1. Low-x Measurements @ LHC Prof. Brian A. Cole Columbia University

2. Overview: Accessible x, Q 2 @ LHC Kinematically can reach x ~ 10 -6 for Q 2 > 100 GeV 2 Near mid-rapidity, x < 10 -3 for Q 2 <~ 100 GeV 2  s = 5.5 TeV

3. Three Experiments: ALICE Central tracking, particle identification, EM calorimetry, forward muon detection, zero-degree calorimetry ALICE

4. Three Experiments: ATLAS Central tracking, EM and hadronic calorimeters, muon spectrometers, forward calorimeter, zero- degree calorimeter

5. Three Experiments: CMS CMS Central tracking, EM and hadronic calorimeters, muon spectrometers, forward tracking and hadronic calorimeter, zero-degree calorimeters

6. Experimental Acceptance (e.g.) CMS ATLAS

7. Overview: Categories of measurements Inclusive particle production, multiplicity, dn/d  –A+A and p+A collisions –Test of saturation interpretation of RHIC Au+Au data Single moderate p T hadrons, jets, prompt  /Zs –p+A with p+p baseline measurements –p T spectra sensitive to shadowing / saturation effects Di-jet,  /Z-jet, di- , Drell-Yan, … –p+p and p+A (all), A+A (di-  and Drell-Yan) –Clean initial-state kinematics (esp. DY and di-  ) –Acoplanarity and Q T sensitive to BFKL, saturation  + A in ultra-peripheral A+A (Mark) Generally: study deviations from collinear pQCD at low x and large g(x, Q 2 ) with many hard processes

8. Inclusive Particle Production in A+A Observe geometric scaling also w/ nuclear targets Use k T factorized formula for gluon production and parton-hadron duality  Calculate A+A dn/d  Compares very well to RHIC (e.g. PHOBOS) data Definite(ive?) prediction for A+A dn/d  @ LHC –Test in 1 st Pb+Pb run Armesto, Salgado, Wiedemann Phys. Rev. Lett. 94 :022002,2005

9. Broadening OnlyIncluding Quantum Evolution Proton-Nucleus: Heavy Quark Production Modest effects of saturation @ intermediate p T –but extend over a long range in p T Measured by: –Single electron/muon, D meson (ALICE) @ lower p T –Tagged b jets at higher p T

10. Forward Jets (e.g. CMS) Measure jets using forward hadronic calorimeter in CMS –3 < |  | < 5 –15% energy resolution for jet E T ~ 20 GeV –Better resolution @ larger  due to smaller underlying event (?) –Even better in p-A For jet p T > 20 GeV –Reach x down to 10 -4 From Quark Matter 2006 talk by D. d’Enterria Energy resolution %

11. Q=100 GeV Q=10 GeV Q=2 GeV Frankfurt, Guzey, Strikman: Phys. Rev. D71:054001, 2005. p-A: Nuclear Shadowing Compilation by Armetso, hep-ph/0604108 Pb Large LT shadowing in Pb –Persisting to high Q 2 (> 10 4 GeV 2 ) But: –Many shadowing fits/extrapolations –Producing wide range R g A values In principle: –Can be resolved w/ high-p T singles, jets In practice: –How well?

12. Heavy Quark: Acceptance Heavy quark measurements will extend to x ~ 10 -5 (ALICE & LHCb), few x 10 -4 for ATLAS & CMS  Will be sensitive to shadowing or saturation Beware definition sign of 

13. ALICE: Forward  in p-Pb

14. Heavy quark pairs: Measuring R g A Eskola, Kolhinen and Vogt, Nucl. Phys. A696:729-746, 2001. Comparison of p+A/p+p cross- section ratio (solid) to (dashed) Conclude: double semi-leptonic decays of heavy quarks give sensitive measurement of

15. LHC: Low-mass Drell-Yan From ATLAS std model WG talk by E. Rizvi. DY  +  - in ATLAS, |   | 6 GeV/c –Triggerable Very preliminary study, but suggests access to x values down to 10 -5 M  +  - (GeV/c 2 ) counts Trigger efficiency does not produce strong x bias

16. The LHC Schedule (Nominal + Guesses) p-p –Commissioning run @  s = 0.9 TeV, late 2007 –First full-energy run @  s = 14 TeV, spring 2008 –Heavy ion base-line @  s = 5.5 TeV, 2011 or 2012 ? Pb+Pb –Short commissioning run, late 2008 (?) –Full (3 weeks) run late 2009 p-Pb –Full run @  s = 8.8 TeV, likely before 2014 By 2010-2011: –We should know if LHC Pb+Pb dn/d  data are consistent with production from saturated initial state. –Have extensive p-p data to test for evidence of BFKL Then: – p-Pb run  unique studies @ low x in nucleus

17. Low-x: LHC, e-RHIC At the LHC –We will be able to access unprecedented low x values –With both protons and nuclei –With a wide variety of probes –Over a wide range of Q 2  But, only for Q 2 > 100-1000 GeV 2 for observables with clean kinematics We will be able to: –Study QCD physics at very low x in both p and A –Measure/constrain nuclear PDFs –Measure pre-cursors of saturation (geometric scaling) –Evaluate whether canonical saturation picture applies e-RHIC needed for precision, universality, low Q 2

18. Counts ATLAS: Low-x Physics w/ ZDC Can access x 4 GeV/c) –Correlate with jets in ATLAS calorimeters (|  | < 5) –Study acoplanarity vs  12 π 0 acceptance From P. Steinberg Quark Matter 2006 ATLAS heavy ion physics plenary talk

19. A+A Multiplicity vs Energy LHC measurements will provide an essential test of whether we understand the mechanism responsible for bulk particle production. –e.g. does saturation correctly extrapolate? RHIC 200 GeV Saturation? Something else?

20. CGC: Gluon Shadowing Given a saturation parameterization of proton DIS data and Q s A  prediction of shadowing. Saturation calculation by Kugeratski et al, hep-ph/058255 Au Ca

21. Saturation: proton(deuteron)-A p/d-A collisions provide an alternative way to study low-x processes in nuclei. –hadron production @ large rapidity (small x A ) Smaller x A Central to peripheral ratio hadron CentralPeripheral Brahms Data@ RHIC

22. CGC w/ Evolution Compared to Data Larger , Lower x Effect of evolution Are we seeing evolved CGC in d-A @ RHIC? Unclear

23. (Very) low x / Saturation @ LHC Can reach x < 10 -4 with p T > 20 GeV jets,  < -3 with 5 GeV single hadrons  ~2. This does not take into account p-A rapidity shift Minimum accessible x (collinear kinematics) Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 94:022002 (2005) e.g. ATLAS Main detector

24. Low-x Physics: Motivation CGC + Quantum evolution provides compelling framework for analyzing low-x physics –New perturbative regime of QCD –Quantum evolution from non-Abelian classical fields a fundamental problem in field theory –Calculation of heavy ion initial conditions!(?) But, we can’t yet be certain that it applies to any system that we will study in the laboratory  p-p, p-A, A-A measurements @ LHC »Will reach lower x (10 -7 ), but imprecise kinematics »Test application of BFKL evolution  e-A measurements @ e-RHIC »Will reach less extreme x, but precise kinematics

25. Low-x Physics: Motivation (2) This cannot be allowed to persist … If not e-RHIC, then how, where ????

26. K. Itakura (QM 2005)

27. Single Moderate p T Hadron Measurements Measurable shadowing even at 100 GeV. Modest effects at mid-rapidity (but going away slowly) Q=100 GeV Q=10 GeV Q=2 GeV Frankfurt, Guzey, Strikman: Leading twist Shadowing Armesto, Salgado, Wiedemann, Phys. Rev. Lett. 94:022002 (2005)