Why is double proton tagging potentially useful for detecting new physics?

Brian Cox

Small x and Diffraction, FNAL Sept 2003

It isn’t … unless this diagram exists, and has a large enough cross section:

This :degrades the beam energy for minimal or no gain at all.

Advantages of exclusive central production

• tagging the protons means the mass of the central system is known, irrespective of decay channel, up to the resolution of the taggers …

… if there is no other particle production in the central region

• only 0++ central systems can be produced, if the protons remain intact and scatter through small angles …

… QCD bbar background is greatly surpressed

… if you see a resonance, you know it’s a CP-even, spin 0 particle

Very schematically it’s a glue – glue collider where you know the beam energy of the gluons

For s >> M2,t

For colour singlet bbar production, the born level contributions of a) and b) cancel in the limit mb -> 0

V. A. Khoze, A. D. Martin and M. G. Ryskin Eur.Phys.J. C23 (2002) 311-327

Exclusive central production

gluon screens the colour

• The cross section ~ factorises …

Hard subprocess cross section

V. A. Khoze, A. D. Martin and M. G. Ryskin Eur.Phys.J. C23 (2002) 311-327

Effective luminosity for production of mass M at rapidity y

… so can be checked by measuring higher rate processes in Run II (-> Angela Wyatt’s talk)

Sudakov factors ‘cut off’ Qt integral

(as Qt-> 0 probability for NO emission -> 0, and hence there is no contribution to the exclusive cross section)

An example in which double tagging may be the only way

In the CPX scenario, the three neutral MSSM Higgs bosons, (CP even) h0 and H 0, and

(CP odd) a mix to produce 3 physical mass eigenstates H 1 , H

2 and H

3 with mixed CP

A. Pilaftsis, Phys. Rev. D58 (1998) 096010; Phys. Lett. B435 (1998) 88.

Then, (very schematically), the lightest Higgs could be predominantly CP odd, and not couple strongly to the Z.

Imagine a light scalar which couples predominantly to glue, and decays to b jets … would we see it at LEP, Tevatron or LHC?

Light neutral Higgs bosons in the CPX scenario

“there are small regions of parameter space in which none of the neutral Higgs bosons can be detected at the Tevatron and the LHC”

Medium grey

Dark grey

Plot from M. Carena, J. Ellis, S. Mrenna, A. Pilaftsis and C. E. M. Wagner, hep-ph/0211467

CPX scenario: M. Carena, J. Ellis, A. Pilaftsis and C. E. M. Wagner , Nucl. Phys. B586 (2000) 92.

Central exclusive CPX Higgs cross sections

LHC

B.C., J. R. Forshaw, J. S. Lee, J. W. Monk and A. Pilaftsis hep-ph/0303206

At LHC, 120 GeV Higgs ~ 3 fb (KMR Eur. Phys. J. C23 (2002) 311.)

Tevatron

Another example: very preliminary results for Radion production

D. Dominici et. al. hep-ph/0206192

Radion field doesn’t couple to Z, BUT mixing can occur

Signal to Background

A. De Roeck, V. A. Khoze, A. D. Martin, R. Orava and M. G. Ryskin Eur.Phys.J. C23 (2002) 311-327

A detailed study for 120 GeV Standard model Higgs at LHC (De Roeck et. al.)

• Leading order ‘bbbar’ backgrounds

• order contribution from massive b quarks

B/S ~ 0.06 • NLO backgrounds

• bbar glue in which gluon goes undetected down beam pipe

Removed by requiring

• bbar glue in which gluon co-linear with b jetB/S ~ 0.06

• NNLO shown to be negligibly small

Assuming 1% b misidentification probability B/S ~ 0.06

• misidentification of large glue – glue subprocess

B/S ~ 0.08

• non-forward ( i.e. non-zero t) JZ = 2 admixture

“Inclusive” (Central-inelastic) process

p+p p + gap + H + X + gap + p

Soft pomeron – pomeron background

Removed by requiring

A. De Roeck, V. A. Khoze, A. D. Martin, R. Orava and M. G. Ryskin Eur.Phys.J. C23 (2002) 311-327L = 30 fb-1

Signal to Background for light CPX Higgs

• Cross sections ~ order of magnitude larger, so more chance of seeing it, right?

Mass resolution of pots (1 GeV)

• JZ = 0 selection rule less effective at small masses

Cross section for exclusive bb production ~

> 2 for MH > 20 GeV at LHC

Probably no chance at Tevatron, even though cross section is potentially large

The Experimental Strategy • Tag both protons at low t, in the appropriate range (Missing mass)

Tevatron: 10 GeV < MH < 50 GeV, 0.005 < < 0.025

p p

D SQ2Q3Q4S A1A2

P1U

P2I

P2O

P1DD2 D1

233359 3323057

VetoQ4Q3

Q2

CDF and D0 ~ 0.035 < < 0.095

D0 ~ all , |t| > 0.6 GeV

• Tag b jets

• Impose approximate mass equality Mmissing = Mbbar

M. G. Albrow and A. Rostovtsev hep-ph/0009336

A. De Roeck, V. A. Khoze, A. D. Martin, R. Orava and M. G. Ryskin Eur.Phys.J. C23 (2002) 311-327

LHC: 10 GeV < MH < 50 GeV, 0.0007 < x < 0.0035

Conclusions

• There may be light scalars in nature, and we should be prepared to look for them

• Proton tagging can in principle deliver the required mass resolution and background suppression

• Proton tagging gives us a high energy ‘pure’, ‘colourless’ glue – glue collider with variable beam energy (for a very small cost)

Physics with forward proton taggers at the Tevatron and LHC

Manchester 14, 15, 16 December 2003

glodwick.hep.man.ac.uk/conference

• BUT !! The cross section predictions must be verified experimentally at the Tevatron - see new CDF results (Angela Wyatt),