Joint Theory-CMS Seminar, Fermilab, May 31 st 2007 Peter Skands Fermilab / Particle Physics Division / Theoretical Physics Early LHC Physics: Theory Preparations

Joint Theory-CMS Seminar, Fermilab, May 31st 2007

Peter Skands

Fermilab / Particle Physics Division / Theoretical Physics

Early LHC Physics: TheoryPreparations for

Peter Skands Event Generator Status 2

OverviewOverview► Introduction

• A recipe for hadron collisions

• Models by the number, tunes by the score, and none the wiser?

► Theoretical implications of early LHC physics once …

• I got my tracker working

• I got my muon chamber working

• I got my jets working

• I got my photons working (+ electrons)

• I got my mojo working

► Who is who


► Main Tool

• Matrix Elements in perturbative Quantum Field Theory

• Example:

QQuantumuantumCChromohromoDDynamicsynamics

Reality is more complicated


Non-perturbativehadronisation, colour reconnections, beam remnants, non-perturbative fragmentation functions, pion/proton ratio, kaon/pion ratio, Bose-Einstein correlations ...

Soft Jets + Jet StructureMultiple collinear/soft emissions (initial and final state brems radiation), Underlying Event (multiple perturbative 22 interactions + … ?), semi-hard separate brems jets

Resonance Masses …

Hard Jet TailHigh-pT wide-angle jets

& W

idths

+ “UNPHYSICAL” SCALES:+ “UNPHYSICAL” SCALES:

• QF , QR : Factorisation(s) & Renormalisation(s)

sInclusive

Exclusive

Hadron Decays

A Recipe For Hadron CollisionsA Recipe For Hadron Collisions


A recipe for hadron CollisionsA recipe for hadron Collisions► Experimental Approach (naïve view)

• Inclusive σpp(14 TeV) Trigger(s) + Calibrations Analysis cuts

• Measure distributions on the accepted events• Taking statistical fluctuations and the risk of systematic mismeasurement into

account to the best of one’s ability (Correlated) Uncertainty Estimate

► Now you want to compare to ‘theory’ but what is that?

• Prohibitive to calculate entire S matrix path integral directly!

► Divide and conquer: calculate small parts of the S matrix at a time

• Cross section = sum over “subprocesses” (w/o interference)• 1 subprocess = [sum at amplitude level]2 (i.e. w/ interference)

o Signalo Other processes which are known to contribute “significantly”o “Unknown” unknowns: mis-ID, “reducible backgrounds”, detector effects, (BSM?), …

• At each step, do further expansions, further approximations, …• Even the most sophisticated calculation can be disastrously wrong


Models by the number, Tunes by the score …Models by the number, Tunes by the score …UA5 @ 540 GeV, single pp, charged multiplicity in minimum-bias

events

Simple physics models ~ Poisson

Can ‘tune’ to get average right, but

much too small fluctuations

inadequate physics model

More Physics:

Multiple interactions +

impact-parameter

dependenceMorale (will return to the models later):

1) It is not possible to ‘tune’ anything better than the underlying physics model allows

2) Failure of a physically motivated model usually points to more, interesting physics

Sjöstrand & van Zijl PRD36(1987)2019











► Who is who


DisclaimerDisclaimer► I’m not an expert, so will not focus on:

• CMS: triggers, calibrations, analyses, systematics, etc.

• Theory: PDF constraints, BSM physics, discovery potential, etc.

• All of these aspects are vital, and will be carried out by abler people

► Instead, I will try to:

• Give you a pick of some interesting early possible measurements

• Take a look ‘under the hood’ of these and discuss their role, if any, in understanding the universe better (including but not restricted to better calibrations)

► Note: this is not ancient material

• It is fresh off the farm, reflects the uncertainties of current state-of-the-art (or at least the subset obtainable with Pythia)

• All plots below are shown for the first time in this talk


► Theory “predictions” for tracker occupancy (idealized 4π tracker):

► 106 events will immediately yield a lot of information

► This was theory – how related to what is more realistically measured?

• Restrict to |η| < 2.5, pT > 0.5 GeV

Counting TracksCounting Tracks

A bunch of models

and tunes

<Nch> ~ 80-120


Counting TracksCounting Tracks► Theory “predictions” for tracker occupancy:

► 106 events will still immediately yield a lot of information, but not all

► Extrapolation to inclusive distribution quite model-dependent, interesting to extend measurements as far as they can possibly go (e.g. ATLAS talks about possibly ~ 150 MeV using Silicon, but this probably does not fall under “early” LHC physics …)

<Nch> ~ 13-20


The things that tracks trackThe things that tracks track

Under the hood (theory)

Spacetime Matter overlap vs. Nch (~ transverse density

profile folded on itself)

Average number of “colour sparks” per pp collision vs. Nch

<Nint> ~ 4 - 11


Knowns and UnknownsKnowns and Unknowns► The Nch distributions are sensitive to

• The spatial distribution of the proton mass

• QCD cross sections folded with multiparton densities

• Hadronization (+ string interactions? Other infrared physics?)

• Energy / Collider extrapolation of all of above

► A multi-dimensional theory space better precision

• Look at many variables successively rule out models• Pedestal effect

• Forward-Backward multiplicity correlations• Underlying Event, Rick’s “Transverse” minijets, … (active min-bias similar to UE)

• LEP, HERA, …, but then additional uncertainties from extrapolation to different initial states

Earliest on: pT and η distributions for charged particles


Multiplicity vs pMultiplicity vs pTT correlation correlation

► Extremely interesting already at Tevatron and RHIC (even UA5)

• An independent / incoherent set of strings flat distribution

► first indication of interacting strings?

• If so, is this a consequence of the area law for classical strings?


Multiplicity vs pMultiplicity vs pTT correlation correlation

► Persists (but again changed) when restricting to |η| < 2.5, pT > 0.5 GeV

• An independent / incoherent set of strings flat distribution

► first indication of interacting strings?

• If so, is this a consequence of the area law for classical strings?


D. B. Leinweber, hep-lat/0004025

The (QCD) LandscapeThe (QCD) Landscape

Anti-Triplet

Triplet

pbar beam remnant

p beam remnantbbar

from

tbar

deca

y

b from

t d

ecay

qbar fro

m W

q from W

hadroniza

tion

?

q from W

Structure of a high-energy collision

In reality, this all happens on top of each other

(only possible exception: long-lived colour singlet)











► Who is who


Drell-YanDrell-Yan► Muons are a direct light into the heart of the process

• Very clean, highly controllable probe of Initial-State Radiation (ISR)

• Feed back into photon + jet improve jet calibration

► Muons + tracking can also study Underlying Event here (minijets, fragmentation, …)

► Evolution of UE and ISR as a function of Q2 good model constraints


Drell-YanDrell-Yan► Muons are a direct light into the heart of the process

• Very clean, highly controllable probe of Initial-State Radiation (ISR)

• Feed back into photon + jet improve jet calibration

► Muons + tracking can also study Underlying Event here (minijets, fragmentation, …)

► Evolution of UE and ISR as a function of Q2 good model constraints

K. Lassila-Perini (Moriond 2006) hep-ex/0605042


OverviewOverview► Who is who

► Introduction










Jet ratesJet rates► Jet rates: 106 min-bias + 105 pThat > 20 GeV

► First: no huge “hard” differences for first couple of jets.

► Second: Shapes and ratios, e.g. N3/N4, do have information

► Third: isolate special regions, group jets pairwise, j-j angles, etc …

MB

2J20


Got My Mojo WorkingGot My Mojo Working► Once everything is working, put it

all together

• Missing ET, HT Dark Matter

• And all the other fun stuff

► It may be a (long) while till the next collider Turn LHC into a precision

machine

• Example: beautiful Tevatron measurements such as the W and top masses are already providing crucial information


The Road to Higher PrecisionThe Road to Higher Precision► Go further into perturbative series

• Matching + better-understood resummations

• Very high precision on perturbative part can be expected in the long term

less ‘wriggle room’ for non-perturbative part?

► Beyond LEP fragmentation models

• K-not-so-short? • Is there a strange light at the end of the tunnel?

• Is it the LEP tunnel?

• Lambdas: • How do baryons get made?

• And did people at LEP make them the same way we make them?

• Tracking at very low momenta (150 MeV?)• How much charged energy sits at low momenta?

STAR: neither strangeness nor baryon

production appears to be well described

Rick (Field): the charged/neutral ratio does not appear to be

well described

RHIC Anomaly

RICK Anomaly


ConclusionsConclusions► When you see the first LHC collisions, take a moment to have

pity on the theorists who will have to wait

• At 14 TeV, everything is interesting

• When you see that first event with 300 charged tracks in it, pause a moment to try to visualize what it must have looked like up close

► Early LHC Physics: theory

• Even if not a dinner Chez Maxim, rediscovering the Standard Model is much more than bread and butter.

• Real possibilities for real surprises

• It is both essential, and I hope possible, to ensure timely discussions on “non-classified” data, such as this allow rapid improvements in QCD modeling (beyond simple retunes) after startup

Both these and Kevin’s slides will be availabe on the theory seminar web page

theory.fnal.gov/seminars


(p(pTT))


(eta)(eta)


Who is WhoWho is Who(obviously somewhat sketchy, floating borders …)

► B Physics, flavor physics, SCET, HQET, form factors, …

• Thomas Becher, Andreas Kronfeld

• Enrico Lunghi, Richard Hill, Jack Laiho, Ruth van de Water

► Lattice QCD, chiral perturbation theory, hadron spectra, …

• Bill Bardeen, Estia Eichten, Andreas Kronfeld, Paul MacKenzie, Chris Quigg, Jim Simone

• Ruth van de Water, Jack Laiho, Elizabeth Freeland

► Perturbative QCD, NLO, jet physics, event generators• R. Keith Ellis (MCFM), Walter Giele (VECBOS, VINCIA), Peter Skands (PYTHIA, VINCIA)

• Steven Mrenna (CMS, PYTHIA, …)

• Jan Winter• Leaving this fall

• Arriving this fall

• Often here on Thursdays

In terms of current work


Who is WhoWho is Who► SM Higgs & MSSM phenomenology

• Bill Bardeen, Marcela Carena, Estia Eichten, Joe Lykken, Chris Quigg, Peter Skands*

• Stephen Martin, Ayres Freitas*, Carlos Wagner

► non-SM Higgs, non-MSSM BSM (Xtra Dim, Techni, Little H, …)

• Bill Bardeen, Marcela Carena, Bogdan Dobrescu, Chris Hill, Joe Lykken, Chris Quigg, Estia Eichten , Tim Tait

• Richard Hill, Jay Hubisz, K.C. Kong, Rakhi Mahbubani, Jose Santiago

► Neutrino Physics

• Carl Albright, Boris Kayser, Stephen Parke

• Andre de Gouvea,

► + Ties to astro / cosmo / dark matter

In terms of current work

• Leaving this fall

• Arriving this fall

• Often here on Thursdays

Documents

Joint Theory-CMS Seminar, Fermilab, May 31 st 2007 Peter Skands Fermilab / Particle Physics Division / Theoretical Physics Early LHC Physics: Theory Preparations