# Presentation: 587755

## Prospects for Diffractive and Forward Physics with CMS

- Edmundo García
- University of Illinois at Chicago
- On behalf of the CMS and TOTEM
- Diffractive and Forward Physics Working Group
- Fermilab, March 2007

- 6th Small-x and Diffraction Workshop** **

## Outline

- Introduction
- Experimental Capabilities
- Forward and near beam detectors
- Example: Central exclusive production SM-H
- Acceptance
- Background (pile-up)
- Triggering
- Forward Physics
- Final Remarks

## Introduction

- HERA:  epeXp diffractive deep inelastic scattering (DIP)

- no colour
- ~ no effective
- spin interchange

- X

- rapidity gap

- Z or W

- diffractive structure functions

- D0 Run I sample

**(or  p)**

**( or p)**

- 

- 

## Low-x Interactions: A Proton Structure Probe 

- Eur. Physs. J. C48 (2006)

-   Hard DIP obeys QDC factorization at HERA energies.

- Eur. Physs. J. C48 (2006)

-   Strong rise at low-x: partons resolved in DIP ~ gluons (+ quark singlets).

-   ~ 70% of exchanged momentum carried by gluons

## Exclusive Diffractive processes

- hep-ph/0311028. ZEUS Data

- HERA:  *pVp  “elastic” vector meson production and *pp deeply virtual Compton scattering

- hep-ph/0511047

-   Energy dependence of these processes becomes steep in the presence of hard scale

-   This reflects the x scale dependence of the gluon density in the proton.

-   Cross section of these processes is ~ proportional to the square of gluon density.

## Low-x Physics at the Tevatron

- SD rates of *W* and *Z*- bosons, dijet, b-quark and J/MESON meson production in the Tevatron are about 10 times lower than expectations based diffractive parton distribution functions (dPDF) determined at HERA

- Breakdown of dPDF QCD factorization
- Likely due to the filling of the rapidity gap (RG) and slowing (breaking) of the proton due to soft scattering of the two interacting hadrons (quantified by  RG survival probability)

- Phys. Rev. Lett. 84 (2000) 5043

- hep-ex / 0606024

## Low-x Physics at the LHC

**LHC**

**Tevatron**

- Based on Stirling’s Eur. Phys. J. C 14, 133 (2000)

- LHC  will probe forward rapidities y ~ 5, Q2 ~ 100 GeV and x down to ~10-5
- Some of the available processes:

| Inclusive single diffraction (SD) and double “pomeron” exchange (DPE) | pppX 
pppXp |
| --- | --- |
| SD and DPE production of dijets, vector bosons and heavy quarks | pppjjX
pppW(Z)
pppqq |
| Central exclusive production | pp pHp
with 
H (120GeV) bb |
| High energy photon interactions | pppWX
pp(pp)pWHX |

## Low-x Physics with CMS, Totem, and the Forward Detectors 

- central CMS
- apparatus

- Near-beam
- detectors - Roman Pots (F420)

- rap gap  - veto on T1,T2, HF, ZDC (CASTOR)

**X**

- Double Pomeron exchange (DPE):

- central CMS
- apparatus

- Near-beam
- detectors

- Near-beam
- detectors

- rap gap

- rap gap

- leading proton

- leading proton

- leading proton

## Central CMS detector - ||<2.4

- CALORIMETERS

** **

**ECAL**

-  Scintillating
- PbWO4 crystals

**HCAL**

- Plastic scintillator/brass
- sandwich

- MUON BARREL

- Drift Tube

- Chambers (**DT**)

- Resistive Plate

- Chambers (**RPC**)

## Forward Region

- (5.2 _<__<_ 6.7)

- T1,T2

- Forward HCal

- (3 _<__<_ 5.2)

- CASTOR

- (5.2 _<__<_ 6.6)

- ZDC

- HAD

- EM

- (z =  140 m)

- (8.3 _<_)

- FP420

- likely addition

- possible
- addition

- (z =  147,  220 m)

- Roman Pots

- (z =  420 m)

**CMS IP**

**T1/T2, Castor**

**ZDC**

**RP**

**RP**

**FP420**

## Central exclusive production pp pHp     H (120GeV)  bb

- In non-diffractive production very hard, signal swamped  with QCD dijet background
- Selection rule in central exclusive production (CEP)  ~ JPC = 0++   improves S/B for SM Higgs dramatically
- Key: Detection of diffractively scattered protons inside of beam pipe. Plus: two collimated jets in central detector, consistent values of central mass from central detectors

-  With √s=14TeV, M=120GeV on average: ****** ******** 0.009 ******** 1%**
-  Nominal LHC beam optics: Low * (0.5m), Lumi 10**33**-10**34**cm**-2**s**-1 ****   **
-     at 220m:     0.02 <  < 0.2
-     at 420m:     0.002 <  < 0.02

**** fractional momentum loss of the proton

## TOTEM (+ FP420): coverage for leading protons

- TOTEM (+ FP420): coverage for leading protons

**x****L****=P’/P****beam****= ******

**FP **
**420**

- At nominal LHC optics, *=0.5m

- TOTEM

- Nucl. Phys. B658 (2003) 3

- Example:
- Central exclusive production
- pp p*H*p with *H* (120GeV)  bb

- For acceptance of Higgs events, both scatter protons have to be detected at opposite sides of the interaction point
- This can happen in different combinations of the 220m and 420m near beam detectors

## Pile-up Background

- Number of pile-up events with protons within acceptance of near-beam detectors on either side:  ~2 % with p @ 420 m , ~6 % with p 220 m
- Translates into a probability of obtaining a fake DPE signature caused by protons from pile-up
- Reduced by correlation between  ξ measured in the central detector and ξ measured by the near-beam detectors
- Reduced by tight cut in the vertex  (vertex resolution FP420 ~ 3mm)

- Example:
- Central exclusive production
- pp p*H*p with *H* (120GeV)  bb

- Selection cuts:
- Jet cuts: >2 jets ET>30/45 GeV
- both jets b-tagged
- 2 protons in near-beam detectors,
- either 220+420 or 420+420

**Notes:**

- e.g. at 1x 1034 cm-2s-1 60% of any signal event one wants to select have a fake DPE signature
- This is independent  of the type of signal.

## Triggering

- Diffractive/forward processes typical values of  pT  lower than most hard processes suited for CMS.
-  The CMS trigger menus now foresee a dedicated diffractive trigger stream
-      with 1% of the total bandwidth on L1 and HLT

- CMS-2006/054   and TOTEM-2006/01

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

- reduction requiring 2 jets in the same  hemisphere
- as the RP detectors that see the proton

- Jet isolation
- criterion

- Dijet trigger + L1 conditions on the near-beam detectors  reduction to lower the dijet threshold to 40 GeV/jet while meeting the CMS L1 bandwidth limits for luminosities up to 2x 1033 cm-1 s-1

## Triggering cont.

- Example: Central exclusive production
- pp p*H*p with *H* (120GeV)  bb

**Level-1:**
-  HT condition ~ rapidity gap of 2.5 with respect to beam direction
-  2-jets (ET>40GeV) & single-sided 220m, efficiency ~ 12%
-  Add another ~10% efficiency by introducing  a 1 jet & 1 (40GeV, 3GeV) trigger condition
**HLT**: Efficiency ~7%
- To stay within 1 Hz output rate, needs to either prescale b-tag or add  420 m detectors in trigger

-  Possible to retain about ~10% of the signal events up to 2 x 1033 cm2 s1 in a special  forward detectors trigger stream** **

-  Reduction of the pile-up background by O(109) can be obtained with relatively simple cuts, further requirements (vertex position) can give a further 10-100 suppression or larger.

-  Yield S/B in excess of unity for a SM Higgs

## Program on diffractive Physics

- The accessible physics is a function of the instantaneous and integrated luminosity
**Low:**
-  Low enough that pile-up is negligible, i.e <1032 cm-2s-1, and integrated lumi a few 100pb-1 to <1 fb-1
-  Measure inclusive SD and DPE cross sections  and their Mx dependence
**Intermediate**** (***=0.5m)
** **Lumi < 1033 cm-2s-1, pile-up starts becoming an issue, integrated lumi 1 to a few fb-1
-  Measure SD and DPE in presence of hard scale (dijets, vector bosons, heavy quarks)
-  Basically follow Tevatron program
**High:**
-  Lumi > 1033 cm-2s-1, pile-up substantial and integrated lumi several tens of fb-1
-  Discovery physics comes into reach in central exclusive production

## Program on Forward Physics

**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
**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

## Forward Jets

-  At Tevatron is the standard tool to study PDFs in global fit analysis
-  At CMS (ET ~20-100 GeV) dijet detection will be possible in  HF for 3<<5  and  -3<<-5
-  This will allow to probe values  as low as x2 ~ 10-4

- Generator level estimates: No detector response, underlying event and hadronization corrections included  for spectrum

- Integreated luminosity 1 pb-1

- –

## Forward Dell-Yan

- Access top the low x regime of the quark(antiquark) distributions requires imbalance of fractional momenta  x1,2 boosted to large rapidities
- CASTOR with 5.3 ≤ || ≤ 6.6 gives
-      access to xBJ~10-7
- Measure electrons angle with T2

## Forward Physics: Gluon saturation region

| Sub detector | Coverage |
| --- | --- |
| Forward HCAL | 3 .0< |  | < 5.2 |
| CASTOR | 5.2 < |  | < 6.6 |
| ZDC | 8.3 < || |

**McLerran, hep-ph/0311028**

- ZEUS data for gluon distribution
- In partons

- Q2

- New regime of QCD is expected to reveal at small x due to effects of large gluon density
- So far not observed in pp  interactions
- Field of common interest with Heivy Ions Physics

- CGC

- saturation

**Notes:**
low x   -> s<<1
At a given Q(x,A) -> gluon density saturation -> saturation models for heavy ions

## Limiting Fragmentation

-  At RHIC limited fragmentation can be
-  explained by gluon saturation (Colour Glass Condensate) model.
- Data PHOBOS 200, 130 and 62 GeV AuAu
- Model McLerran-Venugopalan
- This can be measured also at LHC for larger gluon density region

- Phys. Rev. D 49, 2233 (1994 )

- Castor Coverage

## Photon-mediated processes

- QED theoretical cross section for this process is precisely known
- Reaction can then be used to calibrate the pp-luminosity provided that events can be identified above piule-up
- The calibration for the forward pronton’s energy measurement can be achieved
- Expect ~300 events/100 pb-1 after CMS muon trigger

- rms ~ 10-4

- (di-muons) - (true)

- Resolution of the reconstructed forward proton momentum using exclusive pairs in CMS
- Reconstruction of proton  values with resolution of 10-4, i.e. smaller than beam dispersion

## Validation of Hadronic Shower Models

- 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

## Final Notes

**Diffractive Physics Program**
- Low Luminosity:
-  Measure inclusive SD and DPE cross sections  and their Mx dependence
- Intermediate** (***=0.5m)
-  Measure SD and DPE in presence of hard scale (dijets, vector bosons, heavy quarks)
-  Follow  Tevatron program
- High:
-  Discovery physics comes into reach in central exclusive production
**Forward Physics Program **
** **Heavy-ion and high parton density physics
-  Photon-photon and photon-proton physics
-  Cosmic ray physics

## Backup Transparencies

## Slide 26

## Leading Protons Acceptance

-  Smearing of both transverse vertex position and scattering angle at the IP according to transverse beam size and beam momentum divergence
-  Assume that near-beam detectors are 100% efficient, i..e. assume all protons that reach  220/420m location outside of cutout for beam (1.3mm @220m, 4mm @420m) are detected

- log(-t)

- log()