Physics challenges of the LHC I The LHC

Physics challenges of the LHC I. The LHC machine II. The LHC experiments III. B physics at the LHC 1. CP violation 2. The LHCb experiment 3. Hadron identification 4. Expected results XIV Swieca Summer School, Sao Paolo Roger Forty 29– 31 January 2007 Physics challenges of the LHC (III)

1. CP violation • Recall the arguments for physics beyond the Standard Model (from before): 1. Dark matter may come from indicate that – Astrophysical measurements of. Hints the rotations of galaxies normal “baryonic” matter makestheuphigh-p only ~T experiments 4% of the total energy density of the Universe — what(eg is the rest? Supersymmetry or extra dimensions) 2. Gravity – Gravity is not part of the Standard Model Why is natural scale of gravity m. P = √ħc/G ~ 1019 Ge. V (Planck mass) so much larger than the Electroweak scale ~ 102 Ge. V? Known as the “hierarchy problem” 3. Baryogenesis Connected with the field of B physics – Why is the world we observe made up almost entirely of matter, while it is expected that equal quantities of matter and antimatter were produced in the Big Bang? Roger Forty Physics challenges of the LHC (III) 2

Baryogenesis • Big Bang (~ 14 billion years ago) → matter and antimatter equally produced Followed by annihilation → nbaryon/ng ~ 10 -10 Why didn’t all the matter annihilate (luckily for us)? • No evidence found for an “antimatter world” elsewhere in the Universe • One of the requirements to produce an asymmetric final state (our world) from a symmetric matter/antimatter initial state (the Big Bang) is that CP symmetry must violated [Sakharov, 1967] • CP is violated in the Standard Model, through the weak mixing of quarks For CP violation to occur there must be at least 3 generations of quarks So problem of baryogenesis may be connected to why three generations exist, even though all normal matter is made up from the first (u, d, e, ne) • The way to probe CP violation is through the study of quark mixing In particular, hadrons containing the b-quark show large CP asymmetries However, the CP violation in the SM is not sufficient for baryogenesis Other sources of CP violation expected → good field to search for new physics Roger Forty Physics challenges of the LHC (III) 3

Symmetries • Important concept in physics: if a system remains invariant under a continuous transformation, there is a corresponding conservation law eg invariance under spatial translation ↔ momentum conservation • Other transformations are discrete (eg reflection) Three important discrete transformations: P = parity spatial coordinates x, y, z –x, –y, –z T = time reversal time t –t C = charge conjugation particles ↔ antiparticles • Combined operation of all three discrete symmetries = CPT Conservation of CPT is fundamental property of all field theories Guarantees that particle has exactly the same mass as its antiparticle (tested to 1 part in 1018 by comparison of K 0 and K 0 masses) Roger Forty Physics challenges of the LHC (III) 4

Parity violation • The Strong and Electromagnetic interactions conserve C, P and T • eg pion decay via the electromagnetic interaction: p 0 gg but not ggg Initial state: Final state: C(p 0 + C(gg ( 2 + C(ggg ( 3 p 0 = (uu dd)L=0, S=0 B, E B, E C(p 0 + C(g • Weak interaction violates Parity (experiment of Wu et al in 1957) • Neutrinos are left-handed Antineutrinos are right-handed perhaps weak interaction conserves the combined operation, CP? eg G(p+ m+ n. L) = G(p m n. R) Roger Forty Physics challenges of the LHC (III) 5

CP violation • Weak interaction appeared to conserve CP until the experiment of Christenson et al (1964): KL 3 p (CP = 1) BR = 34 % KL p+ p (CP = +1) BR = 2 10 -3 CP violation • BR (KL p e+ n) = 19. 46 % > BR (KL p+ e n) = 19. 33 % unambiguously differentiates matter from antimatter • In Standard Model, CP violation arises from quark mixing Weak eigenstates are “rotated” combination of flavour states V = unitary CKM matrix (Cabibbo-Kobayashi-Maskawa) Its elements give weak couplings between quark flavours Roger Forty Physics challenges of the LHC (III) 6

• Unitarity of the CKM matrix gives relationships between rows and columns: S Vij Vik* = 0 (j k) • One of these relationships has terms of similar size: Vud Vub* + Vcd Vcb* + Vtd Vtb* = 0 triangle relationship in the complex plane • (3 3) CKM matrix has 4 independent parameters: 3 angles and one non-trivial phase The phase gives rise to CP violation — only present with 3 generations • CKM matrix observed to have a hierarchy of elements Parameterized [Wolfenstein] expanding in powers of the Cabibbo angle l = sin q. C 0. 22 Parameters (l, A, r, h) A 0. 8, measured leaves r and h to be determined h 0 CP violation Roger Forty Physics challenges of the LHC (III) 7

• Rescaling the “Unitarity Triangle” by Vcd Vcb*: h • Many of the measurements made of hadrons containing the b-quark can be presented as constraints on this triangle r eg measurements of their lifetime Vcb The fraction of charmless b decays Vub “Box diagram” for B 0–B 0 oscillation • Neutral B mesons oscillate between their particle and antiparticle states via a second-order weak transition Frequency Dm of this oscillation Vtd • In addition, CP violation in B decays measures the relative phases of the matrix elements measure the angles (a, b, g) (= CP eigenstate) Decay “via mixing” with different phase Depends on phase of B 0–B 0 oscillation arg(Vtd) angle b Roger Forty Physics challenges of the LHC (III) 8

B Factories • The CP violation in B 0 J/ KS decays has recently been beautifully measured by experiments BABAR and BELLE at the B factories These are machines (in the US and Japan) running on the (4 S) resonance: e+e (4 S) B 0 B 0 or B+B • The CP asymmetry A(t) = G(B 0 J/ KS) + G(B 0 J/ KS) A(t) = sin 2 b sin Dm t in the Standard Model • BABAR + BELLE measure sin 2 b = 0. 674 ± 0. 026 • This can be compared with the indirect measurement from other constraints on the Unitarity Triangle Roger Forty Physics challenges of the LHC (III) 9

Triumphant agreement! The Standard Model description of CP violation appears to be correct (at least to the level it has so far been tested) Roger Forty Physics challenges of the LHC (III) 10

2. The LHCb experiment • Cross section for bb production at 14 Te. V: bb ~ 500 mb Enormous production rate at LHCb: ~ 1012 bb pairs per year! much higher statistics than the current B factories • However, bb < 1% of inelastic cross-section more background from non-b events challenging trigger and high energy more primary tracks, flavour tagging more difficult • Expect ~ 200, 000 reconstructed B 0 J/ KS events/year cf current B-factory samples of ~ 4000 events precision on sin 2 b ~ 0. 02 in one year for LHCb (similar to current world average precision) • But in addition, all b-hadron species are produced: B 0, B+, Bs, Bc , Lb … In particular can study the Bs (bs) system, inaccessible at the B factories • ATLAS and CMS are also planning to do B physics but will only have a lepton trigger, and poor hadron identification Roger Forty Physics challenges of the LHC (III) 11

bb events b and b quarks are produced in pairs (mostly in the forward direction) Correlation between the b and b production angle (PYTHIA simulation) • Need to measure proper time of B decay: t m. B L / pc hence decay length L (~ 1 cm in LHCb) and momentum p from decay products (which have ~ 1– 100 Ge. V) • Also need to tag production state of B: whether it was B or B Use charge of lepton or kaon from decay of the other b hadron in the event Roger Forty Physics challenges of the LHC (III) 12

Avoiding pileup • As discussed earlier, at the nominal LHC luminosity of 1034 cm-2 s-1 there are ~ 25 inelastic pp interactions per bunch crossing (every 25 ns) • The superimposed events can mimic the signature of B hadrons: tracks offset from the production vertex • LHCb chooses to run at few × 1032 cm-2 s-1 dominated by single interactions: Inelastic pp collisions/crossing LHCb • Makes it simpler to identify B decays from their vertex structure and will also reduce the radiation dose (which is an issue in the forward region) • Beams are defocussed locally for LHCb can maintain optimal luminosity even when ATLAS/CMS run at 1034 cm-2 s-1 Roger Forty Physics challenges of the LHC (III) 13

LHCb in its cavern Shielding wall (against radiation) Offset interaction point (to make best use of existing cavern) Electronics + CPU farm Detectors can be moved away from beam-line for access Roger Forty Physics challenges of the LHC (III) 14

LHCb detector ~ 300 mrad p p 10 mrad Forward spectrometer (running in pp collider mode) Inner acceptance 10 mrad from conical beryllium beam pipe Roger Forty Physics challenges of the LHC (III) 15

LHCb detector Vertex locator around the interaction region Silicon strip detector with ~ 30 mm impact-parameter resolution Roger Forty Physics challenges of the LHC (III) 16

LHCb detector Tracking system and dipole magnet to measure angles and momenta Dp/p ~ 0. 4 %, mass resolution ~ 14 Me. V (for Bs Ds. K) Roger Forty Physics challenges of the LHC (III) 17

LHCb detector Two RICH detectors for charged hadron identification (discussed in more detail below) Roger Forty Physics challenges of the LHC (III) 18

LHCb detector e h Calorimeter system to identify electrons, hadrons and neutrals Important for the first level of the trigger Roger Forty Physics challenges of the LHC (III) 19

LHCb detector m Muon system to identify muons, also used in first level of trigger Roger Forty Physics challenges of the LHC (III) 20

Vertex detector • Vertex detector has silicon microstrips with rf geometry approaches to 8 mm from beam (inside complex secondary vacuum system) • Gives excellent proper time resolution of ~ 40 fs (important for Bs decays) Beam Vertex detector information is used in the trigger Roger Forty Physics challenges of the LHC (III) 21

LHCb trigger 40 MHz L 0, HLT and L 0×HLT efficiency Detector L 0: high p. T (m, e, g, h) [hardware, 4 ms] 1 MHz HLT: high IP, high p. T tracks [software] then full reconstruction of event 2 k. Hz HLT rate Storage (event size ~ 50 k. B) Event type Calibration Physics 200 Hz Exclusive B candidates Tagging B (core program) 600 Hz High mass di-muons Tracking J/ , b J/ X (unbiased) 300 Hz D* candidates PID Charm (mixing & CPV) 900 Hz Inclusive b (e. g. b m) Trigger B (data mining) Roger Forty Physics challenges of the LHC (III) 22

3. Hadron identification • RICH detectors are the specialized detectors mentioned earlier that allow charged hadrons (p, K, p) to be identified RICH detector Vertex locator • Important for B physics, where there are many hadronic decay modes eg Bs 0 → Ds K+ → K+ K p K+ • Since ~ 7 more p than K are produced in pp events, making the mass combinations would give rise to large combinatorial background unless K and p tracks can be separated • RICH = Ring Imaging CHerenkov Roger Forty Physics challenges of the LHC (III) 23

Cherenkov light • Radiation produced when a charged particle travels faster than the speed of light in the medium it is passing through (refractive index n v = c/n) P. Cherenkov received the Nobel Prize in 1958 for his study of this effect • Like the bow wave of a boat travelling over a lake with speed greater than the wave velocity • Light produced in a cone with cos q. C = 1/b n Can be detected as a ring image if the detector is far from radiator, or if the light is focussed By measuring q. C ( radius of ring) the velocity b of the particle is found Then with knowledge of its momentum the mass of the particle can be found Roger Forty Physics challenges of the LHC (III) 24

Photon detectors • Need to detect single photons on the rings 80 mm • Novel photon detector developed for the RICH detector system of LHCb • The Hybrid Photon Detector (HPD) combines a traditional vacuum phototube with a pixellated silicon anode 1000 pixels Test-beam image of Cherenkov rings from 50 Ge. V e + p beam Roger Forty Physics challenges of the LHC (III) 25

The LHCb RICH system uses three different radiator materials: Cross section of one of the detectors Typical event: complex pattern recognition! Roger Forty Physics challenges of the LHC (III) 26

Hadron ID performance • Performance of the p/K separation determined using simulated events • Example of how the RICH information can help to isolate B 0 → p+ p decays: Roger Forty Physics challenges of the LHC (III) 27

4. Expected results • Example of an early physics measurement that is expected from LHCb: Measurement of Bs–Bs oscillations Use channel Bs Ds p+ • Plot made for one year of data 80, 000 selected events for Dms = 20 ps-1 (SM preferred) Proper time distribution for events produced as Bs (rather than Bs) • Need to take care of flavour tagging, proper-time resolution, background rejection and acceptance correction • Can measure frequency accurately cf recent result Dms = 17. 8 ± 0. 1 ps-1 [CDF] Next step: measure the phase of the oscillation, using Bs J/ f decays (Bs counterpart of B 0 J/ KS), cleanly predicted in the SM: fs = 0. 04 Roger Forty Physics challenges of the LHC (III) 28

Unitarity Triangle • Constraints on the Unitarity Triangle that can be expected after ~ 5 years of LHCb data (10 fb-1), if all measurements agree with the Standard Model: • Accurate measurement (to a few degrees) of the CP angle g from Bs Ds±K±, B 0 DK etc • Angle a from B 0 p+p p 0 • In addition, phase of Bs oscillation fs measured to ± 0. 01, i. e. precisely enough to see SM value and therefore any new physics enhancements Roger Forty Physics challenges of the LHC (III) 29

Penguin decays • These are another category of decays involving loop diagrams New particles might appear in those loops • Some indication from the B factory experiments that their results for penguin decays do not agree with expectations might be a hint of new physics? Experiment Theory • LHCb should reach a precision of ± 0. 04 on the asymmetry of Bs ff Roger Forty Physics challenges of the LHC (III) 30

Rare decays • Profit from the enormous statistics to search for very rare decays such as Bs m+m Branching ratio ~ 3 10 -9 in the Standard Model • BR can be strongly enhanced in SUSY [G. Kane et al, hep-ph/0310042] • LHCb can reach the SM prediction in a few years BR (x 10 -9) SUSY models LHCb 5 SM prediction 3 Integrated Luminosity (fb-1) Roger Forty Physics challenges of the LHC (III) 31

Current status in the LHCb cavern: the experiment is almost complete! Roger Forty Physics challenges of the LHC (III) 32

Summary (Part III) • The mechanism of baryogenesis is one of the open issues in physics It requires CP violation, which is present in the Standard Model • B physics has been a fertile field for checking the Standard Model picture of CP violation, both via constraining the Unitarity Triangle and now by direct measurements • The LHCb experiment is nearing completion, to take these studies to the next level of precision, and to extend them to other B hadron systems It includes RICH detectors that will allow charged hadrons to be identified over a large momentum range • The search for New Physics at LHCb is complementary to ATLAS/CMS, by the precision study of the influence of new particles in loop diagrams • We are eagerly awaiting the first LHC collisions at the end of this year, and the first physics run in 2008 I hope that some of you will join us! Roger Forty Physics challenges of the LHC (III) 33