diffractive studies and forward physics at cms

$: Diffractive studies and forward physics at CMS$
Diffractive studies and forward physics at CMS

Marta Ruspa, Univ. Piemonte Orientale-Novara & INFN-Torino

XIV International Workshop on Deep Inelastic Scattering

April 20-24, 2006, Tsukuba (Japan)

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CMS detectors along beam line:Cal with || ≤ 3, HF with 3 ≤|| ≤ 5Castor calorimeter, behind T2 with 5.2 ≤|| ≤ 6.5Beam Scintillation counters BSCZero-degree calorimeter ZDC

Forward detectors at CMS

CMS IP T1/T2, Castor, BSC ZDC RPs@150m RPs@220m

420m

TOTEM detectors:T1 (CSC) in CMS endcapsT2 (GEM) in shielding behind HFT1 + T2: 3 ≤ || ≤ 6.8Roman pots with Si detectors on 2 sides at up to 220 m

Possible addition: FP420

Unprecedented rapidity coverage at a hadron collider

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Roman pot acceptance

High * (1540m) @ 1028 - 1029cm-2s-1 :

90% of all diffractive protons are seen in TOTEM RPs

Low * (0.5 m) - nominal LHC beam optics @ 1033 - 1034cm-2s-1: 220 m: 0.02 < < 0.2 420 m: 0.002 < < 0.02

Standard optics * = 0.5 m assumed from now on

TOTEM

FP420

see J. Whitmore’s talk

XL= 1 - : longitudinal momentum loss Unprecedented ξ coverage at a hadron collider

marta ruspa

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CMS/TOTEM diffractive physics program

TOTEM and CMS pursue a common diffractive and forward physics program to be described in a common document

A wealth of results already available [see HERA-LHC Workshop proceedings]

Thanks to TOTEM people and to all contributors!

The results presented in the following do not depend on the specific hardware implementation of the T1 and T2 detectors or of the roman pots; they hold for any tracker system with the T1, T2 rapidity coverage in conjunction with RPs at 220 m and 420 m from the IP.

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The accessible physics is a function of the integrated

luminosity

2 gluon exchange with vacuum quantum numbers “Pomeron”

X

Double Pomeron exchange:

X

Single diffraction:

p p p X p p p X p

X

Y

Double diffraction:

p p X Y

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Low lumi Rapidity gap selection possibleHF, Castor, BSCs, T1, T2Proton tag selection optionalRPs at 220m and 420 m

Diffraction is about 1/4 of tot

High cross section processes“Soft” diffractionInteresting for start-up runningImportant for understanding pile-up

Low

lu

mi

Hig

h lu

mi

Map to diffractive/forward physics in CMS

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Number of pileup events per bunch crossing =

= Lumi* cross section * bunch time width * total lhc bunches / filled bunches =

= 1034 cm-2 s-1 * 104 (cm^2/m^2) * 10-28 (m2 / b) * 110 mb * 10-3 (b/mb) * 25 (ns) * 10-9 (s/ns) * 3564 / 2808 35

1x1032 0

1x1033 3.5

2x1033 7

Pile-up: numbers!

1 mb = 100 events/s @ 10 29 cm-2 s-1

PHOJET: ALL PROCESSES 110 mb NONDIF.INELASTIC 51 mb ELASTIC 33 mb DOUBLE POMERON 1.95 mb SINGLE DIFFR.(1) 7.66 mb SINGLE DIFFR.(2) 7.52 mb DOUBLE DIFFRACT. 9.3 mb

Number of pileup events per bunch crossing =

= Lumi* cross section * bunch time width * total lhc bunches / filled bunches =

= 1034 cm-2 s-1 * 104 (cm^2/m^2) * 10-28 (m2 / b) * 51 mb * 10-3 (b/mb) * 25 (ns) * 10-9 (s/ns) * 3564 / 2808 17

This number is valid in the central detector region, but must be corrected for the elastic and diffractive cross section in the forward region!

Selection of diffractive events with rapidity gap only possibleat luminosities below 10 33 cm-2s-1, where event pile-up is absent

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QCD: SD and DPE production of vector bosons, heavy quarks, high ET jets Diff PDFs and generalized PDFs Low-x structure of the proton High-density regime γ γ and γp interactions (QED)Forward energy flow - input to cosmics shower simulation

Low

lu

mi

Hig

h lu

mi


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QCD: SD and DPE production of vector bosons, heavy quarks, high ET jets Diff PDFs and generalized PDFs Low-x structure of the proton High-density regime γ γ and γp interactions (QED)Forward energy flow - input to cosmics shower simulation

Low

lu

mi

Hig

h lu

mi


High lumiNo Rapidity gap selection possibleProton tag selection indispensableRPs at 220 m and 420 m

Central exclusive productionDiscovery physics:Light SM HiggsMSSM Higgs

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Selection rules result in the central systembeing (to good approx) JPC = 0++

I.e. a particle produced with proton tagshas known quantum numbers

Excellent mass resolution (~GeV) from the protons, independent of the decay productsof the central system

CP violation in the Higgs sector manifestsitself as azimuthal asymmetry of the protons

Proton tagging may be the discovery channelin certain regions in the MSSM

Vacuum quantum numbers“Double Pomeron exchange”

shields color charge ofother two gluons

The physics interest of DPE Higgs production

As the delivered luminosity reaches tens of fb-1 the central exclusive production (CEP) processes become a tool to search for new physics

See B. Cox’s talk

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Vacuum quantum numbers“Double Pomeron exchange”

shields color charge ofother two gluons

The physics interest of DPE Higgs production

b jets : MH = 120 GeV · BR = 2 fb (uncertainty factor ~ 2.5)

MH = 140 GeV · = 0.7 fb

MH = 120 GeV : 11 signal / O(10) background in 30 fb-1

after detector cuts

WW* : MH = 120 GeV · BR = 0.4 fb

MH = 140 GeV · BR = 1 fb

MH = 140 GeV : 8 signal / O(3) background in 30 fb-

1

after detector cuts

b-jet channel very important in “intense coupling regime” of MSSM, cross section factor 10-20 larger, discovery channel?

See B. Cox’s talk

As the delivered luminosity reaches tens of fb-1 the central exclusive production (CEP) processes become a tool to search for new physics

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1 2 s = M2

With √s = 14TeV, MH = 120 GeVon average:

0.009 1%

DPE Higgs production: necessary ingredients

Nominal LHC beam optics @ 1033 - 1034cm-2s-1: 220 m: 0.02 < < 0.2 420m: 0.002 < < 0.02

beam

p’

p’roman potsroman pots

dipoledipole

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Trigger studies

“Diffractive Higgs: CMS/TOTEM level-1 trigger studies”M. Arneodo, V. Avati, R. Croft, F. Ferro, M. Grothe, C. Hogg, F. Oljemark, K. Osterberg, M. RuspaProceedings of “ HERA and the LHC: A Workshop on theImplications of HERA for LHC Physics", CERN-DESY 2004/2005, p. 455-460;hep-ph/0601013

Semileptonic WW and tau tau decay channels

(or any final state with high-pT leptons, missing ET): trigger not a problem!

Most challenging case is H (120 GeV) bb

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< 100 Hz < 100 kHz 40 MHz collision

High-Level Trigger HLTLevel-1 trigger

Calo Muon no tracking!

Triggering jets at CMS

ECAL

HCAL

PbWO4crystal veto

patterns

Trigger tower 4x4 trigger towers = region

Search for jets with a sliding 3x3 regions window Jet = 3x3 region with local energy max in middle Reconstructed L1 jet ET on average ~ 60% of real jet ET, thus need for jet ET calibration Jet = 144 trigger towers, with typical jet dimensions: Dh x Df = 1 x 1

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L1 jet trigger signature for a 120 GeV Higgs: 2 jets in CMS Cal, ET < 60 GeV each

Measured L1 jet ET on average only ~60% of true jet ET

L1 trigger applies jet ET calibration and cuts on calibrated value Thus: 40 GeV (calibrated) ~ 20 to 25 GeV measured Cannot go much lower because of noise

The difficulty of triggering on a light Higgs

while considered acceptable: O(1Khz)

Need additional conditions in trigger

Use rate/efficiency @ L1 jet ET cutoff of 40 GeV as benchmark

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+ HT condition = isolation condition for jets:

2 jets in central Cal (|η|< 2.5) with ∑(ET 2 jets)/HT > threshold

HT = scalar sum of ET of all jets in the event with ET(jet) > threshold factor 2 rate reduction

L1 2-jet trigger +…

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2 jets in central Cal (|η| < 2.5) with ∑(ET 2 jets)/HT > threshold


+ Conditions based on TOTEM detectors T1 e T2:• excellent suppression of QCD bacground• useless as soon as pile-up events are present as also signal events

are vetoed (non-diff. component in pile-up events tends to quickly fill in the rapidity gaps).


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2 jets in central Cal (|η| < 2.5) with ∑(ET 2 jets)/HT > threshold


+ Conditions based on TOTEM detectors T1 e T2:• excellent suppression of QCD background• useless as soon as pile-up events are present as also signal events

are vetoed (non-diff. component in pile-up events tends to quickly fill in the rapidity gaps).

+ Topological condition: 2 jets required to be in the same η hemisphere as the RP detectors that

see tthe proton factor 2 rate reduction


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+ Single-arm 220 m condition: mass resolution for CEP

Higgs is worst than with 420 m tag

Integrated QCD rate for events with at least two jets

Integrated QCD rate for events with at least two jets and which satisfy the single-arm 220 m RP condition

kH

zL=1032cm-

2s-1

Plot: Richard Croft

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+ Single-arm 220 m condition: very good reduction of rate in absence of pile-up reduction decreases substantially in the presence of pile-

up+ Single-arm 220 m condition with cut

TOTEM will provide implementation of a cut at L1 (e.g. < 0.1, recall acceptance is 0.02 < < 0.2). Implementation and achievable resolution under study..

Achievable total reduction: 10 x 2 (HT cond.) x 2 (topological cond.) = 40!

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For H (120 GeV, DPE) b bbar, adding L1 conditions on the RPs at 220m is likely to provide a rate reduction sufficient to meet the CMS L1 bandwidth limits at luminosities up to 2x 1033 cm-1 s-1

To go even further up in luminosity need additional handle to stay within bandwidth limits

... So what about triggering with the 420 m RPs ?At the current CMS L1 latency of 3.2 s they are too far away from IP for inclusion in L1

Note: This is a hardware limit - cannot be changed without replacingtrigger pipelines of CMS tracker and preshower detectors with deeper onesShould this however happen (under discussion for SLHC: L1 latency 6.4 s,determined by ECAL pipeline depth) then ....

Triggering on a light Higgs

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+ Asymmetric 220 & 420 condition: in effect means on opposite sides events where values of 2

protons are very different can be used either in L1 after increase in L1 latency or on

HLT!

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L1 efficiency – RP conditionHow much is left of our signal?

Without RP condition Various RP conditions

Plots: Richard Croft

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H bb (120 GeV): relatively muon-rich final state from B-decays - about 20% of events have at least one muon in the final state

How many signal events are being retained by the already foreseen CMS trigger streams, notably the muon trigger?

L1 signal efficiency – muon condition

Half of events with a muon in the final state can be triggered with aa 1 muon + 1 jet trigger (to be implemented)

H WW(140 GeV): about 23% of events have at least one muon in the final state:

70% of events with CMS L1 single muon trigger

Numbers: F. Oljemark

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Single diffractive production of W, Z, dijets

DPE processes constitute only a small part of the diffractive cross section that can be explored by CMS and TOTEM. Exemplary of any process that deposits low ET in the central detector.

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Recall: • RP acceptances at * = 0.5 m:

220m - 0.02 < < 0.2 420m – 0.002 < < 0.02

• Lowest threshold for L1 jet trigger is ET > 40 GeV• Typical loss of factor 2 in efficiency when using 220 m RP cond. (RP acceptance)

Map the parameter space (bandwidth vs efficiency) with ultimate goal of defining a trigger table for a dedicated diffractive trigger stream with target output rates of 1 kHz (L1) and 1 Hz (HLT)

Use POMWIG Monte Carlo

Single diffractive production of W, Z, dijets

Plot: Richard Croft

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At LHC startup, where the luminosity will be low, and where no pile-up is present, CMS can pursue a rich program of rapidity-gap based diffractive and fwd physics

Should the FP420 R&D project result in upgrading CMS with detectors 420 m away from the IP, proton-tag based program and discovery physics becomes possible

Wide proton-tag based program ranging from QCD to the low-x structure of the proton to photon physics already possible by way of a collaboration of CMS with TOTEM

Summary

Key element is trigger, notably at high lumi, when amount of pile-up collisions overlaid to the interesting hard event becomes high. Pile-up events are themselves largely diffractive

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Triggering in absence of pile-up: no problem.

L1 2-jet rate for central with L1 jet ET cutoff of 40 GeV must be reduced to O(1Khz) to accomplish with CMS L1 bandwith restrictions. Therefore using L1 jet trigger alone not an option in the presence of pile-up.

Can trigger with the central detector alone by using the muon trigger Efficiencies with already foreseen CMS L1 thresholds: 10% for H(120GeV) bb, 20% for H(140GeV) WW*

Can also use the L1 jet trigger when combining it with RP condition at (rate of a few kHz achievable at 2x1033 cm-1s-1).

Requires defining a new CMS trigger stream; efficiencies around 10%.

L1 efficiencies for SD production of W’s, Z’s, die-jets available.

A dedicated trigger stream hence feasible, with output rates of O(1) kHz

L1, efficient for selecting CEP, a potential discovery channel for a light

Higgs boson, and hard single diffractive processes.

Triggering diffraction at CMS: summary

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BACKUP

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Beam-halo/beam-gas level numbers produced by TOTEM not a problem as soon as central CMS detector condition is used in L1

Find from PYTHIA pile-up sample:

@220m: 0.012 protons per pile-up event on average, i.e.

at 1034 cm-2s-1: 35*0.055=1.93

@220m: In worst case on average 1.93 tracks from pile-up in addition to track from signal event

@420m: 0.055 protons per pile-up event on average, i.e.

at 1034 cm-2s-1: 35*0.012=0.42

@420m: In worst case on average 0.42 tracks from pile-up in addition to track from signal event

Background in RPs

The reduction factors in the presence of pile-up obtained by scaling the probability per pile-up event to satisfy the relevant RP condition, determined separately, by the average number of pile-up events at the luminosity in question.

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No problem for processes with a lepton in the final state

H (120 GeV) bbbar For luminosities up to 2x1033 cm-2s-1 possible to keep a

reasonable fraction of events At higher luminosities ~ 10% can always be kept by

triggering on muons

MSSM scenario: discovery can be made with lumi at or below 1x1033 cm-

2s-1

at higher luminosities triggering on muons from b-decay

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Other L1 conditions

Effect of combining aready foreseen L1 trigger conditions with conditions on the RP detectors

Large rapidity gap cut at L1 (jets veto in forward calorimeter) Further rate reduction (approx. factor 2) at lumi where pile-up is negligible

Estimated 1kHz Jet Thresholds for various Central / RP conditions

S: single-sided, D: double-sided

C: <0.1 of the leading proton

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+ Double-arm 420 condition: only possible after increase of L1 latency would allow to select events that are gold plated wrt mass

resolution note: single-sided 220 m cond. and asymmetric cond. select

events with worst possible mass resolution

At 420 m & 420 m

500150 30 10


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A) L1: 220 m single-arm condition with a cut

B) Back-to-backness of jets (2.8 < ΔΦ< 2.48 rad) and

(E1T –E2

T)/(E1T + E2

T) < 0.4 and ET> 40 GeV

C) reconstructed from jets in the central detector +(-) = s-½∑ETi exp(-

(+)ηi); cut: difference between 2 values larger than 2σ. No simulation of RP

reconstruction available so far. Assumed resolution of 15% (20%) at 220 (420) m

D) Either one of the 2 jets b-tagged

E) A proton seen at 420 m

No pile-up case: no QCD bgd survives selection. HLT selection cond. A+B+C A+B+D A+B+C+E

HLT rate L=1x1033cm-2s-1 15 Hz 20 Hz < 1Hz

HLT rate L=2x1033cm-2s-1 60 Hz 80 Hz 1 Hz

Signal eff. H bb (120 GeV) 11% 7% 6%

Desired target output rate; no loss in efficiency compared to L1

HLT studies

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FAMOS (Fast CMS Simulation) + RPs acceptance tables

DPEMC MC:

tt production inclusive DPE for the semileptonic channel (tt bb qq μ ν ) good rejection of QCD background obtained for SD the cross section should increase by a factor 30-40

B production SD and DPE production of B-mesons with B J/psi μ μ tens of events for 10 fb-1 in DPE case and several hundreds in SD case

W and WW production DPE inclusive W production: abundant process can be studies at lumi

where pile up is small DPE exclusive WW production: 10 events in 10 fb-1

N.B.: 10 fb-1 collected in 60 days of LHC running @ 2x1033 cm-

2s-1

Hard diffractive QCD studies

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Inclusive dijet production pp pXjjp

Was used by CDF to measure diffractive structure function of the proton: similar measurement possible at CMS, with wider kinematic coverage ( > 0.02 (0.002) compared to > 0.035 at CDF); statistical accuracy of CDF measurement could be reached within a few days of running at ~1032 cm-2 s-1

Comparison of DPE and SD rates for dijet production would give information on the hard diffractive factorisation breaking at the LHC

Exclusive dijet production pp pjjp Cross section for central exclusive production of dijets order of 1 nb

high rate allows precise determination of the off-diagonal un-integrated gluon densities uncertainties in exclusive production cross section of Higgs to be reduced to 1% level

Hard diffractive dijet prod.

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Events with a fast proton in the final state can also originate from the exchange of a vector boson. In particular, tagging one leading proton allows the selection of photon-proton events with known photon energy; likewise tagging two leading protons gives access to photon-photon interactions of well known center of mass energy [PRD 63 070152, hep-ex/0201027].

γp and γγ physics

p

p

e.g.: exclusive 2- γ production of lepton pairs is an important calibration process (forward e+e- pairs in Castor with proton tag, observed cross section 3 pb, μ+μ- would double the statistics)

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SM Higgs with ~120 GeV:

gg H, H b bbar mode has highest BRBut signal swamped by gg b bbar

Best bet with CMS: H , where in 30 fb-1 S/√B 4.4

Light SM Higgs at the LHC (I)

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Production cross section times branching ratio for CEPFrom implementation of KMR model in Exhume MC

Light SM Higgs at the LHC (II)

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Intense-coupling regime of the MSSM: Mh~MA ~ MH ~ O(100GeV): their coupling to, WW*, ZZ* strongly suppressed discovery very challenging at the LHC

Cross section of two scalar (0+) Higgs bosons enhanced compared to SM Higgs

CEP as discovery channel

see Kaidalov et al, hep-ph/0307064, hep-ph/0311023

100 fb

1 fb

120 140

-

“3-way mixing” scenario of CP-violating MSSM: the 3 neutral Higgs bosons are nearly degenerate,mix strongly and have masses close to 120 GeV

Superior mass resolution from tagged proton allows disentangling theHiggs bosons by measuring their production line shape

Explicit CP-violation in Higgs sector visible as asymmetry in the azimuthal distribution of tagged protons (interference of P- and P+ amplitudes) (Khoze et al., hep-ph/0401078)

CEP as CP and line-shape analyzer !

MSSM and proton tagging

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Difference between DPEMC and (EDDE/ExHuMe) is an effect of Sudakov suppression factor growing as the available phase space forgluon emission increases with increasing mass of the central system

Models predict different physics potentials !

DPE Higgs production: models (I)

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More central rapidity in ExHuMe due to gluon distr. falling more sharply than the Pomeron parameterisation in DPEMC N.B: acceptance of forward proton taggers sensitive to the rapidity

distribution of central system.

Cut ξ = 0.1 applied in DPEMC as required by Bialas-Landshoff appr.

DPE Higgs production: models (III)

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DIS06, 20-24/04/06, M. RuspaDPE Higgs event generators

1. DPEMC 2.4 (M.Boonekamp, T.Kucs) - Bialas-Landshof model for Pomeron flux within proton - Rap. gap survival probability = 0.03 - HERWIG for hadronization

2. EDDE 1.2 (V.Petrov, R.Ryutin) - Regge-eikonal approach to calculate soft proton vertices - Sudakov factor to suppress radiation into rap.gap - PYTHIA for hadronization

3. ExHuMe 1.3 (J.Monk, A.Pilkington) - Durham model for exclusive diffraction (pert. calc. by KMR) - Improved unintegrated gluon pdfs - Sudakov factor to suppress radiation into rap.gap + rap.gap survival prob.= 0.03 - PYTHIA for hadronization

All three models

available in the fast

CMS simulation

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DPE Higgs production studies

Models: EDDE, EXHUME, DPEMC, all in FAMOS; RPs acceptance tables

H bb in SM: back-to-backness of the jets, b-tag, two final state protons, consistency of mass reconstruction between RPs and central detector: 2-4 signal events per 30 fb-1

suppression of backgrounds rely on resolution of RP

H WW* in SM [EPJ C45 (2006) 401]: 1-7 events depending on mass range per 30 fb-1

suppression of background does not rely on resolution of RPs irreducible backgrounds small and controllable

N.B.: 30 fb-1 collected in 30 days of LHC running @ 1034 cm-2s-

1

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- Recent versions of DPEMC, EDDE and ExHuMe generators as well as new RP acceptances available in CMS fast simulation

- H->bb: Difficult channel. Cross sections, RP and b-tag efficiencies for

signal well established but the selection cuts still being tuned. BG = ISSUE ! None of the models treats bg properly. The bg issue needs an input from theory side.

- Comparison of generators: Rich resource in HERA-LHC proceedings Non-negligible differences in basic quantities (ξ and yH) influencing RP acceptances

- Comparison to data: Hard task to make a comparison to the only avail.

data (Rjj distr. from RunII). Hope to get a good description, though.


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- H->WW in SM: Solid numbers for signal including L1-trigger

Bg is small and controllable Promising channel for mh>130 GeV

- H->WW (bb,tautau) in MSSM: Idea of Durham group (V.Khoze et al.).

In some scenarios and in some regions of (mA,tanβ)

a much higher yield than in SM case. Especially promising for Higgs -> bb and Higgs -> tautau channels


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Photon fluxes introduced in CALCHEP/COMPHEP,photon events then fed to PYTHIA for decays and

hadronization

CMS full detector simulation + RPs acceptance tables

γγ interactions: 2-γ production of W pairs: studies of quartic gauge couplings

γγWW (LEP limits are weak due to limited cms energy) Exclusive 2- γ production of lepton pairs is an important

calibration process (forward e+e- pairs in Castor with proton tag, observed cross section 3 pb)

γp interactions:

photoproduction of H: significant cross-section at LHC and good signal-to-background ratio; low mass region with Higgs decaying to bb, and W;

W boson production at high transverse momentum, top pair production via photon-gluon fusion, …

DY: qqbar ll

gammagamma ll

γp and γγ physics

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DIS06, 20-24/04/06, M. Ruspa Drell-Yan

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HERA x down to 10-5 ; LHC can probe very low x down to 10-6, 10-7

Low-x studies

Drell-Yan: pp qq */Z e+e- X

Sensitive to very low-x partons in the proton (x~10-6 to 10-7) Detect electrons in CASTOR (5.2 < || < 6.6) Can enhance signal/background ratio by requiring track in T2

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DIS06, 20-24/04/06, M. RuspaSurvey of accessible diff/fwd processes (III)

Inclusive DPE prod of t tbar: (A. Vilela Pereira) semileptonic decay channel: pp p+X+(tt)+X+p; tt bbqq

DPEMC and Pomwig generators Require 2 protons in 220m and/or 420m detectors

Event yield 1-10 per 10fb-1, depending on theoretical model, but taking supression factor of 0.03 into account

DPE and SD prod of B J/ (D. Damiao)DPEMC MCEvent yields per 10 pb-1: DPE: tens of events SD: several hundreds of events

SD and DPE with hard scale: Production of heavy quarks

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DIS06, 20-24/04/06, M. RuspaSurvey of accessible diff/fwd processes (IV)

DPE pp pYWp

DPE excl prod pp pWWpdominated by QED

DPEMC MC require 2 protons, in 220m and/or 420m detectors several thousand events of type W eand W expected per 1 fb-1

together with SD prod of W can be used to measure hard diffractive factorisation breaking with LHC data alone

DPE with hard scale: DPE prod of W and W-pairs (A. Loginov)

DPE prod of W-pairs relatively rare process dominated by QED require 2 protons, in 220m and/or 420m detectors expects about 1 event per fb-1

this small SM expectation would allow detection of anomalous WW prod, as e.g. predicted in theory of supercritical pomeron

diffractive studies and forward physics at cms

Documents

mb elastic

diffractive cross section

mb nondif

p p x ymap

diffractiveforward physics

mb single diffr

mb double pomeron

forward physics program