of data getting to a usable 100pb some steps along the...

2008 UCLA

Getting to a Usable 100pb-1 of dataSome steps along the way

Avi Yagil

UCSD

West Coast Theory Workshop

West Coast Theory Workshop Avi Yagil 22008 UCLA

Not going to tell you how to help us…

• Of course, will tell you anyhow

Obvious things:•Calculate S.M. processes

–Better accuracy–Understand how well you do, uncertainty estimates !!

•Think of things we could miss:–Is it in the trigger? Selection?

•“boosted top”•“Long Lived” particles•…

>> BUT, it will help to know better:~what we do~how we do it


Table of (possible) Contents

• Some startup perspective– Machine (Tevatron)– Detector (CDF)

• Its all about the Trigger– Rates & cross sections– Beam Crossings– What do we trigger on?– Trigger Table (example)

• Some trigger related terms– Trigger paths– Prerequisites– Volunteers– Efficiency/Dead-time– “backup” triggers

• Objects reconstruction– Jets

– Missing Et

– Tracks

– Electrons

– Muons

– Missing Et (revisited)

• Examples of searches for NP– Self calibrating signals

– Counting experiment (easy)

– Counting experiment (tough)

– Shape analysis


Talks to look at

• Ian Hinchlif– Early Physics at LHC (06)

• Zvi Bern– QCD for LHC: What needs to be done? (06)

• Claudio Campagnari– Searches for NP at CMS (06)

• Michelangelo Mangano– Introduction to hadronic collisions: theoretical concepts and practical

tools for the LHC (08)

• Robin Erbacher– What Experimentalists Need from Theorists? (08)

2008 UCLA

Startup

Recent “historical” perspectiveTevatron/CDF RunII


This is not a putdown !

Just a reminder of how hard these things actually are.

Eventually, the Tevatron did break many Lumi records.

It just takes time…


Start of RunII - CDF perspective

Main startup dates:– Engineering run (~6 weeks): Sep-Oct. 2000

– Cosmic running: Nov 2001 - March 2001

– Roll-in: January 2001

– Commissioning with data: April 2001 - February 2002

– Official start of Physics run: February 2002

– First usable offline production release: Fall 2002


What do you have at startup - PR plots


Paper Trail

• First Paper– Ds

+ and D+ mass difference– Submitted March 2003 - two years after we rolled in (January 2001)

• First High PT paper– Top pair cross section in di-lepton– Submitted April 2004 - three years after we rolled in

• First limit paper– H++/-- in dilepton, Submitted June 2004

• First paper with jets and missing energy involved– Single top search, Submitted October 2004

• First Electroweak precision measurement– Mtop = 173.5 +3.9

-3.8 GeV, will be submitted August/September 2005

It takes tim

e

2008 UCLA

First things First,or

Its all about the Trigger


Rates…

Physics Cross Sections:• Inelastic: 109Hz • W l : 102Hz • t t production: 10 Hz• Higgs (100 GeV/c2): 0.1 Hz • Higgs (600 GeV/c2): 10-2Hz

Rejection needed - 1010/11

•250 GeV ET Jets - 1kHz


Time to think.Bunch Spacing…

Less time to think.

MUCH more to think about…


Physics Content of collision-data

• In LEP or a B-factory, ~every event is “important”.

• Hadron collider environment is “Physics poor”, or“dirty”, or “challenging”,… :1. Must have a very sophisticated trigger (or at least harsh)

2. Dangers of very complex, irrevocable online decision process

==>Major trigger challenge!

To make matters worse:

• There is not much time to “think” and “decide”between crossings (consider detector size)

• Physics rate is very low with respect to collisions rate


Trigger - The big picture

• The role of the Level-1 trigger is to take an LHCexperiment from the 25ns to the 10-25μs timescale.– Custom hardware, big switches, Gb/s rates– Simple, coarse & fast algorithms

• Experiments differ on the implementation of the nextlevel of filtering:– Commercial hardware, large networks, Gb/s rates..

• Either: Multi level filtering (Level-2, 3…)• Or: A large software-based High Level Trigger

• Would like to make the High Level Trigger filtering assimilar to the offline analysis software as possible.– Very large PC farms

Monitoring and error detection are crucial and non-trivial!


P. Sphicas, Trigger @LHC talk (06 SSI)


What do we trigger on?Basics

Basics underlying observations (limitations):– Hadron colliders produce mostly low momentum hadrons

– Most interesting Physics has signatures involving large transverse energy(ET) particles.

==> Require high ET on reconstructed particles

• Basic objects used for trigger:– Electrons, Photons, Muons, Jets, Missing Et

• The more complexity one adds to a signature– the lower the rate, and hence the required threshold(s)

• The more complex the signature– the harder it is to understand (measure efficiency, monitor performance)


What do we trigger on?Some numbers

• Objects used for trigger:– Electrons, Photons, Muons, Jets, Missing Et

• A few approximate thresholds and associatedrates:– Single muon with PT>20 GeV - 10kHz

• Double muon PT>6 GeV - 1kHz

– Single em cluster with ET>30 GeV - 10-20 kHz• Double em cluster with ET>20 GeV - 5 kHz

– Single jet with ET>300 GeV - 0.2-0.4 kHz

– Missing Transverse Energy, is another story…

2008 UCLA

Trigger Nomenclatureor

some buzz-words and concepts

•trigger table•trigger path•pre-requisites•Volunteers•efficiency, dead-time•backup triggers•trigger x-section•pre-scales


What is a Trigger Table?

Trigger table is how our trigger menu is called.– It is a list of selection criteria– Each item on the menu:

• Is called Trigger Path• Has a few “courses”: L1, (L2) and HLT recipes:

– Set of cuts, parameters, instructions specific for each level.

– An event is stored if one or more trigger pathscriteria are met.

– Each time data taking starts (a new run), the wholecontent is communicated to the system (run control)

– For bookkeeping, all menus and recipes are stored ina specially designed database.

• Understanding of the data (a.k.a. analysis) depends on it!


Trigger Path

• The “name” of the sequence of requirements thatresults in an event being recorded:– L1 accept– L2 accept (if present)– HLT confirmation of:

• Object type(s)• threshold(s)• ID cut(s)Example:

emu_20_20_iso_met_50hipt_emu_met

• There has to be a bit for each one.– In a “global” trigger summary word(s).

• Defines how events are classified (immutably!)– Implication: complete exploration of all L1 accepts

185 such paths in CDF(not at the start…)


HLT Trigger Table - Example (CMS)

Electron, photon

muon

tau

jets

jets,met, HT

~60%


Pre-requisites, Volunteers

• Pre-Requisite:– Only muons that have a L1 accept are pursued in the HLT.

– Moreover, only that region may be even looked-at(reconstructed).

• Volunteer:– A muon “found” in the HLT, without a corresponding L1 accept

• Possible Convention: such cannot be the cause of a trigger decision(CDF/CMS)

– Cannot happen if only “seeded” (on L1 muon track)reconstruction is pursued in HLT

– Can happen if global reconstruction is performed.

– Very useful in understanding trigger efficiencies


Efficiency and Dead-time

• Goal of trigger is to maximize collection of data forthe physics processes of interest:– Aim for high efficiency !

• For each process, look for:

trigger= Ngood(accepted)/Ngood(Produced)– And watch the dead-time !

• Trigger Dead-time:– Incoming rate is higher than processing rate

==> interactions are rejected due to system being busy

• Buffering of incoming data could reduce dead-time

• But dead-time always incurred if:– <incoming rate> larger than 1/<processing time> !


Trigger Efficiency - How to?

• How does one measure trigger efficiency?– Especially L1 trigger is hard. Why?

– How does one make a plot like this:

– What is the main challenge?

– The solutions:• Special backup triggers

• Usage of volunteers

• MUST be thought out in advance!


Trigger Cross sections

• For any process:rate R= L(L = instantaneous luminosity, = cross section.)

• For a physics process, is independent of L.• For trigger cross sections, we observe:

= A/L + B + CL + DL2

– A,B,C,D are constants depending upon trigger.– High purity triggers typically have C~D~0.– Two effects cause extra powers of L:

• Overlapping objects from different interactions.• Fakes that are luminosity dependent.

• Rates: R=L = A + BL + CL2+ DL3

opt


“Backup” triggers

• Triggers are not only used to look for that specialsignature (signal) one is interested in

• They are also used forcalibration/efficiencies/background studies

• Term backup is misleading

• For example, for top analyses, need to:– Measure L1/L2/L3 signal trigger efficiencies– Develop and tune soft lepton taggers– Calibrate b-tagging efficiency– Calibrate jet energy scale– …


Pre-scales

• Inclusive jet triggers - rate too high to take them all

Sample

• Lower threshold -- higher pre-scale

• Lower threshold at lower trigger level– “sharp” cut at HLT

• “assemble” spectrum

• Fancy:

dynamic prescales

opt

2008 UCLA

The Objects We Use…


Generic Object Detection and ID


General Object Reconstruction

• Interpretation of Digitized Channel Information as Hits– Combine neighboring channels (clustering)– Extract info:

• Position (time-to-distance, local-to-global)• Energy (D to E, scales, pedestals…)

• Establish Associations within Individual Sub-Systems– Pattern Recognition

• Fitting– Determine energy, momentum and/or position

• Correlating Among Sub-Systems - Compound Objects– Track extrapolation to calorimeter, muon systems

• Refining Objects– Best estimate of parameters using all information

Stolen & distorted from M. Shapiro


Missing Et (basic)Be Careful !!

Seems the easiest of all:

• Vector sum all calorimeter cells ==> Missing ET

Also: Sum of all the bad things that happened in the detector…

– After good run selectionfrom DQM

– After a vertexrequirement in trackingdetectors

– After additional pre-selection cuts


Tracks

The most computationally and algorithmically challenging thing we do

• Hit finding– Cluster charge (in individual Si wafers) to form clusters

– Extract position, width

– Determine Global position (from local coordinates)Geometry, alignment…

• Pattern recognition– Associate hits in various “layers” to each other

• Fitting– Extract tracks parameters (PT, Phi, impact parameter…)

• Special cases– Electrons, KS, Lambdas, conversions…

• Vertexing– Primary Vertex finding (multiple interactions)

– Secondary vertex finding (b-tagging)


Jets• Basic approach (used in trigger):

– Calorimeter cells are “particles”– Cluster energy in e.g. fixed circle in , space– Add all cells within radius– Iterate (can merge/split)– Extract PT, , of jet

• Tricky topic! (hadrons to partons, IR issues, color etc.)

• Various working definitions, trade-offs– QCD studies, Jet spectroscopy,…

• LOTS of jets, HUGE range of types of jets, due to fragmentationand Physics content (e.g. Heavy Flavor):– Track multiplicities– Electromagnetic fractions==> Prolific source of fakes to all lepton objects !


Photons

• Most of things that look like photons are jets– Tail of fragmentation function - ~a single 0

– Not separable on event-by-event basis in calorimeter (especially at high Et)

Fake Probability per jet,~10-3 -10-4 D0


Leptons - General

• When we say electron or muon, without qualification, we mean:

”W/Z-like” e or μThat implies:

– High Pt ( >20GeV)

– Isolated (not much tracking or calorimeter activity)

– Prompt (zero impact parameter)

• Main Sources of backgrounds:– Leptons from b/c

– Decay in flight (for μ)

– Conversions, Dalitz decays (for e)

– Hadrons faking e or μ

• Typical fake rates (per jet):~ 1/10,000

For us: all Fakes


Electrons


Muons

• Signature:– Track in inner tracker

– Hits in muon detectors• Maybe a tracklet in the muon

– Calorimeter depositions consistentwith MIP

• Backgrounds:– Decays in flight of p and K

– Punch-Through

– Cosmic Rays

• Much cleaner then electrons (inCMS)


Taus

• Harder– Unlike e and μ, decays to hadrons

• Mostly one charged track (85% of time)– May be accompanied by EM energy ( 0)

• Three “prong” is harder– Looks like narrow jet in calorimeter

• Isolation is key


Missing Et (refined, corrected)

• Needs to be revisited after all else done

• Cleaned up for known detector/beam issues:– Hot/Dead cells– Beam/machine related effects

• Corrected for:– Muons (and other MIPS)– Pions calorimeter nonlinearities (could be big deal)– Jets and possibly jet energy scale

• Inspected for additional problems– Look for oddities (e.g. phi asymmetries, unexpected bumps in eta)

• Important parts of above process are:– Not universal– Signature and hypothesis dependent

2008 UCLA

Examples of few NP search “types”

But first, for something completely different…

2008 UCLA

Search for NP at LHC

and

The Poetry of D.H. Rumsfeld

The Unknown

As we know, There are known knowns. There are things we know we know. We also know There are known unknowns. That is to say We know there are some things We do not know. But there are also unknown unknowns, The ones we don't know We don't know.

—Feb. 12, 2002, Department of Defense news briefing


Looking for known unknowns

We know how to do these…

• Examples:– Higgs search (very similar to top search)

– You could almost write a WBS for this search.

You know all there is to know:- Production

- Decay

- Don’t know only mass

But even this is not so easy- top took ~20 yrs and a number of accelerators (Petra, Tristan, SLC, LEP, SPPS,

Tevatron) and detectors to find.


Looking for unknown unknowns

Not so easy


Possible approaches

- Historical approach- Look for peaks in di-leptons - J/ , . ‘ ??

- Psychic hotline approach- Bet on a model/channel - call your buddy theorist…

- “Maximizing” your chances- Use the tricks you know and the tools you got

- Look under the lamp - at least you’ll see it if its there…

- Hedge - cover as much of phase space as you can


Peak Hunting - High mass resonances

• Issues:– “self calibrating”, “easy”…

– cosmic background rejection

– muons misbehave

– …

• Primary samples:– di-muon

• Backup samples– inclusive muon sample

– lower threshold, prescaled inclusive muon sample

– non-isolated, higher threshold di-muon sample


Counting experiments, distributions

• Not all NP signals are as dramatic as a mass peak

• Need to establish whether data is compatible with SM SM prediction

• In some cases the SM prediction can come entirely from theexperiment (data driven)• Robust

• In other cases the SM prediction relies on theory• Not as robust.

Claims of a discovery based on MC.

• A few examples of some of these issues

Stolen (and distorted) from: C. Campagnari


Reliance on Simulation/Theory inputfor discovery

a lesson on reliance on MC

1984, UA1:– Using isolated high transverse momentum lepton– Requiring 2 or 3 hadron jets– Observed 5 events

(e+ >=2 jets);4 events ( + >=2 jets)

– Expected background: 0.2 events• fake leptons dominate• bb & cc production negligible

– Conclude: results consistent with M top = 40 ±10 GeV

1988, CDF:– x6 the data– much better understanding of backgrounds– Exclusion: M top below 44 GeV


Jet and Missing ET (e.g. monojet)

• An “easy” counting experiment

• Issues:– Sizable SM backgrounds– Understand Missing ET

– Tricky counting experiment!

• “Primary” sample:– Inclusive High Pt jets

• Main SM backgrounds:– Z--> +jet - Irreducible

• Use Z-->μμ sample

– W-->l +jet (lose l) - Real Met, veto leptons• Use W-->e/μ samples

– QCD - Fake missing Et• Use di-jet samples

– Non-Collision• Cosmic, beam

CDF


Comments

• Often purely data driven BG estimates do not work

• SM BG to LO have large normalization uncertainties– Makes counting experiments difficult

• SM LO event generators can have large shape uncertainties– Makes shape analyses difficult

• What are the uncertainties at LO? at NLO?– Often can get handle from data, e.g., W+jets vs Z+jets

• Where does the buck stops?– As an experimentalist, more comfortable if uncertainties are under my

control– As a theorist, you might feel differently– Hybrid approaches - Don't ask how sausages are made…

Stolen (and distorted) from: C. Campagnari


di-leptons and Missing ET

• A tough counting experiment!• Need to study and understand:

– WW, WZ, ZZ, DY, and top• Of course, fakes of all sorts

– As a function of:• Leptons Pt• Jet Et• Missing Et

• Look for deviations in rates– Rediscover top, WW

• Look for deviations in shapes– Missing Et– HT– Leptons pt’s

• What you allow yourself to usedepends on what you wanna use itfor…– Observation Vs. measurement– MC dependence


DY background prediction(s)

• Rate is very high

• Faking Missing ET is possible via:– Miss measurements of leptons PT

– Jets escaping detection or underestimated

– Combinations of the above

• Challenge - How many events left AFTER all cuts– Low statistics

– Poorly understood shapes

– Tails of detector effects and theoretical understanding

– Complex (sometime ugly) procedures


Conclusions

• Hopefully LHC’s recovery from Sep mishap will be quick– Current official estimates: late spring, early summer

– It will take time and we all should be patient and very careful !

• “First collisions” do not imply we are off and running– Forget day-1 physics!

– It takes time to understand, calibrate and debug thesedetectors and we must be patient and careful

• Preparation is key– We don’t know what awaits us

– The extra time we have is not a bad thing

– As long as its not too long…

• Either way, it will be the most exciting time in ourcareers lifetimes

of data getting to a usable 100pb some steps along the...

Documents