Early Physics with the Large Hadron Collider Thomas J. LeCompte High Energy Physics Division Argonne National Laboratory JLAB Users Meeting: 16 June 2008 First Order of Business Thanks very much for the invitation. Ive 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. 2 Second Order of Business
The HEP community likes Mont. You will too. 3 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 4 The Traditional Opening Pitch
Practically every HEP talk starts with this slide. This isnt the way I want to start this talk. 5 Comparing Two Figures Notes Used in Symphony #5 350 300 A histogram of the notes used in Beethovens 5th Symphony, first movement. 250 200 150
100 50 0 A Bb B C C# D Eb E F
F# G Ab 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 Id like to come at this from a different direction. 6 The Twin Pillars of the Standard Model 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 -decay are all decay are all aspects of the same electroweak force. Only three constants enter into it: e.g. , GF and sin2(w). A chiral theory: it treats particles with left-decay are all handed spin differently than particles with right-decay are all handed spin. A beautiful theory. Unfortunately, its broken. 7 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, youll be studying SM physics in 2008 Rate is also an issue Its interesting in and of itself Its predictive power remains extraordinary (e.g. g-decay are all 2 for the electron) We know its incomplete Its a low energy effective theory: can we see what lies beyond it? Weve 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 8 Local Gauge Invariance Part I In quantum mechanics, the probability density is the square of the wavefunction: P(x) = ||2 If I change to , anything I can observe remains unchanged
P(x) = ||2 can be perhaps better written as P(x) = * If I change to ei anything I can observe still remains unchanged. The above example was a special case ( = ) If I cant actually observe , how do I know that its the same everywhere? I should allow to be a function, (x,t). This looks harmless, but is actually an extremely powerful constraint on the kinds of theories one can write down. 9 Local Gauge Invariance Part II The trouble comes about because the Schrdinger equation (and its descendents) involves derivatives, and a derivative of a product has extra terms. d dv du uv u v dx
dx dx At the end of the day, I cant have any leftover 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 s: a massless photon. 10 Massless? A massive spin-decay are all 1 particle has three spin states (m = 1,0,-decay are all 1) A massless spin-decay are all 1 particle has only two. Hand-decay are all wavy argument: Massless particles move at the speed of light; you cant boost to a frame where the spin points in another direction. To cancel all the 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. Were stuck with it now. 11 A Good Theory is Predictiveor at least Retrodictive This is a theoretical tour-decay are all de-decay are all force: starting with Coulombs 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-decay are all 22me) 10-decay are all 22 = concentration of ten molecules of ethanol in a glass of water Roughly the composition of Lite Beer 10-decay are all 22 = ratio of the radius of my head to the radius of the galaxy 10-decay are all 22 = probability Britney Spears wont do anything shameless and stupid in the next 12 months 12 Lets Do It Again A Hamiltonian that describe electrically charged particles also gives you: a massless photon A Hamiltonian that describes particles with color charge (quarks) also gives you: a massless gluon (actually 8 massless gluons) A Hamiltonian that describes particles with weak charge also gives you: massless W+, W-decay are all and Z0 bosons Experimentally, they are heavy: 80 and 91 GeV
Why this doesnt work out for the weak force i.e. why the Ws and Zs are massive is what the LHC is trying to find out. 13 Nobody Wants A One Trick Pony One goal: understand whats 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 (=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, Ws, Zs ) 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-decay are all billion dollar instrument really needs to be able to do more than one thing. All of these require the highest energy we can achieve. 14 The Large Hadron Collider Design Collision Energy = 14 TeV 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. 15 Thermal Expansion and the LHC x T means that the LHC should shrink ~50 feet in radius when cooled down. x The tunnel is only about 10 feet wide. 16 ATLAS = A Toroidal LHC ApparatuS Length = 44m Diameter = 22m Mass = 7000 t
17 CMS = Compact Muon Solenoid 18 How They Work Particles curve in a central magnetic field p Measures their r qB momentum Particles then stop in the calorimeters Measures their energy Different particles propagate differently through different parts of the detector; this enables us to identify them.
Except muons, which penetrate and have their momenta measured a second time. 19 ATLAS Revisited 20 What ATLAS Looks Like Today 21 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 m.
Thermal expansion is enough to cause problems. x x T x T 0.2 K x Beams eye view: d= 22m Instead of keeping the detector in position, we let it flex: Its easier to continually measure where the pieces are than to keep it perfectly rigid. Pictures from Jim Shank, Boston University 22
CMS: The Other LHC Large Detector Different detector technologies e.g. iron core muon spectrometer vs. air core Crystal calorimeter vs. liquid argon Similar in concept to ATLAS, but with a different execution. Different design emphasis e.g. their EM calorimeter is optimized more towards precise measurement of the signal; ATLAS is optimized more
towards background rejection 23 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. 24 QCD vs. QED QED QCD Symmetry Group U(1)
SU(3) Charge Electric charge Three kinds of color Force carrier 1 Photon neutral 8 Gluons -decay are all colored Coupling strength 1/137 (runs slowly) ~1/6 (runs quickly)
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. 25 The Running of s At high Q2, s is small, and QCD is in the perturbative region. Calculations are easy At low Q2, s is large, and QCD is in the non-decay are all 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. 26 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-decay are all quark has charge +2/3 A d-decay are all quark has charge 1/3 The neutron consists of just the opposite: two ds 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 whats missing from this picture?
27 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 earths 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 Theres nothing unusual about the idea of energy stored there Whats unusual is the amount: Energy stored in the field Thunder is good, thunder is impressive; but it is lightning that does the work. (Mark Twain)
Atom 10-decay are all 8 Nucleus 1% Proton 99% 28 The Modern Proton 99% of the protons mass/energy is due to this self-decay are all generating gluon field The Proton Mostly a very dynamic self-decay are all interacting field of
gluons, with three quarks embedded. The two u-decay are all quarks and single d-decay are all 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 cant 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 Like plums in a pudding. 29 The Rutherford Experiment of Geiger and Marsden
particle 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.) particles were the most energetic probes available at the time The electric field the experiences gets weaker and weaker as the enters the Thomson atom, but gets stronger and stronger as it enters the Rutherford atom and nears the nucleus. 30 Results of the Experiment Geiger-Marsden Results Scattering (arbitrary units)
100 1 Data Thomson Model 0.01 0.0001 1E-6 1E-8 1E-10 0 1 2 3 4
5 Degrees 6 7 8 9 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 31 Proton Collisions: The Ideal World 1. Protons collide 2. Constituents scatter 3. As proton remnants separate 32
What Really Happens You dont 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 3 jets 5 jets 3 5 Calorimeter View
Same Events, Tracking View 33 Jets Initial quark 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-decay are all antiquark pair out of the vacuum These quarks and antiquarks ultimately end up as a collection of hadrons We cant calculate how often a jets
final state is, e.g. ten s, three Ks and a . Jet Fortunately, it doesnt matter. Were interested in the quark or gluon that produced the jet. Summing over all the details of the jets 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. 34 Jets after One Week
Jet Transverse Energy ATLAS 5 pb-decay are all 1 of (simulated) data: corresponds to 1 week running at 1031 cm-decay are all 2/s (1% of design) This is in units of transverse momentum. Remember, large angle = large pT 35 Jets after One Week Jet Transverse Energy ATLAS 5 pb-decay are all 1 of (simulated) data: corresponds to 1 week running at 1031 cm-decay are all 2/s (1% of design) New physics (e.g. quark substructure) shows up
here. Number of events we expect to see: ~12 If new physics: ~50 Number we have seen to date worldwide: 0 36 Outrunning the Bear Present limits on 4-decay are all fermion contact interactions from the Tevatron are 2-decay are all 4-decay are all 2.7 TeV This may hit 3 TeV by LHC turn-decay are all on Depends on how many people work on this If we shoot for 6 TeV at the LHC and only reach 5 TeV, weve already made substantial progress
Note that there are ~a dozen jets that are above the Tevatrons kinematic limit: a day at the LHC will set a limit that the Tevatron can never reach. 37 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-decay are all 4 A weeks worth of good data in a 2-decay are all 3 month initial run is much more likely than a months worth
38 Compositeness & The Periodic Table(s) Arises because atoms have substructure: electrons Arises because hadrons have substructure: quarks The 9 lightest spin-decay are all 0 particles The 8 lightest spin-decay are all 1/2 particles 39 Variations on a Theme? Does this arise because
quarks have substructure? 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. 40 The Complication Light quarks arewell, light. Masses of a few MeV Any subcomponents would be heavy At least 1000 times heavier Otherwise, we would have already
discovered them Therefore, they would have to be bound very, very deeply. (binding energy ~ their mass) A -decay are all function potential has only one bound state so the particle periodic table cant be due to them being simply different configurations of the same components. Something new and interesting has to happen. Im an experimenter. This isnt my problem. 41 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, its 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-decay are all d snapshot of the proton. 42
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. 43 Parton Densities 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. Its useful to notate this in terms of x: x = p(parton)/p(proton) The fraction of the protons momentum that this parton carries This is actually the Fourier transform of the position distributions. Calculationally, leaving it this way is best. 44 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 45 Improving the Gluon: Direct Photons DIS and Drell-decay are all 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 q Direct photon Compton process.
46 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 0) Not too wide here. One photon and not two nearby ones (again, a likely 0) Not too much energy here. A schematic of an electromagnetic shower
A GEANT simulation of an electromagnetic shower Indicative of a hadronic shower: probably a neutron or KL. 47 Direct Photons & Backgrounds CMS Before event selection CMS After event selection There are two knobs we can turn Shower shape does this look like a photon (last slide) Isolation if its a fake, its 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. 48 More Variations on A Theme One can scatter a gluon off of a heavy quark in the 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 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 49
Double Parton Scattering Two independent partons in the proton scatter: AB A B Effective might be better characterized by AB A B A s Inelastic 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 Were thinking about bbjj as a good signature Large rate/large kinematic range 105 more events than past experiments Relatively unambiguous which jets go with which other jets. 50 Three Subtleties These densities are not quite universal They depend on the wavelength of your probe of the proton. A large fraction of the protons momentum is carried by gluons at low x There is a halo around the proton of large wavelength gluons (and quark-decay are all antiquark pairs) This sounds a lot like a particle physicists description of a pion cloud
Measurements of heavy flavor in the proton can be interpreted as a cloud of flavored mesons (up to Bs) Its a little paradoxical one needs the highest energy (i.e. shortest wavelength) to probe this large wavelength halo Double parton scattering delineates the breakdown of this simple model. 51 The Problem with Electroweak Theory Here we have the opposite problem than QCD here calculations are easier, but there is a fundamental flaw in the underlying theory. 52 The No Lose Theorem Imagine you could elastically scatter beams of W bosons: WW WW We can calculate this, and at high enough energies the cross-decay are all section violates unitarity
The probability of a scatter exceeds 1 -decay are all nonsense The troublesome piece is (once again) the longitudinal spin state High enough means about 1 TeV A 14 TeV proton-decay are all proton accelerator is just energetic enough to give you enough 1 TeV parton-decay are all parton collisions to study this The Standard Model is a low-decay are all energy effective theory. The LHC gives us the opportunity to probe it where it breaks down. Something new must happen. 53 Spontaneous Symmetry Breaking What is the least amount of railroad track needed to connect these 4 cities? 54 One Option
I can connect them this way at a cost of 4 units. (length of side = 1 unit) 55 Option Two I can connect them this way at a cost of only 3 units. 56 The Solution that Looks Optimal, But Really Isnt This requires only 2 2
57 The Real Optimal Solution This requires 1 3 Note that the symmetry of the solution is lower than the symmetry of the problem: this is the definition of Spontaneous Symmetry Breaking. + n.b. The sum of the solutions has the same symmetry as the problem. 58
A Pointless Aside One might have guessed at the answer by looking at soap bubbles, which try to minimize their surface area. But thats not important right now Another Example of Spontaneous Symmetry Breaking Ferromagnetism: the Hamiltonian is fully spatially symmetric, but the ground state has a non-decay are all zero magnetization pointing in some direction. 59 The Higgs Mechanism Write down a theory of massless weak bosons The only thing wrong with this theory is that it doesnt describe the world in which we live
Add a new doublet of spin-decay are all 0 particles: This adds four new degrees of freedom (the doublet + their antiparticles) 0 *0 Write down the interactions between the new doublet and itself, and the new doublet and the weak bosons in just the right way to Spontaneously break the symmetry: i.e. the Higgs field develops a non-decay are all zero vacuum expectation value Like the magnetization in a ferromagnet Allow something really cute to happen
60 The Really Cute Thing The massless w+ and + mix. You get one particle with three spin states Massive particles have three spin states The W has acquired a mass m = 1 transverse The same thing happens for the w-decay are all and -decay are all m = 0 longitudinal In the neutral case, the same thing happens for one neutral combination, and it becomes the massive Z0. The other neutral combination doesnt couple to the Higgs, and it gives the massless photon. That leaves one degree of freedom left, and because of the non zero v.e.v. of the Higgs field, produces a massive Higgs. 61
How Cute Is It? Theres very little choice involved in how you write down this theory. Theres one free parameter which determines the Higgs boson mass Theres one sign which determines if the symmetry breaks or not. The theory leaves the Standard Model mostly untouched It adds a new Higgs boson which we can look for It adds a new piece to the WW WW cross-decay are all section This interferes destructively with the piece that was already there and restores unitarity In this model, the v.e.v. of the Higgs field is the Fermi constant 62 Searching for the Higgs Boson
Because the theory is so constrained, we have very solid predictions on where to look and what to look for. H H ZZ llll ATLAS Simulation 10 fb-decay are all 1 ATLAS Simulation 100 fb-decay are all 1 63 Two Alternatives Multiple Higgses I didnt have to stop with one Higgs doublet I could have added two This provides four more degrees of freedom: Manifests as five massive Higgs bosons: h0, H0, A0, H+,H Usually some are harder to see, and some are easier
You dont have to stop there either New Strong Dynamics Maybe the WW WW cross-decay are all section blowing up is telling us something: The p p cross-section also blew up: it was because of a resonance: the . Maybe there are resonances among the Ws and Zs which explicitly break the symmetry Many models: LHC data will help discriminate among them. 64 Two of the three necessary measurements are SM measurements. t on W- W-
W+ tex Dir ec ver tO 4W bse r W+ ec Eff vat
ion The Higgs Triangle Loop Effects on m(W) 65 What is the Standard Model? The (Electroweak) Standard Model is the theory that has interactions like: W + Z0
W+ Z0 but not W+ W+ Z0 W Z0
& W- Z0 - Z0 Z0 Z0 Z0 & but not:
Z0 Only three parameters -decay are all GF, and sin2(w) -decay are all determine all couplings. 66 Portrait of a Troublemaker This diagram is where the SM gets into trouble. Its vital that we measure this coupling, whether or not we see a Higgs. W+ W+ W-
W- Yields are not all that great From Azuelos et al. hep-decay are all ph/0003275 100 fb-decay are all 1, all leptonic modes inside detector acceptance 67 A Complication If we want to understand the quartic coupling first we need to measure the trilinear couplings We need a TGC program that looks at all final states: WW, WZ, W (present in SM) + ZZ, Z (absent in SM)
68 The Semiclassical W Semiclassically, the interaction between the W and the electromagnetic field can be completely determined by three numbers: The Ws electric charge Effect on the E-field goes like 1/r2 The Ws magnetic dipole moment Effect on the H-field goes like 1/r3 The Ws electric quadrupole moment Effect on the E-field goes like 1/r4 Measuring the Triple Gauge Couplings is equivalent to measuring the 2nd and 3rd numbers Because of the higher powers of 1/r, these effects are largest at small distances Small distance = short wavelength = high energy 69 Triple Gauge Couplings
There are 14 possible WW and WWZ couplings To simplify, one usually talks about 5 independent, CP conserving, EM gauge invariance preserving couplings: g1Z, , Z, , Z In the SM, g1Z = = Z = 1 and = Z = 0 Often useful to talk about g, and instead. Convention on quoting sensitivity is to hold the other 4 couplings at their SM values. Magnetic dipole moment of the W = e(1 + + )/2MW Electric quadrupole moment = -decay are all e( -decay are all )/2MW2 Dimension 4 operators alter g1Z, and Z: grow as s Dimension 6 operators alter and Z and grow as s These can change either because of loop effects (think e or magnetic moment) or because the couplings themselves are non-decay are all SM 70 Why Center-Of-Mass Energy Is Good For You Approximate Run II Tevatron Reach
Tevatron kinematic limit The open histogram is the expectation for = 0.01 This is a standard deviation away from todays world average fit If one does just a counting experiment above the Tevatron kinematic limit (red line), one sees a significance of 5.5 Of course, a full fit is more sensitive; its clear that the events above 1.5 TeV have the most distinguishing power From ATLAS Physics TDR: 30 fb-decay are all 1
71 Not An Isolated Incident Qualitatively, the same thing happens with other couplings and processes These are from WZ events with g1Z = 0.05 While not excluded by data today, this is not nearly as conservative as the prior plot A disadvantage of having an old TDR Plot is from ATLAS Physics TDR: 30 fb-decay are all 1 Insert is from CMS Physics TDR: 1 fb-decay are all 1 72 Not All Ws Are Created Equal
The reason the inclusive W and Z cross-decay are all sections are 10x higher at the LHC is that the corresponding partonic luminosities are 10x higher No surprise there Where you want sensitivity to anomalous couplings, the partonic luminosities can be hundreds of times larger. The strength of the LHC is not just that it makes millions of Ws. Its that it makes them in the right kinematic region to explore the boson sector couplings. From Claudio Campagnari/CMS 73 TGCs the bottom line Coupling
Present Value LHC Sensitivity (95% CL, 30 fb-decay are all 1 one experiment) g1Z 0.005-0.011 0.01600..022 019 Z 0.06-0.12 0.07600..061 064 0.0023-0.0035 0.02800..020
021 Z 0.0055-0.0073 0.08800..063 061 0.03-0.076 0.027 00..044 045 Not surprisingly, the LHC does best with the Dimension-decay are all 6 parameters Sensitivities are ranges of predictions given for either experiment 74 Early Running Reconstructing Ws and Zs quickly will not be hard Reconstructing photons is harder Convincing you and each other that we understand the efficiencies and jet fake rates is probably the toughest part of this
We have a built in check in the events we are interested in The Tevatron tells us what is happening over here. We need to measure out here. At high ET, the problem of jets faking photons goes down. Not because the fake rate is necessarily going down because the number of jets is going down. 75 Precision EWK:The W Mass I am not going to try and sell you on the idea that the LHC will reach a precision of [fill in your favorite number here]. Instead, I want to outline some of the issues involved.
76 CDF Results: The State of the Art These systematics are statistically limited. These systematics are not. 77 One Way Of Thinking About It 25 MeV 15 MeV If we shoot for 5 MeV, how close might we come? 5 MeV
What needs to happen to get down to 5 (or 15, or 25) MeV? (If you shoot for 5, you might hit 10. If you shoot for 10, you probably wont hit 5) See Besson et al. arXiv :0805.2093v1 [hep-decay are all ex] 8 MeV is 100 parts per million. 78 Difficulty 1: The LHC Detectors are Thicker Detector material interferes with the measurement. You want to know the kinematics of the W decay products at the decay point, not meters later Material modeling is tested/tuned based on electron E/p Thicker detector = larger correction = better relative knowledge of correction needed
CMS material budget X~16.5%X0 (red line on lower plots) ATLAS material budget 79 Difficulty 2 QCD corrections are more important q q W g q W
q No valence antiquarks at the LHC Need sea antiquarks and/or higher order processes NLO contributions are larger at the LHC More energy is available for additional jet radiation At the Tevatron, QCD effects are already of the systematic uncertainty Reminder: statistical and systematic uncertainties are comparable. To get to where the LHC wants to be on total m(W) uncertainty is going to require continuous effort on this front.
80 Major Advantage the W & Z Rates are Enormous The W/Z cross-decay are all sections at the LHC are an order of magnitude greater than the at the Tevatron The design luminosity of the LHC is ~an order of magnitude greater than at the Tevatron I dont want to quibble now about the exact numbers and turn-decay are all on profile for the machine, nor things like experimental up/live time Implications: The W-decay are all to-decay are all final-decay are all plot rate at ATLAS and CMS will be ~ Hz Millions of Ws will be available for study statistical uncertainties will be negligible Allows for a new way of understanding systematics dividing the W sample into N bins (see next slide) The Z cross-decay are all section at the LHC is ~ the W cross-decay are all section at the Tevatron Allows one to test understanding of systematics by measuring m(Z) in the same manner as m(W) The Tevatron will be in the same situation with their femtobarn measurements: we can see if this can be made to work or not One can consider cherry picking events is there a subsample of Ws where
the systematics are better? 81 200 200 150 150 150 100 50 Measurement 200
Measurement Measurement Systematics The Good, The Bad, and the Ugly 100 50 0 50 0 0 2 4
6 8 10 12 Some variable 100 0 0 2 4 6
8 10 12 Some variable Good Bad Masses divided into several bins in some variable Masses are consistent within statistical uncertainties.
Clearly there is a systematic dependence on this variable Provides a guide as to what needs to be checked. 0 2 4 6 8 10 12
Some variable Ugly Point to point the results are inconsistent There is no evidence of a trend Something is wrong but what? 82 So, When Is This Going To Happen? The latest schedule shows the LHC ready for beam in about a month. Beam will be injected into sectors as soon
as they are cold. The plan is to have collisions at 10 TeV for 2-decay are all 3 months in 2008, train the magnets during the winter shutdown, and go to 14 TeV in 2009. 83 LHC Beam Stored Energy in Perspective Luminosity Equation: 2 fE nb N p L n * Luminosity goes as the square of the stored energy. LHC stored energy at
design ~700 MJ Power if that energy is deposited in a single orbit: ~10 TW (world energy production is ~13 TW) Battleship gun kinetic energy ~300 MJ Its best to increase the luminosity with care USS New Jersey (BB-decay are all 62) 16/50 guns firing 84 My Take on The Schedule If we only have the same old problems (i.e.
no new ones) there will beam in fall. Full energy will be in early 2009. We will turn on with very low luminosity and this will grow slowly as we learn to handle the stored energy Luminosity grows as the square of stored energy After maybe a year, the luminosity will shoot up like a rocket Luminosity grows as the square of stored energy 85 Apologies I didnt cover even a tenth of the ATLAS physics program Precision measurements Top Quark Physics Orders of magnitude more events than at the Tevatron Search for new particles
Can we produce the particles that make up the dark matter in the universe? Search for extra dimensions Why is gravity so much weaker than other forces? Are there mini-Black Holes? B Physics and the matter-decay are all antimatter asymmetry Why is the universe made out of matter? Heavy Ions What exactly has RHIC produced? 86 Summary Electroweak Symmetry Breaking is puzzling Why is the W so heavy? Why is the weak force so weak? The Large Hadron Collider is in a very good position to shed light on this The no lose theorem means something has to happen. Maybe its a Higgs, maybe its not. Finding the Higgs is not enough. Precision electroweak measurements are needed to understand whats going on. Any experiment that can do this can also answer a number of other questions
For example, addressing the structure of the proton And the dozens I didnt cover Thanks for inviting me! 87 The LHC: Ready or Not, Here It Comes