Signals And Backgrounds for the LHC

Thomas J. LeCompte

Argonne National Laboratory

-or-

What They Were Thinking When They Designed ATLAS and CMS

2

Lecture Series Outline

Original outline

– A description of the ATLAS and CMS detectors

– A laundry list of potentially interesting signals, followed by their backgrounds, stretching over two hours

This bored me to tears

– I’d hate to think what it would do to you

Revised outline

– I’ll try and explain why the detectors look like they do, in the context of “simple” physics measurements

– I’ll discuss some early and interesting measurements

Zzzzzz

Think of this as one long talk, split over two days.

STOP ME if I go too fast or you have questions!!

3

Outline

Four facts about detectors Representative signals and how they influence experimental design

– High pT muons

– Higgs via H →

– Jets

– Top Quarks

– High pT electrons

– Missing ET and Exotica

Some early physics &future directions

If you come away from this with the perspective that one experiment is better than the other, I haven’t done my job. I hope to outline what choices were made and why – not which choices were “right”

STOP ME if I go too fast or you have questions!!

I’m not kidding…

4

The Most Important Slide I Will Show

Measured cross-sections (except for Higgs) at the Tevatron

How to extrapolate to the LHC

From Claudio Campagnari/CMS

jets

5

“Nobody won any money betting against Michael Jordan”

I’m going to assume here that the Tevatron finds no new physics

– Personally, I wouldn’t take this bet• The Tevatron is running very well• The experiments are experienced,

and also running very well

– Nonetheless, one has to assume something

If the Tevatron does find something, this talk becomes very simple:

– Take whatever the Tevatron found

– Make a zillion of them

– Study the heck out of it

6

Fact One: The basic design of experiments is the same:

Driven by the physics of the interaction of high energy particles with matter.

Because the physics is the same, successful experimental designs are similar

Tracking

Calorimeters

Muon detectors

7

The Compact Muon Solenoid

8

What CMS Looks Like Today

9

ATLAS = A Toroidal LHC ApparatuS

10

What ATLAS Looks Like Today

11

Fact Two: Tracking measures 1/p

Charged particles in a uniform magnetic field move in helices:zBB ˆ0

It’s convenient to work in the transverse plane (i.e. the plane normal to the Z direction)

In this plane, the helices project to circles.

12

Fact Two: Tracking measures 1/p (II)

Radial line from origin to point where the particle exits the tracker:

The sagitta (“arrow”) s is the distance of maximum deflection from a straight line track:

Tp

qBLs

8

2

s

qBLpT 8

2

ps

s

p

p

or

Which leads to the expression

As momentum increases, tracking becomes more difficult.

13

A (constant) fraction of the incoming particle’s energy gets converted to something that we can count (photons, electrons, etc…)

In the approximation that each layer absorbs a constant fraction of the energy, the calorimeter depth grows logarithmically with energy.

Fact Three: Sampling Calorimeters

Absorber layers

Sensitive layers

ENN

N

E

E 11

As energy increases, calorimetry resolution improves.

14

Fact Three: Sampling Calorimeters (II)

EM showers all look the same

Hadronic showers are like snowflakes

– Every one is unique

A schematic of an electromagnetic shower A GEANT simulation of an

electromagnetic shower

15

Fact Four: Compromise is a fact of life

Like it or not, experiments are constrained by resources

– Every dollar that goes into one subsystem is a dollarthat doesn’t go into some other subsystem

– Every inch that goes into one subsystem is an inchthat doesn’t go into some other subsystem

– A collaboration with N members can’t design an experiment that takes 2N members to build or operate.

– Industrial production capacity is finite• Evidence: crystals, silicon wafers, liquid noble gasses

Most experimenters have their own ideas on the best optimization

– Individual interests and experience varies

– The goal of a collaboration is to design a detector that everyone can live with – even if no single person thinks it’s ideal.

16

The Large Hadron Collider

The LHC:

– Collides protons on protons

– Crossing time of 25 (75) ns

– Design Luminosity 1033 cm-2/s, increasing to 1034 after 1-2 years, and hopefully to 1035 (Super LHC) after that

The Tevatron

– Collides protons on antiprotons

– Crossing time of 396 ns

– Design Luminosity 2 x 1032 cm-2/s – exceeded routinely.

17

Why Is The LHC More Luminous?

It has to be

– No valence antiquarks in the proton. If you need an antiquark, you need to get it from the sea, or a gluon induced higher order process

It can be

– It uses protons instead of antiprotons• Protons cost $3/oz.• Antiprotons cost $500,000,000,000,000,000/oz.

– Energy is 7x higher

– More beam bunches• LHC’s bunch intensity is very conservative, compared to what the

Tevatron achieves routinely

*

2

pb

n

NnfEL

18

LHC 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

*

2

pb

n

NnfELLuminosity

Equation:

19

Muon Detection:The Iron Ball

20

What Is The Iron Ball?

Suppose you wanted a detector to look for a very heavy (800 GeV) Higgs via the decay H → ZZ followed by Z → (both Z’s)

– This is a very rare process → increase the luminosity

– Handle this increased luminosity by only looking at muons:

B-field region

21

CMS Muon Detectors

CMS uses the return field of their central solenoid to measure muon momenta

Four planes of detector stations inside the steel measure the muon’s tracks.

Low pT muons range out in the steel, providing an additional measurement.

22

The ATLAS Muon Spectrometer

Pictures from Jim Shank, Boston University

Beam’s eye view

How muon trajectories bend in the magnetic field of the toroids.

Energy stored in the magnetic field is ~1.2 GJ.

Energy stored in a lightning bolt is ~1.5 GJ.

23

Comparing Design Philosophies

CMS uses as their magnetic field the return field through the iron

– Allows one to go to large fields…

– …which means small radii… (Figure of merit is BL2)

– …which means that their calorimeter can be small (and expensive per unit volume).

ATLAS uses air core toroids

– Very good resolution at high momentum• Up to ~TeV scale muons

– Requires a lot of space

B

L2

Neither experiment is an iron ball, but elements of the iron ball design influenced both detectors.

Emphasizes

24

Anticipated Muon Backgrounds

The main background to muons is other muons

What I mean is the main background to muons are muons from a source we are not particularly interested in.

– Example: pion or kaon decays

Improving purity beyond ATLAS or CMS hardly helps.

ATLAS

25

Unanticipated Muon Backgrounds

There is no such thing as a muon detector:

– They are charged particle detectors, behind lots of steel

– If there is any path around the steel, particles will find it

– Since there are a million hadrons per muon, even if this is unlikely, it can be an important background

At the LHC, the cavern backgrounds (secondary particles) might be large.

– The experiments have to worry about particles “raining down” into their detectors.

– This is VERY difficult to predict from first principles. It has to be measured.

The “CMX Ricochet”

26

Dimuon Mass

ATLAS

Requiring two muons is enough by itself to give a clean Z signal.

Note that this is a logarithmic plot

The background is real muons – just from other sources.

27

Dimuon Mass – Post Cuts

It’s possible to remove virtually all of the background in the previous slide.

(Exactly how is not really the subject of this talk)

28

Photon Detection

29

Higgs decays to diphotons The background under the peak

are real events: just not from Higgs decay

– This is the irreducible background

– Note the suppressed zero

To see the peak, there are two things you need to do:

– Get the mass resolution as good as you can• make the peak narrow

– Get the reducible background as small as you can• Keep the background from

getting any worse than it already is

From ATLAS Physics TDR

30

Higgs Event Displays

CMS H → eventATLAS H → event

31

Why Pointing is Helpful (Part 1)

Beam axis

)cos1(2 212 EEm

Can improve resolution

32

Why Pointing is Helpful (Part 2)

Beam axis

Can reduce background

33

Identifying Photons – Basics of Calorimeter Design

A schematic of an electromagnetic shower

A GEANT simulationof 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 KL.

34

ATLAS Electromagnetic Calorimeter

EEE

E GeV2.0%7.0

%10

Design resolution:Technology: uses lead as an absorber and liquid argon as an ionization medium. Energy deposited in the calorimeter is converted to an electrical signal.

35

ATLAS Liquid Argon Calorimeter Module

Highly segmented

– Allows measurement of shower development• Rejects background

– Has some pointing ability

Very good (but not as good as CMS) energy resolution

“Accordion” faster than other LAr calorimeters

– Still slower than crystals

36

ATLAS Calorimeter in Real Life

Before installation – it’s now in a cryostat and impossible to see.

37

CMS Calorimeter Crystals

CMS uses Lead Tungstate crystals

– Scintillator: energy is converted to light

– Exceptional energy resolution, because there are no inert absorbers

The focus is to get the best possible energy resolution, no matter what it takes

– Energy resolution is ~2x better than ATLAS’ in the region where Higgs decay is important

Photo: Ren-yuan Zhu, Caltech

EEE

E GeV16.0%55.0

%7.2

Design resolution:

16X0

22X0

26X0

Another nice feature – low noise

38

CMS EM Calorimeter

Figure: Ren-yuan Zhu, Caltech

39

Comparing Design Philosophies

CMS emphasizes energy resolution– Use PWO crystals

• Expensive – means go to small radius to keep the detector within budget• Only handful of vendors worldwide

ATLAS emphasizes background rejection– Able to go to larger radius: separates showers better– Highly segmented calorimeter allows measurement of shower development

• One photon? Two? A hadron masquerading as a photon?

Both calorimeters are quite thick– Improves resolution (showers are contained)– Degrades electron-hadron separation

• ATLAS measurement of shower development is intended to compensate

40

Jets

When you’re a jet, you’re a jet all the way… S. Sondheim

41

A Two Slide Review of Jets

A “blast” of particles, all going in roughly the same direction.

Calorimeter View Same Events, Tracking View

2 jets 2 jets

3 jets 5 jets

2 2

3 5

42

More on Jets

Where do they come from? 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• Process is called “fragmentation” or

“hadronization”

g

g

g

g

One (of several) processes that produce jets in collisions.

43

Measuring Jets

Lego plot of a top quark event has EM calorimeter energy in green and hadronic calorimeter energy in red.

The prevalence of green comes from two facts:

– Half of the particles (and ~40% of the energy) in a jet are photons from neutral meson decay

– The LHC EM calorimeters are thick, and many hadrons begin their showers inside the electromagnetic calorimeters.

ATLAS

44

ATLAS “Tile” Hadronic Calorimeter

Uses steel as an absorber, and scintillating tiles as the active medium. Energy is converted to light.

45

CMS HCAL (Hadronic Calorimeter) Also a scintillating tile-

based sampling calorimeter

Technology is similar to ATLAS:

– Absorber is brass instead of steel

– Tile orientation is different (more conventional in CMS)

Calorimeter is relatively thin

46

Comparing Design Philosophies

CMS’ calorimeter is inside their magnet– Additional depth is very expensive (since it requires a larger radius magnet)– To increase the effective thickness, it’s made of a denser material (brass)– ATLAS is quite thick – the outer muon detector resembles the “iron ball”

design.

ATLAS would “naturally” use liquid argon for both the hadronic and electromagnetic calorimeter

– Both D0 and H1 designed their detectors this was– This is also very expensive

Both experiments have chosen to economize here– Slightly better performance would cost a lot more money

47

Jets & Hadron Calorimetry

Event with the best calorimeters, you are taking a very clear picture of a very fuzzy object. Economizing on the hadron calorimeter is often the “least bad” option.

Many things besides hadronic calorimeter response affect the jet energy determination: the EM calorimeter response, out of cone corrections, dead material and dead material corrections, hadronic decays or interactions upstream of the calorimeter…

48

Tops

49

Top Quark Pair Production

Top pair events are characterized by the decay of the two W’s in the event.

%99)( WbtBF

50

Early Top Quarks at the LHC

Start with a lepton (e or ) plus four jets sample.

Make all three jet mass combinations, requiring m(jj) = m(W)

Identifying one jet as containing a b quark (“b-tagging”) is not required.

Why is the top signal so clear without b-tagging?

Especially since the Tevatron needed b-tagging to discover the top quark?

51

Top Quark Production

At the Tevatron, the left column diagram dominates.

– The W+jets background is also produced by quark-antiquark annihilation.

At the LHC, the right column diagrams dominate.

52

The Most Important Slide I Will Show (Again!)

Measured cross-sections (except for Higgs) at the Tevatron

How to extrapolate to the LHC

From Claudio Campagnari/CMS

jets

53

Why Bother To B-Tag at the LHC?

A signal to background of 1:2 is fine for a discovery, but one would want to do better for precision physics.

B-tagging keeps 80-90% of the signal, but cuts the blue background by an order of magnitude.

B-tagging also helps with the combinatoric background:

– Without b-tagging, there are 12 combinations in every event

– Tagging one b-jet reduces this to 6

– Tagging both b-jets reduces this to 2 (keeps ~40% of the signal)

54

Fake B Tags

Solution: redundancy, redundancy, redundancy

– More hits reduces your sensitivity to lost/misassigned hits

– More hits increases the standalone tracking capability of your silicon• It takes 5 numbers to parameterize a helix

– More hits improves the resolution of each track

Missing one hitcan turn this

into this:

55

LHC Silicon Vertex Detectors

CMS

ATLAS

ATLAS Pixels

56

Electrons & Other Tracks

The figure of merit for momentum measurement is BL2. A half dozen layers of silicon close in has great impact parameter resolution – but L is tiny.

To improve the momentum resolution, the experiments need to build out from their silicon detectors.

CMS chose to keep going with silicon, and add another half dozen layers

– By far, the largest silicon HEP detector ever built.

CMS Silicon Detector

The key idea: BL2 – relative to what?

Reminder: to tell an electron from a photon, recognize that an electron has a track (of the right momentum) in front of the cluster, and a photon has no track.

57

ATLAS Tracking Philosophy

Instead of adding 6 precise points from silicon, ATLAS adds 36 “pretty good” points from a tracker based on wires inside straws.

The detectors are designed to be sensitive to transition radiation, so can be used to aid in electron identification.

This detector design will not work at the very highest luminosity – when that happens, ATLAS may replace with silicon.

58

Missing ET

59

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)Electron momentum

Missing ET

Momentum of the “underlying event”

eW

60

Pink is the New Black

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

…and neutralinos are the new neutrinos.

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

~

ee~

nnn~

(violates conservation of angular momentum)

(violates conservation of lepton number)

(violates conservation of baryon number)

61

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.

62

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

63

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

64

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 4 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.

65

Why are LHC experiments a little longer than Tevatron experiments?

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 $

D0 detector

66

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…

With apologies to Spinal Tap

67

The Most Important Slide I Will Show (Yet Again)

Measured cross-sections (except for Higgs) at the Tevatron

How to extrapolate to the LHC

From Claudio Campagnari/CMS

jets

68

Triggering – the Oft Overlooked Component

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.

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.

69

Early and/or Interesting Measurements

70

Quark Contact Interactions

New physics at a scale 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.

t - channel

QCD

Quark Contact Interaction

M ~

Quark Compositeness New Interactions

M ~

Dijet Mass <<

q

q

q

q

q

q q

q

Dia

gra

ms:

R.

Ha

rris

, C

MS

71

“Week One” Jet Measurements

Expected limit on contact interaction: (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

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.

72

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

73

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

74

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.

75

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

cos *

QCD Background

Signal

0 1

dN /

dc o

s *

*

Center of MomentumFrame

Parton Parton

Jet

Jet

Dia

gra

ms:

R.

Ha

rris

, C

MS

76

Angular Distribution of a Contact Interaction II

The D0 (hep-ex/980714) dijet ratio: N(|| < 0.5)/N(0.5 < | | < 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.

77

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+

78

What is the Standard Model?

The (Electroweak) Standard Model is the theory that has interactions like:

W+

W+

Z0

Z0

but not

Z0

Z0

W+

W-

Z0

W-

W+

&

but not:

Z0Z0

Z0

&

Z0

Z0

Only three parameters - GF, and sin2(w) - determine all couplings.

79

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

The Semiclassical W

80

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: g1

Z, , 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 = -e( - )/2MW2

– Dimension 4 operators alter g1Z, and Z: grow as s½

– Dimension 6 operators alter and Z and grow as s

Do we live in a Standard Model universe? Or some other universe?

81

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

82

Not An Isolated Incident

Qualitatively, the same thing happens with other couplings and processes

These are from WZ events with g1

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

83

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…

84

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

Coupling Present Value LHC Sensitivity (95% CL, 30 fb-1 one experiment)

g1Z 0.005-0.011

0.03-0.076

Z 0.06-0.12

0.0023-0.0035

Z 0.0055-0.0073

022.0019.0016.0

044.0045.0027.0

020.0021.0028.0

061.0064.0076.0

063.0061.0088.0

85

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 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.

86

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 ’s and Z’s

– At loop level, there are ~10-4 corrections to this

– It is vital that these be explored

87

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.

)~)(~()~)(~(~~ WbWbtttt

But there’s a fly in the ointment…

88

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.

Effective

BAAB

Inelastic

BAAB sA

ˆmight be better

characterized by

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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.

90

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:

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Let’s Do It One More Once…

Measured cross-sections (except for Higgs) at the Tevatron

How to extrapolate to the LHC

From Claudio Campagnari/CMS

jets