road to discovery: lecture 1 sarah eno u. maryland june, 2009 1 cern-fnal hcp summer school please...

CERN-FNAL HCP Summer School 1

Road to Discovery: Lecture 1Sarah Eno

U. Maryland

June, 2009

Please note that this road approaches its destination slowly, for safety reasons.


Credit where credit is dueI stole most of my slides from• Chris Hill,Beate Heinemann,Joe Incandela,Roberto

Tenchini, Andy Parker, Tommaso Lari, Silvano Tosi, Klaus Rabbertz, Kostas Kousouris, Joey Huston, J. Fernandez, Sasha Nikitenko, Saeid Paktinat, Piotr Zalewski, Barbara Clerbaux, Christian Bauer, Roberto Rossin, Alexey Drozdetskiy, Mojataba Najafabadi, Peter Richardson, Fedor Ratnikov, Valerie Halyo, Seong Chan Park (and people I’ve forgotten to mention)

• Also, note specific citations of articles

June, 2009


Outline• Lecture 1: Lessons from an old person,

Standard Model at LHC• Lecture 2: Early Exotica• Lecture 3: SUSY & Higgs

June, 2009


CMS/D0 Bias

June, 2009

I am currently a member of both the D0 and CMS collaborations, so its just easiest for me to get examples. Most of the results are similar between D0/CDF and CMS/ATLAS. I’ll try to note where major differences are expected.

(CDF and AMY in my younger days)


Discover what?

June, 2009

Questions of the standard model• What causes electroweak symmetry breaking?• Where is the higgs?• Why is it light?• Why is there so much more matter than antimatter in the universe??

Question from cosmology• Where is the dark matter particle?• What is dark energy?

And Beyond• Why do the fermions have such different masses?• Why are neutrinos so light?• Why are neutrinos not masses?• Do the forces unify at a high energy scale?

etc. etc. etc.

An exciting time, as there are so very many questions:


Discovery at the LHC*

June, 2009

Simple, right? LO description of a new particle search• think about what distinguishes your signal from background• design some variable or combination of variables (boosted decision tree, NN, etc) that maximizes the differences• cut away most of your background using some criteria that optimizes significance• estimate the remaining number of SM events (production cross section, BR, acceptance, efficiencies)• see how many pass and feed that and your estimated background to RooStat and see what you got

But of course the devil is in the details.• need to use well-understood variables in a well-understood detector for well-understood background processes

Look at past devils. Look at best guess of our own devils.

(* will concentrate on “discovery” through new particle searches in these lectures.Obviously, much important new physics can be done in B physics, QCD, etc as well.)

10 Feb 2009 Colloqium, Boston University 7

It’s been a long time…1983: Tevatron reaches 500 GeV

1985: Tevatron reaches 800 GeV

1987: First collisions in CDF

19821987

20 years!

Installing main ring dipole magnets Our lessons come from long ago. Changing times will affect these lessons.


History of Discoveries

June, 2009

c,τ

Even though we used to cross the energy frontier regularly, major discoveries come about once/decade.

b

Top and W/Z


CERN Z0 1983Tevatron, top, 1995

joint MD-Hopkins Mtg 109 Jan 2008

Very Different?

The SppS turned on at 1% of final instantaneous luminosity in 1983, but in the first run of a few months discovered the W with 8 events.

Z0

sps

Tevatron turned on in 1987. Top discovered 8 years later.


Actually had a lot in common• ALL properties of the signal tightly constrained by standard model •Signal shows up in multiple channels• with FIXED RELATIONSHIPS• not only does the background agree with the background prediction, but THE SIGNAL AGREES WITH THE SIGNAL PREDICTION• searches are easy (in principal) to design (optimize s/root(b), and it’s (relatively) easy to interpret the results


Z to bb

Would you believe this if it weren’t the Z?

When what you are looking for has very well defined properties, you can do remarkable things


Single top

June, 2009

CDF


Tau/charm discovery

June, 2009

When the new physics is unexpected, things are much more challenging

http://www.osti.gov/accomplishments/perl.html


tau/charm

June, 2009


“Discoveries”

June, 2009

That went away• contact interactions• pentaquarks• leptoquarks• 40 GeV top quark• zeta• Mark J events• eeγγ event

That were missed:Many interesting, apocryphal stories involving overflow bins, a frantic sorting of stacks of punch cards … stories people only discuss after much beer. (Could CLEO could have caught this before Babar?)

When looking for something that is not well predicted, it is much easier to make mistakes (missed systematic, cut tuning) or be mislead by a statistical fluctuation.

There are good reasons to be sceptical about new discoveries!


Discrepancy?

June, 2009

When we see disagreement, what can it be? * theory cross section for SM processes wrong? * theory kinematic distributions for SM processes wrong? * particle ID fake rate is wrong? * particle ID efficiency is wrong? * energy scales are wrong? * resolution is wrong?

When you have eliminated the impossible, whatever remains, however improbable, must be the truth (Sherlock Holmes)


How to minimize devils

June, 2009

How can we minimize these errors:• ratios of cross sections• get efficiencies and fake rates energy scales, resolutions from data whenever possible• have a strong group tuning the MC (both theory of background production properties and detector resolution in close contact so as to not mistake one for the other.)• have many data-based methods for checking the background calculation

Most important advice to young particle hunters based on past experiments?• know your detector• know your backgrounds and ALL the sources of their uncertainties

ATLAS CMS

Magnetic field 2 T solenoid + toroid (0.5 T barrel 1 T endcap)

4 T solenoid + return yoke

Tracker Si pixels, strips + TRTσ/pT ≈ 5x10-4pT + 0.01

Si pixels, stripsσ/pT ≈ 1.5x10-4pT + 0.005

EM calorimeter Pb+LAr σ/E ≈ 10%/√E + 0.007

PbWO4 crystalsσ/E ≈ 3%/√E + 0.003

Hadronic calorimeter

Fe+scint. / Cu+LAr (10λ)σ/E ≈ 50%/√E + 0.03 GeV

Brass+scintillator (7 λ + catcher)σ/E ≈ 100%/√E + 0.05 GeV

Muon σ/pT ≈ 2% @ 50GeV to 10% @ 1TeV (ID+MS)

σ/pT ≈ 1% @ 50GeV to 10% @ 1TeV (DT/CSC+Tracker)

Trigger L1 + RoI-based HLT (L2+EF) L1+HLT (L2 + L3)

CERN-FNAL HCP Summer School 20June, 2009

(Nice comparison by Chris Hill)


No detector ever had this much material before the calorimeter. D0 had 0.3 X0

Before Beam: Cosmics

2 Apr 2009 "Shedding Light on Dark Matter", U. MD. 25ATLAS and CMS may be the best understood detectors ever at turn on.

Beam Splash Events

CMS

146 m

TAN

TCTV

TCTH

MBRC

TCLP• Beam with 2x109 protons dumped

onto a target (collimator)150m upstream of CMS– Sept. 7,9,10,18

• Leads to a “tsunami” wave of O(100K) muons coming down the tunnel!– A far cry from the single cosmic muon events…

2 Apr 2009 "Shedding Light on Dark Matter", U. MD. 27

CMS lights upHCAL energy ECAL energy

DT muon chamber hits

debris

Inner tracking systems kept OFF

2 Apr 2009 "Shedding Light on Dark Matter", U. MD. 28

ATLAS also lights up


Well-calibrated

June, 2009


Well-Commissioned Detectors

June, 2009

Atlas and CMS detectors commissioning data: better tuned than any large detector! (at least that I've worked on) however, even well-tuned detectors show surprises D0 Run II Z mass and MET (every body likes to show this, however D0 Run II was not a well-tuned detector)

But probably more interesting is D0 Run I Z mass (this was also a well-tuned detector)


Can still do a lot with a new detector, even with surprises

June, 2009

Peak at 86 GeV.

Run Aug – Oct 92. Some plots from a Nov 92 talk (Madaras)

Don’t panic. Rescale the EM energy scale and move on.


Can still do a lot with a new detector

June, 2009

Run Aug – Oct 92. Some plots from a Nov 92 talk (Madaras)

W events

(MET obviously was quite useable in this brand new detector.)

3326 Jan 07

Can do a lot with new detectors•In 1983, UA1/UA2 discovered the W at 1% of design luminosity within a few months of running.• First publications by UA2 were “Production and Properties of Jets” (1982) and “Discovery of the W” (1983) •The Tevatron (with CDF only) had a small run in 1987 Run, (33 nb-1). The first search presented by CDF was a publication submitted 24 July, 1989 (http://prola.aps.org/abstract/PRL/v63/i14/p1447_1) of a search for heavy stable particles, done with 26.2 nb-1 of data. •The Tevatron (with CDF only) had another run the next year ( June 88 - June 89). The first search was a paper on the search for the top quark, submitted 13 Oct 1989 (see http://prola.aps.org/abstract/PRL/v64/i2/p142_1) •D0 first took data during the Tevatron's "Run I". 1.1 pb-1 of data was taken from August to October 1992 and was presented in November in conferences (see, for example, http://lss.fnal.gov/archive/1992/conf/Conf-92-365.pdf) (albeit with D0 showing a Z peak at 86 GeV!). Results on new particle searches (top quark search) were presented at conferences even after this early running. This run eventually accumulated 200 pb-1 of data. (Note two detectors were running at this time). The first paper was a leptoquark search.

http://prola.aps.org/abstract/PRL/v63/i14/p1447_1

http://prola.aps.org/abstract/PRL/v64/i2/p142_1

http://lss.fnal.gov/archive/1992/conf/Conf-92-365.pdf


Devils

June, 2009

Enough generalities…What will be the challenges specific to the LHC?


Lots of Backgrounds

June, 2009

Most of the backgrounds can be made small by requiring an isolated e or mu, or large MET, in the final state. (apologies for neglecting taus in all these talks)

(However, there can be large uncertainties on the remaining “QCD” background. In general, the simulation can not reliably predict these backgrounds because• fake rates from MC are generally good to within a factor 2 at best• tails of MET distribution even less reliable• need higher order QCD calculations for kinematics, which are not (all) available• due to the small fake rate, would need huge MC samples to do thisData-based background methods are required.)


Cross sections

June, 2009

process order 14 TeV(pb)

10 TeV(pb)

reference

ttbar NLO+NLL 910 430 arXiv: 0804.2800

Single top NLO 320 160

W->eν or μν NNLO 22000 15000 arXiv:0901.0002

Z NNLO 2000 1400 arXiv:0901.0002

WW NLO 112 1

WZ 48

ZZ 15

Also see arXiv:hep-ph/0611148

After lepton or large MET requirement, main backgrounds tend to be top, W, Z, dibosons


Machine Plan

June, 2009

50-300 pb-1 at 9-10 TeV

http://lhc-commissioning.web.cern.ch/lhc-commissioning/luminosity/09-10-lumi-estimate.htm


10 vs 14 TeV

June, 2009

Unfortunately, most of the studies done so far are at 14 TeV. ratio of parton luminosities plots

gg of course down more than qq

Stirling


10 TeV vs 2 TeV

June, 2009

How much luminosity do we need to beat 8 fb-1 (now 5) at the tevatron? The tevatron can probe up to sqrt(s) = 2 TeV, so its not a spent machine.

8000300

=26

O(5) events for discoveryO(8) before acceptanceO(30) fb xsec at these masses

Stirling

26


Devil 1: Measuring Luminosity

June, 2009

How many background events do we expect? Need to know luminosity • relative luminosity by rate of inelastic collisions (measured in some sort of forward detector)• absolute normalization?


Luminosity Measurement

June, 2009

10 TeV

TOTEM experiment will measure this directly, but its not a trivial measurement.


Dead Reckoning

June, 2009

10%


Underlying event

June, 2009

The first thing we will see is a few soft scatters (MB events). Generally, these are not backgrounds for our searches. However, these are useful for our search preparations, as they tell us about the UE. The UE:

• affects efficacy of isolation variables• affects MET resolution, especially for smaller values of MET• may be quicker in the beginning to measure effects from data than try to retune the MC.


Minimum Bias

June, 2009

Model expectations for chargedparticles at |η| = 0 vs. √s:

Pythia: ~ ln2(s) Phojet: ~ ln(s)


ATLAS Track reconstruction

Properties of MB events

CMS

How will you handle the news from these analyses in your search?


Jets

June, 2009

Next we’ll see production of high pT dijets, multijets.• calibration and understanding of jets for searches• checks on PDF’s, ISR, FSR in new environment• searches for new particles decaying to dijets

This is not my generation’s QCD. Much better calculations. But, it will pay to understand them and their limits.

Jets will give us our first indication as to how well these calculations are doing at the new energy.


dijets

June, 2009

Inclusive kT cross section in 6 bins in rapidity y, D = 0.6

NLOJET++, PRL 88 122003 (2002) PR D68 094002 (2003)fastNLO, hep-ph/0609285

NLO code by NLOJET++, Z. Nagy

LHC @ 10 TeV

1 event in 300 pb-1

The LHC will be a very jetty place.

Tevatron limit

1 event in 10 pb-1


Jet Cross Section

June, 2009

A non-trivial measurement: both theory and experiment have non-negligible uncertainties.


Uncertainties

June, 2009


Theory Uncertainties

June, 2009


Jet Calibration

June, 2009

A non-trivial problem• dijet balance to check eta dependence of calibration. Runs out of statististics at high pT• photon plus jets can help towards an absolute calibration. Runs out of statistics at high pT. Non-negligible backgrounds• Z+jets can serve as a cross check at low pT.• very important for compositeness searches. Less accuracy is needed for many other searches.

Dominant: > 80%< 20% LO

Eur. Phys. J. C 53, 49-58 (2008).

CMS photon study:- Photon pT spectrum for 1/fb- Background QCD jets in blue- After photon isolation cuts- Improves S/B > 2 orders of magnitude


Calibration Challenges

June, 2009


Systematics

June, 2009

At high pT, sizeable uncertainties on JES and PDF. At low pT, sizable unsmearing correction.


Z

June, 2009

Cross section is lower than dijets and W, but it’s a very very clean channel that allows full kinematic reconstruction with low systematic errors.

• peak position can be used to check/set EM calorimeter and tracking energy scale• width of peak can be used to check/set resolutions• rate of double tags can be used to estimate lepton identification efficiencies• theory errors on cross section a few percent -> can be used to check/calibrate luminosity• balance between Z transverse momentum can be used to understand jet energy scale, resolutions, MET resolution.• can be used to understand vector boson plus jet production and other vector boson backgrounds to searches (discuss when I discuss susy searches)• usually not a significant background to searches• PROBLEM: low cross section compared to the processes you use it to study

In 300 pb-1 at 10 TeV, expect 420,000 produced per lepton flavorWith typical acceptances of 40%, we expect O(150k) events/lepton flavorZ samples from Tevatron currently at: 50k/fb-1 (with roughly 6 fb-1 on tape->300k/flavor)


Very low backgrounds

June, 2009CMS estimate for 14 TeV, 10 pb-1

14 TeV


Tag and Probe

June, 2009

How will you feed these results into your analysis?


resolutions

June, 2009

D0


MET resolution

June, 2009

Zee MC versus data with and without d0raw2sim: Dzero

D0


Challenges

June, 2009

• limited range of electron ET’s to check detector non-linearity, disentangle offset and slope in energy scale, disentangle various terms in the resolution• lower statistics than W and top


Other di-lepton resonances

June, 2009

1 pb-13 days at 1031 at 30% efficiency

ATLAS preliminary

J/ψ

ϒ

Atlas example, after all cuts:~ 4200 (800) J/ψ (ϒ) evts per day at L = 1031

(for 30% machine x detector data taking efficiency)~ 15600 (3100) events per pb-1

tracker momentum scale, trigger performance, detector efficiency, sanity checks, …

Challenge: if not careful, distributions/efficiencies/ resolutions can be shaped by trigger biases. Needs care.


W

June, 2009

Need to understand well, since a copious source of events with leptons and METLuckily, cross section grows slower with root(s) than top cross section (another great calibration source)

Can be a good calibration source once understood

Challenges • pp is not ppbar (true for Z as well)• larger backgrounds (for top, for example), more difficult to understand• two solutions for pZ of neutrino (and louzy resolution, especially at high <n>)

In 300 pb-1 at 10 TeV, expect 4,200,000 produced per lepton flavorWith typical acceptances of 40%, we expect O(1.5M) events/lepton flavor

W+ vs W-


Hep-ph/9907231arXiv:0901.0002v1

More u’s than d’s in proton so W+ cross section is bigger


Heavy quarks in proton

June, 2009

Note importance ofstrange quarkuncertainty for ratio


W+ vs W-: TeVatron and LHC

June, 2009

arXiv:0812.2571


Rapidity W+

June, 2009 Huston

CERN-FNAL HCP Summer School

Inclusive W’s

June, 2009 66

Ouch!

How big will the backgrounds be?

14 TeV


Inclusive W’s

June, 2009

10 TeV, 10 pb-1


Top

June, 2009

• not just a candle, a candelabra (Ken Bloom, Nebraska)• hadronic W’s -> jet energy scale• b tagging efficiency• b jet energy scale• consistency over many channels gives strong constraint (dilepton, single lepton, all hadronic, with/without b tags)• important background to many searches

Efficiency* acceptance O(20%) in leptonic channelsExpect 17K l+jets and 2k ll+jets (l=e,mu)


top

June, 2009

(J. Huston, Acta Physica Polonica B, 38, 2279)

Challenges• as with the W, in channels with leptons, reconstruction not trivial• its jetty at the LHC


Top dileptons

June, 2009

No b tagging10 pb-1

14 TeVCMS

2 iso l PT > 202 jets PT > 30

Mee, −MZ >15

MET > 30

MET > 0.6rPT

ll or MET not along -rPT

ll


Add b tagging

June, 2009

100 pb-1


semi-leptonic top

June, 2009

cms top-08-005


Single Leptons

June, 2009

mu10 pb-1No b taggingtight selection


ATLAS

June, 2009

ADD B TAGGING


Light Quark calibration

June, 2009


Calibrate using W

June, 2009


Single top

June, 2009

Cross sections (pb)

ttbar Single top ratio

Tevatron 7.60 3.94 1.9

LHC (10 TeV) 446 160 2.8

LHC (14 TeV) 910 320 2.8

240 pb @ 14 TeV 10 pb

60 pb


dibosons

June, 2009

Channel Xsect (14 TeV) (pb)

BR >1 e or mu

Events in 300 pb-1

WW 112 0.4 13k

WZ 48 .26 4k

ZZ 15 .12 0.5k

“at least 1 lepton”

WZ with 3 leptons, 350 pb-1


Devils

June, 2009

However, just remember that just because we can quickly use our data to see if we basically understand the major backgrounds, there can always be corners of phase space that are still incorrect in the MC. It’s in these corners that we will often be looking for our signal.

road to discovery: lecture 1 sarah eno u. maryland june, 2009 1 cern-fnal hcp summer school please...

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