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Signals And Backgrounds for the LHC: Part 2

Signals And Backgrounds for the LHC: Part 2. -or- What They Were Thinking When They Designed ATLAS and CMS. Thomas J. LeCompte Argonne National Laboratory. Outline. Four facts about detectors Representative signals and how they influence experimental design High p T muons

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Signals And Backgrounds for the LHC: Part 2

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  1. Signals And Backgrounds for the LHC: Part 2 -or- What They Were Thinking When They Designed ATLAS and CMS Thomas J. LeCompte Argonne National Laboratory

  2. Outline • Four facts about detectors • Representative signals and how they influence experimental design • High pT muons • Higgs via H →gg • Jets • Top Quarks • High pT electrons • Missing ET and Exotica • Some early physics &future directions Reminder… STOP ME if I go too fast or you have questions!!

  3. The Most Important Slide I Will Show jets From Claudio Campagnari/CMS Measured cross-sections (except for Higgs) at the Tevatron How to extrapolate to the LHC

  4. Missing ET

  5. Missing Transverse Energy • We know momentum is conserved. • An apparent imbalance of momentum can be due to an escaping neutrino • Calculated by adding up all the other momenta and reversing the sign • We work only in the xy (transverse) plane • Many particles escape unmeasured down the beampipe (this will be important later) Missing ET Momentum of the “underlying event” Electron momentum

  6. Pink is the New Black …and neutralinos are the new neutrinos. • In Supersymmetry, every fermion has a boson that’s its partner and vice versa • The spin-1 photon’s partner is the spin-½ photino • The lightest supersymmetric particle is stable • This is called “R parity conservation” and it keeps your supersymmetric theory from violating experimental limits, such as that for proton decay • It leads to a common signature in SUSYmodels: particles that exit your detectorwithout interacting, leading to missing momentum (violates conservation of angular momentum) (violates conservation of lepton number) (violates conservation of baryon number) Footnotes:1. The g, Z and H all have partners, and these partners have the same quantum numbers, so they mix. 2. There are ways to contrive an R-parity violating theory that evades experimental bounds

  7. Variations on a Theme • More exotic: theories with extra dimensions can also have missing ET signatures • The entire standard model is replicated (so-called Kaluza-Klein modes) • The “KK graviton” is one candidate for a particle that gives large missing ET. • Depending on the model, you might get more. • Even more exotic: theories with names like “Hidden Valleys” and “Shadow Matter”. The point is not whether these particular theories are right or wrong. The point is that Missing ET is a common signature present in multiple models, so it should be looked for.

  8. Hermiticity • A fancy way of saying “holes are bad” • Particles escape down holes and cracks, and generate missing ET • “Real missing ET, because it truly is missing • “Fake missing ET” in the sense that it wasn’t what you were looking for • Difference between an undetected and an undetectable particle • Holes, gaps and cracks are necessary • Minimum of two holes (for the beam) • Cables need to come out somewhere • Cooling and cryogens need to go in • If they go in, they have to come out

  9. Improving Missing ET • To keep particles from escaping, one can: • Make the holes smaller • There’s a limit to this • Make the detector longer • The hole is the same size, but subtends a smaller angle

  10. Why Are Detectors as Long As They Are? • Why not make detectors longer and longer and longer? • Resource limitations • Making it twice as big costs twice as much money • …and takes twice as many people • …or takes twice as long • Relativistic kinematics affects the design of a detector • Heavy objects are produced almost at rest • Their decay products populate all 4p of solid angle • What matters is solid angle • Light objects are produced uniformly across rapidity • In principle, argues for a long detector • But if the mass is low, the cross-section is high, and you’re making a lot of them – one unit of rapidity is as good as any other • In either case, there’s a natural point where it’s no longer cost effective to keep going forward I wish someone had told me this sooner.

  11. Why are LHC experiments a little longer than Tevatron experiments? D0 detector • One reason is for Missing ET performance (long is good) • Another reason is from kinematics • Quark-antiquark collisions at the LHC are asymmetric. • Even for light objects, extra coverage doesn’t hurt you – it just costs $

  12. With apologies to Spinal Tap Mismeasured Jets • One way to generate fake missing ET is to mismeasure an object in the event. • Jets are the usual suspects: • There are a lot of them • There are several things that can go wrong: • Plain old mismeasurement • Particles down cracks • Particles in dead regions of the detector • Undercorrection and overcorrection are both possible • Particles in the jet decaying with a leading neutrino • And so on… These all sound unlikely. That’s because they are unlikely. The reason that this is important is…

  13. The Most Important Slide I Will Show (Yet Again) jets From Claudio Campagnari/CMS Measured cross-sections (except for Higgs) at the Tevatron How to extrapolate to the LHC

  14. Triggering – the Oft Overlooked Component • At the LHC, there are 40,000,000 beam crossings per second. • Of these, perhaps 200 can be written to tape for analysis • It’s the job of the trigger to select which 200 • There are no do-overs in baseball • There are no do-overs in triggering • Triggers are usually designed in tiers • Low level triggers tend to be hardware-based, fast, and select events for higher trigger levels to look at • Higher level triggers are software based, and can take much more time to decide whether to keep this event, or some other event. • The Three Laws of Triggering • 1. You cannot analyze an event you didn’t trigger on • 2. If you aren’t going to analyze an event, it doesn’t help to trigger on it • 3. If you are going to cut an event, cut it as early in the chain as you can.

  15. Early and/or Interesting Measurements

  16. QCD t - channel Quark Contact Interactions Quark Compositeness New Interactions q q • New physics at a scale L above the observed dijet mass is modeled as an effective contact interaction. • Quark compositeness. • New interactions from massive particles exchanged among partons. • Contact interactions look different than QCD. • QCD is predominantly t-channel gluon exchange. M ~ L M ~ L q q Dijet Mass << L Quark Contact Interaction q q L Diagrams: R. Harris, CMS q q

  17. Expected limit on contact interaction: L(qqqq) > ~6 TeV Rule of thumb: 4x the ET of the most energetic jet you see Present PDG limit is 2.4-2.7 TeV Ultimate limit: ~20 TeV The ATLAS measurement is at lower x than the Tevatron: PDF uncertainties are less problematic “Week One” Jet Measurements Jet Transverse Energy 5 pb-1 of (simulated) data: corresponds to 1 week running at 1031 cm-2/s (1% of design) Note that after a very short time, LHC will be seeing jets beyond the Tevatron kinematic limit.

  18. Making the Measurement • There are only two hard things in making this plot: • The x-axis • The y-axis • The y-axis has two pieces: counting the events, and measuring the luminosity • The first is easy • The second is hard, and I won’t talk about it • The key to the x-axis is correctly measuring the jet energy

  19. Balancing Jets • The problem of setting the jet energyscale can be split into two parts: • 1. Establish that all jets sharethe same scale • 2. Establish that all jets sharethe right scale. • A good start to #1 is to look at dijetevents and show there is no bias tothe jet energy as a function of jetposition, jet composition, energydeposition, pile-up, etc. • A good start to #2 is to use known particles(electrons and Z’s) to set the overall scale. Getting the jet energy scale right to 20% is easy. Getting it right to 2% is hard – and will take time.20% in JES = a factor of 2 in data

  20. Jet Energy Scale Job List • See that the Z decay to electrons ends up in the right spot • Demonstrates that the EM calorimeter is calibrated • Balance jets with high and low EM fractions • Demonstrates that the EM and hadronic calorimeters have the same calibration • Balance one jet against two jets • Demonstrates that the calorimeter is linear • Balance jets against Z’s and photons • Verifies that the above processes work in an independent sample • Demonstrates that we have the same scale for quark and gluon jets • Use top quark decays as a final check that we have the energy scale right • Is m(t) = 175 and m(W) = 80? If not, fix it! Note that most of the work isn’t in getting the jet energy scale right. It’s in convincing ourselves that we got the jet energy scale right – and that we have assigned an appropriate and defensible systematic uncertainty to it.

  21. Center of Momentum Frame Jet q* Parton Parton Jet QCD Background dN/ dcos q* Signal 0 1 cos q* Angular Distribution of a Contact Interaction • It’s harder to grossly mismeasure a jet’s position than its energy. • Contact interaction is often more isotropic than QCD • QCD is dominated by t-channel gluon exchange. • c.f. Eichten, Lane and Peskin (Phys. Rev. Lett. 50, 811-814 (1983)) for distributions from a contact interaction • CMS (and D0) compress this distribution into a single ratio of central-to-forward jets Diagrams: R. Harris, CMS

  22. Angular Distribution of a Contact Interaction II • The D0 (hep-ex/980714) dijet ratio: N(|h| < 0.5)/N(0.5 < | h | < 1) • This is essentially a measurement of the position of the leading jet. • CMS plans to do the same thing (see plot) • ATLAS is leaning more towards a combined fit of energy and angle. • Same idea, different mathematics New physics changes the shape of this plot. You aren’t counting on having a precise prediction of the QCD value.

  23. Changing Gears • Why did we build the LHC? • Wrong Answer: To find the Higgs Boson • Right Answer: To study the electroweak sector at high energies / small distances. • There might be a Higgs • There may not be a Higgs • There may not be only one Higgs • Finding something other than a single scalar Higgs is not failure.It may even be better than finding a single scalar Higgs. D- A+

  24. g g W+ Z0 W+ Z0 Z0 W+ W+ g Z0 Z0 Z0 Z0 & & W- Z0 g g Z0 g W- Z0 What is the Standard Model? The (Electroweak) Standard Model is the theory that has interactions like: but not but not: Only three parameters - GF, a and sin2(qw) - determine all couplings.

  25. 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. W+ W+ Yields are not all that great W- W- From Azuelos et al. hep-ph/0003275 100 fb-1, all leptonic modes inside detector acceptance

  26. Good News and Bad News • The good news: • The reason all three W’s had to decay leptonically is because top backgrounds are ~1000x larger than the signal. • Top gives you W+W- - + 2 jets • Top never gives you W+W+ + 2 jets • Only the two same sign W’s have to decay to leptons • This turns a 100 fb-1 measurement into a 12 fb-1 measurement • The bad news: • We don’t detect couplings. We detect events.

  27. You Have To Walk Before You Can Run 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, Wg (present in SM) + ZZ, Zg (absent in SM)

  28. The Semiclassical W • 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/r2 • The W’s magnetic dipole moment • Effect on the H-field goes like 1/r3 • The W’s 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

  29. Triple Gauge Couplings • There are 14 possible WWg and WWZ couplings • To simplify, one usually talks about 5 independent, CP conserving, EM gauge invariance preserving couplings: g1Z, kg, kZ, lg, lZ • In the SM, g1Z = kg = kZ = 1 and lg = lZ = 0 • Often useful to talk about Dg, Dk and l instead. • Convention on quoting sensitivity is to hold the other 4 couplings at their SM values. • Magnetic dipole moment of the W = e(1 + kg + lg)/2MW • Electric quadrupole moment = -e(kg - lg)/2MW2 • Dimension 4 operators alter Dg1Z,Dkg and DkZ: grow as s½ • Dimension 6 operators alter lg and lZ and grow as s Do we live in a Standard Model universe? Or some other universe?

  30. Why Center-Of-Mass Energy Is Good For You Approximate Run II Tevatron Reach Tevatron kinematic limit • The open histogram is the expectation for lg = 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 5.5s • 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

  31. Not An Isolated Incident • Qualitatively, the same thing happens with other couplings and processes • These are from WZ events with Dg1Z = 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-1Insert is from CMS Physics TDR: 1 fb-1

  32. 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. Here’s Claudio’s plot again…

  33. 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

  34. 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 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. Early Running

  35. Things I Left Out and Really Shouldn’t Have • Angular distributions have additional resolving power • Remember, the W decays are self-analyzing • Different couplings yield different angular distributions • Easiest to think about in terms of multipole moments • Neutral Gauge Couplings • In the SM, there are no vertices containing only g’s and Z’s • At loop level, there are ~10-4 corrections to this • It is vital that these be explored

  36. Putting it all together • Complex signatures break down into simpler ones • Suppose you were looking for stop squarks: • Signal would be a lepton + 4 jets (2 b-tagged) + lots and lots of missing ET. • One background is real top events + a mismeasured jet leading into large missing ET. • Many backgrounds are similar – additional jets from QCD radiation, which may or may not be reconstructed correctly • Could be misreconstructed as a photon, a b-jet, missing ET, etc… • These jets are often correlated with some other object in the event • You have a radiator, and a radiatee. • Signal usually does not have this correlation – allows discrimination. But there’s a fly in the ointment…

  37. 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. • The jet cross-section is very high at the LHC, so this is proportionally a larger background than at lower energies • We’re thinking about bbjj as a good signature to measure this • Large rate/large kinematic range • Relatively unambiguous which jets go withwhich other jets. might be bettercharacterized by

  38. Comments on Double Parton Scattering • The naïve parton model assumes independence • We don’t expect partons to be completely independent of each other • Quarks are confined, after all. • This is very difficult to calculate • We need to measure this • DPS looks a lot like pileup • Cuts that kill pileup also kill DPS • This may be necessary at high luminosity, so the issue of DPS may be moot.

  39. Just for Fun – Black Holes • In models with extra dimensions, gravity becomes strong at small distances • Allows for production of a Black Hole solong as the impact parameter is less thanthe Swartzchild Radius (s ~ 100 pb) • The Black Hole evaporates via Hawking Radiation • Distribution is spherical (except for boost) • Particle production is very “democratic” • Spectrum is thermal, with caveats • Expectation on limits: • M(BH) > 4 TeV (1 day) • M(BH) > 6 TeV (1 year)

  40. A Black Hole This is a ~6 TeV Monte Carlo Black Hole Is this a multijet event? It looks kind of jetty.

  41. The Same Black Hole Doesn’t look very jet-like.

  42. Summary • I hope to have given you some insight on why the LHC detectors look like they do: • Why the design choices are what they are • What signals they are intended to accept • What backgrounds they are intended to reject • Of course, this is incomplete - doing this right would take all week • I hope to have given you some idea on which strings we will be tugging at to unravel the Standard Model:

  43. Let’s Do It One More Once… jets From Claudio Campagnari/CMS Measured cross-sections (except for Higgs) at the Tevatron How to extrapolate to the LHC

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