atlas week feb 2000 j. huston michigan state university understanding higgs production and...

ATLAS week Feb 2000J. Huston

J. Huston

Michigan State University

Understanding Higgs Production and Backgrounds at the LHC Using Tevatron Data

For more details on Higgs production studies see:

•Proceedings of the Les Houches workshop on Physics at TeV Colliders, currently at http://www.pa.msu.edu/~huston/leshouches/

•C. Balazs, J. Huston, I. Puljak, Higgs Production: A Comparison of Monte Carlo and Resummation, hep-ph/0002032

Work on Higgs production withC. Balazs and I. Puljak; work on Higgs backgrounds withS. Murgia and B. Blair


Modelling the effects of initial state gluon radiation

(see also C. Balazs, J. Huston, I. Puljak, hep-ph/0002032

+ Workshop on Physics at TeV Colliders; http://www.pa.msu.edu/~huston/leshouches/)

J. Huston



Reducible background for inclusive H->Comparison with Tevatron data

J. Huston


Work done with Simona Murgiaand Bob Blair


Higgs-> and Backgrounds One of the most useful search modes for

the discovery of the Higgs in the 100-150 GeV mass range at the LHC is in the two photon mode

Unless the Higgs boson is non-standard, with an enhanced coupling to the two photon mode, there is no discovery potential at the Tevatron for a decay into two photons

Nonetheless searches are going on at the Tevatron in the Run 1 data (Simona Murgia thesis, Michigan State U. 2000; D0 paper) and will continue until the LHC turns on

Even if no discovery takes place, these exercises will be useful experience for the LHC, especially in understanding the backgrounds to Higgs searches, both and o and oo

S. Mrenna and J. Wells, hep-ph/0001226

enhanced2 BR


Diphoton Production at the Tevatron

QCD diphoton production is an interesting process in its own right new physics can show up in the two photon channel (extra dimensions,

SUSY, …) the final state kT can be measured very precisely; thus diphoton production is

a great place to test resummation formalisms

The backgrounds to diphoton production come from o and oo production

both backgrounds are much larger than the signal, unless an isolation cut is made

an isolation cut discriminates greatly against jets fragmenting into leading o’s at CDF (in Run 1B), require that additional energy in a cone of radius 0.4

about photon direction is < 1 GeV saturated by minimum bias energy

no explicit isolation cut in ATLAS, but an implicit one


Photon Measurements

The photon fraction of a sample can be determined on a statistical basis using CES and CPR weights

look for photon conversions in CPR (probability is greater for o)

look at shower width in CES (wider for o)


Photon Fractions for Single Photon Production

Photon fraction from Run 1A agrees roughly with leading order prediction

Photon fraction from Run 1B consistent with 1Abut may be flattening out(?)

fragmentation z values > 0.99

CDF Preliminary


Diphoton Fractions Background subtraction more

complicated for diphoton case both photons can fail CES/CPR test, both

can pass, or one can pass and one fail; end up with 4X4 matrix to invert with

output number of , go, o, oo pairs

•As expected diphoton events are more isolated than background

•consistent with min bias energy

CDF Preliminary


Diphoton Fractions as Function of Isolation

For these studies, a diphoton ntuple was used in which the leading photon was required to have > 22 GeV; no requirement on the 2nd photon

Trigger level isolation energy cuts kick in at about 4 GeV in a cone of radius 0.4; examine photon, background fractions up to that point

Diphoton fraction decreases as isolation cut energy is raised

oo is dominant background;is this reasonable?

/o ratio is < 1in this range; but might have expected closer ratio

o fraction seems to increase with increasing isolation

o is poorly determined at very low isolation (can end up with fit preferring a negative fraction)

black triangles: oo open circles: o

open triangles:


Number of Signal and Background Events

Look at signal fraction compared to background fraction as function of isolation

Increasing isolation energy increases number of events almost linearly, but little effect on signal


Diphoton Backgrounds in ATLAS Again, for a H-> search at the

LHC, face irreducible backgrounds

from QCD and reducible

backgrounds from o and oo

in range from 70 to 170 GeV

jet-jet cross section is estimated

to be a factor of 2E6 times the cross section and -jet a factor

of 8E2 larger Need rejection factors of 2E7 and

8E3 respectively PYTHIA results seem to indicate

that reducible backgrounds are

comfortably less than reducible

ones …but how to normalize PYTHIA

predictions for very high z fragmentation of jets; fragmentation not known well at high z and certainly not for gluon jets


/o ratio at the LHC Use same leading order program as was used

at the Tevatron to look at the /o ratio as a function of the isolation cut

Inclusive /o ratio is extremely small but the imposition of a reasonable isolation cut brings the ratio to the order of 1 or larger in the range of interest

expect /o to be of same order as oo

Picking out the tail of the isolation energy distribution

• /o as a function of isolation energy;uses a collinear approximation to fragmentation•no explicit isolation cut in ATLAS; would be veryinteresting to study effective isolation cut

inclusive

<1 GeV

<2 GeV<3 GeV

<5 GeV

isolation energy for o’s with ET>25 GeV


Study in Progress

Generate -jet, events in PYTHIA, pass them through CDF fast detector simulation program QFL, and trigger/reconstruct as data

qqbar-> ISUB=18 gg-> ISUB=114 qqbar->g ISUB=14 qg->q ISUB-29 gg->g ISUB=115

Compare PYTHIA predictions for background/signal to what is observed in CDF

Job started in Oct ‘99; 18.5 million events so far (~15.7 pb-1)

Just doing -o background for now given large number of events needed (and we haven’t tried to be clever)

o

in the data o is equalto at an isolation cut of 3 GeV


Pythia Study

only o backgrounds being considered


Isolation Energy in Pythia


Isolation in Data and PythiaPythia Data


Isolation in Data and Pythia

Pythia Data


Comparison of o Isolation Energy Distributions from Pythia and Data

Data o-oPythia -o


QCD/EW at the Tevatron

The Tevatron Collider serves as arena for precision tests of QCD and EW with photons,electrons, W/Z’s, jets

Highest Q2 scales currently achievable (searches for new physics at small distance scales)

Sensitivity to parton distributions over wide kinematic range

2 scale problems: test effects of soft gluon resummation

Diffractive production of W/Z, jets, heavy flavor

Data compared to NLO, resummed, leading log Monte Carlo, fixed order calculations

Overall, the data from CDF and D0 agree well with NLO QCD

Some puzzles resolved: W + jet(s): R10

Some puzzles remain: Jet excess at high ET/mass??

Gluon/d quark distribution at large x?

630 GeV jet cross section and xT scaling

Some theory work needs to be done: Inclusive photon cross section

Some searches still continue: BFKL effects (D0 looks forET

decorrelation in dijet events as rapidity separation of jets increases)

No effects not “observed in Herwig”


Theoretical Predictions

There are a variety of programs available for comparison of data to theory and/or predictions. Tree level Leading log Monte Carlo

NnLO

Resummed

Important to know strengths/weaknesses of each.

In general, agree quite well…but before you appeal to new physics, check theME.

Perhaps biggest effort…include NLO MEcorrections in Monte Carlo programs…correct normalizations.

Resummed description describes soft gluoneffects (better than MC’s)…has correct normalization (but need HO to get it); resummed predictions include non-perturbative effects correctly…may have to be put in by hand in MC’sthreshold kT

W,Z, Higgsdijet, direct

b space

qt space


Theoretical Predictions

Determination of the Higgs signal requires an understanding of the Higgs pT distribution

for example, for gg->HX->X, the shape of the signal pT distribution is harder than that of the background; this can be used to advantage

To reliably predict the Higgs pT distribution, especially for low to medium pT region, have to include effects of soft gluon radiation

can either use parton showering a la Herwig, Pythia, ISAJET or kT resummation a la ResBos

parton showering resums primarily the (universal) leading logs while an analytic kT resummation can resum all logs with Q2/pT

2 in their arguments; but expect predictions to be similar and Monte Carlos offer a more useful format

Where possible it’s best to compare pT predictions to a similar data set to insure correctness of formalism; if data is not available, compare MC’s to a resummed calculation

Note the large difference between PYTHIA versions5.7 and 6.1


Change in PYTHIA Older version of PYTHIA has more events at

moderate pT

Two changes from 5.7 to 6.1 A cut has been placed on the combination of z

and Q2 values in a branching: u=Q2-s(1-z)<0 where s refers to the subsystem of hard scattering plus shower partons

corner of emissions that do not respect this requirement occurs when Q2 value of space-like emitting parton is little changed and z value of branching is close to unity

necessary if matrix element corrections are to be made to process

net result is substantial reduction in amount of gluon radiation

In principle affects all processes; in practice only gg initial states

Parameter for minimum gluon energy emitted in space-like showers is modified by extra factor corresponding to 1/ factor for boost to hard subprocess frame

result is increase in gluon radiation

The above are choices, not bugs; which version is more correct?

->Compare to ResBos

S. Mrenna80 GeV Higgs generated at the Tevatron with Pythia


Diphoton production at the Tevatron and LHC

gg

resummation at 14 TeV

gg scattering is an important process for diphoton production at both the Tevatronand the LHC. May be able to constrain resummation formalism for gg processes.


Comparison of CDF Z pT to Resbos and Pythia

Use high statistics precision Z data at the Tevatron to compare PYTHIA and ResBos predictions

from Willis Sakumoto

Note agreement with data

and Pythia at high pT due to

matrix element matching


Low pT region and kT

A blowup of the low pT region shows that an additional “intrinsic” kT (of about 2 GeV) is needed for agreement with

data and ResBos; implemented by a

Gaussian smearing at the start of

the parton shower ResBos needs no additional kT

The 2 GeV needed by PYTHIA

does not mean that the intrinsic

kT inside of a proton is 2 GeV,

just that this amount is needed

to compensate for the cutoff

imposed in the parton shower;

this amount of non-

perturbative kT is automatically

generated by ResBos given the

non-perturbative parameters

determined by fits to fixed target data

(and the evolution to Tevatron

kinematics)


Need gg initial state

Z production primarily from qqbar initial state; would like to have to similar gg process to test resummation/Monte Carlo formalism

A little less than half of Tevatron diphoton cross section (with a pT cut of 12 GeV) is due to gg scattering


Compare PYTHIA and ResBos diphoton predictions

ResBos is larger than PYTHIA at high pT for qqbar scattering

ResBos switches to the appropriate Y piece (exact matrix element) at high pT; Monte Carlo matrix element matching not yet available for Higgs production

ResBos agrees with PYTHIA for gg initial state, even though ResBos has exact ME at high pT and PYTHIA does not

this shows the smallness of the Y piece for gg->g


Compare two versions of PYTHIA

pT distributions look similar between the two versions for both subprocesses


Comparison to CDF data

Diphoton statistics limited from Run 1 but in good agreement with resummed predictions from ResBos

kT distribution in disagreement with NLO predictions, indicating need for soft gluon resummation

More precise comparisons, out to higher pT, will be possible the >20 times the statistics in Run 2


Diphoton Production at the LHC…again, a large fraction of continuum diphoton production from gg scattering

…better agreement for gg subprocess than for qqbar for the same reasons as the Tevatron


Comparison of two versions of PYTHIA

Starting to see more differences for gg initial state due to larger phase space for gluon emission at the Tevatron


Comparison of PYTHIA and ResBos for Higgs Production at LHC

ResBos agrees much better with the more recent version of PYTHIA

Suppression of gluon radiation leading to a decrease in the average pT of the produced Higgs

Affects the ability of CMS to choose to the correct vertex to associate with the diphoton pair

Note that PYTHIA does not describe the high pT end well unless Qmax

2 is set to s (14 TeV)

Again, ResBos has the correct matrix element matching at high pT; setting Qmax

2=s allows enough additional gluon radiation to mimic the matrix element


Similar behavior at the Tevatron Differences between the two

versions are smaller, though (again less phase space for gluon radiation)


Comparisons with Herwig

HERWIG (v5.6) similar in shape in PYTHIA 6.1 (and perhaps even more similar in shape to ResBos)

Is there something similar to the uhat cut that regulates the HERWIG behavior?


“Intrinsic kT” for Higgs Production

How much intrinsic kT to use with PYTHIA for Higgs production at the LHC?

Amount of intrinsic or primordial kT chosen does not matter for Higgs production at LHC since most of kT is “radiated away” before hard collision

Note that peak is above 10 GeVcompared to 3 GeV for the Z atthe Tevatron; larger color factorfor gg state and more phase spacefor gluon emission


Intrinsic kT at the Tevatron

Similar effect (but smaller) for Higgs production at the Tevatron


Z Production at the LHC

Sensitivity to kT still present for Z production


Diphoton Measurements at CDF

2 aspects:•QCD measurements of •exotic searches with diphotons,e.g. Higgs->: looser cuts to maximizeefficiency

Require:•ET

1,2 > 12 GeV/c•Isolation energy in cone of 0.4 < 1 GeV/c

saturated by MB energy forN.B. backgrounds come from jets withzo (=Eo/Ejet) > Eo/(Eo+1)

•zmin~0.95 for ET=20 GeV/c

•fragmentation functions not welldetermined here, especially notwith gluons and especially notin Monte Carlos

Note that distributions that are functionsat LO are not well-described at NLO

->need resummed predictions


Jet Production and the Gluon Distribution

@LO, jet cross section is proportional to s

2g(x,Q)g(x’,Q) and s2g(x,Q)q(x’,Q)

•flexibility in gluon allows for increase intheoretical cross section at high ET

Note that if jet cross section increases by 20%, gluon distribution must double (fraction of gluon jets also almost doubles)


Conclusions

Preliminary conclusion: PYTHIA overestimates backgrounds in CDF->will be investigated in more detail

ATLAS note will be written

New version of PYTHIA in relatively good agreement with both HERWIG and ResBos for predictions of Higgs pT distribution at the LHC

Don’t rely totally on Monte Carlos and certainly not on one Monte Carlo alone

Verify, if possible, theoretical predictions/formalisms with data exisiting Run 1 data/Run 2 data “background” data to be taken at the LHC

If no data, then verify with more complete theoretical treatments

atlas week feb 2000 j. huston michigan state university understanding higgs production and...

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