Presentation on theme: "Early Physics with the Large Hadron Collider Thomas J. LeCompte High Energy Physics Division Argonne National Laboratory JLAB Users’ Meeting: 16 June 2008."— Presentation transcript:
Early Physics with the Large Hadron Collider Thomas J. LeCompte High Energy Physics Division Argonne National Laboratory JLAB Users’ Meeting: 16 June 2008
2 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.
3 Second Order of Business The HEP community likes Mont. You will too.
4 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
5 The Traditional Opening Pitch Practically every HEP talk starts with this slide. This isn’t the way I want to start this talk.
6 Comparing Two Figures 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 A histogram of the notes used in Beethoven’s 5 th Symphony, first movement. I’d like to come at this from a different direction.
7 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 aspects of the same “electroweak” force. –Only three constants enter into it: e.g. , G F and sin 2 ( w ). –A chiral theory: it treats particles with left-handed spin differently than particles with right-handed spin. A beautiful theory. Unfortunately, it’s broken.
8 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
9 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 e i anything I can observe still remains unchanged. –The above example was a special case ( = ) If I can’t actually observe , how do I know that it’s 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.
10 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 ’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.
11 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 ’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.
12 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 -22 m e ) –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
13 Let’s 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 - and Z 0 bosons –Experimentally, they are heavy: 80 and 91 GeV 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.
14 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 (= h/p ) means high energy Third goal: search for new heavy particles –This also means large energy ( E=mc 2 ) 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.
15 The Large Hadron Collider 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. Design Collision Energy = 14 TeV
16 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.
17 ATLAS = A Toroidal LHC ApparatuS Length = 44m Diameter = 22m Mass = 7000 t
19 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.
22 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. 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. Pictures from Jim Shank, Boston University Beam’s eye view: d= 22m
23 CMS: The Other LHC “Large” Detector Similar in concept to ATLAS, but with a different execution. 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
24 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.
25 QCD vs. QED QEDQCD Symmetry GroupU(1)SU(3) ChargeElectric chargeThree kinds of color Force carrier1 Photon – neutral8 Gluons - colored Coupling strength1/137 (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.
26 The Running of s At high Q 2, s is small, and QCD is in the perturbative region. –Calculations are “easy” At low Q 2, s 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.
27 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?
28 Energy is Stored in Fields We know energy is stored in electric & magnetic fields –Energy density ~ E 2 + B 2 –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 E 2 + B 2 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) Energy stored in the field Atom10 -8 Nucleus1% Proton99%
29 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 Mostly a very dynamic self-interacting field of gluons, with three quarks embedded. Like plums in a pudding. The Proton
30 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.
31 Results of the Experiment At angles as low as 3 o, 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
32 Proton Collisions: The Ideal World 1. Protons collide 2. Constituents scatter 3. As proton remnants separate
33 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. Calorimeter View Same Events, Tracking View 2 jets 3 jets 5 jets 2 2 3 5
34 Jets 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 ’s, three K’s and a . 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 Initial quark Jet What makes the measurement possible & useful is the conservation of energy & momentum.
35 Jets after “One Week” Jet Transverse Energy 5 pb -1 of (simulated) data: corresponds to 1 week running at 10 31 cm -2 /s (1% of design) ATLAS This is in units of transverse momentum. Remember, large angle = large p T
36 Jets after “One Week” Number of events we expect to see: ~12 If new physics: ~50 Number we have seen to date worldwide: 0 Jet Transverse Energy 5 pb -1 of (simulated) data: corresponds to 1 week running at 10 31 cm -2 /s (1% of design) ATLAS New physics (e.g. quark substructure) shows up here.
37 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.
38 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
39 Compositeness & The Periodic Table(s) The 9 lightest spin-0 particles The 8 lightest spin-1/2 particles Arises because atoms have substructure: electrons Arises because hadrons have substructure: quarks
40 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 (10 5 :1) –Nuclei: nucleons (few:1) –Quarks (>10 4 :1) will become (~10 5 :1) There are some subtleties: if this is substructure, its nature is different than past examples. Does this arise because quarks have substructure?
41 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 already discovered them Therefore, they would have to be bound very, very deeply. (binding energy ~ their mass) A -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.
42 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.
43 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.
44 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. 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.
45 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
46 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 q g Direct photon “Compton” process.
47 Identifying Photons – Basics of Calorimeter Design A schematic of an electromagnetic shower A GEANT simulation of an electromagnetic shower Not too much or too little energy here. Not too wide here. Not too much energy here. You want exactly one photon – not 0 (a likely hadron) or 2 (likely 0 ) One photon and not two nearby ones (again, a likely 0 ) Indicative of a hadronic shower: probably a neutron or K L.
48 Direct Photons & Backgrounds 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. CMS Before event selection After event selection
49 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…
50 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 10 5 more events than past experiments –Relatively unambiguous which jets go with which other jets. might be better characterized by
51 Three Subtleties These densities are not quite universal –They depend on the wavelength of your probe of the proton. A large fraction of the proton’s momentum is carried by gluons at low x –There is a halo around the proton of large wavelength gluons (and quark- antiquark pairs) This sounds a lot like a particle physicist’s description of a pion cloud Measurements of heavy flavor in the proton can be interpreted as a cloud of flavored mesons (up to B’s) –It’s 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.
52 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.
53 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-section violates unitarity –The probability of a scatter exceeds 1 - nonsense –The troublesome piece is (once again) the longitudinal spin state “High enough” means about 1 TeV –A 14 TeV proton-proton accelerator is just energetic enough to give you enough 1 TeV parton-parton collisions to study this The Standard Model is a low-energy effective theory. The LHC gives us the opportunity to probe it where it breaks down. Something new must happen.
54 Spontaneous Symmetry Breaking What is the least amount of railroad track needed to connect these 4 cities?
55 One Option I can connect them this way at a cost of 4 units. (length of side = 1 unit)
56 Option Two I can connect them this way at a cost of only 3 units.
57 The Solution that Looks Optimal, But Really Isn’t This requires only
58 The Real Optimal Solution This requires 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.
59 A Pointless Aside One might have guessed at the answer by looking at soap bubbles, which try to minimize their surface area. But that’s not important right now… Another Example of Spontaneous Symmetry Breaking Ferromagnetism: the Hamiltonian is fully spatially symmetric, but the ground state has a non-zero magnetization pointing in some direction.
60 The Higgs Mechanism Write down a theory of massless weak bosons –The only thing wrong with this theory is that it doesn’t describe the world in which we live Add a new doublet of spin-0 particles: –This adds four new degrees of freedom (the doublet + their antiparticles) 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-zero vacuum expectation value Like the magnetization in a ferromagnet –Allow something really cute to happen
61 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 The same thing happens for the w - and - In the neutral case, the same thing happens for one neutral combination, and it becomes the massive Z 0. The other neutral combination doesn’t 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. m = ±1 “transverse” m = 0 “longitudinal”
62 How Cute Is It? There’s very little choice involved in how you write down this theory. –There’s one free parameter which determines the Higgs boson mass –There’s 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-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
63 Searching for the Higgs Boson H → ATLAS Simulation 100 fb -1 ATLAS Simulation 10 fb -1 H → ZZ → llll Because the theory is so constrained, we have very solid predictions on where to look and what to look for.
64 Two Alternatives Multiple Higgses –I didn’t 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: h 0, H 0, A 0, H +,H - – Usually some are harder to see, and some are easier –You don’t have to stop there either… New Strong Dynamics –Maybe the WW → WW cross-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 W’s and Z’s which explicitly break the symmetry Many models: LHC data will help discriminate among them.
65 The Higgs Triangle Direct Observation Loop Effects on m(W) Effect on 4W vertex W+W+ W-W- W+W+ W-W- Two of the three necessary measurements are SM measurements.
66 What is the Standard Model? The (Electroweak) Standard Model is the theory that has interactions like: W+W+ W+W+ Z0Z0 Z0Z0 but not Z0Z0 Z0Z0 W+W+ W-W- Z0Z0 W-W- W+W+ & but not: Z0Z0 Z0Z0 Z0Z0 & Z0Z0 Z0Z0 Only three parameters - G F, and sin 2 ( w ) - determine all couplings.
67 Portrait of a Troublemaker This diagram is where the SM gets into trouble. It’s vital that we measure this coupling, whether or not we see a Higgs. From Azuelos et al. hep-ph/0003275 100 fb-1, all leptonic modes inside detector acceptance W+W+ W-W- W+W+ W-W- Yields are not all that great
68 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)
69 Semiclassically, the interaction between the W and the electromagnetic field can be completely determined by three numbers: –The W’s electric charge Effect on the E-field goes like 1/r 2 –The W’s magnetic dipole moment Effect on the H-field goes like 1/r 3 –The W’s electric quadrupole moment Effect on the E-field goes like 1/r 4 Measuring the Triple Gauge Couplings is equivalent to measuring the 2 nd and 3 rd numbers –Because of the higher powers of 1/r, these effects are largest at small distances –Small distance = short wavelength = high energy The Semiclassical W
70 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: g 1 Z, , Z, , Z –In the SM, g 1 Z = = 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 + + )/2M W –Electric quadrupole moment = -e( - )/2M W 2 –Dimension 4 operators alter g 1 Z, and Z : grow as s ½ –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-SM
71 Why Center-Of-Mass Energy Is Good For You The open histogram is the expectation for = 0.01 –This is ½ a standard deviation away from today’s 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; it’s clear that the events above 1.5 TeV have the most distinguishing power From ATLAS Physics TDR: 30 fb -1 Approximate Run II Tevatron Reach Tevatron kinematic limit
72 Not An Isolated Incident Qualitatively, the same thing happens with other couplings and processes These are from WZ events with g 1 Z = 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 -1 Insert is from CMS Physics TDR: 1 fb -1
73 Not All W’s Are Created Equal The reason the inclusive W and Z cross-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 W’s. It’s that it makes them in the right kinematic region to explore the boson sector couplings. From Claudio Campagnari/CMS
74 TGC’s – the bottom line Not surprisingly, the LHC does best with the Dimension-6 parameters Sensitivities are ranges of predictions given for either experiment CouplingPresent ValueLHC Sensitivity (95% CL, 30 fb-1 one experiment) g1Zg1Z 0.005-0.011 0.03-0.076 Z 0.06-0.12 0.0023-0.0035 Z 0.0055-0.0073
75 Early Running Reconstructing W’s and Z’s 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 E T, 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.
76 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.
77 CDF Results: The State of the Art These systematics are statistically limited. These systematics are not.
78 One Way Of Thinking About It 5 MeV 15 MeV 25 MeV If we shoot for 5 MeV, how close might we come? 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 won’t hit 5) 8 MeV is 100 parts per million. See Besson et al. arXiv:0805.2093v1 [hep-ex] arXiv:0805.2093v1
79 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 ATLAS material budget X~16.5%X 0 (red line on lower plots)
80 Difficulty 2 – QCD corrections are more important 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 ¼ 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. q q W q g q W
81 Major Advantage – the W & Z Rates are Enormous The W/Z cross-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 don’t want to quibble now about the exact numbers and turn-on profile for the machine, nor things like experimental up/live time Implications: –The W-to-final-plot rate at ATLAS and CMS will be ~½ Hz Millions of W’s 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-section at the LHC is ~ the W cross-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 W’s where the systematics are better?
82 Systematics – The Good, The Bad, and the Ugly 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. Point to point the results are inconsistent There is no evidence of a trend Something is wrong – but what? Good Bad Ugly
83 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 2-3 months in 2008, train the magnets during the winter shutdown, and go to 14 TeV in 2009.
84 LHC Beam Stored Energy in Perspective 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 It’s best to increase the luminosity with care USS New Jersey (BB-62) 16”/50 guns firing Luminosity Equation:
85 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
86 Apologies I didn’t 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-antimatter asymmetry Why is the universe made out of matter? –Heavy Ions What exactly has RHIC produced?
87 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 it’s a Higgs, maybe it’s not. –Finding the Higgs is not enough. Precision electroweak measurements are needed to understand what’s 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 didn’t cover Thanks for inviting me!
Your consent to our cookies if you continue to use this website.