Diffractive group: experimental summary

$Page 1: Diffractive group: experimental summary$
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Diffractive group: experimental summary

Michele ArneodoUniversità del Piemonte Orientale, Novara, Italy

o) Diffractive studies at the LHC, including the Higgs

o) Input/results from HERA

Not an exaustive summary !

See talks by Pierre van Mechelen(*) yesterdayand by Halina Abramowicz on Tuesday !

(*) covers material by K. Borras, A. Bunyatyan, A. Panagiotou

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Central exclusive production of the Higgs

b, W

b, WH

• Khoze, Martin, Ryskin hep-ph 0111078

• Central system is (to a good approx) 0++

• If you see a new particle produced exclusively with proton tags you know its quantum numbers

• Proton tagging may be the discovery channel in certain regions of the MSSM

• Measuring the protons means excellent mass resolution (~ GeV) irrespective of the decay products of the central system

• Attractive for MH=120-250 GeV

ξ: fractional momentum loss of proton – for 120 GeV Higgs, x~ 1%

t: 4-momentum transfer squared at proton vertex

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How to measure the protons • At CMS: TOTEM, Roman Pots at 150 and 220m from I.P. Excellent coverage in and t at low luminosity optics (*=90, 1540m)Coverage 0.02<<0.2 at high luminosity optics (*=0.5m) [K.Oesterberg, H.Niewiadomski]

• At ATLAS: FP220Roman Pots at 220 m Coverage similar to TOTEM at high luminosity optics [Ch. Royon]

• At CMS and ATLAS: FP420 R&D project, aim to instrument region at 420m from I.P. 0.002<<0.02 (high luminosity optics only) [S. Watts]

detectors@420m

FP420

TOTEM-

FP220

xL=P’/Pbeam=

Log

Logt

*=0.5

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TOTEM (or FP220 at ATLAS)FP420

How to measure the protons

K.OesterbergM.GrotheCh. RoyonH. NiewiadomskiA. Pilkington

• Cold region of LHC• Too far for L1 trigger

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FP420

S. Watts

Replacement ofcryostat @ 420m designed

Moving beam pipe

• R&D phase essentially over

• Proposals to Atlas and CMSimminent• Installation >2009

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FP420 S. Watts

3D Si detectors: edgeless, rad-hard Successful beam test of 3D detectorsat CERN

Timing detectors to reconstruct vertex

Tracking code available in ATLAS, CMSframeworks (W. Plano, X. Rouby)

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FP220

Ch. Royon Proposal to Atlas imminent

Acceptance

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

possibly detectors@420m

T1 (CSC) 3.1 ≤ || ≤ 4.7 HF 3 ≤|| ≤ 5T2 (GEM): 5.3 ≤ || ≤ 6.6Castor 5.3 ≤ || ≤ 6.6

CMS+TOTEM: unprecedented coverage in

Carry out a program of diffractive and forward physics as integral part of the routine data taking at the LHC, i.e. at nominal beam optics and up tothe highest available luminosities.

K.OesterbergM.Grothe

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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

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

Central exclusive productionDiscovery physics:Light SM HiggsMSSM HiggsExtra dimensions

Gamma-gamma and gamma-proton interactions (QED)Forward energy flow - input to cosmics shower simulationQCD: Diffraction in presence of hard scale Low-x structure of the proton High-density regime (Color glass condensate) Diff PDFs and generalized PDFs Diffractive Drell-Yan

Low

lu

mi

Hig

h lu

mi

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

K.OesterbergM.GrotheCMS+TOTEM: physics map

1. Trigger is a major limiting factor for selecting diffractive events

2. Background from non-diffractive events that mimic diffractive events because of protons from pile-up events

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→ CMS trigger thresholds for nominal LHC running too high for diffractive events

→ Use information of forward detectors to lower jet trigger thresholds

→ The CMS trigger menus now foresee a dedicated diffractive trigger stream with 1% of the total bandwidth on L1 and HLT (1 kHz and 1 Hz)

NB Information from 220 detectors crucial for triggering

Much less of a problem is triggering with muons, where L1 threshold for 2-muons is 3 GeV

single-sided 220m conditionwithout and withcut on

Achievable total reduction: 10 (single-sided 220m) x 2 (jet iso) x 2 (2 jets same hemisphere as p) = 40

CMS-TOTEM: diffractive trigger

M.Grothe

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Trigger is a major limiting factor !

Level-1:~12% efficiency with 2-jets (ET>40GeV) & single-sided 220 m condition

HLT: Jet trigger efficiency ~7%To stay within 1 Hz output rate, needs to either prescale b-tag or add 420 m detectors in trigger

Additional ~10% efficiency by introducing a 1 jet & 1 (40GeV, 3GeV) trigger condition

H(120 GeV) → b bbar

L1 trigger threshold [GeV]

Eff

icie

ncy 420m

220m

420+420m

420+220m

pp pWX1-jet trigger

pp p jj X2-jet trigger

Eff

icie

ncy

L1 trigger threshold [GeV]

no fwd detectorscondition

single-arm 220m

single-arm 420m

Eve

nts

per

pb

-1

CMS-TOTEM: diffractive trigger M.Grothe

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Eg at 2x 1033 cm-2s-1 10% probability of obtaining a fake 2-proton signature because of pile-up.

Pile-up

• Average number of pile-up events overlaid to any hard scatter 7 @ 2x1033 cm-2s-1, 35 @ 1x1034 cm-2s-1 (not 20…)

• 25% of these events are diffractive, i.e. have a fast proton

• Example: pile-up background to diffr H bb comes from non-diffractive bb production, superimposed to two single- diffractive pile-up events

K.OesterbergM.GrotheM. TasevskyA. PilkingtonV. Khoze

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Can be reduced by:

Requiring correlation between ξ, M from central detector andξ, M from near-beam detectors

Fast timing detectors to determine if protons came from same vertex as hard scatter(TOF with 10 ps resolution !)

; 1 2 s = M2

(from protons)(

jets

)

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

M(2-jets)/(Missing Mass)

Pile-up


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Pile-up

• S/B for SM H bb of order 1 at 2 x 1033 cm-2 s-1

• S/B for MSSM H bb as large as 100-1000

• Pile-up significantly less severe for H WW

H bb


A. Pilkington

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Further upgrades in CMS forward region

A. Bunyatyan, K. Borras

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• Measurements of diffractive structure function F2D

• QCD fits to F2D and extraction of dPDFs

• How well does QCD hard-diffractive factorisation work ie can use dPDFs to predict cross section of diffractive production of jets or charm ?

• Can we quantify rapidity gap survival probability ? • Leading neutrons and the survival probability

HERA

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Input from HERA: dPDFs

Diffractive PDF: probability to find a parton of given x in the proton undercondition that proton stays intact – sensitive to low-x partons in proton, complementary to standard PDFs

Obtained from QCD fits to F2D data

IP

p’p

p p’

IP

b, jet

b, jet

dPDF

dPDF

p’p

e

e’

IP dPDF

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New measurements of F2D from ZEUS

Three methods to select diffraction at ZEUS:

i) Require a leading proton (leading proton spectrometer, LPS)

ii) Require a large rapidity gap (LRG)

iii) Exploit the different shape of MX

for diffractive and non-diffractive events (MX method)

p’p

ee’

IP

X

For the first time, analyse the same set of data with the three methods and try and understand the differences

J. Łukasik Inclusive Diffraction at HERA from the ZEUS experiment

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ZEUS MX 99-00, ZEUS MX 99-00 (prel.), ZEUS LRG 00 (prel.)

xIPF2D(3) vs. Q2

● reasonable agreement● work on understanding

remaining differences is continuing

ZEUS MX 98-99

ZEUS MX 99-00 (prel.)

ZEUS LRG 00 (prel.)

ZEUS MX vs LRG results


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ZEUS LRG 00 (prel.), ZEUS LPS 00 (prel.)

LPS/LRG=0.82±0.01(stat.)±0.03(sys.)independent of Q2 and β

A measure of the contamination byproton dissociative events in the LRGsample

About 10% normalization uncertainty of the LPS measurement not shown

ZEUS LRG vs LPS results

Np

ee’

IP

X


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xIP = 0.003ZEUS LRG 00 (prel.), H1 LRG

xIP = 0.01ZEUS LRG 00 (prel.), H1 LRG

● Fraction of proton dissociation events for ZEUS and H1 detectors is different

● The ZEUS LRG data are normalized to the H1 LRG data

ZEUS vs H1

H1: (0) = 1.118 ± 0.008 +0.029-0.010

‘ = 0.06 +0.19 -0.06

ZEUS: (0) = 1.117 ± 0.005 +0.024-0.007

' = -0.03 ± 0.07 +0.04 -0.08

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H1 FH1 F22DD

In best regions, precision ~5% (stat), 5% (syst), 6% (norm),…well described by fit

P.Newman

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`Fit A’ and `Fit B’ DPDFs (linear z scale)`Fit A’ and `Fit B’ DPDFs (linear z scale)

• Lack of sensitivity tohigh z gluon confirmedby dropping (high z) Cg

parameter, so gluon is a constant at starting scale!

•Fit B 2 ~164 / 184 d.o.f.

• Quarks very stable• Gluon similar at low z • Substantial change to gluon at high z

P.Newman

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QCD factorisation OK

M. Mozer

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QCD factorisation OK

R. Wolf

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jet

jet

hard scattering

IP LRG

CDF data

Extrapolationfrom HERA

GPDs and diffractive PDFs measured at HERA cannot be used blindly at LHC or Tevatron:

Digression: rapidity gap survival probability

P.Newman

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• Proton and anti-proton are large objects, unlike pointlike virtual photon

• In addition to hard diffractive scattering, there may be soft interactions among spectator partons. They fill the rapidity gap and slow down the outgoing p,p – hence reduce the rate of diffractive events. Quantified by rapidity gap survival probability (underlying event !)

CDF data

Predictions basedon rescattering assuming HERA diffractive PDFs

F2D

Kaidalov, Khoze, Martin, Ryskin (2000)

Rapidity gap survival probability

Can we see a similar suppression at HERA by usingresolved photons at Q2=0 ?

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QCD factorisation OK(but mainly direct component)

I. Melzer (+R. Wolf for H1)

31Unexpected, notunderstood

Hadron-like

QCD factorisationnot OK

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Leading Neutrons

Kaidalov, Khoze, Martin & Ryskinhave used these data to derivethe rapidity gap survival probability(one pion exchange+absorption+migration)

B. Schmidke

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Grand summaryLHC:• Diffraction/forward physics has generated several new detectors now on the way – added value for ATLAS and CMS. Diffractive group in CMS !

• Experimental challenges being addressed, eg trigger, pile-up

• Different experiments joining forces: CMS+TOTEM, ATLAS and CMS in FP420

HERA:• Precious input: dPDFs, rapidity gap survival probability, GPDs, experience in operating near-beam detectors

Wish list: • DVCS, J/psi, Y (including t dependence !) for constraining GPDs• H1+ZEUS F2

D, dPDFs• Understanding of gap survival• Leading proton spectra at LHC (crucial for pile-up)

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RESERVE

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CDF: evidence for exclusive processes at Fermilab

Search for exclusive 3 candidate events found 1 (+2/-1) predicted from ExHuME MC* background under study

Same type of diagrams as for Higgs Validation of KMR model !D. Goulianos, V. Khoze

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Diffractive Structure Function:Q2 dependence

ETjet ~ 100 GeV !

Small Q2 dependence in region 100 < Q2 < 10,000 GeV2

Pomeron evolves as the proton!D. Goulianos

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Diffractive Structure Function:t- dependence

No diffraction dips No Q2 dependence in slope from inclusive to Q2~104 GeV2

Fit d/dt to a double exponential:

Same slope over entire region of 0 < Q2 < 4,500 GeV2

across soft and hard diffraction!

D. Goulianos

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Diffractive group: experimental summary