes it was. Thursday’s plan called for collisions by Saturday morning. They are right on schedule.

    “In fact the physical process of colliding heavy ions is almost identical to colliding protons.”

    No it’s not. The curvature of the path is the same, but otherwise quite different.
    – injection takes place thru LINAC3 and LEIR, not LINAC2.
    – larger energy loss from synchrotron radiation
    – large beam losses from peripheral nuclear collisions
    – larger beam-vacuum collisions
    – collision optics is different at IP2. Zero crossing angle (head-on) and smaller beta*.

    As far as LCHb goes, everything I’ve read indicates that LCHb is not planning to have ion collisions. Has this changed?

    • Lawrence B. Crowell says:

      Since Brehmstrahlung or synchrotoron radiation scales as 1/m^4 I would think there would be less synchrotron loss.

    • Philip Gibbs says:

      Bill, the 1100 TeV will only be reached when they have full energy. I already corrected that on the previous post.

      The differences you mentioned are real, but they do not effect the setup a great deal. I mean they do not need to go through the full process of moving the collimators for example.

      Good point that it is more like 14 times previous records, I will correct that.

    • Bill K says:

      Lawrence, I’m getting my information from the excellent chapter on ion collisions in the LHC specs,


      Regarding synchrotron radiation, they indicate that it depends on the charge as well, going as Z^5/A^4, making it about twice as much for heavy ions as for protons.

      • Lawrence B. Crowell says:

        This is a bit odd for this radiation goes as γ^n where for acceleration perpendicular to the velocity n = 4 and n = 6 for a || v. So for E = γmc^2 the scaling with mass follows, and this is consistent with A^{-4}. Yet the dependency with charge scales as q^2, where the Z^5 is a bit mysterious to me.

      • Bill K says:

        Lawrence, This is getting complicated, and I’m not even sure I can write a gamma! Well here goes.
        Liénard tells us the rate of energy loss due to synchrotron radiation is U ~ Z^2 γ^4. The key point is that the γ’s for proton and ion are different. Protons are 3.5 TeV while Pb ions are only 1.38 TeV/nucleon. The ratio here is 82/208 = Z/A. In other words

        γ{Pb} = Z/A γ{p}

        Consequently U{Pb} ~ Z^6/A^4 U{p}.

        On the other hand, the total energy E for the ion is ~ Aγ, so
        E{Pb} ~ Z E{p}. Divide one by the other and we find the synchrotron radiation damping time is τ = U/E ~ Z^5/A^4 as advertised.

      • Lawrence B. Crowell says:

        What you say makes sense, given the gamma for the Pb is different than the proton. This just looked counter intuitive for a moment. I have to confess that I am not that knowledgeable of accelerator physics.

    • Philip Gibbs says:

      The plan was to start ion physics with stable beams on Thursday. It is now Monday and already we have stable beams. I think that confirms that they are indeed ahead of schedule.

      With stable beams they will now be able to turn on the full tracker systems in ALICE. I hope we soon have some pictures of that from today’s run.

      We should also see some increases in luminosity using more bunches soon, but they don’t want to go too high because they certainly won’t want any pileup with these kind of collisions.

  5. Lawrence B. Crowell says:

    Heavy ion collisions have not only energy, but within transient moment there is a plasma of quarks and gluons with pressure. This is then a many body system or large N process which permits some phase change not as available for observation as a p-p collision.

    I hope to see some ALICE calorimetry images soon. It will be interesting to see how this matches with the apparent quark gluon plasma or glasma observed in the p-p events last September.


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