Prospects for diffractive and forward physics at the LHC

$: Prospects for diffractive and forward physics at the LHC$
Monika Grothe, Prospects for diff and fwd physics at the LHC, March 2007 1

Prospects for diffractive and forward physics at the LHC

Monika GrotheU Turin/ U Wisconsin

CALTECH, March 2007

Brief introduction to diffraction and the PomeronIts interest at the LHCExperimental situation around the CMS interaction pointThe CMS/TOTEM/FP420 program


Diffraction ?! Pomeron ?!Esoterica ?!


Brief introduction to diffraction and the Pomeron:

Diffraction


220

2

21 )(k

4R

-1x(x)][2J

0)I()I( θθ

θ ≈==

θθ 00 kR)sin(kRx ≈=

λπ2

k =

o) Forward peak for θ=0 (diffraction peak)

o) Diffraction pattern related to size of target and wavelength of beam


Diffraction in optics


2)(- 1)exp(0)(/dtd

)/dt(d θσσ pbbt

tt ≈≈=

2)(|| θpt ≈

momentumincident

angle scattering

==

pθ

€

b = R2 4

hadrons ginteractin theof sizes of sum quadratic ≈R

p

p

p

p


Diffraction in hadron scattering


b bElastic

b YDouble Dissociation

Diffraction is a feature of hadron-hadron interactions (30% of σtot):

o) Beam particles emerge intact or dissociated into low-mass states. Energy beam energy (within a few %)

o) Interaction mediated by t-channel exchange of object with vacuum quantum numbers (no colour): the Pomeron (IP)

o) Final-state particles separated by large polar angle (or pseudorapidity, ln tan(θ/2)): Large Rapidity Gap (LRG)

LRG

a a a Xa X

b bSingle Dissociation

vacuum quantumnumbers

IPIP

IP


Diffraction in hadron scattering (II)


2-gluon exchange:LO realisation of vacuum quantum numbers in QCD

pp

pp

IP

In diffractive events look at the proton constituents through a lens that filters out all parton combinations except those with the vacuum quantum numbers


Pomeron !? - A new way to probe the proton

Pomeron goes back to the ‘60s: Regge trajectory, ie a moving pole in complex angular momentum plane (huh ?!) Thanks to HERA and the Tevatron, we now understand diffraction in terms of the theory of strong interactions, QCD, and of the constituents of the proton – quarks and gluons (need a hard process)


The Pomeron as little helper in tracking down the Higgs ?!

Great. And why bother at the LHC ?


Why bother with diffraction at the LHC ?

Suppose you want to detect a light SM Higgs (say MH=120 GeV) at the LHC...

SM Higgs with ~120 GeV:gg H, H b bbar highest BRBut signal swamped by gg jet jetBest bet with CMS: H

Vacuum quantum numbers“Double Pomeron exchange”

shields color charge ofother two gluons

Central exclusive productionpp pXpSuppression of gg jet jetbecause of selection rules forcingcentral system to be JPC = 0++


Why bother with diffraction at the LHC - Detecting CEP of a light Higgs: Necessary ingredients

In addition: Rich QCD program

In addition: Rich program of gamma-gamma mediated processes

Central exclusive production pp pXp: Discover a light (~120 GeV) Higgs

In non-diffractive production hopeless, signal swamped with QCD dijet background

Selection rule in CEP (central system is JPC = 0++ to good approx) improves S/B for SM Higgs dramatically

In certain MSSM scenarios the signal cross section is orders of magnitude higher than for the SM case

• CP violation in the Higgs sector can be measured via azimuthal asymmetry of the protons


CEP of a light Higgs: Necessary ingredients (II)

=0 (beam)

=0.002

=0.015

1 2 s = M2

With √s=14TeV, M=120GeVon average:

0.009 1%

With nominal LHC optics:

fractional momentum loss of the proton

beam

p’

p’roman potsroman pots

dipoledipole

Proton spectrometer using the LHC beam magnets:Detect diffractively scattered protons inside of beam pipe


CEP of a light Higgs: Necessary ingredients (III)

Nominal LHC beam opticsLow * (0.5m): Lumi 1033-1034cm-2s-1 @220m: 0.02 < < 0.2 @420m: 0.002 < < 0.02

1 2 s = M2

With √s=14TeV, M=120GeV on average:

0.009 1%

Detectors at 420m •complement acceptance of 220m detectors•needed to extend acceptance down to low values, i.e. low M(Higgs)

Detectors at ~220m•needed in addition to optimize acceptance (tails of distr.)•needed because 420m too far away for detector signals to reach L1 trigger within latency


Have 220 m detectors, want 420 m ones:Current experimental situation at the CMS

IP


+TOTEM +FP420

pp interactions at 14TeV


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

possibly detectors@420m

Experimental situation at the CMS IP

CMS + TOTEM (+ FP420): Coverage in

Possible addition FP420

TOTEM: An approved experiment at LHC for measuring σtot and σelastic, uses same IP as CMS

TOTEM’s trigger and DAQ system will be integrated with those of CMS , i.e. common data taking CMS + TOTEM possible

TOTEM aims at start-up at the same timescale as CMS (second half of 2007)

Castor Castor

ZDC ZDC


TOTEM

xL=P’/Pbeam=

FP420

Experimental situation at the CMS IP

CMS + TOTEM (+ FP420): Coverage in

Note: Totem RP’s optimized for special optics runs at high *β* is measure for transverse beam size at vertexTOTEM coverage in improves with increasing *

At nominal LHC optics, *=0.5m

Points are ZEUS data

diffractivepeak


Experimental situation at the CMS IP Additional option: The FP420 R&D project

The aim of FP420 is to install high precision silicon tracking and fast timing detectors close to the beams at 420m from ATLAS and / or CMS

FP420 R&D fully funded (~1000K CHF) Proposal to ATLAS and CMS in first half of 2007 Detector installation could take place during first long LHC break (~2009)

Technological challenge:420m is in the cryogenic region of the LHC


FP420 project:

How to integrate detectors into the cold section of the LHC

420m from the IP is in the cold section of the LHC !Need to modify LHC cryostat: Use Arc Termination Modulesfor cold-to-warm transition such that detectors can be operated at ~ room temperature

Scattered protonsemerge here

Movable beam-pipe (pipelets)with detector stations attachedMove detectors toward beam envelopeonce beam is stable


FP420 project:

Which technology for the detectors ?3D edgeless Silicon detectors: Edgeless, i.e. distance to beam envelope can be minimized Radiation hard, can withstand 5 years at 1035 cm-2 s-1

Protons

PMT

Lens? (focusing)

MirrorCerenkov medium (ethane)

~ 15 cm~ 5 cm

(Flat or Spherical?)

Aluminium pump

Injection of gas (~ atmospheric pressure)

Ejection of gas

~ 10 cm

Time-of-Flight detectors:Time resolution of ~10ps would translate in z-vertex resolution of better than 3mm

GASTOF (UC Louvain)Cherenkov medium is a gas

QUARTIC (U Texas Arlington)Cherenkov medium is fused Silica


CRCLouvain-la-NeuveCRCLouvain-la-Neuve

Integration of the moving beampipe and detectors

Benoît Florins, Krzysztof Piotrzkowski, Guido Ryckewaert

ATM

Vacuum Space

BPM

Pockets

ATM

Line X

Bus Bar Cryostat

BPM

Vacuum Space

Transport side

QRLFixed Beampipe


The goal:

Carry out a program of diffractive and forward physicsas integral part of the routine data taking at CMS, i.e. at nominal beam optics and up to the highest available luminosities.This program spans the full lifetime of the LHC.


Joint CMS/TOTEM program

A step on the way ...

submitted to the LHCC in Dec ‘06

Central experimental issues in measuringfwd and diffractive physics at the LHC addressed for the first time - by way of a number of exemplary processes.

Included already as option: Near-beam detectors @420m


Prospects for diffractive

and forward physics at the LHC

Central experimental issues for selecting diffraction at the LHC

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


X

Double Pomeron exchange (DPE):

X

Single diffraction (SD):

o) If X = anything: Measure fundamental quantities of soft QCD Contributes significantly to pile-up

Inclusive SD 15 mb, inclusive DPE 1 mb, where 1 mb = 100 events/s @ 10 29 cm-2 s-1

o) If X includes jets, W’s, Z’s, Higgs (!): Hard processes, calculable in perturbative QCD. Measure proton structure, QCD at high parton densities, discovery physics

central CMSapparatus

central CMSapparatus

Near-beam detectors Near-beam

detectors

Near-beam detectors

IP

IP

IP

rap gap

“Prospects for diffractive and forward physics at the LHC” Part I: “Diffractive” part

The processes we are concerned with

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

Trigger (I)

pp p jj X2-jet trigger

Attention: Gap survival probability not taken into account; normalized to number of events with 0.001 < < 0.2 and with jets with pT>10GeV

Eff

icie

ncy

L1 trigger threshold [GeV]

no fwd detectorscondition

single-arm 220m

single-arm 420m

Eve

nts

per

pb

-1

At 2x 1033 cm-1 s-1 without any additional condition on fwd detectors:L1 1-jet trigger threshold O(150 GeV)L1 2-jet trigger threshold O(100 GeV)

2 x 1033 cm-2 s-1 L1 jet trigger rates

L1 outputbandwidth:100 kHz



Trigger (II)

Adding L1 conditions on the near-beam detectors provides a rate reduction sufficient to lower the 2-jet threshold to 40 GeV per jet while still meeting the CMS L1 bandwidth limits for luminosities up to 2x 1033 cm-1 s-1

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

→ Use information of forward detectors to lower in particular CMS 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)

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


Central exclusive production pp pHp with H (120GeV) bb

Trigger is a major limiting factor !H(120 GeV) → b bbar


Trigger (III)

Level-1:2-jets (ET>40GeV, ||<3) & single-sided 220m condition results in efficiency ~12% (L1)

HLT: Efficiency of above 2-jet condition ~7%To stay within 1 Hz output rate, needs to either prescale or add 420 m detectors in trigger

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

Eff

icie

ncy

L1 trigger threshold [GeV]


TOTEM

xL=P’/Pbeam=

det@420

dσ(

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-1 10% probability for a fake DPE signature.This is independent of the type of signal.


• 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. Fill rapidity gap & slow down outgoing protons Hence reduce the rate of diffractive events.

• Quantified by rapidity gap survival probability.

CDF data

Predictions based on HERA diffractive PDFs

F2D

without and with rescattering effects

jet

jet

hard scattering

IP dPDF

•Diffractive PDFs: probability to find a parton of given x under condition that proton stays intact – sensitive to low-x partons in proton

dPDF

Closely related to the underlying event at the LHC

Side remark:

Rapidity gap survival probability


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 (planned in FP420)


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 Higgsmight be in reach


Low Luminosity (Low Luminosity (≤≤10103232 cm cm-2-2ss-1-1): low & high ): low & high **

Measure inclusive SD and DPE cross sections and t, MX dependence Rapidity Gap selection possible

High Luminosity (> 10High Luminosity (> 103232 cm cm-2-2ss-1-1) : low) : low **

• Measure SD and DPE in presence of hard scale (dijets, vector bosons, heavy quarks)

• and p phyics

Highest Luminosity (> 10Highest Luminosity (> 103333 cm cm-2-2ss-1-1) : low) : low **

Discovery physics in central exclusive production (SM or MSSM Higgs, other exotic processes)

Program on diffractive and forward physics

1. Diffraction

Running with TOTEM optics: large Running with TOTEM optics: large proton acceptanceproton acceptance

No pile-upNo pile-up

Pile-up not negligible:Pile-up not negligible:main source of backgroundmain source of background

Need additional forward protonNeed additional forward protondetectordetector


Prospects for diffractive

and forward physics at the LHC

Castor Castor

ZDC ZDC


Program on diffractive and forward physics

2. Forward physics

Cosmic ray physics

→ Models for showers caused by primary cosmic rays (PeV = 1015 eV range) differ substantially

→ Fixed target collision in air with 100 PeV center-of-mass E corresponds to pp interaction at LHC

→ Hence can tune cosmic ray shower models at the LHC

Study of the underlying event at the LHC:

→ Multiple parton-parton interactions and rescattering effects accompanying a hard scatter

→ Closely related to gap survival and factorization breaking in hard diffraction

Heavy-ion and high parton density physics:

Proton structure at low xBj → saturation → Color glass condensates

Photon-photon and photon-proton physics:

Also there protons emerge from collision intact and with very low momentum loss

Multiple connection points to other areas in High-Energy-Physics !


known

“Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” part Photon-mediated processes:

Exclusive μμ production

Calibration process both for luminosity and energy scales of near-beam detectors

Striking signature: acoplanarity angle between leptonsAllows reco of proton values with resolution of 10-4, i.e. smaller than beam dispersion

Expect ~300 events/100 pb-1 after CMS muon trigger

(di-muons) - (true)

rms ~ 10-4


pQCD

No

n p

ertu

rbat

ive

reg

ion

Saturation(Colour glass condensate)

Qs2(x)

1/x

Q2 [GeV2]

•When x 0 at Q2 > a few GeV2 DGLAP predicts steep rise of parton densities

•At small enough x, this violates unitarity

•Growth is tamed by gluon fusion: saturation of parton densities at Q2=Qs

2(x)

So far not observed in pp interactions

“Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” part Low-x QCD


“Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” part Low-x QCD: Forward jets

Inclusive forward (3<|| < 5) low-ET (20-100GeV) jet production

Large expected yields (~107 at ~20 GeV) !

PYTHIA ~ NLO

1 pb-1

Di-jets sensitive to gluons with: x

2 ~ 10-4, x1 ~ 10-1

Inclusive single jet cross sectionwould be reduced by saturation

At the LHC, accessible xBJ(min) decreases by ~10 every 2 units of rapidity


“Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” part Low-x QCD: Forward Drell-Yan

Gives access to low-xBJ quarks in proton in case of large imbalance of fractionalmomenta x1,2 of leptons, which are then boosted to large rapidities

CASTOR with 5.3 ≤ || ≤ 6.6 gives access to xBJ~10-6 to 10-7

Measure angle of electrons with T2

Pdf’s known at large xBJ, hence can extract pdf’s at low xBJ

DY pairs suppressed in saturated PDF

saturated PDF

Sensitivity to saturation:


→ Models for showers caused by primary cosmic rays (PeV = 1015 eV range) differ substantially

→ Fixed target collision in air with 100 PeV center-of-mass E corresponds to pp interaction at LHC

→ Hence can tune shower models by comparing to measurements with T1/T2, CASTOR, ZDC

“Prospects for diff and fwd physics at the LHC” - Part 2: “Forward physics” part Validation of hadronic shower models in

cosmic ray physics


Summary

CMS aims at carrying out a program of diffractive and forwardphysics as integral part of its scientific program during routinedata taking at the LHC and up to the highest available luminosities

The combination of CMS (CASTOR, ZDC) with TOTEM (T1, T2,near-beam detectors at 220m) and with FP420 offers idealconditions for carrying out a rich program of diffractive and forward physics as integral part of the routine data taking at the LHCand up to the highest achievable luminosities.

CMS aims at a formal agreement with TOTEM on this collaboration.

Feasibility of the detectors suggested by FP420 essentially proven.Decision on building detectors at 420m, as suggested by FP420,as part of CMS to be addressed soon.


Last words...

“QCD at a hadron collider is always a ghetto, and there the worst neighborhood is diffraction.” - W.Smith

Recently the diffraction and forward physics group was promoted to one of the principal physics analysis groups of CMS.

We are on the way to respectability !

Prospects for diffractive and forward physics at the LHC

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