Signals And Backgrounds for the LHC: Part 2

Thomas J. LeCompte

Argonne National Laboratory

-or-

What They Were Thinking When They Designed ATLAS and CMS

2

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

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

Reminder…

3

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

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)Electron momentum

Missing ET

Momentum of the “underlying event”

eW

6

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)

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 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.

11

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

12

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

13

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

14

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.

15

Early and/or Interesting Measurements

16

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

17

“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.

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

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

22

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.

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

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.

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.

From Azuelos et al. hep-ph/0003275

100 fb-1, all leptonic modes inside detector acceptance

W+

W-

W+

W-

Yields are not all that great

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, W (present in SM) + ZZ, Z (absent in SM)

28

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

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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?

30

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

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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

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

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

34

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.

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 ’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.

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

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.

Effective

BAAB

Inelastic

BAAB sA

ˆmight be better

characterized 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 ( ~ 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…

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

How to extrapolate to the LHC

From Claudio Campagnari/CMS

jets