Early Physics with the Large Hadron Collider Thomas J. LeCompte High Energy Physics DivisionArgonne National Laboratory JLAB Users’ Meeting: 16 June 2008
First Order of Business Thanks very much for the invitation.I’ve wanted to visit Jefferson Lab for a long time, both for the rich scientific program, and because Nate Isgur was very kind to me when I was an ignorant graduate student.
Second Order of Business The HEP community likes Mont.You will too.
Outline • The Standard Model • QCD • Electroweak Theory • The Large Hadron Collider and Why You Might Want One • The problem with Electroweak Theory • Detectors: ATLAS and CMS • The problem with QCD • More on the EWK problem • Summary
The Traditional Opening Pitch • Practically every HEP talk starts with this slide. • This isn’t the way I want to start this talk.
Both plots focus on the constituents of a thing, rather than their interactions. While there is meaning in both plots, it can be hard to see. A plot of a composition by A. Schoenberg would look different Comparing Two Figures A histogram of the notes used in Beethoven’s 5th Symphony, first movement. I’d like to come at this from a different direction.
Quantum Chromodynamics Quarks carry a charge called “color” carried by gluons which themselves also carry color charge. A strong force (in fact, THE strong force) Confines quarks into hadrons Electroweak Unification The electric force, the magnetic force and the weak interaction that mediates b-decay are all aspects of the same “electroweak” force. Only three constants enter into it: e.g. a, GF and sin2(qw). A chiral theory: it treats particles with left-handed spin differently than particles with right-handed spin. The Twin Pillars of the Standard Model A beautiful theory.Unfortunately, it’s broken.
Why Study The Standard Model? • Understanding it is a necessary precondition for discovering anything beyond the Standard Model • Whatever physics you intend to do in 2011, you’ll be studying SM physics in 2008 • Rate is also an issue • It’s interesting in and of itself • It’s predictive power remains extraordinary (e.g. g-2 for the electron) • We know it’s incomplete • It’s a low energy effective theory: can we see what lies beyond it? • We’ve lived with the SM for ~25 years • Long enough so that features we used to find endearing are starting to become annoying • Think of the LHC as “marriage counseling” for the SM
Local Gauge Invariance – Part I • In quantum mechanics, the probability density is the square of the wavefunction: P(x) = |Y|2 • If I change Y to –Y, anything I can observe remains unchanged • P(x) = |Y|2 can be perhaps better written as P(x) = YY* • If I change Y to Yeif anything I can observe still remains unchanged. • The above example was a special case (f = p) • If I can’t actually observe f, how do I know that it’s the same everywhere? • I should allow f to be a function, f(x,t). • This looks harmless, but is actually an extremely powerful constraint on the kinds of theories one can write down.
Local Gauge Invariance – Part II • The trouble comes about because the Schrödinger equation (and its descendents) involves derivatives, and a derivative of a product has extra terms. • At the end of the day, I can’t have any leftover f’s – they all have to cancel. (They are, by construction, supposed to be unobservable) • If I want to write down the Hamiltonian that describes two electrically charged particles, I need to add one new piece to get rid of the f’s: a massless photon.
Massless? • A massive spin-1 particle has three spin states (m = 1,0,-1) • A massless spin-1 particle has only two. • Hand-wavy argument: Massless particles move at the speed of light; you can’t boost to a frame where the spin points in another direction. • To cancel all the f’s, I need just the two m = ± 1 states (“degrees of freedom”) • Adding the third state overdoes it and messes up the cancellations • The photon that I add must be massless m = ±1 “transverse” m = 0 “longitudinal” Aside: this has to be just about the most confusing convention adopted since we decided that the current flows opposite to the direction of electron flow.We’re stuck with it now.
A Good Theory is Predictive…or at least Retrodictive • This is a theoretical tour-de-force: starting with Coulomb’s Law, and making it relativistically and quantum mechanically sound, and out pops: • Magnetism • Classical electromagnetic waves • A quantum mechanical photon of zero mass • Experimentally, the photon is massless (< 10-22me) • 10-22 = concentration of ten molecules of ethanol in a glass of water • Roughly the composition of “Lite” Beer • 10-22 = ratio of the radius of my head to the radius of the galaxy • 10-22 = probability Britney Spears won’t do anything shameless and stupid in the next 12 months
Let’s Do It Again • A Hamiltonian that describe electrically charged particles also gives you: • a massless photon J • A Hamiltonian that describes particles with color charge (quarks) also gives you: • a massless gluon (actually 8 massless gluons) J • A Hamiltonian that describes particles with weak charge also gives you: • massless W+, W- and Z0 bosons • Experimentally, they are heavy: 80 and 91 GeV L Why this doesn’t work out for the weak force – i.e. why the W’s and Z’s are massive – is what the LHC is trying to find out.
Nobody Wants A One Trick Pony • One goal: understand what’s going on with “electroweak symmetry breaking” • e.g. why are the W and Z heavy when the photon is massless • Another goal: probe the structure of matter at the smallest possible distance scale • Small l (=h/p) means high energy • Third goal: search for new heavy particles • This also means large energy (E=mc2) • Fourth goal: produce the largest number of previously discovered particles (top & bottom quarks, W’s, Z’s …) for precision studies “What is the LHC for?” is a little like “What is the Hubble Space Telescope for?” – the answer depends on who you ask.A multi-billion dollar instrument really needs to be able to do more than one thing. All of these require the highest energy we can achieve.
The Large Hadron Collider is a 26km long circular accelerator built at CERN, near Geneva Switzerland.The magnetic field is created by 1232 superconducting dipole magnets (plus hundreds of focusing and correction magnets) arranged in a ring in the tunnel. The Large Hadron Collider Design Collision Energy = 14 TeV
Thermal Expansion and the LHC means that the LHC should shrink ~50 feet in radius when cooled down. The tunnel is only about 10 feet wide.
ATLAS = A Toroidal LHC ApparatuS Length = 44mDiameter = 22mMass = 7000 t
How They Work • Particles curve in a central magnetic field • Measures their momentum • Particles then stop in the calorimeters • Measures their energy • Except muons, which penetrate and have their momenta measured a second time. Different particles propagate differently through different parts of the detector; this enables us to identify them.
The ATLAS Muon Spectrometer – One Practical Issue • We would like to measure a 1 TeV muon momentum to about 10%. • Implies a sagitta resolution of about 100 mm. • Thermal expansion is enough to cause problems. • Instead of keeping the detector in position, we let it flex: • It’s easier to continually measure where the pieces are than to keep it perfectly rigid. Beam’s eye view: d= 22m Pictures from Jim Shank, Boston University
CMS: The Other LHC “Large” Detector • Different detector technologies • e.g. iron core muon spectrometer vs. air core • Crystal calorimeter vs. liquid argon • Different design emphasis • e.g. their EM calorimeter is optimized more towards precise measurement of the signal; ATLAS is optimized more towards background rejection Similar in concept to ATLAS, but with a different execution.
The Problem with QCD Calculations can be extraordinarily difficult – many quantities we would like to calculate (e.g. the structure of the proton) need to be measured.
QCD vs. QED a changes by about 7% from Q=0 to Q=100 GeV. This will change the results of a calculation, but not the character of a calculation.
The Running of as • At high Q2, as is small, and QCD is in the perturbative region. • Calculations are “easy” • At low Q2, as is large, and QCD is in the non-perturbative region. • Calculations are usually impossible • Occasionally, some symmetry principle rescues you • Anything we want to know here must come from measurement From I. Hinchliffe – this contains data from several kinds of experiments: decays, DIS, and event topologies at different center of mass energies.
An Early Modern, Popular and Wrong View of the Proton • The proton consists of two up (or u) quarks and one down (or d) quark. • A u-quark has charge +2/3 • A d-quark has charge –1/3 • The neutron consists of just the opposite: two d’s and a u • Hence it has charge 0 • The u and d quarks weigh the same, about 1/3 the proton mass • That explains the fact that m(n) = m(p) to about 0.1% • Every hadron in the Particle Zoo has its own quark composition So what’s missing from this picture?
Energy is Stored in Fields • We know energy is stored in electric & magnetic fields • Energy density ~ E2 + B2 • The picture to the left shows what happens when the energy stored in the earth’s electric field is released • Energy is also stored in the gluon field in a proton • There is an analogous E2 + B2 that one can write down • There’s nothing unusual about the idea of energy stored there • What’s unusual is the amount: Thunder is good, thunder is impressive; but it is lightning that does the work. (Mark Twain)
The Modern Proton • 99% of the proton’s mass/energy is due to this self-generating gluon field • The two u-quarks and single d-quark • 1. Act as boundary conditions on the field (a more accurate view than generators of the field) • 2. Determine the electromagnetic properties of the proton • Gluons are electrically neutral, so they can’t affect electromagnetic properties • The similarity of mass between the proton and neutron arises from the fact that the gluon dynamics are the same • Has nothing to do with the quarks The Proton Mostly a very dynamic self-interacting field of gluons, with three quarks embedded. Like plums in a pudding.
The “Rutherford Experiment” of Geiger and Marsden The electric field the a experiences gets weaker and weaker as the a enters the Thomson atom, but gets stronger and stronger as it enters the Rutherford atom and nears the nucleus. aparticle scatters from source, off the gold atom target, and is detected by a detector that can be swept over a range of angles(n.b.) a particles were the most energetic probes available at the time
At angles as low as 3o, the data show a million times as many scatters as predicted by the Thomson model Textbooks often point out that the data disagreed with theory, but they seldom state how bad the disagreement was There is an excess of events with a large angle scatter This is a universal signature for substructure It means your probe has penetrated deep into the target and bounced off something hard and heavy An excess of large angle scatters is the same as an excess of large transverse momentum scatters Results of the Experiment
Proton Collisions: The Ideal World 1. Protons collide 2. Constituents scatter 3. As proton remnants separate
What Really Happens You don’t see the constituent scatter. You see a jet: a “blast” of particles, all going in roughly the same direction. 2 jets 2 jets 2 2 5 3 3 jets 5 jets Same Events, Tracking View Calorimeter View
Jets Initial quark Jet • The force between two colored objects (e.g. quarks) is ~independent of distance • Therefore the potential energy grows (~linearly) with distance • When it gets big enough, it pops a quark-antiquark pair out of the vacuum • These quarks and antiquarks ultimately end up as a collection of hadrons • We can’t calculate how often a jet’s final state is, e.g. ten p’s, three K’s and a L. • Fortunately, it doesn’t matter. • We’re interested in the quark or gluon that produced the jet. • Summing over all the details of the jet’s composition and evolution is A Good Thing. • Two jets of the same energy can look quite different; this lets us treat them the same What makes the measurement possible & useful is the conservation of energy & momentum.
Jets after “One Week” Jet Transverse Energy ATLAS 5 pb-1 of (simulated) data: corresponds to 1 week running at 1031 cm-2/s (1% of design) This is in units of transverse momentum. Remember, large angle = large pT
Number of events we expect to see: ~12 If new physics: ~50 Number we have seen to date worldwide: 0 Jets after “One Week” Jet Transverse Energy ATLAS New physics (e.g. quark substructure) shows up here. 5 pb-1 of (simulated) data: corresponds to 1 week running at 1031 cm-2/s (1% of design)
Outrunning the Bear • Present limits on 4-fermion contact interactions from the Tevatron are 2-4-2.7 TeV • This may hit 3 TeV by LHC turn-on • Depends on how many people work on this • If we shoot for 6 TeV at the LHC and only reach 5 TeV, we’ve already made substantial progress • Note that there are ~a dozen jets that are above the Tevatron’s kinematic limit: a day at the LHC will set a limit that the Tevatron can never reach.
The Big Asterisk • The first run will be at 10 TeV, not 14 TeV • Magnet training took longer than anticipated • CERN wisely decided to give the experiments something this year rather than to wait. • This increases the running time for a given sensitivity by a factor of 3-4 • A week’s worth of good data in a 2-3 month initial run is much more likely than a month’s worth
Compositeness & The Periodic Table(s) Arises because atoms have substructure: electrons Arises because hadrons have substructure: quarks The 9 lightest spin-0 particles The 8 lightest spin-1/2 particles
Variations on a Theme? • A good question – and one that the LHC would address • Sensitivity is comparable to where we found “the next layer down” in the past. • Atoms: nuclei (105:1) • Nuclei: nucleons (few:1) • Quarks (>104:1) will become (~105:1) • There are some subtleties: if this is substructure, its nature is different than past examples. Does this arise because quarks have substructure?
The Complication • Light quarks are…well, light. • Masses of a few MeV • Any subcomponents would be heavy • At least 1000 times heavier • Otherwise, we would have alreadydiscovered them • Therefore, they would have to bebound very, very deeply. (binding energy ~ their mass) A d-function potential has only one bound state – so the“particle periodic table” can’t be due to them being simply different configurations of the same components.Something new and interesting has to happen. I’m an experimenter. This isn’t my problem.
The Structure of the Proton Even if there is no new physics, the same kinds of measurements can be used to probe the structure of the proton. Because the proton is traveling so close to the speed of light, it’s internal clocks are slowed down by a factor of 7500 (in the lab frame) – essentially freezing it. We look at what is essentially a 2-d snapshot of the proton.
The Collision What appears to be a highly inelastic process: two protons produce two jets of other particles… (plus two remnants that go down the beam pipe) … is actually the elastic scattering of two constituents of the protons.
What looks to be an inelastic collision of protons is actually an elastic collision of partons: quarks and gluons. In an elastic collision, measuring the momenta of the final state particles completely specifies the momenta of the initial state particles. Different final states probe different combinations of initial partons. This allows us to separate out the contributions of gluons and quarks. Different experiments also probe different combinations. It’s useful to notate this in terms of x: x = p(parton)/p(proton) The fraction of the proton’s momentum that this parton carries This is actually the Fourier transform of the position distributions. Calculationally, leaving it this way is best. Parton Densities
Parton Density Functions in Detail • One fit from CTEQ and one from MRS is shown • These are global fits from all the data • Despite differences in procedure, the conclusions are remarkably similar • Lends confidence to the process • The biggest uncertainty is in the gluon • The gluon distribution is enormous: • The proton is mostly glue, not mostly quarks
Improving the Gluon: Direct Photons • DIS and Drell-Yan are sensitive to the quark PDFs. • Gluon sensitivity is indirect • The fraction of momentum not carried by the quarks must be carried by the gluon. • Antiquarks in the proton must be from gluons splitting • It would be useful to have a direct measurement of the gluon PDFs • This process depends on the (known) quark distributions and the (unknown) gluon distribution q g g q Direct photon “Compton” process.
Identifying Photons – Basics of Calorimeter Design Not too much or too little energy here. You want exactly one photon – not 0 (a likely hadron) or 2 (likely p0) Not too wide here. One photon and not two nearby ones (again, a likely p0) Not too much energy here. Indicative of a hadronic shower: probably a neutron or KL. A GEANT simulation of an electromagnetic shower A schematic of an electromagnetic shower
Direct Photons & Backgrounds CMS CMS • There are two “knobs we can turn” • Shower shape – does this look like a photon (last slide) • Isolation – if it’s a fake, it’s likely to be from a jet, and there is likely to be some nearby energy • Different experiments (and analyses in the same experiment) can rely more on one method than the other. After event selection Before event selection
More Variations on A Theme • One can scatter a gluon off of a heavy quark inthe proton as well as a light quark • This quark can be identified as a bottom or charmed quark by “tagging” the jet • This measures how much b (or c) is in the proton • Determines backgrounds to various searches, like Higgs • Turns out to have a surprisingly large impact on the ability to measure the W mass (ask me about this at the end, if interested) • Replace the g with a Z, and measure the same thing with different kinematics • Replace the Z with a W and instead of measuring how much charm is in the proton, you measure how much strangeness there is …and so on…
Double Parton Scattering • Two independent partons in the proton scatter: • Searches for complex signatures in the presence of QCD background often rely on the fact that decays of heavy particles are “spherical”, but QCD background is “correlated” • This breaks down in the case where part of the signature comes from a second scattering. • Probability is low, but needed background reduction can be high • We’re thinking about bbjj as a good signature • Large rate/large kinematic range • 105 more events than past experiments • Relatively unambiguous which jets go withwhich other jets. might be bettercharacterized by