Monika Grothe, FP420 meeting, Arlington March 2007 1

CMS/Totem document

Some suggestion for treating PU background

Some news from the CMS diff group

Signals/backgrounds etc

Monika GrotheWisconsin/Turin

Monika Grothe, FP420 meeting, Arlington March 2007 2

Presented to the LHCC in the LHCC open session in January 2007

“Prospects for diffractive and forward physics at the LHC”, The CMS and TOTEM diffractive and forward physics group, CERN/LHCC 2006-039/G-124,CMS Note 2007/002, TOTEM Note 06-5, December 2006

Monika Grothe, FP420 meeting, Arlington March 2007 3

From Valentina Avati’s talk in the open LHCC session

Monika Grothe, FP420 meeting, Arlington March 2007 4

Conclusions

The CMS and TOTEM collaborations intend to carry out a joint diffractive and forward physics program with an unprecedentedrapidity coverage. The document outlinessome aspects of this physics program.

The document addresses,for the first time at the LHC, the central experimental issuesin measuring fwd and diffractivephysics by way of a number ofexemplary processes

A worthwhile program and we are optimistic that it can be done

from my talk in the open LHCC session

Monika Grothe, FP420 meeting, Arlington March 2007 5

CMS/Totem document is not a TDR Was given to the LHCC just for their information No LHCC response was required, nor provided

Document presented to the LHCC in January

Document was used as argument in the subsequent request from Totem to the LHCC for 90m optics running

CMS position concerning this request not yet decided

Summary CMS/Totem document

Monika Grothe, FP420 meeting, Arlington March 2007 6

TOTEM

xL=P’/Pbeam=

det@420

d(

ep

eXp

)/d

x L [n

b]

Number of PU events with protons within acceptance of near-beamdetectors on either side:

~2 % with p @ 420m

~6 % with p @ 220m

Translates into a probability of obtaining a fake DPE signature caused by protons from PU:

Pile-up background

Depends critically on the leading proton spectrum at the LHC which in turn depends on size of soft rescattering effects (rapidity gap survival factor) !

Eg at 2x 1033 cm-2s-110% of the non-diff background that looks like the signal in the central detector will appear to have DPE signature. This is independent of the type of signal.S/B_PU hence depends on the relative cross sections of diff signal and non-diff backgroundthat looks like the signal in the central detector.

Monika Grothe, FP420 meeting, Arlington March 2007 7

Can be reduced by:

Requiring correlation between ξ, M measured in the central detector andξ, M measured by the near-beam detectors

Fast timing detectors that can determine whether the protons seen in the near-beam detector came from the same vertex as the hard scatter (currently R&D project)

“Prospects for diff and fwd physics at the LHC” - Part I: Diffractive part

Pile-up background (II)

; 1 2 s = M2

(220m)(

jets

)

CEP H(120) bb incl QCD di-jets + PU

M(2-jets)/M(p’s)

CEP of H(120 GeV) → b bbar:

Possible to retain O(10%) of signal up to 2 x 1033 cm-2 s-1 in a special forward detectors trigger stream

S/B in excess of unity for a SM Higgs and up to 1000 for a MSSM Higgsappears achievable

Monika Grothe, FP420 meeting, Arlington March 2007 8

Effect of cuts on the S/B for the background caused by PU for 120GeV Higgs

Beware: Uncertainty of the leading proton spectrum normalization because of uncertainty in rap gap suppression factor and its energy dependence

Beware: Further handles not yet employed against PU are track multiplicity cuts and morestringent vertex cuts which may lead to an additional reduction of PU by a factor 10-100

Part I: “Diffractive” part - High lumi

Impact of pile-up background

0.6/10 0.9/30 0.6/20 0.9/100 0.6/180 0.9/320 0.6/900 0.9/1900MC overestimates leadingproton spectrum in acc region of 420m detectors

Monika Grothe, FP420 meeting, Arlington March 2007 9

Regular phone meetings between Andrew and Arnab in Arlington, Antonio in Turin, Marek in Prague and Andy Pilkington in Manchester

Antonio will present his results on H-> WW tomorrow

Results for H-> WW should be brought to some conclusion soon such that these results can beincluded in any write-up we want to provide to ATLAS/CMSIn particular need to substantiate that PU background is indeed less of a problem in this channelcompared to H->bb

One suggestion for a new approach to treating PU background: next slide

Goal of pile-up background studies

Monika Grothe, FP420 meeting, Arlington March 2007 10

Suggestion:All our attempts so far to reduce PU background based on more and harder cutsProblem: Starve signal

PU background is combinatorialHence in invariant mass should be signal bump sitting on a smooth combinatorial pedestalfrom PU

Select signal in the same way one selects a D0 signal:Use 2 distributions in fit:• mass distribution• Delta(xi) distribution (instead of Delta(M) as for D0)

- Signal bump should be dominated by resolution, i.e. is essentially a delta function aroundHiggs mass convolved with a resolution function which can probably be described withone or two Gaussians- Pedestal from PU combinatorial background should be describable with a polynomialDelta(xi) distribution can be used to compute a weight for each event that describes itsprobability to be a signal event based on where it is located in the delta(xi) distribution

Once one has a good fit to the PU background, can subtract it statistically, which is a loteasier on the signal statistics

Statistical subtraction of PU bckgrd !?

Monika Grothe, FP420 meeting, Arlington March 2007 11

What is currently going on in CMS on diffraction and forward physics

For 2008 data-taking, define rap gap based program Prepare trigger accordingly

In parallel, keep preparing, on a lower level, possible joint data-taking with Totem

Prepare our tools: Integrate all fwd detectors simulation/reconstruction into standard CMS software such that all fwd detectors can be included in the default CMS MC generation chain

We are in the process of interfacing HECTOR to the CMS software Sasha’s FP420 detector simulation/reconstruction code is already part of CMS software (see Sasha’s talk earlier today) Work on-going to interface with HECTOR and validate whole setup

Have asked the Protvino group to do simulation of beam-beam, beam-halo, beam-gas background for ZDC location, 220m (and 420m, see Marta’s talk)

Monika Grothe, FP420 meeting, Arlington March 2007 12

What is currently going on in CMS in diffraction and forward physics (II)

Topics foreseen for first data in 2008:

Measurement of the ratio of events with LRG and a hard scale at 14TeV, where hard scale:WZdijetsheavy quarks

Measurement of the ratio of di-jet events with a LRG between the jets

Measurement of rapidity gap survival probability from SD and DPE dijet events at 14TeV

Study of the underlying event in diffractive topologies

Measurements of forward jets and forward Drell-Yan electrons

Study of gamma-gamma and gamma-proton interactions

Definition of rapidity gap triggersMinimum bias trigger using forward detectors (CASTOR, possibly T1, T2)

see: https://twiki.cern.ch/twiki/bin/view/CMS/DiffractionAndFwdPhysics

Monika Grothe, FP420 meeting, Arlington March 2007 13

What is currently going on in CMS in diff & fwd physics: Rapidity gap trigger

Assume that our first analyses in 2008 will be based on rap gap selectionAs a figure of merit, assume we can get at most a few hundred pb-1 of single-interaction data

Our request to the trigger group for diff trigger bits:

1. Exclusive two-lepton trigger bit -- calib channel for near-beam detectors at 220/420m Two isolated leptons (di-electrons or di-muons) AND (no jets with ET>10GeV)2. Rap gap trigger bit. Since neither Castor nor ZDC software is available for rate studies in standard CMSSW, this has to be based, for the time being, on the HF. Two conditions are possible: a. 2-jets with ET>40GeV AND (no jet in HF with ET>10GeV on one side) b. 2-jets with ET>40GeV AND (sum ET in HF on one side < 50GeV)3. Gap-between-jets trigger bit (Two jets, one in HF-right, one in HF-left, with ET>40GeV) AND (no central jets with ET>10GEV

Desirable: Min bias trigger with CASTORCurrently foreseen min bias trigger requires activity in HF on both sides, i.e. is veto against rap gapsRequire activity in CASTOR just on one side + prescale